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Photomechanical Materials,Composites, and Systems

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Photomechanical Materials,Composites, and Systems

Wireless Transduction of Light into Work

Edited by Timothy J. White

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This edition first published 2017© 2017 John Wiley & Sons, Ltd.

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Names: White, T. (Timothy), editor.Title: Photomechanical materials, composites, and systems : wireless

transduction of light into work / edited by Timothy J. White.Description: Hoboken, New Jersey : John Wiley & Sons, Inc., [2017] | Includes

bibliographical references and index. | Description based on print versionrecord and CIP data provided by publisher; resource not viewed.

Identifiers: LCCN 2017001840 (print) | LCCN 2017012541 (ebook) | ISBN9781119123293 (Adobe PDF) | ISBN 9781119123286 (ePub) | ISBN 9781119123309| ISBN 9781119123309(cloth; pbk.) | ISBN 1119123305(cloth; pbk.)

Subjects: LCSH: Smart materials. | Polymers–Optical properties. |Polymers–Mechanical properties. | Nanocomposites (Materials)

Classification: LCC TA418.9.S62 (ebook) | LCC TA418.9.S62 P46 2017 (print) |DDC 620.1/9204295–dc23

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Contents

List of Contributors xiPreface xv

1 A Historical Overview of Photomechanical Effects inMaterials, Composites, and Systems 1Toru Ube and Tomiki Ikeda

1.1 Introduction 11.1.1 Initial Studies of Photomechanical Effects in Materials 11.1.2 Research of Photomechanical Effects in Materials – 1950–1980 21.1.3 Research of Photomechanical Effects in Materials – 1980–2000 61.1.4 Photomechanical Effects Observed in Cross-Linked

Liquid-Crystalline Polymers – 2001–Present 91.1.5 Photomechanical Effects in Polymeric Materials and Composites

Systems since 2000 191.1.6 Classification 23

References 25

2 Photochromism in the Solid State 37Oleksandr S. Bushuyev and Christopher J. Barrett

2.1 Molecular Photoswitches in the Solid State 372.2 Molecular and Macroscopic Motion of Azobenzene

Chromophores 392.3 Photomechanical Effects 412.3.1 Photomechanical Effects in Amorphous Azo Polymers 422.3.2 Actuation in Liquid-Crystalline Polymers 432.3.3 Photosalient, Photochromic, and Photomechanical Crystals 492.4 Solid-State Photochromic Molecular Machines 542.4.1 Nanostructure Functionalization 552.4.2 Two-Dimensional Assemblies and Surface Functionalization 592.5 Surface Mass Transport and Phase Change Effects 622.6 Photochromic Reactions in Framework Architectures 65

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2.7 Summary and Outlook 68References 69

3 Photomechanics: Bend, Curl, Topography, and Topology 79Daniel Corbett, Carl D. Modes, and Mark Warner

3.1 The Photomechanics of Liquid-Crystalline Solids 813.2 Photomechanics and Its Mechanisms 823.2.1 Absorption, Photomechanics, and Bend Actuation 863.2.1.1 Photostationary Dye Populations and Mechanical Response 873.2.1.2 Dynamical Intensity and Dye Populations 883.2.1.3 Polydomain Photosolids 903.2.1.4 Photomechanics versus Thermal Mechanics upon Illuminating

Photosolids 913.3 A Sketch of Macroscopic Mechanical Response in LC Rubbers and

Glasses 923.4 Photo- and Heat-Induced Topographical and Topological

Changes 973.5 Continuous Director Variation, Part 1 973.6 Mechanico-Geometric Effects, Part 1 1003.7 Continuous Director Variation, Part 2 1003.8 Continuous Director Variation, Part 3 1033.9 Mechanico-Geometric Effects, Part 2 1063.10 Director Fields with Discontinuities–Advanced Origami! 1073.11 Mechanico-Geometric Consequences of Nonisometric

Origami 1103.12 Conclusions 110

References 112

4 Photomechanical Effects in Amorphous and SemicrystallinePolymers 117Jeong Jae Wie

4.1 Introduction 1174.2 Polymeric Materials 1194.3 The Amorphous Polymer State 1194.4 The Semicrystalline Polymer State 1214.5 Absorption Processes 1244.6 Photomechanical Effects in Amorphous and Semicrystalline

Azobenzene-Functionalized Polymers 1264.6.1 Influence of Crystallinity on Photomechanical Response of

Polyimides 1264.6.2 Backbone Rigidity 1284.7 Molecular Alignment 1324.8 Annealing and Aging 138

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4.9 Sub-Tg Segmental Mobility 1424.10 Cross-Link Density 1454.11 Concluding Remarks 146

References 148

5 Photomechanical Effects in Liquid-Crystalline PolymerNetworks and Elastomers 153Timothy J. White

5.1 Introduction 1535.1.1 What Is a Liquid Crystal Polymer, Polymer Network, or

Elastomer? 1535.1.2 How Are Liquid-Crystalline Polymer Networks and Elastomers

Prepared? 1545.1.2.1 Polysiloxane Chemistries 1545.1.2.2 Free Radical or Cationic Photopolymerization 1575.2 Optically Responsive Liquid Crystal Polymer Networks 1595.2.1 Historical Overview 1595.2.2 Photochromic and Liquid Crystalline 1625.2.3 Photomechanics 1645.3 Literature Survey 1655.3.1 Photomechanical Effects in Polysiloxane Materials and

