Gas Permeability Measurements

In Figure 3, we have presented the experimentally obtained reciprocal values of (Di )t.ff of oxygen in a sample of the nano-porous hydrophobic material as a function of the total pressure P of gas mixture (02-N2) when the oxygen concentration in the mixture is 21%. From the intercept of the straight line with the ordinate the value of the Knudsen diffusion coefficient can be also determined. It must be underlined that the value of Knudsen diffusion coefficient obtained by these diffusion measurements (2,86.10"2 cm2/s) is in very good coincidence with the value obtained by the gas permeability measurements. 

The changes in surface properties were analysed both by contact angles and gas permeabilities measurements. The contact angles values for a family of typical liquids are reported on Table 1. The resulting surface tensions, ys, for PE, EVOH and EVO-NO (34,... 

Then, NO treatment results in a significant increase of the surface tension of ethylene-vinyl alcohol copolymers. This point was checked by gas permeability measurements. As shown on Table 2, both permeabilities and permeability coefficients (independent of thickness) increase significantly for "polar gases" (02 and C02). 

The permeability of gases through membranes is most commonly measured in Barrer, defined as 10-10 cm3(STP)/cm2 s cmHg and named after R.M. Barrer, a pioneer in gas permeability measurements. The term ji/ pio — pit), best called the pressure-normalized flux or permeance, is often measured in terms of gas permeation units (gpu), where 1 gpu is defined as 10 6 cm3(STP)/cm2 s cmHg. Occasional academic purists insist on writing permeability in terms of mol m/m2 s Pa (1 Barrer = 0.33 x 10-15 mol m/m2 s Pa), but fortunately this has not caught on. 

The membranes were analysed by single gas permeability measurements in order to characterise their properties. 

As a result of these shortcomings, porosity characterizations generally involve both gas permeability measurements to provide estimates of the avasege pore size and liquid displacement to measure maximum pore size in the membrane surface. The techniques rely on a number of assumptions for their results to heve physical menning. Depending on the stiucture of die asymmetric membrane, some of the assumptions may be satisfied only merginally. In such a case, the characterizations are primarily useful as empirical indices for comparison of differenl samples, and the fundamental meaning of the numbers derived from such analyses is questionable. 

Voluma Permeability Cell—Comparison of Gas Permeability Measurements by the Variable Volume and Varieble Pressure Methods, J. Appl. Polym. Sci., 7, 2035 (1963). 

The mechanical problem is considered elastic but the intrinsic permeability variations induced by deformations have been introduced. This permits a coupling from mechanical to hydraulic and in turn to thermal. Gas permeability measurements undergo important variations during the test due to both moisture redistribution and to deformations. The hydrological effect is more important than the mechanical. It is in fact, difficult to separate both effects. The model is only able to reproduce correctly gas permeability variations at some points. Heterogeneity may play an important role but it has not been introduced in this model. 

No change of the sorption properties of the amorphous phase is observed by thermal treatment. A low gas permeability measured at the biaxially stretched films is related to both a change of the free volume sizes distribution and a tortuosity effect. The barrier properties of biaxially stretched films are kept even after annealing the film at 250°C. 

A very large body of data on the gas permeability of many rubbery and glassy polymers has been published in the literature. These data were obtained with homopolymers as well as with copolymers and polymer blends in the form of nonporous dense (homogeneous) membranes and, to a much lesser extent, with asymmetric or composite membranes. The results of gas permeability measurements are commonly reported for dense membranes as permeability coefficients, and for asymmetric or composite membranes as permeances (permeability coefficients not normalized for the effective membrane thickness). Most permeability data have been obtained with pure gases, but information on the permeability of polymer membranes to a variety of gas mixtures has also become available in recent years. Many of the earlier gas permeability measurements were made at ambient temperature and at atmospheric pressure. In recent years, however, permeability coefficients as well as solubility and diffusion coefficients for many gas/polymer systems have been determined also at different temperatures and at elevated pressures. Values of permeability coefficients for selected gases and polymers, usually at a single temperature and pressure, have been published in a number of compilations and review articles [27—35]. 

