FRACTAL PROPERTIES AND SWELLING BEHAVIOR OF POLYMER NETWORKS

The swelling pressure of randomly crosslinked polymer networks is rela ted to the structural properties of the crosslink topology. Using the assumption that the network structure exhibits fractal properties with in the correlation length xi, a scaling relation between the swelling pressure and the polymer volume fraction has been derived. The exponen t obtained depends on the internal fractal dimension d(i) of the netwo rk and is in general different from the corresponding exponent for lin ear chains. The later can be obtained as the special case d(i)=1. As a consequence, a significant difference in mixing entropy between the n etworks and the corresponding uncrosslinked system is predicted. This explains the experimental results obtained by several authors, which a re in contradiction to the Flory-Rehner assumption. Computer simulatio ns based on the bond fluctuation model support the scaling predictions presented. The exponents obtained for the density dependence of the o smotic or swelling pressure are somewhat larger than expected from the theoretical work for both the linear and the crosslinked system.