國立台灣大學化學工程研究所 博士學位論文
Department of Chemical Engineering
National Taiwan University Doctoral Dissertation
新型無乳化劑乳化聚合技術開發:製備活性共聚乳膠及均一粒 徑有機/無機混成乳膠
Development of Novel Surfactant-Free Emulsion Polymerization Techniques: Preparation of Living Block Copolymer and
Monodisperse Organic/Inorganic Hybrid Latexes
周奕辰 I-Chen Chou
指導教授: 邱文英 博士 Advisor: Wen-Yen Chiu Ph.D.
中華民國 102 年 7 月
July 2013
誌謝
感謝口試委員邱文英老師、謝國煌老師、許克瀛老師、黃延吉老師、陳崇 賢老師、戴子安老師、李佳芬老師對本論文的疏漏提出寶貴的意見，俾使本論文 能夠更加完善。在此特別表達感謝之意。
感謝邱文英教授多年來在學業上以及生活上的悉心指導及照顧。最重要的 是，邱老師在不給我任何壓力的狀況下讓學生自由發揮的進行研究及發現，對學 生來說著實是相當難得的經驗及學習。藉由從邱老師的觀點出發思考以及看事 情，學生更明白多元思考及跳脫自身思維的重要。雖然這些年的日子裡有高有低，
但終究我能明白箇中道理及邱老師的用心良苦。老師的慈祥以及寬恕之心是我永 遠的典範及學習的目標。另外要感謝佳芬學姊不辭辛勞的從台南到台北與我們 meeting，並給予我們實驗上很多細節需要注意的事項。
我要特別感謝盈達學長在學生剛進入實驗室時給予我的指導以及協助，對 於從一些基本實驗的技巧、實驗設計、問題討論以及日後的文章撰寫，學長總是 不厭其煩的耐心指導。如果沒有學長在那初始數個月的幫助，自我摸索的日子相 信是會更久而更費力的。在此要表達學弟由衷的感謝。
感謝粉粒體實驗室/表面分析實驗室美麗氣質兼備的許曉萍學姊，教會了我 很多台儀器、並永遠耐心解答我的任何問題。我的研究因為您而更加完善而能更 上層樓。感謝粉粒體實驗室的林瀚、婉柔、家豪、綉雲在操作儀器上的幫忙。也 謝謝高分子所謝明國先生、共同儀器室的林莉峰小姐及譚丹瑋小姐對於實驗上以 及所有雜務上的大力協助。
謝謝這些年來實驗室的學長姐、同學、與學弟妹們豐富了我的研究生涯。
謝謝仲揚學長、盈達學長、國輝學長、瑞宏學長、文如學姐、嘉甫學長、暉恩學 長、以安學姊、玫伶學姊、晨綱學長、美斐學姊、乃允學長、柏年、雅君、庭章、
晨帆、于誠、凱翔、修瑋、志宇、岱甫、士傑、劉皓、世琪、竹軒、學永、致豪、
修平、芳慈、彥賢、秦瑋等。與每一個人的互動對小弟來說，都是難得的人生經 驗以及學習。特別謝謝吳乃立老師實驗室的均潔學姊，從學弟大一開始到博士班 畢業這九年中不停的照顧著我，給予我許許多多的協助以及方向。
謝謝清大化工系馬振基教授、奕釧學長、士億學長在學生學士專題期間給
予的指導以及幫助。在此特別感謝奕釧學長帶領我進入學術研究的大門，在生活 上、課業上、及大小瑣事上都對我倍加關懷。沒有您們的幫助，今日我無法在這 展開另一段研究生涯，小弟永誌不忘。
謝謝我敬愛的父親母親，盡心竭力的撫養兒子長大，並無怨無悔的作我的 後盾。這個階段的結束，終於能讓您們些許卸下心中的擔子。謝謝爸媽，您們辛 苦了！養育之恩，昊天罔極。最後，我要謝謝以安這些年來的陪伴以及彼此的交 流、成長、互敬互愛。你我因交會而改變豐富了彼此的生命、激盪出美麗的火花，
研究之路這幾年來的相互扶持，是我心中無可取代的美好回憶。
周奕辰 于台灣大學志鴻館219室/舊數學館300-1室 一百零二年八月
摘要
在本研究中，吾人以各種無乳化劑之乳化聚合配合各種新型無乳化聚合技術 之開發，製備活性共聚乳膠及均一粒徑有機/無機混成乳膠，並探討其反應機制。
本論文共分為三大部分。第一部份使用 1,1-對二苯乙烯 (1,1-diphenylethylene，DPE) 控制活性自由基聚合以及製備其團聯共聚物，並使用 Pickering 乳化進行油相內之 活性自由基聚合研究。第二部分為研究不穩定狀態下 Pickering 乳化聚合時所生成 的均一粒徑乳膠顆粒之研究。此外並使用無乳化劑固體粒子穩定聚合法一步驟製 備均一粒徑之高分子/二氧化矽之 (核/殼)奈米複合乳膠。第三部份為使用分散聚合 (dispersion polymerization)，在無乳化劑的狀態下開發新型製備均一微米級乳膠顆 粒之技術及分析，並使用無乳化劑分散聚合包覆碳黑奈米粒子。
第 一 部 份 包 含 第 二 、 三 章 。 在 第 二 章 中 利 用 1,1- 對 二 苯 乙 烯 (1,1-diphenylethylene，DPE) 控制活性自由基聚合。實驗發現在較低溫的聚合系統 中，不論改變 1,1-對二苯乙烯的量、起始劑的量，分子量皆不隨著單體轉化率升 高而成長。藉由提高反應溫度且預熱起始劑的處理，在 DPE 控制活性自由基聚合 的系統中也能得到傳統活性自由基聚合的特徵。反應溫度、預熱步驟等變因對於 分子量、分子量分佈的影響將在這個部分討論。最後利用 DPE-capped PMMA 當 巨起始劑，成功的製備 聚(甲基丙烯酸甲酯)-聚(丙烯酸丁酯) 團聯共聚物，並探討 引入溶劑的量及對反應速率、活性自由基聚合控制效果、分子量及其分佈之影響。
第三章將此 DPE 控制活性自由基聚合引入 Pickering 乳化系統中進行反應，在 水 熱 法 環 境 下 利 用 親 水 性 高 分 子 電 解 質 聚 苯 乙 烯 磺 酸 鈉 鹽 (Poly(sodium 4-styrenesulfonate, PSS-NA) 協 同 合 成 具 懸 浮 性 的 親 水 奈 米 氧 化 鋅 粒 子 作 為 Pickering 乳化之穩定粒子。藉由使用不同分子量的 PSS-NA 和改變其用量，研究
對於Pickering 乳化聚合時之型態、懸浮穩定性、油水介面性質、及 DPE 活性自由
基聚合行為之影響。
第二部份包含第四章以及附錄。吾人發現在不穩定的 Pickering 乳化聚合中，
特定條件下能得到部分穩定的次微米級均一粒徑乳膠顆粒。吾人對於此現象進行 一系列研究並提出一個凝聚誘導(Coalescence induce)的聚合機制用於闡述此均一 乳膠例子之合成路徑，並延伸此研究至沒有穩定粒子存在之無乳化劑系統，對於 諸多變因進行探討。因生成的乳膠粒子其產率(yield)較低，故安排於附錄內。第四 章中則是探討使用市售表面負電荷之二氧化矽奈米粒子，使用無乳化劑乳化聚合 由帶正電起始劑 AIBA 起始一步驟合成均一粒徑之高分子乳膠/二氧化矽之核/殼奈 米複合乳膠粒子，並探討二氧化矽粒子濃度、水相酸鹼度等對此核殼結構合成之 影響。
第三部份包含第五、第六章。第五章中，吾人在醇相中進行無乳化劑之分散 聚合，使用帶電之起始劑合成無乳化劑包覆之乳膠顆粒。然而，單獨使用帶電起 始劑並無法獲得完全均一粒徑以及大於微米級之乳膠粒子。為了克服此問題，使 用了帶電/不帶電的混合起始劑系統，發現藉由加入此不帶電的起始劑能得到數微 米大小之均一粒徑乳膠顆粒。吾人對於此新型研究路徑之各種變因作一系列探討。
第六章中，使用無乳化劑醇/水相分散聚合，製備包覆奈米碳黑粒子之複合材料。
首先使用溶膠凝膠法對市售碳黑表面做反應官能基之接枝反應同時並增加其長時 間分散性使其均勻分散於溶劑中，隨後進行無乳化劑之分散聚合，成功製備高分 子乳膠顆粒包覆之碳黑/高分子之核/殼奈米複合乳膠，並探討碳黑表面接枝量、使 用起始劑種類、溶劑極性等因素對包覆之影響。
關鍵字: Pickering 乳化聚合、1,1-對二苯乙烯、控制活性自由基聚合、有機/無機 複合乳膠顆粒、均一粒徑、分散聚合、無乳化劑乳化聚合、核/殼結構
Abstract
In this research, we developed several kinds of novel surfactant-free polymerization along with various known surfactant-free polymerization techniques for preparing living block copolymer organic/inorganic hybrid latexes as well as monodisperse latexes. This research was divided into three main parts. In the first part, 1,1-diphenylethene(DPE) controlled radical polymerization was performed and used to synthesize living block copolymer, further extended the system to Pickering emulsion polymerization. In the second part, we studied the formation of monodisperse latexes derived from unstable Pickering emulsion polymerization. Besides, monodisperse polymer/silica core/shell nanocomposites were prepared by emulsifier-free solid stabilized polymerization. In the third part, surfactant-free alcoholic dispersion polymerization was conducted for monodisperse latexes as well as nanoparticles encapsulation.
