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Ionic Polyimides: Hybrid Polymer Architectures and Composites with Ionic Liquids for Advanced Gas Separation Membranes Max S Mittenthal, Brian S Flowers, Jason E. Bara, John William Whitley, Scott K Spear, John David Roveda, David A Wallace, Matthew S Shannon, Rob Holler, Rich Martens, and Daniel T. Daly Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00462 • Publication Date (Web): 10 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017

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Industrial & Engineering Chemistry Research

Ionic Polyimides: Hybrid Polymer Architectures and Composites with Ionic Liquids for Advanced Gas Separation Membranes Max S. Mittenthal,1^ Brian S. Flowers,1^# Jason E. Bara1,*, John W. Whitley,1 Scott K. Spear,2 J. David Roveda,1 David A. Wallace,1 Matthew S. Shannon,1 Rob Holler,3 Rich Martens3 & Daniel T. Daly2 1

Department of Chemical & Biological Engineering, University of Alabama, Tuscaloosa, AL USA 354870203

2

Alabama Institute for Manufacturing Excellence, University of Alabama, Tuscaloosa, AL USA 354870204

3

Central Analytical Facility, University of Alabama, Tuscaloosa, AL USA 35487-0164

*corresponding author: [email protected] ^MSM and BSF contributed equally to this work. #Current affiliation: Dept. of Mathematics, Physics & Engineering, Midland College, Midland, TX USA 79705 Abstract Polyimides and ionic liquids (ILs) are two classes of materials that have been widely studied as gas separation membranes, each demonstrating respective advantages and limitations. Both polyimides and ILs are amenable to modification/functionalization based on selection of the requisite precursors. However, there have been but a handful of reports considering how polyimides and ILs could be integrated to obtain fundamentally new materials with synergistic properties. In this manuscript, we demonstrate a new and versatile way to synthesize polyimides with imidazolium cations directly located within the polymer backbone to form polyimide-ionene hybrids, or “ionic polyimides”. Our strategy for synthesizing ionic polyimides does not require the use of amino-functionalized ILs. Instead, the imidization reaction occurs prior to polymerization in the formation of an imidazole-functionalized diimide monomer. This monomer is then polymerization via step-growth (condensation) polymerization with p-dichloroxylene via Menshutkin reactions, simultaneously linking the monomers and creating the ionic components. The resultant ionic polyimide is amenable to melt processing (e.g., extrusion, meltpressing) and capable of forming thin films. Upon soaking thin films of the ionic polyimide in a widely used IL, 1-butyl-3-methylimidazolium bistriflimide ([C4mim][Tf2N]), a stoichiometric absorption of the IL into the ionic polyimide was observed, forming an ionic polyimide + IL composite. The gas separation performances of ionic polyimide and ionic polyimide + IL composite membranes were studied with respect to CO2, N2, CH4 and H2. The neat ionic polyimide exhibits low permeability to CO2 and H2 (~0.9 and ~1.6 barrers, respectively) and very low permeability to N2 and CH4 (~0.03 barrers for both). For the ionic polyimide + IL composite, the permeabilities of CO2, N2 and CH4 increase by 1800-2700%, while H2 permeability only increased by ~200%. The large increases in permeability for CO2, N2 and CH4 are due to greatly increased gas diffusivity through the material, with gas solubility essentially unchanged with the IL present. The ionic polyimide and ionic polyimide + IL composite were characterized using a

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number of techniques. Most interestingly, X-ray diffractometry (XRD) of the films reveals that the ionic polyimide + IL composite displays a sharp peak, indicating that the ionic polyimide may experience supramolecular assembly around the IL. Although the performances of these first ionic polyimide and ionic polyimide + IL composite membranes fall short of Robeson’s Upper Bounds, this work provides a strong foundation on which ionic polyimide materials with more sophisticated structural elements can be developed to understand the structure-property relationships underlying the ionic polyimide platform and ultimately produce high-performance gas separation membranes.

Keywords: Polyimide, ionic liquid, gas separations, polymer membranes, ionene


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Biosketches Max Mittenthal received a B. S. in Chemical Engineering from The University of Alabama in 2016 and is currently finishing a Master’s in Business Administration at the University of Alabama. Prior to working in Bara’s group, he was involved with creating and testing novel ionic liquids with Prof. Robin D. Rogers. Next year, he will be working as a Product Development Analyst with General Motors.

Brian S. Flowers received his B.S. M.S. and Ph.D. degrees in Chemical Engineering from the University of Alabama in 2011, 2013, and 2016, respectively. His dissertation focused on the characterization of new solvents for pre- and post-combustion CO2 capture. He is currently teaching at, and overseeing the growth and development of, the engineering program at Midland College in Midland, Texas.

Jason E. Bara received a B.S. in Chemical Engineering from Virginia Commonwealth University in 2002 and a Ph.D. in Chemical Engineering from the University of Colorado – Boulder in 2007. After working as Senior Research Associate at CU-Boulder from 2007-2009, he started his academic career an Assistant Professor at the University of Alabama in 2010 and was promoted with tenure to Associate Professor in 2015. He has published over 75 peer-reviewed journal articles and book chapters, and named as an inventor on 10 issued US and international patents. He has also developed several iPhone/iPad apps for chemical engineering that are is use in over 190 countries. Jason has also founded two companies: ION Engineering, which is commercializing process technologies for CO2 capture and Vector 254, which develops software for smartphones and tablets.



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John W. Whitley received his B.S. and Ph.D. degrees in Chemical Engineering from the University of Alabama in 2012 and 2016, respectively. His dissertation focused on the use of ionic liquids and coordinated ionic liquids as media in polymerization reactions and applications for poly(IL) materials. He is currently a postdoctoral researcher performing joint work at 525 Solutions, Inc. and the Department of Chemistry at the University of Alabama working with Prof. Robin D. Rogers.


