Enhancing the performance of polybenzimidazole membranes ...

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Enhancing the performance of

polybenzimidazole membranes for

organic solvent nanofiltration

A dissertation submitted to The University of Manchester for the degree

of

Master of Research in the Faculty of Science and Engineering

2018

Gergo Ignacz

Supervisor: Dr Gyorgy Szekely

School of Chemical Engineering and Analytical Science

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Contents

List of Figures ............................................................................................................................ 4

List of Tables.............................................................................................................................. 4

Abstract ...................................................................................................................................... 8

Declaration ................................................................................................................................. 9

Copyright Statement .................................................................................................................. 9

Acknowledgements .................................................................................................................. 11

List of abbreviations ................................................................................................................. 12

1. Introduction ............................................................................................................... 14

1. Literature review ............................................................................................................... 18

1.1. Membrane processes ................................................................................................. 18

1.2. Organic Solvent Nanofiltration (OSN) .................................................... 19

1.3. Advantages and disadvantages of OSN ................................................... 21

1.4. Membrane characteristic and performance .............................................. 21

2. Crosslinking ............................................................................................................... 24

2.1. Crosslinking of polymers .......................................................................................... 24

2.2. Membrane crosslinking............................................................................ 26

2.3. Polybenzimidazole ................................................................................... 29

2.4. Polymers of Intrinsic Microporosity ......................................................................... 32

3. Objectives ......................................................................................................................... 38

4. Experimental ..................................................................................................................... 39

4.1. Materials .................................................................................................................... 39

4.2. Synthesis .................................................................................................................... 39

4.2.1. Synthesis of PIM-1 ............................................................................... 39

4.2.2. Synthesis of PIM-COOH ..................................................................... 41

4.3. Membrane fabrication ............................................................................................... 41

4.3.1. Preparation and crosslinking of PBI–PIM-COOH membranes ........... 41

4.3.2. Preparation and crosslinking of PBI–PIM-amine membranes ............. 43

4.4. Solubility test ............................................................................................................. 44

4.5. Nanofiltration procedure ........................................................................................... 45

4.6. Chemical characterisation ......................................................................................... 46

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4.6.1. Attenuated Total Reflection (ATR) Fourier-transform infrared

spectroscopy (FTIR) ........................................................................................... 46

4.6.2. Atomic Force Microscopy (AFM) linked Infrared spectroscopy ........ 46

4.6.5. Morphological characterisation ............................................................ 48

4.6.6. Brunauer–Emmett–Teller (BET) adsorption measurement ................. 48

4.6.7. Scanning Electron Microscopy (SEM) ................................................ 48

4.6.8. Contact angle measurement ................................................................. 48

5. Results and discussion ...................................................................................................... 49

5.1. Solubility test and membrane crosslinking ............................................................... 49

5.2. Nanofiltration testing ................................................................................................. 52

5.3. Exploring the structure of the PIM-amine membranes ............................................. 58

Conclusion ............................................................................................................................... 67

Appendix .................................................................................................................................. 68

Final word count: 17035

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List of Figures

Figure 1. Total energy consumption distribution by the four biggest factors., .......... 14

Figure 2. Energy consumption of the different separation techniques by the industry.6

.................................................................................................................................... 15

Figure 3. The two most common rig separation techniques. Cross-flow filtration (a)

and dead-end filtration (b). ......................................................................................... 19

Figure 4. Different filtration techniques by size. ....................................................... 20

Figure 5. Rejection profile as a function of molecular weight and visual

representation of MWCO 90% with black dashed line. ............................................. 23

Figure 6. Visual representation of a membrane screening system. ............................ 24

Figure 7. Schematic representation of a crosslinking procedure between two identical

polymer chains. The orange lines are the polymer chains with a chemically reactive

side. The green parts are the chemical crosslinkers. Intramolecular crosslinking can

also happen. ................................................................................................................ 26

Figure 8. Schematic representation of an integrally skinned asymmetric polymer

membrane. A) Compact active layer. B) Porous sub layer with macro voids. C) Non-

woven backing. .......................................................................................................... 27

Figure 9. The structure of PBI; a) schematic model, b) 3D model. ........................... 30

Figure 10. Crosslinking of PBI with DBX as reported by Valtcheva et al.40 ............ 31

Figure 11. Crosslinking of PBI with DEO as reported by Xing et al.70 ..................... 32

Figure 12. Visual representation of the PIM-1’s structure.73 ..................................... 33

Figure 13. Schematic representation of the PIM-1 synthesis from its monomers. .... 35

Figure 14. Hydrolysis of PIM-1 as reported by Weng et al. The reaction takes place

at high temperature. .................................................................................................... 36

Figure 15. Reduction of PIM-1 using Me2S.BH3 complex as a reducing agent. ....... 36

Figure 16. Synthesis of PIM-1. .................................................................................. 39

Figure 17. Synthesis of PIM-COOH from PIM-1. ..................................................... 41

Figure 18. Schematic representation of the crosslinking between a carboxylic acid

and the benzimidazole. ............................................................................................... 42

Figure 19. Reduction of PIM-1 to PIM-amine. .......................................................... 44

Figure 20. PIM-amine based membranes fabricated.................................................. 44

Figure 21. Schematic representation of the AFM-IR. ................................................ 47

Figure 22. ATR-FTIR image of the M1, M2, M3, M4 membranes and the PIM-

COOH and PBI polymer. ........................................................................................... 51

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Figure 23. ATR-FTIR image of the M6, M7, M8, M9, M10 membranes and the

PIM-amine and PBI polymer. .................................................................................... 52

Figure 24. Permeance values of M1, M2, M3, M4 represents the average of three

independent samples. ................................................................................................. 53

Figure 25. 4 markers testing with M1 (black), M2 (pink), M3 (blue), M4 (yellow). 53

Figure 26. Measured permeance in different solvents. M5 (black); M6 (blue); M7

(pink); M8 (yellow). The relative permittivity of the listed solvents is increasing

from the left to the right. For details, see Appendix Table 1. .................................... 55

Figure 27. Measured MWCO in different solvents. M5 (black); M6 (blue); M7

(pink); M8 (yellow). For details, see Appendix Table 2. The error bars represent the

deviations of three independent measurements.......................................................... 56

Figure 28. Permeance dependence in the function of temperature in case of M8. For

details, see Appendix Table 3. The error bars represent the deviations of three

independent measurements. ....................................................................................... 57

Figure 29. MWCO dependence as a function function of temperature in case of M8.

For details, see Appendix Table 3. ............................................................................. 57

Figure 30. AFM-IR measurement of the M9 membrane. A) height; b) IR-amplitude;

c) IR-peak. The assorted colours denote different depth. For a full-size picture with

colour bars see Appendix Figure 9-12. ...................................................................... 59

Figure 31. SEM image of M6; a) surface (x1000), b) surface (x5000), c) surface

(x10000), d) cross-section (x1000), e) cross-section (x5000), f) cross-section

(x10000). .................................................................................................................... 60



(x10000). .................................................................................................................... 61



(x10000). .................................................................................................................... 61



(x10000). .................................................................................................................... 62



(x10000). .................................................................................................................... 63

Figure 36. BET surface area plot with respect to the PIM-amine concentration. ...... 64

Figure 37. ssNMR spectra of the PBI (red), M5 (pink) and the M8 (blue)

membranes. The highlighted area between 40–60 ppm shows the aliphatic carbon

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peaks in case of M8. 13C NMR (101 MHz) δ 150.52, 141.89, 134.06, 128.31,

120.89, 111.53, 110.49. .............................................................................................. 65

Figure 38. 1H-NRM of PIM-1. ................................................................................... 69

Figure 39. 13C-NMR of PIM-1. .................................................................................. 69

Figure 40. 1H-NRM of PIM-COOH. ......................................................................... 69

Figure 41. 13C-NMR of PIM-COOH. ........................................................................ 70

Figure 42. AFM height map of M4 with colour bar. ................................................. 72

Figure 43. AFM-IR IR-amplitude map of M4 with colour bar. ................................. 72

Figure 44. AFM-IR IR-peak map of M4 with colour bar. ......................................... 73

Figure 45. Height mapping of M4.............................................................................. 73









Figure 54. Contact angle picture of M1. .................................................................... 77




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List of Tables

Table 1. Advantages and disadvantages in OSN technology.20 ................................. 21

Table 2. Polar aprotic solvents in OSN processes in chronological order. ................ 28

Table 3. Different type of PIMs by structure in the literature. ................................... 34

Table 4. Modified PIM-1 polymers by structure in the literature. ............................. 35

Table 5. Organic solvent nanofiltration membranes used in the literature in

chronological order. ................................................................................................... 37

Table 6. PBI–PIM-COOH based membranes prepared. ............................................ 42

Table 7. Properties of polar aprotic solvents used in solubility tests. *Relative

permittivity; measured by refractive index measurement **Calculated data. ........... 45

Table 8. Prepared membranes and the specific crosslinking agents used in the

procedure. One asterisk denotes light swelling on the membrane surface. Double

asterisks denote heavy swelling on the surface. ......................................................... 50

Table 9. Solubility test of the different membranes. Negative sign denotes the

insolubility of the membrane while positive sign denotes the solubility of the

membrane in the specified solvent. ............................................................................ 50

Table 10. Predicted and measured BET surface areas of M5, M6, M7 and M8. The

prediction based on a simple percentage calculations respect to the PIM content in

the membrane. ............................................................................................................ 64

Table 11. Measured contact angles for M5, M6, M7 and M8 respectively. .............. 66

Table 12. Properties of polar aprotic solvents used in solubility tests. *Relative

permittivity; measured by refractive index measurement. ......................................... 70

Table 13. Solubility test of the different membranes. Negative sign denotes the

insolubility of the membrane while positive sign denotes the solubility of the

membrane in the specified solvent. ............................................................................ 70

Table 14. Permeance summary of M5, M6, M7 and M8. .......................................... 71

Table 15. MWCO summary of M5, M6, M7 and M8................................................ 71

Table 16. Permeance and MWCO dependence in the function of temperature of M8.

.................................................................................................................................... 71

Table 17. Contact angle measurement of M5, M6, M7 and M8, respectively. .......... 76

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Abstract

Organic solvent nanofiltration is a thriving alternative liquid-liquid membrane

separation method. However, the process faces different challenges and one of them

is the weak solvent resistance of the membranes and the low permeance of the

crosslinked membranes. The aim of this work to enhance the permeance of OSN

membranes in dipolar aprotic solvent. To overcome this problem, chemical

stabilisation of membranes has been carried out on a PBI-PIM type polymer-blend

membrane. The high internal volume of the PIM polymer increased the originally

low permeance of the PBI.

The permeance results range between 0.5–2.5 L m-2 h-1 bar-1 and the MWCO results

range between 190–600 g mol-1. In terms of permeance and MWCO, the PIM-amine

containing PBI polymer blends show superior performance over the PIM-COOH

containing ones. The 12% PIM-amine containing membrane has an outstanding

permeance value with 2.0 L m-2 h-1 bar-1 in DMF, which is slightly higher than the

previously reported crosslinked PBI membranes.

Using a model reaction and IR spectroscopy, the chemical crosslinking has been

rejected and the stability of the membrane assigned to salt formation. AFM-IR, SEM

and contact angle measurement have been used to characterise the imperfect

membranes. On top of this, they have been used to characterise the morphology of

the membranes and provided further evidence to explain the improved properties.

