Model Elastomeric Networks

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Abstract
Tetrafunctional and trifunctional model polymer networks of known crosslink density were prepared by end-linking a variety of hydroxyl terminated chains both in the undiluted state and in solution. The polymers employed were poly(dimethylsiloxane), atactic poly(propylene oxide), poly(ethylene oxide), poly(tetramethylene oxide), and poly(e-caprolactone), and the end-linking agents were alkoxy silanes and an aromatic triisocyanate. The resulting networks were studied (i) in elongation at 25°C, both in the unswollen state and swollen with a nonvolatile diluent, and (ii) in equilibrium swelling in benzene. The values of the elongation modulus in the limits at large and small deformations were found to be in good agreement with recent theoretical predictions for nonaffine and affine deformations, respectively (the latter corresponding to the case in which the displacements of the network chain vectors are a simple linear function of the macroscopic strain). Comparisons between the tetrafunctional and trifunctional siloxane networks showed that both of these moduli have very nearly the expected dependence on network functionality. The swelling equilibrium results were found to be in very good agreement with a recent theory of network swelling in which the closeness of the approach to the affine limit depends on both the network structure and the degree of swelling. None of the results provided evidence for very significant contributions from interchain entanglements. Several of the polymers, however, have large values of the "plateau modulus" in the uncrosslinked state, and are therefore generally considered to have high densities of entanglements of this type. The present results thus contradict the widely held notion that interchain entanglements should contribute to equilibrium elastomeric properties to nearly the same extent as they do to the dynamic, visco-elastic properties.

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