Chapter 2 and 3 were contained in the first part. In chapter 2, controlled free radical polymerizations by 1,1-diphenylethene(DPE) were demonstrated. In our previous study, the DPE controlled radical polymerization with constant molecular weight throughout the polymerization was caused by the intrinsically low reactivation rate constant (k2) of DPE-capped dormant chains at lower temperatures. By using a preheating treatment of initiators followed by a living polymerization of monomers at higher temperatures, a continuous growing of polymers with unimodal molecular weight distribution and a relatively small polydispersity index was observed. The reaction temperature, and preheating treatment to the molecular weight also molecular weight distribution were discussed. Moreover, we prepared poly(methyl methacrylate-block-n butyl acrylate) (PMMA-b-PBA) block copolymers using DPE-capped PMMA as a macroinitiator through bulk and solution polymerization. The influences of solvent and polymerization methods on the polymerization rate, controlled living character, molecular weight (Mn) and molecular weight distribution (PDI) throughout the polymerization were studied and discussed. In chapter 3, Pickering emulsion polymerization in the presence of a novel suspension of zinc oxide/ Poly(sodium 4-styrenesulfonate) (ZnO/PSS-)
nanocomposite particles was applied to prepare ZnO/living block copolymer latexes. In the emulsion system, 1,1-diphenylethene(DPE) controlled radical polymerization of poly(methyl methacrylate)-b-poly(butyl acrylate) was proceeded in oil phase. The influences of hydrophilicity of the ZnO/PSS- nanocomposite as well as the ratio between ZnO/PSS- and oil phase on the polymerization, controlled living character, molecular weight (Mn), and molecular weight distribution (PDI) of living block copolymers throughout the Pickering emulsion polymerization has also been elaborated. To the best of our knowledge, this was the first solid stabilized emulsion with a controlled/living radical polymerization inside.
Chapter 4 and Appendix were contained in the second part. We discovered that monodisperse latexes (MLs) ranging from nanometers to micrometers with a clean surface and acceptable colloidal stability can be obtained under unstable Pickering emulsion polymerization conditions. A coalescence-induced Pickering emulsion polymerization route for the MLs was proposed. Furthermore, we extended this polymerization system to coalescence induced surfactant-free emulsion polymerization in the absence of any particulate stabilizers and studied various parameters thoroughly. Due to the relatively low latex production yields with respect to the monomer input from the coalescence follows creaming, we arranged this part into Appendix. In chapter 4, we conducted the emulsifier-free polymerization in the presence of commercial grade negatively charged colloidal silica. Well-defined vinyl polymer core/silica shell with near monodisperse distributions could be readily obtained in a single step using cationic AIBA as initiator. Various syntheses parameters such as the pH of the solution, the kind of initiator employed into the polymerization, the amount of silica, and the ratio between the monomer and the silica were studied.
Chapter 5 and 6 were contained in the third part. In chapter 5, we propose a new strategy for preparing high solid content, surfactant-free charge stabilized monodisperse latex particles via alcoholic dispersion polymerization by means of a mixed ionic/non-ionic initiator system, which produces hundreds of nanometer up to several micron sized latex particles with a clean
surface in a single batch process. It was observed that no truly monodisperse latexes could be obtained using ionic initiators alone in alcoholic dispersion polymerization, therefore a mixed initiation approach was proposed. The effect of various factors on this new approach is investigated, and a specific mechanism is also presented. In chapter 6, we describe a new method based on surfactant-free aqueous/alcoholic dispersion polymerization, which enables the polymeric encapsulation of nanoparticles. Mechanism of this approach is investigated in the context of the preparation of polystyrene encapsulated carbon black (CB) nanocomposite latex particles. Commercial grade MOGUL® L CB is first grafted with reactive silane coupling agents through sol-gel reaction and is finely dispersed in the polar medium with dissolved monomer, then the ionic initiator is added to the system to start the polymerization. The reactive functional groups introduced onto the CB nanoparticles enable its participation into the nucleation also surface polymerization, leading to the well-defined latex-encapsulated nanocomposite structure. Various synthesis parameters such as the grafting amount of silane coupling agent, the initiator employed into the polymerization, and solvency to the encapsulation were investigated.
Key words: Pickering emulsion polymerization, 1,1-diphenylethene, controlled radical polymerization, organic/inorganic composite latex, monodisperse, dispersion polymerization, surfactant-free emulsion polymerization, core/shell structures.
Contents
摘要... I Abstract ...III List of Tables ... X List of Figures... XII
Chapter 1 Introduction ...1
1-1 Introduction of controlled/living polymerization ...1
1-2 Introduction of Monodisperse Latex...5
1-2.1 Emulsion polymerization...5
1-2.2 Emulsifier-free emulsion polymerization...7
1-2.3 Dispersion polymerization...9
1-2.4 Precipitation polymerization...10
1-2.5 Other techniques ...11
1-2.6 Applications of monodisperse latexes ...11
1-3 Introduction of Pickering (solid-stabilized emulsion) emulsion ...13
1-4 Flow chart of this work ...15
References ...16
Chapter 2 DPE Controlled/Living Radical Polymerization – Preheating Method and Preparation of Block copolymers... 20
2-1 Introduction ...20
2-2 Experimental...23
2-2.1 Materials ...23
2-2.2 Bulk Polymerization...23
2-2.3 Preparation of DPE-capped macro-initiator ...23
2-2.4 Preparation of PMMA-b-PBA block copolymer from DPE-capped PMMA macro-initiator ...24
2-2.5 Characterization ...24
2-3 Results and Discussion...25
