Positron Annihilation Lifetime Measurements of Free Volume in Wholly Aromatic Copolyesters and Blends

Abstract

The temperature dependence of free volume in random copolyesters of hydroxybenzoic acid (HBA) and hydroxynaphthoic acid (HNA) has been studied from -50 to +350 °C by measuring the average lifetime, τ 3 , and intensity, I 3 , of ortho-positronium (o-Ps) annihilation. These parameters have also been used to determine the fractional free volume, h ps . Significant changes in the temperature coefficients of τ 3 , I 3 , and h ps are observed at the α-transition temperature, T α , and at the melt transition temperature, T m . The results indicate that both the average size and the number of free volume cavities occupied by positronium increase at T α , continue to increase uniformly with temperature up to T m , and then level off. A miscible blend of 75/25 and 30/70 copoly(HBA/HNA) with overall monomer ratio 60/40 exhibits a single melt transition well below those of the component copolymers or a random copolymer of the same composition, suggesting that the chains are less ordered in the blend. Comparison of PALS data for these 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, and a substantially larger temperature coefficient of h ps between T α and T m . In the nematic melt, the free volume of the blend decreases to that of the pure copolyesters, indicating that h ps in the nematic phase is independent of composition. Compared to other amorphous or semicrystalline polymers, the HBA/HNA copolymers have both fewer detectable free volume cavities and smaller average cavity sizes, presumably due to the quenched nematic morphology of the noncrystalline regions.