Heat of Fusion of Crystalline Polypropylene by Volume Dilatometry and Differential Scanning Calorimetry

Abstract

Heat of fusion measurements on stereoregular forms of polypropylene have been the subject of a great number of investigations. The techniques most commonly used have been volume dilatometry, specific heat, DTA, TGA, DSC, empirical calculations, and copolymer studies. Furthermore, the published values for the heat of fusion of isotactic polypropylene using these methods have ranged all the way from 15.5 to 62 cal./g. Thus, an accurate value of ΔH_U for the hypothetical 100% crystalline polypropylene remains a subject of some uncertainty. The results of this work present information on a simultaneous study of identical samples using two of the techniques, namely, volume dilatometry and differential scanning calorimetry. The melting points of the pure homopolymer and binary mixtures of it with high purity transdecalin have been carefully determined in a series of dilatometers using mercury as the confining fluid. At the same time, a DSC technique has been employed to obtain the melting point depression as a function of the volume fraction of diluent. The Flory theory for melting point depression is applied to both data and results for ΔH_U thus obtained are compared to other estimates based on crystallinity calculated from density measurements and the actual heat of melting for the semi-crystalline samples.