E L S E V I E R Colloids and Surfaces

A: Physicochemical and Engineering Aspects 108 (1996) 159-171

COLLOIDS AND A SURFACES

The adsorption of polystyrene saturated-polydiene block copolymers on silica substrates

S.M. King a,,, T. Cosgrove b, A. Eaglesham c,1 a ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 OQX, UK

b Department of Physical Chemistry, University of Bristol, Cantocks Close, Bristol, BS8 1 TS, UK c ICI plc, The Heath, Runcorn, Cheshire WA7 4QD, UK

Received 8 February 1995; accepted 5 September 1995

Abstract

The adsorption of AB- and ABA-type block copolymers of polystyrene (the A or anchor blocks) and either poly(ethylene-random-propylene) or poly(ethylene-random-butylene) (the B or buoy blocks) onto normal (hydroxy) and surface-modified (alkylated) colloidal silica particles has been investigated and compared with that of the constituent polymers. Markedly different behaviour has been observed depending on the solvency of the anchor blocks and on the type of silica surface. In all cases the buoy blocks were well solvated but non-adsorbing. The results are discussed in the context of the Marques-Joanny (M J) and Marques-Joanny-Leibler (MJL) scaling descriptions of block copolymer adsorption from non-selective and selective solvents respectively. For the case of adsorption onto normal silica from a non-selective solvent, high-affinity polymer adsorption isotherms were obtained. As the polystyrene content of the polymer increased, the affinity of that polymer for the surface was generally enhanced and a higher adsorbed amount was recorded. The corresponding surface density of polymer shows reasonable agreement with the MJ scaling prediction for the anchor regime. Similar trends were observed in selective solvents, although the adsorption regime was somewhat ambiguous and so there was less satisfactory agreement with MJL scaling predictions. In addition, the highest adsorbed amount did not occur in the most selective solvent. This is thought to be due to extensive, and largely irreversible, micellisation. The thicknesses of the adsorbed polymer layers in selective solvents again showed semi-quantitative agreement with the MJL predictions but it was not possible to differentiate conclusively between normal adsorption from solution and micellar adsorption. By comparison, very low affinity polymer adsorption isotherms were obtained with the modified silicas, the shape of the isotherm being dependent on the polystyrene content of the polymer and on the nature of the surface modifying group.

Keywords: Adsorption; Block copolymers; Saturated-polydiene block copolymers; Silica substrates

1. Introduction

Probably the most effective method of sterically stabilising a colloidal dispersion with adsorbing

* Corresponding author. Formerly at the Department of Physical Chemistry, University of Bristol, UK.

1 Formerly on secondment at the ISIS Facility, Rutherford Appleton Laboratory, Didcot, UK.

0927-7757/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0927-7757(95)03397-1

polymer is to use block copolymers. Ideally, one block, the anchor (or A block), is strongly adsorb- ing whilst another, the buoy (or B block), remains non-adsorbing but well solvated so as to provide an effective steric barrier. AB-type copolymers tend to be more effective in this role than ABA-type copolymers of the same overall or the same buoy block length, since the non-adsorbing segments in the latter are predominant ly in the form of loops.

160 S.M. King et aL/Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 159 171

These loops possess less configurational entropy than the tails present in a layer formed from AB-type copolymers. At the same time, when AB- and ABA-type block copolymers are dissolved in solvents which are selectively poor for one block but selectively good for the other, copolymer micelles usually form. Such solvents can therefore lead to competition between the processes of adsorption and micellisation providing that the polymer micelles are not formed irreversibly.

In recent years a number of scaling [1,2] and self-consistent field (SCF) [3,4] descriptions of block copolymer adsorption from both non-selec- tive [1,3] and selective [2-4] solvents have appeared, as having scaling [5] and SCF [6] descriptions of block copolymer micellisation. Qualitatively the scaling and SCF approaches give similar predictions. Experimental tests of these predictions exist for both the non-selective [7] and selective [-8] solvent environments.

1.I. Scaling theories of diblock Copolymer adsorption

The scaling description of adsorption from a non-selective solvent, due to Marques and Joanny (MJ) [ 1 ], envisages an adsorbed layer consisting of a swollen anchor region and a more dilute and extended buoy region. The structure of the layer is principally determined by the block asymmetry parameter (actually the ratio of the Flory radii of the blocks), flnon =NA3/SN~/Sa, where NA is the degree of polymerisation of the anchor block and a is the ratio of the monomer sizes (aB/aa). Increasing the fraction of anchor blocks VA= NA/(NA + NB), initially enhances F (the amount of adsorbed polymer per unit area) because of the greater adsorption energy per chain but it is moder- ated by the lateral repulsion between the buoys. This is known as the buoy-dominated regime and is found when fi,on > NaA/2. The surface density, p, of adsorbed chains where

p[nm -2] = (F[mg m-Z] /mw[g mol 1])

x NAv x 10 -21 (1)

is expected to scale as p ~ N6/SNB 6Is (~f12) in this regime and the layer thickness as L ~ v2/s v3/5 ~ A ~ V B •

Mw is the weight-average molecular weight of the polymer and NAv is Avogadro's number. When 1 < flnon < N~A/2 lateral repulsion between the buoy blocks is much less important and adsorption is limited only by saturation of the surface by the anchor blocks. This is the anchor-dominated regime. In this regime p ,-~ N A 1 and L ~ NA1/3NB .

