Study of fabrication of PMMA microspheres for applications in large scale manufacturing

The seed particles were prepared by dispersion polymerization using 500-ml reaction containers, and the average size of microspheres is 7.5 µm. For the reaction co[r]

VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY

HOANG MINH KIEN

STUDY OF FABRICATION OF PMMA MICROSPHERES FOR APPLICATIONS

IN LARGE SCALE MANUFACTURING

VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY

HOANG MINH KIEN

STUDY OF FABRICATION OF PMMA MICROSPHERES FOR APPLICATIONS

IN LARGE SCALE MANUFACTURING

MAJOR: NANOTECHNOLOGY CODE: 8440140.11QTD

RESEARCH SUPERVISOR: Dr NGUYEN TRAN THUAT

Dr PHAM TIEN THANH

ACKNOWLEDGEMENTS

I would like to express my gratefulness to my first supervisor Dr Nguyen Tran Thuat, Nano and Energy Center, Hanoi University of Science, Vietnam National University, for giving me guidance and help me finalize the thesis During the research, I have learned from him the way of processing results after each experiment and important skills when working in scientific field

I would like express my gratitude to my second supervisor Dr Pham Tien Thanh, Vietnam Japan University, for giving me support and encouragement His ideas and suggestions were useful for not only my current but also future work

I also would like to show my appreciation to MK Group for the fund of the project for most of the chemicals and equipment

I would like to sincerely thank Nano and Energy Center for providing me the working space and allow me to use their equipment for my research

I would like to thank my project group members Nguyen Ngoc Anh, Bui Thi Nga and Chu Hong Hanh for all the assistance and help me accomplish these results I learned a lot from working with them, especially how to work in a team and discuss with others to advance the work

Finally, I am deeply grateful for the knowledge, the encouragement and support from lecturers and my friends from Vietnam Japan University It has been two meaningful years of studying, training myself and working in this Master program I will bring all these experiences with me and utilize them for my career in the future

TABLE OF CONTENTS Page

1. INTRODUCTION 1

1.1 Poly(methyl methacrylate) microspheres and anisotropic conductive films 1.2 Aim of work

1.2.1. Particles quality requirement 4

1.2.2. Large scale manufacturing 4

1.3 Poly(methyl methacrylate) fabrication

1.3.1. Polymerization 5

1.3.2. Dispersion polymerization 7

1.3.3. Suspension polymerization 8

1.3.4. Microwave-assisted polymerization 10

1.3.5. Seeded polymerization 10

1.3.6. Polymerization using microfluidic system 11

2. EXPERIMENTS 15

2.1 Dispersion polymerization 15

2.2 Suspension polymerization 16

2.2.1. Conventional heating polymerization 16

2.2.2. Microwave-assisted polymerization 16

2.3 Seeded polymerization 17

3. RESULTS AND DISCUSSION 18

3.1 Dispersion polymerization 18

3.1.1. 250-ml reaction container 18

3.1.2. 500-ml reaction container 21

3.1.3. 2000-ml reaction container 24

3.2 Suspension polymerization 29

3.2.1. Conventional heating polymerization 29

3.2.2. Microwave-assisted polymerization 30

3.3 Seeded polymerization 32

3.4 ACF demonstration 34

3.4.1. Silver-plating of PMMA 34

3.4.2. ACF Fabrication 36

4. CONCLUSION 39

LIST OF ABBREVIATIONS

ACF BPO DI ICC MeOH MMA PCB PMMA PVA PVP RPM RT SEM

Anisotropic Conductive Film Benzoyl peroxide

Deionized

Integrated Circuit Card Methanol

Methyl methacrylate Printed Circuit Board Poly (methyl methacrylate) Polyvinyl alcohol

Polyvinylpyrollidone Round per Minute Room Temperature

LIST OF TABLES

LIST OF FIGURES

Page Figure 1.1 Particles comparison of a) PMMA; b) PS products sharing the same

polymerization process (solvent, initiator and surfactant)

Figure 1.2 Bonding mechanism of a conventional ACF

Figure 1.3 a) Model for bonding a flip chip and a substrate in a smart card; b) Schematic for connection between bumps of integrated circuit chip and terminals of a glass substrate formed by an ACF

Figure 1.4 Three main steps in polymerization A: Initiation, B: Propagation, C: Termination

