Rheology of Hydrophobically Modified Polyacrylamide-co-poly(acrylic

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Rheology of Hydrophobically Modified Polyacrylamide-co-poly(acrylic acid) on Addition of Surfactant and Variation of Solution pH Yan Li and Jan C. T. Kwak* Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J3 Received December 10, 2003. In Final Form: March 31, 2004 Constant shear and shear dependent viscosity measurements are reported in aqueous systems of coand terpolymers of acrylamide (AM), N-n-alkylacrylamide (C10, C12, and C14 alkyl groups), and acrylic acid (AA) with added anionic surfactant sodium dodecyl sulfate (SDS). The results are presented as threedimensional plots of viscosity vs surfactant concentration and pH at constant shear rate or viscosity vs shear rate and surfactant concentration at constant pH. For terpolymers incorporating AA, a strong viscosity maximum is observed at intermediate pH values (pH 4-6) where the AA groups are partially ionized and at SDS concentrations close to the critical micelle concentration. At high pH, all AA incorporating terpolymer solutions with SDS are strongly shear thinning, but at pH 3-4 the systems of terpolymers with SDS are strongly shear thickening at low shear, followed by a shear-thinning region at high shear. These results are explained in terms of surfactant-mediated network formation with polymer coil expansion and hydrogen bonding between partially ionized AA groups as additional factors.

Introduction The unusual rheology behavior of hydrophobically modified (HM) water-soluble polymers has been the focus of considerable research.1-4 The solution properties of these polymers can be controlled and manipulated by polymer design. A small amount of hydrophobic group substitution in the polymer chain can lead to dramatic changes in the associative behavior in solution. Incorporation of ionic groups can result in improved solubility of the polymer and increased response of solution properties to ionic strength and pH.4-10 Due to flexibility in synthesis and molecular design, terpolymers based on polyacrylamide (pAM) and poly(acrylic acid) (or acrylate) (pAA) containing a small amount of hydrophobic groups are often employed in studies of structure-property relationships. When the terpolymers are dissolved in aqueous solution, the hydrophobic groups * To whom correspondence may be addressed. Tel: (902) 494 3425. Fax: (902) 494 1310. E-mail: [email protected]. (1) (a) Glass, J. E., Ed. Polymers in Aqueous Media: Performance through Association; Advances in Chemistry Series 223; American Chemical Society: Washington, DC, 1989. (b) Schulze, D. N., Glass, J. E., Eds. Polymers as Rheology Modifiers; ACS Symposium Series 462; American Chemical Society: Washington, DC, 1991. (2) Shalaby, S. W., McCormick, C. L., Buttler, G. B., Eds. WaterSoluble Polymers Synthesis, Solution properties and Applications; ACS Symosium Series 467; American Chemical Society: Washington, DC, 1991. (3) Goddard, E. O., Ananthapadamanabham, K. P., Eds. Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL, 1993. (4) Kwak, J. C. T. Ed. Polymer-Surfactant Systems; Surfactant Science Series 77; Marcel Dekker: New York, 1998. (5) McCormick, C. L.; Middleton, J. C.; Cummins, D. F. Macromolecules 1992, 25, 1201. (6) McCormick, C. L.; Middleton, J. C.; Grady, C. E. Polymer 1992, 33, 4184. (7) (a) Biggs, S.; Salb, J.; Candau, F. Langmuir 1992, 8, 838. (b) Biggs, S.; Salb, J.; Candau, F. Polymer 1993, 34, 580. (c) Biggs, S.; Hill, A.; Salb, J.; Candau, F. J. Phys. Chem. 1992, 96, 1505. (8) Branham, K. D.; Shafer, G. S.; Hoyle, C. E.; McCormick, C. L. Polymer 1994, 35, 4429. (9) (a) Valint, P. L., Jr.; Bock, J.; Schulz, D. N. (b) Bock, J.; Siano, D. B.; Valint, Jr.; Pace, S. J. (c) McCormick, C. L.; Johnson, C. B. (d) Lochhead, R. Y.; Davidson, J. A.; Thomas, G. M. In ref 1a. (10) Li, Y.; Kwak, J. C. T. Langmuir 2002, 18, 10049.

