Article pubs.acs.org/Macromolecules
Mechanically Tough, Thermally Rearranged (TR) Random/Block Poly(benzoxazole-co-imide) Gas Separation Membranes Yongbing Zhuang,†,§,∇ Jong Geun Seong,†,∇ Won Hee Lee,† Yu Seong Do,† Moon Joo Lee,‡ Gang Wang,†,∥ Michael D. Guiver,*,⊥,#,† and Young Moo Lee*,† †
Department of Energy Engineering, College of Engineering, and ‡School of Chemical Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea § College of Chemistry and Chemical Engineering, Hunan University of Arts and Science, Changde, Hunan 415000, P.R. China ∥ College of Chemistry and Chemical Engineering, Henan University of Technology, Zhengzhou 450001, P. R. China ⊥ State Key Laboratory of Engines, School of Mechanical Engineering, Tianjin University, Tianjin 300072, P. R. China # Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China S Supporting Information *
ABSTRACT: Insufficient mechanical properties are one of the major obstacles for the commercialization of ultrahigh permeability thermally rearranged (TR) membranes in largescale gas separation applications. The incorporation of preformed benzoxazole/benzimidazole units into o-hydroxy copolyimide precursors, which themselves subsequently thermally rearrange to form additional benzoxazole units, were prepared for the first time. Using commercially available monomers, mechanically tough membranes prepared from random and block TR poly(benzoxazole-co-imide) copolymers (TR-PBOI) were investigated for gas separation. The effects of the chemical structures, copolymerization modes, and thermal holding time of o-hydroxy copolyimides on the molecular packing and properties, including gas transport, for the resulting TR-PBOI membranes have been examined in detail. After treatment at 400 °C, tough TR-PBOI membranes exhibited tensile strengths of 71.4−113.9 MPa and elongation at break of 5.1−16.1%. Moreover, they presented higher or comparable gas transport performance as compared to those tough/robust TR membranes reported previously. Reported for the first time is a comparative investigation of the copolymerization mode (random or block) on membrane properties. The novel polymer architecture and systematic property studies promote a better understanding of the materials and process development of commercial TR membranes for gas separation applications.
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INTRODUCTION Membrane technologies have been widely used in gas separation, water treatment, polymer electrolytes, etc.,1−6 due to their overall advantages in terms of energy consumption, small footprint, and high operational flexibility for use on a large scale. In particular, for gas separation applications, tough new membrane materials with a combination of high permeability, selectivity, and excellent tolerance to adverse/ harsh environments are needed to meet the requirements for more effective gas separation processes. Polymeric membranes are now routinely used industrially for hydrogen recovery, natural gas purification (CO2/CH4), and on-site nitrogen production from air (O2/N2) and are being explored for other process separations such as carbon dioxide capture (CO2/N2). Typical membrane materials investigated or adopted for gas separation applications include cellulose acetate, polyamides, polysiloxanes, polysulfone, polyimides, polybenzimidazoles (PBIs), polybenzoxazoles (PBOs), polybenzthiozoles (PBZs), and poly(trimethylsilyl-1-propyne) (PTMSP).7,8Among the obstacles inhibiting the adoption of some polymers for use in commercial membranes is the simultaneous realization of high © XXXX American Chemical Society
permeability and high selectivity, as described by the Robeson upper bound.9 Over the past decade, high free volume membranes derived from microporous polymers, such as polymers of intrinsic microporosity (PIMs)4,10−22 (e.g., PIM111) having rigid ladder-type moieties and thermally rearranged (TR) polymers2,23−36 from o-hydroxy polyimide precursors, have been intensively investigated for gas separation. This research has been undertaken due to the overall performance advantages of these materials, especially in terms of their ultrahigh permeability combined with moderate to high selectivity for the separation of certain gas pairs (e.g., CO2/ CH4). Typical thermally rearranged (TR) membranes have been fabricated by the high-temperature conversion of o-hydroxy polyimide precursors into polybenzoxazoles (PBOs).2,28,30−34 TR membranes exhibit extraordinary gas permeabilities with similar permselectivities compared with o-hydroxy polyimide Received: May 1, 2015 Revised: July 17, 2015
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DOI: 10.1021/acs.macromol.5b00930 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Scheme 1. Procedures for the Syntheses of Random o-Hydroxy Copolyimide Precursors and the Corresponding Thermally Rearranged Random TR-PBOIs
our laboratory to overcome the mechanical toughness disadvantages of conventional TR-PBO membranes.25,32,35,38 Several strategies can be utilized to develop acceptably tough TR-PBO membranes. One attempted approach employs polymer structural modification. The conversion temperature (TTR) for thermal rearrangement is mainly influenced by the chain rigidity of the o-hydroxy polyimide precursors and exhibits a linear relationship with the glass transition temperatures (Tgs) of the precursor polyimides.26 Lower Tgs of the ohydroxy polyimide precursor allow the rearrangement and decarboxylation to occur at a faster rate and at a lower temperature; this has the potential to reduce the probability of chain decomposition during the rearrangement process, which would improve the mechanical toughness. Full conversion of the poly(ether imide) precursor into TR-PBO can be performed under relatively low temperatures (e.g., 400 °C), when ether linkages are present in the backbone, due to improved chain flexibility.32 Incorporating ether linkages into the TR-PBO backbone results in higher elongation at break (up to 12%32) compared with conventional TR-PBO (around 3− 4%2) that has been treated under the same conditions. Also, it is noteworthy that the mechanical toughness can be
precursors and other polyimide membranes. This is mainly due to the formation of microporosity and the pore size distributions with in these membranes.2 The high temperatures used to drive the thermal rearrangement (usually above 400 °C) result in an additional advantage of rendering the resulting membranes insoluble through cross-linked networks. While this improves the membrane plasticization and physical antiaging properties, the mechanical properties deteriorate compared to the o-hydroxy polyimide (HPI) precursor membranes, with the possibility of chain decomposition occurring at high temperatures. When applied in industrial gas separation plants, useful membrane materials must be capable of facing high operating pressures and also of being formed into thin defect-free membranes (0.1−1.0 μm thick) and packaged into large area membrane modules, e.g., hollow-fiber modules.37 Therefore, the mechanical property of a membrane material is a very important parameter to assess the stability and durability of a membrane material for industrial gas separation. Insufficient mechanical properties are one of the major obstacles for the commercialization of the ultrahigh permeability TR membrane materials. Recently, some significant progress has been made in B
DOI: 10.1021/acs.macromol.5b00930 Macromolecules XXXX, XXX, XXX−XXX
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Scheme 2. Procedures for the Syntheses of Block ortho-Hydroxy Copolyimide Precursors and the Corresponding Thermally Rearranged TR-PBOIs: (a) Anhydride End-Capped o-Hydroxy Imide Oligomer Solution A from 6FDA and APAF (Ah-OH); (b) Amine End-Capped Imide Oligomer Solution B from 6FDA with BOA (An-BOA) or BIA (An-BIA); (c) Block o-Hydroxy Copolyimide Precursors Prepared by Mixing Solution A and Solution B and the Corresponding Thermally Rearranged Block TR-PBOIs
C
DOI: 10.1021/acs.macromol.5b00930 Macromolecules XXXX, XXX, XXX−XXX
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unit) was selected to incorporate polyimide portions into the backbone to improve the toughness and gas selectivity of the resulting TR-PBOI membranes. For these syntheses, the mole fraction of o-hydroxy diamine (APAF) was equal to that of the non-o-hydroxy diamine (BOA or BIA). The TR-PBOI (5:5) membranes can be easily obtained upon thermal rearrangement of their corresponding copolyimide precursors (Scheme 1). Furthermore, in order to investigate the influence of the copolymerization modes on the properties (including gas transport) for the TR-PBOI (5:5) membranes, one anhydrideend-capped o-hydroxy imide oligomer (Ah-OH) and two amine-end-capped imide oligomers, derived from BOA (AnBOA) and BIA (An-BIA), respectively, were synthesized for the preparation of the corresponding block copolyimide precursors (shown in Scheme 2a−c) and the block TR-PBOI membranes. The goal of this study was to improve the toughness of TR membrane materials to develop the improved membrane materials for industrial gas separation applications. If improved TR membrane with high toughness could be developed, they would also be used more widely in many other fields (e.g., fuel cell48 and energy materials49). Meanwhile, we also wished to investigate the influence of incorporating non-TR BOA or BIA units as well as the influence of the copolymerization modes (random/block) on the gas transport properties of TR-PBOI membranes. Systematic investigation on the relationship between the chemical structures of the o-hydroxy copolyimide precursors and the gas transport performance of the resulting TR-PBOI membranes would guide optimization and modification for development of new membrane materials with better comprehensive performance for current and future membrane applications.
