Barrier Coatings: Permeation of Molecules through Polymers
A molecule that is soluble in the film and can diffuse through the film can permeate a plastic film. As such, in an amorphous film, small-sized molecules find sites to occupy in loosely entangled bundles of molecules or occupy in the spaces between the molecules. A successful diffusion is marked with free movement of the diffusing molecules from site to site. In case of the disruptive polymer chain or bundle of chains, until the natural vibration of the chain opens up a hole, the molecule must wait. Besides, the rest of the chain and holes open rather easily for a relatively flexible polymer molecule, providing ready accommodation for the movement of one part of the chain. In contrast, stiff and bulky polymer molecules demonstrate relatively slow and infrequent movement. As such, amorphous (without a clearly defined shape or form) polymers with a high diffusion rate for small molecules are poor barriers. And this barrier increases substantially for stiff and bulky polymers, like, polyarylates containing large aromatic units.
To produce a high barrier to small molecules, crystallinity serves as an effective way because the diffusion rate of small molecules is reduced by any mechanism that reduces the mobility of the polymer system. The density and order of polymer crystallites limit the diffusion and dissolution of small molecules through them. This suggests the perfect barrier disposition of a polymer with a completely crystalline structure. This paper will later discuss the impracticability of achieving these structures and their limitations in yielding commercially useful coatings. All real crystalline polymers comprise amorphous or disordered regions which lead to the structure’s permeability. Thus, the level of crystallinity determines the level of barrier.