Network Recovery from Uniaxial Extension: II. The Origin of the Mullins Effect

Abstract

Abstract The Mullins effect refers to the dissipation in crosslinked rubber of mechanical energy beyond that due to linear relaxation processes or irreversible structural changes. Physically, Mullins softening probably arises from adjustment of local imbalances in segment density and from contraction of the primitive path of network chain ends. The resulting more random chain configuration contributes to the relaxation of the stress, but at a sufficiently rapid rate that it is omitted from the experimentally measured linear relaxation spectrum. This omission is made apparent upon reversal of the applied strain. The mechanism underlying the hysteresis can be modeled empirically by the introduction of irreversibility into the damping function. Although the Mullins effect is a reflection of nonaffine network motion during deformation of rubber, it is independent of the extent to which the junctions of the network are affinely displaced at elastic equilibrium. Randomly crosslinked networks of widely varying structure exhibited comparable degrees of Mullins softening. Corroboration that the network chain ends are a major source of Mullins softening could be obtained by a similar study using end-linked networks. Such experiments on end-linked polytetrahydrofuran rubber are currently in progress.