Application of random walk model to the glass transition of unsaturated polyester resins cured with different styrene contents A. Saiter a , E. Bureau a , H. Zapolsky b , S. Marais c , J.M. Saiter a, * a Laboratoire d’Etude et de Caract erisation des Amorphes et des Polym eres (LECAP), Faculte des Sciences, Universit e de Rouen, 76821 Mont Saint-Aignan cedex, France b GPM, UMR CNRS 6634, Universit e de Rouen, 76821 Mont Saint-Aignan cedex, France c UMR CNRS 6522, Facult e des Sciences, Universit e de Rouen, 76821 Mont Saint-Aignan cedex, France Abstract Dielectric measurements have been performed on unsaturated polyester resins, cured with different styrene contents. Based on the random walk model (RWM), we have determined, as a function of the styrene content, the width of the density of states kT 0 , the average energy barrier height hE A i that a structural unit must crossover in order to relax, the fragility index a and the value of T c at which qualitative changes in the dynamics occur. The RWM data are in good agreement with the strong-fragile concept. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 61.20.Lc; 64.70.Pf; 66.20.þd; 64.60.Ht 1. Introduction The mechanisms by which a non-ergodic glassy state is obtained from an ergodic liquid state are at the centre of extensive research. Of particular in- terest is a better understanding of the temperature dependence of the average relaxation time s, which characterises the molecular mobility in these dif- ferent states. The transition to a non-ergodic state is observed by an increase of the relaxation time value along with an increase of the average vis- cosity. For typical cooling rates s is of order 10 2 s at the glass transition temperature, T g . Some years ago, a phenomenological approach based on the variations with temperature of the viscosity or relaxation time have shown that all glass-forming liquids exhibit ln s versus T g =T variations included between two bordering limits. This approach, known as the strong-fragile concept, and proposed by Angell [1], allows the definition of a fragility index m introduced by Plazek and Ngai [2]. A low value for m (16) characterises a strong glass- forming liquid, which exhibits an Arrhenius tem- perature dependence of the viscosity (observed for example for rigid network systems [3,4]), while a fragile glass-forming liquid with a high m Journal of Non-Crystalline Solids 307–310 (2002) 738–743 www.elsevier.com/locate/jnoncrysol * Corresponding author. Tel.: +33-2 35 14 66 32; fax: +33-2 35 14 68 82. E-mail addresses: jean-marc.saiter@univ-rouen.fr, allison. saiter@univ-rouen.fr (J.M. Saiter). 0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0022-3093(02)01555-7