Physical Aging

Chain Mobility

The unoccupied space between the molecules, known as free volume, slowly disappears as the material rests at room temperature. During physical aging, the decrease in free volume restricts the movement of the polymer chains even further. This leads to a shift in the relaxation times of the material, which can be measured using thermal analysis or creep tests.

Because the material becomes denser, its response to external forces changes, often making it more prone to cracking under impact.

Property Drift

Precision instruments must account for these changes to maintain their accuracy over many years of service. For example, an optical lens made of plastic might experience a slight change in its focal length due to physical aging. In microelectronics, the internal stresses within a molding compound may increase as the material shrinks, potentially damaging the chip it is supposed to protect.

Engineers use the struik model to predict the rate of this aging and its impact on the long term reliability of the component. The process can be reset by heating the material above its glass transition temperature and then cooling it again.

Temporal Limit

Properties often stabilize after a certain period as the rate of change decreases logarithmically over time. This means the most substantial alterations occur shortly after the material is fabricated.