Analogs 1655.3.2 Photomechanical Effects in Poly(meth)acrylate Materials and

Analogs 1665.4 Outlook and Conclusion 169

References 171

6 Photomechanical Effects in Polymer Nanocomposites 179Balaji Panchapakesan, Farhad Khosravi, James Loomis, and Eugene M.Terentjev

6.1 Introduction 1796.2 Photomechanical Actuation in Polymer–Nanotube

Composites 1806.3 Fast Relaxation of Carbon Nanotubes in Polymer Composite

Actuators 1866.4 Highly Oriented Nanotubes for Photomechanical Response and

Flexible Energy Conversion 1916.4.1 Highly Oriented Nanotubes/Nanotube Liquid Crystals 1916.4.2 Photomechanical Actuation of Oriented Nanotube

Composites 1976.4.3 Relaxation Behavior of Nanotube–Liquid Crystal Elastomers 2006.5 Photomechanical Actuation Based on 2-D Nanomaterial

(Graphene)–Polymer Composites 205

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6.6 Applications of Photomechanical Actuation inNanopositioning 213

6.6.1 Principle of GnP/Elastomer Photothermal Actuation 2146.6.2 Photomechanical-Actuation-Based Nanopositioning System 2186.6.3 GNP/PDMS Actuator Fabrication and Characterization 2186.6.4 Nanopositioner System Integration 2196.6.5 Kinetics of Photothermal Nanopositioners 2216.6.6 Useful Displacement versus Maximum Displacement 2226.6.7 Accuracy and Resolution 2236.7 Future Outlook 224

Acknowledgments 225References 225

7 Photomechanical Effects in Photochromic Crystals 233Lingyan Zhu, Fei Tong, Rabih O. Al-Kaysi, and Christopher J. Bardeen

7.1 Introduction 2337.2 General Principles for Organic Photomechanical Materials 2347.3 History and Background 2347.4 Modes of Mechanical Action 2407.4.1 Partial Reaction and Bimorph Formation 2407.4.2 Complete Transformation and Crystal Reconfiguration 2417.5 Photomechanical Molecular Crystal Systems 2427.5.1 Intramolecular Photochemical Reactions 2427.5.1.1 Ring-Opening/Closing Reactions 2427.5.1.2 Photoisomerization 2447.5.1.3 Photodissociation 2477.5.2 Intermolecular Photochemical Reactions 2487.5.2.1 [2 + 2] Photodimerization 2487.5.2.2 [4 + 4] Photodimerization 2507.5.3 Nonequilibrium Charge Distribution and Electronic Heating 2577.6 Future Directions 2607.6.1 Reaction Dynamics in Molecular Crystals 2607.6.2 New Materials 2617.6.3 Interfacing Molecular Crystals with Other Objects 2627.7 Conclusion 264

Acknowledgments 264References 264

8 Photomechanical Effects in Piezoelectric Ceramics 275Kenji Uchino

8.1 Introduction 2758.2 Photovoltaic Effect 2768.2.1 Principle of the Bulk Photovoltaic Effect 277

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8.2.1.1 “Bulk” Photovoltaic Effect 2778.2.1.2 Experimental Setup 2798.2.1.3 Current Source Model 2798.2.1.4 Voltage Source Model 2828.2.2 Effect of Light Polarization Direction 2858.2.3 PLZT Composition Research 2868.2.4 Dopant Research 2878.3 Photostrictive Effect 2888.3.1 Figures of Merit 2888.3.2 Materials Considerations 2898.3.3 Ceramic Preparation Method Effect 2908.3.3.1 Processing Method 2908.3.3.2 Grain Size Effect 2908.3.3.3 Surface/Geometry Dependence 2918.4 Photostrictive Device Applications 2948.4.1 Displacement Amplification Mechanism 2948.4.2 Photo-Driven Relay 2958.4.3 Micro-walking Machine 2958.4.4 “Photophone” 2978.4.5 Micro-propelling Robot 2978.5 Concluding Remarks 299

References 300

9 Switching Surface Topographies Based on Liquid CrystalNetwork Coatings 303Danqing Liu and Dirk J. Broer

9.1 Introduction 3039.2 Liquid Crystal Networks 3049.2.1 Photoresponsive Liquid Crystal Networks 3079.2.2 Photoinduced Surface Deformation 3079.2.3 Photoinduced Surface Deformation Preset by Patterned Director

Orientation 3119.2.4 On the Mechanism of Surface Deformation 3189.3 Conclusions 322

References 322

10 Photoinduced Shape Programming 327Taylor H. Ware

10.1 One-Way Shape Memory 32910.1.1 Photothermal 33110.1.2 Photochemical 33610.2 Two-Way Shape Memory 34310.2.1 Photothermal 344

