Segmented Polyimides with Poly(ethylene oxide) Blocks Exhibiting Liquid Crystallinity

Polyimides based on BPDA and alpha,omega-bis-(4-aminophenoxy)oxyethylene units with ethylene oxide (EO) sequences in the diamine moiety were readily prepared from biphenyl dianhydride and diamines with terminal aminophenoxy units. Such polymers with three to six EO units exhibit incontrovertible liquid-crystalline behavior at elevated temperatures. The temperature regimes for liquid-crystalline behavior for the polymers with sequences of three and four EO units are close to the thermal decomposition. The polymers with five or six EO units exhibit liquid-crystalline phases stable over temperature ranges of 50 °C or more. The liquid crystalline morphology was smectic A in all cases. Enthalpies of transition between the crystalline and liquid-crystalline phases and liquid crystalline and isotropic liquid phases were on the order of 20 J g-1 and 10 J g-1, respectively. SAXS experiments established that the polymers with five or six EO units adopt layered morphologies and it was concluded that in order to accommodate the polymer repeat units within the observed d-spacings the EO sequences must be in a coiled conformation, and that, in order to achieve the decrease in d-spacing on cooling from the liquid crystalline to the crystalline phase, the aromatic bis(N-phenylphthalimide) units must tilt from their orientation perpendicular to their layers, possibly and rapidly passing through an intermediate smectic C structure. The several observations established that it is possible to prepare segmented polyimides containing oxyethylene sequences that exhibit liquid crystalline phases at elevated temperatures of about 300 °C and which are stable over a temperature range of about 70 °C.