Thermosetting polymers are characterized by what?

• A. Low cross-link density and reversible melting
• B. High cross-link density leading to hard, brittle, and chemically set plastics
• C. Ability to be remelted and reshaped repeatedly
• D. Incompatibility with any cross-linking

Answer: (B)

Thermosetting polymers harden into a highly cross-linked network during curing, which locks the molecular chains in place and prevents them from flowing. This high cross-link density makes the material very rigid and brittle, and once cured, it is chemically set and does not melt upon heating (it may decompose instead). That combination of a dense, permanent network and resulting hardness and brittleness is exactly what characterizes thermosetting polymers, so the description of a hard, brittle, and chemically set plastic with high cross-link density is the best match.

The other ideas describe thermoplastics: they have lower cross-link density, soften or melt when heated, and can be remelted and reshaped repeatedly, or they lack cross-linking altogether, which wouldn’t yield a cured, thermoset network.