Meaning
Glassy polymers that have been cooled rapidly from their molten state undergo a slow, time-dependent relaxation toward thermodynamic equilibrium. This structural recovery is the molecular process by which the polymer’s volume and enthalpy slowly decrease over time below the glass transition temperature. It sets the baseline for the drift in physical properties that occurs as the material adjusts to its storage temperature.
This relaxation bounds the material’s mechanical stability during long-term storage.
Equilibrium Convergence
When a polymer is cooled below its glass transition temperature, it is trapped in a non-equilibrium state with excess thermodynamic properties. The rate of structural recovery depends strongly on how far the material is from its equilibrium volume and the temperature of storage. As the material relaxes, the relaxation rate decreases because the reduced free volume restricts molecular motion.
This self-retarding behavior makes the convergence to equilibrium highly non-linear.
Measurement Method
Tracking this process requires high-precision thermal and dilatometric tests that measure changes in heat capacity or volume over long times. Differential scanning calorimetry is typically used to measure the change in enthalpy during relaxation. It provides the data needed for material modeling.
Mechanical Consequence
The progress of this molecular rearrangement causes the polymer to become stiffer and more brittle. This drift must be monitored during quality checks of stored parts.
