Water-Soluble Polymers - American Chemical Society

Chapter 22

Rheological Properties of Hydrophobically Modified Acrylamide-Based Polyelectrolytes

Downloaded by NORTH CAROLINA STATE UNIV on August 10, 2013 | http://pubs.acs.org Publication Date: July 18, 1991 | doi: 10.1021/bk-1991-0467.ch022

John C. Middleton, Dosha F. Cummins, and Charles L . McCormick Department of Polymer Science, The University of Southern Mississippi, Hattiesburg, M S 39406-0076

Associative polymers of acrylamide and n-decylacrylamide with sodium-3-acrylamido-3-methylbutanoate, sodium acrylate, or sodium-2-acrylamido-2-methylpropanesulfonate have been prepared by a micellar technique. Low shear rheometry was used to obtain plots of apparent viscosity as functions of ionic group type and mole percent incorporation into the backbone, polymer concentration and solution ionic strength. Results indicate that these polymers maintain high viscosity i n N a C l concentrations of up to 0.514 M by intermolecular association of the hydrophobic groups. The amount of aggregation is dependent on the type of ionic group incorporated as well as the distance of the charged group from the backbone. Maximum increases i n apparent viscosity are observed for the terpolymers containing carboxylate groups close to the polymer backbone. A conceptual model based on placement of both charged and hydrophobic groups along the macromolecular backbone is proposed consistent with rheological behavior. Water-soluble polymers containing non-polar groups which aggregate through hydrophobic interactions i n a polar medium were first discovered while studying the conformations of proteins (1). Such polymers contain both ionic or nonionic water-soluble groups and hydrophobic or amphiphilic groups. The unique solution behavior of these hydrophobically modified, water-soluble macromolecules has led to the synthesis and characterization of several synthetic analogs. Novel rheological properties have made them attractive for a variety of applications such as aqueous thickeners i n latex coatings (2,3), and mobility control agents i n enhanced oil recovery (4J>). Synthetic, hydrophobically modified polyelectrolytes may also serve as models of biopolymers for studying how structure and activity are related i n proteins and biomembranes (6). The term "hydrophobic bond" was originally developed to describe the grouping of non-polar side chains i n proteins and has been found to be an important factor i n stabilizing folded conformations (1). Although no bond actually exists the term "hydrophobic bond" or "hydrophobic interaction" has received wide acceptance i n the literature and several articles and books have 0097-6156/91/0467-0338$06.00/0 © 1991 American Chemical Society

In Water-Soluble Polymers; Shalaby, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


22.

M I D D L E T O N E T AL.

Acrylamide-Based

Polyelectrolytes

339

been devoted to the subject (7^). When non-polar groups are introduced into a polar medium such as water, the water molecules hydrogen bond around the hydrophobic group creating what have been called "icebergs" (9). These icebergs are quasi-crystalline structures i n which there is less randomness and slightly better hydrogen bonding than in ordinary liquid water at the same temperature. The ordering of water molecules around a hydrophobic group results i n an energetically unfavorable decrease i n entropy. If a sufficient number of non-polar groups are present, micellar structures will spontaneously form as the hydrophobic molecules are expelled from the solvent creating a more favorable entropie environment and an increase i n free energy. The addition of external electrolytes such as NaCI promotes hydrophobic associations by increasing the polarity of the medium. Studies in our laboratories have focused on developing macromolecules that can maintain or increase the viscosity of aqueous systems i n the presence of mono- or multivalent electrolytes (10-19). Recent work concentrated on the synthesis and characterization of copolymers of acrylamide with nalkylacrylamides with alkyl lengths of 8, 10, and 12 carbons. These polymers show unique solution behavior with the incorporation of less than 1 mol % of the n-alkylacrylamide group (20,21). Polymer association occurs intermolecularly through the pendent hydrophobic groups above a critical polymer concentration (C*). Above C* a rapid increase i n apparent viscosity is observed as the polymers form networks with the hydrophobic groups acting as transient crosslinks (Figures 1 and 2). While these macromolecules show increased viscosity i n the presence of small molecule electrolytes above a critical concentration, they are slow to dissolve from the dry state (22). In order to enhance dissolution and provide electrolyte character, a series of terpolymers containing acrylamide (AM), 0.5 mole % of N-n-decylacrylamide (C-10 AM) as the hydrophobic group, and sodium-3-acrylamido-3-methylbutanoate (NaAMB), sodium acrylate (NaA), or sodium-2-acrylamido-2-methylpropanesulfonate (NaAMPS) have been synthesized (23.24). Rheological properties were determined by low shear viscometry i n deionized water and sodium chloride solutions. Experimental Monomer Synthesis. Acrylamide and A M P S were obtained commercially from Aldrich Chemical Co. and purified by recrystallization from acetone. Acrylic acid was also obtained commercially from Aldrich and purified by vacuum distillation to remove inhibitor before use. N-n-decylacrylamide (20) and N a A M B (25) were synthesized by previously reported methods. The structures of all the monomers appear i n Figure 3. Polymer Synthesis. A series of terpolymers was prepared with monomer feeds of 0.5 mole percent of the N-n-decylacrylamide and 5, 10, 25, and 40 mole percent of each of the ionizable groups. The remaining polymer backbone was composed of acrylamide. The incorporation of water-soluble and water-insoluble monomers into a polymer backbone was accomplished using a micellar polymerization method (4). This technique utilizes a surfactant to solubilize the hydrophobic monomer. Sodium dodecyl sulfate was the surfactant i n this instance. A water-soluble initiator, potassium persulfate, was used to induce free-radical polymerization. Viscometry. The appropriate amount of dried polymer was weighed into a glass container and solvent added. The polymers were dissolved by gentle shaking


