Volume 15, Issue 4 p. 641-656
Article

Studies on the α and β forms of isotactic polypropylene by crystallization in a temperature gradient

Andrew J. Lovinger

Andrew J. Lovinger

Department of Chemical Engineering and Applied Chemistry, Columbia University, New York, New York 10027

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Jaime O. Chua

Jaime O. Chua

Department of Chemical Engineering and Applied Chemistry, Columbia University, New York, New York 10027

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Carl C. Gryte

Carl C. Gryte

Department of Chemical Engineering and Applied Chemistry, Columbia University, New York, New York 10027

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First published: April 1977
Citations: 388

Abstract

Samples of isotactic polypropylene (PP) were zone-solidified in temperature gradients up to 300°C/cm at growth rates down to 3 μm/min. Oriented α-type spherulites were obtained only by nucleation. While β nucleation is extremely rare, the β phase is easily initiated by growth transformations along the oriented α front. Since the β phase was found to grow considerably faster than the α phase, the α-to-β transformation points diverge across the sample, interrupting growth of the oriented α fibrils. This causes subsequent nucleation to yield teardrop-shaped α spherulites.

Differential scanning calorimetry (DSC) studies of zone-solidified PP show the β-phase to be favored by slow growth rates, high temperature gradients, and large degrees of superheat in the melt—all of which tend to suppress nucleation. Differential thermograms of largely β-PP obtained at a heating rate of 1°C/min show the actual melting and recrystallization of the β spherulites into the α form.

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