The kinetics of crystallization of poly(ethylene terephthalate) measured by FTIR spectroscopy

The kinetics of crystallization of poly(ethylene terephthalate), PET, have been measured by FTIR spectroscopy using the absorbances of the crystalline and amorphous phase carbonyl bands at 1717 and 1727 cm^-1 to determine the fractional crystallinity as a function of time and isothermally over the temperature range 230-240 C. Both primary and secondary crystallization could be measured with similar accuracy limited only by the length of time over which the measurements were made. The primary and secondary crystallization time dependences were fitted to Avrami equations with n values of 2.0 and 1.0, respectively. The initial primary process was interpreted as due to two-dimensional spherulites restricted early in the crystallization by the thickness of the film sample, 1.5-3.0 μm. Secondary crystallization was considered to be due to one dimensional thickening of lamellae produced in the initial production of the spherulites. FTIR spectroscopy was considered to have some advantages in measuring the development of crystallinity over other techniques, such as DSC and DTA, which measure the rate of crystallization and are limited by the sensitivity of the calorimeter to measure the rate of heat evolution.