Characterization and thermal degradation of polyimide and polyamide liquid crystalline polymers

The wholly aromatic thermotropic liquid crystalline polymers, polyimide [1,2,4,5-benzentetracarboxylic dianhydride (PMDA) and 1,3-bis [4-(4′-aminophenoxy) cumyl] benzene (BACB)] and polyamide [terephthaloyl chloride (TPC) and BACB] were characterized by FTIR-ATR, and DSC measurements. Three endothermic peak temperatures observed (274, 284, 307 °C) for the polyamide are lower than those observed (277, 297, 337 °C) for the polyimide liquid crystalline polymers. The major difference between the polyamide and polyimide is that their infrared peaks are at 1644 and 1722 cm^-1, respectively. The thermal stability, apparent activation energy and evolved gas analysis of these LCPs was done using thermo-gravimetric analysis (TGA) coupled with Fourier transform infrared (FTIR) spectroscopy. Decomposition results show that the polyamide is much less thermally stable than the polyimide. The evolved gases are found to be H₂O, CO, CO₂ and various hydrocarbon fragments. A substantial amount of CO₂ detected during the decomposition is due to degradation of the carbonyl functional groups from the polyimide liquid crystalline polymers. Activation energies for the initial thermal degradation of this polyimide in nitrogen and air are 236 and 201 kJ/mol, and those for polyamide are 207 and 219 kJ/mol, respectively. A jump in the activation energy is observed around 40% weight loss, beyond which it decreases in the case of polyimide. However, an unusual observation was made during the degradation of polyamide. The apparent activation energy values are found to be higher under air environment than in the nitrogen environment.