Birefringence and rubber-elastic properties of crosslinked polymers in uniaxial deformation

Measurements of birefringence and stress on well characterized endlinked PDMS, randomly crosslinked NR/IR elastomers, and polyether networks are reported in both uniaxial extension and compression. The experimental results are compared with predictions from modem theoretical concepts. The reduced birefringence and the reduced stress are found to be dependent on the inverse deformation ratio 1/lambda with a slight increase of both properties when going from extension to compression. Nevertheless, birefringence and stress seem to have the same dependence on extension respectively compression resulting in a stress-optical coefficient (SOC) independent of the type and the amount of uniaxial deformation in the range 1.5>lambda>0.7 according to the classical Kuhn-Grun theory. Furthermore, the stress-optical coefficients of endlinked PDMS and randomly crosslinked NR/IR networks in the unswollen state are constant over a wide range of crosslink densities and junction functionalities. A detailed analysis of the birefringence data and the mechanical properties completed by the results of swelling tests indicates a considerable contribution of topological interactions (trapped entanglements) in a certain range of crosslink densities. This contribution seems to depend on the actual conditions of the network formation. The consistency of the above conclusions is supported by transversal IH-NMR relaxation experiments on the crosslinked samples.