Asymmetric glassy membrane

The first, and currently only, successful solvent-permeable hyperfiltration membrane is the Starmem series of solvent-resistant membranes developed by W.R. Grace [40]. These are asymmetric polyimide phase-inversion membranes prepared from Matrimid (Ciba-Geigy) and related materials. The Matrimid polyimide structure is extremely rigid with a Tg of 305 °C and the polymer remains glassy and unswollen even in aggressive solvents. These membranes found their first large-scale commercial use in Mobil Oil s processes to separate lube oil from methyl ethyl ketone-toluene solvent mixtures [41-43], Scarpello et al. [44] have also achieved rejections of >99 % when using these membranes to separate dissolved phase transfer catalysts (MW 600) from tetrahydrofuran and ethyl acetate solutions. 

The development of asymmetric membrane technology in the 1960 s was a critical point in the history of gas separations. These asymmetric structures consist of a thin (0.1 utol n) dense skin supported on a coarse open-cell foam stmcture. A mmetric membranes composed of the polyimides discussed above can provide extremely high fluxes throuj the thin dense skin, and still possess the inherently hij separation factors of the basic glassy polymers from which they are made. In the early 1960 s, Loeb and Sourirajan described techniques for producing asymmetric cellulose acetate membranes suitable for separation operations. The processes involved in membrane formation are complex. It is believed that the thin dense skin forms at the... 

Although no commercial examples exist currently in the gas separation field, thin film composite membranes such as those pioneered by Cadotte and co-workers (10) may ultimately permit the use of novel materials with unique transport properties supported on standard porous membranes. Therefore, the focus in this paper will be on suggesting a basis for understanding differences in the permeability and selectivity properties of glassy polymers. Presumably, if such materials prove to be difficult to fabricate into conventional monolithic asymmetric structures, they could be produced in a composite form. Even if thin film composite structures are used, however, the chemical resistance of the material remains an important consideration. For this reason, a brief discussion of this topic will be offered. 

Polysulfone hollow fibers are usually asymmetric in cross-section, with either an internal skin (for use in blood filtration) or an external skin (for use in gas separation). The ability to form asymmetric structures with divergent permeabilities attributable to the skin and supporting structures makes glassy polymers, such as the polysulfones, attractive for use in the development of separation devices. In contrast to membranes having a uniform cross-section, these asymmetric structures permit much higher filtration rates with equivalent sieving spectra. 

The most Important requirements of high selectivity and high permeability for the more permeable CO. gas seem to be met by the asymmetric CA blend membranes. They elnilblt the permeability rates of rubbery materials and the selectlvities of a glassy polymer with an Intermediate high glass transition temperature. 

Further approaches to meet the requirement of high selectivity may Include the blending of glassy and rubbery polymers, the chemical alteration of the dense skin-layer of Integral-asymmetric membranes and morphological variations of dense polymer films by proper post-treatment—as exemplified In this paper for CA blend membranes. 

Membrane type asymmetric homogeneous or microporous composite of a homogeneous polymer film on a microporous substructure or symmetrical homogeneous or porous polymer film. Driving force for the rate of separation hydrostatic pressure concentration. Membrane nonporous membrane elastomer or glassy. 

Most commercial membrane separations use natural or synthetic, glassy or rubbery polymers. To achieve high permeability and selectivity, nonporous materials are preferred, with thicknesses ranging from 0.1 to 1.0 micron, either as a surface layer or film onto or as part of much thicker asymmetric or composite membrane materials, which are fabricated primarily into spiral-wound and hollow-fiber-type modules to achieve a high ratio of membrane surface area to module volume. 

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]. 

Plots of the product permeability versus time on a log-log coordinate system are often linear over relatively long time periods, as shown in Fig. 20.5-2. Similar behavior is observed in asymmetric reverse osmosis membranes. The log-log plotting approach provides a simple and reasonably satisfactory means of predicting the performance change of fibers under long-term operation by exuapolation of short-term data. Mechanical creep and volume recovery in glassy polymers aftw an initi perturbation also ate known to be reasonably represented on such log-log plots. ... 

In this study, the industrial asymmetric membrane based on high permeable glassy polymer PVTMS (poly-[vinyltrimethylsilane]) was used as the membrane contactor... 

Sample Dimension Dependent Effects. Recently, it has been noted that the permeability of glassy polymers decreases over time and the selectivity increases over time. Interestingly, the decreases in permeability and increases in selectivity are accelerated for thin films, on the order of a micrometer in thickness. The study of the gas transport properties in thin films is not only academic because film thicknesses of this order are often used in semiconductors, in adhesives, and in packaging. Furthermore, the skin thicknesses of asymmetric hollow fibers, the most common industrial membrane geometry, are usually 1 /u.m or less. 

Polymer matrix selection determines minimum membrane performance while molecular sieve addition can only improve membrane selectivity in the absence of defects. Intrinsically, the matrix polymer selected must provide industrially acceptable performance. For example, a mixed matrix membrane using silicone rubber could exhibit properties similar to intrinsic silicone rubber properties, O2 permeability of 933 Baiters and O2/N2 permselectivity of 2.1 (8). The resulting mixed matrix membrane properties would lie substantially below the upper boimd trade-off curve for gas permeability and selectivity. In contrast, a polymer exhibiting economically acceptable permeability and selectivity is a likely candidate for a successful polymer matrix. A glassy polymer such as Matrimid polyimide (PI) is an example of such a material because it exhibits acceptable properties and current technology exists for formation of asymmetric hollow fibers for gas separation (10). 

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