Langmuir 2002, 18, 10049-10051
pH-Dependent Viscosity Enhancement in Aqueous Systems of Hydrophobically Modified Acrylamide and Acrylic Acid Copolymers Yan Li and Jan C. T. Kwak* Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J3 Received August 23, 2002. In Final Form: October 4, 2002
Introduction Solution properties of acrylamide-based co- and terpolymers have been extensively studied, based on their interesting characteristics that can be controlled through design of the polymer composition. Introduction of even small fractions of hydrophobic groups such as N-nalkylacrylamide can lead to dramatic changes in polymer solution rheology. Incorporation of ionic groups by copolymerization with charged monomers can produce increased solubility of the polymer and responses of the polymer solution to ionic strength and pH.1-4 The properties of aqueous solutions of such terpolymers, that is, polymers containing acrylamide, N-n-alkylacrylamide, and acrylic acid monomers, can be directly related to their microstructure in the aqueous environment. The hydrophobes tend to form intra- or interpolymer hydrophobic domains.5 It is expected that intrapolymer association will be dominant at low polymer concentrations, whereas interpolymer association becomes prominent at high polymer concentrations accompanied by an expected increase in the viscosity of the solution due to network formation. On the other hand, incorporation of ionic groups (such as acrylic acid) usually leads to a decrease in hydrophobic associative behavior due to the increased overall hydrophilic character of the polymer. However, because of the repulsion between ionic groups, the polymer chain will undergo an expansion in the solution. This conformation change will directly affect the solution properties. Depending on the polymer concentration, the combined effect of the two opposite influences by altering the amount of ionic group in the polymer often produces a complicated solution viscosity profile.3 Typically, at low polymer concentrations McCormick et al.6 reported an initial decrease in viscosity when base was added to an acidic terpolymer consisting of acrylic acid, methacrylamide, and a twin-tailed hydrophobic monomer, while upon addition of base leading to full ionization the viscosity increased again. However, such viscosity changes were relatively small. In their earlier studies on a series of hydrophobically modified maleic anhydride and ethyl vinyl * To whom correspondence should be addressed. Tel: (902) 494 3425. Fax: (902) 494 1310. E-mail:
[email protected]. (1) McCormick, C. L.; Middleton, J. C.; Cummins, D. F. Macromolecules 1992, 25, 1201. (2) McCormick, C. L.; Middleton, J. C.; Grady, C. E. Polymer 1992, 33, 4184. (3) Biggs, S.; Salb, J.; Candau, F. Polymer 1993, 34, 580. (4) Branham, K. D.; Shafer, G. S.; Hoyle, C. E.; McCormick, C. L. Polymer 1994, 35, 4429. (5) (a) Valint, P. L., Jr.; Bock, J.; Schulz, D. N. In Polymers in Aqueous Media: Performance through Association; Glass, J. E., Ed.; Advances in Chemistry Series 223; American Chemical Society: Washington, DC, 1989. (b) Bock, J.; Siano, D. B.; Valint, P. L., Jr.; Pace, S. J. In Polymers in Aqueous Media: Performance through Association; Glass, J. E., Ed.; Advances in Chemistry Series 223; American Chemical Society: Washington, DC, 1989. (6) Smith, G. L.; McCormick, C. L. Macromolecules 2001, 34, 918.
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ether based terpolymers at relatively higher polymer concentration, an initial decrease in solution viscosity was not observed, but the solution viscosity increased when the pH was raised from 4 to 9, attributed to Coulombic repulsion of the carboxylic groups on the polymer chain.7 The different behavior of hydrophobically modified acrylamide-co-acrylate polymers below and above the critical overlap concentration was also noted by Candau et al.3 In this communication, we report a remarkably large increase in viscosity at intermediate pH values for a series of hydrophobically modified (HM) acrylamide (AM) and acrylic acid (AA) terpolymers synthesized by keeping the hydrophobe substitution constant and varying the amount of hydrophilic AA group incorporation. Experimental Section Materials. Triethylamine (BDH) was purified by refluxing for several hours in the presence of KOH 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 an 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 use as a solvent for recrystallization and polymerization. tert-Butyl alcohol (Aldrich) was distilled before use. Water was purified using a Milli-Q system (Millipore). Synthesis of Hydrophobe Monomers and Polymers. The hydrophobe monomers n-dodecylacrylamide (C12) and n-tetradecylacrylamide (C14) were prepared following a method described by Effing et al.8,9 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.8,9 Finally, the purity of the polymer sample was established by NMR spectroscopy. The molecular weights of these polymer samples determined by dilute solution viscometry are in the range of 1.5 × 105 to 3 × 105 g/mol. The calculated degree of hydrophobe and AA mol % substitution based on AM ) 100 are presented in columns 2 and 4 of Table 1, while the mol % based on total polymer monomeric content is listed in columns 3 and 5. Viscosity Measurement. Viscosity-shear data were obtained using a Haake Rotovisco RV 12 rheometer with a coaxial cylinder sensor system. The viscosity measurements were carried out at a constant shear rate of 9.36 s-1. The temperature was kept at 25 ((0.2) °C. No time dependence of the viscosity of the polymer solutions was observed during the experiments. The polymer concentration was kept at 2 wt % for all systems, which is above the critical overlap concentrations of these samples.10,11 The solution pH was controlled by titration of small aliquots of concentrated NaOH (2 M). pH was measured using a Fisher Accumet (620) pH meter and was reproducible to (0.02 pH units. (7) McCormick, C. L.; Chang, Y. Macromolecules 1994, 27, 2151. (8) Effing, J. J.; McLennan, I. J.; Kwak, J. C. T. J. Phys. Chem. 1994, 98, 2499. (9) Effing, J. J.; McLennan, I. J.; van Os, N. M.; Kwak, J. C. T. J. Phys. Chem. 1994, 98, 12397. (10) Howley, C. M.Sc. Thesis, Dalhousie University, Halifax, Canada 1996. (11) Howley, C.; Marangoni, D. G.; Kwak, J. C. T. Colloid Polym. Sci. 1997, 275 (8), 760-768.