A：Preparation of DPE-containing homopolymers by Two-stage Preheating Method ...25
2-3.1A Reaction temperatures and polymerization without preheating treatment ...25
2-3.2A Preheating treatment ...26
2-3.3A Influence of monomer concentration in the step of preheating treatment ...27
2-3.4A Influence of preheating time period ...28
2-3.5A Influence of DPE ...29
B：Preparation of PMMA-b-PBA block copolymer...30
2-3.1B Preparation of DPE-capped macro-initiator...30
2-4 Conclusions ...33
A：Preparation of DPE-containing homopolymers by Two-stage Preheating Method ...33
B：Preparation of PMMA-b-PBA block copolymers ...33
2-5 References and Notes...47
Chapter 3 Preparation Novel Suspensions of ZnO/Living Block Copolymer Latex Nanoparticles via Pickering Emulsion Polymerization... 48
3-1 Introduction ...48
3-2 Experimental Section ...51
3-2.1 Materials ...51
3-2.1 Synthesis of ZnO/PSS- Nanoparticles ...51
3-2.3 Preparation of PMMA-b-PBA Living Block Copolymer Latexes from Pickering Emulsion Polymerization ...51
3-2.4 Fourier Transform Infrared (FTIR) Spectroscopy Experiments...52
3-2.5 Thermogravimetric Analysis (TGA)...52
3-2.6 X-ray diffraction (XRD)...53
3-2.7 Evaluation of Dispersion Stability ...53
3-2.8 Dynamic Light Scattering Measurements (DLS) ...53
3-2.9 Zeta Potential Measurements...54
3-2.10 Transmission Electron Microscopy (TEM)...54
3-2.11 Interfacial tension measurements...54
3-2.12 Polymer Characterizations...54
3-3 Results and Discussion...56
3-3.1 Synthesis and Characterization of ZnO/PSS- Nanocomposite Suspensions ...56
3-3.1 Oil/Water Interfacial Properties of ZnO/PSS- Nanocomposites ...57
3-3.3 Preparation of DPE-Capped Macroinitiator ...59
3-3.4 Living Block Copolymer Latexes from Pickering Emulsion Polymerization...59
3-3.5 The Controlled Living Characters in Pickering Emulsion Polymerization...61
3-3.6 Long Term Colloidal Stability ...63
3-4 Conclusions ...65
3-5 References and Notes...82
Chapter 4 Emulsifier-Free Solid Stabilized Polymerization for the Preparation of Monodisperse Polymer/Silica Core/Shell Structure ... 86
4-1 Introduction ...86
4-2 Experimental Section ...88
4-2.1 Materials ...88
4-2.2 Preparation of Polymer/Silica Core/Shell Nanocomposite Latexes by Emulsifier-Free Solid Stabilized Polymerization...88
4-2.3 Characterization of Polymer/Silica Core/Shell Nanocomposite Latexes...89
4-3 Results and Discussions ...91
4-3.1 Formation Mechanism of Emulsifier-Free Solid Stabilized Polymerization ...91
4-3.2 Effect of the pH...92
4-3.3 Effect of the silica concentration ...93
4-4 Conclusions ...96
Chapter 5 Novel Synthesis of Multi-Scaled, Surfactant-Free Monodisperse Latexes via Alcoholic Dispersion Polymerization in a Mixed Ionic/Non-ionic Initiation System... 107
5-1 Introduction ...107
5-2 Experimental Section ... 110
5-2.1 Materials ... 110
5-2.2 Latex Preparation ... 110
5-2.3 Characterization ... 111
5-3 Results and Discussion... 113
5-3.1 Soap-free Dispersion Polymerization in a Solely Ionic-Charged Initiator System ... 113
5-3.2 Mixed Initiation Approach ... 115
5-3.3 Effect of the Solvency ... 117
5-3.4 Effect of the Initiator ... 119
5-3.5 Effect of the Reaction Temperature ...121
5-3.6 Preparation of large, stabilizer-free monodisperse Latexes in a One Step Process ...122
5-4 Conclusions ...124
5-5 References and Notes...143
Chapter 6 Surfactant-free dispersion polymerization as an Efficient Synthesis Route to a Successful Encapsulation of Nanoparticles... 147
6-1 Introduction ...147
6-2 Experimental Section ...152
6-2.1 Materials ...152
6-2.2 Silyation of Carbon Black ...152
6-2.3 Preparation of Carbon Black/ Polymer Composite Latex by Soapless Dispersion Polymerization ...153
6-2.4 Fourier Transform Infrared (FTIR) Spectroscopy Experiments...153
6-2.5 Thermogravimetric Analysis (TGA)...153
6-2.6 X-ray photoelectron spectroscopy (XPS)...154
6-2.7 29Si MAS Nuclear Magnetic Resonance (NMR) spectroscopy...154
6-2.8 Dynamic Light Scattering Measurements (DLS) ...155
6-2.9 Zeta Potential Measurements...155
6-2.10 Scanning Electron Microscopy (SEM)...155
6-2.11 Transmission Electron Microscopy (TEM)...155
6-3 Results and Discussion...156
6-3.1 Characterization of silane-modified carbon black ...156 6-3.2 Preparation of Carbon Black/Polymer Core/Shell Nanoparticles via Surfactant-free
Dispersion Polymerization ...157
6-3.3 Effect of the MSMA grafting amount ...159
6-3.4 Effect of the Solvency ...160
6-3.5 Effect of the monomer/CB ratio ...162
6-3.6 Preparation of Other Kinds of Polymer-Encapsulated Nanoparticle Composite Latexes by Surfactant-free Dispersion Polymerization ...163
6-4 Conclusions ...165
6-5 References and Notes...179
Chapter 7 Conclusions ... 185
Appendix--Preparation of Monodisperse Latex from Coalescence Induced Pickering (Surfactant-Free) Emulsion Polymerization... 190
A-1 Introduction...190
A-2 Experimental Section ...194
A-2.1 Materials ...194
A-2.2 Typical Coalescence Induced Pickering Emulsion Polymerization procedure ...194
A-2.3 Dynamic Light Scattering Measurements ...195
A-2.4 Static Light Scattering Measurements...195
A-2.5 Zeta Potential Measurements...195
A-2.5 Transmission Electron Microscopy (TEM) ...196
A-2.6 Scanning Electron Microscopy (SEM)...196
A-2.7 Thermogravimetric Analysis (TGA) ...196
A-2.8 Pyrene Fluorescence Measurements (FL) ...196
A-3 Results and Discussion...197
A-3.1 The Criteria for Coalescence Induced Pickering Emulsion Polymerization ...197
A-3.2 Influence of solid particles concentration...199
A-3.3 Variation of the Dispersed Phase Volume Fraction...200
A-3.4 Effect of the agitation rate ...200
A-3.5 Monodisperse Latexes from Coalescence Induced Surfactant-Free Emulsion Polymerization ...201
A-3.6 How do the surfactant-free droplets/latexes remained colloidal stable and effect of pH in the aqueous phase ...202
A-3.7 The particle size evolutions in coalescence induced surfactant free emulsion polymerization ...204
A-3.8 Effect of the hydrophobe ...204
A-3.8 The choice of oil-soluble initiators and latex conductivity measurements ...207
A-4 Conclusions...210
A-4 References and Notes ...230
List of Tables
Table 2-1A. Symbols and recipes of the DPE controlled bulk polymerization of MMA.
