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

Reaction conditions for studying effect of MMA on PMMA products using 250-ml reaction container .... Reaction conditions for studying effect of BPO on PMMA products using 500-ml reaction

VIETNAM NATIONAL UNIVERSITY, HANOI

VIETNAM JAPAN UNIVERSITY

HOANG MINH KIEN

STUDY OF FABRICATION OF PMMA

MICROSPHERES FOR APPLICATIONS

IN LARGE SCALE MANUFACTURING

MASTER'S THESIS

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

Hanoi, 2020

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 1.2 Aim of work 3

1.2.1 Particles quality requirement 4

1.2.2 Large scale manufacturing 4

1.3 Poly(methyl methacrylate) fabrication 5

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

REFERENCES 40

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

Polyvinylpyrollidone Round per Minute Room Temperature Scanning Electron Microscope

LIST OF TABLES

Page Table 3.1 Reaction conditions for studying effect of MMA on PMMA products using 250-ml reaction container 18Table 3.2 Reaction conditions for studying effect of BPO on PMMA products using 250-ml reaction container 20Table 3.3 Reaction conditions for studying effect of BPO on PMMA products using 500-ml reaction container 21Table 3.4 Reaction conditions for studying effect of MMA on PMMA products using 500-ml reaction container 21Table 3.5 Reaction conditions for studying effect of MMA on PMMA products using 2000-ml reaction container 24Table 3.6 Polymerization efficiency when varying the monomer amount 25Table 3.7 Reaction conditions for studying effect of BPO on PMMA products using 2000-ml reaction container 27Table 3.8 Reaction conditions for suspension polymerization 29Table 3.9 Reaction condition for microwave-assisted suspension polymerization 30Table 3.10 Reaction condition for seeded polymerization 32Table 3.11 Electrical resistance results of conductive sample 36Table 3.12 Electrical resistance result of the fabricated ACF 38

LIST OF FIGURES

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

polymerization process (solvent, initiator and surfactant) 2

Figure 1.2 Bonding mechanism of a conventional ACF 2

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 4

Figure 1.4 Three main steps in polymerization A: Initiation, B: Propagation, C: Termination 6

Figure 1.5 Particle size ranges of different polymerization methods 7

Figure 1.6 Schematic for dispersion polymer mechanism 8

Figure 1.7 Schematic for suspension polymer mechanism 9

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

Figure 3.8 Microscopic images of samples a) A% MMA; b) 0.3A% MMA 30

Figure 3.9 Microscopic images of microwave-assisted polymerization samples after a) 2 h of reaction; b) 3 h of reaction 31Figure 3.10 Microscopic images of conventional heating polymerization samples after a) 2 h of reaction; b) 3 h of reaction 32Figure 3.11 a) Microscopic image of PMMA seeding particles; b) Microscopic image of products after 60 min; c) Microscopic image of products after 100 min 33Figure 3.12 Sample fabricated using 2000-ml reaction container, with MMA% = 1.1A% and BPO = 2B % a) optical microscopic image and b) SEM image 34Figure 3.13 Silver-plated PMMA particles a) optical microscopic image; b) SEM image 35Figure 3.14 Schematic for electrical resistance measurement system 36Figure 3.15 Optical microscope of anisotropic conductive films after fabrication using conductive silver-plated PMMA microspherical particles 37Figure 3.16 Schematic for measuring anisotropic conductivity of fabricated ACFs 38

ABSTRACT

Monodispersed poly(methyl methacrylate) microspheres with the size from 5 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

or food preservation With the growth of microtechnology, polymer microparticles have attracted huge attention due to their flexibility, spherical shape and low-cost production Among the types of polymer particles, PMMA and Polystyrene (PS) microspheres are the most popular The chemicals used for the production of these two polymers are cheap and commercially available as well as the polymerization yields high efficiency Comparing PMMA with PS, the particles size overall for PS

is smaller, mostly smaller than 5 µm, while size of PMMA can vary from submicron size to hundreds of micro in diameter Figure 1.1 compares the particles fabricated by polymerization of PMMA and PS using the same reaction condition, size of PMMA microspheres is significantly larger than size of PS microspheres PMMA can yields larger particles size of 8 µm, and even larger than 10 µm with some modification In contrary, for PS polymerization, large particles of 7 or 8 µm can be hardly obtained, even with modified conditions to increase the particles radius PMMA can be observed to be more flexible in terms of creating polymer particles having micro size, indeed, PMMA particles can as large as 100 µm

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

ACF is used to create a mechanical and electrical connection between components

in electrical device Figure 1.2, shows the bonding mechanism of an ACF Heat and

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

Polymer fabrication is the first step of our main project, which is fabrication ACF for integrated circuit card (ICC) One of the reason why PMMA microspheres is a good candidate for the making of ACF is due to the glass transition temperature of the atactic PMMA is 105oC and melting point is 160oC5, which makes the particles flexible and able to soften during the connection process using heat and pressure for compression Furthermore, the surface of PMMA microspheres can be modified with the use of chemicals absorption, then use reduction oxidation reaction to perform electroless metal plating on the surface of particles making the particles conductive In order for the ACF to work in the best condition, quality of particles inside the film should be taken into consideration Moreover, with the purpose for industrial application, large scale manufacturing of PMMA will be studied in the main factor of scalability Figure 1.3 shows the model for the integrated circuit chip and printed conductor runs on substrate for an ICC and schematic of using ACF for the connection of these two components

