In the case of the microporous membranes, the pores, as stated, are formed from the open intersegmental areas prefigured in
the casting solution. They are hypothesized to be of various polygonal shapes, framed by polymeric struts and walls, and to
be, like the zeolites, interconnected by openings in their common walls. It is these openings that are seen to be the metering
and retaining pores of the membranes. As stated, casting solutions of various polymer concentrations give rise to separate
but rather similar intersegmental distances. It is these spaces that are ultimately transformed into the membrane pores that,
in consequence, are also similar in their dimensions—except for the pore-size distribution that results from the casting solutions'
deviation from an ideal homogeneity.
Figure 3: Reticulated polyurethane foam.
Filtration is not a simple sieving process, except perhaps in the case where the filter is a track-etched membrane with pores
passing straight through a much thinner film (~10 μm). As stated however, for the membranes prepared by the casting process
(thinness ~150 μm), a fluid on passing through encounters pores of different diameters and surface areas. Particles are separated
from the fluid by adsorptive attachments to the pore surfaces as well as by the size exclusion mechanism. Thus, the meaning
of the average pore size as reflecting the size of a restrictive diameter within a pore passageway is necessarily an oversimplification.
Nevertheless, the events taking place at the pores are depicted as if the pores were continuous and integral paths through
the depth of the membrane.
Membrane characteristics are assessed because of their pertinence to aseptic processing. However, the pore-size distribution,
despite being an important structural feature that influences both flow and retention, is not among them. It is seldom known
or investigated although an ASTM method based on airflow rates enables its assay (9). The reason for not assaying the pore-size
distributions of filters is that complete organism removal is dependent upon the largest pores of the pore-size distribution
retaining the smallest particle of the particle-size distribution. This is the singular circumstance wherein an absolute filtration
can eventuate. Given this felicitous situation, only the size of the largest pore has pertinence. In fact, however, the distributions
of neither the pore nor organism sizes are likely to be known by the filtration practitioner. Depending on the relative numbers
of pores and particles and their sizes, particles may encounter the larger pores of the distribution to escape removal. These
larger pores of a distribution are assayed by bubble-point measurement recorded in pressure units (psi or bar) rather than
in dimensional units (μm). The accompanying smaller size pores are ordinarily not quantified or measured because they are
not seen to influence retentions.
Methods of pore-size rating
By porosimetry. Numerous methods have been used to assess pore size (10). Early on, mercury porosimetry was the method of choice (11–13).
In this procedure, mercury is forced under pressure to penetrate into membrane pores. This process is performed at increasing
differential pressures, ΔP. The higher the value of ΔP, the smaller the pores that the fluid metal can intrude. Quantitation of the different pore sizes relates to the volume of
mercury that is intruded in filling the pores at each of the progressively increasing differential-pressure stages. Airflow
porosimetry studies also have been performed (14, 15).
The porosimetry method is flawed by the assumptions necessary to its application. It is not suited to polymeric materials
that are at temperatures above their glass-transition points and whose pores are, therefore, liable to stretching and distortion
under the pressures of the intruding fluid. Also, the averaging of volume changes required by the technique may mask the true
dimensions of the "pores."