Gas Permeation Measurements

It is known that fluorinated compounds dissolve considerable amounts of oxygen and that membranes containing ultrahydrophobic moieties show a high selectivity in gas-separation processes, e.g., in O2/N2 separation. 

Encouraged by the X-ray and contact angle results, we performed some preliminary gas-permeation measurements. Here, a self-supporting film is required and only the longer block copolymers were used. Fluoro-PSB-II and Fluoro-triblock were coated on porous Celgard 2400 membranes the measurements were taken at room temperature at a driving pressure of 5 bars. Since no absolute polymer layer thickness has been determined, only relative values of the permeability are given (Table 10.6). In the case of the separation of CO2 from 

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Permeability measurements for polymer blends prepared by mixing different latices have been reported by Peterson (14). Interpreting transport data for such heterogeneous systems as polymer blends is extremely difficult, however (3, 9,15). The main purpose of the present investigation is, therefore, to study the applicability of gas permeation measurements to characterize polymer blends and not to evaluate the different theoretical models for the permeation process in heterogeneous polymer systems. 

When the gas or vapor feed stream contains a component that is highly soluble in the polymer membrane and causes plasticization, then the selectivity as defined by Equation 4.6 will depend on the partial pressure or the amount of the plasticizing component sorbed into the membrane. Furthermore, pure-gas permeation measurements are generally not a good indicator of the separation performance, and mixed-gas permeation measurements will be needed [21-23]. Often, the mixed-gas selectivity is less than predicted from pure-gas measurements [8] however, the opposite has been observed [24], Competitive sorption effects can also compromise the prediction of mixed-gas behavior from pure-gas measurements [25], For gas pairs where each component is less condensable than C02, like 02/N2, it is generally safe to conclude that the selectivity characteristics can be accurately judged from pure-gas permeabilities at all reasonable pressures. When the gas pair involves a component more condensable than C02, plasticization is likely to be a factor and pure-gas data may not adequately reflect mixed-gas selectivity. When C02 is a component, the situation depends on the partial pressures and the nature of the polymer. 

Gavalas et al. [7] prepared ZSM-5 membranes onto porous a-alumina disks by in-situ hydrothermal synthesis at 175°C. The zeolite layers were formed on the bottom face of disks placed horizontally near the air-liquid interface of clear synthesis solutions. The films grown at the optimized conditions were about 10 pm thick and consisted of well-intergrown crystals of about 2 pm in size Pure gas permeation measurements of the best preparations yielded hydrogen isobutane and butane isobutane ratios of 151 and 18 at room temperature and of 54 and 31 at 185°C, respectively. 

These considerations regarding the membrane permeation properties have been summarized by Gasteiger and Mathias in Figures 27.11 and 27.12, with literature data of gas permeation measurements of Nafion membranes. Following their evaluation, the present membranes are close to the upper limit of gas permeation, which is acceptable, but a further increase in these properties at higher temperatures cannot be tolerated. 

Kaptejm et al. [88,89] and Vroon et al. [72,74] could model and describe their single gas permeation measurements for CH4 and n-butane rather well with Eqs. (9.41)-(9.48) taking the thermodynamic factor in the Langmuir regime from adsorption measurements. 

Zeolite NaA and FAU crystals were grown under the same hydrothermal condition using the same composition of clear solution. The only difference was the type of seed crystal used. This result suggested that the crystal growth did not occur by the attachment of nano-crystals. Zeolite films/membranes were prepared by the growth of seeded crystals. Based on the SEM and TEM observations and single gas permeation measurements, densification model of film/membrane is presented. 

Based on the gas permeation measurement results on a variety of silica membranes, we have also reported similar findings [7]. We reported that the pore size distribution of sol-gel silica membranes appear to be bimodal with majority pores with sizes around 0.3 nm and some pores with much larger sizes formed due to the opaqueness of silica polymer clusters. Figure 16.5 shows the permeation results of silica membranes. The four membranes used in the study had been tailored with different porosity values. In spite of the differences in porosity values between the membranes, all the membranes showed ability to separate He from other molecules but the ability to separate between molecules of sizes greater than 0.3 nm was poor. Figure 16.6 shows a schematic representation of the separation behavior exhibited by silica membranes in comparison to a zeolite ZSM-5 membrane [7, 16, 36]. 

Two types of gas permeation measurements are generally performed through zeolite membranes to identify their mass transport properties the pressure drop and the concentration gradient methods. 

For the gas permeation measurements the diffusion coefficient was determined by the well-known time lag procedure, based on the penetration theory. If a penetrant-free membrane is exposed to the penetrant at the feed side at t = 0 and the penetrant concentration is kept very low at the permeate side, then the total amount of penetrant, Q passing through the membrane in time t is given by [37] ... 

For gas permeation measurement, there are three concepts of conditions test methods. First is a constant volume, second is a constant pressure and third is a carrier gas method. 

Gas barrier properties were evaluated to access the potential of isotropic all-cellulosic based composites for packaging applications. The mechanical properties, fiexibility and biocompatibility of these materials can be useful, in particular, for food packaging. Due to the nature of the envisaged application insoluble materials are required. This requisite is only fulfilled by cross-linked composites and, therefore, we have decided to subject only cross-Unked films for gas permeation measurements. 

Fig. 8.58 COj/CH permeation properties of MMMs using Ultem 1000 and Matrimid 5218 as matrices for various loadings of CMS particles. Pure gas permeation measurements were done at 50 psia upstream pressure and at 35 °C...

Materials. Poly(ethylene carbonate), was kindly supplied by PAC Polymers, Incorporated. Reagent grade methylene chloride (T p = 40 C) was obtained from J. T. B er (purity 99.8%) and used without further modification. Breathing quality air (measured to be 20.7% oxygen) was purchased from Air Products and was used for gas permeation measurements as received. 

The extreme difliculty in creating and handling delicate thin films for gas permeation measurement early on limited the study of single-layer free standing films to a minimum thickness of 300 00 nm. Microscopic pinhole defects that destroy the selective of thin polymer membranes form with increasing frequency as film thickness is decreased." To overeome this ehahenge, a... 

In a recent paper, Kruczek and Matsuura reported significant differences in gas transport properties of identical HMW HSPPO-18.5 films determined in a constant volume (CV) and a constant pressure (CP) testing systems [41]. Considering that permeate side in the latter system was open to atmosphere and that permeation rates of gases through the dense films were relatively low, they suggested that gas permeation measured in the CP system could be influenced by the conditions in laboratory, in particular by the water vapor pressure. On the other hand, in the CV system, permeate-side was at vacuum so that the water vapor pressure in laboratory should not have any influence on gas permeation rates measured in this system. Table 6 presents the summary of gas transport properties of identical HMW HSPPO-18.5 films determined at different levels of the water vapor pressure in the laboratory, represented by the relative humidity [41]. The data obtained in the CV... 

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