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Thesis for the Degree of Doctor of Philosophy
Synthesis of water-insoluble methyl cellulose by graft
polymerization and membrane application
그라프트 공중합을 이용한 비수용성 메틸셀룰로오스의
합성 및 막응용
February 2014
By
Hye-Ryun Ahn
Department of Biosystems & Biomaterials
Science and Engineering
Synthesis of water-insoluble methyl cellulose by graft
polymerization and membrane application
그라프트 공중합을 이용한 비수용성 메틸셀룰로오스의
합성 및 막응용
지도교수 탁 태 문
이 논문을 농학박사 학위논문으로 제출함
2013년 12월
서울대학교 대학원
바이오시스템·소재학부
바이오소재전공
안 혜 련
안혜련의 박사 학위논문을 인준함
2014년 2월
위 원 장 현 진 호 (인)
부위원장 탁 태 문 (인)
위 원 박 영 환 (인)
위 원 이 기 훈 (인)
위 원 한 성 수 (인)
Department of Biosystems & Biomaterials
Science and Engineering
SEOUL NATIONAL UNIVERSITY
Supervisory Committee Approval
of Thesis submitted by Hye-Ryun Ahn
This thesis has been read by each member of the following
supervisory committee and has been found to be satisfactory.
Chairman of Committee :
Vice Chairman of Committee :
Member of Committee :
Member of Committee :
i
Abstract
The development of novel polymer materials with high functionality has received attention in recent decades. Among the natural polymers, cellulose and its derivatives are widely used, because they are inexpensive, most abundant, biodegradable and renewable. Methyl cellulose (MC) is a highly functional cellulose derivative with properties such as being non-ionic and having pH stability, hydrophilicity and high water retention. But since MC is water soluble, some applications are limited, for example, use as a water treatment membrane. Therefore, in this study, high functional MC-based graft copolymers were prepared by the reaction of redox system polymerization with ceric ion initiation. In order to graft the water insoluble copolymer with a soluble organic solvent, water soluble MC as a main backbone was used by changing the molar ratio of the grafted vinyl monomer without the destruction of the inherent MC properties. The various vinyl monomers such as, acrylonitrile (AN), methyl methacrylate (MMA) and styrene (ST) were grafted onto the MC backbone by ceric (IV) ion- initiated free radical polymerization in aqueous medium. Ceric (IV) ion-initiated polymerization was a useful method to obtain high molecular weight molecules. The reaction was homogeneous and the grafting percentage (%G) could be easily be controlled. The hydrophilicity, thermal stability, molecular weight, zeta-charge and mechanical strengthe were investigated using Fourier transform-infrared spectroscopy (FT-IR),
ii 1
H nuclear magnetic resonance (1H NMR), thermogravimetric analysis (TGA), gel chromatography (GPC), zeta-potential, water contact angle and universal testing machine (UTM), respectively. The produced graft copolymers showed an improvement in thermal stability and hydrophilicity, maintaining the properties of MC. In addition these graft copolymers could be applied for antifouling water-treatment membranes. The feasibility of these copolymers as water-water-treatment membranes was investigated by flux, rejection and antifouling test using a dead-end filtration system. The prepared copolymer membranes showed different properties, varying by the grafted monomer. The AN graft copolymer represented remarkable antifouling properties compared with the two other grafted copolymers.
Keywords: Methyl cellulose, vinyl monomer, Acrylonitrile, Graft copolymer,
Antifouling membrane.
iii
TABLE OF CONTENTS
Abstract ………..………..…...…….. i
Table of Contents ………..…….……….……iii
List of Figures ……….………..………..vii
List of Tables .……….………….…….….……...……. xvi
1. Introduction ………..………...………..… 1
2. Literature survey ………..…… 5
2.1. Methyl cellulose ……….…….... 5
2.2. Graft copolymerization of cellulose polymers ……….…….….... 10
2.2.1. Free radical graft polymerization ……… 13
2.2.2. The kinetics of ceric (IV) ion-initiated reaction ….……… 21
3. Experimental ………...……… 23
iv
3.2. Synthesis of MC-based copolymers ……….……...….. 24
3.3. Characterizations of MC-based graft copolymers ….…...….. 28
3.4. Membrane fabrication using MC-based graft copolymers … 30
3.5. Characterization and evaluation of membranes ………. 33
4. Results and Discussion ………...…….……..… 36
4.1. Graft reaction and structural analysis of MC-g-MMA
copolymer ………...………...……. 36
4.1.1. Optimization of reaction conditions ………....….. 36
4.1.2. Graft yield and efficiency of MC-g-MMA copolymer …..….. 41
4.1.3. Structural analysis of MC-g-MMA copolymer …... 43
4.2. Graft reaction and structural analysis of MC-g-ST copolymer
………..………….……...……...……. 48
4.2.1. Optimization of reaction conditions ………....….. 48
v
4.2.3. Structural analysis of MC-g-ST copolymer …...….. 55
4.3. Graft reaction and structural analysis of MC-g-AN copolymer
………..………….……...……...……. 60
4.3.1. Optimization of reaction conditions ………....….. 60
4.3.2. Graft yield and efficiency of MC-g-AN copolymer …….….. 65
4.3.3. Structural analysis of MC-g-AN copolymer …...….. 67
4.4. Properties of MC based graft copolymers ………. 72
4.4.1. Solubility of MC-based graft copolymers ….…………... 72
4.4.2. Effect of grafted monomer on hydrophilicity …………... 74
4.4.3. Effect of grafted monomer on thermal stability ………... 76
4.4.4. Performance of membranes ………..…. 78
4.4.5. Effect of grafted monomer on antifouling membranes…... 83
4.5. Effect of the %G on physical and chemical properties of MC-g-
AN graft copolymer ………... 90
vi
4.5.1. Contact angle ……….………... 90
4.5.2. Zeta-potential ……….………... 92
4.5.3. XPS .……….………... 93
4.5.4. Antifouling property ...……….... 96
4.5.5. Morphological structure ……..……… 105
4.5.6. Molecular weight ……...……..……….. 105
4.5.7. Mechanical strength …...……..……….. 106
5. Conclusion ……….….. 111
6. References ……… 113
vii
List of Figures
Figure 1. The chemical structure of cellulose raw material ……… 8
Figure 2. The etherification route from cellulose to methyl cellulose …………..… 9
Figure 3. A schematic representation of cellulose graft copolymer ……… 11
Figure 4. A schematic representation of the “grafting-from” approach ………..… 12
Figure 5. The general mechanism of cellulose graft copolymerzation via ferrous reagent initiation ……… 18
Figure 6. The simplified mechanism of ceric (IV) ion-initiated cellulose graft copolymerization ………...……… 19
Figure 7. Reaction mechanism of MC-based graft copolymer using ceric (IV) ion- initiation process (vinyl monomer: AN, MMA, ST)………..…… 27
Figure 8. Scheme of dead-end membrane filtration system (Test conditions - 25°C, stirring rpm; 350-400 rpm) ……… 35
viii
Figure 9. The relative viscosity of MC-g-MMA copolymer solution during free radical polymerization as a function of CAN concentration (HNO3: 7.5x10-2 M, MMA:10.0 x10-2 M, 25°C, 1 h) ………...… 38
Figure 10. The relative viscosity of MC-g-MMA copolymer solution during free radical polymerization as a function of reaction time (HNO3: 7.5x10
