Block Copolymers near the Microphase Separation Transition. 4. Dielectric Spectroscopy

In this paper, we report evidence of local concentration fluctuations in thermodynamically homogeneous diblock copolymers near the microphase separation transition (MST). Dielectric spectroscopy has been used to probe the local structure of a series of 1,4-polybutadiene/1,2-polybutadiene diblock copolymers with morphologies that ranged from well-ordered to disordered. Four transitions, corresponding to the primary (glass) and secondary transitions of each block, are observed in the dielectric loss spectrum of each copolymer studied. Observation of a primary transition for each block provides evidence of local concentration heterogeneities, large enough to exhibit an apparent glass transition, even in samples that are homogeneous on the molecular length scales probed by small-angle neutron-scattering and rheological measurements. To characterize these heterogeneities, the dielectric loss data have been fit with four discrete transitions, representing each of the glass and secondary transitions. Model fits indicate that the disordered block copolymers are characterized by large-amplitude concentration fluctuations which diminish in magnitude with decreasing N, where x is the segment-segment interaction parameter and N is the degree of polymerization. These results are consistent with recent fluctuation corrections by Fredrickson and Helfand to Leibler's mean-field theory for diblock copolymers near the MST.