Precise Analysis of Thermal Volume Expansion of Crystal Lattice for Fully Aromatic Crystalline Polyimides by X‑ray Diffraction Method: Relationship between Molecular Structure and Linear/Volumetric Thermal Expansion

Coefficients of thermal linear and volumetric expansion (CTE, CVE) of crystal lattice for 13 fully aromatic crystalline polyimides (PIs) were evaluated from lattice parameters measured from variable-temperature (VT) synchrotron X-ray diffraction patterns, and the effects of chemical structure on CTE and CVE are discussed. The smallest CVE (116 ppm K^–1) was observed for PMDA-PPD with the simplest rigid-rod structure, and the largest CTE anisotropy was observed for PMDA-ODA containing an ether linkage with an extraordinarily negative CTE_a (−44 ppm K^–1). The values and anisotropy of the CTEs strongly depended on the crystalline structure, whereas the CVEs were negatively correlated with the weight density, regardless of the PI type. The correlation was explained using the Grüneisen equation, (∂V/∂T)_P/V = γC_v,interχ, assuming that isothermal compressibility χ dominates the equation. An increase in the weight density and/or molecular weight of repeating units effectively suppresses the CVEs of crystalline PIs.