The description of adsorption from a selective solvent is that formulated by Marques, Joanny and Leibler (MJL) [2(a)]. Once again the forms of the scaling laws depend on the parameter fl, but suitably modified to allow for the different Flory radii of the blocks in the selective solvent: flsel=NA1/2Ng/Sa. In the MJL description the anchor blocks are taken to be in a non-solvent environment and so the anchor layer may be likened to a pure "melt-like" film. In addition, polymer micelles are present in the bulk. Once more two regimes are defined: a buoy-dominated regime, where the thickness of the adsorbed layer is determined solely by the stretching energy (the osmotic repulsion) of the buoy blocks, when flsel ~ 1; and the van der Waals (VdW)-buoy regime, where the layer thickness depends on a balance between the stretching energy of the buoy blocks and the VdW energy of interaction of the anchor blocks with the surface, when flsel > 1. The MJL theory actually defines two length scales in the adsorbed layer, the thickness of the anchor block "film", d, and the thickness of the buoy block "brush", L. In the buoy-dominated regime the expected scaling behaviour is p~N12/ZSNB 3°/25, d ~ N3712SNB 30/25 and L ~ -~,AA/4/25 ~'BVlS/25 whilst in the VdW-buoy regime it predicts p ~ NA12/g3N~ 6/23, d ~ NIA1/23NB 6/23 and L ~ N A 4 / 2 3 N 21/23.

1.2. Triblock copolymers

The MJ and MJL theories were derived for diblock copolymers and have not as yet been extended to the case of triblock copolymers, such as are used in this work. However, triblock copoly- mers may be treated as "double-diblock" copoly- mers by halving the NA values in the case of a BAB triblock or by halving the NB values in the case of an ABA triblock. In the latter instance it is assumed that the degree of stretching of a loop formed by the buoy block in a triblock copolymer

S.M. King et al./Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 159-171 161

is the same as that of a tail of length NH/2 in an equivalent diblock copolymer.

2. Experimental

2.1. Substrates

Approximately spherical colloidal silica particles with a narrow particle size distribution were pre- pared by the alkaline hydrolysis of silicon tetra- ethoxide (tetraethyl orthosilicate, G.P.R. Grade, B.D.H. Ltd), 8 i ( O C 2 H 5 ) 4 , in alcoholic solution [-9]. The physical characteristics of these particles are shown in Table 1 along with those of a commer- cially produced pyrogenic silica, Aerosil A130 (Degussa), which was also used in this work. Particle sizes were determined by photon correla- tion spectroscopy (PCS) in the mother liquor for silicas $1-$3 and by transmission electron micro- scopy (TEM).

The silicas S1-$3 were rendered organophilic by condensing the surface silanol (SiOH) groups with the hydroxyl functions of selected primary alcohols. Esterification reactions were conducted with cyclohexanol (G.P.R. Grade, B.D.H. Ltd.), neo-pentyl alcohol (2,2-dfmethylpropan- 1-ol, Aldrich Chemical Co. Ltd.) and stearyl alcohol (octadecanol, Aldrich Chemical Co. Ltd.) in the chosen dispersion medium, which was in turn dehydrated by extensive fractional azeotropic dis- tillation [ 10]. The surface concentration of grafted chains was estimated from the carbon content of the modified silicas as determined by microgravi- metric analysis. The values obtained were 2.8-5.8 gmol m -2 (8.3 t.tmol m -2 = 0.82 mg m 2) for the cyclohexyl silicas, ~ 8.8 gmol m-2

(7.1 gmol m -2 = 0.62 mg m -z) for the neo-pentyl silicas and ~ 2 5 . 4 g m o l m -z (19.3 ~tmolm-Z= 5.20mgm -2) for the stearyl silicas, where the values in parentheses represent estimates of the maximum possible surface concentration in each case [10]. These estimates were determined by considering the molecular dimensions of the carbon skeletons of each surface modifying group (SMG). For comparison, controlled wettability studies with fl-tridymite (a polymorph of silica) suggest that the average concentration of silanol groups on unmodified silica is around 7.6 ~tmol m - 2 o r 0.34 mg m - 2 [11]. However, it is not clear what ratio of vicinal to geminal silanol groups this value relates to, or even if the same ratio would apply to synthetic silica before or after surface modification.

2.2. Polymers

The polymers used in this work were diblock and triblock copolymers of polystyrene (PS) and either poly(ethylene/butylene) (PEB) or poly- (ethylene/propylene) (PEP) and were prepared by the catalytic hydrogenation of the polydiene blocks in PS-polybutadiene or PS-polyisoprene precursors respectively. The EB and EP blocks are therefore random sequences of either polyethylene (PE) and polybutylene or PE and polypropylene (PP) with a small degree of branching as shown in Scheme 1.

CH3 I

H H CH2 H I I I I

PEB is - - C C - - C - - C - - and I I I I

H H H H

CH3 I

H HC--CH2--CH 3 I t

- - C - - C ~ I I

H H

Table 1

Cha rac t e r i s t i c s o f the silica subs t r a t e s

Silica D i a m e t e r (nm) G e o m e t r i c sur face a r e a ( m 2 g 1)

P C S T E M

S1 57_+4.0 52.1_+11.9 58.4 $2 86.1 _+ 19.0 71.5 _+ 10.3 38.7

$3 228.1 _+ 58.5 13.8

A130 16.0 187.4

(a)

H H CH 3 H I I I [

PEP is - - C - - C ~ C - - - - ~ C ~ and I I I I

H H H H

(b)

CH3 I

H HC~CH 3 I I

~ C - - C ~ I I

H H

(c) (d)

Scheme 1. S t ruc tu re s of P E B a n d P E P .

