Abstract
Polarized Rayleigh-Brillouin Spectroscopy (BS) has been used to probe the local segmental dynamics at hypersonic frequencies in a homogeneous l,4-polyisoprene-l,2-polybutadiene diblock copolymer far from the microphase separation transition (MST) and the corresponding homopolymers. An Arrhenius temperature dependence along with an exponential decay of the structural relaxation times τ8 provides an adequate fit of the experimental longitudinal loss modulus M′′, which reveals a similar dynamics in the diblock and 1,4-polyisoprene but at variance with the dynamics in 1,2-polybutadiene (poly(vinylethylene)). On the contrary, recent photon correlation spectroscopy (PCS) data obtained in the same materials near and above Tg indicate a much broader distribution of relaxation times in the diblock than in the constituent homopolymers. Moreover, a Vogel-Fulcher-Hesse-Tammann equation cannot describe reasonably both the low-temperature (PCS) and high-temperature (BS) data. The combined results of the PCS and BS measurements in the present homogeneous diblock copolymer with a single Tg indicate that a difference in segmental mobilities and shape parameters ß (with regard to the conventional Kohlrausch-Williams-Watts equation) may explain the behavior of the diblock near and above Tg, while at high temperatures where BS applies, a fast single relaxation time process seems to be responsible for the similar hypersonic dispersion in the diblock and homopolymer. These measurements represent a first, to our knowledge, application of dynamic light scattering methods to probe the dynamic behavior of homogeneous diblocks. © 1991, American Chemical Society. All rights reserved.