No abstract

Pressure-induced variations in the crystalline structures of polyimides (PIs) that have flexible ether or thioether linkages along the main chain were analyzed by synchrotron wideangle X-ray diffraction at high pressures up to 2.0 GPa. The crystalline lattices of poly(4,4oxidiphenylene pyromellitimide) (PMDA/ODA) and poly(4,4'-diamino-diphenyl sulfide pyromellitimide) (PMDA/SDA) PIs that have one flexible linkage in their repeating unit showed negative linear compressibility along the main-chain direction (c-axis), with a large anisotropy in the compressibilities between the two interchain directions (a-and b-axes). In contrast, polyimides prepared from bis(4-aminophenyl) ether (PMDA/TPE) and 4,4'-bis(4aminophenoxy) biphenyl (PMDA/BAPB) that have two flexible linkages in their repeating units exhibited positive compressibility along the main-chain direction, with a small difference in compressibility between the interchain directions. The different compression behaviors of PI crystalline lattices can be explained by the intrinsic Λ-shaped conformation of PMDA/ODA and PMDA/SDA, specifically the rotational fluctuations of the phenyl groups of the diamine moiety that were induced by the applied pressure.