Why can thermoplastics be softened and reshaped by heating?

Thermoplastics can be softened and reshaped by heating because of their unique molecular structure, which consists of individual, separate molecules that are not chemically cross-linked. Unlike thermosets, which form an infusible, insoluble network through chemical reactions, the molecules in thermoplastics are free to move and flow past one another when sufficient energy is applied.123

At low temperatures, these polymers exist in a glassy state, where molecular motion is restricted. As the material is heated above its glass transition temperature (T_g), the molecules gain increased mobility, allowing the polymer to enter a rubbery state where it becomes plastic and stretchable. For semicrystalline thermoplastics, further heating to the melting temperature (T_m) destabilizes crystalline regions, allowing the material to flow and be molded or extruded into new shapes. Upon cooling, the polymer regains its dimensional stability.425