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How do sulfur cross-links prevent rubber molecules from flowing past each other?

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Sulfur cross-links prevent rubber molecules from flowing past each other by chemically bonding the long, flexible polymer chains into a permanent, three-dimensional network.12

In their natural state, rubber molecules are like entangled spaghetti; while they can stretch, they are essentially a viscous liquid that will disentangle and flow under stress, causing the material to become soft and sticky when warm. During the process of vulcanization, sulfur reacts with the polymer chains—specifically at sites near carbon-carbon double bonds—to form covalent bridges (cross-links) between adjacent molecules.12

Because these molecules are now permanently tied together by these sulfur bridges, they cannot slide past one another even when heated. This transformation converts the material into an elastic solid that maintains its shape and structural integrity, allowing the rubber to revert to its original configuration once any applied tension is released.13