Building Nanostructured Polystyrene Latex Beads Covered

BUILDING NANOSTRUCTURED POLYSTYRENE LATEX BEADS COVERED

WITH POLYOXOMETALATE CLUSTERS

A Thesis

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

Xinyue Chen

May, 2015 BUILDING NANOSTRUCTURED POLYSTYRENE LATEX BEADS COVERED

WITH POLYOXOMETALATE CLUSTERS

Xinyue Chen

Thesis

Approved: Accepted:

______Advisor Dean of the College Dr. Tianbo Liu Dr. Eric J. Amis

______Faculty Reader Interim Dean of the Graduate School Dr. Mesfin Tsige Dr. Rex Ramsier

______Department Chair Date Dr. Coleen Pugh

ii ABSTRACT

Polyoxometalates (POMs) are a class of metal oxide frameworks, whose structures, morphologies, and elecrionic properties can be designed and tuned to meet with different demands.1 POM-organic hybrids are usually made by linking organic part onto the active site of POMs. They have been widely applied in the field of catalysis, electronics, medicine and biology because they have some extraordinary properties.2 As far as making POMs into catalysts, there are some problems: 1) poor processability and recyclability for homogenous catalyst; 2) low efficiency for the heterogeneous catalyst.3

In this work, we developed a new way to fabricate POM based latex particles via emulsion polymerization. These nanoparticles are formed with core-shell structure with polystyrene as core and POMs covered the surface and can be used as quasi-homogenous catalyst, which are easy to recycle and also can maximize the catalytic efficiency. This work explored a new idea to make POM-base catalysts.

iii ACKNOWLEDGEMENTS

I would like to thank my adviser, Dr. Tianbo Liu, for providing me the opportunity to conduct this project, and thanks to Dr. Panchao Yin, who help me a lot with the experiments and knowledge, and thank Dr. Mesfin Tsige to be the committee member and reader. I also would like to thank all the group members in Dr. Tianbo Liu’s group for their help with my experiment and helpful discussion of this project. Finally, I would like to thank all my friends and my families for supporting me in everything.

iv TABLE OF CONTENTS Page

LIST OF FIGURES ...... vii

LIST OF TABLES ...... ix

CHAPTER

I. INTRODUCTION ...... 1

1.1Introduction to Macroions ...... 1

1.2 Introduction to Polyoxometalates ...... 2

1.3 Introduction to Polyoxometalate-Organic Hybrid Materials ...... 4

1.4 Solution Behavior of Polyoxometalates and Their Hybrids ...... 6

1.5 Introduction to Polyoxometalate-based catalyst ...... 8

1.6 Nanostructured Polyoxometalate-Polymer Latex Beads ...... 10

II. EXPERIMENTS AND PROCEDURES ...... 12

2.1. Chemicals and Equipment ...... 12

2.2 Instrumentation ...... 13

2.3 Synthesis of Sodium Tungstophosphate, Na9[A-PW9O34] (Tri-lacunary Keggin POM) ...... 13

2.4 Synthesis of Amphiphlic POM-MMA hybrid, K3[A-(RSiO)3(SiR)] (R =

{H2C=C(CH3)O(CH2)3}) ...... 14

v 2.5 Emulsion Copolymerization of Amphiphilic POM-based hybrid and Styrene .. 16

2.6.1 Laser Light Scattering ...... 17

2.7 1H, 13C, 31P Nuclear Magnetic Resonance (NMR) Spectroscopy ...... 18

2.8 Fourier Transform Infrared Spectroscopy (FT-IR) ...... 18

2.9 Transmission Electron Microscope ...... 18

2.10 Thermogravimetric (TGA) Analysis ...... 19

2.11 Zeta Potential Analysis ...... 19

2.12 Measurement of Critical Micelle Concentration (CMC) ...... 19

III. RESULTS AND DISCUSSION ...... 20

3.1 Micelle Formation of POM-MMA hybrid ...... 20

3.2 The Measured Critical Micelles Concentration (CMC) ...... 22

3.3 Characterization of POM-PS Particles ...... 24

3.4 Zeta Potential Measurement ...... 30

3.5 Thermal Stability and Composition Analysis ...... 30

IV. CONCLUSION...... 33

REFERENCES ...... 34

APPENDIX ...... 36

vi LIST OF FIGURES Figure Page

1. Illustration of the three categories of electrolyte solutions...... 2

2. POM anions with different topologies and sizes...... 3

3. Main coordination modes of organic groups covalently linked to POM units via p-block elements...... 5

4. Molecular structures for hexavanadate-based hybrid surfactants, 1 and 2...... 8

5. The equation for the catalytic oxidation reaction of thiophene...... 9

6. a) Picture of the emulsion catalytic system of surfactant in aqueous solution at pH 6. b) Models for the catalytic system. c) Model for the biphasic catalytic reactions...... 9

