Effect of reaction conditions on poly(N-isopropylacrylamide) gels synthesized by post-polymerization crosslinking system

In this paper, the effect of gelation solvent and temperature on poly(N-isopropylacrylamide) (PNIPAAm) gels synthesized by post-polymerization crosslinking (PPC) was discussed in detail. PPC was two-step synthesis performed by (1) radical copolymerization of NIPAAm and activated ester monomer (NHSA) and (2) crosslinking reaction of the obtained prepolymer and diamine crosslinker. In PPC, various solvents could afford gels, while the gelation time was strongly dependent on solvent polarity. The obtained gels showed similar swelling curves in water, indicating that the network structure of PPC gels was dominantly determined by the prepolymer structure. It was quite contrasting to conventional divinyl crosslinking (DVC), by which gelation could be observed only in water and not in organic solvents under the same concentration condition as PPC gelation system. We also examined the effect of gelation temperature in PPC synthesis in water. The PPC-PNIPAAm gel prepared in water at high temperature was consistently white turbid as same as DVC-PNIPAAm gel, suggesting that this turbidity was derived from polymer aggregation and entanglement fixed by chemical crosslinking.