Crystallization study of a thermoplastic polyimide (new‐TPI)

Crystallization, melting, and morphology of a thermoplastic polyimide (New‐TPI) containing pyromellitic dianhydride (PMDA) and 3,3'‐bis(4‐aminophenoxy) biphenyl diamine (33BAPB) moieties have been studied. This material showed a glass transition temperature (T_g) of 250°C, an equilibrium melting temperature (T°_m) of 406°C and a heat of fusion (ΔH) for 100% crystallinity of 6.38 kJ/mol. Measurements of the crystallization bulk rate (by DSC) and spherulite growth rate (by optical microscopy) indicated that the maximum crystallization temperature was about 320°C and the crystallization growth process was three‐dimensional under thermal nucleation (the Avrami exponent n ca. 4). The rate of nucleation density was estimated to decrease with increasing temperature, and the product of two crystal surface free energies σ_eσ_o was calculated to be 1176 erg²/cm⁴. The meltgrown spherulite consistently showed a Maltese cross pattern with negative birefringence under cross‐polars. The calculation of polarizability along the three unit cell axes suggested that the crystal b axis may be along the spherulite growth (radial) direction. Two scattering maxima were seen in small‐angle x‐ray scattering (SAXS) profiles. The dominant peak indicated a long period of ca. 20 nm which varied as a function of crystallization temperature. The weak peak at a d‐spacing of 2.5 nm was independent of temperature and has been attributed to the chemical repeat distance determined by Okuyama et al. (indexed as 001). The lamellar thickness l_c, estimated by the correlation function analysis of the SAXS data, was found to be similar to that determined by the Scherrer analysis of the 001 reflection peak. © 1994 John Wiley & Sons, Inc.