significantly improved in the TR membranes (even after 100% conversion) by introducing spirobisindane moieties into the backbone (with the highest elongation at break reaching 20%).35 However, the monomer used for this synthesis (spirobisindane-containing bis(amino)phenol) is not commercially available due to the complexity of the syntheses routes and the low yields.35 Another possible solution to overcome the deteriorated mechanical properties for these TR-PBO membranes is the use of a copolymer poly(benzoxazole-co-imide) (TR-PBOI), prepared from a commercial dianhydride and a commercial o-hydroxy diamine with a commercial non-ohydroxy diamine.25 For this strategy, the TR-PBO and non-TR polyimide portions in the backbone are simply constructed from the dianhydride units with o-hydroxy diamine moieties and non-o-hydroxy diamine moieties, respectively, under high temperatures. For the resulting TR-PBOI membranes, the TRPBO portion imparts excellent permeability properties, and the PI portion simultaneously enhances the selectivity properties and improves the membrane toughness. It is well-known that increasing the polymer backbone rigidity improves the gas separation performance.3 Both benzoxazole and benzimidazole moieties are considered to be highly rigid-rod-like structures. It has been reported that polyimides that incorporate benzoxazole moieties exhibit excellent membrane toughness.39,40 The incorporation of benzimidazole moieties into the polyimide chain backbone can effectively improve the oxidative stability, which should decrease the possibility of chain decomposition during the thermal rearrangement process and result in an improved mechanical toughness. Accordingly, the incorporation of benzoxazole or benzimidazole moieties into the main chains is expected to be an effective strategy for improving toughness for TR-PBOI membranes. However, no related research on this approach has been reported. The influences of polyimide precursor synthesis routes,41 membrane thickness,42,43 blend,44 purge environment,45 thermal treatment,46 copolymerization,30,33 and chain chemical structures (e.g., monomer isomerism47 and ortho-position functional group41) on the gas separation performance of TR-PBO membranes have been investigated in detail. In particular, for TR-PBOI, we have clarified the effects of nonTR-able diamine chemical structures on the transport performance.25 However, there has yet to be an investigation related to the effects of the copolymerization modes (such as random or block copolymerization) of the o-hydroxy copolyimide precursors on the gas transport properties of the resulting TR-PBOI membranes. Taking into consideration the aforementioned factors, we hypothesize that incorporating benzoxazole or benzimidazole moieties into the backbone of TR-PBOI will likely be beneficial for improving the toughness of membranes. In this study, random copolymer precursors were synthesized from a dianhydride, 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with an ohydroxy diamine, 2,2′-bis(3-amine-4-hydroxyphenyl)hexafluoropropane (APAF), and a non-o-hydroxy diamine (either 5-amino-2-(4-aminobenzene)benzoxazole (BOA) or 5-amino2-(4-aminobenzene)benzimidazole (BIA)), as shown in Scheme 1. All of these monomers are commercial products. APAF containing the bulky 6F group (−C(CF3)2) was selected as the o-hydroxy diamine to incorporate a TR-PBO unit into the backbone in order to impart excellent separation performance, while either BOA diamine (containing a preformed benzoxazole unit) or BIA diamine (containing a benzimidazole
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EXPERIMENTAL SECTION
Materials. 2,2′-Bis(3-amine-4-hydroxyphenyl)hexafluoropropane (APAF) was purchased from Central Glass Co. Ltd. (Tokyo, Japan) and purified by sublimation before use. 5-Amino-2-(4-aminobenzene)benzoxazole (BOA) and 5-amino-2-(4-aminobenzene)benzimidazole (BIA) were obtained from Changzhou Sunlight Medical Raw Material Co. Ltd. (Jiangsu, China) and used as received. 4,4′-(Hexafluoroisopropylidene)diphthalic anhydride (6FDA) was obtained from Sigma-Aldrich Chemical Co. (Milwaukee, WI) and purified by sublimation before use. Toluene (99.8%) and N-methyl-2-pyrrolidinone (NMP, >99.0%) were purchased from Sigma-Aldrich Chemical Co. (Milwaukee, WI). Synthesis of Random o-Hydroxy Copolyimide Precursor. The random o-hydroxy copolyimide precursors were prepared, using 6FDA with APAF and BOA/BIA as monomers, via a two-step azeotropic imidization process, as shown in Scheme 1. The molar amount of the o-hydroxy diamine (APAF) was equal to that of non-ohydroxy diamine (BOA or BIA). Using the preparation of R-OH-PIa as an example, in a four-necked flask, APAF (1.8313 g, 5 mmol) and BOA (1.1263 g, 5 mmol) were dissolved in NMP (42 mL). 6FDA (4.4424 g, 10 mmol) was then added. After stirring at room temperature for more than 12 h, 14 mL of toluene was added to the reaction solution as an azeotropic agent to remove water. The solution mixture was refluxed with the removal of water with a Dean− Stark trap under a dry N2 atmosphere for at least 12 h. The resulting solution was poured into a mixture of water and methanol (1 L, v/v = 1:1) under vigorous stirring. The resulting fibrous precipitate was filtered off, washed with cold water and methanol, and dried at 120 °C in a vacuum oven to produce a white fibrous powder (7.00 g, yield 99.4%). Random o-Hydroxy Copolyimide Precursor from BOA (R-OHPIa). 1H NMR (300 MHz, chloroform-d6): 10.42 (s, −OH), 8.39 (s, Har), 8.23−8.20 (d, Har), 8.15−8.12 (d, Har), 7.99−7.94 (m, Har), 7.79−7.71 (m, Har), 7.54 (s, Har), 7.21 (s, Har), 7.07 (d, Har). ATRD
DOI: 10.1021/acs.macromol.5b00930 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules FTIR (membrane, ν, cm−1): 1786 (imide CO symmetric stretching), 1718 (imide CO asymmetric stretching), 1619 (cyclic CC stretching), 1518, 1504, 1480 (skeletal vibrations in conjugated phenyl/benzoxazole ring), 1370 (imide −C−N), 1058 (Ar−C−O symmetric stretching of benzoxazole ring). Molecular weight: Mw = 64 × 103 with a polydispersity of 2.5. Elemental analysis calculated for C33H13.5F9N2.5O5.5: C, 56.30; H, 1.93; N, 4.97. Found: C, 54.78; H, 2.00; N, 5.10. Random o-Hydroxy Copolyimide Precursor from BIA (R-OH-PIb). 1 H NMR (300 MHz, chloroform-d6): 13.26 (s, −NH−), 10.42 (s, −OH), 8.34 (s, Har), 8.23−8.20 (d, Har), 8.16−8.12 (d, Har), 7.97− 7.94 (m, Har), 7.79−7.66 (t, Har), 7.21 (s, Har), 7.07 (s,Har). ATRFTIR (membrane, ν, cm−1): 1785 (imide CO symmetric stretching), 1716 (imide CO asymmetric stretching), 1619 (cyclic CN/CC stretching), 1519, 1492 (skeletal vibrations in conjugated phenyl/benzoxazole ring), 1372 (imide −C−N). Molecular weight: Mw = 143 × 103 with a polydispersity of 4.4. Elemental analysis calculated for C33H14F9N3O5: C, 56.34; H, 2.01; N, 5.97. Found: C, 53.69; H, 2.05; N, 5.72. Synthesis of Block o-Hydroxy Copolyimide Precursor. Two diamines (one o-hydroxy diamine (APAF) and one non-o-hydroxy diamine (BOA or BIA)) were separately reacted with the dianhydride 6FDA to form anhydride-end-capped and amine-end-capped imide oligomer solutions (Schemes 2a and 2b), respectively. The two solutions were combined into one solution to prepare the block ohydroxy copolyimide precursors, as shown in Scheme 2c. Anhydride-End-Capped o-Hydroxyimide Oligomer Solution A (Ah-OH). 