Figure 1.5 Particle size ranges of different polymerization methods

Figure 1.6 Schematic for dispersion polymer mechanism

Figure 1.7 Schematic for suspension polymer mechanism

Figure 1.8 Schematic for seeded polymerization process 11

Figure 1.9 Schematic of the channel layouts 12

Figure 1.10 Patterns of droplet formation observed in the T-junction/pocket 13

Figure 1.11 Microwave heating and conventional heating integrated microfluidic systems 13

Figure 2.1 Apparatus for 250-ml and 500-ml Reaction Containers a) Schematic; b) Real system for 500-ml Reaction Container 15

Figure 2.2 Apparatus for 2000-ml Reaction a) Schematic; b) Real system 15

Figure 2.3 Apparatus for microwave-assisted polymerization a) Schematic; b) Real system 17

Figure 2.4 Schematic of apparatus for seeded polymerization 17

Figure 3.1 Microscopic images of samples a) A% MMA; b) 1.3A% MMA and c) 1.5A% MMA 19

Figure 3.2 Microscopic images of damples a) B% BPO; b) 1.3B% BPO 20

Figure 3.3 Microscopic images of samples a) 1.3B% BPO; b) 2B% BPO and 3B% BPO 22

Figure 3.4 Microscopic images of samples a) 0.5A% MMA; b) 0.8A% MMA and c) A% MMA 23

Figure 3.5 Microscopic image of the sample A% MMA and 2B% BPO 25

Figure 3.6 Microscopic images of samples a) 1.1A% MMA; b) 1.3A% MMA and c) 1.5A% MMA 26

Figure 3.7 Microscopic images of samples a) 2B% BPO; b) 1.7B% BPO; c) 2.3B% BPO and d) 2.7B% BPO 28

ABSTRACT

Monodispersed poly(methyl methacrylate) microspheres with the size from µm to 13 µm were prepared by using a dispersion polymerization method Three reaction containers with volume of 250 ml; 500 ml and 2000 ml were used to study the scalability of the fabrication process In the case of 2000-ml reaction container, more than 80 g of microspheres were synthesized with high efficiency, which could be utilized for the fabrication of more than 100 m2 of anisotropic conductive film The products were proved to have a good quality and to be ready for carrying forward to the silver-plating process and the fabrication of anisotropic conductive films Other two methods including microwave-assisted polymerization and seeded polymerization were also performed to decrease the reaction time and to increase particles size However, the two methods still need more optimization to yield better

1

1. INTRODUCTION

1.1. Poly(methyl methacrylate) microspheres and anisotropic conductive films

2

Therefore, for the purpose of providing the polymer microspheres for the use of most of current technology, PMMA is chosen to be the main material

Figure 1.1 Particles comparison of a) PMMA; b) PS products sharing the same polymerization process (solvent, initiator and surfactant)

One of the main reason making PMMA microspheres attracting great attention is that the surface of PMMA particles can be easily modified using chemical process Acrolein was used by Songjun Li for the modification of PMMA surface during the polymerization for the investigation on the polymer immobilization2 With the existence of ester group having Oxygen carrying negative charge from free electron pair, the surface of PMMA particles can also be modified with metal cation, for instance, stannous (II) cation can be absorbed on the PMMA surface in the pretreatment of silver-plating process3 Therefore, PMMA can be used as cores for the preparation of conductive particles, which is one of the essential part in anisotropic conductive film (ACF) fabrication process

Figure 1.2 Bonding mechanism of a conventional ACF4

3

compression is used to create connection between electrodes on two opposite substrates Upon heating and compression, adhesion is created and mechanically connect the two substrates Also, conductive particles inside the ACF contact respective electrodes and create electrical connection between them The main feature of the ACF is that the electricity is allowed only along the vertical direction (one direction) from an electrode to the respective one, but not along the horizontal direction In other word, only the contact between two electrodes is conductive; the non-contact regions are insulated Because of this feature, ACFs are commonly used for the connection of the driver electronics on glass substrate in liquid-crystal devices and for flex-to-flex or flex-to-board of many electronic devices such as smartphones or laptops With the development of technology nowadays, ACFs are more and more in high demand Especially for the advances in technology in Vietnam in this industrial age, the importance for the ACF increases and the procedure for ACF should be developed to reduce the dependence on foreign products

