1995_revievghfjmvhgfw_PEO-PPO-PEO Block Copolymer Surfactants in Aqueous Solutions and at Interfaces

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C L LS A

AND

Colloids and Surfaces SURFACES

LS VI R A: Physicochemical and Engineering Aspects 96(1995) I 46

e v i ew

Poly ethy lene ox ide)-poly pro py lene oxide ) -poly ethylene oxide)

block copolymer surfactants in aqueous solutions and at interfaces:

thermodynam ics, structure, dynam ics, and m ode ling

Paschalis Alexandridis, T. Alan Hat ton

Department ~?/Chemical Engineering, Massachusetts Institute of Teclmolo~y, 77 Massachusetts At:emte, Camhrid:,,e,

MA 02139,

USA

Received 18 July 1994: accepted 17 September 1994

bstract

The association properties of poly(ethylene oxide)-b/ock-poly(propylene oxidel block-poly(ethylene oxidel IPEO

PPO PEOI copolymers (commercially available as Poloxamers and Pluronics) in aqueous solutions, and the adsorpt ion

of these copolymers at interfaces are reviewed. At low temperatures and/'or concentrations the PEO PPO PEO

copolymers exist in solution as individual coils (unimers). Thermodynami cally stable micetles are formed with increasing

copolymer conce ntrat ion and/or solution temperature, as revealed by surface tension, light scattering, and dye

solnbili zation experiments. The unimer-to-micelle trans ition is not sharp, but spans a concentr ation decade o1 10 K.

The critical micellization concen trat ion (CMC) and temperature ( CMTt decrease with an increase in the copolymer

PPO content or molecular weight. The dependence of CMC on temperature, together with differential scanning

calorimetry experiments, indicates that the micellization process of PE O- PP O PEO copolymers in water is endothermic

and driven by a decrease in the polarity of ethylene oxide (EO) and propylene oxide (PO) segments as the temperature

increases, and by the ent ropy gain in water when unimers aggregate to form micelles (hyd rophobic effect I. The free

energy and enthalpy of micellization can be correlated to the total numb er of EO and PO segments in the copolymer

and its molecular weight. The micelles have hyd rodyna mic radii of approximately 10 nm and aggregation numbers in

the order of 50. The aggregation number is thought to be independent of the copolymer concentration and to increase

with temperature. Phenomenological and mean-field lattice models for the formation of micetles can describe

qualitatively the trends observed experimentally. In addition, the lattice models can provide information on the

distr ibution of the EO and PO segments in the micelle. The PEO PPO-- PE() copolymers adsorb on both air water

and solid water interfaces: the PPO block is located at the interface while the PEO block extends into the solution,

when copolymers are adsor bed at hydrophobic interfaces. Gels are formed by certain PEO P PO PEO block

copolymers at high concent rations, with the micelles rema ining apparent ly intact in the form of a 'crystal . -Vhe

gelation onset temperature and the thermal stability range of the gel increase with increasing PEO block length. A

comparison of PEO -P PO copolymers with PEO -P BO and P EO PS block copolymers and C~E/surfactants is made,

and selected applications of PEO PPO -P EO block copolymer solutions (such as solubilization of organics, protection

of microorganisms, and biomedical uses of micelles and gels) are presented.

Kevword,s: Block copolymer surfactants; Dynamics: Interfaces: Modeling: Structure: Thermodynamics

* Corresponding author.

092%7757:95 $29.00 ,c: 1995 Elsevier Science B.V. All rights reserved

S,S I)I 0927-7757( 94 )03028-6



2 P. Alexandridis, T.A . Hatton/Colloids Surfaces A: Physicochem. Eng . Aspects 96 1995) 1 46

1 . I n t r od u c t i on

Copolymers are synthesized by the simultaneous

polymerization of more than one type of monomer.

The result of such a synthesis is called a block

copolymer if the individual monomers occur as

blocks of various lengths in the copolymer mole-

cule. The different types of blocks within the copol-

ymer are usually incompatible with one another

and, as a consequence, block copolymers self

assemble in melts and in solutions. In the case of

amphiphilic copolymers in aqueous solutions, the

copolymers can assemble in microstructures that

resemble micelles formed by low-molecular-weight

surfactants.

Water-soluble triblock copolymers of poly(eth-

ylene oxide) (PEO) and poly(propylene oxide)

(PPO), often denoted PEO PP O- PE O or (EO),I

(PO)m(EO),2, are commercially available non-

ionic macromolecular surface active agents.

Variation of the copolymer composition (PPO/

PEO ratio) and molecular weight (PEO and PPO

block length) during synthesis leads to the

production of molecules with optimum properties

that meet the specific requirements in various areas

of technological significance. As a result, PEO

PPO PEO block copolymers are an important

class of surfactants and find widespread industrial

applicat ions in [-1,2] detergency, dispersion stabili-

zation, foaming, emulsification, lubrication, and

formulation of cosmetics [3,4] and inks [5,6], etc.,

along with more specialized applications in, for

example, pharmaceuticals (drug solubilization and

controlled release [7 10] and burn wound cover-

ing [11]), bioprocessing (protecting microorga-

nisms against mechanical damage [12-14]), and

separations (solubilization of organics in aqueous

solutions [15 17]). Commercial names for these

surfactants are Poloxamers (manufactured by ICI)

and Pluronics (manufactured by BASF).

2 . S y n t h e s i s n o m e n c l a t u r e a n d p h y s i c a l p r o p e r t ie s

The PEO-PPO-PEO triblock copolymers are

synthesized by the sequential addition of first pro-

pylene oxide (PO) and then ethylene oxide (EO)

to a low molecular weight water-soluble propylene

glycol, a poly(propylene oxide) oligomer (note tha t

propylene glycol changes from water soluble to

water insoluble as the molecular weight increases

beyond about 740). The oxyalkylation steps are

carried out in the presence of an alkaline catalyst,

generally sodium or potassium hydroxide. The

catalyst is then neutralized and removed from the

final product [1]. The equations representing the

two steps in the synthesis of the PE O- PP O- PE O

copolymers are shown in Fig. 1.

The Pluronic PEO-PPO PEO block copoly-

mers are available in a range of molecular weights

and PPO/PEO composition ratios [1,2,18]. The

Pluronic copolymers are presented in Fig. 2,

arranged in the so-called Pluronic grid [2,18].

The copolymers along the vertical lines have the

same PPO/PEO composition, while the copoly-

mers along the horizontal lines have PPO blocks

of the same length. The notation for the Pluronic

triblock copolymers starts with the letters L (for

liquid), P (for paste), or F (for flakes). The first

one or two numbers are indicative of the molecular

weight of the PPO block, and the last number

signifies the weight fraction of the PEO block. For

example, Pluronics P104 and F108 have the same

molecular weight of PPO (in the order of 3000),

but P104 has 40 wt.% PEO and F108 80 wt.%

PEO. The notation of Fig. 2 will be used in the

ensuing discussion.

Addit ion of PO to form the PPO middle block

H O 'C H -C H 2" O HI + ( m I ) C H 3 C H ? H 2

C H 3 O

HO[-ClH-CH2-O-]rnH

CH 3

Addit ion of EO to form the PEO s ide block

HO[-CH-CH2-O-]mH + (2n)CH2-CH 2

\ /

OH 3 O

HO[-CH2-CH2-O-]n[-CH-CH2-O-]m[-CH2-CH2-O-]nH

CH 3

Fig. 1 . Equ at ions represent ing the synthes is of the P E O

P P O P E O c o p o l y m e r s .



P . A l e x a n d r i d i s , T . A . H a t t o n / C o l l o i d s

> ; u r /u c e s

A : P h v s i c o c h e m . E n g .

A,v~ects

9 6 ( 1 9 9 5 ) 1 4 6 3

4 ~

36 5

3 2 5 0 ~ ' - - - - - -

~ , 2 7 5 , 0

o 2 2 5 0

2 0 5 1 3 1 - -

~ 1 7 5 0

E

E

t .~_

c ~

>

~ 2 0 0

9 5 0 ' - - - - - - - -

° /o ~ 0

- - - 4 p 85 't

J

i , ~ / /

I

_ _ _ _

1 0 2 0 3 0 q O 5 0 6 0 7 0 8 0

Percen tage o f e thy len e ox ide in a mo lecu le - the P lm on ic grM

Fig. 2 . P luronic PE O -P PO PE O copo lymers arranged in the Pluronic grid . The copolym ers ahm g the vert ical lines have t h e

s a m e P P O , / P E O c o m p o s i t io n r a ti o, w h i l e t h e c o p o ly m e r s a h m g t h e h o r i z o n t a l li ne s

h a v e P P O b l o ck s o f

the sa me length. I Reprinted

with permission from

R e f, [ 2 ] ; c o p y r i g h t C a r l

Hanser Verlag,

1991.

S o m e o f t h e p h y s i c a l p r o p e r ti e s ( av e r a g e m o l e c u -

l a r w e i g h t , m e l t i n g / p o u r p o i n t , v i s c o s i t y , s u r f a c e

t e n s io n , fo a m h e i gh t , c l o u d p o i n t a n d h y d r o p h i l i ~

l ip o p h i li c b a la n c e ) o f t h e P l u r o n i c P E O - P P O -

P E O c o p o l y m e r s a r e li st e d i n T a b l e 1 ( d a t a s u p -

p l ie d b y t h e m a n u f a c t u r e r [ 1 8 ] ) . T h e c l o u d p o i n t ,

t h e te m p e r a t u r e a t w h i c h t h e c o p o l y m e r s p h a s e

s e p a r a t e f r o m w a t e r , r a n g e s f r o m a p p r o x -

i m a t e l y 1 0 C f or c o p o l y m e r s w i t h l o w P E O

c o n t e n t to a b o v e 1 0 0 C f o r c o p o l y m e r s w i t h h i g h

P E O c o n t e n t. T h e P E O c o n t e n t a l so i n fl u en c e s

t h e r a t e o f d i s s o l u t i o n : t h i s r a t e d e c r e a s e s a s t h e

r e l a t iv e p r o p o r t i o n o f t h e P E O b l o c k i n c r e a s e s .

T h e d i s s o l u t i o n r a t e a l s o d e c r e a s e s a s t h e c o -

p o l y m e r m o l e c u l a r w e i g h t i n c r e a s e s f o r P l u r o n i c

c o p o l y m e r g r o u p s w i t h t h e s a m e P P O / P E O c o rn -



4 P. Alexandridis , T .A . Ha t ton/Co l lo ids Sur fa ces A: Phys icochem. Eng. Asp ects 96 1995) 1 46

T a b l e 1

P r o p e rt i es o f t h e P l u r o n ic P E O - P P O - P E O c o p o l y m e r s

A B C D E F G H I

L 3 5 1 9 0 0 5 0 7 3 7 5 4 9 2 5 7 3 1 8 - 2 3

F 3 8 4 7 0 0 8 0 4 8 2 6 0 5 2 35 > 1 0 0 > 2 4

L 4 2 1 6 3 0 2 0 - 2 6 2 8 0 4 6 0 3 7 7 - 1 2

L 4 3 1 8 5 0 3 0 - 1 3 1 0 4 7 0 4 2 7 - 1 2

L 4 4 2 2 0 0 4 0 1 6 4 4 0 4 5 2 5 6 5 1 2 - 1 8

L 6 2 2 5 0 0 2 0 - 4 4 5 0 4 3 2 5 3 2 1 - 7

L 6 3 2 6 5 0 3 0 1 0 4 9 0 4 3 3 0 3 4 7 - 1 2

L 6 4 2 9 0 0 4 0 1 6 8 5 0 4 3 4 0 5 8 1 2 - 1 8

P 6 5 3 4 0 0 5 0 2 7 1 8 0 4 6 7 0 8 2 1 2 - 1 8

F 6 8 8 4 0 0 8 0 5 2 1 0 00 5 0 3 5 > 1 0 0 > 2 4

L 7 2 2 7 5 0 2 0 - 7 5 1 0 3 9 1 5 2 5 1 - 7

P 7 5 4 1 5 0 5 0 2 7 2 5 0 4 3 1 0 0 8 2 1 2 - 1 8

F 7 7 6 6 0 0 7 0 4 8 4 8 0 4 7 1 0 0 > 1 0 0 > 2 4

P 8 4 4 2 0 0 4 0 3 4 2 8 0 4 2 9 0 7 4 1 2 - 1 8

P 8 5 4 6 0 0 5 0 3 4 3 1 0 4 2 7 0 8 5 1 2 - 1 8

F 8 7 7 7 0 0 7 0 4 9 7 0 0 4 4 8 0 > 1 0 0 > 2 4

F 8 8 1 1 4 0 0 8 0 5 4 2 3 0 0 4 8 8 0 > 1 0 0 > 2 4

F 9 8 1 3 0 0 0 8 0 5 8 2 7 0 0 4 3 4 0 > 1 0 0 > 2 4

P 1 0 3 4 9 5 0 3 0 3 0 2 8 5 3 4 4 0 8 6 7 - 1 2

P 1 0 4 5 9 0 0 4 0 3 2 3 9 0 3 3 5 0 8 1 1 2 - 1 8

P I 0 5 6 5 0 0 5 0 3 5 7 5 0 3 9 4 0 9 1 1 2 - 1 8

F 1 0 8 1 4 6 0 0 8 0 5 7 2 8 0 0 4 1 4 0 > 1 0 0 > 2 4

L 1 2 2 5 0 0 0 2 0 2 0 1 7 5 0 3 3 2 0 1 9 1 - 7

P 1 2 3 5 7 5 0 3 0 3 1 3 5 0 3 4 4 5 9 0 7 - 1 2

F 1 2 7 1 2 6 0 0 7 0 5 6 3 1 0 0 4 1 4 0 > 1 0 0 1 8 - 2 3

A : c o p o l y m e r . B : a v e r a g e m o l e c u l a r w e i g h t . C : P E O w t . .

D : m e l t i n g p o u r p o i n t (' ~C ) . E : v i s c o s i t y ( B r o o k f i e l d ) ( c p s ;

l i q u i d s a t 2 5 ~ C , p a s t e s a t 6 0 '~ C , s o l i d s a t 7 7 : C ) . F : s u r f a c e

t e n s i o n a t 0 .1 , 2 5 '~ C ( d y n c m ~ ). G : f o a m h e i g h t ( m m ) ( R o s s

M i l e s , 0 . 1 a t 5 0 : C ) . H : c l o u d p o i n t i n a q u e o u s 1 s o l u t i o n

( : C ) . l : H L B ( h y d r o p h i l i c - l i p o p h i l i c b a l a n c e ) .

posit ion ratio [- 1]. This is probably a result of the

degree of hydrogen bonding between the copoly-

mer molecules, and is also reflected in the physical

form of the copolymers liquid for low molecular

weight, low PEO content; solid for high molecular

weight, high PEO content copolymers). The PEO

PPO-PEO copolymers exhibit maximum foam

height at a PP O/ PEO ratio of 40 : 60; foam proper-

ties of each copolymer series increase and then

decrease slightly as the molecular weight of the

PPO segment increases; copolymers with high

PPO content are effective defoamers [, 1]. Attempts

to correlate the emulsification properties with

PPO/PEO ratio and PPO molecular weight have

not been very successful; copolymers with PPO

blocks of high molecular weight are generally

better emulsifiers than the lower-molecular-weight

homologs [ 1 ]. The thickening power of each series

of copolymers increases as the PP O block molecu-

lar weight increases and as the PPO/PEO ratio

decreases.

3 . M i c e ll e f o rm a t io n in P E O P P O P E O b lo ck

copolym er aqueous solutions

3 1 T e c h ni q ue s f o r d e te c ti n g C M C a n d C M T

The critical micellization concentration CMC),

the amphiphile concentration at which micelles

thermodynamically stable polymolecular aggre-

gates) start forming, is a parameter of great fun-

damental value [19,20]. The micellization of

amphiphilic block copolymers is inherently more

complex than that of conventional, low-molecular-

weight surfactants. The composition polydispersity

could be appreciable even for a copolymer with a

narrow molecular weight distribution and, accord-

ingly, no sharp CMC or CMT critical micell-

ization temperature, the copolymer solution tem-

perature at which micelles form) have been

observed for block copolymers. In practice, a cer-

tain CMC range with some notable uncertainty is

usually detected. A large difference is often noted

between the CMC values determined by different

methods because the sensitivity of the techniques

to the quantity of molecularly dispersed copoly-

mers unimers) present in solution may vary [21],

For common surfactants, a considerable amount

of CMC data has been collected and summarized

by Mukerjee and Mysels [,22], whereas for block

copolymers only scarce CMC data have been

available in the literature so far see, for example,

Refs. [-23] and [-24]), Further, the values reported

in the literature differ substantially: the lack of

sufficient temperature control, in conjunction with

batch-to-batch variations, may be responsible for

the observed variations [25].

Light scatter ing and fluorescence spectroscopy

experiments have shown that PEO-PPO-PEO

block copolymers of suitable PPO/PEO composi-

tion and molecular weight do indeed form polymo-

lecular aggregates in solution. As discussed below,

the CMC of aqueous PEO PPO PEO copolymer



P . . 4 1 e . w m d r i d is . T . A . H a t t o n / C o l h f i d ~ S u d ~ t c e s A : Phvs i c ochem. Eng . ,4.~pect.v 96 17 95 ) 1 4:~

s o l u t i o n s d e c r e a s e s w i t h i n c r e a s i n g t e m p e r a t u r e

[ 2 1 , 2 6 - 2 8 ] . M i c e l l a r g r o w t h w a s o b s e r v e d f o r

P l u r o n i c L 6 4 w i t h a n i n c r e a s e i n t h e c o p o l y m e r

c o n c e n t r a t i o n a t 2 5 < C [ 2 9 ] ; m i c e ll e f o r m a t i o n b y

P l u r o n i c P 8 5 w a s i n v e s t i g a t e d b y B r o w n e t a l .

[ 2 7 ] u s i n g s t a t i c a n d d y n a m i c l i g h t s c a t t e r i n g . I t

w a s f o u n d t h a t u n i m e r s , m i c e l l e s , a n d " ' m i c e l l a r

a g g r e g a t e s " c o e x i st e d ; t h e r e la t iv e p r o p o r t i o n s o f

e a c h s p e c i e s d e p e n d e d s t r o n g l y o n t e m p e r a t u r e

a n d c o p o l y m e r c o n c e n t ra t io n . M i c e ll a r g ro w t h

o c c u r r e d w i t h i n c r e a s i n g t e m p e r a t u r e a s e v i d e n c e d

w h e n t h e s c a t t e r i n g i n t e n s i t y o f a 4 .8 % P 8 5 s o l u t i o n

w a s r e c o r d e d a s a f u n c t i o n o f t e m p e r a t u r e ; a s h a r p

i n c r e a s e i n t h e s c a t t e r i n g i n t e n s i t y a t a p p r o x i m a t e l y

3 0 ' C d e n o t e d t h e f o r m a t i o n o f m i c e ll e s. A s ig n i fi -

c a n t t e m p e r a t u r e d e p e n d e n c e w a s al s o o b s e r v e d i n

t h e m i c e l li z a ti o n b e h a v i o r o f P l u r o n i c I :' 6 8 [ 2 1 ] .

U s i n g s t a t i c a n d d y n a m i c l i g h t s c a t t e r i n g , Z h o u

a n d C h u [ 2 1 ] d e t e c t e d t h r e e t e m p e r a t u r e r e g io n s ,

w h i c h t h e y c a l l e d t h e u n i m e r , t r a n s i t i o n , a n d

m i c e l le r e g i o n s , re s p e c t i v e ly . A t r o o m t e m p e r a t u r e ,

" p a r t ic l e s " w i t h a h y d r o d y n a m i c r ad i u s o f a r o u n d

2 .3 n m a n d a b r o a d p o l y d i s p e r s i t y w e r e d e t e c t e d .

A b o v e 5 0 C , m i c e l le m o l e c u l a r w e i g h t s w e r e

f o u n d t o i n c r e a s e l i n e a r l y w i t h t e m p e r a t u r e , w h i l e

t h e h y d r o d y n a m i c r a d i u s re m a i n e d c o n s t a n t , a t

a p p r o x i m a t e l y 8 n m [ 2 1 ] . W a n k a e t a l. [- 30 ] co n -

f i rm e d t h e f in d i n g o f Z h o u a n d C h u , t h a t a g g r e g a -

t i o n n u m b e r s i n c r e a s e w i t h t e m p e r a t u r e , w h i l e t h e

m i c e l l e r a d i u s r e m a i n s a p p r o x i m a t e l y c o n s t a n t .

M o r e i n f o r m a t i o n o n P l u r o n i c c o p o l y m e r m i c el le

s iz e a n d a g g r e g a t i o n n u m b e r s , d e t e r m i n e d b y s c a t-

t e r i n g t e c h n i q u e s , i s p r e s e n t e d i n S e c t i o n 5 . 1 .

S p e c t r o s c o p i c te c h n i q u e s , b a s e d e i t h e r o n o p t i c a l

a b s o r p t i o n o r o n e m i s s io n o f l ig h t fr o m s o m e

" ' p r o b e " m o l e c u l e , a r e n o w w e l l e s t a b l i s h e d f o r

i n v e s t i g a t i n g a w i d e r a n g e o f p h y s i c a l p r o p e r t i e s

o f m i c e l l a r s o l u t i o n s [ 1 9,3 1 ] . F l u o r e s c e n c e p r o b e s

w e r e e m p l o y e d b y T u r r o a n d C h u n g [ 3 2 ] i n

s t u d y i n g t h e b e h a v i o r o f a p o l y ( e t h y l e n e o x i d e) -

p o l y ( p r o p y l e n e o x i d e ) b l o c k c o p o l y m e r i n w a t e r .

T h e p o l y m e r th e y u s e d h a d a t o ta l m o l e c u l a r

w e i g h t o f 3 0 00 , w i t h a P E O / P P O m o l a r r a ti o o f

0 .8 ; i t wa s no t spe c i f i e d i n t he pa pe r whe the r i t

w a s a d i - o r t ri b l o c k c o p o l y m e r . R e g i o n s o f n o

a g g r e g a t i o n , " m o n o m o l e c u l a r ' " m i c el le s ( a t t r ib u t e d

t o a c h a n g e i n t h e c o p o l y m e r se g m e n t c o n f o r m a -

t i o n t o a m o r e c o m p a c t s t r u c t u r e ) , a n d p o l y -

m o l e c u l a r m i c e l l e s w e r e i d e n t i f i e d w i t h i n c r e a s i n g

p o l y m e r c o n c e n t r a ti o n . T h e a g g r e g a t io n n u m b e r

o f t h e m i c el le s f o r m e d a t h i g h p o l y m e r c o n c e n -

t r a t i o n w a s d e t e r m i n e d t o b e 5 2 . W i t h a n i n c r e a s e

i n te m p e r a t u r e , a h y d r o p h o b i c f l u o r e s c e n ce p r o b e

( e x p e c t e d t o r e s id e i n t h e m i c e l le c o r e ) e x p e r i e n c e d

a m o r e h y d r o p h i l i c e n v i r o m n e n t , w h e r e a s a h y d r o -

p h i l i c p r o b e ( e x p e c t e d t o r e s i d e i n t h e i n t e r f a c i a l

r e g i o n ) e x p e r i e n c e d a m o r e h y d r o p h o b i c e n v i r o n -

m e n t . T h i s o b s e r v a t i o n w a s a t t r i b u t e d t o a g r e a te r

m i x i n g o f Pl - O c h a i n s i n th e h y d r o p h o b i c m i c e l le

c o r e a t h i g h e r t e m p e r a t u r e s o w i n g t o i n c r e a s e d

t h e r m a l a g i t a t i o n .

A h y d r o p h o b i c f l u o r e s c en c e p r o b e ( d i p h e n y l h e x -

a t r i e n e , D P H ) w a s u s e d b y A l e x a n d r i d i s e t a l. [ 2 8 ]

t o d e t e r m i n e t h e o n s e t o f m i c e l l i z a ti o n f o r a

n u m b e r o f P l u r o n i c b l o c k c o p o l y m e r s

i n

a q u e o u s

s o l u t i o n s . T h e f l u o r e s c e n c e ef f ic i e n c y o f D P H is

z e r o

i r i

a h y d r o p h i l i c e n v i r o n m e n t a n d u n i t y in

a

h y d r o p h o b i c e n v i r o n m e n t ( s u ch a s t h e c o r e o f a

m i c e l l e ) , t h u s p r o v i d i n g a s e n s i t i v e i n d i c a t o r o f

m i c e l l e f o r m a t i o n w i t h i n c r e a s i n g s o l u t i o n t e m p e r -

a t u re a n d c o p o l y m e r c o n c e n t r at i o n . C M C a n d

C M T d a t a w e r e o b t a i n e d a n d c o r r e l a t e d t o th e

P l u r o n i c m o l e c u l a r w e ig h t a n d P P O ' P E O r a ti o

[ 2 8 ] . F i g. 3 s h o w s t h e i n t e n s i t y o f a b s o r b e d l ig h t

( a r b i t r a r y u n i t s ) d u e t o D P H a s a f u n c t i o n o f

t e m p e r a t u r e [ o r a q u e o u s P h i r o n i c

P 1 0 4

s o l u t i o n s

g

03

g

E

o

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O..

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/ / ~ / / , / o , , , ' " 0 2 5 ~< ~ • 0 ' :' ,~

/ . 5 mt

,,,,, , ,,

.

0 05o,, j

/ , j ' . / +

/ ' 1 T ' /

/ , ' / , " / ' . o o ~ 5 ~ : ' :

i / /% I / ' : ~ i { ) 0 1 r,~

5 . S e t . / / ,' J , ....

2 5 = ~ x / . . I / . A / . / ~ / - - i ~ , -

• ~ " ~ ' < ~ , ~ : : - 0 ,0 0 5% 'i

i i i i L L . . . . Z . . . . .

20 25 30 35 40 45 50

5

T e m p e r a t u r e ( C )

F i g . 3 . L i g h t a b s o r p t i o n h l r b i t r a r y u n i t s l d u c t o D P l t a s a

f u n c t i o n o f l e m p e r a t u r e f o r a q u e o u s P l u r o n i c P I 0 4 s o l u t i o n s

a t v a r i o u s c o p o l y m e r c o n c e n t r a t i o n s : t h e c ri t ic a l m i c e l l i z a ti o n

t e m p e r a t u r e v a l u e s w e r e o b t a i n e d l 'r o m th e f ir s t b r e a k i n th e

s i g m o i d a l c u r v e s . ( A d a p t e d w i t h p e r m i s s i o n t 'r o m R e [. [ 2 8 ] :

c o p ) r i g h t A m e r i c a n C h e m i c a l S o c i e t y , 1~ )9 4.1



6 P. Alexandridis , T .A . Ha t ton/Co l lo ids Sur fac es A: Phys icochem. Eng. Asp ects 96 1995) 1 46

at various copolymer concentrations [28]; the

critical micelle temperature values were obtained

from the first break in the intensity vs. temperature

sigmoidal curves. Critical micelle concentrations

can be obtained in a similar manner from intensity

vs. log(concentration) plots. Chattopadhyay and

London [33] used the fluorescence emission of

DPH to determine the CMC of various surfactants,

and obtained values tha t were within 10 of those

determined by other techniques. Surface tension

results showed that probe molecules, at the levels

used to determine the CMC by fluorescence (probe

concentration < 10 6 kmol m-3), did not notice-

ably affect the surfactant properties, notably sur-

face tension, nor the surface tension-derived CMC

value [34]. Edwards et al. [35] reported that, in

general, the CMC values they obtained from the

surface tension data for solutions of deionized

water and non-ionic surfactants showed fairly close

agreement with the CMC values inferred from

polyaromatic hydrocarbon solubilization data.

Similar values for the CMC of the PEO-

PPO -P EO block copolymers measured by surface

tension and dye solubilization methods have been

reported by Schmolka and Raymond [36]. The

use of surface tension measurements for the deter-

mination of CMC in PEO PPO PEO copolymer

solutions is discussed in detail in Section 7.1.

Alexandridis et al. [37] reported the effects of

temperature on the micellization properties and

the structure of the micelles for Pluronics P104

and F108, PEO-PPO-PEO block copolymers

having similar size hydrophobic (PPO) block and

different size hydrophilic (PEO) blocks. CMC and

CMT data for aqueous copolymer solutions were

obtained from a dye solubilization method [28]

and corroborated with differential scanning calo-

rimetry (DSC), surface tension, density, light scat-

tering intensity, and fluorescence spectroscopy

experiments. Best agreement between the different

techniques was achieved when the critical micelli-

zation data were estimated from (i) the onset of

solubilization of a hydrophobic dye, (ii) the onset

of the endothermic transition observed in DSC,

(iii) the high-concentration break in the surface

tension vs. polymer concentration curve, (iv) the

first break in the partial specific volume vs. temper-

ature curve, (v) the onset of increased light scatter-

ing intensity, and (vi) the inflexion in the pyrene

fluorescence emission intensity 1 1 / 1 3 ratio (see

Section 5.2.) vs. temperature sigmoidal curve [37].

These guidelines should facilitate the reporting of

consistent CMC and CMT data for amphiphilic

copolymer solutions.

3 2 Ef fe ct o f copolymer com pos i t ion and molecular

we igh t on m ice ll e fo r m a t ion

It is apparent from the experimental studies that

PEO-PPO-PEO copolymers which are relatively

less hydrophobic, either due to a high PEO content

or a low molecular weight, do not form micelles

at room temperature, but do start to aggregate at

higher temperatures. This can be explained by the

fact that water becomes a poorer solvent for both

ethylene oxide and propylene oxide segments at

higher temperatures. As shown by Alexandridis

et al. [28], the micellization process is strongly

driven by entropy, and the free energy of micelliza-

tion is mainly a function of the PPO block, the

hydrophobic part of the copolymer.

For groups of PEO-PPO PEO copolymers

with PEO blocks of constant molecular weight

and PPO blocks of varying molecular weight, the

CMC values for the Pluronic solutions (at a given

solution temperature) decreased with increasing

number of PO segments, indicating that polymers

with a larger hydrophobic (PPO) domain form

micelles at lower concentrations [28]. Higher tem-

peratures resulted in lower CMC values, with the

slope of the log(CMC) vs. PO-segment-number

curve increasing with temperature. The CMT

values for the Pluronic copolymer solutions (at a

given copolymer concentration) also decreased as

a function of the number of PO segments, indicat-

ing that polymers with a larger hydrophobic

domain form micelles at lower temperatures. The

CMC and CMT values for groups of Pluronic

copolymers with hydrophobic blocks of constant

length and hydrophilic blocks of varying length,

e.g. with 30 (L64, P65, F68), 40 (P84, P85, F88),

and 60 (P104, P104, F108) PO segments, showed

a small increase in the CMC and CMT with

increasing number of EO segments. This would

indicate that micelle formation becomes more

difficult the more hydrophilic the molecules,



P. Alexandridis , T .A . Hat ton/ Col lo id~ Sur / iw es A: Phy,~icochem. En~ . A,~pects 96 (19 95 ) 1 46 7

I.--

: s

o

60 i

5 5 5 0 %

P E O

5 0 ) 0 1 %

4 5 , ~

4 0 01 '~k

3 5 \

1 0 % - -~ Q

3 0

o 1

25 40 \

20 P E O ~ k 1 0 ~,;

%

50 1 O 0

0 . 1 %

1 0 % %

100:o ~.. ..

I

1 5 0

8 0 % P E O

\

\

•

" ~

x \ \ \ •

\.j,

,

- - •

2 0 0 2 5 0

N u m ber o f E O s eg m en t s , N EO

3 0 0

Fig. 4. Critical micellization temperatures at various PEO

PPO PgO copolymer concentrations, as a function of the

number of EO segments, for copolymers with 40, 50, and 80

PEO content. IReprinted with permission from Ref.[28]:

copyright American Chemical Society, 19943

a l t h o u g h t h e ef fe ct o f P E O o n t h e C M C a n d C M T

is le ss p r o n o u n c e d t h a n t h a t o f P P O , i n t i m a t i n g

t h a t P P O is a p r i m a r y f a c t o r in t h e m i c e l li z a ti o n

p r oc e ss . T h e C M C a n d C M T v a lu e s f o r c o p o l y -

m e rs o f c o n s ta n t P P O / P E O c o m p o s i t i o n r at io

d e c r e a s e d w i t h i n c r e a s i n g m o l e c u l a r w e i g h t . T h e

C M T v a l u e s w e re in f l u e n c e d m o r e b y m o l e c u l a r

w e i g h t t h e l o w e r t h e r e l a t i v e P E O c o n t e n t a n d t h e

l o w e r t h e c o p o l y m e r c o n c e n t r a t i o n , a s i s e v i d e n c e d

b y t h e s t e e p e r s l o p e s f o r t h e c u r v e s f o r t h e s e

c o n d i t i o n s . T h e s e t r e n d s a r e s h o w n i n F ig . 4 [ 2 8 ] .

4 M i ce l li za t io n therm o dy na m i cs

4.1. Asso c iat io n model , d f ferent ial scan ning

c a lor ime t ry

I f i s w e l l e s t a b l i s h e d t h a t b l o c k c o p o l y m e r s o f

the A B or A B A typ e f o r m m ic e l l e s i n se l e c ti ve

s o lv e n ts w h i c h a r e t h e r m o d y n a m i c a l l y g o o d

s o l v e n l s f o r o n e b l o c k a n d p r e c i p i t a n t s f o r t h e

o t h e r , i n g e n e r a l , m i c e l l i z a t i o n o f b l o c k c o p o l y -

m e r s , a s in t h e c a s e o f c o n v e n t i o n a l s u r f a c t a n t s ,

o b e y s t h e c l o s e d a s s o c i a ti o n m o d e l , w h i c h a s s u m e s

a n e q u i l i b r i u m b e t w e e n m o l e c u l a r l y d i s p e r s e d

c o p o l y m e r ( u n i m e r ) a n d m u l t i m o l e c u l a r a g g r e g a t es

( m ic el le s) [2 4 , 3 8 ] . T h e r e a r e t w o m a i n a p p r o a c h e s

t o t h e t h e r m o d y n a m i c a n a l y s is o f t h e m i c e l li z a ti o n

p r o c e s s : t h e p h a s e s e p a r a t i o n m o d e l , i n w h i c h t h e

m i c e l l e s a r e c o n s i d e r e d t o f o r m a s e p a r a t e p h a s e

a t th e C M C , a n d t h e m a s s - a c ti o n m o d e l t h a t

c o n s i d e r s m i c e l l e s a n d u n a s s o c i a t e d u n i m e r s t o b e

i n a n a s s o c i a t i o n - d i s s o c i a t i o n e q u i l i b r i u m [ 3 9 ] .

I n b o t h a p p r o a c h e s , t h e s t a n d a r d f re e e n e r g y

c h a n g e f o r th e t r a n s f e r o f 1 t o o l o f a m p h i p h i l e f r o m

s o l u t i o n t o t h e m i c e l la r p h a s e , A G ( t h e f r ee e n e r g y

o f m i c e l l iz a t i o n ) , in t h e a b s e n c e o f e l e c t r o s t a t i c

i n t er a c ti o n s ( th e P E O P P O P E O c o p o l y m e r s a re

non - ion i c ) i s g ive n b y [ 19 ,39]

A G = R T lnlXcM c) ( 1 )

w h e r e R i s t h e g a s la w c o n s t a n t , 7 is t h e a b s o l u t e

t e m p e r a t u r e , a n d X c M c is t h e c r i ti c a l m i c e l l i z a t io n

c o n c e n t r a t i o n i n m o l e f r a c t i o n u n i t s . I n o r d e r t o

o b t a i n t h i s s i m p l e e x p r e s s i o n , t h e a s s u m p t i o n h a s

b e e n m a d e t h a t t h e c o n c e n t r a t i o n o f f re e s u r f a c t a n t

( u n i m e r ) i n t h e p r e s e n c e o f m i c e ll e s is c o n s t a n t a n d

e q u a l t o t h e C M C v a l u e , i n t h e c a s e o f t h e p h a s e -

s e p a r a t i o n m o d e l , o r t h a t t h e m i c e ll e a g g r e g a t i o n

n u m b e r i s l a r g e f o r t h e m a s s - a c t i o n m o d e l .

