Thermal, dielectric, and mechanical relaxation in poly(benzimidazole)/poly(etherimide) blends

Thermal, dielectric, and mechanical relaxation measurements on miscible blends of polybenzimidazole (PBI) and a polyetherimide (PEI, Ultem 1000) prepared by solution casting from dimethylacetamide (DMAc) reveal a number of structurally related features. Annealing below the glass transition temperature induces an enthalpy relaxation process typical of single‐phase glasses of nonequilibrium structure. Dielectric relaxation experiments on samples annealed at ambient conditions reveal two relaxation processes below 400°C. At lower temperatures (50–200°C), the desorption of water is observed. Above 200°C in the first run, a composition‐dependent relaxation is seen at the highest frequencies (100 kHz) while a relaxation approximately independent of composition appears in the second run. The latter corresponds to the glass transition of the PEI phase. The glass transition of PBI at this frequency is above the degradation temperature. At lower frequencies a strongly dispersive electrode polarization process masks the high‐temperature relaxations. Dynamic mechanical results show similar features with respect to phase separation; the main difference is that the tan δ curves in the first run are complicated by the desorption of residual solvent.