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10.2.2 Photochemical 35310.3 Summary and Outlook 358

References 358

11 Photomechanical Effects to Enable Devices 369M. Ravi Shankar

11.1 Introduction 36911.2 Analog Photomechanical Actuators 37111.3 Discrete-State (Digital) Photomechanical Actuators 37311.3.1 Binary Actuators 37411.3.2 Latency of Binary Actuators and Repetitive Actuation 37511.3.3 Multistable Implementations 38011.3.4 Beyond Bistable, Buckled Rods 38411.4 Photomechanical Mechanisms and Machines 387

References 388

12 Photomechanical Effects in Materials, Composites, andSystems: Outlook and Future Challenges 393Timothy J. White

12.1 Introduction 39312.2 Outlook and Challenges 39312.2.1 Breadth and Depth 39312.2.2 Beyond Bending: Mechanics Implementations 39412.2.3 Harvesting and Harnessing Light 39612.2.4 Speed is Limited 39612.2.5 Systems Design and Implementation 39812.2.6 Applications 39812.2.6.1 Optical Elements 39812.2.6.2 Morphing Shapes and Surfaces 40012.2.6.3 Actuation 40012.3 Conclusion 401

References 401

Index 405

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xi

List of Contributors

Rabih O. Al-KaysiDepartment of Basic Sciences,College of Science and HealthProfessionsKing Saud bin Abdulaziz Universityfor Health SciencesRiyadhSaudi Arabia

and

Ministry of National Guard HealthAffairsKing Abdullah International MedicalResearch CenterRiyadhSaudi Arabia

Christopher J. BardeenDepartment of ChemistryUniversity of California, RiversideRiverside, CAUSA

Christopher J. BarrettDepartment of ChemistryMcGill UniversityMontrealCanada

Dirk J. BroerDepartment of ChemicalEngineering and ChemistryInstitute for Complex MolecularSystemsTechnical University of EindhovenEindhovenNetherlands

Oleksandr S. BushuyevDepartment of ChemistryMcGill UniversityMontrealCanada

Daniel CorbettSchool of Chemical Engineering andAnalytical ScienceThe University of ManchesterManchesterUK

Tomiki IkedaResearch and Development InitiativeChuo UniversityTokyoJapan

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xii List of Contributors

Farhad KhosraviSmall Systems Laboratory,Department of MechanicalEngineeringWorcester Polytechnic InstituteWorcester, MAUSA

Danqing LiuDepartment of ChemicalEngineering and Chemistry, Institutefor Complex Molecular SystemsTechnical University of EindhovenEindhovenNetherlands

James LoomisDepartment of MechanicalEngineeringUniversity of AucklandAucklandNew Zealand

Carl D. ModesCenter for Studies in Physics andBiologyThe Rockefeller UniversityNew York, NYUSA

Balaji PanchapakesanSmall Systems Laboratory,Department of MechanicalEngineeringWorcester Polytechnic InstituteWorcester, MAUSA

M. Ravi ShankarDepartment of IndustrialEngineeringUniversity of PittsburghPittsburgh, PAUSA

Eugene M. TerentjevCavendish LaboratoryDepartment of PhysicsUniversity of CambridgeCambridgeUK

Fei TongDepartment of ChemistryUniversity of California, RiversideRiverside, CAUSA

Toru UbeResearch and Development InitiativeChuo UniversityTokyoJapan

Kenji UchinoInternational Center for Actuatorsand TransducersElectrical Engineering and MaterialsResearch InstituteThe Pennsylvania State UniversityUniversity Park, PAUSA

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List of Contributors xiii

Taylor H. WareDepartment of BioengineeringThe University of Texas at DallasRichardson, TXUSA

Mark WarnerCavendish LaboratoryDepartment of PhysicsUniversity of CambridgeCambridgeUK

Timothy J. WhiteDayton, OHUSA

Jeong Jae WieDepartment of Polymer Science andEngineeringInha UniversityIncheonSouth Korea

Lingyan ZhuDepartment of ChemistryUniversity of California, RiversideRiverside, CAUSA

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xv

Preface

Transduction of energy is pervasive within our modern society – examplesinclude the conversion of chemical energy to power the motion of an auto-mobile, harvesting wind to provide electric power to our homes, or capturingsolar radiation to power a communications satellite. The focus of this book isthe transduction of light (photons) into a mechanical output.Photomechanical effects in materials or composites are a subcategory of the

broader class commonly referred to as stimuli-responsive or “smart” materi-als. The focus of this book is on materials and composites that are sensitive tolight as the input energy stimulus. Light is compelling as an input energy sourcefor many reasons. Foremost of these reasons is the potential for speed. Youngstudents around the world are taught that nothing moves faster than light – itis the speed limit that defines our universe. Daily, we rely on the transmissionof light over long distances, which is a distinguished method for wireless andremote control of a system or subsystem in a device. Light can also be readilymanipulated to be polarized (linear or circular) as well as complex and evolvingpolarization vortices. Synthetic light, generated by lasers or LED, is increasinglydiverse inwavelength, spanning theUV to the infrared at ever-increasing powerlevels. All the aforementioned properties can very easily be turned on or off,reoriented, or spatially varied.These variations allow for a unique and unprece-dented level of control in generating distinguished mechanical responses. Putsuccinctly, light is a “smart” stimulus for “smart” materials.As detailed by the international collection of authors assembled here,