In terms of the increased permeance and stability result can offer alternative

solutions for industrial separation processes.

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Declaration

No portion of the work referred to in the dissertation has been submitted in support

of an application for another degree or qualification of this or any other university or

other institute of learning.

Copyright Statement

a) The author of this dissertation (including any appendices and/or schedules to

this dissertation) owns certain copyright or related rights in it (the

“Copyright”) and s/he has given The University of Manchester certain rights

to use such Copyright, including for administrative purposes.

b) Copies of this dissertation, either in full or in extracts and whether in hard or

electronic copy, may be made only in accordance with the Copyright,

Designs and Patents Act 1988 (as amended) and regulations issued under it

or, where appropriate, in accordance with licensing agreements which the

University has from time to time. This page must form part of any such

copies made.

c) The ownership of certain Copyright, patents, designs, trademarks and other

intellectual property (the “Intellectual Property”) and any reproductions of

copyright works in the dissertation, for example graphs and tables

(“Reproductions”), which may be described in this dissertation, may not be

owned by the author and may be owned by third parties. Such Intellectual

Property and Reproductions cannot and must not be made available for use

without the prior written permission of the owner(s) of the relevant

Intellectual Property and/or Reproductions.

d) Further information on the conditions under which disclosure, publication

and commercialisation of this dissertation, the Copyright and any Intellectual

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Property and/or Reproductions described in it may take place is available in

the University IP Policy, in any relevant Dissertation restriction declarations

deposited in the University Library, The University Library’s regulations and

in The University’s policy on Presentation of Dissertations.

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Acknowledgements

I would like to express my sincere gratitude to my supervisor, Dr. Gyorgy Szekely

who provided the opportunity to work in his research group. I would also like to

thank him for his support, help and advice throughout this year. Furthermore, I

would like to thank Mr. Levente Cseri for all the help and advice during the project. I

would like to thank Ms. Hai Anh Le Phuong for all the received motivation and help

during the entire year. Without Them, the dissertation would be much less in content

and in style.

I would like to thank Mr. András Németh with the kind help in the synthesis of the

polymers. I would like to thank Dr. Rupesh Bhavsar for his kind help with the BET

and GPC measurement. I would like to thank Mr. Fan Fei for his kind help with the

AFM-IR measurement.

Finally, I would like to express my profound gratitude to my loved ones for their

continuous support and encouragement throughout my studies. This achievement

would not have been successful without them.

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List of abbreviations A Area

AFM Atomic force microscope

ATR Attenuated total reflectance

CF Feed concentration of the solute (or the marker)

COMU (1-Cyano-2-ethoxy-2-oxoethylidenaminooxy)dimethylamino-

morpholino-carbenium hexafluorophosphate

CP Permeate concentration of the solute (or the marker)

CPMAS Cross polarisation magic angle-angle spinning (NMR)

CR Retentate concentration of the solute (or the marker)

DBX α,α’-Dibromo-p-xylene

DCM Dichloromethane

DEO 1,2,7,8-Diepoxyoctane

DIC N,N′-Diisopropylcarbodiimide

DMAc N,N’-Dimethylacetamide

DMF N,N’-Dimethylformamide

DMSO Dimethyl-sulfoxide

EDX energy-dispersive atomic X-ray spectroscopy

FDA Food and Drug Administration

FTIR Fourier-transform infrared spectroscopy

GPC Gel permeation chromatography

HMPT Hexamethyl phosphoramide

HPB Hexaphenylbenzene

ISA Integrally skinned asymmetric (membrane)

J Flux

LogP Represent the affinity of a molecule or a moiety for a lipophilic

environment

MeCN Acetonitrile

MEMs Membranes with extrinsic microporosity

MeOH Methanol

MeTHF 2-Methyltetrahydrofurane

MIMs Membranes with intrinsic microporosity

MMM Mixed matrix membrane

MW Molecular weight

MWCO Molecular weight cut off

NMP N-methyl-2-pyrrolide

NMR Nuclear magnetic resonance spectroscopy

P Permeance

P84 Brand name of the polyimides manufactured by Evonik Fibres

PAN Polyacrylonitrile

PBI Polybenzimidazole

Pc Phtalocyanin

PDA Polydopamine

PDDA Poly(diallyldimethylammonium chloride)

PEBAX Brand name for Polyether block amide polymer (Arkema)

PI Polyimide membrane

PIM Polymers of intrinsic microporosity

pKa The negative base (-10) logarithm of the acid dissociation constant

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(Ka) of a solution

pKb The negative base (-10) logarithm of the base dissociation constant

(Kb) of a solution

PSS Poly(styrenesulfonate)

PTFE Poly(tetraflouroethane)

PVS Polyvinyl siloxane

SEM Scanning electron microscope

SPEEK Sulfonated poly(ether etherketone)

ssNMR Solid-state nuclear magnetic resonance spectroscopy

OSN Organic solvent nanofiltration

t Time

TFC Thin layer composite (membrane)

THF Tetrahydrofurane

�̇� Volume flow rate

V Volume

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1. Introduction

Classic thermal separation techniques including crystallisation, distillation and

adsorption require between 40–70% of the total energy consumption for the

industrial production of chemicals and fuels.1 To avoid such methods with high

energy consumption, new greener separations could be utilised. With the rising

pressure to find a suitable separation and purification method for chemical

compounds, organic solvent nanofiltration (OSN) turned out to be a thriving

alternative liquid-liquid separation technique in the pharmaceutical and chemical

industry.2 The rising demand for a comprehensive utility of OSN puts scientists

under the pressure to develop chemical processes which could be used under harsh

environments, including high temperature and pressure, reactive chemicals and

different solvents. Due to the intensive research in the field, today’s membranes can

separate molecules with a difference only 1.05–1.5x in size.3 Membrane

technologies are state of the art for seawater desalinations and also could be

considered as an uprising technology for gas separation technologies in the oil and

gas industry.

Figure 1. Total energy consumption distribution by the four biggest factors.4,5

Commercial

19%

Residential

21%Industrial

32%

Transportation

28%

Total energy

consuption

29 400 TWh

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According to Sholl and Lively, the total energy consumption is more than

29 400 TWh globally.6 This can be divided to four main subparts, namely the energy

consumed by transportation (28%), commercially used (19%), residentially

used (21%), and industrially used energy (32%), respectively (Figure 1). Around 45–

50% of the used energy by the industry comes from the separation processes which

mostly contains thermally driven processes such as distillation, drying and

evaporation.4 Figure 2 shows the energy consumption of different separation

consumption. The three main parts are called the thermally driven separation

processes namely the distillation (49%), drying (20%) and evaporation (11%). The

three thermally driven separation consumes more than 3300 TWh in total, although,

using membrane based separation this value would be only 330 TWh which is less

than 90% used energy in total.6 Thermodynamics defines the lowest amount of

energy needed to separate contaminants from a solution and distillation uses 50 times

more energy than the calculated minimum.5

Figure 2. Energy consumption of the different separation techniques by the industry.6

Thinking in on industrial scale sometimes other factors have to be taken into

account. For example, the well-known reverse osmosis technology uses 25% less

energy than the evaporation technology.7 Nevertheless, the limited water production

Destillation

49%

Drying

20%

Evaporation

11%

Non-thermal

20%

Separation processes

4200 TWh

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rate requires large plants to fulfil the supply and demand in metropolitan cities such

as Jerusalem. Other problems, such as increased corrosion rate from the salty water,

the so called “membrane fouling” are also considerable aspects when it comes to

design a plant. Nonetheless, in the Middle East (Israel Desalination Enterprises)8 and

in Australia (Perth Seawater Desalination Plant) reverse osmosis of seawater is

already done on commercial scales.

However, there are just a few applications from the pharmaceutical industry since

organic solvent nanofiltration processes are labelled as new technologies hence the

industry’s trust is quite low.9 Although, introducing the OSN to the pharmaceutical

industry would relish great improvements because this sector uses the most solvents

and also requires the purest final product compared to other industries.10 A

concerning problem is that, according to the Food and Drug Administration (FDA)

of the United States, dipolar aprotic solvents are commonly used in the

pharmaceutical industries. Dipolar aprotic solvents, such as N,N-dimethylformamide

(DMF), N,N-dimethyl acetamide (DMAc), N-methyl-2-pyrrolidone (NMP) or

dimethyl sulfoxide (DMSO) are listed as one of the least green solvents due to their

toxicity, high boiling point and high water solubility.11 In recent times, the safety and

environmental health concerns put the engineers and scientists under the pressure to

develop reactions and technologies which covers the required green chemistry

aspects. Mostly, within the pharmaceutical and fine industry, to replace an existing

solvent with a new one is challenging. The legalisation and registration procedures

are strictly forbidding to switch to another solvent, on the other hand, to make the

new solvent to be accepted in the registration book cost enormous amount of money

and time. Typical bio-based solvent are the bio-ethanol and bio-glycerol12 or the

more aprotic 2-methyltetrahydrofurane.13 Commonly used hydrocarbon solvents are

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the terpenoids.14 There are only few examples in the literature for the replacement of

the classic highly polar aprotic solvents.15,16 Until a green and suitable green solvents

will be found for replacement, the researchers are keen to develop new techniques

for the safe handling and reuse for the “old” ones. For example, NMP is a widely

used in pharmaceutical downstream processing and in formulation techniques,

however, due to its toxicity NMP is on the list of the European candidate list of

substance of very high concerns.17 In 2011, Sherwood et al. proposed Cyrene as a

bio-based alternative solvent to replace NMP as a dipolar aprotic solvent.16

Unfortunately, due to the difficult production, the price of Cyrene is extremely high.

For example, one unit of NMP costs around £13/L meanwhile one unit of Cyrene

costs around £3000/L (industrial scale; December 2017).18 Therefore, a grand

engineering challenge is to find suitable technologies not just to develop new

solvents but to find acceptable route to reuse the old ones.

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1. Literature review

1.1. Membrane processes

Membrane separation is a filtration based mechanism where the solutions are

separated by a barrier, called a membrane.19 The membrane is responsible for the

actual separation procedure and the ability to distinguish between different shaped

and sized molecules makes them usable in industrial and research applications.2

These factors which can affect the membrane properties are the shape and size of the

solute (usually classified in terms of the molecular weight, MW) which covers the

length and width of the molecule as well.20 Different physicochemical aspects such

as hydrophilicity (or hydrophobicity) of the solvated molecule, acid-base properties

of the molecules (pKa-pKb) also influence the membrane performance. Less well-

known factors which can affect the permeability of the molecule, are the number of

the heteroatoms and the cyclic moieties in the molecule (these factors also changes

the lipophilicity of the molecule, logP etc).21

Figure 3 shows the difference between the two most common filtration methods.

When the system is operated in cross-flow mode the solvent is continuously entering

to the unit (Feed stream) where the actual filtration happens. When the system

operates in dead-end method, one specific volume of the solution filtered on the

membrane.20 Since OSN is based on pressure (energy required to reach an exact

separation effectiveness) the concentration difference near the membrane surface

will require a higher energy to reach the same separation factor. Due to the unequal

separation factor towards different molecules the concentration difference between

different molecules on the two side of the membranes will increase the osmotic

pressure which has a negative effect on the separation. This phenomenon is called

concentration polarisation.22

19

Retentate stream is the solution which could not permeate the membrane. The

permeate stream is the solution which could permeate the membrane. According to

the mass balance the feed stream is equal to the sum of the retentate and the

permeate solutions when the system operates in steady state and no reaction happens

on the membrane material. Increasing the pressure of the system the permeate flux

will increase. In the case of polymeric membranes this increase is not linear with the

pressure since the membrane material also can be pressurized which will decrease

the flux. This phenomenon is called membrane compacting and is biggest if the

original membrane has comparatively low density.