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1. Overview & Introduction 1.1 Polyimide and Ionic Liquid-based Membranes Membranes can offer improved energy efficiency in separations processes such as CO2 capture from combustion point sources (CO2/N2), natural gas sweetening (CO2/CH4), syngas processing (CO2/H2), olefin/paraffin (e.g., C3H6/C3H8) and air (O2/N2) separations. In recent years, intensifying research on a number of advanced polymer, inorganic and hybrid materials has targeted enhanced performance and efficiency.1, 2 Several distinct material classes have emerged, each with its respective set of benefits and limitations. Specifically, polyimides,2-9 ionic liquid (IL)-based polymers and composites,10-16 polymers of intrinsic microporosity (PIMs),1, 17-21 metal-organic frameworks (MOFs)22-33 and thermally rearranged (TR) polymers34-46 are at the forefront of advanced membrane materials. In this manuscript, the discussion will focus on polyimide and IL-based materials and opportunities for the realization of new polyimide-IL hybrids. Polyimides are among the most desirable polymers for gas separation membranes due their high permeability, intrinsic selectivity and excellent physical properties.2, 47 Wholly aromatic polyimides are synthesized via condensation of an aromatic diamine with a dianhydride at near ambient temperature to form a poly(amic acid), followed by thermal imidization at elevated temperatures. Matrimid® and Upilex® are two of the most relevant polyimides for gas separations in established commercial gas separations applications (e.g., H2/CH4, CO2/CH4, O2/N2) based on their ability to form high quality films and fibers, even though their CO2 permeabilities tend to be < 20 barrers, and wellbelow Robeson’s Upper Bounds.2, 48 Figure 1 presents examples of “Robeson Plots” appended with relevant data for the aforementioned separations, illustrating that advanced polyimides, PIMs and PIMpolyimides define the current state-of-the-art in key, energy-intensive gas separations.



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Figure 1: Robeson Plots illustrating “upper bounds” and performances of advanced polyimide and PIM membranes materials in energy-related separations. Reprinted with permission from Swaidan, R.; AlSaeedi, M.; Ghanem, B.; Litwiller, E.; Pinnau, I., Rational Design of Intrinsically Ultramicroporous Polyimides Containing Bridgehead-Substituted Triptycene for Highly Selective and Permeable Gas Separation Membranes. Macromolecules 2014, 47, 5104-5114. Copyright 2014 American Chemical Society.

Recent developments in the design of polyimide gas separation membranes have revolved around the use of the fluorinated dianhydride, 4,4'-(Hexafluoroisopropylidene)diphthalic anhydride (more commonly known as 6-FDA) in combination with bulky aromatic diamines.2 These materials have been observed to have much higher CO2 permeabilities (500-700 barrers) due to the large disruptions in chain packing (and increased FFV) caused by -CF3 groups on 6-FDA and multiple -CH3 groups introduced on aromatic species such as durene diamine. The inclusion of very bulky triptycene linkages within the


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polyimide backbone has recently yielded materials with CO2 permeabilities approaching 3000 barrers.4951

Such materials are found at or near Robeson’s 2008 Upper Bound for CO2/CH4 separation.52 ILs offer seemingly endless possibilities for the design of materials for gas separation

membranes.10, 11, 13, 14, 53, 54 Among the cations commonly used to form ILs, the imidazolium platform represents the most versatile and tunable substrate, as it possesses up to five points of possible functionalization/derivatzation.55 Initial interest in the use of ILs within gas separation membranes focused on supported liquid membranes (SLMs) since the negligible vapor pressure of the IL would prevent evaporation into the gas stream.56-60 Scovazzo, et al. showed that ILs could have CO2 permeabilities as high as 1000 barrers, and that ILs were more promising for separation of CO2/N2 (αij = 20 – 40) than for CO2/CH4 compared to conventional polymer membranes on Robeson Plots.52, 56, 57, 61-63 However, a limitation of SLMs containing ILs is the stability of the membrane, since the IL is held in place via weak capillary forces, and it can be easily “blown out” at pressure differentials well below 1 atm.16, 64 Recognizing this inherent limitation of ILs in SLMs, Bara, et al. focused on the design of polymeric IL or “poly(IL)” membranes wherein a polymerizable IL monomer was impregnated into a porous support (vis-à-vis SLMs) but then photopolymerized in situ to form a mechanically robust polymer membrane.65-67 These poly(IL) membranes retained, or even improved upon, the CO2/N2 and CO2/CH4 selectivities exhibited by analogous SLMs, although CO2 permeability was reduced to the range of 10-40 barrers. Reduced permeability in poly(IL) membranes was associated with reduced gas diffusivity, since a large volume fraction of poly(styrene) was now present in the membrane.65-67 To remedy this reduction in permeability, poly(IL)-IL composite membranes were subsequently developed as a means to use the poly(IL) as a support framework to hold non-polymerizable “free” IL in the membrane.11, 68-70 By incorporating 20 mol% “free” IL within a poly(IL) matrix, CO2 permeability improved ~400% compared to the neat poly(IL) while CO2/N2 selectivity improved 35%. As first recognized by Lodge and co-workers, the non-covalent, Coulombic interactions between ions in the



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poly(IL)-IL composite prevent “blow out” of the free IL due to pressure.16 Further work has shown that the poly(IL)-IL concept is highly tunable and logical structure-property relationships can be used to understand membrane performances.3, 71-76 Furthermore, the free IL content in the composite can exceed 50 mol% while maintaining a mechanically stable material, and the permeabilities of these poly(IL)-IL composites can approach those achieved by SLMs.71-73 Other polymer membrane materials, such as block copolymers (BCPs), have incorporated ILs into their structures.15, 77 In addition to their compatibility with poly(IL) materials as well as many conventional polymers,78 ILs have been shown to be effective agents for promoting self-assembly, either as amphiphiles themselves79-82 or as a secondary phase around which liquid crystals (LCs) organize,83-89 giving rise to uniquely nanostructured materials. The use of imidazoles/imidazolium cations as key building blocks for polymers remains an attractive approach to membrane design because it presents a modular and tailorable architecture.53, 90 Although seemingly two disparate classes of materials in terms of molecular structure and physical properties, ILs and polyimides can be covalently integrated through efficient chemical reactions to form “ionic polyimides”. The concept of ionic polyimides is dramatically different from simply encapsulating or blending conventional ILs with conventional polyimides.91, 92 Ionic polyimides may offer unprecedented access to arrays of new materials building on the growing, yet separate, knowledge bases of ILs and polyimides. Ionic polyimides are based upon the “ionene” motif (Figure 2, right) wherein the cation is directly in the polymer backbone,93, 94 rather than the poly(IL) or “ionomer” motif where the cation is present as a pendant group (Figure 2, left).

Figure 2: Representations of the general structures of cationic ionomers (left) and ionenes (right).