1.8313 g (5.0 mmol) of APAF was dissolved in 70.8 mL of NMP with stirring for about 30 min. 2.4433 g (5.5 mmol) of 6FDA was then added into the solution. After stirring at room temperature for more than 12 h, 23.6 mL of toluene was added to the reaction solution as an azeotropic agent. The solution mixture was refluxed with the removal of water through a Dean−Stark trap under a dry N2 atmosphere for at least 12 h. The reaction solution was cooled to room temperature to form the anhydride-end-capped hydroxy imide oligomer (solution A). Amine-End-Capped Imide Oligomer Solution B (An-BOA or AnBIA). Using the synthesis of An-BOA as an example, 1.1263 g (5.0 mmol) of BOA was dissolved in 51.6 mL of NMP with stirring for about 30 min. 1.9991 g (4.5 mmol) of 6FDA was then added into the solution. After stirring at room temperature for more than 12 h, 17.2 mL of toluene was added to the reaction solution as an azeotropic agent. The solution mixture was refluxed by removing water through a Dean−Stark trap under a dry N2 atmosphere for at least 12 h. The reaction solution was cooled to room temperature to form the amineend-capped imide oligomer An-BOA (solution B). The synthetic method of An-BIA (solution B) was similar to that used for the AnBOA except that BIA was used as the diamine. Then, solution A (Ah-OH) and solution B (An-BOA or An-BIA) were combined into one solution to prepare the block o-hydroxy copolyimide precursors (B-OH-PIa and B-OH-PIb). For example, for the B-OH-PIa, the mixed solution was stirred for another 3 h under a N2 atmosphere. 40.8 mL of toluene was then added to the reaction solution as an azeotropic agent. The solution mixture was heated under reflux for at least 12 h under a N2 atmosphere. While the toluene was refluxing, water was removed using a Dean−Stark trap. After cooling, the solution was poured into a mixture of water and methanol (1 L, v/v = 1:1) under vigorous stirring. The resulting fibrous precipitate was filtered off, washed with cold water and methanol, and dried at 120 °C in a vacuum oven to produce white fibrous powder (6.84 g, yield 97.2%). Ah-OH. 1H NMR (300 MHz, DMSO-d6): 10.42 (s, −OH), 8.14− 8.12 (d, Har), 7.94 (d, Har), 7.72 (s, Har), 7.50 (s, Har), 7.20 (s, Har), 7.08−7.06 (d, Har). FTIR (powder, ν, cm−1): 3432 (−OH stretching, broad), 1859 (CO asymmetric stretching of anhydride ring), 1788 (imide CO symmetric stretching), 1724 (imide CO asymmetric stretching), 1617 (cyclic CC stretching), 1520 (skeletal vibrations in conjugated phenyl ring), 1381 (imide −C−N). Molecular weight: Mn = 7.4 × 103, Mw = 12.0 × 103 with a polydispersity of 1.6.
An-BOA. 1H NMR (300 MHz, DMSO-d6): 8.41−8.39 (d, Har), 8.24 (s, Har), 8.01 (t, Har), 7.95 (s, Har), 7.89−7.87 (d, Har), 7.78−7.74 (m, Har), 7.56−7.54 (d, Har), 7.40−7.37 (d, Har), 6.71−6.69 (d, Har), 6.07 (s, -NH2). FTIR (powder, ν, cm−1): 3491, 3380, 3232 (N−H stretching), 1786 (imide carbonyl symmetric stretching), 1726 (imide carbonyl asymmetric stretching), 1372 (imide −C−N), 1058 (Ar−C− O symmetric stretching of benzoxazole ring). Molecular weight: Mn = 6.6 × 103, Mw = 15.7 × 103 with a polydispersity of 2.4. An-BIA. 1H NMR (300 MHz, DMSO-d6): 13.29 (s, −N−H in nonterminal benzimidazole ring), 12.67 (s, −N−H from terminal benzimidazole ring), 8.34 (s, Har), 8.23 (t, Har), 8.00 (s, Har), 7.80− 7.78 (m, Har), 7.69−7.66 (m, Har), 7.33−7.27 (q, Har), 6.68−6.66 (d, Har), 5.67 (s, −NH2). FTIR (powder, ν, cm−1): 3488, 3381, 3226 (N− H stretching), 1784 (imide carbonyl symmetric stretching), 1722 (imide carbonyl asymmetric stretching), 1371 (imide −C−N). Molecular weight: Mn = 17.1 × 103, Mw = 41.4 × 103 with a polydispersity of 2.4. Block o-Hydroxy Copolyimide Precursor from BOA (B-OH-PIa). 1 H NMR (300 MHz, chloroform-d6): 10.42 (s, −OH), 8.39 (s, Har), 8.24−8.22 (d, Har), 8.14−8.11 (d, Har), 7.99−7.94 (m, Har), 7.79−7.71 (m, Har), 7.54 (s, Har), 7.21 (s, Har), 7.07 (d, Har). ATR-FTIR (membrane, ν, cm−1): 1786 (imide CO symmetric stretching), 1720 (imide CO asymmetric stretching), 1619 (cyclic CC stretching), 1518, 1504, 1480 (skeletal vibrations in conjugated phenyl/ benzoxazole ring), 1370 (imide −C−N), 1058 (Ar−C−O symmetric stretching of benzoxazole ring). Molecular weight: Mn = 36.0 × 103, Mw = 125.6 × 103 with a polydispersity of 3.5. Elemental analysis calculated for C33H13.5F9N2.5O5.5: C, 56.30; H, 1.93; N, 4.97. Found: C, 56.04; H, 2.13; N, 4.93. Block o-Hydroxy Copolyimide Precursor from BIA (B-OH-PIb). 1H NMR (300 MHz, chloroform-d6): 13.27 (s, −N−H from nonterminal benzimidazole ring), 10.42 (s, −OH), 8.34 (s, Har), 8.24−8.22 (d, Har), 8.14−8.12 (d, Har), 7.99−7.93 (m, Har), 7.79−7.66 (m, Har), 7.19 (s, Har), 7.06 (s, Har). ATR-FTIR (membrane, ν, cm−1): 1785 (imide CO symmetric stretching), 1716 (imide CO asymmetric stretching), 1619 (cyclic CN/CC stretching), 1519, 1492 (skeletal vibrations in conjugated phenyl/benzoxazole ring), 1372 (imide −C−N). Molecular weight: Mn = 39.2 × 103, Mw = 118.9 × 103 with a polydispersity of 3.0. Elemental analysis calculated for C33H14F9N3O5: C, 56.34; H, 2.01; N, 5.97. Found: C, 54.99; H, 2.63; N, 5.95. Membrane Formation. The o-hydroxy copolyimide precursor fibrous powders were dissolved in NMP to form ∼20 wt % polymer solutions and were cast onto clean glass plates after filtering with 1.0 μm nylon (NY) filter cartridges. The glass plates were placed in a vacuum oven and slowly heated from 60 to 250 °C to evaporate the solvent by successive heating at 60, 100, 150, 200, and 250 °C for 1 h at each temperature. This process is the same as that reported previously.25,40 The resulting membranes were removed from the glass plates by immersion in hot water, washed with deionized water, and dried overnight in a vacuum oven at 120 °C. The dry membranes were further heated up to 300 °C in a muffle furnace (Lenton, London, UK) at a rate of 5 °C/min with a Eurotherm controller and were maintained at 300 °C for 1 h to obtain the fully imidized o-hydroxy copolyimide precursor membranes (random: R-OH-PIa/PIb; block: B-OH-PIa/PIb). The thickness of all of the films after the heating treatment was approximately 40−70 μm. For the thermal rearrangement step, each sample was heated further to 400 °C and maintained for 1 or 2 h in a high-purity argon atmosphere. After the furnace was slowly cooled to room temperature, the TR-PBOI membranes were obtained. To simplify the description, the corresponding thermally rearranged membranes from o-hydroxy copolyimide precursor, OHPIa/PIb, maintained for 1 and 2 h at 400 °C in this study are denoted as TR-PBOIa/PBOIb-1h and TR-PBOIa/PBOIb-2h, respectively. Characterization. 1H NMR was measured by a Mercury Plus 300 MHz spectrometer (Varian, Inc., Palo Alto, CA) using dimethyl-d6 sulfoxide (DMSO-d6) or chloroform-d6 as the solvent, depending on the chemical structure. ATR-FTIR spectra of the membranes or FTIR spectra of the polymer powders were obtained by an infrared microspectrometer (IlluminatIR, SensIR Technologies, Danbury, CT). E