10.1021/la026463v CCC: $22.00 © 2002 American Chemical Society Published on Web 11/09/2002
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Langmuir, Vol. 18, No. 25, 2002
Notes
Table 1. Concentrations of Hydrophobic and Hydrophilic Contents in the Polymers and in 2 wt % Solution
polymer
hydrophobe content relative to AM in polymer (mol %)
actual hydrophobe content in polymer (mol %)
AA concn relative to AM in polymer (mol %)
actual AA concn in polymer (mol %)
actual AA concn in 2 wt % polymer solution (mM)
pAM-C10-2% pAM-C10-2%-AA-5% pAM-C10-2%-AA-10% pAM-C10-2%-AA-20% pAM-C10-2%-AA-40% 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% pAM-C14-2%-AA-5% pAM-C14-2%-AA-10% pAM-C14-2%-AA-20% pAM-C14-2%-AA-40%
2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
1.96 1.87 1.79 1.64 1.41 1.96 1.87 1.79 1.64 1.41 1.96 1.87 1.79 1.64 1.41
0 5 10 20 40 0 5 10 20 40 0 5 10 20 40
0 4.67 8.93 16.39 28.17 0 4.67 8.93 16.39 28.17 0 4.67 8.93 16.39 28.17
0.0 12.7 24.3 44.6 76.9 0.0 12.6 24.1 44.3 76.5 0.0 12.5 23.9 44.1 76.1
Figure 1. Viscosity as a function of solution pH for (4) pAMC12-2%-AA-20%, (2) pAM-C14-2%-AA-20%, and ([) pAMC14-2%-AA-40% at 2 wt % concentration.
Figure 2. Viscosity as a function of the degree of AA neutralization in 2 wt % polymer solutions of (4) pAM-C122%-AA-20%, (2) pAM-C14-2%-AA-20%, and ([) pAM-C142%-AA-40%.
Results and Discussion Figure 1 presents the solution viscosity as a function of the solution pH for pAM-C12-AA-20%, pAM-C14AA-20%, and pAM-C14-AA-40% at 2 wt % polymer solution concentration. In Figure 2, the viscosity is plotted as a function of the percent neutralization of the AA groups. The initial pH value for all polymer solutions is 3.5 ( 0.07. The polymer solution exhibits very large viscosity changes when the pH is varied over a fairly narrow range between 4 and 7. A sharp viscosity increase is observed when the pH is raised from 3.5 to 4.8. This viscosity
enhancement effect increases as the AA incorporation in the polymer is increased from 20 to 40 mol %. The large effect of the hydrophobe is seen from the difference between the C12- and C14-substituted polymers. Even for the 20 mol % AA incorporation and C12-substituted terpolymer, the viscosity of a 2 wt % solution still increases 100-fold between pH 3.5 and 4.8, but on the graph this major effect still seems small when compared to the even larger viscosity change for the C14-substituted terpolymer. McCormick et al., in a series of systematic investigations on HM copolymers containing AA, reported an initial decrease in viscosity as pH increases.6,12,13 This phenomenon was attributed to the formation of hydrogen bonding between adjacent COOH and COO- groups, causing a decrease in the unimer hydrodynamic volume. These studies were generally carried out at relatively low polymer concentrations, where intrapolymer interactions are prominent. In the present study, the polymer concentration is well above the possible critical overlap concentration,10,11 where we expect interpolymer association to be dominant leading to interpolymer chain entanglement. Thus, if there is a hydrogen bonding formation, it will be primarily interpolymer. When the pH is increased from 3.5 to 4.8, partial ionization of the AA groups (0-25% neutralization, see Figure 2) results in expansion of the polymer chain and the entangled polymer coil due to charge-charge repulsion, allowing for interpolymer hydrogen bonding between the ionized AA and nonionized AA groups. As a result, the interpolymer hydrophobic domains can be formed more effectively, leading to network formation and the observed large viscosity increase. In addition, the hydrogen bonds may stiffen the entangled polymer complex.14,15 This effect would also contribute to the viscosity increase in this pH range. The interpolymer interaction is still largely hydrophobic in nature through the association between hydrophobes from different polymer chains. Therefore, the interpolymer association is strongly dependent on hydrophobe size. In Figure 1, it is evident that the viscosity maximum increases significantly as the hydrophobe is changed from C12 to C14 while the AA incorporation is kept constant. Partial ionization of the terpolymer’s hydrophilic AA group, on the other hand, promotes the hydrophobic interactions by unfolding the polymer coil and allowing (12) Hu, Y.; Smith, G. L.; Richardson, M. F.; McCormick, C. L. Macromolecules 1997, 30, 3526. (13) Branham, K. D.; Shafer, G. S.; Hoyle, C. E.; McCormick, C. L. Macromolecules 1995, 28, 6175. (14) Branham, K. D.; Snowden, H. S.; McCormick, C. L. Macromolecules 1996, 29, 254. (15) Kulicke, W. M.; Horl, H. H. Colloid Polym. Sci. 1985, 263, 530.
