Permeation Through Polymers

Permeation of small molecules through polymers takes place in four steps. In the first stage, the permeating molecules, know as the diffusants, wet or adsorb onto the polymer s surface. Secondly, the diffiisant molecules dissolve in the polymer. In the third step, the molecules diffuse down a concentration gradient towards the opposing surface. Finally, the diffiisant molecules desorb or evaporate from the surface, or are absorbed into another material. 

We can express the permeability of a difiusant through a polymer in terms of its diffusion rate and solubility according to Eq. 8.8. 

We report permeability in terms of volume of diffiisant passing through a film of unit thickness in a given time under a given pressure differential, such as  

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Campion, R.P., Permeation through Polymers for Process Industry Applications, MTI Publication No. 53, Materials Technology Institute of the Chemical Process Industries, St Louis, MO distributed by Elsevier Science, Amsterdam, The Netherlands, 2000. 

GM Zentner, JR Cardinal, J Feijen, S-Z Song. Progestin permeation through polymer membranes. IV. Mechanism of steroid permeation and functional group contributions to diffusion through hydrogel films. J Pharm Sci 68 970-975, 1979. 

K Burczak, T Fujisato, M Hatada, Y Ikada. Protein permeation through polymer membranes for hybrid-type artificial pancreas. Proc Jpn Acad B 67 83-88, 1991. 

Figure 14 The reversibilities of insulin permeation through polymer membranes in a two-compartment diffusion cell AH20 ( ), AS 15 (A), AS20 ( ), H ( ). Numbers indicate the content of styrene or HEMA in feed compositions in moles. H represents a cross-linked poly(2-hydroxyethyl methacrylate) (HEMA). (From Ref. 34.)... 

The theoretical aspects of permeation through polymers have been considered in some detail in a very comprehensive review by Lomax1,2 and by Hands in Handbook of Polymer Testing3. Only the basic concepts are necessary here and Lomax s review and the chapter by Hands are recommended for a more detailed consideration of the subject. The review describes and comments on virtually all known test methods at the time as well as considering the theory and providing a bibliography of almost 100 references, whilst the book chapter has the same scope and provides more recent references. 

The values of permeability coefficients for He, O2, N2, CO2, and CH4 in a variety of dense (isotropic) polymer membranes and the overall selectivities (ideal separation factors) of these membranes to the gas pairs He/N2,02/N2, and CO2/CH4 at 35°C have been tabulated in numerous reviews (Koros and Heliums, 1989 Koros, Fleming, and Jordan et al., 1988 Koros, Coleman, and Walker, 1992). Moreover, several useful predictive methods exist to allow estimation of gas permeation through polymers, based on their structural repeat units. The values of the permeability coefficients for a given gas in different polymers can vary by several orders of magnitude, depending on the nature of the gas. Thevalues oftheoverall selectivities vary by much less. Particularly noteworthy is the fact that the selectivity decreases with increasing permeability. This is the well-known inverse selectivity/permeability relationship of polymer membranes, which complicates the development of effective membranes for gas separations. 

Julian, T.N. Zentner, G. Ultrasonically mediated solute permeation through polymer barriers. J. Pharm. Pharmacol. 1986, 38, 871-877. 

Recent evidence indicates that the influence of molecular structure on gas permeation through polymers is complex. For example, reports investigating series of structurally varied polyimides (5-7), polyacetylenes (2), polystyrenes (2) and silicone polymers (12) show that gas transport rates within a particular polymer class can vary dramatically depending upon the structure of the monomer present. These observations on materials where the monomer changes while the functional "link" remains constant suggest that structural factors other than the polymer class are significant in determing gas transport properties. 

The way in which water permeates through polymers is much the same as other vapors and has been well characterized (5,6). As is the case with non-condensible gases, the permeability is a function of the sorption level and the diffusivity of the permeating substance ... 

Fabricate a "bench-top" model of a tri-layer, electrochemically-active protection device that greatly reduces hydrogen permeation through polymer "substrates"... 

Small molecules can penetrate and permeate through polymers. Beeause of this property, polymers have found widespread use in separation technology, protection coating, and controlled delivery [53]. The key issue in these applications is the selective permeability of the polymer, which is determined by the diffusivity and the solubility of a given set of low-molecular-weight compounds. The diffusion of a small penetrant oeeiu s as a series of jitmps... 

Adhesion is a more difficult problem. There are many chemically different interfaces in a laminant stack glass-EVA, EVA-solar cell surface (oxidized silicon or some anti reflection (AR) coating), cell back surface metallization-EVA, EVA-back sheet, and EVA-bus bars (copper or tinned copper). Each of these interfaces is important, because mechanically good adhesive bonds will often fail by delamination after exposure to humidity in the field. Water vapor will permeate through polymers and, if there is a non-chemically bonded surface, water may collect and cause failure by displacement. Tables I, II, and III provide details of some of the research efforts in adhesion. 

According to Guizard et al. [28], different approaches like the models proposed by Machado et al. [26] and Bhanushali et al. [22] stress the complexity of mechanisms involved in organic solvent permeation through polymer membranes. 

Rooney, M.L. and Holland, R.V. (1979) Singlet oxygen an intermediate in the inhibition of oxygen permeation through polymer films. Chem. Ind. 1979, 900-901. 

Cover image is adapted from Steward, P.A., Hearn, J. 8c Wilkinson, M.C. (1995) Studies in permeation through polymer latex films. 11. Permeation modification by sucrose addition. Polymer International 38 (1), 13-22. Copyright Society of Chemical Industry. Reproduced with permission. Permission is granted by John Wiley 8c Sons Ltd on behalf of the SCI. 

Steady-state permeability is defined as the flux of penetrant per unit pressure difference across a sample of unit thickness. Permeation through polymers is generally a three step process absorption of penetrant into the polymer matrix, diffusion of penetrant through the matrix and desorption of penetrant at the other side (Kirwan and Strawbridge, 2003). Thus, permeability is influenced both by the dissolution and the diffusion of the penetrant in the polymer matrix. In this sorption-diffusion model of penetrant transport across the polymer, permeability (P) is given as the product of diffusion coefficient () and solubility coefficient () (Callister and Rethwisch, 2010), that is ... 

The aims of this chapter are to outline the basic principles of the process of permeation through polymers and to demonstrate how the permeation properties can vary with both the nature of the polymer and those of the permeant. Indications are given of those areas where current research interest lies, along with brief outlines of the main areas of current commercial applications. 

Hubbell, W. H., Jr. Brandt, H. Munir, Z. A., Transient and steady-state water vapor permeation through polymer films. Journal of Polymer Science, Polymer Physics Edition 1975, 13, (3), 493-507. 

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