Bio-Inspired Robust Membranes Nanoengineered from

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Bio-Inspired Robust Membranes Nanoengineered from Interpenetrating Polymer Networks of Polybenzimidazole/Polydopamine Dan Zhao, Jeong F. Kim, Gergo Ignacz, Peter Pogany, Young Moo Lee, and Gyorgy Szekely ACS Nano, Just Accepted Manuscript • Publication Date (Web): 03 Jan 2019 Downloaded from http://pubs.acs.org on January 3, 2019

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Bio-Inspired Robust Membranes Nanoengineered from Interpenetrating Polymer Networks of Polybenzimidazole/Polydopamine Dan Zhao,†$ Jeong F. Kim,‡§$ Gergo Ignacz,† Peter Pogany,∥ Young Moo Lee,‡ Gyorgy Szekely†*

†School

of Chemical Engineering & Analytical Science, The University of

Manchester, The Mill, Sackville street, Manchester, M13 9PL, United Kingdom ‡WCU

Department of Energy Engineering, Hanyang University, Seoul 04763,

Republic of Korea §Research

Centre for Membranes, Advanced Materials Division, Korea Research

Institute of Chemical Technology, Daejeon, 34114, Republic of Korea

1 ACS Paragon Plus Environment

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Department of Inorganic & Analytical Chemistry, Budapest University of Technology

∥

& Economics, Szent Gellert ter 4, Budapest, 1111, Hungary $

These authors contributed equally

*Corresponding

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

ACS Paragon Plus Environment

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Abstract Marine mussel inspired polydopamine (PDA) has been receiving increased attention due to its good thermal and chemical stability as well as strong adhesion on most materials. In this work, high performance nanofiltration membranes based on interpenetrating polymer networks (IPN) incorporating PDA and polybenzimidazole (PBI) were developed for organic solvent nanofiltration (OSN). Generally, in order to obtain solvent stability, polymers need to be covalently crosslinked under harsh conditions, which inevitably leads to losses in permeability and mechanical flexibility. Surprisingly, by in situ polymerization of dopamine within a PBI support, excellent solvent resistance and permeance of polar aprotic solvents were obtained without covalent crosslinking of the PBI backbone due to the formation of an IPN. The molecular weight cut-off (MWCO) and permeance of the membranes can be fine-tuned by changing the polymerisation time. Robust membrane performance was achieved in conventional and emerging green polar aprotic solvents (PAS) in a wide temperature range covering –10 °C to +100 °C. It was successfully demonstrated that the in situ polymerization of PDA — creating an IPN — can provide a simple and green alternative to covalent crosslinking of membranes. To elucidate the nature of the


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solvent stability, a detailed analysis was performed that revealed that physical entanglement along with strong secondary interaction synergistically enable solvent resistance with as low as 1–3% PDA content.

Keywords bio-coatings, surface modification, nanofiltration, temperature effect, polar aprotic solvents, in situ polymerisation, polydopamine


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Liquid-liquid separation techniques share a key role in the chemical and pharmaceutical industry. Increasing environmental and safety regulations have driven attention towards greener separation methods. Organic solvent nanofiltration (OSN) is a sustainable technology for molecular level separation.1,2 One of the key challenges in membrane-based separation is the stability of polymers in harsh organic solvents, in particular polar aprotic solvents (PAS).1

PASs are widely used in chemical industries due to their excellent solubility properties, which make them indispensable. Despite their several drawbacks, such as toxicity and high boiling point, PASs are hard to replace due to their strong advantages, for example in Sn2 type reactions, coupling reactions or amide formations. Moreover, due to their mid-range lipophilicity with both acidic and basic moieties, PASs are often used for the dissolution of active pharmaceutical ingredients; however, they are of high concern by the European Chemical Agency.3 Green alternatives such as Cyrene® and propylene carbonate (PC) to replace the undesired conventional PASs are emerging in the literature;4–8 however, their industrial viability is yet to be demonstrated. Generally, the industry is reluctant to accept emerging technologies until fully proven


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safe, and therefore conventional PASs will remain as first choice. Hence, we have selected both conventional and green PASs in our attempt to improve membrane performance (Table 1).


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Table 1. Properties of polar aprotic solvents used for nanofiltration. Data obtained from Perry's Chemical Engineers' Handbook9 unless otherwise stated.

Dielectric

MW

Boiling point Viscosity

Dipole moment

constant

(g mol-1)

(°C)

(cP @ 25 °C)

(D)

NMP

34.1

99

203

1.65

4.09

DMF

36.7

73

152

0.92

3.86

DMAc

37.8

87

165

0.945

3.72

DMSO

47

78

189

1.996

3.96

HMPT

47.4

179

232

3.34

5.37

PC

64

102

242

0.625

4.90

Cyrene

37.3a

128

227

14.510

4.08b

Solvent

aDetermined

via

electrochemical

impedance

spectroscopy

(see

Supporting

Information); bCalculated on levels B3LYP/cc-pVTZ and M062X-D3/6-311G** (see Supporting Information).

