Miscibility in polybenzimidazole/polyimide sulphone blends: a comparison of blends containing fluorinated and non-fluorinated polyimide sulphone

A comparison of miscibility is presented for blends containing a fluorinated and a non-fluorinated polyimide sulphone, of otherwise comparable structure, with polybenzimidazole [PBI, poly(2,2′-( m-phenylene)-5,5′-bibenzimidazole]. The miscibility behaviour was studied using Fourier transform infra-red ( FTi.r.) spectroscopy and thermal analysis. The NH and CO absorbance regions in the FTi.r. spectra show that the fluorine-containing group in the hexafluoroisopropylidene-containing polyimide (PI1) decreases the miscibility of this polyimide with PBI, whereas the carbonyl group in the benzophenone moiety in the second polyimide (PI2) enhances miscibility. In blends with low PBI content, PI2 forms uniform hydrogen bonds with PBI, and the strength of the hydrogen bonding is maximized at a blend composition of 50 50 wt% . FTi.r. spectra of the blends were also obtained as a function of temperature. Above the glass transition temperature the PBI/PI2 blends contain two phases: pure PI2 (present in small amounts) and the predominant miscible phase. Analysis of the carbonyl band could be related to glass transition behaviour in the blend. Finally, an enhanced degradation of PI2 in the presence of PBI was observed and is discussed.