Control of Physical Aging in Super-Glassy Polymer Mixed Matrix Membranes

Abstract

Conspectus Since the discovery of polymers of intrinsic microporosity (PIMs) in 2004, the fast size-selective interconnected pore cavities of the polymers have caused the upper bound of membrane performance to be revised, twice. Simultaneously, porous materials have meant that mixed matrix membranes (MMMs) are now a relatively simple method of enhancing transport properties. While there are now reliable routes with mixed matrices to improve the fundamental transport properties of membrane materials, many of the other properties crucial for separation applications remain largely unaddressed. Physical aging severely affects membrane performance over time, especially for those prepared from high fractional free volume polymers. Gradual densification of the glassy polymer chains causes the connected pore channels present in these materials to constrict. Studies now suggest that aging of superglassy polymer materials is a two-step process; a rapid densification occurs within the first few days, followed by a gradual rearrangement of packed chains over longer time frames toward a theoretical equilibrium state. Although advantageous in terms of size selectivity, the considerable drop in permeation over the days and weeks after manufacture greatly impacts material applicability. While often still permeating faster than traditional membrane materials, the continuous gradual collapse of cavities in these polymers are a significant challenge in the application of high free volume polymer membranes. In 2014, we discovered that the porous aromatic framework PAF-1 not only greatly improved the membrane’s void space and speed of gas transport but also seemingly froze several glassy polymers in a low-density state, holding the polymer’s pore channels open, a process termed as Porosity Induced Side chain Adsorption (PISA). This discovery of PISA fundamentally challenged the conventional wisdom at the time that the aging rate could only be addressed by densification of the polymer. Unlike other high-performance glassy polymers, membranes containing PAF-1 can retain their high permeability for more than a year. Several other examples of antiaging behavior have been subsequently reported by the team, where control of aging rate as a function of gas penetrant, selectivity increases, and stability at higher pressures was reported. These works also demonstrate that these mixed matrix systems had applicability for several other separations, including pervaporation, solvent nanofiltration, and as separators for energy applications. In our subsequent studies, the antiaging mechanism has been elucidated as an effect of the interaction between the polymer’s accessible pendant methyl group and the aromatic pore surface of PAF-1 or other antiaging additives. In otherwise identical MMMs, where this hypothesized methyl−π interaction is either absent or interrupted, we find that the antiaging behavior expected by the fixation of the polymer chains to the pore surface and PAF-1 does not occur. As a design approach for mixed matrix membranes, targ...

Ion-capture electrodialysis using multifunctional adsorptive membranes

“…S18). These observations confirm that PAF composite membranes are uniform and robust, likely due to favorable van der Waals forces between the PAFs and polymer, π-π stacking between aromatic groups, and polymer pore plugging into PAF mesopores (27).…”

“…PAFs conversely display exceptional hydrothermal stability and chemical compositions similar to those of polymer matrices (16). Few reports, however, have demonstrated PAF-incorporated membranes, with applications mostly limited to anti-aging gas separation membranes and thin-film composites (16,27). PAF membranes originated herein exhibit many useful properties for new directions in IC-ED processes and general water treatment, electrochemical, and adsorptive membrane applications.…”

Hybrid Microporous Polymeric Materials with Outstanding Permeability and Increased Gas Transport Stability: PTMSP Aging Prevention by Sorption of the Polymerization Catalyst on HCPS

“…However, a significant disadvantage of glassy polymers is physical aging over time and, as a result, a significant decrease in the permeability coefficients. This poses a serious problem for the commercial application of these polymers [15][16][17][18][19][20]. The effect of physical aging on the gas permeability of various membrane materials is actively studied in the literature both from a theoretical and empirical point of view.…”

“…One of the simplest and most effective ways to reduce membrane aging is to introduce a filler phase to produce so-called mixed-matrix membranes (MMM) [21][22][23]. Molecular sieves [24], structures based on metal-organic frameworks (MOF) [25][26][27][28], porous aromatic frameworks (PAF) [16,[29][30][31][32][33], hyper-crosslinked polystyrene (HCPS) [34][35][36][37][38], graphene, and graphene oxide (GO) [39] are used as such additives. MOF, such as zeolitic imidazolate frameworks (ZIF-7, -8), can increase the gas permeability, selectivity, and mechanical properties of membranes [26,27].…”

“…However, these improvements are short-lived, as these nanocomposites undergo accelerated physical aging [25]. PAF particles initially reduce the mechanical strength of glassy polymer films, but they stabilize over time and are mechanically stronger than aged mixed-matrix membranes impregnated with MOF [16]. The introduction of PAF into highly permeable glassy polymers improves their gas permeability characteristics due to an increase in the fractional free volume of the polymer and significantly suppresses physical aging due to a decrease in the mobility of macrochains as a result of partial sorption/incorporation of polymer segments into porous fillers.…”