Differential scanning calorimetry has been used to study the kinetics of the crystallization processes of polyethylene terephthalate over a wide temperature range, 95–110°C and 230–240°C. The rates of crystallization of the two processes are in agreement with earlier dilatometric measurements. Nonintegral values of the A VRAMI exponent, n = 4.5 and n = 3.4, found for the low temperature and pre‐melt processes, respectively, are not in agreement with reported values of n = 2.0 and n = 4.0 for these processes. The value of n = 4.5 for the low temperature crystallization is surprising and not easily accounted for; one possible explanation is that the time dependence of the nucleation process is greater than first order. Because of their fractionality the n values yield little information on the mechanism of crystallization. Some mechanistic information is available since the low temperature crystallization isotherms are superposable along the log‐time axis, whereas the pre‐melt isotherms are not. Additional X‐ray data have indicated the existence of one crystalline form. The X‐ray data show no significant crystallite size increase for samples crystallized at 150°C over those crystallized at 105°C. Samples crystallized at 235°C show a larger and more ordered crystalline structure than those crystallized at 105 and 150°C. The relationship between the crystalline melting points and the temperatures of crystallization is as predicted by M ANDELKERN .
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Fielding‐Russell et al. (1970) studied this question.