Thermal Expansion Behavior of the Ordered Domain in Polyimide Films Investigated by Variable Temperature WAXD Measurements

Abstract

Coefficients of thermal expansion (CTEs) along the main chain direction in the ordered domain of aromatic and semi-aliphatic polyimide (PI) films (α_c) were determined in order to investigate the thermal expansion behaviors of self-standing PI films in the direction parallel to the film plane. Variable temperature wide-angle X-ray diffraction (VT-WAXD) measurements were performed using a synchrotron radiation facility in the temperature range of 60-360°C. All the PI films showed characteristic WAXD profiles to liquid-crystalline-like ordered domain overlapped with broad and intense amorphous halos, and diffraction peaks corresponding to the repeating unit along the main chain (c-axis) and the inter-chain ordering (Ch-pack) were clearly resolved. The α_cs along the c-axis of PIs having rigid-rod structure are negative, whereas those for PIs containing bent linkages, such as ether (-O-) or methylene (-CH₂-), in the main chains are large and positive. The α_c values coincide well with the in-plane CTEs of the PI films estimated by thermal mechanical analysis (TMA). This indicates that the negative in-plane CTEs observed for the rigid-rod PI films originates from the negative α_c in the ordered domain, which originates from the extended main chains with appreciable orientation in the film plane.