Stress Softening of Reinforced Vulcanizates by Swelling in Solvents
A model, by means of which it is possible to describe the strain-stress behavior of carbon black-reinforced elastomers, has been established by Rigbil based on a qualitative description by Dannenberg.2 The model also appears to describe adequately the phenomenon of stress softening and, after suitable modification, the behavior a t high rates of strain.*
Careful inspection of the model indicates that a phenomenon similar to stress softening should also appear following swelling of the loaded vulcanizates in solvents. In the original model, stress softening is postulated to arise as the result of the movement of a crosslink relative to a carbon-black agglomerate, this being the microscale result of an overall strain; the stress developed in the molecular chain then results in a net increase in the chain length and, consequently, a fall in modulus. Similar behavior must also appear in a swollen vulcanizate, in which the crosslinks and agglomerates move affinely in three dimensions rather than uniaxially.
However, swelling of reinforced vulcanizates has not, in the past, been observed to produce stress softening. When such studies were made, a hardening was almost always noted due to the removal of plasticizing oils or of curative adjuncts such as fatty acids.
This note gives details of the experiments carried out in order to detect stress softening and the theoretical reasoning behind them.
Theory
As described earlier, stress softening and its recovery are highly temperature-sensitive processes which, presumably, cease completely a t the glass transition temperature. However, at temperatures low enough even if they are well above To, these processes must be extremely slow, since it has been shown4 that at O' C, the recovery from stress softening virtually disappears in compounds for which T, is of the order of -40Β°C.