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 alpha,omega-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 (chi), compression modulus (G) and effective crosslinking density (v(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 v(e) being greater than v(1) (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-, chi- and v(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 degrees C showed maxima corresponding to 0.20 and 0.50 of methanol volume fractions, respectively. It was assumed that the maxima on G, v(e) and chi 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.