Small-angle neutron scattering study of interdiffusion in polymer blends.

Abstract

Earlier investigations on the theory of scattering from diffusing polymer blends are generalized to apply to non-ideal blends which are homogeneous at thermodynamic equilibrium. A significant difference between the procedure used here and that of previous authors is the emphasis on the different time dependence of the intramolecular and intermolecular contributions to the scattering. The generalized results include the original results and the Cook-Binder equation as limiting cases. The importance of the theory is its applicability to extract very small diffusion coefficients of the order of 10$\sp{-16}$cm$\sp2$/sec from scattering data. This domain is difficult to reach by other experimental methods. The method is particular useful for studying slow diffusion in polymer blends near the glass transition temperatures (Tg) of the blends. Binary polymer blends prepared in nonequilibrium states were allowed to interdiffuse. The general features of the resulting scattering curves taken at different annealing times followed the predictions from the theory. At low q, the scattering intensity decreases with increasing annealing time while at high q the scattering intensity increases with annealing time. The crossover region was clearly observed. For blends of polystyrenes with mismatched molecular weights, interdiffusion coefficients of the order of 10$\sp{-17}$cm$\sp2$/sec were extracted from the scattering data. The interdiffusion was Fickian. The rates of interdiffusion were enhanced at short annealing times. This anomaly was explained based on the reptation theory. The results also indicate that the rates of diffusion were controlled by the fast moving species as predicted by the "fast theory". Interdiffusion between polystyrene and poly(xylenyl ether) at temperatures 20$\sp\circ$C above Tg was also studied by SANS. Fick's law was not applicable to the system due to a coupling between interdiffusion and structural relaxation; however, interdiffusion coefficients and relaxation effects are obtainable using a generalized diffusion equation proposed by Jackle and Frisch. The rate of interdiffusion was found to be comparable to the rate of structural relaxation for diffusion near Tg.