Meaning
A thermodynamic concept explains the diffusion of penetrant molecules through a polymer by focusing on the unoccupied space within the amorphous regions. For polymer films, the free volume theory relates the mobility of the polymer chains directly to the rate of gas and moisture permeation.
Molecular Mechanism
Thermal fluctuations generate transient micro-voids that allow small molecules to jump from one site to another. In the context of the free volume theory, the frequency of these jumps depends on the size of the penetrant and the flexibility of the polymer backbone. As temperature rises, the unoccupied space increases and accelerates the rate of diffusion.
Process Application
Extruders use these principles to modify the barrier properties of packaging materials by adjusting the crystallinity of the resin. When applying the free volume theory, increasing the crystalline fraction of a polymer reduces the unoccupied space because crystalline domains are densely packed and impermeable. This adjustment reduces the transmission rate of organic compounds and gases through the film.
Structural Limitation
The approach is less accurate when used for highly polar polymers where strong intermolecular forces restrict chain movement. For these materials, the free volume theory fails to account for specific polymer-penetrant interactions that alter the swelling of the matrix.