162 S.M. King et al./Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 159-171

Structures (a) and (c) have been shown to be the principal constituent (typically > 90 tool%) in each polymer [12]. Although these copolymers are commercially available (as Shell Kraton TM elasto- reefs), those used in this work were prepared by Revertex Ltd. and were fractionated in order to give narrow molecular weight distributions.

For the purposes of comparison, a random copolymer of PE and PP (EMBV 8716, supplied by Exxon Chemicals), which we designate EP1, and two PS homopolymers, designated PS1 and PS2, were also studied. The latter were prepared at Bristol by anionic polymerisation [13]. The characteristics of the polymers are listed in Table 2. The molecular weights of PS1, PS2 and the styrene copolymers were determined by gel permeation chromatography (GPC) in trichloromethane at 40°C. The calibration was against PS standards. The residual unsaturation of the ethylene-butylene copolymers was determined by 1H F T - N M R in ortho-dichlorobenzene at 140 °C.

To obtain estimates for aa and aB we have used the cube roots of the molecular volumes of each monomer. This gives a a = 0 . 5 5 n m and aB = 0.45 nm.

2.3. Techniques

The surface-modified silicas were variously dis- persed to about 2% w/w in tetrachloromethane (carbon tetrachloride, AnalaR Grade, B.D.H. Ltd.), a good solvent for PS, cyclohexane (Aldrich Chemical Co. Ltd.), a slightly worse than theta solvent for PS at room temperature (the theta temperature is 34.5°C in C6H12 and 40.2°C in

C6D12 ) and hexane (G.P.R. Grade, B.D.H. Ltd.), which had been purified by standard laboratory techniques [14], a non-solvent for PS. All three solvents are good solvents for PEB and PEP. The amount of adsorbed polymer per unit area, F, was determined from the change in the bulk polymer concentration more than 48 h after addition of a polymer solution of known concentration to an aliquot of one of the silica dispersions. During this time the sample bottles were tumbled end-over- end. The change in concentration was determined by UV spectrophotometry with reference to a previously determined calibration graph. With the dry Aerosil silica a known mass was added directly to solutions of the polymers in the same three solvents. In the case of polymer EP1, the change in bulk concentration was determined by high- resolution 1H FT-NMR. In calculating adsorbed amounts we have, throughout this paper, used the geometric specific surface areas of the substrates given in Table 1 on the basis that there are no pores large enough to permit widespread polymer absorption. We believe this to be a more rational approach than using surface areas determined by nitrogen adsorption measurements.

The adsorption of PS2, SEB1, SEBS4 and SEBS5 onto Aerosil silica from cyclohexane was also investigated by small-angle neutron scattering (SANS) using the diffractometer L O Q at the ISIS Spallation Neutron Source, Didcot, U K [ 15]. One advantage of using a time-of-flight instrument like L O Q for these measurements is that all of its Q range (where Q = 4~z/2 sin(0/2) is the modulus of the scattering vector, 2 is the neutron wavelength and 0 is the scattering angle) is accessible in a

Table 2 Characteristics of the polymers

Polymer Block Composition Residual Mn Mpeak Mw/Mn sequence C-C

EP1 PE-r-PP 65% PE n/a 76400 2.35 PS 1 PS 100% PS n/a 11500 1.41 PS2 PS 100% PS n/a 28000 2.10 SEP1 PS-PEP 37% PS 142300 180000 1.12 SEB1 PS-PEB 15% PS 0.12% 19100 27500 1.08 SEBS4 PS-PEB-PS 30% PS 0.17% 20300 26300 1.15 SEBS5 PS-PEB-PS 50% PS 0.00% 20900 20600 1.14

S.M. King et al./Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 159-171 163

single measurement, thereby removing the need to overlap datasets covering different Q ranges. This is ideal for model-fitting. LOQ uses a "white" beam of neutrons with wavelengths between 2 and 10 ,~ to obtain its simultaneous Q range of 0.006-0.22 ,~- 1. (An additional high-angle detector bank is being installed on LOQ. This will signifi- cantly widen the available Q range.)

Cyclohexane was used for these measurements because the solvency of the anchor blocks could be altered by varying the temperature (also, chlo- rine has a high neutron absorption cross-section making measurements in tetrachloromethane difficult). However, in this paper we only report the results of measurements made below the theta temperature for PS. The scattering from the silica was contrast-matched to that of the dispersion medium using a mixture of 50.4% C6H12 and 49.6% C6D12 and any extraneous scattering from polymer in solution was minimised by adsorbing the polymers at an initial concentration corre- sponding to a point slightly below the start of the pseudoplateau region on the respective adsorption isotherms. The observed scattering (after subtrac- tion of a small background) therefore only origi- nated from the adsorbed polymer layer. In the low Q limit, where Qtr ~< 1 (a is the second moment about the mean, ~z2) 1/2 - ~z ) if z is the extent of the adsorbed layer, of the polymer density distribu- tion normal to the particle surface) this scattering has been shown to have the following form [16]:

