What characteristic of thermosets prevents them from softening when heat is applied?

Thermosets are a category of polymers that, once cured through a chemical reaction, form a rigid structure that does not soften upon heating. The key characteristic that enables this behavior is the extensive covalent bonding present in their molecular structure. During the curing process, these polymers undergo a transformational network formation that creates strong cross-links between the polymer chains.

These covalent bonds are much stronger than the weaker van der Waals forces found in thermoplastics, which do soften and can be reshaped upon heating. Because of the strong covalent bonds and the three-dimensional cross-linked structure of thermosets, they maintain their integrity at elevated temperatures and do not flow or soften, which is a significant advantage in many applications where heat resistance is critical. This feature is what distinguishes thermosets from thermoplastics, which can be reheated and reshaped.

In contrast, other factors listed, such as low molecular weight, lack of cross-linking, and the design to melt at high temperatures, do not accurately represent the fundamental properties of thermosets, further affirming the importance of extensive covalent bonding in ensuring their thermal stability.