Energetics analysis of polyethylene crystallization: single crystal growth surfaces

Abstract

Semiempirical energy calculations have been used to investigate the structure and energetics of molecular attachment and mobility on the {110} and {200} growth faces of polyethylene single crystals. The results indicate that, while the crystallographic position of the first nucleating segment on the {110} face is a low energy position, other dispositions of the molecule have somewhat lower energy. Growth layers nucleated by molecules in the lowest energy position are substantially different from the crystallographic structure. After inclusion of a few molecules however, this growth layer must undergo a rearrangement requiring the cooperative motions of several molecules in order to assume the now lower energy crystalline structure. Comparisons of the energies and mobility of molecular segments on the {110} and {200} growth faces indicate that there is a greater attraction of the segments to the {200} face, but that chain mobility is substantially greater on the {110} face. Correlation of these factors with the observed dependence of degree of truncation of polyethylene single crystals on crystallization temperature and the concentration of polymer in solution suggests that the nucleation energetics take precedence over factors of chain mobility, but the total truncation behaviour observed is probably the result of an interplay between these two factors. Energetics calculations also provide an explanation for the observed lower melting temperature of the {200} sectors.