6 x 10-227zRpl~bpm 2 I(Q)obs [cm- 1] = C Q2

x exp( - QZa2) + Bi,o (2)

where C is the calibration factor which puts the observed scattering on an absolute scale, Rp is the particle radius (A), ~bp is the volume fraction of particles, Binc is the incoherent background signal which is largely contributed by the hydrogen nuclei present and M is a parameter directly related to theamount of adsorbed polymer through

M D [-gcm -3 ] F [mg m -2] = (3)

l(6pol -- 6sol) [-cm-2] I

where D is the bulk density (of the polymer in this equation), 6po~ is the (coherent) neutron scattering

length density of the polymer and fisol is that of the dispersion medium (or silica, since the two are at contrast match), see Table 3. This expression is analogous to that used by Auroy et al. [17(a)] (These authors give FEmg m -z] = 0.17J-AID [g cm-3], where 7 is directly related to the area under the polymer density profile) in SANS studies of poly(dimethylsiloxane) grafted onto silica. This methodology ignores any contribution to the scat- tering from spatial density fluctuations, though the effect of these is expected to be minimal in the Q range of interest here [17(b,c)].

Eq. (2) was least-squares fitted to the observed scattering to obtain values for a and F by way of Eq. (3).

In Table 3, Y bi is the sum of the atomic neutron scattering lengths over all the atoms in the mole- cule or monomer unit [-18] and m is the corre- sponding formula weight. The scattering length densities of the copolymers were calculated as a weighted linear combination of the values for the constituent polymers. The weighting factors used were the ratios of the numbers of monomers.

3. Results and discussion

3.1. Adsorption on normal silica f r o m non-selective solvents

The adsorption isotherms for PS1, SEB1, SEBS4 and SEBS5 onto the high surface area Aerosil silica from tetrachloromethane at 25 °C are shown in Fig. 1. Solid lines have been added to aid the eye. At very low concentrations all of the polymers are seen to have a high affinity for the substrate, while at higher concentrations there is a very slow but continual increase in the adsorbed amount. This is in contrast to the behaviour of homo- polymers where a similar rapid increase at low concentrations is followed by a levelling off into a "pseudoplateau" region. Plateau adsorbances of around 0.8-1.0mgm -2 are typical in systems where the polymer molecules physically adsorb as single chains. Close inspection of the data shows that the adsorbed amount at a given equilibrium concentration increases as the PS content of the

164 S.M. King et al./Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 159 171

Table 3 Coherent neutron scattering length densities

Molecule 6 = (E b i • D" NA)/m Molecule 6 = (Y~ bi" D" NA)/m (cm ~} (cm 2)

Silica + 3.18 x 101° Polystyrene Cyclohexane -0 .28 x 101° SEBI Cyclohexane-dlz +6.70 x 10 l° SEBS4 Poly(ethylene/butylene) -0 .32 x 10 l° SEBS5

+ 1.42 x 10 a° +0.10 x 101° +0.60 x 101° + 1.27 x 101°

9.5-

E

1.0-

<(

~] 0.5-

0.0

o--

I 10100 20100 50100 4000

Equilibrium Polymer Concentration [ppm] 50~00

Fig. 1. Adsorption isotherms on Aerosil silica from tetra- chloromethane at 25°C: (11), SEBS5; (©), PS1; (I ') , SEBS4; (O), SEB1.

polymer increases. This behaviour is consistent with that reported in the literature for the adsorp- tion of both PS homopolymers and PS-polybuta- diene copolymers onto silica from cyclohexane at the theta temperature [19], The mechanism of adsorption in that work was attributed to a specific interaction between the surface silanol groups (Si6+-OH 6-) and the aromatic ring in the PS blocks.

The data show that the copolymer SEBS5 and the homopolymer PS1 have very similar adsorp- tion isotherms. If SEBS5 and PS1 were both PS homopolymers then we would expect the higher molecular weight SEBS5 to adsorb much more than PS1 [20]. SEBS5 is however a copolymer with almost exactly twice the molecular weight of PS1 but with a composition of only 50% PS. Whilst the PS and PEB blocks will have broadly similar energies of mixing in tetrachloromethane they will have rather different adsorption energies.

Experimentally though we found no evidence for the adsorption of EP1 onto hydroxy-silica from tetrachloromethane or from any of the other solvents we have studied and we assume that a random copolymer of ethylene-butylene would behave the same. The fact that EPI does not adsorb obviously supports the idea that adsorption is due to a specific interaction between the PS blocks and the silica surface.

Table 4 shows quite clearly that all of the copoly- mers satisfy the MJ anchor regime condition that fl,on < N~/2. This is true irrespective of whether the buoy block degrees of polymerisation for the two triblock copolymers are halved (the values marked with an asterisk) or not. The surface density of the polymers featured in Fig. 1 should therefore scale as NA 1. In calculating p, values of F at an equilib- rium concentration of 2500 ppm have been used. In addition we have halved the surface densities of the ABA copolymers SEBS4 and SEBS5 in an attempt to make a more valid comparison with the AB copolymer SEB1. This is because treating a triblock copolymer as a "double-diblock" gives rise to twice as lrmny molecules per unit surface area. Fig. 2 shows p plotted as a function of N 21 and although it is difficult to make any definitive conclusions, it is clear that the data do not contra- dict the scaling predictions as is shown by the linear least-squares fit.