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1.2 Known Examples of Polyimides Bearing Ionic Functionalities To date, there have been just a handful of reports considering the incorporation of imidazolium cations within a polyimide backbone. Anderson’s and Coleman’s groups first explored this concept by reacting 6-FDA with both 4,4’-methylenedianiline (MDA) and diamino-functionalized ILs (Figure 3).95, 96

H2 C

m H2N

NH2

O +

n

H2N

N

N

m+n O

O

N F3C CF3

O

O

O

N

O

CF3

NH2

Tf2N O

F3C

O

O

O H2 C

N

O

O

N F3C CF3

O

N

N Tf2N

m

n

Figure 3: Random copolymer with imidazolium cation present in backbone produced by Li, et. al.

Yet, the “neat” ionic polyimide (e.g., m = 0, Figure 3) exhibited a low number-average molecular weight (MN) of 4700 g/mol with a number-average degree of polymerization (XN) of ~ 4 (i.e., m = 0, n = 4 in Figure 3). This resulted in poor thermal/mechanical properties and reduced gas permeability compared to the neat 6-FDA/MDA polyimide without the ionic segment (e.g., m > 0, n = 0, Figure 3).95, 96 It appears that the use of diamino-functionalized ILs as the diamine source must present some inherent barrier to achieving a tractable ionic polyimide when m = 0, n > 0. The difficulty in achieving higher MN polymers when using a diamino-functionalized IL is curious, as aliphatic amines should be much more reactive toward anhydrides and amenable to imide formation (i.e., ring closure) than aromatic diamines such as MDA. Li’s papers95, 96 took a different synthetic approach toward amine-functionalized ILs than that outlined in Davis’ original amine-functionalized “task-specific” IL work.97 The low MN of the resulting polymer is attributed to a stoichiometric imbalance between diamine and dianhydride due to



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the presence of residual NaBr in the diamino-functionalized IL associated with a neutralization step in its synthesis.95 However, even taking great care to remove NaBr yielded no marked improvement in MN. More recently, the material in Figure 2 has been produced in the form of block copolymers with both “short” and “long” blocks.98 For all materials based on Figure 2 with m, n > 0, reported CO2 permeabilities were modest (7-15 barrers) with CO2/N2 selectivities ranging from 20-30.95, 96, 98 Interestingly, work by Lee, et al. also experienced similar challenges with ionic polyesters produced via the condensation of α,ω-dihydroxy-functionalized ILs and aliphatic diacid chlorides.99 In a series of 13 ionic polyester products, MN ranged from 6,000 – 44,000 g/mol, with 6 samples < 15,000 g/mol, 6 samples > 20,000 g/mol and 1 sample of indeterminate MN. Based on the apparent challenges using di-functionalized ILs in condensation polymerizations, can alternate strategies be formulated to achieve the same ionic polyimide (or ionic polyester) product without the use of a diamino-functionalized IL? Simply put, are diamino-functionalized ILs the most appropriate precursors for ionic polyimides? Several examples exist in the literature that demonstrate the synthesis of ion-containing polyimides without the use of IL-based monomers. Shaplov, et al.100 modified a 6-FDA/benzimidazole polyimide to form a benzimidazolium cation within the backbone (Figure 4). This approach requires two post-polymerization alkylkations of the benzimidazole component followed by exchange from halide to the bistriflimide (Tf2N-) anion, and is limited to starting materials that will yield a benzimidazole (or similar) functionality within the backbone.


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O

O N

N

O

F3C CF3

O

H N N n

Alkylation 1

Ion-exchange

O

O N O

Alkylation 2

N F3C CF3

O

N N Tf2N

n

Figure 4: Three-step modification of 6-FDA/benzimidazole polyimide employed by Shaplov to form an ionic polyimide.

Because the ionic polyimide in Figure 4 arises from modification of a polyimide produced via conventional dianhydride-diamine chemistry, the resultant ionic polyimide had MN = 97,000 g/mol. As such, this material was amenable to forming mechanically stable films from solvent casting. CO2 permeability of ~29 barrers was achieved, which rose to 85 barrers when one equivalent of [C2mim][Tf2N] per polymer repeat unit was blended with the ionic polyimide. Interestingly, ideal selectivities for CO2/N2 and CO2/CH4 were both very low in the ionic polyimide membrane without the IL (αij = 4-5), yet the inclusion of IL caused selectivities to rise to ~30 for both gas pairs. There have also been polyimides bearing pendant imidazolium cations recently reported by Kammakakam, et al.101 This approach relies on modification of a 6-FDA/durene-based polyimide (Figure 5) wherein one or more -CH3 groups associated the durene moiety is converted to a benzylic bromide in the presence of N-bromosuccinimide (NBS) and benzoyl peroxide. A poly(ethylene glycol) (PEG)functionalized imidazole102 then reacts with the polymer to form the pendant imidazolium cation. The location of the cation is distinct contrast to the structures in Figures 3 and 4 as it is not within the



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polymer backbone and as such, the structure in Figure 4 does not meet the definition of an ionene (Figure 2),94 and might be better thought of as a “polyimide ionomer”.

Figure 5: 6-FDA/durene polyimide ionomer with pendant PEG-functionalized imidazolium cations.

The 6-FDA/durene material (i.e. no modification to include pendant cations) exhibited CO2 permeability of 495 barrers with CO2/N2 and CO2/CH4 selectivities of ~12 and ~13, respectively. For materials with the pendant cation and Br- anion, theCO2 permeability decreased from 485 to 54 barrers as the length of the PEG-substituent increased from p = 2 to 12. CO2/N2 and CO2/CH4 selectivities rose from ~27 and ~37 to ~40 and ~49, respectively. The decrease in diffusivity is likely due to increased chain packing due to entanglements between pendant PEG side chains, cation-anion interactions, as well as intermolecular associations between Lewis basic ether oxygen atoms and the “acidic” H atom at the imidazolium C(2) site.103-105 A similar modification strategy has been employed by Chen, et al. to form a polyimide ionomer with a dihydrogen phosphate anion (H2PO4-) for use as a proton-exchange membrane in fuel cells (Figure 6).106 Here the polyimide is first formed from 2,2’,6,6’-Tetraphenyl-4,4’-oxydianiline (4-PhODA) and 6FDA, followed by chloromethylation using paraformaldehyde with Me3SiCl/SnCl4 to introduce reactive sites at one or more of the pendant benzene rings. These benzylic chlorides were then reacted with 1methylimidazole to form the pendant imidazolium cation followed by exchange of Cl- to H2PO4-.