tend to favor intra- and interpolymer associations to minimize their exposure to water. Intrapolymer association may dominate at low polymer concentrations, whereas interpolymer association becomes prominent at high polymer concentrations.7,9 It is generally established that, above the polymer overlap concentration (C*), the dramatic viscosity enhancement stems from the associative nature of the hydrophobic groups forming an interpolymer crosslinked network in solution. Therefore an increase in the amount of hydrophobe substitution, or in hydrophobicity of the hydrophobe, leads to an increase in hydrophobic association and an elevated solution viscosity. At high degrees of hydrophobe incorporation, and depending on hydrophobe type, parent polymer solubility, and solvent conditions, the polymer may become insoluble. An increase in ionic (AA group) incorporation in the polymer is generally expected to hinder hydrophobic association, due to the increased overall hydrophilic character of the polymer resulting in a decrease in solution viscosity. However, because of repulsion between the ionic groups, the polymer coil will undergo an expansion in aqueous solution resulting in an increase in solution viscosity. These two opposite influences of the ionic groups often produce a complicated solution viscosity behavior.7,10 In the case of acrylic acid incorporation, at intermediate degrees of ionization a large viscosity increase is observed, attributed to interpolymer hydrogen bonding formation between the partially ionized carboxylic groups.10 In the presence of surfactant, interpolymer association is enhanced by the interaction between the hydrophobes and surfactant molecules in solution.7,11-16 A drastic (11) Effing, J. J.; McLennan, I. J.; Kwak, J. C. T. J. Phys. Chem. 1994, 98, 2499 (12) Effing, J. J.; McLennan, I. J.; van Os, N. M.; Kwak, J. C. T. J. Phys. Chem. 1994, 98, 12397. (13) Howley, C.; Marangoni, D. G.; Kwak, J. C. T. Colloid Polym. Sci. 1997, 275 (8), 760. (14) Wang, Y.; Lu, D.; Long, C.; Han, B.; Yan, H.; Kwak, J. C. T. Langmuir 1998, 14, 2050. (15) Chang, Y.; Lochhead, R. Y.; McCormick, C. L. Macromolecules 1994, 27, 2145. (16) Kopperud, H. M.; Hansen, F. K.; Nystrom, B. Macromol. Chem. Phys. 1998, 199, 2385.

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increase in solution viscosity at concentrations around the surfactant critical micelle concentration (cmc) is attributed to interpolymer cross-linking through the formation of mixed micelles involving the surfactant molecules and the hydrophobes from different polymer chains. Polymer-surfactant association is found to increase with increasing hydrophobicity of the hydrophobe and degree of the hydrophobic modification in the polymer.7,17,18 Notably, significant polymer-surfactant association still occurs even if the surfactant ion and the ionic group of the HM polymer are of the same charge,19-21 indicating that the hydrophobic effect is the driving force in this type of system. A subsequent decrease in viscosity with further increase of the surfactant concentration above the cmc is ascribed to the breakdown of the cross-linking network as sufficient surfactant is available to form micelles with each individual polymer hydrophobe. In recent studies on series of alkylacrylamide modified copolymer of acrylamide and acrylic acid, we have demonstrated the associative behavior of these terpolymers in the presence of various types of surfactants and at two different pH values,22 and a strong pH dependence of the terpolymer solution viscosity over a fairly narrow pH range in the absence of surfactant due to the formation of hydrogen bonding resulting from partially ionization of the carboxylic groups.10 In a continuation of these studies, we describe here a combined effect of interpolymer hydrophobic associations enhanced by the presence of surfactant SDS and ionization of the acrylic acid groups in the polymer. Yang et al. presented a literature review on the effects of polymer architecture on the macroscopic phase behavior of hydrogel-forming hydrophobically modified watersoluble polymers.23 The shear dependence of the viscosity of associative polymer systems has attracted a great deal of interest.7-9,16,18 For example, Candau et al., in a recent study on a series of dihexylacrylamide-modified polyacrylamides, indicated that a shear-thinning behavior is observed when the polymer concentration is much higher than C*, whereas a shear-thickening followed by a shearthinning profile is often observed in systems where the polymer concentration is higher than, but close to, C*.18 Furthermore, for the effect of added surfactant, these authors found that the shear thickening is not suppressed by the presence of SDS, but is even enhanced. The viscoelastic properties of the polymer solutions above the overlap concentration C* provide a measure of intra- and interpolymer association. There is a paucity of experimental data on terpolymers that contain both hydrophobic and ionic groups. The solution properties of such polymers can be adjusted very sensitively by control of the degree of substitution of both functional groups. In this report, the shear dependent behavior of a series of such terpolymers is presented as function of the degree of AA substitution and the chain length of the hydrophobe in the terpolymer, the concentration of surfactant (SDS), and the solution pH. The results are discussed in terms of the influence of shear (17) Magny, B.; Iliopoulos, I.; Zana, R.; Audebert, R. Langmuir 1994, 10, 3180. (18) (a) Jimenez-Regalado, E.; Selb, J.; Candau, F. Macromolecules 1999, 32, 8580. (b) Jimenez-Regalado, E.; Selb, J.; Candau, F. Langmuir 2000, 16, 8611. (19) Iliopoulos, I.; Wang, T. K.; Audebert, R. Langmuir 1991, 7, 617. (20) Philippova, O. E.; Hourdet, D.; Audebert, R.; Khokhlov, A. R. Macromolecules 1996, 29, 2833. (21) Zana, R. In ref 4. (22) Li, Y.; Kwak, J. C. T. Colloid Surf., A 2003, 225, 169. (23) Yang, Y.; Schulz, D.; Steiner, C. A. Langmuir 1999, 15, 4335.

