The Use of Iptycenes in Rational Macromolecular Design for Gas

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The Use of Iptycenes in Rational Macromolecular Design for Gas Separation Membrane Applications Jennifer R. Weidman and Ruilan Guo* University of Notre Dame, Department of Chemical and Biomolecular Engineering, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: Iptycene molecules, featuring three-dimensional [2,2,2]-ring configurations with 120° contortion sites and unique internal free volume (IFV) elements, have recently been identified for their great potential in the design of polymers for gas separation membranes. Some of these novel iptycene-containing polymers have already displayed unprecedented separation performance. This review summarizes the progression of the gas separation membranes, utilizing iptycene structures over the past five years by dividing the polymers into three categories: nonladder polymers, semiladder polymers, and ladder polymers. The focus lies on examining macromolecular design strategies of these novel polymers and consequential effects on chain packing, free volume architecture, and gas transport properties to provide useful guidance on the design of new membrane materials. Finally, we provide some perspectives and future research directions for designing and utilizing iptycene-containing polymers for gas separation membranes.

1. INTRODUCTION 1.1. Major Materials-Related Challenges for Membrane Gas Separations. The development of polymeric membranes as an energy-efficient route for gas separations has gained a lot of attention from researchers over the past several decades. In comparison to conventional techniques, such as distillation and chemical absorption, membrane modules offer many proven advantages, including no necessary phase change, low capital investment, small space requirements, and reduced carbon footprint.1−4 Central to membrane separation performance is the polymer material used to make a membrane. It is always desired to develop materials that allow for a high gas throughput, or permeability, to lower the membrane surface area required for a given separation and have a high sieving capability, or selectivity, to ensure high separation efficiency and high-purity products with proper process design. Unfortunately, an empirical trade-off relationship generally exists between these two properties for most polymers that perform separations following the diffusion− solution mechanism;5−7 that is, as the permeability increases, the selectivity decreases, and vice versa. This trade-off relationship was graphically described by Robeson8,9 through “upper bound” plots based on experimental data and was later theoretically validated by Freeman.10 The position of the permeability/ selectivity data relative to the upper bounds is now widely used as a performance gauge to assess the potential of a polymer material for gas separations, although the upper bound analysis is based on homogeneous membranes utilizing pure gas permeation data. Over the last few decades, the search for better polymeric membrane materials with maximized permeability/selectivity combinations has spurred a huge effort by many academic and © 2017 American Chemical Society

industrial research groups focusing on sophisticated macromolecular design. One common strategy to enhance the gas permeability is to create high fractional free volume (FFV) in the material to increase the number of low barrier diffusion pathways for the gas molecules. This has been typically achieved through the introduction of bulky substituent groups and/or integrating contortion sites into the polymer backbone with high rigidity to disrupt chain packing and limit chain segment rotation, creating a less-dense polymer matrix.11 Some distinctive examples of highFFV polymers with enhanced permeabilities are the highly substituted polyacetylene, poly(1-trimethylsilyl-1-propyne) (PTMSP), 12,13 which is known for very high oxygen permeabilities, because of its rigid, twisted shape; the perfluoropolymer, Teflon AF series,14,15 which has a high rotation barrier between dioxolane rings that gives rise to high free volume; and polymers of intrinsic microporosity (PIMs), such as PIM-1,16−18 which is a ladder polymer composed of fused rings with a spiro-center as a contortion site, creating a very rigid, kinked structure. While these polymer structures boast superiorly high permeabilities due to high fractional free volume, the selectivities are always less than satisfactory, mainly due to the lack of precise control on the size distribution of the free volume elements. Thermally rearranged (TR) polymers are among the few materials that show separation performance well above the upper-bound limits.19−27 TR polymers are fabricated by Received: Revised: Accepted: Published: 4220

February 7, 2017 March 24, 2017 March 29, 2017 March 29, 2017 DOI: 10.1021/acs.iecr.7b00540 Ind. Eng. Chem. Res. 2017, 56, 4220−4236

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Figure 1. (a) The triptycene molecule with an internal free volume element of 31 Å3 highlighted, labeled A; (b) the pentiptycene molecule with two internal free volume elements of 31 Å3 and 133 Å3 highlighted, labeled A and B, respectively. The values of internal free element volume for cavities A and B were reported by Tsui et al.51 and Luo et al.,52 respectively; (c) interactions of iptycene clefts and blades via π−π stacking,53 adapted with permission from ref 53 (Copyright 2017, American Chemical Society, Washington, DC); and (d) supramolecular chain threading and molecular interlocking,51,54 adapted with permission from ref 51 (Copyright 2006, American Chemical Society, Washington, DC).

processability of the cross-linked polymers. Lastly, it should be mentioned that other materials properties such as chemical stability and processability should be carefully considered in new material design, as these properties, if not emphasized, would prevent the translation of a novel membrane material from the research laboratory to real-world industrial processes. 1.2. Iptycenes: A Unique Class of Molecules. A macromolecular design motif that has garnered attention in the past 5 years for gas separation membranes is the use of iptycenes as an architectural element to construct tailorable free-volume architecture that has great potential to address the issues mentioned above. Iptycenes are a class of shape-persistent, threedimensional (3D) molecules built on [2,2,2]-ring systems.43−47 The two simplest members of the iptycene family are triptycene and pentiptycene, named for their configurations of 3 and 5 arene planes, respectively, as shown in Figure 1. Triptycene was first synthesized in 1942 by Bartlett et al.,43 and the synthesis procedure was later simplified through the reaction of anthracene with benzyne formed in situ.48,49 It was later in 1981 that Hart et al. opened the door to the concept of iptycenes, with the development of pentiptycene and other extended-triptycene derivatives.50 The rich structural hierarchy and the versatile chemistry possibilities associated with iptycene units offer great opportunities for developing sophisticated polymers that are both structurally and chemically functional for a wide spectrum of applications. Iptycene molecules contain a few unique characteristics that are particularly beneficial to gas transport when built into polymer backbones for fabrication of membranes. First, because of the bulky, rigid nature, iptycenes, when incorporated into polymer backbones, are highly efficient in disrupting chain packing and increasing overall free volume, and thus gas permeabilities. More importantly, the unique molecular configurations of iptycenes introduce a fundamentally new type of free volume “void” called internal free volume (IFV) elements that are located in the clefts of the benzene “blades”.51,55,56 The fraction of IFV elements is highlighted in Figure 1 for triptycene and pentiptycene molecules. Not only do these IFV elements add to the material’s overall free volume for further gas permeability enhancement, but, because the IFV is intrinsically integrated with the molecular configurations of triptycene and pentiptycene, it also is not susceptible to collapse

thermally converting a soluble aromatic polyimide precursor that contains hydroxyl groups ortho to the imide ring at high temperatures (usually 400 °C or higher) in an inert atmosphere to complex polybenzoxazole (PBO) structures. This imide-tobenzoxazole rearrangement has been demonstrated to cause an increase in the average size of the free volume elements and a narrowing of their size distribution, which allows for exceptional combinations of permeability and selectivity. However, because of the high temperature required for the TR process, these materials often suffer from poor mechanical integrity,27,28 creating the necessity for continued research into more suitable alternatives for industrial needs. In this regard, developing robust polymer membranes with synergistically improved gas permeability and selectivity remains arguably the most challenging materials problem in the field of membrane gas separation, and transformative new macromolecular designs are in demand to address this problem. While the permeability/selectivity tradeoff is one of the most fundamental barriers to utilizing membranes for gas separations, other materials-related challenges exist. One issue to which high fractional free volume polymers are particularly susceptible is physical aging.29−32 Physical aging is the long-term densification of the polymer membrane due to short-range, local motion of the chains, which causes the collapse of free-volume elements and, consequently, a loss in permeability.33 For example, PIM-1 suffered an 80% decrease in nitrogen permeability after aging 1380 days.34 The key material design factor to address this challenge is to construct noncollapsible free-volume “voids” based on molecular configurations rather than conformations, because the configuration-based free volume is truly intrinsic to the macromolecular structure, similar to those present in inorganic molecular sieve materials, while conformations are subjected to constant changes, depending on chain dynamics. Another major challenge is membrane plasticization in the presence of condensable gases such as CO2.35−37 As the gas sorbs into the membrane at higher concentrations, the polymer swells and increases the free volume available for diffusion and sorption. In turn, this causes an upturn in permeabilities with increased upstream partial pressure and a loss in mixed-gas selectivities. Chemical cross-linking seems to represent the main current material strategy to address this issue;38−42 however, the crosslinking often leads to reduced gas permeability and poor 4221

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Figure 2. Structures of iptycene-based diamines and dianhydrides used in nonladder iptycene-based polyimides as follows: DATRI,60 DAT2,61 triptycene-1,4-diamine monomers,62 triptycene-1,4-ortho diamine,53 triptycene-2,6-para diamine,53 pentiptycene-diamine monomers,52 and TPDAn,63 along with the commercial dianhydride 6FDA.

