Dielectric study of subchain dynamics and dimension: butadiene-isoprene block copolymers

Abstract

Dielectric normal mode relaxation was studied for dilute and semidilute hexane solutions of butadiene (B)-isoprene (I) block copolymers in which the polyisoprene block has the component of dipole moment aligned parallel to the chain contour (Type-A dipole moment) while the polybutadiene block does not. Thus, the fluctuation of the end-to-end vector of a portion of a chain labeled by type-A dipoles was observed. The results indicate that BI diblock copolymers exhibit a single loss peak while BIB triblock copolymers exhibit double peaks. This behavior can be qualitatively explained by the Rouse-Zimm theory. In dilute solutions, the loss maximum frequency and the shape of the loss curve agree well with the Zimm theory. In semidilute solutions, the loss curve broadens more than the theoretical loss curve calculated by either the Rouse theory or the Doi-Edwards theory. The broadening of the loss curve can be explained by the coupling theory proposed by Ngai and his co-workers. The mean square end-to-end distance, 〈 r 2〉, of the PI block was estimated from the relaxation strength, Δϵ. The expansion factor, α, for 〈 r 2〉 agreed approximately with that of homo PI solution in heptane.