-2 M, CAN: 15.0x10-3M, MMA: 10.0x10-2 M, 25°C) ……...………… 39
Figure 11. The relative viscosity of MC-g-MMA copolymer solution during free radical polymerization as a function of reaction temperature temperature (HNO3: 7.5x10-2 M, CAN: 15.0x10-3M, MMA: 10.0x10-2 M, 1 h) ……….……… 40
Figure 12. The effect of molar ratio of MMA on the percentage of grafting (HNO3:7.5x10-2 M, CAN: 15.0 x10-3M, 25°C, 1 h) ………...… 42
Figure 13. The FT-IR transmittance spectra of base polymer MC and the MC-g-MMA copolymer (upside one: control MC, down side one: MC-g-MMA grafted copolymer) ………..………… 45
ix
Figure 14. 1H nuclear magnetic resonance peaks of base polymer MC and the MC-g-MMA copolymer (upper one: MC-MC-g-MMA copolymer, down: MC base polymer, NMR solvent: DMF) ………..……… 46
Figure 15. The C 1s level of control MC and MC-g-MMA graft copolymer (X-ray source: monochromatic Al-Kα, 15 kV, 100W, 400 micrometer) ……... 47
Figure 16. The relative viscosity of MC-g-ST copolymer solution during free radical polymerization as a function of CAN concentration (HNO3: 3.5 x 10
-2 M, STY: 1.5 x 10-2 M,25 °C, 1h) ………...……….……… 50
Figure 17. The relative viscosity of MC-g-ST copolymer solution during free radical polymerization as a function of reaction time (HNO3: 3.5 x 10-2M, STY: 1.5 x 10-2 M, CAN:1.5 x 10-3, 25 °C) ….…………..…… 51
Figure 18. The relative viscosity of MC-g-ST copolymer solution during free radical polymerization as a function of reaction temperature (HNO3: 3.5 x 10-2M, STY: 1.5 x 10-2 M, CAN:1.5 x 10-3, 1h) ……...………… 52
Figure 19. The effect of molar ratio of ST on the percentage of grafting grafting (HNO3: 3.5 x 10-2M, CAN:1.5 x 10-3, 25 °C) ………..………...…… 54
x
Figure 20. The FT-IR spectra of control MC and MC-g-ST graft copolymer (upper one: MC backbone peak, down: Mg-ST copolymer peak; aromatic C-C, C=C-C, C-H paek) ……… 57
Figure 21. 1H nuclear magnetic resonance peaks of MC and the MC-g-ST graft copolymer (NMR solvent: DMF) ……… 58
Figure 22. The XPS spectra of MC-g-ST graft copolymer (X-ray source: monochromatic Al-Kα, 15kV, 100W, 400 micrometer, no ion etching condition) ………..……… 59
Figure 23. The relative viscosity of MC-g-AN copolymer solution during free radical polymerization as a function of reaction time (HNO3: 7.5 x 10-2 M, CAN: 7.5 x 10-3 M, AN: 60.0 x 10-2 M, 25 °C) ……… 62
Figure 24. The relative viscosity of MC-g-AN copolymer solution during free radical polymerization as a function of CAN concentration (HNO3: 7.5
x 10-2 M, AN: 60.0 x 10-2 M, 25 °C, 1 h) ……… 63
Figure 25. The relative viscosity of MC-g-AN copolymer solution during free radical polymerization as a function of reaction temperature (HNO3: 7.5 x 10-2 M, CAN: 7.5 x 10-3 M, AN: 60.0 x 10-2 M, 1 h) ……...……… 64
xi
Figure 26. The effect of molar ratio of acrylonitrile monomer on the percentage of grafting (HNO3: 7.5 x 10
-2
M, CAN: 7.5 x 10-3 M, 25 °C, 1 h) …..… 66
Figure 27. The FT-IR spectra of control MC and MC-g-AN copolymer (upper : MC backbone polymer peak, down : MC-g-AN copolymer peak) ……..… 69
Figure 28. 1H NMR spectra of control MC and MC-g-AN graft copolymer (NMR solvent: DMF) ………...……… 70
Figure 29. The XPS spectra of the control MC (a) and MC-g-AN (b) copolymer (X-ray source: monochromatic Al-Kα, 15kV, 1000W, 400 micrometer, no ionic etching ………..………… 71
Figure 30. Contact angle images and values of MC-based graft copolymer varying on grafted monomer (AN, MMA and ST) and control PAN membrane (%G 100 copolymer, average value 5 times, measurement : 25 °C, 20 % humidity) .…...………..……… 75
Figure 31. TGA analysis of MC-based graft copolymers (MC-g-AN, MC-g-MMA, MC-g-ST) and MC ………..….……. 77
xii
Figure 32. The PWF and rejection of MC-g-MMA graft copolymer membrane (Feed:DI water, test cell area: 28.5 cm2, 25°C, 30min level off before test) ……….. 80
Figure 33. The water flux and BSA rejection of MC-g-ST copolymer membrane (Feed: DI water, 25°C, at 2 bar 30 min level off before test, BSA: 1000ppm) ……….... 81
Figure 34. The %G dependent PWF and BSA rejection of MC-g-AN copolymer membranes during dead-end filtration process (Feed : DI water, 1000ppm BSA, 2bar 30 min level off before test, 25°C, 20 min collect permeant) ………. 82
Figure 35. Time-dependent normalized flux of MC-based copolymer membranes varying on monomer, during dead-end filtration with BSA solution (Feed: 1000ppm, 25°C, collect: every 1min for 60 min automatically) ………..……… 84
Figure 36. Flux recovery ratio of MC-based copolymer membranes varying on monomer, during dead-end filtration with BSA solution (cleaning: DI water, 1 bar, 30 min) ……… 85
xiii
Figure 37. Time-dependent normalized flux of MC-based copolymer membranes varying on monomer, during dead-end filtration with SA solution (Feed: 1000ppm, 25°C, collect: every 1min for 60 min automatically) …… 86
Figure 38. Flux recovery ratio (FRR) of MC-based copolymer membranes varying on monomer, during dead-end filtration with SA solution (cleaning: DI water, 1 bar, 30 min) ………..……… 87
Figure 39. Time-dependent normalized flux of MC-based copolymer membranes varying on monomer, during dead-end filtration with HA solution (Feed: 1000ppm, 25°C, collect: every 1min for 60 min automatically) …… 88
Figure 40. Flux recovery ratio (FRR) of MC-based copolymer membranes varying on monomer, during dead-end filtration with HA solution (cleaning: DI water, 1 bar, 30 min) ………..……… 89
Figure 41. The zeta-potential of MC-g-AN copolymer membranes varying on percentage of grafting ……….……… 94
Figure 42. Time-dependent flux of MC-g-AN copolymer membranes varying on %G, during BSA filtration (Feed : 1000ppm, 25 °C, 20 % humidity, collect : every 1 min for 60 min automatically) ……… 97
xiv
Figure 43. Flux recovery ratio (FRR) of MC-g-AN copolymer membranes using different %G membrane filtration; BSA 1000ppm (cleaning condition : 1bar 30 min water clening, 25 °C, 20 % humidigy) …………..…….... 98
Figure 44. Time-dependent flux of MC-g-AN copolymer membranes varying on %G, during SA filtration (Feed : 1000ppm, 25 °C, 20 % humidity, collect : every 1 min for 60 min automatically) ……… 99
Figure 45. Flux recovery ratio (FRR) of MC-g-AN copolymer membranes using different %G membrane filtration; SA 1000ppm (cleaning condition : 1bar 30 min water clening, 25 °C, 20 % humidigy) ……….... 100
Figure 46. Time-dependent flux of MC-g-AN copolymer membranes varying on %G, during HA filtration (Feed : 1000ppm, 25 °C, 20 % humidity, collect : every 1 min for 60 min automatically) ……… 101
Figure 47. Flux recovery ratio (FRR) of MC-g-AN copolymer membranes using different %G membrane filtration; HA 1000ppm (cleaning condition : 1bar 30 min water clening, 25 °C, 20 % humidigy) ……….... 102
xv
