Brillouin Scattering from a Homogeneous 1,4-Polyisoprene-Poly(vinylethylene) Diblock Copolymer and Its Constituent Homopolymers

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 T_g 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 T_g 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 T_g, 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.