Structural investigations and swelling behavior of 6FDA copolyimide thin films

The effects of swelling on chain distance, free volume size and orientation of functional groups of copolyimides containing carboxyl groups were examined using wide‐angle X‐ray diffraction (WAXD), positron annihilation lifetime spectroscopy (PALS) and Fourier transform infrared (FTIR) spectroscopy. A 2,3‐dimethylbenzothiophene/ n ‐dodecane mixture (0.3 wt%/99.7 wt%) was used as a swelling agent, as it is a binary model mixture to investigate membrane‐based desulfurization of kerosene. Thin film samples were prepared from copolyimide structures which had been synthesized via a polycondensation reaction of 4,4′‐hexafluoroisopropylidene di(phthalic anhydride), 3,5‐diaminobenzoic acid, 2,3,5,6‐tetramethyl‐1,4‐phenylenediamine and 2,4,6‐trimethyl‐1,3‐phenylenediamine. A relationship between a lower amount of 3,5‐diaminobenzoic acid present in the copolyimide backbone and a higher amount of solvent sorption in the film was found. This was attributed to the fact that the carboxyl groups on the polymer backbone can form hydrogen bonds, reducing sorption and swelling phenomena. PALS measurements for the different structures showed significantly smaller hole sizes for the free volume sites of the copolyimide containing fewer carboxyl groups. Although WAXD of film samples after time‐dependent swelling indicated that the chain distance initially decreased slightly, FTIR experiments using polarized light did not show any significant orientation changes in the polymer chains.