Polymers in Aqueous Media - American Chemical Society

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Poly(acrylic acid) Thickeners The Importance of Gel Microrheology and Evaluation of Hydrophobically Modified Derivatives as Emulsifiers Robert Y. Lochhead, John A. Davidson, and G. M. Thomas BFGoodrich Company, Avon Lake Technical Center, Avon Lake, OH 44012

The gel microstructure of poly(acrylic acid) thickeners is a key variable that defines the rheology conferred to and end-use applications of these products. The microstructure can be probed by using a quasi-elastic light-scattering technique to measure the Stokes-Einstein microdiffusion coefficient of colloidal gold sol particles immersed within the thickened system. The microstructures of two such thickeners have been determined. The structure of swollen microgels packed in intimate contact is postulated for an efficient thickener that confers pseudoplastic rheology. A less efficient thickener that confers viscoelastic rheology on aqueous systems is shown to be a true solution of polymer molecules. The structure of this compound is important in end-use applications such as the prevention of wicking in the printing of textiles. This chapter also discusses hydrophobic modification of poly(acrylic acid) thickeners to yield products that are useful as primary emulsifiers for oil-in-water systems. These emulsions are stable for years, but they break and coalesce almost instantly when electrolyte is introduced into the aqueous phase.

HlGH-MOLECULAR-WEIGHT POLY(ACRYLIC ACID) THICKENERS have been used successfully in many aqueous systems for more than a quarter of a century. The general structure of these thickeners is shown in structure 1. When dissolved in water in their native form, these polymer molecules adopt 0065~2393/89/0223-0113$09.75/0 © 1989 American Chemical Society

Glass; Polymers in Aqueous Media Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

POLYMERS IN A Q U E O U S M E D I A

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Structure 1. Poly(acrylic acid).

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the relatively relaxed configuration shown in Figure 1. Upon neutralization with a suitable base, the carboxylate groups ionize, and mutual ionic re pulsion between these groups causes the molecule to adopt a greatly ex panded configuration (Figure 2) (1,2). In principle, however, mutual electrostatic repulsion between neigh boring carboxylate groups only partially explains the swelling of the polyelectrolyte molecule, because this repulsion is screened by other ions in the system, including hydrated protons and hydroxyl ions from dissociated water and the polyelectrolytes own counterions. Each polyelectrolyte molecule can be regarded as a microscopic ionic network (3). The counterions are exchanged between the swollen ionic network and its surrounding external solution. Diffusion of the mobile counterions away from the immediate vi cinity of the polyelectrolyte molecule leads to a net negative charge within the polymer molecule, which increases the electrical potential within the molecule relative to its surroundings. Such conditions favor a higher con centration of counterions within the domain of the polymer molecule than in the external solution. In reality, a state of equilibrium is reached where the ionic attraction of the counterions by the polyanion is just balanced by diffusion into the external solution, which is driven by the chemical potential gradient, which in turn arises from the difference in counterion concentrations between the two domains. The overall effect closely resembles Donnan membrane equi libria (4) (Figure 3). As a consequence of the difference in counterion con centrations, the osmotic pressure inside the polymer domain exceeds that of the external solution, and the expansion of the polyelectrolyte can be equated to the difference in osmotic pressures of the intramolecular and intermolecular solutions. The quantitative treatment of the force of expansion can be carried out by assuming either pure ionic repulsion or that osmotic pressure is entirely responsible for the expansion. These two models yield identical results (3) because they are mutually related. No net charge would develop if the counterions were immobile, and no excess of mobile ions would be present to generate an osmotic pressure difference if they did not carry a charge of opposite sign to that of the polyion. Addition of a microion salt, such as sodium chloride, to the solution causes a decrease in the extent of polymer swelling. This effect can be



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Poly(acrylic acid) Thickeners

115

Figure 1. Schematic diagram of poly (acrylic acid) in relaxed configuration.

explained as arising from a reduction in the osmotic force due to a lessening of the difference in counterion concentration between the domains (3). A l ternatively, collapse of the polymer could be ascribed to collapse of its ionic double layer upon addition of excess salt (5). Typical thickening curves for poly(acrylic acid) thickeners are shown in Figure 4. Neutralization to p H 5 causes polyion expansion. Measured vis cosity reaches a plateau between p H 5 and 10. Above p H 10 the effective increase in counterion concentration causes a successive reduction in the swelling of the polyelectrolyte molecules. Figure 4 shows four different types of commercial poly(acrylic acid) thickeners. These products differ in molecular weight, but there are also subtle differences in molecular architecture that lead to very different rheo logical characteristics and end-use application behavior. These polymers differ not only in bulk rheology, but also in microrheology. The microrheology is important in many applications and is a manifestation of the microstructure of gels thickened by these polymers. Previous work has shown that gels formed from cross-linked polyacryl amide (6), alginate (6), hydrolyzed starch-polyacrylonitrile (7), and a crosslinked poly(acrylic acid) (8), consist of discontinuous structures with microregions of extremely low viscosity. Such microscopic heterogeneity has been attributed to permanent or diffusing fluctuations within the gel (I) or to a structure of closely packed, swollen microgels in a continuous water phase (2, 3). This study was aimed at probing the microstructure of two poly(acrylic acid) thickeners, which will be called type A and type Β polymers. Polymer type A has a reported number-average molecular weight of 4 million, and polymer type Β has a reported number-average molecular weight of 500,000