1.2. Aim of work

4

Figure 1.3 a) Model for bonding a flip chip and a substrate in a smart card6; b) Schematic for connection between bumps of integrated circuit chip and terminals of

a glass substrate formed by an ACF7

1.2.1 Particles quality requirement

The requirement for the PMMA microspheres used for ACF is that the particles should have spherical shape and the size distribution should be narrow in order to ensure the contact of most conductive particles to both the chip bump of integrated circuit chip and terminals of printed conductor wires on a glass substrate Another requirement for the particles size is that the particles diameter should be from to 30 µm The particles should not be smaller than this range in order to prevent the agglomeration during the film making and ensure the one layer of particles connecting terminals and chip bumps Moreover, the size is kept under 30 µm to match the thickness of ACFs which is from 30 to 40 µm – the best thickness for good adhesion Therefore, particles quality, regarding spherical shape and size, was investigated in the research

Concerning the synthesis of microspherical polymer particles, four common methods are often applied, which are emulsion, dispersion, suspension and precipitation polymerization These methods are presented in details in the next section For each desired size of particles, polymerization methods are chosen In our research, desired range size of particles is from to 30 µm

1.2.2 Large scale manufacturing

5

ICC, 10mmx10mm of ACF is used, hence, tape of ACF can make the connection of 10,000 cards Imagine if we can produce 60 g or 600 g of PMMA per day, 100 tapes or 1000 tapes can be fabricated, which can be used for million or 10 millions of ICCs This number can be considered as large scale manufacturing, and the goal of our research is to prove the polymer synthesis method not only able to reach this amount but also has the potential to produce more for other industrial purpose

1.3. Poly(methyl methacrylate) fabrication

1.3.1 Polymerization

6

Figure 1.4 Three main steps in polymerization A: Initiation, B: Propagation, C: Termination

7

suspension polymerization to be two main techniques for our PMMA microspheres fabrication

Figure 1.5 Particle size ranges of different polymerization methods8

1.3.2 Dispersion polymerization

Figure 1.6 shows the schematic for dispersion polymerization mechanism In dispersion polymerization, during the initial state, the reaction mixture is homogenous where the monomer and the initiator both dissolve in the polymerization solvent During propagation step, macroradicals or oligomers are formed, to a critical point the molecules are insoluble in the solvent and hence, phase separation occurs (Figure 1.6B) These macroradicals then gather for the nucleation to form primary particle, and the polymerization continues within individual particles (Figure 1.6C) Then, the particles grow until reaching stabilization (Figure 1.6D)

8

process has been published for fabrication of microspherical particles and with the development of microtechnology and nanotechnology, the demand for these polymers grew and grew In our research, we will prove whether the procedure is applicable for large scale production purpose

Figure 1.6 Schematic for dispersion polymer mechanism A: initial step, B: formation of macroradicals – phase transition, C: Nucleation process and particles

growth, D: particles reach stabilization8

1.3.3 Suspension polymerization

9

Figure 1.7 Schematic for suspension polymer mechanism

In order to control the size of droplets, according to Equation 1, stirring speed, and volume ratio of monomer to liquid matrix, viscosity of the two phases and concentration of stabilizer should be taken in consideration.8

̅

where ̅ is average particle size, k includes parameters related to the reaction vessel design, Dv is the reaction vessel diameter, Ds is the diameter of the stirrer, R is the

volume ratio of the droplet phase to medium, N is the stirring speed, νm and νd are

the viscosity of the monomer phase and liquid matrix respectively, ε is the interfacial tension of the two phases, and Cs is the concentration of stabilizer

