Effect of crystal texture on the anisotropy of thermal expansion in polyethylene naphthalate: measurements and modelling

Abstract

This paper reports measurements of thermal expansion in polyethylene naphthalate films and relates them to the crystal texture. X-ray diffraction shows that the naphthalate ring planes are distributed with a strong maximum in the plane of the film. The crystal c axes are aligned in the machine direction for uniaxially drawn films but have a bimodal distribution for biaxially oriented samples. Parameters extracted from the distributions were combined with experimental and estimated values of the thermal expansion and stiffness constants for the isotropic amorphous and anisotropic crystalline phases to obtain predictions of thermal expansion coefficient using Ward's aggregate model. The Voigt average (crystals and amorphous regions in parallel) is very close to the uniaxial experimental expansion coefficients. In the biaxial samples, the model predicts the widthwise variation of principal directions of the expansivity tensor well. A linear combination of the Voigt and Reuss (series arrangement) averages reproduces the measured values.