Enhanced crystallization kinetics in poly(ethylene terephthalate) thin films evidenced by infrared spectroscopy Monica Bertoldo a, * , Massimiliano Labardi b , Cinzia Rotella c, 1 , Simone Capaccioli b, c a PolyLab-INFM-CNR, Via Risorgimento 35, I-56126 Pisa, Italy b PolyLab-INFM-CNR, Largo Pontecorvo 3, I-56127 Pisa, Italy c Department of Physics, University of Pisa, Largo Pontecorvo 3, I-56127 Pisa, Italy article info Article history: Received 28 January 2010 Accepted 20 May 2010 Available online 9 June 2010 Keywords: Poly(ethylene terephthalate) Crystallization Ultrathin films abstract The cold crystallization process in poly(ethylene terephthalate) (PET) spin-coated ultrathin films was studied by infrared spectroscopy. The conformational change associated to the formation of crystal phase during annealing at 107  C was measured in real time, by monitoring both intensity and frequency shift of trans and gauche conformer bands of the PET glycol segment. Enhancement of crystallization kinetics was observed in thin films deposited on amorphous silicon, with respect to a 20 mm thick free standing film used as reference, where the fastest kinetics was observed for the thinnest (35 nm) film. Experi- mental findings were interpreted in terms of scarce interaction between PET films and silicon substrate, which does not provide slowing down of crystallization kinetics as observed on different substrates. This results in a dominant effect of the polymer/air interface, where faster kinetics is observed, as also confirmed by atomic force microscopy imaging, particularly on the thinnest film. Additionally, Avrami and Avramov analyses evidence a decrease of both the Avrami exponent, related to growth dimen- sionality, and induction time, related to delay of nucleation start, when decreasing film thickness. Therefore, the reported results enrich the description of confinement and substrate interaction effects on the cold crystallization process taking place in PET ultrathin films. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The behavior of polymers in nano-pores, nano-channels, nano- layers as well as polymer nano-particles, nano-rods and nano-films is of wide interest to the scientific and applicative development of soft matter nanotechnologies since all the mentioned geometries are examples of confined spaces. Indeed, properties of polymers such as the glass transition, the melting temperature and the crystallization kinetics may differ in confined geometry in more or less extent from the corresponding properties of the unperturbed materials in the bulk [1]. It is widely accepted that polymer crystallization at surfaces and interfaces differs somehow from bulk crystallization, while the reasons for this behavior are still under debate. Nucleation from impurities concentrated at the exposed surface has been often invoked in the past [2,3], while more recently, phenomena connected to the different dynamics of the polymer at the interface have been considered [4,5]. Among polymers, poly(ethylene terephthalate) (PET) can be chosen as a model material to study crystallization in confined geometry, because its crystallization rate is slow enough to allow isothermal studies also under accelerated conditions such as those that may be found in confined geometries [6]. Moreover, PET is an important material for large scale applications such as packaging films and bottles as well as for electronics, due to its good optical, electrical and gas barrier properties, as well as to its quite high melting temperature. For all the mentioned applications and in particular in view of combining packaging and electronics, the properties of polymers at the interface with air, as well as with those metals that are typically used for electronics, may be of crucial importance. Several works have dealt in the last years with the behavior of PET in confined geometries [7e10]. In particular, its glass transition temperature in ultrathin films on gold was found to decrease monotonically with the film thickness when the latter is smaller than 100 nm [7]. Crystallization of PET at the air interface studied by AFM, grazing incidence X-ray diffraction and XPS was found in all cases to be faster than the bulk one [11e 13]. * Corresponding author. Tel.: þ39 (0)502219413; fax: þ39 (0)502219320. E-mail address: monicab@dcci.unipi.it (M. Bertoldo). 1 Present address: Department of Physics and Astronomy, Leuven University, Celestijnenlaan 200d - bus 02416, B-3001 Leuven, Belgium. Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2010.05.040 Polymer 51 (2010) 3660e3668