Figure 48. Time-dependent flux of MC-g-AN copolymer membranes varying on %G, during HA filtration (Feed : 1000ppm, 25 °C, 20 % humidity, collect : every 1 min for 60 min automatically) ……… 103
Figure 49. The flux recovery ratio (FRR) of MC-g-AN copolymer membranes using different model foulants filtration; HA, SA, BSA (cleaning condition : 1bar 30 min water clening, 25 °C, 20 % humidigy) ………..……… 104
Figure 50. The cross-section of MC-g-AN graft copolymer membranes varying on %G (a) 100 %, (b) 140 % and (c) 180 % (FE-SEM mode, gold coating) ……… 107
Figure 51. The molecular weight of MC-g-AN graft copolymers varying on %G (before measurement at least 4 h level off, DMF column, RI detector) ………..……… 108
Figure 52. The effect of %G on tensile strength of the MC-g-AN graft copolymer (test condition: 25 °C, 20 % humidity) ……… 109
xvi
List of Tables
Table 1. The summary of monomer reactivities under various conditions ….…. 20
Table 2. The composition of the MC-based graft copolymers membrane casting solutions ……… 32
Table 3. The solubility of MC-based graft copolymers .….…….………. 73
Table 4. The contact angle values of the MC-g-AN graft copolymers …...……… 91
Table 5. Elemental surface composition of membranes of various %G of MC-g-AN graft copolymers ……….…………..…….. 95
1
1. Introduction
Nowadays, according to increasing demand for environmentally friend and biocompatible materials, a number of research studies have reported the development of high functional novel materials. Among the many kinds of polymer materials, cellulose is the most abundant natural polymer material, and it has a fascinating structure and desirable properties[1,2]. Cellulose and cellulose derivatives are widely used in various applications, such as membranes, pharmaceuticals, coating films, food processing, and so on. Many properties of cellulose, both physical and chemical are significantly different from those of synthetic polymers. Despite all its well established and interesting properties, cellulose lacks some of the versatile properties of synthetic polymers[3]. Recently, modification of cellulose and cellulose derivatives by graft copolymerization has been studied to increase the functionality and the scope of cellulose use[3-5]. Depending on the chemical structure and property of the monomer grafted onto a cellulosic polymer, graft copolymers gain new properties such as hydrophilic, hydrophobic character, water absorption, and heat resistance[6-8].
Methyl cellulose (MC) is one of the cellulose derivative made by etherification of cellulose; the hydroxyl functional groups of the cellulose backbone are replaced by alkyl groups (methyl group). Many properties of biodegradable, inexpensive,
2
non-ionic and pH stability can be applied to food processing, construction industry and pharmaceutical area. Despite all its properties, methyl cellulose used additives and restricted the using to wide application because of water soluble property of MC. Until now, there have been few reports on the grafting of MC-based graft copolymer and its applications so in this study, MC-based graft copolymers were synthesized by a one-step redox polymerization[9, 10].. Ceric (IV) ion-initiated graft free radical copolymerization is a well-known, suitable and the simplest method for incorporation of vinyl monomers onto cellulose derivatives without any destruction of the original properties and with minimized undesirable side-reactions compared to the photosensitizer, magnetic field, and radiation methods[11-13]. This study concentrate on the properties and characterization of the MC and MC-based graft copolymers further, explain the optimization of the reaction conditions for grafting vinyl monomers for example, acrylonitrile (AN), methyl methacrylate (MMA) and styrene (ST) monomers onto MC obtained by etherification of cellulose. And report on the effects of the reaction parameters, such as initiator concentration, reaction temperature, reaction time and molar ratio of monomers. The grafting percentage (%G) was controlled by molar ratio of monomer. The %G with significant parameter, change the physical and chemical properties of graft copolymers. These structural and chemical properties of the graft copolymers are characterized via several analytical processes. Fourier transform infrared spectroscopy (FT-IR), 1H Nuclear magnetic resonance (1H NMR) and X-ray photoelectron spectroscopy (XPS) were used for structural and chemical properties
3
of graft copolymers. The Gel chromatography (GPC), Differential scanning calorimetry (DSC) and Thermo gravimetric analysis (TGA) were used to confirm the molecular weight and thermal properties of graft copolymers.
Additionaly, the synthesized MC-based graft copolymers were applied to the antifouling water-treatment membranes. Membrane technology has been considered as a useful tool in separation, purification and fractionation process and polymeric membranes play an irreplaceable role in the membrane technology due to their versatile and tuneable properties, ease of manufacture, and cost effectiveness[14, 15]. However, membrane fouling caused by the intrinsic hydrophobic character of polymeric membranes is a major obstacle for their wide industrial applications. The adsorption and/or deposition of many foulant molecules on membrane surface increases the hydraulic resistance, thereby reducing the filtration flux and inducing a serious impact on the efficiency and economics of the membrane process[16-20]. Many studies have been investigated to solve the fouling problems, including development of high functional new polymeric materials. In this point, synthesized MC-based graft copolymer in this study, have advantage to apply antifouling membrane because of the MC-based copolymer which have hydroxyl groups and form intra- and intermolecular hydrogen-bonds between hydroxyl groups thereby, MC-based graft copolymer excellent hydrophilicity, despite non-soluble in water. The synthesized copolymers also showed high hydrophilicity and electrical neutrality, which can resist non specific foulants adsorption effectively with graft
4
copolymer membrane[21-23]. The MC-based graft copolymer membranes were fabricated via traditional phase-inversion method to apply as water-treatment membranes[24]. The instrumental analysis was performed to confirm the hydrophilicity and electrical neutrality of MC-based copolymer membranes by contact angle measurement and zeta-potential analysis. And morphological structure was showed by Scanning electron microscope (SEM) observation. The antifouling properties of prepared membranes were estimated with model foulant solutions. Bovine serum albumin (BSA), Sodium alginate (SA) and Humic acid (HA) were used as model foulants. The time-dependent flux and normalized flux were evaluated in dead-end flow mode filtration system[25, 26].