7. Molecular structure of the hybrids...... 14

8. Molecular structure of Tri-lacunary Keggin POM...... 14

9. Molecular structure of POM-based hybrid...... 16

10. Typical experiment for emulsion polymerization...... 17

11. DLS results of the assemblies of POM-hybrid surfactant in water...... 21

12. The size estimate of the hybrid...... 22

13. Plot of conductivity to concentration of the hybrid water solution...... 24

14. Model for hybrid stabilized emulsion and the particles after polymerization...... 25

15. CONTIN analysis of DLS studies on POM-Polymer nanobeads with 5mg mL-1 and vii 15mg mL-1 POM-hybrid surfactant concentration...... 25

16. TEM images of the PS nanoparticle (surfactant 15 mg/mL)...... 26

17. TEM images of the PS nanoparticle (surfactant concentration 5 mg/mL) ...... 27

18. top) TEM images of the POM-polymer latex; bottom) EDS results of the surface area of the latex beads...... 28

19. FT-IR of POM-Polymer nanobeads (KBr)...... 28

20. The appearance blue color of the suspension of the latex after UV radiation...... 29

21. The recycling of the latex sample from its suspension by using centrifugation...... 30

22. TGA curves of (1) Pure POM, (2) POM-Polymer nanobeads (POM surfactant concentration is 15 mg mL-1)...... 32

23. FT-IR of Tri-lacunary Keggin POM ...... 36

24. ESI-MS results for POM-organic hybrid. (a) Clusters with 3 negative charge; (b) Clusters with 2 negative charge; (c) Clusters with 1 negative charge...... 38

25. 1H-NMR spectroscopy of POM-organic hybrid...... 39

26. 13C-NMR spectroscopy of POM-organic hybrid...... 40

viii LIST OF TABLES Table Page

1. Formulas and molecular structures of the hybrid surfactants studied in our group ...... 6

2. Information on the structure, charge density, and self-assembly behavior of macro-polyoxoanions in aqueous solutions ...... 7

3. The experiment data for measuring CMC ...... 23

ix CHAPTER I

INTRODUCTION

1.1 Introduction to Macroions

Macroions are highly charged ions with large size, which may range from 1 nm to

10 nm.4 Small simple ions such as NaCl, whose size is less than 1 nm, and large colloidal

suspensions, whose size is within the range of 10 nm to 100 nm, are well studied by

Debye-Hückel theory and Derjaguin-Landau-Verwey-Overbeek theory (DLVO theory)

respectively.5,6 There are mainly three groups of compounds in the field of macroions： nanocages, bio-macromolecules, and polyoxometalates (POMs).4

Why it is important to study the macroions? Their size range falls in between the simple ions and the colloidal system, which is a missing part for people to understand.

That is, there are no well-established theories for the understanding of macroions. Hence, it is of great importance to do some research on macroions and build up basic theories.

1.2 Introduction to Polyoxometalates

Polyoxometalates (POMs), which belong to the class of macroions, are made up of early transition metals (e.g. Wo, M, V), oxo ligands (M-O-M and M=O, M is metal or hetero atom) and sometimes heteroatoms (e.g. Si, P, and As).7,8 They are actually a kind of metal oxide frameworks, formed by sharing oxygen atoms between metal or hetero atoms. As macroions, they are usually large in size. Large amount of oxo ligands presented in POMs makes them negatively in charge and bear hydrophilic properties.1

Therefore, on one hand, just like small ions, POMs are highly soluble in water and are able to form uniform real solution, which makes them different from colloidal suspensions. One the other hand, the large size of POM macroions makes them behave

2 completely differently in solutions. In other words, because of the properties mentioned before, POM anions are able to provide a simple way for people to look into the behavior of macroions. This will be very promising to help to understand complex polyelectrolytes and some of biomolecules.1

In terms of the structure, there are various types of POMs, such as Keggin, Lindqvist,

Anderson, and Dawson. They are all simple POMs, but widely studied and functionalized in literatures.

