Photomechanical Materials, Composites, and Systems E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
An exhaustive review of the history, current state, and future opportunities for harnessing light to accomplish useful work in materials, this book describes the chemistry, physics, and mechanics of light-controlled systems. * Describes photomechanical materials and mechanisms, along with key applications * Exceptional collection of leading authors, internationally recognized for their work in this growing area * Covers the full scope of photomechanical materials: polymers, crystals, ceramics, and nanocomposites * Deals with an interdisciplinary coupling of mechanics, materials, chemistry, and physics * Emphasizes application opportunities in creating adaptive surface features, shape memory devices, and actuators; while assessing future prospects for utility in optics and photonics and soft robotics
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Veröffentlichungsjahr: 2017
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Table of Contents
Cover
Title Page
Copyright
List of Contributors
Preface
Chapter 1: A Historical Overview of Photomechanical Effects in Materials, Composites, and Systems
1.1 Introduction
References
Chapter 2: Photochromism in the Solid State
2.1 Molecular Photoswitches in the Solid State
2.2 Molecular and Macroscopic Motion of Azobenzene Chromophores
2.3 Photomechanical Effects
2.4 Solid-State Photochromic Molecular Machines
2.5 Surface Mass Transport and Phase Change Effects
2.6 Photochromic Reactions in Framework Architectures
2.7 Summary and Outlook
References
Chapter 3: Photomechanics: Bend, Curl, Topography, and Topology
3.1 The Photomechanics of Liquid-Crystalline Solids
3.2 Photomechanics and Its Mechanisms
3.3 A Sketch of Macroscopic Mechanical Response in LC Rubbers and Glasses
3.4 Photo- and Heat-Induced Topographical and Topological Changes
3.5 Continuous Director Variation, Part 1
3.6 Mechanico-Geometric Effects, Part 1
3.7 Continuous Director Variation, Part 2
3.8 Continuous Director Variation, Part 3
3.9 Mechanico-Geometric Effects, Part 2
3.10 Director Fields with Discontinuities–Advanced Origami!
3.11 Mechanico-Geometric Consequences of Nonisometric Origami
3.12 Conclusions
References
Chapter 4: Photomechanical Effects in Amorphous and Semicrystalline Polymers
4.1 Introduction
4.2 Polymeric Materials
4.3 The Amorphous Polymer State
4.4 The Semicrystalline Polymer State
4.5 Absorption Processes
4.6 Photomechanical Effects in Amorphous and Semicrystalline Azobenzene-Functionalized Polymers
4.7 Molecular Alignment
4.8 Annealing and Aging
4.9 Sub-
T
g
Segmental Mobility
4.10 Cross-Link Density
4.11 Concluding Remarks
References
Chapter 5: Photomechanical Effects in Liquid-Crystalline Polymer Networks and Elastomers
5.1 Introduction
5.2 Optically Responsive Liquid Crystal Polymer Networks
5.3 Literature Survey
5.4 Outlook and Conclusion
References
Chapter 6: Photomechanical Effects in Polymer Nanocomposites
6.1 Introduction
6.2 Photomechanical Actuation in Polymer–Nanotube Composites
6.3 Fast Relaxation of Carbon Nanotubes in Polymer Composite Actuators
6.4 Highly Oriented Nanotubes for Photomechanical Response and Flexible Energy Conversion
6.5 Photomechanical Actuation Based on 2-D Nanomaterial (Graphene)–Polymer Composites
6.6 Applications of Photomechanical Actuation in Nanopositioning
6.7 Future Outlook
Acknowledgments
References
Chapter 7: Photomechanical Effects in Photochromic Crystals
7.1 Introduction
7.2 General Principles for Organic Photomechanical Materials
7.3 History and Background
7.4 Modes of Mechanical Action
7.5 Photomechanical Molecular Crystal Systems
7.6 Future Directions
7.7 Conclusion
Acknowledgments
References
Chapter 8: Photomechanical Effects in Piezoelectric Ceramics
8.1 Introduction
8.2 Photovoltaic Effect
8.3 Photostrictive Effect
8.4 Photostrictive Device Applications
8.5 Concluding Remarks
References
Chapter 9: Switching Surface Topographies Based on Liquid Crystal Network Coatings
9.1 Introduction
9.2 Liquid Crystal Networks
9.3 Conclusions
References
Chapter 10: Photoinduced Shape Programming
10.1 One-Way Shape Memory
10.2 Two-Way Shape Memory
10.3 Summary and Outlook
References
Chapter 11: Photomechanical Effects to Enable Devices
11.1 Introduction
11.2 Analog Photomechanical Actuators
11.3 Discrete-State (Digital) Photomechanical Actuators
11.4 Photomechanical Mechanisms and Machines
References
Chapter 12: Photomechanical Effects in Materials, Composites, and Systems: Outlook and Future Challenges
12.1 Introduction
12.2 Outlook and Challenges
12.3 Conclusion
References
Index
End User License Agreement
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Guide
Cover
Table of Contents
Preface
Begin Reading
List of Illustrations
Chapter 1: A Historical Overview of Photomechanical Effects in Materials, Composites, and Systems
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, parabolic reflector; B, battery; T, electroacoustic transducer.