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 5 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 5 to 30 µm

1.2.2 Large scale manufacturing

In terms of the PMMA microspheres fabrication using for large scale manufacturing, another main factor studied in the research is the scalability For example, in order to produce 1 roll (or 1 tape) of ACF with 100 m in length and 10

mm in width, or 1 m2 of film, 0.6 g of PMMA is needed For the creation of one

ICC, 10mmx10mm of ACF is used, hence, 1 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 1 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

by breaking the weak double bond to create a new bond and a new radical Then the second step is the propagation, where this new radical attaches to another monomer to form a longer free radical molecule This molecule undergoes the same process, and the chain reaction continues until two radicals interact (hydrogen transferring) together (recombination) to eliminate the free radical, in the final step

of termination

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

Termination There are four common techniques employed for the manufacturing of polymer, which are emulsion polymerization, dispersion polymerization, precipitation polymerization and suspension polymerization The main factors that distinguished the polymerization method are the initial state of polymerization mixture (regarding the solubility of initiator and monomer in solvent); kinetics of polymerization; mechanism of particles formation and shape/size of final particles Figure 1.5 shows the particle size range for each type of polymerization method For emulsion polymerization, the synthesized particles are mostly spherical and have a small size varying from 50 nm to 500 nm Meanwhile, particles having size ranging from 500

nm to 15 µm can be prepared by dispersion and precipitation polymerization However, comparing these two techniques, dispersion polymerization yields monodispersed and spherical polymer particles while particles prepared by precipitation method are mostly polydispersed as well as have irregular shape Regarding suspension polymerization, spherical particles with size larger than 15

µm and up to 1 mm can be fabricated; however, the monodispersity is difficult to achieve By the resulted particles for each technique, we choose dispersion and

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)

Dispersion polymerization of PMMA were already reported in past literature Effect of reaction temperature, initiator concentration and type, monomer concentration and stabilizer were studied by S.Shen Conditions for synthesize particles with narrow size distribution ranging from 0.4 µm to 10 µm were illustrated9 However, for the large scale production or industrial manufacturing, no

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

In suspension polymerization, the initiator is soluble in the monomer, however, both of them are insoluble in the polymerization medium Figure 1.7 illustrates the mechanism for suspension polymerization When the monomer and the initiator are mixed in the solvent, mini droplets of monomer are formed upon high speed of stirring When energy is provided for the initiation, polymerization occurs within each individual droplets Instead of continue growing until reaching stabilization like dispersion polymerization, the droplet will directly turned into the polymer particles when finishing polymerization Therefore, to control the size of particles, size of droplets should be considered

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

Even though the suspension polymerization can produce a wide range size of particles, the narrow size distribution is quite difficult to achieve due to the versatile of polymer droplets, where the droplets can easily be agglomerated or separated during polymerization The most common way to control the size is to increase the speed of stirring, which is the most efficient to ensure stabilized state

of droplets Moreover, the apparatus design and reaction vessel diameter is one of the main factor decides the size of particles according to Equation 1 Therefore, changing, for instance, small reaction chamber to lager reaction chamber, new conditions for reaction should be re-optimized, which makes the process not flexible

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

It is not easy to synthesize particles larger than 15 µm with narrow size distribution via one-step classic dispersion or suspension polymerization One of the techniques can be used to increase the size of particles while still keeping the monodispersity

is seeded polymerization Miliang Ma already succeeded in forming 14 µm monodispersed cross-linked P(GMA-St-EGDMA) from monodispersed 7 µm P(GMA-St) as seed particles13 For seeded polymerization, the most important

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

When using classical polymerization, mostly, the monodispersity is obtained for synthesis of particles smaller than 15 µm For larger size of monodispersed particle,

as mention before, seeded polymerization can be used; however, the process is quite complicated as it is a multiple steps method which requires the first step to be the fabrication of seed particles With the development of microfluidic systems, one step polymerization for these large particles is attainable as the system allow organic droplets with same size formed continuously in the flow of aqueous phase Monodispersed particles of poly(1,6-hexanediol diacrylate) with mean diameter around 43 µm was made by Takashi Nishisako with the use of T-shape channel for feeding in microfluidic system14 (Figure 1.9) In Takashi research, feed rate of

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

Figure 1.9 Schematic of the channel layouts

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

Figure 1.11 Microwave heating and conventional heating integrated microfluidic

systems11

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

In our research, in order to satisfy the particles quality requirement having a monodispersed size in the range from 5 to 30 µm, two main polymerization methods were performed, which were dispersion polymerization – which can produce spherical particles size range from 0.5 to 15 µm – and suspension polymerization which produced spherical particles larger than 20 µm In addition,

to study the scalability factor, three types of reaction container with increasing in reaction volume are investigated, which are 250 ml, 500-ml and 2000-ml reaction containers

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

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