An Investigation into the influence of crosslinker type and solvent composition on physical properties and phase transition behavior of poly(N-isopropylacrylamide) hydrogels

Poly(N-isopropylacrylamide) (PNIPAAm) hydrogels have been synthesized by free radical polymerization in 1,4-Dioxane using N,N'-methylenebisacrylamide (BIS) and α,ω-acryloxyorganofunctional poly(dimethylsiloxane) (VTPDMS) as crosslinkers. The disc samples equilibrated by swelling in water, water-methanol mixtures and methanol were examined by volumetric and stress-compression measurements to afford the values of swelling ratio (V/ V_o), polymer-solvent interaction parameter (χ), compression modulus (G) and effective crosslinking density (?_e). It was observed that the incorporation of hydrophobic VTPDMS chains, instead of hydrophilic BIS molecules, as crosslinker into the structures of neutral NIPAAm hydrogels improved their mechanical strengths. The values of ?_e, being greater than ?_t (theoretical crosslinking density) also indicated the presence of hydrophobic physical interactions, i.e., physical crosslinkings, between dimethyl siloxane groups in the PNIPAAm hydrogels crosslinked with VTPDMS. Further, the G-, χ- and ?_e-solvent composition curves of both NIPAAm/VTPDMS and NIPAAm/BIS hydrogels swollen to equilibrium in water/methanol mixtures (80/20, 50/50 and 20/80, in v/v %) at 23°C showed maxima corresponding to 0.20 and 0.50 of methanol volume fractions, respectively. It was assumed that the maxima on G, ?_e and χ versus solvent composition curves resulted from the competitions between interaction of methanol and water together versus their individual interactions with PNIPAAm/VTPDMS and PNIPAAm/BIS polymers.