10

1.3.4 Microwave-assisted polymerization

Regarding normal polymerization using heat transfer from a heat source, the time for the reaction varies from 24 to 48 h, which requires resources to maintain the reaction and quite time consuming Therefore, many studies were carried out with the goal of reducing the time of polymerization, which includes the use of microwave J Jacob had used microwave with different power of 500 W; 300 W and 200 W for the polymerization of MMA and obtained the reaction rate enhance comparing to the thermal method which are 275%; 200% and 138%, respectively10 Not only the reaction rate is faster, but the conversion of polymer using microwave-assisted process was higher than conventional heating, which has been studied by Liu Z in the polymerization of butyl acrylate (BA) to PBA11 The mechanism of microwave on the acceleration of reaction has not yet been demonstrated, but two effects of microwave irradiation have been proposed: specific microwave heat and nonthermal microwave effect12 When using normal conventional heating, the heat transfers from outside to inside solution, and the hottest part is the glass container directly contacting the heat source Meanwhile, the microwave can pass through every part of reaction mixture including solvent, surfactant, monomer and initiator, allow heat to be generated across the entire reaction volume and most part of the reaction will reach reaction point much faster Regarding nonthermal process, intermediate can absorb the microwave energy and be accelerated or some molecule under microwave can be enhanced from ground state to transition state and be more active However, when nonthermal microwave effects are discussed, they are generally invoked as the inaccuracy in comparison with conventional heating effects12

1.3.5 Seeded polymerization

11

process is the swelling process of seed particles in monomers Figure 1.8 shows the phenomena occur during seeded polymerization process When polymer particles contact with monomer in the solution, the monomer concentration is not high for dissolving the polymer, but the monomer still affect the polymer, surround the polymer and make the polymer particle become larger We call this phenomenon the swollen effect When the swollen particles reach the point of stabilization, energy was provided for the initiator in the solution to initiate the polymerization of monomer and the monomer will form the new polymer shell outside the seed particles, hence, results in larger size particles In seeded polymerization, polymerization condition such as the amount of seed particles or swollen time should be carefully controlled in order to stop the formation of new particles

Figure 1.8 Schematic for seeded polymerization process

1.3.6 Polymerization using microfluidic system

12

aqueous phase were varied why the organic was kept constant, with the result showed in Figure 1.10, gives different size of polymer particles This means that the factor of feed rate is most essential when concerning microfluidic system Liu Zhendong also compared two heating strategies using conventional heating and microwave heating to observe the polymerization rate of organic droplets during the process Overall, with the assistance of microwave, the polymerization process not only faster but the conversion is also higher A typical example of microfluidic system using two heating strategies is illustrated in Figure 1.11

13

Figure 1.10 Patterns of droplet formation observed in the T-junction/pocket when the flow rate of aqueous phase (Qc) was varied at a fixed monomer flow rate (0.1

ml/h): (a, b) Qc = 0.5 ml/h; (c, d) Qc =1.0 ml/h; (e, f) Qc =2.0 ml/h

14

Despite a good control in size of monodisperse polymer microspheres, the limit of this method was the amount of particles made The microfluidic system is a continuous flow system concerning the feed rate of phases rather than the amount of chemicals used as in batch system And this feed rate is optimized and kept constant during the process, which results in a fixed amount of particles formed in a period of time Meanwhile for batch system, more chemicals can be used, reaction container volume can be changed, and makes the scalability of batch reaction is achievable

15

2. EXPERIMENTS

2.1. Dispersion polymerization

Three types of reaction containers, a 250-ml and a 500-ml two-neck, round-bottom flasks and a 2000-ml three-neck, round-bottom flask, were employed for studying scalability of the process A condenser was equipped to the system along with a thermometer for heat control and observation For 250-ml and 500-ml flasks, magnetic stirrer were used while mechanical agitation containing Teflon paddle were applied for 2000-ml flask system Reaction systems are presented in Figure 2.1 and Figure 2.2

Figure 2.1 Apparatus for 250-ml and 500-ml Reaction Containers a) Schematic; b) Real system for 500-ml Reaction Container

16

A solution containing X% PVP surfactant dissolved in Methanol solvent was prepared in the reaction flask and heated to 65oC while stirring at 600 rpm The second solution containing BPO as initiator dissolved in MMA monomer and this mixture was poured into the reaction flask all at once Then, the reaction was carried out at 65oC with stirring speed 600 rpm for 24 h After the reaction, PMMA particles product was then washed by centrifugation, decantation and redispersion in deionized (DI) water three times to eliminate the residual MMA and PVP The particles were obtained and dried in air ambient and observed using an optical microscope

2.2. Suspension polymerization

2.2.1 Conventional heating polymerization

A solution prepared by dissolving X% PVA in 1000-ml DI Water was added to the 2000-ml reaction container and heated to 70oC while stirring at 600 rpm After that, a mixture of BPO dissolving in MMA monomer was poured into reaction flask all at once The reaction was carried out at 70oC for 24 h with constant agitation Finally, the PMMA particles was washed by centrifugation, decantation and redispersed in DI Water and dried in air ambient

