Order-disorder transition in a poly(styrene-b-isoprene) diblock copolymer at hypersonic frequencies

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.