The above-mentioned unique characteristics and properties of iptycenes have provided motivation to researchers over the past five years to investigate their integration into polymer backbones for use in gas separation membranes. Undoubtedly, research efforts on this topic continue to grow as some of these iptycenecontaining polymers have already displayed unprecedented separation performance. This review provides an overview of the state-of-the-art iptycene-containing polymeric membranes for gas separations, with an emphasis on examining macromolecular design motifs using iptycenes and the fundamental structure/property relationships correlating microscopic chain packing and free volume architecture with macroscopic gas transport behavior. We have grouped the iptycene-containing polymers reported in the literature for gas separation membranes into three categories based on the polymer backbone architecture, i.e., iptycene-containing nonladder polymers, semiladder polymers, and ladder polymers. Review and discussion are provided via comprehensively comparative studies on the relevant iptycene polymers within and across each category to provide insight into respective macromolecular design concepts and fundamental structure−property principles. Finally, we will provide some perspectives and future directions for designing and utilizing iptycene-containing polymers for gas separation membranes.

over time, unlike nonequilibrium conformation-based free volume caused by chain packing disruption found in all other non-iptycene-containing high-FFV polymers. Furthermore, the IFV elements have well-defined sizes that are comparable to common gas penetrant molecules, which allows the formation of tailorable microporosity and contributes to great molecular sieving potential when incorporated into polymer membrane materials. Another unique phenomenon resulting from the IFV elements in iptycenes is the supramolecular interactions of iptycene clefts and blades that involve chain threading and molecular interlocking (Figure 1c), which can be beneficial to the mechanical properties of polymer membranes and in regard to regulating chain organization. It has been shown that triptycene molecules have a tendency to orient themselves in a manner that minimizes their internal free volume, such as aligning liquid crystals in the clefts.55,56 When incorporated into polyesters with long aliphatic segments, triptycene directed the organization of the polymer chains by threading the aliphatic segments through the clefts, driven by minimization of internal free volume. In addition, under high strain, the triptycene moieties in the polyesters demonstrated an interlocking effect (Figure 1c), which greatly improved the mechanical properties, compared to an analogue polyester lacking triptycene.51,57 These chain-threading and molecular interlocking effects could be used to enhance the mechanical properties of certain gas separation membranes, such as rubbery membranes. Moreover, interactions between iptycene arene “blades” on neighboring chain segments, such as π−π stacking (Figure 1c), can also help to enhance mechanical properties.58 Furthermore, these sorts of interactions can act as a means to combat plasticization, because both interchain and intrachain interactions suppress polymer swelling.

2. IPTYCENE-CONTAINING NONLADDER POLYMERS 2.1. Aromatic Polyimides Containing Iptycene Units. Glassy aromatic polymers such as polyimides represent the prevailing gas separation membrane materials due to their good mass exchange characteristics and excellent thermal and mechanical properties, which are important for industrial applications such as hydrogen purification and natural gas sweetening.2,59 Polyimides are prepared through polycondensa4222

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Figure 3. Compilation of iptycene-based nonladder polyimides, with a few commercial membrane materials for reference, against the Robeson upper bounds (solid lines) for gas pairs (a) CO2/CH4, (b) H2/N2, and (c) O2/N2, respectively. The recently proposed 2015 upper bounds65 for O2/N2 and H2/N2 are also shown (denoted by the dashed line). Polymers are labeled as follows: 1, 6FDA-DATRI;60 2, 6FDA-DAT2;61 3, 6FDA-1,4-trip_CF3;62,66 4, 6FDA-2,6-trip_para;53 5, 6FDA-PPDA(CF3);52,67 6, TPDAn-TPDAm;63 7, TPDAn-PPDAm;63 8, TPHA-TC400;68 9, TPHI-TR450;68 10, Matrimid;69−71 11, cellulose acetate;72 12, TB-BisA-PC;73 and 13, polysulfone.74

6FDA-based polyimides (e.g., 6FDA-pODA = 16.4%, 6FDAmTMPD = 18.2%),64 indicating the effectiveness of triptycene at increasing polymer free volume, because of the bulkiness of triptycene moieties, as well as the highly tortuous backbone structure derived from the kinked 2,6-connection of DATRI diamine (Figure 2). Despite this high FFV, the average interchain spacing of 5.54 Å found using wide-angle X-ray diffraction (WAXD) was lower than other 6FDA-based polyimides (e.g., 6FDA-mTMPD = 6.15 Å, 6FDA-pODA = 5.60 Å). This supports the idea that a portion of the overall FFV must be attributed to the internal free space in the triptycene clefts, rather than solely from chain packing disruption. The 6FDA-DATRI film showed a high CO2 permeability (PCO2) of 189 Barrer and a good CO2/ CH4 selectivity of 30.5, which outperforms any commercial polymer (e.g., for Matrimid, PCO2 = 8.7 Barrer and αCO2/CH4 = 36) and performs well against the permeability/selectivity tradeoff, as shown in Figure 3 (indicated by the royal blue triangle) (also see Table S4 in the Supporting Information). In addition, while many 6FDA-based polyimides suffer from plasticization in mixed-gas studies, 6FDA-DATRI showed excellent resistance to plasticization with no significant loss of selectivity in a 50:50

tion reactions between a diamine and dianhydride; using different diamine−dianhydride pairs can afford enormous structural variants of polyimides with tailored transport properties. Because of these promising attributes, polyimides were the subject of the first studies investigating iptycenes for use in gas separation membranes. A list of the structures of all of the iptycene-based diamine and dianhydride monomers reported for polyimides synthesis is shown in Figure 2. In all polyimides, when an iptycene-based diamine was used, the aromatic dianhydride 6FDA (i.e., 4,4-(hexafluoroisopropylidene) diphthalic anhydride) was always used, because of its proven usefulness in providing considerably higher selectivity than other dianhydrides. The structures of corresponding polyimides, along with other nonladder iptycene polymers and some of their physical properties, are summarized in the Supporting Information (Table S1). 2.1.1. Triptycene-Based Nonladder Polyimides. One of the first triptycene-based polymers for gas separation applications was a polyimide synthesized from 6FDA and 2,6-diaminotriptycene (DATRI), reported by Yoon Jin Cho and Ho Bum Park in 2011.60 This polyimide, 6FDA-DATRI, was found to have a high fractional free volume (FFV) of 22.6%, compared to other 4223

