Osmotic Swelling of Polyacrylate Hydrogels in Physiological Salt Solutions

Abstract

The swelling behavior of fully neutralized sodium polyacrylate gels was investigated in aqueous solutions of alkali metal (LiCl, NaCl, KCl, CsCl) and alkaline earth metal salts (CaCl2, SrCl2, BaCl2). The total salt concentration and the ratio of monovalent to divalent cations were varied in the biologically significant range. It is found that the concentrations of both monovalent and divalent cations vary continuously and smoothly in the gel despite the abrupt change in the gel volume. The individual elastic, mixing, and ionic contributions to the free energy of the gel were separately determined as a function of the degree of network swelling to elucidate the thermodynamics of swelling. Shear modulus measurements performed at different Ca2+ concentrations suggest that Ca2+ does not form stable cross-links between the polymer chains. At low and moderate swelling ratios the concentration dependence of the shear modulus follows a power law behavior, G variation of phi n, with n = 0.34 +/- 0.03. At high swelling degrees, however, the shear modulus increases with increasing swelling. The value of the Flory-Huggins interaction parameter, chi, determined from osmotic swelling pressure and shear modulus measurements, strongly depends on the ionic composition of the equilibrium solution and increases with increasing Ca2+ concentration.

Molecular thermodynamics for swelling of a mesoscopic ionomer gel in 1 : 1 salt solutions

“…Further addition of salt to the external solution leads again to noticeable swelling of an ionic gel 6,10 . Swelling behavior is not sensitive to the nature of added monovalent cations 3,13 . Divalent 7,13 and trivalent 11 cations have a stronger effect on swelling of a gel than the monovalent cations.…”

“…1 to ca. 100 mM [3][4][5][6][7][8][9][10][11][12][13][14] . At sufficiently large ionic strength, swelling is no longer sensitive to the polymer's degree of ionization or to ionic strength.…”

Effect of calcium neutralization on elastic and swelling properties of crosslinked poly(acrylic acid) - correlation to inhomogeneities and phase behaviour

“…Furthermore, it has previously been shown that phase separation is a phenomenon commonly occurring in crosslinking polymerization [18,25,26] and that increasing crosslinker content is an inducing factor [21,22]. In the literature it has been suggested that the solubility for PAA gels in Ca 2+ containing solution is reduced [10], and that precipitation of PAA occurs in presence of high Ca 2+ concentration [14]. It is therefore likely that the neutralization of AA with calcium during the synthesis may induce changed phase behaviour of the mixture and thus alter the structure of the gel product.…”

“…It has previously been reported that the equilibrium degree of swelling for both PAA and sodium neutralized PAA decreases strongly in solvents containing divalent cations compared to in solvents containing monovalent cations [10,11]. Horkay et al [10] stated that the reduced swelling is not due to Ca 2+ ions acting as stable crosslinks between chains, but rather due to a change in the interactions between the solvent and the polymer chains.…”

“…Horkay et al [10] stated that the reduced swelling is not due to Ca 2+ ions acting as stable crosslinks between chains, but rather due to a change in the interactions between the solvent and the polymer chains. However, it has been shown that Ca 2+ can stabilize PAA hydrogels by preventing the dissolution of the gel by crosslinking the PAA.…”