Effect of cross-links on the phase separation behavior of a miscible polymer blend

The effect of radiation cross-linking on the phase diagram and scattering function for a compatible polymer blend of deuteriated polystyrene and poly(vinyl methyl ether) has been examined by small angle neutron scattering. The scattering curves for the cross-linked blends exhibit a maximum at a nonzero q vector, the position of which depends linearly on the square root of the radiation dose or inversely on the square root of the number of repeat units between cross-links, N exp 0.5 sub c . This dependence was predicted by de Gennes, but the measured position of the maximum is smaller than predicted. The spinodal temperature can be determined by plotting the inverse of the scattered intensity at the maximum, S(q*) exp --1 , vs. inverse temperature and extrapolating to the point where S(q*) exp --1 = 0. The inverse of the measured spinodal temperature depends linearly with radiation dose or with N exp --1 as predicted. This increases the size of the single phase region of the phase diagram with the extrapolated spinodal temperature increasing from 149 deg C for the uncross-linked blend to 430 deg C for the blend with a radiation dose of 125 Mrad. The theory by de Gennes predicts that the scattered intensity at q = 0 equals 0 for the cross-linked blends, which is not observed experimentally. 31 ref.--AA