A new group of miscible mixtures (molecular composites) has been studied by means of differential scanning calorimetry, dielectric relaxation and dynamic mechanical techniques. Three kinds of rigid aromatic polyimides (PI) and a commercial-grade poly(ether imide) (PEI) were mixed by dissolving them in m-cresol and subsequently coagulating in methanol. The mixtures show a single glass transition temperature (Tg) in all compositions in which the Tg increases with increasing PI content. These Tg values are reproducible in repeated heating cycles, suggesting the true miscibility of the blends. This miscibility may be attributed to the similarity of imide structure between the rigid PI and PEI matrix. The bulk PI/PEI indeed shows improved modulus and tensile strength as compared to the pure PEI matrix.
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