Figure 3. The two most common rig separation techniques. Cross-flow filtration (a) and dead-end

filtration (b).

1.2. Organic Solvent Nanofiltration (OSN)

OSN technology (also called as solvent resistant nanofiltration) is an emerging

pressure driven membrane separation method based on the original nanofiltration

technology which is able to separate molecules between the range 150–2000 g mol-1

in specific organic solvents. Organic solvents are commonly used in the

pharmaceutical and in the chemical industry. Owing to the comparatively high value

and toxicity they should be recovered and also removed from the reaction mixture

and from the product as well.23 Figure 4 shows the different filtration techniques by

size. Particle filtration includes particles above 1 µm (sand, turf, dust etc).

Microfiltration techniques are able to remove different living microorganism such as

bacteria and viruses although individual molecules, molecule clusters and proteins

20

are able to permeate the microfiltration membranes.24 The aforementioned molecules

are in the range of the ultrafiltration and OSN. Ultrafiltration membranes can

separate molecules in the range 0.1–0.01 µm while OSN based processes can

separate molecules in the range of 1–10 nm. Reverse osmosis membranes are able to

separate the clean water from different ions which are usually sodium chloride ions

thus it can be used desalination.25 The used pressure is in inverse proportion to the

particle size and the pressure for OSN is between 5–40 bar. It is worth to mention

that an OSN membrane might be used for example in a microfiltration process, but

the efficiency will decrease due to the high fouling. Further on this, the used higher

pressure also will lead to increased operating costs.

Figure 4. Different filtration techniques by size.26

Due to the ideal separation range, OSN is commonly used in various applications

such as solvent recovery,27 active pharmaceutical ingredient purification,23 catalyst

recovery28 or continuous solvent exchange method.29

21

1.3. Advantages and disadvantages of OSN

The benefits and drawbacks of using OSN are summarized in Table 1. The main

advantage is that the OSN is based on a pressure driven mechanism therefore

requires lower energy to operate compared to traditional separation methods such as

crystallisation and distillation.

Table 1. Advantages and disadvantages in OSN technology.20

Advantages Disadvantages

Not thermally driven separation Comparatively low separation factor

Easy to scale-up Membrane fouling

Easy to connect to other technologies Limited solvent/pH range

Does not require additives Limited temperature range

Recovery of solvent/catalyst High solvent consumption

Can operate in continuous mode Low product yield respect to target

compound

Comparatively long procedure

One of the main disadvantages is that OSN membranes are usually limited to

specific solvents. Typically, dipolar aprotic solvents are able to dissolve the

membranes, therefore they are not suitable to use in OSN. On the other hand, dipolar

aprotic solvents are among of the most frequently used solvents in different

industries since their superior solvent properties and stability at high temperature. It

should be mentioned that different type of membranes are stable in different solvents

so in theory one would be able to find specific type of membranes for any kind of

purpose. On the other hand, due to their variant structure different type of

membranes dispose rather dissimilar characteristic (see Chapter 1.4).

1.4. Membrane characteristic and performance

The amount of matter flowed through a specific area (here volume) in a given time

when there is no reaction happening, called flux (referred as 𝐽).

𝐽 =𝑉

𝐴∙𝑡 Eq. 1.

22

Where 𝑉 is the volume of the liquid permeating through the membrane. 𝐴 is the

active area of the membrane and 𝑡 is the time component. In membrane technology,

a convenient unit for the flux is L m-2 h-1.

As flux is dependent on pressure, the more convenient permeance is advised to use

in the characterisation of the membrane performance. Permeance is the amount of

matter (here volume) flowed through a specific area in a given time at a given

pressure when there is no reaction happening. Herein, 𝑃 denotes the permeance with

the following formula:

𝑃 =𝑉

𝐴∙𝑡∙𝑝=

𝐽

𝑝 Eq. 2.

Where 𝑉 is the volume of the permeate. 𝐴 is the active area of the membrane, 𝑡 is the

time component and 𝑝 is the trans pressure. In membrane technology, a convenient

unit for the permeance is L m-2 h-1 bar-1. Permeance allows us for the direct

comparison of different membranes.

Rejection is a parameter that shows the ability of a membrane to retain a specific

molecule. Rejection is expressed in terms of percentage.

𝑅 = 100% ∙ (𝐶𝐹−𝐶𝑃

𝐶𝐹) ≅ 100% ∙ (1 −

𝐶𝑃

𝐶𝑅) Eq. 3.

Where 𝑅 is the rejection, 𝐶𝐹 is the feed concentration of the solute, 𝐶𝑃 is the

permeate concentration of the solute, 𝐶𝑅 is the retentate concentration of the solute.

If �̇�𝐹 ≫ �̇�𝑃 then 𝐶𝐹 ≅ 𝐶𝑅, which is a substantive approximation.

Molecular weight cut off (MWCO) is parameter to specify the selectivity of a

membrane. MWCO is the molecular weight of the smallest reference compound that

23

is rejected by 90% or 80%, depending on the definition. Rejection can be derived

from the rejection curves of solutes representing the membrane’s rejection of

components with increasing molecular weight (Figure 5). Here, MWCO always

refers to the 90% rejection model. The purple line has an ideal MWCO at 500 g mol-

1 which is a theoretical case; the yellow line has a MWCO with 300 g mol-1. The

membranes denoted by blue and orange lines both have an MWCO around

750 g mol-1, however, the gradient of the slope is significantly higher than in the case

of the “blue” membrane. Therefore, the separation will be more efficient in the case

of the “orange” membrane.

Figure 5. Rejection profile as a function of molecular weight and visual representation of MWCO 90%

with black dashed line.

The MWCO depends on the parameters which can affect the permeability of the

membrane. In addition, MWCO strongly depends on the measurement methods and

therefore the used reference molecule to measure the MWCO. Comparing

membranes just based on their MWCO can lead to false results.

Figure 6 shows a typical membrane screening setup. A pump provides pressure and

flow rate to the membrane where the separation happens. Both the retentate and the

24

permeate streams are taken back to the feed tank. As mentioned before, the

difference between the retentate and the feed concentration can be disregarded if the

flow rate provided by the pump is significantly higher than the permeate flow rate.

This simplification allows to measure just the concentration difference between the

permeate and the feed concentration and use these two data to calculate the rejection.

Figure 6. Visual representation of a membrane screening system.

2. Crosslinking

2.1. Crosslinking of polymers

In this dissertation the term “crosslinking” or “crosslinked” always refers to the

synthetic polymer chemical crosslinking or chemically crosslinked polymer.

Connection of one polymer chain to another via a specific bond is called

crosslinking.30 The crosslinking can form during or after the polymerisation

procedure. The crosslinking bond can be both covalent or ionic depending on the

actual characteristic of the reaction and the reagents. Crosslinking changes the

polymer properties depending on the crosslinking efficiency (or crosslinker density)

and the type of the crosslinker. After the crosslinking the polymers chains partially

or totally lose their ability to move as individual molecules which will cause a

25

difference in the physical features of the polymer usually affecting the chemical

properties.32 Highly crosslinked polymers usually have rigid structure with an

increased glass transition temperature. The features of the medium and low

crosslinked polymers can vary depending on the actual material. A typical example

is the vulcanisation procedure of the natural rubber or the similar compounds into a

more durable polymer by addition of sulfur. The sulfur reacts with the specific part

of the cis-polyisoprene to form a non-viscous and stable polymer called vulcanised

rubber.31 Figure 7 shows the schematic representation of a general crosslinking

procedure between two chemically identical polymer chains and the crosslinking

agent. The most used chemical crosslinking agents are usually the symmetrical

bifunctional reagents with the general molecular structure of X—X (where the X

denotes the reactive part of the molecule).32 Since the reagent is significantly smaller

than the polymer chain, therefore in case of small loading of crosslinking agent the

weight difference will be negligible before and after the crosslinking. However, the

difference between the structure of the polymer and crosslinking agent will cause a

change in the hydrophilic character of the membrane/polymer material. Specific

separation techniques, including the OSN, will sense the difference in terms of

permeance and rejection between the membranes before and after the crosslinking.

Also, this difference can be measured via contact angle measurement.33

26

Figure 7. Schematic representation of a crosslinking procedure between two identical polymer chains. The

orange lines are the polymer chains with a chemically reactive side. The green parts are the chemical

crosslinkers. Intramolecular crosslinking can also happen.

2.2. Membrane crosslinking

OSN is still facing different problems which need to be overcome to fulfil the

requirements from the industry.20,34 Among them, is to produce membranes which

can endure the operation environment- in harsh solvents. To address this

problem,35,36,37 researchers are trying to find new materials38 or improving the

existing ones.39 The crosslinking of different membranes for OSN has been

extensively studied for different membrane materials, such as integrally skinned

asymmetric polyimide membranes (P84 PI),43,49,50 polyacrylonitrile membranes,51

polybenzimidazole based membranes.40

Table 2 summarizes the different membranes used in polar aprotic solvents in the

literature. DMF is widely used since it is the cheapest dipolar aprotic solvent and

also commercially available.41 Integrally skinned asymmetric membranes (ISA),

which are in the focus of this dissertation, are tested the most.42 Integrally skinned

membranes: the pore structure gradually changes from very large pores to very fine

pores, essentially forming a “skin” on top of the membrane, giving rise to the name

“integrally skinned” (Figure 8.).

27

Figure 8. Schematic representation of an integrally skinned asymmetric polymer membrane. A) Compact

active layer. B) Porous sub layer with macro voids. C) Non-woven backing.

1

Table 2. Polar aprotic solvents in OSN processes in chronological order.

