Meaning
Mathematical framework describes the transport of small molecules through a polymer matrix by linking mobility to the unoccupied space between chains. The free volume diffusion model assumes that penetrants can only move when a local fluctuation in the polymer structure creates a gap of sufficient size. It is widely used to predict the shelf life of packaged goods and the efficiency of gas separation membranes.
Permeation Rate
Size of the penetrant molecule relative to the available gaps determines how quickly it passes through the material. Under the free volume diffusion model the rate of movement increases as the temperature rises because thermal energy creates more frequent and larger gaps. This relationship helps engineers select resins that provide an effective barrier against oxygen or moisture.
Larger molecules like aromatic hydrocarbons require much more space to move, resulting in lower measured permeation rates.
Molecular Mobility
Chain flexibility and the presence of side groups influence the amount of empty space in the glassy or rubbery state. A more rigid backbone reduces the applicability of the free volume diffusion model because the gaps are smaller.
Theoretical Framework
Predictions made using this approach rely on the fractional amount of space not occupied by the atoms themselves. While the free volume diffusion model provides a reliable estimate for many systems, it must be adjusted for polymers with specific chemical interactions like hydrogen bonding. This ensures that the calculated values align with the measured data from laboratory tests.
