Thermal Cycloimidization Study of Polyimides and Poly(amide imide)s Synthesized from Low-Temperature Solution Polymerization

Polyimides (PI)s and Poly(amide imide)s (PAI)s containing di/tri-structural arrangements of monomer constituents were synthesized from 1,2,4,5 benzenetetracarboxylic anhydride/3,3′,4,4′ benzophenonetetracarboxylic dianhydride, diamines and trimellitic dianhydride chloride, i.e., a mixture of dianhydrides and diamines, using low-temperature solution polymerization. The majority of the resulting polymers were readily soluble in polar aprotic solvents such as dimethylformamide, dimethylsulfoxide, dimethylacetamide, etc. Thermal cycloimidization studies by thermogravimetic analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR) revealed that the rate of cycloimidization was much faster during the initial 15 min, and then occurred at much slower rates till the completion. The activation energies (Ea) for thermal cycloimidization of poly(amic acid)s was calculated from the rate of mass loss of the polymer heated at different temperatures according to the Coats and Redfern method. PI and PAIs derived from diamines containing single cyclic/benzene ring structures, such as m-phenylenediamine and 1,3-cyclohexanebis(methylamine), showed less amount of char yield than those having two benzene rings and separated by methylene, sulfone, or ether linkages. The structure of both diamine and trimellitic chloride had a profound effect on the polymer chain mobility, as indicated by the big variation in the glass transition temperatures. Thermally stable polymers were developed into membranes and tested for their mechanical strength by dynamic mechanical analysis(DMA).