Variations in the crystal lattice of PEEK and related para-substituted aromatic polymers: 1. Thermal expansion

Abstract Uniaxially oriented films of PEEK have been examined by X-ray diffractometry over the temperature range 20 to 290°C in order to determine the expansion of the crystal lattice. The a and b lattice parameters were measured from equatorial scans using the 110, 200 and 020 reflections and the c parameter from meridional scans using the 002 reflection. The expansion rate in the a direction is about three times that of the b direction, while the change in c is almost negligible. Close to the T g at around 145°C, there is an increase in the expansion coefficient of a and a reduction in the b coefficient. The overall expansion of the cell volume is essentially linear with a volume expansion coefficient of 1.65 × 10 −4 °C −1 .