Abstract
The order-to-disorder transition (ODT) has been studied in a
poly(styrene-bisoprene) diblock copolymer with a polystyrene (PS) volume
fraction f(PS) of 0.22 at hypersonic frequencies using a tandem
Fabry-Perot interferometer. Because of the asymmetric composition we are
effectively studying the sound dispersion in a composite material made
of hard PS spheres embedded in a soft polyisoprene matrix. All
measurements are made in the long-wavelength limit, qD < 1, where q is
the scattering wave vector and D is the sphere diameter. The transition
temperature (T-ODT) was identified earlier with rheology and SAXS at 112
degrees C. We have investigated the sound velocity and attenuation at
temperatures below and above the transition. We find the signature of
T-ODT from the broad hypersonic attenuation reflecting the enhanced
segmental mobility due to mixing at temperatures near the T-ODT.