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(I). These polymers display different bulk rheology. Type A shows viscoelastic characteristics, and type Β displays plastic rheology. Microstructures were probed by measuring the microdiffusion coeffi cients of bimodal gold sols within the polyelectrolyte gel structure by using quasi-elastic light scattering. Gold sols were ideal for this purpose because of their high refractive index, and because any flocculation of the sol caused by interaction with the polymer would be signaled by a change in the color Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 13, 2016 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0223.ch007

of the sol. The colloidal stability of gold sol-gel systems was first noted by Faraday (9). Faraday prepared soluble "ruby jellies" 130 years ago by the reduction of gold chloride solution with elemental phosphorus dissolved in carbon disulfide in the presence of gelatin. Such gels did not change color with time, even in the presence of relatively high salt concentrations. The bimodality of the gold sols used in this study allowed examination of two size regions of the microstructure centered around 10 nm in one case and 100 nm in the other.

Experimental Details Sols were prepared by the Zsigmondy technique. "Colloid-free" water (quartz-dis tilled, 120 mL), 15 mg of AuHCl · 3 H 0 , and 37 mg of K C 0 were heated to boiling, and a few drops of formaldehyde were added. The sol was run through a mixed-bed ion-exchange column to remove any unreacted reagents. The sol used in these experiments was bimodal with distributions centered around 10 and 100 nm. Mucilages of poly(acrylic acid) in the gold sols were prepared by dispersing polymer powder in the rapidly agitated sol. Each mucilage was neutralized directly in the light-scattering cuvette by addition of the appropriate amount of sodium hydroxide solution. The mucilages were neutralized to a pH within the range 7.2 to 7.6. In all cases, the color of the gold sol was unchanged upon the addition of poly(acrylic acid) and subsequent neutralization. Quasi-elastic light scattering (QELS) experiments were conducted on both neu tralized and unneutralized sols, in a Brookhaven model BI-90 particle sizer. This instrument measured the autocorrelation function, C(i), and fit this function to 4

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For the Brookhaven instrument, the scattering angle, Θ, is constant at 90°; the wavelength of the laser light, λ , is 0.6328 μπί; and η is the refractive index of the solution. A diffusion coefficient, D, is used to calculate particle size (assuming spherical geometry) from the Stokes-Einstein equation 0

where fc is the Boltzmann constant (1.38054 Χ I O ergs/K); Τ is absolute tem perature; η is the viscosity of liquid in which the particle is moving; and d is the particle diameter. Alternatively, if the particle diameter of the scattering particles is known, the microviscosity encountered by these particles in Brownian motion can be measured. The microviscosity measurements are described in this chapter. The Brookhaven instrument did not measure the angular dependence of scat tering. However, it has been shown (6) for diffusion of bovine plasma albumin in polyacrylamide gels, that the calculated value of D was independent of scattering angle. Gold sols dispersed within poly(acrylic acid) mucilages would be expected to show similar constancy of the measured diffusion coefficient with scattering angle.


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Results of Viscosity Measurements The shear dependence of Brookfield viscosity is shown in Figures 5 through 8 for unneutralized (type A) and neutralized (type B) poly(acrylic acid)s. Type Β showed little change in viscosity with rate of shear, whereas the highly efficient thickener displayed shear-thinning behavior. The concentration de pendence of microviscosities measured from the Brownian motion of 10- and 100-nm gold particles was compared with Brookfield viscosities of the poly(acrylic acid) mucilages in Figures 9 and 10 for type A polymer and in Figures 11 and 12 for type Β polymer.

Discussion Type A Polymer. Type A polymer is an efficient gelling and sus pending agent for cosmetic and pharmaceutical gels. For these end uses, plastic rheology is desired, and the gel must display a yield value that is sufficiently high to allow permanent suspension of the particles that are formulated into the gel. Shear-thinning behavior is desirable for ease of spread of the gel in its end-use. Type A polymer confers these desirable rheological characteristics (Figures 5 and 6). The concentration dependence of the Brookfield viscosity (Figures 9 and 10) indicates a rapid drop in viscosity upon dilution below a critical concen tration. Bagley (7) attributed such rheological behavior to a structure of swollen, deformable gel particles closely packed in intimate contact. Dav idson (8) later attributed the thickening efficiency of a cross-linked poly(acrylic acid) to the dispersed rather than the continuous phase. In general, pseu doplastic and viscoplastic rheology is characteristic of dispersions with low



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The neutralized polymer is more hydrophilic than its unneutralized counterpart, and the resulting enhanced compatibility with water would be expected to diminish the driving force for adsorption at the oil-water interface. The emulsion instability at higher p H values for systems containing 0.2% polymer can probably be explained by depletion of hydrophobe at the interface due to the enhanced hydrophilicity of the polymer at these p H values. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 13, 2016 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0223.ch007

At the highest polymer concentration studies (0.6%), however, this mechanism cannot apply because stable emulsion can be reproducibly prepared at higher p H values with 0.4% polymer. A possible explanation for the instability at the highest concentrations may be that these concentrations lie in the semidilute region, where the polymer coils just touch (23). Vincent (24) has shown that in this concentration range, dimensional collapse of the polymer chains occurs, and stabilization is lost.

