Determination of the equilibrium melting point of the beta-form of polypropylene

Juhasz, P. and Varga, József and Belina, Károly and Marand, H. (2002) Determination of the equilibrium melting point of the beta-form of polypropylene. JOURNAL OF THERMAL ANALYSIS AND CALORIMETRY, 69 (2). pp. 561-574. ISSN 1388-6150

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Abstract

The melting behavior of the beta-form of isotactic polypropylene (beta-iPP) was investigated as a function of crystallization time and temperature. Calcium suberate, a selective beta-nucleating agent was used to produce samples that consist entirely of beta-form i-PP. The experimental melting points were recorded at different crystallization times and were extrapolated to the start of the crystallization process in order to eliminate the effect of lamellar thickening. Using the non-linear Hoffman-Weeks approach to correlate these extrapolated experimental melting temperatures with the corresponding crystallization temperatures, an equilibrium melting point of 209degreesC was obtained for beta-iPP. The equilibrium melting point estimated through the non-linear Hoffman-Weeks analysis is about 30degreesC higher than that (T-m(o) =177degreesC) obtained on the basis of the linear extrapolation. These results are consistent with earlier claims that a linear extrapolation of T-m-T-c data leads to an underestimation of the equilibrium melting point. The results obtained for beta-iPP exemplify the importance of accounting for both the isothermal lamellar thickening effects and the non-linearity in the T-m-T-c correlation, when the determination of an equilibrium melting point is carried out using a procedure based on the predictions of the Lauritzen-Hoffman secondary nucleation theory.

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