Effects of CO2 treatments on the dual glassy relaxations and dual melting peaks in poly(ethylene terephthalate)

In this study, poly(ethylene terephthalate) (PET), an important packaging material for carbonated beverages, was investigated on its glassy relaxations and melt crystallizations in CO2 in a high-pressure differential scanning calorimeter (PDSC) and a high-pressure thermostat chamber as a function of CO2 pressure, time, CO2 depressurization rate, and PET crystallinity. DSC measurements found a low glass transition temperature (TgL) at near 50 °C in the wholly amorphous PET and a high glass transition temperature (TgH) at near 70 °C in the PET sample with a fairly high crystallinity (Xc 50%). Both TgL and TgH decreased with increasing time in CO2, attributed to plasticization by CO2. PET sample with a moderate crystallinity (Xc 42%), however, exhibited both TgL and TgH corresponding to the relaxations of the dual amorphous phases as confirmed by dynamic mechanical analysis, with the TgL assigned to the free amorphous phase and the TgH to the constrained amorphous phase. The TgL is much farther apart from TgH than those obtained in N2. The TgL and TgH in PET with a moderate crystallinity unexpectedly appeared to be insignificantly varied with time in CO2; however, the magnitude of the TgL signal increased but the TgH signal decreased with increasing time in CO2, attributed to disentanglement of polymer chains by CO2. Dual melting peaks in PET were found after nonisothermal crystallization from the melt in CO2. Temperature-modulated DSC (TMDSC) analysis indicated that melting-recrystallization model and double lamellar thickness model were both responsible for the appearance of the dual melting peaks.

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