Spontaneous Chain Orientation of Aromatic Polyimides Evolved during Thermal lmidization from Shear-Oriented Glassy Liquid Crystalline Precursors

摘要

A series of poly(amic ester)s, PMm, based on pyromellitic acid derivatives with long n-alkylene side chains [-(CH2),14], were synthesized to prepare low-temperature lyotropic liquid crystalline (LC) precursors for a fully aromatic rigid-rod polyimide (PI). Highly oriented LC precursor films (m > 8) were prepared by shearing at ambient temperature. Although the polydispersity of PMm was not controlled, the PMm solution exhibited a lyotropic smectic phase in which the main chains were parallel to the layer normal. After prebaking the lyotropic solution (70 degrees C), uniaidal orientation order parameter of the main chain was maintained at similar to 0.5 during heating from 70 to 250 degrees C before imidization. Synchrotron radiation wide-angle X-ray diffraction and polarized Fourier transform infrared spectroscopy revealed that the orientation order spontaneously increased (0.5-0.9) during imidization (>270 degrees C), even for free-standing films. This unique phenomenon distinctly differs from the behavior of conventional amorphous precursors. In the present case, the rigid PI segments with longer persistence length spontaneously grow along the n-director of the LC matrix of the precursor to form highly oriented homogeneous PI films.