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Figure 6: Imidazolium-functionalized polyimide used by Chen, et al. for proton-exchange membrane fuel cells.106

Similarly, anionic polyimide ionomers bearing pendant sulfonate groups and “free” 1-ethyl-3methylimidazolium ([C2mim]) counterions were reported by Imazumi, et al. for use as polymer actuators (Figure 6)107 and structurally analogous materials (Figure 7, n > 0, m = 0) with H+ counterions for use in fuel cells.108 N O

O

N

N

O

O

N O3S

SO3

O

O

N

N

O

O

O

n N

O S O

O m

N

Figure 7: Anionic polyimide ionomer with pendant sulfonate anions and “free” [C2mim] cations synthesized by Imaizumi, et al.107

It is worth noting that the anionic polyimide ionomers required no post-synthetic modification, other than a simple exchange of the cation from a protonated ammonium salt to [C2mim].



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1.3 Developing a Versatile Synthetic Methodology for Ionic Polyimides Having recognized that the combination of polyimides and ILs presents new opportunities for membrane science, we have developed a modular and versatile approach to the synthesis of ionic polyimides. Unlike conventional polyimide synthesis, our methods do not rely on the near universally employed polycondensation of dianhydrides with diamines. Instead, the synthesis of an imidazoleterminated diimide occurs via reaction of two equivalents of amine-functionalized imidazole and a dianhydride. In the current work, N,N'-bis(3-imidazol-1-yl-propyl)pyromellitic diimide (Figure 8), synthesized from inexpensive and commercially available 1-(3-aminopropyl)imidazole (API) and pyromellitic dianhydride (PMDA) is used to demonstrate our methodology. The imidazole endgroups are then reacted with an dihalide (e.g., p-dichloroxylene) via Menshutkin reactions,109, 110 giving rise to ionic polyimides without the use of IL precursors and without post-polymerization modification.

Figure 8: N,N'-Bis(3-imidazol-1-yl-propyl)pyromellitic diimide synthesized from API and PMDA.

There is only one reference to such an imidazole-based diimide compound in the literature.111 Lü, et al. synthesized and observed it adopted triple-stranded helices and “plywood” arrays when coordinated with Ag+ salts and small solvents to form a coordination polymer. In this work, we utilize this imidazole-based diimide in combination with p-dichloroxylene and lithium bistriflimide (LiTf2N) as the basis of a new ionic polyimide material. This ionic polyimide was successfully synthesized at a large scale (~200 g), and the material was extruded and melt-cast into thin films. Soaking the ionic polyimide films in an excess of 1-butyl-3-methylimidazolium bistriflimide ([C4mim][Tf2N]), a widely used IL, resulted


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in a stoichiometric absorption of IL corresponding to 1 equivalent of IL for every repeat unit in the ionic polyimide. Several standard techniques were employed to characterize the properties of the ionic polyimide and the ionic polyimide + IL composite, and their performances as gas separation membranes were studied with respect to CO2, N2, CH4 and H2. The ionic polyimide + IL composite exhibited > 2000% increases in CO2, N2 and CH4 permeability compared to the neat ionic polyimide, which can be attributed entirely to increased gas diffusivity, as gas solubility remained largely unchanged with the IL present. While H2 permeability also increased in the ionic polyimide + IL composite, the increase was more modest (~200%). Furthermore, while H2 was the most permeable gas in the neat ionic polyimide, CO2 was the most permeable gas in the ionic polyimide + IL composite. The composite material exhibited a much more ordered as evidenced by XRD, which indicates that the ionic polyimide chains self-assemble around the IL as it is absorbed. Although the performances of the ionic polyimide and the ionic polyimide + IL composite membranes fall below Robeson’s Upper Bounds, the proof of concept established through this work forms a foundation by which the various components of the ionic polyimide can be selected to create new polymer materials with enhanced properties.

2. Experimental 2.1 Materials API (98%), PDMA (>95%) and p-dichloroxylene (>98%) were purchased from TCI America. 1methylimidazole and 1-bromobutane were purchased from N,N-dimethylformamide (DMF) (ACS Grade) was purchased from BDH/VWR Analytics. Lithium bistriflimide (LiTf2N) (99%) was purchased from IoLiTec.



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2.2 Synthesis The ionic polyimide was prepared in a two-step sequence as illustrated in Scheme 1. First, the bis(imidazole) diimide monomer was prepared from API and PMDA according to a literature procedure. The condensation polymerization was carried out in DMF solution by reacting equimolar amounts of the bis(imidazole) diimide monomer and p-dichloroxylene in the presence of excess LiTf2N.

Scheme 1: Synthesis of the bis(imidazole) diimide monomer and ionic polyimide. O

O

2

N

N

+ O

NH2

O

O

O

N n

N Cl

N O

DMF, LiTf2N

H2O

150oC, 16h

RT

O

O N

Cl +n

N

O

N

120oC, 16h

N

N N

O

O

O

N

O

N N

O

N DMF

N Tf2N

N O

Tf2N =

O N O

N

N

O O F3C S N S CF3 O O

Tf2N n

2.2.1 Synthesis of N,N’-Bis(3-imidazol-1-yl-propyl)-pyromellitic Diimide (“Diimide Monomer”) The following procedure is based on that previously published by Lü.111 API (68.85 g, 550 mmol) and PMDA (54.53 g, 250 mmol) were dissolved in 150 mL of DMF in a 500-mL heavy walled round bottom-flask (Ace Glass) equipped with a magnetic stir bar. The flask was sealed with a threaded PTFE cap and DuPont Kalrez® o-ring. The reaction was heated while stirring at 120oC using an oil-free magnetic hot plate stirrer (Heidolph HEI-Tec). A white precipitate began to form within 1 h, and the reaction was allowed to continue overnight. After this time, the reaction was stopped and cooled to


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ambient temperature. The contents of the reaction were poured into 500 mL deionized H2O and placed in a refrigerator for 3 h to further cool. The solids were then filtered and washed with 3 x 200 mL Et2O. The solids were collected and dried overnight in a vacuum over at 120oC. Yield = 53.3 g (49.3%) as an off-white powder. This reaction was repeated a second time with the same conditions resulting in a similar yield. 1H NMR (500 MHz, DMSO-d6) δ 8.19 (d, J = 1.0 Hz, 2H, Ar-H), 7.65 (t, J = 1.1 Hz, 2H, Im-H), 7.20 (t, J = 1.2 Hz, 2H, Im-H), 6.88 (t, J = 1.1 Hz, 2H, Im-H), 4.05 (t, J = 7.0 Hz, 4H Im-CH2CH2CH2N), 3.62 (t, J = 6.7 Hz, 4H, Im-CH2CH2CH2N), 2.08 (p, J = 6.9 Hz, 4H, Im-CH2CH2CH2N). 1H NMR values were consistent with published results.111 1H NMR spectrum image provided in the Supporting Information.

