A simple method for studying internal migration of liquids in rubber

When swollen rubber sheets are bent, they swell more on the tension side and less on the compression side. When the bending constraint is removed, immediate recovery towards the flat state is only partial, followed by slow further recovery as the swelling liquid migrates internally. The rate of this slow recovery is directly related to the internal diffusion rate of the swelling liquid [1]. Experiments are described here for sharply bent sheets of lightly-crosslinked natural rubber, swollen with decane. The initial recovery when the bending constraint was removed was determined from the angle θ between the two arms of a bent strip. Slow further recovery was followed as liquid migrated internally and the strip straightened (θ increased towards 180°). The amount of "set" following release from bending was in reasonable accord with swelling theory and the time dependence of slow further recovery was consistent with the observed rate of diffusion of decane into rubber. Thus, recovery from "fold-back" bending appears to be a simple way of studying the internal mobility of absorbed liquids in polymers, provided that the amount of liquid absorbed depends on applied strain and that the polymer is reasonably elastic.