Free volume in carbon-black-filled isoprene rubber investigated by positron annihilation lifetime spectroscopy

The free volume in the amorphous phase of a simple carbon-black-filled peroxide cross-linked isoprene rubber (IR) is investigated by positron annihilation lifetime spectroscopy to clarify the structural changes in the rubber phase by the carbon black (CB) fillers. The ortho-positronium (o-Ps) pick-off annihilation lifetime measured in the rubber without CB is around 2.8 ns and is found to be almost constant even after CB filling. This result indicates that in a portion of the rubber phase the free volume remains similar in size to that in the cross-linked IR without CB. The o-Ps intensity drastically decreases from 37% without CB to 15% with a CB loading as high as 80 phr. This decline is revealed to exceed the contribution, which is expected only from the decrease in the fraction of the amorphous phase by CB addition, by a factor of 0.37 at the highest CB loading. The additional reduction is related to the generation of a bound rubber layer surrounding the CB fillers with low molecular-chain mobility where positronium formation is inhibited. The decrease rate is found to represent the volume of this rigid rubber phase and strongly depend on the formation of the CB network structure.