THE DYNAMICS OF SYMMETRICAL 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 copol ymer melts. Above but close to the order-disorder transition, four mod es contribute to the time-dependent correlation function. In the depol arized geometry there is a very fast, broad relaxation typifying the s egmental reorientational dynamics and a slower, single-exponential mod e related to chain orientation and stretching. In the polarized experi ments two additional processes are seen: the heterogeneity mode relate d to the self-diffusion of individual copolymer molecules and a slow m ode representing the translational diffusion of large (congruent to 10 0 nm) clusters. Just below the order-disorder transition (ODT), the sl owest mode is shifted to very long times. The heterogeneity mode gives rise to depolarized scattering below the ODT because of the anisotrop y induced by the lamellar structure. With the lowest molar mass copoly mer above the ODT, it could be seen that the amplitude of the mode rel ated to chain orientation and stretching decreases the deeper the samp le is in the disordered state. A new subtraction technique was used fo r data analysis. (C) 1996 American Institute of Physics.