Glass transition temperature of copolyesters of PET, PEN and PHB as determined by dynamic mechanical analysis

Abstract Binary and ternary copolymers of poly(ethylene terephthalate) (PET), poly(ethylene-2,6-naphthalene dicarboxylate) (PEN) and poly( p -hydroxybenzoic acid) (PHB) were synthesized and dynamic mechanical measurements at temperatures ranging from −140°C up to the melting point were performed. A linear dependence of the glass transition temperature of the isotropic materials on composition was obtained. Copolymers containing more than about 30 mol% PHB are partly liquid-crystalline, and those containing more than 50 mol% PHB are completely liquid-crystalline. The PET/PEN/PHB ternary copolyester containing equal parts of each of the three components can be obtained at room temperature as a liquid-crystalline glass and as an isotropic glass, depending on thermal history. The glass transition temperature T g of the material in the liquid-crystalline state was found to be about 30°C lower than that of samples in the isotropic state. By extrapolation to 100 mol% PHB, a glass transition temperature of about 120°C was estimated for the PHB homopolymer. Samples containing more than 70 mol% PHB show an additional peak in tan σ, which is attributed to a relaxation process in a disturbed hexagonal crystalline phase. At temperatures below T g in all copolymers investigated several β relaxation maxima were observed. The temperature positions of these maxima do not depend on the composition.