Thermodynamic Progression
Gradual release of excess energy from a glassy solid as its molecular structure slowly rearranges toward a lower energy state. Measuring enthalpic relaxation requires differential scanning calorimetry to detect the endothermic peak that appears at the glass transition during a heating scan. This phenomenon occurs when a material is stored below its transition temperature for an extended period.
Detection Method
Calorimetric analysis identifies the magnitude of energy loss by integrating the area under the recovery peak. Heating rate affects the perceived position and height of this peak, necessitating a standardized scan speed for consistent reporting. Comparison with a freshly quenched sample provides a baseline for calculating the total energy lost during aging.
Material Property
Physical aging often results in increased brittleness and a reduction in free volume. While the chemical composition remains unchanged, the structural densification alters the mechanical response to external loads. Brittle failure becomes more likely as the relaxation progresses.
Condition Boundary
Annealing above the glass transition temperature completely erases the thermal history of the specimen. Once the polymer chains regain mobility, the previous energy loss is reversed and the material returns to a disordered state. Industrial storage protocols aim to limit this progression by controlling ambient temperatures.
