Thermally rearranged polymeric membranes for gas separation

63CHAPTER 4: Effect of degree of thermal rearrangement conversion on physicochemical and gas transport properties of thermally rearranged polyhydroxyamide amic acid.... 100 CHAPTER 5: Ef

THERMALLY REARRANGED POLYMERIC MEMBRANES FOR

THERMALLY REARRANGED POLYMERIC MEMBRANES FOR

GAS SEPARATION

by WANG HUAN

(B.Eng (Hons.)), National University of Singapore

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

DEPARTMENT OF CHEMICAL & BIOMOLECULAR

ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2013

This thesis has also not been submitted for any degree in any university previously

Student: Wang Huan

Signature:

Date: 05, Apr, 2013

ACKNOWLEDGEMENTS

Completing a Ph.D degree is never an easy job After 4-year learning and working, I am approaching the successful end of this challenging task Without the help, support, encouragement and guidance of the following important people, my dream of being a doctor will never come true

I sincerely thank my supervisors Professor Tai-Shung Chung and Professor Donald R Paul for their supervision, guidance and patience throughout my Ph.D study I am grateful that I had the chance to work with these two knowledgeable membrane scientists and to work in a well-equipped research laboratory Professor Chung and Professor Paul have provided me with numerous valuable advice and great encouragement whenever I encountered difficulties Their attitude towards science and logical research has greatly influenced both my research and my life

My appreciation also goes to my mentors Dr Songlin Liu and Dr Bee Ting Low They opened the window for my research and they let me understand what a scientific work should be Dr Low was always very helpful when I consulted her about my problems, though later she was not assigned as my mentor She has not been only a very responsible mentor but also a precious friend who I can share my joy and sorrow with Dr Liu is full

of knowledge about polymer and membrane science He gave me lots of guidance in my research directions He taught me how to design an experiment and how to study a topic deeply Their dedication to me makes a far-reaching significance to my whole life

I also extend my gratitude to other people who helped me in one way or another: Dr Rongyao Wang and Ms Yanfang Xu for teaching me to do FT-IR; Dr Kaiyu Wang for helping repair my permeation system; Dr Hangzheng Chen and Ms Tingxu Yang for assisting building up the high temperature constant-pressure variable-volume permeation system; Dr Pei Li and Dr Youchang Xiao for their advice and assistance in setting up the new gas chromatography system My other group mates Dr Yin Fong Yeong, Ms Mei Lin Chng, Ms Xue Li, Dr Dingyu Xing, Dr Sui Zhang, Dr Honglei Wang, Dr Natalia Widjojo, Dr Jincai Su, Mr Yi Hui Sim, Ms Mei Ling Chua and all other past and present members are also gratefully acknowledged for the friendship, support and joy

This work also involves many efforts of people from other organizations Professor Jerry Jean and Dr Hongmin Chen from University of Missouri - Kansas City are appreciated for teaching me the positron techniques and for the warm hospitality during my visit to their UMKC lab Thanks to Dr Lili Cui, Dr Rajikiran Tiwari, Dr Norman Horn and Mr Grant T Offord from the University of Texas at Austin for teaching me fabricating and characterizing thin films and sharing the gas chromatography design I am also thankful

to Dr Pramoda from IMRE for helping conducting NMR analysis Professor Young Moo Lee from Hanyang University is also thanked for the sharing and discussion on thermally rearranged polymers

I appreciate the National Research Funding (R-279-000-261-281), A*STAR 044-305) and Tan Chin Tuan Foundation (C-279-000-019-101) for their financial support

(R-398-000-to this work

Last but not least, I give my infinite thanks to my parents and my husband, Peng Thank

my parents for instilling in me a strong motivation to do research and the value of research ethics Thank Peng for his continuous support, encouragement, understanding and the useful discussion about membrane formation during last 4 years