Li and Kwak Chart 1

and terpolymer microstructure on the intra- and interpolymer interactions. Experimental Section Materials. Materials for synthesis of the terpolymers are described as follows. Triethylamine (BDH) was purified by refluxing in the presence of KOH for several hours and followed by distillation. Acryloyl chloride (Aldrich) was distilled under vacuum before use. N-n-Dodecylamine and N-n-tetradecylamine were obtained from Aldrich and were used as received. AM (Aldrich) was recrystallized twice from chloroform (CHCl3) and refrigerated prior to use. AA (Aldrich) was purified by vacuum distillation. The initiator azobis(isobutyronitrile) (AIBN) (Aldrich) was recrystallized twice from ethanol. Quinol used as inhibitor was purchased from BDH. SDS (Fluka, >99%) was recrystallized twice from ethanol. CHCl3 was run over an aluminum oxide (neutral, activity grade 1) column before used as solvent for recrystallization and polymerization. tert-Butyl alcohol (Aldrich) was distilled before use. Synthesis of Polymers. The hydrophobe monomers n-decylacrylamide (C10), n-dodecylacrylamide (C12), and n-tetradecylacrylamide (C14) were prepared following a method described by Effing et al.11,12 Acrylamide copolymer and terpolymer were prepared by free radical polymerization of AM, n-alkylacrylamides, and, in the case of the terpolymer, AA in tert-butyl alcohol. The mole fraction of hydrophobe was kept constant at 2 mol % of the AM monomer composition, while AA varies from 0 to 40 mol %, again relative to the AM content. The detailed synthesis procedure can be found elsewhere.11,12 Finally, the purity of the polymer sample was established by NMR spectroscopy. The molecular structure of the terpolymer is shown in Chart 1. Preparation of Sample Solutions. SDS (Fluka, 99%) was recrystallized twice from ethanol. Water was purified using a Milli-Q system (Millipore). For each of the polymers, two stock solutions, a polymer (2wt %) solution and a mixed surfactant/ polymer (2 wt %) solution containing concentrated SDS, were prepared. Prolonged stirring (2-3 days) and gentle heating were applied to ensure complete dissolution of the polymer. A series of sample solutions was made by mixing different amounts of the two stock solutions to obtain desired SDS concentrations between 0 and 30 mmol/L. The polymer concentration was kept at 2 wt % for all sample solutions, which is above the critical overlap concentrations of the polymers in this study.10,13,24 Sample solutions were then stirred at least for 24 h and left overnight to equilibrate before the viscosity measurements. For pH dependent viscosity experiments, the sample solution pH was increased gradually using very small aliquots of concentrated NaOH solution, minimizing the dilution effect. The solution was then allowed to reach equilibrium before the pH and viscosity measurements. For shear dependent viscosity experiments at pH 8-9, the solution pH was adjusted and fixed at the required pH. At natural pH, the solution pH values were all in the range of 3-4 without adjustment (except for the neutral polymer pAMC12-2% solution with natural pH of 7-8). All pH data were measured using a Fisher Accumet (620) pH Meter and were reproducible to (0.02 pH units. Rheology Measurement. Viscosity-shear data were obtained using a Haake Rotovisco RV 12 rheometer with a coaxial cylinder stainless steel sensor system (MV I St). For the pH-dependent behavior, the viscosity data at a constant shear rate of 9.36 s-1 were recorded. For the shear-dependent behavior, the viscosity measurements were carried out at shear rates ranging from 100 (24) Howley, C. M.Sc. Thesis, Dalhousie University, Halifax, Canada, 1996.

