Synthesis and properties of polyimides containing hexaisobutyl-substituted T8 cages in their main chains

Polymerization of a para-substituted bis(3-aminopropyl) hexaisobutyl-substituted T-8 cage with pyromellitic dianhydride (PMDA) and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) resulted in corresponding polyimides containing T-8 cages in the main chains, denoted as T-8/PMDA polyimide and T-8/6FDA polyimide, respectively. The optical transmittance of the yellow-colored freestanding film of T-8/PMDA polyimide was over 90% in the visible region between 780 and 475 nm with a film thickness of 0.1 mm and the absorption edge was observed at approximately 590 nm. On the other hand, the film of T-8/6FDA polyimide showed excellent transparency in the visible region with an excellent optical transparency of over 90% even at 360 nm. DSC analysis of both the polyimides showed no glass transition and melt behavior between room temperature and 400 degrees C. Contact angles of water for the films of T-8/PMDA polyimide and T-8/6FDA polyimide were 97 +/- 2 degrees and 96 +/- 5 degrees, respectively, and both are significantly higher than that of poly(pyromellitic dianhydride-oxydianiline) (PMDA-ODA) polyimide and comparable to that of polyethylene. Martens' hardness and coefficient of thermal expansion (CTE) of T-8/PMDA polyimide were significantly lower and higher than that of PMDA-ODA polyimide, respectively, and these values are comparable to those of polyethylene. The replacement of PMDA with 6FDA significantly improved Martens' hardness and CTE. The XRD analysis for the T-8-containing polyimides indicates a highly packed nanostructure of the hexaisobutyl-T-8 unit in the polymer filmsand the isobutyl-T-8 unit in T-8/6FDA polyimide has a more packed structure than that in T-8/PMDA polyimide.