Covalent crosslinking is used to increase the stability of membranes under demanding environments, and to prevent their dissolution in PASs. However, most methods employ toxic substances such as diols, diamines, or heteroaromatic- type crosslinkers in toxic solvents at high temperatures.1,11 In the membrane separation field, musselinspired polydopamine (PDA) coated membranes have been receiving attention due to their excellent features and potential ability to improve the stability and rejection of membranes.12 Zhao et al.13 first explored PDA to prepare composite membranes


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through the self-polymerization of dopamine and interfacial polymerization of dopamine and trimesoyl chloride (TMC) on polyethersulfone (PES) substrate. However, attempts to fabricate PDA-based membranes in OSN have been scarce, though this field is expected to rapidly evolve. Uniform and stable PDA membranes have been prepared using an H2O2/CuSO4 system and were tested in OSN.14,15 Peinemann

and

his

co-workers

reported

for

the

first

time

the

use

of

dopamine/terephtaloyl chloride (TPC) for the fabrication of an ultrathin film composite membrane for solvent-resistant nanofiltration.16 The optimized dopamine/TPC membrane showed good solvent permeance of 5 L m-2 h-1 bar-1 combined with a molecular weight cut-off above 800 g mol-1. Uniform PDA coatings on polyacrylonitrile (PAN) substrate have been fabricated via meticulous regulation of dopamine selfpolymerization.14 Interestingly, crosslinked and PDA-coated PAN membranes oxidized by CuSO4/H2O2 showed robust stability in organic solvents such as methanol, acetone, and tetrahydrofuran. Recent developments have focused on the challenge of speeding up the deposition time.12 All of these works have employed chemical crosslinking to achieve stability in organic solvents.


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Herein, we report on the fabrication, characterisation, separation performance and long-term stability of a robust nanofiltration membrane based on an interpenetrating polymer network (IPN) formed via in situ polymerisation of dopamine in a polybenzimidazole (PBI) support (Figure 1). IPNs are blends of polymers characterized by molecular scale penetration of a linear polymer by a branched polymer. Membranes based on IPNs have been proposed for gas separation,17 ion exchange,18 small molecule transport19 and pervaporation,20 but never used for OSN before. PBI-based OSN membranes have recently been developed by the Livingston and Chung groups,21–23 and they have been successfully used for various applications.24,25 To the best of our knowledge, neither PDA-coated non-crosslinked membranes nor IPNs have been reported for separation in PASs.

Figure 1. Schematic overview of membrane fabrication methodology for in situ polymerised polydopamine (PDA)/polybenzimidazole (PBI) interpenetrating polymer network (IPN): A) casting of a dope solution comprising PBI and dopamine monomers, B) phase inversion in water, C) immersion of membrane in aqueous NaIO4 initiator,


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and D) in situ polymerisation of dopamine in Tris buffer solution. IPN of linear PBI and branched PDA (E). Molecular level interactions between PBI and PDA polymers (F).

Results/Discussion The solvent stability of pristine PBI membrane (M0) and PBI-PDA membranes (M1– M4) were measured in DMF at 25 C and 100 C (Table 2). Expectedly, M0 without any PDA dissolved in DMF, as reported in the literature.23 Interestingly, when dopamine monomers were encapsulated in PBI membrane followed by in situ polymerization, the resulting membranes (M1–M3) exhibited excellent solvent stability, even at 100 C. As a control experiment, we deposited dopamine monomers on PBI membrane following a conventional technique, i.e. without incorporating dopamine monomers into the PBI dope solution (M4). Similar to M0, the M4 membrane also dissolved in DMF. The solubility data allowed us to deduce that the encapsulation of the dopamine monomer in the PBI support, i.e. in situ polymerisation, is the key to achieve robust solvent resistant membranes.


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Table 2. Determination of the stability of in situ versus post-casting polymerised PDA/PBI IPN membranes through percentage weight loss of membranes after treatment with DMF for 1 week.

T (C) 25 100

M0 100±0. 0 100±0. 0

M1

M2

M3

0.7±0.8 0.1±1.0 0.3±1.4 0.1±0.8 0.2±1.3 0.4±0.1

M4 96.2±2. 8 100±0. 0

We propose the following mechanism to explain the formation of the high-performance membrane. The dopamine monomer present in the dope solution acts as an additive during the phase inversion process. The in situ polymerisation of the entrapped dopamine monomers in the PBI support results in branched PDA, which encompasses the linear PBI chains, forming an IPN. The strong adhesion characteristic of the PDA and the formation of the IPN are jointly responsible for the excellent stability of the obtained membranes. Both the PDA and IPN structures are highly controversial, and therefore full structural elucidation of these membranes is a challenge yet to be solved. Nonetheless, the importance of in situ polymerisation was verified through comparison


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of the stability of M1–3 versus M4 (Table 2). The IPN membranes were put to the test in a wide range of PASs (Figure 2A).

