Composite membranes crystallinity

The concept of the pH electrode has been extended to include other ions as well. Considerable research has gone into the development of these ion-selective electrodes over the years, especially in studying the composition of the membrane that separates the internal solution from the analyte solution. The internal solution must contain a constant concentration of the analyte ion, as with the pH electrode. Today we utilize electrodes with 1) glass membranes of varying compositions, 2) crystalline membranes, 3) liquid membranes, and 4) gas-permeable membranes. In each case, the interior of the electrode has a silver-silver chloride wire immersed in a solution of the analyte ion. 

It is evident that the ceramic membrane, which is represented in the XRD pattern (see Figure 10.6) by the amorphous component of the XRD profile, was covered by the AlP04-5 molecular sieve, since the crystalline component of the obtained XRD pattern fairly well coincides with the standards reported in the literature [107]. Consequently, the porous support was successfully coated with a zeolite layer, which was shaped by the hydrothermal process as previously described. Thus, a composite membrane, that is, an AlP04-5 molecular sieve thin film zeolite-based ceramic, was produced. 

For the unique properties of PBs to be exploited, PBs must be deposited properly onto a solid support. It is highly desirable to prepare mechanically robust PBs films with controlled thickness, chemical composition and crystallinity, having ion-sieving membranes and electrochromic devices in mind [6], or to create regular patterns of PB-based single molecule magnets [13],... 

Sulfuric acid and sodium hexametaphosphate (SHMP) are injected in the feedwater line upstream of the cartridge filters. The sulfuric acid is injected to adjust the feedwater pH to a level of between 4 and 6. The purpose of acid injection is twofold. The primary purpose is to mitigate the possibility of calcium carbonate deposition by conversion of bicarbonate to carbon dioxide. Coincidentally, the rejection performance of the thin film composite membrane is pH sensitive and the optimum performance is at the operating pH level. SHMP is added to the feedwater as a threshold inhibitor to inhibit the crystalline growth of sparingly soluble salts such as calcium sulfate. 

Monomeric amines have two advantages over polymeric amines in interfacial composite membrane fabrication. First, monomeric amines can be obtained in most cases as pure crystalline compounds, identical in lot after lot. Polymeric amines, on the other hand, will show variations in purity, molecular weight, chain branching and viscosity from lot to lot. This adds an element of variability to the membrane fabrication process. Second, monomeric amines lead to thicker barrier layers, which consequently tend to show better abrasion resistance and greater tolerance to chemical attack. By contrast, a membrane such as PA-300 is normally overcoated with a protective layer of water-soluble polyvinyl alcohol to minimize abrasion and salt rejection losses during spiral element assembly. 

An appropriate amount of Nafion ionomer (5% wt/wt, Aldrich) was mixed with 3% wt/wt Si02 (Aerosil 200, Degussa) in an ultrasonic bath for 30 min. This solution was cast [19,20] in a Petri dish and heated at 80°C for 30 min. The recast composite Nafion film was detached from the Petri dish by addition of distilled water and allowed to dry for 15 hrs at room temperature. Afterwards, it was cut to obtain a regular shape and then hot pressed between two PTFE foils at a few bars and increasing temperatures. The final treatment was 160°C for 10 min. The latter step allowed to increase the crystalline fraction inside the composite membrane with consequent improvement of the mechanical properties. The... 

Accordingly, Fig. 4 shows the X-ray diffraction profiles of Nafion 117 and the composite membranes, prepared as described in the experimental section, in the protonic form and dry state. The broad diffraction peaks at 26 = 12-20° result from a convolution of amorphous (20 = 16) and crystalline (20 = 17.50) scattering from the polyfluorocarbon chains of Nafion . 

From that point of view it seems natural that special consideration should be given to the composite membranes in which one component consists of liquid crystal. The transitions from crystalline to mesophase state are connected with mobility increase thus influencing the transport properties of these composite systems. However, it was only recently that a discontinuous jump of permeability to liquids, gases and vapours in the vicinity of transition temperature of the liquid crystal phase has been discovered. Membranes of this type form a new class of composites which deserve special consideration due to their particular properties. 

In the case of pure PC membrane, the values of permeability coefficients are larger for CH4 than for C3H8. For the composite membrane containing 45 wt.% the reverse is true, i.e. for molecules with greater number of carbon atoms higher permeability coefficients were found above the transition temperature of the liquid crystalline component. These effects appear more clearly in membranes containing 60 wt.% of EBBA. From the In P - 1/T plots it can be seen that below the transition of the liquid crystalline component the values of permeability coefficients are lower for bigger molecules than for smaller ones. These studies lead to the conclusion... 

Zeolite membranes indicate inorganic membranes with a selective/cata-lytic layer composed of a zeolite which is crystalline aluminosilicate with the feature of a high ordered porous structure with size comparable to molecular dimension. An example of the use of zeolites as a catalyst in a multi-phase membrane reactor can be found in Shukla and Kumar (2004) who have immobilized a lipase on a zeolite-clay composite membrane by using glu-taraldehyde as a bifunctional ligand in order to carry out the hydrolysis of olive oil. An application of a zeolite-based membrane in a three-phase membrane reactor has been reported by Wu et al. (1998), where TS-1 zeoUte crystallites were embedded in a polydimethylsiloxane (PDMS) membrane in order to catalyse the oxyfunctionalization of n-hexane (from a gas phase) with hydrogen peroxide (from a liquid phase). 

The wide angle x-ray diffraction study makes it clear that amphiphile molecules aggregate in a crystalline form in the composite membrane. Also, the small angle x-ray scattering investigation tells us existence of the multilamellar liposome composed of bimolecular lamellae. 

It has been proposed that hybrid membranes are more proton conductive at low humidity than the parent polymer membranes. For example, zirconia phosphate (ZrP) was formed in the sulfonated poly(arylene ether sulfone). Nanoparticles of zirconia phosphate were homogeneously distributed in the membranes as crystalline ot-zirconium hydrogen phosphate hydrate [96]. The composite membrane (with ZrP up to 50 wt%) showed 3.7 x 10 S/cm of the proton conductivity at 90" C, 30% RH, which was ca. 5 times higher than that of the parent polymer membrane under the same conditions. It is claimed that the composite effect depends upon interfacial contact between the polymer matrix and additives, and inappropriate preparation procedure could result in opposite effects. 

Nam et al. [26] used organic-inorganic nanocomposite material like Nafion/ poly(phenyhnethyl silsequioxane, PPSQ). Incorporation of PPSQ improved initial degradation temperature of Nafion membrane and increased the crystallinity of the recast composite membrane. The membrane was reported to have lower methanol permeability as compared with bare Nafion due to interruption of organic filler. 

Apart from DMFCs, Nafion /silicon oxide (Si02)/phosphotungstic acid (HPW) and Nafion /silicon oxide composite membranes were also used for the H2/O2 PEMFCs to be operated above 100°C [96]. The composite membranes were recast from Nafion solution mixed with Si02 and HPW/Si02 mixtures, and the incorporation of the Si02 and HPW into the Nafion membrane increased the crystallinity and improved the initial degradation temperature of the Nafion membrane. The proton conductivity of the composite membranes appeared to be similar to that of the native Nafion membrane at high temperatures at 100% relative humidity (RH) ... 

Park, Y.-S., and Yamazaki, Y. (2005b). Novel Nafion/hydroxyapatite composite membrane with high crystallinity and low methanol crossover for DMFCs. Polym. Bull. 53, 181. 

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