DOI: 10.1021/acs.macromol.5b00930 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 1. Molecular Weights and Mechanical Properties of the o-Hydroxy Copolyimide Precursors
a
polymer
Mwb (×10−3)
PDI (Mw/Mn)
Tga (°C)
R-OH-PIa R-OH-PIb B-OH-PIa B-OH-PIb
64.5 143.3 125.6 118.9
2.5 4.4 3.5 3.0
328 357 331 362
tensile strength (MPa) 121.4 131.7 113.6 131.1
± ± ± ±
3.9 5.3 4.6 3.4
tensile modulus (GPa) 0.91 0.95 0.88 0.95
± ± ± ±
0.01 0.03 0.03 0.02
elongation at break (%) 29.3 24.6 24.2 23.0
± ± ± ±
0.6 1.6 2.0 2.7
Measured by DMA at 1 Hz at a heating rate of 10 °C/min in nitrogen. bRelative to polystyrene standards. where θ is the “time lag” and l is the membrane thickness. The solubility coefficient (S) was obtained indirectly via the following equation:
Molecular weights were measured by gel permeation chromatography (Waters GPC Systems, Milford, MA) with polystyrene as an external standard and NMP as the eluent. Elemental analyses were performed by a Thermofinnigan EA1108 (Fisions Instrument Co., Milan, Italy) elemental analyzer. Mechanical properties were obtained using a Universal Testing Machine (UTM, AGS-J, Shimadzu, Kyoto, Japan) with specimens prepared according to ASTM D638-Type 5 recommendations; the average value was determined from at least five specimens. Thermogravimetric analyses (TGAs) were performed on a TA Q-500 thermobalance (TA Instruments, New Castle, DE) coupled with mass spectroscopy (MS, ThermoStar GSD 301T, Pfeiffer Vacuum GmbH, Asslar, Germany). Dynamic mechanical analysis (DMA) was performed by using a DMA Q800 (TA Instruments). The membrane samples (area: 0.9 × 4 cm2) were cut by a punch die and tested in the tensile mode at 1 Hz. The test temperature was in the range of 50−500 °C with a heating rate of 10 °C min−1. Wide-angle Xray diffractometry (WAXD) spectra were recorded in the reflection mode at room temperature by using a Rigaku Denki D/MAX-2500 (Rigaku, Tokyo, Japan) diffractometer with Cu Kα (wavelength λ = 1.54 Å) radiation. Densities of the membranes were measured in 2,2,4-trimethylpentane (Sigma-Aldrich Chemical Co.) using a density measurement kit (Sartorius LA 120S, Sartorius AG, Goettingen, Germany) by the buoyancy method. Fractional free volume (FFV, Vf) was calculated as follows:
Vsp =
Vf =
M0 ρ
S = P /D
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RESULTS AND DISCUSSION Synthesis of Random Hydroxy Copolyimide Precursor. The random o-hydroxy copolyimide precursors were easily prepared using 6FDA with APAF, and either BOA or BIA monomers by a conventional two-step procedure, as shown in Scheme 1. The polymerization results for these syntheses are summarized in Table 1. The weight-average molecular weights (Mws) of o-hydroxy copolyimide precursors from BOA and BIA were 64.5 × 103 and 143.3 × 103, respectively. The random o-hydroxy copolyimide precursor from BOA exhibited a much lower Mw in comparison with the precursor made from BIA. This is probably due to the lower nucleophilicity of the BOA diamine than that of the BIA diamine, which can be inferred from their 1H NMR spectra, as shown in Figure S1. Two amine proton signals of the BOA diamine (located at 4.99 and 5.80 ppm, respectively) underwent downfield shifts relative to the amine proton signals of the BIA diamine (at 4.83 and 5.37 ppm), implying that the nucleophilicity of BOA was lower than that of BIA. The chemical structures of the random o-hydroxy copolyimide precursors were confirmed by elemental analyses, 1H NMR recorded in chloroform-d6 (Figure 1), and ATR-FTIR spectroscopy (Figure 2). The elemental analyses values of the two random o-hydroxy copolyimide precursor powders were in agreement with the expected values for carbon, hydrogen, and nitrogen (shown in the Experimental Section). As shown in Figure 1, the proton signals of the −O−H groups appear as singlets at 10.42 ppm. The R-OH-PIb copolyimide exhibited an additional signal at 13.26 ppm (H-b) due to −N−H in the benzimidazole rings, which is absent in the R-OH-PIa. In the ATR-FTIR spectra (Figure 2), characteristic imide absorption bands were observed at around 1786 cm−1 (imide carbonyl symmetric stretching), 1718 cm−1 (imide carbonyl asymmetric stretching), and 1370 cm−1 (−C−N stretching). The copolyimide containing BOA (R-OH-PIa) exhibited an additional characteristic absorption at ∼1058 cm−1 due to Ar−C−O symmetric stretching of benzoxazole units. Syntheses of Oligomers and Block Hydroxy Copolyimide Precursors. As shown in Scheme 2a, the anhydride-endcapped hydroxy imide oligomer block segment (Ah-OH) was synthesized by using excess dianhydride 6FDA monomer with hydroxy diamine (APAF) at a molar ratio of 11/10 of m(6FDA)/ m(APAF). This was accomplished by a two-step procedure via the corresponding amic acid intermediate. GPC evaluation showed that the resulting anhydride-end-capped hydroxy imide oligomer exhibited a Mw of 12.0 × 103. The structure was confirmed by FTIR spectroscopy (Figure 3a) and 1H NMR recorded in DMSO-d6 (Figure 4).
(1)
Vsp − 1.3VW Vsp
(2)
where Vsp is the molar volume of the polymers determined by the measured density and VW is the van der Waals molar volume based on Bondi’s group contribution theory. Gas permeation properties were obtained from a custom-made instrument using the constant volume method (i.e., the time-lag method). All tests were conducted three times on single films for each polymer, and the data were averaged if the deviation among the results was less than 10%. Otherwise, we performed additional measurements with a different film until the deviation was under 10%. The permeability of all gases was measured at 1 atm at 35 °C and was calculated with the equation P=
273.15Vl dp 76T ΔpA dt
(3)
where P (Barrer), l (cm), V (cm ), T (K), Δp (cmHg), and A (cm2) are the gas permeability, membrane thickness, downstream chamber volume, measurement temperature, pressure difference between the upstream and downstream, and effective membrane area, respectively; dp/dt is the rate of the pressure rise in the downstream chamber at steady state. The effective membrane area was 1.0 cm2 with a thickness of 60−70 μm. The ideal selectivity (αx/y) for components x and y was defined as the ratio of the pure gas permeability of the two components. The diffusion coefficient (D) can be calculated from the time-lag apparatus with the equation 3
D=
l2 6θ
(5)
(4) F
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Figure 1. 1H NMR spectra of the random o-hydroxy copolyimide precursors from BOA (R-OH-PIa) and BIA (R-OH-PIb).
