A REEXAMINATION OF THE RELATION BETWEEN THE MELTING TEMPERATURE AND THE CRYSTALLIZATION TEMPERATURE - LINEAR POLYETHYLENE

Analysis of the dependence of the melting temperature on the crystalli zation temperature has been frequently used to estimate the equilibriu m melting temperature of a polymer. In the present paper the underlyin g basis for this extrapolative method is examined in detail. Two dista ntly different types of nucleation are considered. One is the Gibbs tw o-dimensional nucleus; the other, certain kinds of three-dimensional n uclei. Thickening, subsequent to nucleation, is also taken into accoun t. Appropriate experiments with linear polyethylenes that have most pr obable molecular weight distributions, as well as with a molecular wei ght fraction, were carried out over a wide range in crystallization te mperatures. Three distinct regions are observed. The high-temperature region is curved and precludes an extrapolation of the data to the equ ilibrium melting temperature. Reasons for this behavior are presented. Evidence for its being a general phenomenon is also given. A melting temperature of 141 degrees C was directly observed for linear polyethy lene.