Abstract
Nine poly(amic acid)s (PAAs) were synthesized by reacting butane-1,4-diyl bis(1,3-dioxo-1,3-dihydroisobenzofuran-5-carboxylate) dianhydride with various diamine monomers, including m-xylylenediamine, p-xylylenediamine, 4,4′-oxydianiline, bis(3-aminophenyl) sulfone, bis[4-(3-aminophenoxy)phenyl] sulfone, 2,2′-bis(trifluoromethyl)benzidine, N,N′-[2,2′-bis(trifluoromethyl)-4,4′-biphenylene]bis(3-aminobenzamide), 2,2-bis[4-(4-aminophenoxy)phenyl]propane, and 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane. These PAAs were subsequently converted into colorless and transparent polyimide (CPI) films via thermal imidization under various heat treatment conditions. To achieve CPI films with suppressed charge transfer complex formation, the selected diamine monomers featured bent molecular structures, strong electron-withdrawing substituents such as –CF3 or –SO2–, or ether/ketone functional groups incorporated within the backbone. The thermomechanical properties, optical transparency, and solubility of the resulting CPI films were systematically evaluated, and the structure–property relationships between the monomers and CPI film performance were elucidated. Overall, CPI films derived from aromatic or linear main-chain structures exhibited excellent thermal and mechanical properties. In contrast, films incorporating bent structures with polar functional groups or electron-withdrawing substituents in the main chain showed superior optical transparency and solubility.