2.2.2 Microwave-assisted polymerization

17

Figure 2.3 Apparatus for microwave-assisted polymerization a) Schematic; b) Real system

2.3. Seeded polymerization

PMMA seed particles having size of 7.5 µm was added to the 500-ml reaction flask containing a solution of 270 ml Methanol with X% PVP The particles were dispersed in the solution using Ultrasonic and then the mixture were heated to 50oC while stirring at 600 rpm The second mixture of MMA and BPO were added drop wise to the reaction mixture at feed rate of ml/min After the addition, the mixture was let stir at 50oC for 30 for the swollen of particles Then, the temperature was increased to 65oC and the reaction was monitored every 10 Figure 2.4 below presents the apparatus set up for seeded polymerization

18

3. RESULTS AND DISCUSSION

3.1. Dispersion polymerization

3.1.1 250-ml reaction container

In this study, after running a number initial reactions to study the behavior of polymerization, we choose the most optimized conditions for the reaction, which includes the methanol (MeOH) solvent capable of dissolving both monomer and initiator, the amount of surfactant PVP fixed at X% comparing to solvent weight, the reaction temperature kept at 65oC – the boiling point of methanol, and the stirring speed around 600 rpm Two main factors are studied in the polymerization are the amount of MMA and BPO used for the reaction Table 3.1 and Table 3.2 shows the reaction formulas for changing the amount of MMA and BPO respectively

Table 3.1 Reaction conditions for studying effect of MMA on PMMA products using 250-ml reaction container

Sample name MeOH

(ml)

PVP (w.t.% to MeOH)

MMA (w.t.% to MeOH)

BPO (w.t.% to MMA)

1 90 X A B

2 90 X 1.3A B

3 90 X 1.5A B

19

20

Table 3.2 Reaction conditions for studying effect of BPO on PMMA products using 250-ml reaction container

Sample name

MeOH (ml)

PVP (w.t.% to MeOH)

MMA (w.t.% to MeOH)

BPO (w.t.% to MMA)

1 90 X A B

4 90 X A 1.3B

It can be observed from Figure 3.2, when increasing BPO amount from B% to 1.3B%, the size distribution of particles becomes narrower and the particles size decrease to average size of 7.5 µm We obtain the monodispersity using A% MAA in methanol and 1.3B% BPO in MMA, and we carry this condition in the study of larger reaction container volume of 500-ml

21

3.1.2 500-ml reaction container

In the case of 500-ml reaction container, we also study the effect of monomer and initiator amount to the polymerization The volume of methanol solvent was fixed at 360 ml and the PVP surfactant at X% of methanol

Table 3.3 Reaction conditions for studying effect of BPO on PMMA products using 500-ml reaction container

Sample name

MeOH (ml)

PVP (w.t.% to MeOH)

MMA (w.t.% to MeOH)

BPO (w.t.% to MMA)

5 360 X A 1.3B

6 360 X A 2B

7 360 X A 3B

When carrying the previous optimized condition to the new reaction chamber, the monodispersity is no longer attainable The results can be seen from Figure 3.3a, where the particles size varying from µm to 13 µm The same concept of increasing initiator amount to obtain monodispersed particles was carried out and when using 2B% of BPO in MMA, monodispersity of particles reached From Figure 3.3b, the particles in this condition have the average size around 9.5 µm And when we tried to increase this amount to 3B%, smaller amount of monodispersed particles is obtained with average size µm (Figure 3.3c) It can be seen at this point that increasing initiator amount can lead to a monodispersity and small products

Table 3.4 Reaction conditions for studying effect of MMA on PMMA products using 500-ml reaction container

Sample name

MeOH (ml)

PVP (w.t.% to MeOH)

MMA (w.t.% to MeOH)

BPO (w.t.% to MMA)

6 360 X A 2B

8 360 X 0.8A 2B

22

Figure 3.3 Microscopic images of samples a) 1.3B% BPO; b) 2B% BPO and 3B% BPO

23

monodispersity is still achieved, and the average size of particles of both samples were around 8.5 µm

Figure 3.4 Microscopic images of samples a) 0.5A% MMA; b) 0.8A% MMA and c) A% MMA

