Why can thermoplastics be reformed while thermosets cannot?

The ability of thermoplastics to be reformed while thermosets cannot is primarily due to the molecular structure and bonding characteristics of these materials. Thermoplastics consist of long chains of polymers that are held together by relatively weak van der Waals forces and are generally not cross-linked. This means that when heat is applied, thermoplastics can soften and melt, allowing them to be molded, shaped, and reformed multiple times without undergoing any significant change to their chemical structure.

In contrast, thermosets are characterized by their strong covalent cross-links that form during the curing process. Once these cross-links are established, the material becomes rigid and cannot return to a liquid state, even when heat is applied. This irreversible transformation limits thermosets to a single shaping process, after which they cannot be remolded or reshaped.

The other options presented relate to various properties of materials but do not directly address the fundamental reason behind the ability to reform thermoplastics versus the rigidity of thermosets. Therefore, the explanation focusing on the non-cross-linking nature of thermoplastics accurately captures the essential difference between these types of polymers.