2 The Art of Making Polymeric Membranes
2.5 Characterization of Membrane by Different Techniques
2.5.1 Conventional Physical Methods to Determine Pore Size and Pore Size Distribution
2.5.1.1 Bubble Gas Transport Method
This method is based on the measurement of pressure necessary to blow air through a water-filled porous membrane [9,85]. This technique is also called the bubble point method. This method is only able to determine maximum pore size present in the pore distribution, corresponding to the minimum pressure necessary to blow the air bubble first observed. Figure 2.8 shows a schematic drawing of the test apparatus. The top of the filter is placed in contact with water. When the membrane is wet, all the pores are filled with liquid. The bottom of the filter is in contact with air. When the air pressure is gradu-ally increased, bubbles of air penetrate through the membrane at a certain pressure.
The relationship between pressure and pore radius is given by the Laplace equation:
rp =
(
2ϒ ∆/ P)
cosq
(2.1)where rp is the radius of a capillary shaped pore, ° is the surface tension at the liquid/air interface, ΔP is the pressure and q is contact angle (usually, contact angle θ = 0o (or cos θ = 1) is used in the calculation). Penetration will first occur through the largest pores and since the pressure is known, the pore radius can be calculated from Equation 2.1.
This method can only be used to measure the largest active pores in a given membrane
Figure 2.8 Schematic drawing of a bubble-point test apparatus.
56 Handbook of Polymers for Pharmaceutical Technologies
and has therefore become the standard technique used by suppliers to characterize their (dead-end) microfiltration membranes.
2.5.1.2 Mercury Intrusion Porosimetry
Mercury intrusion porosimetry is used extensively for the characterization of various aspects of porous media, including porous membranes and powders, and is applicable to pores from 30 Å to 900 Å in diameter. It is well commercialized.
Mercury intrusion porosimetry involves placing the sample in a special sample cup (penetrometer), surrounding the sample with mercury. Mercury is a non-wetting liq-uid to most materials and resists entering voids, doing so only when pressure is applied.
The pressure at which mercury enters a pore is inversely proportional to the size of the opening to the void. As mercury is forced to enter pores within the sample material, it is depleted from a capillary stem reservoir connected to the sample cup. The incremental volume depleted after each pressure change is determined by measuring the change in capacitance of the stem. This intrusion volume is recorded with the corresponding pressure or pore size. Both pore size and pore-size distribution can be determined by this technique.
In this technique there are a few disadvantages:
i. It is expensive and not widely used;
ii. It needs high pressure which could damage the surface;
iii. It measures all the pores present in the structure, including dead-end pores.
2.5.1.3 Gas Liquid Equilibrium Method (Permporometry)
Permporometry is the only method suitable for the determination of the size distri-bution of the active pores with diameters ranging from about 1.5 nm to 0.1 μm in porous media, particularly those with an asymmetric (and/or composite) structure.
Permporometry, a relatively new technique, is based on the controlled blocking of the pores by capillary condensation and simultaneous measurement of the gas diffusional flux through the remaining open pores [86]. There are two different approaches of the method.
i. Liquid Displacement Permporometry (LDP)
This method is commonly used to determine pore sizes and the pore size distributions of a membrane because it is close to UF filtration practice;
dead-end pores are not evaluated. The membrane is characterized in wet conditions. In this technique, the pressure is kept as low as possible and thus no alteration of the membrane occurs.
ii. Diffusional Permporometry (DP)
With this technique, the permeation rate is measured at various trans-membrane pressures and used to calculate pore-size distribution. Thus, this technique is the extension of the bubble point method.
The Art of Making Polymeric Membranes 57 2.5.1.4 Adsorption-Desorption Method: Barett-Joyner-Halenda (BJH)
Method [87]
The adsorption-desorption method is a popular and commonly used method for characterization of surface and structural properties of porous materials, allow-ing the determination of their surface area, pore-size distribution, pore volume and adsorption energy distribution. Nitrogen is often used for the adsorbent gas but other adsorbent gases such as argon can also be used. According to this method, adsorption-isotherm (amount of adsorbed gas versus relative pressure [pressure/saturation vapor pressure of the adsorbent]) is drawn and the data are analyzed by assuming capillary condensation.
This method is well documented in literature and textbooks and well commercialized.
2.5.1.5 Permeability Methods
Figure 2.9 shows a schematic diagram for the permeability measurements.The cell con-tains a homogeneous membrane of known thickness and is pressurized using a certain gas. The extent of gas permeation through the membrane is measured by means of a mass-flow meter or by a soap-bubble meter.
The gas permeability coefficient P can be determined from the steady-state gas flow if the membrane thickness l is known, since
J = /P l (2.2)
where J is the pressure normalized gas flux, called permeance, (cm3.cm–2.s–1.cmHg–1) and l the membrane thickness (cm). P is permeability (cm3.cm.cm–2.s–1.cmHg–1 or m3.m.
m–2.h–1.bar–1) and is considered intrinsic to the polymeric material. Often, the perme-ability is expressed in Barrer (1 Barrer = 10–10 cm3 cm.cm–2.s–1.cmHg–1) [9]. This method is often called the constant pressure method.
Figure 2.9 Gas permeability apparatus.
58 Handbook of Polymers for Pharmaceutical Technologies
The gas permeability of the membranes also can be measured by means of a two-chamber cell as shown in Figure 2.10 [88].
A sample membrane is fixed by porous carbon plates with the membrane edges sealed by gaskets. During the measurement, a pressure difference of up to 1 x 106 Pa is applied to the membrane. The gas pressure of each side of the membrane is monitored by pressure sensors. The temperature is controlled with a temperature controlled bath by immersing the system into it. One chamber of the cell is filled with pressurized gas while the other is always kept under vacuum. The gas passes through the membrane under the driving force of pressure. The mole number of the gas, n, passed through the membrane can be calculated from the decreased pressure Pd on the upstream side of the membrane with a certain time (t in s) by using the equation:
n P V RT= d. /
( )
mol (2.3)where R is the gas constant, T the temperature in Kelvin and V is the volume of the gas chamber (upstream side). The gas permeability coefficient, P, can be calculated:
P n l A t P= . / . . mol cm cm s Pad