... 34 Table 2-2A. Symbols and recipes of the DPE controlled bulk polymerization of Styrene. ... 34 Table 2-3B. Preparation of DPE-capped PMMA macro-initiator ... 34 Table 2-4B. Symbols and recipes of the DPE-controlled bulk polymerization of BA using sample DPE-PMMA as macroinitiator... 34 Table 3-1. The Recipe, Apparent Sedimentation Speed and Zeta Potential of the ZnO/PSS- Nanocomposites and Latexes... 66 Table 3-2. Symbols and recipes of the DPE-controlled Minemulsion polymerization and Pickering emulsion polymerization using DPE-capped PMMA as macroinitiator and ZnO/PSS- nanocomposites as emulsion stabilizer. ... 67 Table 4-1.The recipe for emulsifier-free solid stabilized polymerization of
polystyrene/silica composite latexes. ... 97 Table 5-1.The decomposition rate constants of the initiators employed into the latex synthesis...125 Table 5-2. The recipe for dispersion polymerization of styrene in solely AIBA
initiation system ...126 Table 5-3. The recipe for soapless dispersion polymerization of Methyl methacrylate in solely AIBA initiation system ...127 Table 5-4. The recipe for dispersion polymerization of styrene in ionic/non-ionic mixed initiation system ...128 Table 5-5. The recipe for soapless dispersion polymerization of styrene in
ionic/non-ionic mixed initiation system ...129 Table 6-1. Recipe for the Silane-Modified Carbon Black Nanoparticles. ...166 Table 6-2. Recipe for Soapless Dispersion Polymerization for Encapsulation of
Carbon Black (CB) into Polymeric Latexes. ...167 Table 6-3. Surface Compositions Obtained from X-ray Photoelectron Spectroscopy for Selected CB and CB/polymer Nanocomposites. ...168 Table 6-4. Recipe for the Silane-Modified SiO2 Nanoparticles for Encapsulation. ..169 Table 6-5. Recipe for the Silane-Modified TiO2 Nanoparticles for Encapsulation. ..169 Table 6-6. Recipe for Soapless Dispersion Polymerization for Encapsulation of
nanoparticles (NPs) into Polymeric Latexes...170 Table A-1. The recipe for coalescence induced Pickering emulsion polymerization in the presence of Ludox® TM40 silica. ...211 Table A-2.The recipe for coalescence induced Pickering emulsion polymerization in the presence of SYTON® HT50 silica...212
Table A-3. The recipe for coalescence induced surfactant-free emulsion
polymerization of styrene using AIBN as initiator...213 Table A-4. The recipe for coalescence induced surfactant-free emulsion
polymerization of styrene using ACHN as initiator. ...214
List of Figures
Figure 1-1. Schematic presentation of early stages of emulsion polymerization
(adapted from B. Vollmert, Polymer Chemistry, Springer Verlag, New York 1973). . 7
Figure 1-2. Schematic illustration of dispersion polymerization. (Kawaguchi, S.; Ito, K. Adv. Polym. Sci. 2005, 175, 299–328.)... 10
Figure 1-3. General kinetic features and particle size ranges of heterogeneous polymerization processes... 13
Figure 1-4. Mechanisms of Solid stabilized emulsions. (a) Barrier from full coverage of monolayer particles. (b) Sparsely covered droplets through bridging stabilization. ... 14
Figure 2-1A. The DPE mechanism in CRP polymerization. ... 35
Figure 2-2A. GPC traces of sample A0, A1, B1, B1-1... 36
Figure 2-3A. GPC traces of sample A2. ... 37
Figure 2-4A. GPC traces of sample A7. ... 37
Figure 2-5A. Molecular weights and PDI versus conversion of sample A3-A5... 38
Figure 2-6A. Molecular weights and PDI versus conversion with various preheating time periods of MMA system. ... 39
Figure 2-7A. Molecular weights and PDI versus conversion with various preheating time periods of styrene system... 40
Figure 2-8A. Molecular weights and PDI versus conversion of sample A4,A8. ... 41
Figure 2-9A. Molecular weights and PDI versus conversion of sample B2,B3. ... 42
Figure 2-10A. –ln(1-x) versus polymerization times for samples A2-1 to A8. ... 43
Figure 2-11A. –ln(1-x) versus polymerization times for samples B1-B4. ... 43
Figure 2-12B. The monomer conversion versus polymerization time from bulk/solution polymerization with various amounts of solvent... 44
Figure 2-13B. GPC traces of PMMA-b-PBA block copolymer from bulk/solution polymerization at various monomer conversion (Toluene/BA=0)... 44
Figure 2-14B. GPC traces of PMMA-b-PBA block copolymer from bulk/solution polymerization at various monomer conversion (Toluene/BA=1)... 45
Figure 2-15B. GPC traces of PMMA-b-PBA block copolymer from bulk/solution polymerization at various monomer conversion (Toluene/BA=4)... 45
Figure 2-16B. Molecular weight versus monomer conversion of PMMA-b-PBA block copolymer from bulk/solution polymerization with various amounts of solvent. ... 46
Figure 2-17B. PDI versus monomer conversion of PMMA-b-PBA block copolymer from bulk/solution polymerization with various amounts of solvent. ... 46
Figure 3-1. Synthesis of Poly(sodium 4-styrenesulfonate) (PSS-Na)-assisted Zinc Oxide Nanoparticles (ZnO/PSS-). ... 68 Figure 3-2. Schematic diagram of the formation mechanism of PMMA-b-PBA block
copolymer latex from Pickering emulsion polymerization using ZnO/PSS- as stabilizer.
... 69
Figure 3-3. The mechanism of DPE-controlled radical polymerization... 69
Figure 3-4. TEM photographs of ZnO/PSS- nanocomposite particles.(a)Sample A1 70 Figure 3-5. FTIR spectra of (a)PSS-Na, (b)PSS-Na chelation with zinc cation, and (c)ZnO/PSS- nanocomposite... 71
Figure 3-6. The XRD pattern of ZnO/ PSS- nanocomposite A1. ... 71
Figure 3-7. TGA curves of ZnO/ PSS- nanocomposites and pristine PSS-Na... 72
Figure 3-8. Aqueous ZnO/ PSS- nanocomposite suspension/Toluene interfacial tension as a function of time for various recipes. ... 72
Figure 3-9. Size distributions of the droplets and corresponding latex particles. ... 73
Figure 3-10. Size distributions of the droplets and the corresponding latex particles. ... 74
Figure 3-11. TEM images of (a) Pickering emulsion droplets after ultrasonication in C3. (b) Pickering emulsion latexes after polymerization in C3. (c)Pickering emulsion latexes after polymerization in C4. ... 75
Figure 3-12. (a) Monomer conversion and –ln(1-X) versus polymerization time of B1, C1 to C3.(b) Molecular weight and PDI of block copolymers versus monomer conversions of B1, C1 to C3.(c) GPC traces of sample C3... 77
Figure 3-13. (a) Monomer conversion and –ln(1-X) versus polymerization time of C3, C4.(b) Molecular weight and PDI of block copolymers versus monomer conversions of C3, C4.(c) GPC traces of sample C4... 79
Figure 3-14. Photographs of sample A1 to A3 and C3 in sedimentation experiments after (a) 0 days (b) 7 days (c) 14 days (d) 3 months. From left to right: A1 A2 A3 C3. Photographs of sample C4 in sedimentation experiments after (e) 7 days... 80
Figure 4-1. Schematic Representation of core/shell nanocomposite latexes synthesized from emulsifier-free solid stabilized polymerization using negatively charged silica and positively charged initiator (EFS-PS2, 3, 4). ... 98
Figure 4-2. (a), (b) TEM, and (c), (d) SEM images of EFS-PS3. ... 99
Figure 4-3. (a) TEM, (b) SEM image of emulsifier-free polymerization in the presence of ...100
Figure 4-4. Zeta potentials as a function of pH values of Ludox® TM40 silica nanoparticles. ...101
Figure 4-5. (a) TEM image of EFS-PS4. (b) TEM image of EFS2...102
Figure 4-6. (a) TEM image of EFS-PS5. (b) SEM image of EFS6...102
Figure 4-7. (a) SEM, (b) TEM image of EFS-PS7...103
Figure 4-8. TEM image of EF-PS1. ...103 Figure 4-9. TEM images of (a) polystyrene/ Ludox® SM30. (b) Poly(methyl
methacrylate)/ Ludox® SM30. (c) Poly(methyl methacrylate)/ Ludox® TM40. ...104 Figure 5-1. A schematic representation of particle formation and growth in soap-free alcoholic dispersion polymerization in solely AIBA initiation system. ...130 Figure 5-2. A schematic representation of particle formation and growth in soap-free alcoholic dispersion polymerization with AIBA/AIBN mixed initiation system. ...130 Figure 5-3. Representative SEM images of AIBA-PS latexes synthesized in pure methanol via dispersion polymerization. Scale bar: 3μm. (a)A-PS2 (b)A-PS1
(c)A-PS3 (d)A-PS6 ...131 Figure 5-4. The plots of conversion versus time, diameter, and Np evolution of
AN-PS4 (60 oC) and AN-PS8 (45 oC). ...132 Figure 5-5. Representative SEM images of (a) AIBA-PS latexes synthesized in pure methanol via dispersion polymerization (A-PS1). Scale bar: 3μm. (b) The particle bridging in AN-PS6 at 70% conversion, and (c) 75% conversion. Scale bar: 5 μm..133 Figure 5-6. SEM images at various monomer conversions of nucleation and growth of the PS latexes in system AN-PS4. Scale bar: 2μm. (a) 0.23%, (b)1.82%, (c)7.48%, (d)11.07%, (e)19.76%, (f)29.35%, (g)38.39%, (h)54.55%, (i)71.43%, (j)89.85%....133 Figure 5-7. Plot of conversion versus time, diameter, and particle number in
AIBA/AIBN mixed initiation system in pure methanol (AN-PS4) and methanol/water system (AN-PS1 series)...134 Figure 5-8. Zeta potential as function of conversion for latex particles synthesized in pure methanol (AN-PS4) and methanol/water medium (AN-PS1). (No added
electrolyte apart from ionic initiator) ...135 Figure 5-9. Plot of conversion versus time, diameter, and particle number in
AIBA/AIBN mixed initiation system at AIBN/AIBA=0/1wt% (A-PS1), 4/1wt%
(AN-PS3), and 10/1 wt% (AN-PS5) with fixed AIBA concentration...136 Figure 5-10. Plot of Molecular weight versus conversion in (a) A-PS1. (b)AN-PS3.
