Inhomogeneity in Hydrogels Synthesized by Thiol-Ene Polymerization

Hydrogels synthesized via thiol-ene polymerization were characterized with respect to their macroscopic properties via elasticity measurements and to their microscopic properties, in particular inhomogeneity, by dynamic light scattering. For comparison, identical studies were made on polyacrylamide (PAAm) hydrogels prepared by free-radical chain-growth cross-linking copolymerization. Mechanical measurements prove that for both types of gels the degree of network imperfections is widely varied by varying concentration, while the macroscopic properties observed at particular concentrations are very similar. However, the fraction of static scattering which is a measure of gel inhomogeneity depends significantly on the type of network studied when the measurement is performed in the state of preparation. The majority of the thiol-ene gels appear homogeneous, contrary to the PAAm gels. The situation is completely different in the equilibrium swollen state, where all gels exhibit extremely high and almost identical fractions of static scattering intensity, irrespective of the type of reaction or the concentration employed in their synthesis. These observations imply that (i) there is no clear correlation between network imperfections and homogeneity and (ii) the type of cross linking reaction controls the concentration inhomogeneity in the state of preparation but not (iii) the inhomogeneity of network density. The latter is seen when the gels are swollen.