Photochromic cross-links in thermoresponsive hydrogels of poly(N-isopropylacrylamide):: Enthalpic and entropic consequences on swelling behavior

The preparation and properties of thermosensitive gels of poly(N-isopropylacrylamide) (PNIPAM) crosslinked, with azoaromatic chromophores were investigated. Polymer gels were prepared with different concentration of the cross-linking agent, 4,4'-di[2-methacryloylamide]azobenzene (DMAAB). The gels were optically transparent and possessed a high absorption for ultraviolet light. Characterization of the equilibrium swelling in water revealed that an increase in cross-linking density caused a shift in the volume phase transition to lower temperatures. This shift in the transition temperature (T-c) was interpreted to arise from an increase in the unfavorable interactions between the solvent (water) and the polymer network due to the azoaromatic cross-links. A high cross-linker concentration also resulted in a decrease in the degree of equilibrium swelling due to an increase in the topological constraints of the PNIPAM chains. Below T-c, the shrinking dynamics of the PNIPAM gels with photochromic cross-links followed a Fickian diffusion behavior, and above T-c, the gels collapsed completely within 20 min. During the deswelling, irradiation of the gel and trans-cis photoisomerization of the cross-links were found to increase the expulsion of water from the swollen gels and decrease the water content of the gels by as much as 20-30%. This result contrasts with past studies on PNIPAM gels in which the azochromophore was present in the side chains and irradiation increased the swelling ratio. The largest decrease in water content (DeltaR(s.max)) due to irradiation was observed near T-c, and the magnitude of the change, DeltaR(s.max), decreased with increase in cross-link density. The changes in swelling due to irradiation were attributed to entropic changes in the network due to irradiation.