Membrane type Membrane material Solvent Permeance Marker Marker MW Rejection Ref. Year

Lm-2h-1bar-1 g mol-1 %

Polymeric ISA Cross-linked P84 DMF 8.3 Styrene oligomers 236-1800 95 (1800 g mol-1) 39 2007

Polymeric ISA Cross-linked P84 PI DMF 1.6 Styrene oligomers 236-1200 95 (236 g mol-1) 43 2008

Polymeric TFCs via coating Polypyrrole/PAN-H DMF 0.05 Rose Bengal 1017 98 44 2008

Polymeric TFCs via coating Segmented polymer networks / PAN-H DMF 2.7 Rose Bengal 1017 96 45 2008

MMM ISA Cross-linked P84 Pl/TiO2 DMF 2.1 Styrene oligomers 236-1200 90 (236 g mol-1) 46 2009

Polymeric TFCs via coating (PS-b-PEO/PAA)/alumina DMF 0.02 PEG 200–900 78 (370 g mol-1) 47 2010

Polymeric TFCs via coating (SPEEK/PDDA)/ PAN DMF 0.05 Rose Bengal 1017 92 48 2010

Polymeric ISA Cross-linked HT PI DMF 1.6 Styrene oligomers 236-1200 92 (236 g mol-1) 49 2011

Polymeric ISA Crosslinked PI DMF 3.7 Styrene oligomers 236-1200 92 (420 g mol-1) 50 2011

Polymeric ISA Cross-linked PANI DMF 0.6 Styrene oligomers 236-1200 95 (300 g mol-1) 51 2012

Polymeric TFC via interfacial

polymerisation

PA/cross-linked P84 PI DMF 1.5 Styrene oligomers 236-1200 91 (236 g mol-1) 52 2012

Polymeric TFCs via coating (PDDA/PSS)/PAN (PDDA/PVS)/PAN DMF 0.2 Rose Bengal 1017 99 53 2012

Polymeric TFCs via coating Cross-linked PI/cross-linked PI DMF 0.7 Sudan Blue II 350 46 54 2013

Polymeric TFCs via coating (PDDA/SPEEK)/PAN-H/Si DMF 0.07 Rose Bengal 1017 89 55 2013

Ceramic Inopor TiO2/Alumina ACN 6.0 3-nitro-2-pyridinethanol 156 38 56 2013

Polymeric ISA Crosslinked PBI DMF 0.4 Styrene oligomers 236-1200 99 (236 g mol-1) 57 2013

Polymeric ISA Cross-linked Matrimid DMF 3.92 Tetracycline 444 90 58 2014

Polymeric ISA Cross-linked P84 PI DMF 0.2 Styrene oligomers 236-1200 90 (236 g mol-1) 59 2015

Polymeric ISA Crosslinked poly(thiosemicarbazide) DMF 5.2 Rose Bengal 1017 99 60 2015

Polymeric ISA Cross-linked PEEK DMF 0.07 Styrene oligomers 236-1200 90 (500 g mol-1) 61 2015

Polymeric ISA Cross-linked P84 DMF 5.0 Methyl Orange 327 90 62 2016

Polymeric ISA Crosslinked PBI DMSO 0.31 Remazole Brilliant Blue 626 91 70 2016


polymerisation

Cross-linked PAN/PDA DMF 3.1 Rose Bengal 1017 92 63 2016


polymerisation

Cellulose composite DMF 0.53 Remazol Brilliant Blue 626 90 64 2016

NMP 0.18 94

DMSO 0.11 93


polymerisation

Cross-linked PAN/Tannin NMP 0.09 Different dyes 350-1017 92 (800 g mol-1) 65 2017

Other Poly(ether block amide) – Pebax – graphene oxide DMF 1.0 Styrene oligomers 236-1200 90 (236 g mol-1) 66 2017

1

Crosslinked membranes tend to have lower permeance than the non-crosslinked

ones. A well-reasoned explanation can be that the dipolar aprotic solvents have

usually higher viscosity which will lead the permeance drop. Unfortunately, the

acetone permeance will also drop after the crosslinking procedure. This will lead to

the conclusion that the membrane properties are changing during the crosslinking

procedure. Since most of the crosslinking procedures are chemical modifications of

the membrane material, they will induce the change in the physical properties of the

membrane (e.g. pore size, hydrophilic properties).

In Table 2, the permeance of the polymeric ISA membranes is ranging from 0.07 to

8.3 L m-2 h-1 bar-1, however, with different MWCO values. It is worth to mention that

there are only two systems that use crosslinked PBI (see Chapter 2.3) as membrane

with a comparatively low permeance value of 0.4 and 0.31 L m-2 h-1 bar-1.57,70

Polymeric thin film composite (TFC) membranes made via interfacial

polymerisation sharing the same low permeance values as the ISA membranes. The

permeance ranging from 0.9 to 3.1 L m-2 h-1 bar-1. Coated polymeric TFC

membranes share the lowest permeance values, usually under 1 L m-2 h-1 bar-1.

Ceramic type membranes are hardly tested in polar aprotic solvents; however, they

are not soluble in the aforementioned solvents.

2.3. Polybenzimidazole

Poly[2,2’-(m-phenylene)-5,5’-bisbenzimidazole] will be referred in this article as

polybenzimidazole or PBI.

Polybenzimidazole is a thermally stable high-performance polymer with numerous

different applications. It was developed alongside with different polyimide and

30

polyamide polymers in the early ’60 to supply competent materials for military

purposes.67 Polybenzimidazole based polymers are expensive, therefore their

application confined to military, astronautic and high-end engineering purposes.

Despite their high cost, PBI received significant interest due to their outstanding

features such as high temperature stability with high glass transition temperature

(>400°C), excellent chemical resistance but good miscibility and solubility with

other polymers (polyimide, polyacrylate, polyamide imide) and solvents.68 The

structure of polybenzimidazole contains two benzimidazole moieties connected to

each other by the 5C atom and they are also connected to a benzene ring (meta

position) by the 2C atoms. The chemical modification therefore limited only to the

nitrogen groups, although the molecule would be able to undergo aromatic

electrophilic substitution reaction as well. The first synthesis of polybenzimidazole

polymer was performed by Vogel and Marvel at the University of Illinois and later at

DuPont which later commercialized PBI under the trade name Celazole® by

Celanese. They are prepared from an aromatic tetraamine and an aromatic

dicarboxylic acid via condensation reaction at elevated temperature (350–400 °C)

Figure 9 shows the structure of the PBI as two repeating unit as drawn and as a 3D

model.

Figure 9. The structure of PBI; a) schematic model, b) 3D model.

Recently, crosslinked polybenzimidazole membranes showed superior performance

and stability in dipolar aprotic solvents and has been used in organic solvent

b) a)

31

nanofiltration processes. Valtcheva et al. reported a chemically crosslinked PBI with

α,α′-dibromo-p-xylene (DBX) (Figure 10).40 The reaction of DBX with PBI is an

electrophilic substitution reaction where the substituent is the alkyl-bromine

compound. The reaction starts with an N-alkyl-benzimidazole moiety which will

lose a proton by the presence of a basic N in the benzimidazole molecule. Hence, the

alkylation can go further to give specific bis-substituted compounds. The reaction

monitoring is difficult and usually results in a mixture of different substituted

compounds. A hydrogen bromide molecule will be the side product of the reaction

which makes it slightly inefficient in terms of environmental factors or reaction mass

efficiency.69

Figure 10. Crosslinking of PBI with DBX as reported by Valtcheva et al.40

Xing et al. reported a chemically crosslinked PBI with 1,2,7,8-diepoxyoctane (DEO)

(Figure 11).70 Compared to the DBX, DEO is milder alternative reagent without the

possible HBr salt formation reaction. Although, the additional aliphatic –OH groups

will increase the possibility of different complexation reactions.

32

Figure 11. Crosslinking of PBI with DEO as reported by Xing et al.70

If the membrane crosslinking was successfully carried out, the resultant material

could not be dissolved physically in any kind of solvent, which makes the chemical

analysis more difficult. Different analytical methods are used to distinguish the

efficiency of the crosslinking such as elemental analysis and surface elemental

analysis,71 attenuated total reflectance (ATR) Fourier-transform infrared

spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR)72, surface

tension measurement, scanning electron microscope measurement (SEM)71

combined with energy-dispersive atomic X-ray spectroscopy (EDX).71

2.4. Polymers of Intrinsic Microporosity

With an initial strategy to extend the aromatic components to mimic the graphene

sheets of activated carbons, within a rigid polymer network, McKeown and Budd

reported a network polymer with an amorphous microporous structure but a well-

defined surface chemistry.73 These type of polymers called polymers of intrinsic

microporosity or PIMs.73 PIMs are a class of microporous materials with a specific

structure with straight sections followed by cambered section by the axle. Thus, a

large amount of free volume is inefficiently compassed by the chains of the polymer.

Due to the interconnected clusters in the polymer, PIMs could be considered as

microporous materials. The innovative work by McKeown and Budd showed that

the porphyrin and phtalocyanine type PIMs (Pc-Network-PIMs) membranes does not

33

provide sufficient materials due to their high price and difficult synthesis. Instead,

rigid spirocyclic linking groups were introduced via the efficient dioxane-forming

reaction. This idea led them to develop different novel PIM polymers including the

PIM-1 which has been the most studied PIM polymer due to its simple synthesis and

easy-to-modify structure.

Figure 12. Visual representation of the PIM-1’s structure.73

The pore size distribution of PIMs is usually below 0.7 nm with the average surface

area between 500–1000 m2g-1. Meanwhile the molecular weight of PIMs is usually

between 104–105 g mol-1. Despite the rigid structure they are well soluble in some

organic solvent. For example, PIM-1 is soluble in THF and CHCl3 but not in the

mixture of the two solvents. PIM-1 contains an electron withdrawing nitrile group

which alongside the electron donating ether-oxygen connected to the aromatic ring,

generates a push-pull mechanism. Therefore, the PIM-1 polymer is highly

fluorescent.74

34

Table 3. Different type of PIMs by structure in the literature.

Polymer Structure Ref. Polymer Structure Ref.

PIM-1

73 PIM-4

73

PIM-7

73

PIM-

EA-TB

75 PIM-

SBI-TB

75

Table 3 shows the different synthesised PIMs from the literature. The rigid planar

structure with the restricted angles can be observed in every molecule. In case of

PIM-1 and PIM-4 the aromatic nitrile groups can be converted into different groups

(see Table 4). Due to the basic nitrogen in case of the PIM-EA-TB and PIM-SBI-TB

the modification is limited only to salt formations.

PIM-1 could be synthetized from commercially available and cheap monomers.

Namely, 3,3,3',3'-tetramethyl-2,2',3,3'-tetrahydro-1,1'-spirobis[indene]-5,5',6,6'-

tetraol and 2,3,5,6-tetrafluoroterephthalonitrile. The aromatic nucleophilic SN2 type

reaction takes place at high temperature due to the decreased reactivity of the

aromatic-halogen bond. The DMAc-Toluene solvent mixture is used to provide good

solubility for the reactants and stabilize the Ar-O- anion while the potassium-

carbonate is used to facilitate the removal of HF from the reaction mixture (Figure

13).

35

Figure 13. Schematic representation of the PIM-1 synthesis from its monomers.

PIM-1 has a surface area of 760 m2 g-1 and the molecular weight could reach

2·105 g mol-1. Nevertheless, the polymer contains only non-reactive groups. Due to

the PIMs’ rigid fused-ring backbone which promotes inefficient packing in the solid-

state, they are interesting candidates for membrane-based gas separation.75

Table 4. Modified PIM-1 polymers by structure in the literature.

Structure Ref. Structure Ref.

O

O

O

O

CN

CNn

72

O

O

O

On

HNHN

NH

NHHO

OH

76

77

O

O

O

O

COOH

COOHn

78

O

O

O

O

CONH2

CONH2n

79

80

81

82

36

Weng et al. hydrolysed the aromatic nitrile group in acidic environment in the PIM-1

which resulted in a carboxylate functionalized PIM (PIM-COOH). In their study

PIM-COOH membrane was used to separate enantiomers.78 However, this is the

only reported application of PIM-COOH in the literature.

O

O

O

On

OHO

HO O

O

O

O

O

CN

CNn

H2SO4

WaterAcetic acid

Figure 14. Hydrolysis of PIM-1 as reported by Weng et al. The reaction takes place at high temperature.

Mason et al. reduced the PIM-1 into amine-PIM-1 using borane complexes. The

resulted polymer was insoluble in any solvent meanwhile the average surface area

remained the same. These properties make the amine-PIM-1 promising use it in OSN

based processes with dipolar aprotic solvents.

Figure 15. Reduction of PIM-1 using Me2S.BH3 complex as a reducing agent.

The proposed procedure for the PIM-1 modification to amine-PIM-1 uses borane

dimethyl sulfide complex as a reducing agent in dry diethyl ether. Diethyl ether is a

nonsolvent for both the PBI and the PIM-1 therefore the reduction step could be

performed after the formation of the membrane.

