The degree of crystallinity of monoclinic isotactic poly(propylene)

The degree of crystallinity of a set of monoclinic (alpha) isotactic poly(p ropylenes), prepared by a metallocene-type catalyst, were determined at roo m temperature. Three different methods were used: density, enthalpy of fusi on, and wide-angle X-ray scattering, and the results compared. The relation between the heat effusion and the specific volume of these poly(propylenes ) was found to be nonlinear, thus precluding any linear extrapolation to ob tain the heat of fusion of the pure crystal (Delta H-u). The value of Delta H-u obtained from depression of the melting temperature by diluents is use d. Based on the unit cell density of monoclinic crystals formed from a low defected fraction, the density obtained crystallinity levels were found to be between 0.15-0.25 higher than those calculated from the heat of fusion. This relatively large difference holds for the isothermally crystallized an d quenched isotactic poly(propylenes), and reflects the contribution of the interphase to the density determined crystallinity, which does not contrib ute to the heat of fusion. Paralleling results found in other systems, the crystallinity levels obtained from wide-angle X-ray scattering agree with t hose obtained from density, indicating a significant contribution of the pa rtially ordered phase to the total diffraction. Emphasis is given on the ne ed to account for the large differences in the crystallinities of poly(prop ylene) measured by different techniques when evaluating the dependence of p roperties on this quantity. (C) 1999 John Wiley & Sons, Inc.