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which are responsible for size sieving. The larger microcavities that have an average diameter ∼6.9 Å are generated by the chain packing disruption from the bulky nature of triptycene and contribute toward fast diffusion. An intriguing structure/property relationship between the size and nature of the substituent group and the overall free volume in triptycene polymers was also uncovered in the studies on the 6FDA-1,4-triptycene polyimide series.62,66 The substituent groups adjacent to the triptycene units were systematically varied in their size and nature, i.e., H (no substitution), CH3, and CF3, to examine their influence on the overall free volume architecture. As demonstrated in these studies, the free volume fraction and permeation results showed an unexpected trend due to the proposed partial filling mechanism, whereby relatively small-sized substituent groups, such as CH3 (occupied volume = 29.5 Å3),75 could reside (partially) in the clefts of the triptycene units, leading to decreased fractional free volume. According to this mechanism, the 6FDA-1,4-trip_CH3 polymer showed lower FFV and permeabilities than those of the unsubstituted 6FDA1,4-trip_para polymer. Conversely, the 6FDA-1,4-trip_CF3 polymer had the highest FFV and gas permeabilities due to the incapability of such bulky CF3 groups to fill the internal clefts of triptycene units.62,66 This unique trend demonstrates how the free volume architecture of triptycene-based polymers can be manipulated to fine-tune chain packing and gas transport behavior by making minute adjustments to the polymer backbone. Furthermore, these 1,4-triptycene-based polyimides exhibited unprecedented substituent-dependent physical aging behavior.66 The reference, unsubstituted polymer, 6FDA-1,4trip_para, showed a typical decrease in permeability with age; however, the addition of CH3 and CF3 substituent groups caused increases in permeabilities, by an average of 23% and 40%, respectively. This is ascribed to synergic effects of the flexible polyimide backbone, which allows for relatively dense packing that reduces the driving force of physical aging, and local segmental motion that likely promotes interconnectivity and/or unblocking of cleft microcavities. Importantly, stable or increased selectivity accompanied the increase in permeability with age, which can be attributed to the intrinsic sieving capability of the triptycene molecule. This combination allowed the performance of the substituted 6FDA-1,4-triptycene polyimides to move toward the upper bound with age.66 More recently, a comparative study that further explores the tunability of free volume architecture in iptycene-containing polymers was reported by investigating the effect of variations in the linkage geometry surrounding the triptycene unit.53 In this study, the effects of varying the triptycene orientation from the 1,4-connection position to the 2,6-connection position (i.e., 6FDA-1,4-trip_para vs 6FDA-2,6-trip_para; see Table S1 for structures) and varying the linkage geometry from para to ortho in 1,4-connected iptycene polyimides (6FDA-1,4-trip_para vs 6FDA-1,4-trip_ortho; see Table S1 for structures) on polymer free volume and gas transport properties were comprehensively examined.53 For consistency, the 6FDA-2,6-trip_para polymer mirrors the structures of the 6FDA-1,4-trip series62,66 with the flexible ether linkages neighboring the triptycene moiety. Similar to the 6FDA-DATRI polyimide discussed earlier,60 6FDA-2,6trip_para integrates the contortion site (∼120° bend) into the polymer backbone. Generally, this would be expected to cause a highly tortuous polymer backbone with high FFV; however, the flexibility of the ether linkages veils this effect, causing 6FDA-2,6trip_para to have similar FFV and lower permeabilities, compared to the 6FDA-1,4-trip_para polyimide. In addition,

CO2/CH4 feed gas mixture up to a CO2 partial pressure of 8 atm. This plasticization resistance is attributed to the π−π interactions of the triptycene benzene blades perpendicular to the polymer backbone (see Figure 1c). Furthermore, the “permanent” IFV elements of the shape-persistent triptycene unit allowed for stable gas transport performance, even after 150 days of aging.61 Based on the success of 6FDA-DATRI, an extended triptycene structure (i.e., benzotriptycene) was utilized as the basis for a diamine monomer, DAT2, to give an iptycene-based 6FDA polyimide, named 6FDA-DAT2, in which the “blade” that extrudes from the backbone is composed of two stacked benzene rings (Figure 2).61 This extended structure proved to be more effective at disrupting chain packing, as evidenced by a 40% increase in BET surface area. However, the corresponding increases in permeabilities were only ∼10%. The extended triptycene moiety was efficient at sieving, despite the enhanced chain packing disruption and larger internal free volume cavities, as indicated by a similar selectivity as 6FDA-DATRI (CO2/CH4 selectivity = 30; see Figure 3 and Table S4). Specifically, although the high permeability of 6FDA-DAT2 is attractive, its mixed gas separation performance is not satisfactory. For example, under a 50:50 CO2:CH4 mixed gas feed, the selectivity drops to ∼25 at 10 bar, making it fall short of the target selectivity of 40 that is of industrial interest for natural gas sweetening. An attractive feature of iptycenes in macromolecular design is that the chemistry of these multiarene molecules is diverse, allowing for versatile functionalization options. In this regard, via judiciously varying substituent groups, manipulating iptycene linkage geometry, or adjusting the content or distribution of iptycene units, macromolecules with finely tailored backbone tortuosity and chain packing can be readily produced. In turn, the size and size distribution of free volume elements can be feasibly fine-tuned, which is vital to achieve both fast transport and high selectivity. While 6FDA-DATRI and 6FDA-DAT2 made use of the 2,6-connection of triptycene with one benzene blade extruding from the backbone, a new polyimide series was designed utilizing the 1,4-triptycene connection, which connects the triptycene unit by a single benzene ring (Figure 2).62,66 The 6FDA-1,4-triptycene series (see Table S1 in the Supporting Information) was designed with two distinctive features: (1) incorporating relatively flexible ether linkages to improve solvent solubility and processability, and (2) introducing various substituent groups neighboring the triptycene to investigate the tunability in free volume architecture.62 In these studies, it was found that these 1,4-connected triptycene polyimides had moderately high fractional free volumes, ranging from 15.1% to 18.3%, and permeabilities, ranging from 0.41 to 0.99 Barrer for nitrogen. Interestingly, it was found that permeabilities of the 6FDA-1,4-triptycene polymers were higher than those of commercial materials with similar FFV (e.g., Matrimid FFV = 17% and PN2 = 0.32 Barrer), highlighting the importance of the preferable free volume size distribution induced by triptycene. In addition, the small IFV cavities in the clefts of the triptycene moieties allowed for concurrent high selectivities; for example, αH2/N2 ranged from 60 to 90 for the triptycene polyimides, compared to only 56 for Matrimid. Quantitatively, the free volume and its size distribution were analyzed by positron annihilation lifetime spectroscopy (PALS), which clearly showed a bimodal size distribution of free volume in these polymers due to the integration of triptycene in the polymer backbone.66 The smaller microcavities with an average diameter of ∼2.9 Å correspond to the internal free volume of the triptycene clefts, 4224

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Figure 4. Conversion routes to triptycene polybenzoxazoles: (a) thermal rearrangement of TPHI poly(hydroxyimide) precursor and (b) thermal cyclodehydration of TPHA poly(hydroxyamide) precursor.68

possible intrachain filling of the triptycene clefts occurred in the 2,6-connected polyimide that compromised its sieving capability, as evidenced by somewhat lower selectivities of 6FDA-2,6trip_para, compared to 6FDA-1,4-trip_para. When varying the imide linkage geometry from symmetric para orientation to the asymmetric ortho orientation, more-efficient chain packing was obtained, causing decreased permeabilities. Although ortho linkages generally increase steric hindrance,59,76 the relatively flexible ether linkages are believed to allow for a compact chain configuration in 6FDA-1,4-trip_ortho. All these results point to an important macromolecular design principle that, in order to realize the full sieving capabilities of triptycene, a highly rigid polymer backbone (such as that in 6FDA-DATRI60) and/or bulky moieties/substituents larger than the internal cleft microcavities of triptycene unit (as evidenced by the improved properties of 6FDA-1,4-trip_CF362,66) are indispensable. 2.1.2. Pentiptycene-Based Polyimides. As observed with triptycene, pentiptycene is a rigid, shape-persistent moiety with internal free volume microcavities.47,77 However, the H-shaped scaffold, which is composed of five fused arene rings, is even bulkier and contains four IFV microcavities per molecule rather than three: two microcavities identically sized to triptycene’s clefts (31 Å3)51 and two larger ones (∼133 Å3), as shown in Figure 1.52 Therefore, the total volume of IFV microcavities in pentiptycene is more than three times that of triptycene, which makes pentiptycene a highly attractive structural unit in the design of novel gas separation membranes. A series of polyimides prepared from custom-synthesized pentiptycene-diamines (PPDA series; see Figure 2) and 6FDA were studied by Luo et al. for their use in gas separation membranes.52,67 Similar to the 6FDA-1,4-tripycene series,62,66 the pentiptycene polyimide series was systematically varied in pendent substitution groups: 6FDA-PPDA(H), 6FDA-PPDA(CH3), and 6FDA-PPDA(CF3) (see structures in Table S1). These pentiptycene polyimides demonstrated higher FFV values than the triptycene-based counterparts, because of the bulkier nature of pentiptycene and its higher amount of IFV. As analyzed by PALS, the pentiptycene-based polyimides FFV follows a similar bimodal size distribution as observed in triptycene-based polyimides; however, both groups of microcavities in pentiptycene polyimides (3.7 and 7.8 Å in diameter) are larger than those of the triptycene analogues (2.9 and 6.9 Å).67 The pentiptycenebased polyimides also follow the partial filling mechanism, previously discussed for the triptycene-based polyimides,62,66 in