2.2.2 Synthesis of Ionic Polyimide The diimide monomer (86.6 g, 200 mmol), p-dichloroxylene (35.01 g, 200 mmol) and LiTf2N (172.30 g, 600 mmol) were dissolved in 400 mL of DMF in a mechanically stirred batch reactor with digital temperature control (IKA LR 1000 Control). The stirring mechanism featured PEEK blades for scraping the walls of the reactor vessel, and the working volume of the reactor vessel is 1500 mL. The reactor vessel seals provided by the manufacturer were replaced with Kalrez® o-rings. The solution was heated to 120oC while stirring. The solution turned a bright red/orange color, followed by precipitation of an off-white solid after ~20 min. As precipitate continued to build over the next 20 min, the reactor was no longer able to continue stirring. At this point, the reaction was stopped and allowed to cool. The reactor vessel was then filled with deionized H2O (~1250 mL) and left to soak for ~72 h. After this time, the solid product was scraped from the reactor vessel and collected in a 2-L Erlenmayer flask. The flask was filled with ~1 L of deionized H2O and the solid product pulverized into a homogeneous powder using a dispersing instrument (IKA T-18 Digital Ultra-Turrax®). Photographs of the polymerization in batch reactor are included in the Supporting Information.



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The off-white homogenized powder was filtered and dried overnight in a vacuum oven at 180oC. Collected yield = 210 g (88.8%). The actual yield is higher, but losses of product occurred during cleaning of the reactor. 1H NMR (360 MHz, DMSO-d6) δ 9.30 (s, 2H, Im+-H), 8.25 (s, 2H, C2NO2-C6H2-C2NO2), 7.79 (d, J = 13.8 Hz, 4H, Im+-H), 7.47 (s, 4H, Im-CH2-C6H4-CH2-Im), 5.44 (s, 3H, Im+-CH2-C6H4-CH2-Im+), 4.28 (s, 4H, Im+-CH2CH2CH2N), 3.68 (s, 3H, Im+-CH2CH2CH2N), 2.20 (s, 4H, Im+-CH2CH2CH2N). 1H NMR spectrum image provided in the Supporting Information.

2.2.3 Synthesis of [C4mm][Tf2N] [C4mim][Tf2N] was prepared at a ~25 g scale as a clear, colorless oil from 1-methylimidazole, 1bromobutane and LiTf2N according to well-established procedures (Scheme 2).105, 112 Scheme 2: Synthesis of [C4mim][Tf2N].

1

H NMR (500 MHz, DMSO-d6) δ 9.11 (d, J = 1.9 Hz, 1H, Im+-H), 7.72 (dt, J = 32.4, 2.0 Hz, 2H, Im-H), 4.17

(td, J = 7.2, 1.4 Hz, 2H, Im+-CH2CH2CH2CH3), 3.86 (d, J = 1.5 Hz, 3H, Im+-CH3), 1.78 (pd, J = 7.5, 1.4 Hz, 2H, Im+-CH2CH2CH2CH3), 1.28 (hd, J = 7.3, 1.4 Hz, 2H, Im+-CH2CH2CH2CH3), 0.91 (td, J = 7.4, 1.4 Hz, 3H, Im+CH2CH2CH2CH3). Consistent with published data.113 1H NMR spectrum image provided in the Supporting Information.

2.3 Membrane Preparation The ionic polyimide produced in the condensation polymerization and subsequent washing/homogenization is in the form of a fine powder. While we did experiment with solution-casting


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this powder into thin films, we were also able efficiently produce films suitable for membrane testing from melt-casting methods using the following procedure: Ionic polyimide powder was fed to a dual-screw benchtop extruder (LE-075, Custom Scientific Instruments) operating at 220oC and extruded into ~2-mm-diameter filaments (Figure 9). These filaments were then manually cut into ~1/8-1/2” long pieces (Figure 9). While the ionic polyimide was fed to the extruder as an off-white/tan powder it emerged as a dark brown filament.

Figure 9: Photographs of extrusion of ionic polyimide filament (left) and chopped pieces (right).

A Carver® hot-press (Model 4386) set to 220oC was used to melt-cast defect-free films of the ionic polyimide. First, the lower plate of the hot-press was covered by a 6” x 6” square of Kapton® film. Then, ~1.8 g of the ionic polyimide pellets were placed in the center of the Kapton film. A second 6” x 6” square of Kapton film was placed on top and the hot-press was raised so that the pressure on the material was ~1600 psi and was held at this pressure for 2 min. The press was released and the “sandwiched” film was removed. After cooling for several minutes, the first Kapton layer was easily peeled away, exposing one side of the ionic polyimide film. The exposed side was then placed face down on a hard, smooth surface, and the second Kapton film was slowly and carefully peeled away,



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liberating the melt-pressed ionic polyimide film. In this manner, defect-free ionic polyimide films > 4” in diameter were produced. The films were golden/amber in color, translucent, flexible and exhibited a sheen. Using a 47 mm diameter steel punch, the bulk films were cut to produce discs of the exact size of the membrane systems. The typical thickness of the resultant film was ~90 µm. Figure 10 presents some photographs of the ionic polyimide bulk film and a membrane cut from the bulk.

Figure 10: Images of a large area melt-pressed film (left), film with 47 mm disc punched (center) and relative scale and transparency of membrane (right).

Several discs were placed in Petri dishes, covered in a large excess of [C4mim][Tf2N] and allowed to equilibrate with the IL for at least 24 h. The films were then wiped with an absorbent paper to remove any excess IL from the surface. Measuring the mass gain of the film as a function of IL immersion time confirmed that the ionic polyimide ceased absorbing additional IL within 24 h. A number of replicate experiments (~10) confirmed that the ionic polyimide consistently absorbed 0.38 – 0.43 g IL per g of neat ionic polyimide. The MW of [C4mim][Tf2N] is 419.36 g/mol, and the MW of the ionic polyimide repeat unit is 1096.86 g/mol. Thus, the equilibrated absorption of 0.38-0.43 g IL / g ionic polyimide corresponds almost precisely to 1 molecule of “free’ IL introduced (i.e. 1 cation – anion pair) for every 1 repeat unit of the ionic polyimide (2 cation – anion pairs) (e.g., 419.36/1096.86 = 0.382 g IL /


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g of ionic polyimide). The ionic polyimide + IL composite is stable like other reported poly(IL)-IL composites,70 and the IL cannot be squeezed out of the composite under applied pressure (i.e. the composite is not like a sponge). The ionic polyimide + IL composite is more flexible than the neat ionic polyimide, but the composite is not mechanically fragile. Dissolution of the ionic polyimide into the IL is not observed at ambient temperature. However, if heated to higher temperature (~150oC), the ionic polyimide is observed to reversibly gel the IL phase upon cooling (Figure 11), with results resembling those obtained when ILs are gelled by 12-hydroxy stearic acid.114

150oC

22oC

Figure 11: Progression of [C4mim][Tf2N] gelation by an ionic polyimide filament when the two components were heated to 150oC together and cooled. The vial in the rightmost image was inverted to illustrate that the gel did not flow against gravity.

