Ion-Stabilized Membranes for Demanding Environments Fabricated

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Ion-Stabilized Membranes for Demanding Environments Fabricated from Polybenzimidazole and its Blends with Polymers of Intrinsic Microporosity Gergo Ignacz, Fan Fei, and Gyorgy Szekely ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01563 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Nano Materials

Ion-Stabilized Membranes for Demanding Environments Fabricated from Polybenzimidazole and its Blends with Polymers of Intrinsic Microporosity Gergo Ignacz,† Fan Fei,‡,§ Gyorgy Szekely*,† †

School of Chemical Engineering and Analytical Science, University of Manchester, The Mill,

Sackville Street, Manchester, M1 3BB, United Kingdom ‡

School of Materials, University of Manchester, Oxford Road, Manchester, M13 9PL,

United Kingdom §

Manchester Institute of Biotechnology, University of Manchester, 131 Princess Street, Manchester,

M1 7DN, United Kingdom *

Corresponding author: [email protected], +44 (0) 161 306 4366

ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Robust, ion-stabilized membranes from polybenzimidazole (PBI) and its blends with polymer of intrinsic microporosity (PIM-1) were fabricated for organic solvent nanofiltration in polar aprotic solvents (PAS). Flat sheet membranes comprising of polybenzimidazole and 4–12 wt% PIM-1 were fabricated via conventional phase inversion followed by the reduction of nitrile PIM-1 to amine PIM-1. Stabilization of the membranes were achieved via ion formation (chloride salt) under acidic condition. Such simple ion-stabilization methodology could replace cumbersome crosslinking. The aging of the membranes and the their performance at room and elevated temperatures, in a wide range of pH, in the presence of organic bases, in 6 conventional and 2 green PAS, were investigated. The PIM-1 content enabled fine-tuning of the molecular weight cut-off (MWCO) of the membranes between 190 and 650 g mol-1.

KEYWORDS Polymer of intrinsic microporosity, organic solvent nanofiltration, polar aprotic solvent, temperature effect, ion stabilization.


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INTRODUCTION Organic solvent nanofiltration (OSN) is a promising sustainable technology for molecular level separations.1 One of the key challenges in membrane-based separations is the stability of polymers in harsh organic solvents, in particular polar aprotic solvents (PAS).2 Despite their drawbacks such as toxicity and high boiling point, they are widely used in the chemical industries due to their excellent solubility properties in the Sn2 type reactions, coupling reactions and amide formations. They are also often used for the dissolution of active pharmaceutical ingredients (API) due to their mid-range lipophilicity,

i.e.

having both

acidic

and

basic

moieties.

Green

alternatives

such as

2-methyltetrahydrofuran (MeTHF) and propylene carbonate (PC) to substitute the undesired conventional PASs are emerging in the literature,3 however their industrial viability is yet to be demonstrated. Consequently, it is necessary to develop membranes that are stable in PAS, which could facilitate the implementation of OSN by the industrial sector. Chemical crosslinking aims to increase the stability of polymer membranes under harsh environments such as PAS. However, most of these methods employ toxic substances such as aliphatic diols,4 aliphatic and aromatic diamines,2,5,6 halogenated,13 epoxy7 or heteroaromatic8 crosslinkers, often at elevated temperature. Moreover, the crosslinking process can significantly compromise the permeance of the membranes.9,10 Polybenzimidazole (PBI) membranes for OSN are emerging in the literature, which have been successfully used for various applications.11,12 Livingston et al. developed the first stable PBI membranes using a dibenzyl crosslinker at elevated temperature,10,13 while Chung et al. focused on more sustainable approaches via reducing the crosslinking temperature and employing green solvents.9,14 This work aims to explore ion-stabilization via salt formation of the amine groups of the benzimidazole moiety, which eliminates the need for the undesired chemical crosslinking (Figure 1). Lively et al. has recently critically assessed the potential of microporous materials for OSN.15 Microporous membranes comprising conjugated polymers on polyacrylonitrile (PAN) with rigid backbones with ultrafast permeation were recently reported.16 Polymers of intrinsic microporosity (PIM) were used to fabricate thin film composite OSN membranes on PAN17,18,19 and polyimide (PI)20 porous supports. PIMs are a class of microporous materials with a specific polymer structure with straight



sections followed by cambered section by the axle, which results in large free volume.21 We hypothesized that the incorporation of PIMs into a PBI membrane could loosen its structure and increase permeance. Albeit, nitrile PIM-1 is soluble in many solvents, insoluble amine PIM-1 can be obtained through the treatment with reducing agent borane dimethyl sulphide complex [(CH3)2S·BH3].22 To the best of our knowledge, this is the first report on i) polymer blend approach for OSN, and ii) the application of PIM membranes in PAS.

