Azo Polymers (eBook)

Xiaogong Wang received his M.Sc. and Ph.D. degrees from Tsinghua University under the supervision of Professor Qixiang Zhou in 1984 and 1994, respectively. From 1985 to 1986, he worked as a visiting scholar with Professor Isao Ando at the Department of Polymer Chemistry, Tokyo Institute of Technology, Japan. From 1994 to 1997, he worked as a postdoctoral fellow and visiting scientist with Professor Sukant K. Tripathy at the Center for Advanced Materials, Department of Chemistry, University of Massachusetts at Lowell, U.S.A. He was promoted to full professor at the Department of Chemical Engineering, Tsinghua University in 1998. His main research interest is to develop azo polymers with novel photoresponsive functions and elaborate their mechanisms. His research is also dedicated to polymer self-assembly, polymer surface modification and liquid crystal polymer.

Preface 6
Acknowledgment 9
Contents 10
Chapter 1: Introduction 13
1.1 Background and Terminology 13
1.2 Azo Functional Structure 16
1.3 Electronic Excitation and Spectroscopy 18
1.4 Classification of Azo Functional Structures 22
1.5 Photoisomerization 24
1.6 Azo Polymers in Retrospect 26
References 27
Chapter 2: Trans-Cis Isomerization 30
2.1 An Overview 31
2.2 Trans-/Cis-azobenzene and Thermal Isomerization 33
2.3 Excited States, Intermediates, and Pioneering Models 35
2.4 Photoisomerization Mechanism of Azobenzene 38
2.4.1 Rotation vs Inversion Controversy 39
2.4.2 Experimental Reexamination 42
2.4.3 Theoretical Studies 47
2.4.4 Simulation of Photoisomerization Dynamics 52
2.5 Substituted Effect 54
2.5.1 Spectroscopic Characteristics 55
2.5.2 Thermal Isomerization of Substituted Azobenzenes 56
2.5.3 Photoisomerization of Substituted Azobenzenes 58
References 61
Chapter 3: Azo Polymer Syntheses 68
3.1 Synthetic Method Classification 69
3.2 Azo Homopolymer and Random Copolymer 70
3.2.1 Chain Polymerization 70
3.2.2 Step Polymerization 72
3.2.3 Post-polymerization Modification 77
3.2.4 Controlled Radical Polymerization 83
3.2.5 Cross-Linked Azo Polymer 83
3.3 Azo Block and Graft Copolymer 84
3.3.1 Atom Transfer Radical Polymerization (ATRP) 85
3.3.2 Reversible Addition-FragmentationChain Transfer Polymerization 91
3.3.3 Azo Graft Copolymer Synthesis 93
3.4 Azo Dendritic and Star Polymers 95
3.4.1 Azo Dendron and Dendrimer 95
3.4.2 Azo Hyperbranched Polymer 100
3.4.3 Other Azo Polymers with Nonlinear Architecture 104
3.5 Azo Polymers with pi-Conjugated Backbone 106
3.5.1 Conjugated Polymer with Side-Chain Azo Groups 106
3.5.2 Main-Chain Conjugated Azo Polymers 107
3.5.3 Conjugated Dendrimer with Azobenzene Core 110
3.6 Azobenzene-Containing Polypeptide 110
3.6.1 Polymerization of Azo Monomers 111
3.6.2 Azo Functionalization of Polypeptide 111
3.7 Synthesis, Structures, and Functions 113
References 114
Chapter 4: Photoinduced Orientation and Anisotropy 127
4.1 Background and Terminology 128
4.2 Photoinduced Orientation 129
4.2.1 Polarized Light 129
4.2.2 Dichroism and Birefringence 130
4.2.3 Orientation Induced by Linearly Polarized Light 131
4.2.4 Out-of-Plane Orientation Induced by Unpolarized Light 134
4.2.5 Chirality Induced by Circularly Polarized Light 134
4.3 Mechanism and Models 136
4.4 Azo Polymers as Photoanisotropic Materials 138
4.4.1 Liquid Crystalline Azo Polymer 138
4.4.2 Amorphous Azo Polymer 141
4.4.3 Amphiphilic Azo Polymer 144
4.4.4 Azo Polyelectrolyte and Supramolecular Assembling 146
4.4.5 Azo Block Copolymer 147
4.5 Optical Grating and Holography 150
References 153
Chapter 5: Photoinduced Mass Transport 160
5.1 SRG Formation and Characterization 161
5.1.1 Materials 161
5.1.2 Optical Setup for SRG Inscription 164
5.1.3 SRG Characterization 166
5.2 Factors Affecting SRG Formation 167
5.2.1 Influences of Molecular Structures 167
5.2.2 Roles of Optical and Grating Parameters 171
5.3 Correlation with Polarization Grating 172
5.4 Models and Theories 174
5.4.1 Thermal Gradient Model 175
5.4.2 Isomerization-Driven Free Volume Expansion Model 176
5.4.3 Photoinduced Translation Diffusion of Azo Dyes 178
5.4.4 Electromagnetic Gradient Force Model 180
5.4.5 Mean-Field Model 182
5.4.6 Other Models and Theories 185
5.5 More Observations and Insights 185
5.5.1 Concomitant Bulk Variations 185
5.5.2 Photoinduced Mechanical Property Variations 187
5.5.3 Real-Time Imaging of SRG Formation 188
5.6 Photoinduced Mass Transport Beyond SRG 190
5.7 Spontaneous Patterning with a Uniform Light Field 194
References 197
Chapter 6: Photoresponsive Liquid Crystal Elastomers 205
6.1 Liquid Crystal Elastomers: A Brief Introduction 205
6.2 LCEs Containing Azo Functional Groups 208
6.2.1 Polysiloxane-Based Azo LCEs 209
6.2.2 Polyacrylate-Based Azo LCEs 212
6.2.3 Side-On Nematic Elastomer 214
6.3 Molecular Theory of Light-Induced Deformation 216
6.3.1 Deformation Caused by Order-Disorder Transition 216
6.3.2 Deformation Caused by Photoinduced Orientation 217
6.4 Photomechanical Responses and Their Mechanisms 218
6.4.1 Deformation Modes 218
6.4.2 Reversible Bending Deformation 219
6.4.3 Nonlinear and Complex Deformation Modes 222
6.4.4 Mechanics of Photoinduced Deformation 223
6.5 Prototype Devices and Possible Applications 224
6.5.1 Miniature Swimming Devices 224
6.5.2 Light-Driven Plastic Motors 225
6.5.3 High-Frequency Photodriven Oscillator 228
6.5.4 Printed Artificial Cilia 229
6.5.5 Shape-Memory Applications 231
References 233
Index 237