Material BehaviourEffect of molecular weight on crystallization, melting behavior and morphology of poly(trimethylene terephalate)

The crystallization behavior of poly(trimethylene terephthalate) (PTT) as a function of molecular weight was investigated under non-isothermal and isothermal conditions using differential scanning calorimetry (DSC), temperature-modulated differential scanning analysis (TMDSC) and polarized light optical microscopy (PLOM). The overall rate of bulk crystallization decreased with increasing molecular weight. The enthalpy for non-isothermal cold crystallization (ΔHc), the melting enthalpy (ΔHm), the melting point (Tm) and the enthalpy of melting crystallization (ΔHm,c) decreases with increasing molecular weight of PTT. This indicates that the cold-crystallization rate and the crystallization rate of PTT during cooling decrease as the molecular weight of PTT increases. The effective energy barrier for the isothermal crystallization process and both the t0.5 and t0.5−1 values suggest that the relative crystallization ability for these PTTs decreases with increasing molecular weight. The initialization crystallinity and equilibrium crystallinity of PTT decreases as the molecular weight increases, this also confirmed that the crystallization ability of PTT decreases with increasing molecular weight. Triple-melting endotherms were evident: the peaks were labeled as I, II and III for the low-, middle- and high-temperature melting endotherms, respectively. The positions of both peaks I and II steadily increase with increasing crystallization temperature, heating rate and molecular weight of PTT. The morphology studies indicated that all of the spherulites are in rings of alternating yellow and white bands, the spherulites apparently became coarser with increasing crystallization temperature. The spherulitie size decreases with increasing molecular weight.