Figure 1. Schematic overview of the fabrication methodology for PBI and amine PIM-1 polymer blend membranes: a) casting of a dope solution comprising of PBI and PIM-1; B) phase inversion in water followed by BH3·SMe2 treatment to reduce the PIM-1 and HCl treatment for ion-stabilization of the membrane; c-d) polymer network of protonated PBI and amine PIM-1.

EXPERIMENTAL Materials and methods Novatexx 2471 polypropylene non-woven backing was purchased from Freudenberg Filtration Technologies, Germany. PBI dope solution (26.2 wt% polymer, 72.3 wt% DMAc, 1.5 wt% LiCl,


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27 kg mol-1) was purchased from PBI Performance Products, USA. All the used solvents including acetone, acetonitrile (MeCN), borane dimethylsulfide, chloroform, dichloromethane, N,Ndimethylacetamide (DMAc), N,N-dimethylformamide (DMF), diethyl-ether, dimethyl sulfoxide (DMSO), ethyl acetate, N-methyl-2-pyrrolidone (NMP), 2-methyltetrahydrofuran (MeTHF), methanol (MeOH), propylene carbonate (PC), and trimethylamine (TEA) were purchased from Sigma-Aldrich. Acetophenone, benzophenone, triazabicyclodecene (TBD), tetrafluoroterephthalonitrile, 5,5',6,6'tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirobisindane, potassium tert-butoxide (tBuOK), and potassium carbonate (K2CO3) were purchased from Alfa. Polystyrene markers for solute rejection evaluation were purchased from Agilent Technologies. All solutions were prepared using Milli-Q Type II water with a resistivity of 18.2 MΩ cm at 25 °C. Synthesis of PIM-1 Synthesis of PIM-1 was performed according to literature procedure.21 3,3,3',3'-tetramethyl-2,2',3,3'tetrahydro-1,1'-spirobis[indene]-5,5',6,6'-tetraol (15 g, 14.7 mmol, 1 eq.), tetrafluoroterephthalonitrile (8.8 g, 14.7 mmol, 1 eq.) and K2CO3 (18.5 g, 44.1 mmol, 3 eq.) were weighted into a round-bottomed flask equipped with Dean-Stark Apparatus. DMAc (44 mL) and toluene (88 mL) were added under argon. The mixture was stirred at reflux temperature until the nascent water was removed from the system. The excess K2CO3 was removed by filtration, and the crude product was precipitated using methanol (100 mL) as anti-solvent, filtered, washed, and dried at room temperature. The powder was dissolved in chloroform, precipitated with methanol, filtered, washed and dried at room temperature yielding a yellow powder as final product (15.0 g, 71%). 1H NMR (400 MHz, chloroform-d) δ 6.83 (s, 2H), 6.45 (s, 2H), 2.35 (s, 2H), 2.19 (s, 2H), 1.39 (s, 3H), 1.34 (s, 3H). 13C NMR (101 MHz, chloroformd) δ 149.7, 146.92, 139.2, 112.3, 110.5, 109.4, 94.1, 58.8, 57.1, 43.6, 31.3, 29.9. Molecular weight (GPC, in CHCl3): 23 kg mol-1. Preparation and ion-stabilization of PBI/PIM membranes Polymer dope solutions were prepared by dissolving PIM-1 and PBI in DMAc in specific ratios given in Table 1, followed by mechanical stirring for 2 hours at 50 °C. The dope solutions were degassed for



