Direct visualization of microstructural deformation processes in polyethylene

Abstract

The deformation morphology of high density polyethylene was investigated by electron microscopy of oriented thin films. A melt-drawing process was used to produce chain axis oriented and planar textured thin film, which was subsequently deformedin situ at room temperature in a scanning transmission electron microscope. Both as-drawn and annealed films were studied. Deformation along the orientation direction is initially accomodated by the interlamellar regions, which cavitate and form microfibres. For annealed films it is possible to directly observe strain-induced crystallization at about 300% strain in the fibrils. With increased deformation, suitably oriented lamellar crystals deform by two clearly visualized chain slip systems: {1 0 0}, 〈0 0 1〉 and {0 1 0}, 〈0 0 1〉. Thesec axis shear processes could be further distinguished as fine slip or as block shear. Still higher deformation causes more breakup of blocks by shear; when the block size is less than some critical size, the blocks decrystallize. The deformation leads toward a fibrillar morphology consisting of oriented crystals from crystallized amorphous material at high elongations, crystal blocks broken out of lamellae, and chains drawn out of lamellae and recrystallized. The deformation behaviour of the as-drawn films is somewhat different from that of the annealed films. Initially as for the annealed films the lamellae are separated with increasing strain as the interlamellar regions are deformed but there is less voiding. Higher deformation causes the lamellae in the as-drawn films to shear apart at significantly lower strain levels (50% as opposed to 300%) than in the annealed films. At about 100% deformation, the as-drawn film no longer has a recognizable lamellar structure. Although generalization is tempered by the simplicity of this model texture, these deformation results are highly relevant to the current microstructural understanding of lamellar deformation in different regions of a spherulite, to the morphology of commercial extruded and blown films, and to specially prepared textured polymers, such as rolled and annealed films or capillary melt flow and solidification methods which can produce texture approaching that of a single crystal.