There are continues efforts spent on studying POM chemistry. They can be designed and synthesized with tunable morphologies, structures and electronic properties.10 So

POMs can find application in a lot of fields such as catalysis11 and conductors12. 3 1.3 Introduction to Polyoxometalate-Organic Hybrid Materials

POMs are very promising material with well-designed structures and properties.

However, POMs are relatively rigid metal oxide cluster, which renders them very poor processability. Besides, highly charged POMs are completely hydrophilic, not compatible with organic solvents, and therefore limit their use in organic solvents. These two limitations greatly prevent the widely use of POMs. In order to solve the problems, POM based hybrid materials come into being and attract more and more attention recently. The organic chains present in the hybrids can help to increase flexibility and solvent compatibility.

There are two major classes of POM-organic hybrids. One is to incorporate organic parts into POMs through non-covalent interaction, e.g. electrostatic interaction, van der

Waals forces and hydrogen bonding.13 Another is to covalently link organic chains on some specific sites of POMs. There are several ways to covalently functionalize POM with some specific groups (Figure 3).14

There are many kinds of POM-based hybrids studied previous in our group (Table

1). In this thesis, the functionalization of Keggin type POMs is presented in the following section.

1.4 Solution Behavior of Polyoxometalates and Their Hybrids

In aqueous solution, POMs are found to slowly self-assemble into hollow spherical structure, like a “blackberry” (Figure 4).15,16 The self-assembly process is proved to charge regulated process and the driving force for the self-assembly is believed to be counter-ion mediated interaction.17

6 Table 2. Information on the structure, charge density, and self-assembly behavior of macro-polyoxoanions in aqueous solutions1 Reprinted with permission from ref.1. Copyright 2012 Royal Society of Chemistry.

POM-Organic hybrids are also able to self-assemble into large structures. POMs are completely hydrophilic, and organic molecules are usually hydrophobic, so the formed

POM-Organic hybrids are mostly amphiphilic molecules. Thus, the driving force for their self assembling into large structures is usually solvophobic interaction, for example, hydrophobic interaction.

7 1.5 Introduction to Polyoxometalate-based catalyst

For centuries, people keep trying to fabricate functional material with POMs. They are widely applied in catalysis, electronics / magnetics, and biomedicine. During recent years, POMs are used as catalyst even in the realm of industry. They are considered to have rapid and reversible redox properties and high stability, able to catalyst a wide range of reactions such as desulfurization, water oxidation, and carbon dioxide conversion.2

There are two POM-base amphiphilic hybrids (Figure 4).18 They are able to form micelles in water. The POM head has active electrons so that can catalyze oxidation reaction.19,20 The hybrids are used to catalyst the oxidation reaction of thiophene to convert the inert sulfur containing species in oil to sulfur-containing water soluble species

This reaction is critical for petroleum industry to remove toxic sulfur containing species(Figure 5). When two reactants meet at the interface of the micelles formed by the hybrid surfactants, they can react with each other with POM head as catalyst (Figure 6).

Figure 6. a) Picture of the emulsion catalytic system of surfactant in aqueous solution at pH 6. b) Models for the catalytic system. c) Model for the biphasic catalytic reactions.18 Reprinted with permission from ref.1. Copyright 2012 Wiley-VCH.

However, there are two major limitations for POM based catalyst: 1) poor processability and recyclability for homogenous catalyst; 2) low efficiency for the heterogeneous catalyst.3 So in this thesis, the quasi-homogenous POM-based catalyst is designed and synthesized to solve the two problems.

9 1.6 Nanostructured Polyoxometalate-Polymer Latex Beads

Covalently functionalized POM hybrids have the advantage of good stability and controllability, making it easier to handle POM hybrids and fabricate them into functional nanomaterials. The covalently functionalization of POMs is introduced 1.3. POM-organic hybrid showed in Figure 7 is synthesized by linking four propyl methacrylate groups into the pocket of lacunary phosphotungstate through multiple C-Si-O bonds.21 Look at its molecular structure, the four propyl methacrylate tails are organic part, which form the relatively hydrophobic domain in the whole molecule, while the POM part are quite hydrophobic. So the whole molecule exhibits amphiphilic properties. In this project, amphiphilic hybrid was utilized in the emulsion polymerization of styrene system. The amphiphilic POM hybrids can form micelle in aqueous solution. In order to immobilize the POM onto some kind of nanostructured frame, the amphiphilic POM hybrids are used as surfactants in the emulsion polymerization of styrene in water. So when micelles form in water, then styrene monomers are added to the system, and fuse into the micelles.