Figure 3. (a) FTIR spectra of the anhydride-end-capped ohydroxyimide oligomer (Ah-OH) and amine-end-capped oligomers from either BOA (An-BOA) or BIA (An-BIA) and (b) ATR-FTIR spectra of the block hydroxy copolyimide precursor membranes from BOA (B-OH-PIa) and BIA (B-OH-PIb) and their corresponding block TR-PBOI membranes treated at 400 °C for 1 or 2 h for thermal rearrangement.
were confirmed by FTIR (Figure 3a) and 1H NMR (Figure 4). As shown in Figure 3a, the imide absorption bands for the two amine-end-capped imide oligomers appeared at around 1784 cm−1 (imide carbonyl symmetric stretching), 1722 cm−1 (imide carbonyl asymmetric stretching), and 1371 cm−1 (−C−N stretching). Also, three −N−H stretching absorption bands, caused by the amine groups of the two amine-end-capped imide oligomers, were clearly observed in the range between 3491 and 3226 cm−1 (Figure 3a). For the An-BIA, shown in 1H NMR spectra (Figure 4), there were two downfield proton signals at 13.29 (H-3) and 12.67 (H-4), arising from the −NH groups in nonterminal benzimidazole rings and terminal benzimidazole rings, respectively. Because of their asymmetric structures, BOA and BIA diamine monomers each have two distinct amine proton signals located at 5.8−5.5 (H-b) and 5.0−4.8 ppm (H-a) (Figure S1). These signals correspond to the amine protons linked to the phenyl ring and benzoxazole/ benzimidazole ring, respectively. Interestingly, the amine-endcapped imide oligomers An-BOA and An-BIA each showed only one proton signal at 6.07 and 5.67 ppm (H-2), respectively (Figure 4), from the amine end-groups. This indicates that only the amine groups having lower nucleophilicity (i.e., aniline-type in this study) survived as terminal groups in the amine-endcapped imide oligomers because of the preferential nucleophilic reaction between higher activity amine groups (where the amine proton signals have a upfield shift in the 1H NMR) and anhydride rings. This is consistent with our previous findings.40 The block o-hydroxy copolyimide precursors were synthesized by combining one anhydride-end-capped hydroxy imide
Figure 2. ATR-FTIR spectra of the random o-hydroxy copolyimide precursor membranes from BOA (R-OH-PIa) and BIA (R-OH-PIb). The corresponding random TR-PBOI membranes treated at 400 °C for 1 or 2 h for thermal rearrangement are also shown.
The characteristic absorption bands of Ah-OH due to the imide units were found at 1788 cm−1 (imide carbonyl symmetric stretching), 1724 cm−1 (imide carbonyl asymmetric stretching), and 1381 cm−1 (−C−N stretching), as shown in Figure 3a. A broad −OH stretching vibration and characteristic CO asymmetric stretching vibration were observed at about 3432 and 1859 cm−1 (Figure 3a), respectively, indicating the presence of hydroxy groups and anhydride rings in the backbone. Furthermore, the presence of the o-hydroxy proton (H-1) in Ah-OH was confirmed by a singlet at 10.42 ppm in the 1H NMR spectrum (Figure 4). Similar to the preparation method reported previously,50 the amine-end-capped imide oligomer block segments were produced by the reaction of excess diamine monomer (BOA or BIA) with dianhydride monomer (6FDA) at a molar ratio of 10/9 of m(BOA or BIA)/m(6FDA). The resulting amine-end-capped imide oligomers had Mws in the range of (15.7−41.4) × 103 and polydispersity of 2.4 (by GPC). Their chemical structures G
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oligomer at 5.67 ppm (H-2, end-capped −NH2) and 12.67 ppm (H-4, −NH in terminal benzimidazole ring) were absent (Figure 4). The signals at 10.42 ppm (H-1, o-hydroxy proton) observed in the anhydride-end-capped oligomer Ah-OH and at 13.29 ppm (H-3, −NH in nonterminal benzimidazole ring) observed in the An-BIA were retained in the block o-hydroxy copolymer B-OH-PIb (Figure 4). The block o-hydroxy copolymer B-OH-PIa also had the predicted 1H NMR spectrum, suggesting that the block copolymerization was performed successfully, as shown in Scheme 2c. Both types of block copolyimide precursors B-OH-PIa and B-OH-PIb exhibited high Mws in the range of (125.6−118.9) × 103 (Table 1), indicating successful coupling of the amine/ anhydride-end-capped oligomers (Mw = (12.0−41.4) × 103). The random o-hydroxy copolyimide R-OH-PIa from BOA had a comparatively much lower Mw of 64.5 × 103 compared with the corresponding B-OH-PIa (Mw of 125.6 × 103); this was caused by the lower nucleophilicity of the BOA diamine. Properties of Hydroxy Copolyimide Precursors. All of the o-hydroxy copolyimide precursors were readily soluble in NMP, N,N-dimethylacetamide (DMAc), and chloroform at room temperature, as shown in Table 2. Also, the o-hydroxy copolyimide precursor membranes exhibited excellent mechanical properties with tensile strengths of 113.6−131.7 MPa and elongation at break of 23.0−29.3%, as shown in Table 1. DMA measurements (Figure S2) provided the glass transition temperatures (Tg), which were between 328 and 362 °C and varied depending on the structures and copolymerization modes (Table 1). As expected, the copolyimides with benzimidazole units exhibited relatively higher Tgs compared to those with benzoxazole units because of stronger interchain hydrogen-bonding interactions between the N−H groups and the carbonyl groups of the imide rings.39 Thermal Rearrangement Studies of Hydroxy Copolyimide Precursors. The random/block TR-PBOI membranes, including R-TR-PBOIa-1h, R-TR-PBOIa-2h, R-TR-PBOIb-1h, R-TR-PBOIb-2h, B-TR-PBOIa-1h, B-TR-PBOIa-2h, B-TRPBOIb-1h, and B-TR-PBOIb-2h can be obtained by thermal rearrangement of the corresponding o-hydroxy copolyimide precursors, as shown in Schemes 1 and 2c. ATR-FTIR spectra of the membranes show evidence for the chemical structural changes occurring during thermal rearrangement of the ohydroxy copolyimide precursors, as shown in Figures 2 and 3b. After being held at a high temperature for 1 h, the random/ block TR-PBOI membranes from BIA and BOA exhibited new and/or stronger benzoxazole characteristic absorption at ∼1058 cm−1 (Figures 2 and 3b). This is caused by Ar−C−O symmetric stretching of benzoxazole units, indicating the formation of new benzoxazole rings from the o-hydroxy group thermal rearrangement reaction. Thermogravimetric analysis with mass spectroscopy (TGAMS) was used to elucidate the thermal conversion character-
Figure 4. 1H NMR spectra of the anhydride-end-capped ohydroxyimide oligomer (Ah-OH), amine-end-capped oligomers from BOA (An-BOA) and BIA (An-BIA), and the block hydroxy copolyimide precursors from BOA (B-OH-PIa) and BIA (B-OH-PIb).
