Cross-linking in polymers is typically achieved by what kind of bonds?

• A. Physical entanglements that are easily reversible
• B. Irreversible covalent bonds between chains
• C. Ionic bonds that can be reversed by water
• D. Hydrogen bonds that melt upon heating

Answer: (B)

Cross-linking in polymers relies on actual bonds that connect different polymer chains to form a network. This network restricts chain mobility, boosting stiffness, heat resistance, and solvent stability. Physical entanglements aren’t bonds; they’re just chains interwoven, so they can rearrange or disentangle under heat or solvent—offering less permanence. Ionic bonds can provide cross-links, but they’re often sensitive to the environment (water can screen or disrupt them), making them less robust as permanent links. Hydrogen bonds are even weaker and tend to break with heating, so they don’t create a stable, permanent network. Irreversible covalent bonds between chains give the durable, cross-linked structure that’s typical in thermoset-like polymers.