ACKNOWLEDGEMENTS ii

SUMMARY xii

LIST OF TABLES 1

LIST OF FIGURES 3

CHAPTER 1: Introduction & Objective 9

1.1 Membranes for gas separation 10

1.2 Highly permeable polymer membranes for gas separation 11

1.3 Thermally rearranged polymeric membranes 13

1.4 Research objectives and organization of this dissertation 14

1.5 References 16

CHAPTER 2: Literature review & Background 22

2.1 Solution-diffusion model 23

2.2 Dual-mode sorption model 25

2.3 Precursors for thermal rearrangement 26

2.4 Literature review on thermally rearranged membranes for gas separation 30

2.5 References 40

CHAPTER 3: Materials and experimental procedures 45

3.1 Materials 46

3.2 Polymer syntheses 47

3.3 Characterization procedures 47

3.3.1 Gel permeation chromatography (GPC) 48

3.3.2 Inherent viscosity (IV) 48

3.3.3 Nuclear magnetic resonance spectroscopy (NMR) 48

3.3.4 Fourier transform infrared spectroscopy (FTIR) 49

3.3.5 X-ray photoelectron spectroscopy (XPS) 49

3.3.6 Elemental analysis 50

3.3.7 Thermogravimetric analysis (TGA) 50

3.3.8 Thermogravimetric analysis - infrared spectroscopy (TGA-IR) 50

3.3.9 Differential scanning calorimetry (DSC) 51

3.3.10 Dynamic mechanical analysis (DMA) 51

3.3.11 Tensile properties measurement 51

3.3.12 Density measurement and fractional free volume (FFV) 51

3.3.13 X-ray diffraction (XRD) 52

3.3.14 Positron annihilation lifetime spectroscopy (PALS) 53

3.4 Measurements of gas transport properties 54

3.4.1Pure gas permeation properties 54

3.4.2 Mixed gas permeation properties 56

3.4.2.1 Constant-volume variable-pressure system 56

3.4.2.2 Constant-pressure variable-volume system 58

3.4.3 Gas sorption isotherm 62

3.5 References 63

CHAPTER 4: Effect of degree of thermal rearrangement conversion on physicochemical and gas transport properties of thermally rearranged poly(hydroxyamide amic acid) 66

4.1 Introduction and objective 67

4.2 Experimental 67

4.2.1Synthesis of poly(hydroxyamide amic acid) 67

4.2.2 Membrane casting and thermal rearrangement procedures 68

4.2.3 Characterization 70

4.3 Results and Discussion 71

4.3.1 Polymer synthesis and membrane fabrication 71

4.3.2 Characterization results 74

4.3.3 Gas transport properties 94

4.4 Conclusions 100

4.5 References 100

CHAPTER 5: Effect of thermal history and thermal crosslinking on physicochemical and gas transport properties of thermally rearranged polyhydroxyamide 107

5.1 Introduction and objective 108

5.2 Experimental 108

5.2.2 Membrane fabrication and thermal rearrangement procedures 109

5.2.3 Characterization 111

5.3 Results and Discussion 111

5.3.1 Characterization results 111

5.3.2 Gas transport properties 128

5.4 Conclusions 134

5.5 References 135

CHAPTER 6: Effect of purge environment on thermal rearrangement of ortho-functional polyamide and polyimide 139

6.1 Introduction and objective 140

6.2 Experimental 141

6.2.1 Syntheses of ortho-functional polyamide and ortho-functional polyimide 141

6.2.2 Thermal rearrangement procedures 141

6.2.3 Characterization 144

6.3 Results and Discussion 145

6.3.1 Polymer synthesis 145

6.3.2 Characterization results 146

6.3.2.1 DSC analyses 147

6.3.2.2 TGA isotherms 151

6.3.2.3 FTIR and XPS analyses 155

6.3.2.4 Tensile properties 162

6.3.3 Pure gas permeation properties 164

6.4 Conclusions 167

6.5 References 168

CHAPTER 7: Effect of incorporation of cardo moiety on thermal rearrangement of ortho-functional polyimide 173

7.1 Introduction and objective 174

7.2 Experimental 175

7.2.1 Synthesis of polyimide and cardo-copolyimide 175

7.2.2 Membrane casting and thermal rearrangement procedures 176

7.2.3 Characterization 177

7.3 Results and discussion 178

7.3.1 Preparation of polymers and membranes 178

7.3.2 Characterization results 180

7.3.2.1 Structural analyses 180

7.3.2.2 Thermal properties 187

7.3.2.3 Intersegmental distance 190

7.3.2.4 Physical properties 192

7.3.3 Pure gas permeation properties 194

7.3.4 Mixed gas 201

7.3.5 Gas sorption and diffusion coefficients 202

7.4 Conclusions 205

7.5 References 206

CHAPTER 8: Conclusions and recommendations 210

8.1 A short summary of the dissertation 211

8.2 Conclusions 212

8.2.1 The effect of thermal rearrangement temperature 212

8.2.2 The effect of purge environment for thermal rearrangement 214

8.2.3 The effect of incorporation of cardo moiety 215

8.3 Recommendations 216

8.3.1 Aging and plasticization characteristics of TR thin films 216

8.3.2 Fabrication of hollow fibers by using cardo-copolyimide 217

8.3.3 Metal-Organic Framework (MOF)/TR polymer mixed matrix membrane 218

8.3.4 The effect of film thickness on thermal rearrangement 219

8.4 References 220

Appendix: Publications, Patent & Conference presentations 223

SUMMARY

This research work was designed to investigate the effects of thermal rearrangement

