Viscoelastic properties of networks with low concentration of pendant chains Leandro E. Roth a , Daniel A. Vega b , Enrique M. Valle ´s a , Marcelo A. Villar a, * a Department of Chemical Engineering, Planta Piloto de Ingenierı ´a Quı ´mica, Camino La Carrindanga Km. 7, UNS-CONICET, CC 717, 8000 Bahı ´a Blanca, Argentina b Departamento de Fı ´sica, Universidad Nacional del Sur, Av. L.N. Alem 1253, 8000 Bahı ´a Blanca, Argentina Received 26 November 2003; received in revised form 15 June 2004; accepted 22 June 2004 Abstract A good knowledge of the molecular structure of polymer networks allows to relate rheological properties with different molecular parameters. In this work we analyze the influence of low concentrations of pendant chains on the viscoelastic properties of polymer networks. Model networks, with a well defined structure, were synthesized by reacting a commercial a,u-divinyl poly(dimethylsiloxane) (B 2 ) with a trifunctional cross-linker bearing silane groups (A 3 ) and known amounts of an anionic u-vinyl poly(dimethylsiloxane) (B 1 ). The structure of the networks was predicted with a molecular model based on a mean field approach (recursive model) taking into account the initial composition of the reactants. Rheological characterization was carried out in a rotational rheometer by dynamic and stress relaxation test. Viscoelastic properties of the networks depend on both concentration and molecular weight of pendant chains. Relaxation modulus was adjusted by the empirical Chasset–Thirion equation. It was found that it provides a very good fit to the behavior of these networks prepared by end-linking. The fitting parameter m in the Chasset–Thirion equation shows a strong dependence with the molecular mass of pendant chains, but it is rhougly independent of concentration. The results agree remarkably well with the predictions of a theoretical model previously reported by our group. q 2004 Elsevier Ltd. All rights reserved. Keywords: Model networks; Pendant chains; Viscoelastic properties 1. Introduction Mechanical properties of a polymeric material can be directly related to its structure. In the particular case of polymer networks, chemical or physical cross-linking points gave distinctive characteristics to these materials such as high elongation, better thermal stability and insolubility. Model silicone networks have been extensively studied in order to explain the influence of molecular structure on mechanical properties. However, almost all the effort with these networks was focused on the verification of the theory of rubber elasticity and the contribution of entanglements to elastic equilibrium modulus [1–8]. There are few works in which the influence of network defects on the non-equilibrium properties was investigated [9,10]. Free chains and pendant chains are the more common defects in real networks. In particular, pendant chains have a strong influence on the viscoelastic properties of these networks, changing considerably the spectrum of relaxation times. Fig. 1 is a schematic representation of a network where defects, such us free and dangling chains are shown. Most of the studies concerning pendant chains have been focused on polymer networks obtained by random cross- linking reactions. However, in those networks, little is known about the structure of the dangling chains, such as their molecular weight, molecular weight distribution, degree of branching, etc. Then, an accurate description of the relationships between rheological properties and the structure of those defects becomes extremely difficult. In such sense, model PDMS networks with controlled amounts of well-characterized defects offer an ideal system to reveal 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.06.043 Polymer 45 (2004) 5923–5931 www.elsevier.com/locate/polymer * Corresponding author. Tel.: C54-291-4861700; fax: C54-291- 4861600. E-mail address: mvillar@plapiqui.edu.ar (M.A. Villar).