Polymer International 41 (1996) 345-350

lnterpolymer Complexes of Poly(acry1amide) and Poly(N-

isopropylacrylamide) with Poly( acrylic acid) : a Comparative Study

Georges Staikos,* Georges Bokias? & Katerina Karayanni

Department of Chemical Engineering, University of Patras and Institute of Chemical Engineering and High Temperature Chemical Processes, ICE/HT-FORTH, PO Box 1414, GR-26500 Patras, Greece

(Received 24 May 1996; accepted 3 July 1996)

Abstract: The interpolymer complexation, through successive hydrogen bonding, between poly(acry1amide) (PAAm) and poly(N4sopropylacrylamide) (PNiPAAm) with poly(acry1ic acid) (PAA) in aqueous solution has been visco- metrically and potentiometrically investigated. The stoichiometry of the com- plexes formed was determined. By comparing the strength of the two complexes the very important contribution of the hydrophobic interaction in their forma- tion has been indicated.

Key words: interpolymer complexes, poly(acry1ic acid), poly(acrylamide), poly(N-isopropylacrylamide), hydrogen bonding, hydrophobic interaction.

I NTRO D U CTlO N During past decades, the interpolymer complexation between a polyacid (e.g. poly(acry1ic acid), PAA) and a

Poly(N-isopropylacrylamide) (PNiPAAm) is a water- PolYbase, i s . a Proton-accePtor Polymer (e.g. soluble polymer that has recently attracted great inter- PolY(ethYlene glYCOl), PEG Or PAAm), in water SOlU- est.1 What makes PNiPAAm especially interesting is its tion, has been extensively ~ t u d i e d . ~ - ' ~ This complex- inverse solubility upon heating, so that its aqueous solu- ation has been generally attributed to successive tions exhibit a lower critical solution temperature at 32- hydrogen bonding. Nevertheless, hydrophobic inter- 34"CV2 This behaviour is closely related to the action seems to make a major contribution to the for- hydrophobicity of PNiPAAm chains due to their iso- mation of these complexes. Such a hypothesis is, for propyl side groups. As a result, PNiPAAm forms associ- instance, supported by the fact that PolY(methacrYlic ation complexes with surfactants, such as sodium acid) (PMAA) forms stronger interpolymer complexes dodecylsulphate, through hydrophobic interaction^.^-^ than PAA with PEG.'4 This behaviour has been On the other hand, poly(acry1amide) (PAAm) is soluble explained by the hydrophobic interactions between the in water at all temperatures and scarcely any inter- methyl side groups of PMAA and the ethylene back- actions with surfactants are reported.6 This behaviour is bone of PEG. due to the fact that PAAm is an exceptionally hydro- In this work we have studied interpolymer complex- philic polymer. ation in the ternary systems PAAm/PAA/H,O and

PNiPAAm/PAA/H,O by viscometry and poten- * To whom all correspondence should be addressed. tiometry. We have followed a recently introduced t Present address : Laboratoire de Physico-Chimie Macro- approach,' according to which we can measure the molkculaire, Universitk Pierre et Marie Curie, CNRS URA intrinsic viscosity ([?I) of the polymer mixture, avoiding 278, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France the appearance of the polyelectrolyte effect. The dilution

Polymer International 0959-8103/96/$09.00 0 1996 SCI. Printed in Great Britain 345

346 G. Staikos, G. Bokias, K. Karayanni

solvent used is water adjusted to the same pH and ionic strength as the polymer mixture solution being mea- sured, following the isoionic dilution m e t h ~ d . ' ~ , ~ ~ Potentiometry leads to an apparent complexation con- stant (KJ, independent of the polymer mixture com- position, directly characterizing the strength of the interpolymer complexes formed. The results obtained show the important contribution of hydrophobic inter- actions to the formation of interpolymer complexes in aqueous solution.

EXPERIMENTAL

The PAA sample was a 25wt% aqueous solution obtained from Polysciences. It was diluted to a 10 wt% solution with 0.01 M HCl and then purified by dialysis in water and freeze-dried. Its relative molecular mass was found viscometrically to be 7.2 x lo4. The PAAm sample was synthesized in a 5% water solution of acryl- amide (Sigma) using hydrogen peroxide as initiator at 50"C.17 It was precipitated in methanol, dissolved in water and freeze-dried. Its molecular mass was found viscometrically to be 1.48 x lo5. The preparation of the PNiPAAm sample has been described el~ewhere.~ Its molecular mass was found viscometrically to be

The water used was reagent grade water from a Seral- pur Pro 90 C apparatus. The partially neutralized PAA sample was prepared by neutralization with 0.01 M NaOH solution.

