| Abstract: | The effect of varying molecular weight distribution on the self-seeding phenomenon was investigated by using high molecular weight polyethylene fractions prepared by the stirring-induced crystallization method of Pennings. The numbers of self-seeding nuclei per gram were determined by measuring crystal dimensions, and were found to increase with increasing molecular weight of the polymer, in accord with previous findings. In another experiment, self-seeded single crystals were grown from materials of various molecular weights, prepared by blending two samples of differing molecular weight in various proportions. The concentration of nuclei varies linearly with the weight fraction of high molecular weight polymer in the mixture. This result is shown to be consistent with the proposition that each nucleus contains on the average an identical number (most plausibly one) of molecules of very high molecular weight. The application of this finding to the determination of molecular weights is discussed, and it is shown that the technique provides a method of unprecedented sensitivity for the characterization of the very high end of the molecular weight spectrum. Some morphological results are also presented. In particular, direct observations of the nuclei were found to be consistent with the loosely connected multiple nucleus structure, which had been proposed previously to account for certain light-scattering results. |