A p p l y i n g t h e G i b b s H e l m h o l t z e q u a t i o n , w e c a n

e x p r e s s t h e s t a n d a r d e n t h a l p y o f m i c e l l i z a ti o n ,

A H ' , a s [ 1 9 , 3 9 ]

A H = - R T 2 [ ? l n (X c M c ) /T T ] p

= R [ ? l n ( X c M c ) / ? ( l / T ) ] e (2)

F i n a ll y , t h e s t a n d a r d e n t r o p y o f m i c e ll i za t i o n p e r

m o l e o f s u r f a c t a n t , A S ', c a n b e o b t a i n e d f r o m

A S = ( A H - A G ' ) /T ( 3)

I t h a s b e e n s h o w n f o r b lo c k c o p o l y m e r m i c e l li z a-

t i o n [ 4 0 ] t h a t , w i t h i n e x p e r i m e n t a l e r r o r ,

? l n ( X c M c ) / ( ( I / T ) = (~ ln(X ) : ? ( l /T c~ lr ) (41

w h e r e X i s t h e c o n c e n t r a t i o n e x p r e s s e d a s a m o l e

f r a c t i o n , a n d Tc Mr i s t he c r i t i c a l m ic e l l i z a t i on t e m -

p e r a t u r e ; t h u s E q . ( 2 ) b e c o m e s

AH = R [ ~ I n ( X l /? ( l /TcMr t ie {5 }

I n a c c or da nc e w i th E q . ( 5 ) , t he i nve r se T oM ,

v a l u e s a r e s h o w n a s a f u n c t i o n o f t h e l o g a r i t h m o f

c o p o l y m e r c o n c e n t r a t i o n ( m o l e f r a c t i o n u n i t s ) f o r

a n u m b e r o f P l u r o n i c c o p o l y m e r s in F i g . 5 [ 2 8 ] .

The To,M y d a t a , p l o t t e d i n t h i s m a n n e r , g e n e r a l l y

s h o w e d l i n e a r i t y . T h e u s e o f t h e c l o s e d a s s o c i a t i o n



8 P. Alexandridis, T. A. Hatton/Colloids Surfaces A: Physicochem. Eng. Aspe cts 96 1995) 1-46

0 . 0 0 3 5 ~ , ~ f

P , ~ 4

~ 0 . 0 0 3 3

o

0 . 0 0 3 2

0 . 0 0 3 1

F 8 8 F 6 8

0 . 0 03 I I L J

- 1 7 - 1 5 - 1 3 - 1 1 - 9 - 7

In C (mole fraction)

Fig. 5. R eciproc al TCMT VS. copolym er concentration plots for

various Pluronic PE O-P PO PEO copolym ers; he plots were

used for determination of the micellization enthalpy in terms

of the closed association model. (Reprinted with permission

from Ref. [-28]; copyrigh t American C hemical So ciety, 1994.)

m o d e l a p p e a r e d w e ll j u s t if i ed b a s e d o n t h e g o o d

fit; s u c h a m o d e l w a s c o n s e q u e n t l y e m p l o y e d f o r

e s t i m a t in g A H ° a n d A S ° o f t h e m i c e l l iz a t io n p r o -

c e s s [ 2 8 ] . N o t e t h a t t h e m a j o r a s s u m p t i o n

i n v o l v e d i n d e r i v in g E q s . ( 1 ) - ( 5 ) i s b a s e d o n t h e

m i c e l l e a g g r e g a t i o n n u m b e r ( N ) b e i n g s u f f i c i e n t l y

l a r g e t h a t t h e t e r m s R T N ~ I n N a n d

R T N - ~ In

Xmic w her e Xmic

i s t h e m o l e f r a c t i o n o f

m i c e l l e s ) c a n b e o m i t t e d f r o m t h e r i g h t - h a n d - s i d e

o f E q . (1 ) [ 1 9 ] . F o r a t y p i c a l a g g r e g a t i o n n u m b e r

o f 5 0, N 1 I n N ~ 0 .0 8 , in d e e d n e g l i g i b l e c o m p a r e d

to t he v a lue o f [ ln Xc M c [ ( i n t he r a ng e 10 15) .

U s i n g t h e s a m e a g g r e g a t i o n n u m b e r a n d c o n c e n -

t r a t i o n s X c M c = 6

x 10 -6

a n d

Xmi c=6 x 10 -1°

t h e

m a g n i t u d e o f t h e I N - 1 I n X m i¢ [ t e r m is 0 .4 , a p p r o x i -

m a t e l y 3 % o f th e [ln X cM c [ ( ~ 1 2 ) t e r m ; t h e t e m -

p e r a t u r e d e r i v a ti v e s o f t h e R T N - 1 1 n N a n d

R T N - 1 1 n X m i c t e r m s t h a t s h o u l d a p p e a r i n E q .

( 2 ) a r e a l so ne g l ig ib l e .

T h e s t a n d a r d e n t h a l p y o f m i c e l li z a ti o n ,

A H ~ ,

w a s c a l c u l a t e d f r o m t h e i n v e r s e s lo p e o f t h e l i n e a r

f i t t o t he 1/TcMT v s . l n ( m o l e f r a c t i o n ) d a t a , i n

a c c o r d a n c e w i t h E q . (5 ) . A H ° v a l u e s , t o g e t h e r w i t h

A G ° a n d A S ° ( c a l c u l a t e d f r o m E q s . ( 1 ) a n d ( 3 ) ,

r e s p e c t i v e l y , a t t h e c r i t i c a l m i c e l l i z a t i o n t e m p e r -

a t u r e f o r 1 % c o p o l y m e r s o l u t i o n s ) , a r e l i s t e d i n

R ef. [ 2 8 ] f o r v a r i o us P l u r o n i c P E O - P P O - P E O

c o p o l y m e r s . T h e t a b u l a t e d v a l u e s o f A H ' - , A G ° ,

a n d A S ~ r a n g e d f r o m 1 69 t o 3 3 9 k J m o 1 - 1 , - 2 4 . 5

t o - 2 8 . 8 k J m o 1 - 1 , a n d 0 . 6 3 8 t o 1 .2 44 k J t o o l - 1

K - 1 , r e sp e c ti v e ly . T h e h i g h e r v a l u e s o f A H °, A G ' 2

a n d A S ° w e r e o b s e r v e d f o r P l u r o n i c s P 1 0 3 a n d

L 6 4 , w h i l e t h e l o w e r v a l u e s w e r e o b s e r v e d f o r t h e

r e l a t i v e l y h y d r o p h i l i c P l u r o n i c c o p o l y m e r s F 6 8

a n d F 8 8 . T h e s t a n d a r d e n t h a l p y o f m i c e l li z a ti o n ,

A H ° , is p o s i t i v e , i n d i c a t i n g t h a t t h e t r a n s f e r o f

u n i m e r s f r o m s o l u t i o n t o t h e m i c e l le i s a n e n t h a l p i -

c a l l y u n f a v o r a b l e e n d o t h e r m i c p r o c e s s . T h e f r e e

e n e r g y , A G ° , i s n e g a t i v e , s i n ce t h e r m o d y n a m i c a l l y

s t a b l e m i c e l l e s a r e f o r m e d s p o n t a n e o u s l y . T h u s , i t

b e c o m e s c le a r t h a t a n e g a t i v e e n t r o p y c o n t r i b u t i o n

m u s t b e t h e d r i v i n g f o r c e f o r m i c e l l iz a t i o n o f t h e

b l o c k c o p o l y m e r s ; n o t e t h a t , i n c o n t r a s t t o t h e

e n t r o p y - d r i v e n m i c e l l i z a t i o n i n w a t e r , t h e m i c e l l i -

z a t i o n o f c o p o l y m e r s i n n o n - p o l a r s o l v e n t s o r ig i -

n a t e s f r o m e n t h a l p y i n t e r a c t i o n s b e t w e e n t h e

c o p o l y m e r se g m e n ts a n d t h e s o lv e n t [ 2 4 ] . T h e

t r a d i t i o n a l v i e w o f m i c e ll e f o r m a t i o n 1 -1 9,4 1] is

b a s e d o n t h e h y d r o p h o b i c e f fe c t [ 4 2 , 4 3 ] . T h e

p r e s e n c e o f h y d r o c a r b o n m o l e c u l e s in w a t e r c a u s e s

a s i g n i f i c a n t d e c r e a s e in t h e e n t r o p y o f t h e l a t te r ,

s u g g e s t i n g a n i n c r e a s e in t h e d e g r e e o f s t r u c t u r i n g

o f t h e w a t e r m o l e c u le s . W h e n h y d r o c a r b o n r e s i-

d u e s a g g r e g a t e i n a q u e o u s s o l u t i o n t o f o r m a

m i c e l l e , t h e h y d r o g e n b o n d i n g s t r u c t u r e i n t h e

w a t e r i s r e s t o r e d a n d t h e w a t e r e n t r o p y i n c r e a s e s ,

o v e r c o m i n g t h e e n t r o p y l o ss d u e t o t h e l o c a l i z a ti o n

o f th e h y d r o p h o b i c c h a i n s i n t h e m i c el le s . T h e

e n t r o p y c o n t r i b u t i o n u s u a l ly d o m i n a t e s t h e m i c el li -

z a t i o n p r o c e s s i n a q u e o u s s u r f a c t a n t s o l u t i o n s ,

w i t h th e e n t h a l p y p l a y i n g a m i n o r r o l e [ 1 9 ] . T h e

m a g n i t u d e o f th e h y d r o p h o b i c e f fe c t i n c re a s e s w i th

t e m p e r a t u r e E 4 3 ] , i n a g r e e m e n t w i t h t h e o b s e r v e d

i n c r e a s e d t e n d e n c y f o r m i c e l l e f o r m a t i o n w i t h

i n c r e a si n g t e m p e r a t u r e ; h o w e v e r , s u ch a n e x p l a n a -

t i o n d o e s n o t t a k e i n t o a c c o u n t t h e s p e ci fi c n a t u r e

o f t he P E O - P P O - P E O c o po ly m e rs .

S o l u t e - s o l v e n t [ 4 4 , 4 5 ] o r s o l u t e - s o l u t e i n t e r -

a c t i o n s [ 4 6 ] h a v e a l s o b e e n p r o p o s e d i n o r d e r t o

i n t e r p r e t t h e ~ m o l e c u l a r- l ev e l m e c h a n i s m b e h i n d

t h e t e m p e r a t u r e d e p e n d e n c e o b s e r v e d i n t h e m i c el -

l iz a ti o n o f P E O - P P O P E O c o p o l ym e r s in w a te r

( n o t e t h a t t h e h y d r o p h o b i c e ff e ct o u t l i n e d a b o v e i s

b a s e d o n s o l v e n t s o l v e n t i n t e r a c t i o n s ) . K j e l a n d e r

a n d F l o r in [ 4 4 ] a t t e m p t e d t o r e p r o d u c e t h e n e g a-

t iv e e n t r o p y a n d e n t h a l p y o f w a t e r - P E O m i x in g

a n d t h e p h a s e d i a g r am o f t h e P E O - w a t e r s y st em ,



P. A lexan dr id is . T .A . Ha t ton /C ol lo i ds Sur lac es A . Phys icochem . Eng . A ,v~ec ts 96 ( 1995 , 1 46 9

a s s u m i n g a z o n e w i t h i n c re a s e d s t r u c t u r i n g o f

w a t e r t o e x is t a r o u n d t h e P E O c h a i n . T h e p h a s e

s e p a r a t i o n t h a t t a k e s p l a c e a t h i g h t e m p e r a t u r e s

w a s a t t r i b u t e d t o a b r e a k d o w n o f t h e z o n e s o f

e n h a n c e d w a t e r s t ru c t u r e . K j e l a n d e r a n d F l o r i n

[ -4 4 ] c l a im t h a t w h e n P P O is i n t r o d u c e d i n t o

w a t e r i t a l s o d e v e l o p s a h y d r a t i o n s h e l l w i t h a n

e n h a n c e d w a t e r s t r u c t u r e , b u t s i n c e t h e m e t h y l

g r o u p s o f P P O p r o v i d e s t e ri c h i n d r a n c e , t h e w a t e r

s t r u c t u r e i s w e a k a n d l e a d s t o p h a s e s e p a r a t i o n .

K a r l s t r o m [ - 4 6 ] p r e d i c t e d t h e P E O w a t e r p h a s e

d i a g r a m u s i n g F l o r y H u g g i n s t h e o r y a n d a s s u m -

i n g t h a t e a c h s e g m e n t o f t h e P E O c h a i n c a n e x i st

i n t w o f o r m s , o n e p o l a r w i t h a l o w e n e r g y a n d a

l o w s t a t i s t i c a l w e i g h t , a n d o n e l e s s p o l a r , o r n o n -

p o l a r , w i t h a h i g h e r e n e r g y a n d s t a t i s t i c a l w e i g h t .

T h e p o l a r c o n f o r m a t i o n s d o m i n a t e a t lo w t e m p e r -

a t u r e s m a k i n g t h e s o l u t e - s o l v e n t i n t e ra c t i o n f a v o r -

a b le , w h e r e a s a t h i g h e r t e m p e r a t u r e s t h e n o n - p o l a r

c l a ss o f s t a t e s b e c o m e s i n c r e a s i n g l y p o p u l a t e d .

r e n d e r i n g t h e s o l u t e s o l v e n t i n t e r a c t i o n l e s s f a v o r -

a b l e [ 4 7 ] . T h e m o d e l f o r p r e d i c t in g t h e s o l u t i o n

b e h a v i o r o f b l o c k c o p o l y m e r m i c e ll e s d e v e l o p e d

by Hur t e r e t a l . [ - 48 ,49] a nd L inse [ - 50 ,51]

( se e S e c t i o n 6 . 2. f o r a d e s c r i p t i o n o f th i s m o d e l )

i n c o r p o r a t e d K a r l s t r o m ' s i d ea s t o a c c o u n t f o r t h e

c o n f o r m a t i o n a l d i s tr i b u ti o n in P E O a n d P P O ; t h e

m o d e l p r e d i c t i o n s a g r e e w i t h t h e t r e n d s o b s e r -

v e d e x p e r i m e n t a l l y , t h u s s u p p o r t i n g t h e p o l a r -

n o n - p o l a r s t a t e m o d e l a s a n e x p l a n a t i o n o f t h e

e ff ec t o f t e m p e r a t u r e o n t h e s o l u t i o n b e h a v i o r o f

P E O a n d P P O .

A n o t h e r m e t h o d f o r o b t a i n i n g e s t i m a t e s o f t h e

e n t h a l p y o f m i c e l li z a ti o n , A H ~ is D S C . D S C m e a s -

u r e m e n t s f o r a q u e o u s P l u r o n i c c o p o l y m e r s o l u -

t i o n s s h o w e n d o t h e r m i c p e a k s [ 3 0 , 5 2 ] , t y p i c a l f o r

a f i r s t - o r d e r p h a s e t r a n s i t i o n , a t c o n c e n t r a t i o n -

d e p e n d e n t c h a r a c t e r i s t i c t e m p e r a t u r e s a s s h o w n i n

F ig . 6 [ 3 7 ] . T h e p e a k s y i e l d ra t h e r h i g h e n t h a l p y

v a l u e s a n d a r e b r o a d , e x t e n d i n g m o r e t h a n 1 0 K .

T h e l a t t e r o b s e r v a t i o n h a s b e e n a t t r i b u t e d t o t h e

f a ct t h a t t h e c o p o l y m e r s a r e n o t p u r e c o m p o u n d s

b u t s h o w a b r o a d m o l a r w e i g h t d i s t r i b u t i o n ; i t i s

k n o w n t h a t m e l t i n g p e a k s b e c o m e b r o a d i n th e

p r e s e n c e o f i m p u r i t ie s [ 3 0 ] . I t s h o u l d b e p o i n t e d

o u t t h a t t h e e n t h a l p y c h a n g e m e a s u r e d b y th e

p e a k a r e a i n D S C i s n o t t h e s t a n d a r d e n t h a l p y

c h a n g e , b u t d e p e n d s u p o n t h e r e al s t at e s o f t h e

0 3 - - ~ i i i T ~ i - -

1 0 %

0 2 5 P 1 0 4

G

~ ~

0 . 1 5

0 0 5

0 ~ ~ - - -

- 0 . 0 5 i i ,

10 15 20 25 30 35 4 ) 45 50

Temperature (C)

Fig. 6. Differential scann ing ca lorimet ry curves fl~r Pluronic

P104 aqueous solutions at various copolymer concentrations:

the endothermic peak is indicative of miccllc formation.

(Adapted from Rcf. [31].}

c o p o l y m e r m o l e c u le s b e f o r e a n d a f t er m i c e l li z a ti o n .

T h e s t a n d a r d s t a t e e n t h a l p y c h a n g e i s d e f i n e d f o r

t r a n s f e r o f 1 t o o l o f c o p o l y m e r s f r o m t h e i d e a l ly

d i l u t e s o l u t i o n t o t h e s o l v a t e d m i c e l l a r s t a t e . I n

t h e i d e a l l y d i l u t e s o l u t i o n , c o p o l y m e r s e g m e n t s

i n t e r a c t o n l y w i t h t h e s o l v e n t , w h e r e a s i n r e a l

s o l u t i o n s s e g m e n t s a l s o i n t e r a c t w i t h e a c h o t h e r ,

a n d t h i s m a y c a u s e d i s c r e p a n c i e s b e t w e e n m i c e l l i -

z a t i o n e n t h a l p i e s o b t a i n e d f r o m D S C a n d t h o s e

d e r i v e d f r o m a n a n a l y s i s s i m i l a r t o t h a t o f

A l e x a n d r i d i s e t a l . [ 2 8 ] . I t h a s a l s o b e e n r e p o r t e d

b y H i e m e n z [ 4 1 ] t h a t A H ' v a l u e s c a l c u l a te d b y a

m i c e l li z a ti o n t h e r m o d y n a m i c s m o d e l g e n e r a l l y gi v e

p o o r a g r e e m e n t w i th t h o s e d e t e r m i n e d c a l o r i m e t r i -

c a l l y , a t l e a s t f o r i on i c su r f a c t a n t s .

F r o m a r a t h e r l i m i te d s e t o f d a t a ( P l u r o n i c s

P 1 2 3 , F 1 2 7 , a n d P 1 0 4 ) , W a n k a e t a l . [ 3 0 ] o b s e r v e d

n o p r o p o r t i o n a l i t y b et w e en t h e D S C e n t h a l p y

v a l u e s a n d t h e s iz e o f t h e P E O b l o c k o f t h e

m o l e c u l e s , b u t n o t e d t h a t , t o a f i r s t a p p r o x i m a t i o n ,

t h e r e w a s p r o p o r t i o n a l i t y b e t w e e n t h e s e w d u e s a n d

t h e n u m b e r o f P O s eg m e n ts . T h e y c o n c l u d e d f r o m

t h i s t h a t t h e t r a n s i t i o n w a s p r o b a b l y d u e t o t h e

d e h y d r a t i o n o r m e l t i n g o f t h e P O s eg m e n t s .

S i m i l a r t r e n d s w e r e o b s e r v e d i n t h e

l l

d a t a o f

A l e x a n d r i d i s e t a l. [ 2 8 ] , a l t h o u g h a w e a k e f fe c t o f

P E O b l o c k si ze o n A H w a s a p p a r e n l f r o m t h e

l a t te r d a t a s et , F o r P l u r o n i c c o p o l y m e r s w i th

t h e s a m e s iz e h y d r o p h o b i c ( P P O ) b l o c k a n d v a r y -

i n g s iz e h y d r o p h i l i c ( P E O ) b l o c k ( i.e . P I 0 3 , P I 0 4 ,



10 P. A lexandridis, T .A . H atton/Colloids Surfaces A. Physicochem. E ng. Aspe cts 96 (1995) 1 46

P 1 0 5 , a n d F 1 0 8 ) , A H '~ d e c r e a s e d b y a p p r o x i m a t e l y

1 5 % as th e n u m b e r o f E O s e g m e n t s in c r e a s e d f r o m

2 x 7 t o 2 x 1 32 . F o r P l u r o n i c c o p o l y m e r s w i th t h e

s a m e s iz e h y d r o p h i l i c ( P E O ) b l o c k a n d v a r y i n g

s iz e h y d r o p h o b i c ( P P O ) b l o c k ( i.e. P 6 5 , P 8 4 , a n d

P 1 2 3 ) , A H ° i n c r e a se d b y a p p r o x i m a t e l y 1 0 0% a s

t h e n u m b e r o f P O s e g m e n t s i n c re a s e d f r o m 3 0 t o

7 0 . T h e P P O e f fe c t w a s m o r e s i g n i fi c a n t, le a d i n g

t o t h e c o n c l u s i o n t h a t P P O i s m a i n l y r e s p o n s i b l e

f o r t he m ic e ll iz a ti o n o f P E O - P P O - P E O c o p o ly -

m e r s i n w a t e r . D S C w a s a l s o u s e d b y B e e z e r e t al .

[ 5 3 ] a n d M i t c h a r d e t al. [ 5 4 ] t o s tu d y P E O -

P P O - P E O b l o ck c o p o l y m e r a q u e o u s so lu ti on s .

T h e o b s e r v e d p h a s e t r a n s i t io n s w e r e i n t h e

1 0 0 3 0 0 k J m o l - ~ r a n g e a n d w e r e i n it ia l ly t h o u g h t

t o r e s u l t f r o m c h a n g e s i n t h e p o l y m e r s o l v a t i o n a s

t h e t e m p e r a t u r e c h a n g e d . B e e z e r e t al . [ 5 3 ] a n d

M i t c h a r d e t a l . [ 5 4 ] c l a i m e d t h a t , a t t h e c o n c e n -

t r a t i o n u s e d ( 5 g 1 ~ ), o n l y u n i m e r s w e r e p r e s e n t

i n s o l u t i o n . I n a m o r e r e c e n t p a p e r f r o m t h e s a m e

g r o u p , h o w e v e r , A r m s t r o n g e t al . [ 5 2 ] c o n c l u d e d

t h a t t h e o b s e r v e d e n t h a l p y c h a n g e i s i n d i c a t i v e

o f a n a g g r e g a t i o n p r o c e s s , a c c o m p a n i e d b y d e s o l -

v a t i o n a n d c h a n g e i n c o n f o r m a t i o n o f t h e

h y d r o p h o b e . A l t h o u g h d i r e c t c o m p a r i s o n b e t w e e n

t h e c a l o r i m e t r y A H v a l u e s o f R efs . [ 5 2 ] - [ - 5 4 ] a n d

t h e A H :' o f R ef. [ 2 8 ] c a n n o t b e m a d e a s t h e

c o p o l y m e r s u s e d w e r e n o t t h e s am e , th e d a t a o f

t h e l a t t e r p a p e r s t r o n g l y s u g g e s t t h a t t h e m i c e l l i z a -

t i o n p r o c e s s is t h e m a j o r c o n t r i b u t o r t o t h e c a l o r i -

m e t r i c a l l y - o b s e r v e d e n t h a l p y c h a n g e .

4 2 E f f ec t o f copo lym er com pos i t ion and m o lecu lar

we igh t on m ice l l iz a t ion ther m odyna m ics

F r e e e n e rg i e s o f m i c e l li z a t io n p e r m o l o f P l u r o n i c

c o p o l y m e r i n s o l u t i o n , A G : ( c a l c u l a te d a t t h e C M T

f o r t h e v a r i o u s c o p o l y m e r s a t d i f f e r e n t s o l u t i o n

c o n c e n t r a t i o n s ) , e x h i b i t s o m e s c a t t e r w h e n p l o t t e d

a s a fu n c t i o n o f c o p o l y m e r m o l e c u l a r w e i g h t , b u t

t h e y d o i n d i c a t e a t e n d e n c y f o r h ig h e r m o l e c u l a r

w e i g h t c o p o l y m e r s t o h a v e m o r e n e g a t i v e f r e e

e n e r g ie s o f m i c e l l iz a t i o n [ 2 8 ] . T h e A G ~ d a t a c o l -

l a p s e i n t o a s i n g l e s m o o t h c u r v e , i n d e p e n d e n t o f

t h e c o p o l y m e r c o m p o s i t io n r a ti o P P O / P E O , w h e n

n o r m a l i z e d w i t h r e s p e c t t o t h e t o t a l n u m b e r o f

m o n o m e r s e g m e n t s i n t h e p o l y m e r , N ~ o + N p o ,

a s s h o w n i n F i g . 7 [ 2 8 ] . I t c a n b e s e e n c l e a r l y

t h a t l o w e r m o l e c u l a r w e i g h t , m o r e h y d r o p h o b i c

P l u r o n i c s y i e ld m o r e n e g a t iv e A G ° v a l u e s p e r

m o n o m e r s e g m e n t. T h e d e p e n d e n c e o f th e t h e r-

m o d y n a m i c p a r a m e t e r s A H ° a n d A G ° o n t h e

P P O / P E O c o m p o s i t io n r a ti o w as al so e x a m i ne d .

T h e n o r m a l i z e d A H ( e x p re s s e d i n k J m o l - ~ o f

a v e r a g e m o n o m e r s e g m e n t ) a p p r o a c h e d z e r o a s

t h e P P O / P E O r a ti o w e n t t o z e r o [ 2 8 ] . I t c a n th u s

b e i n f e r r e d t h a t t h e m i c e l l i z a t i o n p r o c e s s i s d o m i -

n a t e d b y t h e P P O ( h y d r o p h o b i c ) p a r t o f t h e c o p o l-

y m e r . A s i m i l a r t r e n d w a s o b s e r v e d b y A r m s t r o n g

e t a l. [ 5 2 ] b a s e d o n D S C e n t h a l p y c h a n g e s o f I C I

P o l o x a m e r s o l u t i o n s , a n d b y W i l l i a m s e t a l . [ 5 5 ]

u s i n g t h e i n c r e m e n t i n a p p a r e n t m o l a r v o l u m e o n

t h e r m a l t r an s i t io n o f P o l y S c ie n c e s P E O - P P O

c o p o l y m e r s a s a f u n c ti o n o f P P O / P E O . T h e f ac t

t h a t t h e n o r m a l i z e d A H ° v a lu e s a re a p p r o x i m a t e l y

t h e s a m e f o r P l u r o n i c c o p o l y m e r s o f t h e s a m e

P P O / P E O c o m p o s i t io n r a ti o a n d d i ff er en t m o l e c u -

l a r w e i g h t s w o u l d i n d i c a t e t h a t t h e m i c e l l i z a t i o n

e n t h a l p y p e r m o n o m e r s e g m e n t i s i n d e p e n d e n t o f

m o l e c u l a r w e i g h t.

I n c o n t r a s t t o t h e t r e n d o f d e c r e a s in g A G ° w i t h

i n c r e a s i n g P l u r o n i c m o l e c u l a r w e i g h t , t h e r e i s n o

d e f i n i t e d e p e n d e n c e o f A H ~' o n m o l e c u l a r w e i g h t

o v e r t h e r a n g e c o v e r e d i n t h e s t u d y o f A l e x a n d r i d is

e t a l. [ 2 8 ] . T h e A H '~ d a t a p r e s e n t e d i n R e f. [ 2 8 ]

r e v e a le d t w o m a i n g r o u p s o f c o p o l y m e r s : t h e r e la -

t iv e l y h y d r o p h o b i c P l u r o n i c s P 1 0 3 , P 1 0 4 , P 1 0 5 ,

z

4 - 0

0

E

2

0

- 0 . 1

- 0 . 2

- 0 . 3

- 0 . 4

- 0 . 5

- 0 . 6

- 0 . 7

- 0 . 8

2 0 0 0

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L 6

4 0 0 0 6 0 0 0 8 0 0 0 1 0 0 0 0 1 2 0 0 0 1 4 0 0 0

1 6 0 0 0

Copolymer molecu lar weight

F i g . 7 . E f fe c t o f P E O - P P O P E O c o p o l y m e r m o l e c u la r w e ig h t

o n t h e f r e e en e r g i e s o f m i c e l l i z a ti o n f o r v a r i o u s c o p o l y m e r

c o n c e n t r a t i o n s e x p r e s s e d a s AG : / ( N Eo + N p o ) k J m o l 1 o f

m o n o m e r s e g m e n ts ) . R e p r i n t e d w i t h p er m i s s i o n f r o m

R e f. [ 2 8 ] ; c o p y r i g h t A m e r i c a n C h e m i c a l S o c i e ty , 1 9 94 .)



P. Alexandridis, T .A. Hatton/Colloids SutJiwes A. P hysicochem. Etlg. Aspects 96 /g? 5; l 4 5 11

a n d P 1 2 3 w i t h A H '~' i n t h e 3 0 0 - 3 5 0 k J m o l

r a n g e , a n d t h e r e la t iv e l y h y d r o p h i l i c c o p o l y m e r s

L 64 , P65 , P84 , a nd P85 wi th A H '~ i n t h e

1 8 0 - 2 3 0 k J m o l ~ r a n g e : A H '~ f o r t h e o t h e r f o u r

c o p o l y m e r s r a n g e d b e t w e e n 1 7 0 a n d 2 7 0 k J t o o l ~.

A p l o t o f A G o f m i c e l l i z a t i o n ( e x p r e s s e d i n

k J t o o l ~ o f a v e r a g e m o n o m e r s e g m e n t ) a s a

f u n c ti o n o f P P O / P E O s h o w e d t he A G - '/ s eg m e n t

v a lu e s a p p r o a c h i n g z e r o as th e P P O / P E O r a ti o

w e n t t o z e r o [ 2 8 ] . T h e s e g m e n t f re e e n e r g y , h o w -

e v e r , d e c r e a s e d ( b e c a m e m o r e n e g a t i v e ) w i t h a

d e c re a s e in m o l e c u l a r w e i gh t f o r P E O P P O P E O

c o p o l y m e r s h a v in g t he s am e P P O / P E O r at io . In

a d i f f e r e n t i nve s t i ga t ion , Re ddy e t a l . [ - 26] e s t i -

m a t e d t h e A G o f m i c e l li z a ti o n a t - 2 0 k J t o o l -

a n d t h e A H ' o f m i c e l l i z a t i o n a t 2 0 0 k J t o o l 1 f o r

a s o l u t i o n o f p u r i f i e d P l u r o n i c L 6 4 , u ti l iz i n g

I n ( C M C ) v s . l I T d a t a ( o n l y th r e e d a t a p o i n t s,

t h o u g h ) . T h e s a m e g r o u p [ - 4 0 ] r e p o r t e d t h e A H '

o f m i c e l l iz a t i o n a t 3 1 6 + 2 0 k J t o o l - ~ f o r a s o l u t i o n

o f p u r i fi e d F 1 2 7 ( f r o m In ( c o n c e n t r a t i o n ) v s. 1 /T cM r

d a t a ) . T h e s e v a l u e s v a r y b y a p p r o x i m a t e l y 2 0

f ro m A G = - 2 4 . 5 k J m o l - ~ , a n d

A H ' =

2 3 0 k J t o o l 1 f o r L 6 4 a n d A H - = 2 5 3 f o r F 1 2 7

r e por t e d i i a Re f . [ 28] .

5 Structure of block copolym er micel les

5 .1 . A¢ greg a t ion munbe ; mice l l e s i ze and shape

T h e a s so c ia ti o n o f P E O P P O - P E O b l oc k

c o p o l y m e r s i n t o m i c e ll es a n d t h e s t r u c t u r e o f t h e s e

m i c e l l e s h a v e b e e n i n v e s t i g a t e d b y m a n y r e s e a r c h -

e r s [ -2 1,2 6 3 0 , 3 7 , 4 0 , 5 6 - 7 0 ] . W e p r e s e n t b e l o w

i n f o r m a t i o n o n m i c e l l e s f o r m e d b y P l u r o n i c s L 6 4 ,

P 8 5, F 8 8 , F 6 8 , a n d F 1 2 7 P E O - P P O P E O b l o c k

c o p o l y m e r s , f o r w h i c h a c o n s i d e r a b l e n u m b e r o f

s t u d i e s h a v e b e e n p u b l i s h e d . S i g n i f i ca n t i n f o r m a -

t i o n h a s b e e n o b t a i n e d f r o m l i g h t a n d n e u t r o n

sc a t t e r ing s tud i e s . I n ge ne r a l , t he un im e r s i z e i s

f o u n d t o b e a p p r o x i m a t e l y 1 n m a n d t h e m i c el le

s iz e 1 0 n m , i n d e p e n d e n t o f c o p o l y m e r c o n c e n -

t r a t i o n ( a l t h o u g h , t h e c o n c e n t r a t i o n s u s e d w e r e

u s u a ll y h ig h ( a b o v e a p p r o x i m a t e l y 2 ) t o a l lo w

g o o d s i g n a l - t o - n o i s e r a t i o i n t h e s c a t t e r i n g e x p e r i -

m e n t s ) . T h e e f f e ct o f t e m p e r a t u r e i s i n te r e s t in g : a n

i n c r e a s e i n t h e a g g r e g a t i o n n u m b e r w i t h t e m p e r -

a t u r e h a s b e e n o b s e r v e d , w h i l e t h e m i c e l l a r r a d i u s

r e m a i n s c o n s t a n t . T h e c o n c l u s i o n s a r e c o m p l i c a t e d

b y t h e r a t h e r b r o a d C M T t r a n s it i o n , a n d t h e fa c t

t h a t d y n a m i c l i g h t s c a t t e r i n g d e t e c t s t h e m i c e l l e

h y d r o d y n a m i c r a d i u s w h i c h in c l u d e s t h e w a t e r

h y d r a t i n g t h e E O s e g m e n t s .

P l u r o n i c L 6 4 s h o w e d d e t e c t a b l e a g g r e g a t e s a t

2 5 ' C o n l y a t co n c e n t r a t io n s a b o v e a p p r o x i m a t e l y

6 : th e m i c e l le s i ze i n c r e a s e d w i t h c o n c e n t r a t i o n

( 1 0 n m a t 8 1 2 . 5 n m a t 2 0 ) a n d ex h i b i te d

s i g n i f ic a n t p o l y d i s p e r s i t y , p r o b a b l y i n d i c a t i n g a

m u l t i p l e a s s o c i a t i o n p r o c e s s [ 2 9 ] . A t 3 5 C , h o w -

e v e r , e s s e n t i a l l y i n v a r i a n t v a l u e s f o r t h e h y d r o d y -

n a m i c r a d iu s w e r e f o u n d o v e r a w i d e c o n c e n t r a t i o n

r a n g e a n d t h e m i c e l l e s w e r e m o n o d i s p e r s e : t h e s e

s y s t e m s a r e m o r e l i k e l y r e p r e s e n t e d b y a c l o s e d

a s s o c i a ti o n m o d e l [ -2 9 ]. Z h o u a n d C h u [ 5 6 ] f o u n d

t h a t m i c e ll a r m o l e c u l a r w e i g h t i n c r ea s e d e x p o n e n -

t i a l l y w i t h i n c r e a s i n g t e m p e r a t u r e ; c o m p o s i t i o n

h e t e r o g e n e i t y ( p r e s e n c e o f h y d r o p h o b i c i m p u ri ti es )

c a u s e d s t r o n g a n g u l a r a s y m m e t r y o f s c a t t e re d l i g h t

b y L 6 4 c o p o l y m e r s o l u t io n s . T h e m o d e o f a ss o c ia -

t i o n o f a p u r i fi e d L 6 4 s a m p l e i n a q u e o u s s o l u t i o n

h a s b e e n e x a m i n e d b y R e d d y e t a l . [ 2 6 ] ; a s s o c i a -

t i o n a t 34 .5 a n d 4 0 C w a s d e s c r i b e d b y a c o o p e r a -

t i v e a s s o c i a t i o n m o d e l w h i c h a s s u m e s a g g r e g a t e

g r o w t h b y s t e p w is e a d d i t i o n o f u n i m e r s ; n o t e t h a t

t h i s i s i n c o n t r a s t t o t h e r e s u l t s r e p o r t e d b y

A1- Sa de n e t a l . [ 29] . A lm gr e n e t a l . [ - 58] c a l c u l a t e d

( f r o m s t a t i c l i g h t s c a t t e r i n g d a t a ) t h e a g g r e g a t i o n

n u m b e r s o f L 6 4 a t t h e t e m p e r a t u r e s 2 1 . 0 , 2 5 . 9 ,

40.0. an d 6 0.0 C to be 2, 4 .

19,

a nd 85 r e spe c t ive ly ,

i n d i c a t i n g a s m o o t h i n c r e a s e w i t h t e m p e r a t u r e .

M o r e r e c en t ly , th e e f f ec t o f t e m p e r a t u r e in a q u e o u s

s o l u t i o n s o f P l u r o n i c L 6 4 w a s e x a m i n e d b y m e a n s

o f v i sc o s i ty , s e d i m e n t a t i o n , s c a t t e r i n g . a nd s o u n d

v e l o c i t y m e a s u r e m e n t s [ -6 7 ] . T h e m i c e l le s g r e w

l a r g e , p a r t i c u l a r l y a t t e m p e r a t u r e s n e a r t h e c l o u d

p o i n t . T h e v i s c o s i t y d a t a fl )r L 6 4 w e r e a n a l y z e d

t o e s t i m a t e v a r i o u s p a r a m e t e r s , i n c l u d i n g t h e

h y d r a t e d m i c e l l a r v o l u m e , h y d r a t i o r l n u m b e r ,

h y d r o d y n a m i c r a d i u s , e t c .

