Compression and Thermal Expansion Behaviors of Highly Crystalline Polyimide Particles Prepared from Poly(amic acid) and Monomer Salts

This study discusses the lattice deformation with an increase in the pressure or temperature of a rigid-rod aromatic polyimide (PI), poly(p-phenylene pyromellitimide), which was enhanced with an increase in the weight density of the lattice. This unique behavior is caused by the suppression of lattice deformation, by means of "misfit strain", which is induced by the mismatch of the interchain distances between the crystalline and noncrystalline domains. For lattice compression under hydrostatic pressure, the interchain compression was suppressed with a decrease in the crystallinity and/or an increase in the number of PI chains penetrating the interfaces between the crystalline and noncrystalline domains, which is caused by an enhancement of the misfit strain in the crystalline lattice. In contrast, the lattice strain along the main-chain direction (c axis) increased with an increase in the volume fraction of the compressible space near the PI chain ends included in the crystalline domain. Moreover, the thermal expansion of the crystalline lattice was inversely related to the lattice compression, and the coefficient of volumetric expansion increased with an increase in the volumetric compressibility. Accordingly, these results show that the compression and thermal expansion behaviors of the crystalline lattice of the PI are mainly determined by the higher-order structures at the mesoscopic scales, such as the crystallinity and segregated structures between the crystalline and noncrystalline domains, rather than by the crystalline lattice density.