The viscosity measurements were carried out at 25 0*02"C with an automated viscosity measuring system (Schott-Gerate AVS 300) equipped with an Ubbelohde type viscometer. Kinetic energy corrections were applied.

The potentiometric measurements were carried out at 25°C with a Metrohm 713pH meter equipped with a separate pH electrode suitable for measurements in ion- deficient solutions and a Ag/AgCl reference electrode with sleeve diaphragm and bridge electrolyte with outer filling, 0.01 M KC1.

2.8 x 105.

RESULTS AND DISCUSSION

Viscomerry

We performed intrinsic viscosity measurements for the polymer mixtures PAAm/PAA and PNiPAAm/PAA in water solutions at various compositions; the results obtained are presented in Fig. 1 as a function of the unit mole fraction of the polyacid, N p A A . The pH of the PAA solution used for the study of the ternary system PAAm/PAA/H,O was 3.00 as a result of the PAA con- centration chosen (5.0 x ~O- 'M). The same pH value was also obtained for the PAAm solution by using HCl.

1.00

L

n E' U

6 Y ._ -w

>\ -w .- 0.50

V cn > 0 cn C

.-

._

.- I c, c .-

0.00 I , 0.00 0.50 1

mole fraction,NpM Fig. 1. Variation of the intrinsic viscosity ratio, [ q ] , , versus the polyacrylic acid unit mole fraction, N,,, , for the ternary systems PAAm/PAA/H,O (0) and PNiPAAm/PAA/H,O

(0).

The solvent used for the dilution of each mixture was an HC1 solution of the same pH as that of each polymer mixture. Therefore, the pH of the dilution solvent was 3.00 for the solutions of the two pure constituents, but somewhat higher for the solutions of their mixtures, because of the polyacid-polybase interactions through hydrogen bonding. These pH variations are discussed in detail later in the Potentiometry section. As a result of this experimental procedure, the pure PAA solution and the solutions of PAAm/PAA mixtures exhibited a visco- metric behaviour in agreement with Huggins' law; by extrapolation this gives an intrinsic viscosity value,

without the behaviour characteristic of poly- electrolyte solutions.'*

In Fig. 1 the intrinsic viscosity ratio, [q], = [?]/[?]id,

for the two polymer mixtures is shown as a function of the unit mole fraction of PAA, N p A A = nPAA/nPA + nPB, where npAA is the unit moles of PAA and nPB the unit moles of PAAm or PNiPAAm. [?lid is the weight average of the intrinsic viscosities [?I1 and [q]' of the two pure components, according to the equation :

[?lid = w 1 C ~ 1 1 + w ~ C t l 1 2 (1)

where w1 and w2 are their weight fractions. In other words, [?]id expresses an ideal intrinsic viscosity value that would occur for each polymer mixture in the absence of any interaction between the two polymers. [q] is the experimentally determined value.

The viscosity measurements for the ternary system PNiPAAm/PAA/H,O were performed by using a PAA

POLYMER INTERNATIONAL VOL. 41, NO. 3, 1996

Interpolymer complexes: a comparative study

6.0 -

4.0 -

2.0 -

347

sample 5% neutralized with NaOH in a dilute aqueous solution (1.25 x M) exhibiting a pH value equal to 4.15. The PNiPAAm solution used was adjusted with HC1 to the same pH value. The ionic strength of both solutions was adjusted to 1.0 x 1 0 - 3 ~ with NaC1, so that it coincided with the ionic strength of the system studied above at pH 3.00. This pH region has been chosen to ensure clear PNiPAAm/PAA aqueous solu- tions and to avoid any phase separation, as happens at lower pH values."

From the results shown in Fig. 1 it is concluded that PAAm and PAA form an interpolymer complex having a compact structure ([q], < 1 is lower than unity for all compositions) and obeying a 1 : 1 unit mole stoichio- metry ([q], takes its minimum value at N p A A = 0.50). The other polymer pair, PNiPAAm/PAA, forms a much more compact interpolymer complex, even if the pH of the two pure components is considerably higher, 4.15 instead of 3.00. At higher pH the polyacid contains a considerable number of carboxylate groups, COO -, that do not favour complex formation. Nevertheless, the structure of the complex formed is much more compact, implying that PNiPAAm forms much stronger com- plexes with PAA than PAAm does. Moreover, from the mixture composition, where [qIr takes its minimum value, the stoichiometry of the complex formed seems to vary in the range of 1.5 to 2 PNiPAAm monomer units for each PAA monomer unit.

The observed differences in the strength and the stoi- chiometry of the complexes formed could be attributed to the influence of the isopropyl side groups of PNiPAAm. Their presence on the polymer chain seems to favour hydrophobic interactions, which make a major contribution to complex formation.