Multilayers may also be used for their permeation properties. Accordingly, membranes covered by multilayers have been employed for gas permeability measurements and for pervaporation studies [88,181,185,333-335]. These measurements showed, for example, 02/H20,C02/02, or toluene/ heptane selectivity. The permeation properties of polyelectrolyte multilayers are also important when they are used for the encapsulation of enzymes [210] or living cells [336,337]. The deposition of polyelectrolyte multilayers or of hybrid polyelectrolyte/inorganic multilayers on latex particles, and the subsequent dissolution or calcination of the latex beads leads to the fabrication of hollow spheres [94-96,338-340]. Potential applications of such hollow spheres are numerous, for example, for the controlled release and targeting of drugs. 

With the isostatic method, the pressure in each chamber is held constant by keeping both chambers at atmospheric pressure. In the case of gas permeability measurement, there must again be a difference in permeant partial pressure or a concentration gradient between the two cell chambers. The gas that has permeated through the film into the lower-concentration chamber is then conveyed to a gas-specific sensor or detector by a ... 

Cell Gasket Porous carbon paper Membrane Gas switch valve Figure 2.10 Cell for gas permeability measurements [87]. 

Fig. 6.9 The cell configuration for gas permeability measurement. Reproduced from [29] with permission of Elsevier...

The procedures for membrane casting and gas permeability measurement were the same as above. The permeability of water vapor was measured separately according to GB 1037-70 (China). The gas permeation properties of PEK(S)-L and DIDMPEK(S)-C are shown in Table IV. 

Gas permeability measurements were performed on the blended films as presented in Fig. 33.14. The blend shows a definite increase in permeability for all gases tested, relative to polyimide and the base form of polyaniline [60J. Ordinarily, there is an inverse relationship between permeability of two gases and the separation factor for those gases [61-64]. However, this blend does not hold to this axiom. Figure 33.15 illustrates the separation factors for the blend and the homopolymers. The separation factors for the blend are comparable to those of polyaniline (as-cast) for H2/N2 (a = 200) and O2/N2 (a = 9) and closer to that of polyimide for CO2/CH4 (a = 58). Thus, this blend appears to have achieved an improved combination of properties compared to its parent polymers, with enhanced permeability and good selectivity. 

Table 2.1(C) shows the physical properties of HP-LCP inflation film and sLCP solvent casting film. In this case, the thickness of HP-LCP film and sLCP film were both 25 pm. In particular, an unique property of the HP-LCP film is its excellent gas barrier. Permeability tests were performed imder hydrogen gas, oxygen gas and water vapor with the following conditirais 23 °C, 50 %RH and 0.1 MPa (1 bar), 23 C, 60 % RH and 0.1 MPa (1 bar) and 40 °C, 90 % RH and 0.1 MPa (1 bar), respectively. All gas permeabilities measured were extremely low and belonged to the lowest values known for thermoplastic films. The reason for this effect should be the hindered motion of the naphthalene moiety in HP-LCP that restricts penetration mobility of gas molecules. The combination of the excellent gas barrier properties and the low water absorption rate are key properties and great advantages of HP-LCP film.

In addition to characterizing the influence of aging on density, results from aging studies using ellipsometry have also been shown to correlate well with gas permeability measurements. ° Direct comparison of volumetric aging rates determined by ellipsometry to rates of permeability decline shows a strong correlation to the properties probed using these two different techniques. 

PAS-8 were greater than those on SILASTIC 500-1, PAS-41, and 71. Although the surfaces of PASs and SILASTIC 500-1 were hydrophobic, the contact angle for PAS-8 was slightly lower than those for SILASTIC 500-1, PAS-41, and 71 (see Table 9). It was thought that the PAS-8 surface might have an influence on the aramid block. The phenomenon of the cell adhesion onto PAS-41 and 71 could not be explained by hydrophobicity alone. In former sections, the results of the gas-permeability measurement and the dynamic thermomechanical analysis of PAS implied the presence of a microphase-separated structure between PDMS and aramid phases in the PASs containing over 26 wt% of PDMS [14]. Moreover, a TEM study indicated that PAS films possessed microdomain structures in their bulk phases. That is, not only the hydrophobicity of the surfaces, but also the presence of a microphase-separated structure in the PASs may influence cell adhesion. 

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