(c)AN-PS5. ...137 Figure 5-11. (a) Effect of total initiator concentration on particle diameter with
various AIBN/AIBA ratio. (b) The logarithmic plot of AIBN concentration on the particle diameter with three fixed AIBA concentration...138 Figure 5-12. The conversion versus diameter with reaction temperatures at 45 oC (A-PS2, AN-PS8, AN-PS9) and 60 oC (A-PS1, AN-PS4, AN-PS5). ...139 Figure 5-13. SEM images of (a) 4.1μm cationic polystyrene latex (VD-PS2). Scale bar: 5μm. (b) 1.8μm anionic polystyrene latex (KN-PS1). Scale bar: 5μm. (c) 1.7μm cationic poly(isobutyl methacrylate) latex. Scale bar: 10μm. (d) 2.2μm cationic poly(styrene-co-divinylbenzene) latex. Scale bar: 3μm. (e) 3.4μm cationic
poly(styrene-co-butyl acrylate) latex.Scale bar: 3μm. ...140 Figure 5-14. The particle size distribution and SEM image of AN-PS1 obtained from
DLS measurement. ...141 Figure 5-15. The size distribution of cationic poly(styrene-co-divinylbenzene) latex particles obtained from laser light scattering measurement and its SEM image
(VD-P(S-DVB)1). ...142 Figure 6-1. A schematic representation of 3-(Trimethoxysilyl)propyl methacrylate (MSMA) modified carbon black via sol-gel reation. ...171 Figure 6-2. (a) The XPS survey spectra of MOGUL® L CB and MSMA modified CB (CB3). (b) FT-IR spectra of MOGUL® L CB and MSMA modified CB (CB3). (c) 29Si solid state NMR spectrum of MSMA modified CB (CB3). ...172 Figure 6-3. The photographs of MOGUL® L carbon black (left) and MSMA modified carbon black (CB3) (right) dispersion in methanol. (a) As prepared. (b) After 2 hours.
(c) After 2 weeks. (d) TEM image of MSMA modified carbon black (CB3). Scale bar:
200nm. ...173 Figure 6-4. Schematic illustration of particle formation and growth of
polymer-encapsulated carbon black composite latexes via surfactant-free dispersion polymerization using anionic initiator...174 Figure 6-5. TEM images of polystyrene encapsulated carbon black nanocomposite latexes synthesized by surfactant-free dispersion polymerization (PS-CB5) at various monomer conversions. Scale bar: 200nm. (a) 10% conversion. (b) 59% conversion. (c) A higher magnification of the selected area in (b). (d) 87% conversion...174 Figure 6-6. Representative SEM images of polystyrene encapsulated carbon black (CB) nanocomposite latexes prepared by surfactant-free dispersion polymerization, without purification of excess CB nanoaprticles. (a) PS-CB6. Scale bar: 0.5μm. (b) PS-CB5. Scale bar: 0.5μm. (c) PS2. Scale bar: 1μm. (d) PS-CB4. Scale bar: 1μm. (e) PS-CB7. Scale bar: 1μm. (f) PS-CB8. Scale bar: 0.5μm...175 Figure 6-7. Thermogravimetric analysis (TGA) curves of polystyrene encapsulated carbon black nanocomposite latexes. ...176 Figure 6-8. 29Si solid state NMR spectra of 3-(Trimethoxysilyl)propyl methacrylate (MSMA) modified MOGUL® L CB. (a) CB 1. (b) CB 2. (c) CB 5. (d) CB 7. ...177 Figure 6-9. Thermogravimetric Analysis (TGA) analysis of pristine MOGUL® L CB and MSMA modified CB3, CB5...177 Figure 6-10. Representative TEM images of (a) Polystyrene encapsulated carbon black nanocomposite latexes (PS-CB6). (b) 890nm polystyrene encapsulated TiO2 nanocomposite latexes with cationic surface charges (PSTI5). (c) Ultramicrotomed 433nm polystyrene encapsulated SiO2 nanocomposite latexes with anionic surface charges (PSSI3)...178 Figure A-1. Schemetic representation of monodisperse latexes produced from
coalescence induced Pickering emulsion polymerization. ...215
Figure A-2. (a) SEM image of polystyrene/silica core/shell structure derived from stable Pickering emulsion polymerization (TPS1). (b) SEM image of 560nm polystyrene latexes from coalescence induced Pickering emulsion polymerization (TPS3)...216 Figure A-3. (a)(b) TEM (c) SEM images of coalescence induced Pickering emulsion polymerization in the presence of ZnO nanorods. Condition: Water 100g, ZnO nanorod 0.01g, styrene 0.75g, Hexadecane 0.25g, AIBN 0.01g, sonication
amplitude/time 100%/ 60mins, reaction temperature 80 oC. ...217 Figure A-4. SEM images of (a) HPS1. (b) HPS2. (c) HPS3. (d) HPS4. ...218 Figure A-5. SEM images of (a) HPS5. (b) HPS6. (c) HPS7. (d) HPS8. ...219 Figure A-6. SEM images of near-monodisperse polystyrene latexes from coalescence induced Pickering emulsion polymerization (Sample TPS5). ...220 Figure A-7. SEM images of polystyrene latexes from coalescence induced
surfactant-free emulsion polymerization. (a)IBPS1. (b)IBPS2. (c)IBPS3. (d)IBPS4.
...221 Figure A-8. SEM images of polystyrene latexes from coalescence induced
surfactant-free emulsion polymerization. (a)IBPS6. (b)IBPS11. (c)IBPS9. (d)IBPS10.
...222 Figure A-9. Monomer conversion and PDI versus time in trial CHPS3. ...223 Figure A-10. Droplet/latex size distributions with various monomer conversions in trial CHPS3. ...223 Figure A-11. Emission Spectra for styrene:hexadecane =3:1 surfactant-free emulsion at different times after ultrasonication for 5 minutes. ...224 Figure A-12. IE/IM(t) for surfactant-free emulsion of styrene:hexadecane =3:1 and 9:1 as a function of time after ultrasonication for 5 minutes...225 Figure A-13. The chemical structure of two kinds oil-soluble azo-initiator employed into the coalescence induced surfactant-free emulsion polymerization (a) AIBN. (b) ACHN...225 Figure A-14. SEM images of polystyrene latexes from coalescence induced
surfactant-free emulsion polymerization. (a)CHPS2. (b)CHPS3. (c)CHPS5. (d)CHPS6.
...226 Figure A-15. SEM images of polystyrene latexes from coalescence induced
surfactant-free emulsion polymerization. (a)CHPS9. (b)CHPS11. (c)CHPS13.
(d)CHPS14...227 Figure A-16. (a) Illustration of the reactor and the methodology to investigate the initial period of heterophase polymerizations with on-line monitoring of transmission and conductivity (not to scale); 1 and 2 denote possibilities to inject AIBN; the stirrer speed is just enough to avoid concentration gradients in the continuous phase and
does not cause the formation of monomer droplets. (b) On-line record of the changes in transmission (solid lines) and conductivity (dashed lines) during AIBN-initiated surfactant-free styre-ne heterophase polymerize-tion; the curves represent averages of 5 repeats; 1, 2 AIBN addition into the monomer and the water phase, respectively.
(Tauer, K.; Mukhamedjanova, M.; Holtze, C.; Nazaran, P.; Lee, J. Macromol. Symp.
2007, 248, 227-238.) ...228 Figure A-17. The equipments for emulsification using in our experiments for
coalescence induced surfactant-free emulsion polymerization (a) Ultrasonicator. (b) High shear homogenizer. ...229
Chapter 1 Introduction
1-1 Introduction of controlled/living polymerization
Free radical polymerization is extensively used in view of its industrial production and applications. This technique can be readily performed since the polymerization condition is not stringent. However, it possesses drawbacks of lack of control over the molecular weight, molecular weight distribution, and macromolecular architecture. Controlled/living free radical polymerization (CRP), which provides a versatile route to the synthesis of well-defined polymer architecture and promises facile control of polymer morphology as well as microstructure, has attracted much attention in the past two decades. Up to know, these techniques can be roughly categorized into three types of synthesis pathways as follows: Nitroxide-mediated polymerization (NMP),1-5reversible addition-fragmentation transfer (RAFT),6-10 and atom transfer radical polymerization (ATRP).11-15These three main CRP techniques are rather mature in academic research and are used widespread globally.
