Composite semi-interpenetrated polymer network (semi-IPNs) hydrogels were synthesized by free-radical polymerization at different temperatures, and cross-linker ratios, using dextran sulfate (DxS) (M(v) = 100 kDa) as an ionic physical entrapped polymer, acrylamide (Mm) monomer and N,N'-methylene-bis(acrylamide) (BAAm) as cross-linking agent. Their mechanical, morphological and swelling properties were evaluated as a function of gel preparation conditions. It was found that the stability of the DxS into composite hydrogels has been influenced by the gel preparation temperature as well as by the crosslinker ratio, the lowest amount of DxS being released from the hydrogels obtained at -18 degrees C with a crosslinker ratio of 1/40. The mechanical analysis revealed that the composite hydrogels formed at -18 degrees C, with a cross-linker ratio of 1/40, were very tough and could be compressed up to about 100% strain, without any crack development, while those formed at +20 degrees C were fragile. The swelling ratio of the semi-IPN composite hydrogels has been evaluated as a function of time, the values of diffusion exponent n indicating the swelling was controlled by a non-Fickian mechanism. (C) 2011 Elsevier Ltd. All rights reserved.