photomechanical effects in materials or material composites have beenobserved since ancient times in the various versions of the sundial. More than100 years ago, the American inventor Alexander Graham Bell was captivatedin part by the aforementioned properties of light and focused years of researchinto the “photophone,” after his earlier invention of the telephone. Seminalpapers that appeared in the 1960s and 1970s initiated a renaissance in the topicwhich has steadily grown into the practicing research community of today. In2016, more than 900 papers were published using the term “photomechanical”(or variants thereof )!

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xvi Preface

As will be evident throughout the book – photomechanical effects inmaterials and composites are a complex interplay of light, photochemistry,polymer chemistry and physics, and mechanics. Due to the breadth of thefundamental subject matter, the book begins with three introductory chapters.Chapter 1, by Ikeda and Ube, gives a high level and introductory survey ofthe generally topic to emphasize the historical evolution of the topic and tellthe unfolding story of the development and employment of these materials.Subsequently, Chapter 2 by Bushuyev and Barrett details the basics of pho-tochromism in the solid state. The foundational chapters are completed witha contribution from Corbett, Modes, and Warner, detailing the interplay ofphotochemistry and mechanics, with specific emphasis on anisotropic andpatterned material systems prepared from liquid-crystalline polymers.Thereafter, the book transitions into detailed treatments of the subclasses

of photomechanical materials including conventional polymers (Chapter 4by Wie), liquid-crystalline polymer networks and elastomers (Chapter 5 byWhite), crystalline solids (Chapter 7 by Bardeen and coauthors), and ceramics(Chapter 8 by Uchino) as well as a chapter on photomechanical effectsin nanocomposites (Chapter 6 by Panchapakesan, Khosravi, Loomis, andTerentjev). The book concludes with chapters detailing cross-cutting topics ofrecent interest including photoinduced topographical features (Chapter 9 byLiu and Broer), shape programming (Chapter 10 by Ware), actuating devices(Chapter 11 by Ravi Shankar), and an outlook (Chapter 12 by White).I am forever grateful to the wonderful collection of authors for taking their

time and spending their expertise on the chapters that follow. I would beremiss not to thank the editorial staff at Wiley for their help and assistancein navigating an endeavor such as this. Most of all, I thank my wife Jaymieand children Avery, Micah, and Beckett for their sacrifice in allowing for thisproject to go forward in my personal time away from an already overscheduledand full work week.I andmany of the authors of this book believe that these materials are quickly

defining and finding unique potential application opportunities. It is my hopethat this bookwill captivate aspiring scientists and peers in other research com-munities to join in this pursuit to further realize the promise that has captivatedso many for so long.

Tim WhiteDayton, OH

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A Historical Overview of Photomechanical Effectsin Materials, Composites, and SystemsToru Ube and Tomiki Ikeda

Research and Development Initiative, Chuo University, Tokyo, Japan

1.1 Introduction

Photomechanical effects in materials are a topic of considerable recentresearch. Many papers are continually appearing in top-ranked journalsreporting novel materials, demonstrations of distinctive mechanical outputs,and initial demonstrations of device utility. This book is a comprehensivereview of the material development, fundamental science (photochemistry,optics, and mechanics), and application of photomechanical effects in mate-rials. This chapter provides an overview of the historical development of thesimple yet captivating idea of photomechanical energy conversion inmaterials.In this way, the reader will have a general awareness of the interrelated natureof the topics and themes discussed throughout the subsequent chapters.

1.1.1 Initial Studies of Photomechanical Effects in Materials

Historians might argue that the first implementation of photomechanicaleffects in materials was the invention of the sundial by the ancients. It isinarguable, however, that humankind has sought to harvest this plentifulresource. Many of these pursuits have found their inspiration in nature inwhich countless species have adapted to use and leverage light-inducedmotility (photomechanical effects) to harvest more energy (sunflower), protectsensitive leaves (circadian rhythm plants), or even camouflage (chameleon,cephalopods).The emergence of the potential utility of photomechanical effects in the

modern era can largely be attributed to the famous American inventor Alexan-der Graham Bell and his work in the late 1800s [1]. After Bell invented thepractical telephone, he shifted his focus on the development of a photophoneto enable communication without the necessity of a conducting wire between

Photomechanical Materials, Composites, and Systems: Wireless Transduction of Light into Work,First Edition. Edited by Timothy J. White.© 2017 John Wiley & Sons, Inc. Published 2017 by John Wiley & Sons, Inc.