temperature, purge environment and precursor structure on thermal conversion of

ortho-functional polymers to polybenzoxazole (PBO) The physicochemical properties of the films before and after thermal rearrangement were studied by thoroughly characterizing the membrane chemical structure changes, thermal transitions, mechanical properties and physical properties Various characterization techniques including elemental analysis, TGA, TGA-IR, DSC, GPC, ATR-FTIR, XPS, XRD and Positron Annihilation Lifetime Spectroscopy (PALS) were applied The films have been used for gas separation The gas permeability and selectivity are presented for comparison

Thermal rearrangement temperature is a key process parameter Its effects were examined

by thermally rearranging poly(hydroxyamide amic acid) (PHAA) and polyhydroxyamide (PHA) films at elevated temperatures up to 450 C The thermally induced structure alteration and polymer chain rearrangement of PHAA derived from 2,2-bis(3-amino-4-hydroxyphenyl)hexafluropropane (BisAPAF) and trimellitic anhydride chloride (TAC) were firstly examined by treating the pristine PHAA films at different temperatures in the temperature range of 200 C to 400 C for 2 h Owing to the bi-functionality of PHAA, stepwise thermal rearrangement takes place At a temperature lower than 300 C, the PHAA structure transforms to poly(imide benzoxazole) (PIBO); at a temperature higher than 300 C, the PIBO structure is converted to PBO The XPS analyses show that the degree of conversion towards PBO structure increases with the increase of thermal rearrangement temperature A high conversion of about 90 % has been attained in the

PHAA film treated at 400 C Following the chemical structure changes, physical properties of the films and gas transport properties also show the stepwise changes The significant increase of gas permeability is well correlated to the increase of polymer inter-chain distance and the enlargement of the free volume radius Besides the degree of conversion, high treatment temperature also introduces other changes that could influence the gas permeation properties The PHA films were thermally rearranged at different temperatures from 300 C to 450 C ATR-FTIR and XPS results show that the PHA films can be cyclized to PBO with a similar conversion degree of around 90% at any temperature above 300 C; however, the gas permeability increases significantly in the film cyclized at higher temperature Analyses based on membrane physical properties and glass transitions prove that the aforementioned phenomenon is attributed to the different thermal history and the different degrees of thermal crosslinking reaction occurred simultaneously to the thermal conversion of PHA towards PBO

The effects of nitrogen and air purge during thermal rearrangement of an ortho-functional polyamide (o-PA) and an ortho-functional polyimide (o-PI) towards PBO structure have

been investigated The o-PA polymer was prepared from BisAPAF and dicarbonyl chloride (BPDC) while the o-PI polymer was derived from 4,4-(hexafluoro-

4,4‘-biphenyl-isopropylidene) diphthalic anhydride (6FDA) and 3,3‘-dihydroxybenzidine (HAB) Experimental results show that the purge environment for the conditions used does not

affect the thermal rearrangement of the o-PA film but significantly affects the thermal conversion of the o-PI film Nearly identical chemical structures and pure gas permeability values are observed for o-PA films thermally treated at 300 C under air or

N2 The o-PI film was thermally rearranged at 425 C because its thermal conversion takes place at a higher temperature range of 300-450 C The o-PI film thermally

rearranged in air exhibits improved gas permeation properties but significantly deteriorated mechanical properties The air purge interrupts the thermal conversion of the

ortho-functional imide to benzoxazole by oxidatively degrading the imide structure and

forming the imine structure As a result, both polymer structure and film properties change

In the last part of this study, the precursor structure was also designed for improved gas separation properties A series of cardo-copoly(hydroxyimide) have been synthesized by polycondensation of 6FDA with various molar ratios of two different bisaminophenols: 3,3‘-dihydroxybenzidine (non-cardo) and 9,9-bis(3-amino-4-hydroxyphenyl)fluorene (cardo) It is found that the incorporation of cardo group has increased the gas permeability significantly Thermally rearranged copolybenzoxazole membrane with the addition of 10 mol% of cardo moiety has shown the largest enhancement in gas permeability As compared to the CO2 permeability in the non-cardo PBO, an increment

of five times in CO2 permeability has been achieved The excellent features of the copolybenzoxazole shows a promising approach for the future cavity engineering of high performance materials

cardo-LIST OF TABLES

Table 2.1: Effect of degree of cyclization on FFV and gas transport properties Gas

permeability, solubility and diffusivity were measured at 50 C and 10 atm 31

Table 2.2: summary of the gas separation performance of thermally rearranged polymer membranes 38

