A polymer can contain amorphous regions, crystalline regions, or both. Their temperature-driven changes are therefore distinct.

The glass transition $T_g$ concerns segmental mobility in amorphous material. Below the relevant glass-transition range, chain rearrangement is slow and the polymer tends to respond more glass-like; above it, amorphous segments can rearrange on shorter time scales and the response becomes more compliant.

The melting temperature $T_m$ instead applies to crystalline regions. At melting, ordered polymer crystals lose their structure and become a disordered melt.

An amorphous polymer has a $T_g$ but no crystalline melting point. A semicrystalline polymer can exhibit both $T_g$ in its amorphous fraction and $T_m$ for its crystals.

Polymer architecture strongly shifts these transitions. Stiff backbones, bulky side groups, strong intermolecular interactions and crosslinks can restrict segmental motion and raise $T_g$. Chain regularity and crystal stability influence $T_m$ and the achievable crystalline fraction.

Design therefore depends on service temperature relative to both transitions. A polymer that is tough and flexible above $T_g$ may become brittle when cooled below it, while approaching $T_m$ can destroy the dimensional stability supplied by crystalline regions.