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(styr ene-bisoprene) diblock copolymer with a polystyrene (PS) volume fracti on f(PS) of 0.22 at hypersonic frequencies using a tandem Fabry-Perot interferometer. Because of the asymmetric composition we are effective ly studying the sound dispersion in a composite material made of hard PS spheres embedded in a soft polyisoprene matrix. All measurements ar e 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 temperatu res below and above the transition. We find the signature of T-ODT fro m the broad hypersonic attenuation reflecting the enhanced segmental m obility due to mixing at temperatures near the T-ODT.