Table 4.1: Solubility parameters of PHAA and selected organic solvents 73

Table 4.2: Mechanical properties of the working polymer film 73

Table 4.3: Elemental analyses for thermally rearranged membranes 74

Table 4.4: d-spacings calculated for film samples treated at different temperatures A bimodal distribution is observed for PBO350 and PBO400 88

Table 4.5: o-Ps lifetime (τ3), intensity (I3) and free volume radius calculated from PATFIT program 91

Table 4.6: Pure gas permeation properties of thermally rearranged membranes 95

Table 5.1: Elemental analyses results for precursor and thermally rearranged membranes 120

Table 5.2: Thermal properties of PHA, PBO300, PBO350, PBO425 and PBO450 films measured under nitrogen atmosphere 122

Table 5.3: Physical properties of PHA and PBO films at 25 °C 126

Table 5.5: Pure gas permeation properties of thermally rearranged membranes at 35 °C and 10 atm 128

Table 5.6: Gas transport parameters of thermally rearranged membranes at 35 °C and 10 atm 130

Table 5.7: Dual-mode sorption parameters of the pristine and the thermally rearranged membranes at 35 °C 134 Table 6.1: Mechanical properties of the APBO films 163Table 6.2: Pure gas permeation properties of the APBO and IPBO films at 35 °C and 10 atm 164 Table 7.1: Precursors synthesized in the present work 179Table 7.2: XPS nitrogen (1s) curve resolution results for thermally cyclized membranes 187Table 7.3: Physical properties of thermally cyclized membranes 193Table 7.4: Pure gas permeation performance of thermally cyclized membranes 195

Table 7.5: Separation performance obtained from binary CO

2/CH

4 (50:50) gas test at 35

°C and 3.5 atm 202Table 7.6: Comparison of gas transport parameters in PBO, CPBOc and CPBO

membranes at 35 °C and 3.5 atm 204

LIST OF FIGURES

Figure 2.1: Schematic of the solution-diffusion process (p 1 and p 2 is the gas pressure at

the upstream and downstream of the film, respectively; C 1 and C 2 represent the gas

concentration at the two sides of the film.) 23Figure 2.2: Reaction mechanism for thermal conversion of hydroxyamide to benzoxazole [6] 27Figure 2.3: Reaction scheme for thermal conversion of hydroxyimide to benzoxazole [13] 28Figure 2.4: Proposed reaction mechanism of the conversion of hydroxyl-imide to

benzoxazole Reproduced from Park et al [14] 29Figure 2.5: Possible reaction scheme for the conversion of hydroxyl-containing

polyimide proposed by Hodgkin and Dao [15] 30Figure 2.6: Possible reaction scheme for the conversion of hydroxyl-containing

polyimide to lactam structure proposed by Rusakova et al [18] 30Figure 2.7: Robeson upper bound (2008) for CO2/CH4 separation [24] The blue dots represent the performance of the TR polymers reported by Park et al [20] Reproduced from Robeson [24] 32 Figure 3.1: Chemical structures of monomers used for polymer syntheses 47Figure 3.2: Schematic for a constant volume-variable pressure gas permeation chamber for testing pure gas permeation properties of a flat membrane 55Figure 3.3: Schematic for a constant volume-variable pressure gas permeation chamber for testing mixed gas permeation properties of a flat membrane 57Figure 3.4: Schematic of mixed gas permeation system [15] 59

Figure 3.5: Membrane cell design and schematic representation of gas flows [15] 60Figure 3.6: Schematic diagram of the dual-volume Cahn D200 microbalance apparatus.62 Figure 4.1: Reaction scheme for PHAA synthesis and its structure changes during

thermal rearrangement 70Figure 4.2: 1H NMR analysis result for polymer precursor 72Figure 4.3: TGA curves of PHAA130, PIBO300 and PBO400 films under nitrogen

atmosphere 75Figure 4.4: TGA-IR spectra of the exhausts released at different temperatures when PHAA130 film was tested 75Figure 4.5: DSC thermograms (first heating curve) of films before and after being

cyclized at different temperatures 79 Figure 4.6a: ATR-FTIR spectra of (a)PHAA130, (b)PIBO200, (c)PIBO220, (d)PIBO300, (e) PBO350 and (f) PBO400 from 900 to 2000 cm-1 80Figure 4.6b: ATR-FTIR spectra of (a) PHAA130, (b) PIBO200, (c) PIBO220, (d)