Potentiometry

This method has been widely used for the study of inter- polymer complexes formed through hydrogen bonding.' 3*20-23 The interpolymer complexation between a polyacid, such as PAA, and a polybase (PB), such as PAAm or PNiPAAm, takes place through a complexation equilibrium :

PAA + P B e C (2) characterized by an apparent complexation constant, K , , given by the equation:

rc1 K c = [PAA][PB] (3)

where [C] is the concentration of the complex formed, and [PAA] and [PB] are the concentrations of the uncomplexed polyacrylic acid and polybase, respec- tively. This complexation proceeds through successive hydrogen bonding interactions, following the relation :

COOH + B COOH- *B (4)

and induces a displacement of the PAA dissociation equilibrium :

COOH e COO- + H +

to the undissociated form, resulting in an increase of the pH of the PAA/PB mixture compared with the pH of the pure PAA and PB solutions.

In Fig. 2a the pH variation of the system PAAm/ PAA as a function of N p A A is shown. The pure solutions of PAAm and PAA are the same as those used for vis- cometry. We observe that the pH of the mixtures is clearly higher than that of the two pure components. It shows that a hydrogen-bonding interaction, according to relation (4) has taken place.

To calculate the apparent complexation constant, K , , given by eqn (3) we have to determine the complex con- centration, [C], and the uncomplexed concentrations of the polyacid and of the polybase, [PAA] and [PB], respectively. For this reason we use the apparent disso- ciation constant of the polyacid, corresponding to the equilibrium (5), given by the equation :

( 5 )

K , = [COO-][H+]/[COOH]

It is known that K , depends on the degree of ionization of the p ~ l y a c i d , ~ ~ but for a relatively narrow pH range

30 A C 7

A

I W

P)

0

X 7

ij U

b

0.0 - a

3.05 -

3.00 0.00 0.50 1.

m o I e fraction, NpM Fig. 2. Potentiometric results for the ternary system PAAm/PAA/H,O. The unit mole concentration of the pure

polymer solutions is 5.0 x lo-' M.

POLYMER INTERNATIONAL VOL. 41, NO. 3, 1996

348

c A 15 'L

E" i a 1

I

W rc)

0 5 7

5 y o

3.0

A

3 W

"o 2.0

G 7

X

U

1 .o

4.30

$ 4.20

4.10 C

C T

b

t a

1 1 0.50 1.1

mole fraction,NpM Fig. 3. Potentiometric results for the ternary system PNiPAAm/PAA/H,O. The unit mole concentration of the

pure polymer solutions is 1-00 x 1 0 - ~ M.

it can be obtained from the Kern empirical equation:25

pKd = -log,, Kd = c1 -k bpH (7) Taking into account the electroneutrality of the solu- tions, given by the equation:

[COO-] = [H'] - [Cl-] (8) where [H'] is measured by potentiometry and [Cl-] is calculated by the contribution of the polybase solution of which the pH has been adjusted with HCl, we can calculate the concentration of carboxylates, [COO -1, due to the dissociation of the uncomplexed PAA. We can then calculate from eqn (6) the concentration of carb- oxylic acid units, [COOH], and hence the complex concentration:

[C] = [PAA], - [COOH] - [COO-] (9) where [PAA], is the initial polyacid concentration in each polymer mixture, given by C O N p A A , where C, is the concentration of the pure polyacid solution and N p A A is the unit mole fraction of PAA in each polymer mixture, [COOH] is the uncomplexed carboxylic acid concentration and [COO -1 is the uncomplexable car- boxylate concentration. In Fig. 2b, [C] is shown as a function of N p A A for the PAAm/PAA mixture. It takes its maximum value at NPAA = 0.50, indicating a 1 : 1

G. Staikos, G. Bokias, K. Karayanni

n 7 I

1.5 1

4.20

I a

4.10 / I I 1

0.00 0.50 1 mole fraction,NpM

Fig. 4. Potentiometric results for the ternary system PNiPAAm/PAA/H,O. The unit mole concentration of the pure polymer solutions is the same as that of Fig. 3, but the reactive unit for the PNiPAAm has been taken as equal to 1.5

monomer units.

stoichiometry for the complex formed. To calculate the apparent complexation constant, K,, from eqn (3) we need [PAA] and [PB]. It is easily concluded that:

[PAA] = [COOH] + [COO-]

CPBl = CPBI, - CCl

(10)

(11) where [PB], is the initial polybase concentration in each polymer mixture, given by C,(1 - NpAA), where C, is the concentration of the pure polybase solution, the same as that of the polyacid.

