Abstract
The polymer synthesis of 2,3,5,6-tetrafluoro-4-pyridinecarbonitrile-3,3,3′,3′-tetramethyl-1,1′-spirobisindane-5,5′,6,6′-tetrol copolymer, termed PIM-Py, was investigated under different solvent (dimethylformamide (DMF) and dimethylacetamide/dichlorobenzene) and temperature (65–160 °C) conditions to produce a range of topologically different polymer samples. Characterization of the polymers, particularly with proton NMR spectroscopy and multiple detector SEC analysis, indicated that, like PIM-1, the polymerizations proceeded with a degree of polymer chain branching. This is attributed to the occurrence of monosubstitution reactions, instead of disubstitution, which eventually leads to a significant proportion of colloidal network formation. However, all polymer samples remained soluble/dispersible in chloroform at the concentration required to cast self-standing films. This work reports the first examination of PIM-Py as a membrane for gas separation applications. The most structurally diverse PIM-Py samples produced films that exhibited selectivity/permeability balances in single gas permeation studies above the 2008 Robeson upper bound for the CO2/N2 gas pair. Indeed, a film cast from the highest colloidal network content sample surpassed the recently introduced 2019 CO2/N2 upper bound. After 143 days of aging, a 40 μm self-standing membrane still exhibited a single gas CO2 permeability of 4480 barrer and an ideal CO2/N2 selectivity of 45. The polymers produced in lower temperature reactions in DMF exhibited gas separation performances very similar to a structurally regular “normal” PIM-1 polymer, sitting on or around the 2008 Robeson upper bound line. Single gas permeation measurements to determine CO2/CH4 selectivity showed similar trends across the range of polymer samples, without generally reaching high selectivities as for the CO2/N2 pair. Mixed gas CO2/CH4 permeation measurements with aging were also completed for PIM-Py membranes, which indicated similar gas separation performance to a structurally regular PIM-1 polymer. This study would suggest that, like PIM-1, gas separation performance of PIM-Py is greatly influenced by the topological balance toward branched and network material within the polymer sample.
Highlights
Polymers of intrinsic microporosity (PIMs) have created a great deal of interest since the first of its type, PIM-1, was reported in 2004.1 The original, ladder-like PIM structures, which contained spiro centers providing sites of contortion, proved soluble in some common organic solvents for filmcasting or coating purposes, and were found as membranes to exhibit impressive permeability and good selectivity for certain gas pairs
We have reported blending it in mixed matrix membranes (MMMs) with synthesized high cross-link density and low cross-link density network versions of PIM-1,11,12 seeking to gain greater compatibility between the two components in films
High molar mass polymers of PIM-1 and PIM-Py were obtained in good yields (>90%) from all the reactions (1−8), after the purification processes (Table 1)
Summary
Polymers of intrinsic microporosity (PIMs) have created a great deal of interest since the first of its type, PIM-1, was reported in 2004.1 The original, ladder-like PIM structures, which contained spiro centers providing sites of contortion, proved soluble in some common organic solvents for filmcasting or coating purposes, and were found as membranes to exhibit impressive permeability and good selectivity for certain gas pairs. Seek to increase further the interchain rigidity and spacing of the polymer chains to produce even greater gas-sieving performance in membranes Incorporation of groups, such triptycene, ethanoanthracene, Troger’s base, and tetramethyltetrahydronapthalene[5−7] in polymeric structures, have meant that new revised upper bounds for membrane performance for most common gas separation pairs of interest have been introduced either in 20158 or 2019.9. The moderate molecular weight polymers obtained were suitable for the intended study but would not be sufficient to cast self-standing membranes for gas separation studies Both solvents used have very high boiling points, which would mean that even if the polymerizations were optimized to produce high molar mass material, it would prove difficult to remove from the products. All PIM-Py polymer samples remained solution processable and formed self-standing films suitable for testing as membranes for gas separation.
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