Single-component materials, permeability

Single Component System. In a single component system, the drug is encapsulated in its pure form and release rates are essentially zero order (72). Polydimethylsiloxane and polyethylene are the materials most often chosen for encapsulation. Table HI shows some typical release rates reported by Kind, et al. (14), for various steroids through silicone rubber. Clearly, when a solution-diffusion mechanism controls the drug release rate, drug permeabilities can be expected to vary widely. 

Polymer blends have been categorized as (1) compatible, exhibiting only a single Tg, (2) mechanically compatible, exhibiting the Tg values of each component but with superior mechanical properties, and (3) incompatible, exhibiting the unenhanced properties of phase-separated materials (8). Based on the mechanical properties, it has been suggested that PCL-cellulose acetate butyrate blends are compatible (8). Dynamic mechanical measurements of the Tg of PCL-polylactic acid blends indicate that the compatability may depend on the ratios employed (65). Both of these blends have been used to control the permeability of delivery systems (vide infra). 

The design of cover systems is site-specific and depends on the intended function of the final cover—components can range from a single-layer system to a complex multilayer system. To minimize percolation, conventional cover systems use low-permeability barrier layers. These barrier layers are often constructed of compacted clay, geomembranes, geosynthetic clay liners, or combinations of these materials. 

The item here called a conductivity factor has various names— permeability, diffusivity, etc.— that sometimes emphasize the host material (e.g., permeability of sandstone ) and sometimes emphasize the traveling material (e.g., diffusivity of hydrogen ). The factor in reality always depends on both host and traveler it is a property of the transport situation as a whole. Sometimes it is useful to separate out two components such as mobility of the diffuser and tortuosity of the matrix but for present purposes we shall stay with a single comprehensive factor. The terms permeability and diffusivity may be used from time to time, but we shall try to maintain the view that any conductivity factor is acceptable, under whatever name, as long as its units are clearly in view. 

Table 20.2-S presents the system parameters determined by Ward et al. for producing 10 x 10 SCFD of 30% oxygen with single-stage 1000 A ultrathin silicone-polycarbonate copolymer membranes. In addition to their thinness, these membranes had high permeabilities due to their silicone rubber component (57% on a mole basis) (see Fig. 20.2-7). The separation factor for the family of silicone-polycaibonate materials shown in Fig. 20.2-8 increases as the fraction of flexibilizing silicone decreases. If one considers the ratio of permeabilities at 0% silicone and at 57% silicone, it is clear that an aqrproximately lOO-foU increase in permeation area is required to achieve the same oxygen productivity far pure polycarbonate membranes as for the 57% copolymer. Thus, roughly 7.8 x 1(T ft of membrane area widi a 1(K)0 A thick separating layer would be required to supply the same absolute amoum of oxygen in the product gas for the polycarbonate case as compared to 78,CW0 ft for the copolymer case.

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