Abstract
Coefficients of thermal expansion (CTEs) along the main chain direction in the ordered domain of aromatic and semi-aliphatic polyimide (PI) films (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 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 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 c in the ordered domain, which originates from the extended main chains with appreciable orientation in the film plane. Keyword: Polyimide / WAXD / thermal expansion / in-plane orientation / -transition
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