Effects of chain packing and structural isomerism on the anisotropic linear and volumetric thermal expansion behaviors of polyimide films

• Published in: Polymer (Guilford) Vol. 146; pp. 386 - 395
• Main Authors: Okada, Tomohiro, Ishige, Ryohei, Ando, Shinji
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 20.06.2018; Elsevier BV
• Subjects: Anisotropy; Chain packing; Glass transition temperature; Interferometry; Linkages; Local motion; Low temperature; Packing; Polyimide; Polyimide resins; Polymerization; Polymers; Refractivity; Structural isomerism; Thermal expansion; Transition temperatures; Volumetric thermal expansion; Structural isomerism; Polyimide; Chain packing; Anisotropy; Volumetric thermal expansion; Local motion
• ISSN: 0032-3861; 1873-2291
• DOI: 10.1016/j.polymer.2018.05.059

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 (Tg). 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 Tgs because π-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. [Display omitted] •Linear & volumetric thermal expansion behavior of isomeric polyimides were examined.•PI film with high refractive index exhibited smaller CVE due to dense chain packing.•PI films with bent m-phenylene exhibited smaller CVE than those with linear p-phenylene.