The dielectric properties of liquid-crystal random copolyesters of 4-hydroxybenzoic acid and 2-hydroxy 6-naphthoic acid

Dielectric measurments made from 1 Hz to 10 kHz and from ‐80 to 150°C on random copolyesters of 4‐hydroxybenzoic acid (HBA) and 2‐hydroxy 6‐naphthoic acid (HNA) reveal three relaxation processes at low frequencies rather than the two seen at audio frequencies. The higher temperature α process is similar to a glass transition, with a high activation energy ∼ 170 kcal/mol, but the two lower temperature processes are identified as local mode processes associated with the HNA and HBA moieties, respectively. It is shown that the observed value of the permittivity above the glass transition is lower than that which would be expected for ester groups rotating freely about the local chain axis, and it is suggested that this may be due to several factors including immobilization of dipoles in the crystalline regions, correlated motion of dipoles, and librational rather than rotational motion.