Poly(ionic liquid), which had a strong carbon dioxide (CO(2)) absorption ability, was introduced into polystyrene (PS) by the radical copolymerization between styrene and an ionic liquid monomer, i.e. [2-(methacryloyloxy)ethyl]-trimethylammonium tetrafluoroborate ([MATMA][BF(4)]). The [MATMA][BF(4)] segments in the copolymer greatly increased the solubility of CO(2), which was demonstrated by the significant glass transition temperature (T(g)) reduction of the copolymer under high pressured CO(2). The copolymer microcellular foams were then prepared by a batch foaming process using supercritical CO(2) as the blowing agent. The morphology characterization indicated that the copolymer microcellular foams had the much higher cell density and the much smaller cell size compared with the pure PS foams. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.