PIM membranes are usually used in gas separation techniques.83 However, only a

few recent applications have been proposed in organic solvent nanofiltration

37

processes. Two scientific work have been carried out regarding the usage of PIM

membranes in OSN (Table 5). Fritsch et al covered the surface of a polyacrylonitrile

membrane with PIM-1 and crosslinked the system with different methods. The

resulted membrane showed good performance with comparatively low permeance.

On the other hand, Gorgojo et al. prepared a sub-micron thick membrane containing

only PIM-1 with an outstanding 18 L m-2 h-1 bar-1 permeance. The authors explained

the high permeance with the fact that the membrane thickness was under 100 nm,

therefore the additional resistance - which decreases the flux - was the lowest. They

introduced first the concept, membranes with intrinsic microporosity (MIMs) and

membranes with extrinsic microporosity (MEMs).

Table 5. Organic solvent nanofiltration membranes used in the literature in chronological order.

Polymer material Solvent Permeance Marker MW Rejection Ref. Year

L m-2 h-1 bar-1 g mol-1 %

PIM-1/PAN TFCs n-heptane 5.2 HPB 534 97 74 2012

PIM-1 n-heptane 18 HPB 534 90 84 2014

38

3. Objectives

The increasing demand from the industrial separation sector prompts the researchers

to develop new materials and methods which can endure harsh environments. On top

of that, this modern technology should fulfil the health and safety requirements by

the governmental sector and should operate with a reduced cost compared to the

other technologies. Organic solvent nanofiltration has already proven to be a cheap

and green liquid-liquid separation method. However, OSN suffers from several

drawbacks and one of them is that the membrane material has low solvent resistance

towards polar aprotic solvents and the permeance values are low in such solvents.

Therefore, the aim of this dissertation to propose a suitable membrane which has

improved permeance properties and can be used in polar aprotic solvents. The

starting hypothesis is that the specific PIM polymers might increase the internal free

volume of the originally packed structured PBI. The resulting polymer blend will be

crosslinked, and the solubility tests will be carried out in dipolar aprotic solvents.

Two, chemically different PIM-type polymers are targets, namely the carboxyl

functionalised PIM-1 (PIM-COOH) and the amine functionalised PIM-1 (PIM-

amine). Different polymer blends with different PIM-PBI ratios will be prepared,

crosslinked and tested. To analyse the membrane material, several different

analytical methods are used as characterisation. Membranes with the best

performance will be tested in a real-life membrane separation performance test.

39

4. Experimental

4.1. Materials

Polypropylene non-woven backing was supplied by Viledon, Germany. PBI dope

solution was purchased from Celazole®, Germany. All the used solvents including

acetone, acetonitrile (ACN), butyl formate, chloroform, chloroform-d, Cyrene®,

dichloromethane (DCM), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide

(DMF), diethyl-ether, dimethyl sulfoxide (DMSO), ethyl acetate, isobutyronitrile,

hexamethyl phosphoramide (HMPT), nitromethane, N-methyl-2-pyrrolidone (NMP),

methanol (MeOH), propylene carbonate, sulfolane, tetrahydrofurane (THF) are

purchased from commercial suppliers. Acetophenone, benzophenone,

tetrafluoroterephthalonitrile, 5,5',6,6'-Tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-

spirobisindane and potassium carbonate (K2CO3) were purchased from Alfa Aesar

and used without further purification. Borane dimethylsulfide and thionyl chloride

were purchased from Sigma-Aldrich. Mometasone furoate was donated by Hovione,

Portugal. All the solvents and reagents listed before were used without further

purification.

4.2. Synthesis

4.2.1. Synthesis of PIM-1

OH

OH

HO

HO

CN

CN

F

F

F

F

O

O

O

O

CN

CNn

DMAc - Toluene

K2CO3reflux

Dean-Stark apparatus

+

Figure 16. Synthesis of PIM-1.

Synthesis of PIM-1 was performed according to the procedure first reported by Budd

et al.73 3,3,3',3'-tetramethyl-2,2',3,3'-tetrahydro-1,1'-spirobis[indene]-5,5',6,6'-tetraol

40

(15 g, 1.47·10-2 mol, 1 eq), tetrafluoroterephthalonitrile (8.82 g, 1.47·10-2 mol, 1 eq)

and K2CO3 (18.5 g, 4.41 ·10-2 mol, 3 eq) were measured into a suitable round

bottomed flask (three necked) with Dean-Stark Apparatus then DMAc (44 mL) and

toluene (88 mL) was added under continuous argon flow. The mixture was stirred

and heated up to reflux temperature and maintained until the nascent water was

removed from the system (2 hours). Then the suspension was filtered on Buchner

filter to remove the excess K2CO3. The PIM-1 was precipitated from the warm

solution using methanol (100 mL) as anti-solvent and left for overnight. The

precipitate was filtered on Buchner filter and washed with methanol (3 times, 30

mL), water (3 times, 50 mL) and acetone (3 times, 30 mL) and left for drying

overnight at room temperature. The yellow powder was dissolved in CHCl3 (100

mL) then MeOH (150 mL) was added when yellow precipitation occurred which was

filtered on Buchner and left for drying overnight at room temperature. The powder

was suspended in water and boiled for overnight and filtered on Buchner and washed

with methanol (2 times, 30 mL) and acetone (2 times, 30 mL) and left for drying

overnight at room temperature. The resulted product yielded as a yellow powder

(15.0 g, 71%). 1H NMR (400 MHz, Chloroform-d) δ 6.83 (s, 2H, 1), 6.45 (s, 2H, 2),

2.35 (s, 2H, 3), 2.19 (s, 2H, 3’), 1.39 (s, 3H, 4), 1.34 (s, 3H, 4). For detailed 1H NMR

see Appendix Figure 1. 13C NMR (101 MHz, Chloroform-d) 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. For detailed 13C NMR see

Appendix Figure 2. Molecular weight (GPC, in CHCl3): 23,000 g/mol.

41

4.2.2. Synthesis of PIM-COOH

O

O

O

On

OHO

HO O

O

O

O

O

CN

CNn

cc. H2SO4

WaterAcetic acid

105 °C46h

Figure 17. Synthesis of PIM-COOH from PIM-1.

Synthesis of PIM-1 was performed according to the procedure first reported by

Weng et al.78 In-house prepared PIM-1 (3 g, 6.12·10-2 mol, 1 eq), concentrated

sulfuric acid (90 mL), glacial acetic acid (10 mL) and water (90 mL) was measured

into a round bottomed flask (250 mL) equipped with reflux condenser. The mixture

was heated up to 105 °C under constant mixing with magnetic stirrer. After 46 h the

mixture was filtered on a Büchner filter, washed with water several times and dried

on air. The residue was dissolved in THF (50 mL) and heated up to 50 °C which was

followed by the addition of methanol (140 mL). The solid precipitate was filtered on

Büchner filter and washed with methanol, water, acetone and diethyl ether (3 times

15 mL each) and left for drying on air overnight. PIM-COOH product was as brown

solid, yielded in 2.55 g (82%).

4.3. Membrane fabrication

4.3.1. Preparation and crosslinking of PBI–PIM-COOH membranes

A polymer dope solution was prepared by dissolving 9–30% (w/w) PIM-COOH and

26% (w/w) PBI dope solution in DMAc in specific proportions and stirring for half

an hour. The viscous solutions were incubated and shaken for 2 hours to remove the

bubbles at room temperature. The dope solution was then cast on a polypropylene

(PP) support tightly taped to a plain glass surface using a casting knife. The casting

thickness was set to 250 µm. To make the membrane repeatable and uniform in

42

performance, the non-woven PP support was coated with dope at a casting speed of

0.05 m s-1 using a continuous casting machine located in a room held at constant

temperature (22 °C) and humidity (45–48%). Immediately after casting, the

membrane was immersed in water (22 °C) where phase inversion occurred. After 30

min, the membrane sheet was transferred to fresh water bath then left for an hour.

Using a surgical blade, a circular membrane sheet was cut with the area of 52.8 cm2.

Then the membrane was immersed in a solvent exchange bath according to the

procedure described previously85 and it was repeated 3 times to minimize the excess

water left in the membrane material. The membrane then was immersed in to dry

acetonitrile and treated with the crosslinking method with the following procedure.

Table 6. PBI–PIM-COOH based membranes prepared.

Membrane type Sign PIM-COOH-content

(%)

PBI–PIM-COOH M1 10




The crosslinking between the PBI’s basic type nitrogen and the PIM-COOH’s

carboxyl groups can occurn with an aromatic amide formation. The reaction requires

high temperature or activating agent such as N,N′-diisopropylcarbodiimide (DIC),86

(1-cyano-2-ethoxy-2-oxoethylidenaminooxy)dimethylamino-morpholino-carbenium

hexafluorophosphate (COMU),87 thionyl chloride (SOCl2)88 or excessive heating up

to 140 °C.89

R O

OHHN

N

N

N

OR

Activating agent+ + H2O

Figure 18. Schematic representation of the crosslinking between a carboxylic acid and the benzimidazole.

43

In case of the heat induced crosslinking method, the membrane was immersed in an

annealing solution of 2-propanol and polyethylene glycol with an average molecular

weight of 400 g mol-1 (PEG 400) in 1:1 volume fraction and left there for 4 hours.

After the annealing the membrane was left to dry on air until the 2-propanol was

evaporated from the surface and heated up to 160 °C using an electric oven.

4.3.2. Preparation and crosslinking of PBI–PIM-amine membranes

A polymer dope solution was prepared by dissolving 6.5% (w/w) PIM-1 and 26%

(w/w) PBI dope solution in DMAc in specific proportions and stirring for 2 hours at

50 °C. The viscous solution was incubated and shaken for 2 hours at 50 °C to

remove the bubbles. The dope solution was then cast on a polypropylene support

tightly taped to a plain glass surface using a casting knife. The casting thickness was

set to 250 µm. To make the PP support repeatable and uniform in performance, the

non-woven was coated with dope solution at a casting speed of 0.05 m s-1 using a

continuous casting machine held at constant temperature (45 °C) and humidity (36–

52%). Immediately after casting, the membrane was immersed in water (21–21.5 °C)

where phase inversion occurred. After 30 min, the membrane sheet was transferred

to fresh water bath then left for an hour. Using a surgical blade, a circular membrane

sheet was cut with the area of 52.8 cm2. The membrane was then immersed in a

solvent exchange bath according to the procedure described previously90 and it was

repeated 3 times to minimize the excess water left in the membrane material. The

membrane then immersed in into diethyl ether and treated with BH3.SMe2 according

to the literature when the reduction of PIM-1 occurred.77 Using one-pot method,

excess SOCl2 (2 eq. calculated to the PBI in the membrane) was added to the mixture

to crosslink the PBI and washed with acetone, water and DMF (3 times 100 mL

each). The prepared membranes were tested within one week in every occasion.

44

O

O

O

O

CN

CNn

O

O

O

On

H2N

NH2

Et2O

Me2S.BH3, reflux, 24 h

Figure 19. Reduction of PIM-1 to PIM-amine.

Name Sign PIM-amine cont. (%)

PBI benchmark crosslinked M5 0

PBI-PIM-amine crosslinked M6 4





Figure 20. PIM-amine based membranes fabricated.

4.4. Solubility test

To test the crosslinking efficiency, solubility test was performed on each membrane.

