Stress-strain behaviour of model networks in uniaxial tension and compression

Abstract

Elasticity measurements performed on well-defined poly(dimethylsiloxane) networks (PDMS) reveal that there is a direct proportionality between the small-strain modulus, G, and the chemical network density, v_ch, only at high network densities, whereas G is quite constant and in the order of the plateau modulus, G ^o_N , at low network densities. This indicates that topological interactions, e. g. entanglements, contribute to the modulus in a certain range of network densities.

PDMS networks haying well-defined topologies were prepared by endlinking fractionated PDMS chains (¯ M _n ranging from 2000 to 62000 g mol⁻¹) with a pentafunctional cyclic siloxane. Generally, the sol fraction of the samples was below 1.5% suggesting that the crosslinking reaction was quite complete.

Stress-strain isotherms in uniaxial tension and compression were measured at 333 K for these networks utilizing only one specimen in the same apparatus for the whole deformation range covered. At small and medium deformations, the reduced stress increases monotonically as a function of reciprocal elongation when going from extension to compression. A maximum in the Mooney-Rivlin plot may occur, if at all, in the compression range at λ ≦ 0.7, in qualitative accord with some theoretical approaches.