D y n a m i c l i g h t s c a t t e r i n g e x p e r i m e n t s i n

P l u r o n i c P 8 5 s o l u t i o n s i n d i c a t e d t h e c o e x i s t e n c e

o f u n i m e r s ( h y d r o d y n a m i c r a d i u s 1 .8 n m ) , m i c el le s

( h y d r o d y n a m i c r a d i u s 8 n m ) , a n d m i c e l la r a g g re -

g a te s , at c o p o l y m e r c o n c e n t r a t i o n s l e ss t h a n 1 0

a n d a t l o w t e m p e r a t u r e s ( 2 5 C ) [ - 2 7 ] . M i c e l l e s



2 P. Alexandridis, T. A. Hatton/Colloids Surfaces A: Physicochem. En g. Aspects 96 1995) 1 .4 6

w e r e f o rm e d a t a c o p o l y m e r c o n c e n t r a t i o n o f

a p p r o x i m a t e l y 5 % a t 2 5 ° C ; a t t e m p e r a t u r e s o f

4 0 ° C a n d h i g h e r , m i c e l l e s w e r e p r e s e n t a t a l l t h e

c o n c e n t r a t i o n s s t u d i ed (C > 0 . 3 % ) . T h e h y d r o d y -

n a m i c r a d i u s o f t h e m i c e ll e s ( c a l c u l a t e d a t i n f i n i te

d i l u t io n ) w a s a p p r o x i m a t e l y c o n s t a n t ( 8 n m ) o v e r

t h e t e m p e r a t u r e r a n g e 1 5 - 5 0 ° C ( a g g re g a ti o n

n u m b e r 2 0 - 4 0 ) . A t fi n it e c o n c e n t r a t i o n s t h e a p p a r -

e n t m i c e l l a r r a d i u s i n c r e a s e d w i t h i n c r e a s i n g t e m -

p e r a t u r e . T h e h a r d - s p h e r e r a d i u s o f a c o p o l y m e r

m o l e c u l e o f M w = 4 5 0 0 w o u l d b e 1 .1 5 n m ; t h u s ,

f r o m t h e 1 .8 n m r a d i u s o b s e r v e d e x p e r i m e n t a l l y

o n e m a y c o n c l u d e t h a t t h e u n i m e r i s h i g h l y c o m -

p a c t , p o s s ib l y w i th t h e P E O c h a i n s f o r m i n g a t ig h t

s h el l a r o u n d t h e n o n - h y d r a t e d P P O c o r e [ 2 7 ] .

I n t r i n s i c v i s c o s i t y m e a s u r e m e n t s ( c a p i l l a r y v i s c o m -

e t ry ) w e r e m a d e o n d i l u t e P 8 5 s o l u t i o n s i n o r d e r

t o o b t a i n m o r e i n f o r m a t i o n o n t h e m o l e c u l a r

d i m e n s i o n s . T h e i n t ri n s ic v i s c o s i t y d e c r e a s e d f r o m

1 6 m l g - 1 a t 1 5 ° C t o 6 . 5 m l g - 1 a t 5 0 ° C . T h e

i n t r i n s i c v i s c o s i t y o f 6.5 m l g 1 a t 5 0 ° C s u g g e s t e d

a v e r y c o m p a c t p a r t i cl e ( a lt h o u g h w i t h s o m e d i s-

s y m m e t r y a n d / o r s o l v a t i o n ); th e r a d i u s o f g y r a t i o n ,

R g , w a s e s t i m a t e d a t 8 . 3 n m .

V a l u es f o r th e h y d r o d y n a m i c r a d i i o f t h e m i c e l la r

c o m p o n e n t f r o m s e l f - d i f f u s i o n c o e f f i c i e n t s ( m e a -

s u r e d b y p u l s e d - f ie l d - g r a d ie n t N M R a n d e x t r a p o -

l a t e d t o i n f i n i t e d i l u t i o n ) v a r i e d b e t w e e n 6 . 2 a n d

9 .0 n m i n t h e te m p e r a t u r e r a n g e 2 1 .3 3 5 . 7 ° C , w i th

a n a v e r a g e v a l u e o f 7.7 r im . T h i s v a r i a t i o n w i t h

t e m p e r a t u r e c o n t r a d i c t e d t h e c o n s t a n t m i c e l l e

r a d i u s d e t e c t e d u s i n g d y n a m i c l i g h t s c a t t e r i n g . T h i s

w a s p o s s i b l y t h e r e s u l t o f p o l y d i s p e r s i t y , s in c e t h e

n u m b e r - a v e r a g e q u a n t i t y o b t a i n e d w i t h N M R

w o u l d b e v e r y s e n s i t i v e t o e v e n s m a l l a m o u n t s o f

m o n o m e r [ 2 7 ] ; t h e d if f er e n c e b e t w e e n t h e h y d r o -

d y n a m i c r a d i i d e t e r m i n e d f r o m p u l s e d f i e l d g r a d i -

e n t ( P F G ) N M R a n d d y n a m i c li g ht s ca t te r in g w a s

s m a l l e r a t t h e h i g h e r t e m p e r a t u r e s a t w h i c h t h e

s u s p e n s i o n i s n e a r l y m o n o d i s p e r s e . T h e s e l f -

d i f fu s i o n o f P l u r o n i c P 8 5 c o p o l y m e r i n a q u e o u s

s o l u ti o n w as in v e s ti g a te d w it h P F G - N M R

[ -6 5 ,6 6 ]. T h e h y d r o d y n a m i c r a d ii o f th e u n i m e r

a n d t h e m i c e l l e s w e r e d e t e r m i n e d t o b e 1 . 5 a n d

4 .5 n m , r e sp e c t iv e l y , a n d f o u n d t o b e i n d e p e n d e n t

o f t e m p e r a t u r e a n d c o n c e n t r a t i o n i n t h e c o n c e n -

t r a t i o n r a n g e b e t w e e n 1 a n d 1 0% . M o r t e n s e n a n d

c o - w o r k e r s [ , 61 , 6 2 ] s tu d i e d t h e s t r u c t u r e o f P 8 5

a q u e o u s s o l u t i o n s u s i n g n e u t r o n s c a t t e r i n g . T h e

r a d i u s o f g y r a t i o n o f t h e f r ee c o p o l y m e r w a s 1 .7 n m .

T h e m i c e l l a r s i z e s ( m i c e l l e c o r e r a d i u s a n d h a r d -

s p h e r e i n t e r a c t io n r a d iu s ) a p p e a r e d t o b e i n d e p e n -

d e n t o f p o l y m e r c o n c e n t r a t i o n , b u t s h o w e d a s m a ll

t e m p e r a t u r e d e p e n d a n c e r e fl e ct in g c h a n g e s i n

a g g r e g a t i o n n u m b e r . T h e m i c e ll e c o r e r a d iu s a n d

h a r d - s p h e r e i n t e r a c t i o n r a d i u s w e r e 3 .8 a n d 6 .0 n m

a t 2 0 ° C , r e s p e c t i v e l y , a n d i n c r e a s e d t o 5 .1 a n d

7 . 5 n m a t 5 0 C ; t h e a g g r e g a t i o n n u m b e r w a s 3 7 a t

2 0 ° C , i n c r e a s i n g t o 7 8 a t 4 0 ° C [ 6 4 ] .

B r o w n e t a l. [ 5 7 ] a n d M o r t e n s e n a n d B r o w n

[ 6 3 ] p u b l i s h e d s tu d i e s c o m p a r i n g t h e P l u r o n i c

P E O - P P O - P E O c o p o l ym e r s P 8 5, F 8 7, a n d F 8 8.

R e l a x a t i o n t i m e d i s t r i b u t i o n s o b t a i n e d b y L a p l a c e

i n v e r s io n o f t h e d y n a m i c l ig h t s c a t ti n g ( D L S )

c o r r e l a t i o n f u n c t i o n s d e m o n s t r a t e d c o m p l e x s t a t e s

o f a g g r e g a t i o n i n s o l u t i o n [ -5 7 ] . U n i m e r , m i c e ll e s,

a n d l a r g e r ag g r e g a t e s c o e x is t e d i n p r o p o r t i o n s t h a t

d e p e n d e d s t r o n g l y o n t e m p e r a t u r e a n d c o n c e n -

t r a t i on , a s se e n in F ig . 8 f o r P85 so lu t ions [ - 27].

T h e u n i m e r s h a d h y d r o d y n a m i c ra d i i i n t h e si ze

r a n g e 1 . 5 - 3 . 0 n m ; t h e m i c e ll e r a d i i w e r e 8 1 3 n m

( in s e q u e n c e o f i n c r e a s i n g P E O b l o c k l e n g t h ) ,

w h e r e a s t h e r a d i i o f t h e c l u s t e r s w e r e g r e a t e r t h a n

8 0 n m . T h e m i c el le a g g r e g a t i o n n u m b e r f o r F 8 8

w a s 1 7 a t 40 ~ 'C , a n d t h e h y d r o d y n a m i c r a d i i 1 3

a n d 1 0 . 5 n m a t 4 0 a n d 5 0~ 'C , r e s p e c t iv e l y .

M o l e c u l a r w e i g h t s , g y r a t i o n r a d i i , e t c . , c o u l d n o t

b e d e t e r m i n e d f r o m s t a t i c l i g h t s c a t t e r i n g m e a s u r e -

m e n t s o w i n g t o t h e h i g h l y p o l y d i s p e r s e c h a r a c t e r

o f t h e s o l u t i o n s . T h e i n v e r s e o s m o t i c c o m p r e s s i b i l -

i t y w a s e v a l u a t e d f r o m t h e a b s o l u t e s c a t t e r i n g

i n t e n s i t y a t z e r o a n g l e, p r o v i d i n g a m e a s u r e o f t h e

r e l a t iv e s t r e n g t h s o f s o l u t e / s o l v e n t a n d s o l u t e /

s o l u t e i n t e r a c t i o n s ; f o r m a t i o n o f m i c e l le s a n d

a g g r e g a t e s w a s c o n s i s t e n t w i t h l o w v a l u e s o f t h e

i n v e r s e o s m o t i c c o m p r e s s i b i l i t y . P l u r o n i c s P 8 5 ,

F 8 7 , a n d F 8 8 h a d a p p r o x i m a t e l y th e s a m e te n -

d e n c y f o r m i c e l l e f o r m a t i o n w h e n c o m p a r e d a t t h e

s a m e m o l a r c o n c e n t r a t i o n ; f o r t h e s a m e w e i g h t

c o n c e n t r a t i o n , P 8 5 f o r m e d m i c e l l e s m o r e r e a d i l y

[ - 5 7 ] . M o r t e n s e n a n d B r o w n [ - 6 3 ] f o u n d t h a t

t h e c r i t i c a l m i c e l l i z a t i o n t e m p e r a t u r e v a l u e s o f

P l u r o n i c s L 8 1 , P 8 5 , P 8 7 a n d F 8 8 f ell o n a c o m m o n

l i n e w h e n p l o t t e d a g a i n s t t h e c o n c e n t r a t i o n o f

P P O i n s o l u t i o n ; i t t h u s a p p e a r e d t h a t t h e P P O

c o n c e n t r a t i o n w a s t h e r e l e v a n t p a r a m e t e r i n t h e



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f o r P l u r o n i c P 8 5 a t l o w c o n c e n t r a t i o n s ( < 1 0 % ) a n d a t

t e m p e r a t u r e s b e t w e e n 5 a n d 5 0 C [ A( r) i s t h e a u t o c o r r e l a t i o n

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d e t e r m i n a t i o n o f t h e C M T . P l u r o n i c L 8 1 f o r m e d

a g g re g a te s a t t e m p e r at u r es b e l o w t h e c o m m o n

c u r v e , p r e s u m a b l y d u e t o t h e s i g n i f i c a n t l y g r e a t e r

h y d r o p h o b i c n a t u r e o f L 8 1 ( L 8 1 c o n t a i n s 9 0

P P O ) : t h e re w a s a l s o s o m e d e v i a t i o n f r o m t h e

c u r v e f o r P 8 5 a t lo w P P O c o n c e n t r a t i o n s .

T h e h a r d - s p h e r e i n t e r a c t i o n r a d i u s , Rh s f o r

P l u r o n i c s P 8 5 , P 8 7 a n d F 8 8 , o b t a i n e d b y f it t in g

t o t h e e x p e r i m e n t a l s c a t t e r i n g d a t a , is s h o w n i n

F i g . 9 [ 6 3 ] . T h e r a d i i a r e p l o t t e d a s a f u n c t i o n

F i g . 9 . H a r d - s p h e r e i n t e r a c t i o n r a d i u s , R h ~ , f o r P l u r o n i c P 8 5 ,

P 8 7, a n d F 8 8 P E O P P O P E O c o p o l y m e r s , o b t a i n e d by f i tt in g

t h e h a r d - s p h e r e P e r c u s Y e v ic k m o d e l t o e x p e r i m e n t a l n e u t r o n

s c a t t e r i n g d a t a : t h e r a d ii a r e p l o t t e d a s a f u n c t i o n o f t e m p e r a t u r e

f o r d i f f e re n t c o p o l y m e r c o n c e n t r a t i o n s . I R e p r i n t e d w i t h p e r m i s -

s i o n f r o m R e f. [ 6 3 ] ; c o p y r i g h t A m e r i c a n C h e m i c a l S o c i e ty ,

1993.1

o f t e m p e r a t u r e f o r d if fe r en t c o p o l y m e r c o n c e n -

t r a t i o n s . T h e m i c e l l a r c o r e r a d i u s , Re a n d t o s o m e

e x t e n t a l s o t h e h a r d - s p h e r e i n t e r a c t i o n r a d iu s , R h s,



14 P. Alexandridis , T .A. Ha t ton/Co l lo ids Sur fac es A. Phys icochem. Eng. Asp ects 96 (1995) 1 46

appeared to be essentially independent of copoly-

mer concentration but showed a significant

increase with temperature. The difference between

R c and Rh, remained practical ly unaffected by the

temperature. When Rc was plotted against reduced

temperature T-TcMv, the data for solutions of

P85, P87, and F88 followed a common master

curve. A double logarithmic plot of R¢ vs. T - Tcuc

gave the empirical scaling relation R~

T- TCMT) 2 as seen in Fig. 10 [63]. The aggrega-

tion number was very small, close to TcMx, and

increased continuously following a N

T- TcMT) 6 relationship to N ~ 200 at the highest

temperature, where spherical micellar aggregates

were present. From the hard-sphere da ta analysis,

it appears that the main difference between P85,

P87, and F88 micelles at a given temperature is

the size of the micelles; the larger the PEO block,

the smaller the core and the aggregation number

[63]. The dependence of the core radius and the

aggregation number on the degree of polymeriza-

tion of both EO and PO [63] contradicted the

Halperin [71] star model of polymeric micelles,

which was recently shown to be valid for the

system PEO-PS-PEO [72]. The relationship of

increasing micellar core with decreasing number

of EO segments would lead to a limitation in the

formation of spherical micelles; this would happen

if the number of EO segments were so small that

the core radius extended the length of a stretched

PPO chain. Extrapolation of the core radii data

40 .

~ z

g % 2 O X

v •

F 8 8

20 . 1 0 .

1o . 1oo.

T T c r n I

F i g . 1 0. D o u b l e l o g a r i t h m i c p l o t o f R c v s . r e d u c e d t e m p e r a t u r e ,

T - T c M c ( d a t a s h o w n fo r 9 a n d 20 a q u e o u s s o l u ti o n s o f

P l u r o n i c P 8 5 , F 8 7 , a n d F 8 8 ) . ( R e p r i n t e d w i t h p e r m i s s i o n f r o m

R e f . [ 6 3 ] ; c o p y r i g h t A m e r i c a n C h e m i c a l S o c i e t y , 19 93 .)

for Pluronics P85, F87, and F88 to the number of

EO segments corresponding to L81 would lead to

a value for the core approaching or exceeding the

maximum value given by the length of a fully

extended PPO chain; this is probably the reason

why Pluronic L81 does not form spherical micellar

aggregates [63].

Static and dynamic light scattering experiments

on Pluronic F68 aqueous solutions revealed three

temperature regions, those of unimer, transition,

and micelle [21]. The apparent hydrodynamic

radii of F68 solutions at three different copolymer

concentrations are presented in Fig. ll a) [21].

A transition corresponding to the CMT was

observed; below the CMT, low scattering intensity

and small particle size 2.3nm) were detected,

1

(a)

/ /

, - ' . /

2

i i I i i

3 0 4 0 5 0 6 0 7 0

Temperature ('C)

6

E

o

I

@ -

0 i t ~ i i

5 0 5 5 6 0 6 5 7 0 7 5

(b) Tem perature ( 'C)

16

12

8

B

0

8 0

F i g . 11 . ( a t T e m p e r a t u r e d e p e n d e n c e o f a p p a r e n t h y d r o d y n a m i c

r a d i u s o f P l u r o n i c F 6 8 s o l u t i o n s a t t h r e e d i f f e r e n t c o n c e n -

t r a t i o n s : ~ , 5 1 .7 m g m l - 1; @ , 2 5 . 0 m g m l 1 ; I I , 1 2 . 5 m g m l ~ .

( b ) L i n e a r i n c r e a s e o f F 6 8 m i c e l l a r w e i g h t s w i t h i n c r e a s i n g

t e m p e r a t u r e , w i t h t h e h y d r o d y n a m i c r a d i u s o f t h e m i c e ll e s

r e m a i n i n g a p p r o x i m a t e l y c o n s t a n t . ( R e p r i n t e d w i t h p e r m i s s io n

f r o m R e f . [ 2 1 ] ; c o p y r i g h t A c a d e m i c P r e s s , 1 9 8 8. )



P. A h ' xa n 'i d i s , T . A . H a t t o n / Co l l o i d s S u rJa cc s ,4: Ph y s i co ch em . EH k '. A S lWCt S 9 6 ( 1 9 ~ 5 ~ 1 4 t5 1 5

s h o w i n g l i tt le t e m p e r a t u r e d e p e n d e n c e . O n l y u n i m -

e rs w e r e p r e s e n t b e l o w t h e C M T ; m i c e ll e f o r m a t i o n

b e c a m e a p p r e c i a b l e a b o v e t h e C M T . I n t h e m ic e ll e

r e g i o n , t h e m i c e l l a r w e i g h t s m e a s u r e d w e r e o f t h e

o r d e r o f 1 05 g m o l ~ ( a v e ra g e a g g r e g a t i o n n u m b e r

w a s 6 5 ) a n d i n c r e a s e d l i n e a r l y w i t h t e m p e r a t u r e ,

w h i le t h e h y d r o d y n a m i c r a d ii o f t h e m i c el le s

r e m a i n e d n e a r l y c o n s t a n t ( 8 . 0 n m ) a s s e e n i n

F i g . l l ( b ) [ 2 1 ] ( n o t e , h o w e v e r , t h a t A 1 - S a d e n e t a l.

[ 2 9 ] r e p o r t e d th e a v e ra g e h y d r o d y n a m i c ra d iu s t o

i n c re a s e w i t h t e m p e r a t u r e f o r a g i v e n c o n c e n -

t r a t i o n ) . S u c h a d u a l e f f e c t o f t e m p e r a t u r e w a s

i n t e r p r e t e d in t e rm s o f e n h a n c e d d e h y d r a t i o n o f

t h e m i c e ll e c o r o n a ( c o n s is t in g m a i n l y o f P E O ) w i t h

t e m p e r a t u r e [ 2 1 ] . I n s t a t i c l i g h t s c a t t e r i n g m e a s -

u r e m e n t s , t h e s c a t t e r e d l i g h t f ie l d i s a m e a s u r e o f

t h e p o l a r i z a b i l i t y d i f f er e n c e b e t w e e n t h e d i s p e r s e d

p a r t i c l e a n d t h e s u r r o u n d i n g m e d i u m , t h u s p r o v i d -

i n g u s e fu l i n f o r m a t i o n c o n c e r n i n g t h e ' d r y d i s-

p e r s e d p a r t i c l e s . I n t h e d y n a m i c l i g h t s c a t t e r i n g

e x p e r i m e n t s , t h e B r o w n i a n m o t i o n o f t h e w e t

d i s p e r s e d p a r t i c l e s i s m o n i t o r e d t o p r o v i d e s i z e

i n f o r m a t i o n o n t h e s o l v a t e d p a r t i c l e s . I t i s w e l l

e s t a b li s h e d t h a t d e h y d r a t i o n o f n o n - i o n i c s u r f a c-

t a n t s g r a d u a l l y o c c u r s w i t h i n c re a s i n g t e m p e r a t u r e ,

w h i c h r e s u l t s i n a n e n h a n c e d t e n d e n c y t o s e p a r a t e

f r o m t h e s o l v e n t e n v i r o n m e n t a n d , c o n s e q u e n t l y ,

i n a n in c r e a s e i n th e a g g r e g a t i o n n u m b e r . S u c h a

d u a l e f fe c t o f t e m p e r a t u r e i n c r e a s e o n a n i n c r e a s e d

a g g r e g a t i o n n u m b e r a n d a d e c r e a s e d m i c e l l a r

h y d r a t i o n m a k e s i t p o s s i b l e f o r t h e h y d r o d y n a m i c

r a d iu s o f P E O P P O P E O c o p o l y m e r m i ce ll es t o

r e m a i n a h n o s t i n d e p e n d e n t o f t e m p e r a t u r e [ 2 1 ] .

T h e h y d r o d y n a m i c r a d ii o f m i c el le s f o r m e d b y

P l u r o n i c F 1 2 7 i n w a t e r , a s c a l c u l a t e d f r o m d i f f u -

s i o n ( Q E L S ) d a t a a t t h e C M C , r e m a i n e d c o n s t a n t

a t 1 0. 2 n m , o v e r t h e 3 5 4 5 C t e m p e r a t u r e r a n g e

[ 7 3 ] . V i s c o m e t r ic s tu d i es , h o w e v e r , s h o w e d a p r o -

g r e s s iv e d e h y d r a t i o n o f t h e m i ce l le s w i t h t e m p e r -

a t u r e i n c r e a s e : t h e a g g r e g a t i o n n u m b e r w a s 3 a t

3 5 C , 9 a t 4 0 C , a n d 1 2 a t 4 5 C [ 7 3 ] . M a l m s t e n

a n d L i n d m a n [ 5 9 ] u s ed N M R t o s t u d y d i ff u si o n

o f m o l e c u l e s in F 1 2 7 s o l u t i o n s . T h e d i f f u s i o n o f t h e

p o l y m e r m o l e c u l es w a s s l ow

D p = 1 0 - 1 1 1 0 - 1 2 m 2

s ~) a n d d e c r e a s e d w i t h i n c r e a s i n g c o p o l y m e r c o n -

c e n t r a t i o n ( r o u g h l y a s D p ~ c ~) u p t o 2 0 w t. % :

the d i f f us ion c oe f f i c i e n t o f wa te r wa s Dw = 10 - 9 m - '

s 1 [5 91 ]. W a t e r s e l f- d i ff u s io n d e c r e a s e d m o n o t o n i -

c a ll y w it h i n c r e a s in g c o p o l y m e r c o n c e n t r a t i o n ,

g i v i n g D / D o = 0 .5 a t 4 0 w t . % . T h e d e c r e a s e i n D / D .

w i t h in c r e a s in g c o p o l y m e r c o n c e n t r a t i o n c o u l d b e

r e p r o d u c e d b y' t a k i n g i n t o a c c o u n t t h e o b s t r u c t i o n

d u e t o e x c l u d e d v o l u m e a n d t h e h y d r a t i o n o f t h e

p o l y m e r m o l e c u le s . T h e a n a l y s is y i e l d e d t h a t s o m e -

w h e r e b e t w e e n t w o a n d f i v e w a t e r m o l e c u l e s p e r

E O s e g m e n t w e r e p e r t u r b e d . T h e w a t e r d i f f u s i o n

i n c r ea s e d w i th t e m p e r a t u r e . T h e d a t a o f M a l m s t e n

a n d L i n d m a n [ 5 9 , 6 0 ] a r e c o n s i st e n t w it h a g r a d u a l

d e h y d r a t i o n o f t h e p o l y m e r m o l e c u l es w i th i n c re a s -

i n g t e m p e r a t u r e , a n d a r e i n a g r e e m e n t w i t h t h o s e

o f A t t w o o d e l a l. [ 7 3 ] . S u c h a d e h y d r a t i o n , h o w -

e v e r , d i d n o t c o r r e l a t e w i t h t h e o c c u r r e n c e o f t h e

g e l r e g i o n [ 6 0 ] .

5 .2 . M i c e l l e m i c r o e n v i r o m m n t

F l u o r e s c e n c e s p e c t r o s c o p y t e c h n i q u e s h a v e b e e n

d e v e l o p e d a n d o p t i m i z e d o v e r t h e p a s t 1 5 y e a r s

f o r t h e s t u d y o f c o l l o i d a l s o l u t i o n s , a n d m a n y

f l u o r e s c e n t m o l e c u l e s a re n o w a v a i l a b l e f o r p r o b i n g

s t r u c t u r a l i n f o r m a t i o n i n s u c h s y s t e m s [ 3 4 , 7 7 ] .

P y r e n e , a m o n g s t o t h e r s , i s a w e l l - c h a r a c t e r i z e d

p o l a r i t y - s e n s i t i v e p r o b e . T h e p y r e n e f l u o r e s c e n c e

e m i s s i o n s p e c t r u m c o n s i s t s m a i n l y o f fi ve b a n d s

r e f e r r e d to a s , 1 1 , I,_ . . . . . I s , f r o m s h o r t e r t o l o n g e r

w a v e l e n g th s . T h e

I 1 / I 3

i n t e n s i t y r a t i o o f t h i s v i b r a -

t i o n a l f in e s t r u c t u r e d e p e n d s s t r o n g l y o n t h e p o l a r -

i t y o f t h e m e d i u m : t h e l a r g e r t h e r a t i o , t h e m o r e

p o l a r t h e m e d i u m . E l e c t r o n i c a l l y e x c i t e d p y r e n e is

a r e p o r t e r o f th e a v e r a g e m i c r o p o l a r i t y o f th e

e n v i r on m e n t i t v i si t s dur in g i ts l i f e t im e ( v 300 ns) .

r e n d e r i n g p y r e n e a t t r a c t i v e f o r s t u d y i n g r e s t r i c t e d

m i c r o e n v i r o n m e n t s s u c h a s m i c e l l a r s y s t e m s

[ 3 4 , 7 7 ] .

A l e x a n d r i d i s a n d c o - w o r k e r s [ 3 7 , 7 0 , 7 6 ] u s e d

p y r e n e t o p r o b e t h e m i c r o p o l a r i t y in P l u r o n i c

c o p o l y m e r s o lu t i o n s, b y c o n d u c t i n g a s y s t e m a t i c

i n v e s t i g a t i o n o f t h e v a r i a t i o n o f t h e p y r c n e f l u o r e s -

c e n c e e m i s s i o n i n t e n s i t y r a t i o I~ 13 i n d i f f e r e n t

m e d i a a s a f u n c t i o n o f t e m p e r a t u r e . S p e c i fi c a ll y ,

s t u d i e s w e r e c o n d u c t e d i n w a t e r , o r g a n i c s o l v e n t s ,

b u lk P E O a n d P P O h o m o p o l y m e r s , a n d a q u eo u s

s o l u ti o n s o f P E O P P O P E O c o p o l y m e r s . A

s h a r p d e c r e a s e i n

I1 , , I ,~

w a s o b s e r v e d f o r P E O

P P O P E O t r i b l o c k c o p o l y m e r s o l u ti o n s , a t t e m -

p e r a t u r e s c h a r a c t e r i s t i c o f e a c h p o l y m e r , f o l l o w e d



16 P. Alexandridis, T.A . Hatton/Colloids Surfaces A: Physicochem. En g. Aspe cts 96 1995) 1 46

by a less dramatic linear decrease as the temper-

ature was further increased (see Figure 12(a)). This

sharp decrease is attributed to the formation of

micelles with a well-defined hydrophobic core into

which pyrene partitions preferentially. The temper-

ature dependence of the

11/13

intensity ratio can

be used for the determination of the CMT [37],

as with the I 1 /13 vs. concentration plots in the

determination of the CMC in surfactant systems

[34]. The smoother linear decrease observed at

temperatures higher than the CMT reflects a less

polar microenvironment with increasing temper-

(a)

2 . 1

2

1 . 9

1 . 8

1 . 7

1 . 6

1 . 5

1 . 4

1 . 3

i i i r

t

1 5 2 5 3 5 4 5

Temperature ( 'C)

5 5

2 6 i i i i i

• PPO 425

.4

\

~ % . "m - m - P PO 2 0 0 0

2 . 2

\

" ~ . \ q . - -* - F 1 0 8 1 %

1 8

1 4

1.2 t I I I a

3 4 3 7 4 0 4 3 4 6 4 9 5 2

(b) Tempe rature ( 'C)

F i g. 12 . (a ) M i c r o p o l a r i t y i n P E O P P O P E O c o p o l y m e r s o l u-

t i o n s : p y r e n e f l u o r e s c e n c e e m i s s i o n i n t e n s i t y r a t i o 11/13) a s a

f u n c t i o n o f t e m p e r a t u r e f o r 1 a q u e o u s s o l u t i o n s o f P l u r o n i c s

P 1 0 5 , P 8 5 , a n d P 6 5 . ( A d a p t e d f r o m R e f . [ -7 6 ]; c o p y r i g h t

M a s s a c h u s e t t s I n s t i t u t e o f T e c h n o l o g y , 1 99 4.) ( b t M i c r o v i s c o s i t y

i n P E O - P P O P E O c o p o l y m e r m ic e ll es : d i p y m e f lu o r es c e n ce

e m i s s i o n m o n o m e r / e x c i m e r i n te n s i ty r a ti o IM/IE) a s a f u n c t i o n

o f te m p e r a t u r e f o r P l u r o n i c F 1 0 8 m i c e ll e s; a l s o s h o w n i n t h e

s a m e g r a p h f o r c o m p a r i s o n p u r p o s e s a r e d a t a f or b u l k P P O

h o m o p o l y m e r . ( A d a p t e d f r o m R e f . [ 7 9 ] . )

ature. This was attributed to a temperature-

induced change in the solution micropolarity [70].

The influence of molecular weight on the micro-

polarity afforded by copolymer micellar solutions

is depicted in Fig. 12(a) for a series of Pluronic

copolymers (P105, P85, P651 with the same

PP O/PE O composition ratio (50%). The highest

molecular weight P105 exhibited the lowest CMT,

in accordance with the findings of Ref. [28]. At

temperatures higher than the CMT, the

11/13

ratios

decreased linearly with increasing temperature for

all copolymers, but their values remained in the

order: 1 1 /1 3 P 6 5 ) > I 1 / I 3 P 8 5 ) > I 1 / I 3 (P105).

This is an indication that micelles assembled from

higher-molecular-weight Pluronics (maintaining

constant PPO/PEO copolymer composition)

exhibit more hydrophobic microdomains [70]. In

addition to probing the microenvironment, time-

resolved fluorescence quenching from pyrene has

been used to determine the aggregation number of

PEO-PPO-PEO micelles [70,75].

The microviscosity in the interior of block copol-

ymer micelles can be probed using the hydrophobic

molecule bis(1-pyrenylmethyl)ether (dipyme) that

exhibits intramolecular excimer fluorescence

(intensity Is) in competition with fluorescence from

the locally excited pyrene chromophore ( mon-

omer emission, intensity IM; 15 is used here to

represent IM) [78]. The extent of excimer emission

in dipyme depends upon the rate of conformational

change, the motion of the pyrene segments of

dipyme being restricted by the local friction

imposed by the environment. As a consequence,

the intensity ratio I M / I E provides a measure of the

microviscosity of its environment (the larger the

IM/I~ ratio, the more viscous the environment in

which the probe is located), and is a particularly

powerful means of monitoring changes in microvis-

cosity as the system is subjected to external stim-

uli [78].

Quali tative changes in the microviscosity experi-

enced by dipyme are reported by Nivaggioli et al.

[79] for PPO homopolymers and PEO-

PP O- PE O copolymer micellar solutions, as a

function of a copolymer composition and sample

temperature. Values of the monomer/excimer

intensity ratio I M / I E ) of dipyme in bulk PPO

homopolymers and Pluronic F108 micelles are



P. Alexandridis. T. ,4. Hatton/Colloi& Surliwes A: Phvsicochem . EiJg.,4.g~ects96 (1995) 1 4~, 17

s h o w n i n F i g . 1 2 ( b l [ 7 9 ] . I M / I E d a t a a r e r e p o r t e d

f o r th e 3 5 5 0 ' C t e m p e r a t u r e r a n g e , s o t h a t

m i c e l l e s a r e p r e s e n t i n w h i c h d i p y m e c a n b e s o l u b i -

l iz e d [ t h e s o l u b i l i ty o f d i p y m e i n w a t e r i s v e r y

l im i t e d) . M i c r o v i s c o s i ty d e c r e a s e d w i th t e m p e r -

a t u r e f o r a l l t h e s y s t e m s s t u d i e d : h i g h e r m i c r o v i s -

c o s i t y v a l u e s w e r e o b s e r v e d f o r t h e h i g h e r M w

P P O . T h e l u / I F v a l u e s f o r t h e E l 0 8 m i c e l l e s a r e

c o m p a r a b l e w i t h th o s e fo r b u l k P P O , s u g g e s ti n g

t h a t t h e e n v i r o n m e n t i n w h i c h d i p y m e i s l o c a t e d

is s i m i l a r t o b u l k P P O . N o t e t h a t t h e m i c r o v i s c o s -

i ty e x p e r i e n c e d b y d i p y m e i n t h e i n t e r i o r o f F I 0 8

m i c e l l e s a p p e a r s t o b e s i m i l a r t o t h a t s e n s e d i n l o w

Mw ~7 25 ) P P O a t 3 5 C , w h i l e i t e x c e e d s t h e

m i c r o v i s c o s i t y in h i g h M 2 ( 2 0 0 0 ) P P O a t 5 0 C .

T h i s c o u l d b e a n i n d i c a t i o n t h a t t h e m i c e l le c o r e

b e c o m e s m o r e c o m p a c t a s t h e t e m p e r a t u r e

i n c r e a s e s . S u c h a n o b s e r v a t i o n c a n b e r e l a t e d t o

t h e fa c t t h a t t h e a g g r e g a t i o n n u m b e r o f P l u ro n i c

c o p o l y m e r m i c e l l e s i n c r e a s e s w i t h t e m p e r a t u r e ,

w h i l e t h e h y d r o d y n a m i c r a d i u s r e m a i n s a p p r o x i -

m a t e l y c o n s t a n t ( s e e S e c t i o n 5 . 1 ) . A s e m i l o g p l o t

o1 IM~I~ VS. t e m p e r a t u r e c a n b e f i t te d t o a l i n e a r

r e l a t i o n s h i p [ 7 9 ] : a c t i v a t i o n e n e r g y ( E a ) v a l u e s f o r

t h e m i c r o v i s c o s i ty e x p e r ie n c e d b y d i p y m e c a n b e

e s t i m a t e d f r o m s u c h a p l o t. I n c r e a s e o f t h e c o p o l y -

m e t m o l e c u l a r w e i gh t a t c o n s t a n t P P O / P E O c o m -

p o s i t i o n , a n d i n c r e a s e o f t h e P P O b l o c k s i z e f o r

c o n s t a n t P E O b l o c k s iz e , r e s u l t e d in in c r e a s i n g

m i c r o v i s c o s i t y a n d a c t i v a t i o n e n e r g i e s . T h e e f f e c t

o f P E O b l o c k o n m i c e ll e m i c r o v i s c o s i ty w a s l e ss

s i g n i f i c a n t .

T h e h y d r a t i o n a n d m i c r o v i s c o s i t y i n th e m i c e l la r

s o l u l io n s a n d g e ls f o r m e d b y P l u r o n i c F 1 2 7 h a v e

b e e n s t u d i e d u s i n g t h e f l u o r e s c e n t p r o b e m o l e c u l e s

p y r e n e a n d 8 - a n i l i n o - l - n a p h t h a l e n e s u lf o n ic a ci d

[ 7 4 ] : t h e r e s u l t s i n d i c a t e d t h a t m i c r o v i s c o s i t y

d e c r e a s e s w i t h i n c r e a s i n g t e m p e r a t u r e . R e c e n t l y ,

N a k a s h i m a e t a l . [ 8 0 ] r e p o r t e d f l u o r e s c e n c e

a n i s o t r o p y d a t a f r o m o c t a d e c y l r h o d a m i n e B in

P l u r o n ic F 6 8 P E O P P O P E O m i ce ll es ; t he m i c r o -

v i s c o s i t y o f t h e l a t t e r w a s e s t i m a t e d f r o m t h e s e

m e a s u r e m e n t s . T h e m i c r o v i s c o s i t y i n c r e a se d f r o m

1 .9 c P a t 2 0 'C t o 1 6 c P a t 4 0 ': C : t h is i n c r e a s e w a s

a t t r i b u t e d t o a c o n f o r m a t i o n a l c h a n g e o f t h e

m i c e l l e s f r o m a l o o s e l y c o i l e d a g g r e g a t e t o a m o r e

c o m p a c t s tr u c t u re . N o t e h o w e v e r , th a t o c t a d e c y l

r h o d a m i n e B is a c a t io n i c a m p h i p h i l i c m o l e c u l e

t h a t w i l l m o s t l i k e l y a s s o c i a t e w i t h t h e m i c e l l e

c o r o n a , t h u s c o m p l i c a t i n g t h e m i c r o v i s c o s i t y

s tud ies .

