Composite hydrogel films are prepared by photocrosslinking of partially polymerized poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) prepolymer solution dispersed with aragonite precipitated calcium carbonate (PCC) particles. The effects of PCC aspect ratio (AR, 4.26.6) and volume fraction (?PCC) are studied with respect to the prepolymer viscosity, the maximum packing fraction (?m), and optical properties. ?m obtained from KreigherDougherty fitting is found to decrease with increasing AR of PCC particles, from ?m = 0.33 (AR = 4.2) to ?m = 0.21 (AR = 6.6). Scanning electron microscopy images revealed that the particles are uniformly dispersed and randomly orientated in the polymer network when ?PCC < ?m. Optically transparent neat hydrogel films become opaque with increasing particle concentration and AR because of random light scattering from the PCC particles. A whiteness up to 80 is obtained from 100 mu m thick composite hydrogel film in the wet state with PCC AR = 6.6 and ?PCC = 0.19 (near ?m). POLYM. ENG. SCI., 2012. (C) 2012 Society of Plastics Engineers