PIBO300, (e) PBO350 and (f) PBO400 from 1000 to 4000 cm-1 Figure 4 6 82Figure 4.7: 13C NMR spectrum (solid state) of PBO350 sample 83Figure 4.8: N 1s spectra of the surfaces of (a) PHAA130, (b) PIBO200, (c) PIBO220, (d) PIBO300, (e) PBO350 and (f) PBO400 85Figure 4.9: Mole fractions of correlated functional groups based on the deconvoluted XPS N1s spectra 86Figure 4.10: XRD curves of films with thermal treatment at different temperatures (a) PHAA130, (b) PIBO200, (c) PIBO220, (d) PIBO300, (e) PBO350 and (f) PBO400 87

Figure 4.11: Normalized positron annihilation lifetime spectra of the TR films treated at different temperatures One channel corresponds to 49.5 ps 90Figure 4.12: Free volume size distribution obtained from MELT analysis of PALS data for (a) PHAA130, (b) PIBO220, (c) PIBO300, (d) PBO350 and (e) PBO400 93Figure 4.13: The relationship of gas permeability increment and thermal cyclization temperature 96

Figure 4.14: Comparison of CO2/CH4 separation performance of the current thermally rearranged membranes with the Robeson‘s upper bound (1991 and 2008) 99Figure 4.15: Comparison of O2/N2 separation performance of the current thermally

rearranged membranes with the Robeson‘s upper bound (1991 and 2008) 99 Figure 5.1: Reaction scheme for thermal rearrangement of PHA 110Figure 5.2:1H NMR spectrum of PHA 112Figure 5.3: ATR-FTIR spectra of (a) PHA ,(b) PBO300, (c) PBO350, (d) PBO425 and (e) PBO450 films 114Figure 5.4: TGA isothermal experiments simulating thermal rearrangement process 116Figure 5.5: TGA-IR spectra of the exhausts released during the isothermal experiments simulating the thermal rearrangement process for PBO450 sample films 116Figure 5.6: Possible reactions during the thermal rearrangement of PHA 118Figure 5.7: XPS N 1s spectra of the surfaces of (a) PBO300, (b) PBO350, (c) PBO425 and (d) PBO450 119Figure 5.8: TGA curves of PHA and PBO membranes 123Figure 5.9: TGA-IR spectra of the exhausts evolved at different temperatures by testing PHA films with a ramping rate of 15 °C/min under nitrogen atmosphere 123

Figure 5.10: XRD curves of films with thermal treatment at different temperatures (a) PHA, (b) PBO300, (c) PBO350, (d) PBO425 and PBO450 125Figure 5.11: Comparison of CO2/CH4 separation performance of the thermally rearranged membranes with the Robeson‘s upper bound (2008) 129Figure 5.12: (a) CH4 sorption isotherms and (b) CO2 sorption isotherms in pristine and thermally cyclized membranes at 35 °C 132

Figure 6.1: Reaction schemes for the synthesis of the ortho-functional polyamide and its

thermal rearrangement 143

Figure 6.2: Reaction schemes for the synthesis of the ortho-functional polyimide and its

thermal rearrangement 144

Figure 6.3: DSC thermograms (first heating curve) of (a) o-PA films under N2 purge, (b)

o-PA films under air purge 148

Figure 6.4: DSC thermograms (first heating curve) of (a) o-PI films under N2 purging, (b)

o-PI films under air purge 150

Figure 6.5: TGA curves of (a) o-PA films under N2 or air purge, (b) o-PI films under N2 or air purge 151

Figure 6.6: TGA isothermal experiments simulating the thermal rearrangement process of

o-PA films held at 300 C in different purge gases 153Figure 6.7: TGA isothermal experiments simulating the thermal rearrangement process of

o-PA films held at 425 C in different purge gases 154Figure 6.8: TGA isothermal experiments simulating the thermal rearrangement process of

o-PI films at 425 C in different purge gases 155

Figure 6.9: ATR-FTIR spectra of (a) APBO300-N2, (b) APBO300-air, (c) APBO425-N2

and (d) APBO425-air films 156Figure 6.10: Comparison of XPS raw spectra of films thermally rearranged under

different purge atmospheres: (a) APBO300 films, (b) APBO425 films and the

deconvoluted spectra of (c) APBO300 films, (d) APBO425 films 157Figure 6.11: ATR-FTIR spectra of (a) IPBO425-N2 and (b) IPBO425-air films 159Figure 6.12: Comparison of (a) XPS raw spectra of IPBO425 films thermally rearranged under different purge atmospheres and (b) their deconvoluted spectra 160Figure 6.13: Possible nitrogen functional forms with a binding energy of 398.6 eV 161Figure 6.14: Comparison of CO2/CH4 separation performances of the resultant

