Permeability is a general term used to describe the property of a material to allow the passage of some substance through it. Permeation takes place by diffusion. Measurements of permeability refer to the rate of transmission through a film.
Diffusion can be defined as the movement of atoms, molecules or ions through a gas, liquid or solid. In diffusion, gases, vapours (i.e. water vapour) and liquids pass through permanent or transient voids between polymer molecules. The diffusion rate therefore depends considerably on the size of the small molecules and the size of the gaps. It also depends on the partial pressure gradient across the film and intermolecular forces (hydrogen bonding, Van der Waals forces). For polymers, the size of the gaps depend on the physical state of the polymer, that is, whether it is glassy, rubbery or crystalline. In the case of amorphous polymers above Tg, molecular segments have considerable mobility and there is an appreciable 'free volume' in the polymer mass. Also, because of segment mobility, there is a good chance that a molecular segment will at some stage move out of the way of a diffusing small molecule. Below Tg, the segments have little mobility and there is a reduction in free volume. There will be fewer voids and a diffusing molecule will have to take a much more tortuous path to get through the polymer. Around the Tg there are often complicating effects as the diffusing molecule may plasticize the polymer thus effectively reducing the Tg (e.g. PVAL and water). Crystalline structures have a much greater degree of packing and the individual lamellae can be considered to be almost impermeable. Diffusion can therefore only take place through amorphous zones or through zones of imperfection. Hence, crystalline polymers tend to resist diffusion more than either rubbery or glassy polymers. Unfortunately penetration of solvent into the polymer mass will be similarly restricted and these materials are difficult to formulate and use as coatings. Crosslinked polymers tend to resist diffusion but are generally unsuitable for conservation coatings.
Permeation can be defined as a three-part process. It involves dissolution of small molecules in the polymer, migration or diffusion through the polymer according to the concentration gradient and emergence of the small particle at the other side. Hence, permeability is a product of chemical compatibility and diffusion. Henry's law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas at a given temperature. Most atmospheric gases have relatively low solubilities and obey Henry's law but diffuse freely through amorphous regions of solid polymer. Vapours of organic substances with similar solubility parameters to the polymer have relatively high solubilities, with deviations from Henry's law, but diffuse more slowly as a result of comparatively larger molecular size and often strong interactions with polymer chains.
Polymers with low permeability to both gases and vapours include poly(vinylidene chloride) and co-polymers, acrylonitrile-styrene co-polymers, epoxides, poly(vinylidene fluoride), poly(ethylene terephthalate) (polyester), poly(vinyl chloride). These may be encountered in museum objects but are not suitable for use as conservation materials as they either have restricted solubility or are thermosetting.
In general, coatings are not complete vapour barriers. They do not prevent the transmission of vapours but reduce the amount transmitted in a given time. This property is valuable in the absence of stable environmental conditions. The effect of coatings in reducing the rate of dimensional change in hygroscopic materials, especially wooden panels, has been well demonstrated (Buck, 1961). Unfortunately, many sensitive objects are coated only on one side. When environmental conditions are stable, water vapour will pass through most coatings until equilibrium is established and in most cases this will happen surprisingly quickly. Some waxes, for example paraffin wax, possess good barrier properties to water vapour and are better in this respect than the synthetic thermoplastic polymers used as conservation coatings. The moisture exclusiveness of finishes on wood is reviewed by Feist et al. (1985).
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