Besides conventional NMP, DMF, DMSO, DMAc and HMPT, emerging green alternatives, namely propylene-carbonate (PC) and Cyrene®, were also used in the membrane performance tests. The selected solvents have high dielectric constants ranging from 34.1 (NMP) to 64 (PC), and they have wide range of viscosity and dipole moment (Table 1). The rejection profile was measured and the molecular weight cutoff (MWCO) was determined for M1, M2 and M3. Interestingly, the results show no exact correlation between the solvent properties and the MWCO, which is in sharp contrast to the membrane permeance, which can be correlated to the solvent properties. On the other hand, the MWCO decreases with increasing PDA polymerization time, meaning the membranes get progressively tighter and the pores get smaller. The decreasing MWCO from M1 to M3 can be explained by the increased polymerization time of the PDA on the membranes, which will result in smaller internal pores. Owing to the in situ polymerisation of the dopamine in the membrane, it is not


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just the surface of the PBI that is covered with PDA; rather, PDA is an integral part of the entire PBI polymer matrix.

Figure 2. Dependence of molecular weight cut-off (MWCO) of membranes on solvent (A) and temperature (B). Various polar aprotic solvents were tested at 25 °C, while the temperature was varied between –10 °C to 100 °C in DMF at 20 bar. Refer to Supporting Information for rejection values and MWCO curves.

In previous works, PDA coatings were found to enhance the thermal stability of materials.26 The thermal stability of M1–3 and the effect of temperature on the separation performance were tested in DMF covering a range of –10 °C to 100 °C (Figure 2B). It is speculated that higher temperature leads to higher MWCO as a result of membrane swelling and higher solute solubility in the polymeric backbone. By increasing the temperature from –10 °C to 100 °C, the MWCO for M1, M2 and M3 gradually increased from 410, 340 and 250 g mol-1 to 1330, 1020 and 610 g mol-1, respectively. The temperature-dependent change in MWCO was found to be smaller


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with longer PDA polymerization times. The MWCO and subsequently the separation performance of the membranes can be fine-tuned by changing the temperature of the filtration.

The obtained values of permeance in the PASs and a comparison of these values with the literature data are shown in Figure 3A. The PDA/PBI IPN membranes significantly outperformed all reported membranes in NMP and DMSO. In particular, the tightest membrane (M3), with 4 L m-2 h-1 bar-1 for DMSO, shows an order of magnitude higher permeance than other PBI-based membranes reported in the literature.22,27 The M2 and M3 membranes showed results similar to those of the 33 permeance values in DMF reported in the literature, while M1 outperformed the best reported membrane by nearly 50%. Refer to Table S1 in the Supporting Information for a detailed comparison with the literature. The solvent permeance can be correlated to the physical properties (solubility parameter, viscosity, molecular diameter), as suggested by Livingston

et al.28 for thin film composite membranes. Similarly, Figure 3A shows a reasonable correlation, suggesting that the solvent permeation mechanism is similar to that in dense thin film composite membranes. The highest permeance of 19.2 L m-2 h-1 bar-1


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was obtained for M1 in PC. Besides the fact that PC is an emerging green solvent alternative to conventional PASs, its high permeance (9.6–19.2 L m-2 h-1 bar-1) and low MWCO (230–320 g mol-1) could further justify and facilitate its industrial uptake. Table 3 compares the performance of PDA-containing OSN membranes. To further investigate the stability of the membranes, permeance has been measured at different temperatures in DMF (Figure 3B). The permeance progressively increases with increases in temperature. The tighter the membrane the less sensitive the permeance is to temperature-change. The permeance ranges for M1, M2 and M3 are 8–26, 5–18, 3–9 L m-2 h-1 bar-1, respectively.

The

fabricated

PDA/PBI

IPN

membranes

also

showed

competitive

nanofiltration performance in common organic solvents such as ethanol, hexane, ethyl acetate, acetonitrile, tetrahydrofuran, and toluene. The membrane permeance and MWCO data are summarized in the Supporting Information (Figures S7 and S8).

Table 3. Performance comparison for PDA-containing OSN membranes for which both solvent permeance and solute rejection were measured.


