Poly(ethylene terephthalate) (PET) sheets of different crystallinity were obtained by annealing the amorphous PET (aPET) sheets at 110 C for various times. The peaks of enthalpy recovery and double cold-crystallization in the annealed aPET samples with different crystallinity were investigated by a temperature-modulated differential scanning calorimeter (TMDSC) and a dynamic mechanical analyzer (DMA). The enthalpy recovery peak around the glass transition temperature was pronounced in TMDSC nonreversing heat flow curves and was found to shift to higher temperatures with higher degrees of crystallinity. The magnitudes of the enthalpy recovery peaks were found to increase with annealing times for samples annealed 30 min but to decrease with annealing times for samples annealed !40 min. The nonreversing curves also found that the samples annealed short times ( 40 min) having low crystallinity exhibited double cold-crystallization peaks (or a major peak with a shoulder) in the region of 108-130 C. For samples annealed long times (!50 min), the cold-crystallization peaks were reduced to one small peak or disappeared because of high crystallinity in these samples. The double cold-crystallization exotherms in samples of low crystallinity could be attributed to the superposition of the melting of crystals, formed by the annealing pretreatments, and the cold-crystallizations occurring during TMDSC heating. The ongoing crystallization after the cold crystallization was clearly seen in the TMDSC nonreversing heat flow curves. DMA data agreed with TMDSC data on the origin of the double cold-crystallization peaks.

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