Positron annihilation spectroscopy study of rubber-carbon black composites

Abstract

The focus of this research was to use Positron Annihilation Spectroscopy (PAS) to investigate the response of rubber and rubber-carbon black composites subjected to different physical conditions. The work examined the effect of deforming rubber and rubber filled with carbon black. The results showed that deformation of the rubber depends on whether the sample is filled with carbon black (CB) or not. CB, we propose impedes the aligning of the rubber chains during deformation. Aging of rubber was done and natural rubber was found to exhibit reversion property of its chains from a vulcanized state to un-vulcanized gum state as opposed to synthetic rubbers. This shows how vulnerable the natural rubber chains are at high temperature. We also found that heat can induce crystallization in the rubber chain network. The most common type of rubber crystallization inducement is through strain, which has been studied in detail.^In our investigation, we have found that when rubber is heated and allowed to cool slowly to room temperature, its chains can align themselves in an orderly fashion many times leading to crystal growths.^Heat also favors oxidation of the rubber chains, hence causing their quick degradation We studied the effect of sulfur in the cross-linking of rubber. We found that during vulcanization, sulfur cross-links rubber chains by tying them together in a network like structure reducing the chains' mobility. The work also explored the positronium formation in liquids and some common polymers then compared the results with those found from rubber. It was found that Ps formation depends on the nature of the liquid. We found that the results for rubber were similar to those of liquids and concluded that rubber behaves more like a liquid. At room temperature, rubber is far away from its glass transition temperature hence has soft and flexible chains.^Ps atom can thus dig itself a cavity within the rubber chains and live longer in it. This explanation was explored through the bubble model.