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Publication

Membran

PDA

e

incorporation

Page 16 of 49

Solvents

Permeancea

MWCO

(L m-2 h-1 bar-1)

(g mol1)

Zhao et al.,29

PDA/PES

2014 Pérez

Interfacial

ethanol

7.5

452

DMF

3.1

800

Interfacial

heptane,

1.8

380-

polymerization

toluene,

polymerization

et PDA/PAN

al.,16 2016 Mu et al.,30

Interfacial polymerization

PDA/PAN

2017

695

isopropanol, ethyl acetate Xu et al.,31

PDA/P84

Crosslinking

2017

alcohols,

0.5 (DMF)

974

9 (PC)

230

acetone, DMF, toluene

This work

PDA/PBI

In situ

HMPT,

DMF,

polymerization

DMAc,

DMSO,

NMP,

PC,

Cyrene® aIn

cases of using several solvents for filtration, the listed permeance denotes the

highest value in a polar solvent.


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Figure 3. Permeance for M1, M2 and M3 membranes in various polar aprotic solvents at 25 °C (A), and in DMF at temperatures ranging from –10 °C to 100 °C (B). Refer to the Supporting Information for solvent permeance correlation analysis.

Longer PDA polymerization times (M1  M2  M3) led to lower permeance due to decreased pore size and increased hydrophobic properties (Table 4). For the IPN and pristine PBI membranes, contact angle measurement was performed; this method has been proven a suitable technique to characterize membrane surface properties (Table 4). The contact angle increased with the polymerization time, resulting in membranes with increased hydrophobicity. Similar contact angle values were observed after PDA deposition on PE surfaces.32 Nitrogen sorption analysis for the pristine PBI showed values similar to those reported in the literature.33 The in situ PDA polymerization led to a two-fold increase in the pore volume and surface area for M1, which explains the high flux for PBI-based membranes. However, the increase in polymerization time gradually decreased the pore volume and surface area in tandem. Nanoindentation and tensile testing revealed a progressive improvement in the mechanical properties


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of the membranes as a result of increasing PDA content. The nanoindentation-derived hardness increased by 33–38%, while the Young’s modulus increased by 7–50%.

Table 4. Effect of in situ polymerisation of PDA on membrane hydrophilicity, BET surface area and pore volume, hardness from nanoindentation, Young’s modulus from tensile strength measurements and calculated pore size.

Parameters

M0

M1

M2

M3

Contact angle ()

48±1

60±2

73±3

76±1

PDA contenta (%)

0

1.11±0.0004

2.01±0.0011

3.10±0.0017

PDA growth rateb (%

0

0.36

0.49

0.61

Pore volume (mm3 g-1)

10.2±0.5

18.7±0.9

15.6±0.7

12.8±0.7

Surface area (m2 g-1)

22.4±0.7

42.1±1.4

34.2±1.1

28.7±0.9

Hardness (GPa)

0.24±0.0

0.32±0.03

0.33±0.03

0.35±0.03

day-1)

2 Young’s modulus (GPa)

1.4±0.1

1.5±0.1

1.7±0.2

2.1±0.2

Mean pore size (nm)c

N/A

1.4

1.3

0.8


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aPDA

content of membranes was determined by elemental microanalysis. Refer to

Supporting Information for calculations. b The

polymerization kinetics of PDA, calculated from % increase of PDA per day.

cMembrane

mean pore size was calculated from experimental MWCO data. Refer to

Supporting Information for calculations.

In addition, the calculated pore size also showed a decreasing trend with increasing PDA polymerization times (See Supporting Information for detailed calculations). Interestingly, it was possible to fine-tune the membrane pore size in the nano-range by controlling the extent of polymerization.

For instance, the pore size of the

membranes progressively decreased from 1.4 nm (M1) to 1.3 nm (M2) and 0.8 nm (M3), which corresponds to MWCO of 700 Da, 450 Da and 300 Da, respectively. It should be emphasized that controlling the MWCO of OSN membranes is one of the key challenges to improving the design of membrane-based processes,34 and this method offers a convenient way to precisely control the membrane separation properties. These results are in line with the changes in permeance presented in Figure 3.


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Elemental microanalysis showed an increasing PDA content with increasing polymerization time (Table 4). The growth rate of PDA also increased with time. It is hypothesised that the increasing PDA concentration on the surface acts as a selfcatalytic system for further PDA polymerization. PBI is soluble in PASs, while PDA is not. It is worthwhile to mention that as low a value as 1% of PDA (M1) prevents dissolution of the PBI membrane in PASs, even at temperatures as high as 100 C. All previous reports have employed PDA deposition techniques without the presence of monomers during membrane casting, and have performed crosslinking to achieve stability in PASs. On the contrary, our methodology featuring in situ PDA growth does not require covalent crosslinking. It is hypothesised that the presence of monomers in the dope solution eventually leads to better distribution of PDA at the molecular level, and subsequently to a larger number of secondary interactions between PDA and PBI. The exact structure of PDA and its interaction with surfaces is a highly debated topic.35 PDA can react with primary amines via the Schiff base reaction; however, the secondary aromatic amine group of PBI is not reactive enough.36,37 The pKa value of the 1[H] hydrogen on the benzimidazole moiety of PBI is 12.8 and basic nitrogen has a pKb of 5.6; and this low acidic behaviour prevents Schiff base formation.