which the CH3-substituted structure showed reduced free volume, compared to the unsubstituted structure due to CH3 groups residing in the pentiptycene clefts. However, the partial filling effect in the pentiptycene polymers is less dramatic than the effect in the triptycene polymers, because of the larger cleft sizes of pentiptycene.52 The 6FDA-PPDA series is found to be highly permeable, because of the high free volume imparted by the pentiptycene moiety. This effect was especially pronounced for gases with larger diameters, such as methane, which saw a 10fold increase in permeability from the triptycene-based series. Despite lower selectivities for the pentiptycene series compared to the triptycene series, the combinations of high permeability and decent selectivity for the 6FDA-PPDA series allowed for lateral movement toward the upper bound (see Figure 3 and Table S4), indicating high sieving potential for pentiptycene.52 Based on the encouraging results of polyimides based on iptycene−6FDA combinations, a series of entirely iptycenebased polyimides were made by replacing the 6FDA dianhydride with a triptycene-based dianhydride (TPDAn; see the structure in Figure 2) in the polycondensation reactions with a iptycenebased diamine (TPDAm or PPDAm; see the structure in Figure 2) to study the effect of iptycene concentration/distribution in the polymer backbone.63 By placing iptycene structures in both diamine and dianhydride monomers, this study aimed at constructing an “hourglass”-like free volume architecture utilizing the feature of bimodal size distribution in both triptycene and pentiptycene moieties, in which the ultrafine microcavities (∼3 Å in diameter) act as the narrow neck, allowing for fine sieving capabilities and the larger micropores (∼7 Å in diameter)63,66,67 allow fast adsorption/desorption at the surface.63 As revealed by PALS, the fully triptycene-based polymer, TPDAn-TPDAm, had slightly smaller pore sizes for both domains (2.87 and 6.84 Å), compared to TPDAn-PPDAm (3.10 and 7.36 Å), the polymer with alternating triptycene and pentiptycene moieties. This is because of pentiptycene’s bulkier structure and higher amounts of IFV. Because the small micropores in these polyimides are slightly larger than the kinetic diameter of hydrogen, but smaller than the kinetic diameters of other common penetrant gas molecules, these TPDAn-based polymers performed very well in hydrogen separations (i.e., H2/N2 and H2/CH4), as shown in Figure 3 and Table S4. Systematic comparisons between 6FDA- and TPDAn-based polyimides revealed that, although the TPDAnbased polymers contain twice as many bulky iptycene moieties as 4225

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membranes provide motivation for continued research in integrating iptycene moieties into PBO-based polymers for gas separation membranes. In particular, this first attempt proved that the triptycene structure is instrumental in maintaining/ improving size-sieving properties in TR polymers. A rational macromolecular design would be to further boost their permeabilities via incorporating a bulky, thermally labile group to replace the hydroxyl group in the precursor structures. Previously reported acetate-type thermally labile groups79−82 are likely the most promising candidates, since they not only promoted permeabilities but also likely removed the requirement of an inert atmosphere during thermal treatment. However, thermally labile groups such as glucoses and cyclodextrins83−85 have also been attempted. 2.3. PEO-Based Copolymer Membranes Reinforced by Iptycene Structure. Some special-case iptycene-based polymers for gas separation membranes are also worth mentioning, since they may shed some light on exploring fundamentally new mechanisms to regulate chain packing and finely tune transport properties to meet various separation needs. The special-case study of merit is the investigation on supramolecularly reinforcing poly(ethylene oxide) (PEO) copolymers with pentiptycene-containing polyimide hard segments.54 PEObased membranes are attractive for carbon dioxide separations, because the polar ethylene oxide (EO) moieties make them selective toward CO2; however, these membranes generally suffer from poor mechanical properties and high crystallinity.1,54,86 In this study, segmented PEO-rich copolymer membranes were developed by incorporating hard segments composed of a pentiptycene-based polyimide (6FDA-PPDA(CF3)) to induce supramolecular chain threading and interlocking interactions between PEO segments and pentiptycene units (see Figure 1c) for mechanical reinforcement.54 It was demonstrated that these copolymers were diffusivity-selective until the PEO content reached 40 wt %, at which point they became solubility-selective, leading to a nonlinear permeability trend. Promising gas transport properties for hydrogen purification (CO2/H2) and carbon capture (CO2/N2) were obtained for these polymers. For example, the copolymers with the longest PEO sequence (2000 g mol−1) and highest PEO content (60 wt %) demonstrated a CO2 permeability of 30 Barrer and selectivities of 4.1 and 4.6 for CO2/H2 and CO2/N2, respectively. The ability of pentiptycene to improve the properties of PEO-based membranes demonstrates the potential of integrating iptycene moieties in rubbery polymers. The extensive work carried out on incorporating triptycene and pentiptycene moieties in nonladder polymers provides evidence for the promise of utilizing iptycenes as design elements as a route to outstanding gas separation properties. The combination of interchain free volume elements and internal free volume (IFV) elements uniquely present in these nonladder iptycene polymers creates a valuable “hourglass”-type microcavity architecture, corroborated by the superior sieving capabilities of these polymers. In addition, the fine-tunability of membrane properties through the partial filling mechanism and linkage geometry adjustment was discovered, demonstrating the high versatility of iptycene-based polymers for various separation needs. Although not emphasized in this review, iptycene-based polymers showed very good solvent solubility, allowing for good processability. Based on comprehensive examination of these reported nonladder iptycene polymers, it can be concluded that chain rigidity is of vital importance in constructing microporous structures that allow both high permeability and high selectivity.

their 6FDA-based analogues, they display lower permeabilities. This unexpected result was attributed to the reduced overall chain rigidity in the TPDAn-based polymers that lack the chain stiffening effect of the hexafluoro-substituted carbon in 6FDA1,78 and have the two flexible ether linkages in the TPDAn moiety, allowing for tighter chain packing. Although the tighter polymer lattice around the iptycene moieties in the TPDAn-based polyimides created excellent sieving properties, the diminished permeabilities once again demonstrate the significant role of polymer backbone flexibility or rigidity on chain packing and gas transport properties. 2.2. Iptycene-Based Polybenzoxazoles (PBOs) and Thermally Rearranged (TR) Polymers. Polymers with heterocylic rings such as polybenzoxazoles (PBOs) are attractive membrane materials for gas separations, because of their superior separation performance and excellent resistance toward plasticization and chemical contaminants. A distinctive class of PBO-based membranes is widely known as thermally rearranged (TR) polymers, which are generally prepared via high-temperature conversion of aromatic polyimide precursors with orthopositioned hydroxyl groups to complex PBO structures in inert atmosphere (i.e., TR process). Comprehensive reviews on TR polymer membranes can be found elsewhere.2,20 A recent study reported the first introduction of the triptycene structure into TR polymers and relevant PBOs and examined their potential for gas separation membranes.68 In this paper, two triptycene-based PBO polymer series were prepared from two different precursors, following different conversion mechanisms (Figure 4). The first series made use of the previously reported TPDAn6FAP poly(hydroxyimide)63 as a soluble precursor (referred to as TPHI in this study), which underwent a high-temperature TR process (350−450 °C) to obtain the final triptycene TR polymers, referred to as TPHI-TR series. The second series used a triptycene-containing poly(hydroxyamide) precursor, TPHA, which underwent a thermal cyclodehydration (TC) process at lower temperatures (300−400 °C) to form the final PBO structures, referred to as TPHA-TC series. Each series comes with its own benefits and drawbacks. Generally, TR polymers undergo more significant chain conformation changes, leading to higher permeabilities, compared to the TC polymers. However, because of the lower thermal requirements of the TC polymers to achieve full conversion, these polymer membranes are much more mechanically robust than the TR counterparts, because of no thermal degradation.19,26,28,79 In this study, the triptycenecontaining TR polymers showed outstanding gas transport performance, and an unprecedented trend in permeability/ selectivity tradeoff was observed in the TR process. In all previously reported TR systems, ideal selectivities experience a significant decrease with thermal treatment. However, the triptycene-based TR polymersparticularly, the TPHI-TR450 sampleshowed an upturn in selectivities, along with the expected significant increase in permeabilities, which was ascribed to the excellent sieving capabilities of triptycene units in these TR polymers. As a result, the performance of TPHI-TR450 sample is positioned well above the 2008 upper bounds, as shown in Figure 3 and Table S4, and outperform most of noniptycene-containing TR polymers.68 On the other hand, the TPHA-TC series showed decent separation performance, although still below the upper bound. However, the TPHA-TC polymer membranes were far more robust than the TPHI-TR films.68 The outstanding gas transport performance and improved mechanical properties of these triptycene-based PBO 4226