(1) Nitroxide-mediated polymerization (NMP)
In mid 1980s, Solomon et al. first introduced that introducing stable free radical such as nitroxides into polymerization system the free radical polymerization could be controlled. Afterward in the 1990s, George et al. utilized (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (2,2,6,6-TEMPO) as a stable free radical, using benzoyl peroxide (BPO) as an initiator to perform the styrene polymerization at 123oC. The molecular weight of polymer was found to increase with the monomer conversion in a linear fashion with acceptable polydispersity (PDI) lower than 1.3, presenting a controlled living character.1 The NMP mechanism is shown below.
Where Pn-X is the dormant species, Pn• is the active radical and X• is the stable free
radical, ka and kd are the dissociation and recombination reaction rate constant, respectively. These stable free radicals are not tended to undergo mutual terminations, but to deactivate active propagating centers of radical polymerization through a reversible termination reaction. The most typical stable free radical polymerization system used among the early research is 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) controlled styrene system. However, the restriction of the NMP is that monomer was limited to styrene derivatives under a high polymerization temperature (>120 oC). Therefore, modifying the chemical structure of TEMPO to adapt mild experimental conditions of CRP has been widely studied afterward.
Pn-X Pn
.
X.
+
(2) Atom transfer radical polymerization (ATRP)
Atom transfer radical polymerization (ATRP) or transition metal-mediated living radical polymerization is another commonly used controlled/living radical polymerization. In 1995, Matyjaszewski11 and Sawamoto14 independently performed a controlled/living polymerization by atom transfer radical polymerization (ATRP).
Similar to its counterpart, ATRA or atom transfer radical addition, it forms a carbon-carbon bond through a transition metal catalyst. The atom transfer step is the key step in the reaction which responsible for narrowly distributed polymer chain growth. 1-phenylethyl chloride was used as the initiator and CuCl and 2,2-bipyridine were used as catalyst for styrene CRP at 130oC. The monomer conversion reached 95% after 3hr polymerization and molecular weight of polymer increased with monomer conversion in a linear fashion.11 The radicals or the active species are generated through a reversible redox process which catalyzed by a transition metal complex (Mtn
-Y/Ligand, where Y may be another ligand or the counterion), that
undergoes a one electron oxidation with concomitant abstraction of a (pseudo)halogen atom, X, from a dormant species, R-X. Polymer chains grow by the addition of the intermediate radicals to monomers similar to a conventional radical polymerization with the rate constant of propagation kp. The generally known mechanism for ATRP is shown below.
(3) Reversible addition-fragmentation transfer (RAFT)
Rizzardo et al. first reported addition–fragmentation chain transfer (RAFT) polymerization for CRP in 1998.16 The mechanism of RAFT polymerization is a sequence of addition–fragmentation equilibrium as shown in the scheme. Initiation and radical mutual termination occur as the same manner compared with conventional radical polymerization. In the early stages of the polymerization, addition of a propagating radical (P•n) to the thiocarbonylthio compound [RSC(Z)S], followed by fragmentation of the intermediate radical gives rise to a polymeric thiocarbonylthio compound [PnS(Z)CS,] and a free radical (R•). New propagating radicals (P•m) were formed by the primary radical (R•) and monomer. Rapid equilibrium between the active propagating radicals (P•n and P•m) and the dormant polymeric thiocarbonylthio compounds provides almost equal probability for chain growing, led to the formation of polymers with narrow polydispersity. When the polymerization is completed, most of chains retain the thiocarbonylthio end group and can be easily isolated.
Although these three techniques of controlled/living polymerization were relatively mature, some limitation and disadvantages hindered their application in industrial production. Drawbacks in three main techniques of CRP impede the mass production on an industrial scale. For example, the NMP system has to be carried out at high temperatures (>120C), due to its inherently slow reaction rate and it works better in styrene derivatives. The difficulties of catalyst removal from the polymer for ATRP and relatively complicated synthesis pathway as well as the unpleasant odor in RAFT are known defects. In 2001, Nuyken and co-workers developed a new additive for controlled radical polymerization.17 Most of the radical polymerizations were controlled if a small amount of 1,1-diphenylethene (DPE) was added, and the method was compatible with the choice of monomers and solvents. Although the reported polydispersity indexes of DPE-controlled systems were relatively large compared to the three well known CRP methods, facile synthesis of block copolymers by heating DPE-capped macroinitiator with the presence of second monomer was achievable on an industrial scale.18 Low PDI value was of minor concern in these cases. Moreover, the number of publications in DPE-mediated polymerization systems was very limited in the literatures. 19-25
1-2 Introduction of Monodisperse Latex
Monodisperse latexes (ML) are commercially and biologically important products in diagnostic analysis, medical treatment, electronics, and standard for calibration as well as standard for virus particles,26,27 which is widely used industrially and in laboratories. They can be prepared with desired characteristics such as particle size, degree of crosslinking, surface functional group, and porosity from various synthesis approaches. The first ML was discovered accidentally in 1947 at the Dow Chemical Co. A 10 micron ML was also the first commercial product made in space to prevent creaming and settling due to the absence of gravity.28 Due to the high profit margin, ML is also sometimes called “liquid gold”.
These monosize polymer beads are typically made from heterogeneous polymerization processes,29 including emulsion,30,31 membrane emulsification,32,33 emulsifier-free,34–36 dispersion,37–40 and precipitation polymerization.41–43 Nearly monosize beads with moderate polydispersity were also achieved by miniemulsion44–48 and microemulsion polymerization.49 Recent research has drawn attention to highly uniform irregularly-shaped latex nanoparticles with optical anisotropy for their potential applications in photonic materials.50,51 However, it turns out to be particularly difficult to prepare ML in micrometer to millimeter size by a single step process, therefore they are generally prepared by successive seeded emulsion polymerization and multistep suspension polymerization. Following are the introduction of commonly used stratagies for producing ML.
1-2.1 Emulsion polymerization
In classical emulsion polymerization, the monomer is not dissolved (or only to a negligible extent) in the polymerization medium, but emulsified in it with the aid of an emulsifier. Under there conditions, the monomer is present in the mixture partly in
the form of droplets (about 1-10 micron or even larger), and partly in the form of surfactant-coated micelles (ca. 50-100 Å), depending on the concentration of the surfactant. A small percentage of the monomer is also dissolved in the medium. For example, solubility of styrene in water at 70C is about 4g/L.
The volume ratio of the monomer phase to the medium in emulsion polymerization is usually within 10%-50%, and the polymerization is carried out at 40-80C. The state of the polymerization mixture in the early stages of emulsion polymerization is illustrated in Figure1-1. Since the initiator is present only in the medium, the initial locus of polymerization is in the medium, that is, outside the droplets or micelles. The oligo- radicals formed in the polymerization medium are either surrounded by the dissolved monomer and surfactant molecules, or the already present monomer micelles. In either case, the initially formed oligoradicals produce stabilized nuclei. Subsequently, surfactant-stabilized polymer nuclei become the main loci of polymerization by absorbing further oligoradicals and monomer from the medium, or from the monomer droplets. In this manner, the nuclei/particles grow gradually until the monomer is completely consumed. The size of the latexes thus produced is usually in the range of 50-300nm.
For oil in water emulsion polymerization (e.g., those of styrene or methyl methacrylate in water), potassium peroxydisulfate (KPS) and sodiumdodecylsulfate (SDS) are commonly used as initiator and surfactant, respectively. Combinations of ionic and nonionic surfactants may also be used. An interesting example is the use of SDS and Triton X-100, as reported by Woods et al.30 for the preparation of monodisperse polystyrene latexes around 250nm.
Figure 1-1. Schematic presentation of early stages of emulsion polymerization (adapted from B. Vollmert, Polymer Chemistry, Springer Verlag, New York 1973).