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Secondary interactions between PDA and PBI must be strong enough to overcome the thermodynamically favoured solvation process of PBI. Using the NMR titration method, the self-association constant of PBI was approximated to be 5×10-4, which can be considered negligible (see Supporting Information). On the other hand, the self-association constant for the PDA system was found to be orders of magnitude higher (5.6). Investigation of the association between the different components of the system revealed that PDA–PBI interaction forms a stronger complex than do either PDA–solvent or PBI–solvent, with association constants of 131, 62 and 12, respectively (Table 5). These results support the hypothesis that PDA is attached to the surface of PBI and acts as a glue that prevents the dissolution of PBI in PASs. Theoretical modelling (B3LYP-D3 with basis set 6-311++G**) was performed on the model system to check whether the calculations verify the interaction differences for the three systems. The calculated relative energies were shifted with a constant value, as presented in Table 5, to make the comparison easier. The calculations reproduced the energy orders of the three systems. Consequently, the complex between PBI and PDA is more stable than the complexes between each compound and PAS.


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Table 5. Association constants (K), and measured and calculated Gibbs free energy (ΔG) values.

Parameters

PBI–PDA

PDA–DMAc

PBI–DMAc

K

131

62

12

-12.07

-10.23

-6.18

-14.73

-14.55

-5.55

ΔGmeasured [kJ mol1]

ΔGcalculated [kJ mol1]

FTIR was used to investigate possible chemical bond formations or secondary interactions occurring between PDA and PBI (Figure 4A). For pristine PBI, free N–H stretch and N–H···H hydrogen bond interaction can be found in the region of 3500– 2500 cm-1. The absorption peaks situated at 3415 cm-1 were attributed to stretching vibration of the isolated non-hydrogen bonds of the N–H group. The very broad peak located between 3100 cm-1 and 3350 cm-1, approximately centred at 3145 cm-1, can be assigned to self-associated hydrogen bonded N-H groups. Due to the planar cyclic and aromatic parts, the spectrum is composed of narrow peaks between 2000–


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1000 cm-1 as the NH and CN deformation modes (δNH and δCN). The characteristic aromatic C=C and C=N stretching modes are found between 1650 cm-1 and 1400 cm-1 as medium wide peaks. The corresponding peaks show similarity to those of the M3 membrane, except in the highlighted region between 1700–1500 cm-1. Cyclic unsaturated α,β-ketones and vicinal diols can be observed around 1500–1620 cm-1, which overlaps with the PBI peaks and increases the relative absorbance of the peaks. The characteristic peaks of the PDA at 1510 and 1596 cm-1 are beneath the abovementioned peaks of PBI.38 The absence of any difference between the spectra can be explained as resulting from the similar chemical structures of PDA and PBI. The same pattern can be observed between 3300–2600 cm-1, which correlates to the aforementioned different N–H vibrational wavenumbers. Owing to their similar chemical characteristics and small quantity of PDA (1–3%), the IPN and pristine PBI membranes, show no differences in their FTIR spectra. Therefore, solid-state NMR analysis was performed and, again, showed no difference between the IPN and pristine PBI membranes (see Supporting Information). It is worth noting that neither FTIR nor NMR revealed the formation of covalent bonds. The surface roughness of the membrane was not affected by the presence of PDA (Figure 4B and 4C), which


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can be explained by the low amount of 1–3 wt% PDA (see Table 4) in the membrane. In line with these observations, surface (Figure 4D and 4F) and cross-section SEM images (Figure 4E and 4G) of the pristine PBI and IPN membranes are virtually the same. The membrane thickness was found to be about 50 m.

Figure 4. FTIR spectra for PDA/PBI IPN (blue) and pristine (red) PBI membrane, and PP support (green) (A); Surface roughness of PDA/PBI IPN (B) and pristine (C) PBI membranes measured via AFM. SEM images of the PDA/PBI IPN (D-E) and pristine (F-G) PBI membranes, where panels D) and F) show the surface area, while panels E) and G) show cross-sections of the membranes at 5000 magnification. The presented data are typical for all IPN membranes (M1–M3).