Afterwards, initiators are introduced to formed emulsion and the polymerization starts.

The POM hybrids not only act as surfactants in the process, but also randomly copolymerize with styrene, because the end MMA group have similar reactivity ratio to styrene.

10 In a typical emulsion polymerization experiment, POM surfactants can be immobilized onto the surface of the PS latex nanoparticles, which provide a cheap and convenient protocol to make quasi-homogenous POM-based catalysts. In this way, the recyclability is increased and the efficiency is maximized.22

Figure 7. Molecular structure of hybrid..

11 CHAPTER II

EXPERIMENTS AND PROCEDURES

2.1 Chemicals and Equipment

The following chemicals are used as received: Sodium tungstate dehydrate (Sigma,

Aldrich), Phosphoric acid (H3PO4, 85%, Fisher Scientific,), Glacial acetic acid (GR),

3-(Trimethoxysilyl) propyl methacrylate (Sigma-Aldric), Acetonitrile (Sigma-Aldric),

Hydrochloric Acid (HCl, Fisher Scientific), Potassium chloride (KCl , Sigma-Aldric,),

Styrene (Sigma-Aldric,), Potassium chloride (KCl, Sigma-Aldric), Potassium persulfate

(K2S2O8, Sigma-Aldric).

Equipment: Disposable Scintillation Vials (20 ml, Brookhaven Instrument

Corporation, Holtsville, NY), Pasteur pipettes (9”, Flint Glass/Non-Sterile, Thermo

Fisher Scientific Inc., Pittsburgh, PA), glass beakers (50~1000 ml, Thermo Fisher

Scientific Inc. Pittsburgh, PA), plastic syringe (25ml), hydrophilic PTFE filter (200nm,

450 nm, EMD Millipore Corporation, Billerica, MA).

12 2.2 Instrumentation

Excalibur Series FT-IR spectrometer (DIGILAB, Randolph, MA), Varian Mercury

300 MHz NMR (Liquids), Varian NMRS 500 NMR (Liquids), vacuum oven (Fisher

ISOTEMPOR Model 281, Thermo Fisher Scientific Inc. Pittsburgh, PA), ultrasonic

(Model 1510R-MTH, Branson Ultrasonic, Danbury, CT), analytical balance (Model

Ax105, SARTORIUSOR , NY), dynamic light scattering and static light scattering (DLS,

BI-200SM goniometer, version 2.0, Brookhaven Instruments Corporation, Holtsville,

NY), Transmission Electron Microscope (JEM 1200XII TEM, University of Akron),

Waters Synapt HDMSTM Q/ToF tandem mass spectrometer coupled with an ultra-high pressure LC, ESI.

2.3 Synthesis of Sodium Tungstophosphate, Na9[A-PW9O34] (Tri-lacunary Keggin POM)

The procedure was based on previous literature.23 A mixture of 120 g (0.36 mol) of

sodium tungstate dihydrate Na2WO42H2O and 150 g of water is stirred in a 300 mL beaker with a magnetic stirring bar until the solid is completely dissolved. Phosphoric acid (85%) is added dropwise with stirring (4.0 ml, 0.06 mol). Glacial acetic acid (22.5 ml, 0.40 mol) is added dropwise with vigorous stirring. Large quantities of white precipitate form during the addition. The solution is stirred for 1h, and the precipitate is collected and dried by suction filtration.

13 Yield: 87%. FTIR (KBr): 2135 (w, broad), 1637 (m), 1577 (m), 1461 (m), 1057 (s), 1016

(m), 937 (s), 888 (m), 760 (s, broad), 645 (m), 594 (m), 508 (m), 462 (m). TGA:

Na9[A-PW9O34]·xH2O, measured from TGA, x = 14. 31P NMR (500 MHz, D2O) δ

10.78.

Figure 8. Molecular structure of Tri-lacunary Keggin POM.

2.4 Synthesis of Amphiphlic POM-MMA hybrid, K3[A-(RSiO)3(SiR)] (R =

{H2C=C(CH3)O(CH2)3})

3-(Trimethoxysilyl) propyl methacrylate (1.5 mL, 6.3 mmol) was added to 200 mL of a solvent mixture of CH3CN/H2O (150 mL/50 mL, v/v). The solution was acidified with 5 mL of 6 M aqueous HCl solution. To it was slowly added the precursor Na9[A-

PW9O34]14H2O (it was synthesized in the previous step) (2.7 g, 1.0 mmol) with a small portion. The resulting colorless clear solution was concentrated to ca. 60 mL in

14 volume with a water bath at 80 °C. To it was added KCl (3.0g, 40.2 mmol). After keeping in the fridge for 12h, the water was poured out and evaporated at reduced pressure. The obtain compound was dissolved in 50 ml acetonitrile followed by centrifugation at 6000 rpm for 30 min. The supernatant was poured into a glass petri dish and put into hood. After all the solvent evaporated, the resulted transparent product was kept in fridge.