In this study, synthesis and characterization of novel hydrophilic MC-based graft copolymers by ceric (IV) ion-initiated free radical copolymerization with AN, MMA and ST monomers also, presenting the application of these graft copolymers with antifouling water-treatment membranes.
5
2. Literature survey
The preceding parts deal with the synthesis of polymers by polymerization reactions and explain the detailed methyl cellulose properties. These include the etherification of cellulose and grafting of various monomers.
2.1. Methyl cellulose
The modern chemical industry has an ever-increasing demand for polymeric materials with improved or novel properties, high functionality requires and environmentally friend properties to develop the appropriate applications. Recently, most research is devoted to development of new polymer materials that are, for example, applied as textile, membranes, electric device and medical polymer, food processing, pharmaceutical, and so on[4, 27].
Cellulose is a natural polymer, a long chain made by the linking of smaller molecules. The links in the cellulose chain are a type of sugar; β-D-glucose (in Figure 1)[28, 29]. As the cellulose from woody plants, the most abundant organic raw materials and used the wide area applications and versatile starting material for subsequent chemical conversion, in order to the production of artificial cellulose-based threads and films as well as of a variety of stable soluble cellulose derivatives
6
to be used in many areas of industry and people’s life. In recent year, to improve the functionality and the scope of their use increase the attention of cellulose derivaties. Cellulose etherification is a very important branch of commercial cellulose derivatization that started considerably later than the conversion of the polymer to esters. Methyl cellulose (MC) is one of the cellulose derivative made by etherification of cellulose; the hydroxyl functional groups of the cellulose backbone are replaced by methoxide groups. The methylation of cellulose, which is usually classified as an SN2 reaction, is the result of the nucleophilic attack of the cellulosic alkoxido group on the acceptor C atom of the methyl chloride[30-32]. In Figure 2 showed the methyl cellulose production mechanism. The first step, cellulose is treated with approximately 50 % aqueous sodium hydroxide at 60 °C for about 20 minutes. The resulting alkali cellulose is reacted with methyl chloride (CH3Cl). The degree of etherification of the MC produced be regulated by control of the reaction conditions; commercial products with a degree of substitution (DS) between 1.5 and 2.0 are obtained by a Williamson reaction of alkali cellulose with gaseous or liquid CH3Cl. The most relevant applicational properties of methyl cellulose are the solubility and solution properties. In the cold, aqueous solution of methyl cellulose obtained because of the polymer chains are hydrated by water molecules which allow the chains to move over one another so that a viscous solution is obtained. On heating to about 60 °C the water molecules break away, their lubricating action is lose and the chains interlock; the solution therefore gels[30, 33-35].
7
dispersion stabilizing agent, food products and pharmaceuticals in many areas of industry due to MC has anionic charge, water retention, pH stability, non-toxicity and biocompatibility and compatible with other ionic and non-ionic additives in aqueous solutions while providing a stable combination with them dissolved in water. And MC provides excellent viscosity stability in long-term storage due to its resistance against fungi and bacteria attacks.
8
9
10
2.2. Graft copolymerization of cellulose polymers
The graft copolymerization is one of the best way of improving the physical and chemical properties of cellulose materials[3, 36, 37]. This synthesis usually employed for a covalent attachment of polymer side chains onto a cellulosic backbone, but free radical polymerization of vinylic compounds initiated by a redox system or by high-energy radiation dominates by far. In Figure 3 shows the schematic structure of a cellulosic graft copolymer. The graft copolymerizations of cellulosic polymer are carried out heterogeneous as well as homogeneous systems and the methods are based on various approaches[3, 38, 39]. The first, the “grafting-to” approach, an end functional pre-formed polymer with its reactive end-gorup is reacted with the functional groups that are located on the cellulose backbone. The “grafting through” is convenient but requires the synthesis of a macromonomer. The most commonly use procedure is the “grafting-from” approach. This approach can be achieved high grafting density because of the easy access of the reactive groups to the chain ends of the growing polymers and the growth of polymer chains occurs from initiating sites on the cellulosic backbone (see in Figure 4)[3, 40, 41].
11
12
13
2.2.1. Free radical graft copolymerization
The free radical polymerization has received the greatest amount of attention among all of the polymerization methods. Indeed, about 60 % of all available polymers are still obtained by this method and remarkable among chain polymerization processes in that they can be conveniently conducted in aqueous medium further advantages of free radical polymerization following[2, 42, 43].
1. Its applicability to the polymerization of a wide range of monomers such as methacrylates, styrene, acrylonitrile and vinyl acetate.
2. Its ability to provide an unlimited number of copolymers.
3. Its tolerance to a wide range of functional groups (OH, NR2, COOH) and reaction conditions (bulk, solution, emulsion, suspension).
4. Its tolerance to water or other impurities in contrast to the great sensitivities of ionic and coordination polymerization.
5. The convenient, mild reaction conditions and wide temperature range under which it can be conducted.
In the free radical copolymerization of cellulosic polymer, the mechanism generally involves the “grafting-from” approach. The reaction starts the formed free radical onto the cellulosic backbone. Free radicals on the polymer backbone can be formed by chemical initiators such as peroxide, potassium persulfate and direct
14
oxidation via ceric (IV) ions. Among the various chemical initiation methods, the formation of free radicals on the cellulose molecules by direct oxidation with ceric (IV) ions has gained considerable importance, because of its ease of application and its high grafting efficiency compared with other redox systems.
The graft copolymerization of many monomers onto polymer backbone has been carried out. Monomer reactivity depended to a large extent on the grafting conditions employed. The findings of several investigators are summarized in Table 2. In general, the rate of polymerization with acrylate monomers decreased with increasing alkyl length. This was presumably due to changes in diffusion, steric hinderance, polarity and solubility with increasing monomer size. The low reactivity of some monomers, such as acrylic acid, and to a lesser extent acrylonitrile, was attributed to the sorption of ceric (IV) ions by the graft copolymer. The influence of temperature on grafting yields depended upon the type of initiator, generally used cerium ammonium nitrate (CAN) decrease experienced in grafting at high temperature. Due to the CAN is not stable at elevated temperature and increased rate of termination[43-45]. A desirable change in a specific property was accompanied by grafting copolymerization; improvement of mechanical strength, water sorption and penetration, thermal stability and acid resistance, etc.
The redox initiation is directly formed the free radicals on the cellulosic polymer backbone by oxidation-reduction reactions. A prime advantage of redox initiation is that radical production occurs at reasonable rates over a very wide range of temperatures, depending on the particular redox system, including initiation at
15
moderate temperatures of 0 to 50 °C and even lower and they are frequently used for initiation of emulsion polymerization. This allows a greater freedom of choice of the polymerization temperature than is possible with the thermal homolysis of initiators[46, 47]. The mechanism of redox initiation is usually bimolecular and involves a single electron transfer as the essential feature of the mechanism that distinguishes it from other initiation processes while others involve the intermediate formation of reductant-oxidant complexes; the latter are charge-transfer complexes in some cases. Common components of many redox systems are peroxide and a transition metal ion or complex. Various transition metal salts or complexes oxidize or reduce cellulosic polymer substrates by single electron transfer and radicals formed from the cellulosic polymer compound may initiate polymerization[48-50].
