Compliance-Time-Temperature Relationships from Indentation Measurements on a Pure-Gum Rubber Vulcanizate

Previous work on the calculation of shear compliance J (limit of the ratio of strain to stress at zero deformation) from observations of the indentation of a flat natural rubber surface by a rigid sphere has been extended over greater ranges of time, temperature, and state of vulcanization. The extreme ranges of time were 5 sec to 23.5 hr and of temperature - 67 to +100 °C. The curves of JT against log t _-60 (where t _-60 is the equivalent time at - 60 °C and T is in °K) were sigmoid with a region of maximum slope and continuing through a region of minimum slope extending over about 7 decades. In contrast with the predictions of the Williams, Landel, and Ferry equation, the empirically determined shift of abscissa necessary to give a single JT curve was found to be a linear function of T with a slope corresponding to an "apparent activation energy" of 36.9 kcal (°K)^-1 (mole)^-1 (154.5 kJ (°K)^-1 (mole)^-1).