Rheology of Water-Soluble Polymers

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Table 1. Molecular Weights and Hydrophobic/ Hydrophilic Content of the Terpolymers

polymer

MW (g/mol) ×105

pAM-C12-2% pAM-C12-2%-AA-5% pAM-C12-2%-AA-10% pAM-C12-2%-AA-20% pAM-C12-2%-AA-40% pAM-C14-2%-AA-10% pAM-C14-2%-AA-20% pAM-C14-2%-AA-40%

1.37 1.34 1.02 2.01 2.18 1.05 1.37 2.0

hydrophobe content in polymer (% mol) rel to AM actual 2 2 2 2 2 2 2 2

1.96 1.87 1.79 1.64 1.41 1.79 1.64 1.41

AA content in polymer (% mol) rel to AM actual 0 5 10 20 40 10 20 40

0 4.67 8.93 16.39 28.17 8.93 16.39 28.17

to 103 s-1 for the series of C12 and C14 hydrophobe modified terpolymers containing different amounts of AA incorporation (0-40%). Temperature was kept at 25 ((0.2) °C during all experiments. All rheology data were taken after 5 min of equilibration under a certain shear rate to minimize a possible time dependence, although no significant time effect was detected for the polymer solutions used in this study. All viscosities reported are apparent viscosities.

Results and Discussions The hydrophobic/hydrophilic character of the co- and terpolymers in this study can be systematically adjusted by varying the alkyl chain length of the hydrophobe (n ) 10, 12, 14) and controlling the molar fractions of the acrylic acid group (0-40%) in the polymerization. In the viscosity experiments, the very weak response to SDS, pH, and shear rate changes observed for the low viscosity C10 hydrophobe modified series could not be determined with any accuracy due to the sensitivity limit of the viscometer. For those C14 hydrophobe modified polymers with 0-5% AA substitution, 2 wt % solutions of the polymer alone were already very viscous approaching the upper limit of the viscometer measuring range, and the viscosities were too high to be measured experimentally on addition of SDS or variation of solution pH. Therefore attention will be focused on the rheology data of the C12-modified series and some of the C14-modified polymers with a relatively high ratio of AA substitution. The molecular weights of these polymer samples are in the range of 1 × 105 to 2.2 × 105 g/mol, determined by dilute solution capillary viscometry. The calculated degrees of hydrophobe and AA mol % substitution are reported in Table 1. For these terpolymers, note the need to distinguish between the hydrophobe and AA content relative to the AM content (columns 3 and 5) and the hydrophobe and AA fractions in the total polymer (columns 4 and 6). Effects of pH and SDS. Figures 1 and 2 show the apparent viscosity (at 9.36 s-1) as a function of surfactant concentration and solution pH for 2% C12 and C14 hydrophobe modified terpolymers in 2 wt % solutions. Consistent with our previous studies,10,22 a sharp increase in solution viscosity at surfactant concentrations around its cmc is observed, attributed to enhanced interpolymer network formation involving mixed micelles of the surfactant and the hydrophobes from different polymer chains. The subsequent viscosity decrease at high surfactant concentration is the result of the breakdown of the cross-linking network as the excess of surfactant molecules in the solution form micelles with individual hydrophobes. At a fixed surfactant concentration, a remarkable viscosity increase can be seen at pH 4-6. At these intermediate pH values the partial ionization of the AA groups leads to strong interpolymer hydrogen bonding