2.4 Membrane Testing A “time-lag”115 apparatus was used to characterize the performances of the neat ionic polyimide and the ionic polyimide + IL composite membranes with respect to CO2, N2, CH4 and H2. Prior works provide details describing the construction and operation of this instrument.67, 116 All measurements



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were ideal (i.e., single-gas) and performed at 22oC. The feed pressure was ~3 atm (~45 psia) against initial vacuum (< 0.01 psia) downstream. Pressures and temperatures were measured and recorded using LabView® (National Instruments). After each experiment, the system was held under dynamic vacuum (< 0.01 psia) for at least 16 h at ambient temperature. The permeability (Pi) of each gas was calculated from the steady-state pressure rise in the downstream volume with time. Assuming the solution-diffusion (S-D) mechanism is applicable, the diffusivity (Di) of each gas was calculated from the time lag (Θ) and membrane thickness (l) and the solubility (Si) of each gas was calculated as the quotient of Pi and Di. Ideal permeability, solubility and diffusivity selectivity (αi,j) for a given gas pair (e.g., CO2/N2) were calculated as Pi/Pj, Si/Sj and Di/Dj, respectively.

2.5 Ionic Polyimide Characterizations 2.5.1 MW Analysis 1

H NMR endgroup analysis was employed to search for chemical shifts associated with the

expected imidazole and Cl-CH2-Ar endgroups respectively arising from the diimide monomer and pdichloroxylene used in the condensation polymerization (Scheme 1). 1H Chemical shifts in (DMSO-d6) for protons associated with terminal unreacted imidazole endgroups would be expected at ~7.65 ppm (C(2)H), ~7.20 (C(4)-H) and 6.88 (C(5)-H), while those for the Cl-CH2-Ar endgroup would be expected at 4.77 ppm. However, no clear evidence of these peaks was present in the 1H NMR spectrum of the ionic polyimide, and they cannot be discerned from the baseline. Using a conservative estimate of a signalto-noise ratio of 50:1 for the 1H NMR spectrum, the fraction of undetectable endgroup signals in the baseline could be as high as 2% relative to polymer signals. Thus, a value of XN = 50 was calculated, corresponding to MN = 54,843 g/mol for the ionic polyimide.


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Matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) was conducted on the ionic polyimide using a Bruker Ultraflex instrument. The ionic polyimide sample was dissolved in methanol (MeOH) at 5 mg/mL and mixed with a trans-2-[3-(4-tert- butylphenyl)-2methyl-2-propenylidene]malononitrile (DCTB) matrix. The results displayed two broad overlapping peaks at ~40,000 m/z and ~60,000 m/z. Signals were detected from as low as 25,000 m/z to as large as 120,000 m/z. The MALDI-TOF MS experimental results are provided in the Supporting Information. These results are consistent with the 1H NMR endgroup analysis estimate of an MN for the ionic polyimide of ~55,000 g/mol. Gel permeation chromatography (GPC) of the ionic polyimide was performed using a Malvern Viscotek VE 2001 Triple-Detector Gel Permeation Chromatograph equipped with an automatic sampler, a pump, an injector, an inline degasser, a column oven (60 °C), and two in-series Malvern T6000M SEC columns with DMF as the elution solvent. Detection was conducted by means of a photodiode array detector operating between 190 nm and 500 nm. GPC calibration was performed using polystyrene standards and conventional calibration techniques. Upon addition of the sample to the column, little signal was found in the eluent. An increase in the pressure of the system at the guard column indicated that there was a stoppage in the column. Based on these observations, it was concluded that the sample did not travel through the column and that the sample was the source of the stoppage. No further attempts were made to use GPC with the ionic polyimide.

2.5.3 Fourier-Transform Infrared Spectroscopy (FT-IR) FTIR spectra of the ionic polyimide and ionic polyimide + IL composite were obtained using a Perkin-Elmer Spectrum 2 with an attenuated total reflectance (ATR) accessory.



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2.5.4 Differential Scanning Calorimetry (DSC) Thermal properties of the neat ionic polyimide and the ionic polyimide with [C4mim][Tf2N] were examined using DSC performed with a TA Instruments DSC Q2000 differential scanning calorimeter equipped with a liquid nitrogen cooling system (LNCS). Tzero™ hermetic aluminum pans containing between 10 and 15 mg sample were equilibrated with empty pans at 25°C and heated at a rate of 2°C/min to 240°C for both the ionic polyimide and ionic polyimide + IL composite samples. Following the heating segment, samples were cooled at a rate of 5° C/min and equilibrated at 25°C. An N2 purge was maintained at a flowrate of 20 mL/min for the duration of the tests. Thermal transitions of the polymer samples were determined using the TA Universal Analysis software with endothermic and exothermic peak positions reported as the onset temperatures of the transitions.

2.5.5 X-ray Diffractometry (XRD) Wide-angle powder X-ray diffractometry (WAXD) profiles (2Θ = 8.8 – 104.2) of the ionic polyimide and ionic polyimide + IL composite films were obtained using a Bruker D8 Discover instrument equipped with a general area detector diffraction system (GADDS). The radiation source was Co-Kα.

2.5.6 Scanning Electron Microscopy (SEM) Cross-sections of ionic polyimide and ionic polyimide + IL composite membranes were produced via freeze-fracturing in liquid N2 and analyzed using a TESCAN LYRA focused ion beam field emission scanning electron microscope (FIB-FESEM). SEM helped to determine membrane thicknesses, which with the cross sectional area (17.35 cm2) and membrane mass used to estimate the density of the ionic polyimide and the ionic polyimide + IL composites as 1.66 and 1.75 g/cm3, respectively.


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3. Results & Discussion 3.1 Membrane Performance Table 1a summarizes the results of gas permeation tests in both the neat ionic polyimide and the ionic polyimide + IL composite membranes. Each value is presented as the average obtained from at least three replicate experiments and uncertainties (+/-) are presented as one standard deviation Table 1b presents the relative change in the value of each property in the ionic polyimide + IL composite relative to the neat ionic polyimide.

