Soluble and processable thermoplastic hybrid polyimides containing POSS in main chains

Abstract

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 T₈ 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 N₂ and air. In particular, 6b exhibited the highest value of 1% weight loss among the tested polyimides, at 460 °C under N₂ 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.