Swelling of rubber under nonuniform stresses and internal migration of swelling liquid when the stresses are removed

As shown by Treloar, the degree of swelling of a rubber sample is strongly affected by applied stress. His work suggests that the degree of swelling will vary from point to point in a sample, in accordance with the local stress field. Thus, when rubber sheets are bent, they are expected to swell more on the tension side and less on the compression side, and when the bending constraint is removed, recovery toward the initial flat state is expected to be only partial at first and then followed by a slow further recovery. These effects are explored here for natural rubber sheets swollen by dodecane. The measured "set" following release from bending is found to be in accord with simple swelling theory. The time dependence of later recovery is shown to be in agreement with the rate of diffusion of the swelling liquid. It is concluded that internal migration of compatible liquids will cause temporary delays in deformation kinetics and make a significant contribution to energy losses. Also, the bending experiment itself appears to provide a simple and generally applicable method for determining the internal diffusion coefficients of absorbed liquids.