Thermal conversion rates of the polyamic acid derived from pyromellitic dianhydride and 4,4′‐diaminodiphenyl ether are presented. Formation of the imide ring proceeds by fast and slow first‐order processes. Tertiary amines give ring closures faster by a factor of 10 than the free acid. The enhancement is an activation entropy rather than an activation energy effect.

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Crystalline homopolyimides are shown to occur by DSC from 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and either 1,3-bis(4-aminophenoxy)benzene (134APB) or 1,12-dodecanediamine (C12). Additionally, copolyimides synthesized from these monomers are crystalline in all compositions studied. Each composition was found to have a singular glass transition temperature, which is rectilinearly dependent on the percent of 134APB/C12. A similar relationship is found for the crystallization temperature but not for the apparent melting temperature. Upon heating, a triple-melting behavior was observed when materials were crystallized at large degrees of supercooling. Otherwise, a conventional double-melting behavior was seen. The equilibrium melting temperature (Tm°) was estimated using the Hoffman-Weeks method. Results indicate that compositions with 134APB composition >40% are dominated by the 134APB/BPDA polyimide phase, and below it by the C12/BPDA polyimide phase. The value of Tm°for the 134APB/BPDA polyimide is about 410 °C and for the C12/BPDA polyimide is about 243 °C.

A study of crystallization, melting, and morphology is carried out for homopolyimides derived from 3,3‘,4,4‘-biphenyltetracarboxylic dianhydride (BPDA) and either 1,3-bis(4-aminophenoxy)benzene (134APB) or 1,12-dodecanediamine (C12), as well as copolyimides containing different compositions of 134APB and C12. For copolyimides, although a singular glass transition was seen in each composition, the crystallization and melting behavior of all compositions is dominated by either 134APB/BPDA or C12/BPDA homopolyimide crystals. The bulk crystallization rate is significantly reduced in compositions with high C12 content. Compositions of copolyimides with 40/60 and 20/80 134APB/C12 ratios additionally exhibited dual maxima in crystallization rates which can be attributed to the phase separation of the two crystals. Since no large depression in the spherulite growth rate was found, the decrease in the bulk crystallization rate is primarily due to the decrease in nucleation density. The last endotherm in the triple-melting behavior is associated with a reorganization process rather than the polymorphism, as verified by DSC and X-ray diffraction. Finally, two major spherulite appearances were identified:  (1) Maltese-cross spherulites with negative birefringence in 134APB/BPDA dominant systems; (2) ringed-like spherulites with no preferred birefringence in C12/BPDA dominant systems. Both morphologies consist of lamellar crystals with the 134APB/BPDA crystals having a broader thickness than C12/BPDA, as was determined by small angle X-ray scattering.