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Figure 5. Compilation of iptycene-based semiladder polymers against the Robeson upper bounds for gas pairs (a) CO2/CH4, (b) H2/N2, and (c) O2/ N2. Recently proposed 2015 upper bounds65 for O2/N2 and H2/N2 also are shown (as a dashed line). Polymers are labeled as follows: 1, i-C3 KAUST-PI1; 2, i-C3 KAUST-PI-2; 3, i-C3 KAUST-PI-3; 4, i-C3 KAUST-PI-4; 5, i-C3 KAUST-PI-5; 6, i-C3 KAUST-PI-6; and 7, i-C3 KAUST-PI-7;87,88 8, n-C3 KAUST-PI-1′ and 9, n-C3 KAUST-PI-5′;88 10, n-C2 KAUST-PI-5″;88 11, PBIBI-TPD; and 12, PBIBI-PPD.89

Figure 6. Reaction scheme for the synthesis of the iptycene-based PBIBIs.89

3. IPTYCENE-CONTAINING SEMILADDER POLYMERS To further boost permeabilities, macromolecular designs focusing on polymer backbone rigidity have rapidly gained attention in iptycene-containing polymer research. By removing flexible linkages and incorporating fused rings into the backbone, chain packing becomes more frustrated, increasing the FFV. In addition, chain rigidity can enhance sieving capabilities and potentially membranes’ resistance to plasticization. The following section summarizes the research on iptycenecontaining semiladder polymers. Here, a polymer is defined as

a semiladder structure when there is at least one non-single-bond connection present in the repeat unit structure of the polymer. Structures and some physical properties of all semiladder iptycene-containing polymers are listed in Table S2 in the Supporting Information. Gas separation performance of select semiladder polymers is displayed in the upper bound plots, shown in Figure 5, and is listed in Table S4. 3.1. Iptycene-Containing Poly[bis(benzimidazobenzisoquinolinones)] (PBIBIs). A study by Mao and Zhang investigated the incorporation of triptycene and 4227

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membranes. Detailed KAUST-PI structures can be found in Table S2 in the Supporting Information. The first generation of triptycene PIM-PIs, KAUST-PI-1 and KAUST-PI-2,87 utilize short, branched isopropyl groups at triptycene bridgehead positions. In addition, methyl groups ortho to the imide bond are included from the diamine monomer to restrict chain rotation and further stiffen the polymer backbone. It was found that KAUST-PI-1 has an extremely high BET surface area for a polyimide (752 m2 g−1) and high gas uptakes, similar to those of PIM-1. In addition, similar to previously discussed iptycene-based nonladder polyimides,63,66,67 a bimodal distribution of pore sizes was also revealed in these semiladder PIM-PIs from pore-size distribution (PSD) analyses, in which larger micropores enhance permeabilities and smaller micropores improve selectivity. The microstructure of KAUST-PI-1 and KAUST-PI-2 allow for extraordinary gas separation performance, well above the 2008 upper bound for O2/N2, H2/N2, H2/CH4, and CO2/CH4 gas pairs. This excellent performance was attributed to high diffusivity selectivities imparted by the 9,10-diisopropyl-triptycene moieties producing molecular-sieving behavior. This is in combination with permeabilities that are comparable to or even higher than notable ladder-like PIMs (e.g., the CO2 permeability is 2389 Barrer for KAUST-PI-1,87 2300 Barrer for PIM-1,16 and 1100 Barrer for PIM-716). Quickly following the study on the KAUST-PI-1 and KAUSTPI-2, a broader family of PIM-PIs was developed, covering three groups of triptycene-based PIM−PIs. The first series, including KAUST-PI-1 and KAUST-PI-2, were based on the 9,10diisopropyl-substituted triptycene and contained seven members with differing aromatic diamine moieties.87,88,93 The second group utilized a 9,10-dipropyl bridgehead-substituted triptycene moiety and two different diamines (tetramethylphenyl diamine (TMPD) and 6FpDA).88 Lastly, a PIM-PI containing 9,10diethyltriptycene and the 6FpDA was synthesized.88 All polymer structures are summarized in Table S2. From the various polymer designs, detailed insight into the structure/property relationships in triptycene-based PIM-PIs was established. Looking first at the effect of the diamine structure, larger surface areas were displayed for those polymers based on bulky and rigid diamines, with substituent groups restricting rotation around the imide bond.88 Specifically, KAUST-PI-7, containing a pair of two stacked benzene rings in the diamine moiety, showed the highest BET surface area of 840 m2 g−1; however, this excessively open structure led to low selectivities. Generally, excellent combinations of permeability and selectivity, especially for air and hydrogen separations, are found for the KAUST-PIs composed of rotation-restricted diamine moieties (KAUST-PI-1, KAUST-PI-2, and KAUST-PI-3). Many other polymers designed with these diamines that aimed to increase intrachain rigidity saw consequential losses in selectivity, alongside the gain in permeability (e.g., 6FDA-based polyimides and spirobisindane-based PIM-PIs). However, it was shown by incorporating 9,10-diisopropyltriptycene, that selectivity can be recovered and significantly increased, leading to great performance against the Robeson upper bound, as shown in Figure 5 and Table S4. Polymers utilizing more-flexible diamine moieties (KAUST-PI-4 and KAUST-PI-6) show high selectivities, especially for CO2/ CH4, but diminished permeabilities, because of more efficient chain packing. Effects of varying the triptycene bridgehead substituent on chain packing and transport properties were also analyzed, mainly by comparing KAUST-PI-1 and KAUST-PI-5 with

pentiptycene in polybenzimidazole(PBI)-type polymers, referred to as PBIBIs.89 These polymers were prepared via a onestep, high-temperature polycondensation reaction between iptycene-based tetraamine monomers and 4,4-binaphthyl-1,1− 8,8-tetracarboxylic dianhydride (see Figure 6). As shown, these PBIBIs are composed of many fused ring moieties, which qualifies them as semiladder polymers, even though they still contain flexible ether linkages on either side of the iptycene unit. While PBIBIs typically suffer from poor solvent solubility, causing difficult membrane fabrication,90−92 the pentiptycene moiety in PBIBI-PPD allowed for polymer solubility in organic solvents, N-methyl-2-pyrrolidone (NMP) and trichloroethylene (TCE), because of the interruption of strong interchain interactions.89 However, the triptycene-based PBIBI-TPD was only soluble in strong acids and phenolic solvents, indicating that the smaller three-ring triptycene is less effective at disrupting interchain interactions. The incorporation of iptycene moieties in the PBIBIs also led to increased FFV (e.g., PBIBI-PPD has a FFV of 17.1%), compared to non-iptycenebased analogues (e.g., PBIBI-DOD has an FFV value of 14.8%), because of chain packing disruption and the internal free volume of the iptycene moieties. In addition, this study shows evidence of interlocking of the phenyl rings perpendicular to the polymer chain (see the “π−π stacking” in Figure 1c) by the observed higher glass-transition temperatures of iptycene-containing PBIBIs, compared to the non-iptycene-containing ones. The combination of chain packing disruption, internal free volume, and interlocking of the benzene blades led to favorable permeation properties. For instance, the carbon dioxide permeability was 137.2 Barrer for PBIBI-PPD and 92.1 Barrer for PBIBI-TPD, compared to 70.8 Barrer for the non-iptycenecontaining PBIBI-DOD. Because of its high FFV, PBIBI-PPD showed reduced selectivities, compared to the other PBIBI membranes; however, the triptycene-based PBIBI displayed equal or higher selectivities, compared to the non-iptycenecontaining PBIBI membranes, which put the performance of PBIBI-TPD near the upper bound for the O2/N2 separation, as shown in Figure 5 and Table S4. 3.2. Triptycene-Based PIM-Polyimides (PIM-PIs). With a goal of bridging the gap between low-free-volume, highly selective polymers, such as traditional polyimides, and highfree-volume, highly permeable ladder-like polymers, such as PIMs, researchers at KAUST initiated extensive research efforts in developing a series of triptycene-based PIM-polyimides (PIMPIs), named KAUST-PIs.87,88,93−95 Generally, these polymers fuse the triptycene molecule into the dianhydride moiety (Figure 7) to limit chain rotation and introduce various bridgehead substituent groups on the triptycene moiety to further tune free volume and chain packing properties for gas separation

Figure 7. Structure of the triptycene-based diamine (TPDA) used in the KAUST-PI series.87,88 4228