oligomer from solution A (Scheme 2a) with one amine-endcapped imide oligomer from solution B (Scheme 2b), as shown in Scheme 2c. In order to obtain high molecular weight block copolymers, the molar quantity of the anhydride-end-capped ohydroxy imide oligomer should be exactly equal to that of the amine-end-capped imide oligomer. The polymerization results for these materials are summarized in Table 1. Their chemical structures were confirmed by elemental analyses, ATR-FTIR spectroscopy (Figure 3b), and 1H NMR recorded in chloroform-d6 (Figure 4). The elemental analysis of the resulting block o-hydroxy copolyimide precursors for carbon, hydrogen, and nitrogen also agree well with the expected values (Experimental Section). In the ATR-FTIR spectra, the block o-hydroxy copolyimide precursors exhibited characteristic absorption bands for imides (Figure 3b), which were very similar to those of the random o-hydroxy copolyimide precursors (Figure 2) and the anhydride/amine-end-capped imide oligomers (Figure 3a). In the block copolymers, the cyclic anhydride CO asymmetric stretching band at 1859 cm−1 was absent (Figure 3b). In the block o-hydroxy copolymer B-OH-PIb, the proton signals that were present in the An-BIA
Table 2. Solubility of the o-Hydroxy Copolyimide Precursor Fibrous Powdersa solvent
a
polymer
acetone
THF
DMSO
NMP
DMAc
methanol
ethanol
chloroform
R-OH-PIa R-OH-PIb B-OH-PIa B-OH-PIb
− − − −
+ +− + +−
+ + + +
+ + + +
+ + + +
− − − −
− − − −
+ + + +
Symbols: +, soluble at room temperature; + −, partially insoluble at room temperature; −, insoluble at room temperature. H
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Figure 5. Thermogravimetric analyses combined with mass spectroscopy (TGA-MS) of o-hydroxy copolyimide precursors: (a) R-OH-PIa, (b) ROH-PIb, (c) B-OH-PIa, and (d) B-OH-PIb.
Table 3. Mechanical Properties and Packing Parameters of the TR-PBOI Membranes Treated for 1 or 2 h at 400 °C polymer R-TR-PBOIa-1h R-TR-PBOIa-2h R-TR-PBOIb-1h R-TR-PBOIb-2h B-TR-PBOIa-1h B-TR-PBOIa-2h B-TR-PBOIb-1h B-TR-PBOIb-2h poly(ether−benzoxazole)-400-1h32 poly(ether−benzoxazole)-400-2h32 spiroTR-PBO-6F-425-2h35 PBO-MCDEA-400-2h25 PBO−DAM-400-2h25
tensile strength (MPa) 113.2 ± 71.4 ± 112.6 ± 106.1 ± 112.5 ± 108.6 ± 113.9 ± 81.9 ± 81 84 82.3 ± 84.57 64.32
5.4 11.9 1.4 6.0 3.5 3.0 12.3 8.2
tensile modulus (GPa) 1.42 1.68 1.58 1.61 1.50 1.48 1.59 1.64
± ± ± ± ± ± ± ±
elongation at break (%) 16.1 ± 5.1 ± 13.2 ± 10.3 ± 16.1 ± 13.3 ± 13.4 ± 6.2 ± 7.3 6.5 20.0 ± 13.32 8.27
0.08 0.09 0.08 0.07 0.05 0.02 0.19 0.20
1.3
2.2 1.1 0.8 1.3 2.5 0.9 3.0 1.7
4.0
density (g/cm3) 1.4386 1.4225 1.4471 1.4323 1.4357 1.4243 1.4366 1.4259 1.398 1.389 1.12
FFV 0.179 0.184 0.171 0.179 0.180 0.183 0.177 0.179 0.174 0.180 0.27 0.173 0.169
conversion (%)
d-spacing (Å)
76.2 81.8 81.0 88.1 82.5 88.0 77.0 84.3
6.05 6.07 6.03 6.09 5.86 6.08 5.98 6.07 6.3 6.2 6.42
Since the o-hydroxy copolyimide precursor containing BIA had a higher Tg compared with that containing BOA, higher starting temperatures for thermal rearrangement as well as higher temperatures for maximizing the reaction were required for the former copolymer precursor. These results are in agreement with the previous work of Calle et al.26 and Soo et al.25 Furthermore, the random and block o-hydroxy copolyimides had very similar thermal rearrangement temperatures, implying that the reaction is more closely related to the chemical structures and interchain interactions than it is to the copolymerization modes. FFV and Chain Packing of TR-PBOI Membranes. The percentage of rearrangement conversion into TR-PBO units
istics of the o-hydroxy copolyimide precursors and determine evolved gases; these results are shown in Figure 5. All of the thermograms showed two weight loss regions at high temperatures. The first conversion regions were between 330 and 500 °C for R-OH-PIa and B-OH-PIa and between 350 and 510 °C for R-OH-PIb and B-OH-PIb. The amount of CO2 released reached a maximum value at ∼430 and ∼440 °C for the copolyimides derived from BOA and BIA, respectively. The differences in the start and maximum thermal rearrangement temperatures were closely related to the glass transition temperature (Tg) of the material. Thermal rearrangement becomes more difficult with higher Tg because additional kinetic energy must be supplied to drive the rearrangement. I
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Macromolecules was evaluated by using an isothermal TGA method.25 The FFV values can then be estimated by using eq 2. These results are summarized in Table 3. The FFV values were in the range between 0.171 and 0.184 and were dependent on the chemical structures, copolymer types, and the thermal treatment time. In the case of similar copolymerization modes and treatment times, the copolymers containing BOA exhibited slightly higher FFV values than those containing BIA. For the same copolymers, the FFV increased with the treatment time increased, indicating that the thermal conversion reaction can continue to evolve, even at relatively low temperatures (400 °C) for thermal rearrangement. This is consistent with previous work on poly(ether−benzoxazole) membranes.32 However, the random TR-PBOI membranes showed almost the same FFV values as the corresponding block copolymer membranes, implying that the copolymer type has no obvious influence on the FFV values. Out-of-plane wide-angle X-ray diffraction (WAXD) measurements were also conducted to further examine the polymer chain packing within the membranes, as shown in Figure 6. All
similar to our previous report.32 In addition, the copolymerization mode has no obvious influence on the interchain distance (d-spacing) values (Table 3), which was similar to the results from FFV values of these membranes. Mechanical Properties of TR-PBOI Membranes. The mechanical properties are one of the key performance parameters for determining whether or not a membrane material can be developed industrially for applications in gas separation. Poor mechanical properties are a major obstacle for the commercialization of thermally rearranged membranes. In this study, after treatment at 400 °C, tough TR-PBOI membranes can be obtained, as confirmed by their mechanical property values (Table 3) and tensile stress−strain curves (Figure S3). These membranes have tensile strengths of 71.4− 113.9 MPa, elongation at break of 5.1−16.1%, and initial modulus values of 1.42−1.68 GPa, as shown in Table 3. The tensile strengths and elongation at break of the thermally rearranged membranes (Table 3) are lower as compared with their corresponding o-hydroxy copolyimide precursor membranes (Table 1), whereas the tensile modulus of the thermally rearranged membranes were much higher than the corresponding ones from their precursor membranes due to the chain structural transformation induced by the solid state thermal rearrangement reaction. Also, as the thermal treatment time is increased from 1 to 2 h, the mechanical properties of the thermally rearranged membranes clearly deteriorate, as shown in Table 3. For example, the R-TR-PBOIa-1h membrane showed an elongation at break of 16.1%, which is much higher than that of R-TR-PBOIa-2h (5.1%). Surprisingly, the copolymerization mode (random and block) was not observed to have a consistent effect on mechanical properties of the membranes. If treated for 1 h, the random TR-PBOI membranes exhibited very similar mechanical strengths. However, for an extended thermal rearrangement time of 2 h, the membrane tensile properties of R-TR-PBOIa-2h were lower than B-TR-PBOIa-2h and similarly B-TR-PBOIb-2h were lower than R-TR-PBOIb-2h. This is likely due to the relatively lower Mws of the corresponding precursors (see Table 1), which may have poorer tolerance to decomposition at high temperatures or have less entanglement for the more rigid chains formed. Of great significance it that R-TR-PBOIb-2h and B-TR-PBOIa-2h exhibited excellent mechanical properties with good elongation at break (above 10.3%) and high tensile strength (up to 108.6 MPa), which are much higher than the values for any previously reported thermally rearranged membranes, including poly(ether−benzoxazole)-400-2h (84 MPa),32 spiroTR-PBO-6F425-2h (82.3 MPa),35 PBO-MCDEA-400-2h (84.57 MPa),25 and PBO−DAM-400-2h (64.32 MPa).25 It is well-known that the deterioration of the mechanical properties for these TR membranes is largely due to the decomposition that occurs during the thermal rearrangement process at high temperatures. The introduction of aromatic heterocyclic units (e.g., benzoxazole and benzimidazole, which have excellent thermal stability) and nonthermally rearranged imide portions into the backbone effectively inhibit chain decomposition, thereby improving the mechanical properties of the resulting thermally rearranged membranes. Single Gas Permeability. The gas permeabilities (P) of six gases (He, H2, CO2, O2, N2, and CH4) and the ideal selectivities of TR-PBOI membranes and their copolyimide precursor membranes for H2/N2, O2/N2, CO2/CH4, CO2/N2, H2/CO2, and H2/CH4 gas pairs are presented in Table 4. For comparison, the reported values for tough/robust thermally
Figure 6. WAXD curves of the TR-PBOI membranes from (a) BOA and (b) BIA treated for 1 and 2 h at 400 °C.
