Hydrogels of N-vinylimidazole and sodium styrenesulfonate were synthesized by radical crosslinking copolymerization. Transient swelling measurements in water, at room temperature, reveal an unusual behavior. For some gel compositions, the swelling time dependency is about a ␦ function: dry hydrogel pellets swell up readily to reach a high degree of swelling and, spontaneously, they deswell to collapse. Such peculiarity was interpreted in terms of several competing events in the swelling mechanism: water diffusion toward the gel, chain disentanglement, sodium-proton interchange through the gel-external bath boundary, approaching of chains to allow interaction of sulfonate groups with neighboring protonated imidazole moieties and diffusion of water outside the gel. The results of cyclic swelling-deswelling following abrupt changes of composition in the external bath, from water to sodium hydroxide solution, support that mechanism.
Several hydrogels of N-vinylimidazole and sodium styrenesulfonate have been prepared by radical cross-linking copolymerization in aqueous solution, using N,N 0 -methylene-bisacrylamide as crosslinker. Depending on composition, these hydrogels were neutral, amphoteric, cationic or anionic. Compression-strain measure-ments were performed on samples as-synthesized and swollen in deionized water or in acid aqueous solutions, with and without salt. It was thus found that the cross-linking densities determined by compression measurements on assynthesized sam-ples are in good accordance with those calculated by means of the model of polymer networks with pendant vinyl groups. A non-Gaussian parameter (b) was introduced to explain that the elastic moduli (G) of samples swollen at equilibrium are larger than predicted by the Gaussian model. The b values of the neutral or ionized systems increase with swelling and fall into a single curve, which denotes a common behav-ior. Swelling has two opposite effects on G; on the one hand G decreases because the polymer volume fraction diminish and the system shifts from the affine limit to the phantom one; on the other, b increases and contributes to increasing G. The balance of those two opposite effects determines the variation of G with swelling. The possi-ble contribution of ionic crosslinks to m e for the polyampholyte and for the polycation wearing divalent counteranions was discussed.A peculiar system is poly(sodium styrenesulfonate), whose cross-linking density is much lower than expected.
The compression modulus of poly(N‐vinylimidazole) (PVI) hydrogels synthesized by cross‐linking polymerization in aqueous solution, was measured at room temperature in several related systems: i) just after polymerization, ii) swollen at equilibrium in deionized water, iii) swollen in HCl (aq) (pH=2.5), iv) swollen in HCl (pH=2.5) and 1 M NaCl (aq) solution and v) swollen in H2SO4 (pH=2.5) (aq) solution. Samples of the first and second groups are neutral whereas hydrogels of the other three groups are ionic because of protonation of basic imidazole groups. The experimental results were fitted with the Erman‐Monnerie theory, applied to compression measurements for the first time, to determine the phantom modulus, [fph*], and the parameter κG which measures the constraining role of entanglements on the fluctuations of chains between knots.
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