Macroporous poly(N-isopropyl)acrylamide networks: formation conditions

The formation conditions of macroporous poly(N-isopropyl)acrylamide (PNIPA) networks by free-radical crosslinking copolymerization are described. The networks were prepared in water at an initial monomer concentration of 20% using N,N′-methylene(bis)acrylamide (BAAm) as the crosslinker. The synthesis parameters varied were the crosslinker concentration and the gel preparation temperature Tprep. Depending on these parameters, the variations of the swelling, mechanical and texture properties of the networks were investigated. It was shown that, after passing a critical crosslinker concentration (2–5% BAAm), the network structure changes from homogeneous to heterogeneous ones. Further increase of the crosslinker content increases both the rate of swelling and the porosity of the networks. Heterogeneous PNIPA networks consist of spherical globules called microspheres of 0.1–0.5 μm in diameter aggregated to large, unshaped, discrete clusters with dimensions of a few μm. At 30% BAAm, the structure looks like cauliflowers, typical for a macroporous network. The dependence of the elastic moduli of PNIPA gels on the crosslinker content shows three different regimes depending on the structure of the networks. Increasing Tprep from 9 to 50°C at a fixed BAAm content increases both the rate of swelling and the swelling capacity of the networks, while the total porosity decreases.