Table 1a: P, D, and S values of gases neat ionic polyimide and ionic polyimide + IL composite membranes. Gas

P (barrer)

H2 N2 CH4 CO2

1.62 0.027 0.028 0.871

H2 N2 CH4 CO2

5.18 0.517 0.793 20.4

D (cm2/s) +/S (cm3gas/cm3membrane/atm) Ionic Polyimide 0.01 2.56E-07 1.00E-07 0.052 0.001 8.54E-09 6.29E-09 0.033 0.004 1.38E-09 2.4E-10 0.152 0.003 3.08E-09 4E-11 2.15 Ionic Polyimide with [C4mim][Tf2N] 0.04 6.74E-07 1.56E-07 0.061 0.023 1.33E-07 5.2E-08 0.025 0.023 4.05E-08 5.3E-09 0.154 0.1 7.56E-08 3.4E-09 2.01 +/-

+/0.020 0.020 0.005 0.03 0.016 0.004 0.026 0.19

Table 1b: Percent change in P, S and D for ionic polyimide + IL composite (1) relative to neat ionic polyimide (0) membrane. Gas H2 N2 CH4 CO2

100%*(P1-P0)/P0 220% 1815% 2732% 2242%

100%*(D1-D0)/D0 163% 1457% 2835% 2355%

100%*(S1-S0)/S0 17% -24% 1% -7%

Table 1a reveals that the neat ionic polyimide membrane displays very low permeabilities for all gases tested, and only H2 exhibited permeability > 1 barrer. The low permeability is clearly attributable to slow diffusion of the gases through the material as diffusivities of CO2, N2 and CH4 are ~10-9 cm2/s and two orders of magnitude less than that of H2. Compared with conventional and advanced polyimides,



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the diffusion coefficients for CO2, N2 and CH4 in the neat ionic polyimide are very low. They are even lower than values reported by Carlisle, et al. for an imidazolium-based ionene with n-decyl linkers with Tf2N- anions.93 This behavior suggests that the neat ionic polyimide backbone is very rigid, and that strong interchain interactions create a very tight polymer matrix with pores through which only the smallest gas, H2, can move with (relative) ease. The solubility of these gases, however, largely align with expectations for imidazolium-based poly(IL) materials and IL solvents.11, 12, 105, 117 Significant changes occur in the behaviors of CO2, N2 and CH4 within the ionic polyimide + IL composite membrane. Tables 1a and 1b show that increased diffusivity is responsible for increased permeability, as the solubility of each gas was essentially the same in both membrane types when accounting for the uncertainty of the measurement. This increase in diffusivity created increases of ~2200%, ~1800% and ~2700% in the respective permeabilities of CO2, N2 and CH4. A much more modest increase (220%) was observed in the permeability of H2. Thus, although the diffusivity of H2 increases, the presence of IL within the ionic polyimide did not have the same influence on H2 as the other gases. As will be shown in Section 3.2, the nanostructure of the ionic polyimide + IL composite is more ordered than that of the neat polyimide. Thus, it may be that this ordering creates polymer chain or IL domain alignment and/or reduces tortuosity in the ionic polyimide + IL composite through which CO2, N2 and CH4 diffuse more rapidly. However, the cavities may already be sufficiently large such that H2 does not experience a much different environment diffusion pathway than it does in the neat ionic polyimide. Table 2 presents selectivity data for the six gas pairs for the ionic polyimide and ionic polyimide + IL composite membranes as the ratio of ideal permeabilities with the individual diffusivity and solubility contributions.


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Table 2: Ideal selectivities in terms of permeability (Pi/Pj), diffusivity (Di/Dj) and solubility (Si/Sj) and diffusivity of selected gas pairs tested in membranes of neat ionic polyimide and ionic polyimide soaked in [C4mim][Tf2N]. Ionic Polyimide Gas Pair CO2/H2 CO2/N2 CO2/CH4 CH4/N2 H2/CH4 H2/N2 Gas Pair CO2/H2 CO2/N2 CO2/CH4 CH4/N2 H2/CH4 H2/N2

Pi/Pj Di/Dj 0.538 0.012 32.3 0.361 31.1 2.23 1.04 0.162 57.9 186 60.0 30.0 Ionic Polyimide with [C4mim][Tf2N] Pi/Pj Di/Dj 3.94 0.112 39.5 0.568 25.7 1.87 1.53 0.305 6.53 16.6 10.0 5.07

Si/Sj 41.3 65.2 14.1 4.61 0.342 1.58 Si/Sj 33.0 80.4 13.1 6.16 0.396 2.44

As was already shown in Table 1a, with the IL present, CO2 not H2, was the most permeable gas. This is reflected in Table 2 where for the CO2/H2 gas pair, the neat ionic polyimide is H2-permselective while the ionic polyimide + IL composite is CO2-permselective. This is due to the disproportionate increase in the diffusivity of CO2 relative to that of H2 with the IL present. Likewise, this effect also causes the permselectivity of H2 relative to CH4 and N2 to decrease from ~60 for both H2/CH4 and H2/N2 to 6.53 and 10.0, respectively. For both materials and all relevant gas pairs tested, CO2 solubility is always favored, while H2 diffusivity is always favored. CO2/N2 permselectivity increased ~25% in the ionic polyimide + IL composite, while CO2/CH4 permselectivity decreased ~20%. Robeson Plots52 annotated with data for ionic polyimide and ionic polyimide + IL composite membranes for CO2-based separations are provided in the Supporting Information. All of the tested gas pairs in both the ionic polyimide and ionic polyimide + IL composite fall below Robeson’s upper bounds for these separations. For CO2/N2 and CO2/CH4, the ionic polyimide and ionic polyimide + IL composite membrane performances are found in within the large clusters of membranes typically observed on



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Robeson Plots. For other gas pairs including CO2/H2, the performance of the ionic polyimide and ionic polyimide + IL composite tends to be somewhat outside of these clusters, similar to the Carlisle, et. al imidazolium-based ionene membranes.93

3.2 Materials Characterization

Both the ionic polyimide and ionic polyimide + IL composite were characterized using several techniques in order to examine how the absorbed IL might interact with the ionic polyimide. DSC revealed a distinct melting transition for the ionic polyimide at ~190oC, which was consistent with our expectations based on ionic polyimide melt processing (Section 2.3). A similarly sharp melting transition was not observed for the ionic polyimide + IL composite via DSC. Images of the DSC traces are provided in the Supporting Information. Figure 12 presents an overlay of the FTIR spectra for the ionic polyimide and ionic polyimide + IL composite.

