Time-dependent changes in the morphology and the physical properties of poly(ethylene terephthalate) (PET) were studied using small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). The properties of the crystalline and mobile amorphous fractions exhibit systematic trends with annealing time during the secondary crystallization stage. Our results suggest that secondary lamellar stacks form from the mobile amorphous fraction in interlamellar liquid pockets after spherulitic impingement. The existence of a bimodal distribution of lamellar stacks has been correlated with the observation of a double melting behavior by DSC. The low endotherm which is associated with the melting of secondary lamellae shifts to higher temperatures with annealing time. Evaluations of the change in crystallinity during the secondary stage by DSC and X-ray diffraction showed excellent agreement. Secondary crystallization results in significant constraints on the mobile amorphous fraction, as evidenced by a shift in the glass transition toward higher temperature with increasing annealing times. The parallel increase in the half width of tan (delta) vs. T peaks in the (alpha) relaxation region indicates a shift in the relaxation spectrum to longer times as secondary crystallization proceeds.
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