Application of Sample—Sample Two-Dimensional Correlation Spectroscopy to Determine the Glass Transition Temperature of Poly(Ethylene Terephthalate) Thin Films

Abstract

The glass transition temperatures (T_g) of poly(ethylene terephthalate) (PET) thin films with different thicknesses are determined by analyzing their in situ reflection–absorption infrared (RAIR) spectra measured over a temperature range of 28 to 84 °C. The criterion of standard deviation of the covariance matrices is used as a graphical indicator for the determination of the T_g present in the sample–sample two-dimensional (2D) correlation spectra calculated from the temperature-dependent RAIR spectra. After two data pretreatments of the first derivative of the spectral absorbance versus temperature and the mean normalization over the wavenumbers are sequentially carried out on the RAIR spectra, an abrupt change of the first-derivative correlation spectra with respect to temperature is quickly obtained. It reflects the temperature at which the apparent intensity changes in pertinent absorption bands of PET thin films take place due to the dramatic segmental motion of PET chain conformation. The T_g of the thin PET films is accordingly determined. The results reveal that it decreases with a great dependence on the film thickness and that sample–sample 2D correlation spectroscopy enables one to determine the transition temperature of polymer thin films in an easy and valid way.