Understanding the thermal behavior of polymers is defined by several key transition temperatures. 1. Glass Transition Temperature (Tg): This is the critical temperature at which an amorphous polymer or the amorphous regions of a semi-crystalline polymer transition from a hard, glassy state to a softer, rubbery (high-elastic) state upon heating. It represents the onset of segmental molecular chain motion and is often considered the upper service temperature limit for rigid applications of the material. 2. Melting Temperature (Tm): For crystalline polymers, Tm is the temperature at which the ordered crystalline structure breaks down, and the polymer transitions into a disordered viscous melt. It is the minimum temperature required for processing techniques like injection molding of these materials. 3. Flow Temperature (Tf): This refers to the temperature at which an amorphous polymer transitions from a rubbery state into a viscous flow state, becoming fluid enough for processing under pressure. It marks the lower bound of the practical processing window for amorphous plastics. 4. No-Flow Temperature: This is a practical processing parameter defined as the highest temperature at which a material does not flow under a specific pressure (e.g., 50 MPa in a capillary rheometer). It is determined through a standardized test involving stepwise heating under constant pressure until flow is observed, helping to define the lower temperature limit for molding. 5. Decomposition Temperature (Td): If a polymer in the melt state is heated excessively, the energy causes the molecular chains to break down chemically. Td is the temperature at which this thermal degradation becomes significant and rapid, leading to irreversible loss of properties and the generation of volatile products, thus defining the absolute upper temperature limit for processing.
