Isothermal Microphase Separation Kinetics in Blends of Asymmetric Styrene-Isoprene Block Copolymers

The microphase separation behavior of Kraton D1161, a highly asymmetric styrene-isoprene triblock and diblock copolymer mixture often used as the base polymer in pressure-sensitive adhesives, was studied using time-resolved small-angle X-ray scattering (SAXS) and oscillatory rheological analysis during temperature ramp and temperature quench experiments. The total scattering intensity obtained from SAXS was represented as a sum of the scattering from disordered and ordered polystyrene domains. A modified Percus-Yevick hard-sphere model was used to describe the scattering from disordered spheres, while Gaussian functions were used to model scattering from ordered domains. The temperature- and time-dependent SAXS experiments suggest a sequence of four stages in the development of the copolymer morphology during the isothermal phase separation kinetics experiments. Avrami analysis of the shear modulus evolution following a temperature quench indicates that domain growth is preceded by nucleation process. The ordering of polystyrene domains as observed during rheological and SAXS analysis is dominated by polymer chain mobility, which resulted in slower kinetics at lower temperatures.