Morphology and crystallization kinetics of melt miscible polyolefin blends

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Blends of copolymers of ethylene/hexene (9 CH3/1 000 C) and ethylene/butene (77 CH3/1 000 C) synthesized with metallocene catalysts were prepared by co-precipitation from solution. A phase diagram for this blend had been obtained in a preceding work, where the blends were found to be miscible in the melt with a characteristics upper critical solution temperature (UCST). In this work, the successive self-nucleation and annealing (SSA) thermal fractionation method revealed that both pure copolymers, even though they had been prepared by metallocene catalysis, displayed a very broad distribution of short chain branching. The thermal behavior of the pure copolymers and their blends revealed that co-crystallization effects were present within each phase and that molecular fractionation during crystallization can prevail when the materials are slow cooled from the melt. A crystallization kinetics study was performed by differential scanning calorimetry (DSC) and it revealed a strong competition between crystallization and phase segregation. Upon cooling from a one phase melt, phase segregation precedes crystallization if the crystallization temperature (Tc) is high. On the other hand when low crystallization temperatures are employed, the amount of phase segregation that can be achieved before crystallization is limited. Transmission electron microscopy (TEM) allowed the observation of the lamellar morphology and their aggregation state. In specially prepared samples, the relative lamellar frequency of isothermally crystallized material was employed as ascertain whether the blend under consideration was within the one or the two phase region of the blends phase diagram.

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Macromolecular Chemistry and Physics