340

WATER-SOLUBLE P O L Y M E R S

16.0

OH α

12.0

Hydrophobically Modified PAM 0.75 mole % C - 1 0 AM


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β.ο

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4.0

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9 H

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AM

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3

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341

Polyelectrolytes

CH^-CH I

0=0

L • ONa

ι CH C-CH r

CH

3

2

C—O I• ONa NaAMB

CH3-C-CH3

CH

2

SOgNa*

NaAMPS

NaA

F i g u r e 3. Monomer structures used to prepare terpolymers.


342

WATER-SOLUBLE POLYMERS

on an orbital shaker for 14 days to allow complete hydration before further dilutions of these stock solutions were made. Viscosity experiments were conducted on the Contraves L S 30 low shear rheometer at a shear rate of 6 reciprocal seconds at 30 C. e


Results and Discussion Conceptual Model. Two opposing forces determine the solution behavior of hydrophobically modified polyelectrolytes i n aqueous solution- electrostatic repulsions and hydrophobic associations. The electrostatic interactions of the anionic groups along the backbone tend to increase the hydrodynamic volume and repel polymer segments from another. Hydrophobic moieties aggregate i n aqueous solution and may associate either intramolecularly i n dilute solution or intermolecularly at higher concentration (above C*). The combination of these factors along with other molecular parameters such as molecular weight, polymer microstructure, p H and solvent ionic strength result in a complex, but technologically important system. Solutions Studies. Three series of polymers were synthesized with differing ionic groups. One series contained sulfonate groups (NaAMPS) and the other two contained carboxylate groups (NaAMB and NaA). The effect of ionic group distance from the backbone was also evaluated by comparing the N a A M B and NaA. Ionic group content influence on viscosity was determined by varying the amount of charged group incorporation between 5 and 40 mol percent. Apparent viscosity (in centipoise) was plotted as a function of polymer concentration for each polymer i n six different solvent ionic strengths. The solvents were deionized water and sodium chloride solutions of 0.085 M , 0.17 M , 0.259 M , 0.342 M and 0.514 M . For these studies, the polymers were grouped according to polymer ionic content. The effects of the three different types of electrolyte groups were compared as functions of solvent ionic strength vs apparent viscosity. The polymers containing 10 and 25 mole per cent electrolyte are shown i n figures 4-9 as representative cases. In deionized water all of the polymers have high viscosities typical of polyelectrolytes i n aqueous media (Figures 4 and 5). However, when small molecule electrolyte is added (Figures 6-9) the solution apparent viscosity is greatly reduced below the overlap concentration as the intramolecular ionic repulsions are shielded reducing the hydrodynamic volume of the polymer coils. Above the overlap concentration, however, which varies depending on the polymer composition and solution ionic strength, significant associative behavior is observed only for the NaA and NaAMB polymers. Comparison of the NaAMPS, NaAMB and N a A Terpolymers. At low ionic strength and concentration, the electrostatic repulsive forces dominate the polymer solution behavior and all the polymers act as polyelectrolytes with similar viscosities. Ionic shielding by externally added NaCI both reduces the repulsive forces and increases the polarity of the medium. Therefore hydrophobic association is favoredas the non-polar τι-decyl groups are excluded from the polar environment resulting in network formation. The N a A M P S polymers show an almost linear increase in viscosity with polymer concentration independent of ionic strength. However, for the NaAMB and N a A terpolymers, apparent viscosity increases linearly with sample concentration only in deionized water (Figures 4 and 5). In brine solutions, the viscosity increases exponentially above C*, indicative of intermolecular hydrophobic association


22.

