Abstract
The microstructural reorientation occurring during annealing of various grades of oriented high-and low-density polyethylenes was examined using wide and low angle X-ray diffraction. The differences in the degree of reorientation were interpreted in terms of the relative crystallinities of the different grades. Differential scanning calorimetry was used to relate the melting behaviour of the specimens to their structure. Drawing at elevated temperatures resulted in specimens with a particularly simple texture where the stacks of lamellae were inclined with their normals aligned parallel to the drawing direction. Specimens with this initial texture were annealed and their elastic constants determined by ultrasonic techniques. A study of four different ultrasonic techniques was made to determine their relative accuracy and reproducibility in measuring the velocity of stress waves in oriented polyethylene. The highly anisotropic nature of the specimens led to diffraction and dispersion effects, with the result that two of the techniques were found to be unsuitable for oriented systems. The symmetry of the drawn and annealed specimens was found to be approximately hexagonal rather than orthorhombic. The value of the elastic constant was shown to increase with draw ratio to over 70 GPa for draw ratios of x25. After annealing at temperatures approaching the melting point, the value of C[33] fell to about 10% of its as-drawn value. This behaviour is correlated with the loss of orientation of the molecules in the amorphous phase during annealing. The elastic constants C[11], C[22], C[44] and C[55] were comparatively insensitive to changes in molecular orientation. A simple structural model was proposed and developed and its elastic constants evaluated. The experimentally measured elastic constants were compared with the model elastic constants by using theoretically determined values of the elastic moduli of polyethylene crystals in the expressions developed to describe the elastic properties of the model. Good quantitative correlation was found between the predicted and measured elastic constants, taking into account their relative crystallinities.