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

Abstract

The melting behavior of the β-form of isotactic polypropylene (β-iPP) was investigated as a function of crystallization time and temperature. Calcium suberate, a selective β-nucleating agent was used to produce samples that consist entirely of β-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 209°C was obtained for β-iPP. The equilibrium melting point estimated through the non-linear Hoffman—Weeks analysis is about 30°C higher than that (Tm0=177°C) obtained on the basis of the linear extrapolation. These results are consistent with earlier claims that a linear extrapolation of Tm−Tc data leads to an underestimation of the equilibrium melting point. The results obtained for β-iPP exemplify the importance of accounting for both the isothermal lamellar thickening effects and the non-linearity in the Tm−Tc 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.