Applied force and second order transitions of rubber

The second order transition temperatures of three different cures of natural rubber have been studied as functions of elongation by observing length dependence on temperature. This transition temperature ( T m ) decreases with increasing applied force, and at any given force increases with increased crosslinking; moreover the slopes of the T m vs. force curves become less steep with increased crosslinking. The results are in agreement with the following propositions: ( 1 ) that transitions are caused by the onset of rotations; ( 2 ) that extension of the polymer makes the onset of such rotations easier, thus lowering T m ; ( 3 ) that increased crosslinking hinders rotation so as to increase T m ; and ( 4 ) that increased crosslinking diminishes the effect of a given force on T m . The hypothesis of rotation is also discussed in connection with the effect of swelling agents on the second order transition.