We have also found that adsorption in the hydroxy-silica/tetrachloromethane systems is rev- ersible if the solution is replaced by pure solvent. This process was however rather slow and several solvent-exchange cycles were necessary before desorption ceased. Within the limits of experimen- tal error we are unable to say if all the polymer desorbs.

S.M. King et al./Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 159-171

Table 4 Scaling parameters for the polymers

165

Polymer Na N B VA firton N1/2 flsel p/(N~ 2/2s Nff 3o/25) p/(NA 12/23 NB 6/23 )

PS1 156 0 1.00 0 12.49 0 0 0 PS2 564 0 1.00 0 23.75 0 0 0 SEB 1 30 208 0.13 2.61 5.47 3.67 ( 2.437 ) 0.486 SEP1 566 1432 0.28 1.43 23.79 2.69 n/a n/a SEBS4 68 97* 0.41 1.01 8.25 1.54 0.373 0.349 SEBS5 114 71" 0.62 0.62 10.68 1.00 0.304 (0.637)

20-

~E c 15-

o x

>,10-

o ~ 5 -

L 1.5-

E E"

~ 1.0-

~ 0.5- <

5 1 no 1 [5 210 2J5 3i0 35 4no 1000 / G

Fig. 2. Surface density vs. NT~ 1 (after Ref. [1]): (O), copolymers (SEB1, SEBS4 & SEBS5); ( - - ) , least-squares fit.

3.2. Adsorption on normal silica from selective solvents

The adsorption isotherms for SEB1 and SEBS5 onto Aerosil silica from cyclohexane at 25°C are shown in Fig. 3. Comparison with Fig. 1 shows that over the concentration range depicted higher adsorbed amounts are obtained in cyclohexane, the poorer solvent for PS. These adsorbances can be compared with those obtained from SANS.

Fig. 4 shows the intensity of small-angle neutron scattering, I(Q), from a layer of SEBS5 adsorbed on Aerosil silica from cyclohexane at an initial polymer concentration of 1000 ppm. This is equiva- lent to an equilibrium concentration of ~ 670 ppm. Also shown to the same scale is the scattering from a 1.4% w/w dispersion of "bare" Aerosil in the same solvent mixture. The difference between these two scattering curves is the scattering from just

0.0- [

500 10nO0 15100 2000 Equilibrium Polymer Concentration [ppm]

Fig. 3. Adsorption isotherms on Aerosil silica from cyclohexane at 25°C: (©), SEBS5; ( i ) , SEB1.

A @

5

3.0-

2.5- g

2.0- "~ ,e

~.5- :~ .o

1.0- :o

0.5-

0.0-

-0.5- 0.0

-% oo ,%

"%,.. , . aooooooo * ° ' ° ° ' oo , . . ~@~g~o l l e l l t . e ~ • • e 8 • •

I I I I /

0.02 0 .04 0 .06 0 .08 O. 1

Scattering Vector, Q [Angstroms ']

Fig. 4. SANS intensity from SEBS5 and Aerosil silica in cyclohexane at 25°C at close to contrast match for the silica: (©), polymer and substrate; (O), substrate only.

the adsorbed layer of polymer and this is plotted in Fig. 5. At the neutron contrast (i.e. (•pol -- 0sol) 2) conditions used in this work both blocks are

166 S.M. King et al./Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 159-171

6

8 -

4 -

E

G" 2 -

1-

0 0.0

~ ° ° ° ° ° ° ° ° l . . . . . . . . . . r : : ~ = = ° fl ~ ~ ~ ° ~ q 0.02 0.04 0.06 0.08 0. !

Sc~tterlng Vector, Q [Angst roms- ' ]

Fig. 5. SANS intensity from a layer of SEBS5 physically adsorbed on Aerosil silica from cyclohexane at 25°C: (O), experimental data; (--), fit to Eq. (2).

contributing to the observed scattering but the buoy blocks are approximately four times more "visible" than the anchor blocks. Both Figs. 4 and 5 are representative of the data obtained from the other polymers and the same method of sample preparation was used for all four polymer samples studied by SANS. The results of least-squares fits of Eq.(2) to these data over the range 0.006 ~ Q ~< 0.015 ,~-~ are given in Table 5. Also shown for comparison are the adsorbed amounts measured by the bulk depletion method at an equilibrium concentration of 670 ppm, taken from

Fig. 3. The quantity x/ i2a is an approximation to the hydrodynamic thickness of an adsorbed poly- mer layer with a single block segment density distribution. It therefore provides an estimate of the thickness of a highly stretched layer.

Where comparisons can be made, it can be seen from Table 5 that the two methods of determining the adsorbed amount appear to give broadly sim- ilar results. (The rather high adsorbed amount determined for the homopolymer PS2 could be indicative of muir±layer adsorption. As a rule homopolymers adsorbing from worse than theta solvents do exhibit inflated adsorbed amounts.)

It is also clear that, as in the non-selective solvent case, there is once again a clear increase in the amount of adsorbed polymer as the PS content of the polymer increases. This is reasonable since it will be thermodynamically unfavourable for the PS blocks to stay in solution. The MJL theory sees adsorption in such a system as being a balance between the VdW forces trying to spread the collapsed layer of anchor blocks and osmotic repul- sion inside the solvated layer of buoy blocks (the "brush") opposing this.

