Permeability of siloxane polymers

Another type of membrane designed as an artificial skin coating for bums also exploits the high permeability of siloxane polymers.18,203 The inner layer of the membrane consists primarily of protein and serves as a template for the regenerative growth of new tissue. The outer layer is a sheet of silicone polymer, which not only provides mechanical support, but also permits outward escape of excess moisture while preventing ingress of harmful bacteria. 

Madkour, T. Kloczkowski, A. Mark, J. E. Roe, R. J., Molecular Dynamics Investigations into the High Permeability of Siloxane Polymers. In Computational Methods in Materials Science, Mark, J. E. Glicksman, M. E. Marsh, S. P., Eds. Materials Research Society Pittsburgh, PA, 1992 Vol. 278, pp 33-40. 

The importance of oxygen and moisture permeability of siloxanes has been discussed in Section 3.12.4.9. Among many new siloxane polymers and co-polymers, novel polyamide PDMS multiblock co-polymers were evaluated for gas permeability and thermomechanical properties.600... 

The use of siloxane polymers in applications such as separation membranes, drug release systems, and blood oxygenators requires extensive permeability studies. This, of course, also involves measurements of diffusivity and solubility.16... 

Tsujita Y, Yoshimura K, Yoshimizu H, Takizawa A, Kinoshita T, Furukawa M, Yamada Y, Wada K (1993) Structure and gas permeability of siloxane-imide block copolymer membranes 1 effect of siloxane content. Polymer 34(12) 2597... 

When siloxane chains becomes longer than tetrasiloxane, the polymers become waxy and eventually lose their self-supporting property. The polymers show only one Tg, and the absolute Tg value is lowered significantly by the intr uction of the siloxane linkage. This clearly indicates that oligosiloxanes do not phase-separate from the main-chain component and distribute homogeneously in the membrane. (Jxygen permeability of the polymers are also depicted in Table 1. 

Medical. There are numerous medical applications of siloxane polymers (46), Prostheses, artificial organs, facial reconstruction, and catheters, for example, take advantage of the inertness, stability, and pliability of the polysiloxanes. Artificial skin, contact lends, and drug delivery systems utilize their high permeability as well. 

As expected, the degree of crosslinking in the cured sih-cone has an effect over the permeability of the polymer. A certain degree of crosslinking is needed for a proper stabihty and durability of the membrane but, at the same time, an excess of crosslinking will decrease the permeability. The higher the crosslinking, the lower the siloxane permeability [47,66,68]. 

PDMS based siloxane polymers wet and spread easily on most surfaces as their surface tensions are less than the critical surface tensions of most substrates. This thermodynamically driven property ensures that surface irregularities and pores are filled with adhesive, giving an interfacial phase that is continuous and without voids. The gas permeability of the silicone will allow any gases trapped at the interface to be displaced. Thus, maximum van der Waals and London dispersion intermolecular interactions are obtained at the silicone-substrate interface. It must be noted that suitable liquids reaching the adhesive-substrate interface would immediately interfere with these intermolecular interactions and displace the adhesive from the surface. For example, a study that involved curing a one-part alkoxy terminated silicone adhesive against a wafer of alumina, has shown that water will theoretically displace the cured silicone from the surface of the wafer if physisorption was the sole interaction between the surfaces [38]. Moreover, all these low energy bonds would be thermally sensitive and reversible. 

Interesting polymers have been produced by copolymerization of siloxanes with other polymers. For example, dimethylsiloxane was copolymerized with ethylene oxide and methyl methacrylate to create a series of polymers with controlled permeability to hydrophilic or hydrophobic steroids, enhanced mechanical properties, and improved adhesion to tissues [15-17]. 

Siloxane polymers (better known in the commercial market as silicones) have widespread applications, a few of which are personal care products, greases, sealants, varnishes, waterproofing materials, synthetic rubbers and gas-permeable membranes such as those used in soft contact lenses. Medical applications are increasing in importance, although the use of silicone breast implants is a controversial issue because of claims of the migration of low molecular weight siloxanes from the implant into the surrounding body tissue. 

Other parameters requiring consideration in polymer design are wettability, dimensional stability and refractive index. The enhancement of oxygen permeability in siloxane lens systems is associated with high relative proportions of silicon-oxygen and silicon-carbon bonds. These long bonds lead to a free volume element which in the case of polydimethylsiloxane is 5-6 times greater than that for polymethacrylates [3]. 

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