We investigated the effect of organic filler and the degree of crosslinking on the performance of a hydrogel/filler system to control its diffusion and mechanical properties. A series of poly(acrylic acid) gels filled with polystyrene (PS) nanoparticles at well-controlled filler loading levels was synthesized by free radical polymerization in the presence of a crosslinking agent. Three different concentrations of the crosslinking agent (methylene bis-acrylamide), that is, 5 mg/(g of AAc), 10 mg/(g of AAc), and 20 mg/(g of AAc), were used. The hydrogel crosslinked to the least extent that included a high amount of PS showed fast swelling rates, high equilibrium swelling ratios, and the highest pH sensitivity. Three gel/filler combinations including the gel without filler, gel/filler mixture, and gel/filler copolymerization were considered. The PS particles of approximate to 40 nm in diameter, which were used as the fillers, were prepared by microemulsion polymerization. Copolymerization of styrene and AAc having different reactivities ratios was achieved by a seed-microemulsion polymerization with the crosslinking agent. The gel/filler mixture showed the highest swelling rate and the gel/filler copolymer resulted in highly enforced mechanical properties and the highest pH sensitivity. The incorporation of filler by copolymerization to mechanically enforce the equilibrium swelling hydrogel was more efficient than the increase of crosslinking degree. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3574-3587, 2013