Influence of liquid crystalline order on the dielectric relaxation of random copolyesters of PET, PEN, and PHB

The dielectric constant and dielectric loss values for random copolyesters of poly(ethylene terephthalate) (PET), poly(ethylene 2,6-naphthalenedicarboxylate) (PEN), and poly(p-hydroxybenzoic acid) (PHB) have been determined in the 10(3)-10(6) Hz frequency range. The variation of the dielectric properties with temperature has been related mainly to two processes: (1) The alpha relaxation, which is associated to the glass transition and which has been shown to be dependent on the molecular order of the systems. Copolymers with liquid crystalline order exhibit an alpha relaxation process occurring at lower temperatures than the amorphous ones. (2) The beta process which has been assigned to a local motion of the ester groups attached to both sides of the aromatic rings. The beta process also depends on the molecular order. The existence of a liquid crystalline state increases the dipolar correlation of both the beta and the alpha relaxation processes.