Mechanical properties of polyethylene crystals. II. Deformation process of spherulite

Optical and electron microscopy of the plastic deformation process of polyethylene spherulitic film revealed the variation of the process with the temperature. Even within a single spherulite the mode of deformation differs with the relative position in the overall deformation; hence it is hard to represent with a simple affine model. The morphological change can be classified by dividing a spherulite into a few zones by conical boundaries which incline 45° to the direction of stretching and pass the center of the spherulite. At 20°C, the unfolding of molecules takes place in two sectors; the concentric extinction patterns are disturbed, probably due to rotation or bending of lamellae. At 95°C., the necking or the unfolding of molecules is not detected in any sector but the deformation with the slip between or within lamellae occurs preferentially in two side sectors. The anisotropic parameters corresponding to the tensile and shear compliances of simple lamellar structures are estimated from mechanical properties of tubular extruded film in which lamellae pile up along the machine direction. Applying these values of parameters to the structure model of spherulite, the mechanism of its deformation is analyzed. One of these parameters which relates to shear deformation inducing the “slip” between or within lamellae is most sensitive to temperature and varies widely with the irradiation dose (number of crosslinkings).