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12 h in an incubator shaker at 45 °C. The dope solution was then cast at 45 °C and a speed of 0.05 m s-1 on non-woven polypropylene support using a casting knife set to 250 µm thickness. The wet films were immediately immersed in water at 21±0.5 °C for phase inversion, and then the water was replaced three times (3 × 5 L). For the PIM-1 reduction, membranes of 52.8 cm2 area were treated with BH3·SMe2 in diethyl ether.22 The membrane was washed with 50 mL dry methanol and 5 × 50 mL dry diethyl ether in order to remove any water from the membrane. The membrane was transferred into a reactor equipped with a cooler. Dry diethyl ether (250 mL) and BH3·SMe2 (12.24 mL, 12.24 mmol) were added, and the reaction mixture was stirred at 200 rpm for 12 h. The membrane was washed with dry methanol and for the ion-stabilization, 1M HCl (2 eq. benzimidazole, 10 mL) was added to treat the membranes, followed by washing with 50 mL water. The membranes were stored in Type II water at room temperature (24 °C) at a dark place prior to testing. The viscosity of the dope solutions was measured using an Elcometer 2300 Rotational Viscometer (Elcometer Limited, UK). Table 1. Membrane compositions and membrane treatments. The weight percentages refer to the polymer content of the membrane. All dope solutions were prepared using DMAc as the solvent, and had 18 wt% total polymer content.

M0

PBI content (wt%) 100

PIM-1 content (wt%) 0

Viscosity (mPas) 6550

BH3·SMe2 Treatment n.a.

HCl Treatment -

M1

100

0

6650

n.a.

+

M2

96

4

7600

+

+

M3

92

8

8900

+

+

M4

88

12

9800

+

+

M5*

84

16

10900

n.a.

n.a.

Membrane

n.a. = not applicable *

This membrane was found to be unstable and it was not tested. Refer to Figure 6b for further details.


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Membrane stability test Dissolution in PAS. A membrane piece of 50 mm × 50 mm was dried, weighed, and immersed into 250 mL PAS such as acetone, MeCN, DMAc, DMF, DMSO, NMP, PC or MeTHF. The samples were placed in an incubator shaker for 7 days at 25 °C, followed by rinsing with acetone and drying under vacuum. The solubility experiments were carried out in duplicates using independently prepared membranes. Dissolution at different pH. A membrane piece of 50 mm × 50 mm was immersed into a 250 mL aqueous solution having a pH of 2, 5, 6, 8, 9, 10, 11 or 12. The pH was adjusted by adding 1M HCl or NaOH. The samples were placed in an incubator shaker for 24 hours at 25 °C, followed by rinsing with water, drying and weighing. The dry samples were immersed into 250 mL DMF. The samples were placed in an incubator shaker for 7 days at 25 °C, followed by rinsing with acetone and drying under vacuum. The solubility experiments were carried out in duplicates using independently prepared membranes.

Nanofiltration Nanofiltration experiments were carried out in a cross-flow system (Figure S1) at 30 bar at room temperature with MeTHF, MeCN, DMF, DMSO, PC as solvents to determine permeate flux and molecular weight cut-off (MWCO)23 curves of the membranes. Two independently prepared membrane discs of each type were tested, and the reported results are the mean values of these measurements. To avoid ambiguity of the results, independently prepared membranes were used for each solvent. The filtrations were performed for 24 hours. Steady-state was reached within 6 hours, and samples were taken after 24 hours. The flow rate for the retentate recirculation was set at 1200 mL h-1 to ensure homogeneous solute concentration in the retentate loop. The permeance and the MWCO of M4 has also been tested at 25, 50, 75, 100 and 125 °C in DMSO following the thermal treatment (tt) sequences described in Table 2. Each step was carried out for 24 hours in DMSO at 30 bar.