Thermogravimetric analysis (TGA) provides useful information about membrane structure and thermal stability; coupled with GCMS, it enables elucidation of pyrolysis degradation products (Figure 5). At room temperature, an isothermal weight loss profile was observed, which can be attributed to the loss of solvent or moisture bound


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to the membrane surface. The subsequently measured temperature-dependent weight loss profile revealed polymer degradation for the pristine PBI and PDA/PBI IPN membranes. The 5% decomposition temperature values (Td5) revealed a distinct improvement in the thermal stability of the IPN membranes. The pristine PBI membrane has a Td5 of 175 °C; this value was doubled for the IPN membranes, reaching in a range of 341–360 °C. Improved thermal stability of IPNs has been observed in various fields.39 The α and β gas chromatograms (Figure 5B) and the mass spectra of the corresponding peaks taken at each decomposition step (Figure 5C and 5D) confirmed the presence of PDA. The PDA fragments found (Figure 5C) were identical to the ones reported in the literature.38 The presence of indolequinone with the peak at 147 m/z [M+H] at 8.7 min resident time can be derived only from the PDA. The peak at 119 m/z (8.7 min) can be attributed to the loss of a carbonyl (C=O) group from the indole ring . The characteristic benzimidazolium cation peak at 118 m/z [M+H]+ observed at 6.7 min is a result of PBI fragmentation. A twofold loss of HCN occurs due to fragmentation, which was observed at 91 m/z and 63 m/z.


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Figure 5. TGA weight loss profile of pristine PBI membrane (M0) and PDA/PBI IPN membranes (M1–M3): isothermal weight loss of solvent for 24 h and consecutive temperature domain weight loss of membranes (A). Sample-taking points for GCMS analysis are indicated with stars. Gas chromatograms for pristine PBI (α) and PDA/PBI IPN (β) membranes (B) and corresponding mass spectra (C–D).

In summary, FTIR and solid-state NMR analyses indicate that no covalent crosslinking has occurred; and yet, the unexpected enhancement in thermal stability as suggested by TGA shows that physical crosslinking — characteristic of IPNs — has taken place. Also, the presence of PDA within the membrane was confirmed by elemental microanalysis and TGA-GCMS. Based on the material analysis data, we can draw the conclusion that the membrane solvent stability in polar aprotic solvents without covalent crosslinking can be attributed to strong secondary interaction (PBI–PDA) and the formation of IPN working in tandem.


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Conclusions An in situ PDA polymerization methodology was successfully developed for the preparation

of

solvent-resistant

nanofiltration

membranes

comprising

an

interpenetrating polymer network. The dopamine monomer was encapsulated in PBI backbone followed by its polymerisation. The in situ polymerisation was found to be the key to fabricating solvent-resistant robust membranes. To fine-tune the MWCO values in the range of 190 to 850 g mol-1, the membrane pore size and the separation performance were nano-engineered by controlling the PDA polymerization time. The robustness and long-term stability of the membranes were successfully demonstrated in seven polar aprotic solvents in the temperature range of –10 °C to +100 °C. The proposed PDA polymerization methodology resulted in increased permeance without compromising the MWCO values. The proposed approach contributes to efforts towards sustainable membrane fabrication: PDA is a bio-inspired coating, and the methodology eliminates the need for conventional crosslinking to make PBI membranes stable in harsh environments. Permeance up to 12 L m-2 h-1 bar-1 in DMF was obtained, which is the highest reported value to date. The permeance for DMSO


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reached a value an order of magnitude higher than those of similar type membranes reported in the literature.


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Methods/Experimental

Materials and reagents Dopamine hydrochloride (DA–HCl, 99%) was purchased from Alfa Aesar (Lancashire, England). Tris(hydroxymethyl)aminomethane (Tris, 99.8–100%) was purchased from Acros Organics (Leicestershire, England). N,N-dimethylacetamide (DMAc, 99.9%, extra dry, argon flushed), chloroform-d (99.8%, extra dry), hexamethylphosphoric triamide (HMPT, 98%, purum), propylene-carbonate (PC, 99.7%), N-methyl-2pyrrolidone (NMP, 99%, ACS reagent), dimethyl sulfoxide (DMSO, 99%), N,Ndimethylformamide (DMF, 99.8%, ACS reagent), Cyrene® (99%), benzimidazole (99.9% recrystallised from chloroform), 1,2-dihidroxy benzene (99.9%, recrystallised from chloroform) and sodium periodate (NaIO4, 99.8%) were purchased from Sigma-Aldrich (Dorset, England). Polybenzimidazole dope solution (26 wt% in DMAc) was purchased from PBI Performance Products, Inc (Charlotte, USA). Novatexx 2471 polypropylene non-woven backing was purchased from Freudenberg Filtration Technologies (Germany). Polystyrene markers for solute rejection evaluation were


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purchased from Agilent Technologies. All materials and solvents were used as received without further purification.

