Swelling behavior and network structure of hydrogels synthesized using controlled UV-initiated free radical polymerization

N-vinyl-2-pyrrolidone (VP) and 2-hydroxyethyl methacrylate (HEMA) copolymeric gels have been synthesized using UV-initiated photopolymerization to understand their characteristic behavior for development as a bioengineering material, specifically for tissue expansion. The properties of the gels have been investigated by systematic variation of the monomer feed composition and initiator and crosslinker concentrations as well as UV irradiation intensity, which was controlled by various photomasks. The swelling kinetics and network characteristics for the various hydrogels were investigated through the observation of gel swelling behavior in saline solutions and compression modulus determination of the fully swollen hydrogels. The equilibrium swelling ratio (q(e)) of the gels increased as expected with increasing VP content and decreasing crosslinker concentration. However, it was found that as the amount of initiator or UV intensity increased, unexpectedly q(e) also increased, which indicates a network structure with decreasing effective crosslink density (v(e)) (or increasing average molecular weight between crosslinks (M-c)). Based on this anomalous swelling behavior and thermal analysis of the gels, a molecular structure is proposed consisting of increasing number of dangling chain ends within the polymer network. (C) 2008 Wiley Periodicals, Inc.