Calcium carbonate composite hydrogel films: Particle packing and optical properties

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, 4.2-6.6) and volume fraction (φ _PCC) are studied with respect to the prepolymer viscosity, the maximum packing fraction (φ _m), and optical properties. φ _m obtained from Kreigher-Dougherty fitting is found to decrease with increasing AR of PCC particles, from φ _m = 0.33 (AR = 4.2) to φ _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 φ _PCC < φ _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 μm thick composite hydrogel film in the wet state with PCC AR = 6.6 and φ _PCC = 0.19 (near φ _m).