5 .3 . P h a s e d ia g r a m

T h e p h a s e b e h a v i o r o f P E O P P O P E O tr i bl o c k

c o p o l y m e r s d i ss o l ve d i n w a t e r h a s b e e n s t u d i e d b y

M o r t e n s e n a n d c o - w o r k e r s u s i n g s m a l l - a n g l e n e u t -

r o n s c a t t e r i n g a n d d y n a m i c l i g h t s c a t t e r i n g

[ 6 1 , 6 3 , 6 4 ] . T h e s t r u c t u r a l p r o p e r t i e s h a v e b e e n

s t u d ie d a s a f u n c t io n o f p o l y m e r c o n c e n t r a t i o n a n d

t e m p e r a t u r e f o r a s er ie s o f P l u r o n ic c o p o l y m e r s

w i t h P P O b l o c k s o f c o n s t a n t s i ze a n d P E O b l o c k s

o f v a r y i n g s i te . A t l o w t e m p e r a t u r e [T -< ~ 15 C ) a n d

l o w p o l y m e r c o n c e n t r a t i o n s , t h e u m m e r s w e r e f ul ly

d i s s o l v e d g a u s s i a n c h a i n s w i t h r a d i u s R g = 1 .7 n m

C l o s e t o a m b i e n t t e m p e r a t u r e , t i l e h y d r o p h o b i c

n a t u r e o f P P O c a u s e d a g g r e g a t i o n o f t h e p o l y m e r s

i n t o s p h e r i c a l m i c e l l e s w i t h c o r e s i z e s o f t h e o r d e r

o f 4 5 r a n . s o m e w h a t t e m p e r a t u r e d e p e n d e nt .

A c c o r d i n g t o t h e d a t a a n a l y s i s , t h e c o r e s i z e

i n c r e a s e d w i t h d e c r e a s i n g P E O b l o c k s iz e a n d w i t h

i n c r e a s in g t e m p e r a t u r e . T h e c o p o l y m e r w i th t h e

l a r g e s t P E O b l o c k a g g r e g a t e d in m i c e ll e s w i t h a

c o r e d i a m e t e r w h i c h , w i t h i n t h e w h o l e l e m p e r a t u r e

r e g i m e , w a s s m a l l e r t h a n t h e l e n g th o f a s t r e t c h e d

P P O c h a in . M i c el le s f o r m e d b x c o p o l y m e r s o f

i n t e r m e d i a t e P E O s iz e h a d a c o r e d i a m e t e r w h i c h

a t h i g h t e m p e r a t u r e a p p r o a c h e d t h e si ze o f a fu l ly

s t r e t c h e d P P O c h a i n , t h u s c a u s i n g a n a b r u p t

c h a n g e f r o m a s p h e r i c a l t o a r o d - l i k e s t r u c t u r e ( a s

o b s e r v e d b o l h b y n e u t r o n s c a t t e ri n g a n d d e p o l a r -

i z ed l ig h t s c a t te r i n g ) . I t a p p e a r e d f r o m t h e h a r d -

s p h e r e d a t a a n a l y s is t h a t t h e m i c e l l e - f o r m m g p o l y -

m e r s c o u l d a l l b e s c a l e d t o a c o m m o n p h a s e

b e h a v i o r w h e r e t h e c r i t i c a l m i c e l l i z a t t o n t e m p e r -

a t u r e is d e t e r m i n e d b y th e P P O c o n c e n t r a t i o n .

w h e r e a s t h e c r y s t a l l i z a t i o n ( g e l a t i o n l t e m p e r a t u r e

is d e t e r m i n e d b y t h e to t a l c o p o l y m e r c o n c e n t r a t i o n

[ 6 3 ] . T h e c o n c e n t r a t i o n o f m i c e ll e s i n c re a s e d

r o u g h l y l i n e a r l y w i t h t e m p e r a t u r e u n t i l e i t h e r a

s a t u r a t i o n , a a s r e a c h e d ( w h e r e al l p o l y m e r s w e r e

p a r t o f a m i c e l l el o r t h e v o l u m e d e n s i t y o f m i ce l le s

w a s s o h i g h t h a t t h e y ' l o c k e d i n t o a c r y s t a l l in e

s t r u c t u r e o f h a r d s p h e r e s . I n t h e 60 7(1 C t e m p e r -

a t u r e r a n g e , t h e m i c e l l a r s t r u c t u r e c h a n g e d f l ' o m

s p h e r i c a l f o r m t o p r o l a t c e l l i p s o i d , l e a d i n g t o a



18 P. Alexandridis . T .A. H at ton/C ol lo ids SurJaces A: Phys icochem. Eng. Asp ects 96 1995) 1 46

decreasing intermicelle interaction. At high copoly-

mer concentration this caused melting of the cubic

lattice and led successively to the formation of

a rod-like structure with hexagonal symmetry.

Large aggregates of PE O- PP O- PE O copolymers

ordered in lamellae structure were formed close to

95°C, leading to an opaque suspension [64].

The phase diagram of Pluronic P85 PEO-

PPO-PEO copolymer in water is presented in

Fig. 13, in which the concentration-temperature

regions where the different aggregates (discussed

in the previous paragraph) exist are also depicted

[61]. The micellization boundary (phase diagram)

for a number of Pluronic PEO PP O- PE O copoly-

mers in aqueous solutions is shown in Fig. 14 [76]

(the CMC CMT data are extracted from

Ref. [28]). The free polymer (unimer) is in solution

at copolymer concentrations and temperatures

lower than the CMC-CMT boundary; micelles in

equilibrium with unimers are present above the

boundary. Recently, Wanka et al. [69] reported

the phase diagrams of several PEO-PPO-PEO

copolymers in water. Unimer, micellar, cubic (gel),

hexagonal, and lamellar phases were observed;

8 0 . 0

6 0 . 0

40.0

E

2 0 . 0

0 . 0

0 . 0 0

liquid ~

-c~ -~ ~ hexagonal rods

I I

I 1 1

micellar

~c.c- ryst al

. . . . . . . . ~ . . . . . . . . 1_ ,.~ ¢ __ J ~ . ~

polymer in so lu t ion ~ ~ 0 ~ ~ ~

0 . 1 0 0 . 2 0 0 . 3 0 0 . 4 0

p o l y m e r c o n c e n t r a t i o n

F i g . 1 3. P h a s e d i a g r a m o f a q u e o u s P l u r o n i c P 8 5 s o l u t i o n ,

s h o w i n g t h e f u l ly d i s s o l v e d p o l y m e r s ( u n i m e r s ) a t l o w t e m p e r -

a t u r e s a n d c o n c e n t r a t i o n s, t h e C M C C M T l in e ( ¢ = 0 ) , t h e

¢ ~ = 0 . 5 3 t r a n s i t i o n t o b . c .c , m i c e l l a r c r y s t a l , a n d t h e h i g h -

t e m p e r a t u r e h e x a g o n a l p h a s e ( ¢ i s t h e h a r d - s p h e r e m i c e l l e

v o l u m e f r a c t i o n ). A t T ~ 6 0 ~ C a l i q u i d p h a s e a p p e a r s b e t w e e n

t h e c r y s t a l l i n e b . c . c , a n d h e x a g o n a l p h a s e s . ( R e p r i n t e d w i t h

p e r m i s s i on f ro m R e f . [ 6 1 ] ; c o p y r i g h t E u r o p e a n P h y s i ca l

Soc ie ty , 1992 . )

6O

i i

m i c e l l e s

C ~

f r e e p o l y m e r ~ - - , _ - - ~ -

1 0 . . . . . . . . J . . . . . . . . J . , , L ~ i ,

0 . 0 1 0 . 1 1 1 0

C o p o l y r n e r c o n c e n t ra t i o n ( % w / v )

F i g. 1 4. P h a s e d i a g r a m o f a q u e o u s P l u r o n i c P E O . - P P O P E O

c o p o l y m e r s o l u ti o n s , s h o w i n g t h e C M C C M T m i c e ll i za t io n

b o u n d a r y . T h e f r e e p o l y m e r ( u n i m e r ) i s i n s o l u t i o n a t c o p o l y m e r

c o n c e n t r a t i o n s a n d t e m p e r a t u r e s l o w e r t h a n t h e C M C C M T

b o u n d a r y : m i c e l le s in e q u i l i b r i u m w i t h u n i m e r s a r e p r e s e n t

a b o v e t h e b o u n d a r y . ( R e p r i n t e d w i t h p e r m i s s i o n f r o m R ef . [ 7 6 ] ;

c o p y r i g h t M a s s a c h u s e t t s I n s t i t u t e o f T e c h n o l o g y , 1 9 94 .)

their occurrence depended on the copolymer

PPO/PEO composition ratio. Copolymers with a

small hydrophilic PEO block usually formed a

hexagonal phase as the first liquid crystalline meso-

phase. The lyotropic mesophases exhibited thermo-

tropic behavior: reversible transitions between the

different phases occurred with increasing temper-

ature at constant copolymer concentration. The

phase behavior of a monoolein-rich (89.5 wt.%)

monoglyceride blend based on pine needle oil,

Pluronic F127, and water has been investigated by

small-angle X-ray diffraction and reported recently

by Landh [81]: an extended cubic phase region

was observed.

5 4 E f f e c t o f add i t i v e s on m ic e ll i za t ion

The phase behavior of Pluronic L64 PEO

PPO PEO block copolymer in the presence of

various additives was studied by Pandya et al.

[67] using cloud point and phenol index (i.e. the

amount of phenol added to give a turbid solution)

measurements. Addit ion of electrolytes, having

anions and cations of different sizes and polariz-

abilities, resulted in either an increase or a decrease

in cloud point: the effect of salts was discussed in

terms of salting in and 'salting out and followed

the Hofmeister series [67]. The effect of various



P . A l e . v a m b M i s , T . A . H a t t o n / C o l l o i d v S t o ; l ia ' e s < 4. P h v . s i c o c h e m . E r a ,, . A s p ec t, ~ ~ )6 : 1 7 ~ 5 : 1 4 ~

19

n o n - e l e c t r o l y te s (e .g . h y d r o x y c o m p o u n d s a n d

a m i d e s ) o n t h e c l o u d p o i n t o f L 6 4 w a s a l s o e x a m -

i n e d a n d d i s c u s s e d i n t e r m s o f t h e i r i n fl u e n c e o n

w a t e r s t r u c t u r e [ 6 7 ] . T h e a g g r e g a t i o n b e h a v i o r o f

P l u r o n i c s P 8 5 a n d L 6 4 in a q u e o u s s o l u t i o n h a s

b e e n i n v e s t i g a t e d i n th e p r e s e n c e o f a d d e d s a l ts

{ K C N S , K I , K B r , K C I a n d K F ) b y v is c os it y, c l o ud

p o i n t , l i g h t s c a t t e r i n g , p u l s e g r a d i e n t s p i n e c h o

N M R , a n d s o l u b i l i z a t i o n m e a s u r e m e n t s [ 8 2 ] . T h e

s a l ts h a v e a s t r o n g e f f e ct o n t h e c l o u d p o i n t o f t h e

P E O P P O P E O c o p o l y m e r s . A s l ig h t l in e a r

i n c r e a s e i n c l o u d p o i n t w a s o b s e r v e d w i t h a n

i n c r e a s e i n c o p o l y m e r c o n c e n t r a t i o n i n t h e c a s e o f

L 6 4 , l \ q l o w i n g a n i n i t i a l d e c r e a s e u p t o a c o p o l y -

m e r c o n c e n t ra t i o n o f a p p r o x i m a t e l y 2 . K C N S ,

w h i c h i s k n o w n t o i n c r e a s e t h e c l o u d p o i n t , s h o w e d

a s i m i l a r t r e n d i n b o t h L 6 4 a n d P 8 5 s o l u t i o n s ;

a d d i t i o n o f K B r , K C 1, a n d K F d e c r e a s e d t h e c l o u d

p o i n t . A l i n e a r c h a n g e ( i n c r e a s e o r d e c r e a s e ) i n t h e

c l o u d p o i n t w i t h in c r e a s i n g s a lt c o n c e n t r a t i o n w a s

o b s e r v e d . B o t h P l u r o n i c P 8 5 a n d L 6 4 f o r m e d

m i c e l le s w h i c h i n c r e a s e d in s iz e a n d b e c a m e e l o n -

g a t e d i n s h a p e w h e n t h e c l o u d p o i n t w a s

a p p r o a c h e d . T h e c h a n g e s o f si ze a n d s h a p e o f th e

mi c e l l e s , r e v e a l e d b y t h e i n t r i n s i c v i s c o s i t y a n d

r h e o l o g i c a l p r o p e r t i e s o f t h e s o lu t i o n , s e e m e d t o

o c c u r a t t h e s a m e t e m p e r a t u r e r e la t iv e t o t he c l o u d

p o i n t , i n d e p e n d e n t o f t h e n a t u r e o f t h e s al t. T h e

o n s e t o f m i c e l l e f o r m a t i o n w a s a l s o s h i f te d i n t h e

s a m e d i r e c t i o n a s t h e c l o u d p o i n t b y t h e s a l t s , b u t

t o a l e ss e r d e g r e e [ 8 2 ] .

T h e c l o u d p o i n t o f P h i r o n i c F 6 8 w a s r e d t i c e d

b y 50 C w h e n 1.0 M K F w a s a d d e d t o t h e a q u e o u s

s o l u t i o n [ 8 3 ] . T h e s i z e o f t h e u n i m e r s a n d m i c e ll e s

i n t h e s o l u t i o n , a s e s t i m a t e d f r o m d i f f u s i o n c o e f f i -

c i e n ts ( d e t e r m i n e d b y d y n a m i c l i g ht s c a t t e ri n g a n d

N M R s e lf - di ff u si o n m e a s u r e m e n t s ) o r m i c e ll e

m o l e c u k u w e i g h t s t f r o m s t a t i c l i g h t s c a t t e r i n g ) ,

w a s s i m i l a r in t h e p r e s e n c e a n d a b s e n c e o f sa l ts ,

b u t t h e g r o w t h o f t h e m i c e l le s s t a r t e d a t a l o w e r

t e m p e r a t u r e 1 25 C i n 1 .0 M K F , 4 4 - C i n w a l e r t

a n d c o n t i n u e d o v e r tt w i d e r t e m p e r a t u r e r a n g e i n

t h e s tilt s o l u t i o n [ 8 3 ] . T h e i n f lu e n c e o f t e m p e r a t u r e

o n P h i r o n i c I -'1 27 m i c e t l a r si z e a n d h y d r a t i o n w a s

l es s p r o n o u n c e d i n t h e p r e s e n c e o f 0 .5 m o l d m

N a C I i n t h e a q u e o u s s o l u t i o n [ 7 3 ] . T h e i n f l u e n c e

o f a g a r o s c o n t h e c lo u d i n g b e h a v i o r a n d t h e d if fu -

s i o n ~ f E O i ,~PO ,~< ,E O I, ~ c o p o l y m e r , a s w e l l a s t h e

e ff ec t o f th e c o p o l y m e r o n t h e g e l a t io n p r o p e r t i e s

o f a g a r o s e w e r e r e c e n t l y r e p o r t e d b y P e n d e r s e t al .

[ 8 4 ] . T h e c l o u d p o i n t o f

EOI PO oEOI on

COOl-

i n g a n d t h e s e lf - d if f u si o n o f t h e c o p o l y m e r

d e c r e a s e d i n t h e p r e s e n c e o f a g a r o s e g e l s. T h e

p r e s e n c e o f 1.0 M N a S C N s u p p r e s s e d t h e f o r m a -

t i o n a n d g r o w t h o f E O 1 3 P O 3 n E O I 3 m i c e l l e s [ 8 4 ] .

T h e e f fe c ts o f u r e a o n t h e m i c e l l i z a t io n p r o p e r t i e s

o f P l u r on i c P 105 P E O - P P O P E O c o p o l y m e r , a n d

o n t h e st r u c t u r e a n d m i c r o e n v i r o n m e n t o f t h e

m i c e l le s w e r e i n v e s t i g a t e d b y A l e x a n d r i d i s e t al .

[ 8 5 ] . C M C a n d C M T d a t a f o r P h i ro n i c P 1 05

d i s so l v e d m u r e a / w a t e r m i x t u r e s ( u r ea c o n c e n -

t r a t i o n : 0 , 1 , 2 , a n d 4 M I w e r e o b t a i n e d u s i n g a

d y e s o l u b i ti z a t io n m e t h o d a n d c o r r o b o r a t e d w i th

s u r f a c e t e n s i o n , d e n s i t y , a n d f l u o r e s c e n c e s p e c t r o -

s c o p y e x p e r i m e n t s . A d d i t i o n o f u r e a i n c r e a s e d t h e

C M C a n d C M T o f t he P F, O P P O P E O c o p ol y -

m e t : t h e e f fe c t o f u r e a o n t h e ( ' M T w a s m o r e

p r o n o u n c e d a t lo w c o p o l y m e r c o n c e n t r a ti o n s a n d

d i m i n i s h e d a t c o n c e n t r a t i o n s o f a p p r o x i m a t e l y

2 . 5 ° ; .

T h e t h e r m o d y n a m i c p a r a m e t e i s o f P 1 05

m i c e l l e f o r m a t i o n i n t h e p r e s e n c e o f u r e a w e r e

e s t i m a t e d u s i n g a c l o s e d a s s o c i a t i o n m o d e l : t h e

e n t h a t p y o f m i c e l l i z a t i o n w a s p o s i t i v e ( e n d o t h e r -

m i c l a n d d e c r e a s e d u p o n i n c r e as i n g t h e u r e a c o n -

c e n t r a t i o n . ~ I h e s u r f a c e a c t i v i t y ' a n d t h e p a r t i a l

s p e c if i c v o l u m e o f P l u r o n i c P I 0 5 d e c r e a s e d w i t h

a n i n c r e a s e i n t h e u r e a c o n c e n t r a t i o n , w h e r e a s t h e

h y d r o d y n a m i c r a d i i o f t h e c o p o l y m c r m i c el le s .

d e t e r m i n e d u s i n g d y n a m i c l i g h t s c a t t e r i n g ,

r e m a i n e d u n a f f e c t e d b y t h e p r e s e n c e o f 4 M u r e a

i n t h e s o l u t i o n . T h e m i c r o p o h u i t y i n c o p o l y m e r

s o l u t i o n s i n u r e a / w a t e r w a s p r o b e d a s a f u n c t i o n

o f t e m p e r a t u r e u s in g t h e

l

' I. ~ i n t e n s i t y r a t i o o f

t h e p y r e n e f l u o r e s c e n c e e m i s s i o n s p e c t r a : a s m a l l

d e c r e a s e i n th e m i c r o p o l a r i t y o f t h e m i c e l te c o r e

w a s o b s e r v e d i n t h e p r e s e n c e o f u r e a [ : S 5 ].

S o d i t u n d o d e c y l s u l f a t e I S D S t i n c r e a s e d t h e

c l o u d p o i n t o f P l u r o n i c L 6 4 , w h i c h o t h e r w i s e

d e c r e a s e d i n t h e p r e se n c e o f s m a l l a m o u n t s o f

e l e c t r o ly t e s . T h e c l o u d i n g b e h a v i o r o f a m i x e d

L 6 4 . S D S s y s t e m in th e a b s e n c e a n d p r e s e n ce o f

s a l t w a s d i s c u s s e d b y P a n d y a e t a l. [6 7 11 , p r i m a r i l y

i n t e r ms o f e l e c t r i c a l c h a r g e o n t h e mi c e l l e su r f a c e .

N M R c h e m i c a l s h i f t m e a s u r e m e n t s a n d f l u o r e s -

c e n c e q u e n c h i n g h a v e b e e n c o m b i n e d i n a s t u d y

o f t h e a g g r e g a t i o n b e h a v i o r a n d p r t+ p e r ti e s o f



2

P. Alexandridis , T .A . Ha t ton/C ol lo ids Sur fac es A: Phys icochem. Eng. Aspe cts 96 1995) 1 46

P l u r o n i c s L 6 4 a n d F 6 8 i n t h e p r e s e n c e o f S D S a t

2 0 an d 4 0 ° C [ 8 6 ] . W h i le t h e c o p o l y m e r s d i d n o t

f o r m m i c e l l e s a t 2 0 ° C , t h e y f o r m e d m i x e d m i c e l l e s

w i t h S D S , a t S D S c o n c e n t r a t i o n s a s l o w a s 1 r a M .

T h e a g g r e g a t i o n n u m b e r s o b t a i n e d f r o m f l u o r e s -

c e n c e q u e n c h i n g s t u d i e s in d i c a t e th a t a b o u t t h r e e

S D S m o l e c u l e s p e r c o p o l y m e r m o l e c u l e a r e n e e d e d

t o g l u e t o g e t h e r t h e P P O b l o c k s i n t o a h y d r o -

p h o b i c c o r e at 2 0 ~ C [ 8 6 ] . T h e m i c e ll e s f o r m e d a t

l o w S D S c o n c e n t r a t i o n s w e r e s m a ll ( 1 5 S D S m o l e -

c u le s a s s o c i a te d w i t h f o u r t o f iv e c o p o l y m e r m o l e -

c u l es ) ; t h e m i c e l le s c o n s i s t e d m a i n l y o f S D S a t h i g h

S D S c o n c e n t r a t i o n s . L 6 4 f o r m e d m i c e l l e s a t 4 0 u C

t h a t w e r e l a r g e b u t d e c r e a s e d r a p i d l y i n s i z e u p o n

a d d i t i o n o f S D S . T h e 1 3C c h e m i c a l s h if ts f o r t h e

m e t h y l c a r b o n s i n t h e P P O b l o c k w e r e i n d i c a ti v e

o f a c h a n g e f r o m a c o i l e d c o n f o r m a t i o n i n s m a l l

m i c e l l e s , u n i m e r s , a s w e l l a s i n t h e b u l k P P O

l i q u i d , t o a m o r e e x t e n d e d P P O c o n f o r m a t i o n i n

t h e l a r g e m i c e ll e s. T h e c h a n g e o f th e s h i ft s o f th e

i n d i v i d u a l c a r b o n s i n th e a l k y l c h a i n o f S D S o n

a d d i t i o n o f t h e c o p o l y m e r s t o S D S m i c el le s fo l -

l o w e d a s i m il a r p a t t e r n f o r b o t h L 6 4 a n d F 6 8

P E O P P O - P E O c o p o ly m e rs [ 8 6 ] .

R e c e nt ly , H e c h t a n d H o f f m a n n [ 8 7 ] r e p o r t e d

t h e i n fl u e n c e o f S D S o n t h e a g g r e g a t i o n b e h a v i o r

o f P l u r o n i c F 1 2 7 . F o r b l o c k c o p o l y m e r s o l u t io n s

i n t h e c o n c e n t r a t i o n r a n g e o f 1 - 5 % a n d i n c r e a s i n g

S D S c o n c e n t r a t i o n , t h e l i g h t s c a t t e r i n g i n t e n s i t y

p a s s e d t h r o u g h a d e e p m i n i m u m , t h e e l ec t ri c b i re -

f r in g e n c e p a s s e d o v e r a m a x i m u m , a n d t h e e n d o -

t h e r m i c p e a k o f t h e D S C s ig n a ls ( d u e to t h e

m i c e l l iz a t i o n o f t h e b l o c k c o p o l y m e r ) d i s a p p e a r e d .

T h e s e r e s u l t s i n d i c a t e t h a t S D S b i n d s t o u n i m e r s

o f F 1 2 7 a n d t h e r e b y s u p p r e s s e s c o m p l e t e l y t h e

f o r m a t i o n o f F 12 7 m ic e ll es . A p p r o x i m a t e l y s i x S D S

m o l ec u le s b i n d t o o n e F1 2 7 P E O - P P O P E O

c o p o l y m e r m o l e c u l e at s a t u r a t i o n . T h e b i r e f ri n -

g e n c e d a t a i n d i c a t e d t h a t th e F 1 2 7 / S D S c o m p l e x

e x is ts in a m o r e o r l e ss e x t e n d e d c o n f o r m a t i o n a n d

n o t a s a c o il . T h e b i n d i n g o f S D S o n F 1 2 7 w a s

a l s o c o n f i r m e d b y s u r f a c e t e n s i o n m e a s u r e m e n t s ,

w h i c h s h o w e d t h e s u r f a c e a c t i v e b l o c k c o p o l y m e r s

t o b e d i s p l a c e d f r o m t h e a i r - w a t e r i n t e r f a c e s b y

S D S m o l e c u le s . T h e b i n d i n g o f S D S o n F 1 2 7

c o m m e n c e d a t a c o n c e n t r a t io n f a r b e l ow t h e C M C

o f S D S . A d d i t i o n o f C T A B N-cetyl-N,N,N-tri-

m e t h y l a m m o n i u m b r o m i d e ) t o F 1 2 7 s o l u t i o n s

d e c r e a s e d th e m a g n i t u d e o f t h e D S C p e a k , s h o w i n g

t h a t C T A B ( a c a t i o n i c s u r f a c t a n t ) a l s o s u p p r e s s e s

t h e m i c e l l i z a ti o n o f F 1 2 7 . T h e z w i t t e r i o n i c s u r f a c -

t a n t C a 4 D M A O N,N-dimethyl-l-tetradecanam-

i n e - N - o x i d e ) s u p p r e s s e s F 1 2 7 m i c e l l i z a t i o n a t

c o n c e n t r a t i o n s t e n t i m e s a b o v e i t s

C M C ,

sugge s t -

i n g a d if f e re n t m o d e o f a s s o c i a t i o n t h a n S D S ;

m a y b e , F 1 2 7 m o l e c u l e s b i n d t o t h e s u r f a c e o f

C 1 4 D M A O m i c e l l e s [ 8 7 ] .

6 M o de l i ng o f b l o ck co po l y m er m i ce l l iza t i o n a nd

micel l e s tructure

T h e o r i e s o f m i c e ll e f o r m a t i o n i n s o l u t i o n s o f

b l o c k c o p o l y m e r s h a v e b e e n s u g g e s t e d b y a

n u m b e r o f re s e a r c h e rs o v e r t h e p a s t d e c a d e . L e i b l er

e t a l. [ 8 8 ] , N o o l a n d i a n d H o n g [ 8 9 ] , M u n c h a n d

G a s t [ 9 0 ] , a n d N a g a r a j a n a n d G a n e s h [ 9 1 ] c o m -

p u t e d t h e f r ee e n e r g y o f m i c el le f o r m a t i o n a s s u m i n g

u n i f o r m c o p o l y m e r s e g m e n t c o n c e n t r a t i o n s i n t h e

c o r e a n d c o r o n a r e g i o n s , r es p e c t i v e ly . S c a l i n g t h e o -

r i e s f o r p o l y m e r i c m i c e l l e s w i t h a n i n s o l u b l e c o r e

a n d a n e x t e n d e d c o r o n a w e r e d e v e l o p e d b y

H a l p e r in [ 7 1 ] , M a r q u e s [ 9 2 ] , S e m e n o v [ 9 3 ] , a n d

Z h u l i n a a n d B i r s h te i n [ 9 4 ] . I n a t h ir d a p p r o a c h ,

v a n L e n t a n d S c h e u t j e n s [ 9 5 ] , L i n s e [ 5 0 , 5 1 ] , a n d

H u r t e r e t a l . [ 4 8 , 4 9 ] u s e d a s e l f - c o n s i s t e n t m e a n -

f ie l d t h e o r y t o d e t e r m i n e t h e d e t a i l e d s e g m e n t

d e n s i t y p r o f i l e s w i t h i n a m i c e ll e , m a k i n g n o a p r i o r i

a s s u m p t i o n s a s t o t h e l o c a t i o n s o f t h e m i c e l l a r

c o m p o n e n t s . T h e m i c e l l e s t r u c t u r e s d e t e r m i n e d

f r o m t h i s m o d e l h a v e b e e n c o n f i r m e d r e c e n t l y b y

M o n t e C a r l o s i m u l a t i o n s ( e . g . W a n g e t a l . [ 9 6 ] ) ,

a n d s h o w t h a t t h e i n t e r f a c i a l r e g i o n b e t w e e n t h e

c o r e a n d c o r o n a o f t h e m i c e l l e is d i f fu s e a n d n o t

s h a r p a s i s a s s u m e d i n t h e o t h e r m o d e l i n g

a p p r o a c h e s .

6.1. Phenomenological models

M u n c h a n d G a s t [ 9 0 ] a p p l ie d th e th e o r y o f

L e i b l e r e t al . [ 8 8 ] , w h i c h t r e a t e d m i c e ll e f o r m a t i o n

i n m i x t u r e s o f b l o c k c o p o l y m e r s a n d h o m o -

p o l y m e r s , t o d e s c r i b e t h e m i c e l l iz a t i o n o f d i b l o c k

A B c o p o l y m e r s in s o l u ti o n . T h e m i ce l le c o r e w a s

a s s u m e d t o b e a m e l t o f B c h a in s , w h i l e t h e c o r o n a

c o n t a i n e d A c h a i n s w i t h s o l v e n t S . T h e i n t e r a c ti o n



P . A l e .w m ~ h ' f l li s , T . , 4 . H a t t o # i / C o l l o i d ~ " S u d d c e . ~ A . " P h v s i c o c h c m . E H g , . . ~l .S o c ct s 9 6 ( 1 9 9 5 ~ 4 6 2 1

b e t w e e n t h e s o l v e n t S a n d t h e B s e g m e n t w a s

d e s c r i b e d b y t h e Z us i n t e r a c t i o n p a r a m e t e r , w h e r e a s

a t h e r m a l i n t e r a c t i o n s b e t w e e n t h e A s e g m e n t a n d

s o l v e n t w e r e a s s u m e d ( Z A S = 0 L T h e e n e r g y o f a

s i n g l e m i c e l l e w a s e x p r e s s e d a s a c o n t r i b u t i o n o f

t h e c o r e s o l v e n t i n t e r f a c i a l e n e r g y , t h e e n e r g y d u e

t o d e f o r m a t i o n o f t h e c o p o l y m e r c h a i n s in t h e

m i c e l le , a n d t h e f r e e e n e r g y o f m i x i n g o f s o l v e n t

m o l e c u l e s w i t h t h e A m o n o m e r s i n t h e c o r o n a ( ' / ~ o :

v o l u m e f r a c t i o n o f A in t h e c o r o n a l . T h e t o t a l f r e e

e n e r g y o f t h e s y s t e m i n c l u d e d t h e e n e r g y o f t h e

m i c e l l e s , t h e f r e e e n e r g y o f m i x i n g i n d i v i d u a l

d i b l o c k c o p o l y m e r s ( u ni m e r s ) w it h s o l v e n t m o l e -

c u le s ~ q ~ : v o l u m e f r a c t io n o f c o p o l y m e r s o u t s i d e

t h e m i c e l l e s ) , a n d a n e n e r g y c o n t r i b u t i o n a r i s i n g

f r o m t h e t r a n s l a ti o n a l e n t r o p y o f th e g a s o f

m i c e l l e s . T h e t o t a l f r e e e n e r g y w a s m i n i m i z e d w i t h

r e s p e c t t o t h e m i c e l l e a g g r e g a t i o n n u m b e r p , a n d

t h e w f l u m e f r a c t i o n s qS~ a n d ~0 . O n e u s e f u l f e a t u r e

o f th e t h e o r y o f L e i b l e r e t a l . [ 8 8 ] i s t h a t , f o r a

h i g h d e g r e e o f i n c o m p a t i b i l i ty b e t w e e n B a n d S ,

t h e f r ee e n e r g y ' m i n i m i z a t i o n w i t h r e s p e c t t o ~

r e s u l t s i n a n a n a l y t i c a l e s t i m a t e f o r

C M C a s 4 1

~+0

[ 8 8 , 9 0 ] :

4>: ~

exp{.l: k

T + N -

ZllsNl~ ) ( 13 )

w h e r e

[ , k T

is th e e n e r g y p e r c o p o l y m e r c h a i n i n

a n i s o l a t e d m i c e ll e , N i s t h e t o t a l n u m b e r o f

s e g m e n t s in t h e c o p o l y m e r m o l e c u l e , N , is t h e

n u m b e r o f B t c o r e ) s e g m e n t s , a n d Z R sN ~ is t h e

t o t a l e f f e c t iv e in t e r a c t i o n p e r c h a i n ( s in c e t h e A

c h a i n is in a n a t h e r m a l e n v i r o n m e n t l . T h e l e a d i n g

o r d e r t e r m s i n t h e / / k T e x p r e s s i o n a r e [ 9 0 ]

[ 5 k T ~ { 0 j T 2 . ~ B } 1 6 ( ~ . B S n l g } 1 2 1 .) 1 3

+ N i I + N I , , , 'N ~ , I ~ (1 -q ~oj ~b o ~ l n ( 1 q~o)

( 1 4 )

T h e f i r st t e r m o n t h e r i g h t - h a n d - s i d e o f Eq . ( 1 4 )

a r i s e s f r o m t h e e n e r g y r e q u i r e d t o f o r m t h e m i c e l l e

c o r e s o l v e n t i n t e r f a c e a n d o p p o s e s t h e f o r m a t i o n

o f m i c e l le s , w h e r e a s t h e s e c o n d t e r m , w h i c h o r i g i -

n a t e s f r o m t h e c o n f i g u r a t i o n a l e n t r o p y o f m i x i n g

s o l v e n t m o l e c u l e s w i t h A s e g m e n t s i n t h e c o r o n a ,

f a v o r s m i c e l li z a t i o n . A s Z As w a s a s s u m e d t o b e

z e r o , n o e f f e c t o f b l o c k A o n m i c e l l i z a t i o n c o u l d

b e o b s e r v e d i n t h e m o d e l o f M u n c h a n d G a s t ,

o t h e r t h a n t h e c o n f i g u r a t io n a l e n t r o p y o f m i x i n g

o f s o l v e n t m o l e c u l e s w i t h A m o n o m e r s i n th e

c o r o n a . T h e m i c e ll i z a li o n m o d e l o f N a g a r a j a n a n d

G a n e s h [ 9 1 ] t o o k i n t o a c c o u n t A s o lv e n t in t e r-

a c t i o n s : h o w e v e r , t h is w a s a t t h e e x p e n s e o f a n

a n a l y t i c a l e x p r e s s i o n f o r ( ~c Mc .