Yield: 89%. FTIR (KBr): 2963 (s, -CH3, as), 2936 (m, -CH2-, as), 2877 (w, -CH3, as),

- - 2741 (vw,-OH in H PO4-, s), 2361 (w, CO2 , s), 2342 (w, CO2 , s), 1891 (vw, C=O, s,

coupling), 1716 (vs, C=O in ester, s), 1637 (w, -OH in H2PO4-, s), 1484 (w), 1437 (w,

-CH2-,), 1405 (w, -COO-, s), 1381 (w, -CH2-, w), 1322 (m, -C-C-H, d), 1298 (m), 1167 (s,

P-O, s), 1094 (vs, Si-O, s), 1056 (s, P-O, as), 977 (vs, W=O, as) 954 (vs, W=O, as), 922

(m, P-OH in H PO -,s), 868 (s, W-O -W, as), 818 (m, W-O -W, as) 746 (vs), 670 (w) 593

(vw), 527 (w), 471 (vw). [Note: Oa atoms are shared by two W atoms of different W3O3

1 units; Ob atoms are shared by two W atoms from the same Mo3O13 unit.]. H NMR (300

MHz, [D6]DMSO) δ 5.98 (d, J = 9.3 Hz, 4H), 5.59 (s, 4H), 4.03 (t, J = 15.8 Hz, 7H), 3.26

(d, J = 19.7 Hz, 19H), 2.56 – 2.42 (m, 14H), 2.05 (s, 4H), 2.03 – 1.66 (m, 16H), 1.66 (s,

31 4H), 0.58 (d, J = 5.4 Hz, 10H). P NMR (500 MHz, [D6]D2O) δ 18.19. ESI-MS

(Acetonitrile): m/z: 965.9 ([POM hybrid]3- Calc. 966.2), 1449.8([H+ + POM hybrid]2-

15 + - 13 Calc. 1449.8), 2900.6 ([2H + POM hybrid] Calc. 2900.6). C NMR (500 MHz, D2O) δ

167.30 (s), 135.22 (s), 125.21 (s), 118.29 (s), 65.83 (s), 21.44 (s), 16.99 (d, J = 8.3 Hz),

8.46 (s), 0.00 (s).

2.5 Emulsion Copolymerization of Amphiphilic POM-based hybrid and Styrene

They synthesized POM-based hybrid in precious step is further used in the emulsion copolymerization. The polymer reaction was carried out based on traditional emulsion polymerization protocol except we used POM-based hybrid as surfactant. 150 mg hybrid

was added to the 9 mL H2O, and stirred for 10 min. Then added 1mL styrene to the

16 solution and allow for 30min stirring to form the emulsion (r = 1000 r / min). After

24-hrs polymerization reaction at 70 °C with potassium persulfate as initiator (50 mg), the obtained milk-like solution was centrifuged and obtained solid sample was washed with water.

Figure 10. Typical experiment for emulsion polymerization.

Then, in order to study the influence of amount of surfactant on the size of the PS nanoparticles, the same processed was repeated except that the amount of surfactant was

50 mg.

2.6 Laser Light Scattering

Dynamic light scattering and Static light scattering are used to determine the Rg and

Rh respectively, of micelles or vesicles formed by the POM-based hybrids in water and the polystyrene particles. The DLS data were processed by CONTIN method, and the

SLS data were processed by Zimm model.

17 2.7 1H, 13C, 31P Nuclear Magnetic Resonance (NMR) Spectroscopy

1 13 H NMR spectra were acquired in DMSO (Aldrich, 99.9% D6). C NMR spectra

31 were acquired in CD3CN (Aldrich, 99.9% D3). P NMR spectra were obtained in D2O

(Aldrich, 99.9% D2). Results are analyzed by MestReNova v6.1.0

2.8 Fourier Transform Infrared Spectroscopy (FT-IR)

Infrared spectra were recorded on an Excalibur Series FT-IR spectrometer

(DIGILAB, Randolph, MA) by compress polymer with KBr into tablets from material powder. The data were processed using Win-IR software.

