The degree of in-plane molecular orientation of various polyimides (PIs) and their precursors, poly(amic acid)s (PAAs), were estimated by measuring the visible dichroic absorption at an incidence angle for a rod-like dye (perylenediimide, PEDI) dispersed in the matrices. The effects of PI chain structure, film thickness, heating rate, and residual solvent on a spontaneous in-plane orientation phenomenon were examined to fully understand the mechanism. All PAA films cast on a substrate showed the low degrees of in-plane orientation of the chain axis, nearly independent of the chain structure. Upon thermal imidization of the PAA films adhered on a substrate, a striking spontaneous in-plane orientation behavior was observed for some PI systems with rigid chains; in contrast to that, some flexible PI systems showed no spontaneous behavior. Cure of the PAA films adhered on a substrate induced the spontaneous orientation even if the films were considerably thick (∼50 μm); in contrast to that, the cure of the free-standing thick film did not. However, upon cure of the free-standing thin films (∼10 μm), the spontaneous orientation behavior was observed. For rigid PI systems in which interchain stacking preferentially occurs, thermal cure of the PAA films on a substrate forms liquid-crystal-like highly oriented regions, and simultaneously, apparent stretching (due to constraint of film contraction during imidization) promotes the molecular orientation further. The mechanism is closely associated with a “cooperative effect” in which the neighboring chains enhance the molecular orientation of each other during cure. In addition, structural changes (orientations of the chain axis and a molecular plane and molecular packing) upon stepwise annealing were followed. Polarized infrared absorption spectra measured at an incidence angle demonstrated that the phthalimide molecular plane in the PI film on a silicon wafer aligns somewhat parallel to the film plane, but no significant orientational and conformational changes occurred upon stepwise annealing.

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