Gil R. Hawley

Crystallization, melting and morphology of syndiotactic polypropylene fractions: 1. Thermodynamic properties, overall crystallization and melting☆

Abstract A series of syndiotactic polypropylene (s-PP) fractions with constant syndiotacticities and different molecular weights have been studied through differential scanning calorimetry (d.s.c.), wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering experiments. The molecular weights, molecular weight distributions, syndiotacticities and sequence distributions of this series of fractions have been characterized by gel permeation chromatography, solution nuclear magnetic resonance and Fourier transform infra-red spectroscopy. The equilibrium melting temperature of sufficiently high molecular weight (above 40 000) s-PP with about 94% racemic dyads is 160±1°C, and the heat of fusion is 8.0±0.3 kJ mol −1 . Overall crystallization rates exhibit a molecular weight dependence and a discontinuity with respect to crystallization temperature for the fractions. The temperature at which this discontinuity happens is at an undercooling of ca. 50°C. Based on nucleation theory, this discontinuity may be recognized as a regime III to regime 11 transition. With decreasing undercooling (increasing crystallization temperature) and molecular weight, a doubled crystal unit cell along the b axis becomes increasingly dominant during the crystallization. In this unit cell, opposite handedness of the helical chains exists along both the a and b axes (antichiral packing). Double melting peaks can be observed for all fractions in the high to middle undercooling region (ΔT> 50°C), while only one melting peak can be found in the relatively low undercooling region. Different heating rate experiments after isothermal crystallization in d.s.c. and WAXD indicate that the low-melting crystal may undergo reorganization and melt-recrystallization processes to form the high-melting crystal. During this transformation, doubling of the crystal unit cell along the b axis with an antichiral packing of the chain molecules is obtained.

Crystallization, melting and morphology of syndiotactic polypropylene fractions: 2. Linear crystal growth rate and crystal morphology

Abstract Linear crystal growth rates of two narrow molecular weight fractions of syndiotactic polypropylene having the same syndiotacticity have been measured using polarized light microscopy over a temperature range of > 20°C. It has been found that a regime III to regime II transition at a supercooling of ∼ 50°C exists. Structure analysis via electron diffraction (ED) experiments indicates that no change of growth planes has been found during this regime transition. Nevertheless, a gradual change of the crystal perfection due to a chain packing change from a crystal incorporated with isochiral packing defects to a majority of cell III structure in this supercooling range has been observed. The validity of the nucleation theory applied to s-PP is discussed. For the crystal morphological study, single crystals with rectangularly faceted lamellae can be grown at high crystallization temperatures (low supercooling) in thin s-PP film samples as observed via transmission electron microscopy. Similar to the results reported by Lovinger and Lotz, the ED patterns show that the long axis of the single lamellar crystal is the b -axis. On decreasing the crystallization temperature, spherulites are developed. Cracks on the lamellar crystals have been observed, and they are always perpendicular to the b -axis. This phenomenon has been explained by invoking the observation that the coefficient of thermal expansion along the b -axis is about one order of magnitude larger than that along the a -axis, as measured via wide-angle X-ray diffraction experiments. However, at high crystallization temperatures, the cracks are found less frequently. This is due to the pure cell III crystal packing that forms at these tempratures leading to the incorporation of fewer isochiral packing defects which promote crack initiation.