Glass transition and cold crystallization in carbon dioxide treated poly(ethylene terephthalate)

An amorphous poly(ethylene terephthalate) (aPET) and a semicrystalline poly(ethylene terephthalate) obtained through the annealing of aPET at 110°C for 40 min (aPET‐110‐40) were treated in carbon dioxide (CO 2 ) at 1500 psi and 35°C for 1 h followed by treatment in a vacuum for various times to make samples containing various amount of CO 2 residues in these two CO 2 ‐treated samples. Glass transition and cold crystallization as a function of the amount of CO 2 residues in these two CO 2 ‐treated samples were investigated by temperature‐modulated differential scanning calorimetry (TMDSC) and dynamic mechanical analysis (DMA). The CO 2 residues were found to not only depress the glass‐transition temperature ( T g ) but also facilitate cold crystallization in both samples. The depressed T g in both CO 2 ‐treated poly(ethylene terephthalate) samples was roughly inversely proportional to amount of CO 2 residues and was independent of the crystallinity of the poly(ethylene terephthalate) sample. The nonreversing curves of TMDSC data clearly indicated that both samples exhibited a big overshoot peak around the glass transition. This overshoot peak occurred at lower temperatures and was smaller in magnitude for samples containing more CO 2 residues. The TMDSC nonreversing curves also indicated that aPET exhibited a clear cold‐crystallization exotherm at 120.0°C, but aPET‐110‐40 exhibited two cold‐crystallization exotherms at 109.2 and 127.4°C. The two cold crystallizations in the CO 2 ‐treated aPET‐110‐40 became one after vacuum treatment. The DMA data exhibited multiple tan δ peaks in both CO 2 ‐treated poly(ethylene terephthalate) samples. These multiple tan δ peaks, attributed to multiple amorphous phases, tended to shift to higher temperatures for longer vacuum times. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009