Amorphous materials membranes structure

Carbon molecular sieve membranes. Molecular sieve carbons can be produced by controlled pyrolysis of selected polymers as mentioned in 3.2.7 Pyrolysis. Carbon molecular sieves with a mean pore diameter from 025 to 1 nm are known to have high separation selectivities for molecules differing by as little as 0.02 nm in critical dimensions. Besides the separation properties, these amorphous materials with more or less regular pore structures may also provide catalytic properties. Carbon molecular sieve membranes in sheet and hollow fiber (with a fiber outer diameter of 5 pm to 1 mm) forms can be derived from cellulose and its derivatives, certain acrylics, peach-tar mesophase or certain thermosetting polymers such as phenolic resins and oxidized polyacrylonitrile by pyrolysis in an inert atmosphere [Koresh and Soffer, 1983 Soffer et al., 1987 Murphy, 1988]. 

Positronium lifetime spectroscopy is particularly well suited for stud)hng defects in crystals and structural fluctuations in amorphous materials and can give an estimate of free volumes in condensed matter [116]. It is a useful technique to estimate the free volume of polymeric membranes [117]. In a study on silica gels, the decay lifetime has been found (Fig. 4.16) to be proportional to the pore diameters (measured by N2 adsorption) between 30 and 100 A [118]. Information on pore size distribution and surface area may also be obtained by means of calibration curves. 

General criteria for selection of materials for the processing of hydrogen separation membranes are discussed. Performance and stability standards required for applications in high temperature membrane reactors have been focused. The correlations between pore structure and stability issues of membranes made of amorphous materials, specifically silica membranes are discussed in detail. 

The lack of methods for a fast and reliable assessment of membrane quality is still one of the outstanding issues in zeolite-membrane development. The usual meaning of the term quality relates to the ability of the membrane to carry out a given separation with a reasonable flux therefore, a system-specific property and a universal membrane quality test do not exist. In general, specihc permeation measurements at different temperatures, either of single gases (or vapors) or of multicomponent mixtures in the gas or liquid (pervaporation) phase, provide extremely useful information on the effective pore structure of the membrane, on the existence of intercrystalline defects, and amorphous material and permeation fluxes, as well as information about the main transport controlling effect (adsorption or diffusion). 

The addition of the filler into a polysiloxane matrix will also influence the porosity of the resulting material. Differences in permeabihty between porous and amorphous sihcone membranes have been determined [73]. The authors concluded that the trcuisport of gcises through porous rubber membranes is primarily due to capihary flow-through pores, whereas the transport of gcises through amorphous rubber films is attributed to activated diffusion, hi facf structural analysis of a PMDS-sihca mixture by small-angle neutron... 

Because of the angled structure of poly(arylene ether sulfone)s, they generally do not crystallize. They are thus amorphous and optically transparent with glass transition temperatures between 150-200 °C. They are soluble in some polar solvents, hydrolysis resistant, and inherently flame resistant. Fields of application for these materials are found particularly in the area of electronics and membrane technology. 

Atomistic molecular modeling techniques have proven to be a very useful tool for the investigation of the structure and dynamics of dense amorphous membrane polymers and of transport processes in these materials. By utilizing these methods, information can be obtained that is not accessible by experimental means. 

Most polymers that have been of interest as membrane materials for gas or vapor separations are amorphous and have a single phase structure. Such polymers are converted into membranes that have a very thin dense layer or skin since pores or defects severely compromise selectivity. Permeation through this dense layer, which ideally is defect free, occurs by a solution-diffusion mechanism, which can lead to useful levels of selectivity. Each component in the gas or vapor feed dissolves in the membrane polymer at its upstream surface, much like gases dissolve in liquids, then diffuse through the polymer layer along a concentration gradient to the opposite surface where they evaporate into the downstream gas phase. In ideal cases, the sorption and diffusion process of one gas component does not alter that of another component, that is, the species permeate independently. 

FIGURE 28.11 Site of glycerin-based moisturizing lotion from the 52-year-old female of Figure 28.10. The structure of the intercellular space is unusual, (a) Many areas contain amorphous and fibrous material in the intercellular space, (b) Other areas contain vesicles, membrane-bounded compartments, and a mesh-like material. Bar = 200 nm. 

Amorphous silicas play an important role in many different fields, since siliceous materials are used as adsorbents, catalysts, nanomaterial supports, chromatographic stationary phases, in ultrafiltration membrane synthesis, and other large-surface, and porosity-related applications [16,150-156], The common factor linking the different forms of silica are the tetrahedral silicon-oxygen blocks if the tetrahedra are randomly packed, with a nonperiodic structure, various forms of amorphous silica result [16]. This random association of tetrahedra shapes the complexity of the nanoscale and mesoscale morphologies of amorphous silica pore systems. Any porous medium can be described as a three-dimensional arrangement of matter and empty space where matter and empty space are divided by an interface, which in the case of amorphous silica have a virtually unlimited complexity [158],... 

In other respects, we can consider zeohte membranes as pertaining to the ceramic material category. Indeed zeolites are classified for the most part as microporous, crystalline silico-aluminate stmctures with different alumininum/silicon ratios. Thus, the chemical compositions are close to those of ceramic oxide membranes, in particular of microporous silica and alumina membranes. On the other hand, zeohtes are crystalline materials and they have a structural porosity very different from microporous amorphous silica [124]. Zeohte membranes are well adapted to the separation of gases, in particular H2 from hydrocarbons, but these membranes are not very selective for the separation of mixtures of noncondensable gases. 

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