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 synthe sis parameters varied were the crosslinker concentration and the gel prepar ation temperature T-prep. Depending on these parameters, the variations of the swelling, mechanical and texture properties of the networks were invest igated. It was shown that, after passing a critical crosslinker concentrati on (2-5% BAAm), the network structure changes from homogeneous to heterogen eous ones. Further increase of the crosslinker content increases both the r ate of swelling and the porosity of the networks. Heterogeneous PNIPA netwo rks consist of spherical globules called microspheres of 0.1-0.5 mum in dia meter aggregated to large, unshaped, discrete clusters with dimensions of a few mum. 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 str ucture of the networks. Increasing T-prep from 9 to 50 degreesC at a fixed BAAm content increases both the rate of swelling and the swelling capacity of the networks, while the total porosity decreases. (C) 2001 Published by Elsevier Science Ltd.