Why do network polymers break down instead of melting when reheated?

Network polymers, also known as thermosetting resins, do not melt upon reheating because of their unique molecular architecture. Unlike linear or branched polymers, which consist of separate molecules that can flow past one another when intermolecular forces are overcome by heat, network polymers are characterized by a complex, three-dimensional structure where chains are interconnected by strong covalent chemical bonds.1234

During their initial formation, these polymers undergo a cross-linking reaction that effectively turns the entire finished article into one single, massive molecule. Because these covalent bonds are significantly stronger than the intermolecular forces found in thermoplastics, the molecules cannot move or flow independently. Consequently, when heat is applied to a network polymer, it cannot reach a molten state. Instead, the intense energy eventually causes the chemical bonds to rupture, leading to the chemical decomposition or breakdown of the material.134