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Table 2. Thermal treatment sequences and steps, which were carried out consecutively on the same membrane piece in DMSO at 30 bar. The duration of each step was 24 hours. Temperature (°C)

Sequence 

Step 1

Step 2

Step 3

Step 4

Step 5

1 (tt1)

25

50

75

100

125

2 (tt2)

125

100

75

50

25

3 (tt3)

25

50

75

100

125

4 (tt4)

125

100

75

50

25

Permeate samples for flux measurements were collected at intervals of 1 h, and samples for MWCO evaluations were taken after steady permeate flux was achieved. MWCO values were obtained by using a standard test solution composed of a homologous series of styrene oligomers dissolved in the selected solvent.24 The MWCO values were derived from the MWCO curves via linear interpolation. The styrene oligomer mixture contained 1 g L-1 of PS 580 and PS 1300 and 0.01 g L-1 of α-methylstyrene dimer, and their concentration was determined using a previously reported HPLC method.25 The effective area (A) of each membrane was 52.8 cm2. The permeance was calculated as given in Eq. (1).

𝑃𝑒𝑟𝑚𝑒𝑎𝑛𝑐𝑒 =

𝐽 𝑉 = = [𝐿 𝑚−2 ℎ−1 𝑏𝑎𝑟 −1 ] ∆𝑃 ∆𝑃𝐴𝑡

(1)

The permeance of each membrane was calculated by dividing the solvent flux through the membrane (J) by the transmembrane pressure (∆P). The flux was obtained by measuring the volume of solvent (V) that permeates through the membrane per membrane area (A) per time (t). The rejection of solutes was determined as the ratio of its measured concentration in the permeate (Cp) and the feed (Cf) as defined in Eq. (2).

𝑅𝑒𝑗𝑒𝑐𝑡𝑖𝑜𝑛 = 1 −

𝐶𝑝 𝐶𝑓

(2)


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Analytical methods The infrared spectra were recorded from dry membrane samples using a Thermo Fisher Nicolet iD5 ATR-FTIR spectrometer. Atomic force microscope infrared spectroscopy (AFM-IR) was performed on a NanoIR2 system (Anasys Instruments) in contact mode at a scan rate of 0.3 Hz using a gold-coated silicon nitride probe (Anasys Instruments, 0.07–0.4 N m-1 spring constant, 13±4 kHz resonant frequency). The amplitude of induced cantilever oscillations was mapped using 32 co-averages per 1024 points per 1024 scan lines. The solid-state NMR measurements were recorded on a Bruker Avance III (400 MHz) with 4 mm CPMAS probe with the spinning rate of 10 000 Hz. The liquid phase NMR measurements were recorded on a Bruker Avance III (400 MHz) using chloroform-d as a solvent. The average molecular mass of PIM-1 was measured by gel permeation chromatography (GPC). Analysis was performed in CHCl3 at a flow rate of 1 mL min-1 using a Viscotek VE2001 GPC solvent/sample module with two PL Mixed B columns and a Viscotek TDA302 triple detector array (refractive index, light scattering, viscosity detectors). The data were analysed by the OmniSec program. BET surface area measurement was performed on ASAP 2020 V4.00H machine using nitrogen gas at 77.451 K. Scanning electron microscopy (SEM) measurements were performed on a FEI Quanta 200 field emission scanning electron microscope, and the samples were sputtered 10 nm gold/palladium coating under an argon atmosphere using a Quorum Q150TES. The contact angle measurements were performed on a Krüss DSA 100 drop-shape analysis system. The viscosity for the dope solutions for membrane casting was measured with an Elcometer 2300RV at room temperature. Atomic force microscopy (AFM) images were acquired in tapping mode in air using a Digital Instruments Dimension 3100 with Bruker TESPAV2 probe with a nominal spring constant of 37 N m-1 and a nominal tip apex radius of 7 nm. Samples were prepared by sticking the membranes on a glass slide using double-sided tape. For roughness calculations, three membranes of each type with an area of 25 μm2 were scanned and analyzed with the NanoScope Analysis software.



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RESULTS AND DISCUSSIONS Membrane performance The stability of membranes were probed against polar aprotic solvents, and the weight losses are summarized in Table 3. All the membranes were found to be stable in acetone, MeCN and MeTHF. The non-modified PBI membrane (M0) was dissolved in DMAc, DMF, NMP, DMSO and PC. Surprisingly, the HCl-treated PBI membrane (M1) was not dissolved in any PAS. Literature reports on robust PBI membranes that are stable in PAS, all employed crosslinking with toxic reagents at elevated temperatures,13,26,27 although sustainable attempts are sought-after.28 It was successfully demonstrated that a simple protonation with HCl can provide a simple and green alternative to covalent crosslinking of PBI membranes.

