Hollow Pyramidal Crystals of Polyethylene and a Mechanism of Growth

Abstract

A hollow pyramidal model for diamond‐shaped single crystals of polyethylene is proposed which accounts for the electron microscope observations and electron diffraction effects presented in this paper and in the literature. The pyramidal shape occurs because the rows of folded molecules along the (110) growth face in each quadrant of the crystal are translated along the molecular chain direction by two carbon‐carbon distances relative to the previous row. This translation presumably occurs in order to relieve steric interferences at the folds or loops of the polymer chain. The resulting triclinic cell, of which the familiar orthorhombic cell is in reality a subcell, has a c̄ dimension equal to the constant fold length of the polymer chain, and remaining parameters of ā=7.84 A, b̄=5.56 A, α=(90−27.3)°, β=(90−19.0)° and γ=81.5°. The values 19.0 and 27.3 degrees are, respectively, arctan (c/a) and arctan (c/b) of the orthorhombic subcell, so that the (001) planes of the triclinic cell which contain the folds on the surface of the pyramids are (111) planes relative to the orthorhombic subcell.