Mixed matrix materials, membrane formation

A wide variety of polymeric membranes with different barrier properties is already available, many of them in various formats and with various dedicated specifications. The ongoing development in the field is very dynamic and focused on further increasing barrier selectivities (if possible at maximum transmembrane fluxes) and/ or improving membrane stability in order to broaden the applicability. This tailoring of membrane performance is done via various routes controlled macro-molecular synthesis (with a focus on functional polymeric architectures), development of advanced polymer blends or mixed-matrix materials, preparation of novel composite membranes and selective surface modification are the most important trends. Advanced functional polymer membranes such as stimuli-responsive [54] or molecularly imprinted polymer (MIP) membranes [55] are examples of the development of another dimension in that field. On that basis, it is expected that polymeric membranes will play a major role in process intensification in many different fields. 

Most of the more recent research has focused on developing membrane materials with a better balance of selectivity and productivity (permeability) as that seems the most likely route for expanding the use of this technology. There appear to be natural upper bounds [9,10] on this tradeoff that limit the extent of improvement that can be realized by manipulating the molecular structure of the polymer used for the selective layer of high-flux membranes, at least in many cases. This has led to interest in nonpolymeric and so-called mixed-matrix materials for membrane formation [8] however, at this time, polymers remain the materials of choice for gas-separation... 

These concerns can be addressed partially through the use of mixed-matrix membranes [77-79]. Dispersing the microporous material in the form of small particles within a polymeric matrix simplifies membrane formation dramatically. Mixed matrix materials possess transport properties intermediate between those of the polymer matrix and the microporous particle and operating temperatures are limited by the thermal stability of the polymer matrix. However, proper selection of the matrix, control of particle volume fraction, and development of a membrane formation process can yield materials with properties that approach those of the particles [77-78]. Special attention must be given to the particle-polymer interface. If the interface morphology is uncontrolled, the matrix may 1) not wet the particle leaving a non-selective void around the particle, 2) enter the particle and block pores, or 3) rigidify around the particle and block access to it [79]. 

Mahajan R (2000) Formation, characterization and modeling of mixed matrix membranes materials. Ph.D. Dissertation, The University of Texas at Austin, TX, USA... 

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). 

One way to achieve high permselectivity is to use the so-called mixed-matrix membranes, which can be formed by embedding the high-permselectivity molecular sieving materials into the structure of polymeric materials (Koros and Mahajan, 2000 Kulprathipanja, 2003). Since polymeric materials are commonly used in the formation of a mixed-matrix membrane, it is more appropriate for low-temperature applications. 

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