Rapid swelling and deswelling of thermoreversible hydrophobically modified poly(N-isopropylacrylamide) hydrogels prepared by freezing polymerisation

Rapid response thermally sensitive hydrophobically modified poly(N-isopropylacrylamide) hydrogels have been synthesised successfully using a two-step polymerisation method, the initial polymerisation being carried out at 20 °C, followed by polymerisation at −28 °C for 24 h. The results show that the swelling/deswelling rates of poly[N-isopropylacrylamide-co-(di-n-propylacrylamide)] P(NIPA-co-DPAM) hydrogels prepared by two-step polymerisation are much faster than for the same type of hydrogels prepared via conventional methods (30 °C for 24 h), i.e. the time for the former xerogel to absorb 70 and 90 wt% is just 30 and 240 min, respectively, compared to the latter xerogel which takes 1600 and 2500 min to absorb the same amounts of water. During deswelling (shrinking), the hydrogel loses 95 wt% water in 1 min, compared to a timescale for the corresponding cross-linked copolymers prepared by conventional methods of about 5 h for 50 wt% water loss. Scanning electron microscopy, and flotation experiments together with swelling ratio studies reveal that the polymeric network of the former hydrogel is characterised by an open structure with more pores and higher swelling ratio but lower mechanical strength compared to the latter hydrogels. Such rapid response hydrogels have potential applications in separation and drug release technologies for example.