Rheological, thermal and free volume studies of blends containing two thermotropic liquid crystalline polymers

A melt blend of a rigid-chain copolyester and a terpolyester, thermotropic liquid crystalline polymer is described with reference to its miscibility, rheological and free volume properties. Dynamic rheometry is used to characterize the effect of blend concentration on rheological properties, whilst capillary rheometer is used to quantify the efficacy of the reduction in viscosity of the more viscous component at higher shear rates. Dynamic mechanical thermal analysis indicates that the blend becomes two phase structures, transesterification being intrinsically low or sufficiently suppressed by the addition of inhibitor to prevent blocky or random copolymers forming via interchain reactions. There appears to be a limited miscibility of the copolyester in the other terpolyester phase and it is suggested that this may be due to differences in packing and chemistry of the two components. The density and free volume measurements show that the blends are denser (lower fractional free volume) than may be expected from linear additivity, even though the average size of the free volume pores in the blends is greater. This increase in size may be due to increased free volume at the interfaces between the two phases. The overall decrease in fractional free volume may be due to the slight miscibility of one phase in the other, leading to closer packing within that phase, and thus explaining the greater than average density.