DYNAMICS OF POLYISOPRENE IN STAR BLOCK-COPOLYMERS CONFINED IN MICROSTRUCTURES - A DIELECTRIC-SPECTROSCOPY STUDY

Dielectric spectroscopy has been employed to study selectively the loc al and global dynamics of polyisoprene (PI) in three microphase separa ted star diblock copolymers (SI)(4), where PI and polystyrene (PS) for m the core and corona, respectively, The thermally induced order-to-di sorder transition (ODT) has been identified by theology and small-angl e x-ray scattering (SAXS), All dielectric measurements were made at te mperatures well below the ODT and the polystyrene glass transition tem peratures, where the four PI chain ends are tethered by the glassy pol ystyrene domains. The ordered state morphology, studied by SAXS, revea led the formation of PS spheres (f(PS) = 0.25, where f(PS) is the poly styrene volume fraction), polyisoprene cylinders (f(PS) = 0.68), and a lamellar structure (f(PS) = 0.41) in the three copolymers. Ln contras t to the local segmental motions, the chain orientational dynamics ass ociated with the amplitude and the characteristic relaxation time are strongly influenced by the spatial confinement. The main effects are t he suppressed amplitude and the enhanced chain mobility, which are pre dicted and found to be stronger if the star topology is accompanied by topological (entanglement) effects of the PI block, irrespective of t he ordered star morphology. (C) 1997 American Institute of Physics.