A membrane piece (5 mm × 5 mm) was immersed in polar aprotic solvent (1 mL,

acetone, acetonitrile, butyl formate, dichloromethane, N,N-dimethylacetamide, N,N-

dimethylformamide, dimethyl sulfoxide, ethyl acetate, isobutyronitrile, hexamethyl

phosphoramide, nitromethane, N-methyl-2-pyrrolidone, propylene carbonate,

sulfolane, tetrahydrofurane) and left for 3 days. In case of Cyrene®, the membrane

piece (1 mm × 1 mm) was immersed in the solvent (200 µL) and left for 3 days. The

solubility test was negative if the neither the surface of the membrane nor the solvent

did not show any change after the 3rd day.

The list of the used solvents is given in Table 7 in the order of increasing relative

permittivity. The increasing relative permittivityrelative permittivity generally

reflects the increase of the dipole moment. However, the solvent properties depend

45

on a few additional factors, such the sterical hindrance of the solvent and the solute

or the ability of the solvent to initiate hydrogen bond with the solute.

Name Relative permittivity MW Boiling point Viscosity Dipole moment

g mol-1 °C cP (25°C) D

Ethyl acetate 6 88.1 77.1 0.426 1.78

Butyl formate 6.5 102.1 107 1.002 1.84

2-Methyltetrahydrofurane 6.97 86.1 80.2 0.467 1.38

Tetrahydrofurane 7.5 72.1 66 0.48 1.63

Dichloromethane 9.1 84.9 39.6 0.413 10.6

Acetone 20.7 58.1 56.1 0.295 2.91

Isobutyronitrile 20.8 69.1 103.9 0.35 4.29

NMP 34.1 99.1 203 1.65 4.09

Nitromethane 35.9 61.1 101.2 0.61 3.46

DMF 36.7 73.1 152 0.92 3.86

Acetonitrile 37.5 41.1 81.3 0.35 3.92

DMAc 37.8 87.1 165 0.945 3.72

Sulfolane 44 120.2 285 10 4.35

DMSO 47 78.1 189 1.996 3.96

HMPT 47.4 179.2 232 3.34 5.37

Propylene carbonate 64 102.1 242 0.625 4.9

Cyrene 2.181* 128.1 227 14.591 ?**

Table 7. Properties of polar aprotic solvents used in solubility tests.92 *Relative permittivity; measured by

refractive index measurement **Calculated data.

4.5. Nanofiltration procedure

In case of PBI–PIM-COOH (M1, M2, M3, M4; see Chapter 4.3.1). The prepared

membranes were screened using markers as solute in the range between 120–430

g mol-1 to measure the rejection values (acetophenone, 120 g mol-1; benzophenone,

180 g mol-1; 3,3,3',3'-tetramethyl-2,2',3,3'-tetrahydro-1,1'-spirobi[indene]-5,5',6,6'-

tetraol, 340 g mol-1; Mometasone furoate, 427 g mol-1).93 A screening rig was set up

in cross-flow configuration as described previously.94 A solution containing the

screening solutes (1 M) was circulated around the rig at a flow rate of 100 L·h-1.

Three coupons of each type of membrane with an area of 52.8 cm2 per membrane

were tested. Samples were taken at steady state for pressure of 30 bar.

In case of PBI–PIM-amine (M5, M6, M7 and M8; see Chapter 4.3.2) Nanofiltration

experiments were carried out in a cross-flow system at 30 bar39 with MeTHF,

MeCN, DMF, DMSO, PC as solvents and at 25 °C to determine permeate flux and

molecular weight cut off curves of the membranes. The permeance and the MWCO

of M8 has been tested at 25, 50, 75, 100 and 125 °C. Permeate samples for flux

46

measurements were collected at intervals of 1 h, and samples for rejection

evaluations were taken after steady permeate flux was achieved. MWCO curves

were obtained by using a standard test solution composed of a homologous series of

styrene oligomers dissolved in the selected solvent.95 The styrene oligomer mixture

contained 1 g L-1 of PS 580 and PS 1050 and 0.01 g L-1 of a-methylstyrene dimer.

Concentrations of markers and the styrene oligomers in the permeate were measured

on VWR Hitachi Chromaster instrument with pump (Chromaster HPLC 5160),

autosampler (Chromaster HPLC 5260), column oven (Chromaster HPLC 5310) and

diode array detector (DAD) (Chromaster HPLC 5430); using ACE-5-C18 (150 × 4.6

mm) column; with eluent A: water type I (18.2 MΩ) with 0.1% TFA and eluent B:

acetonitrile with 0.1% TFA.

4.6. Chemical characterisation

4.6.1. Attenuated Total Reflection (ATR) Fourier-transform

infrared spectroscopy (FTIR)

Spectrums were recorded from dry membrane samples with or without a

polypropylene support on it using a Thermo Fisher Nicolet iD5 ATR-FTIR

spectrometer.

4.6.2. Atomic Force Microscopy (AFM) linked Infrared

spectroscopy

Atomic force microscopy combined with infrared spectra has been used to prove the

homogenous mixing of the PIM-amine and the PBI in the membrane material. AFM-

IR is the combination of two analytical equipment methods. The technique can be

used to identify the surface roughness and identify the surface composition.

Measuring the infrared absorption as a function of position on the surface can create

the chemical composition morphological map which shows the spacial distribution

47

of different chemical components.96 A simplified scheme of the AFM-IR can be seen

on Figure 21. The IR-laser exposes the surface of the sample which expands if the

used wavenumber is close to the resonance frequency. This expansion can be

detected by the needle and used to draw an IR-amplitude and IR-peak surface

morphological maps. The resolution of the AFM needle is 10 nm and the resolution

of the laser depends on the used laser’s frequency, usually around 100 nm.

Figure 21. Schematic representation of the AFM-IR.97

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 used mapping frequency is based on the IR results of the polymer; 1373 cm-1 for

the PIM and 1628 cm-1 for the PBI base.

4.6.3. Nuclear Magnetic Spectroscopy (NMR) and solid-state NMR

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.

48

The liquid phase NMR measurements were recorded on a Bruker Avance III (400

MHz) using chloroform-d as a solvent at room temperature (22°C).

4.6.4. Gel permeation chromatography

Average molecular masses of the parent polymers were measured by multidetector

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.

4.6.5. Morphological characterisation

4.6.6. Brunauer–Emmett–Teller (BET) adsorption measurement

All BET surface area measurement was performed on an ASAP 2020 V4.00H

machine. Nitrogen gas was used as analysis adsorptive at 77.451 K. The ambient

temperature was 22.00 °C and the sample automatically degassed for 24 h.

4.6.7. Scanning Electron Microscopy (SEM)

Scanning electron microscopy measurements were performed on a FEI Quanta 200

field emission scanning electron microscope. All membrane samples were taken

after the thionyl chloride treatment. The samples were mounted onto SEM stubs and

sputtered using Emitech K550 platinum sputter coater. SEM conditions were: 10 mm

working distance, inlense detector with an excitation voltage of 15 kV.

4.6.8. Contact angle measurement

49

The contact angle measurement were performed on a Krüss DSA 100 drop shape

analysis system using a sessile drop method, where the drop of the water is placed on

the surface of the membrane and the contact angles were determined using the

software of the machine. In this dissertation, the contact angles were only used to

compare the hydrophilic or hydrophobic properties of the membranes. Actual surface

tension calculation has not taken place.

5. Results and discussion

5.1. Solubility test and membrane crosslinking

Table 8 shows the result of the different crosslinking methods in case of M1, M2,

M3 and M4. COMU and DIC are considered mildly effective, however, it failed to

produce an acceptable membrane. The heat induced crosslinking failed in every case.

Crosslinking with SOCl2 produced acceptable membrane which cannot be dissolved,

and the swelling cannot be observed. Therefore, SOCl2 has been chosen as a

crosslinking agent and membranes crosslinked with SOCl2 has been tested in the

solubility tests.

The general interaction between non-crosslinked polymers only limits to secondary

interaction. Since the NH group is able to act as a hydrogen donor in the amine PIM-

1 and the [3]N in the benzimidazole moiety as a hydrogen acceptor, the possible

secondary interaction between the is the two polymer chains.98 Similar interaction

can be observed in many natural cases such as the RNA.99 However, the PBI is also

able to form hydrogen bond with another PBI chain, therefore the strengths of the

interaction between the amine PIM-1 and the PBI will also depend on the free [3]N

groups in the PBI molecule. To further study this would require molecular modelling

studies which has not the aim of this study.

50

Table 8. Prepared membranes and the specific crosslinking agents used in the procedure. One asterisk

denotes light swelling on the membrane surface. Double asterisks denote heavy swelling on the surface.

Sign Crosslinking method

SOCl2 COMU DIC heat

M1 – –* –* +

M2 – –** –** +

M3 – + + +

M4 – + + +

Table 9 shows the result of the complete solubility tests for all membranes. All the

prepared membranes show good stability in the listed polar aprotic solvents.

Table 9. Solubility test of the different membranes. Negative sign denotes the insolubility of the membrane

while positive sign denotes the solubility of the membrane in the specified solvent.

Solvent Membrane

M1 M2 M3 M4 M5 M6 M7 M8

Ethyl acetate – – – – – – – –

Butyl formate – – – – – – – –

THF – – – – – – – –

DCM – – – – – – – –

Acetone – – – – – – – –

Isobutyronitrile – – – – – – – –

NMP – – – – – – – –

Nitromethane – – – – – – – –

DMF – – – – – – – –

ACN – – – – – – – –

DMAc – – – – – – – –

Sulfolane – – – – – – – –

DMSO – – – – – – – –

HMPT – – – – – – – –

Propylene carbonate – – – – – – – –

Cyrene – – – – – – – –

To prove the chemical crosslinking in the membrane, ATR-FTIR measurements

have been carried out. Figure 23 shows the FTIR spectrum of the M1, M2, M3, M4

membranes and the PIM-COOH and the PBI membrane for comparison. For the pure

PBI, the peaks corresponding to the free N-H stretch and N–H···H hydrogen bond

interaction can be found in the region 3500–2500 cm-1. According to Musto100,101

absorption peaks situated at 3415 cm-1 was attributed to the stretching vibration of

isolated nonhydrogen bonds N–H group. The very broad peak located between 3100

and 3350 cm-1, approximately centred at 3145 cm-1 was assigned to the self-

associated hydrogen bonded N–H groups.102 The highlighted area between 1600–

51

1750 cm-1 are corresponding the acid carbonyl C=O stretching frequencies. Due to

the salt formation of the benzimidazole moiety in the membrane, the broad peak (N–

H stretching) is shifted to the lower wavenumber around 2600 cm-1. An unidentified

peak at 1552 cm-1 also shifted to the left.

Figure 22. ATR-FTIR image of the M1, M2, M3, M4 membranes and the PIM-COOH and PBI polymer.

Figure 23 shows the ATR-FTIR spectra of the M5, M6, M7, M8, M9, M10

membranes and the PIM-amine and the PBI polymers, respectively. Owing to the

planar cyclic and aromatic part, the spectrum is composed with narrow peaks

between 2000–1000 cm-1 as the NH and CN deformation modes (δNH and δCN) in

case of the PIM-amine. 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 aromatic

and aliphatic C–H vibrational modes are found around 3000 cm-1 for the PIM-1

polymer. The characteristic CN stretching frequency mode is found at 2240 cm-1 as a

low compact peak. The aforementioned peaks are missing in the reduced

membranes, which can indicate a successful reduction procedure. The aromatic ether

C-O stretching frequency mode can be found at 1008 cm-1 as a strong narrow peak.

