Thermal analysis and X-ray scattering study of metallocene isotactic polypropylene prepared by partial melting

Autor: Malcolm Capel, Peggy Cebe, Patrick S. Dai
Rok vydání: 2002
Předmět:
Zdroj: Journal of Polymer Science Part B: Polymer Physics. 40:1644-1660
ISSN: 1099-0488
0887-6266
DOI: 10.1002/polb.10220
Popis: In this study, we examine the effects of heating, nucleation, cooling, and reheating on the thermal properties and structure of metallocene isotactic polypropylene (m-iPP) that had been prepared initially in a standard state containing nearly equal amounts of the crystallographic α and γ phases. Heat treatment was achieved through partial melting and annealing by the heating of samples to self-nucleation temperatures (Tn's) that spanned and exceeded the entire range of melting of the standard state, from 122 to 160 °C. The relative amounts of α and γ crystals are determined from the area under the unique wide-angle X-ray reflections. The lower and upper endotherms are caused by the melting of γ and α crystals, respectively. Four distinct regions of Tn were identified on the basis of the thermal and structural parameters of m-iPP. In region I, Tn is below the peak melting temperature of the γ phase. Here, γ crystals are annealed and α crystals are barely affected by Tn. In region II, Tn is above the peak of the lower endotherm but below the peak of the upper endotherm. γ crystals melt, and α crystals anneal. In both regions I and II, the portion of the sample melted at Tn recrystallizes epitaxially with existing parent α lamellae as the substrates, and the amount of α always exceeds the amount of γ. In region III, Tn is above the peak of the upper endotherm, and all γ crystals and some or all α crystals are melted at Tn. The number of α-crystal nuclei steadily decreases as Tn increases, causing systematic depression of the crystallization and melting temperatures seen during cooling. Finally, in region IV, Tn exceeds the upper endotherm, and only small self-nuclei or heterogeneous nuclei remain. Recrystallization is now suppressed to lower temperatures. For regions III and IV, a crossover behavior in the relative amounts of α and γ is observed during cooling from Tn. Because of the effective nucleating ability of α toward γ, as the temperature drops, the amount of γ increases and then exceeds the amount of α. With subsequent reheating, the reverse crossover occurs because of the lower melting point of γ. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1644–1660, 2002
Databáze: OpenAIRE
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