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2 1 A Historical Overview of Photomechanical Effects in Materials, Composites, and Systems

M

L

L

CS

H

PR

S

T

B

FR

LS

Figure 1.1 Schematic illustration of a photophone proposed by A. G. Bell. LS, light source;M, mirror; L, lens; H, heat absorber; S, sound; FR, flexible reflector; C, crystal; PR, parabolicreflector; B, battery; T, electroacoustic transducer.

a transmitter and a receiver (Figure 1.1). To accomplish this, Bell used acrystalline material (selenium) as a component of a receiver, which was con-nected in a local circuit with a battery and an electroacoustic transducer. Thesound emission changes depending on the state of light through a variationin resistance of selenium. The photophone Bell envisioned is the basis ofoptical communication and realized in recent times in practical applicationsenabled by the development of optical fibers and lasers [2]. Bell subsequentlyinvestigated nonelectronic photoresponsive receivers to make light audiblewithout the aid of electricity. He found that diaphragms of various substances(metals, rubbers, paper, etc.) produce sounds when irradiated with light. Thisphenomenon is explained in terms of a vibration of the diaphragm, which iscaused by a local, photoinduced temperature rise and a corresponding changein thermal expansion of the material. Recent examinations of photoacoustictomography extend upon this fundamental tenet pursued by Bell [3]. Accord-ingly, Alexander Graham Bell can be considered as the originator and “father”of photomechanical effects in materials in the modern era.

1.1.2 Research of Photomechanical Effects in Materials – 1950–1980

Stimuli-induced deformation of materials has attracted much attention sincethe 1950s. The most responsive form of these materials is a polymer gel, whichconsists of a cross-linked polymer network and solvent. Kuhn, Katchalsky,and coworkers demonstrated expansion and contraction of hydrogels con-taining carboxyl groups by successive addition of alkali and acid [4]. Thecarboxyl groups ionize and deionize depending on the pH, leading to thechange in intramolecular electronic repulsion and subsequent expansion andcontraction of polymer chains.This conformational change at amolecular scaleis translated to macroscopic deformation. Subsequently, various types of the

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1.1 Introduction 3

so-called smart materials have been developed, which deform when subjectedto stimuli such as heat, electricity, light, magnetic field, and humidity [5].Photoresponsive materials have potential advantages compared to these

other stimuli. Light is a comparably “smart” stimulus allowing for remoteand wireless controllability with spatial selectivity and also direct control ofresponsemagnitude via variation of intensity, wavelength, or even polarization.Initial research activities of photomechanical effects in polymeric materialswere undertaken in the 1960s. The general approach of these initial studiesremains largely unchanged today, focused on incorporating photoresponsivemoieties into polymeric or crystalline materials.By far, the most common approach to sensitizing polymeric materials to

light is to functionalize these materials with azobenzene. Azobenzene isa common dye molecule and widely known to photoisomerize between athermally stable trans isomer and a metastable cis isomer (Figure 1.2) [6].

UV

Vis

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Vis

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Vis

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R1

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R1

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R1 R2

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NO2

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NO2O

O

O

O

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OO

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Figure 1.2 Typical photochromic molecules used to induce photomechanical effects:(a) azobenzene, (b) spiropyran, (c) fulgide, and (d) diarylethene.

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4 1 A Historical Overview of Photomechanical Effects in Materials, Composites, and Systems

Generally, trans-azobenzene isomerizes to the cis isomer upon irradiationwith UV light, whereas cis-azobenzene reverts to the trans isomer uponirradiation with visible light or heating. The isomerization of azobenzeneproduces a variety of changes in properties such as molecular shapes andpolarity. Photochromic behavior and applications of azobenzene derivativeshave been actively studied since the isolation of the cis isomer in 1937 [7]. Thephotochemistry of azobenzene and other chromophores employed to generatephotomechanical effects is exhaustively detailed in Chapter 2.In 1967, Lovrien predicted that light energy could influence the confor-

mation of polymer chains if photochromic molecules such as azobenzenewere parts of polymers or bound to them [8]. In this seminal work, Lovrienproposed four strategies to achieve a conversion of light energy into mechan-ical energy. (i) Use of a polymer electrolyte solution containing azobenzenesin side chains (Figure 1.3a). trans-Azobenzenes in the side chains tend tocontract polymers by hydrophobic interaction. When irradiated with light,the hydrophobic interaction within the side chains decreases with trans–cisisomerization and results in a local expansion of the spacing of the polymerchains driven by Coulombic interaction. (ii) Use of solutions composed ofpolymer and azobenzene electrolytes (Figure 1.3b). In this approach, thepolymer chains are spaced by electronic repulsion between trans isomers,which Lovrien suggested would assemble on the chains. Upon trans–cisisomerization with light irradiation, the polymer chains could organize intoneutral coil conformation upon liberation of azobenzenes from chains. (iii)Incorporation of photoisomerizable groups in the backbone of polymerchains. (iv) Introduction of photoisomerizable cross-links so that light cangovern the distance between chains. Experimentally, Lovrein investigated thefirst two approaches: a polymer electrolyte solution containing azobenzenechromophores in the side chains and a polymer solution blended with azoben-zene electrolytes. In both systems, photoinduced changes in viscosity wereobserved. This effect is ascribed to the conformational change of the materialsystem, which was correspondingly amplified to macroscopic deformation orforce. Thereafter, van der Veen and Prins prepared a water-swollen polymergel containing a sulfonated azostilbene dye (chrysophenine) [9]. The presenceof cross-links enables the translation of microscopic changes in conformationinto macroscopic deformation of gels. These authors observed shrinkage asmuch as 1.2% upon irradiation with UV light.Photomechanical effects of dye-doped polymers were also observed in bulk

