Eur. Phys. J. E 18, 459–465 (2005) DOI: 10.1140/epje/e2005-00045-6 T HE EUROPEAN P HYSICAL JOURNAL E Confined crystallization in phase-separated poly(ethylene terephthalate)/poly(ethylene naphthalene 2,6-dicarboxilate) blends C. ´ Alvarez 1,2 , A. Nogales 1 , M.C. Garc´ ıa-Guti´ errez 1 , A. Sanz 1 , Z. Denchev 3 , S.S. Funari 4 , M. Bruix 5 , and T.A. Ezquerra 1, a 1 Instituto de Estructura de la Materia, CSIC, Serrano 19, 28006 Madrid, Spain 2 Escuela T´ ecnica Superior de Ingenieros Industriales, UPM, Jos´ e Guti´ errez Abascal 2, 28006 Madrid, Spain 3 Department of Polymer Engineering, University of Minho, Campus de Azur´ em, 4800-058 Guimar˜aes, Portugal 4 Max-Planck-Intitute for Colloids and Interfaces, c/o HASYLAB, DESY, Notkestraße 85, 22603 Hamburg, Germany 5 Instituto de Qu´ ımica F´ ısica Rocasolano, CSIC, Serrano119, 28006 Madrid, Spain Received 8 July 2005 / Received in final form 13 October 2005 Published online 15 November 2005 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2005 Abstract. The isothermal cold crystallization of poly(ethylene terephthalate)(PET) in cryogenic mechani- cal alloyed blends of PET and Poly(ethylene naphthalene 2,6-dicarboxilate)(PEN) 1:1 by weight has been investigated by simultaneous small and wide angle X-ray scattering (SAXS and WAXS) and dielectric spectroscopy (DS). For transesterification levels higher than 23% the blends tend to transform into a one- phase system and the crystallization of PET is strongly inhibited due to the significant reduction of the PET segment length. For lower levels of transesterification the blends are phase separated and the overall crystallization behaviour can be explained considering the confined nature of the PET domains in these blends. The formation of a rigid amorphous phase in the intra-lamellar stack amorphous regions is reduced in the blends due to a lower probability of stack formation in the confined PET-rich domains. The more effective filling of the space by the lamellar crystals in the blends provokes a stronger restriction to the amorphous phase mobility of PET in the blends than in pure PET. PACS. 61.10.-i X-ray diffraction and scattering – 61.41.+e Polymers, elastomers, and plastics – 36.20.-r Macromolecules and polymer molecules – 81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials 1 Introduction Blending of poly(ethylene terephthalate) (PET) and poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) has been shown to be an attractive possibility to combine the inherent economics of PET with the superior mechanical, thermal and barrier properties of PEN [1]. The molec- ular structure of PEN is stiffer than that of PET due to the presence in its main chain of naphthalene instead of benzene rings. The glass-transition temperature, T g , of PEN is about 50 ◦ C higher than that of PET contribut- ing to a better performance in terms of thermal, mechan- ical, and gas barrier properties [1–4]. However, PEN is not envisioned to substitute completely PET in the men- tioned applications due to its higher price and more com- plex processing [4,5]. Therefore blending of PEN and PET has received much industrial and scientific attention as an alternative way to obtain polymer materials with im- proved properties [1]. It is well documented that PET and a e-mail: imte155@iem.cfmac.csic.es PEN are immiscible polymers that tend to form separated phases upon blending [6]. However, at extrusion tem- peratures above ≈270 ◦ C certain amounts of PET–PEN block copolymers develop due to transesterification reac- tions [6–10]. The study of the phase structure and dy- namics as a function of transreactions requires uniform PET/PEN blends with a controlled degree of transesteri- fication. A method capable to fulfil these requirements was recently proposed for PET/PEN blends [11] and involves cryogenic grinding [12,13], melt pressing at 300 ◦ C for var- ious times and subsequent quenching. By this procedure, amorphous films of PET/PEN blends with various de- grees of transesterification can be prepared, the latter be- ing controlled by the melt-pressing time. The effect of the transesterification degree on thermal properties [11] and relaxation behaviour [11–14] of PET/PEN films has been characterized by means of differential scanning calorime- try (DSC) and relaxation spectroscopy. For low levels of transesterification two glass-transition steps, two peaks of crystallization and melting and two α-relaxation processes