2.9 Transmission Electron Microscope

TEM images of formed PS particles were provided by JEM 1200XII Transmission

Electron Microcsope. The formed PS latex (2µL) was dilute by using water (6 mL).

Then dropped on drop of water containing PS particles on the copper grid designed for

TEM, and allowed 3 days for the water to evaporate.

18 2.10 Thermogravimetric (TGA) Analysis

The centrifuged POM-PS nanoparticles were put into the 60°C oven for 24 h to allow it fully dried. Thermogravimetric (TGA) analysis was performed on a TA

Instrument Q50 Thermogravimetric Analyzer at a heating rate of 10 °C/min from room temperature to 800 °C under a nitrogen purge.

2.11 Zeta Potential Analysis

The suspension of the POM-latex (synthesized with hybrid concentration as ca. 15 mg/mL) was used for measurement. The experiments were done by using Malvern

Instruments Zetasizer Nano-ZS90 with disposable curvette.

2.12 Measurement of Critical Micelle Concentration (CMC)

The critical micelle concentration was measured by monitoring the conductivity change when changing the concentration of the hybrid surfactant. The conductivity was measured by conductivity meter (OAKLON TDS/ Conductivity meter, CON 10 series).

The concentration was changed through titrating concentrated solution of hybrid (10 mg / mL) into 20ml pure water. A stirring bar was used to make the concentration uniform.

19 CHAPTER III

RESULTS AND DISCUSSION

3.1 Micelle Formation of POM-MMA hybrid

Organic-coupling agent with MMA as ending group has been attached to the Keggin

POM, which renders the POM-MMA hybrid. To test its amphiphilic prosperities, the hybrid was put in the water and dynamic light scattering is used to probe the size of the assemblies formed in aqueous solution.

From the dynamic light scattering (DLS), intensity-time correlation function was provided. The correlation function was analyzed by the constrained regularized CONTIN method, using WINCONTIN software, to yield the information of the size and the size distribution of the assemblies formed by hybrid in the solution. In Figure 11, it is the size

and the size distribution information for the sample. The average Rh is around 5 nm.

20 1.0

0.8

0.6 ) Γ

G( 0.4 Γ

0.2

0.0 1 10 100 1000 R /n m h

An estimate of the size of the hybrid was made. The total length should be around 2

nm (Figure 12). Rh value around 5 nm is pretty small. Thus the micelles should be formed by hybrid in water instead of vesicles.

21 ~0.88nm

~1nm

Figure 12. The size estimate of the hybrid.

3.2 The Measured Critical Micelles Concentration (CMC)

In order to conduct emulsion polymerization, it is necessary to know the critical micelles concentration (CMC) of the POM-MMA hybrids. CMC value was determined by conductmetric measurement at room temperature. The experiment data are list in

Table 3. Then a plot relating the conductivity with concentration was made to figure out the critical point, where there is a suddenly drop in the slope. According to Figure 13,

CMC is ca. 2.02 mg / mL.

22 Table 3. The experiment data for measuring CMC

No. Concentration/(mg/mL) Conductivity/µS 1 0 1.84 2 0.001 2.26 3 0.003 3.98 4 0.006 5.24 5 0.01 6.8 6 0.15 9.24 7 0.025 12.44 8 0.0449 18.47 9 0.0746 25.3 10 0.1139 36.1 11 0.1626 48.1 12 0.2348 64.6 13 0.3294 85.5 14 0.4452 110.7 15 0.5805 141.6 16 0.755 177.3 17 0.9643 313 18 1.242 392 19 1.6103 497 20 1.7831 628 21 2.4087 775 22 3.1871 950 23 4.007 1123 24 4.7912 1288 25 5.3843 1408

23 1600

1400

1200

1000 S µ 800

600

400 C onductivity / 200 C MC

-200 0 1 2 3 4 5 6 C/ (mg/mL)

3.3 Characterization of POM-PS Particles

POM-MMA hybrid is used as the surfactant in a typical emulsion polymerization reaction of styrene. The formed latex was diluted by water and monitored by DLS.

When added the surfactant is 150 mg, which means the concentration is 15 mg / mL,

the hydrodynamic radius (Rh) is ca. 85 nm

-1 0.35 5 mg mL -1 15 mg mL 0.30

0.25

0.20 ) Γ G(

Γ 0.15

0.10

0.05

0.00 1 10 100 1000 10000

R h/n m

Figure 15. CONTIN analysis of DLS studies on POM-Polymer nanobeads with 5mg mL-1 and 15mg mL-1 POM-hybrid surfactant concentration. Reprinted with permission from ref.22. Copyright 2015 Royal Society of Chemistry.

