The dynamics of symmetric polystyrene-polybutadiene diblock copolymer melts studied above and below the order-disorder transition using dynamic light scattering

This communication describes dynamic light scattering measurements on three low molar mass symmetric polystyrene-polybutadiene diblock copolymer melts. Above but close to the orderdisorder transition, four modes contribute to the time-dependent correlation function. In the depolarized geometry there is a very fast, broad relaxation typifying the segmental reorientational dynamics and a slower, single-exponential mode related to chain orientation and stretching. In the polarized experiments two additional processes are seen: the heterogeneity mode related to the self-diffusion of individual copolymer molecules and a slow mode representing the translational diffusion of large (=100 nm) clusters. Just below the order-disorder transition (ODT), the slowest mode is shifted to very long times. The heterogeneity mode gives rise to depolarized scattering below the ODT because of the anisotropy induced by the lamellar structure. With the lowest molar mass copolymer above the ODT, it could be seen that the amplitude of the mode related to chain orientation and stretching decreases the deeper the sample is in the disordered state. A new subtraction technique was used for data analysis.