membranes with the Robeson‘s upper bound (2008) 166 Figure 7.1: Reaction scheme for cardo-copolyimider synthesis and its conversion to cardo-copolybenzoxazole (m and n represent the molar compositions of HAB and BisAHPF, respectively) 177Figure 7.2: Pictures of a precursor membrane and a thermally cyclized copolymer

membrane (*thermally cyclized at 425 °C for 30 minutes) 179Figure 7.3: 1H NMR (DMSO-d6, 300 MHz) spectra of precursor (a) PI, (b)CPIc (90:10) and (c) CPI * Percentage of OH residual calculated based on the signal integration 182Figure 7.4: Solid-state 13C CP/MAS NMR spectra of (a) PI, (b) CPIc (90:10) and (c) CPI 184Figure 7.5: Solid-state 13C CP/MAS NMR spectra of (a) PBO, (b) CPBOc (90:10) and (c) CPBO 185

Figure 7.6: XPS 1N core-level scan spectra for (a) precursor and (b) thermally cyclized membranes 186Figure 7.7: TGA curves of (a) precursor membranes (b) thermally rearranged films 188Figure 7.8: DMA analyses of the precursor films 190Figure 7.9: Wide-angle X-ray diffraction patterns of (a) precursor and (b) thermally cyclized membranes 191Figure 7.10: Effect of BisAHPF molar composition on pure gas permeability 197Figure 7.11: Effect of BisAHPF molar composition on FFV 198Figure 7.12: (a) O2/N2 and (b) CO2/CH4 permeation performance of PBO, CPBOc and CPBO membranes compared with Robeson‘s upper bound [19], cPBO= chemically-imized PBO [16]; PIM= polymers with intrinsic microporosity [20]; PI-g-CD = beta-cyclodextrin grafted polyimide [21] 200Figure 7.13: CO2 plasticization behavior of thermally cyclized PBO, CPBOc (90:10) and CPBO membranes 201

CHAPTER 1: Introduction & Objective

1.1 Membranes for gas separation

A brief but complete definition of ‗membrane‘ describes it as [1]:

‘a phase or a group of phases that lies between two different phases, which is physically and/or chemically distinctive from both of them and which, due to its properties and the force field applied, is able to control the mass transport between these phases.’

After continuous efforts made in the last centuries, membrane science and technology now has become one of the integrated subjects among separation and purification technologies Membranes as a selective barrier have been used in many fields such as water purification, liquid-liquid separation, chiral molecules separation, gas separation and so on Among these applications, membranes for gas separation are still a young topic Currently, the studies on gas separation membranes mainly focus on H2purification [2, 3], CO2 capture [4, 5], natural gas sweetening [6, 7], ammonia production,

N2 enrichment from air and hydrocarbon recovery [8-10], etc

The first published study on membranes for gas separation dates back to 1866 when Sir T Graham measured the permeation rates of gases and proposed what is sometimes called Graham‘s law of diffusion [11] In 1879, based on the Graham‘s model, S von Wroblewski defined the permeability coefficient as the quantitative measurement for gas permeation rates in a membrane [12] The foundation of the model theories for gas transport in a membrane were well established by Barrer [13], Lonsdale [14], Stern [15], Meares [16] and Paul [17], after Fick proposed his laws to describe diffusion phenomena

[18] In 1950s, attributed to the invention of the semicrystalline polyolefins and their

application in packaging industry, the measurement to permeation rate of a film became

of paramount importance This greatly stimulated the research interests in the fields of gas transport phenomena and membrane material design The discovery of asymmetric membranes by Loeb and Sourirajan in the 1960s made the commercialization of membrane technology possible [19] Later, Henis and Tripodi fabricated the first gas separation membrane PRISM® for industrial use by coating a very thin layer of a highly permeable polymer on top of an asymmetric hollow fiber membrane substrate [20] Up to now, many materials have been fabricated into hollow fibers modules or spiral-wound modules for different gas separation processes Cellulose acetate, polyimide and polysulfone are popularly used materials owing to their low cost and/or good stability

1.2 Highly permeable polymer membranes for gas separation

Many materials could be made into a membrane Ceramic and zeolite, carbon, metal and polymer are the four major types of gas separation membrane materials Since polymers are of low cost and exhibit feasible processability as compared to other materials, many polymers have been introduced and attempted for gas separation since 1970s However, polymer materials usually show low permeability, which limits their application