The cellulosic polymer material can be chemically converted into peroxide, it can then decompose into radicals and initiate either graft copolymerzation, or homopolymerization. This homopolymerization can be reduced by the use of a reducing agent. Peroxides in combination with a reducing agent and act a common source of radicals; for example, the reaction of hydrogen peroxide with ferrous ion (in Figure 5). Other reductants such as Cr2+, V2+, Ti3+, Co2+ and Cu+ can be employed in place ferrous ion in many instances[12, 51].
The ceric (IV) ion-initiated polymerization has been described by redox system. The mechanism of redox initiation is usually bimolecular and involves a single electron transfer as the essential feature of the mechanism that distinguishes it from other initiation processes. Redox initiation systems are in common use when
16
initiation is required at below ambient temperature and they are frequently used for initiation of emulsion polymerization. Ceric (IV) ion is common component of redox systems formed transition metal ion or complex[52-54]. Ceric (IV) ions oxidize polymeric materials and the mechanisms typically involve radical intermediates and when conducted in the presence of a monomer these radicals may initiate polymerization and the ceric (IV) ion-initiated in different acid media. The reaction of ceric (IV) ions with polymer-bound functionalities gives polymer-bound radicals. Thus, one of the major applications of ceric (IV) ion-initiation chemistry has been in grafting onto starch, cellulose, polyurethanes and other polymers. The advantage of this over conventional initiating systems is that, ideally, no low molecular weight radicals which might give homopolymer contaminant are formed. The principles of the reaction are illustrated in Figure 6 (ex. Cellulose)[55, 56]. The cellulosic polymer molecule contains several possible sites for oxidation by ceric (IV) salts; the hydroxyl at the C6 position, the glycol group at the C2-C3 position and the hydroxyl at the end of the cellulose chain. Support for grafting at the C2-C3 glycol group with cleavage of the C-C bond was obtained by others. The graft polymerization mechanism is based on between cellulosic polymer and monomer interaction and is applicable to uncatalyzed grafting systems as well as ceric (IV) ion-initiated systems. The cellulosic agent acts as a complexing agent for acceptor monomers which causes a delocalization of electrons in the double bond, which in turn, increases the electron accepting ability of the monomer. Between donor and acceptor complexes undergo spontaneous initiation or may be initiated by oxygen or
17
radicals formed on the cellulosic backbone. The ceric (IV) ion-initiated grafting copolymerization method is useful process to obtain cellulosic polymer-based graft copolymers with high molecular weight[57-60].
18
Figure 5. The general mechanism of cellulose graft copolymerzation via ferrous reagent initiation.
19
Figure 6. The simplified mechanism of ceric (IV) ion-initiated cellulose graft copolymerization[91].
20
Table 1. The summary of monomer reactivities under various conditions.
Order of reactivity Temperature (°C)
MA > AN > MMA > ST 20 MA > AN = MMA > ST 40 MA > MMA > AN > ST 60 MA > EA > MMA 30 EA > AN > AA 30 MA > AN > AA 25 EA > MMA > AN > ST 25
*MA : methylacrylate, AN : acrylonitrile, MMA : methyl methacrylate, ST : styrene, AA : acrylic acid
21
2.2.2. The kinetics of ceric (IV) ion-initiated reaction
The ceric (IV) ion-initiated polymerization was found to proceed without an induction period, and the steady-state rate was attained in a short time. The rate of monomer disappearance was found to bear a square dependence on monomer concentration and is independent of both the ceric (IV) and hydrogen ion concentration[47, 61].
The ceric (IV)-alcohol redox systems generally involved two categories depending on the termination mode. This polymerization proceeds in the same manner as other polymerization in terms of the propagation and termination steps without that the source of radicals for the initiation step. For this polymerization, termination is by bimolecular reaction of propagating radicals. However, some redox polymerizations for example, in aqueous medium reaction, only ceric (IV) ions alone with no reducing agent, involve a change in the termination step from the usual bimolecular reaction to monomolecular termination involving the reaction between the propagating radicals and a component of the redox system. The reaction sequence is followed below.
22 Initiation Termination (1) (2)
Where, Ri and Rp are rate of initiation and polymerization, k(I,p,t) represent rate
constant. By steady-state assumption (Ri = Rt), therefore, polymerization rate as
(3)
Where, [M] is a monomer concentration, this result suggested that the chain lengths of polymers were directly proportional to the monomer concentration[62-64].
23
3. Experimental
3.1. Materials and reagents
Methyl cellulose powder supplied by Samsung Fine Chemicals (Mecellos FMC 60150, Seoul, Korea), with 1.9 degree of substitution and viscosity of 4000 cP was used in 2% solution without further purification. Acrylonitrile (AN, Mn 53 g/mol), methyl methacrylate (MMA, Mn 100 g/mol), styrene (ST, Mn 104 g/mol) and cerium ammonium nitrate (CAN) were purchased from Sigma Aldrich and used as a monomer and initiator, respectively. The nitric acid was used to dissolve the CAN, and hydroquinone (99%, Daejung Chemicals & Metals Co., Ltd, Seoul, Korea) was used as a stopper of the graft copolymerization. N,N-dimethylformamide (DMF), N,N-dimethylacetamide(DMAc) and N-methyl-2-pyrrolidone(NMP) were obtained from Sigma Aldrich, and used as solvents. Methanol, acetone, petroleum ether were purchased from Samchun Chemicals (Korea) for the purification process. Three target foulants, bovine serum albumin, sodium alginate and humic acid were purchased from Sigma Aldrich (USA). All other reagents were analytical grade, and were used as received, without further purification.
24
3.2. Synthesis of MC-based copolymers
The grafting of MMA onto MC was carried out in a nitrogen atmosphere by free radical polymerization (in Figure 7). First, 1.0 g of MC was added to a three-necked flask with a reflux condenser. Then, 100 mL of a solution of cerium ammonium nitrate (5.0 × 10-3 M to 25 × 10-3 M) prepared in 2.0 × 10-2 M nitric acid was poured into the flask. The flask was fitted with an electrically operated stirrer and immersed in 35°C to 60°C water bath. The solution was initiated with nitrogen gas for about 30 min before the MMA monomer (5.0 × 10-1 M to 12.5 × 10-1 M) was added to generate the radicals. The reaction mixture was stirred at a constant rate of 200 rpm, and the reaction carried out for 30 minutes to 180 minutes, after which the reaction product was precipitated in acetone and petroleum ether. The copolymers formed were filtered and washed several times with water and MeOH to remove unpolymerized MC and poly (methyl methacrylate) homopolymers. The graft copolymers synthesized were then dissolved in DMAc and re-precipitated with acetone and petroleum for purification. The purified graft copolymer was dried to a constant weight under vacuum at 50°C for 24 h.
