Longitudinal growth of polymer crystals from flowing solutions V.: Structure and morphology of fibrillar polyethylene crystals

Abstract

Longitudinal fibrillar polyethylene crystals grown from xylene solutions subjected to simple shear flow have been investigated by various X-ray diffraction and electron microscopic techniques. The macrofibers with diameters in the micron range are composed of elementary fibrils of two basically different types. They may consist of either striated fibrils of the shishkebab type or smooth fibrils. The latter morphologies were obtained when the crystallization was carried out at temperatures above approximately 114 °C and the fibrillar crystals were allowed to grow longitudinally at the surface of the rotor of a Couette type instrument at a sufficiently high speed. The average spacings between the lamellae of the shish-kebabs as derived from the maximum in the small-angle X-ray scattering curve correspond remarkably well to the values obtained from transmission electron micrographs. The lateral dimensions of the elementary fibrils were acquired from equatorial line-broadening analysis of electron diffraction spots, wide-angle X-ray diffractograms, dark-field images and Guinier plots. They varied from 260 A for fibrillar polyethylene crystals grown at 103 °C to approximately 150 A for fibers formed at 118 °C The lateral crystallite size was also found to diminish at higher growth rates. Under the latter conditions the growth is accompanied by an extensive coil deformation as reflected by the presence of a considerable amount of the triclinic crystal modification. Dark-field electron microscopy indicates that the lengths of the crystallite blocks are of the order of 250–350 A for fibers grown at 103 °C and 450 A and longer for fibers formed at 118 °C. These values are in agreement with those obtained from the broadening of the (002) electron diffraction spots.