Table 3. Percentage weight loss of membrane in polar aprotic solvents. aFaint discoloration of the solvent occurred, which is speculated to be from the leaching of some smaller oligomers from the membranes. M0

M1

M2

M3

M4

Acetone

00

00

00

00

00

MeCN

00

00

00

00

00

DMAc

1000

0a

00a

00a

00a

DMF

1000

0a

00a

00a

00a

NMP

1000

0a

00a

00a

00a

DMSO

1000

0a

00a

00a

00a

PC

89

00

00

00

00

MeTHF

00

00

00

00

00

Figure 2a shows the permeance of M1–4 membranes in PAS. The permeance increased with the increasing amine PIM-1 content due to its microporosity and increased hydrophilicity. The number of amine groups are increasing with the increasing amine PIM-1 content (M2  M3  M4), which increases the hydrophilic characteristics of the membrane. The lowest permeance has been measured in


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case of M1 in MeTHF, and the highest in case of M4 in PC with 0.370.04 and 2.40.1 L m-2 h-1 bar-1, respectively. PC has the highest permeance since the membrane material favors hydrophilic solvents, and vice versa MeTHF has the lowest permeance. Refer to Table S4 in the Supporting Information for the solvent properties. Figure 2b shows the measured MWCO for M1–4 membranes in PAS. M2 has the lowest MWCO with 190 g mol-1 in PC, however, M2 has an MWCO of 420 g mol-1 in DMSO. M1 has the highest MWCO in each solvent ranging from 510 to 660 g mol-1. Interestingly, the incorporation of only 4 wt% PIM-1 resulted in a significantly tighter membrane (M2) as the MWCO values decreased by up to 67% in all the solvents compared to M1. However, further addition of PIM-1 increased the MWCO but still provided tighter membranes (M3–4) than M1 in all the solvents. Considering all membranes, the lowest polarity solvent (MeTHF) has the narrowest MWCO distribution covering the 400–510 g mol-1 range, while the highest polarity solvent (PC) has the widest MWCO distribution ranging from 200 to 600 g mol-1. These membranes enable a wide operation range for processing PC-based solutions, which could facilitate the uptake of this green solvent by the industrial sector. The aforementioned phenomenon is speculated to be the result of the non-linear change in the microporosity and interconnectivity of the two polymers at different concentrations (4, 8, 12 wt%).



Figure 2. Permeance (a) and MWCO (b) values for M1 (0 wt% amine PIM-1), M2 (4 wt% amine PIM-1), M3 (8 wt% amine PIM-1) and M4 (12 wt% amine PIM-1) in different PAS measured at 25 °C. Refer to the Supporting Information for the rejections and cut-off curves.

The thermal processing and thermal resistance of membranes could open new opportunities for liquid processing. The changes of permeance and MWCO of M4 as a function of temperature in DMSO were investigated (Figure 3). At each temperature a stable but different performance was observed for tt1 (see Table 2). The permeance increased from 1.4 to 2.0 L m-2 h-1 bar-1, as the temperature was gradually increased from 25 to 125 C (Figure 3a). However, the consecutive decrease (tt2) – increase (tt3) – decrease (tt4) in temperature revealed a different but consistent performance. The permeance change was found to have a linear correlation with the solvent viscosity (Figure 3c), which suggests that the system is in accordance with Darcy’s Law and the pore-flow model.29 The thermal treatment resulted in a tighter membrane with lower permeance and MWCO. The MWCO considerably decreased from 620 to 360 g mol-1 during tt1, and then further decreased from 360 to 320 g mol-1 during tt2–4 (Figure 3b). It


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is speculated that these results are due to the combined effects of swelling, accelerated aging and thermal rearrangement of the polymer blend. The literature on thermal treatment of membranes for liquid separations is scare and contradictory as different observations were made.30,31,32,33 Besides varying the PBI/PIM-1 polymer blend composition, the thermal treatment of the membranes enables fine-tuning the separation performance at a cost of decreased permeance. Further fine-tuning of membrane performance could be achieved via altering the phase inversion parameters, such as the temperature.

