Q.Why is untreated natural rubber soft, tacky and of low tensile strength, while vulcanized rubber is much harder, stronger and more elastic? Explain in terms of molecular structure.
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Start your 14-day free trial to unlock the full solution →In raw natural rubber, the cis-1,4-polyisoprene chains are tangled together but held to one another only by weak van der Waals (dispersion) forces; under mechanical stress or warmth, entire chains can slide permanently past their neighbours rather than simply uncoiling and snapping back, so the material deforms permanently, feels soft and tacky, and has low tensile strength. Vulcanization introduces sulfur cross-links -- short disulfide/polysulfide bridges -- connecting the residual double bonds of NEIGHBOURING chains at intervals along their length. These covalent cross-links do not prevent the chains from stretching and uncoiling under load (there are far too few of them to make the network rigid), but they DO prevent the chains from sliding permanently past one another, so once the stress is released the cross-linked network pulls the chains back to their original coiled shape. The result is a much higher tensile strength, better elastic recovery, and the los …
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