25 The morphology of the PS particles was observed by TEM. The spherical shape and the diameter of 120 nm can be got from the images (Figure 16). This is acceptable comparing with the DLS result that the diameter should be around 170 nm.

When using 50 mg surfactant (concentration is 5mg/mL), the Rh value given by DLS result is 130 nm. The size is larger than using more amount of the surfactant, which is because less amount of surfactants will form larger micelles and finally lead to larger PS nanoparticles. The TEM images also conform that the diameter is around 250 nm (Figure

17).

Figure 18 shows the Energy dispersive X-ray spectroscopy analyzed on latex

nanoparticles, suggesting that W element exist in the nanoparticles, which proves the

successful loading of POM onto the surface of nanoparticles. In addition, FT-TR result

indicates the intact POM structure is still present in the latex after polymerization.

In order to test the chemical property of POM on the latex surface, it was irradiated with UV light. The suspension of latex appears a light blue color, which is the characteristic of resulting photochemical reduction of POM framework. Because the latex is opaque so the light blue color just appear in the front (Figure 20).

27 As illustrated in figure 21, the colloidal nanoparticles are not thermodynamically stable system. So after centrifugation, they all come to the bottom and can be collected for reuse.

28 50

Transmisttance (%) 20

10 4000 3500 3000 2500 2000 1500 1000 500

-1 Wavenumber (cm )

3.4 Zeta Potential Measurement

The obtained PS nanoparticles are diluted for Zeta-potential measurements. The result shows zeta potential (ζ) is -50 mV, indicating that the latex particles’ surfaces are highly negatively charged.

3.5 Thermal Stability and Composition Analysis

Thermal stability and composition of POM based latex nanoparticles are characterized by thermogravimetric analysis (TGA) (Figure 22). As a comparison, the

TGA result of pure POM is represented in red and marked as curve 1, whereas the result

30 of latex nanoparticles is showed in red and marker as curve 2. From the TGA curves, the decomposition temperature of nanoparticles is 340 °C. Decomposition of PS core out of nanobeads cause the mass loss of curve around 400 °C. The pure POM is stable at such temperatures. The loss of the weight at the beginning of the pure POM (curve 1) is due to the loss of crystalline water.

Besides the thermal stability, TGA analysis can also give us information about POM content in latex. It is of great importance to quantify the amount of active parts in latex nanoparticles if they will be used as catalysts in the future. The POM content is cal. 10.16 wt.%. If it is assumed that the latex nanoparticles are made by a PS core covered with

POM on their surface, the POM should be almost fully cover the nanoparticles with a center to center distance of 1.05 nm between them. For specific calculation process, please see APPENDIX.

31 100 A 1 80

Weight / % 40

20 B 2

0 0 200 400 600 800 Temperature / °C

Figure 22. TGA curves of (1) Pure POM, (2) POM-Polymer nanobeads (POM surfactant concentration is 15 mg mL-1).

32 CHAPTER IV

CONCLUSION

Polyoxometalates (POMs), with well-defined structures and properties, are promising in making functional nanomaterials in various fields, especially as surfactants and catalysts. Making POMs into hybrids can help to tune their electronic properties and improve their compatibility and processability.48 In our work, we made a kind of

surfactants with POM as the hydrophobic head and organic chain with a MMA group at

the end as the hydrophobic tail. The amphiphilic POM-based surfactants are observed to

be able to self-assemble into micelles due to hydrophobic interaction. Then through

emulsion copolymerization of surfactants with styrene, the POM can be fixed on the

surface of formed PS nanoparticles. This nanoparticles are promising to be used as

quasi-homogenous catalyst for some oxidation reactions.

