SEGMENTAL DYNAMICS IN HOMOGENEOUS 1,4-POLYISOPRENE-1,2-POLYBUTADIENE DIBLOCK COPOLYMERS

The segmental dynamics associated with the primary relaxation process in three homogeneous diblock copolymers of poly(1,4-isoprene-co-1,2-butadiene) (PIP-PVE) with similar degree of polymerization N but different composition were investigated by depolarized Rayleigh scattering (DRS) and dielectric (DS) and mechanical (MS) spectroscopy. The entropically mixed samples are well in the disordered phase with chi(N) approximately 0 (chi being the interaction parameter) and hence exhibit a single glass transition (T(g)). For the symmetric PIP-PVE, the first careful DRS experiment and the complementary DS measurements clearly reveal two primary relaxation processes which are not a simple superposition of the relaxation functions of the two components. This dynamic modification is better documented for the asymmetric PIP-PVE reach in PIP which is selectively probed by DRS. For all copolymer samples, the overall shape of the distribution function is virtually temperature independent. The pertinent experimental results are consistent with the 'cigar" form of a single diblock copolymer chain in the isotropic melt (chi approximately 0) as suggested by recent Monte Carlo simulations.