Membrane fabrication Dope solutions comprising 20 wt% PBI (14 g) and 5 wt% dopamine monomer (3.5 g) were prepared in DMAc (52.5 g); this was followed by mechanical stirring of components at 50 rpm and 30 C for 6 h. The homogenised solution was degassed in an incubator shaker at 400 rpm and 30 C for 14 h. The dope solution was kept under inert atmosphere throughout the procedure. The viscosity of the dope solution prior to casting was 2531 ± 7 Poise. Membranes were cast onto non-woven polypropylene support using an Elcometer 4340 film applicator with a casting knife set to a thickness of 150 μm and casting speed of 5 cm s–1. The membranes were formed by precipitation of film in a coagulation bath containing deionised water (15.0 MΩ) at 20 C. The A4-size membrane sheets were kept in water for 0.3 h. Membrane discs of 100  100 mm size were immersed in 50 mL aq. 5 mM NaIO4 solution (53 mg) and aq. 10 mM Tris buffer (pH=8.5; 61 mg) solution for 3, 4 or 5 days each, resulting in membranes M1, M2 and M3, respectively. A control membrane (M0) was prepared by


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casting a 20 wt% PBI solution with a thickness of 250 μm, followed by phase inversion at 20 C. The M0 membrane was coated with PDA using the conventional PDA deposition technique: M0 was immersed in a 1 g L-1 dopamine monomer (50 mg) solution having a 10 mM Tris buffer (pH=8.5; 61 mg); this was followed by the addition of 5 mM NaIO4 solution (53 mg) to initiate PDA polymerisation and form M4.

Characterisation 1H

NMR measurements were recorded on a Bruker Avance III 400 MHz NMR

spectrometer using chloroform-d as a solvent and internal peak standard. NMR titrations were performed to explore secondary interactions; 1H-benzimidazole and benzene-1,2-diol were used to approximate PBI and PDA, respectively. Attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) spectra of membrane samples were acquired using a Nicolet iS5 run on air, mounted on a diamond plate. The spectra were recorded at a resolution of 4 cm-1 with an average of 32 scans and analysed by OMNIC Spectra Software. The nitrogen sorption measurements were performed on a Quantachrome Autosorb 6B automatic adsorption instrument. For the TGA-GCMS analysis, the membranes were dried using


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a desiccator and placed on a TGA platinum sample pan. For mass and temperature equilibration, ca. 2 min elapsed prior to commencement of data recording. TGA measurements were carried out on a TA Instruments Q500 in two successive steps with the same specimen. In the first step, the isothermal weight loss was recorded over 24 h. The sample was kept at a constant 25 °C under helium atmosphere (40 mL min−1). In the following step, the same specimen was heated to 625 °C at a rate of 20 °C min−1. The evaporated compounds were transferred by helium carrier gas from the TGA to the GCMS. Mixtures of substances retained from the decomposition step were separated on an Agilent DB-624UI column (30 m × 0.53 mm I.D., 3.0 μm film) and the components were individually identified in the MS. GC oven conditions were as follows: 40 °C//20 min//10°C/min//200 °C//20 min; carrier gas, helium 32 cm s−1 set at 40 °C. Membrane surface and cross-sectional microstructure were determined by scanning electron microscopy (SEM) using a Hitachi S-3000N with a tungsten hairpin filament emission gun at an accelerating voltage of 5 kV. Samples were prepared by sticking the membranes on a conductive carbon tab; samples were sputter coated with gold/palladium under an argon atmosphere using a Quorum Q150TES in order to make them conductive. Atomic force microscopy (AFM)


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images were acquired in tapping mode in air using a Digital Instrument Dimension 3100 with a Bruker TESPA-V2 probe with a nominal spring constant of 37 N m-1 and a nominal tip apex radius of 7 nm. Samples were prepared by using double-sided tape to stick the membranes on a glass slide. For roughness calculations, three membranes of each type, with areas of 25 μm2, were scanned and analysed with NanoScope Analysis software. The contact angles of the membranes were obtained by DSA100 contact angle measuring system (Krüss, Germany). Digital images captured by the camera were analysed using the drop analysis system. The final values of contact angle of each membrane were averages of 5 measurements. Elemental microanalysis measurements were performed on a Thermo Scientific Flash 2000 Organic Elemental Analyser. Mechanical strength was tested by nanoindentation on an aluminium substrate using an MTS Nanoindenter XP with a Berkovich tip. An Instron 1122 Universal Testing Machine with a 500 N load cell at 50% humidity was used for the tensile strength measurements.


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Membrane performance tests Membrane performance was tested in a cross-flow nanofiltration apparatus between 10 and 30 bar. Three independently prepared membrane discs of each type were tested, and the reported results are the mean values of these measurements. The effective area (A) of each membrane was 52.8 cm2. The permeance was calculated using Eq (3).