polymeric systems. Merian first reported the photoinduced deformation ofpolymer fibers containing photochromic molecules [10]. Azobenzene is acommon dye molecule, and in the course of using an azobenzene derivative todye hydrophobic fibers, Merian found that the dyed nylon fiber shrank about0.1% upon irradiation with light. He attributed this macroscopic dimensionalchange to the conformational change of the azobenzene moieties. Agolini

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1.1 Introduction 5

UV

Vis

(a)

(b)

trans-Azobenzene cis-Azobenzene

− −

−

−

−−

−

−

−

−

− −

UVVis

−

−−

−

−

−

−

−

−−

− −

Figure 1.3 Systems for photoinduced deformation of polymer chains proposed by Lovrien.(a) Polymer electrolyte functionalized with azobenzene moieties. (b) Blend solutioncomposed of polymer and azobenzene electrolytes.

and Gay observed macroscopic deformation of about 0.5% and measuredphotogenerated stresses when azobenzene-functionalized polyimide filmswere exposed to light [11]. Smets and de Blauwe reported deformation ofpolymer networks containing spirobenzopyran as photochromic cross-linkers,confirming that photomechanical effects in polymeric materials are notlimited to azobenzene chromophores [12]. The photomechanical responseof polymeric materials and gels prepared from conventional morphologies(amorphous, semicrystalline) is detailed in Chapter 4.

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6 1 A Historical Overview of Photomechanical Effects in Materials, Composites, and Systems

In these early examinations of photomechanical effects in polymeric systems,the correspondingmechanismwas solely ascribed to photochemical processes.However, heat generated by nonradiative deactivation process could also causemacroscopic deformations of thesematerials.The importance of photothermalcontributions was first elucidated byMatejka et al. [13].The rise in temperaturewas shown to causemacroscopic deformation ofmaterials due to dilation and achange in elasticmodulus.They carefully investigated the force induced by irra-diation with light under constant strain for a cross-linked copolymer of maleicanhydride and styrene, which contains azobenzene groups in the side chains.The time evolution of the generated force was found to correlate directly withtemperature rather than the isomerization of azobenzene.Thus, photothermalcontributions in these materials, composites, and systems must be considered.Photomechanical effects can also be realized through photoelectrical pro-

cesses within inorganic solids [14]. In 1966, Tatsuzaki et al. reported photoin-duced strain in a single crystal of SbSI, which shows photoconductivity andferroelectricity [15]. This behavior is attributed to the combination of pho-tovoltaic effect and converse piezoelectric effect. When ferroelectric materi-als are irradiated with light, a high voltage is generated, which considerablyexceeds the band gap energy. Subsequently, mechanical strain is induced dueto the converse piezoelectric effect. The photoinduced contraction of this classofmaterials is often called photostriction. Photomechanical effects in ferroelec-tric ceramics of lanthanum-modified lead zirconate titanate (PLZT) have beenextensively studied. In 1983, Brody demonstrated photoinduced bending of abimorph consisting of two PLZT ferroelectric layers with different remanentpolarization [16]. The bending of the material is caused by the expansion ofone layer and the contraction of the other. Uchino applied the photomechan-ical response of PLZT to micro-walking machines driven by light [17, 18], asdetailed in Chapter 7. The machine has two legs of bimorph of PLZT plates,which are fixed to a plastic board. When the legs are alternately irradiated withlight, the machine moves similarly to an inchworm. Photomechanical effectsof inorganic solids have also been observed in polar semiconductors (e.g., CdSand GaAs crystals) and nonpolar semiconductors (e.g., Si and Ge crystals) [14].

1.1.3 Research of Photomechanical Effects in Materials – 1980–2000

In the 1980s and 1990s, considerable effort focused on enhancing themagnitude of the photomechanical output of gels and dry polymers. Largedeformation of photoresponsive gels was reported by Irie and Kungwatchakun[19]. The authors’ strategy was to utilize photoinduced variation in long-rangeelectrostatic (repulsive) forces rather than employ the microscopic shapechanges accompanying the conformational change of chromophores such asazobenzene. Toward this end, polyacrylamide gels functionalized with triph-enylmethane leuco derivatives were employed. These derivatives dissociate

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1.1 Introduction 7

UVVis

(a)

(b) (c) (d)

H2C CH

CH3

CH3CN

CH

−CN

H2C

H3C

H3C CH3

CH3H3C

H3C

NN

N N+

Figure 1.4 Photoinduced bending of an acrylamide gel containing triphenyl methane leucodyes under an electric field. (a) Photochromism of triphenyl methane leucocyanide.(b) Before irradiation. (c) Under irradiation with UV light. (d) Under irradiation in the reverseelectric field to that in (c). (Irie [20]. Reproduced with the permission of American ChemicalSociety.)