The current research on membrane materials is no longer confined to barrier properties of

a polymer membrane A membrane with high permeability and reasonable selectivity is desired in most of the refinery and related industries, because the higher permeation rate could effectively reduce the footprint and operation cost of a membrane system [13, 21, 22]

Polymers like polydimethylsiloxane (PDMS), polyethylene glycol (PEG), polyionic liquid represent highly permeable rubbery polymers In addition, these membranes do not age over time However, their weak mechanical properties and poor thermal stability are always a concern for industrial scale-up In terms of stability, glassy polymers outperform rubbery materials Therefore, in recent years, many glassy materials with improved gas separation performance have been introduced Masuda et al [23] reported on the formation of substituted polyacetylenes, e.g., poly[l-(trimethylsily1)-1-propyne] (PTMSP) by using metal halides as the catalyst This material was found to be ultra permeable to all gases, but at the cost of low selectivity Budd et al [24, 25] introduced a category of materials named polymers of intrinsic porosity (PIM) by incorporating a rigid ladder structure with contortion sites into the polymer backbone The contortion caused

by the spiro-center strongly inhibited the regular alignment of polymer chains As a result, the fractional free volume (FFV) of the film is very high, leading to the very high gas permeability Among various PIMs reported to date, the PIM-1 polymer derived from 5,5‘,6,6‘-tetrahydroxy-3,3,3‘,3‘-tetramethyl-1,1‘-spirobisindane and 1,4-dicyanotetra-fluorobenzene has shown excellent gas permeation properties but its selectivity is relatively low [26] Polyimides are another category of polymers that have been widely investigated, especially with a focus on structure-property relationships [6, 27-33] Among the polyimides investigated, the one based on 4,4‘-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) shows similar high gas permeability but low selectivity [32,

34, 35]

As seen, the high permeability usually trades off with the selectivity in a polymeric membrane Thus, the development of new materials with reasonable selectivity while remaining the high permeability still needs continuous effort

1.3 Thermally rearranged polymeric membranes

As a new category of polymers with microporosity, thermally rearranged (TR) polymers have greatly stimulated the research interests in the fields of polymer science and technology [36, 37] TR polymer films are obtained by thermally cyclizing the ortho-

positioned aromatic precursor at a temperature above 300 C under vacuum or nitrogen

purging condition During thermal cyclization, the ortho-positioned group reacts with the

aromatic ring, which finally results in a more rigid heterocyclic structure Depending on

the functional group at the ortho-position (-OH, -SH or -NH2), the resultant thermally rearranged structures can be in the form of polybenzoxazoles (PBO), polybenzothiazoles (PBT) or polypyrrolones (PPL) [36]

The resultant insoluble heterocyclic membranes are of good thermal resistance, strong chemical resistance, high modulus and strength, low flammability, low water absorption and low dielectric constant [38-41] Thus, they are used widely in microelectronic devices [42] and semiconductor industry [43-45]

Recently, Park et al declared another attractive application of these TR polymers [37]

By thermally rearranging the solid film derived from hexafluropropane (BisAPAF) and 4,4-(hexafluoroisopropylidene) diphthalic anhydride

2,2-bis(3-amino-4-hydroxyphenyl)-(6FDA) at a temperature higher than 400 C, the resultant TR films have shown superior gas separation performance of CO2 permeability at ~2000 Barrers and CO2/CH4

permselectivity of 45 [37] As compared to the pristine polyimide film, the gas permeability of TR films increased by around hundred folds, while there is a minor decrease in selectivity It is proposed that the high free volume nature of the TR polymers

is the cause of the sudden raise in permeability, while the rigid-rod benzoxazole structure and the proposed hourglass-shaped cavities formed during structure rearrangement contribute to the relatively stable selectivity

1.4 Research objectives and organization of this dissertation

The TR polymer membrane has become a promising candidate for future applications in the industrial gas separation processes owing to its remarkable gas separation performance However, besides the aforementioned, there is limited research performed

to explore the various factors that may influence thermal rearrangement and the properties of the resultant films

In this work, we systematically investigated the effect of three factors: thermal rearrangement temperature, purge environment and TR precursor structure, on thermal rearrangement process The chemical structure changes, polymer physicochemical properties and gas separation performance of the thermally rearranged films are the major aspects explored The effect of thermal arrangement temperature was studied by focusing

on degree of chemical structure conversion, film thermal history and the possible thermal crosslinking mechanism Two purge atmospheres, N2 and air, for thermal rearrangement

were compared by using different TR precursor polymers The design of TR precursor structure was also attempted so as to improve the separation performance of the available

TR membranes

Precursors with different functionalities, including poly(hydroxyamide amic acid)

