How can a solvent test distinguish cross-linked rubber from an uncross-linked polymer?

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A solvent test is an effective way to distinguish between cross-linked rubber and uncross-linked polymers because of how their molecular structures interact with liquids.

When you place a polymer in a compatible solvent, the outcome depends on whether the molecules are permanently connected:

Uncross-linked polymers: Because these molecules are not chemically bonded to one another, they will eventually disentangle and dissolve into the solvent, forming a solution.
Cross-linked rubber: Because the molecules are permanently tied together by covalent bonds—a process known as vulcanization—they cannot separate from one another. Consequently, the material will not dissolve; instead, it will merely swell as the solvent penetrates the molecular network.1

This difference is critical because cross-linking is what prevents the material from flowing like a liquid, providing the necessary solvent resistance and thermal stability required for useful rubbery materials.1