Elasticity of uncrosslinked rubbers

Relaxation of stress at constant elongation, instantaneous recovery, delayed recovery, and permanent set, have been measured for four simple rubbery polymers over a wide range of time, temperature, and amount of elongation. The extent of instantaneous recovery is shown to be in satisfactory accord with a two‐network calculation based on the hypothesis that the preceding stress relaxation is due to progressive rearrangement of a constant number of molecular entanglements. Delayed recovery R was found in all cases to follow a simple empirical time function magnified image where ts is the period of prior deformation. This relationship was obeyed for times t and ts varying from 1 to 106 sec, temperatures between 4 and 80°C, and imposed extensions ranging from 30 to 200%. It was also found to hold in some preliminary measurements on carbon black filled rubbers. Thus, it appears to have wide applicability, although this particular time function is not readily accounted for in terms of a theoretical model. Permanent set was found to be in reasonable accord with Newtonian flow behavior under the observed stress history.