MC-g-ST copolymer was synthesized by free radical polymerization in Figure 7. Determined amount of MC (6g) dissolved in 150mL nitric acid solution (1.7 x 10-1 M) and added desired amount (1.0 × 10-2 M to 2.5 × 10-2M) of cerium ammonium nitrate in the three-necked round-bottom flask. The flask was fitted with an electrically operated stirrer and kept in a water bath (25°C to 60°C). The solution
25
was initiated with nitrogen gas for about 30 min before the styrene monomer (5.0 × 10-1 M to 12.5 × 10-1 M) was added to generate the radicals. The reaction mixture was stirred at a constant rate (200 rpm), the grafting reaction was carried out for various time intervals (1 h to 3 h). At the end of the reaction, 1 wt% hydroquinone was added in the reaction mixture to stop the graft copolymerization, and then the reaction product was precipitated in acetone and then washed MeOH / petroleum ether mixture (5 : 5). The purified grafted copolymer was dried under vacuum for 24 hours[76].
The graft copolymerization of polyacrylonitrile onto MC (in Figure 7) was carried out by adding 1.0 g of MC in a three-necked round-bottom flask containing the desired amount (5.0 x 10-3 to 12.5 x 10-3 M) of cerium ammonium nitrate in a 100 mL solution of 7.5 x 10-2 M nitric acid. The flask was fitted with an electrically operated stirrer and kept in a water bath (25°C to 60°C). The solution was purged with nitrogen gas for about 30 min before adding the acrylonitrile monomer (45.0 x 10-2 to 70.0 x 10-2 M) into the flask to initiate the graft copolymerization. The reaction mixture was stirred at a constant rate. The grafting reaction was carried out for various time intervals (1 h to 3 h). At the end of the reaction, 1 wt% hydroquinone was added in the reaction mixture to stop the graft copolymerization, and then the reaction product was precipitated at 80°C. The copolymers formed were filtered, washed several times with MeOH and water, to remove unpolymerized methyl cellulose and other impurities, and then graft copolymers synthesized were dissolved in DMF and reprecipitated with MeOH : water = 1 : 1,
26
for purification. The purified grafted copolymer was dried under vacuum to a constant weight at 50°C.
27 O O O CH2 O O O OCH3 HO H2C OCH3 O OCH3 Ce4+ O O O O CH2 OH Ce4+ complex
Figure 7. Reaction mechanism of MC-based graft copolymer using ceric (IV) ion initiation process (vinyl monomer: AN, MMA, ST).
28
3.3 Characterizations of MC-based graft copolymers
The terms of relative viscosity (ratio of the viscosity of a solution to the viscosity of the solvent) of the copolymer was used in order to determine the optimum reaction conditions by following equation[77, 78]
Relative viscosity = t / to (4)
Where t is the copolymer solution (0.5 g/dl) dropping time and to is the pure
solvent dropping time, when DMF, DMAc and NMP were used as the solvent for the copolymer solution. The relative viscosity of the MC-based copolymer was measured and calculated using an Ostwald viscometer (C606, Cannon, PA, USA). The details of the apparatus are shown in Figure 8. The time taken for the level of the liquid to pass between two marks (one above (A) and one below (B) the upper bulb) is proportional to the kinematic viscosity. The higher relative viscosity values mean a high molecular weight of the copolymers prepared during the grafting process, indicating more monomer grafted to the MC polymer backbone. The degree of grafting copolymerization was evaluated in terms of percentage of grafting (%G) and percentage of grafting efficiency, (%GE) by the following equations[8, 79].
29
In Eq. 5, W1 and (g) W2 (g) are the dry weight of the MC and produced copolymer, respectively. The fourier transform infrared (FT-IR) spectra and the 1H nuclear magnetic resonance (1H NMR) of the synthesized copolymers were recorded with a Nicolet 6700 FT-IR spectrometer (Thermo Scientific, IL, USA) with a KBr pellet and 1H NMR (AVANCE 600, Bruker, Germany) with dissolved DMF. The X-ray photoelectron spectroscopy (Sigma Probe, ThermoVG, U.K.) was used to investigate the chemical composition of the C, N, and O elements in the synthesized copolymer with a monochromatized radiation source (Al-Kα, 15.0 kV, 100 W). The weight percent and the ratio between different peak binding enengies of carbon atom were calculated from Cls core level spectra. The (YL9100 HPLC, Young Lin Instrument Co., Ltd., Korea) and TGA (Q-5000 IR, TA Instruments, USA) was used to investigate the molecular weight with used DMF eluent at the flow rate of 1.0ml / min at 25°C and thermal stability of MC control and MC-based copolymers. The mechanical strength was measured using UTM.
The solubility of MC-based graft copolymers with organic solvent and water was evaluated. The DMF, DMAc and NMP were used as organic solvent with graft copolymers which copolymers were dissolved in each organic solvent.
30
3.4. Membrane fabrication using MC-based graft copolymers
Membranes were prepared using the classical immersion precipitation phase- inversion method with water as the coagulant and under equivalent conditions. Table 3 shows the compositions of the membrane casting solutions. The casting solutions were prepared with various % G by dissolving MC-g-MMA copolymer in DMAc. After complete dissolution in DMAc, the copolymer solutions were cast (200 μm in thickness) onto non-woven fabric supports, and the casts were exposed to the atmosphere (humidity: 30 %, temperature: 25 °C) for 30 sec. The casts were then immersed in a coagulation water bath at 25 °C for membrane formation. After they were peeled off the glass plate, the membranes were rinsed with, and stored in, water for at least 24 h before use[25, 81].
The MC-g-ST copolymer membrane was fabricated by phase-inversion method using NMP as the solvent. Table 3 shows the compositions of the membrane casting solutions with various % G. The casting solutions were cast onto non-woven fabric with a nominal thickness of 200 μm using a lab-developed casting knife. After casting, the membrane was exposed to the atmosphere (humidity: 30 %, temperature: 25 °C) for 30 sec. and then immersed into water bath (25 °C) for at least 24 h until most of the solvent was removed.
The casting solutions for membrane fabrication were prepared with various % G. The compositions of the casting solution were represented in Table 3. The MC-g-acrylonitrile copolymer was dissolved in N-N-dimethylformamide (DMF). After
31
complete dissolution in DMF, the copolymer solutions were cast (200 μm in thickness) onto non-woven polyester fabric, and exposed to the atmosphere (humidity: 30 %, temperature: 25 °C) for 30 sec. The casts were then immersed in a coagulation water bath at 25 °C for membrane formation and rinsed with water for at least 24 h.
32
Table 2. The composition of the MC-based graft copolymers membrane casting solutions.
Membrane Polymer (wt %) Solvent
MC-g-AN 9 91
MC-g-MMA 8 92
MC-g-ST 6 94
*Membrane casting conditions : 25°C, 30% humidigy, thichness: 200µm 25°C water coagulation, 30 sec evaporation.
