Constant Stress Elongation of Soft Polymers: Plasticizer Studies

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Abstract
Constant stress measurements have been made on lightly cross linked GR-S and polyisobutylene of 7 million molecular weight. The modulus values as a function of temperature, time, and concentration of mineral oil plasticizer have been treated according to the ``reduced variables'' principles. Since both plasticized and unplasticized samples showed the expected equivalence of temperature and time, the modulus values could be shifted to produce master curves for each composition. In the case of GR-S the concentration of plasticizer could be similarly reduced to yield a single curve for all the data. The polyisobutylene results, however, showed quite clearly that the plasticizer had a unique effect not paralleled by temperature or time. This effect has been attributed to the pseudoequilibrium modulus which apparently results from chain entanglements. Since the pseudoequilibrium modulus should be proportional to the number of chain entanglements per gram of polymer, it should vary as the square of the volume fraction of polymer. It is shown that a correction of this type improves the shift of the curves for polyisobutylene. Changes in activation energy with the addition of plasticizer are apparently within experimental error (±2 kcal) in all cases.