(PHAA), polyhydroxyamide (PHA) or functional polyamide (o-PA) and functional polyimide (o-PI) were selected for different objectives PHAA comprises both

ortho-hydroxyamide and hydroxy(amic acid); thus, its stepwise thermal conversion attained by varying thermal rearrangement temperature is of the interest The high-temperature treatment could cause many other effects besides the different degrees of conversion Thus, subsequently, PHA was chosen in view of the much lower thermal rearrangement temperature We designed the experiment to convert PHA for the similar degree at elevated temperature The effects of thermal history and possible thermal crosslinking reaction on gas transport properties in the resultant films were explored The TR films reported in the literature are usually obtained under inert atmosphere, which results in additional cost for membrane fabrication In order to examine whether air could be used

in the TR process, two different purge environments (nitrogen versus air) were applied to

treat the o-PA and o-PI films The feasibility of conducting thermal rearrangement in air

was evaluated Additionally, precursor structure design by incorporating rigid planar cardo moiety into polymer backbone via copolymerization was also conducted to improve gas separation performance Various characterization techniques including elemental analysis, TGA, TGA-IR, DSC, ATR-FTIR, XPS, XRD, DMA and Positron

Annihilation Lifetime Spectroscopy (PALS) have been utilized to thoroughly examine the evolution of the polymer properties during thermal rearrangement

This dissertation is divided into eight chapters Chapter 1 gives a brief introduction on membrane technology, highly permeable membrane materials and thermally rearranged polymer membranes Chapter 2 presents the background on the fundamentals of gas transport phenomena in a film and the literature review about thermally rearranged polymers Chapter 3 describes the details about materials used and experimental procedures The investigation on poly(hydroxyamide amic acid) as the TR precursor was presented in Chapter 4 Chapter 5 explores the thermal rearrangement of polyhydroxyamide and the effect of thermal history and thermal crosslinking are illustrated with sufficient evidences from characterization Chapter 6 studies the effect of purge environment The thermal transitions, structure changes, mechanical properties and gas permeation properties of the films were compared In Chapter 7, the success of improving gas permeation properties by incorporating cardo moieties in polymer backbone was reported The enlargement of fractional free volume was closely associated

to the increase of permeability Lastly, Chapter 8 summarizes the whole dissertation by giving conclusions Recommendations for future work will also be presented

1.5 References

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CHAPTER 2: Literature review & Background

2.1 Solution-diffusion model

The solution-diffusion model is well accepted to describe the permeation of gas molecules through a dense polymer film According to this model, the gas molecules firstly dissolve/sorb into the membrane Subsequently, the gas molecules diffuse through the membrane driven by a concentration gradient Finally, the gas molecules desorb at the other side the membrane and diffuse into the bulk [1] Therefore, the separation of a gas mixture is determined by both solubility of a gas in a membrane and diffusion rate at which gas molecules transport through the solid film Figure 2.1 demonstrates the solution-diffusion process

Figure 2.1: Schematic of the solution-diffusion process (p 1 and p 2 is the gas pressure at

the upstream and downstream of the film, respectively; C 1 and C 2 represent the gas

concentration at the two sides of the film.)

According to Fick‘s first law, the flux of a pure gas in a membrane can be represented as:

where N is the flux of a gas, D is the diffusion coefficient of a gas in the membrane,

dC/dx is the concentration gradient of a gas cross the membrane

By assuming D is a constant at any point in the membrane, Eq 2.1 can be integrated to

where C 1 and C 2 are the gas concentrations at the respective upstream surface and

downstream surface of a membrane and l is the membrane thickness

At the early stage of gas permeation measurement, it was observed that the gas flux is

thickness and pressure dependent Thus, the gas permeability coefficient, P, is defined as:

where p is the tran-membrane pressure gradient and l is the membrane thickness

By substituting Eq 2.2 into Eq 2.3, P can be expressed as:

2 1

2 1 2

C C D p

2.2 Dual-mode sorption model

Equilibrium gas sorption in a glassy polymer at a temperature below its glass transition temperature is widely described by the dual-mode sorption model [2-4] It is proposed that two concurrent mechanisms, dissolution and hole-filling, occur simultaneously in gas sorption process [4, 5] The dissolution follows the Henry‘s law and occurs in the so-called Henry sites which consist of the densified regions of the polymer However, the hole-filling process follows the Langmuir isotherm and saturates the Langmuir sites formed by the microvoids or the excess free volume generated from the polymer chain

packing defects The overall sorption is illustrated as a sum of Henry dissolution (C D) and

Langmuir sorption (C H) as shown in Eq 2.8: