Small-Strain Modulus of Model Trifunctional Polydimethylsiloxane Networks

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Abstract A number of studies to test quantitatively the molecular basis of the theory of rubber elasticity have been performed during the last few years on elastomeric networks of relatively well-known structure. Many of these studies have been done on end-linked networks, for which the number of chains between junctions, their average length, and the functionalities of the junctions could be calculated. Several studies have been with poly(dimethylsiloxane) (PDMS) networks. One of the major controversial points up to now has been the effect of topological interactions between network chains on mechanical properties. Two groups of researchers have come to quite different conclusions about the role of entanglements with respect to their contribution to the elastic modulus. The first group led by Flory considers that the entanglement contribution to the elastic equilibrium modulus is due to the restrictions that topological interactions impose on junction fluctuations among their mean equilibrium position. The second group follows the hypothesis of Langley and Graessley who argue that these interactions are also present along the chain contour. Their support of this thesis is based on the experimental observation of the rubbery plateau modulus GN0 in linear polymers, which is evidence of topological interactions between chains. These interactions would also be present during network formation, and they will be “trapped”, or prevented from relaxing, by the crosslinking process. Thus, these chain-chain interactions can contribute directly to the modulus. Recently, one of us has reviewed most of the results obtained with PDMS networks. This showed that all the experimental data to date are quite consistent and give support to the second approach. In this work, we add to these data some new experiments obtained by crosslinking trifunctional PDMS networks in different states of dilution. The results give additional evidence of the contribution of chain entanglements to the modulus.