EFFECT OF THE DISORDER-TO-ORDER TRANSITION IN DIBLOCK COPOLYMERS ON THE SEGMENTAL DYNAMICS - A STUDY USING DEPOLARIZED PHOTON-CORRELATION SPECTROSCOPY

Depolarized light scattering has been applied to study the anisotropy fluctuations in a series of polystyrene/polyisoprene diblock copolymer s. This method is selective for the dynamics of the polystyrene block. The molecular weights of the block copolymers (10500-20800) were chos en such that the microphase separation transition (MST) occurs at temp eratures which are experimentally accessible with photon correlation s pectroscopy. Small-angle X-ray scattering (SAXS) was used to identify the disordered regime, the fluctuation-controlled transition regime, a nd the ordered state. The relaxation time spectrum in the disordered s tate is similar to that of bulk polystyrene. Concentration fluctuation s increasingly broaden the spectrum and lead to a bimodal shape. Close to the MST the spectrum becomes dominated by a slow, nearly single ex ponential contribution. Comparison with the SAXS results suggests that this splitting of the relaxation time spectrum can be related to chai n stretching.