Thermally crosslinkable poly(N-isopropylacrylamide) copolymers: Synthesis and characterization of temperature-responsive hydrogel

Thermally crosslinkable copolymers of N-isopropylacrylamide (NIPAAm) and N-Methylol acrylamide (NMA), a thermal crosslinker, were prepared by redox polymerization. The aqueous solutions of the copolymer were temperature responsive, and they showed phase separation behavior. The corresponding phase transition temperatures were detected by UV–visible spectroscopy and differential scanning calorimetry (DSC). The cloud points increased with increasing content of NMA from 32 °C of pure polyNIPAAm to 48 °C for poly(NIPAAm-co-NMA) with 30 mol% NMA. The crosslinking capability of the resulting polymers with different NMA contents was evaluated by monitoring their gel fractions and swelling ratios after crosslinking. The equilibrium swelling behaviors of the poly(NIPAAm-co-NMA) hydrogels were also investigated as a function of curing time. The results showed the hydrogels cured at shorter times or lower temperatures had lower gel fractions and higher swelling ratios. The introduction of a crosslinking structure into the temperature-responsive polyNIPAAm controlled the swelling capability and the cloud point of the crosslinked hydrogels.