24

3.1.3 2000-ml reaction container

For this reaction container volume, Methanol amount was fixed at 1000-ml with the same X% of PVP surfactant

Table 3.5 Reaction conditions for studying effect of MMA on PMMA products using 2000-ml reaction container

Sample name

MeOH (ml)

PVP (w.t.% to MeOH)

MMA (w.t.% to MeOH)

BPO (w.t.% to MMA)

10 1000 X A 2B

11 1000 X 1.1A 2B

12 1000 X 1.3A 2B

13 1000 X 1.5A 2B

25

Figure 3.5 Microscopic image of the sample A% MMA and 2B% BPO The trend when increasing the amount of MMA can be seen in figure 3.6, where the size distribution becomes wider, which agrees with the overall trend when increasing MMA in previous case Increasing MMA to 1.1A% in methanol, the size of particles varies from 10 to 34 µm (Figure 3.6a) When increasing MMA to 1.3A% and 1.5A%, the small particles < µm is formed and becomes majority (Figure 3.6b and c) The efficiency of the polymerization is shown in Table 3.6 Overall, the polymerization has a high yield of products Even though the particles not have quality satisfying the requirement, when using 1.5A% of MMA in Methanol, the conversion is still 86%, the weight of product is 136 g Because of this high yield, we tried to study if we could maximize the amount of MMA by changing concentration of BPO in MMA to obtain more monodispersed particles

Table 3.6 Polymerization efficiency when varying the monomer amount

Sample name MMA (w.t.% to MeOH) PMMA obtained (g) Efficiency

10 A 84 81%

11 1.1A 94 77%

12 1.3A 115 85%

26

27

Table 3.7 Reaction conditions for studying effect of BPO on PMMA products using 2000-ml reaction container

Sample name

MeOH (ml)

PVP (w.t.% to MeOH)

MMA (w.t.% to MeOH)

BPO (w.t.% to MMA)

13 1000 X 1.5A 2B

14 1000 X 1.5A 1.7B

15 1000 X 1.5A 2.3B

16 1000 X 1.5A 2.7B

28

29

3.2. Suspension polymerization

3.2.1 Conventional heating polymerization

In suspension polymerization, both monomer and initiator is insoluble in the medium In our experiment, DI water is used as solvent as it cannot dissolve MMA and BPO Polyvinyl alcohol (PVA) surfactant was used instead of PVP because PVP tended to give bigger particles with size from 100 µm to mm in our optimization experiment

Table 3.8 Reaction conditions for suspension polymerization

Sample name

DI water (ml)

PVA (w.t.% to MeOH)

MMA (w.t.% to MeOH)

BPO (w.t.% to MMA)

17 1000 X A 2B

18 1000 X 0.3A 2B

30

Figure 3.8 Microscopic images of samples a) A% MMA; b) 0.3A% MMA

3.2.2 Microwave-assisted polymerization

For the microwave, same condition as using conventional heat was used in order to compare the reaction rate of the two heating strategies Microwave with power 600 W was used, which means the heating to polymerization point is very fast using microwave

Table 3.9 Reaction condition for microwave-assisted suspension polymerization

Sample name

DI water (ml)

PVA (w.t.% to MeOH)

MMA (w.t.% to MeOH)

BPO (w.t.% to MMA)

19 1000 X 0.3A 2B

31

heating, the number of big particles is small, while the small particles are majority Meanwhile by using microwave heating, after h, a large number of particles are formed and after h, the reaction finished and the conversion of particles is high at around 82% Comparing to the normal method taking 24 h for the reaction to finish, using microwave increase the reaction rate drastically However, with the use of suspension polymerization, the obtained particles have a wide size distribution, and most of particles are larger than the required size from to 30 µm In further research, by changing the power of microwave to lower amount (100 W; 200 W or 300 W), we can apply this heating strategy for methanol as using lower power microwave, boiling point of methanol is not exceeded significantly lead to unstable reaction

32

Figure 3.10 Microscopic images of conventional heating polymerization samples after a) h of reaction; b) h of reaction

3.3. Seeded polymerization

The seed particles were prepared by dispersion polymerization using 500-ml reaction containers, and the average size of microspheres is 7.5 µm For the reaction condition, monomer is drop wisely rather than all at once in order to prevent the deform effect of monomer to the polymer particles Moreover, after finishing adding, the mixture was kept stirring for 30 for the seed particles become swollen by monomer, which is an essential point of this method

