How do sulfur cross-links change rubber's polymer chains?

Sulfur cross-links transform rubber by chemically bonding its long, flexible polymer chains into a stable, three-dimensional network.12

In its raw state, rubber (such as cis-polyisoprene) consists of independent polymer molecules that can slide past one another, causing the material to flow like a viscous liquid when warm or crystallize and harden when cold. During the process of vulcanization, sulfur atoms react with carbon atoms near the double bonds of the polymer chains, creating sulfur bridges (—S${}_n$—) between adjacent molecules.132

These cross-links have several critical effects:
* Structural Stability: By permanently tying the molecules together, the sulfur bridges prevent the chains from sliding, which stops the rubber from flowing or becoming soft at high temperatures.1
* Improved Elasticity: The network allows the material to remain resilient and elastic across a much wider range of temperatures.12
* Reduced Crystallization: The addition of sulfur makes the molecular structure sufficiently irregular to impede the crystallization that otherwise causes rubber to harden at low temperatures.1
* Enhanced Durability: The resulting solid gains greater tensile strength and improved resistance to swelling, abrasion, and solvents.2