Figure 1. Viscosity as a function of SDS concentration and solution pH at constant shear rate of 9.36 s-1 for a series of 2 mol % C12 modified terpolymers containing (a) 10%, (b) 20%, and (c) 40% AA, polymer concentration 2 wt %, and temperature 25 °C.

which in turn promotes interpolymer hydrophobe association resulting in a large increase in solution viscosity. The subsequent sharp fall in viscosity at higher pH is attributed to the combined effects of a decrease in hydrogen bonding and an increase in charge repulsion among polymer chains due to the high degree of AA ionization at this stage. There are some noteworthy features of the solution viscosity related to the role of added surfactant and solution pH. A viscosity maximum is observed in the pH range 4-6 combined with a surfactant concentration 5-10 mmol/L, for all of the polymers in Figures 1 and 2. For a given hydrophobe modification, the value of the viscosity maximum seems independent of the amount of the AA content (10-40%) in the terpolymer in these graphs. We

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Figure 2. Viscosity as a function of SDS concentration and solution pH at constant shear rate of 9.36 s-1 for a series of 2 mol % C14-modified terpolymers containing (a) 10%, (b) 20%, and (c) 40% AA, polymer concentration 2 wt %, and temperature 25 °C.

believe this observation is a result of two complementary interpolymer associative mechanisms both due to the AA groups. When the degree of AA incorporation in the polymer is low (Figures 1a and 2a), the interpolymer hydrophobic association induced by interpolymer hydrogen bonding between the partially ionized carboxylic groups with increasing pH is limited. In this circumstance, however, interpolymer cross-linking promoted by the formation of hydrophobe/surfactant mixed micelles is much more favored due to increased hydrophobicity of the terpolymer and the decreased ionic repulsion between the ionized AA group and the surfactant ion. When the AA content is increased (Figures 1c and 2c), interpolymer hydrophobe association induced by hydrogen bonding at the intermediate pH becomes dominant over that pro-

Li and Kwak

moted by surfactant binding. Even though the formation of hydrophobe/surfactant mixed micelles and hydrogen bonding are different in their origins, both lead to an enhanced interpolymer network resulting in the same final effectsan increased solution viscosity. The two effects appear to complement each other, and the viscosity maximum can be described as a sum of the two effects, leading to optimum interpolymer cross-linking. In these three-dimensional (3-D) graphs for terpolymers with low AA substitution (Figures 1a, 2a, and 2b), the high viscosity observed at high pH is another characteristic feature of these systems. This phenomenon reveals that the unfavorable factors at high pH including decreased hydrogen bonding and increased ionic repulsions among the carboxylic groups have little negative effect on the interpolymer network formation because for the C12- and especially the C14-substituted polymers the hydrophobic interaction is the dominant process. We believe that in this pH range the extended polymer confirmation due to the ionization of the AA groups makes only a minor contribution to the increase of the solution viscosity, as shown in Figures 1c and 2c at high pH for the 40% AA substituted polymers. Instead, the unfolding of the polymer chain exposing more hydrophobes for interpolymer association leads to the sustained high viscosity at high pH, as demonstrated by the high viscosity at high pH for the 10% AA substituted polymers in Figure 1a and 2a. In other words, the increased polymer chain entanglement probably only makes a minor contribution to the viscosity enhancement. It should be noted that the molecular weights of the C14 polymers may also affect the observed viscosities, however at C . C* with extensive network formation this effect may be considered minor. Interpolymer association due to the chain expansion exposing more hydrophobes may account for the major viscosity effect at high solution pH. This result also indicates that, even for a polyelectrolyte type polymer possessing the same sign of charge as the surfactant ion, enhanced network formation through surfactant mixed micelles not only is possible but even dominates over a wide pH range in the solution, provided the polymer is made hydrophobic enough by modification. In addition, there seems to be a synergistic viscosity effect of the addition of surfactant and the ionization of the polymer carboxylic groups. In Figure 1 we note that for polymers with added SDS at natural pH (pH approximately 3) and for polymers at varying pH but in the absence of SDS, the solution viscosity is low and relatively unaffected by SDS or pH variation. In contrast, only the combination of the two, i.e., varying surfactant concentration as well as solution pH, can have a very large effect. Interesting future challenges may include the quantitative treatment of these two mutually related effects and viable explanations for the synergistic mechanism. Effects of Shear Rate and SDS. The shear-dependent experiments were conducted by increasing the shear rate in double proportional steps at 5-min intervals. Figure 3 shows the viscosity shear dependence of the C12 hydrophobe modified parent copolymer, pAM-C12-2% with added SDS at the natural pH of a 2 wt % solution. Due to the absence of AA content in the polymer, pH values of the solutions are close to the neutral range of pH 7-8. The viscosity of the terpolymers containing 5-40% AA substitution at solution pH 3-4 (natural) and pH 8-9 (adjusted) as a function of both surfactant concentration and shear rate are shown in two series of 3-D plots in Figures 4 and 5, respectively. The effect of changing the size of hydrophobe at a given degree of AA substitution and natural pH is presented in Figure 6.


