POSITRON-ANNIHILATION LIFETIME MEASUREMENTS OF FREE-VOLUME IN WHOLLY AROMATIC COPOLYESTERS AND BLENDS

The temperature dependence of free volume in random copolyesters of hy droxybenzoic acid (HBA) and hydroxynaphthoic acid (HNA) has been studi ed from -50 to +350 degrees C by measuring the average lifetime, tau(3 ), and intensity, I-3, of ortho-positronium (o-Ps) annihilation. These parameters have also been used to determine the fractional free volum e, h(ps). Significant changes in the temperature coefficients of tau(3 ),I-3 and hps are observed at the alpha-transition temperature, T-alph a, and at the melt transition temperature, T-m. The results indicate t hat both the average size and the number of free volume cavities occup ied by positronium increase at T-alpha, continue to increase uniformly with temperature up to T-m, and then level off. A miscible blend of 7 5/25 and 30/70 copoly(HBA/HNA) with overall monomer ratio 60/40 exhibi ts a single melt transition well below those of the component copolyme rs or a random copolymer of the same composition, suggesting that the chains are less ordered in the blend. Comparison of PALS data for thes e systems indicates that the blend has a larger h(ps) below T-m, which is primarily due to a larger number of cavities accessible to o-Ps, a nd a substantially larger temperature coefficient of h(ps) between T-a lpha and T-m. In the nematic melt, the free volume of the blend decrea ses to that of the pure copolyesters, indicating that h(ps) in the nem atic phase is independent of composition. Compared to other amorphous or semicrystalline polymers, the HBA/HNA copolymers have both fewer de tectable free volume cavities and smaller average cavity sizes, presum ably due to the ''quenched nematic'' morphology of the noncrystalline regions.