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Industrial & Engineering Chemistry Research KAUST-PI-1′, KAUST-PI-5′, and KAUST-PI-5″.88 It was demonstrated that switching from a branched isopropyl chain to the linear alkyl chains caused large reductions in BET surface area. PSD analyses showed that a significantly higher fraction of small, molecular-sized pores existed in the polymers containing the isopropyl bridgehead substituents. The reduced surface areas led to lower permeabilities for KAUST-PI-1′, KAUST-PI-5′, and KAUST-PI-5″ with linear bridgehead substituent groups. The extent of the effect of switching the bridgehead substituent from linear propyl to branched isopropyl groups is dependent on the diamine used. The polymers containing the TMPD experienced a higher increase (5−7 fold) in permeabilities for small gases (He, H2, O2) compared to the 6FpDA-based polymers (3−4 fold), while the 6FpDA-based polymers showed a more significant increase (4−5 fold) in permeability for large gases (N2 and CH4) than the TMPD (3 fold). This suggests that, in the TMPD-based polymer, switching to the branched groups creates more ultrafine microporosity, as evidenced by improved diffusivity selectivity. Conversely, the relatively more freely rotating 6FpDA can better accommodate the larger gases. When comparing KAUST-PI-5′ and KAUST-PI-5″, which share the same 6FpDA moiety but have linear bridgehead substituents of different length (propyl vs ethyl), it was determined that the shorter ethyl groups allowed for increased permeability, because of increased solubility coefficients. However, it was also found that, despite lower permeabilities, the longer propyl groups led to higher diffusivity coefficients due to improved mobility of the longer, more-flexible chains. In summary, the study of structure/ property relationships of KAUST-PIs allowed the development of a few design principles to balance intersegmental spacing and intrasegmental mobility to obtain high-sieving ultramicroporous polymers, including rigidly integrating bridged bicyclic contortion sites into the polymer backbone, using short diamines to restrict imide bond rotation, incorporating branched isopropyl substituent groups at the triptycene bridgehead, and increasing contortion center concentration.88 Select KAUST-PI materials were evaluated for their resistance toward physical aging and plasticization. In the study of gas transport properties in KAUST-PI-1 and KAUST-PI-5 over time, both microporous polymers displayed an “aging” knee.88,93 Further experiments with recovery-aging cycles showed that excess free volume dissipated over the first 10−15 days. It is thus recommended to store membranes for ∼15 days before evaluating them for their gas transport properties in order to obtain properties in the quasi-steady-state phase. In addition, long-term aging effects of KAUST-PI-1, KAUST-PI-2, and KAUST-PI-7 were evaluated for H2/N2 and O2/N2 over 150 days.34 Generally, it was found that oxygen was affected more significantly by physical densification induced by aging, meaning that the permeability losses outweighed selectivity gains, relative to the smaller H2, for which selectivity gains outweighed permeability losses. For example, KAUST-PI-1 suffered a 34% decrease in PO2 with a 26% increase in O2/N2 selectivity and an 18% decrease in PH2 with a 56% increase in H2/N2 selectivity, over 150 days. Even after aging, these combinations of permeability and selectivity still allow for their performance above the upper bound of H2/N2 and air separations.34 The triptycene-based PIM-PIsKAUST-PI-1 and KAUSTPI-5were evaluated for their plasticization resistance.93 As discussed earlier, KAUST-PI-1, containing the TMPD moiety, displayed outstanding gas separation performance, because of the high amount of ultramicroporosity induced by the

diisopropyltriptycene moiety. However, this same characteristic caused it to be susceptible to accelerated and extensive plasticization, because a high fraction of micropores in this polymer were ∼3−4 Å in diameter (evidenced by the superior O2/N2 separation performance), which, when dilated, highly facilitated the transport of similarly sized CO2 and, likewise, the transport of the larger CH4. This led to a 300% increase in PCH4, a continuous increase in PCO2, and a 50% decrease in CO2/CH4 selectivity for KAUST-PI-1 under mixed-gas conditions (50:50 CO2/CH4 feed) tested over a range of 4−30 bar. Conversely, KAUST-PI-5, which contains the more-flexible 6FpDA moiety, showed much higher plasticization resistance, because of the ability of the chains to pack more efficiently at the initial stage, facilitating in coplanarization, which improves interchain interactions (e.g., charge transfer complex or CTC). This shows that, although intrachain rigidity is efficient in producing impressive combinations of permeability and selectivity, it is not enough on its own to combat the issue of plasticization. 3.3. Hydroxyl-Functionalized, Triptycene-Containing PIM-PIs. The need for a focus on promoting interchain interactions, in addition to the attention to intrachain rigidity in microporous polymers, becomes apparent from the plasticization study on KAUST-PI-1 and KAUST-PI-5 described above.93 Besides chemical cross-linking, an arguably easier way to effectively suppress polymer dilation upon the adsorption of condensable gases is through the introduction of interchain interactions. This has been achieved through introducing hydroxyl groups to promote hydrogen bonding and by (subTg) thermal annealing, which promotes interchain dipole−dipole interactions (charge transfer complexes (CTCs)).72,94,96,97 Both of these approaches were investigated on triptycene-based PIMPIs,94,95 and their effects on gas separation performance are summarized in Table S4 in the Supporting Information and in Figure 8 in an upper bound plot, with polymer structures shown in Table S2. In these studies, the same 9,10-diisopropyltriptycene dianhydride moiety was utilized, while the diamine monomer with or without substituent groups for hydrogen bonding was used for comparative studies. In particular, a new triptycene PIMPI, TPDA-APAF, was prepared using a hydroxyl-functionalized diamine, 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane

Figure 8. Effect of hydroxyl-functionalization of triptycene-based semiladder polymers,94,95 utilizing i-C3 TPDA, against the Robeson upper bounds for the CO2/CH4 gas pair. Polymer are labeled by diamine moiety. Square points represent reference polymer; circular points represent OH-functionalized polymer. 4229

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Figure 9. Compilation of iptycene-based ladder polymers against the Robeson upper bounds for gas pairs (a) CO2/CH4, (b) H2/N2, and (c) O2/N2. The recently proposed 2015 upper bounds65 for O2/N2 and H2/N2 are also shown (dashed line). Circular data points represent TPDA-based ladder polymers;99 square points represent TB-based ladder polymers.100,101

indicates that even with added thermal energy, a structure without strong interchain interactions cannot form desirably packed microstructures, demonstrating the vital role of interchain interactions in the form of hydrogen bonding in improving sieving capabilities. In mixed gas tests (1:1 CO2/ CH4), the annealed TPDA-APAF film showed a resistance to plasticization up to 50 bar, along with even higher CO2/CH4 selectivities (10%−20% higher) under mixed-gas conditions than pure-gas conditions, which suggests the ultramicroporous structure can co-permeate CO2 and effectively “block” the transport of CH4. Further investigation of hydroxyl-functionalized triptycenebased PIM-PIs was carried out through a pair of polymers still utilizing the diisopropyltriptycene dianhydride moiety (TPDA), but employing a shorter m-phenylenediamine moiety, mPDA, and a dihydroxyl-functionalized 4,6-diaminoresorcinol, DAR (see Table S2).95 The motivation of this design is to reduce the number of flexible bonds in the overall polymer backbone to further improve selectivities of triptycene-based PIM-PIs. Similar to the previously discussed set of polymers, the inclusion of the two OH groups in TPDA-DAR caused strong interchain interactions due to hydrogen bonding and CTC formation, leading to reduced surface area, loss of free volume, and size reduction of the micropores, when compared to its nonhydroxyl

(APAF), which is an analogue structure to previously discussed KAUST-PI-6 (referred to as TPDA-ATAF in this study) with the exception of having hydroxyl substituents in place of the methyl groups.94 As determined by solid-state fluorescence emission spectroscopy, the hydroxyl groups induced hydrogen bonding, drawing the polymer chains closer and promoting the development of CTCs. In addition, comparisons between the polymer films thermally treated at sub-Tg temperatures of 120 and 250 °C showed that the higher thermal treatment supplied enough thermal energy to assist polymer chains in overcoming rotational energy barriers to undergo coplanarization for enhanced CTC formation. Conversely TPDA-ATAF, with bulky CH3 groups, displayed inefficient chain packing and steric interference disrupting coplanarization. These effects were seen in the poresize distribution analysis; TPDA-ATAF shifted toward higher fractions of large pores, because of the reduced interchain interactions. Pure gas permeation results showed that both TPDA-ATAF and TPDA-APAF displayed reductions in gas permeability (∼66% for CH4) with thermal treatment, because of polymer densification (similar to the aging phenomena); however, TPDA-APAF with hydrogen-bonding interchain interactions showed a significant increase in selectivity (αCO2/CH4 increased from 38 to 53), compared to TPDA-ATAF (30 to 33). This 4230

