Interaction and Transport Mechanisms

Diffusion is based on the attempt of molecules to spread out over the entire space available. 

The transport of low-molecular substances through polymers is based mainly on activated diffusion that depends on polymer structure, temperature, and on the molecular size of the diffusing substance and interaction between the substance and the polymer. However, it can also proceed via submicroscopic capillaries present as micropores in the polymers [86]. 

The possibility and ease of free volume formation depend on the mobility of the macromolecules. The amount of free volume can be calculated from the density in the solid and the molten state, and is approx. 2.5 vol.% at glass transition. Only when there is more than 2.5% free volume, macromolecules can move sufficiently [1]. If the polymer chains are mobile and flexible, very little energy is required to move individual segments away from each other [32]. 

The absorption of a medium may lower glass transition temperature considerably, depending on the type and amount of medium, which will also change diffusion behavior noticeably. For instance, in amorphous plastics, the diffusion coefficient is very small below glass transition temperature. Medium absorption will increase the diffusion coefficient by entire orders if it lowers the glass transition temperature. 

The rate of molecular diffusion can be calculated by Pick s first law  

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