I n t h e m i c e l l i z a t i o n t h e o r y d e v e l o p e d b y

N a g a r a j a n f in d G a n e s h [ 9 1 ] t h e m i c e ll e s w e r e a l s o

a s s u m e d t o h a v e a c o m p l e t e l , , , s e g r e g a t e d c o r e

r e g i o n , c o n s i s t i n g o f o n l y t h e B b lo c k , a n d a c o r o n a

r e g i o n c o n s i s l i n g o f th e s o l v e n t S a n d t h e s o l v e n t -

c o m p a t i b l e A b l o c k . T h e f i'e e e n e r g y o f m i c e l li z a -

t i o n w a s e x p r e s s e d i n t e r m s o f c h a n g e s in t h e s t a t e

o f d i l u t i o n o f b l o c k s A a n d B , c h a n g e s i n th e s t a t e

o f d e f o r m a t i o n o f b l o c k s A a n d B . l o c a l i z a t i o n o f

t h e c o p o l y l n e r mo l e c u l e s f i t t h e mi c e l l e i n t e r f a c e ,

a n d f o r m a t i o n o f t h e m i c e l l a r c o r e s o l v e nt

i n t e r f a c e . T h e h y d r o p h o b i c i n t e r a c t i o n s , w h i c h

s t r o n g l y i n f l u e n c e t h e m i c e l l i z a t i o n i n w a t e r a s

d i s c u s s e d i n S e c t i o n 4 . w e r e i m p l i c i t l y a c c o u n t e d

f o r in t h e e x p e r i m e n t a l l y d e t e r m i n e d Z v a l u e s u s e d

f o r t h e P E O w a t e r a n d P P O w a t e r in t e r a c t io n

f re e e n e r g y t e r m s . T h e m i c e l l iz a t io n p a r a m e t e r s

w e r e o b t a i n e d f i 'o m m i n i m i z a t i o n o f t h e f re e e n e r g y

e x p r e s s i o n w i t h r e s p e c t t o t h e r a d i u s o f t h e m i c e l le

c o r e a n d t h e c o r o n a t h i c k n c s s . T h e m o s t i m p o r t a n t

c o n t r i b u t i o n l a v o r i n g m i c e l l i z a t i o n a r i s e s f r o m t h e

c h a n g e in s t a l e o f d i h it i o n o f b l o c k B a s i t g o e s

f r o m s o l v e n t S in t o t h e m i c e l l e c o r e . N o t e t h a t t h e

m u t u a l i n c o m p a t i b i l i t y o f b l o c k s A a n d B is a l s o

e s s e n t ia l f o r m i c e l li z a t i o n , o t h e r w i s c t h e i n c o m p a t i -

b i l i t y b e t w e e n t h e s o l v e n t a n d p o l y m e r b l o c k s w i l l

g i v e r i s e t o m a c r o s c o p i c p h a s e s e p a r a t i o n : t h e

i n c o m p a t i b i l i t y o f b l o c k s A a n d B w a s i m p l i c i t ly

t a k e n i n to a c c o u n t b y th e c o m p l e t e l y s e g r e g a t e d

c o r e s t r u c t u r e a s s u m e d f o r t h e m i c e l l e . T h e m o s t

i m p o r t a n t

c o n t r i b u t i o n

o p p o s i n g m i c e l l i z a t i o n

o r i g i n a t e s f r o m t h e f o r m a t i o n o f t h e m i c e ll e c o r e

s o l v e n t i n t e r f a c e . T h e o p t i m a l a g g r e g a t i o n n u m b e r

i s d e t e r m i n e d p r i m a r i l y b y b a l a n c i n g t h e h ' e e

e n e rg y , c o n t r i b u t i o n s d u e t o c h a n g e i n s t a t e o f

d i h i ti o n a n d d e f o r m a t i o n o f t he A ( m o r e i m p o r t a n t

i n t h e c a s e o f t he P E O P P O , w a t c r s y s t e m t a n d B

b l o c k s , fa w ~ r in g l o w a g g r e g a t i o n n u m b e r s , a n d t h e

f re e e n e rg y c o n t r i b u t i o n s d u e t o f o r m a l i o n o f t h e

m i c e l le c o r e s o l v e n t i n t e r t h c e t h a t f a v o r h i g h

a g g r e g a t i o n n u m b c r s . T h e t h e o r y o f N a g a r a j a n

f in d G a n c s h , d e v e l o p e d f o r d i b lo c k c o p o l y m e r s ,

s u c c e e d s m p r e d i c t i n g q u a l i t a t i v e l 5 l h c c o n t r i b u -



P. Alexandrhl i s . T .A. Ha t ton/Co l lo ids Sur[ aces A: Ph) s icochem. Eng. Aspects 96 (1995) 1 46

tions of the copolymer composition and molecular

weight on the micellization of triblock copolymers.

The trends are the same in both theory and experi-

ment, although the theory overpredicts the experi-

mental ly obtained free energies of micellization by

a factor of eight [76], resulting in unrealistically

low CMC values. Part of the numerical discrepancy

could be eliminated by adding extra terms in the

expression for the free energy of micellization, to

account for the entropy penalty involved in joining

two AB diblock copolymer molecules to create a

ABzA triblock, and in having loops in the micelle

core.

Prochazka et al. [97] modified Nagarajan's

model to treat the association of ~ B A triblock

copolymers in solvents selective for A blocks, by

introducing two additional terms into the free

energy expression discussed above. These terms

accounted for the reduction in entropy due to loop

formation of the middle (B) block, and the localiza-

tion of the two A-B joints of the copolymer at the

interface between the core and the corona of the

micelle. This modification resulted in less negative

AG ' values (in the correct direction with respect

to agreement between experiment and theory), but

the decrease in the absolute value of the predicted

AG was approximately 10%, not enough to

account for the eight-fold overprediction of the

experimental AG-' values. A comparison of the

model calculations by Nagarajan and Ganesh [91]

for a diblock copolymer with the results of

Prochazka et al. [97] obtained for a triblock

copolymer (of the same molecular weight and

composition as the diblock) showed the aggrega-

tion number of the diblock copolymer micelles to

be three to five times higher than that for the

triblock micelles. Linse [50,51] concluded, using a

mean-field lattice micellization model, that there is

a five-fold increase in CMC and a four-fold

decrease in aggregation number on going from a

di- to a triblock PEO-PPO copolymer. Prochazka

et al. [97] also made an interesting observation

regarding the effect of the deformation energy

contribution on the model AG =expression: model

calculations using the deformation energy terms of

Nagarajan and Leibler et al. [88] resulted in

different aggregation numbers, although both

terms were based on Flory's [98] theory.

Recently, Izzo and Marques [99] studied theo-

retically, using scaling concepts, the formation of

dilute phases of spherical, cylindrical, and planar

aggregates of diblock A-B and triblock ~B-A

copolymer chains in solvents selective to B. They

treated the micelle formation of the triblock copol-

ymers in the same way as that of the diblocks, the

only difference being that the core was composed

of twice as many chains of half the length of B.

This resulted in AG triblock ~2AG:~diblock, which is

counterintuitive and opposite to the micellization

theories examined above. It thus appears that the

entropy loss associated with the formation of

loops is essential for describing the micellization

of triblock copolymers. The loop (backfolding)

entropy term was first introduced by ten Brinke

and Hadziioannou [ 100] in their model of B A-B

copolymer micelle formation in A-homopolymer,

and modified by Balsara et al. [101] in a similar

model. Balsara et al. [101] commented on the

applicability of the theory of Leibler et al. [88],

originally proposed for micellization of A-B block

copolymer in A-homopolymer, in the theories of

Munch and Gast [90] and Nagarajan and Ganesh

[91], that treat micellization in selective solvents.

They noted that, in the presence of solvents, the

description of the core corona interface is compli-

cated owing to the three-component (owing to the

presence of solvent at the interface) nature of the

problem [101]. These points, in addition to the

hydrogen bonding interactions between both PEO

and PPO, and water, may account for the quantita-

tive discrepancy between predictions of phenome-

nological block copolymer micellization theories

and the experimental results of Ref. [28] for the

system PEO- PPO PEO/water.

6.2. Mean f ie ld lat t i ce mod els

Another approach to modeling block copolymer

micelles has used the Scheutjens-Fleer theory

[ 102], an extension of the Flory-Huggins analysis

of homogeneous polymer solutions [98], in which

the polymer chains are allowed to assume different

conformations on a lattice. A first-order Markov

approximat ion (i.e. the position of a segment

depends only on that of the preceding segment in

the chain) is used in the Flory-Huggins analysis



P. . 4 h ' . va mh ' id is , T . A . H a t to n / ( ' o l l o id s S u Ja ~ e s A . Ph r , v i co ch em. EHg . . 4. V~ec l. v 9 6 r 1 ~ )9 5 , I 4r~ 2 3

a n d , a s a r e s t l l t , t h e c h a i n c o n f o r m a t i o n f o l l o w s

t h e p a t h o f a r a n d o m w a l k . I n a d d i t i o n , a m e a n -

f i e l d a s s u m p t i o n i s i n v o k e d t o d e s c r i b e t h e i n t e r -

a c t i o n s b e t w e e n d i f f e r e n t p o l y m e r s e g m e n t s . T h e

f r e e e n e r g y o f th e s y s t e m is m i n i m i z e d i n o r d e r t o

c a l c u l a t e t h e e q u i l i b r i u m t h e r m o d y n a m i c p r o p e r -

t ie s o f t h e p o l y m e r s o l u t io n . F o r s p a t i a l l y i n h o m -

o g e n e o u s s y s t e m s s u c h a s t h o s e c o n t a i n i n g i n t e r -

f a c e s , S c h e u t j e n s a n d c o - w o r k e r s [ 9 5 , 1 0 2 , 1 0 3 ]

r e s t r i c t e d t h e m e a n - f i e l d a p p r o x i m a t i o n t o t w o

d i m e n s i o n s l i. e. w i t h in p a r a l l e l o r c o n c e n t r i c l a t t i c e

l a y e rs l a n d a p p l i e d a s t e p - w e i g h t e d r a n d o m w a l k

t o a c c o u n t f o r t h e in h o m o g e n e i t i e s n o r m a l t o t h e

l a y e r s . T h e p o l y m e r a n d s o l v e n t m o l e c u l e s a r e

a s s u m e d t o b e d i s t r i b u t e d o v e r a l a t ti c e , s u c h t h a t

s o l v e n t m o l e c u l e s a n d p o l y m e r s e g m e n t s o c c u p y

o n e l a t t ic e s it e e a c h . E a c h p o l y m e r c h a i n c a n

a s s u m e a l a rg e n u m b e r o f p o s s ib l e c o n f o r m a t i o n s ,

d e f i n e d b y t h e l a y e r n u m b e r s i n w h i c h s u c c e s s i v e

s e g m e n t s a r e f o u n d . T h e r e c a n b e m a n y d i f f e r e n t

a r r a n g e m e n t s f or e ac h c o n f o r m a t i o n : if t he n u m b e r

o f p o l y m e r c h a i n s i n e a c h c o n f o r m a t i o n is sp e c if ie d ,

t h e c o n f i g u r a t i o n a l e n t r o p y c o n t r i b u t i o n t o t h e

s y s t e m f r ee e n e r g y c a n b e e v a l u a t e d . T h e o t h e r

c o n t r i b u t i o n s t o t h is f re e e n e r g y , d u e t o t h e i n t e r -

a c t i o n s b e t w e e n t h e p o l y m e r m o l ec u l e s, s o h e n t

m o l e c u l e s a n d t h e s u r f a c e , a r e c h a r a c t e r i z e d b y

F I o r y H u g g i n s 7 - p a r a m e t e r s . I f t h e f re e e n e r g y o f

t h e s y s t e m is m i n i m i z e d w i t h r e s p e c t to t h e n u m b e r

o f p o l y m e r c h a i n s i n e a c h c o n f o r m a t i o n , it is

p o s s i b l e t o c a l c u l a t e t h e e q u i l i b r i u m s e g m e n t d e n -

s i t y p r o f i l e s . Se l f - c o n s i s t e n t f i e l d t h e o r y c a n b e u s e d

t o c a l c u l a t e t h e s e g m e n t d e n s i t y p r o f i l e s in a m i c e l l e

o n c e t h e a g g r e g a t i o n n u m b e r o f t h e m i c e ll e is

k n o w n [ 9 5 , 1 0 3 , 1 0 4 ] . T o fi n d t h is a g g r e g a t i o n

n u m b e r , s m a l l s y s te m t h e r m o d y n a m i c s c a n b e u s ed

[ 1 0 5 1 0 7 ] , a c c o r d i n g t o w h i c h th e c h a n g e i n f r ee

e n e r g y d u e t o t h e c h a n g e i n t h e n u m b e r o f m i c e ll e s

( a t c o n s t a n t t e m p e r a t u r e , p r e s s u r e a n d n u m b e r o f

m o l e c u l e s l m u s t b e z e r o a t e q u i l i b r i u m . T h i s

' e xc e ss f r ee e n e r g y is t h e s u m o f t h e e n e r g y

r e q u i r e d t o c r e a t e m i c e l le s a n d t h e e n e r g y d u e t o

t h e t r a n s l a t i o n a l e n t r o p y o f t h e m i c e ll e s. S i nc e t h e

s e g m e n t d e n s i t y p r o f l e s a r e r e q u i r e d i n o r d e r t o

c a l c u l a t e t h e e n e r g y o f m i c e l l e f o r m a t i o n , a n i t e r a -

t i r e p r o c e s s i s f o l l o w e d . T h e s e l f - c o n s i s t e n t m e a n -

f ie l d l a t ti c e t h e o r y o f S c h e u t j e n s a n d F l e e r [ 1 0 2 ]

h a s b e e n u s e d t o s t u d y m a n y c o l l o i d a l sy s t e m s ,

i n cl u di n g a d s o r p t i o n o f h o m o p o l y m e r s a n d b lo c k

c o p o l y m e r s o n s u rf a c e s [ 1 0 8 ] , i n t e r a c ti o n s b e t -

w e e n a d s o r b e d p o l y m e r la y e r s [ 1 0 9 ] , a n d t h e

f o r m a t i o n o f m i c e l le s [ 1 0 3 ] , v e s ic l e s [ 1 0 4 ] a n d

m e m b r a n e s [ 1 1 0 ] .

R e c e n t l y , t h e S c h e u t j e n s a n d F l e e r t h e o r y h a s

b e e n e x t e n d e d b y H u r t e r e t a l . [ 4 8 , 4 9 ] t o s t u d y

t h e f o r m a t i o n o f m i c e ll e s b y l in e a r P E O

P P O P E O a n d b r a n c h e d , s t a r- l ik e , a m p h i p h i l i c

b l o c k c o p o l 5 m e r s : t h e s o l u b i l iz a t io n o f n a p h t h a -

l e n e i n t h e s e mi c e l l e s h a s a l s o b e e n a n a l y z e d a s a

f u n c t io n o f p o l y m e r s t r u ct u r e , c o n l p o s i t i o n a n d

m o l e c u l a r w e i g h t [ 4 8 , 4 9 ] . T h e d e t a i l e d d e n s i t y

p r o f i le s o f t h e p o l y m e r s e g m e n t i n t h e m i c e l l e s

w e r e o b t a in e d , a n d m a c r o s c o p i c q u a n t it i e s s u c h a s

c r i t i c a l m i c e l l i z a t i o n c o n c e n t r a t i o n , a g g r e g a t i o n

n u m b e r , a n d m i c e l l e s i z e w e r e c a l c u l a t e d . T h e

c a l c u l a t i o n s s h o w t h a t h i g h e r - m o l e c u l a r - w e i g h t

p o l y m e r s f o r m l a r g e r m i c e ll e s. T h e a d d i t i o n o f t h e

s o l u t e c a u s e s t h e m i c e l l e t o b e c o m e l a r g e r , w i t h a

l o w e r c o n c e n t r a t i o n o f w a t e r a n d a h i g h er c o n c e n -

t r a t i o n o f P P O i n t h e m i c e l le c o r e . T h e s i m p l e

l a t t i c e t h e o r y f o r fl e x i b le c h a i n mo l e c u l e s , h o w e v e r ,

c a n n o t c a p t u r e e f l' ec ts s u c h a s t h e p h a s e b e h a v i o r

o f P E O a n d P P O i n w a t e r , w h e r e a h > w e r c r i t i c a l

s o l u t i o n t e l n p e r a t n r e is o b s e r v e d . T o p r e d i c t s u c h

b e h a v i o r , t h e g a u c h e a n d t r a n s b o n d o r i e n t a t i o n s

o f t h e p o l y l n e r c h a i n m u s t b e a c c o u n t e d f o r.

L e e r m a k e r s [ 1 1 1 ] c o m b i n e d t h e r o t a t i o n a l i so -

m e r i c s t a t e s c h e m e ( w h i c h a c c o u n t s f o r t h e g a u c h e

t r a n s o r i e n t a t i o n s i n a c h a i n a n d e l i m i n a t e s b a c k -

f o l d i n g l w i t h s e l f - c o n s i s t e n t l i e l d t h e o r y t o p r e d i c t

t h e f o r m a t i o n o f l ip id b i l a y e r m e m b r a n e s a n d l ip id

v e si cl es . A n a p p r o a c h w h i c h i s c o m p u t a t i o n a l l y

m o r e s i m p l e t h a n t h e r o t a t i o n a l i s o m e r i c s t a te

s c h e m e , b u t n e v e r t h e l e s s a c c o u n t s f o r t h e t e m p e r -

a t t i re a n d c o m p o s i t i o n d e p e n d e n c e c ,f t h e in t e r -

a c t i o n p a r a m e t e r Z in a p h y s i c a l ly a c c e p t a b l e

m a n n e r , h a s r e c e n t l y b e e n p r e s e n t e d b y K a r l s t r o m

[ 4 6 ] . T h i s m o d e l f o r P E O r e c o g n i z e d t h a t c e r t a i n

s e q u e n c e s o l t h e g a u c h e t r a n s o r i e n t a t i o n s i n a n

E O s e g m e n t w o u l d l e a d

I t L I. p O I L l r

c o n f o r m a t i o n ,

w h i l e o t h e r s w o u l d b e e s s e n t ia l l y n o n - p o h u : t h e

p r o b a b i l i t y o f n o n - p o l a r c o n f o r m a t i o n s i n c r e a se d

w i t h t e m p e r a t u r e , m a k i n g t h e p o l y m e r m o r e

h y d r o p h o b i c . T h e s o l u b i l i t y g a p i n P E O w a t e r

a n d P P O w a t e r p h a se d i a g r a m s c o u ld b e re p r o -

d u c e d u s i n g t h i s m o d e l .



4 P, Alexandridis , T .A. Hat ton/ 'Col lo ids Sur /aces A. Phys icochem. Eng. Aspects 96 (1995) 1 46

A m o d e l f o r b l o c k c o p o l y m e r m i c e l le s h a s b e e n

d e v e l o p e d [ 4 8 , 5 0 ] w h i c h i n c o r p o r a t e s K a r ls t r om s

i d e a s t o a c c o u n t f o r t h e c o n f o r m a t i o n a l d i s tr i b u -

t i o n i n P E O a n d P P O . A n i n c r e a s e i n t e m p e r a t u r e

c a u s e d t h e m o d e l - p r e d i c t e d m i c e l l e a g g r e g a t i o n

n u m b e r t o i n c r e a s e , a n d t h e C M C c o n c e n t r a t i o n

t o d e c r e a s e . T h e a g g r e g a t i o n n u m b e r i n c r e a s e d

r a p i d l y w i t h p o l y m e r c o n c e n t r a t i o n u n d e r d i l u t e

c o n d i t i o n s , b u t w a s a p p r o x i m a t e l y i n v a r i a n t t o

p o l y m e r c o n c e n t r a t i o n a t c o n c e n t r a t i o n s f a r f r o m

t h e C M C . R a i s i n g t h e P P O c o n t e n t o r t h e m o l e c u -

l a r w e i g h t o f t h e c o p o l y m e r i n c r e a s e d t h e a g g r e g a -

t i o n n u m b e r , a n d d e c r e a s e d t h e C M C . T h e m o d e l

p r e d i c t s t h a t t h e m o s t h y d r o p h o b i c p o l y m e r ,

P l u r o n i c P 1 0 3 , d o e s n o t f o r m s p h e r i c a l m i c e l l e s a t

h i g h e r p o l y m e r c o n c e n t r a t i o n s , o w i n g t o p h a s e

s e p a r a t io n o r f o r m a t i o n o f a gg r e g a te s o f n o n -

s p h er i ca l s t ru c tu re . F i g . 1 5 (a) s h o w s th e e f fec t o f

P E O b l o c k s i z e o n t h e s e g m e n t d e n s i t y p ro f i le a n d

m i c el le si ze f or P E O P P O - P E O c o p o l y m e r s w i th

t h e s a m e P P O b l o c k [ 4 8 ] . A n in c r e a se in P E O

b l o ck s i ze re s u l t ed i n a s m a l l er m i ce l l e co re , w h i l e

t h e c o r o n a b e c a m e m o r e e x t e n d e d . T h i s i s r e f l e c t e d

i n t h e d e c r e a s i n g a g g r e g a t i o n n u m b e r f o r t h e

m i c e l l e s s h o w n i n F ig . 1 5 (b ) . T h e c o n f o r m a t i o n o f

t h e p o l y m e r w a s a f f e c t e d b y b o t h t h e t e m p e r a t u r e

a n d t h e c o m p o s i t i o n o f th e s u r r o u n d i n g s o l u t io n ;

b o t h P E O a n d P P O h a d a lo w e r fr a c ti o n o f p o l a r

s e g m e n t s i n t h e c o r e o f t h e m i c e ll e s, a n d t h e p o l a r

f r a c t io n d e c r e a s e d w i t h a n i n c r e a s e i n t e m p e r a t u r e .

T h e m i c e l l e a g g r e g a t i o n n u m b e r w a s f o u n d t o

i n c r e a s e w i t h a n i n c r e a s e in t h e s o l u t e b u l k c o n c e n -

t r a t i o n . T h i s i s i n a g r e e m e n t w i t h t h e e x p e r i m e n t a l

r e s u lt s o f A 1 - S a d e n e t a l. [ 2 9 ] , w h o f o u n d t h a t t h e

i n c r e a s e in t h e h y d r o d y n a m i c r a d i u s o f L 6 4 a g g r e -

g a t e s u p o n h e x a n e s o l u b i l i z a t i o n w a s g r e a t e r t h a n

c o u l d b e e x p l a in e d b y t h e a d d i t i on a l v o l u m e o f

th e s o l u te . I t w a s p o s tu l a ted th a t th e i n crea s e i n

h y d r o d y n a m i c r a d i u s r e s u l t e d f r o m a c o m b i n a t i o n

o f a n i n c r e a se i n th e a g g r e g a t i o n n u m b e r a n d t h e

i n c o r p o r a t i o n o f t h e s o l u t e ( h e x a n e i n t h e st u d y o f

R ef . [ 2 9 ] ) .

T h e p h a s e b e h a v i o r o f a q u e o u s s o l u t i o n s o f

b l o c k p o l y m e r s co n t a in i n g P E O a n d P P O h a s

b e e n m o d e l e d b y L i n se [ 5 1 ] o n t h e b a s is o f a

m e a n - f i e ld l a t t i c e t h e o r y ( s im i l a r t o t h a t u s e d b y

H u r t e r e t a l. [ 4 8 ] ) d e v e l o p e d f or m u l t i c o m p o n e n t

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~ m

8 ~

c o

g - , o

~ i i ; 0

2 0 40 60 80 100

P E O C o n t e n t

(wt%)

Fig. 15. (a) Effect of PEO block siz e on the segm ent density

profiles in micelles form ed by PEO PPO PEO copolyme rs

having the sam e PPO block , as p redicted by a self-consistent

mean-field model. The volum e fraction of PEO in the m icelle

is represented by the dotte d lines, the v olum e fraction of PPO

by the solid lines, and the volume fraction of water by the

broken lines. An increase in PEO block size results in a smaller

micelle core, wh ile the corona becom es mo re extended. This is

reflected in the decreasing aggregation num ber for the m iceles

as shown in (b). (Reprinted with permission from Ref. [49]:

copyright Am erican Chem ical Society, 1993.}



P . A h x and r i d i s , T . A . Ha t t on / C o l l o i d s Sur /m e .~ A . P i t v s i c oc l t em. E H ~ . A sp e c t s 96 I 1995 J 1 46

25

r i n g i n h e t e r o g e n e o u s s y s t e m s . T h e r e g i o n s f o r

u n i m e r s o l u t i o n , s p h e r i c a l m i c e l l e s , a n d e l o n g a t e d

r o d s h a v e b e e n e x a m i n e d fo r t h re e P E O - P P O -

P E O t r ib l o c k c o p o l y m e r s . A s e m i q u a n t i t a t iv e

d e s c r i p t i o n o f t h e s tr o n g t e m p e r a t u r e d e p e n d e n c e

o f t h e p h a s e b e h a v i o r w a s o b t a i n e d . A t l o w p o l y -

m e r c o n c e n t r a t i o n s a n d t e m p e r a t u r e s a u n i m e r

s o l u t i o n w a s f o u n d , w h e r e a s a t e i t h e r h i g h e r t e m -

p e r a t u r e s o r h i g h e r c o n c e n t r a t i o n s a s o l u t i o n o f

s p h e r i c a l m i c e l l e s w a s p r e s e n t . A t r a n s i t i o n f r o m

s p h e r i c a l t o i n f i n i t e l y l o n g r o d - l i k e a g g r e g a t e s

o c c u r r e d a t e v e n h i g h e r t e m p e r a t u r e s , a n d e v e n t u -

a l l y t h e s y s t e m s e p a r a t e d i n t o t w o p h a s e s . F i g . 1 6

s h o w s t h e p h a s e d i a g r a m o f P l u r o n i c s P I 0 5 , P 9 5 ,

a n d P 1 0 4 a s p r e d i c t e d b y th e m o d e l o f L i n s e [ 5 l ] .

A c o m p a r i s o n o f t h e p h a s e d i a g r a m s i n d i c a t e d t h a t

a r e d u c t i o n o f th e c o p o l y m e r m o l e c u l a r w e i g h t b y

a p p r o x im a t e l y 2 0 a t c o n s ta n t P P O / P E O c o m -

pos i t i on r a t i o ( i . e . f r om P105 to P95) l e d t o a n

i n c re a s e in C M C f o r a g iv e n t e m p e r a t u r e ,

a

s m a l l e r

m i c e l l a r r e g i o n , a n d a r e g i o n o f ro d s o f a s i m i l a r

r a n g e b u t s l i g h t l y s h i f t e d t o l o w e r t e m p e r a t u r e s .

D e c r e a s i n g t h e P E O b l o c k s iz e , w h i l e k e e p i n g t h e

P P O b l o c k c o n s t a n t ( i . e . f r o m P 1 0 5 t o P 1 0 4 1 ,

r e s u l t e d i n a s h if t o f t h e C M C c u r v e t o l o w e r

c o p o l y m e r c o n c e n t r a t i o n a t a g iv e n t e m p e r a t u r e ,

a s m a l l e r m i c e l l a r r e g i o n , a n d a s m a l l e r r e g i o n o f

r o d - l i k e a g g r e g a t e s w h i c h i s s h i f t e d c o n s i d e r a b l y

t o lo w e r te m p e r a t u r e s [ 5 1 ] . C o m p a r i s o n o f t h e

m o d e l i n g p r e d i c t io n s f o r t h e p h a s e d i a g r a m o f

L i n s e [ 5 1 ] w i t h t h e e x p e r i m e n t a l l y d e t e r m i n e d

p h a s e d i a g r a m o f M o r t e n s e n [ 6 1 ] r e v ea l s q u a l i t a -

t iv e a g r e e m e n t w i th r e s p ec t t o t h e n u m b e r o f

p h a s e s o c c u r r i n g a n d t h e i r r e l a t i v e l o c a t i o n .

H o w e v e r , t h e c a l c u l at e d C M C c u r v e o c c u r r e d a t

t e m p e r a t u r e s 2 0 3 0 K h i g h e r t h a n t h o s e e x p e r i-

m e n t a l l y o b s e r v e d , a n d t h e c a l c u l a t e d r e g i o n f o r

t h e h e x a g o n a l p h a s e o c c u r r e d a t a l o w e r t e m p e r -

a t u r e , t h u s l e a d i n g t o a t e m p e r a t u r e r a n g e f o r t h e

m i c e l l a r s o l u t i o n p h a s e w h i c h i s t o o n a r r o w .

S e g m e n t d e n s i t y p r o f i l e s i n d i c a t e d t h a t , a t a g i v e n

c o n c e n t r a t i o n , t h e r a d i a l e x t e n s i o n i n c r e a s e d

w h e r e a s t h e h e a d g r o u p a r e a d e c r e a s e d w i t h

i n c r e a s i n g t e m p e r a t u r e f o r b o t h m i c e l le s a n d r o d s .

A t t h e t r a n s i t i o n f r o m m i c e l l e s t o r o d s t h e r a d i a l

e x t e n s i o n w a s r e d u c e d a b r u p t l y b y a p p r o x i m a t e l y

10 1151

.

I n a d d i t i o n l o p h a s e b e h a v i o r . L i ns e [ 5 0 ] u s e d

3 4 0

" x :

3 2 O

l a )

[

I w o p n a s e r e g ,o , 1

r o d s

rni ~lle~

3 0 0 =

O 0 0 . 1 0 2

t ) t O

0 3

Y

t b )

34 0 ;, , i

3 2 0

i

two

p h a s e

ro j,on

\

-. rods i

m~celres i

3 0 0 ~ • : •

0 . 0 0 . 1 0 . 2

O l c

0 . 3

3 4 0

3 2 0

t w o p h a s e r e g io n

3 0 0 . . . . .

0 . 0 0 . 1

- - : = - r o d s

m l c e l l e s I

m o n o m e r s I

0 2 0 . 3

O o t

F i g . 1 6. P h a s e d i a g r a m s o f l a ) P I 0 5 , I b l P 9 5 , a n d l c) P I 0 4

P l u r o n i c P E O P P O P E O c o p o l y m e r s I p r e d i c t e d b .~ a se lf -

c o n s i s t e n t m e a n - f ie l d m o d e l m o d e l ) i n d i c a t i n g r e g io n s o f

u n i m e r s , m i c e l l e s , r o d s , a n d p h a s e s e p a r a t i o n . ( R e p r i n t e d w i t h

p e r m i s s i o n f r o m R e f . [ 5 1 ] : c o p y r i g h t A m e r i c a n ( h e r o i c a l

S o c i e t y , 1 9 9 3 . t

t h e m o d e l t o p r e d i c t m i c e l l i z a t i o n p r o p e r t i e s l i .e .

C M C , a g g r e g a ti o n n u m b e r , h y d r o d y n a m i c r a d iu s )

a n d t h e i r t e m p e r a t u r e d e p e n d e n c e f o r P l u r o n i c

c o p o l y m e r s o l u t io n s . F i g . 1 7 (a ) s h o w s t h e C M C

a n d t h e c o n c e n t r a t i o n o f t h e f re e p o l y m e r a s a f u n c -

t io n o f t e m p e r a t u r e fo r v a r io u s P E O P P O P E O



26 P. Alexandridis , T .A. Ha t ton/Co l lo ids Sur lac es A. Phys icochem. Eng. ,4apects 96 (1995) 1 46

( a )

3 3 0

3 2 0

3 1 0

3 0 0

0 . 0

'.~ ~i. -- ' ~ - -

• \ " " " \ ' - . ~ P g 5 P 8 5

" " \ . " ~ P 1 0 4

, . ~ ~ " ~ . . ~ ° ~ , ~ ~ ~

0 , 1 0 . 2 0 , 3 0 . 4 0 . 5

3 0

20

P 1 0 3

oa

Z

/

P 1 0 5

%

P 8 5

0 . 0 0 . 2 0 . 4 0 . 6

(b)

~,n,

F i g . 1 7. (a } C a l c u l a t e d ( b y a s e l f - c o n s i s t e n t m e a n - f i e l d m o d e l

m o d e l } C M C ( ) a n d t h e c o n c e n t r a t i o n o f f re e p o l y m e r ( . )

a s a f u n c t i o n o f t e m p e r a t u r e f o r P l u r o n i c s P 8 5 , P 9 5 , P 1 0 5 ,

P 1 0 4, a n d P 1 03 . ( b ) E ff e ct o f v a r y in g t h e c o p o l y m e r P E O / P P O

c o m p o s i t i o n { a t a g i v e n t e m p e r a t u r e } o n t h e m i c e l l a r a g g r e g a -

t i o n n u m b e r • ( R e p r i n t e d w i t h p e r m i s s i o n f r o m R e f . [ 5 0 ] ;

c o p y r i g h t A m e r i c a n C h e m i c a l S o c i e ty , 1 9 93 •)

mers, and provide detailed information on the

microstructure of the micelles. Copolymers of vary-

ing blockiness and block sequence could be han-

dled, and no a priori assumptions with respect to

the core-corona interface were necessary. The

model calculations showed qualitative agreement

with experimental predictions on the effect of

temperature, concentration, hydrophobicity, and

molecular weight of the polymer; quantitative

agreement was also good, considering that all

the model input parameters were obtained from

independent experiments. The phenomenological

theory of Nagarajan and Ganesh [91], developed

originally for diblock copolymers and extended to

triblocks by Prochazka et al. [97], was also suc-

cessful in predicting qualitatively the contributions

of copolymer composition and molecular weight

to the micellization of triblock PEO PPO-PEO

copolymers (although the predicted CMCs were

unreasonably low). A sharp interface had to be

assumed, however, between the core and the

corona, and variation of the number of blocks or

block sequence was not straightforward. Further,

the effects of temperature on micellization were

not examined. An advantage of the Nagarajan and

Ganesh theory is that the various contributions to

the micellization free energy were easily quantified,

leading to a better appreciation of their magnitude

and influence on the micellization.

copolymers [50]. Both the PPO/PEO ratio and

the copolymer molecular weight influence micelli-

zation, in qualitative agreement with experimental

CMC data [28]. The effect of varying the copoly-

mer PPO/PEO composition (at a given temper-

ature) on the micellar aggregation number is

presented in Fig. 17(b) [50]. For PE O- PP O PEO

polymers with the same PPO/PEO composition,

increasing the molecular weight resulted in increas-

ing the aggregation number. Note also that the

model predicted an

' i n f i n i t e

aggregation number

for Pluronic P103, similar to the predictions of

Hurter et al. [48,49].

The self-consistent mean-field theory used by

Hurter et al. [48,49] and Linse [50,51] was able

to reproduce the anomalous phase behavior of

PEO and PPO homopolymers, predict the micelli-

zation behavior of PEO-PP O PEO block copoly-

7 . S u r face ac ti v ity o f P E O P P O c op o l ym e r s

7 .1 . A d s o r p t i o n a t t h e a i r w a t e r in t er J ~ w e

Surface tension data for various Pluronic P EO-

PPO PEO copolymer aqueous solutions have

been reported by Alexandridis et al. [ 112] at two

temperatures, 25 and 35'~C. Fig. 18 shows surface

tension values for Pluronics P105 and P85, plotted

semilogarithmically with respect to the copolymer

concentration in the solution [112]. At low con-

centrations the surface tension decreased with

increasing concentration for all copolymers, in

accordance with the Gibbs adsorption isotherm

[113]. A change in slope (break) was observed in

the surface tension curve at a characterist ic concen-

tration, after which the surface tension values



P. A h' .vamtridis, T. A. Ha tton/C olloi & Sm:l~lces ,4. Pit rsicoclw m. /z ) l t~ . A ,v* ec ts 9 ,5 f 19 95 .J 1 4r~ 27

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7

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0 9

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, , @ 5

5 . 5 " C " X " 1 .

II

. . . . . . . . r . . . . . . . . i . . . . . . . . i . . . . . . . . i . . . . . . . . i . . . . . .

I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

f - , - t . i

5 " C \ . • o . . o ]

~ : ~

A . ' e ' o

• • o . •

" A " • ¥

I ' " A Y A , t 1 . . . l . . z T l ~ l

/

. . . . . . . . i . . . . . . . . t . . . . . . . . i . . . . . . . . i . . . . . . . . i . . , , , , I

~ 0 0 0 0 1 0 . 1 1 0

C o n c e n t ra tio n % w / v )

f -i g. I S. S u r fa c e t e n s i o n f o r P l u r o n i c P 1 0 5 a n d P 8 5 a q u e o u s

s o l u t i o n s , p l o t t ed a s a f u n c t i o n o f c o p o l y m e r c o n c e n t r a t i o n a t

2 5 a n d 3 5 C . ( A d a p t e d w i t h p e r m i s s i o n f r o m R e f . [ l l 2 ] :

c o p y r i g h t A m e r i c a n C h e m i c a l S o c i e t y , 1 99 4 .)

c o n t i n u e d t o d e c r e a s e u n t i l a p l a t e a u w a s r e a c h e d

( s e c o n d b r e a k ) : t h e s u r f a c e t e n s i o n v a l u e s r e m a i n e d

c o n s t a n t w i th f u r t h e r i n c r e a se in c o p o l y m e r c o n -

c e n t r a t i o n [ 1 1 2 ] . T h i s b e h a v i o r (t w o b r e a k s ) w a s

o b s e r v e d f o r m o s t c o p o l y m e r s st u d i e d a n d w a s

v e r y r e p r o d u c i b l e . T h e f i r s t ( l o w - c o n c e n t r a t i o n l

b r e a k o c c u r r e d a t a c o p o l y m e r c o n c e n t r a t io n o f

a p p r o x i m a t e l y 0 .0 0 1% , w h i c h w a s r o u g h l y t h e

s a m e f o r b o t h t e m p e r a t u r e s s t u d ie d a n d f o r a

n u m b e r o f c o p o l y m e r s . T h e s u r f a c e t e n s io n v a l u e

a t t h e o n s e t o f t h i s b r e a k v a r i e d f r o m 4 0 t o 5 0 m N

m ~. T h e p r e s e n c e o f t w o b r e a k s h a s l e d t o s o m e

c o n f u s i o n i n t h e l i t e r a t u r e o n P E O P P O P E O

c o p o l y m e r s r e g a r d i n g t h e i n t e r p r e t a t i o n o f t h e

s u r f a c e t e n s i o n d e p e n d e n c e o n c o n c e n t r a t i o n , a n d

t h e e x t r a c t io n o f t h e C M C f r o m s u c h a s e t o f d a t a

[ 3 0 . 1 1 4 , 1 1 5 ] . I n m o s t c a s e s i t h a s b e e n a s s u m e d

t h a t t h e f ir s t c h a n g e i n s l o p e s ig n i f ie d t h e C M C

[ 1 1 5 , 1 1 6 ] , a l t h o u g h o t h e r i n t e r p r e t a t i o n s , s u ch a s

f o r m a t i o n o f ' u n im o l e c u l a r' m i c e ll e s [ 1 1 4 ] , h a v e

b e e n g i v en . r h e r e g i o n o f d e c r e a s i n g s u r f a c e t e n -

s i o n t h a t f o l l o w s t h e b r e a k a t 0 . 0 0 1 % , a n d t h e

s e c o n d b r e a k h a v e o f t e n b e e n i g n o r e d [ 1 1 4 , 1 1 5 ] .

b u t i t h a s b e e n s u g g e s t e d t h a t t h e y w e r e d u e t o

b r o a d c o p o l y m e r m o l e c u l a r w e ig h t d i s t r ib u t i o n

a n d t o t h e p r e s e n c e o f i m p u r i t ie s [ 3 0 t .

