Structural aspects of high temperature thermosets - Bismaleimide/propargyl terminated resin system-polymerization and degradation studies

Bispropargyl ether of bisphenol-A (BPEBPA), 4,4′-bismaleimido diphenyl ether (BMIE) and a blend consisting 60 mol% of BPEBPA and 40 mol% of BMIE are prepared. The materials are structurally characterized by FTIR. The curing characteristics of the monomers are measured by FTIR and DSC. The results indicated that BPEBPA-BMIE blend has low ΔH_cure (J g⁻¹) for the thermal polymerization and the whole temperature window for the exothermic curing reaction is shifted to lower temperature compared to BPEBPA. Borchardt and Daniels method is used to study the cure kinetics of the materials. The thermal curing of BMIE requires activation energy of 156.0 kJ mol⁻¹ whereas BPEBPA needs slightly higher activation energy (177.2 kJ mol⁻¹). From the TG studies, it can be concluded that the cured BPEBPA exhibits higher thermal stability than the cured BMIE due to the more complex network structure that are formed during thermal polymerization of BPEBPA. Dharwadkar and Kharkhanavala equation is employed to calculate the activation energy needed for the thermal degradation of the thermally cured materials. BPEBPA shows much higher activation energy (65.5 kJ mol⁻¹) for thermal degradation indicating the higher thermal stability over the other two materials (BMIE: 42.5 kJ mol⁻¹ and BPEBPA-BMIE blend: 46.9 kJ mol⁻¹). The isothermal degradation of cured materials is effected in nitrogen atmosphere for constant time interval (10 min). The detailed analysis of the degradation products by GC-MS revealed the formation of phenols and several substituted phenols. This finding hints that the competitive C-C and C-O scissions of the chromene ring units formed via the Claisen rearrangement of the aryl propargyl ether system present in BPEBPA is operative.