Acrylamide-Based

M I D D L E T O N E T AU

140.00

343

Polyelectrolytes

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AA

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80.00 -j

60.00

0)

(d PU Λ

40.00 H 20.00 -| 0.00 0.00

0.06

0.12

0.

Concentration (g/dL) F i g u r e 4. Apparent viscosity vs polymer concentration for the 10 mol polymers i n deionized water.

240.00

••••α 2555 ΑΜΒΑ

^220.00 H Δ Δ Δ Δ Δ 2 5 » ^200.00 A •^180.00 A -4-3

1

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£« 60.00 -I

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20.00 40.000.00 Η

0.06

0.12

0.18

Concentration (g/dL) F i g u r e 5. Apparent viscosity vs polymer concentration for the 25 mol polymers i n deionized water.



344

WATER-SOLUBLE POLYMERS

Concentration (g/dL) F i g u r e 6. Apparent viscosity vs polymer concentration for the 10 mol % polymers i n 0.085 M NaCI.

40.00 • α ο α ο 2 5 » AM ΒΑ

CL. Ο

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0.24

Concentration (g/dL) F i g u r e 7. Apparent viscosity vs polymer concentration for the 25 mol % polymers i n 0.085 M NaCI.


22.

Acrylamide-Based

MIDDLETON ET AL.

345

Polyelectrolytes

200.0 M 80.0

PL, ϋ

160.0

• • • • • 1 0 » AMBA

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AA

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I

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Concentration (g/dL)

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F i g u r e 9. Apparent viscosity vs polymer concentration for the 25 mol % polymers i n 0.342 M NaCI.



346


(Figures 6-9). The terpolymers containing N a A have lower overlap concentrations and higher viscosities than the other systems investigated. The terpolymers containing NaAMPS are the least affected by changing ionic strength because the sulfonate anion binds cations weakly and is not affected strongly by the added electrolyte. Polymer viscosity is actually lowered with increasing NaAMPS concentration i n solvents of high ionic strength. As sulfonate group incorporation is increased from 10 to 25 mol %, all aggregation is prevented. The weaker carboxylate acid analogs, N a A M B and NaA, do show aggregation i n brine solutions as charge is more effectively shielded through stronger site binding. Increasing the ionic group content from 10 to 25 mol % for the carboxylate polymers increases apparent viscosity. The presence of the carboxlyate groups and their counterions give the polymers added hydrodynamic volume through nearest neighbor steric and short-range electrostatic interactions without disrupting hydrophobic interactions. The N a A M B and NaAMPS mers are farther from the polymer backbone and may interfere with hydrophobic association more than N a A mers which do not protrude as far, accounting for the much higher viscosities of the N a A terpolymers. Additionally, the ^e/n-dimethyl groups of the N a A M B and N a A M P S may have sufficient hydrophobic character to intramolecularly stabilize the n-decyl groups through nearest neighbor interactions making them less thermodynamically driven to associate with other decyl groups (Figure 10). The individual, flexible polymer coils may be represented as spheres containing charged groups i n the interior as well as externally covering the surface (Figure 11). At low solution ionic strength the coils are expanded due to intramolecular electrostatic repulsions, but hydrophobic interactions are prevented by intermolecular repulsions. Introduction of NaCI ionic shielding lessens both intra- and intermolecular repulsions reducing the hydrodynamic volume of the individual coil. However, apparent viscosity increases due to hydrophobic aggregation in the absence of long range intermolecular repulsions. Conclusions Series of associative terpolymers with ionizable carboxyl and sulfonyl sites have been prepared and their solution properties evaluated. The N a A M P S terpolymers display typical polyelectrolyte behavior. The sulfonate groups are not well shielded by the Na counterions; the resulting ionic repulsions prevent hydrophobic aggregation. The NaAMB and N a A terpolymers, on the other hand, exhibit strong associative properties due to effective hydrophobic associations among the decyl groups. These polymers show increases in viscosity in brine solutions, apparently due to increased hydrophobic association and reduced intermolecular electrostatic repulsions. Distance of the ionic group from the backbone influences hydrophobic association. The N a A M B and N a A M P S monomers extend charged groups farther from the polymer backbone preventing associations among the hydrophobic decyl groups. This is not observed for the NaA monomer where the charge is much closer to the backbone. A hydrophobically modified, charged-sphere model is used to illustrate how the rheological behavior may be rationalized by the balancing the electrostatic repulsions and hydrophobic attractions in solutions of differing ionic strength.


