Study of the cold crystallization of poly(ethylene terephthalate) at the air interface by ATR spectroscopy Carlo Andrea Massa a , Simone Capaccioli a,b , Antonella Manariti c , Monica Bertoldo a,⇑ a Istituto per I Processi Chimico-Fisici del Consiglio Nazionale delle Ricerche, Area della Ricerca, via G. Moruzzi, 1, Pisa, Italy b Dipartimento di Fisica, Università di Pisa, Largo Pontecorvo, 3, Pisa, Italy c Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via Risorgimento 35, Pisa, Italy article info Article history: Received 22 May 2014 Received in revised form 8 August 2014 Accepted 13 August 2014 Available online 23 August 2014 Keywords: Air interface Poly(ethylene terephthalate) ATR (attenuated total reflectance) Crystallization kinetics Malkin model abstract The cold crystallization at the air interface in fairly thick films of poly(ethylene terephthal- ate) (PET) was studied by combining attenuated total reflectance (ATR) infrared spectros- copy 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 inter- face and in the bulk. The method is based on the use of two Malkin equations, one describ- ing 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 crystalli- zation rate at the surface is faster because of the higher nucleation rate in this region than in the bulk. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction and model background Poly(ethylene terephthalate) (PET) is an important material for packaging applications such as films and bot- tles as well as for flexible electronic device fabrication, due to its good optical, electrical and gas barrier properties and its quite high melting temperature [1]. All the mentioned properties depend more or less on the crystallinity degree of the polymer. For instance, the transparency is strongly worsened with the increase of the crystallinity degree, whereas the gas barrier is improved [2]. However, there is not a simply correlation between these properties and the crystallinity degree, since other parameters such as the morphology and distribution of the crystal phase play a crucial role [2]. Actually, the crystal phase at the surface more affects the surface reactivity [3], adhesion and gas permeability than those imbibed in the bulk. While the optical properties are scarcely affected if the crystal domain are smaller than half the wavelength of light. The crystallinity degree of PET, the crystal phase distribution and morphology are well known to change as result of any thermal treatment at temperatures exceed- ing the glass transition temperature of the polymer (343 K) [4–7]. This process is typically known as cold crystalliza- tion and can occur in amorphous PET films, for instance http://dx.doi.org/10.1016/j.eurpolymj.2014.08.012 0014-3057/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: monica.bertoldo@ipcf.cnr.it (M. Bertoldo). European Polymer Journal 60 (2014) 286–296 Contents lists available at ScienceDirect European Polymer Journal journal homepage: www.elsevier.com/locate/europolj