DSC thermograms show that PET fibers upon thermal treatment develop an endotherm peak between 130°C and 234°C, depending on the temperature and the time of the treatment. The available experimental data suggest strongly that the middle endotherm peak (MEP) is the result of melting of very small crystallites (nuclei) located in the highly extended anisotropic, amorphous fraction of the fiber. The energy involved in the MEP can reach a value of 2 cal/g, compared to 17 cal/g for the major melting endotherm. The nuclei are formed in the highly strained, anisotropic amorphous fraction of the fiber as a result of two opposing and competitive processes, one operating under the driving force to increase the entropy of the system, the second one governed by the tendency to decrease the enthalpy. The formation of nuclei prevents the complete disorientation of the amorphous phase and controls the amount of shrinkage the yarn will undergo at temperatures above the glass transition temperature.
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Oswald et al. (1977) studied this question.