33
3.5. Characterization and evaluation of membranes
In order to investigate hydrophilicity of the fabricated membranes, the static water contact angle values were measured using a drop shape analyzer (DE/DSA100; KRUSS), and an average of at least five measurements were recorded.
All filtration experiments were conducted using a dead-end filtration cell (Model 8200; Millipore Co.). The details of the apparatus are shown in Figure 8. Following fouling experiments a cleaning step is conducted to characterize cleaning efficiency of membranes. The working volume of the cell was 200 mL, and it was fitted with a membrane disc with an effective diameter of 63.5 mm within the cell and an effective membrane surface area of 28.7 × 10-4 m2. All experiments were performed at room temperature, and the membranes were initially compacted at a 0.20 MPa transmembrane pressure (TMP) for 30 min. Then, pure water flux (PWF) was measured at 0.10 MPa TMP and calculated using the following equation[18, 82]:
Jw1= V/(A × t) (6)
where Jw1 (L / (m2h)) is the PWF, V (L) is the volume of permeated water, A (m2) is the effective membrane area, and t (h) is the permeation time. Rejection was calculated by measuring the concentration of BSA (1000 mg / L) in the feed and the
34
permeate solution using UV-vis spectroscopy (S-3100; SCINCO) and with the following equation:
%R = (1- Cp / Cf) × 100 (7)
where Cp and Cf are the BSA concentrations of the permeate and feed solutions,
respectively. In order to evaluate the antifouling resistance of the prepared membranes, 1000 mg/L BSA, sodium alginate and humic acid solutions were tested as a model protein, polysaccharide and organic matter. After measuring initial water flux and rejection, model foulant solution was measured. The flux decline was recorded for 1 h using the software name of “Ohaus balance talk” automatically. At the end of fouling test, the solution in the feed reservoir was discarded and cleaning of the fouled membrane was performed. Cleaning of fouled membrane with distilled (DI) water for 30 minutes and applied 0.1 Mpa pressure and the PWF of the cleaned membranes (Jw2) (LMH) measured again. The flux recovery test was performed to
investigate the cleaning efficiency of membranes.
Flux recovery ratio (FRR) was calculated using the following expression:
FRR (%) = ) 100 (8)
Where JW1 is initial water flux, JW2 is water flux after membrane cleaning[17, 21,
35
Figure 8. Scheme of dead-end membrane filtration system (Test conditions : 25°C, stirring rpm; 350-400 rpm)[83].
36
4. Results and Discussion
In this chapter, synthesis of MC-based graft copolymers reaction conditions were optimized and characterized the properties, such as hydrophilicity, thermal stability, mechanical strength, morphological structure and chemical compositions.
4.1. Graft reaction and structural analysis of MC-g-MMA
copolymer
4.1.1. Optimization of reaction conditions
The graft copolymerization of methyl methacylate onto methyl cellulose has been studied at different reaction conditions such as CAN concentration, reaction time and reaction temperature. On varying the concentration of CAN, the relative viscosity represents improvement with CAN concentration until 15.0 x 10-3 M and then decreased beyond 15.0 x 10-3 M (in Figure 9). This can be explained that the CAN concentration increases, the number of free radicals produce on the methyl cellulose backbone which is apparent from the increasing trend in relative viscosity. Thereafter, the decrease in relative viscosity at higher concentration of CAN at constant concentration of nitric acid has been due to the decrease in ratio of nitric
37
acid to CAN. Therefore the hydrated forms of ceric (IV) ions were produced, which were not able to produce active sites onto the methyl cellulose backbone as a result, decrease in relative viscosity of reaction product. The evident of effect of reaction time on the grafting polymerization is shown Figure 10. The relative viscosity increases with time up to 1 h and then levels off. The leveling of relative viscosity after 1 h can be attributed to the depletion of initiator and monomer concentration with the progress of the reaction. It is also explained that as hydroxyl groups are occupied by the graft chains and the methyl cellulose backbone is not amenable for grafting anymore. The effect of reaction temperature was observed from 25 to 60 in Figure 11. The relative viscosity has shown an increasing trend up to 50 °c. On increase of the temperature, the kinetic energy of monomer molecules has increased which has increased the concentration of monomer molecules nearby to the active sites onto the methyl cellulose because of the enhanced rate of diffusion of monomer molecules. This trend decreased on higher reaction temperature beyond 50 °c due to the increase in the rate of chain transfer and chain termination reactions between grafted chains and monomer molecules.
38 CAN concentration (10-3M) 5 10 15 20 25 R e la ti v e v is c o si ty ( t / t o ) 1.5 2.0 2.5 3.0
Figure 9. The relative viscosity of MC-g-MMA copolymer solution during free radical polymerization as a function of CAN concentration (HNO3: 7.5x10-2 M, MMA:10.0 x10-2 M, 25°C, 1 h).
39 Time (min) 0 50 100 150 R e la ti v e v is c o si ty ( t / t o ) 1.0 1.5 2.0 2.5 3.0
Figure 10. The relative viscosity of MC-g-MMA copolymer solution during free radical polymerization as a function of reaction time (HNO3: 7.5x10
-2
M, CAN: 15.0x10-3M, MMA: 10.0x10-2 M, 25°C).
40 Temperature (℃) 35 40 45 50 55 60 R e la ti v e v is c o si ty ( t / t o ) 0.5 1.0 1.5 2.0 2.5 3.0
Figure 11. The relative viscosity of MC-g-MMA copolymer solution during free radical polymerization as a function of reaction temperature (HNO3: 7.5x10
-2 M, CAN: 15.0x10-3M, MMA: 10.0x10-2 M, 1 h).
41
4.1.2. Graft yield and efficiency of MC-g-MMA copolymer
As shown in Figure 12, the percentage of grafting increased up to 1 : 10 (molar ratio of MC : MMA) and then decreased with increasing concentrations of the MMA monomer. This indicates that the maximum fraction of monomers was consumed during the formation of graft chains. The %G decreases beyond a molar ratio of 1 : 10 (MC : MMA) because of the increase in the viscosity of the medium because of the high concentration of MMA at a molar ratio of 1 : 10 (MC : MMA) and the static hindrance created by the grafted MMA chains, which retarded the reaction between the MC backbone and the monomer. The maximum graft percentage (% G) was at a molar ratio of 1 : 10 (MC : MMA), and it showed a decreasing trend with an increase in the concentration of the MMA monomer. This result indicates that a large number of monomer participated in the homopolymerization and the generated homopolymers were removed during the purification process.
42
MMA / MC molar ratio
6 8 10 12 % G 50 100 150 200 250
Figure 12. The effect of molar ratio of methyl methacylate monomer on the percentage of grafting (HNO3: 7.5x10
-2
43
4.1.3. Structural analysis of MC-g-MMA copolymer
In Figure 13 shows the FT-IR spectra of control MC and MC-g-MMA graft copolymer. Since the MMA grafted copolymer contains carboxylic group, ether bond and methoxy group, etc functional group, these peaks are confirmed in FT-IR spectra. In this result, successful grafting was confirmed by characteristic absorption of MMA monomer. In Figure 17, at 2949 cm-1, 1724 cm-1, 1266 cm-1 and 1189 cm-1 peaks were represented -CH2, C=O, C-O-C and -OCH3 of MMA, respectively. The peak 1143 cm-1 (C-C of MMA) and 750 cm-1 (-CH rocking of MMA), these peak were not observed for MC. These results indicated that MMA monomer was successfully attached to the MC backbone.