Figure 12: FTIR spectra of neat ionic polyimide (blue) and ionic polyimide + IL composite (red).


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A comparison of the FTIR spectra for the neat ionic polyimide and the ionic polyimide + IL composite shows that the presence of the IL only enhances signals associated with the ionic moieties (i.e., imidazolium cations and Tf2N- anions). No changes in absorbance are observed for the characteristic carbonyl (C=O) stretch at ~1720 cm-1 or the imide band between 740-730 cm-1. Instead, the absorption only increases for signals that are shared between the ionic polyimide and the [C4mim][Tf2N] IL. For the cation, these include the prominent imidazolium ring system peaks between 1180-1050 cm-1 and at ~1360 cm-1. With respect to the anion, the relevant signals include the 790 cm-1 and ~570 cm-1 bands corresponding to the respective C-S/N-S and -CF3 deformations in the Tf2N anion.118-120 A larger image of the full FTIR spectra (4000 – 450 cm-1) is provided in the Supporting Information. Figure 13 presents the WAXD patterns for both the ionic polyimide and ionic polyimide + IL composite.

Figure 13: WAXD patterns of films of ionic polyimide before (lower, blue) and after (upper, red) absorption of [C4mim][Tf2N]. Although offset, both data sets have been scaled to the same relative intensity.



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Figure 13 clearly illustrates that while the neat ionic polyimide (blue) shows only a broad aggregation between 2Θ = 18-25o, the ionic polyimide + IL composite (red) has a distinct ordering at 2Θ = 20o (0.515 nm). A sharp peak in this region is typically associated with polyamides/polyaramids (e.g., Nylon® and Kevlar®) that feature strong H-bonding.121, 122 Thus, it may be that the absorption of IL into the ionic polyimide also facilitates organization of the ionic polyimide through creation of intermolecular ionic interactions and/or H-bonds between imidazolium cations and “free” [Tf2N]- anions.123-127 As the ionic polyimide was synthesized using PMDA, the dianhydride used to produce Kapton, the broad signal in the neat ionic polyimide x-ray diffraction pattern is likely associated with the Kaptonlike “subunit” of the ionic polyimide based on comparison to a published x-ray diffraction pattern of Kapton.128 For the ionic polyimide + IL composite, the broad signal also appears, with another relatively broad signal occurring at 2Θ = 12-14o, perhaps indicating a secondary mode of aggregation with the IL present. Figure 14 presents photographs of the cross-sections of the ionic polyimide and ionic polyimide + IL composite taken during SEM analysis.


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Figure 14: SEM Images of membrane cross-sections for ionic polyimide (left) and ionic polyimide with [C4mim][Tf2N].

The cross-section of the neat ionic polyimide is a dense, largely homogeneous structure, while that of the ionic polyimide + IL composite shows morphological changes throughout the bulk. The SEM image of the composite clearly shows an exfoliation, which may be a visual indication that the IL has rearranged the ionic polyimide structure when taken in tandem with the XRD patterns (Figure 13). The absorption of IL into the ionic polyimide also results in an increase in thickness.

4. Conclusions The concept of integrating polyimides and ILs represents a new direction in the design of polymer materials for use as gas separation membranes. While there have been a few prior reports exploring combinations of polyimides with ILs, we have introduced a versatile synthetic method to produce hybrid polyimide-ionenes that we now believe can be applied to many types of related precursors to make more sophisticated ionic polyimide materials. Much like ILs, the possible combinations of precursors that can be used means that there are likely 10,000’s of unique ionic polyimide structures possible. Further work on ionic polyimides is clearly warranted, and we believe



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that the next logical steps are to extend our studies to other dianhydrides (e.g., 6-FDA) and the effects of regiochemistry (i.e., use of m-dichloroxylene in the polymerization). This work also demonstrates an important concept: ionic polyimides spontaneously absorb ILs into their structures at ambient temperature, likely undergoing an ordering/self-assembly process as a result. This behavior presents another new opportunity by which to tailor/control polymer properties and nanostructure. The integration of cations within the backbones of condensation polymers is an underexplored area and we foresee its extension to not only polyimides, but also PIMs and TR polymers. Using ILs to structure high-performance polymers for gas separation membranes may present a key to developing ordered pore structures as well as preventing collapse of free volume and slowing/stopping aging.

Supporting Information 1

H NMR spectra images for diimide monomer, ionic polyimide and [C4mim][Tf2N]. Photographs of the

batch reactor and polymer preparation. Experimental procedure for vapor pressure measurements. Robeson Plots annotated with data for ionic polyimide and ionic polyimide + IL composite membranes. DSC traces for ionic polyimide and ionic polyimide + IL composite. MALDI-TOF MS report for ionic polyimide. Larger version of the FTIR spectra for the ionic polyimide and ionic polyimide + IL composite. This material is available free of charge from the authors or at http://pubs.acs.org

Acknowledgment This invited contribution is part of the I&EC Research special issue for the 2017 Class of Influential Researchers.


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JEB acknowledges partial support from the National Science Foundation (NSF) (CBET-1605411). JEB, SKS and DTD acknowledge support from NSF I-Corps (IIP-1450923). JEB and DAW acknowledge partial support from NSF in the form of an REU supplement to CBET-1139597. This work utilized resources owned and maintained by the Central Analytical Facility, which is supported by The University of Alabama. The authors thank Prof. Paul A. Rupar and Mr. Ian A. Brettell-Adams from the University of Alabama Dept. of Chemistry for their assistance in attempting to measure polymer molecular weight using GPC. The authors thank Dr. Kenneth A. Belmore from the University of Alabama Dept. of Chemistry for obtaining 1H NMR spectra. The authors thank Dr. Qiaoli Liang from the University of Alabama Dept. of Chemistry for running MALDI-TOF MS experiments. The authors thank Prof. Anwar Haque and Mr. Md. Easier Arafat Papon from the Department of Aerospace Engineering at the University of Alabama for access to, and assistance with, the benchtop extruder. JEB thanks Lakshmi Krishnamurthy, Kenneth Hancock, Gilberto Lunardi, John Legare and Mark B. Shiflett from DuPont for the generous donation of Kalrez® o-rings used to seal the chemical reactors.



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Ionic Polyimides: Hybrid Polymer Architectures and ... - ACS Publications

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