Contrary to the non-selective solvent situation examined in Section 3.1, the copolymers are now predicted to belong to different adsorption regimes. From Table 4 it can be seen that in the case of SEB1 (and SEP1)/~sel > 1 and so these copolymers should obey the MJL scaling laws from the VdW- buoy regime. However, in the case of SEBS5/3se~ = 1 and so it should obey the scaling laws for the buoy regime. SEBS4 is clearly intermediate in character but it is not clear which regime it would necessarily prefer because although the theory pre- dicts distinct adsorption regimes, in reality there is likely to be some degree of overlap. All this serves to complicate the interpretation of our data from selective solvent environments.

We have used the values of FsAys in Table 5 to

Table 5 Comparison of adsorbed amounts determined by SANS and the bulk depletion method and comparison of adsorbed layer thicknesses determined by SANS with theoretical predictions for the thickness of the buoy layer, L, in two different regimes, the radius of a surface hemi-micelle, Lm~o,.e, and the radius of a spherical micelle in solution, Rmi~ell,

Polymer /"SANS /~expt ff ~/12cr L B Zvdw_ B Lmicell e Rmiceli e (mg m-2) (mg m 2) (nm) (nm) (nm) (nm) (nm) (nm)

SEP1 n/a 114 102 122 PS2 2.4 ± 0.2 7.3 ± 2.5 25.4 -+- 8.6 0 0 0 0 SEB1 0.7 ± 0.1 0.8 ± 0.2 13.7 ± 0.7 48.1 ± 2.5 (19) 33 21 23 SEBS4 0.9 ± 0.1 8.8 ± 1.8 30.8 ± 6.4 14 30 20 23 SEBS5 1.4 + 0.1 1.0 ___ 0.1 6.9 + 2.3 24.0 + 7.9 12 (21) 16 23

S.M. King et al./Colloids Surfaces A." Physicochem. Eng. Aspects 108 (1996) 159-171 167

test the MJL predictions for the scaling of p. Once again we have halved the surface densities of the ABA copolymers. The values of flsel in Table 4 show that for each regime at least one of the copolymers lies at the limit of the theoretically predicted range of validity. The final two columns in Table 4 show p divided by the appropriate scaling relation for F for each regime, and for clarity we have in each case bracketed the result for the polymer which should not belong to that regime. Clearly, if our data follow one of the scaling predictions then the numbers in one or other of the last two columns of Table 4 should be constant. In fact it can be seen that both scaling relations give approximately constant values, particularly if the bracketed ones are ignored. We are therefore unable to unambiguously assign adsorption in this system to any particular adsorption regime.

In a selective solvent like cyclohexane the picture of adsorption is complicated by the fact that the bulk solution is usually above the critical micelle concentration and is therefore micellar when it comes into contact with the adsorbent. Adsorption therefore either requires disentanglement of the micelles or the formation of micellar or multilayer surface phases [21]. MJL find that providing NA < N~ the equilibrium structure of the adsorbed layer is essentially independent of the nature of the copolymer in solution. With the exception of SEBS5, this condition is true of all the copolymers we have used.

The hydrodynamic radii of SEP1 molecules in tetrachloromethane and SEP1 micelles in hexane and octane at a concentration of 1000 ppm have been determined by PCS and pulsed field gradient (PFG) 1 H FT-NMR respectively [ 10]. The results are shown in Table 6 and it can be seen that the

Table 6 Hydrodynamic radii of SEPI molecules and micelles in different solvents at 25°C and a concentration of 1000 ppm. The data in brackets are taken from Ref. [22]

Solvent Rh (nm) Rh (nm) PCS PFG-NMR

Tetrachloromethane 5.6 9.9 Hexane 72.4 Octane (60 + 2) 65.0

two experimental techniques give broadly similar estimates.

Since cyclohexane at 25 °C is a marginally better solvent for PS than an alkane, the hydrodynamic radii of SEP1 micelles in cyclohexane can reason- ably be expected to lie between the values obtained in tetrachloromethane and hexane or octane, approximately 8 _+ 2 nm and 66 + 6 nm respec- tively. Using these data, and allowing for the difference in the numbers of monomers between SEP1 and the other copolymers, we expect the micellar radii of SEB1, SEBS4 and SEBS5 in cyclohexane to be in the range 2-13 nm and proba- bly skewed towards the upper limit. (In their paper MJL show that the radius of a micelle core scales as ~ N 3/5 and that the extent of the micelle corona

~T4/25 ~T3/5 This is the origin of Eq. (7). scales as ~ ~, A 1 • B "

The overall radius of a micelle should therefore scale as the sum of these two contributions. Using the data in Table 4 leads one to the prediction that SEP1 micelles are approximately five times larger than those of the other copolymers.)

The values of the second moment, ~ (which will principally reflect the extent of the non-adsorbing buoy blocks because of the contrast conditions) determined from the SANS data lie inside the range of micellar sizes predicted above. Three trends towards smaller layer thicknesses are evi- dent: one with increasing VA; one with decreasing NB; and another with increasing F. The first two trends are actually predicted by the MJL theory but only for constant overall and anchor block lengths respectively. The former condition is almost met by SEBS4 and SEBS5. In contrast, the observed trend with F is contrary to both the theoretical and experimentally observed behavi- ours of homopolymers.

