Thermal degradation mechanism and thermal mechanical properties of two high-performance aromatic polyimide fibers

Two organo-soluble aromatic polyimides have been synthesized by introducing pendant groups of trifluromethyl or methyl onto the 2- and 2'-positions of a biphenyl diamine and reacting each of them with biphenyltetracarboxylic dianhydride (BPDA): 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (PFMB) o r 2,2'-dimethyl-4,4'-diaminobiphenyl (DMB). These two polyimides, BPDA-PFMB and BPDA-DMB, are prepared via a one-step polycondensation method, and the y can be fabricated into fibers, films, and other application forms. Our re search shows that in fiber applications, these two fibers exhibit excellent mechanical properties and outstanding thermal and thermooxidative stabilit y. Their longterm mechanical tensile performance at high temperatures is fo und to be critically associated with the type of side pendant groups at the 2- and 2'-positions of the diamines. High-resolution pyrolysis-gas chromat ography/mass spectrometry (PyGC-MS) results show that the pyrorgams of thes e two polyimides possess more than 16 pyrolyzates. Based on the composition s and distributions of the pyrolyzates at different temperatures, the therm al degradation mechanisms for both of the polyimides are found to be the sa me.