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The IR spectra of representative miscible aromatic polybenzimidazole/aromatic polyimide (PBI/PI) blends have been studied. Composition-dependent frequency shifts of up to 55 cm-1 in the N-H stretching band of PBI and up to 6 cm-1 in the carbonyl stretching bands of Pl were observed. These shifts were removed by thermally induced phase separation. The results suggest that the miscibility of these blends derives from specific intermolecular interactions involving the > NH and carbonyl groups. 

Copolymers of or tho (para )fluorostyrene and ortho(para )bromostyrene with a range of copolymer compositions were prepared by free radical polymerization in toluene solution using azobis­(isobutyronitrile). The miscibility and phase behaviour of these copolymers in blends with poly(2,6-dimethyl-l,4-phenylene oxide) (PPO) have been studied by differential scanning calorimetry. Of the four possible copolymer systems, miscibility was observed only for PPO/poly(o-fluorostyrene­co-p-bromostyrene) blends in which the copolymer contains between 11 and 73 mo!% p-bromostyrene. High temperature phase separation in the miscible blends is a function of copolymer composition and of the thermal history. The results can be explained on the basis of the …