G o o d a g r e e m e n t b e t w e e n t h e C M C s d e t e r m i n e d

f r o m d y e s o l u b il i z a ti o n [ 2 8 ] a n d t h e c o n c e n t r a t i o n

a t t h e s e c o n d b r e a k o f t h e s u r f a c e t e n s i o n c u r v e

h a s b e e n s h o w n [ 1 1 2 ] , l e a d i n g t o t h e c o n c l u s i o n

t h a t th e h i g h e r - c o n c e n t r a t i o n b r e a k c o r r e s p o n d s

t o th e C M C a n d s i g ni fi es th e f o r m a t i o n o f p o l y m o -

l e c u l a r m i c e ll e s w i t h a w e l l - d e f in e d h y d r o p h o b i c

i n t e r i o r ( i . e . a b l e t o s o l u b i l i z e o r g a n i c s o l u t e s ) . I t

s h o u l d b e n o t e d t h a t th e c o p o l y m e r c o n c e n t r a t i o n s

a t t h e s e c o n d b r e a k , e x t r a c t e d f r o m t h e s u r f a c e

t e n s i o n d a t a o f W a n k a e t al . [ 3 0 ] ( 0 ,3 % f o r F 1 2 7 .

a n d 0 .0 1 e2 f o r P1 0 4 , b o t h a t 25 C) a n d P r a s a d

e t a l. [ 1 1 4 ] . a r e al s o c o m p a r a b l e w i t h t he C M C

v a l u e s f o u n d i n t h e d y e s o l u b i l i z a t i o n s t u d y o f

A l e x a n d r i d i s e t a l . [ 2 8 ] . T w o o t h e r o b s e r v a t i o n s

s u p p o r t i n g t h e c o n c l u s i o n t h a t : h e h i g h e r -

c o n c e n t r a t i o n b r e a k i s t h e C M C a r e (i ) t h e p l a te a u

o b s e r v e d i n t h e s u r f a c e t e n s i o n v a l u e s a f t e r t h e

h i g h - c o n c e n t r a t i o n b r e a k , a n d l i ll t h e e f fe c t o f

t e m p e r a t u r e o n t h e c o p o b m e r c o n c e n t r a t i o n a t

w h i c h t h e s e c o n d b r e a k o c c u r s. A t t a i n m e n t o f a

c o n s t a n t s u r f a c e t e n s i o n v a l u e i s a n i n d i c a t i o n o f

m i c e ll e f o r m a t i o n ( C M C ) in s o l u t io n s o f t y p ic a l

s u r f a c t a n t s 1 7 1 1 3] . W h i l e t h e s u r f a c e t e n s i o n o f

P l u r o n i c c o p o l y m e r s o l u t i o n s k ee p,,, d e c r e a s i n g

w i t h i n c r e a si n g c o p o l y m e r c o n c e n t r a t i o n a f te r t h e

f i r s t l o w - c o n c e n t r a t i o n b r e a k , i t r e m a i n s c o n s t a n t

a f t e r t h e s e c o n d b r e a k ( a d i p i n t h e s u r f a c e t e n s i o n

a f t e r t h e b r e a k o b s e r v e d f o r P l u r o n i c P 8 5 is m o s t

l i k e l y d u e t o i m p u r i t i e s [ 2 6 ,4 0 , 11 7 1 1 . R e g a r d i n g

t h e e ff ec t o l t e m p e r a t u r e , t h e c o p o l y m e r c o n c e n -

t r a t i o n a t w h i c h t h e s e c o n d c h a n g e i n s l o p e o c c u r s

d e c r e a s e d s i g n i f i c a n t l y w i t h i n c r e a s i n g t e m p e r -

a t u r e , i n g o o d a g r e e m e n t w i t h t h e C M C s o b t a i n e d

f r o m d y e s o lu b i l iz a t i o n [ 2 8 ] a n d o b s e r v a t i o n s i n

o t h e r s t u d i e s o f P l u r o n i c c o p o l y m e r s [ 2 7 , 6 4 ] ,

w h e r e a s t h e c o n c e n t r a t i o n a t t h e l i r s t b r e a k d i d

n o t w t r y s i g n i f i c a n t l y [ 1 1 2 ] .

H a v i n g a t t r i b u t e d t h e h i g h - c o n c e n t r a t i o n b r e a k

{ c h a n g e i n t h e s l o p e ) o f t h e s u r f a c e t e n s i o n c u r v e

t o t h e f o r m a t i o n o f m i c e ll e s in t h e b u l k I C M C ) ,

w e e x a m i n e t h e r e g i o n b e l o w t h e o r i g in o f t h e f ir s t



28 P. Alexandridis, T.A . Hatton/Colloids Surjaces A: Physicochem. En g. Aspects 96 1995) 1 46

( l o w - c o n c e n t r a t i o n ) b r e a k . T h e w i d e m o l e c u l a r

w e i g h t d i s t r ib u t i o n o f t h e c o p o l y m e r s s tu d i e d , a n d

t h e p r e s e n c e o f h y d r o p h o b i c i m p u r i ti e s , h a v e b e e n

s i n g l e d o u t i n t h e p a s t a s p o s s i b l e f a c t o r s c a u s i n g

d i ff e re n c e s i n t h e C M C v a l u e s o b t a i n e d b y s u r f a c e

t e n s i o n a n d d y e s o l u b i l i za t i o n m e t h o d s . T h e e f f ec t

o f m o l e c u l a r w e i g h t d i s t r ib u t i o n o n t h e s u r fa c e

t e n s i o n o f n o n - i o n i c s u r f a c t a n t s h a s b e e n n i c e l y

d e m o n s t r a t e d b y C r o o k e t al. [ ,1 1 8 ] , w h o s t u d i ed

t h e s u r fa c e t e n s io n o f n o r m a l ( P o i s s o n ) d i s t r i b u t i o n

a n d s i n g l e s p e c i e s o c t y l p h e n o x y - e t h o x y e t h a n o l s

( O P E l l o, w h e r e 1 - 1 0 i s t h e n u m b e r o f e t h y l e n e

o x i d e s e g m e n t s ) , a n d m i x t u r e s o f s i n g le s p e c ie s

a n d n o r m a l d i s tr i b u ti o n O P E 4 a n d O P E l o . T h e

a u t h o r s f o u n d p r e f e re n t i a l a d s o r p t i o n o f m o l e c u l e s

o f s h o r t e r P E O - c h a i n l e n g t h (O P E 3 _ 5) at t h e a i r

w a t e r i n t e r f a c e ; t h i s w a s d e m o n s t r a t e d b y a l e s s

s h a r p ( b u t s t i l l , o n l y s i n g l e l b r e a k i n t h e s u r f a c e

t e n s i o n c u r v e , a n d a d i p i n t h e s u r f a c e t e n s i o n

v a l u e s i n t h e v i c in i t y o f th e b r e a k . S u c h b e h a v i o r

w a s a t t r i b u t e d t o t h e r e p l a c e m e n t o f h i g h l y s u r fa c e

a c ti v e , s h o r t P E O - c h a i n - l e n g t h m o l e c u l e s ( c o n t r ib -

u t i n g t o a l o w s u r f a c e t e n s i o n v a l u e ) o n t h e i n t e r -

f a ce b y t h e m a j o r c o m p o n e n t , w h e n t h e se h i g h ly

s u r f a c e a c t i v e m o l e c u l e s w e r e s o l u b i l i z e d i n m i c e l l e s

f o r m e d b y t h e m a j o r c o m p o n e n t . N o m a n i f e s t a -

t i o n s o f th e a b o v e w e r e s e e n i n t h e s u r f a c e t e n s i o n

c u r v e s f o r P l u r o n i c s [ 1 1 2 ] . A s f o r t h e i n f lu e n c e o f

i m p u r i t i e s , a d e c r e a s e i n s u r f a c e t e n s i o n w i t h t i m e ,

o b s e r v e d i n s o m e e x p e r i m e n t s a t l o w c o p o l y m e r

c o n c e n t r a t i o n s , w o u l d i n d i c a t e t h e p r e s e n c e o f

h y d r o p h o b i c i m p u r i t ie s t h a t a d s o r b s l ow l y o n t h e

s u r f a c e [, 1 1 7 ] . H o w e v e r , t h e p r e s e n c e o f s u c h i m p u -

r i ti e s s h o u l d r e s u l t i n a l o w e r i n g o f t h e s u r f a c e

t e n s i o n ( d e s c r i b e d a b o v e a s a d i p ) b e l o w t h e

s t e a d y v a l u e o b t a i n e d a f te r t h e C M C . S u c h a d i p

w a s n o t o b s e r v e d i n t h e v i c i n it y o f t h e l o w -

c o n c e n t r a t i o n b r e a k i n t h e s u r f a c e t e n s i o n c u r v e ,

b u t c a n b e s e e n in s o m e o f o u r e x p e r i m e n t s ( e.g .

P l u r o n i c P 8 5 ) a t t h e h i g h - c o n c e n t r a t i o n b r e a k . I t

c a n t h u s b e c o n c l u d e d t h a t n e i t h e r p o l y d i s p e r s i t y

o f th e c o p o l y m e r s iz e n o r h y d r o p h o b i c i m p u r i ti e s

c a u s e t h e l o w - c o n c e n t r a t i o n b r e a k o b s e r v e d i n

t h e su r fa c e t e n si o n c u rv e s o f P E O P P O - P E O

s o l u t i o n s .

I t h as b e e n p r o p o s e d t h a t t h e lo w - c o n c e n t r a ti o n

b r e a k i n t h e s u r fa c e t e n s i o n v s. c o p o l y m e r c o n c e n -

t r a t i o n c u r v e s i s d u e t o a c h a n g e i n c o n f i g u r a t i o n

( s t ru c t u r a l t r a n s i t io n ) o f th e c o p o l y m e r m o l e c u l e s

a t t h e a i r - w a t e r i n t e r f a c e [ , 1 1 2 ] . C o n s t a n t s u r f a c e

c o v e r a g e h a s b e e n a t t a i n e d a t b u l k c o p o l y m e r

c o n c e n t r a t i o n s r a n g i n g f r o m 10 -6 to 10 3 ,

w i th t he P E O - P P O - P E O c o p o l y m e r m o l ec u le s

a d s o r b e d a t t h e i n t e r f a c e p o s s i b l y a s a n i n v e r t e d

U ' ( t h is is o n e o f t h e p r o p o s e d o r i e n t a t i o n s f o r

h o m o p o l y m e r P g O a t t h e i n te r fa c e [, 1 1 9 ] ) a n d t h e

P E O c h a i n s l o c a t e d a t t h e a i ~ w a t e r i n t e r f a c e . A t

a b u l k c o n c e n t r a t i o n o f a p p r o x i m a t e l y 1 0 3 % , a

s t r u c t u r a l t r a n s i t i o n o c c u r s a n d t h e c o p o l y m e r

l a y e r b e c o m e s m o r e c o m p a c t ; w a t e r i s e x p e l l e d

[ 1 2 0 ] a n d P E O s e g m e n t s p r o t r u d e i n t o t h e a q u e -

o u s s o l u ti o n [ , 1 21 ] o r f o ld a r o u n d P P O . M o r e

c o p o l y m e r m o l e c u l e s c a n f it a t t h e i n t e r f a c e (c a u s -

i n g i t t o b e c o m e t h i c k e r [ , 1 2 2 ] ) a n d t h e s u r f a c e

t e n s i o n c o n t i n u e s t o d e c r e a s e w i t h i n c r e a s i n g b u l k

c o p o l y m e r c o n c e n t r a t i o n , b u t a t a s l o w e r r a t e (n o t e

a l so t h a t a b u l k c o n c e n t r a t i o n o f 10 3 % is c o m p a -

r a b l e t o t h e b u l k P E O c o n c e n t r a t i o n r e q u i r e d f o r

f ul l c o v e r a g e o f th e a i r - w a t e r i n t e rf a c e a c c o r d i n g

t o G l a s s [ 1 2 3 ] a n d t h e b u l k P P O c o n c e n t r a t i o n

a t w h i c h p h a s e s e p a r a t i o n o c c u r s [ 1 2 4 ] ) . O n f o r -

m a t i o n o f m i c e ll e s i n t h e b u l k , a f u r t h e r i n c r e a s e

i n th e n u m b e r o f c o p o l y m e r m o l e c u le s in t h e b u l k

i s a c c o m m o d a t e d b y a n i n c r e a s e i n t h e n u m b e r o f

m i c e ll e s, t h e a c t i v i t y o f t h e c o p o l y m e r i n t h e b u l k

r e m a i n s a p p r o x i m a t e l y c o n s t a n t , a n d t h e s u r f ac e

t e n s i o n a t t a in s a s t e a d y v a l u e t h a t d o e s n o t c h a n g e

w i t h f u r t h e r i n c r e a s e i n t h e b u l k c o p o l y m e r

c o n c e n t r a t i o n .

T h e v a l u e s fo r s u rf a c e a re a p e r c o p o l y m e r m o l e -

c u le , c a l c u l a te d t h r o u g h t h e G i b b s a d s o r p t i o n i so -

t h e r m [ 1 1 2 , 1 1 4 ] , a r e g e n e r a l l y s m a l l c o m p a r e d t o

t h o s e o f n o n - i o n i c s u r f a c ta n t s w i t h a n a l ip h a t i c

c h a in a n d P E O h e a d g r o u p ( o f c o m p a r a b l e s iz e t o

t h e P E O b l o c k o f P l u r o n i c s ) [ 1 2 5 , 1 2 6 ] , i n d i c a t in g

t h a t t h e r e is c o n s i d e r a b l e f o l d i n g o f th e p o l y m e r s

a t t h e a i r - w a t e r i n t e rf a c e [ 1 1 4 ] a n d / o r d e s o r p t i o n

o f P E O s e g m e n t s in t h e w a t e r p h a s e [ 1 2 7 , 1 2 8 ] .

T h e a r e a p e r c o p o l y m e r m o l e c u l e i n c r ea s e d w i t h

t h e n u m b e r o f E O s e g m e n t s i n t h e m o l e c u l e f o r a

f am i ly o f P l u r o n i c c o p o l y m e r s w i th t h e s a m e P P O

a n d v a r y i n g P E O b l o c k l e n g t h , a s se e n i n F i g. 19

[ - 1 1 2 ] . A l s o s h o w n o n t h e s a m e p l o t a r e a r e a s p e r

m o l e c u l e f o r a s e ri es o f o c t y l p h e n o x y - e t h o x y e t h a -

n o l s u r f a c t a n t s ( a r e a v a l u e s w e r e e x t r a c t e d f r o m

C r o o k e t a l . [ 1 1 8 1 1 . A n i n c r e a s e i n t h e l e n g t h o f



P. A l e x a n d r i d i s T . 4 . Ha t t o n / C o l l o i d s S u t J a c e s A : P i t v s i c o c h e m . EH ~ . A v w c t s 9 6 i l ?95 , 1 4 t ~ 29

_e

e

o

E

<D

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1 0 0

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NEO12

C q~(EO),

i

5

~ P 1 0 4

P 1 0 3

10 . . . . i J h i J I

1 0 1 0 0

Num ber of EO seg men ts , NEO

F i g. 19 . A r e a p e r P E O P P O P E O c o p o l y m e r m o l e c u l e p l o t te d

a s a f u n c t io n o f t h e n u m b e r o f E O s e g m e n t s . A l s o s h o w n o n

t h e s a m e p l o t a r e a r e a s p e r m o l e c u l e f o r a s e r i e s o f

o c t y l p h e n o x y ~ t h o x y e t h a n o l s u r f a c t a n t s ( a r ea v a l u e s w e r e

e x t r a c t e d f ro m R e f. [ l l 8 ] k I R e p r i n t e d w i t h p e r m i s s i o n f ro m

R c f. [ 1 1 2 ] : c o p y r i g h t A m e r i c a n C h e m i c a l S o c i e ty . 1 9 9 4 3

t h e h y d r o p h o b i c ( P P O ) b l oc k , f o r a g r o u p o f

c o p o l y m e r s t h a t h a v e t h e s a m e P E O b l o c k s i z e ,

r e s u l t e d i n a d e c r e a s e i n t h e a r e a t h a t e a c h m o l e c u l e

o c c u p i e s a t t h e a i r w a t e r i n t e r f a c e , i n a g r e e m e n t

w i t h P r a s a d e t a l . [ 1 1 4 ] . T h i s f i n d i n g s u g g e s t s a

m o r e c o m p a c t i n te r f a ci a l l a y e r f o r l a r g e r P P O

s e g m e n t s i z e , w i t h t h e c o p o l y m e r m o l e c u l e s o r i -

e n t e d o n t h e s u r f a c e i n a c o i l e d m a n n e r h a v i n g

P P O o u t o f t h e a q u e o u s p h a s e , a n d t h e h y d r o p h i l ic

P E O s e g m e n t s at t h e e x t r e m i t ie s o f t h e c o p o l y m e r

p a r t l y a n c h o r i n g t h e p o l y m e r in t h e a q u e o u s p h a s e

a n d p a r t l y f o l d in g a r o u n d P P O .

T h e a r e a s p e r m o l e c u l e r e p o r t e d b y Y e a t e s e t a l .

[ 1 2 9 ] w e r e h ig h e r f o r C I E s C i t h a n f o r C i E 1 5 C i

( C = m e t h y l g r o u p , i = 5 12, E = e t h y l e n e o x id e

g r o u p ) , i n d i c a t i n g a g a i n t h a t t h e h y d r o p h o b i c

m o i e t y m a k e s t h e i n te r r ac i a l la y e r m o r e c o m p a c t .

P h i p p s e t a l. [ 1 2 1 ] s t u d ie d t h e c o n f o r m a t i o n o f

P l u r o n i c F 1 2 7 a t t h e h e x a n e - w a t e r i n t e r f a c e

e m p l o y i n g a n e u t r o n r e f l e c ti v i ty t e c h n i q u e . U s e o f

h e x a n e w a s n e c e s s a r y t o a c h i e v e c o n t r a s t v a r i a t io n ;

h o w e v e r , t h e a u t h o r s r e p o r t i n d i c a t i o n s t h a t t h e

p r e s e n c e o f h e x a n e h a d l it tl e e f fe c t o n e i t h e r t h e

a d s o r b e d a m o u n t o r th e c o n f o r m a t i o n o f t he c o p o l -

y m e r o n t h e w a t e r s i de o f th e i n t e r f a c e : in t h i s

r e s p e c t , t h e i r f i n d i n g s s h o u l d a l s o b e a p p l i c a b l e

t o c o p o l y m e r s a d s o r b e d a t a i r w a t e r i n t e rf a c es .

P l u r o n i c 17 12 7 w a s f o u n d t o a d o p t a c o n f o r m a t i o n

t h a t e x t e n d e d b e y o n d t h e m i c e l l a r r a d i u s i n w a t e r

a n d , t h u s , a p p e a r e d q u i t e s t r e t c h e d . T h e e s t i m a t e d

( u n p e r t u r b e d ) r a d i u s o f g y r a t i o n o f a P E O b l o c k

is 2.0 n m a n d t h a t o f t h e P P O b l o c k 1 .6 n m : t h e

v o l u m e f r a c t i o n p r o f i l e o f 17 12 7 a t t h e i n t e r f a c e

{ f i t t e d f r o m t h e n e u t r o n s c a t t e r i n g d a t a ) s h o w e d

t h e P E O s e g m e n t s t o p r o t r u d e 9 n m i n to t h e w a t e r

s u b p h a s e a n d P P O t o e x t e n d 4 n m i n to h e x a n e

[ 1 2 1 ] . O n t h e co n t r a r y , a r a n d o m c o p o l y m e r of

p o l y ( v i n y l a l c o h o l - c o - a c e t a t e ) w a s f o u n d t o a d o p t

a v e r y f i a t c o n f o r m a t i o n a t t h e in t e r f a c e , f o r m i n g

a d e n s e l a y e r o f a p p r o x i m a t e l y 2 n m t h ic k n e s s. T h e

e x p e r i n a e n t a l f i n d i n g s r e g a r d i n g t h e c o n f o r m a t i o n

o f t h e P l u r o n i c c o p o l y m e r a t ti le i n t e r fa c e w e r e

b o r n e o u t b y a S c h e u t j e n s F l e e r s im u l a t i o n ( s im -

i l a r t o t h e o n e u s e d f o r m o d e l i n g b l o c k c o p o l y m e r

m i c e ll e s [4 8 51 ] ) t h a t s h o w e d t h e c o p o l y m e r

a t t a c h e d t o t h e i n te r f a ce b y t h e h y d r o p h o b i c P P O

b l o c k a n d t h e P E O b l o c k s h a v i n g a - p a r a b o l i c

s e g m e n t d e n s i t y p r o f i le , s i m i l a r t o a p o l y m e r c h a i n

g r a f t e d t i t a s o l i d s u r f a c e . Bo t h t h e p r o t r u s i o n o f

P E O i n t h e w a t e r s u b p h a s e a n d t h e r e s e m b l a n c e

o f P E O t o a g r a f t e d c h a i n a g r e e w i t h t h e f i n d i n g s

o f th e s u r f a c e t e n s i o n s t u d y o f A l e x a n d r i d i s e t a l.

[ l i e ]

7 .2 . A d s o r p t i o n a t s o l i d w a t e r i n t er l i tc e s

T h e a d s o r p t i o n o f s e v e ra l P l u r o n i c P E O

P P O P E O c o p o l y m e r s (L 6 1 , L 6 2 , L 6 4 , 1 -'3 8. F 6 8 ,

F 8 8 , a n d F I 0 8 ) o n t o p o l y s t y r e n e l a t e x p a r t i c l e s

w a s d e s cr i b e d b y K a y e s a n d R a w l i n s [ 1 3 0 ] .

L a n g m u r i a n i s o t h e r m s w e r e o b t a i n e d a n d , in g e n e -

r a l, m a x i m u m a d s o r p t i o n o c c u r r e d a f t e r t h e m e a -

s u re d a p p a r e n t C M C . T h e a r e a s o c c u p ie d b y a

c o p o l y m e r m o l e c u l e a t t h e i n t e r fa c e in t h e p l a t e a u

r e g i o n o f t he a d s o r p t i o n i s o t h e r m w e r e f o u n d t o

b e g r e a t e r t h a n t h o s e o b s e r v e d a t t h e a i r w a t e r

i n t e r f a c e [ 1 1 2 , 1 1 4 ] , a n d c o n s i d e r a b l y l es s t h a n

t h o s e a t a q u a r t z w a t e r i n t e r f a c e { r e p o r t e d i n a n

e a r l y s t u d y b y H e y d e g g e r a n d D u n i n g [ 1 3 1 ] ) ,

i n d i c at i n g t h e i m p o r t a n c e o f th e n a l u r e o f t h e

i n t e r f a c e o n a d s o r p t i o n . T h e a r e a - p e r - m o l e c u l e

vah tes m ea su re d we re 2 .85 , 3 .20 , 5 .90 , 6.51 , 15 .10 ,

1 7 .5 2 , a n d 2 4 . 2 6 n m 2 f o r P l u r o n i c s L 6 1 , L 6 2 , L 6 4 .

F 3 8 , F 6 8 , F 8 8 , a n d F 1 0 8, r e s p e c t i v e ly . E x a m i n a t i o n

o f t h e m o l e c u l a r a r e a s a t t h e i n t e r D c e s h o w e d t h a t

t h e a d s o r b e d P P O b l o c k s f o r m e d s l n al l l o o p s o r



30 P. Alexandridis . T .A. H at ton/C ol lo ids Sur laces A. Phys icochem. Eng. Aspe cts 96 (1995) 1 46

were tightly coiled on the surface; this was con-

firmed by determination of the adsorbed layer

thickness from light scattering and electrophoresis

experiments [- 130].

Baker and Berg [132] investigated the adsorp-

tion configuration of PEO and its copolymers with

PPO (Pluronic triblock copolymers P75, P85, F68,

F98, F108, and random PEO-PPO copolymers

50-HB-260, 50-HB-2000, 50-HB-5100} on model

polystyrene latex dispersions. Adsorption iso-

therms and adsorbed layer thickness (determined

from dynamic light scattering) for a number of

Pluronics on latex particles are presented in Fig. 20

[132]. The adsorption isotherms exhibited the

expected behavior, both adlayer thickness and

1 6

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, ~ " , - - ~ - F 9 8

[ ; 4 , F 6 8

- - ', P 8 5

c

- -~ - P 7 5

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0 1 2 3 4 5

E q u i l l i b r iu m p o l y m e r c o n c e n t r a t i o n ( g / m L x 1 0 4 )

i 1 i i i

. . . . o

_ A - - - - -o - F 1 0 8

o - o- -£-- . . . . - -~ - F98

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Fig. 20. (al Adsorp tion isotherms and (b) ad sorbed layer

thickness (determined from dynamic light scattering) for a

numb er of Pluronic copolymers on latex particles: 3, P75;

X, P85; O, F68: A, F98; ) , FI08. (Adapt ed with permi ssion

from Ref. [ 132]; copyright American Chemical Society, 1988.)

specific adsorption increasing with the PEO block

size and the bulk polymer concentration. The

thickness isotherms have essentially the same shape

as conventional high-affinity specific adsorption

isotherms, rising rapidly at low concentrations and

leveling off at bulk polymer concentrations corre-

sponding to the plateau adsorption coverage. The

specific adsorption values for Pluronics reach their

plateau values at bulk copolymer concentrations

slightly higher than the copolymer CMCs. In

addition, the concentration corresponding to the

adsorption plateau is well above the concentration

required to create a monolayer of polymer seg-

ments at the particle surface (assuming all segments

are in contact with the surface). These data suggest

that the polymers adsorb in the extended configu-

ration, with thin PPO loops and trains attached

to the surface and longer PEO tails extending

away from the surface [ 132]. Berg and co-workers

[133] also studied the effect of particle radius on

the hydrodynamic thickness of adsorbed PEO

homopolymers and PEO PPO PEO copolymers.

In the latter case, the adlayer thickness increased

with particle radius, but the dependence was more

complex than that of PEO adsorption.

The effect of surface modification of polystyrene

latex particles on subsequent protein adsorption

was studied by gee et al. [134]; the surface modi-

fiers used were Pluronic PEO PPO-PEO block

copolymers. The hydrodynamic thickness 5h of the

adsorbed (on the particles) copolymers is plotted

against the number of EO segments in Fig. 21

[134]. Shown for comparison as curves A and B

are the dimensions for a rod-like chain and a coil-

like chain. The scaling exponent of the fit to the

experimental points suggested a coil-like chain

conformation for the PEO blocks of the adsorbed

copolymers. Fig. 21 shows that the dimensions of

the adsorbed Pluronic copolymers are smaller than

those of the extended rod-like chain (curve A), but

larger than those of the adsorbed PEO homo-

polymer (curve B). Those 6h values are also larger

than the corresponding free surfactant unimer

dimensions, suggesting that the picture of qoops

and trains for the hydrophobic PPO block and

'loops and tails for the hydrophilic PEO blocks

is a suitable description for the adsorbed copoly-



P . A l c x a n d r i d is , T . A . H a n o n , C o l l o i d s S u r l u c e s A: P hrsic<,chem. Er ie. ,4.V~ect.v96 r 1995,, 1 4r, 31

1 0 0 0

<.%

03 1 0 0

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~ . 1 0 '

R

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10 100 1000

a ( n u m b e r o f E O s e g m e n t s )

F i g . 2 1. H y d r o d y n a m i c t h i c k n e s s ' h o f a d s o r b e d P h t r o n i c

P E O P P O P E O c o p o l y m e r s p lo t t ed a g a i n st t h e n u m b e r o f

E ( ) s e g t n e n t s . C u r v e A i s t h e e x t e n d e d c h a i n d i i n e n s i o n

a c c ord ing to D i , a I3 .7 ~ i ) . Cu rv e B re p re se nt s ~ t~ vs . de g re e

o f p o l y m e r i z a t io n l \) r a d s o r b e d P E O h o m o p o l y m e r . [ A d a p t e d

w i t h p e r m i s s i o n f r o m R e f. [ - 1 3 4 ]: c o p y r i g h t A c a d e m i c P r e s s .

1989.1

m e r s [ 1 3 4 ], i n a g r e e m e n t w i th t h e s t u d y o f B a k e r

a n d B e r g [ 1 3 2 ] .

K i l l m a n n e t a l. [ 1 3 5 ] r e p o r t e d o n t h e h y d r o d y -

n a m i c l a y e r t h ic k n e ss o f P E O h o m o p o l y m e r a n d

P E O P P O P E O c o p o l y m e r s a d s o r b e d o n p o ly -

s t y r e n e l a t e x a n d p r e c i p i t a t e d s i l i c a p a r t i c l e s . T h e

c o n f o r m a t i o n o f t he a d s o r b e d p o l y m e r w a s d e t e r-

m i n e d b y t h e s u rf a c e p o l y m e r i nt e r a c ti o n s : h y d r o -

p h o b i c i n t e r a c t i o n s e x i s t o n t h e a p o l a r l a t e x

s u r fa c e , w h e r e a s h y d r o g e n b o n d s d o m i n a t e o n t h e

p o l a r s ili ca s u rf a ce . P E O P P O P E O c o p o l y m e r s

a d h e r e d t o t h e la t ex v ia t h e h y d r o p h o b i c P P O

s e g m e n t s : t h e P E O b l o c k s e x t e n d e d i n t o t h e s o l u -

t i o n a s ta il s, a n d t h e l a y e r t h i c k n e s s d e p e n d e d o n

t h e m o l a r m a s s o f t h e P E O b l oc k s. T h e a d s o r p t i o n

m e c h a n i sm o f P E O P P O - P E O c o p o ly m e r s o n

s il ic a r e s e m b l e d th a t o f P E O h o m o p o l y m e r : t h e

a d l a y e r t h ic k n e s s e s o f t h e b l o c k c o p o l y m e r s w e r e

v e r y s m a l l a n d c o m p a r a b l e t o t h e t h i c k n e s s o f

P E O a d l a y e r s o n s i l i c a [ 1 3 5 ] .

T h e a d so r p t i o n o f P E O P P O - P E O b lo c k

c o p o l y m e r s o n s i l i c a , a s w e l l a s t h e a d s o r p t i o n o f

P E O h o m o p o l y m e r , h a s b e e n st u di e d b y T i b e r g

e t a l. [ 1 3 6 ] a n d M a l m s t e n e t a l . [ 1 3 7 ] . F o r a

n u m b e r o f p o l y m e r s w it h a to t a l m o l e c u l a r w e i g h t

o f a p p r o x i m a t e l y 1 5 00 0 , it w a s f o u n d t h a t t h e

a d s o r b e d a m o u n t is r a t h e r l o w ( 0 .3 5 0 . 40 m g m e )

a n d i n d e p e n d e n t o f t h e P P O c o n t e n t i n t h e r a n g e

0 3 0 % P P O . F o r a c o p o l y m e r w i th a t o t a l m o l e c u -

l a r w e i g h t o f 4 0 0 0 a n d 5 0 % P P O c o n t e n t , t h e

a d s o r b e d a m o u n t w a s a p p r o x i m a t e l y 0 .2 0 m g m e.

T h i n a d s o r b e d l a ye r s , w i t h h y d r o d y n a m i c t h ic k -

n e ss e s o f a p p r o x i m a t e l y 2 5 a m , w e r e f o r m e d b y

a l l p o l y m e r s i n v e s t i g a t e d . T h e p H d e p e n d e n c e o f

t he a d s o r b e d a m o u n t a n d t h e h y d r o d y n a m i c t h ic k -

n e s s w e r e s i m i h t r t o t h a t d i s p l a y e d b v P E O h o m o -

p o l y m e r s . E l l i p s o m e t r y e x p e r i m e n t s p r o v i d e d

i n f o r m a t i o n o n b o t h a d s o r p t i o n a n d d e s o r p t i o n

k i n e t i c s , b o t h o f w h i c h a r c f a s t p r o c e s s e s i n t h e s e

s y s t e m s a n d o c c u r o % rer a n u m b e r o f m i n u t e s .

F i n a ll y , e l l ip s o m e t r y e x p e r i m e n t s s h o w e d t h a i a n

a b r u p t i n c r e a s e i n t h e a d s o r b e d a m o u n t o c c u r r e d

p r i o r t o m i c e l l e f o r m a t i o n i n t i l e s o l u t i o n . A t

t e m p e r a t u r e s a b o v e t he C M T . t he a d s o r b e d

a m o u n t r e m a i n e d i n d e p e n d e n t o f t e m p e r a t u r e ( in

t h e t e m p e r a t u r e r a n g e s t u d i e d 1. T h e h y d r o d y n a n a i c

t h i c k n e s s w a s l n t l c h s m a l l e r t h a n t h e h y d r o d y -

n a m i c d i a m e t e r o f t h e s o l u t i o n m i c e l le s f o r a ll

t e m p e r a t u r e s [ 1 3 7 ] . N o a b r u p t i n c re a s e in t h e

a d s o r b e d a m o u n t w a s o b s e r v ed o n h y d r o p h o b i c

s u r f a c e s, a l t h o u g h a s t r o n g i n c r e a s e i ll t h e a d s o r b e d

a m o u n t w a s o b s e r v e d a s t i l e s y s t e m a p p r o a c h e d

t h e p h a s e b o u n d a r y . T h e q u a l i t a t i v e a s p e c t s o f l h is

f i n d i n g w e r e p r e d i c t e d t h e o r e t i c a l h r ; is b e i n g a

c o n s e q u e n c e o f a p a r ti a l p h a s e s e p a r a t i o n p h e l l o m -

e n o n d u e t o e l e v a te d c o p o l y m e r c o n c e n t r a t i o n in

t h e s u r f a c e z t, n e [ 1 3 6 ]. T h e e x p e r i m e n t a l t i n d m g s

w e r e i n t e r p r e t e d u s i n g a r o o d • l i e d m e a n - l i e l d

t h e o r y w h i c h t a k e s t h e r e x e r s e t e m p e r a t u r e p h a s e

b e h a v i o r o f th e s v s t e m i n t o a c c o u n t [ 1 3 6.1 3 7 1-

8. G e l s f o rm e d b y P E O - P P O - P E O b lo ck

c o p o l y m e r s

P E O P P O P E O c o p o l y m c r s o lu t io n s of h ig h

c o p o l y m e r c o n c e n t r a t i o n e x h i b it a d r a m a t i c

c h a n g e i n x i s c o s it y a t t e m p e r a t u r e s c l o s e t o

a m b i e n t , r e v e a l i n g a ' t h e r m o r e v e r s i b l e g e l a t i o n

[ 2 7 , 3 0 ,4 0 , 1 3 8 , 1 3 9 ]. S e v e r a l m e c h a n i s m s h a v e b e e n

p r o p o s e d a s d r i v i n g f o r c e s f o r t h i s t h e r n - ml g e h t t i o n :

R a s s i n g a n d A t t w o o d [ 1 4 0 ] r e l a te d t h e g e l t r a n s -

i t i o n t o i n t r i n s i c c h a n g e s i n t h e m i c e l l a r p r o p e r t i e s ,

V a d n e r e e t a l , [ 1 4 1 ] d i s c u s s e d t h e g e l a t i o n i n t e r ms

o f e n t r o p i c c h a n g e s i n v o l v i n g lo c a ll y o r d e r e d w a t e r



32

P. Alexandridis. T.A . Hatton/Colloids Sur/ itces A: Physicochem. Eng . Aspe cts 96 1995) 1 46

m o l e c u le s cl o se t o t h e h y d r o p h o b i c P P O s e gm e n ts ,

w h e r e a s W a n k a e t a l. [ 3 0 ] a n d W a n g a n d J o h n s t o n

[ 1 3 8 ] s p e c u l a t e d o n t h e p o s s i b i li t y o f a n o r d e r e d

t h r e e - d i m e n s i o n a l s t r u c t u r e d s t a t e o r n e t w o r k .

