HIGH-PRESSURE EFFECTS ON THE DISORDERED PHASE OF BLOCK-COPOLYMER MELTS

We have measured the effect of high pressure on the scattering from th e disordered state of a styrene-isoprene diblock copolymer containing 48 wt % styrene (M(W) similar to 16 500. Small-angle X-ray scattering measurements performed at the Cornell High Energy Synchrotron Source ( CHESS) show an increase in the aspect ratio of the scattering maximum and a decrease in the angle at which the maximum occurs with increasin g pressure. Within the context of the mean-field theory of block copol ymer phase behavior originally developed by Leibler,(1) this behavior is due to an increase in both the radius of gyration of the molecule ( R(g)) and the degree of segregation of the system (N-chi). Examination of the data using the theory of Tang and Freed(2) reveals an identica l quantitative dependence of R(g) and N-chi on pressure without indica ting a role for the free volume fraction used by these authors to trea t compressibility effects. We therefore propose that the observed beha vior arises primarily from shifts in the structural characteristics of the blocks, rather than from a reduction in free volume. From compari son of the pressure-induced changes in the structural characteristics of the system with the corresponding temperature-induced ones, we pred ict the pressure dependence of the order-disorder transition temperatu re in this material, provided that the phase state of this polymer is determined solely by the structural characteristics examined in this e xperiment.