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Why do hydrocarbon oils swell and weaken many hydrocarbon elastomers?

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Hydrocarbon oils cause swelling and weakening in many hydrocarbon elastomers because they act as compatible solvents. When a liquid is chemically compatible with a polymer, the elastomer molecules absorb the liquid, causing the material to swell.

Because many hydrocarbon elastomers are held together by physical entanglements rather than strong covalent bonds, they are susceptible to this process. When these oils penetrate the polymer matrix, they act as internal lubricants, increasing the distance between polymer chains and reducing the intermolecular forces that hold the material together. This results in a loss of structural integrity and mechanical strength.

To prevent this, elastomers are often subjected to vulcanization, a process of cross-linking that ties polymer molecules together into a permanent network. Covalently interlinked molecules are resistant to dissolving and are significantly more stable when exposed to solvents, as the network prevents the material from flowing or losing its shape even when swelling occurs.1