R e c e n t l y , n e u t r o n s c a t t e r i n g s t u d i e s s h o w e d t h a t

t h e o b s e r v e d c h a n g e i n v i s c o s i t y is d u e t o a h a r d -

s p h e r e c r y s ta l l i z a ti o n a s t h e m i ce l le c o n c e n t r a t i o n

a p p r o a c h e s t h e c r it i ca l v o l u m e f r a c t i o n o f 0 .5 3

( m i c e l l e s c l o s e - p a c k e d ) [ 6 1 ~ 6 4 ] . A t e v e n h i g h e r

t e m p e r a t u r e s t h e g e l ' d is s o lv e s a g a in . T h e P E O

c h a i n s i n t h e m i c e ll a r m a n t l e i n t e r p e n e t r a t e e x t e n s -

i v e l y i n t h e g e l , y i e l d i n g a d y n a m i c c o r r e l a t i o n

l e n g t h o f m a g n i t u d e 0 .8 n m a t t h e h i g h e s t c o n c e n -

t r a t i o n u s e d ( a b o u t 0 .3 5 g m 1 - 1 ) [ 5 7 ] . O s c i l l a t o r y

s h e a r m e a s u r e m e n t s s h o w t h a t t h e g e l a t i o n o n s e t

t e m p e r a t u r e a n d t h e t h e r m a l s t a b i l it y r a n g e o f t h e

g e l i n c r e a s e w i t h i n c r e a s i n g P E O b l o c k l e n g t h

[ 5 7 3 .

U l t r a s o n i c r e l a x a t i o n a n d 1 3C N M R s p e c t r a

w e r e r e p o r t e d f o r P l u r o n i c F 1 2 7 d i s s o lv e d i n w a t e r

a n d

D 2 0

r e s p e c t i v e ly , b y R a s s i n g e t a l. [ 1 4 2 ] .

T h e p o l y m e r s o l u t i o n e x h i b i t e d r e v e r s e t h e r m a l

b e h a v i o r : a 2 0 % w / w s o l u t i o n f o r m e d a g e l a t

r o o m t e m p e r a t u r e . T h e u l t r a s o n i c r e l a x a t i o n , a s

w e l l a s th e m e t h y l c a r b o n r e s o n a n c e , o f t h e P P O

b l o c k s h o w e d d i s t in c t a n o m a l i e s o v e r t h e t e m p e r -

a t u r e r a n g e i n w h i c h t h e g e l f o r m a t i o n t a k e s p l a c e .

S u c h a n o m a l i e s , h o w e v e r , w e r e a l s o o b s e r v e d i n

d i l u t e s o l u t i o n s w h e r e a g e l i s n o t b e i n g f o r m e d . I t

w a s d e d u c e d , o n t h e b a s i s o f t h e r e s u l ts o b t a i n e d ,

t h a t o b s e r v e d r e l a x a t i o n s a r o s e p r e d o m i n a n t l y

f r o m c o n f o r m a t i o n a l c h a n g e s r e l a t e d t o a l t e r -

n a t i o n s i n t h e o r i e n t a t i o n o f th e m e t h y l g r o u p s id e

c h a i n s in t h e P P O b l o c k o f t h e p o l y m e r . T h e s e

c o n f o r m a t i o n a l c h a n g e s w e r e t h o u g h t t o b e

i n d u c e d b y e x t ru s i o n o f h y d r a t e d w a t e r w i t h

i n c r e a s i n g t e m p e r a t u r e f r o m t h e e x i s t i n g m i c e l l e s ,

t h e i n te r i o r s o f w h i c h c o n s i s t o f t h e P P O s e g m e n ts .

I t w a s f u r t h e r p r o p o s e d t h a t t h e d e h y d r a t i o n ,

w h i c h c a u s e s a h i g h e r f r i c t i o n b e t w e e n t h e e n d

g r o u p s i n t h e p o l y m e r c h a in , w a s r e s p o n s i b l e f o r

t h e r e v e r s e t h e r m a l b e h a v i o r o f t h e v i s c o s it y a n d ,

h e n c e , f o r th e g e l fo r m a t i o n i n c o n c e n t r a t e d c o p o l -

y m e r s o l u t i o n s [ 1 4 2 ] .

T h e t e m p e r a t u r e - i n d u c e d g e l a t i o n i n P l u r o n i c

F 1 2 7 c o p o l y m e r s o l u t i o n s w a s r e f l ec t e d i n t h e

r a p i d i n c r e a se o f t h e s t o r a g e m o d u l u s b y t w o o r

m o r e o r d e r s o f m a g n i t u d e [ 3 0 ] . T h e s t o ra g e m o d u -

l u s G ' a p p r o a c h e d a s t e a d y v a l u e a f t e r g e l f o r m a -

t i o n , w h i c h i n c r e a s e d w i t h i n c r e a s i n g c o p o l y m e r

c o n c e n t r a t i o n [ 3 0 ] . T h e g e l f o r m a t i o n t e m p e r a t u r e

d e c r e a s e d w i t h i n c r e a s i n g p o l y m e r c o n c e n t r a t i o n .

T h e c o p o l y m e r g e ls e x h i b i t e d a c h a r a c t e ri s t ic y i e ld

s t re s s a t w h i c h t h e y f l o w w h e n s t re s s is a p p l i e d .

T h e y i e l d v a l u e w a s z e r o f o r s o l u t i o n s b e l o w t h e

g e l t e m p e r a t u r e , a n d i n c r e a s e d a b o v e t h e g e l t e m -

p e r a t u r e w i t h f u r t h e r i n c r e a s e i n t h e t e m p e r a t u r e .

T h e y i e ld v a l u e in c r e a s e d w i t h p o l y m e r c o n c e n -

t r a t i o n ; t h e h i g h e s t y i e l d v a l u e s w e r e a b o v e 1 0 0 0

P a . W a n k a e t a l . [ 3 0 ] a l s o n o t e t h a t t h e g e l s w e r e

n o t b i r e f r in g e n t , in d i c a t i n g t h a t t h e y w e r e n o t

a n i s o t r o p i c l i q u i d c r y s t a l l i n e p h a s e s . B r o w n a n d

c o - w o r k e r s [ 2 7 , 5 7 ] m e a s u r e d t h e v i s c o el a st i c p r o p -

e r t ie s o f P l u r o n i c s P 8 5 , P 8 7 , a n d F 8 8 a s a f u n c t i o n

o f t e m p e r a t u r e . T h e y d e f i n e d t h e g e l a t io n t e m p e r -

a t u r e a s t h e t e m p e r a t u r e a t w h i c h s t o r a g e m o l u l u s ,

G , e q u a l s t h e l o s s s h e a r m o d u l u s , G . T h e g e l a t i o n

t e m p e r a t u r e f o r P l u r o n i c P 8 5 i s p r e s e n t e d a s a

f u n c t i o n o f c o n c e n t r a t i o n i n F ig . 2 2 ( a ) [ 2 7 ] . P 8 5 ,

P 8 7 , a n d F 8 8 s h o w e d q u a l i t a t i v e l y s i m i l a r g e l l i n g

b e h a v i o r , b u t w i t h i m p o r t a n t q u a n t i t a t i v e d i f f e r -

e n c e s a s s h o w n i n F ig . 2 2 ( b l [ 5 7 ] . P 8 5 f o r m e d a

g e l a t 3 5 C a t 2 0 % c o p o l y m e r c o n c e n t r a t i o n . I t

c a n a l s o b e s e e n in F i g . 2 2 ( b ) t h a t P 8 5 f o r m e d a

g el o v e r a r a t h e r n a r r o w t e m p e r a t u r e i n t e r v a l (u p

t o 4 8 : : C t , w h e r e a s P 8 7 a n d F 8 8 r e m a i n e d a s g e l s

u p t o 8 0 - 'C . T h e m a x i m u m v a l u e s o f G ' a re s i m i l ar

f o r F 8 7 a n d F 8 8 , a n d s o m e w h a t s m a l l e r f o r P 85 .

B r o w n e t a l . [ 5 7 ] c o n c l u d e d t h a t t h e g e l a t i o n

o n s e t t e m p e r a t u r e a n d t h e t h e r m a l s t a b i l i t y r a n g e

o f t h e g e l i n c r ea s e d w i t h i n c r e a s in g P E O b l o c k

le ng th .

T h e s o l - g e l t r a n s i t i o n t e m p e r a t u r e w a s m e a -

s u r e d a s a fu n c t i o n o f c o p o l y m e r c o n c e n t r a t i o n f o r

n in e P l ur on ic P E O - P P O - P E O c o p ol ym e r s b y

V a d n e r e e t a l. [ 1 4 1 ] . F i g . 2 3 ( a ) s h o w s a p l o t o f t h e

l o g a r i t h m o f t h e c o p o l y m e r c o n c e n t r a t i o n v s.

l I T

f o r s o m e o f t h e p o l y m e r s s t u d i e d i n R e f. [ 1 4 1 ] .

T h e e n t h a l p y o f g e l a ti o n , A H ~ ge l, w a s e s t i m a t e d

f r o m a r e l a t i o n s h i p s i m i l a r t o t h a t u s e d f o r o b t a i n -

i n g A H ° o f m i c e l li z a ti o n f r o m C M C v s. t e m p e r -

a t u r e d a t a ( s e e S e c t i o n 4 . 1 ) . A p o s i t i v e e n t h a l p y

c h a n g e b e t w e e n 5 a n d 1 0 k c a l m o l . 1 o f c o p o l y m e r

w a s o b s e r v e d i n a l l c a s e s . U n l i k e t h e g e l a t i o n o f

g e l a t i n e , w h e r e t h e l a r g e e n t h a l p y c h a n g e ( - 6 7

k c a l m o 1 - 1 ) f a v o r s t h e g e l a t i o n p r o c e s s , t h e



P. Ah . \amh idi,s T .A . Ha t tom Co l loids Sul:lbces A: PhY.~icoHlem. El le. Aspect,s 96 (1 09 5) 1 4r5

33

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F i g . 2 2 . a ) G c l a t i o n t e m p e r a t u r e d e t e r m i n e d fr o m o s c i l l a t o r y

s h e a r m e a s u r e m e n t s } i t s

a f u n c t i o n o f P l u r o n i c P 8 5 c o n c e n -

t r a t i o n a d a p t e d w i t h p e r m i s s i o n f r o m R e f . [ 2 7 ] : c o p y r i g h t

A m e r i c a n C h e m i c a l S o c ie t y , 1 99 1.1 b ) C o m p a r i s o n o f t e m p e r -

a t u r e - d e p c n d e n t g e l a t io n

o f 0. 25 g m l ~ s o l u t i o n s o f P l u r o n i c s

P 8 5 . P 8 7 . a n d F 8 8 .

A d a p t e d w i t h p e r m i s s i o n

f r o m R e f. [ 5 7 ] :

c o p y r i g h l A m e r i c a n C h e m i c a l S o c i e t y , 1 9 9 2 . t

e n t h a lp y c h a n ge in th e c as e o f P E O - P P O - P E O

g e l a t i o n i s u n f a v o r a b l e . T h e c o p o l y m e r c o n c e n -

t r a t i o n r e q u i r e d t o g i v e a g e l t r a n s i t i o n t e m p e r a t u r e

o f 2 5 C i s p l o t t e d i n F i g . 2 3 b 1 a s a f u n c t i o n o f

t h e P P O / P E O c o m p o s i t io n r a tio [ 1 4 1 ] . T h e

f o l l o w i n g t r e n d s w e r e n o t e d : t h e c o n c e n t r a t i o n

r e q u ir e d f o r g e l f o r m a t i o n w a s a p p r o x i m a t e l y t h e

s a m e f o r c o p o l y m e r s w i t h t h e s a m e s i ze P P O

b l o ck : f o r a g i v e n P P O / P E O r a ti o , t h e p o l y m e r

c o n c e n t r a t i o n r e q u ir e d fo r f o r m a t i o n o f a g e l a t

a g i v e n t e m p e r a t u r e ) d e c r e a s e d w i t h i n c r e a s i n g

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F i g . 2 3 . a ) L o g a r i t h m o f c o p o l y m e r c o n c e n t r a t i o n v s . i n v e r s e

g e h U i o n t e m p e r a t u r e f o r v a r i o u s P l u ro n i c P E O P P O P E O

c o p o l y m e r s . b ) C o n c e n t r a t i o n o f c o p o l y m e r

r e q u i r e d t o g i v e a

g e l t r a n s i t i o n t e l n p e r a t u r e o f 2 5 C , p l o t t e d a s a f u n c t i o n o f t h e

c o p o l y m e r P P ) / P E O c o m p o s i t i o n r a ti o . I A d a p t e d f ro m

Ref . [ 141] . )

o b s e r v e d b e t w e e n l o g M w a n d 1 T tk~r c o p o l y m e r s

w i t h t h e s a m e P P O / P E O r a ti o. T h e p r e s e n ce o f

N a C 1 , K C 1 , a n d N a S O 4 d e c r e a s e d t h e g e l t r a n s i t i o n

t e m p e r a t u r e , w h e r e a s t h e o p p o s i t e e f fe c t w a s

o b s e r v e d w i th u r e a , a l c o h o l a n d s o d i u m d o d e c y l -

s u l fa t e . T h e e n t h a l p y o f g e l f o r m a t i o n w a s n o t

s i g n i f ic a n t l y a l t e r ed b y t h e a d d e d s u b s t a n c e s , s u g -

g e s t i n g t h a t e n t r o p y p l a y s t h e m a j o r r o l e i n t h c

g e l a t i o n p r o c e s s [ 1 41

]

T h e p o t e n t i a l o f u s i n g P l u r o n i c F 1 2 7 g e l s f o r

c o n t r o l l e d d r u g d e l i v e r y w a s s t u d i e d b y G i l b e r t

e t a l. [ 1 4 3 ] : t h e e ff e c t o f s o l u t e s a n d p o l y m e r s o i l

t h e g e l a t i o n p r o p e r t i e s w a s a l s o r e p o r t e d . T h e



34 P. Alexandridis. T.A. Hatton/'Colloids Surfaces A. Physicochem. Eng. Aspects 96 1995) 1 46

g e l a t i o n t e m p e r a t u r e w a s d e t e r m i n e d f o r a r a n g e

o f F 12 7 c o n c e n t r a t i o n s ( 2 4 - 3 4 % ) w i t h b e n z o i c a c i d

l o a d i n g i n c r e a s i n g f r o m 0 t o 2 % w / v . T h e g e l s o l

t r a n s i t i o n t e m p e r a t u r e d e c r e a s e d a s t h e b e n z o i c

a c i d c o n c e n t r a t i o n i n c r e a s e d [ 1 4 3 ] . T h e h o m o l o -

gou s se r i e s

o f p a r a h y d r o x y b e n z o a t e

e s t e rs , me t h y l ,

e t hy l , p ropyl , a nd bu t y l , we re a l so c ons i de re d : t he

m o r e l i p o p h i l i c c o m p o u n d s r e s u l t e d i n a g r e a t e r

d e c r e a s e i n t h e g e l - s o l t e m p e r a t u r e f o r a l l c o p o l y -

m e r c o n c e n t r a t i o n s s t u d i e d . A d d i t i o n o f P E O

h o m o p o l y m e r i n cr e a se d t h e g e l a ti o n t e m p e r a t u r e ;

t h e e x t e n t o f t h i s i n c r e as e d e p e n d e d o n t h e a d d e d

P E O c h a i n l e n g t h a n d c o n c e n t r a t i o n . T h e e f f ec t o f

a d d i t i v e s o n g e l f o r m a t i o n i n a q u e o u s F 1 2 7 s o l u -

t i o n s h a s a l s o b e e n r e p o r t e d b y M a l m s t e n a n d

L i n d m a n [ 5 9 , 6 0 , 1 4 4 ] . T h e s t a b i l it y ra n g e o f t h e

g e l p h a s e d e p e n d e d s t r o n g l y o n t h e p r e s e n c e o f

sa l t s . NaC1, which i s of ten re ferred to as a typica l

sa l t i ng ou t c oso l u t e , d i sp l a c e s t he wh ol e ge l

re g i on , a s we l l a s t he c l oud po i n t , t o l owe r t e mpe r -

a t u r e s , a s s h o w n i n F i g . 2 4 [ 5 9 ] . N a S C N , a t y p ic a l

sa l t i ng i n c oso l u t e , d i sp l a c e s t he whol e gel r e g i on

a n d t h e c l o u d p o i n t u p t o h i g h e r t e m p e r a t u r e s ( s e e

F i g. 2 5 ) [ 5 9 ] . T h e e ff ec ts o f a d d e d P E O a n d P P O

h o m o p o l y m e r s w e r e al s o i n v e s t ig a t e d . I t w a s f o u n d

t h a t P E O o f i n t e rm e d i a t e m o l e c u l a r w e ig h t c au s e d

t h e g el t o m e l t ' a t a c e r t a in h o m o p o l y m e r a m o u n t

w h i c h d e p e n d e d o n t h e c o p o l y m e r c o n c e n t ra t i o n .

T h e e f fi c ie n c y o f P E O i n i n d u c i n g m e l t i n g o f t h e

g e l i n c r e a s e d w i t h P E O m o l e c u l a r w e i g h t , b u t a t

v e r y h i g h P E O m o l e c u l a r w e ig h t s p h a se s e p a r a t i o n

( r a t h e r t h a n g e l m e l t i n g ) o c c u r r e d . T h e g e l m e l t i n g

b e h a v i o r w a s a l s o o b s e rv e d u p o n a d d i t i o n o f a

c a t i o n i c p o l y e l e c t r o l y t e , p o l y ( d i a l l y l d i m e t h y l a m -

m o n i u m c h l o r i d e )( P D A D M A C ) . A d d i ti o n of P P O

h o m o p o l y m e r , h o w e v e r , t e n d e d t o i n c r e a s e t h e

s t a b i l i t y r e g i o n o f t h e g e l , a l t h o u g h t h i s d e p e n d e d

o n t h e P P O m o l e c u l a r w e i g h t [ 1 4 4 ].

9 . C o m p a r is o n o f P E O - P P O c o p o ly m e r s w i th

o t h e r n o n - i o n ic c o p o l y m e r s a n d s u r f a c t a n t s

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p r e s e n c e o f 1 .0 M N a C I . A l s o s h o w n i s t h e p h a s e d i a g r a m f o r

t h e p o l y m e r / w a t e r s y st e m . ( b t G e l a t i o n p o i n t ( G P ) , m e l t i n g

p o i n t ( M P ) , a n d c l o u d p o i n t {C P ) o f a 30 F 1 2 7 s o l u t i o n a s

a f u n c t i o n o f N a C I c o n c e n t r a t i o n . [ A d a p t e d w i t h p e r m i s s i o n

f r o m R e f. [ 5 9 ] : c o p y r i g h t A m e r i c a n C h e m i c a l S o c i e t y , 1 99 2.1

( P B O ) h a s b e e n r e p o r t e d i n t h e l a s t f e w y e a r s b y

t h e g r o u p s o f A t t w o o d , P r ic e , a n d B o o t h f r o m t h e

U n i v e r s i t y o f M a n c h e s t e r . A c c o r d i n g t o t h e m

P E O - P B O P E O c o p o l y m e r s h a v e t he a d v a n t a g e

o f g r e a te r c o m p o s i t i o n a n d c h a i n l e n g th u n i f o r m i t y

o v e r c o m p a r a b l e P E O P P O P E O c o p o ly m e r s.

Luo e t a l . [ -145] no t e t ha t t he p re pa ra t i on o f

P E O P P O P E O c o p o l y m e r s i n v ol v es t h e a n i o n i c

p o l y m e r i z a t i o n o f p r o p y l e n e o x i d e ( P O ) t o f o r m

d i f u n c t i o n a l P P O , f o l l o w e d b y p o l y m e r i z a t i o n o f

e t h y l e n e o x i d e to f o r m t h e o u t e r b l o c k s : t h e a n i o n i c

p o l y m e r i z a t i o n o f P O is c o m p l i c a t e d b y a s id e

r e a c t i o n , i n v o l v i n g h y d r o g e n a b s t r a c t i o n f r o m t h e

m e t h y l g r o u p o f P O t o f o r m a l ly l a l c o h o l , w h i c h

l e ad s t o i n i ti a t i o n o f a d d i t i o n a l m o n o f u n c t i o n a l

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F i g . 2 5 . { a) P h a s e b e h a v i o r o f t h e F 1 2 7 / w a t e r s y s t e m m t h e

p r e s e n c e o f 2.4 M N a S C N . A l s o s h o w n i s th e p h a s e d i a g r a m

f o r t h e p o l y m e r w a t e r s y s t e m . ( b ) G e l a t i o n p o i n t ( G P ) , m e l t i n g

p o r e I M P ) , a n d c l o u d p o i n t IC P I o f a 3 0 % F 1 2 7 s o l u t i o n a s

a f u n c t io n o f N a S C N c o n c e n t r a t i o n . (A d a p t e d w i t h p e r m i s s i o n

f r o m R c f . [ 5 9 ] : c o p y r i g h t A m e r i c a n C h e m i c a l S o c i e t y , 1 9 9 2 . )

w i t h a d i ff e re n t P P O c o n t e n t . T h e a n i o n i c p o l y m e r -

i z a t i o n o f b u t y l e n e o x i d e , h o w e v e r , is m o r e

s t r a i g h t f i ~ r w a r d , s i n c e t r a n s f e r d o e s n o t o c c u r a n d

d i f u n c t i o n a l P B O c a n b e e x c lu s i v el y p r o d u c e d .

L uo e t a l . [ 1145] p r e pa r e d , us ing se que n t i a l

a n i o n i c c o p o l y m e r i z a t i o n , a p o l y ( e t h y l e n e o x id e }

p o l y ( b u t y l e n e o x i d e j - p o l y ( e t h y l e n e o x i d et t r i b lo c k

c o p o l y m e r , d e n o t e d E O s s B O t , E O s s , w h e r e E O

r e p r e se n t s e t h y l e n e o x id e a n d B O r e p r e s e n ts b u t y l -

e n e o x i d e , a n d d e t e r m i n e d i t s m i c e l l a r a n d g e l a t i o n

p r o p e r t i e s in a q u e o u s s o l u t i o n . S u r f a c e t e n s i o n ,

l i g h t s c a t t e r i n g i n t e n s i t y , p h o t o n c o r r e l a t i o n

s p e c t r o s c o p y , a n d g e l a ti o n m e a s u r e m e n t s w e r e

m a d e a t v a r io u s t e m p e r a t u r e s o v e r c o p o l y m e r

c o n c e n t r a t i o n s r a n g i n g f r o m d i l u t e s o l u t i o n t o

t h e ge l ( > 2 0 % ) . S u r f a c e t e n s io n m e a s u r e m e n t s a t

3 0 C o v e r t h e 1 0 3 l o g I ~ c o n c e n t r a t i o n r a n g e

sho we d a s i ng l e b r e a k a t 0 .3 g 1 ~ , i nd i c a t i v e o f

t h e C M C . T h e C M C o f t h e E O s sB O 1 7 E ( )s ~ c o p o l y -

m e r w a s l o w e r th a t t h e C M C o f P l u r o n i c F 6 8 , a

P E O P P O P E O c o p o l y m e r o f c o m p a r a b l e m o le c-

u l a r w ei g h t a n d P E O c o m p o s i t i o n . T h e m i ce ll e

r a d i u s w a s a p p r o x i m a t e l y 6 n m . w h i l e t h e m i c e ll e

' m o l e c u l a r w e i g h t i n c r e a s e d w i t h t e m p e r a t u r e :

t h e a g g r e g a t i o n n u m b e r w a s 1 3 a t 30 C a n d 2 0 at

5 5 C . T h e m i c e l l iz a t i o n e n t h a l p y . AH was es t i -

m a t e d t o b e 3 4 k J m o l - 1 f i' o m a n a l y z i n g C M ( ' v s.

t e m p e r a t n r e d a t a . T h e l o w e r a n d u p p e r t e m p e r -

a t u r e b o u n d a r i e s f o r t h e g e l r e g i o n w e r e i d e n t i t i e d .

a n d t h e o c c u r r e n c e o f s y n e r e s i s ( s e p a r a t i o n o f

wa te r f r om the ge l ) wa s no t e d . N ic h ,~ l a s e t a l .

[ 1 4 6 ] c o m p a r e d t h e m ic e l la r a n d g e l a t i o n p r o p e r -

t ie s o f E O s ~ B O t ; E O s s , E O ~ t B O 2 s E O 7 1 , an d

E OI 32BOs .~E Ol .~2 c opo lym e r s i n a que ous so lu -

t io n s : t h e c o p o l y m e r s h a d s i m il a r P E O c o m p o s i -

t i ons [ 75 go wt .Ck PE O) bu t d i f f e r e n t c ha in l e ng ths

( t h e c o p o l y m e r m o l e c u l a r w e i g h t s w e r e 7 0 0 0. 8 8 0 (/.

a n d 1 7 5 00 , , e s p e c t i v e l y l. M c a s u r e m e n t s w e r e m a d e

a t 3 0 C o v e r a w i d e c o n c e n t r a t i o n r a n g e . C r i t i c a l

m i c e l l i z a t i o n a n d g e l c o n c e n t r a t i o n s d e c r e a s e d

m a r k e d l y a s t h e c o p o l y m e r c h a in l e n g th i n c re a s e d .

whi l e m ic e l l a r we igh t s a nd s i z e s i nc r e a se d . T he

C M C s f o r E O = t B O 2 s E O = t a n d t ~ () l~ e B ( )5 3 E O t 3 e

w e r e 0 .1 6 a n d < 0 . 0 2 g 1 ~ r c s p e c l iv e l y : t h e

a g g r e g a t i o n n u m b e r s w e r e 4 0 a n d 6 0 0 t\ ~ r

E O - 1 B O e s E O - t a n d E O t _ L ~B O 5 3 [ X )I 3 ., , r e s p e c -

t i v e l y . P h a s e s e p a r a t i o n w a s o b s e r v e d i n d i l u t e

s o l u t i o n s o f th e c o p o l y m e r o f h i g h e r m o l e c u l a r

w e i g h t ( E O ] 3 , B O s 3 E O I 3 2 1 . T h e m i c e l l a r , g e l a t i o n

a n d d r u g r e le a se p r o p e r t i e s o f a q u e o u s

E O a u B O l s E O a u s o l u t i o n s w e r e i n x . c s t i g a l e d b y L u o

e t al. [ 1 4 7 ] . . \ b r i e f i n v e s t ig a t i o n n f t h e r a te o f

r e l e a se o f s a li c y l i c a c id f r om the , ,e l s e r ve d t o

i l l us t r a t e use f u l sus t a ine d r e l e a se . T he ge l a t i on

b e h a v i o r , i n t e r p r e t e d v i a t h e h a r d - s p h e r e m o d e l .

wa s c ons i s t e n t w i th t he m e a sur e d m ic e l l a r s i z e s .

D i b lo c k P E O P B O c o p o l y m e r s ha v e a ls o be e n

s y n t h e s iz e d a n d c h a r a c t e ri z e d b y t h e M a n c h e s t e r

g r o u p . S u n e t a l. [ 1 4 8 ] r e p o r t e d p r e l i m i n a r y r e s u lt s

o n t h e p r e p a r a t i o n o f

E O I 3 5 B ( ) 3 3

a nd i t s m ic e l l i z a -

t i o n a n d s u r l a c e p r o p e r t i e s i n d i h t t e a q u e o u s

s o l u ti o n s . S ix P E O P B O d i b l o c k c n p o l y m e r s

( EO~,eB O : ,

I { O s o B O 1 3, l 7,O 4 ,~ B O s , | { O 5 n B O 4 ,

E O _~ 4 BO to , a n d E O _ , : B O s ) h a v e b e e n p r e p a r e d b y

Be d de l s e t a l. [ 14 9] . a nd t he i r m ic e l l i z a t i on i n



6 P. Alexandridis, T.A. Hatton/Colloids Surfaces A: Physicochem. Eng. Aspects 96 1995) 1-46

aqueous solution investigated. Surface tension,

static and dynamic light scattering and gel permea-

tion chromatography techniques were used to

study solutions at temperatures in the range

20-50 :C over a wide concentration range (up to

100 g 1- x . CMCs, micellar molar masses and radii

have also been reported [ 149]. The the rmodynam-

ics of micellization of EOmBO, diblock copolymers

was discussed in relation to that of related triblock

copolymers (EOmBO,EO,, and EOmPO,EO,,).

Tanodekaew et al. [150] prepared a series of

diblock copolymers EO3oBO, (with n in the range

3-16) by sequential anionic polymerization, and

investigated the association behavior of the copoly-

mers in aqueous solution and the gelation of their

concentrated micellar solutions. At temperatures

in the range 30-50°C, a minimum PBO block

length of 4 to 5 units was required for micellization.

Gelation of micellar solutions of the copolymers

was observed for copolymers with PBO block

lengths of 6 to 7 units or more. The relationships

between (i) standard Gibbs energy of micellization

and molecular characteristics and (ii) critical gela-

tion concentration and micellar characteristics

were explored. Diblock PEO-PBO copolymers

(allyl-BOlzEO15, allyl-BO12EO25, and allyl-

BO~zEO35) were investigated by Hatton and

co-workers [151]. The formation of micelles was

studied using fluorescence methods and the

thermodynamic parameters of micellization esti-

mated from a closed association model; the micelle

hydrodyn amic radii (Rh) were also determined; a

small decrease in Rh was observed with an increase

in temperature.

9.2. PEO PS block copolymers

Evidence for the formation of micelles in aque-

ous solutions of poly(styrene)-poly(ethyleneoxide)

(PS-PEO) block copolymers has been reported by

Bahadur and Sastry [152], and Riess and Rogez

[153]. The polymers were synthesized with ethy-

lene oxide as the major component in order to

make the copolymers water soluble, the polysty-

rene being extremely hydrophobic. The apparent

molecu lar weight of PS PEO micelles was found

by Riess and Rogez [ 153] and Xu and co-workers

[72,154] to increase with increasing copolymer

molecular weight and decreasing PEO content.

Diblock copolymers formed larger micelles than

triblock copolymers. Bahadur and Sastry [152]

found the micelle size to decrease with an increase

in PS content, which shows that the micelle size

also depends on the insoluble block. Increasing the

temperature resulted in the formation of micelles

with a smaller hydrodynamic radius. This is proba-

bly because water becomes a poorer solvent for

ethylene oxide at higher temperatures, so the

corona becomes less swollen with water, resulting

in a decrease in the hydrodynamic radius; similar

temperature effects have been observed in PEO

PPO-PEO copolymer micelles (see Section 5.1).

Critical micellization concentrations of these

PS-PEO block copolymers were estimated from

the increase in the fluorescence intensity of pyrene

with an increase in polymer concentration, and

found to be in the order of 1-5 mg 1 ~ [155]. The

micelle-water partition coefficient of pyrene was

found to be in the order of 3 × 105 , based on the

polystyrene volume [155]. Gallot et al. [156]

investigated micelle formation in methanol-water

mixtures for several series of PEO- PS block copol-

ymers, each having a constant PS chain length and

varying lengths of the PEO block. Light scattering

and small angle neutron scattering measurements

carried out in methanol showed that the CMCs

were primarily dependent on the molecular weight

of the PS block and that, for a given molecular

weight of the PS block, the aggregation number

decreased with an increase in the PEO chain

length. Despite the decrease in aggregation

number, the hydrodynamic radius of the micelles

was found to increase with PEO chain length. The

SANS results showed the polystyrene core to be

compact and small compared to the overall dimen-

sions of the micelles, and the micelles to be

spherical.

9.3. CiE surfactants

It is interesting to compare the micellization of

PEO PP O- PE O block copolymers to that of CiEj

(j-ethyleneglycol i-alkyl ethers) low molecular-

weight non-ionic surfactants [157,158], where

both amphiphiles have a PEO headgroup. In the

latter systems, it was found that the CMC



P. Alexandridis, T .A . Hatton/Colloids Sur/}wes A. Ptu'sicochem. En g.

spects

96 (1995) l 4V~ 37

d e c r e a s e d a n d t h e m i c e l l a r s i z e i n c r e a s e d s t r o n g l y ,

b o t h w i t h i n c r e a s i n g t e m p e r a t u r e . T h u s , i n t h e s e

r es p ec ts , t h e a n a l o g y b e t w e e n P E O - P P O c o p o l y -

m e r s a n d C ~ E,i s e e m s v a l id . H o w e v e r , t h e p r o -

n o u n c e d s e n s it iv i ty o f t h e C M C t o t e m p e r a t u r e ,

o r o f t h e C M T t o c o n c e n t r a t i o n o b s e r v ed f o r m a n y

P E O P P O P E O c o p o l y m e r s ( se e S e c t io n s 4 .1 , 5 .1

a n d 5 . 2 ) i s n o t e v i d e n t w i t h t h e n - a l k y l p o l y ( e t h y l -

e n e g l y c o l ) e t h e r s , C i E j [ 1 5 9 ] . T h e d e c r e a s e i n

h y d r o p h i l i c i t y o f t h e P E O b l o c k s w i t h in c r e a s in g

t e m p e r a t u r e h a s c o m p a r a t i v e l y l it tl e e f fe c t o n t h e

C M C f o r C ~ E j [ 1 5 9 ] : i n th e c a s e o f P E O -

P P O P E O c o p o l y m e r s o l u t io n s , a 1 0 K d e c r e a se

i n t e m p e r a t u r e r e s u l t e d i n c h a n g e s i n t h e C M C o f

m o r e t h a n a n o r d e r o f m a g n i t u d e [ 2 0 , 2 8 ]. T h e

n u m b e r o f E O s e g m e n t s a l s o h a s a s m a l l e f fe c t o n

t h e C M C o f C i E j s u r fa c t a n t s [ 1 5 7 ,1 5 9 ] ; th e c h a n g e

in C M C p e r E O s e g m e n t f o r P l u r o n i c c o p o l y m e r s

w a s s m a l le r t h a n a n e q u i v a l e n t c h a n g e f o r t h e

C ~E i ,

i n d i c a ti n g t h a t t h e i n f lu e n c e o f t h e P E O

b l o c k o n C M C d i m i n i s h es a s t h e s iz e o f t h e

s u r f a c t a n t i n cr e as e s. T h e m a i n d e t e r m i n a n t o f t h e

C M C f o r C /E .i w a s f o u n d t o b e t h e l e n g t h o f t h e

a lk y l ( h y d r o p h o b i c ) c h a in . T h e l o g a r it h m o f C M C

f o r t h e

C i E s , i = l O

15 . s e r i e s de c r e a se d l i ne a r ly

w i t h i n c r e a s i n g c a r b o n n u m b e r i n t h e a l k y l c h a i n

( C M C v a r ie d b y a p p r o x i m a t e l y 2 .5 o r d e r s o f m a g -

n i t u d e o v e r t h e c a r b o n n u m b e r r a n g e s t u d i e d )

[ 1 5 S ] .

T h e f re e e n e r g y o f m i c e l li z a t io n ,

A G

, d e c r e a s e d

w i t h i n c re a s i n g n u m b e r o f c a r b o n a t o m s in t h e

a l k y l c h a i n , a n d w i t h i n c r e a s i n g t e m p e r a t u r e f o r

. / - e t h y l e n e g l y c o l - i - a l k y l e t h e r s w i t h t h e s a m e

h e a d g r o u p ( P E O ) a n d v a r y i n g h y d r o p h o b i c t ai l

[ 1 5 8 , 1 6 0 ] . F r e e e n e r g y c h a n g e s f o r m i c e ll e f o r m a -

t i o n p e r m e t h y l e n e g r o u p w e r e e q u a l t o 0 .6 9

k c a l m o l ~, o r - 2 . 8 8 7 k J m o l ~, t o b e c o m p a r e d

w i t h a A G p o- o f a p p r o x i m a t e l y - 0 . 4 5 k J m o l - l

[ 3 0 ,6 9 , 76 ] f o r t h e P E O - - P P O P E O c o p o l y m e r s .

A H d e c r e a s e d w i t h i n c r ea s i n g c a r b o n n u m b e r ,

whi l e

T A S ='

i n c r e a s e d , i n d i c a t i n g t h a t A S c o n t r i b -

u t e s m a i n l y t o m i c e l l i z a ti o n , w h i le A H

c o u n t e r a c t s m i c e ll i za t io n . T h e h e a d g r o u p c o n t r i b u -

t i o n t o A G ~ d e c r e a s e d w i t h i n c r e a s i n g t e m p e r a t u r e .

a t t r i b u t e d t o d e h y d r a t i o n o f t h e P E O c h a in . F o r

. / -e t h y l e n e g l y c o l i - al k y l e t h e r s w i t h t h e s a m e

h y d r o p h o b i c t a i l a n d v a r y i n g P E O h e a d g r o u p { e . g .

f o r t he sys t e m C~¢ ,E i . . j= 17 , 32 , 44 , 63 [ 157] ) A G '

i n c r e a s e d ( b e c a m e l e s s n e g a t i v e t w i t h i n c r e a s i n g

n u m b e r o f E O s e g m e n t s in t h e h y d r o p h i l i c h e a d -

g r o u p , a n d d e c r e a s e d w i t h i n c r e a s i n g t e m p e r a t u r e .

