Abstract
In order to obtain a comprehensive understanding of polyethylene crystals, n-alkanes have been examined as model compounds. It has become apparent that the phase transition and related thermal behavior of n-alkane samples have remarkably been affected by the existence of homologues. At first, it is described on the crystalline structure of low molecular weight polyethylene fractions formed in dilute solution. By the presence of molecular weight distribution in the fractions, bicomponent crystals were obtained; one was extended-chain crystal, the other was once folded-chain crystal. Thermal analysis was proved to be useful in characterization of the bicomponent crystals. In order to get a precise information of chain folding phenomena, it was necessary to prepare the monodisperse samples having exact molecular weights corresponding to n-alkanes, although it was difficult to synthesize pure and long n-alkanes. It has been known that folded-chain crystals exhibit thickening when they are maintained at a temperature below their melting point. It was suggested that this phenomena seemed to correlate to solid-solid transitions of higher n-alkane crystals. The high temperature phase showed a gradual structural changes during heating process by X-ray measurement, although it was difficult to discern the change on DSC curves. Transition temperatures and entropies of pure odd n-alkanes from heneicosane to tritetracontane are given as a function of carbon number per molecules.