Figure 3. Permeance (a) and MWCO (b) values for M4 as a function of temperature ranging from 25 °C to 125 °C measured in DMSO at 30 bar. Refer to Table 2 for the thermal treatment sequences (tt1–4). Permeance as a function of viscosity (c) from 25 °C to 125 °C measured in DMSO at 30 bar. Physical aging takes place in every polymer, however, for glassy polymers with high free volume such as PIM-1, accelerated physical aging occurs.34 The aging of PIM-based membranes were studied for 3– 8 days in heptane, ethanol and THF.17,18,19,20 The aging of M4 in DMSO was studied over 3 weeks of



continuos operation with a 3 months shelf storage in DMSO. Over this time, the MWCO and the permeance decreased approximately 6% and 1%, respectively. Refer to the Supporting Information for the experimental details and the data. Owing to the ion-stabilization methodology, acids and basis in the solution could adversely affect the stability of the membranes. The pKa value of each amine groups in the membranes, and therefore their protonation/deprotonation, were estimated using MarvinSketch® (refer to the Supporting Information). Figure 4 shows the effect of pH on the protonation of the polymers as well as the results for the stability test. In line with the predictions, the membranes were found to be stable under both acidic (pH < 6) and basic (pH > 11) conditions. However, at pH = 6–10 the membranes were dissolved due to the solubility of the neutral PBI, which is the main polymer component of the membranes. Organic bases are often used in PAS to perform various synthetic transformations, and therefore the stability of membranes were also demonstrated in the presence of TBD, TEA and tBuOK in DMSO. Refer to the Supporting Information for the MWCO and permeance data in these solutions.

Figure 4. The effect of pH on the protonation and stability of the membranes. The latter is expressed as percentage weight loss after immersion into DMSO. Refer to the Supporting Information for the details of the pKa analysis.


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Chemical and morphological characterization Figure 5a shows the FTIR spectra for the PIM-1 and the M0–4 membranes. Owing to the planar cyclic and aromatic part of PBI, the spectra of M0–4 is composed of narrow peaks between 2000–1000 cm-1 corresponding to the N–H and C–N deformation modes (δN–H and δC–N). The characteristic aromatic C– H stretching frequencies were found between 2900 cm-1 and 3050 cm-1 as weak compact peaks (νC-H). The characteristic aromatic and C=N stretching modes were found around 1650 cm-1 as medium wide peaks (νC=N). The compact peaks between 1200 cm-1 and 1550 cm-1 indicate the C–C and C–H of the in-plane ring vibrational and breathing vibrational frequencies of the aromatic rings (ν C-H and νC-C). The strong peak at 802 cm-1 belongs to the heterocyclic ring vibration (νC-H). The characteristic C≡N stretching frequency mode was found at 2240 cm-1 as a low compact peak (νC≡N). The aforementioned peaks are missing in the membranes containing amine PIM-1 (M2–4) confirming the successful reduction of the nitrile group. The aromatic ether C–O stretching frequency mode was found at 1008 cm-1 as a strong narrow peak (νC-O). The highlighted part on Figure 5a indicates the main difference between the original PBI and its polymer blend with the amine PIM-1. Owing to the salt formation, characteristic new peaks can be found between 2500 cm-1 and 2750 cm-1. Figure 5b shows the solid-state NMR measurements for M0, M1 and M4. The spectrum of the PBI (M0) correlates to the NMR spectra reported in the literature. As a result of the HCl treatment, the peaks at 110, 121 and 128 ppm were slightly shifted downfield, and the original 150 and 141 ppm peaks convoluted. The spectrum of the M4 membrane is similar to M1, with the additional peaks at 47 and 57 ppm corresponding to the aliphatic carbon atoms in the amine PIM-1.



Figure 5. Chemical characterization of the membranes: a) FTIR spectra of PIM-1 and M0–4 membranes, b) solid-state NMR of M0, M1 and M4. The asterisks, (s), and (b) denote the peaks caused by the spinning, the stretching and bending frequencies, respectively.