Figure 3. Viscosity as a function of SDS concentration and shear rate for the copolymer pAM-C12-2% at natural pH (7-8), 2 wt % solution, and temperature 25 °C.

A comparison of Figures 3 and 4 reveals a striking difference between the parent copolymer and the AA incorporated terpolymers, showing that even a small amount of AA incorporation can result in profound changes in polymer solution behavior. This interesting phenomenon may be related to the increased solvation of the primarily hydrophobic parent copolymer brought about by the incorporation of carboxylic groups. (The terpolymers dissolve much easier than the nonionic copolymers in water during the sample preparation.) The incorporation

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of AA groups in the polymer appears to improve the compatibility (quality) of water as solvent for the polymer, favoring polymer-solvent contact and, consequently, weakening both intra- and interpolymer associations. A pronounced change in intrinsic viscosity for a similar polymer, i.e., hydrolyzed HM polyacrylamide,9b was also found in dilute solutions where the improved polymersolvent contact resulted in a decrease of intrapolymer associations. Since in the data presented here the polymer concentration is above C*, interpolymer association is more affected by this change and likely to be disrupted by the improved polymer-solvent contact. These results show that the incorporation of AA groups in a HM polymer leads to drastic changes in solution properties of a magnitude comparable to that of hydrophobically modification, but in an opposite effect. These changes are also dependent on the polymer concentration relative to C*. The high viscosity of AA-incorporated terpolymers at high pH is attributed to ionization of the AA groups, expanding the polymer coil and favoring interpolymer association on the addition of surfactant. This phenomenon is clearly demonstrated by comparing the magnitudes of viscosities in Figures 4 and 5 at natural pH (3-4); even the highest viscosity values for 5% and 10% AA terpolymers are an order of magnitude lower that the corresponding values at higher pH when the ionic groups are fully ionized. In addition, the hydrophobic nature of the interpolymer interaction is also illustrated by the viscosity decrease as the AA content increases from 5% to 40% and

Figure 4. Viscosity as a function of SDS concentration and shear rate for a series of C12-modified terpolymers containing (a) 5%, (b) 10%, (c) 20%, and (c) 40% AA, natural pH (3-4), polymer solution 2 wt %, and temperature 25 °C. Note the small viscosity scale compared to Figure 1-3 and 5.

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Figure 5. Viscosity as a function of SDS concentration and shear rate for a series of C12-modified terpolymers containing (a) 5%, (b) 10%, (c) 20%, and (d) 40% AA, pH (8-9), polymer solution 2 wt %, and temperature 25 °C.

a substantial rise in viscosity as the hydrophobe chain length is increased from C10 to C14 (Figure 6). We now consider the details of the viscosity versus shear rate data in Figures 4-6. Before discussing the individual cases, we wish to point out that the overlap concentration C* for a water-soluble polymer is a variable parameter strongly dependent on a range of factors,7,9,16,18,19 such as the composition of the polymer (including the ratio of hydrophobic/hydrophilic contents, structures of both moieties, and molecular weight), solution pH and salinity, etc. Because interpolymer association is often induced and promoted by addition of surfactant, C* and chain overlap for a polymer/surfactant mixed system decrease at surfactant concentrations before and around the cmc. Therefore it is not surprising to find a wide variation in C* in a series of analogous polymer or mixed surfactant/polymer systems, which in turn strongly affects the solution rheological properties. In practice, C* or the apparent chain overlap concentration in the presence of surfactant is at lower solution concentration with increasing hydrophobe concentration and size of the polymer.7,25,26 This effect can also be recognized in the present data. A qualitative evaluation of the influence of C* on solution rheology is presented in the following discussion. In the absence of surfactant, associating polymer systems often exhibit a shear thinning viscosity response, which is attributed to the shear-induced disruption of the (25) McCormick, C. L.; Nonaka, T.; Johnson, C. B. Polymer 1988, 29, 731. (26) Flynn, C. E.; Goodwin, J. W. In ref 1b.

