Structural changes during deformation of high molecular weight and low molecular weight polyethylene

Abstract

AbstractStructural changes caused by cold drawing low and high molecular weight polyethylene (M = 7 × 104 and 1.5 × 106) were investigated by using wide‐angle x‐ray diffraction, light and electron microscopy. Results obtained from drawn samples subjected to stress and samples at zero stress, revealed important differences in the behavior of these two polymers. In contrast with the low molecular weight polyethylene, the high molecular weight samples were deformed homogeneously without neck formation; the orthorhombic lattice, when subjected to stress, is transformed into a monoclinic one, not only in the initial stage, but over the whole deformation range up to breaking. The spherulitic structure in both polymers changes into the fibrillar one, while the degree of orientation attainable for the low molecular weight polyethylene is higher than that for the high molecular weight sample. During deformation of the low molecular weight polymer, cracks form between fibrils, and the fibrils become separated. Such an effect has not been observed with the high molecular weight material. After the external stress has been removed, a considerable contraction and decrease in orientation characterize the high molecular weight polyethylene. The differences in behavior of the two types of polymers are discussed in terms of different structural models.