Effect of Electrolyte.

Emulsions stabilized with hydrophobically

modified poly(acrylic acid) are sensitive to electrolytes. Upon contact with a brine solution, emulsion stability is immediately lost, and rapid coalescence of the oil droplets ensues (Figure 23). This instability can be understood by consideration of the Donnan equilibrium of counterions in polyelectrolytes (discussed earlier in this chapter). Addition of salt causes collapses of the polyelectrolyte microgels that are adsorbed at the oil-water interface. Shrinkage of the microgels could conceivably lead to immediate loss of stability, as depicted schematically in Figure 24.

Figure 23. The effect of electrolyte on emuhions stabilized with HMPAA. Continued on next page.




Figure 23.—Continued.



Figure 24. Schematic illustration of destabilization of HMPAA emukion by addition of salt.


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Summary


Measurement of the Stokes-Einstein microdiflusion coefficient by quasielastic light scattering of aqueous gels of two high molecular weight poly(acrylic acid) thickeners has shown vast differences in their mierorheology. Type A polymer displays extremely high bulk viscosities, but the mi eroviscosities of its gels approach the viscosity of water. A structure of swollen gels packed in intimate contact is postulated. Such a structure would explain the pseudoplastic rheology of these gels. This rheology is more often asso ciated with dispersions than with true polymer solutions. F o r systems thick ened with type Β polymer, the mieroviscosities closely approach the measured bulk viscosities. These systems, therefore, are true polymer so lutions, and the tenacity with which these polymer molecules hold water (down to the 10 nm level measured in this study) lends a plausible explanation to the use of this resin as an antiwicking agent in textile printing applications. Hydrophobically modified poly(acrylic acid) thickeners do show efficacy as primary emulsifiers. These emulsions, although stable for long periods will break and coalesce almost instantly when electrolyte is introduced into the aqueous phase.

References 1. Carbopol Water Soluble Resins, technical bulletin GC-67, BFGoodrich Com pany, Cleveland, Ohio. 2. Rice, S. Α.; Nagasawa, M. Polyelectrolyte Solutions; Academic: New York, 1961. 3. Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. 4. Hermans, J. J.; Pals, D. T. F. J. Polym. Sci. 1950, 5, 733. 5. Verwey, J.; Overbeek, J. Th. Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. 6. Sellen, D . B. J. Polym. Sci., Polym. Lett. Ed. 1987, 25, 699. 7. Taylor, N. W.; Bagley, E . G . J. Appl. Polym. Sci. 1974, 18, 2747. 8. Davidson, J. Α.; Collins, E . A. J. Colloid Interface Sci. 1975, 55, 163. 9. Faraday, M . Philos. Trans. R. Soc. London 1857, 145, 23. 10. Cheng, D . C . - H . Chem. Ind. (London) 1980, 10, 311. 11. Huang, C . ; Nieding, D . C . Book Pap., Natl. Tech. Conf.—AATCC (American Association of Textile and Color Chemists), 1982, 137. 12. Gillespie, T. J. Colloid Sci. 1958, 13, 32. 13. Kissa, E . J. Colloid Interface Sci. 1981, 83, 265. 14. Emulsion Science; Danielli, J. F.; Pankhurst, K. G . Α.; Riddiford, A. C., Eds.; Academic Press: New York, 1968. 15. Davies, J. T. Rec. Prog. Surf. Sci. 1964, 2, 129. 16. Friberg, S. Food Emulsifiers; Marcel Dekker: New York, 1976. 17. Friberg, S.; Mandell, L . ; Larsson, K. J. Colloid Interface Sci. 1969, 29, 155. 18. Friberg, S.; Larsson, K. Adv. Liq. Cryst. 1976, 2, 173. 19. Osmond, D . W. J.; Waite, F. A. Dispersion Polymerization in Organic Media; Barret, Κ. E . J., E d . ; John Wiley and Sons: New York, 1975; Chapter 2, p 24. 20. Napper, D . H . Trans. Faraday Soc. 1968, 64, 1701. 21. Napper, D. H . J. Colloid Interface Sci. 1969, 29, 168.


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22. Napper, D. H . Polymeric Stabilization of Colloidal Dispersions; Academic Press: New York, 1983. 23. Tadros, Th. F. The Effect of Polymers on Dispersion Properties; Academic Press: New York, 1982. 24. Vincent, B.; Whittington, S. Colloid and Surface Science; Matijevic, E . , E d . ; Plenum Press: New York, 1982.


RECEIVED for review March 3, 1988. A C C E P T E D revised manuscript January 5, 1989.

American Chemical Society Library 16th t , N.W. Glass;1155 Polymers in S Aqueous Media Advances in Chemistry;Washington, American Chemical Society: Washington, DC, 1989. D.C. 20036

Polymers in Aqueous Media - American Chemical Society

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