Elemental analysis was used to reveal the elemental composition and the degree of protonation (Table 4). Chlorine was not found in M0, which confirms the complete removal of the LiCl stabilizer found in the commercial PBI used to prepare the dope solution. The degree of protonation () is defined as the ratio of protonated amine groups and its theoretical maximum. When all the protonizable groups are protonated with HCl, the degree of protonation is 100%. Refer to the acidic pH region of Figure 4


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and Schemes S3–4 for the protonated structures.  was found to be 98–100% for all the HCl-treated membranes (M1–4), which was expected based on the pKa predictions and previous literature.22

Table 4. Elemental composition and degree of protonation () for the membranes. The first value is calculated for pristine PBI (M1) and assuming 100% protonation for M1–4, while the second value is measured with elemental analysis. Membrane M0 M1 M2 M3 M4

C (wt%) 77.91/77.86 63.01/63.25 63.06/63.17 63.11/63.07 63.17/63.12

H (wt%) 3.92/3.95 3.70/3.71 3.78/3.81 3.85/3.87 3.93/3.97

N (wt%) 18.17/18.14 14.70/14.75 14.31/14.33 13.93/13.96 13.55/13.57

Cl (wt%) 0.00/0.00 18.60/18.25 18.38/18.22 18.16/18.14 17.94/17.91

 (%) 0 98 99 100 100

The membrane morphology was characterized by SEM surface, cross section and AFM measurements (Figure 6). Both AFM height images and SEM images revealed similar morphologies for M0–4 membranes (Figure 6a, 6c and 6e). However, the SEM revealed the presence of nano- and microspheres in M5, which contains 16 wt% amine PIM-1 (Figure 6d and 6f). The phase separation behavior of M5 was further investigated with AFM-IR at 1373 cm-1 infrared wavelength (one of the characteristic peaks of amine PIM-1), which confirmed the spheres to be amine PIM-1, as it revealed that the spheres have higher IR intensity (Figure 6b). The aggregation of PIM-1 at higher concentrations limits its addition in the polymer blend. Employing different types of solvent or co-solvents, or increasing the temperature for the preparation of the dope solution and the film casting, could increase the solubility of PIM-1, which would allow membranes with higer PIM-1 content.



Figure 6. Membrane morphology: a) typical AFM height image of M0–4 with scan size of 10 μm × 10 μm; b) AFM-IR of M5, generated from the IR intensity at 1373 cm-1 as a characteristic peak of amine PIM-1 (color; red represents higher intensity while blue represents lower intensity) superimposed on the AFM height image (contour) of the same area; c–d) SEM surface images for typical M0–4 membranes (c) and for M5 (d); e–f) SEM cross-section images for typical M0–4 membranes (e) and M5 (f). All SEM images were taken at 10k magnification. Refer to the Supporting Information for the characterization of each membrane. The uniqueness of the PIM-based materials is the high surface area derived from the rigid structure of the polymer. Therefore, surface area measurement was performed to further investigate the effect of PIM content on the membranes (Table 5). The surface area of M1 was found to be  m2 g-1 which gradually increased with the incorporation of PIM- into the membrane, and reached 56.1 m2 g-1 (M4). The surface area shows a linear correlation with the increasing amount of amine PIM-1 in the membrane, which could explain the increasing permeance (Figure 2a). Based on the surface area of PIM-1 (705 m2 g-1) and M1 (18.9 m2 g-1), the theoretical surface area for M2–4 was estimated to be 46, 73 and 101 m2 g-1, respectively. The observed lower values are speculated to be the result of the PIM-1 reduction, protonation and entanglement of the PBI and PIM-1 polymer chains. Higher amine PIM-1


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content increased the hydrophilicity of the membranes, which was demonstrated by the decreasing contact angle from 61° (M1) to 36° (M4). The increase in both the hydrophilicity and surface area resulted in a higher transport rate of polar molecules from the bulk to the surface, and subsequently increased the permeance of PAS (see Figure 2a). Decoupling of these effects is a challenge yet to be solved. Table 5. The effect of PIM-1 addition on the BET surface area and hydrophilicity of the membranes. The PIM-1 has 705 m2 g-1 surface area; refer to the Supporting Information for further characterization. Membranes 

