Orientation and Strength of Semi-Crystalline Polymers

A study has been carried out of the tear strength and tensile strength of two simple partially-crystalline polymers: trans-polyisoprene and polyethylene. The materials were lightly crosslinked in the molten state, or uncrosslinked, and then oriented either in the molten or in the crystalline state. The tear energy was found to depend strongly upon the thickness t of the sheet, increasing linearly over the range of 0.03-1.2 mm. This dependence is attributed to plastic yielding at the crack tip in a zone having a cross-sectional area proportional to t('2). A threshold tear energy was obtained by extrapolation to zero thickness. It is suggested that these values represent the strength in the absence of large scale plastic yielding. They are relatively large, however: 4-8 kJ/m('2) for unoriented sheets. Expressed in terms of the inherent sharpness of the propagating tear, they correspond to values of the tip diameter of 40-90 (mu)m. The tear strength was also found to depend upon the natural draw ratio of these materials, being smallest for trans-polyisoprene which has the lowest natural draw ratio, about 2. Oriented sheets showed a lower draw ratio. The variation was consistent with a model of maximum extensibility. In consequence, the threshold tear energy varied strongly with the degree of prior orientation of the test pieces, changing by several orders of magnitude. Comparison of the values of the threshold tear energy and the work of plastic deformation suggests that the work of fracture is mainly expended in plastic yielding of the material at the tear tip. The size of the plastic zone was estimated from the dependence of the tear energy upon the torn thickness and was found to increase linearly with extension ratio perpendicular to the fracture plane.