22.

Acrylamide-Based Polyelectrolytes

MIDDLETON ET AL.

347

0=0 0=0 0=0 NH (CH ) 3

2

I NH

I c I CH


I NH

ONa

I Ç(CH)

3 2

2

I oo I

CTNa* i

CH

2

0=0

I 0"Na

+

F i g u r e 10. A schematic comparision of N a A M B and N a A relative to the C-10 group.

NaCI

F i g u r e 11. The shielding of electrostatic repulsions by added electrolyte results i n polymer coil contraction due to reduced intramolecular repulsions (i) and polymer aggregation due to reduced intermolecular repulsions (e).

American Chemical Society library 1155 16th St, N.W. Washington, O.C 20036 In Water-Soluble Polymers; Shalaby, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

348


Acknowledgments Financial support for this work was provided by the Department of Energy, the Office of Naval Research and the Defense Advanced Research Projects Agency and is gratefully acknowledged. Literature Cited


1.

Advances in Protein Chemistry; Kauzmann, W., Ed.; Academic Press: New York, 1959. 2. Landoll, L. M. J. Poly. Sci., Poly. Chem. Ed. 1982, 20, 443. 3. Glass, J. E . Advances in Chemistry Series No. 213; J. E . Glass. 4. Turner, S. R.; Siano, D. B.; Bock, J. (to Exxon Research and Engineering) U.S. Patent 4 520 182, 1985. 5. Evani, S. (to Dow Chemical Co.) U.S. Patent 4 432 881, 1984. 6. Miyamoto, S. Macromolecules 1984, 14, 1054. 7. Tanford, C. A. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley-Interscience: New York, 1973. 8. Ben-Naim, A. Hydrophobic Interactions; Plenum Press: New York, 1980. 9. Shinoda, K. J. Phys. Chem. 1977, 81(13), 1300. 10. McCormick, C. L.; Elliott, D. L.; Blackmon, Κ. P. Macromolecules 1986, 19, 1516. 11. McCormick, C. L.; Blackmon, Κ. P. J . Macromol. Sci. Chem. 1986, A25, 1451. 12. McCormick, C. L.; Elliott, D. L. J. Macromol. Sci. Chem. 1986, A23, 1469. 13. McCormick, C. L.; Blackmon, K. P. Polymer 1986, 27, 1971. 14. McCormick, C. L.; Elliott, D. L.; Blackmon, K. P. Polymer 1986, 27, 1976. 15. McCormick, C. L.; Blackmon, K. P. Angew. Makromol. Chem. 1986, 144, 73. 16. McCormick, C. L.; Elliott, D. L.; Blackmon, K. P. Angew. Makromol. Chem. 1986, 144, 87. 17. McCormick, C. L.; Elliott, D. L. Polym. Sci., Polym. Chem. Ed. 1986, A25, 1329. 18. McCormick, C. L.; Johnson, C. B. Macromolecules 1988, 21, 686. 19. McCormick, C. L.; Johnson, C. B. Macromolecules 1988, 21, 694. 20. McCormick, C. L.; Johnson, C. B. Polym. Mater. Sci. Eng. 1986, 55, 366. 21. McCormick, C. L.; Johnson, C. B.; Tanaka, T. Polymer 1988, 29, 731. 22. McCormick, C. L.; Johnson, C. B. In Polymers in Aqueous Media; Glass, J. E., Ed.; Advances in Chemistry Series No. 223; American Chemical Society: Washington, DC, 1989. 23. McCormick, C. L.; Middleton, J. C. Polym. Mater. Sci. Eng. 1986, 55, 700. 24. McCormick, C. L.; Middleton, J . C.; Cummins, D. F. Polym. Preprints 1989, 30(2), 348. 25. McCormick, C. L.; Blackmon, Κ. P. J. Polym. Sci., Polym. Chem. 1986, A24, 2635. RECEIVED June 4, 1990


Water-Soluble Polymers - American Chemical Society

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