Notes
formation of interpolymer hydrogen bonds. Thus, when the hydrophobe component is kept constant, an increase in AA incorporation in the terpolymer increases interpolymer hydrophobic associations. At pH 4.8, ionization of AA reaches a critical degree beyond which electrostatic repulsion between ionized AA dominates over the hydrophobic interactions of the hydrophobes between different polymer chains. As a result, the entangled polymer chains will disengage and network formation through the hydrophobic domain entanglement will decrease. In addition, as the degree of ionization increases, the fraction of protonated carboxyl groups needed for hydrogen bond formation decreases. Both factors then lead to the observed solution viscosity decrease when the pH increases above the viscosity maximum at pH 4.8. Finally, at high pH almost all of the AA groups are ionized (AA neutralization, 90-100%) and hydrogen bonding between the COOH and COO- groups is no longer a contributing factor. The charge repulsion between the now anionic polymers is sufficient to disentangle the polymer complexes and hinder interpolymer hydrophobe association. As a result, the solution viscosity levels off at a low value above pH 7, but due to the same charge repulsion effect, the polymer chains are in an extended conformation, leading to solution viscosities higher than those at the original acidic pH before any AA deprotonation, where the polymers are in a coiled conformation. McCormick et al.6 noted a viscosity decrease in solutions of terpolymers containing acrylic acid, methacrylamide, and a twin-tailed hydrophobic monomer above pH 10 and attributed this to an ionic strength effect. This is not the cause for the much stronger decrease observed in the present study above pH 4.8, as demonstrated by the addition of NaCl to the gel at pH 4.8, which even at an
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added NaCl concentration of 0.07 M lowers the viscosity by only 20%. Conclusion The viscosity behavior as a function of pH for hydrophobically modified terpolymers of acrylamide and acrylic acid is strongly dependent on the polymer solution concentration. In dilute polymer solution, hydrophobic associations are mainly intrapolymer, whereas at high polymer concentration, polymer chains form entangled complexes and interpolymer interactions are prominent. Previous reports in the literature for dilute solution conditions have demonstrated a decrease in viscosity of similar polymer systems when the pH is raised from acidic conditions. On the other hand, for the relatively concentrated polymer solutions studied in this work, when the degree of ionization of the AA fraction is raised by raising the pH of the solution, interpolymer interactions and network formation lead to a remarkably large viscosity increase due to a combination of hydrophobic interactions and hydrogen bonding. At pH 7, depending on the size of the hydrophobe, gel-like systems are obtained. Although solution pH clearly plays an important role in this very large viscosity enhancement, the hydrophobic effect is the driving force for the formation of hydrophobic domains cross-linking different polymer chains. In accordance with reports in the literature,3,6,7,13 we conclude that hydrogen bonding between the partially ionized carboxylic groups also plays an important role. At higher pH, due to a combination of charge repulsion and decreased hydrogen bonding, the viscosity falls sharply but remains higher than that under acidic conditions, due to expansion of the charged polymer chain. LA026463V
