Crystallization, Morphological Structure, and Melting of Polymer Blends

When the melt of a crystalline polymer is cooled to a temperature between the glass-transition and the equilibrium melting point, the thermodynamic requirement for crystallization is fulfilled.

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CRYSTALLIZATION, MORPHOLOGICAL STRUCTURE, AND MELTING OF POLYMER BLENDS

G. Groeninckx, M. Vanneste and V. Everaert

Catholic University of Leuven, Heverlee, Belgium

3.1

Abstract

When the melt of a crystalline polymer is cooled to a temperature between the glass-transition and the equilibrium melting point, the thermodynamic requirement for crystallization is fulfilled. In a crystallizable miscible blend, however, the presence of an amorphous component can either increase or decrease the tendency to crystallize depending on the effect of the composition of the blend on its glass-transition and on the equilibrium melting point of the crystallizable component. The type of segregation of the amorphous component, influenced by parameters such as crystallization conditions, chain microstructure, molecular weight and blend composition, determine to a large extent the crystalline morphology of a crystallizable binary blend. Separate, concurrent or co-crystallization can occur in a blend of two crystallizable components. The spherulite growth of the crystallizable component in miscible blends is influenced by the type and molecular weight of the amorphous component, the former affecting the intermolecular interactions between both components and the latter the diffusion of the amorphous component. The blend composition, the crystallization conditions, the degree of miscibility and the mobility of both blend components, the nucleation activity of the amorphous component are important factors with respect to the crystallization kinetics. The melting behavior of crystallizable miscible blends often reveals multiple DSC endotherms, which can be ascribed to recrystallization, secondary crystallization, or liquid-liquid phase separation. For the discussion of the crystallization and melting behavior in immiscible polymer blends, a division into 3 main classes is proposed. In blends with a crystallizable matrix and an amorphous dispersed phase, both the nucleation behavior and the spherulite growth rate of the matrix can be affected. Nucleation of the matrix always remains heterogeneous; however, the amount of nuclei can be altered due to migration of heterogeneous nuclei during melt-mixing. Blending can also influence the spherulite growth rate of the matrix. During their growth, the spherulites have to reject, occlude or deform the dispersed droplets. In general, the major influence of blending is a change in the spherulite size and semicrystalline morphology of the matrix.

L.A. Utracki (Ed.), Polymer Blends Handbook, 203-294. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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G. Groeninckx, M. Vanneste and V. Everaert

A completely different behavior is reported for blends in which the crystallizable phase is dispersed. Fractionated crystallization of the dispersed droplets, associated with different degrees of undercooling and types of nuclei is the rule. The most important reason is a lack of primary heterogeneous nuclei within each crystallizable droplet. An important consequence of fractionated crystalliz