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.
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 1: Free-floating soap bubbles.
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.
Figure 2: Detergent foam between two glass plates.