Deformation of polypropylene single crystals

The plastic deformation modes of single crystals of polypropylene have been studied as a function of applied uniaxial strain and as a function of orientation of the lathlike crystals relative to the draw direction. The crystals were drawn on a Mylar substrate and the morphology of the drawn crystals was observed by electron microscopy for various imposed uniaxial strains varying from nominal values of 3 to 100%. For crystals oriented in, or close to, the direction of draw, microcracks developed at low strains and grew progressively larger as the draw ratio increased. Fibrils, about 50 A in diameter, were found to extend across the apparent cracks. The fibril diameter appeared to be independent of the imposed strain but the fibril length was found to be proportional to the strain. For crystals oriented at an appreciable angle to the strain, additional modes of plastic deformation were found to occur. These included simple and multiple shear, slip, and buckling. Crystallographic twinning also appeared to be present in some of the deformed crystals. The results obtained are compared and contrasted with results of similar tests on polyethylene.