52

The highlighted part on Figure 23 indicates the main difference between the original

PBI and the mixed PBI–PIM-amine membranes. The strong narrow peaks at

1108 cm-1 and 1008 cm-1 cannot be observed, therefore the shifted peak at 1373 cm-1

has been chosen as an indicator resonance frequency for the AFM-IR measurement.

With the increasing PIM-1 content in the membranes, the IR spectra shows little or

no difference between the different membranes. Owing to that original PIM-1

content is low the detectable peaks of the PIM-amine are under the PBI peaks.

In case of M9 and M10 the NH signals around 3200 cm-1 showing significantly less

intensity than the other membranes. With an increasing content of the amine PIM-1

in the membranes will lower the density of the NH groups in the membrane and this

will also cause the loss of intensity in the IR frequencies.

Figure 23. ATR-FTIR image of the M6, M7, M8, M9, M10 membranes and the PIM-amine and PBI

polymer.

5.2. Nanofiltration testing

Figure 25 shows the 4 markers testing results for the M1, M2, M3, M4 membranes,

respectively. The rejection values are generally low with a consecutive trend from

M1 to M4. M1 shows the highest rejection around 50% and the difference between

53

340 and 427 g mol-1 marker. This trendline can be observed with all the other

membranes as well. Increasing the PIM-COOH concentration in the membrane, the

rejection drops drastically in case of M4. Meanwhile, the permeance values are

slightly increasing from 3.8 L m-2 h-1 bar-1 to 4.2 L m-2 h-1 bar-1 from M1 to M3,

respectively. However, M4 has an outstanding permeance value with 8.0 L m-2 h-

1 bar-1. The explanation behind the low rejection values and the increased permeance

values is that the intramolecular crosslinking between the PIM-COOH and the PBI

molecules were just partially formed.

Figure 24. Permeance values of M1, M2, M3, M4 represents the average of three independent samples.

M1 M2 M3 M4

Permeance (L m-2 h-1 bar-1) 3.8±0.07 4.1±0.12 4.2±0.18 8.0±0.11

Based on the low rejection values, the further experiments with the M1, M2, M3, M4

has been rejected and morphological characterisation experiments have not been

carried out.

Figure 25. 4 markers testing with M1 (black), M2 (pink), M3 (blue), M4 (yellow).

Based on the SEM pictures and the AFM-IR results (see Chapter 5.3) only

membrane M5, M6, M7 and M8 has been chosen for further testing.

0

10

20

30

40

50

60

0 50 100 150 200 250 300 350 400 450

Rej

ecti

on

(%

)

Molecular weight (g mol-1)

54

The various properties (permeance, MWCO) has been measured for M5, M6, M7,

M8 in different solvent, namely acetonitrile (MeCN), 2-merthyltetrahydrofuran

(MeTHF), propylene carbonate (PC), N,N-dimethylformamide (DMF) and dimethyl

sulfoxide (DMSO). MeCN represents a low viscosity (0.35 cP), medium relative

permittivity (37.5) solvent with a medium dipole moment (3.92 D). MeTHF

represents a low viscosity (0.467 cP), low relative permittivity (6.97) solvent with a

low dipole moment (1.38 D). PC represents a medium viscosity (0.625 cP), high

relative permittivity (64) solvent with a high dipole moment (4.9 D). DMF

represents a high viscosity (0.92 cP), medium relative permittivity (36.7) solvent

with a medium dipole moment (3.86 D). DMSO represents a high viscosity (1.996

cP), high relative permittivity (47) solvent with a medium dipole moment (3.96 D).

The comparison is based on Table 7.

Figure 29 shows the permeance of the M5, M6, M7 and M8 membranes,

respectively. The permeance always increasing with the increasing PIM-amine

content. The lowest permeance has been measured in case of M5 and the highest in

case of M8. Interestingly, neither just with the increasing relative permittivity nor

with just the increasing viscosity of the solvents, the permeance cannot be explained.

On the other hand, simultaneously using both, the permeance result can be

explained. The MeTHF has the lowest permeance, which comes from the fact that

the membrane material favours hydrophilic molecules. (the relative permittivity of

the water is 80.1 and the viscosity is 0.890 cP). The number of –NH2 groups are

increasing in case of M6, M7, M8 respectively which increase the hydrophilic

characteristic of the molecule. DMF and the MeCN share similar relative

permittivity, however, the viscosity of the DMF is more than twice of the MeCN.

Therefore, the permeance will be lower. The permittivity of the DMSO cannot

55

overcome the high viscosity of the DMSO, therefore the permeance is lower than the

other three. It should be noted, that the permeance difference between M5, M6, M7

and M8 measured in DMSO is more compact than in the case of the other four

membranes. This phenomenon can be contributed to the high viscosity as well.

Figure 26. Measured permeance in different solvents. M5 (black); M6 (blue); M7 (pink); M8 (yellow). The

relative permittivity of the listed solvents is increasing from the left to the right. For details, see Appendix

Table 1.

Figure 27 shows the measured MWCO in different solvents for M5, M6, M7 and M8

membranes. There is no exact trend between the MWCO and the solvents properties

or the permeance. M6 has the lowest MWCO with 190 g mol-1 in PC, however, M6

has an MWCO 420 g mol-1 in DMSO. M5 has the highest MWCO in every solvent

with 500 g mol-1 in MeTHF and 650 g mol-1 DMF. Adding PIM to the membrane

material increases the MWCO in every occasion; the highest difference is between

the M5 and M6, for example in DMF M5 has an MWCO around 650 g mol-1. On the

other hand, M6 has an MWCO around 295 g mol-1 in DMF. The increasing amount

0.0

0.5

1.0

1.5

2.0

2.5

Pe

rme

ance

(L

m-2

h-1

bar

-1)

MeCNMeTHF PCDMF DMSO

56

of PIM dilutes the original PBI dope solution, which leads to a loose membrane with

higher MWCO and higher permeance. The permeance results from Figure 26

confirm the aforementioned trend.

Figure 27. Measured MWCO in different solvents. M5 (black); M6 (blue); M7 (pink); M8 (yellow). For

details, see Appendix Table 2. The error bars represent the deviations of three independent measurements.

Increased temperature will increase the permeance due to the decreasing viscosity.103

Although, the increasing temperature will decrease the relative permittivity which

will lead to increased permeance (see the permeance results in Figure 26).104 Figure

28 shows that overall the permeance will increase with the increasing temperature

following a depressed plot. At 125 °C, the measured permeance is the highest with

9.85 L m-2 h-1 bar-1.

0

100

200

300

400

500

600

700

MW

CO

(g

mo

l-1

)

M5

M6

M7

M8

MeCNMeTHF PCDMF DMSO

57

Figure 28. Permeance dependence in the function of temperature in case of M8. For details, see Appendix

Table 3. The error bars represent the deviations of three independent measurements.

Figure 29 shows the changes of the MWCO in the function of temperature in case of

M8. The MWCO decreasing with a linear trend starting from 600 g mol-1 to

340 g mol-1 at 125 °C. Similar trend was observed by Sairam et al. using a

polyaniline film membrane. However, in their work the difference was just few

percentages.105

Figure 29. MWCO dependence as a function function of temperature in case of M8. For details, see

Appendix Table 3.

Generally, PIM-amine shows a superior performance over the PIM-COOH

membranes, since the permeance and the MWCO results are better (the MWCO

6.5

7

7.5

8

8.5

9

9.5

10

10.5

0 20 40 60 80 100 120 140

Per

mea

nce

(L

m-2

h-1

bar

-1)

Temperature (°C)

y = -2.64x + 664R² = 0.9918

0

100

200

300

400

500

600

700

0 20 40 60 80 100 120 140

MW

CO

(g

mo

l-1)

Temperature (°C)

58

could not be measured in case of M1, M2, M3, M4). The best solvent to use from the

list is the PC. PC has similarly high relative permittivity as water which is favourable

by the hydrophilic membrane. M6 showed the best performance (4% PIM-amine) in

terms of MWCO and M8 showed the best performance in terms of permeance. There

is a retrograde trend between the PIM-amine content and the permeance results

which can be explained with the diluted original dope solutions.

5.3. Exploring the structure of the PIM-amine membranes

To further understand the structure and the characteristic of the M5, M6, M7, M8,

M9, M10 membranes, several morphological and modelling tests has been carried

through. Since M5, M6, M7 and M8 showed the best nanofiltration properties, these

membranes were tested the most.

Figure 30 shows the topography and 3D IR spectra of the M9 membrane. The

height–topography image shows a rough surface with different sized particles. The

particles represent some of PIM-amine particles in the membrane. This is proven by

both the IR-amplitude–topography and the IR-peak–topography images. The rough

surface indicates that the PIM-1 has reached solubility boundaries in case of the M9

membrane. Based on AFM and the SEM results M9 has been proposed not to test in

the nanofiltration experiments.

a) b)

c)

59

Figure 30. AFM-IR measurement of the M9 membrane. A) height; b) IR-amplitude; c) IR-peak. The

assorted colours denote different depth. For a full-size picture with colour bars see Appendix Figure 9-12.

Small molecule model reaction used to confirm and provide further evidence of the

crosslinking method with SOCl2. The applied model reagents were 2-phenyl-1H-

benzo[d]imidazole, benzoic acid and benzyl amine. The chosen model reagents are

representing the chemical functionalities in the membrane material. The reaction was

performed in dry DMF at room temperature with excess amount of SOCl2. Reaction

only occurred between the two 2-phenyl-1H-benzo[d]imidazole meanwhile the

benzyl amine and the benzoic acid could be fully recovered.

The theoretically resulted 1,1'-sulfinylbis(2-phenyl-1H-benzo[d]imidazole is highly

unstable. It immediately reacts with water or any protic type reagents resulting SO2

and the chloride salt of 2-phenyl-1H-benzo[d]imidazole.106 The decomposition might

take place when the molecule is not stabilized in a solution. This conclusion might

take place upon the fact that the proposed 2-phenyl-1H-benzo[d]imidazole molecule

could not be synthetized in this way.

This result indicates the fact that there is no actual crosslinking happening in the

membrane material. The high stability of the PBI membrane can be derived from the

salt form of the polybenzimidazole in the membrane.

To further investigate the morphological characteristic of the prepared membranes,

SEM testing has been carried out. Figure 31 shows the SEM pictures (surface and

60

cross-section) of the M6 with different magnification. The surface of the membrane

is the smoothest compared to the other membranes and shares the highest similarity

with the normal PBI membrane since it contains the lowest amount of PIM-amine in

the sample. However, several solid particles can be observed on the surface images

of the membrane but negligible on the cross-section images.

Figure 31. SEM image of M6; a) surface (x1000), b) surface (x5000), c) surface (x10000), d) cross-section

(x1000), e) cross-section (x5000), f) cross-section (x10000).

Figure 32 shows the SEM pictures (surface and cross-section) of the M7 with

different magnification. The amount of solid PIM-amine particles in the sample is

higher in the case of M6 due to the increased amount of PIM-amine in the mixture.

