Morphologies and rheological properties of ethylene-propylene block copolymers

Ethylene-propylene block copolymer (EPBC) is mainly composed of two component polymers, polypropylene (PP) and ethylene-propylene rubber (EPR). The dependence of morphologies as well as dynamic moduli of EPBC on the molecular characteristics of the components were investigated for four EPBC samples. From the results of the temperature dependence of dynamic moduli and the morphologies, it was suggested that the samples have heterogeneous structure composed of a dispersed EPR phase and a PP matrix phase in which a part of the EPR component dissolves. The concentration of dissolved EPR in the matrix increases with increasing propylene content of the EPR component. For the melts of the EPBC samples containing the EPR component of higher than about 70 wt% propylene contents, a secondary plateau was observed in the long time region on the dynamic viscoelasticity curves. To clarify the origin of this slow relaxation, the dynamic viscoelasticity were compared with three theories: Palierne's emulsion model, the thermal diffusional model and the blending rule. The comparison suggested that the blending rule can well explain the secondary plateau behavior originated from the existence of the high molecular weight EPR dissolved in the PP matrix.