TR-PBOI membranes exhibited two broad diffraction peaks (A and B) located at 2θ = ∼14.7° and ∼25.7°, respectively. These peaks are very similar to the ones found in typical, nonthermally rearranged poly(benzoxazole-co-imide) membranes40 and also to the peaks found in thermally rearranged poly(ether−benzoxazole) membranes.32 The WAXD results indicated the existence of ordered packing domains combined with primarily amorphous domains. Peak A (2θ = ∼ 14.7°) likely corresponds to the average interchain packing in the ordered domains, whereas peak B (2θ ∼ 25.7°) can be assigned to the π−π stacking order.39,40,50−55 As shown in Table 3 and Figure 6, as the holding time at 400 °C increased from 1 to 2 h, the interchain distances (d-spacing values) in the ordered domains was almost unchanged or slightly increased, which was J
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Table 4. Single Gas Permeabilities (P), Diffusion Coefficients (D), Solubility Coefficients (S), and Ideal Selectivities (α) for Random/Block TR-PBOI Membranes and o-Hydroxy Copolyimide Precursor Membranes ideal selectivity (α)b code
He
H2
N2
R-OH-PIaa P 80 57 1.0 D 4975 1541 30 S 0.12 0.28 0.25 R-OH-PIba P 73 51 0.6 D 8766 2041 7 S 0.06 0.19 0.71 B-OH-PIaa P 75 53 0.7 D 2456 1989 15 S 0.23 0.20 0.37 B-OH-PIaa P 81 60 0.8 D 2570 1273 7 S 0.24 0.36 0.84 R-TR-PBOIa-1ha P 154 143 3.3 D 2255 817 96 S 0.52 1.33 0.26 R-TR-PBOIa-2ha P 192 191 5.0 D 5388 1027 46 S 0.27 1.40 0.82 R-TR-PBOIb-1ha P 130 111 1.8 D 3539 1499 17 S 0.28 0.56 0.81 R-TR-PBOIb-2ha P 149 135 2.6 D 3574 1827 85 S 0.32 0.56 0.24 B-TR-PBOIa-1ha P 152 133 2.9 D 1635 3146 66 S 0.71 0.32 0.33 B-TR-PBOIa-2ha P 150 136 3.4 D 1704 4398 1321 S 0.67 0.23 0.02 B-TR-PBOIb-1ha P 140 122 2.1 D 6395 2790 447 S 0.17 0.33 0.04 B-TR-PBOIb-2ha P 169 167 4.0 D 873 1318 106 S 1.40 0.96 0.29 poly(ether−benzoxazole)-400-2ha 32 P 74.9 64.5 1.98 PBO-MCDEA-400-2ha 25 P 65.9 66.5 1.36 PBO−DAM-400-2ha 25 P 56.0 53.2 0.79 spiroTR-PBO-6F-425-2ha 35 P 318 429 30
O2
CH4
CO2
H2/N2
O2/N2
CO2/CH4
CO2/N2
H2/CO2
H2/CH4
4.4 41 0.82
0.27 1.3 1.5
19 7.7 18
57 51.4 1.1
4.4 1.4 3.3
65 5.9 12.0
19 0.3 72.0
3.1 200.1 0.02
202 1185 0.2
3.9 33 0.88
0.23 0.74 2.4
15 6.5 18
84 314.0 0.3
6.3 5.1 1.2
65 8.8 7.5
25 1.0 25.4
3.4 314.0 0.01
220 2758 0.1
4.6 63 0.55
0.34 1.2 2.2
18 8.2 17
74 132.6 0.5
6.4 4.2 1.5
53 6.8 7.7
26 0.5 45.9
2.9 242.6 0.01
154 1657 0.1
4.9 33 1.10
0.27 0.8 2.6
19 6.4 22
75 179.3 0.4
6.1 4.6 1.3
70 8.1 8.5
24 0.9 26.2
3.2 198.9 0.02
222 1611 0.1
17 176 0.71
1.80 6.3 2.2
75 25.0 23
44 8.5 5.1
5.0 1.8 2.7
42 4.0 10.5
23 0.3 88.5
1.9 32.6 0.06
80 129 0.6
25 176 1.10
2.70 5.8 3.6
114 37.0 23
38 22.3 1.7
5.0 3.8 1.3
42 6.4 6.4
23 0.8 28.0
1.7 27.7 0.06
70 177 0.4
11 222 0.36
0.93 2.0 3.4
44 16.0 21
60 86.6 0.7
5.8 12.8 0.4
48 8.0 6.2
24 0.9 25.9
2.5 93.7 0.03
120 749 0.2
14 692 0.16
1.20 3.6 2.5
66 23.0 22
51 21.5 2.3
5.4 8.1 0.7
56 6.4 8.8
25 0.3 91.7
2.1 79.4 0.03
115 507 0.2
16 245 0.48
1.50 14.0 0.8
68 33.0 16
47 47.6 1.0
5.4 3.7 1.5
46 2.4 20.3
24 0.5 48.5
2.0 95.3 0.02
90 224 0.4
17 902 0.14
1.70 3.9 3.4
79 26.0 23
40 3.3 11.5
4.9 0.7 7.0
46 6.7 6.8
23 0.02 1150
1.7 169 0.01
79 1127 0.07
12 2361 0.04
0.93 3.6 2.0
52 22.0 18
57 6.3 8.3
5.6 5.3 1.0
56 6.1 9.0
25 0.05 450
2.3 126.8 0.02
132 775 0.2
20 557 0.27
2.00 5.2 2.9
93 30.0 23
42 12.4 3.3
5.0 5.3 0.9
47 5.8 7.9
23 0.3 79.3
1.8 43.9 0.04
85 253 0.3
10.7
1.35
27.0
32.6
5.4
20.0
13.6
2.39
47.8
7.34
0.86
35.3
48.9
5.4
41.0
26.0
1.88
77.3
4.99
0.43
23.5
67.3
6.3
54.7
29.7
2.26
123.7
120
34
675
14.3
4.0
19.9
22.5
1.6
12.6
Units: P (Barrer),10−10 cm3 (STP)/(cm s cmHg); D, 10−9cm2/s; and S, cm3 (STP)/cm3 atm measured at 1 atm at 35 °C. bIdeal selectivity α = P1/ P2. a
K
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Figure 7. Robeson plots relevant to R-TR-PBOIa-1h (R1), R-TR-PBOIa-2h (R2), R-TR-PBOIb-1h (R3), R-TR-PBOIb-2h (R4), B-TR-PBOIa-1h (B1), B-TR-PBOIa-2h (B2), B-TR-PBOIb-1h (B3), and B-TR-PBOIb-2h (B4) for (a) H2/CH4, (b) H2/N2, (c) H2/CO2, (d) CO2/CH4, (e) CO2/ N2, and (f) O2/N2 gas pairs (solid lines represent the 2008 upper bound). Data points from commercial Matrimid membrane and reported tough/ robust thermally rearranged membranes are shown for comparison: Matrimid (1),56,57 spiroTR-PBO-6F-425-2h (2),35 poly(ether−benzoxazole)400-1h (3),32 poly(ether−benzoxazole)-400-2h (4),32 PBO-MDA-400-2h (5),25 PBO-MOCA-400-2h (6),25 PBO-MCDEA-400-2h (7),25 PBO− DAM-400-2h (8),25 PBO-OT-400-2h (9),25 PBO-BAP-400-2h (10),25 PBO-TPE-R-400-2h (11),25 and PBO-BAPP-400-2h (12).25
2 h. It was found that after treatment for only 1 h at 400 °C the TR-PBOI membranes with BOA units showed slightly higher permeabilities than the corresponding TR-PBOI membranes with BIA units. For example, the CO2 permeability of R-TRPBOIa-1h (75 Barrer) was higher than that of R-TR-PBOIb-1h (44 Barrer). This difference is due to the fact that these materials have nearly the same solubility coefficients ((21−23) × 109 cm2/s), but R-TR-PBOIa-1h has a higher diffusion coefficient (25 × 109 cm2/s) compared to that of R-TR-PBOIb1h (16 × 109 cm2/s). These results correlate well with the FFV values. The copolymerization mode appears to have no consistent effect on membrane gas permeability; R-TR-PBO membrane containing BOA units exhibited higher permeabilities than the corresponding block copolymer membrane, whereas R-TR-PBO containing BIA units showed lower permeabilities than the corresponding block copolymer membrane. In this study, the TR-PBOI membranes with BOA or BIA units displayed higher or comparable permeabilities as
