Meaning
Kinetic process describing the change in unoccupied space between molecules or atoms as a material undergoes aging or deformation. In polymers and amorphous metals, free volume evolution directly influences the mobility of the constituent particles and the overall mechanical response of the system. It governs the transition from a ductile to a brittle state as the material densifies over time.
Mobility Mechanism
Molecular motion requires enough local clearance to allow segments to jump or rotate. As a glass relaxes, the free volume evolution leads to a reduction in these voids, which increases the viscosity and decreases the diffusion rate. This densification is often measured by tracking changes in density or using positron annihilation lifetime spectroscopy.
Mechanical Performance
Toughness in many engineering plastics depends on the presence of sufficient free volume to facilitate plastic flow. If the process of aging proceeds too far, the loss of interstitial space makes the material prone to crazing and sudden failure under load. Designers must account for this shift when calculating the long-term load-bearing capability of structural components.
Processing Window
Injection molding cycles involve rapid changes in temperature that freeze in a high initial void fraction. Controlled reheating can accelerate the free volume evolution to reach a stable state before the part enters service. This step prevents later dimensional changes that could compromise tight tolerances.
