Extrusion-blended and injection-molded PEI/PBT blends were found to be miscible whatever the composition. The processability of the blends clearly improved with the presence of PBT. The melt pressure at the exit was seen to be a parameter as representative of the processability of the blends as the torque of blending. In the blends with 80 and 90% PBT, a positive volume of mixing and the maintenance of the crystallinity of PBT were seen. However, in the rest of the blends, negative volumes of mixing and important decreases in the crystallinity of PBT were found. These solid state features gave rise to a ductility similar to that of the pure PEI and to a synergism of the modulus of elasticity and of the yield stress in the 90/10 and 80/20 blends such that the values were higher than those of either of the pure components.

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Blends of two thermotropic copolyesters, poly(ethylene terephthalate/p‐hydroxy benzoic acid) (20/80), EB, and poly(p‐hydroxy benzoic acid/2,6‐hydroxy naphthoic acid) (73/27), BN, were obtained in the melt state by direct mixing in an injection machine throughout the full composition range. They comprised an almost pure EB phase and a BN‐rich phase that contained some reacted EB. Important synergisms with most of the values above those of the two pure blend components were found at most compositions in the modulus of elasticity and the tensile strength. The synergisms appeared to be due to the concomitant effect mainly of an orientation of the components higher in the blends than in the pure components, and to a lesser extent, of a blending‐induced specific volume decrease.

Blends of two different thermotropic liquid crystalline polymers, poly(p‐hydroxy benzoic acid‐co‐2, 6‐hydroxy naphthoic acid), VA, and poly(p‐hydroxybenzoic acid‐co‐terephthalic acid‐co‐aminophenol), VB, were obtained by direct injection molding throughout the full composition range. The blends were almost fully immiscible apart from a low degree of reaction. This, as well as the nature of the blends, probably influenced their mechanical properties. Enhanced properties were obtained both in the modulus of elasticity and in the tensile strength, at all the compositions studied, with absolute maxima at VB contents between 50% and 75%. These synergisms were mainly due to a greater orientation of the components in the blends than in the pure components.

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