INFLUENCE OF THERMAL IMIDIZATION ON THE CRYSTALLIZATION AND MELTING BEHAVIOR OF THE AROMATIC POLYIMIDE, LARC CPI-2

The development of crystallinity during the thermal imidization of a rigid aromatic polyimide synthesized from 1,4-bis(4-aminophenoxy-4'-benzoyl)benzene (1,4-BABB) and oxydiphthalic dianhydride (ODPA) was investigated. Film samples were imidized through a stepwise procedure involving isothermal staging for I h each at 100, 200 and 300 degrees C. Differential scanning calorimetry (d.s.c.), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and thermogravimetric analysis (t.g.a.) were employed to monitor changes in the crystalline state of the films during imidization. The development of crystallinity during thermal imidization was shown to progress through stages that depended upon the time and temperature of the process, wherein the final crystalline index was approximately 46%. This relationship was explained in terms of chain mobility. SAXS analysis suggests that a two-stage, lamellar in-filling process exists for the development of the final crystalline superstructure where the final long spacing is approximately 130 Angstrom. Fourier transform infra-red spectroscopy (FTi.r.) analysis does not reveal the presence of anhydride groups at any stage of the imidization process, indicating that significant chain scission did not occur during processing. Transmission electron microscopy (TEM) analysis of the stepwise imidized films reveals that crystalline growth begins at the glass surface of the film and progresses toward the air surface with increasing thermal staging. TEM analysis of a series of fully imidized films reveals that an amorphous layer remains at the air surface of the films, the thickness of which is dependent upon molecular weight.