The estimation of phase modifications in polymers by the method of “molecular probes” in gas chromatography—VI. The study of the transitions in poly(ethylene terephthalate)

Multiple transitions have been identified by inverse gas-chromatography for fractions of poly(ethylene terephthalate) with $\text{M}$_n between 1.47 and 2.69 × 10⁴. Depending on the thermal treatment, at least two liquid-liquid transitions T_ll⁽¹⁾ and T_ll⁽²⁾ have been observed in addition to T_g and T_m; this feature is generally considered as characteristic of semi-crystalline polymers. The similarity of the dependence of T_g and T_ll on the molecular weight of the polymer probes suggests a common molecular origin of these phenomena; the fact that T_ll >T_g indicates that the T_ll transitions involve longer chain segments. The melting temperature decreases with increasing molecular weight, suggesting again that the crystallinity and/or the size distribution will enlarge as the molecular weight of the polymer is made higher.