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N2 performance was unsatisfactory. This was partially attributed to the turnstile-like rotatory motion of the CH3 groups, which could hinder selectivity. To improve performance, the methyl groups were removed and the ethanoanthracene unit was extended to the full triptycene moiety, creating the triptycene polymer of microporosity, PIM-Trip-TB (see Table S3).100 Sorption isotherms of CO2, CH4, and N2 in PIM-Trip-TB revealed relatively higher sorption at low pressures, compared to PIM-EA-TB, while relatively lower CO2 and CH4 sorption were noted at higher pressures, indicating that PIM-Trip-TB has a larger fraction of small free volume elements. This, in addition to the lack of methyl groups, allowed for enhanced sieving capabilities. In addition, PIM-Trip-TB saw much improved permeabilities, compared to PIM-EA-TB for all gases, except H2 and He (which are similar). In particular, the PCO2 and αCO2/N2 for PIM-Trip-TB was 9709 Barrer and 15.9, compared to 7696 Barrer and 13.3 for PIM-EA-TB. In addition, this polymer performed high above the O2/N2 2008 upper bound, because of its impressive diffusivity selectivity introduced through the highly sieving triptycene moieties. PIM-Trip-TB was evaluated for its aging performance as well. Although it showed significant decreases in permeability over time (e.g., 60% loss of PO2 in 100 days), the improvements in selectivity balanced or outweighed this effect. This allows the material to move parallel to or even further above the upper bound with age.100 Another triptycene- and TB-based ladder polymer, PIMBTrip-TB (Table S3), was developed shortly after PIM-Trip-TB, which makes use of the extended triptycene structure, benzotriptycene.101 Similar to the motive behind the design of 6FDA-DAT261 (Table S1), the goal of this polymer design was to capitalize on the bulkier structure of the extended benzotriptycene and the larger internal free volume elements that it possesses. PIM-BTrip-TB was shown to have a BET surface area (870 m2 g−1) similar to that of PIM-Trip-TB (899 m2 g−1) and increased CO2 adsorbed at 1 bar (3 mmol g−1 and 2.1 mmol g−1 for BTrip and Trip, respectively). As expected, the benzotriptycene moiety generated much higher permeabilities compared to the triptycene moiety. For instance, the carbon dioxide permeability increased to 13 200 Barrer for PIM-BTripTB from 9709 Barrer for PIM-Trip-TB. However, the large voids induced by the bulkier moiety with larger IFV moieties caused a reduction in selectivities (e.g., O2/N2 selectivity is 3.6,101 compared to 4.3 and 4.0 for PIM-Trip-TB and PIM-EA-TB,100 respectively). PIM-BTrip-TB performed above the upper bound for O2/N2, H2/N2, and H2/CH4, and on the upper bound for CO2/CH4 (Figure 9 and Table S4). However, because of the reduced selectivities, the performance of the benzotriptycene PIM is slightly hindered against the upper bound, compared to the triptycene PIM. In contrast to PIM-Trip-TB, PIM-BTrip-TB aged gracefully, with substantial increases in selectivity which outweighed the losses in permeability, leading to improved performance against the upper bound for air and hydrogen gas pairs. The incredible gains in selectivity for both PIM-Trip-TB100 and PIM-BTrip-TB101 demonstrate the critical role of the iptycene internal free volume, which is insusceptible to collapse over time and facilitates in size sieving. 4.2. Triptycene-Based PIMs. The effect of bridgehead substituent on the triptycene moiety on membrane properties of ladder polymers was investigated with a set of triptycene-based PIMs: TPIM-1 and TPIM-2 (see Table S3).99 Both polymers were synthesized from A-B-type monomers, with dihydroxylfunctionalization on one side and difluoro- functionality on the

analogue, TPDA-mPDA. As a result, TPDA-DAR was less permeable than TPDA-mPDA (e.g., PCO2 = 215 and 349 Barrer for TPDA-DAR and TPDA-mPDA, respectively). However, because of the favorable hydrogen-bonding interactions, TPDADAR showed a very high CO2/CH4 selectivity of 46, compared to 32 for TPDA-mPDA and 33 for commercial cellulose triacetate. In addition, TPDA-DAR outperformed previously discussed TPDA-APAF with a 2-fold higher CO2 permeability and a 20% higher CO2/CH4 selectivity (see Figure 8 and Table S4).94,95 Furthermore, TPDA-DAR also showed promising mixed-gas performance (1:1 CO2/CH4), with no evidence of plasticization, a CO2 permeability of 140 Barrer, and a CO2/CH4 selectivity of 38 at a partial CO2 pressure of 10 atm.95 Great strides in the rational design of iptycene-based polymers for use in gas separation membranes were made through the development of semiladder polymers. Several new iptycenebased polymers (i.e., KAUST-PI-1, KAUST-PI-2, KAUST-PI-3, KAUST-PI-5, KAUST-PI-7; TPDA-DAR) perform above the 2008 upper bound. In addition, general design principles were developed to guide the research in the area of high-sieving ultramicroporous polymers. Methods to combat plasticization in PIM-PIs were also established, including the incorporation of strong interchain interactions like hydrogen bonding and sub-Tg annealing. In particular, it seems a natural extension of these hydroxyl-substituted PIM-PIs to subject them to TR and convert them from semiladder to highly rigid ladder structures, which could lead to attractive properties, as suggested by the success of the triptycene-based PBO polymers68 previously discussed.

4. IPTYCENE-BASED LADDER POLYMERS The arguably most attractive trend of macromolecular design using the iptycene structure for gas separation membranes is to construct fully ladder-like polymer backbones with high contortion, where chain packing is severely frustrated and no rotational segmental motion is allowed due to complete elimination of single-bond connection along the backbone. Representative ladder polymers for gas separation membranes are PIMs.11,16,98 Although highly permeable, these polymers often suffer from relatively low selectivities, because of their large-sized free volume elements. Iptycenes are considered to be especially useful in constructing ladder polymers, because of their intrinsic three-dimensional molecular configuration. Moreover, as discussed previously, iptycene moieties with unique internal free volume elements bring out ultrafine microporosity, enabling superior sieving capability in membranes. Therefore, integrating the rigid, highly sieving iptycenes and using the molecules’ intrinsic 120° angle as the site of contortion is a promising design route. Structures and some physical properties of iptycene-based PIMs are summarized in Table S3 in the Supporting Information. Their upper bound plots for select gas pairs are shown in Figure 9, and the gas transport properties are listed in Table S4. 4.1. Tröger’s Base PIMs Containing Iptycene Moieties. One of the first triptycene-based ladder polymer studies aimed to remove the relatively flexible spiro-center and dioxin linkages found in conventional PIMs. The study was an extension on the success of a previously studied Tröger’s base (TB) PIM, which contained a rigid ethanoanthracene moiety with bridgehead methyl groups (PIM-EA-TB).102 Tröger’s base is a bridged bicyclic diamine, making it a good candidate for creating shapepersistent polymers. In addition, this monomer can be easily synthesized. While PIM-EA-TB displayed good sieving capabilities for hydrogen and air separations, the CO2/CH4 and CO2/ 4231

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backbone creates a high degree of intrachain rigidity, crucial to inefficient chain packing and high selectivities. This was demonstrated by its much increased selectivities from nonladder and semiladder polymers: for nonladder 6FDA-DATRI with the freely rotatable triptycene, αH2/N2 = 32;60 for semiladder KAUSTPI-1, αH2/N2 = 37;87 and for TPIM-1 with the fully fused backbone, αH2/N2 = 50.99 In fact, the performance of highly rigid triptycene-based polymers is so impressive that a new set of 2015 upper bounds was suggested by Swaidan et al.65 for O2/N2, H2/ N2, and H2/CH4 based on TPIM-1,99 KAUST-PI-1,87 and PIMTrip-TB,100 along with the ethanoanthracene-based PIM, PIMEA-TB.102 This is displayed, as indicated by the dashed line, on the permeability/selectivity trade-off plots in Figures 3, 5, and 9.