rearranged PBO or PBOI membranes have also been included in Table 4. The sequence of permeability coefficients from highest to lowest for all of the membranes was He > H2 > CO2 > O2 > N2 > CH4, which follows the gas kinetic diameter order, i.e., He (2.60 Å) < H2 (2.89 Å) < CO2 (3.30 Å) < O2 (3.46 Å) < N2 (3.64 Å) < CH4 (3.80 Å). The permeability coefficients of the membranes strongly depend on the chemical structures, holding time at high temperature, and copolymer types. As expected, each of the o-hydroxy copolyimide precursor membranes exhibited much lower gas permeabilities compared with the corresponding TR-PBOI membranes (Table 4), which is consistent with the results of previous research.32,35 For example, the CO2 permeability of the R-TR-PBOIa-1h membrane was nearly 4 times higher than the corresponding R-OH-PIa membrane. Also, the permeabilities increased as the thermal holding time was increased from 1 to 2 h for each of the o-hydroxy precursor membranes. For example, for R-TRPBOIa, the permeability of CO2 increased by about 52% from 75 to 114 Barrer when the holding time was extended from 1 to L
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Macromolecules
membranes. The copolymers prepared with BOA exhibited slightly higher FFV values compared to those prepared with BIA. However, the copolymerization modes in the random and block copolymers did not exhibit a consistent effect on the Tg, thermal rearrangement behavior, FFV values, molecular packing, and transport properties of the resulting TR-PBOI membranes.
compared to commercial polyimides and those reported tough/ robust thermal rearrangement membranes. For example, the CO2 permeability coefficient of the tough B-TR-PBOIa-2h membrane was 79 Barrer, which is about 12 times higher than that of the commercial membrane Matrimid (6.5 Barrer56). Also, the CO2 permeability of the tough B-TR-PBOIa-2h membrane was ∼2.9 and ∼2.2 times higher than the CO2 permeability of the tough poly(ether−benzoxazole)-400-2h (27.0 Barrer32) and PBO-MCDEA-400-2h (35.3 Barrer25) membranes, respectively. However, it should be noted that its CO2 permeability is significantly lower than those of the reported robust spiro TR-PBO-6F-425-2h membrane (675 Barrer35) and several high FFV 6FDA-based polyimides, such as PIM-6FDA-OH (263 Barrer47), 6FDA-SBF (182 Barrer45), and 6FDA-DATRI (189 Barrer45). The overall gas transport properties for these thermally rearranged membranes containing preformed benzoxazole or benzimidazole units can be further summarized by their Robeson permeability/selectivity trade-off plots, as shown in Figure 7. The present membranes are more permeable than many previously reported tough/robust thermally rearranged membranes, including poly(ether-benzoxazole)-400-2h,32 PBOMCDEA-400-2h,25 and PBO−DAM-400-2h,25 with slightly decreased selectivity for all of the listed gas pairs (H2/N2, O2/ N2, CO2/CH4, CO2/N2,H2/CO2, and H2/CH4). However, the performances of the present membranes are still below that of the robust, thermally rearranged (TR) spiroTR-PBO-6F membrane.35 The introduction of preformed benzoxazole or benzimidazole units into the TR-PBOI backbone also clearly improves the transport properties of the membranes.
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ASSOCIATED CONTENT
S Supporting Information *
1
H NMR spectra of BOA and BIA diamines, DMA curves the o-hydroxy copolyimides, and tensile stress−strain curves the TR-PBOI membranes. The Supporting Information available free of charge on the ACS Publications website DOI: 10.1021/acs.macromol.5b00930.
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of of is at
AUTHOR INFORMATION
Corresponding Authors
*(M.D.G.) Tel +86-22-2740-4479; e-mail michael.guiver@ outlook.com. *(Y.M.L.) Tel +82-2-2220-0525; fax +82-2-2291-5982; e-mail
[email protected]. Author Contributions ∇
Y.B.Z. and J.G.S contributed equally.
Notes
The authors declare no competing financial interest. M.D.G. is a BK21-PLUS visiting professor at Hanyang University.
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ACKNOWLEDGMENTS This research was supported by the Korea Carbon Capture & Sequestration R&D Center (KCRC) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2014M1A8A1049305). Y.B.Z. appreciates the financial support of the Hunan Provincial Natural Science Foundation of China (Grant 14JJ2126) and the Scientific Research Fund of the Hunan Provincial Education Department (Grant 13C618).
CONCLUSIONS Four novel, random/block o-hydroxy copolyimide precursors, with Mws ranging between 64.5 × 103 and 143.3 × 103, were prepared using 6FDA with APAF and diamine monomers containing preformed benzoxazole or benzimidazole units. The block precursors were synthesized by copolymerizing anhydride-end-capped hydroxyl oligomers with amine-end-capped oligomers. For the BOA copolyimides, block copolymerization resulted in higher Mws than random copolymerization due to the lower nucleophilicity diamine BOA. All the o-hydroxy copolyimide precursors showed good solution processability in NMP, DMAc, and chloroform. The thermal rearrangement step, signaled by CO2 release, reached a maximum value at ∼430 and ∼440 °C for the o-hydroxy copolyimide precursors from BOA and BIA, respectively, and depended on the glass transition temperature (Tg) of the material (ranging from 328 to 362 °C). By conducting density measurements and thermogravimetric analysis, the FFV values of these materials were determined to be in the range between 0.171 and 0.184. After treatment at 400 °C, the tough TR-PBOI membranes exhibited tensile strengths of 71.4−113.9 MPa, elongation at break of 5.1−16.1%, and initial modulus values of 1.42−1.68 GPa. The permeability/selectivity trade-off performance of the eight random/block TR-PBOI membranes exceeded a variety of commercial polyimide membranes and also many of the tough/robust thermally rearranged (TR) membranes that have been reported in the literature. The transport performance clearly improved as the holding time at 400 °C was increased for all the TR-PBOI membranes. The introduction of preformed benzoxazole or benzimidazole units into the TRPBOI backbone enhanced the mechanical properties while simultaneously providing good gas transport properties in the
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