The Relationship among Pore-Size Ratings, Bubble Points, and Porosity - Pharmaceutical Technology

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The Relationship among Pore-Size Ratings, Bubble Points, and Porosity

Pharmaceutical Technology

The membrane pores

Microporous membranes can be prepared by various methods. Among these is the track-etch process wherein a polymeric, dielectric film is bombarded with heavy-mass fission fragments, followed by an alkali etch of the radiation-damaged pathway. This process creates pores that are straight through and regularly cylindrical in shape. Their diameters can be accurately measured with scanning electron microscopy. Despite the relative regularity of their pore structures, these membranes do not usually find application in pharmaceutical processing. The membranes used in pharmaceutical processes are almost exclusively prepared by the phase-inversion technique, generally referred to as the "casting method" (4).

Little is known about the numbers, sizes, and shapes of the pores of microporous membrane so prepared. The membrane structure usually is pictured as being analogous to that of a polymeric sponge. A hypothesized oversimplification of the pore passageways is that of irregular and tortuous capillaries that are, therefore, more extended in length than the filter's surface-to-surface thickness. The pores are marked by irregularly restricted diameters that provide the choke-points that interfere with particle passage. However complex, the pores are pictured as being essentially cylindrical and composed of interconnected spaces extending through the depth of the polymer matrix.

The very concept of a definable "pore" is an artificiality when applied to microporous membranes other than the straight-through columnar pores that characterize the track-etched variety. The complex geometry of the sponge-like membrane results in the pores having ratios of cross-sectional areas to perimeters, called the "hydraulic parameters." These vary over the entire thickness of the membrane (5). A membrane's depth can be constructed of several superimposed unit planes that in their aggregate impose their effect on retention and flow rates (6). The "pores" so considered are presumably connected throughout the unit planes to constitute pathways for fluid flows. However, where particle retentions interfere flow redistributions may result through new "pore" alignments. The "pore" concept arises as a hypothetical construct useful in understanding filter performance. Unlike the track-etched pores, they are not integral, structural pathways for fluid flow.

Pore architecture

Figure 1: Free-floating soap bubbles.
The pore structure derives from a cast polymer solution wherein the polymer chain segments are separated from one another by distances that reflect the degree of dilution. It is the inter-segmental distances among the polymeric chains that in their interconnections prefigure the "pores" of the finished membrane. Formulae of various polymer concentrations give rise to different intersegmental separations, ultimately to different porosities, when by proper manipulations the polymer is precipitated as a gel, to be washed and dried to its solid, microporous membrane state. There is inevitably a pore-size distribution and some anisotropic pore shape formation (4).

Figure 2: Detergent foam between two glass plates.
It is hypothesized that the formation of the microporous membrane structure accords with the known phenomenon of "soap bubble clustering" (7). The reasons for this resemblance is that in both cases there is the coming together of spheres whose spatial clustering is under the influence of area-minimizing forces. The geometric consequences of these forces is known from the study of soap bubbles (8). Polygonal facets characterize the resulting spaces of a free-floating cluster of soap bubbles (see Figure 1). In the pore formation, the nonsolvent of the casting solution takes the place of the air of the bubbles. In support of such structures, Figure 2 is that of a detergent foam confined between glass plates. The polyhedral spatial structures are obvious. Figure 3 is of a reticulated polyurethane (polymeric) foam. The cellular pores can be seen, in fact, to be polygonal in shape. The phenomenon of clustering through polyhedral spatial arrangements is manifested in other settings as well. It is a trait of zeolitic molecular sieves whose interconnection is through the open panels common to contiguous polyhedra, albeit caused by crystal-packing rather than area-minimizing forces (7). There is, therefore, technical support for the concept of polyhedral microporous structures.


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