1-2.2 Emulsifier-free emulsion polymerization
In emulsifier-free emulsion polymerization there exist some similarities to the classical emulsion polymerization. The monomer is present in the mixture in the form of large droplets (1-10 micron in diameter). These droplets act as reservoirs of monomer for particle growth. In addition, a small amount of the monomer is present in the polymerization medium (usually water) where the polymerization reaction starts. The polymerization mechanism depends on the reactivity and solubility of the monomer in the medium. For slightly water-soluble monomers, such as methyl methacrylate, oligomer radicals are no longer soluble in water and they precipitate in small primary particles (nuclei) which become the main loci of polymerization (some
claimed homogeneous nucleation). On the other hand, oligomer radicals of highly water-insoluble monomers, such as styrene, terminate first with the formation of surface-active oligomers and then precipitate (some claimed micelle nucleation). The nuclei and surface-active oligomers collide and form larger latex particles until the electric charge on their surface reaches a value that resists further aggregation. Latex particles are stabilized by orientation of their own polymer chain ends originating from initiator molecules. For example, in the case of potassium peroxydisulfate initiator (KPS), the chains end with sulfate groups. Subsequently, the polymerization occurs in the monomer-swollen latex particles, in which free-radical oligomers enter from the polymerization aqueous medium. As a result, the original latex particle is transformed into a particle of much greater size. As the monomer in a particle is consumed, additional monomer diffuses from the droplets through the aqueous phase to the site of polymerization. Latex particles grow until the supply of monomer or radicals is exhausted. Since the number of latex particles becomes constant within a short time after the start of the polymerization, the resulting beads are uniform in size.
This is the main difference from classical emulsion polymerization, in which latex particles are formed over rather a long period. The reason for the absence of polymerization in the monomer droplets is that the droplets are too large, and hence the total droplet surface too small, compared to that of the latex particles, to be able to compete for the radicals formed in the aqueous medium. The degree of monodispersity of the latex particles increases during the growth as they adsorb the newly formed nuclei, which at the same time stabilize them. The average number of growing free radicals within the particle is not constant but increases with increasing particle size. The molecular weight of the polymer in the latexes formed in surfactant-free systems is generally lower than that found in latexes prepared in the presence of emulsifier. The control of particle size is affected by polymerization
temperature. By raising the temperature, smaller particles were obtained. A wide range of monomers can be emulsion-polymerized in the absence of emulsifier, e.g., styrene, methyl methacrylate, divinylbenzene, and vinyl acetate. In most cases, the concentration of monomer dissolved in aqueous medium must be less than 5 wt% in order to produce good quality latexes without agglomeration. Emulsifier-free emulsion polymerization typically yields monosized beads with diameter in the range 0.5-1 micron. These beads can be used for the multi-step swelling and polymerization method.
1-2.3 Dispersion polymerization
Dispersion polymerization is an attractive and promising alternative to other polymerization methods that affords micron-size monodisperse particles in a single batch process. Dispersion polymerization may be defined as a type of precipitation polymerization in which one carries out the polymerization of a monomer in the presence of a suitable polymeric stabilizer soluble in the reaction medium. The solvent selected as the reaction medium is a good solvent for both the monomer and the steric stabilizer polymers, but a non-solvent for the polymer being formed.
Dispersion polymerization, therefore, involves a homogeneous solution of (co)monomer with initiator and dispersant, in which sterically stabilized polymer particles are formed by the precipitation of the resulting polymers. As a continuous medium, the properties of the solvent also change with increasing monomer conversion. Under favorable circumstances, the polymerization can yield, in a batch step, polymer particles of 0.1–15 micron in diameter, often of excellent monodispersity. This dispersant polymer can be formed as a reactive, polymerizable macromonomer. It can be a block copolymer in which one block has an affinity for the surface of the precipitated polymer, or it can be a soluble polymer (a “stabilizer
precursor”) to which grafting is thought to occur during the polymerization reaction.
In all instances, this soluble dispersant polymer – a hairy layer – plays a crucial role in the dispersion polymerization process. By adsorbing or becoming incorporated onto the surface of the newly-formed precipitated polymers, it acts as a steric stabilizer, directing the particle size and colloidal stability of the system. This feature of dispersion polymerization is widely appreciated and well understood (Fig. 1-2).
Figure 1-2. Schematic illustration of dispersion polymerization. (Kawaguchi, S.; Ito, K. Adv. Polym. Sci. 2005, 175, 299–328.)
1-2.4 Precipitation polymerization
In precipitation polymerization, the initial state of the reaction mixture is essentially the same as that in dispersion polymerization, i.e., a homogeneous solution.
Therefore the particle formation and growth mechanism resemble those in dispersion polymerization. The only difference, which is to advantage, is that it does not require a stabilizer which would remain in the product as a contaminant. Beads resulting from precipitation polymerization are thus pure. The absence of stabilizer, on the other hand, restricts the number of monomers from which monosized beads can be formed.
Monosized poly (divinylbenzene) beads with diameters between 2 and 5 micron are
prepared by precipitation polymerization in acetonitrile initiated with azobisisobutyronitrile or dibenzoyl peroxide. On the other hand, the initiator with a high decomposition rate (2,2’-azobis(2,4-dimethylvaleronitrile)) form beads with broad size distribution. Depending on the selection of a suitable solvent (ethyl propionate, butyl acetate or methyl butyl ketone), monosized poly (glycidyl methacrylate-co-2- hydroxyethyl methacrylate-co-triethylene glycol dimethacrylate) beads of the size ranging from 0.5 to 5 micron were prepared. Precipitation copolymerization of acrylamide with methylenebisacrylamide in ethanol and isopropyl alcohol affords coarse and bulky particles having a diameter of about 100 micron. Methacrylic acid added to the polymerizing mixture contributes to the stabilization of the particles formed at the initial stage of polymerization and to the enhancement of swelling of the particles by monomers and alcohols resulting in fine monosized beads having a diameter around 1 micron.
1-2.5 Other techniques
Monosized polyamide beads were prepared by rapid cooling of nylon solution in a theta solvent above the theta temperature;52 the phase separation mechanism was proposed for their formation.53 Monosized polymer beads were also prepared by chemical reactions in aerosols.54,55
1-2.6 Applications of monodisperse latexes
Monosized polymer latexes are finding an ever-increasing number of applications. Non-porous MLs have found widespread use in medicine as a means for clinical diagnosis and detection of antigens and antibodies.56 Besides for applications in various immunoassays, they are used as support materials for cell separation (sorting, labelling, attachment) and cultivating, as spacers in large liquid crystal
displays, fillers in advanced composites, materials for adhesives, paints, electrostratographic toners, calibration standards of various instruments (e.g., electron microscopes, light scattering devices, ultracentrifuges, aerosol and particle size counters), standards for determination of pore size and efficiency in membranes and filters. On the other hand, porous monosized beads are used either underivatized or derivatized. The former are applied in controlled drug release vehicles and in size exclusion chromatography, the latter as ion exchangers (in particular for water treatment, i.e., softening and demineralization chromatographic packings (in high-performance liquid and ion chromatography), carriers for reagents, enzymes and catalysts. Due to uniform column packing, uniform flow velocity profile and high resolution, monosized polymer beads make a great contribution to the improved separation efficiency, fast kinetics, and high flow rates and capacity in chromatography. Advantages of monosized macroporous polymer packing materials compared with silica consist in stability in alkaline conditions and in broader pore size distribution, which enhances the access of protein molecules to the active site and renders polymers with higher capacity. In ion-exchange resins for industrial water treatment, monosized beads have a higher operating capacity and efficiency and lower pressure drop than conventional beads, resulting in significant savings in salt usage and regenerant as well as wastewater consumption.
Figure 1-3. General kinetic features and particle size ranges of heterogeneous polymerization processes.
1-3 Introduction of Pickering (solid-stabilized emulsion) emulsion
From the beginning of the early 20th centry, Ramsden proposed that colloidal particles were able to stabilize emulsion. Afterward, the phenomena had been investigated thoroughly by Pickering, therefore, so-called Pickering emulsion as known to date. The generally well accepted mechanism of Pickering emulsion is the adsorption of solid particles at the oil/water interface, forming solid mono/multi-layer structures. Figure1-4 illustrated the mechanism of solid stabilized emulsions.
As compared to the conventional surfactant stabilized emulsion, Pickering emulsions possess several advantages such as (1) Reducing the usage of molecular surfactant, which is known to be high cost also detrimental to the environment. (2) Low toxcitity to human bodies. (3) Environmental friendly. (4) The emulsion stability is neglegibly affected by the pH, ionic strength, temperature, and the oil components.
Hence, Pickering emulsions have been widely used in the field of food, cosmetics, and medicine. Over last several decades, people fabricated various kinds also shapes of nanoparticles along with the progress in nanotechnology. Deep insights into the solid stabilized emulsions have attracted some attentions. Recently, Pickering emulsions were used as templets for the preparation of colloidosome, core/shell structures. The migration, aggregation as well as self-assembling of colloidal particles at the curved oil/water interfaces were also studied. Up to now, the nanoparticles involved into the studies focused on monodisperse particulate particles such as silica, iron oxide, titania, and organic latexes.
