Which statement correctly describes the typical molecular structures of thermoplastics and thermosets?

• A. Thermosets are linear; Thermoplastics are crosslinked networks
• B. Thermosets are crosslinked networks; Thermoplastics are linear or lightly branched
• C. Both are linear polymers
• D. Thermosets are crosslinked networks; Thermoplastics are linear and lightly branched

Answer: (D)

Molecular architecture and crosslinking determine how a polymer behaves when heated. Thermosets form a crosslinked network during curing, meaning many polymer chains are bonded together in three dimensions. That covalent framework locks the structure in place, so the material does not flow when heated, is insoluble, and cannot be melted to reform. Thermoplastics, by contrast, lack permanent crosslinks; their chains are long molecules that are either linear or only lightly branched. Without a network, these chains can slide past each other when heated, allowing the material to soften or melt and then re-harden on cooling. This combination—a crosslinked network for thermosets and linear or lightly branched chains for thermoplastics—best captures the typical molecular structures and explains their distinct processing behaviors. The other statements misrepresent crosslinking or the fundamental difference, which is why they don’t fit as well.