ELSEVIER PIh S0032-3861(97)10333-0 Polymer Vol. 39 No. 17, pp. 4037-4045, 1998 © 1998 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0032-3861/98/$19.00+0.00 Effect of intramolecular reaction on the properties of polybutadiene-silane rubbers Maria S. Di Nezio, Claudia Sarmoria and Enrique M. Valles* P/anta Piloto de Ingenier[a Qu[mica, UNS-CONICET, 12 de octubre 1842, (8000) Bah[a Blanca, Argentina (Received 24 January 1997;revised 23 June 1997;accepted 25 November 1997) Model networks were prepared by the reaction of monodisperse polybutadiene, containing 8% of double bonds in the 1,2 vinyl position, with p-bis(dimethyl silyl) benzene at different stoichiometric imbalances. Small strain modulus and measurements of the gel fraction indicate important differences with the predictions from ideal network forming reactions and the molecular theories of rubberlike elasticity. Sol extractions are higher than expected and small strain moduli are consistently low. Other macroscopic signs of the departure from ideal behaviour are detected. The onset of the gel point is delayed to a degree that grows with increasing imbalance of the reactive groups. We consider all the possible explanations for the observed behaviour, and suggest that the most likely one is the presence of intramolecular reaction. We also propose a simple theoretical model to confirm that our assumptions are consistent with the experimental results. © 1998 Elsevier Science Ltd. All rights reserved. (Keywords: intramolecular reaction; polybutadiene-silane robbers; model networks) INTRODUCTION Different theories have been proposed to relate the molecular structure of rubber networks to their elastic properties. One of the major controversies in this respect is that concerning the contribution of molecular entangle- ments. The phantom network model considers a network composed of a collection of Gaussian random chains whose conformations are independent of neighbouring chains. In this idealized behaviour the junctions at which the chain ends are connected can fluctuate freely, that is, all chains may adopt any conformation. This 'phantom chain' formulation gives, for the shear modulus G, the following resultl,2: G = (v - tx)RT (1) where p and # are the concentrations of elastically active chains and junctions, respectively, R is the universal gas constant and T is the absolute temperature. In this model the effect of entanglements resulting from the excluded volume due to chain-to-chain interactions is not taken into consideration. Revisions of the phantom theory to take the effect of entanglements into account form the basis of various current molecular theories of rubber elasticity. The so called 'affine deformation' model 3-7 assumes that the deviation of real networks from phantom networks results from restrictions affecting the mobility of the chain junctions. From this point of view the constraints imposed by the presence of entanglements alter the independent fluctuation of the junctions making them move in a fashion that is affine with the macroscopic deformation of the network. Under this supposition the following expression is obtained: G = ~RT (2) * To whom correspondence should be addressed Other research groups consider that chains contribute to the mechanical properties not only through their junction points, but also through contacts along their contour. This is based on the experimental observation of a rubbery pla- teau modulus, G ° , for linear polymers of high molecular weight, something that indicates that chain-chain interac- tions must be present in uncrosslinked samples. Some of those interactions could conceivably become trapped as crosslinks are introduced into the system, and these trapped entanglements would contribute to the shear modulus. Based on these arguments Langley 8 and Graessley9,t° pro- posed that G = (~ - h/z)RT + GeTe (3) where Ge is the 'entanglement modulus', close to ~, T e is the entanglement trapping factor, and h is a factor that may vary between 0 and 1. Equation (3) is the most general because it contains equations (1) and (2) as special cases. We see from equation (3) that in order to get the phantom chain formulation GeTe must be zero. It has been shown that 5 "7 this is very unlikely -. The phantom chain formulation, then, is a lower bound on the admissible values of shear modulus. The initial goal of this work was to investigate the contribution of entanglements with a well-defined experi- mental system. The crosslinking of polybutadienes (PB) of narrow molecular weight distributions was chosen because this polymer has a high plateau modulus. It was expected that this property would be helpful in evaluating the term that accounts for trapped entanglements in equation (3). The pendant vinyl groups from the PB were crosslinked with a bifunctional silane using the hydrosilation reaction. Differ- ent networks were obtained using a range of stoichiometric imbalances. They went from values around unity--which implies perfect crosslinking of all vinyl groups--to the so called 'critical imbalance', beyond which gelation is impossible. The resulting networks exhibited higher soluble POLYMER Volume 39 Number 17 1998 4037