The relationship between the experimentally measured layer thickness, the second moment, and the MJL scaling laws for d and L has been tested. As with the surface density data, the scaling laws for both the buoy and the VdW-buoy regimes were investigated. Despite the paucity of experimental points significantly better coefficients of regression were obtained against the scaling laws for L. This is a reasonable finding given the contrast condi- tions, though once again both regimes appear to explain the data.

168 S.M. King et al./Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 159-171

Table 5 also shows four sets of calculated layer thicknesses. These are L, the thickness of the brush which would be expected by the MJL theory if the anchor blocks spread to form a "film" on the surface, again for both the buoy and VdW-buoy regimes; Lmicene, the thickness of the adsorbed layer which would be expected if the copolymer adsorbs as surface hemi-micelles [21(a)]; and Rmicene , the total radius (i.e. core plus corona) of a spherical copolymer micelle in solution according to MJL. These were calculated according to the following expressions:

M4/25 ~J15/25 ~ L B ~ , A ~'B "B (4)

LVdW-B ~ NA4/23N21/23aB (5)

~1/5, ~r4/25 ~r15/25 _ (4aA/5)N1A5/25 (6) Zmicelle ~ " ~B ~ • A ~ • B

Rmicelle ~'~ t~B~' A ~ ~ h/a/251,B~,/15/25 4- aAN1A 5/25 (7)

In the calculation of LmiceUe it has been assumed that the number of chains in a surface hemi-micelle is half that present in a micelle in solution, a number, p, that MJL give as

p ~ 4nN 4/5 (8)

It has also been assumed that the contact angle of the (wetting) anchor blocks with the surface was n/2. This corresponds to the case of maximum stretching of the buoy blocks in the surface micelle [21(a)]. As in Table 4 parentheses have been used to highlight calculated values for polymers outside their expected adsorption regime.

It can be seen that all four sets of values are very comparable to the experimentally determined layer thicknesses of the copolymers, although Eq. (7) overestimates the radii of SEP1 micelles by about 85% compared to the PCS results and in the case of SEB1, SEBS4 and SEBS5 the theory predicts micellar radii which are larger than the upper limit of our expected range by a similar amount. However, given the approximations involved, to be within a factor of two is rather encouraging.

The calculated values most consistent with the experimental data are LH, Lvaw-a a~a Lmicelle- If we assume that Eqs.(4-6) do indeed overestimate L by the same amount as Eq. (7) overestimates Rmicelle, then remarkably good agreement between

a and LB is obtained. However, we are unable to conclusively differentiate between single chain and micellar adsorption from cyclohexane on the basis of such comparisons, although the former would seem to be the more likely.

Fig. 6 shows the adsorption isotherms for the much higher molecular weight diblock copolymer SEP1 adsorbing onto Aerosil silica from cyclohex- ane and hexane. There is seen to be a much greater degree of adsorption from cyclohexane. Taken alone this is not unreasonable. Micellisation should be much more extensive in hexane that it is in cyclohexane. Also, as the glass transition temper- ature for PS is about 90°C there is unlikely to be much disentangling of the micelle cores at room temperature. Furthermore the SEP1 micelles have more extensive coronas than those of the other copolymers. These screen the silanol-styrene inter- action more effectively and provide a good steric barrier to adsorption, though in actual fact the corona of a micelle in hexane will be slightly collapsed relative to a micelle of the same polymer in cyclohexane on account of the slightly poorer solvency of PEP in hexane (Z ~0.35_+ 0.02 for PEP in cyclohexane but Z ~ 0.43 + 0.03 in hexane).

For copolymers with the same VA, MJL predict that in the VdW-buoy regime p (and hence F/Mw) should decrease (as ~ N n 12/23 Nff 6/23) with increas- ing chain length. Using Table 2, and the data in Table 4, to correct for the difference in VA, it is found that this effect is indeed observed for copoly-

1.5-

E

Z ~ I.0- <E

o.5-

0.0

/

/

/ 7

10'00 20'00 30'00 40;0 50;0 Equilibrium Polymer Concentration [ppm]

Fig. 6. Adsorp t ion i so therms for SEP1 on Aerosi l sil ica at 25°C: (O), from cyclohexane; ( . ) , f rom hexane.

S.M. King et al./Colloids Surfaces A. Physicochem. Eng. Aspects 108 (1996) 159-171 169

mers SEB1 and SEP1. Moreover, this approach predicts that the adsorbed amount for SEP1 in cyclohexane should be approximately 1.4 times that for SEB1 in the same solvent. Comparison of Figs. 3 and 7 shows that a much greater difference is observed experimentally, approximately 3.3 times at an equilibrium concentration of 500 ppm. A possible explanation for this difference may come from comparing the adsorption of SEP1 in cyclo- hexane with that of PS2 (which is the same length as the SEP1 anchor block) in the same solvent using Table 5. As we have already seen, the value of /Wexpt for SEB1 is comparable to the value measured by SANS, yet, even so, FSANS for PS2 is significantly larger than that for any of the copoly- mers studied. It is possible therefore that the stretching energy of the EP buoy blocks in a SEP1 brush is significantly less than that of the EB buoy blocks in a SEB1 brush in the same solvent, something which is not altogether unreasonable given the stereochemistry.