M1

M2

M3

M4

BET surface area (m g )

   

Contact angle (°)



2

-1







CONCLUSIONS Polymer blend membranes, for solvent-resistant nanofiltration in polar aprotic solvents, have been fabricated for the first time using PBI and amine PIM-1. Ion-stabilization of PBI and PBI/PIM-1 membranes via simple HCl treatment was successfully demonstrated. The novel stabilization approach eliminates the use of cumbersome chemical crosslinking. The incorporation of PIM-1 into the membranes improved the permeance of the membranes up to 4 times, and simultaneously decreased the MWCO. Filtration studies covering 3 months revealed a moderate membrane aging with performance change of up to 6%. A comprehensive thermal treatment study of the membranes revealed stable but anomalous performance up to 125 C. Propylene carbonate was identified as a promising green solvent for replacement of conventional PAS in chemical manufacturing due to its facile processing with membranes. Based on the results, ion-stabilization could not only open new possibilities for the fabrication of robust OSN membranes, but could be exploited for the fabrication of polymer coatings,35 batteries36,37 and electromembranes.38,39



ASSOCIATED CONTENT The authors declare no conflict of interest.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. NMR spectra, SEM and AFM images, AFM-IR analysis, pKa predictions, membrane stability test, aging study, reaction schemes, PIM-1 characterization, rejections and cut-off curves.

AUTHOR INFORMATION ORCID Gergo Ignacz: https://orcid.org/0000-0002-7227-3070 Fan Fei: https://orcid.org/0000-0003-0548-5775 Gyorgy Szekely: http://orcid.org/0000-0001-9658-2452

ACKNOWLEDGMENTS The authors would like to express their gratitude to Mr. Levente Cseri (The University of Manchester) for the technical assistance with the solid-state NMR.


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TABLE OF CONTENTS GRAPHICS



Schematic overview of the fabrication methodology for PBI and amine PIM-1 polymer blend membranes: a) casting of a dope solution comprising of PBI and PIM-1; B) phase inversion in water followed by BH3·SMe2 treatment to reduce the PIM-1 and HCl treatment for ion-stabilization of the membrane; c-d) polymer network of protonated PBI and amine PIM-1. 113x69mm (300 x 300 DPI)


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Permeance (a) and MWCO (b) values for M1 (0 wt% amine PIM 1), M2 (4 wt% amine PIM 1), M3 (8 wt% amine PIM 1) and M4 (12 wt% amine PIM 1) in different PAS measured at 25 °C. Refer to the Supporting Information for the rejections and cut-off curves. 98x154mm (300 x 300 DPI)



The effect of pH on the protonation and stability of the membranes. The latter is expressed as percentage weight loss after immersion into DMSO. Refer to the Supporting Information for the details of the pKa analysis. 173x75mm (300 x 300 DPI)


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Permeance (a) and MWCO (b) values for M4 as a function of temperature ranging from 25 °C to 125 °C measured in DMSO at 30 bar. Refer to Table 2 for the thermal treatment sequences (tt1–4). Permeance as a function of viscosity (c) from 25 °C to 125 °C measured in DMSO at 30 bar. 100x153mm (300 x 300 DPI)



Membrane morphology: a) typical AFM height image of M0–4 with scan size of 10 μm × 10 μm; b) AFM-IR of M5, generated from the IR intensity at 1373 cm-1 as a characteristic peak of amine PIM-1 (color; red represents higher intensity while blue represents lower intensity) superimposed on the AFM height image (contour) of the same area; c–d) SEM surface images for typical M0–4 membranes (c) and for M5 (d); e–f) SEM cross-section images for typical M0–4 membranes (e) and M5 (f). All SEM images were taken at 10k magnification. Refer to the Supporting Information for the characterization of each membrane. 342x187mm (300 x 300 DPI)


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Chemical characterization of the membranes: a) FTIR spectra of PIM-1 and M0–4 membranes, b) solid state NMR of M0, M1 and M4. The asterisks, (s), and (b) denote the peaks caused by the spinning, the stretching and bending frequencies, respectively. 125x126mm (300 x 300 DPI)


Ion-Stabilized Membranes for Demanding Environments Fabricated

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