into ion pairs upon irradiation with UV light (Figure 1.4). The electrostaticrepulsion between photogenerated charges led to substantial swelling ofpolyacrylamide gels. Photoinduced reversible bending of rod-shaped gels wasobserved under an electric field applied perpendicular to the rod [20]. Thebending is attributed to the inhomogeneous deformation of the gel, whichdepends on diffusion of free counter ions derived by an electric field. Anothernotable work exploring photoresponsive gel systems was detailed by Suzukiand Tanaka [21], where they employed a poly(N-isopropylacrylamide) gel,which is known to undergo a volume change by thermal phase transition [22].The authors incorporated chlorophyllin in the side chains as a light absorber.Upon irradiation with visible light, the gel collapsed due to phase transitioninduced by a photothermal effect.The enhancement of photomechanical effects in bulk polymer systems

was comparably limited in this time period. Although the photoinduced

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8 1 A Historical Overview of Photomechanical Effects in Materials, Composites, and Systems

deformation was observed in various polymers containing photochromicmoieties in cross-links [23] or main chains [24], the magnitude of strainremained small (typically <1%). Through these studies, it was generallyconcluded that the photoinduced change in molecular shape associated withconformational changes of photochromic groups such as azobenzene tendsto be accommodated by the local motion of flexible polymer chains. Thus, tomore efficiently translate molecular level events into the desired macroscopicmechanical response, the molecular systems should be densely packed andwell organized.During this era, research of thesematerials extended intomonolayer systems,

which are restricted in two dimensions, and the change in molecular shapecan be readily transferred to macroscopic deformation. Various azobenzenepolymers form monolayers at air/water interfaces when organic solutions ofazobenzene polymers are spread on the water surface (Langmuir technique).Photomechanical effects in the monolayers of polymers containing azoben-zene moieties were first reported by Blair et al. in 1980 [25, 26]. They preparedmonolayers of polyamide with azobenzene moieties in the main chain. Theycompared surface pressure–area curves of polyamides under UV-irradiatedand dark conditions.TheUV-irradiatedmonolayer showed a reduction in area,suggesting that polymers aremore contracted in cis forms.The in situ areamea-surement under UV irradiation with constant surface pressure showed rathercomplicated behavior. Depending on the applied surface pressure, monolay-ers exhibited either contraction or expansion. This behavior was understoodto indicate that the conformation of polymer chains strongly depends on thepreparation processes of the monolayers. The polymers with azobenzene moi-eties in the side chains were investigated as well. Malcolm and Pieroni pre-pared monolayers consisting of polypeptides with azobenzene moieties in 40%of the side chains [27]. The samples contracted upon irradiation with UV light.They speculated that the more extended trans form occupies a larger area inthe air/water interface compared to the cis form. On the other hand, Menzelet al. used polypeptide with azobenzene moieties in all of the side chains [28].Themonolayers expanded upon irradiation with UV light, which is opposite tothe result of Malcolm and Pieroni. The trans–cis isomerization of azobenzenemoieties leads to a large increase in dipole moment and a high affinity for awater surface (Figure 1.5a). Therefore, azobenzene moieties move to the watersurface with trans–cis isomerization, resulting in the increase in surface areaper monomeric unit. These two examples clearly indicate that photomechan-ical response can be very sensitive to the architecture of polymers. Seki andcoworkers extensively studied monolayers of poly(vinyl alcohol)s containingazobenzene side chains [30]. Upon irradiation with UV light, the film exhib-ited a rapid threefold expansion from the original area [31]. The clear in situobservation of the photoinduced deformation of the monolayer was enabledby Brewster angle microscopy (Figure 1.5b) [29].

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1.1 Introduction 9

0 s 500 s

UV

UVVis

(a)

(b)

Figure 1.5 Photoinduced deformation of polymer monolayers containing azobenzene.(a) Schematic illustration. (b) Photoinduced expansion of a monolayer of poly(vinyl alcohol)with azobenzene side chain observed by Brewster angle microscopy. (Seki et al. [29].Reproduced with the permission of American Chemical Society.)

1.1.4 Photomechanical Effects Observed in Cross-LinkedLiquid-Crystalline Polymers – 2001–Present

Further improvement of photomechanical effects in bulk polymeric materials,composites, and systems was enabled by the introduction of alignment and ori-entation of the photoresponsive molecules. The alignment and orientation, asin liquid-crystalline (LC) systems, allows for amplification of small changes atthemolecular level to yield largemacroscopic deformations.The science under-lying the improvements detailed in this section is the result of the convergenceof advances in the preparation of cross-linked LC polymers and insights intothe photochemistry of azobenzene in polymeric materials that were separatelypursued in extensive research studies in the past decades.

