CRYSTALLIZATION, MELTING AND MORPHOLOGY OF SYNDIOTACTIC POLYPROPYLENEFRACTIONS .1. THERMODYNAMIC PROPERTIES, OVERALL CRYSTALLIZATION AND MELTING

A series of syndiotactic polypropylene (s-PP) fractions with constant syndiotacticities and different molecular weights have been studied th rough differential scanning calorimetry (d.s.c.), wide-angle X-ray dif fraction (WAXD) and small-angle X-ray scattering experiments. The mole cular weights, molecular weight distributions, syndiotacticities and s equence distributions of this series of fractions have been characteri zed by gel permeation chromatography, solution nuclear magnetic resona nce and Fourier transform infra-red spectroscopy. The equilibrium melt ing temperature of sufficiently high molecular weight (above 40 000) s -PP with about 94% racemic dyads is 160 +/- 1-degrees-C, and the heat of fusion is 8.0 +/- 0.3 kJ mol-1. Overall crystallization rates exhib it a molecular weight dependence and a discontinuity with respect to c rystallization temperature for the fractions. The temperature at which this discontinuity happens is at an undercooling of ca. 50-degrees-C. Based on nucleation theory, this discontinuity may be recognized as a regime III to regime II transition. With decreasing undercooling (inc reasing crystallization temperature) and molecular weight, a doubled c rystal unit cell along the b axis becomes increasingly dominant during the crystallization. In this unit cell, opposite handedness of the he lical 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 (DELTAT > 50-degrees-C), while only one mel ting peak can be found in the relatively low undercooling region. Diff erent heating rate experiments after isothermal crystallization in d.s .c. and WAXD indicate that the low-melting crystal may undergo reorgan ization 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.