Importance of postgel reactions in determining the polymer network structure and properties.: Poly(N-vinylimidazole) hydrogels

We compare the characteristics of polymer networks synthesized in the same or in different times of reaction, always after gelation. Several poly(N-vinylimidazole) hydrogel samples labeled with fluorescein (PVI) were synthesized by radical copolymerization in aqueous solution with N,N'-methylenebis(acrylamide) (BA) as cross-linker, with different times of polymerization, and with different concentrations of VI and BA in the polymerization feeding mixture. The glass transitions of samples synthesized with larger BA concentration were broader and shifted to larger T-g and with smaller heat capacity jump. Throughout postgel reactions, T-g decreases from a value typical of rigid domains enriched in BA to values closer to that of linear PVI, and the average density of permanent cross-links (obtained with the DiMarzio equation) decreases. In apparent contradiction with these facts, the equilibrium swelling degree in water and methanol, gravimetrically determined, decreases significantly upon increasing the time of polymerization. The effective cross-linking density, obtained from swelling measurements, increases throughout postgel reactions, contrary to permanent cross-links. It was concluded that spatial inhomogeneities give a porous structure characterized by high swelling and large T-g which during postgel reactions evolves to a less porous and less rigid material. The characteristics of the fluorescence spectra of labeled samples report on the access of water to polymer material and support that conclusion.