cross-linked network; i.e., the rate of network disruption exceeds the rate of network re-formation. This phenomenon is a characteristic feature for polymer systems at concentrations far above C* (i.e., C . C*), because interpolymer association is dominant and, therefore, more easily disrupted by shear. In the present study, this typical pseudoplastic behavior is observed for those polymers exhibiting pronounced interpolymer hydrophobe association in the presence of surfactant, including the case of the parent hydrophobic associative copolymer (Figure 3), terpolymers with low AA incorporations at high pH (parts a and b of Figure 5), and terpolymers even with relatively high level of AA content but containing the larger C14 hydrophobe (Figure 6c). The compositions of these polymers and the molecular conformations at high pH seem to suggest that critical overlap occurs at very low polymer concentration in these systems. The profile of the viscosity-shear rate curve and the magnitude of the viscosity decrease in these plots also imply that the concentrations in these systems are far above C*. This shear-thinning effect becomes pronounced at the SDS concentrations close to cmc in these 3-D plots, where the most effective interpolymer associating structures were observed in our previous viscosity studies.22 The interpolymer cross-links via hydrophobe/surfactant mixed micelles appear to be weak and can be broken under high shear, similar to polymer solutions in the absence of surfactant. Similar surfactant effects on shear behavior were observed in a recent study on associative copolymers.16 It is clear that the decrease in viscosity is the


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Figure 7. Viscosity as a function of shear rate for PAM-C122%-AA-5% at SDS concentrations (mM) indicated in the insert: (a) viscosity maximum shifts to lower shear rate below 2mM SDS; (b) viscosity maximum shifts to higher shear rates above 2 mM SDS. 2 wt % polymer solution, temperature 25 °C, and natural pH (3-4).

Figure 6. Viscosity as a function of SDS concentration and shear rate for a series of terpolymers modified with (a) C10, (b) C12, and (c) C14 hydrophobes at constant 20% AA incorporation, natural pH (3-4), polymer solution 2 wt %, and temperature 25 °C.

result of the gradual disruption of interpolymer hydrophobe association as the shear rate increases. Shear-Thickening Behavior. Figure 4 shows the strong shear-thickening behavior of AA incorporating terpolymers with added SDS at low shear rates. Some typical plots of the solution viscosity as a function of shear rate are presented in Figure 7 for a given terpolymer at different surfactant concentrations and in Figure 8 at a fixed surfactant concentration for a series of terpolymers with different amounts of AA incorporation. A remarkable feature of the rheology for these associating polymer solutions is the initial shear thickening effect in the low shear region followed by shear thinning at higher shear rates. This interesting effect was already reported for

Figure 8. Viscosity as a function of shear rate for C12-modified terpolymers with degrees of AA incorporation indicated at SDS concentration 1.96 mM. 2 wt % polymer solution, temperature 25 °C, natural pH (3-4).

systems of a copolymer (dihexylacrylamide modified polyacrylamide) with surfactant.18b Earlier we had also reported the shear-thickening behavior for the parent copolymers in the absence of surfactant.24 One of the explanations used for the shear-thickening regime in polymer solutions in the absence of surfactant involves a change in the relative contribution of intra- and interpolymer association when shear is applied to the system. As the polymer chains are extended at low shear, intrapolymer association decreases and the released hydrophobes are free to form interpolymer cross-links. As a result, this shear region favors interpolymer associations and a shear-thickening effect is observed. This phenomenon only occurs in the vicinity of C* (i.e., C g C*) where the intrapolymer associations are significant and, as a result, the relative amounts of intra- and interpolymer associations are sensitive to the applied shear. Further increase of shear rate then results in a subsequent shear-

