Soluble and processable thermoplastic hybrid polyimides containing POSS in main chains

Colorless and soluble thermoplastic polyimides are promising high-performance polymer materials suitable for a wide range of industrial applications. Polyimides with double-decker-shaped silsesquioxanes (DDSQ) in their main chains are beneficial while designing high-performance materials. In this study, we polymerized 3,13-bis(3-aminopropyl)-DDSQ ( 3 ) with several tetracarboxylic dianhydrides and studied their thermal, optical, and mechanical properties. Their properties were compared with those of polyimides using 3,13-bis(4-aminophenyl)-DDSQ ( 1 ) or bis(3-aminopropyl)hexaisobutyl-substituted T 8 cage ( 2 ) instead of 3 . The polymerization of 3 with pyromellitic dianhydride ( 4a ) or 4,4′-(hexafluoroisopropylidene)diphthalic anhydride ( 4b ) resulted in soluble thermoplastic polyimides ( 6a and 6b ) with low glass transition temperatures of 135 and 136 °C, respectively. The thermal stability of the resulting polyimides was estimated via thermogravimetric analysis under N 2 and air. In particular, 6b exhibited the highest value of 1% weight loss among the tested polyimides, at 460 °C under N 2 and 436 °C under air. Freestanding films of 6a and 6b were obtained by casting the polymer solutions in THF onto Teflon sheets. The optical transmittance of a colorless film of 6b was over 80% in the visible region between 780 and 380 nm. The elastic modulus of 6b was significantly lower than that of the fully aromatic polyimide, poly(pyromellitic dianhydride-oxydianiline) (PMDA-ODA), which was also lower than that of 6a . These observations suggest that the combination of 3 with 4b produced colorless, low-elasticity, soluble, and highly heat-resistant polyimides. The combination of 3,13-bis(3-aminopropyl)-double-decker-shaped-silsesquioxane and a fluorinated dianhydride produces a flexible polyimide with a low T g , high heat resistance, and excellent solvent solubility. The fluorinated dianhydride loosened the interchain packing to suppress CT interactions, leading to the formation of a colorless polyimide.