Mixed-matrix glassy membranes

Modelling Gas Solubility Into Mixed Matrix Glassy Membranes... 

Glassy polymers with much higher glass transition temperatures and more rigid polymer chains than rubbery polymers have been extensively used as the continuous polymer matrices in the zeolite/polymer mixed-matrix membranes. Typical glassy polymers in the mixed-matrix membranes include cellulose acetate, polysul-fone, polyethersulfone, polyimides, polyetherimides, polyvinyl alcohol, Nafion , poly(4-methyl-2-pentyne), etc. 

It has been demonstrated by many studies that mixed-matrix membranes with a good match between the permeabihty of proper zeolite materials and these glassy polymers exhibit separahon properties superior to the corresponding pure glassy... 

Mixed matrix membrane materials with glassy polymers. Part 1. Polym. Eng. Sd., 42,1420-1431. 

Pechar, T.W., Tspatsis, M., Marand, E., and Davis, R. (2002) Preparation and characterization of a glassy fluorinated polyimide zeolite mixed matrix membrane. Desalination, 146, 3-9. 

Mixed matrix membranes based on fluorinated high free volume matrices loaded with fumed silica nanoparticles show attractive features, especially if compared to unloaded polymeric membranes. The addition of filler changes the FFV of the glassy polymeric matrix, thus positively affecting both solubility and diffnsivity. A simple procedure has been proposed to calculate the relevant properties of the MMM, which is based on limited experimental data represented by the solubility isotherm of a test penetrant and few values of diffusion coefficients. The nnexpected behaviour shown by the MMM obtained by using nano-filler loadings can be properly described by the method developed. The... 

We believe that the procedure can be properly appUed also for the case of MMM obtained by loading glassy matrices with other nano-fillers and can be a useful tool to develop better mixed matrix membranes. 

M. Valero, B. Zomoza, C. Tellez, J. Coronas, Mixed matrix membranes for gas separation by combination of silica MCM l and MOF NH2-MIL-53(A1) in glassy polymers, Microporous Mesoporous Materials 192 (2014) 23-28. 

In the past 25 years, relatively few attempts to increase gas separation membrane performance with dense film mixed matrices of zeolite and rubbery or glassy polymer have been reported. Table I summarizes practically all of the reported O2/N2 mixed matrix membranes. Permeabilities and permselectivities are specified as a range to encompass the various zeolite volume fractions studied. In general, an increase in permeability is observed with zeolite addition coupled with a slight increase in permselectivity. Despite the wide variety of combinations of zeolites with rubbery and glassy polymers, reported mixed matrix membranes fail to exhibit the desired O2/N2 performance increases. These failures have generally been attributed to defects between the matrix and molecular sieve domains. While this is certainly a possible practical source of failure, our work earlier 8) has addressed a more fundamental source caused by inattention to matching the transport properties of the molecular sieve and polymer matrix domains. This topic is discussed briefly prior to consideration of the practical defect issue noted above. 

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). 

The strategy for the development of mixed-matrix membranes is to combine the advanced features of polymer membrane and inorganic membrane into one composite membrane. As discussed in the previous section, this is done by incorporating dispersed fillers into continuous polymer matrices. As noted in the introduction, there are three main types of mixed-matrix membranes reported in the literature solid-polymer, liquid-polymer, and solid-liquid-polymer mixed-matrix membranes. The polymer matrices providing low cost and easy processability are selected from either glassy polymers (e.g., polyimide, polysulfone, polyethersulfone, or cellulose acetate) or rubbery polymers (e.g., silicone rubber). The dispersed fillers include solid, liquid, or both solid and liquid. 

Polyvinyl acetate (PVAc) is another rubbery polymer used in mixed-matrix research. Its flexible nature helps to prevent void formation at the solid-polymer interface. Although it may not have practical industrial applications, PVAc aids in developing proof-of-concept associated with mixed-matrix membranes. Zeolite 4A-PVAc membranes have been proven to enhance membrane selectivity in mixed-matrix membranes with only 15 vol% zeolite " however, the permeability is lower than predicted presumably due to matrix rigidification. The rubbery nature of PVAc allows for more polymer relaxation at the solid-polymer interface as compared to the case with traditional, glassy polymers." ... 

Several researchers have successfully created glassy polyimide mixed-matrix membranes with separation properties that fall in line with accepted models, if one accounts... 

Nanocomposite mixed-matrix membranes have been investigated for close to a decade. Ti02-poly(amide-imide) membranes showed selectivity improvement but suffered loss of productivity when TiOa was added. Nonporous, nanoscale, fumed silica was embedded in a glassy, amorphous polymer, poly(4-methyl-2-pentyne), which resulted in enhancements in both permeability and selectivity for the mixed-matrix membrane. These membranes were discovered to be reverse selective, so the membrane is selective for the larger penetrant. This phenomenon is attributed to increased free volume in the bulk polymer from chain packing disruption, which occurs when the filler is added. ... 

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