Why must polymer particles be above their glass transition temperature to coalesce easily?

Polymer particles must be above their glass transition temperature ($T_g$) to coalesce because this state transition is essential for particle mobility and film formation.12

Below the $T_g$, polymer molecules are in a rigid, glassy state where they are essentially frozen, with little to no movement of molecules past one another. In this state, the material is stiff and brittle, preventing the particles from flowing together to form a continuous film.321

When the temperature rises above the $T_g$, the polymer enters a rubbery state. In this state, the molecules exhibit increased mobility, allowing them to move past one another and flow together under surface-mediated forces. This process, known as coalescence, enables the individual particles to fuse into a clear, solid film. If a polymer is naturally below its $T_g$ at application temperatures, a coalescent—a solvent that plasticizes the polymer to lower its effective $T_g$—is often added to facilitate this necessary mobility.132