Morphological effects on thermal transitions in immiscible polymer blends

Polymer blend morphology and the interaction between component domains is a critical feature of immiscible blends that strongly influences physical properties. Lacking formal chemical bonding between the components, immiscible blends depend completely on the morphology and mechanical phase interaction developed through melt blending to achieve desirable properties. Even in the absence of chemical bonding, these mechanical interactions can alter the phase transitions of the blend components in significant and important ways. The primary objective of this thesis was to study the degree to which shifts in glass transitions and crystallization occur in selected binary blends and to deduce the mechanisms for these effects and their influence on mechanical properties.; In blends of polystyrene and polypropylene prepared by melt processing the polystyrene Tg revealed a sharp increase at high polypropylene content in the blend, an unexpected result because an invariant glass transition is the primary indicator of blend immiscibility. This novel and unexpected effect was confirmed in other polyolefin-containing blends melt processed in various ways. The variation in glass transition is attributed to the polymer domain interactions resulting from the different morphologies of various blend compositions and qualitative structural models are proposed based on physical interactions between the components. A size-dependent crystallization mechanism was observed in the semi-crystalline polymers in all the blends pairs.; Polycarbonate/polyethylene blends were prepared to alter the relationship between Tc of the semi-crystalline polymer and Tg of the amorphous polymer. Results show that the variation in amorphous component Tg with composition depends strongly on the physical state of the semi-crystalline domains.; Overall, the morphology of the immiscible polymer blends represents a composite structure in which three main effects have been identified in this work that influence the phase transition behaviors of the components: the resolved mechanical stresses that arise from differential contraction during cooling; the spatial pinning of domain structures, particularly in micro-dispersed blends; and the inter-diffusion of mobile modifying constituents that may be present in certain polymers but not formally bonded to the backbone. These effects are important to the evolving scientific understanding of polymer blend structures and are also critical to the fabrication of commercially important materials.