Relaxation in poly-(ethylene terephthalate glycol)/montmorillonite nanocomposites studied by dielectric methods H. Couderc a, * , L. Delbreilh a , A. Saiter a , J. Grenet a , N. De Souza b , J.M. Saiter a a UMR CNRS 6522, L’Equipe de Caracte ´risation des Amorphes et des Polyme `res (L’ECAP), Institut des Mate ´riaux de Rouen, Faculte ´ des Sciences, Universite ´ de Rouen, Site du Madrillet, 76801 Saint Etienne du Rouvray cedex, France b University of North Texas, Department of Material Science, Denton, TX 76203-5310, USA Abstract Samples of polyethylene-1,4-cyclohexylenedimethylene terephthalate glycol (PETG) with different filler contents were prepared by a master batch process. The dispersion state of montmorillonite was characterized using X-ray diffraction. The influence of nanofiller on molecular mobility at the glass transition was studied by dielectric relaxation spectroscopy and thermo stimulated depolarization cur- rents. The use of these two techniques allowed us to determine the fragility index m. For this parameter, we observed a maximum for a 1 wt% montmorillonite content. For higher contents, no significant modification of the fragility index was observed. This behavior has been attributed to nanofiller agglomeration. Ó 2007 Elsevier B.V. All rights reserved. PACS: 64.70.Pf; 81.05.Qk; 82.35.Np Keywords: X-ray diffraction; Dielectric properties, relaxation, electric modulus; Thermally stimulated and depolarization current; Glass transition; Nanocomposites; Polymers and organics; Enthalpy relaxation; Fragility; Structural relaxation 1. Introduction It is known that liquids can be transformed into a glassy state by cooling, when the cooling rate is high enough to avoid the crystallization of the material. Below the crystal- lization (or melting) point, the liquid state is transformed into a metastable state known as supercooled liquid, in which the viscosity increases strongly with decreasing tem- perature. For a viscosity typically between 10 11 and 10 12 Pa s, the supercooled liquid vitrifies. Thermodynami- cally the glass is in a non-equilibrium state. The glassy nat- ure is revealed by a transition occurring at a temperature T g , called the glass transition temperature. Due to the excess of free energy or free volume, structural relaxation phenomena could occur when the glass is maintained at a temperature T < T g . A classification of the glass-forming liquids from the variations of the viscosity with tempera- ture, and more exactly with the normalized reduced T g /T quantity has been proposed by Angell [1]. For the extremes of the behavior, glass-forming liquids that exhibit a depen- dence of viscosity (or relaxation time) on temperature that is approximately Arrhenius are defined as strong glass formers; whereas those which exhibit a non-Arrhenius behavior are declared as fragile glass formers. From the variations of the relaxation time (s), with the temperature a fragility index m can be calculated according to the fol- lowing relationship [2] m ¼ d logðsÞ d T g T               T ¼T g : ð1Þ The value of the fragility index varies from m = 16 [3] for very strong glass-forming liquids like those formed by highly directional bonded molecules, to m = 250 [4] for 0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.03.046 * Corresponding author. Tel.: +33 02 32 955 081. E-mail address: Hugues.couderc@gmail.com (H. Couderc). www.elsevier.com/locate/jnoncrysol Available online at www.sciencedirect.com Journal of Non-Crystalline Solids 353 (2007) 4334–4338