COMPOSITION FLUCTUATION EFFECTS ON DIELECTRIC NORMAL-MODE RELAXATION IN DIBLOCK COPOLYMERS .2. DISORDERED STATE IN PROXIMITY TO THE ODT ANDORDERED STATE

Dielectric relaxation spectroscopy has been used to investigate the bl ock end-to-end vector relaxation in symmetric poly(styrene-b-1,4-isopr ene) diblock copolymers both in the ordered state and in the disordere d state close to the disorder-to-order transition (ODT). Evidence is p resented of two new types of dielectrically active chain relaxations b y approaching the ODT from the disordered state and in the ordered sta te. In the ordered state and besides the usual isoprene subchain overa ll orientation, a new block relaxation process appears with slow dynam ics and amplitude that depends on the sample preparation. The slow pro cess is related to the coherently ordered microstructure and is attrib uted to the relaxation of the conformal interfaces formed in the order ed state; this assignment is supported by computer simulation results. In the disordered state, a bifurcation of the single dielectric relax ation is observed; the total amplitude increases as the temperature ap proaches the ODT, whereas the high-frequency process shows an apparent weak temperature dependence. The fast process may be related to polyi soprene subchains that feel higher polyisoprene concentrations near th e ODT, whereas the slow relaxation mode can be attributed to the induc ed orientation correlations near and above the ODT, as supported also by computer simulations.