Study of the cold crystallization of poly(ethylene terephthalate) at the air interface by ATR spectroscopy

The cold crystallization at the air interface in fairly thick films of poly(ethylene terephthalate) (PET) was studied by combining attenuated total reflectance (ATR) infrared spectroscopy and X-ray diffraction (XRD) analysis methods. In ATR analysis, Ge and ZnSe were both used as internal reflectance elements to gain information at two different penetration depths from the air interface of the films. Samples were crystallized for different time at the selected temperature so to obtain kinetics plots of the crystallization process. The kinetics obtained by X-ray and ATR analysis on Ge and ZnSe clearly indicate that the crystallization at the film surface is faster than that in the bulk. All kinetics plots were analyzed with Avrami and Malkin macrokinetics models. Finally a method based on the deconvolution of the surface contribution from the bulk was developed to deeply investigate the difference between the crystallization at the air interface and in the bulk. The method is based on the use of two Malkin equations, one describing the behavior at the surface and the other in the bulk. It has allowed to roughly estimate the thickness of the surface layer, which was found to be in the submicrometer range. Finally, the Malkin parameters obtained in the analysis clearly indicated that the crystallization rate at the surface is faster because of the higher nucleation rate in this region than in the bulk.