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35 APPENDIX

20 Tansmittance% 10

4000 3500 3000 2500 2000 1500 1000 500

-1 Wavenumber / [cm ]

Figure 23. FT-IR of Tri-lacunary Keggin POM

36 (a)

Intens. x106 965.5 1.0 965.9 Sample)(35)) 0.8 964.9 967.2 0.6

0.4

0.2 962.5 970.3 972.8 0.0

966.2 2500

965.2 967.2 2000 964.9 967.5 964.5 1500 967.9 Calculated)isotope)pa.ern) 964.2 968.2

1000 963.8 968.5 963.5 968.9 500 963.2 969.2 962.8

0 962 964 966 968 970 972 974 m/z

(b)

Intens. 5 x10 1449.8

8 1449.3 1450.8 Sample)(25)) 1448.8

1448.3 1451.3 6 1452.3 1446.8 1452.7 4 1446.3

1454.2 2 1445.2 1455.1 1456.2 1443.7 1442.8

1449.8 2000 1450.3

1448.3 Calculated)isotope)pa.ern) 1451.3 1447.8 1500 1451.8 1447.3 1452.3 1446.8 1000 1452.8 1446.3 1453.3 1445.8 1453.8 500 1445.3 1454.3 1444.8 1455.3 0 1442 1444 1446 1448 1450 1452 1454 1456 m/z

37 (c)

Intens. x104 2899.6 6 2901.6 Sample)(15)) 2903.6 5 2895.5 2897.6 2904.6

2918.5 4 2894.6 2905.6 2892.6 2917.5 2906.6 2920.6 2891.2 2908.6 3 2912.5 2916.5 2888.6 2914.5 2915.6 2 2885.22886.7

2900.6 2000 2901.6

2897.6 Calculated)isotope)pa.ern) 2903.6 1500 2896.6 2904.6 2895.6 2905.6 1000 2894.6 2906.6 2893.6 2907.6 2892.6 2908.6 500 2891.6 2909.6 2890.6 2911.6 0 2885 2890 2895 2900 2905 2910 2915 2920m/z

Figure 24. ESI-MS results for POM-organic hybrid. (a) Clusters with 3 negative charge; (b) Clusters with 2 negative charge; (c) Clusters with 1 negative charge.

38 pw9organic.esp Water

DMSO

1.0 2.07

0.9

0.8

0.7

0.6

0.5 Normalized Intensity Normalized 1.86

0.4

0.3

0.2 4.03 5.61 4.01 6.01 1.68 0.58 6.03 0.1 0.60 0.55 3.38 3.24

14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 -1 -2 Chemical Shift (ppm)

Figure 25. 1H-NMR spectroscopy of POM-organic hybrid.

Figure 26. 13C-NMR spectroscopy of POM-organic hybrid.

Calculation of area that each POM head occupies on the POM-Polymer nanobeads:

In Figure 9, assume that before the point A, the mass loss is only due to the water or other small molecules absorbed on the surface of the nanobeads. Between the A and B, the mass loss is due to the decomposition of polystyrene (Mass loss of tracer of organic tails attached to POM is ignored to simplify the calculation). After point B, only POM part exists.

A (260.83°C, 90.31%), B (489.86°C, 9.984%), Sample size: 0.6700 mg, density of polystyrene: 1.057 g cm-3.

Weight of polystyrene

40 �!" = �!"!#$× �! − �! = 0.6700× 90.31% − 9.984% = 0.5381 (��)

Where �!"!#$ = 0.6700 ��, �! = 90.31%, �! = 9.984%.

Weight of POM

�!"# = �!"!#$×�! = 0.6700×9.984% = 0.06689 (��)

Total volume of polystyrene � � = !" = 0.5381×10!! ÷ 1.057 = 5.091×10!! (cm!) !"!#$ �

Where � = 1.057 � ��!!.

For R=78 nm, the volume of one nanobead

4 4 � = ��! = �×(78×10!!)! = 1.998×10!!" (cm!) !"#$%& 3 3 Total number of nanobeads

!! �!"!#$ 5.091×10 !! �!"!#$ = = !!" = 2.561×10 �!"#$%& 1.998×10

The average mass of POM on each nanobead

!! �!"# 0.06689×10 !!" �!"# = = !! = 2.612×10 (�) �!"!#$ 2.561×10 The average numble of POM on each nanobead

!!" �!"# 2.612×10 !" ! �!"# = ×�! = ×6.022×10 = 6.932×10 �!"# 2269

Where �!"# is the average molecular weight of [A-PW9O34] plus three K

The surface area of each nanobead

! ! ! ! �!"#$%& = 4�� = 4�×78 = 7.645×10 (�� ) 41 The average area occupied by one POM on each nanobead � ! !"#$%& 7.645×10 ! �!"# = = ! = 1.103 (�� ) �!"# 6.932×10 The average distance between the centers of POM on the surface of nanobead

� = �!"# = 1.103 = 1.05 (��)

Hybrid 2 is compatible with the length of lacunary Keggin Na9[A-PW9O34], around 0.7 nm. This indicates the fully coverage of POM on the surface of nanobeads.