There are still several topics remained unsolved in the Pickering emulsion. For instance, (1) Whether the interfacial tension reduce or not by the adsorption of colloidal particles on the oil/water interface. (2) The mechanism of emulsion co-stabilized by surfactants and colloidal particles. (3) Grafting environmental-stumuli chemical bondings on the nanoparticles brings wide development space in this area. (4) Thermodynamically stable Pickering emulsion.
Figure 1-4. Mechanisms of Solid stabilized emulsions. (a) Barrier from full coverage of monolayer particles. (b) Sparsely covered droplets through bridging stabilization.
1-4 Flow chart of this work
References
(1) Georges, M. K.; Veregin, R. P. N.; Kazmaier, P. M.; Hamer, G. K.
Macromolecules 1993, 26, 2987-2988.
(2) Veregin, R. P. N.; Georges, M. K.; Kazmaier, P. M.; Hamer, G. K.
Macromolecules 1994, 27, 5238-5238.
(3) Kuo, K. H.; Chiu, W. Y.; Cheng, K. C. Polym Int 2008, 57, 730-737.
(4) Lee, C. F.; Yang, C. C.; Wang, L. Y.; Chiu, W.Y. Polymer 2005, 46 , 5514-5523.
(5) Hawker, C. J.; Bosman, A. W.; Harth, E. Chem. Rev. 2001, 101, 3661-3688.
(6) Hawthorne, D. G.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1999, 32, 5457-5459.
(7) Mayadunne, R. T. A.; Rizzardo, E.; Chiefari, J.; Chong, Y. K.; Moad, G.; Thang, S. H.; Macromolecules 1999, 32, 6977-6980.
(8) Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.;
Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H.
Macromolecules 1998, 31, 5559-5562.
(9) Moad, G.; Rizzardo, E.; Thang, S. H. Aust. J. Chem. 2005, 58, 379-410.
(10) Barner-Kowollik, C.; Perrier, S. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 5715-5723.
(11) Wang, J. S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117, 5614-5615.
(12) Matyjaszewski, K.; Xia, J. H. Chem. Rev. 2001, 101, 2921-2990.
(13) Matyjaszewski, K.; Patten, T. E.; Xia, J. H. J. Am. Chem. Soc. 1997, 119, 674-680.
(14) Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1995, 28, 1721-1723.
(15) Kamigaito, M.; Ando, T.; Sawamoto, M. Chem. Rev. 2001, 101, 3689-3745.
(16) Krstina, J.; Moad, C. L.; Moad, G.; Rizzardo, E.; Berge, C. T. In A new form of
controlled growth free radical polymerization, 2nd International Symposium on Free Radical Polymerization - Kinetics and Mechanisms, Genoa, Italy, May 26-31, 1996;
Genoa, Italy, 1996; pp 13-23.
(17) Wieland, P. C.; Raether, B.; Nuyken, O. Macromol. Rapid. Commun. 2001, 22, 700-703.
(18) Raether, B.; Nuyken, O.; Wieland, P. C.; Bremser, W. Macromol. Symp. 2002, 177, 25-41.
(19) Viala, S.; Antonietti, M.; Tauer, K.; Bremser, W. Polymer 2003, 44, 1339-1351.
(20) Viala, S.; Tauer, K.; Antonietti, M.; Lacik, I.; Bremser, W. Polymer 2005, 46, 7843-7854
(21) Luo, Y. D.; Chiu, W. Y. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 6789-6800.
(22) Chou, I. C.; Luo, Y. D.; Chiu, W. Y. Polymer 2010, 51, 2527-2532.
(23) Stoeckel, N.; Wieland, P. C.; Nuyken, O. Polym. Bull. 2002, 49, 243-250.
(24) Tasdelen, M. A.; Degirmenci, M.; Yagci, Y.; Nuyken, O. Polym. Bull. 2003, 50, 131-138.
(25) Chen, D.; Fu, Z. Shi, Y. Polym. Bull. 2008, 60, 259-269.
(26) Ugelstadt, J.; Berge, A.; Elingsen, T.; Smid, R.; Nielsen, T. N. Prog. Polym. Sci.
1992, 17, 87–161.
(27) Horak, D. Acta Polym. 1996, 47, 20–28.
(28) Vanderhoff, J. W.; El-Aasser, M. S.; Micale, F. J.; Sudol, E. D.; Tseng, C. M.;
Silwanowicz, A.; Kornfeld, D. M.; Vicente, F. A. J. Dispersion Sci. Technol. 1984, 5, 231–246.
(29) Arshady, R. Colloid Polym. Sci. 1992, 270, 717–732.
(30) Woods, M. E.; Dodge, J. S.; Krieger, I. M.; Pierce, P. E. J. Paint Technol. 1968, 40, 541–548.
(31) Chern, C. S. Prog. Polym. Sci. 2006, 31, 443–486.
(32) Liu, W.; Yang, X. L.; Ho, W. S. W. J. Pharm. Sci. 2011, 100, 75–93.
(33) Yuan, Q.; Cayre, O. J.; Manga, M.; Williams, R. A.; Biggs, S. Soft Matter 2010, 6, 1580–1588.
(34) Wang, P. C.; Lee, C. F.; Young, T. H.; Lin, D. T.; Chiu, W. Y. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 1342–1356.
(35) Nagao, D.; Yokoyama, M.; Yamauchi, N.; Matsumoto, H. Langmuir 2008, 24, 9804–9808.
(36) Wang, L. Y.; Lin, Y. J.; Chiu, W. Y. Synth. Met. 2001, 119, 155–156.
(37) Kawaguchi, S.; Ito, K. Adv. Polym. Sci. 2005, 175, 299–328.
(38) Cheng, C. M.; Micale, F. J.; Vanderhoff, J. W.; El -Aasser, M. S. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 235–244.
(39) Ma, Z. Y.; Guan, Y. P.; Liu, H. Z. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 3433–3439.
(40) Ober, C. K.; Lok, K. P.; Hair, M. L. J. Polym. Sci., Polym. Lett. Ed. 1985, 23, 103–108.
(41) Li, W. H.; Stöver, H. D. H. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 1543–1551.
(42) Downey, J. S.; Frank, R. S.; Li, W. H.; Stöver, H. D. H. Macromolecules 1999, 32, 2838–2844.
(43) Wang, J. F.; Cormack, P. A. G.; Sherrington, D. C.; Khoshdel, E. Angew. Chem., Int. Ed. 2003, 42, 5336–5338.
(44) Capek, I.; Chern, C. S. New Polymerization Techniques and Synthetic Methodologies; Springer: Berlin, 2001; Vol. 155, pp 101–165.
(45) Antonietti, M.; Landfester, K. Prog. Polym. Sci. 2002, 27, 689–757.
(46) Luo, Y. D.; Chiu, W. Y.; Dai, C. A. Polym. Eng. Sci. 2009, 49, 1043–1049.
(47) Cao, Z.; Ziener, U.; Landfester, K. Macromolecules 2010, 43, 6353–6360.
(48) Cheng, S. Q.; Guo, Y.; Zetterlund, P. B. Macromolecules 2010, 43, 7905–7907.
(49) Antonietti, M.; Bremser, W.; Muschenborn, D.; Rosenauer, C.; Schupp, B.
Macromolecules 1991, 24, 6636–6643.
(50) Kim, J. W.; Larsen, R. J.; Weitz, D. A. J. Am. Chem. Soc. 2006, 128, 14374–14377.
(51) Park, J. G.; Forster, J. D.; Dufresne, E. R. J. Am. Chem. Soc. 2010, 132, 5960–5961.
(52) Hou, W. H.; Lloyd, T. B. J. Appl. Polym. Sci. 1992, 45, 1783.
(53) Hou, W. H.; Lobuglio, T. M. J. Appl. Polym. Sci. 1994, 54, 1363.
(54) Partch, R.E.; Matijevic, E.; Hodgson, A. W.; Aiken, B. E. J. Polym. Sci. 1983, A21, 961.
(55) Partch, R.E.; Nakamura, K.; Wolfe, K. J.; Matijevic, E. J. Colloid Interface Sci.
1985, 105, 560.
(56) Awan, M. A.; Dimonie, V. L.; El-Aasser, M.S. Polym. Prepr. 1994, 35, 551.
