Influence of the microstructure of PPO-based glass-formers on their dynamics as investigated by neutron spin echo A. Faivre a, * , C. Levelut a , D. Durand b , S. Longeville c , G. Ehlers d a Laboratoire des verres, UMR CNRS, Universit e Montpellier II, cc69, 34095 Montpellier cedex 5, France b Polym eres, Collo € ıdes, Interfaces, UMR CNRS, Universit e du Maine, 72085 Le Mans cedex 9, France c Laboratoire L eon Brillouin, (CEA–CNRS), CEA-Saclay, 91191 Gif-sur-Yvette cedex, France d Institut Laue-Langevin, 165X, 38042 Grenoble cedex, France Abstract We present a neutron spin echo investigation of the high temperature dynamics of two related glass-forming systems based on poly(oxypropylene): namely the molecular liquid triol and the polyurethane gel. Measurements performed at the first maximum of the static structure factor, on a rather large time window, allow one to analyse precisely the relaxation characteristics of these systems and to compare them with previous neutron time of flight and dielectric results. We discuss the differences in relaxation times and stretching exponents obtained for both samples as a function of their microstructure: molecular nature and architecture (effects of branching or/and three-dimensional network). Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 64.70.pf; 61.41.þe; 78.70.Nx 1. Introduction In the past few years we have been investigating the relaxational processes related to the glass transition in a series of glass-formers mainly based on poly(oxypropylene) (PPO). This polymer fam- ily offers a great diversity in microstructure and molecular architecture. The length and shape of PPO molecules can be varied. Polyurethane (PU) is produced by polycondensation of PPO diol (linear)orPPOtriol(star-like)withadiisocyanate. In the case of star-like PPO molecules, the cross- linked density can be varied, as well as the length between cross-link points. These polymers are consequently good candidates for analysing the relaxation mechanisms associated with the glass transition on a molecular level. Moreover, no spe- cific neutron spin echo (NSE) investigations have been performed in a series of glass-formers, vary- ing the molecular architecture. The dynamics of the PPO-based systems have been investigated both below and above the glass transition temperature by combining different tech- niques.Thestructural a relaxationhasbeenprobed by mechanical, dielectric and ultrasonic measure- ments [1–4]. A slow secondary b-Johari relaxation has also been evidenced by dielectric measure- ments [3,4]. Both light [5] and inelastic neutron Journal of Non-Crystalline Solids 307–310 (2002) 712–718 www.elsevier.com/locate/jnoncrysol * Corresponding author. Tel.: +33-4 67 14 32 84; fax: +33-4 67 54 48 01. E-mail address: alfaivre@univ-montp2.fr (A. Faivre). 0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0022-3093(02)01551-X