𝑃𝑒𝑟𝑚𝑒𝑎𝑛𝑐𝑒 =

𝐽 𝑉 = = [𝐿 𝑚 ―2ℎ ―1𝑏𝑎𝑟 ―1] ∆𝑃 ∆𝑃𝐴𝑡

(3)

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 the membrane per membrane area (A) per time (t). The model system for determining the molecular weight cut-off (MWCO) — the molecular weight of the compound that is 90% rejected — curve comprised a mixture of 1 g L-1 PS580 and PS1300 polystyrene markers and 0.1 g L-1 of divinylbenzene (130 g mol-1) and 0.1 g L-1 of methyl styrene dimer (236 g mol-1) solutions.40 The concentrations were ten times lower for ethanol due to marker solubility limitations. The rejection (R) of solutes was determined according to the ratio of their measured concentrations in the permeate (Cp) and the feed (Cf), as defined in


34

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Eq (4). Experiments were carried out in duplicate using independently fabricated membranes.

(

𝑅= 1―

)

𝐶𝑝 𝐶𝑓

∙ 100 = [%]

(4)

Membrane dissolution test Membrane pieces were dried in a desiccator under vacuum until constant weights were achieved. About 250 mg of dried membrane pieces were immersed in 25 mL DMF. The samples were agitated in an incubator shaker at either 25 °C or 100 °C for a week. The membrane pieces were filtered in a Buchner funnel, followed by extensive washing with acetone and eventual drying in a desiccator under vacuum until constant weight was achieved. The percentage weight loss was calculated based on the weights measured before and after the dissolution test. The experiments were carried out in duplicate using independently fabricated membranes.


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Associated Content

The authors declare no conflicts of interest.

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. Rejection and MWCO data, solvent permeance and MWCO correlation analysis, dielectric constant determination, elemental microanalysis calculations, solid state NMR, NMR titration data, photos of membranes, quantum chemical computations.

Author Information

ORCID

Dan Zhao: https://orcid.org/0000-0003-3822-1418

Jeong F. Kim: https://orcid.org/0000-0002-5575-4374


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Gergo Ignacz: https://orcid.org/0000-0002-7227-3070

Peter Pogany: https://orcid.org/0000-0003-3536-0746

Young Moo Lee: https://orcid.org/0000-0002-5047-3143

Gyorgy Szekely: http://orcid.org/0000-0001-9658-2452


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TABLE OF CONTENTS GRAPHICS

ACKNOWLEDGEMENTS The authors would like to express their gratitude to Dr. Christopher Blanford and Mr. Fan Fei (Manchester Institute of Biotechnology) for the dielectric constant measurements. Training and consultancy provided by Dr. Christos Didaskalou (The University of Manchester) is greatly acknowledged by Dan Zhao. This work was supported by the Biotechnology and Biological Sciences Research Council


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[BB/L013770/1]. This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 201820101066550 and No.20172010106170).

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Figure 1. Schematic overview of the membrane fabrication methodology for in situ polymerised polydopamine/polybenzimidazole interpenetrating polymer network: A) casting of a dope solution comprising of PBI and dopamine monomers, B) phase inversion in water, C) soaking the membrane in aqueous NaIO4 initiator, and D) in situ polymerisation of dopamine in a Tris buffer solution. Interpenetrating polymer network of linear PBI and branched PDA (E). Molecular level interactions between the PBI and PDA polymers (F). 59x54mm (300 x 300 DPI)


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Figure 2. The dependence of the molecular weight cut-off (MWCO) of the membranes on the solvent (A) and the temperature (B). Various polar aprotic solvents were tested at 25 °C, while the temperature was varied between –10 °C to 100 °C in DMF at 20 bar. Refer to the Supporting Information for the rejection values and MWCO curves. 131x264mm (300 x 300 DPI)


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ACS Nano

Figure 3. Permeance for M1, M2 and M3 membranes in various polar aprotic solvents at 25 °C (A), and in DMF at temperatures ranging from –10 °C to 100 °C (B). Refer to the Supporting Information for the solvent permeance correlation analysis. 121x227mm (300 x 300 DPI)


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Figure 4. FTIR spectra for PDA/PBI IPN (blue) and pristine (red) PBI membrane, and the PP support (green) (A); Surface roughness of PDA/PBI IPN (B) and pristine (C) PBI membranes via AFM. SEM images of the PDA/PBI IPN (D-E) and pristine (F-G) PBI membranes, where panels D) and F) show the surface area, while panels E) and G) show the cross-section of the membranes at 5000× magnification. The presented data are typical for all IPN membranes (M1–M3). 104x167mm (300 x 300 DPI)


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Figure 5. TGA weight loss profile of pristine PBI membrane (M0) and PDA/PBI IPN membranes (M1–M3): isothermal weight loss of solvent for 24 h and consecutive temperature domain weight loss of the membranes (A). Sample taking points for GCMS analysis are indicated with stars. Gas chromatograms for pristine PBI (α) and PDA/PBI IPN (β) membranes (B) and the corresponding mass spectra (C–D). 85x112mm (300 x 300 DPI)


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