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10 1 A Historical Overview of Photomechanical Effects in Materials, Composites, and Systems

The development of cross-linked liquid-crystalline polymers (CLCPs)(interchangeably referred to as liquid crystalline polymer networks or LCNsthroughout the book) was a new horizon for the research and developmentof stimuli-responsive materials [32]. LCs have many unique properties asself-organized anisotropic materials [33, 34]. Without any treatment, LCstend to form microdomains: the orientation is locally ordered in each domainbut random among different domains (often referred to as polydomain). Inlow-molecular-weight LCs, the orientational axis (director) can be easilycontrolled by substrates coated with alignment layers such as polyimidesthat are rubbed to induce shallow groves as well as to induce orientationof the aromatic chains. The alignment of LC molecules can be switched byexternal stimuli such as electric and magnetic fields. The employment oflow-molar-mass LC materials in displays is largely attributable to the surfacealignment of LCs and sensitivity of LCs to align to fields [35]. Combinationof the anisotropies of LCs in polymers leads to fascinating materials knownas liquid-crystalline polymers (LCPs). Orientation of mesogens (rod- ordisc-like parts responsible for liquid crystallinity) is strongly coupled with theconformation of polymer main chains. In CLCPs, this relation is extended tothe macroscopic shape of materials as predicted by de Gennes in 1975 [36, 37].It should be noted that CLCPs is a general term referring to materials withglass transition temperature (Tg) below room temperature (elastomers) andabove room temperature (glassy) [32].Finkelmann et al. initially prepared elastomeric CLCPs of polysiloxanes

containing phenyl benzoate mesogens in the side chains with polydomainorientation [38]. Later, this same group oriented the mesogens within thematerial upon uniaxial stretching of the polydomain films, elucidating for thefirst time the coupling between the alignment of mesogens and macroscopicshape [39, 40]. Later, they prepared monodomain films with homogeneouslyaligned mesogens by the two-step cross-linking method now referred to asthe “Finkelmann method” (Figure 1.6a) [41]. Monodomain CLCPs exhibitlarge contractions along the director orientation upon heating through theLC–isotropic phase transition (Figure 1.7). The film restores to the originalshape upon cooling. The deformation ratio (strain) of 40% in the original workhas ultimately been enhanced to 400% by modification of the macromolecularstructure, specifically the inclusion of main-chain mesogenic units [42, 43].Other CLCPs chemistries have been examined, including polyacrylates andpolymethacrylates with side-chain mesogens, initially reported by Zentel andReckert [44]. Low-molar-mass LC monomers and the employment of surfacealignment methods to induce highly oriented polyacrylate CLCPs were furtherdeveloped by Broer et al. (Figure 1.6b) [45–47]. Commonly, CLCPs of thistype are prepared from mixtures of monofunctional and bifunctional acrylatemonomers that are photopolymerized at elevated temperatures where themixtures exhibit LC phases (referred to in this chapter as the in situ method).

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1.1 Introduction 11

(a)

(b)

SubstrateAlignment layer

MesogenInitiator

Polymer

Polymerization

Crosslinking

Crosslinking

Stretching

Figure 1.6 Preparation methods of CLCPs with aligned mesogens. (a) Two-stepcross-linking. (b) In situ polymerization.

In this approach, the orientation of LCmonomers during the polymerization ismemorized and retained after cross-linking. Countless CLCPs have been syn-thesized by the two-step cross-linking, and in situ polymerizationmethods andtheir thermomechanical properties have been thoroughly investigated [42].Concurrent to these developments in the fundamental materials chem-

istry of CLCPs, methods to induce alignment changes of dye moleculesand liquid crystals by light were subject to intense study in the 1980s and1990s [48]. Photoinduced (phototropic) phase transitions were observed inlow-molecular-weight LC systems doped with azobenzene (Figure 1.8a) [49].When irradiated with UV light in a nematic phase, the LC–isotropic phasetransition occurred with trans–cis isomerization of azobenzene moieties.The LC phase was restored by irradiation with visible light or heating. Thisphenomenon is based on the LC nature of azobenzene moieties. Rod-liketrans-azobenzene stabilizes LC phases, whereas bent cis-azobenzene disturbsthem. In low-molecular-weight systems, the photoinduced isotropic phasereadily returns to the LC phase due to either diffusion of cis isomers fromthe irradiated sites or fast cis–trans thermal back isomerization. On the

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12 1 A Historical Overview of Photomechanical Effects in Materials, Composites, and Systems

Isotropic Nematic

Cool

Heat

115 °C 90 °C 20 °C

Figure 1.7 Contraction andextension of a CLCP film inducedby temperature change. (Ohmet al. [42]. Reproduced with thepermission of John Wiley andSons.)

other hand, polymeric systems functionalized with mesogens and azobenzenemoieties show more stable isotropic phases because the diffusion of the cisisomers is limited and the disordered states of chromophores remain evenafter the cis–trans thermal back isomerization [50, 51]. The photoinducedisotropic phase can be maintained stable for a long time (more than 10 years)below Tg of the polymer. Therefore, LCPs functionalized with azobenzeneshave been extensively studied as photomemory materials. Furthermore,polymers with azobenzene moieties in all monomer units were found toshow rapid photoinduced phase transition [52]. Thus, photoisomerizationof azobenzene molecules can be amplified into the alignment change of thewhole system. Photoinduced phase transition has been investigated for various