In Fig. 2c, K , is shown as a function of N p A A . We observe that K, remains constant over a broad com- position range, indicating that the procedure followed and the stoichiometry proposed describe adequately the formation of the PAAm/PAA interpolymer complex. Its relatively low value, K,=(15 f 5)lmol-', indicates that the complex formed is rather weak.

In Figs 3-5 the potentiometric results for the PNiPAAm/PAA/H,O ternary system at three different unit mole polybase/polyacid ratios are shown. We have used a very dilute PAA solution with a unit mole con- centration of 1.00 x 1 0 - 3 ~ and pH value of 4.15. The

and for a 1 : 1 stoichiometry:

POLYMER INTERNATIONAL VOL. 41, NO. 3, 1996

Interpolymer complexes : a comparative study 349

n r 200 1-

; _1

I

v r) 100 0 7

?I

y o

- 4.0 3 W

d. 0 x 3.0 F

ij U

2.0

4.40

4.30

4.20

4.10

I a

0

T x C

b

a

I 0.50 1.00 m o I e f ra c t i o n , N pM

Fig. 5. Potentiometric results for the ternary system PNiPAAm/PAA/H,O. The unit mole concentration of the pure polymer solutions is the same as that of Fig. 3 , but the reactive unit for the PNiPAAm has been taken as equal to

two monomer units.

pH of the PNiPAAm solutions has also been adjusted to 4-15 with HC1. The ionic strength of all pure polymer solutions has been set at 1-0 x 1 0 - 3 ~ with NaC1. Working under the above conditions we have obtained clear solutions for the polymer mixtures, without any complex precipitation. The pH values are in a region (4-15-4.40) where water dissociation need not be taken into account in the calculation, and the degree of com- plexation leaves considerable amounts of uncomplexed polymers, so that their concentration determination can be made within the limits of acceptable relative error.

The results shown in Fig. 3 indicate that a maximum value for the complex concentration is obtained at N p A A = 0.40, suggesting that a stoichiometry ratio of 1-5 PNiPAAm for each PAA in monomer units would probably be more suitable. Moreover, the K , value for the complex formed seems to decrease as N p A A

increases. We repeated our measurements using a PNiPAAm solution with a concentration adjusted according to the hypothesis that the reactive unit is 1.5 times its monomer unit. The results obtained are shown in Fig. 4. The maximum of the complex concentration, [C], moved to N p A A = 0.50 and the K , values obtained

at different compositions fluctuate around a mean value of (20 f 6) x lo3 1 mol- I , i.e. a value a thousand times higher than that determined for the PAAm/PAA complex. In Fig. 5 the potentiometric results are given for the same system, but using a PNiPAAm solution with a concentration adjusted according to a hypothesis that the reactive unit is twice its monomer unit. In Fig. 5b [C] maximum is displaced to more than N p A A = 0.50 while K , (Fig. 5c) seems to increase rapidly with NPAA.

The above potentiometric investigation leads to a reliable interpolymer complex composition. We con- sider that the presence of the hydrophobic isopropyl group in PNiPAAm dramatically increases the strength of the interpolymer complex formed, relative to the PAAm/PAA complex, while simultaneously influencing the stoichiometry of the complex. As a result, even if these interpolymer complexes are hydrogen bonded it is obvious that they are also hydrophobically stabilized to such an extent that hydrophobicity may be said to be the dominant factor in the formation of the complexes.

CONCLUSIONS

In this study it has been shown that PAA forms a rather weak interpolymer complex with PAAm and a very strong one with PNiPAAm. This interaction has been indicated by viscometry and quantitatively expressed by potentiometry. The interpolymer complexation stoichio- metry has been reliably determined from the polymer mixture composition, where the concentration of the complex formed takes its maximum value. The results obtained 1/1 for the PAAm/PAA interpolymer complex and 1.5/1 for the PNiPAAm/PAA in polybase/polyacid monomer units, are in agreement with the viscometric evidence. The constant of the complexation equilibrium, K,, constitutes a direct measure of the strength of the complexes. From the results obtained we conclude that the presence of the isopropyl side groups in PNiPAAm contributes to a major stabilization of the complex formed with PAA through hydrogen bonding, because of an important hydrophobic interaction. The difference in the stoichiometry of this complex in comparison with that formed between PAAm and PAA also seems to be attributable to this effect.

ACKNOWLEDGEMENT

This work has been partially supported by European Union through an HCM Network under Contract No. CHRX-CT94-0655.

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350 G. Staikos, G. Bokias, K . Karayanni

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POLYMER INTERNATIONAL VOL. 41, NO. 3, 1996