However, the cross section shows comparable similarity with the PBI’s cross section

image.

a) b) c)

d) e) f)

a) b) c)

61



Figure 33 presents the SEM pictures (surface and cross-section) of the M8 with

different magnification. The amount of solid PIM-amine particles in the sample

shows similarity with the M7. However, the cross section shows comparable

similarity with the PBI’s cross section image.




different magnification. The surface of the membrane shows a highly rough and

inhomogeneous surface with several visible particles in the membrane. The cross-

section image clearly shows the solid particles in the membrane material as small

spherical object surrounded by the membrane material. This can be explained by the

limited solubility of the PIM-1 in dry DMAc. Considering, that the initial solution of

the PIM-1 was clear the precipitation (or crystallisation) of the PIM-1 happens after

d) e) f)

a) b) c)

d) e) f)

62

mixing the PIM-1 polymer solution with the PBI dope solution. It worth to mention

that the mixing and casting were both performed at50 and 45 °C, respectively. At the

investigated temperature the PIM-1 solubility might be kinetically controlled. The

cross-section image on Figure 34 shows similarity with a normal PBI-membrane

cross-section. Owing to the visible particles in the cross-section image, the PBI-PIM-

1 20% has been decided not to use for further testing.




different magnification. The surface of the membrane contains more solid particles

due to the increased amount of PIM-1 in the membrane material. The cross-section

images show an evidence of cracking of the membrane and the pores of the

membrane are open.

a) b) c)

d) e) f)

63



The uniqueness of the PIM based materials is the high surface area which is derived

from the rigid structure of the PIMs. Therefore, surface area measurement are

required to prove the increased permeance results are based on the increased surface

areas.

Figure 36 presents the correlation between the BET surface area of the membrane

and the PIM-amine concentration. In general, OSN membranes share a packed and

dense structure, therefore the surface area should be low. Extremely high surface

area might indicate imperfections in the membrane. Surface area of the M5

membrane is 3.05 m2g-1 which increases to 3.54 m2g-1 (M6) after the addition of the

PIM-amine. This slight increase can be explained when the polymer blends are

formed, the specific rigid structure of the PIM-1 cannot be formed. Therefore, under

4% PIM content the polymer mostly conserves the structure of the PBI. To prove

this concept, further experiment is recommended where the PIM content is under

a) b) c)

d) e) f)

64

4%. The surface areas are 7.85 and 11.69 m2g-1 in case of M7 and M8, respectively.

The increased surface area shows a linear correlation with the increased amount of

PIM-amine in the membrane. This linear correlation can explain the increased

permeance results. However, the original PIM surface area is 700 m2g-1 which would

have estimated the surface area around 28, 56 and 84 m2g-1 surface areas for the M6,

M7, M8, respectively. Compared to the measured values, the predicted data is

significantly higher, which leads to the conclusion that the PBI-PIM polymer blend

decreases the PIM’s surface area. The drop can be explained that the PBI polymers

partially fills the voids between the PIM-amine moieties.

Table 10. Predicted and measured BET surface areas of M5, M6, M7 and M8. The prediction based on a

simple percentage calculations respect to the PIM content in the membrane.

BET surface area M5 M6 M7 M8

Predicted (m2g-1) - 28 56 84

Measured (m2g-1) 3.05 3.54 7.85 11.69

Figure 36. BET surface area plot with respect to the PIM-amine concentration.

Figure 37 shows the solid-state NMR measurements of the normal PBI membrane

and the M5 and the M8 membranes, respectively. The spectrum of the normal PBI

y = 1.0198x - 0.4653R² = 0.9989

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12 14

BET

su

rfac

e ar

ea (

m2

g-1)

PIM-amine concentration (%)

M5 M6

M7

M8

65

range with other ssNMR spectra reported in the literature.72 The full characterisation

and spectra evaluation of the normal PBI stated on Figure 37. In case of M5 the

peaks 110, 121 and 128 ppm are shifted to higher ppm. Also, instead of two different

peaks at 150 and 141 ppm, only just one peaks can be seen. The peak at 134 ppm can

be observed as a narrow shoulder.

The spectra of the M8 membrane is similar with M5 with the observed addition

peaks at 44 and 68 ppm which are the aliphatic carbon peaks from the PIM molecule.

Figure 37. ssNMR spectra of the PBI (red), M5 (pink) and the M8 (blue) membranes. The highlighted area

between 40–60 ppm shows the aliphatic carbon peaks in case of M8. 13C NMR (101 MHz) δ 150.52,

141.89, 134.06, 128.31, 120.89, 111.53, 110.49.

The increased permeance results could be assigned to the changes in the

hydrophilicity of the membranes. To prove this concept, contact angle measurement

has been carried out with the best four membranes from the nanofiltration procedure

(M5, M6, M7 and M8).

Table 11 shows the results of the contact angle measurements (water in air). The

contact angle decreases from 61.43±2.04° to 35.68±2.01° from M5 to M8,

respectively. The increasing PIM-amine content cause a drop in the contact angle.

The lower contact angle means more hydrophilic surface which will induce the more

water-like molecules to form stronger interactions with the surface. This effect will

cause a higher transport rate from the bulk to the surface ergo will increase the

66

permeance. This hypothesis can be proved by the increasing permeance result from

Chapter 5.2.

For the complete table of the contact angle see Appendix Table 4. For contact angle

pictures see Appendix Figure 17. (M5), Figure 18. (M6), Figure 19. (M7), Figure 20.

(M8).

Table 11. Measured contact angles for M5, M6, M7 and M8 respectively.

Contact angle M5 M6 M7 M8

(°) 61.43±2.04 51.32±1.66 40.85±1.88 35.68±2.01

67

Conclusion

The preparation procedure of a PIM-PBI polymer blend has been successfully

carried out and the chemical stabilization of the membrane with SOCl2 has been

proposed and developed. The prepared membranes show high stability in various

dipolar aprotic solvents. The nanofiltration procedure has been performed and the

permeance and the MWCO has been measured. In one occasion, the permeance and

the MWCO has been measured in the function of temperature. The permeance

results range between 0.5–2.5 L m-2 h-1 bar-1 and the MWCO results range between

190–600 g mol-1. In terms of permeance and MWCO, the PIM-amine containing PBI

polymer blends show superior performance over the PIM-COOH containing ones.

The 12% PIM-amine containing membrane has a permeance value with 2.0 L m-2 h-1

bar-1 in DMF, which is slightly previously reported crosslinked PBI membranes.52,65

Using a model reaction and IR spectroscopy, the chemical crosslinking has been

rejected and the stability of the membrane assigned to salt formation. AFM-IR, SEM

and contact angle measurement have been used to distinguish the imperfect

membranes. On top of this, they have been used to characterise the morphologic of

the membranes and provide further evidence towards the increased properties.

As a future prospective, the thionyl chloride based chemical stabilization method

should be changed to a greener method. On the other hand, the salt formation

stabilization might open a new path for the PBI membranes used in OSN system,

where the deformation of the salt is negligible.

68

Appendix

by

Gergo Ignacz

School of Chemical Engineering and Analytical Science

Supervised by Dr Gyorgy Szekely

2018

69

Figure 38. 1H-NRM of PIM-1.

Figure 39. 13C-NMR of PIM-1.

Figure 40. 1H-NRM of PIM-COOH.

70

Figure 41. 13C-NMR of PIM-COOH.

Table 12. Properties of polar aprotic solvents used in solubility tests. 107 *Relative permittivity; measured

by refractive index measurement.

Name Dielectric constant MW Boiling point Viscosity Dipole moment

- g mol-1 °C cP (25°C) D

Ethyl acetate 6.00 88.1 77.1 0.426 1.78

Butyl formate 6.50 102.1 107 1.002 1.84

2-Methyltetrahydrofurane 6.97 86.1 80.2 0.467 1.38

Tetrahydrofurane 7.50 72.1 66 0.48 1.63

Dichloromethane 9.10 84.9 39.6 0.413 10.6

Acetone 20.7 58.1 56.1 0.295 2.91

Isobutyronitrile 20.8 69.1 103.9 0.35 4.29

NMP 34.1 99.1 203 1.65 4.09

Nitromethane 35.9 61.1 101.2 0.61 3.46

DMF 36.7 73.1 152 0.92 3.86

Acetonitrile 37.5 41.1 81.3 0.35 3.92

DMAc 37.8 87.1 165 0.945 3.72

Sulfolane 44.0 120.2 285 10 4.35

DMSO 47.0 78.1 189 1.996 3.96

HMPT 47.4 179.2 232 3.34 5.37

Propylene carbonate 64.0 102.1 242 0.625 4.9

Cyrene 37.3 128.1 227 14.5108 4.08

Table 13. Solubility test of the different membranes. Negative sign denotes the insolubility of the

membrane while positive sign denotes the solubility of the membrane in the specified solvent.

Solvent Membrane

M1 M2 M3 M4 Non-modified PBI

Ethyl acetate – – – – –

Butyl formate – – – – –

THF – – – – –

DCM – – – – –

Acetone – – – – –

Isobutyronitrile – – – – –

NMP – – – – +

Nitromethane – – – – –

DMF – – – – +

ACN – – – – –

DMAc – – – – +

Sulfolane – – – – +

DMSO – – – – +

DMSO+TEA + + + + +

HMPT – – – – +

71

Propylene carbonate – – – – +

Cyrene – – – – +

Table 14. Permeance summary of M5, M6, M7 and M8.

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

M1 St.Dev. M2 St.Dev. M3 St.Dev. M4 St.Dev.

MeTHF 1.93 0.07 3.16 0.1 5.11 0.23 7.33 0.25

DMF 3.17 0.15 5.33 0.14 7.23 0.19 10.18 0.36

MeCN 3.75 0.05 5.23 0.04 7.06 0.06 9.15 0.05

DMSO 3 0.16 3.8 0.11 5.62 0.23 6.92 0.13

PC 5.58 0.26 7.25 0.13 9.49 0.16 12.89 0.45

M5 M6 M7 M8

BET Surface area predicted (m2g-

1)

- 28 56 84

BET Surface area measured

(m2g-1)

3.05 3.54 7.85 11.69

Contact angle (°) 61.43±2.04 51.32±1.66 40.85±1.88 35.68±2.01

Table 15. MWCO summary of M5, M6, M7 and M8.

MWCO (g mol-1)

M5 M6 M7 M8

MeTHF 500 350 400 440

DMF 650 295 479 550

MeCN 520 280 350 390

DMSO 630 400 500 600

PC 580 190 295 420

Table 16. Permeance and MWCO dependence in the function of temperature of M8.

Temp (°C) Permeance (L m-2 h-1 bar-1) Standard deviation MWCO (g mol-1)

DMSO DMSO

25 6.92 0.13 600

50 8.21 0.2 540

75 9.03 0.19 450

100 9.61 0.15 400

125 9.84 0.18 340

72

Figure 42. AFM height map of M4 with colour bar.

Figure 43. AFM-IR IR-amplitude map of M4 with colour bar.

73

Figure 44. AFM-IR IR-peak map of M4 with colour bar.

Figure 45. Height mapping of M4.

74



75




76




Table 17. Contact angle measurement of M5, M6, M7 and M8, respectively.

Cont. ang. 1 Cont. ang. 2 Cont. ang. 3 Average St.deviation

77

M5 60.2 59.9 58.8

63.6 63.1 63.0 61.43 2.04

M6 50.2 49.8 49.5

53.3 52.5 52.6 51.32 1.66

M7 42.7 43.0 41.9

39.1 39.2 39.2 40.85 1.88

M8 37.1 36.9 36.8

34.1 36.9 32.3 35.68 2.01

Figure 54. Contact angle picture of M1.



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