Table 3.10 Reaction condition for seeded polymerization

Sample name

MeOH (ml)

PVP (w.t.% to MeOH)

PMMA (w.t.% to

MeOH)

MMA (w.t.% to

MeOH)

BPO (w.t.% to

MMA)

33

From Figure 3.11b, it can be seen that after 60 min, larger particles with size from 25 to 35 µm were fabricated However, the number of particles having small size are formed by the polymerization of monomer not surrounding the polymer These small particles is the main reason after 100 of seeding, the particles become very large with wide size distribution (Figure 3.11c) Furthermore, at this period, the particles become non-spherical due to the uncontrolled amount of macroradicals newly formed The reaction needs to be optimized with the chosen monomer amount as well as initiator amount to prevent this new polymer from forming while the seeding process is prioritized

34

3.4. ACF demonstration

3.4.1 Silver-plating of PMMA

For the next step of the project, a silver-plating process, particles which were prepared from the 2000-ml reaction container experiment using 1.1A% MMA in methanol and 2B% BPO in MMA was used The process contains three main steps: pre-treatment with stannous chloride, silver “seeding” and electroless plating The products obtained from this step are the conductive silver-PMMA shell-core particles Figure 3.12 shows the microscopic and SEM images of initial particles having smooth surface After plating, Silver layer surrounding polymer particle can be observed using SEM and light reflection can be observed using the optical microscope (Figure 3.13)

35

Figure 3.13 Silver-plated PMMA particles a) optical microscopic image; b) SEM image

36

The silver-plated PMMA microspherical particles satisfy the conductive particles quality used for ACF fabrication, hence the sample was carried to the next process

Figure 3.14 Schematic for electrical resistance measurement system

Table 3.11 Electrical resistance results of conductive sample

Compression Pressure (bar) Electrical resistance (Ω)

0 32

300 2.6

600 0.55

3.4.2 ACF Fabrication

37

Figure 3.15 Optical microscope of anisotropic conductive films after fabrication using conductive silver-plated PMMA microspherical particles

38

low electrical resistance or good conductivity between respective PCB sections, but no electrical conductivity with others According to Table 3.12, for our ACF sample, even though most parts show good conductivity with low resistance, there are still some connection without conductivity Still, no conductivity with different section numbers is observed, which shows the anisotropic properties of the ACF In conclusion, the fabrication of all processes needs to be optimized in further research to obtain high quality ACFs

Figure 3.16 Schematic for measuring anisotropic conductivity of fabricated ACFs

Table 3.12 Electrical resistance result of the fabricated ACF Compression

pressure (bar)

Section

1 10

0 (before) O.L O.L O.L O.L O.L O.L O.L O.L O.L O.L 300 0.3 Ω 0.5 Ω 0.3 Ω 13.2

Ω

0.4 kΩ

O.L 6.5 Ω O.L 70 MΩ

1.4 kΩ 600 0.2 Ω 0.3Ω 0.4Ω 9.2Ω 10Ω O.L 2.3 Ω O.L 0.75

Ω

17 Ω (after) 0.5 Ω 0.2Ω 0.4Ω 0.4Ω 6.2Ω MΩ 10 Ω 0.9

MΩ 10 MΩ

O.L

39

4. CONCLUSION

With the use of dispersion and suspension polymerization method, PMMA microspheres can be synthesized Regarding dispersion polymerization, scalability and large scale manufacturing is proven to be achievable By increasing the reaction containers volume and optimizing the reaction conditions for each 250-ml; 500-ml and 2000-ml containers, monodispersed particles having size smaller than 13 µm were produced Moreover, the reaction has a high efficiency more than 75% and more than 80 g of particles can be produced in 24 h using the 2000-ml containers reaction condition This amount of particles theoretically can be used for the fabrication of more than 100 tapes of ACF which used for the connection of millions of ICC The synthesized PMMA microspherical particles were carried to the next process of Silver-plating and ACF fabrication process and good results are obtained in terms of electrical conductivity For the next step of the project, bigger container volume such as 10 l or 20 l will be employed for the industrial scale

manufacturing Also, microwave-assisted polymerization and seeded

40

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