Polyethylene unit cell variations with temperature

Abstract

The unit cell dimensions of a linear polyethylene are reported as determined by x‐ray diffraction measurements at five‐degree intervals from −70°C. to +130°C. and at −196°C. The relative expansion coefficient of the a dimension of the orthorhombic unit cell was found to increase from 1.1 × 10⁻⁵/°C. at −196°C. to 22 × 10⁻⁵/°C. at 30°C. to 58 × 10⁻⁵/°C. at 138°C., the melting point of the crystal. The coefficient of the b dimension was found to vary from −2.8 × 10⁻⁵/°C. at −196°C. (questionable) to +3.8 × 10⁻⁵/°C. at 30°C., and then to −7.3 × 10⁻⁵/°C. at 138°C. These values were obtained from series expansion analytical fits to the experimental points, and are less accurate at the ends of the temperature range; the 30°C. values are probably correct to a few per cent.