Thermodynamic Evolution
The gradual reduction of unoccupied space between disordered molecular chains drives amorphous materials toward a denser state below their glass transition temperature. This physical process, known as free volume relaxation, represents the slow evolution of a glass toward thermodynamic equilibrium. The contraction occurs because the molecular structures continuously rearrange over extended periods.
Structural Compaction
Molecular mobility decreases as the packing density of the polymer or metallic glass increases. During free volume relaxation, the local atomic clusters consolidate, restricting further structural movements. This consolidation modifies the localized diffusion rates within the material matrix.
Mechanical Property
The loss of internal unoccupied space alters how glassy materials respond to external loads. As free volume relaxation progresses, the material becomes more brittle and exhibits higher yield strength. These macroscopic changes occur because the molecular chains lack the local space required to slide past each other during deformation, turning a ductile polymer into a brittle one.
Thermal Signature
Differential scanning calorimetry measurements show the energy changes associated with structural aging. When a specimen undergoes free volume relaxation, it exhibits an endothermic recovery peak during subsequent heating scans. Measuring this peak allows researchers to quantify the extent of the structural transition.