other. TPIM-1 contains short, branched isopropyl substituents and TPIM-2 contains flexible, linear propyl groups. The study showed that the isopropyl substituents allowed for surface areas similar to other PIMs (TPIM-1: 862 m2 g−1) while the linear substituents caused reductions in surface area (TPIM-2: 612 m2 g−1). In addition, TPIM-1 showed more prominent ultramicroporosity due to the short, branched isopropyl groups, compared to TPIM-2. These packing properties manifested themselves in the gas transport performance. TPIM-1 showed simultaneous enhancements in permeability and selectivity, compared to TPIM-2. For example, the hydrogen permeabilities were 2666 Barrer and 655 Barrer for TPIM-1 and TPIM-2, respectively, and the H2/N2 selectivities were 50 and 37 for TPIM-1 and TPIM-2, respectively. Similar to the partial filling mechanism discussed previously for triptycene-based nonladder polyimides,52,62,66,67 it is likely that the bulky isopropyl groups add to chain packing disruption and facilitate in the formation of microporosity, while the more flexible propyl groups fill in some of the free volume, reducing both permeability and selectivity.99 Although TPIM-1 shows lower permeabilities than PIM-TripTB and PIM-BTrip-TB, when coupled with outstanding selectivities, TPIM-1 showed the best performance against the Robeson upper bound among all triptycene-based semiladder and ladder polymers (Figure 9 and Table S4). TPIM-1 and TPIM-2 were also evaluated for their performance against physical aging and plasticization.34 Because of the high FFV value for TPIM-1, the driving force for physical aging was high, resulting in significant permeability losses and selectivity gains. For example, the PO2 of TPIM-1 decreased 95% in 780 days (compared to 70% for PIM-1 in 1380 days) and the αO2/N2 increased by 115% (∼40% for PIM-1). Because the selectivity gains outweighed the permeability losses, the aged TPIM-1 performed with unprecedented permeability/selectivity combinations for O2/N2 and H2/N2. In addition, in comparing TPIM-1 to TPIM-2, it was found that aging significantly affected the ultramicroporosity induced by the isopropyl groups in TPIM-1, as demonstrated by the more pronounced enhancement to O2/N2 selectivity in TPIM-1 than TPIM-2, while the effect on H2/N2 selectivity was similar for both polymers. Furthermore, it was hypothesized that such drastic aging effects displayed by TPIMs compared to other PIMs were due to lowered equilibrium specific volumes (i.e., increased driving force for aging) caused by the unique ribbon-like twodimensional geometry, which could pack more efficiently than contorted 3D geometries of many other PIMs. It was also found that TPIM-1 suffered from significant plasticization effects, with an ∼60% decrease in CO2/CH4 selectivity and a 93% increase in methane permeability in mixed-gas conditions, compared to pure-gas conditions. These results support the concept discussed earlier that high intrachain rigidity alone is not sufficient to suppress plasticization of polymer membranes. Interestingly, TPIM-2 was less susceptible to plasticization, with only an ∼10% increase in methane permeability in mixed-gas conditions, compared to pure-gas conditions. This was attributed to the lack of ultramicroporosity (present in TPIM-1) that is highly sensitive to matrix dilation.34 Triptycene-based ladder polymers have shown some of the most attractive combinations of permeabilities and selectivities to date. This success is attributed to the combination of large interchain voids from triptycene’s bulky nature and the sieving capabilities of the triptycene moieties due to the ultramicroporosity they create. In addition, the fully fused ring

5. SUMMARY AND OUTLOOKS Over the past five years, iptycene structures have proven to be transformative design elements in polymeric gas separation membranes. The rigid, bulky structures are efficient in disrupting chain packing and the unique internal free volume elements create microporosity imperative to enhanced sieving capabilities. Central to the macromolecular design involving iptycenes is the utilization of the molecular configuration of iptycenes to construct truly intrinsic microporosity, unlike the conformation-based free volume architecture seen in other conventional microporous polymers. While initial studies on nonladder iptycene-based polymers imparted fundamental knowledge on structure−property properties, the most impressive properties to date have been obtained by integrating the frameworks of iptycenes into polymer backbones with high degrees of intrachain rigidity. These polymersnamely, KAUST-PI-1, TPIM-1, and PIM-Trip-TBare identified as next-generation materials and their performance suggests the construction of a 2015 upper bound for permeability/selectivity tradeoff. Although many of the outstanding gas transport properties of iptycene-based polymers are determined under idealized, puregas conditions, iptycene-based polymers also show promise in aggressive mixed-gas conditions (e.g., 1:1 CO2/CH4, up to 50 bar), more closely resembling harsh industrial gas separation environments. Specifically, 6FDA-DATRI, KAUST-PI-5, TPDAAPAF, TPDA-DAR, and TPIM-2 showed excellent resistance to plasticization in a mixed-gas feed, mostly because of high backbone rigidity. In addition, because of the intrinsic nature of the microporosity imparted by the iptycene moieties, these materials show promising physical aging results, which is a significant obstacle for membrane technology in an industrial setting. For example, 6FDA-1,4-trip_CH3 and 6FDA-1,4trip_CF3 show increased permeabilities with maintained selectivities over time. Also, KAUST-PI-1, PIM-Trip-TB, PIMBTrip-TB, and TPIM-1 show exceptional enhancements in selectivity for O2/N2 and H2/N2 gas pairs that outweigh the effects of reduced permeabilities after aging. While not emphasized in this review, other materials properties, such as processability, scalability, and cost, are indispensable considerations in any new membrane material design. Many of the monomers used in the iptycene-based systems require complicated synthesis procedures, especially those which utilize benzyne in situ in the Diels−Alder reaction, which could present some potential obstacles for scaleup. However, the systems that are derived from commercial anthracene and p-benzoquinone are quite simple and make use of relatively inexpensive starting materials. Nevertheless, the relatively high cost of these novel iptycene-based membrane materials can be justified by their 4232

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Industrial & Engineering Chemistry Research superior separation performance. This is especially true if these ultrahigh performance polymers can be integrated in multilayer composite membrane modules as selective thin layers (∼100 nm), where the bulk porous support layers can be made from a relatively low-cost material. Regarding future endeavors in iptycene-based polymers for gas separation membranes, there seems to be significant potential to integrate iptycenes in thermally rearranged (TR) polymers. The reported hydroxyl-functionalized triptycene-based polyimides, TPDA-APAF and TPDA-DAR (Table S2 in the Supporting Information), can be readily used as precursors to prepare corresponding TR polymers. In addition, while triptycene has been incorporated in various polymer backbone structures, pentiptycene-based polymers remains largely unexplored. Considering its more hierarchical molecular configuration and more internal free volume, pentiptycene holds great promise in constructing intriguing free volume architecture in corresponding polymer membranes for demanding gas separation applications.

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Ruilan Guo is an Assistant Professor of Chemical and Biomolecular Engineering at the University of Notre Dame. She earned her bachelor’s and master’s degrees from Beijing University of Chemical Technology, and completed her Ph.D. from Georgia Institute of Technology. Before joining Notre Dame in 2012, she was a postdoctoral fellow at Virginia Tech working with the late Prof. James E. McGrath. Her research focuses on developing hierarchically functional polymeric materials with applications impacting both energy and the environment, including new membranes for gas separations, polyelectrolyte membranes for fuel cells, nanocomposite membranes, and membranes for desalination and water treatment. Dr. Guo has received several prestigious awards including the Department of Energy (DOE) Early Career Research Award and the ACS Petroleum Research Fund for Doctoral New Investigators.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00540. Tables listing chemical structures, thermal properties (Tg and Td), physical properties (d-spacing, density, FFV, SBET, pore volume), and gas transport properties (P and α) of iptycene-based polymers (PDF)

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ACKNOWLEDGMENTS This invited contribution is part of the I&EC Research virtual special issue for the 2017 Class of Influential Researchers. The authors sincerely thank the Office of Basic Energy Sciences of the U.S. Department of Energy (DOE), under Award No. DESC0010330, and the National Science Foundation (NSF), under Award No. CBET-1603414 for funding support.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +1-574-631-3453. Fax: +1-574-631-0317. E-mail: rguo@ nd.edu. ORCID

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Ruilan Guo: 0000-0002-3373-2588 Notes

The authors declare no competing financial interest.

REFERENCES

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Biographies

Jennifer Weidman is a graduate student at the University of Notre Dame. She earned her bachelor’s degree in chemical engineering from West Virginia University in 2011. She came to Notre Dame in the Fall of 2012 as an Arthur J. Schmitt Presidential Fellow in the Chemical and Biomolecular Engineering department. With the guidance of Professor Ruilan Guo, her doctoral research has focused on the development of novel iptycene-based polymeric membranes for gas separation applications. 4233

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