Figure 14 shows the 1H NMR spectra of the MC-g-MMA copolymer. The MMA groups introduced onto the MC backbone were further confirmed by 1H NMR spectroscopy; the peaks at 3.6 ppm and 3.5 ppm correspond to the peak of – OCH3 of MMA and the MC backbone, respectively (denoted in Figure 18 as a and b). The – CH2 group is indicated at 2.0 ppm, 1.9 ppm and 1.8 ppm, and the peaks at 1.2 ppm, 1.0 ppm, and 0.8 ppm correspond to the – CH3 of MMA. The mole ratio was calculated by peak area ratio, the mole ratio of synthesized MC-g-MMA copolymer’s MC : MMA contents was 1 : 3, however, actual used MC : MMA mole ratio was 1 : 12. The obtained in this 1H NMR peak was lower than the feed composition and % GE was calculated as 24 %. These results prove the successful incorporation of MMA moieties onto MC backbone.
44
The difference of chemical composition between MC and MC-g-MMA copolymer was confirmed by XPS analysis. The C 1s core level spectra of control MC and MC-g-MMA copolymer are shown in Figure 15. The spectra was curve-fitted with peak components with binding energies at 289.0 eV and 285.0 eV for the O-C=O species and C-H species. At 289.0 eV peak is newly represented by grafted MMA monomer however, 289.0 eV peak not shown in MC peak. This result explained that the MMA monomer grafted onto the MC backbone.
45 Wavenumbers(cm-1) 1000 1500 2000 2500 3000 3500 % T MC MCMMA C =O C-C C-O-C -OCH3
Figure 13. The FT-IR transmittance spectra of base polymer MC and the MC-g-MMA copolymer (upside one: control MC, down side one: MC-g-MMA grafted copolymer).
46
Figure 14. 1H nuclear magnetic resonance peaks of base polymer MC and the MC-g-MMA copolymer (upper one: MC-MC-g-MMA copolymer, down: MC base polymer, NMR solvent: DMF).
47 Binding energy(eV) 280 282 284 286 288 290 C o u n ts /s 0 500 1000 1500 2000 2500 3000 Methyl cellulose MC-g-MMA C - H COO C 1s
Figure 15. The C 1s level of control MC and MC-g-MMA graft copolymer (X-ray source: monochromatic Al-Kα, 15 kV, 100W, 400 micrometer).
48
4.2. Graft reaction and structural analysis of MC-g-ST copolymer
4.2.1. Optimization of reaction conditions
The free radical polymerization of MC-g-ST copolymer was carried out various reaction conditions. The effect of CAN initiator concentration was studied from 1.0 x 10-3 M to 2.5 x 10-3 M and degree of grafting polymerization was evaluated in terms of relative viscosity in Figure 16. The relative viscosity increased up to 2.0 x 10-3 M of CAN and then decreased beyond 2.0 x 10-3 M. It is likely that cerium ions exclusively participated in the formation of active sites on the MC until no more active sites were formed on the MC. Then, beyond 2.0 x 10-3 M, cerium ions enhance the possibility of termination of the backbone radicals before grafting takes place. Further, at relatively high concentration of the initiator, the grafting reaction competes with the homopolymerization, thereby leading to a decrease in relative viscosity. The reaction time and reaction temperature also affected the grafting of the ST monomer onto MC backbone. The evident of effect of reaction time and reaction temperature on the grafting polymerization is shown Figure 17 and Figure 18. The relative viscosity increases with time up to 1 h and then levels off. The leveling of relative viscosity after 1 h can be attributed to the depletion of initiator and monomer concentration with the progress of the reaction. It is also explained that as hydroxyl groups are occupied by the graft chains and the methyl cellulose backbone is not amenable for grafting anymore. The increasing reaction
49
temperature, the kinetic energy of monomers increases. The relative viscosity has shown an increasing trend up to 40 °c and then decreased beyond 40 °c. Until 40 °c the kinetic energy increased the concentration of monomers near the active sites onto the methyl cellulose due to the enhanced diffusion rate. However beyond 40 °C this effect was decreased because of chain transfer reaction and homopolymerization reaction might be accelerated in an aqueous medium at high temperatures.
50 CAN concentration (10-3 M) 1.0 1.5 2.0 2.5 R e la ti v e v is c o si ty ( t/ to ) 1.5 2.0 2.5 3.0 3.5
Figure 16. The relative viscosity of MC-g-ST copolymer solution during free radical polymerization as a function of CAN concentration (HNO3: 3.5 x 10
-2
M, ST: 1.5 x 10-2 M,25 °C, 1h).
51
Reaction time (min)
0 30 60 90 120 R e la ti v e v is c o si ty ( t/ to ) 1.0 1.5 2.0 2.5
Figure 17. The relative viscosity of MC-g-ST copolymer solution during free radical polymerization as a function of reaction time (HNO3: 3.5 x 10
-2
M, ST: 1.5 x 10-2 M, CAN:1.5 x 10-3, 25 °C).
52 Reaction temperature (℃) 30 40 50 60 R e la ti v e v is c o si ty ( t/ to ) 2.72 2.74 2.76 2.78 2.80
Figure 18. The relative viscosity of MC-g-ST copolymer solution during free radical polymerization as a function of reaction temperature (HNO3: 3.5 x 10
-2
M, ST: 1.5 x 10-2 M, CAN:1.5 x 10-3, 1h).
53
4.2.2. Graft yield and efficiency of MC-g-ST copolymer
The effect of reaction parameter, such as molar ratio, on graft percentage (%G) was investigated and shown in Figure 19. It can be seen that increasing the ratio of styrene monomer to methyl cellulose raise the degree of %G until 1 : 4 (MC : ST), but the molar ratio beyond 1 : 4 (MC : ST), the %G was decreased. This indicates that the maximum fraction of monomers was consumed during the formation of graft chains because of the %G decreases beyond a molar ratio of 1 : 4 (MC : ST). This might be attributed to the increase in the viscosity of the medium because of the high concentration of MMA at a molar ratio of 1 : 4 (MC : ST) and the static hindrance created by the grafted MMA chains, which retarded the reaction between the MC backbone and the styrene monomer. The maximum graft percentage (% G) was at a molar ratio of 1 : 4 (MC : ST), and it showed a decreasing trend with an increase in the concentration of the styrene monomer. These results can be explained that a large number of monomer participated in the homopolymerization and the generated homopolymers were removed during the purification process.
54
Styrene / MC (molar ratio)
2 3 4 5 6 % G 50 100 150 200 250
Figure 19. The effect of molar ratio of ST on the percentage of grafting (HNO3: 3.5 x 10-2M, CAN:1.5 x 10-3, 25 °C).