3.3. Adsorption on surface-modified silicas from non-selective solvents

neo-pentyl silica Fig. 7 shows the adsorption isotherms for PS1,

SEB1 and SEBS5 onto neo-pemyl silica from tetra- chloromethane. For PS1 and SEBS5 the adsorp- tion isotherms depict very weak adsorption; at low concentrations the polymer shows very little affin-

ity for the surface ( F < 0 . 2 m g m -2) and there is negligible adsorption below some apparent "threshold" concentration. Above this threshold the adsorbed amount increases rapidly with increasing polymer concentration. The rate at which the adsorbed amount increases appears to be greater for the polymer with the higher PS content.

The adsorption isotherm for SEB1 on neo-pentyl silica more closely resembles its adsorption iso- therms on normal silica that have already been seen, but the pseudoplateau adsorbance is still only around 0.3 mg m -2. This is about 60% of the value at an equivalent concentration on normal silica. Clearly the n-pentyl SMG must be influencing the adsorption process.

Stearyl silica The adsorption of EP1, PS1 and SEP1 onto

stearyl silica from tetrachloromethane was also investigated. Within the limits of experimental error EP1 and SEP1 appear not to adsorb on this surface. Only rather weak adsorption of PS1 was detected above a threshold concentration and adsorbed amounts are lower than those for the cyclohexyl or neo-pentyl silicas, see Fig. 8. In the stearyl case the solid line, drawn to aid the eye, has been extrapolated towards another experimen- tal point (not shown) at a higher concentration. From these data we conclude that the mechanism

1.5-

E

Z ~ l . 0 -

g

o 0.5-

0.0 I i 500 t000 15~00 2000 25100 30100

Equilibrium Polymer Concentration [ppm]

Fig. 7. Adsorption isotherms on neo-pentyl silica from tetra- chloromethane at 25°C: (O), PSI; (11), SEBS5, (V), SEB1.

1.5-

E

Z ~ l .0-

g o.5-

0.0 t 0 500 10tO0 15100 20100 2500 30;0

Equilibrium Polymer Concentration [ppm]

Fig. 8. Adsorption isotherms for PS1 from tetrachloromethane at 25°C: (O), on neo-pentyl silica; (n) , on cyclohexy! silica; (V), on stearyl silica.

170 S.M. King et al./Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 159-171

of adsorption is still due to a PS-silica interaction despite the presence of the SMGs. Given that the degree of surface modification was only 34-70% for the cyclohexyl groups and about 56% for the neo-pentyl groups this is not unreasonable.

The picture that emerges is therefore the following. Adsorption of PS1 and the styrene copolymers onto the modified silicas is, as with adsorption onto normal silica, due to a PS-silica interaction but the strength of the interaction is regulated by the surface coverage and steric bulk of the SMGs. Thus, at low equilibrium concen- trations of polymer, much lower adsorbed amounts are observed on the neo-pentyl or cyclohexyl silicas than on normal silica. With increasing polymer concentration the SMGs become less effective at screening the PS-silica interaction. The very rapid increase in adsorbed amount exhibited by PS1 adsorbing on cyclohexyl silica from tetrachloro- methane, even though the theoretical maximum surface coverage of the cyclohexyl SMG is compa- rable to that for the neo-pentyl SMG, can be attributed to the fact that not only does the former SMG possess the least steric bulk (it is essentially two-dimensional) but the actual degree of surface modification is worse than that for the neo-pentyl silica. Consequently the polymer finds it easier to adsorb on the cyclohexyl surface at lower concen- trations. Conversely, the longer stearyl chains screen the surface more effectively and F is sup- pressed as a result.

4. Conclusions

In this paper, we have investigated the adsorp- tion of AB and ABA block copolymers of PS and either poly(ethylene/butylene) or poly(ethyl- ene/propylene) onto various types of silica surfaces from both non-selective and selective solvent envi- ronments for the PS blocks. Only the PS blocks adsorbed.

All of the PS-containing copolymers exhibit high affinity adsorption isotherms when adsorbed onto normal (hydroxy) silica. In general the adsorbed amount increases as the PS content of the polymer increases and as the solvent becomes progressively worse for PS. The surface density of adsorbed

polymer shows qualitative agreement with both the Marques-Joanny and Marques-Joanny- Leibler scaling predictions for diblock copolymer adsorption, despite the fact that we have treated triblock copolymers as "double-diblock" copoly- mers. In selective solvent systems the highest adsorbed amount did not occur in the poorest solvent. This is thought to be a consequence of extensive and largely irreversible micellisation.

We have compared the experimentally deter- mined adsorbed layer thicknesses obtained in a selective solvent system with the equivalent scaling predictions for a "brush-like" adsorbed layer, a layer of surface hemi-micelles and adsorbed spheri- cal micelles. The first two cases correlate best with the experimental data but we are unable to conclu- sively differentiate between them.

The adsorption isotherms obtained with modi- fied (alkylated) silicas in a non-selective solvent environment depict a different type of adsorption. We believe that this is due to the inability of the surface modifying groups to effectively screen the silica surface and prevent PS-silanol interactions.

Acknowledgements

The authors thank Professor Brian Vincent (Bristol) and Dr Iain More (Exxon) for their interest in this work and the Science and Engineering Research Council (now the Engineering and Physical Sciences Research Council) and ISIS for the provision of neutron beamtime. S.M.K. thanks Exxon Chemicals, Abingdon, UK for the award of a postgraduate studentship during part of this work.

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