Processable PMR-type polyimides: Process-property relationships, curing kinetics, and thermooxidative stability

The rheological and structural characteristics of polyimides with enhanced melt flow have been investigated. The polyimides were based on 2,3,3', 4'-biphenyltertracarboxylic dianhydride (PBDA) and a mixture of a diamine, 4,4' (1,4-phenylene-bismethylene) bisaniline (BAX) and a triamine, 1,3,5-Tris (4-aminophenoxybenzene), TAB, where the amount of TAB was 4 and 8%. Melt viscosities of these polymers suggest that they are processable by resin infusion methods. Although curing occurs through the phenylethynyl endcap, the steric and electronic differences of the three amines results in different cure kinetics and heterogeneous crosslinking. This is manifested in multiple T-g values in the differential scanning calorimetry (DSC) scan of the cured samples. Rheological and DSC kinetic studies of the cure behavior indicate that the sample with 4% TAB cures more quickly than the system with 8% TAB and it has a lower activation energy (147 versus 185 kJ mole(-1)). Thermal gravimetric analysis (TGA) scans indicate that both TAB based samples are more thermally stable than PMR-15. The lower activation energy for 8% TAB, relative to 4% TAB (147.0 versus 170.2 kJ mole(-1)) suggests the additional branching present decreases the thermal stability.