In the use of polymer materials, a significant number of research employ what is called “guest diffusion” in order to “dye” the polymer material a different color. This process occurs based on polymer membrane selectivity due to the following factors:
- separation of differential solubility of guest molecules at the surface of the membrane (known as sorption)
- varying diffusion characteristics of different guest molecules or ions across the membrane
- desorption of molecules or ions on the other side of the membrane
For gas and liquids factors 1 and 2 are vital, while in ion transport all three factors are influential. An example of the mechanism described in (1) is that hydrophilic molecules will dissolve in a hydrophilic surface faster than hydrophobic molecules. However, in general, specific functional group determine solubility, attachment and entry into the membrane. When diffusion occurs through a solid polymer, it is dependent on affinity and available space between chains. In these cases, the molecular space can be influenced by whether the polymer is above or below the glass transition temperature. When it is above Tg, greater chain movement is allowed, and therefore more free volume and permeability change for certain guest molecules.
1. Allcock, H. Introduction to Materials Chemistry. Hoboken, NJ: Wiley, 2008.