Why can thermoplastic adhesives be softened by heating?

Thermoplastic adhesives can be softened by heating because of the inherent molecular structure and mobility of thermoplastic polymers.12

At low temperatures, these polymers exist in a glassy state, where molecules are essentially frozen with little movement. As the material is heated to its glass transition temperature ($T_g$), the molecules gain enough energy to transition into a rubbery state, characterized by increased molecular motion.12

For many thermoplastics, further heating leads to the melting temperature ($T_m$). At this stage, the crystalline regions—which provide structural rigidity—become unstable, and the polymer chains are able to flow past one another. Because thermoplastic adhesives consist of linear or branched structures rather than the rigid network structures found in thermosetting plastics, they can transition into a molten, liquid state without degrading, allowing them to be applied as hot liquids.123