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thinning effect that can be attributed to shear-induced disruption of interpolymer association as discussed earlier. This explanation seems to fit our observations for the terpolymer/surfactant mixed systems. At this point we wish to stress that shear can disrupt both intra- and interpolymer association, but with opposite effects. The disruption of interpolymer association results in a viscosity decrease as the cross-linked network breaks down, whereas the disruption of intrapolymer association may lead to a viscosity increase because the coiled polymer chain is stretched and more hydrophobes are released for possible interpolymer associations. The balance of these two viscosity effects results in a remarkable rheology profile: shear thickening followed by shear thinning. It can be seen in Figure 7 that the viscosity maximum first shifts to lower shear rates with increasing surfactant concentration when the surfactant concentration is below 2 mM (Figure 7a, the maximum viscosity reached is a function of AA content of the polymer composition, as will be discussed below) and then shifts to higher shear rates as the SDS concentration is increased further. We consider that shear thinning following the viscosity maximum signals the change of the dominant effect from intra- to interpolymer hydrophobe association. In general, with a stronger pre-existing interpolymer association, the viscosity maximum should be reached at lower shear rates. A similar rheological behavior was observed by Candau et al.18b in SDS/HM polyacrylamide mixed systems. On the basis of the data for the terminal relaxation time TR and plateau modulus G0, these authors also proposed that the viscosity enhancement is not due to the formation of additional mixed micelles but to an increase in the lifetime of the existing cross-links resulting from surfactant binding. In their systems, Candau et al. observed a Newtonian plateau at very low shear. This is not observed in any of the systems studied here, but this may due to the experimental limitations of our measurements at very low shear rates. The 3-D plots in Figure 4, and noting the low viscosity values and extended shear rate range, reveal interesting solution rheology responses to the AA segments in the terpolymer chains. As the AA content in the terpolymer is increased, the viscosity maximum is shifted to higher surfactant concentrations but the maximum viscosity is lower. The shift to higher surfactant concentration may be explained by the fact that the formation of an optimum number of cross-links requires more surfactant for the terpolymer with higher AA incorporation, which is a factor unfavorable to interpolymer hydrophobe association. The lower viscosity value at the maximum applies to all of the systems that we have studied10,22 and may be attributed to the competition between intermolecular Coulombic repulsion and hydrophobic association under the influence of the surfactant.

Li and Kwak

Conclusion The 3-D plots allow a more comprehensive examination of the terpolymer solution rheology as affected by a number of parameters including polymer composition (hydrophobe size and ionic group content), solution pH, and added surfactant. The viscosity experiments suggest that incorporation of AA in the parent copolymer results in improved solvation of the polymer in aqueous solution and leads to dramatic changes in the polymer rheology. At intermediate pH, optimum interpolymer cross-linking is achieved with the terpolymer as a result of hydrophobe association enhanced by both surfactant binding with the hydrophobes and hydrogen bonding among the partially ionized carboxylic groups. The two interpolymer associative mechanisms, both related to the AA content in the terpolymer, appear to complement each other. As the polymer coil is extended due to ionization of the AA groups, the increased polymer chain entanglement may be only a minor effect; in contrast, increased interpolymer association is probably the major effect responsible for the high tailing viscosity profiles at high pH and intermediate SDS concentration. Given the fact that the apparent overlap concentration C* for polymer/surfactant mixed systems is a variable parameter depending on a number of factors (including polymer composition and molecular weight, hydrophobe size, and solution pH), the shear rate-viscosity profile is strongly dependent on the polymer concentration range. Systems with polymer concentrations far above C* (i.e., C . C*) exhibit a monotonic shear thinning behavior because of the predominantly interpolymer association under these conditions. Systems in the vicinity of C* (C > C*), on the other hand, exhibit shear thickening followed by shear thinning due to the changing balance in the relative numbers of intra- and interpolymer association. In this case, the maximum viscosity is dependent not only on the added surfactant concentration but also on the degree of AA incorporation in the terpolymer. Finally, our data seem to support the conjecture that the interpolymer cross-linking network involving binding of surfactant with the polymer hydrophobe is relatively weak16,18,22 and imply that the rheology of the polymer solution and of surfactant/polymer mixed solution is controlled by the same hydrophobic association and shear disruption mechanisms of the polymer cross-linking network. Acknowledgment. This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). LA036331H

Rheology of Hydrophobically Modified Polyacrylamide-co-poly(acrylic

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