A H a n d A S d e c r e a s e d w i t h i n c r e as i n g n u m b e r

o f E O s e g m e n ts , w h il e t h e h e a d g r o u p c o n t r i b u t i o n

t o AG i n c re a s e d w it h i n c r e as i n g n u m b e r o f E O

s e g m e n t s .

1 0 Se l ec t a pp l ica t i o ns o f b lo ck co po l y m er s o l u t i o ns

I t ) .l . S o l u h i l i z a t i o n q l ' o r ~ a n i c s

A n i n t e r e s t i n g p r o p e r t y o f a q u e o u s m i c e l la r sy s -

t e m s i s t h e i r a b i l i t y t o e n h a n c e t h e s o l u b i l i t y i n

w a t e r o f o t h e r w i s e w a t e r- i n s o l u b l e h y d r o p h o b i c

c o m p o u n d s . T h i s o c c u r s b e c a u s e th e c o r e o f t h e

m i c e l l e p r o v i d e s a h y d r o p h o b i c m i c r o e n v i r o n m e n t ,

s u i t a b l e f o r s o l u b i l i z i n g s u c h m o l e c u l e s . T h e p h e -

n o m e n o n o f s o l u b i li z a ti o n f o r m s t h e b a s is f o r m a n y

p r a c t ic a l a p p l i c a ti o n s o f a m p h i p h i l e s . T h e e n h a n c e -

m e n t i n t h e s o l u b i l i t y o f l y o p h o b i c s o l u t e s i n

s o l v e n t s a l f o r d e d b y a m p h i p h i l i c c o p o l y m e r

m i c e l l e s h a s s h o w n p r o m i s e i n m a n y i n d u s t r i a l a n d

b i o m e d i c a l a p p l i c a t i o n s [ 1 7 ] .

T h e s o l u b i l iz a t i o n o f p a r a - s u b s t i tu t e d a c e t a n i-

l i d e s i n a s e r i e s o f P l u r o n i c P E O P P O P E O

t r i b lo c k c o p o l y m e r s d e p e n d e d o n b o t h t h e c o p o l y -

m e r c o m p o s i t i o n a n d t h e n a t u r e o f t h e s o l u b il i za t e

[ 1 6 1 ] . T h e m o r e h y d r o p h o b i c h a l o g e n a t e d a c e t an -

i l i de s e xh ib i t e d a de c r e a se i n so lub i l i t y w i th a n

i n c r e a s e i n t h e c o p o l y m e r P E O c o n t e n t , w h e r e a s

t h e o p p o s i t c t r e n d w a s n o t e d w i t h le ss h y d r o p h o b i c

d r u g s s u c h a s a c e t a n i l i d e a n d t h e h y d r o x y - ,

m e t h o x y - , a n d e t h o x y - s u b s t i tu t e d c o m p o u n d s . T h e

s o l u b i l i ty o f i n d o m e t h a c i n i n a q u e o u , ; s o l u t i o n s o f

P E O P P O P E O c o p o l y m e r s w a s s tu d ie d b y k i n

a n d K a w a s h i m a [ 7 ] . T h i s h y d r o p h o b i c a n ti -

i n f l a m m a t o r y d r u g w a s s o l u b i l i z e d i n s i g n i f i c a n t

a m o u n t s o n l y a b o v e a c e r ta i n t h r es h o l d c o p o l y m e r

c o n c e n t r a t i o n , m o s t l i k e l y r e l a t e d t o t h e C M C .

T h e s o l u b i l i z i n g c a p a c i t y i n c r e a s e d w i t h p o l y m e r

m o l e c u l a r w e i g h t a n d s o h l t i o n t e m p e r a t u r e .

A p p r o x i m a t e l y 0 .5 m o l i n d o m e t h a c i n p e r t o o l o f

c o p o l y m e r c o u l d b e s o lu b i li z e d in t h e c o p o l y m e r

s o l u t i o n s [ 7 ] .

N a g a r a j a n e t a l . [ 1 5 ] s t u d i e d t h e s o l u b i l i z a t i o n

o f a r o m a t i c a n d a l ip h a t i c h y d r o c a r b o n s i n s o lu -



P. ,41exandrMis , T . A . Ha t to n /C ol l o i& Su l ju ces A . Phys icoch em. E l l g . ,4~spec ts 96 ( 1995i 1 46

tl

r a t h e r t h a n m i c e l l a r g r o w t h [ 1 7 ] . T h e s o l u b i l i -

z a t i o n o f 1 , 6 - d i p h e n y l - l ,3 , 5 - h e x a t ri e n e { D P H ) , a

f l u o r es c e n c e d y e , in P l u r o n i c c o p o l y m e r m i c e l la r

s o l u t i o n s w a s i n v e s t i g a t e d b y A l e x a n d r i d i s a n d

c o - w o r k e r s [7 6 , 1 6 2 ] . D P H p a r t i ti o n e d f a v o r a b ly

i n t h e m i c e l l a r p h a s e , w i t h a p a r t i t i o n c o e f f i c i e n t

t h a t w a s h i g h e s t f o r P 1 2 3 ( a P l u r o n i c c o p o l y m e r

w i t h 7 0 % P P O ) a n d d e c r e a s e d i n t h e o r d e r

P 1 2 3 > P 1 0 3 > P 1 0 4 = P I 0 5 > F 1 2 7 > P 8 4 > P 8 5

= F 1 0 8 > F 8 8 > F 6 8 > P 6 5 . I t c a n b e c o n c l u d e d

f r o m t h e s e d a t a t h a t t h e s o l u b i l i z a t i o n w a s i n f l u -

e n c e d b y b o t h t h e r e l a ti v e ( w i th r e s p e c t to P E O )

a n d a b s o l u t e s iz e o f t h e h y d r o p h o b i c P P O b l o c k .

10.2. Protection ql microorganisms

I n s e ct a n d m a m m a l i a n c el ls a r e b e c o m i n g

i m p o r t a n t i n th e b i o t e c h n o l o g y i n d u s t r y f o r t h e

p r o d u c t i o n o f c o m p l e x p r o t e i n s . T h e s e c el ls a r e

s e n s i t iv e t o s h e a r , a n d a r e t h u s d i ff i cu l t t o g r o w t o

h i g h c e l l c o n c e n t r a t i o n s s u c h a s t h o s e f o u n d i n

f e r m e n t a t i o n f o r th e p r o d u c t i o n o f an t i b io t i cs . C e l l

d e a t h o c c u r s d u r i n g s p a r g i n g b e c a u s e o f h i g h s h e a r.

P E O P P O P E O c o p o l y m e r s h a v e b e en s h o w n t o

p r o t e c t m a m m a l i a n c el ls f r o m s u c h d a m a g e .

M u r h a m m e r a n d G o o c h e e [ 1 2 ,1 6 3 ] e x a m in e d

t h e s t r u c t u r a l f e a tu r e s o f n o n - i o n i c p o l y g l y c o l

c o p o l y m e r m o l e c u l e s r e s p o n s i b le f o r t h e p r o t e c t iv e

e f fe c t i n s p a r g e d a n i m a l c e ll b i o r e a c t o r s . I t w a s

s h o w n t h a t t h e s a m e P l u r o n i c c o p o l y m e r s t h a t d i d

n o t i n h i b i t c e l l g r o w t h w e r e t h e o n e s t h a t p r o v i d e d

p r o t e c t i o n f r o m s p a r g i n g i n t h e b i o r e a c t o r s u s e d

i n t h i s s t u d y . T h e p r o t e c t i v e a b i l i t y o f P l u r o n i c s

w a s f o u n d t o c o r r e l a t e w i t h t h e h y d r o p h i l i c - l i p o -

p h i l i c b a l a n c e ( H L B ) ; t h e c o p o l y m e r s w i t h t h e

l a rg e s t H L B ( h i g h e r P E O c o n t e n t ) w e r e t h e p r o t e c -

t iv e a ge nts . T h e P E O P P O - P E O c o p o ly m e r s

s t u d i ed w e r e m o r e e ff e ct iv e t h a n h o m o p o l y m e r

P E O i n p r o t e c t i n g t h e m i c r o o r g a n i s m s . S u p p l e -

m e n t i n g t h e c e ll c u l t u r e m e d i u m w i t h 0 .2 % P l u r o -

n i c 1 _3 5 p r o v i d e d s i g n i f ic a n t l y m o r e p r o t e c t i o n f o r

c e l l s g r o w i n g i n a K o n t e s a i r l i f t b i o r e a c t o r t h a n

a d d i t i o n o f 0 .2 % P l u r o n i c F 6 8 [ 1 2 ] . I t w a s d e m o n -

s t r a t e d , h o w e v e r , t h a t i n c r e a s i n g t h e F 6 8 c o n -

c e n t r a t i o n t o 0 . 5 % , w h i c h r e s u l t s i n a m o l a r

c o n c e n t r a t i o n c o m p a r a b l e t o t h a t o f 0 .2 % L 3 5 ,

p r o v i d e d s i g n i f i c a n t c e l l p r o t e c t i o n ; L 3 5 a n d F 6 8

a p p e a r t h u s t o b e c o m p a r a b l e p r o t e c t i v e a g e n t s o n

a m o l a r b a si s [ 1 6 3 ] . Z h a n g e t a l. [ 1 3 ] m e a s u r e d

t h e m e c h a n i c a l p r o p e r t i e s o f h y b r i d o m a c e ll s t a k e n

f r o m a c o n t i n u o u s c u l t u r e i n t h e p r e s e n c e o r

a b s e n c e o f P l u r o n i c F 6 8 i n t h e c u l t u r e m e d i u m .

T h e m e a n b u r s t i n g m e m b r a n e t e n s i o n a n d t h e

m e a n e l a st i c a re a c o m p r e s s i b i li t y m o d u l u s o f th e

c e l l s w e r e s i g n i f i c a n t l y g r e a t e r i n a m e d i u m c o n -

t a i n i n g 0 . 0 5 ( ' } F 6 8 , c o m p a r e d t o t h a t w i t h o u t

P l u r o n i c c o p o l y m e r . T h e s h o r t - t e r n 1 e f f e c t o f

P l u r o n i c c o p o l y m e r w a s a ls o t e s t ed b y i ts a d d i t i o n

a t v a r i o u s l e v e l s u p t o 0 . 2 % i m m e d i a t e l y b e f o r e

t h e m e c h a n i c a l p r o p e r t y m e a s u r e m e n t s : a s i g n i f i -

c a n t s h o r t - t e r m e ff ec t c o u l d o n l y b e d e t e c t e d a b o v e

a t h r e s h o l d c o p o l y m e r c o n c e n t r a t i o n o f 0 .1 % [ 1 3 ].

O r t o n [ 1 4 ] f o u n d P l u r o n i c F 6 8 t o p r o v i d e s ig n if i-

c a n t p r o t e c t i o n i n m a m m a l i a n c e l l s i n a s p a r g e d

b u b b l e c o l u m n r e a c to r . I t w a s o b s e r v e d t h a t , w h e n

F 6 8 w a s p r e s e n t i n t h e m e d i u m , c e l l s n o l o n g e r

a t t a c h e d t o b u b b l e s a n d t e n d e d t o a g g r e g a t e l e s s

a m o n g t h e m s e l v e s . A s h e d d i n g e f f ec t w a s a l s o

v i s u a l i z e d [ 1 4 ] : o n a b u b b l e f o r m e d i n a m e d i u m

w i t h o u t F 6 8 , t h e c e l l s w e r e f o u n d t o b e a s s o c i a t e d

w i t h t h e b u b b l e s u r f a c e , w h e r e a s c e ll s w e r e q u i c k l y

s h e d f r o m t h e g a s l i q u i d i n t e r f a c e i n t h e p r e s e n c e

of P lu r on i c I - ' 68 .

10.3. Medical applications o[ PEO PPO PEO

copolymers

' M i c r o c o n t a i n e r s f o r d r u g t a r g e t i n g u si n g

P l u r o n i c P E O P P O P E O b l o c k c o p o l y m e r s w e r e

p r e p a r e d b y K a b a n o v a n d c o - w o r k e r s [ 1 0 , 1 6 4 ] .

I l l o r d e r t o t a r g e t s u c h m i c r o c o n t a i n e r s t o a c e r t a i n

c el l, th e c o p o l y m e r m o l e c u l es w e r e c o n j u g a t e d w i t h

a n t i b o d i e s a g a i n s t a t a r g e t - s p e c if i c a n t i g e n o r w i t h

p r o t e i n l i g a n d s s e l e ct i v e ly i n t e r a c t i n g w i t h t a r g e t

c e l l r e c e p t o r s : p r o t e i n s w e r e a t t a c h e d t o a l d e h y d e

g r o u p c o n t a i n i n g P l u r o n i c m o l e c u l e s u s i n g

t h e r e d u c t i v e a l k y l a t i o n r e a c t i o n . T h e o b t a i n e d

c o n j u g a t e s w e r e t h e n i n c o r p o r a t e d i n t o m i c e l l e s

c o n t a i n i n g s o l u b i l i z e d d r u g s ( s u c h a s f l u o r e s c e i n

i s o t h i o c y a n a t e ( F I T C ) a n d h a l o p e r i d o l ) b y si m p l e

m i x i n g o f t h e c o r r e s p o n d i n g c o m p o n e n t s . I t w a s

f o u n d t h a t s o l u b i li z a ti o n o f F I T C in P l u r o n i c

c o p o l y m e r m i ce l le s c o n s i d e r a b l y i n f l u e n c e d t h e di s -

t r i b u t i o n o f F I T C i n a n i m a l ( m o u s e ) ti s su e s , a n d

r e s u l t e d in a d r a s t i c i n c r e a s e o f F I T C f l u o r e s c e n c e

i n th e l u n g . F h e p e n e t r a t i o n e f fi c ie n c y o f F I T C i n



4 P. Alexandridis, T. A. H atton/Colloids Sur]aces A: Physicochent . Eng. Aspects 96 1995) 1-4 6

l u n g t i ss u e s i n c r e a s ed w i t h i n c r e a s in g P E O -

P P O - P E O c o p o l y m e r h y d r o p h o b i c i ty in t he s er ie s

F 6 8 , P 8 5 , L 6 4 . S u c h b e h a v i o r w a s a t t r ib u t e d t o

e i t h e r d i ff e re n t p a r t i ti o n i n g p r o p e r t i e s o f P l u r o n i c

c o p o l y m e r s i n t i s s u e m e m b r a n e s o r d i f f e r e n c e s

i n m i ce ll e s iz e. C o n j u g a t i o n o f F I T C - c o n t a i n i n g

m i c e l l e s w i t h i n s u l i n v e c t o r r e s u l t e d i n i n c r e a s e d

F I T C p e n e t r a t i o n i n a l l t i s s u e s , i n c l u d i n g t h e b r a i n .

S p e c i fi c t a r g e t i n g o f t h e s o l u b i l i z e d F I T C i n t h e

b r a i n w a s o b s e r v e d w h e n a P l u r o n i c c o n j u g a t e

w i t h a n t i b o d i e s t o t h e a n t i g e n o f b r a i n g l i a l c e ll a 2-

g l y c o p r o t e i n ) w a s i n c o r p o r a t e d i n t o t h e m i c el le s .

A c o n s i d e r a b l e i n c r e as e o f F I T C f l u o r es c e n c e i n

t h e b r a i n a n d d e c r e a s e o f it s f lu o r e s c e n c e i n t h e

l u n g s h a d b e e n r e g i s t e r e d u n d e r t h e s e c o n d i t i o n s .

T h e p o s s i b il i ty o f u s in g m i c e l la r m i c r o c o n t a i n e r s

f o r t a r g e t i n g n e u r o l e p t i c s h a l o p e r i d o l ) i n t h e b r a i n

w a s a l so s t u d i e d . I n c o r p o r a t i o n o f a n t i b o d i e s

t o a z - g l y c o p r o t e i n i n t o h a l o p e r i d o l - c o n t a i n i n g

m i c e l le s r e s u l t e d i n a d r a s t i c i n c r e a s e o f t h e

e f f e ct i v e n e ss o f t h e d r u g , i n d i c a t i n g t h a t v e c t o r -

c o n t a i n i n g P l u r o n i c c o p o l y m e r m i ce l le s c a n e f fe c-

t i v e l y t r a n s p o r t s o l u b i l i z e d n e u r o l e p t i c s a c r o s s t h e

b l o o d b r a i n b a r ri e r. T h e s a m e r e s e a rc h g r o u p a l s o

d e m o n s t r a t e d t h a t a l o w - m o l e c u l a r - w e i g h t c o m -

p o u n d A T P ] , s o l u b i l i z e d i n P l u r o n i c m i c e ll e s ,

a c q u i r e s t h e a b i l i t y t o p e n e t r a t e w i t h i n a n i n t a c t

c e l l i n v i t r o [ 1 6 5 ] .

G e ls f or m e d by P l u ro n ic P E O - P P O P E O

c o p o l y m e r s w e r e e v a l u a t e d b y G u z m a n e t al. [ 9 ]

a s m e d i a f o r t h e s u b c u t a n e o u s a d m i n i s t r a t i o n o f

d r u g s ; p h e n o l s u l f o p h t h a l e i n P R ) w a s u s e d a s a

t r ac e r . T h e t y p e o f P l u r o n i c c o p o l y m e r F 1 0 8 o r

F 1 2 7) , t h e i r c o n c e n t r a t i o n s , a n d t h e e f f e ct o f s o l u te s

N a O H , N a C 1 o r P R ) o n g e l a t i o n p r o p e r t i e s w e r e

s t u d ie d ; s o d i u m h y d r o x i d e a n d s o d i u m c h l o r i d e

d e c r e a s e d t h e g e l s o l t r a n s i t i o n t e m p e r a t u r e ,

w h e r e a s t h e o p p o s i t e e f fe c t w a s o b s e r v e d w i t h P R .

T h e i n v i t r o r e l e a s e r a t e s o b t a i n e d f o r P R w e r e

i n v e r se l y p r o p o r t i o n a l t o t h e c o n c e n t r a t i o n o f

c o p o l y m e r u s e d a n d a z e r o - o r d e r r e le a s e r a t e w a s

o b s e r v e d i n a l l p r e p a r a t i o n s a s s a y e d , T h e a m o u n t

r e l e a s e d a t a g i v e n t i m e w a s s m a l l e r a t h i g h p o l y -

m e r c o n c e n t r a t i o n s ; t h i s w a s a t t r i b u t e d t o t h e g e l

m a t r i x b e i n g m o r e r ig i d. P l u r o n i c F 1 2 7 / P R p r e p a -

r a t i o n s w e r e a d m i n i s t e r e d s u b c u t a n e o u s l y t o

W i s t a r r at s; P R p l a s m a l ev e ls w e r e c o m p a r e d w i th

t h o s e r e a c h e d a ft e r s u b c u t a n e o u s o r i n t r a v e n o u s

a d m i n i s t r a t i o n o f a P R a q u e o u s s o l u t i o n T h e g el

f o r m u l a t i o n p r o d u c e d a s u s t a i n e d p l a t e a u l ev e l

w i t h i n 1 5 r a i n t h a t l a s t e d 8 9 h . G o o d f i ts f o r

e x p e r i m e n t a l i n v i v o d a t a f r o m P l u r o n i c g el s w e re

o b t a i n e d u s i n g a z e r o - o r d e r i n p u t a n d f i rs t - o rd e r

o u t p u t t w o - c o m p a r t m e n t p h a r m a c o k i n e t ic s m o d e l;

t h e f it te d p a r a m e t e r s o f th e m o d e l i n d i c a t e t h a t

r e le a s e o f P R f r o m t h e g e l c o n t r o l l e d t h e P R

a b s o r p t i o n p r o c e s s . T h e r e s u l t s o b t a i n e d s u g g e s t

t h a t F 1 2 7 a q u e o u s g el s m a y b e o f p r a c t ic a l u s e a s

m e d i a f o r t h e s u b c u t a n e o u s a d m i n i s t r a t io n o f

d r u g s .

T h e a d s o r p ti o n o f P P O P E O c o p o ly m e r s a t

s u r fa c e s a s a m e a n s o f p r e v e n t in g p r o t e i n a d s o r p -

t i o n a t t h e s e s u r f a c e s h a s b e e n e x p l o r e d b y L e e

e t a l. [ 1 6 6 ] a n d A m i j i a n d P a r k [ 1 6 7 ] . P r o t e i n s

a d s o r b t o a l m o s t a l l s u r f a c e s d u r i n g t h e f i r s t f e w

m i n u t e s o f b l o o d e x p o s u r e . T h e r e h a s b e e n m u c h

e f f o r t i n m i n i m i z i n g o r e l i m i n a t i n g p r o t e i n a d s o r p -

t i o n , a s s u r f a c e s t h a t e x h i b i t m i n i m a l p r o t e i n

a d s o r p t i o n a r e i m p o r t a n t i n m a n y a p p l i c a t i o n s ,

i n c l u d i n g d e v i c e s t h a t c o m e i n t o c o n t a c t w i t h

b l o o d , m e m b r a n e s f o r s e p a r a t i o n p r o c e s s e s , c o n -

t a c t le n s e s, et c. [ 1 6 6 ] . L e e e t a l. [ 1 6 6 ] f o u n d t h e

a d s o r p t i o n o f h u m a n a l b u m i n o n l o w d en s i ty

p o l y e t h y l e n e L D P E ) su r fa c e s t r e a t e d w i t h c o p o l y -

m e r t o b e s i g n i fi c a n t ly le ss t h a n o n t h e u n t r e a t e d

s u r f a c e . T h e p r o t e i n r e s i s t a n c e t o t h e c o p o l y m e r -

t r e a t e d s u r f a c e w a s h i g h l y d e p e n d e n t o n t h e

a m o u n t o f c o p o l y m e r a d s o r b e d a n d o n t h e m o b i l-

i ty o f t h e P E O s e gm e n ts . S ta r -l ik e P E O - P P O

b l o c k c o p o l y m e r s w e r e f o u n d t o h a v e b e t t e r

a d s o r p t i o n p r o p e r t i e s t h a n t r i b l o c k s . D e s p i t e t h e

f a v o r a b l e r e s ul ts , t h e a u t h o r s v o i c e c o n c e r n o v e r

t h e s t a b il i ty o f th e a d s o r b e d c o p o l y m e r l a y e r a n d

t h e p o s s ib l e d e s o r p t i o n o f t h e c o p o l y m e r s i n t o t h e

t r e a t e d m e d i u m [ 1 6 6 ] . N o t e a l so t h a t , b e c a u s e o f

c o m p e t i t i v e a d s o r p t i o n b e t w e e n c o p o l y m e r s a n d

p r o t e i n s , a n d d e s o r p t i o n o f th e c o p o l y m e r b lo c k s ,

a m b i g u i t i es m a y b e i n t r o d u c e d i n p r o t e i n a d s o r p -

t i on s tud i e s .

A d s o r p t i o n o f f i b r in o g e n a n d p l a te l e t a d h e s i o n

o n t o d i m e t h y l d i c h l o r o s i l a n e - t r e a t e d g l a s s a n d

L D P E s u rf ac e s t r e at e d w i th P E O h o m o p o l y m e r

a n d v ar io u s P E O P P O P E O b l oc k c o p o l y m e rs

w a s e x a m i n e d b y A m i ji a n d P a r k [ 1 6 7 ] . P E O

c o u l d n o t p r e v e n t p l a t e le t a d h e s i o n a n d a c t i v a t i o n ,

e v e n w h e n t h e b u l k P E O c o n c e n t r a t i o n fo r a d s o r p -



P . A h 'x and r i d i s , T . A . Ha t t on / C o l l oM s Sur / ~ t c e s A . P h r s i c o c h e m E n ~ . . 4 ,spe ct s 96 ( I c )95 J 1 46 41

t i o n w a s i n c re a s e d t o 1 0 m g m l - ~ . P l u r o n i c c o p o l y -

m e r s c o n t a i n i n g 3 0 P O s e g m e n t s c o u l d n o t p r e v e n t

p l a t e l e t a d h e s i o n a n d a c t i v a t i o n , a l t h o u g h t h e

n u m b e r o f E O s e g m e n t s v a r ie d u p t o 7 6 . P l u r o n i c s

c o n t a i n i n g 5 6 P O s e g m e n t s i n h i b i te d p l a t e le t a d h e -

s io n a n d a c t iv a t i o n , a l th o u g h t h e n u m b e r o f E O

s e g m e n t s w a s a s s m a ll a s 1 9. F i b r i n o g e n a d s o r p t i o n

o n t h e P l u r o n i c - c o a t e d s u r f a c e s w a s r e d u c e d b y

m o r e t h a n 9 5 c o m p a r e d to a d s o r p t i o n o n c o n t r o l

s u r f a c e s . T h e a b i l i t y o f P l u r o n i c c o p o l y m e r s t o

p r e v e n t p l a t e l e t a d h e s i o n a n d a c t i v a t i o n w a s

m a i n ly d e p e n d e n t o n t h e n u m b e r o f P O s e gm e n ts ,

r a t h e r th a n t h e n u m b e r o f E O s e g m e nt s [ 1 6 7 ] .

T h i s w a s c o n t r a r y t o t h e f i n d i n g s o f L u o e t a l.

[ 14 71 ] w h o s u g g e s t e d t h a t c o p o l y m e r s w i t h l a r g e

P P O b l o c k s d o n o t a d s o r b w e l l b e c a u s e th e y fo r m

m i c e l l e s i n s o l u t i o n ; i t s h o u l d b e n o t e d , h o w e v e r ,

t h a t t h e n u m b e r o f P l u r o n i c c o p o l y m e r s s tu d i e d

b y L u o e t a l . w a s r a t h e r l i m i t e d . A c c o r d i n g t o

A m i j i a n d P a r k [ 1 6 7 ] , t h e P O s e g m e n t s w e r e

r e s p o n s i b l e f o r a t t a c h i n g t h e c o p o l y m e r t o t h e

s u r f a ce , a n d t h e E O s e g m e n t s f o r r e p e l l in g f i b r in o -

g e n a n d p l a t e l e t s b y a s t e r i c r e p u l s i o n m e c h a n i s m .

1 1 D y na m i cs o f m i ce l l e a nd g e l f o rm a t i o n

T h e k i n e ti c s o f m ic e ll e f o r m a t i o n i n a q u e o u s

s o l u t i o n s o f l o w - m o l e c u l a r - w e i g h t s u r f a c t a n ts h a v e

a t t r a c t e d c o n s i d e r a b l e a t t e n t i o n . I n g e n e r a l , t w o

r e l a x a t i o n p r o c e s s e s h a v e b e e n o b s e r v e d : a f a s t

p r o c e s s / H s m s t i m e r a n g e ) a t t r i b u t e d t o a s u r f a c -

r a n t m o l e c u l e e n t e r i n g o r e x i t i n g a m i c e ll e , a n d a

s l o w e r p r o c e s s { m s - s t i m e r a n g e ) d u e t o m i c e l l e

a s s e m b l y o r d i s s o c i a t i o n [ 1 6 8 ] . T h e l a r g e m o l e c u -

l a r w e ig h t a n d c h a i n - li k e s t r u c t u r e o f b l o c k c o p o l y -

m e t m o l e c u l e s a r e e x p e c t e d t o c o m p l i c a t e t h e

p r o c e ss e s o f m ic e ll e f o r m a t i o n a n d e x c h a n g e o f

c o p o l y m e r m o l e c u l e s b e t w e e n m i c e l l e s a n d t h e

b u l k s o l u t i o n : d i s e n t a n g l e m e n t o f c o p o l y m e r m o l e -

c u l e s f r o ln t he m ic e l l e s shou ld r e su l t i n t im e - sc a l e s

s l o w e r t h a n t h o s e o b s e r v e d i n c o n v e n t i o n a l s u r f ac -

t al lt s. L i n sc a n d M a l m s t e n [ 2 5 ] a n d M a l m s t e n

a n d L i n d m a n [ 5 9 ] f o ll o w e d t h e t e m p e r a t u r e

d e p e n d e n c e o f th e P l u r o n i c F 1 2 7 m i c e l li z a ti o n

p r o c c s s b y p e r f o r m i n g g e l - p e r m e a t i o n c h r o m a t o g -

r a p h y ( G P C ) e x p e r i m e n t s a t d i f f e r e n t t e m p e r -

a t u r e s . T h e m i c e l l e f r a c t i o n a p p e a r e d a s a s e p a r a t e

p e a k i n t h e c h r o m a t o g r a m : t h i s l e d t h e a u t h o r s t o

t h e c o n c l u s i o n t h a t t h e r e s i d e n c e ti m e o f t h e p o l y -

m e r m o le c ul es in th e F 1 2 7 P E O - P P O P E O

m i c e l l e s i s e x t r e m e l y l o n g ( h o u r s ) . H o w e v e r .

F l e i s c h e r [ 6 5 ] r e p o r t e d t h a t t h e l if e ti m e o f a

c o p o l y m e r m o l e c u l e w i th i n t h e m i ce ll e w as s h o r t e r

t h a n t h e m i n i m u m o b s e r w l t i o n t im e o f t h e e x p e ri -

m e n t s ( ~ 3 m s ) . H e r e a c h e d t h i s c o n c l u s i o n f r o m

P F G - N M R m e a s u r e m e n t s o f F 1 27 c o p o l y m e r se lf-

d i f fu s i o n . S i n c e u n i m e r s a n d m i c e ll e s c o e x i s t i n t h e

s o l u t i o n . F l e i s c h e r e x p e c t e d t o o b s e r v e t w o s e l f -

d i f f u s io n c o e f fi c ie n t s , o n e f o r t h e u n i m e r s a n d o n e

f o r t h e m i c e l le s , i f t h e r e w e r e n o e x c h a n g e o f t h e

m o l e c u l e s b e t w e e n t h e t w o s t a t e s w i t h i n t h e d i f f u -

s ion t im e . A s ing l e d i f f us ion c oe f f i c i e n t wa s , ne ve r -

t h e le s s , o b s e r v e d , a n d t h u s F l e i s c h e r [ 6 5 ] i n f e r re d

t h a t t h e c o p o l y m e r s o lu t i o n w a s m t h e d y n a m i c

r a n g e o f f a st e x c h a n g e .

T h e k i n e ti c s o f g e l a t io n f o r a q u e o u s s o l u t i o n s o f

P l u r o n ic F 9 8 a n d F 12 7 P E O P P O P E O c o p o l y -

m e r s w e r e i n v e s t i g a t e d b y W a n g a n d J o h n s t o n

[ 1 3 8 ] w i t h l h e h e l p o f p ul se s h e a r o m e t r y . T h e

p u l s e s h e a r o m e t e r w a s d e s i g n e d t o m e a s u r e t h e

p r o p a g a t i o n v e l o c it y ( a n d f r o m t h is t h e d y n a m i c

s h e a r m o d u l u s ) o f a s h e a r w a v e t h r o u g h a v i s c o -

e l a s t i c m a t e r i a l t h a t h a s b e e n s u b j e c t e d t o c o n d i -

t i o n s o f n o n - s t e a d y s h e a r . T h r e e d i s t i n c t l i n e a r

p h a s e s w e r e o b s e r v e d f o r t h e l o g ( d y n a m i c s h e a r

m o d u l u s , G ' ) v s . t i m e p r o f i l e s , a s c o p o l y m e r s o l u -

t i o n s o f v a r s i n g c o n c e n t r a t i o n s w e r e a l lo w e d t o

p a s s iv e l y w a r m a t r o o m t e m p e r a t u r e t o a t e m p e r -

a t t i r e e x c e e d i n g t h e s o l - t o - g e l t r a n s i t i o n t e m p e r -

a t t i r e , T ,,~ . T h e i n i t i a l e xp on e n t i a l pha se wa s d ue

t o w a r m i n g , w h i le t h e b e g i n n i n g o f th e s e c o n d

e x p o n e n t i a l p h a s e c o i n c i d e d w i t h t h e o n s e t o f t h e

g e l a t i o n p r o c e s s a s d e t e r m i n e d b y v i s u a l o b s e r v a -

t i o n . T h e t h i r d e x p o n e n t i a l p h a s e w a s a t t r i b u t e d

t o t h e r a t e o f f o r m a t i o n o f t h e p o l y m e r n e t w o r k

( g e l a ti o n a p p e a r e d t o b e c o m p l e t e a t t h e b e g i n n i n g

o f t h is p h a s e ). A c o m p a r i s o n o f F 1 2 7 1 3 0 , 7 ~ , =

10 .9 C) a nd V981 37 , Tm = 11 .1 C ) wo uld sugge s t

t h a t t h e c o n c e n t r a t i o n o f c o p o l y m e r re q u i r e d t o

a c h i e v e a c e r t a i n g e l a t i o n t e m p e r a t u r e d e c r e a s e s

w i t h in c r e a s in g m o l e c u l a r w e ig h t o f t h e P P O b l o c k

o f t h e c o p o l y m e r . C h a r a c t e r i s t ic t im e s f o r g e l at i o n

we re 17 an d 9 min for 20 an d 3() (~; F '127, respec-

t i v el y , a n d 4 3 a n d 3 0 m i n f o r 2 0 a n d 3 0 F 9 8 ,

r e s p e c ti v e l y . T h e g e l a t i o n p r o c e s s w a s m o r e r a p i d



4 2 P. Alexandridis, T. A. Hatton/Colloids SurJktces A: Physicochem. Eng. Aspects 96 1995) 1-46

for F127 (at a given concentration) and the charac-

teristic times decreased with increasing concen-

tration for a given copolymer. Wang and Johnston

[-138] suggested that the rate of gelation for the

Pluronic solutions studied was dependent on the

rate of heat transfer through the polymer solution

(copolymer solutions were allowed to passively

warm at room temperature). If this statement is

true then the attribution by the authors of the

time-dependent results to ~kinetics of gelat ion is

erroneous, and the time-scales they reported were

actually kinetics of heat transfer. It becomes obvi-

ous from the above that the dynamics of block

copolymer micelle and gel formation, although

important from both a theoretical and a practical

point of view, are still unexplored and not well

understood.

1 2 C o n c l u s i o n s

The phase behavior and aggregation proper-

ties of poly(ethylene oxide) -b lock-po ly p ropylene

oxide)-block-poly ethylene oxide) (PEO-PPO-

PEO) copolymers in solution, as affected by the

copolymer molecular composition and concen-

tration, additives, and solution temperature, are

important in understanding the mechanism of

copolymer action underlying the various applica-

tions. While the PEO PPO -P EO copolymers exist

in solution as individual coils (unimers) at low

temperatures and/or concentrations, micelles are

formed on increasing the copolymer concentra tion

and/or solution temperature, as revealed by surface

tension, light scattering, and dye solubilization

experiments. The unimer-to-micelle transition is

not sharp, but spans a concentration decade or

10K. The critical micellization concentration

(CMC) and temperature (CMT) decrease with

increase in the copolymer PPO content or molecu-

lar weight, and in the presence of salts. The depen-

dence of C M C on temperature together with

differential scanning calorimetry measurements

indicate that the micellization process of PEO -

PPO-PEO copolymers in water is endothermic,

and is driven by the entropy gain in water when

unimers aggregate to form micelles (hydrophobic

effect) and the decrease in polarity of EO and PO

as temperature increases. The free energy and

enthalpy of micellization can be correlated to the

number of EO and PO segments in the copolymer

and the copolymer molecular weight. The PEO-

PPO PEO copolymers form micelles with hydro-

dynamic radii of approximately 10 nm and aggre-

gation numbers in the order of 50. The aggregation

number is thought to be independent of concen-

tration and to increase with temperature. Despite

the latter, the micelle hydrodynamic radii are

generally independent of temperature, a result of

dehydration of the PEO segments at elevated

temperatures. Fluorescence molecules have been

used to probe the microenvironment in block

copolymer micelles. Phenomenological and mean-

field lattice models for the formation of micelles

can capture qualitatively the trends observed

experimentally. A self-consistent mean-field theory

was able to reproduce the anomalous phase beha-

vior of PEO and PPO homopolymers, predict the

micellization behavior of PE O- PP O- PE O block

copolymers, and provide detailed information on

the distribution of the copolymer segments in the

micelles. The model calculations showed qualita-

tive agreement with experimental predictions on

the effect of temperature, concentration, hydropho-

bicity, and molecular weight of the polymer: quan-

titative agreement was also good, considering that

all the model input parameters were obtained from

independent experiments. The PEO-PPO PEO

copolymers adsorb on air-water and solid-water

interfaces; for copolymers adsorbed at hydrophobic

interfaces, the PPO block is located at the interface

while the PEO block extends into the solution.

Gels are formed by certain PEO -P PO -P EO block

copolymers at high concentrations, while the

micelles remain apparently intact in the form of a

crystal . The gelation onset temperature and the

thermal stability range of the gel increase with

increasing PEO block length.

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