Anisotropic linear and volumetric thermal expansion behaviors of four types of isomeric polyimides (PIs) were investigated based on thermal mechanical analyses and optical interferometric measurements. Among PI films of poly (p-phenylene 3,3',4,4'-diphenyldiimide) (sBPDA/PPD) with different chain packing densities, a film with a higher refractive index exhibited a smaller coefficient of volumetric thermal expansion (CVE), indicating that a smaller amount of free volume leads to a smaller CVE even at much lower temperatures than the glass transition temperature (T-g). The PI films containing quasi-linear sBPDA structures exhibited smaller CVEs than those containing bent 2,3',3,4'-diphenyldiimide (aBPDA) structures, whereas the PI films with bent m-phenylene (MPD) linkages exhibited smaller CVEs than those containing linear p-phenylene (PPD) linkages. It is noteworthy that sBPDA/MPD exhibited a significantly smaller CVE (113.7 ppm/K) than that of sBPDA/PPD despite its lower packing coefficient. This value could be the smallest CVE among solid polymers. Based on the dynamic mechanical analyses and the consideration of stereochemical structures with conformational energy calculations, the local molecular motions of PIs containing MPD are more restricted in the solid state than those with PPD even below their T(g)s because pi-flip rotation at the m-phenylene linkage is sterically inhibited in the former, while that at the p-phenylene linkage is allowed in the latter. (C) 2018 Elsevier Ltd. All rights reserved.