Figure 1.2 Typical photochromic molecules used to induce photomechanical effects: (a) azobenzene, (b) spiropyran, (c) fulgide, and (d) diarylethene.
Figure 1.3 Systems for photoinduced deformation of polymer chains proposed by Lovrien. (a) Polymer electrolyte functionalized with azobenzene moieties. (b) Blend solution composed of polymer and azobenzene electrolytes.
Figure 1.4 Photoinduced bending of an acrylamide gel containing triphenyl methane leuco dyes under an electric field. (a) Photochromism of triphenyl methane leucocyanide. (b) Before irradiation. (c) Under irradiation with UV light. (d) Under irradiation in the reverse electric field to that in (c).
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.
Figure 1.6 Preparation methods of CLCPs with aligned mesogens. (a) Two-step cross-linking. (b)
In situ
polymerization.
Figure 1.7 Contraction and extension of a CLCP film induced by temperature change.
Figure 1.8 Photomodulation of molecular alignment. (a) Photoinduced phase transition. (b) Photoalignment induced by linearly polarized light (LPL). (c, d) Surface-mediated photoalignment.
Figure 1.9 Photoinduced contraction and extension of a CLCP film.
Figure 1.10 Photoinduced deformation of CLCP films. (a) Bending of a monodomain film. (b) Direction-selective bending of a polydomain film by linearly polarized light.
Figure 1.11 Various three-dimensional motions of CLCP systems induced by light. (a) Oscillation [75]. (b) Swimming [74]. (c) Light-driven plastic motor [73]. (d) Inchworm walk [76]. (e) Robotic arm [76]. (f) Manipulation of an object [77]. (g) Actuation through tissues [78]. (h) Gripper [128]. (i) Crawling up [128]. (j) Adaptive liquid lens [80]. (k) Localized actuation [81]. (l) Tactile device [82]. (m) Heliotropism [83]. (n) Microparticle [138]. (o) Artificial cilia [85]. (p) Pillar array [86]. (q) Size-changeable pores [87]. (r) Fiber [88]. (s) Micropump [89]. (t) Snap-through [90]. (u) Deformation into cone [114]. (v) Accordion folding [115]. (w) Checkerboard pattern [115]. (x) Photoswitchable stripes [116]. (y) Dynamic 3D finger print [79]. (z) Winding of spring [118].
Figure 1.12 Photoinduced deformation of organic crystals. (a) Anthracene ester. (Al-Kaysi
et al
. [148]. Reproduced with the permission of American Chemical Society.) (b) Diarylethene. (Kobatake
et al
. [149]. Reproduced with the permission of Nature Publishing Group.) (c) Cocrystal of diarylethene and perfluoronaphthalene.
Chapter 2: Photochromism in the Solid State
Figure 2.1 Examples of common small-molecule photoswitch isomerization reactions: (a) azobenzenes; (b) diarylethenes; (c) spiropyrans.
Figure 2.2 (a) Series of photographs showing the rotation of the light-driven plastic motor with the LCE laminated film induced by simultaneous irradiation with UV and visible light. (Yamada
et al
. [1]. Reproduced with the permission of Royal Society of Chemistry.) (b) Series of photographs of the flexible “robotic arm” motion of the azo-LCE laminated film induced by irradiation with UV and visible light. Arrows indicate the direction of light irradiation. (Yamada
et al
. [85]. Reproduced with the permission of Royal Society of Chemistry.)
Figure 2.3 The optical protocol for activating the light-powered oscillation of a cantilever. The nematic director (
n
) is positioned parallel to the long axis of the polymer cantilever of dimension 5 mm × 1 mm × 50 mm. When exposed to light polarized orthogonal to
n
(
E
⊥
n
), bending occurs toward the laser source. Cycling the Ar
+
laser from
E
⊥
n
to
E
n
can turn oscillation “on,” while blocking the Ar
+
or returning the polarization of the laser beam to
E
⊥
n
turns the oscillation “off.” (White
et al.
[51]. Reproduced with the permission of Royal Society of Chemistry.)
Figure 2.4 The central kink in the mixed helicity ribbon (a) incorporating azobenzene chromophore undergoes a piston-like shuttling motion upon irradiation with visible (b) and UV (c) light. A magnet is connected to the kink and is transported and transmits the push–pull shuttling motion to a second magnet placed 10 mm below. (Iamsaard
et al.
[98]. Reproduced with the permission of Nature Publishing Group.)
Figure 2.5 Photomechanical systems based on diarylethene crystals that convert light into mechanical work. (a) A rod-like crystal pushes a gold microparticle that is 90 times heavier than the crystal when irradiated with UV light. Bending of the crystal pushes the microparticle up to 30 µm. (Kobatake
et al.
[109]. Reproduced with the permission of Nature Publishing Group.) (b) Rotation of gears facilitated by the reversible bending of a crystalline actuator upon UV and visible irradiation. (Terao
et al
. [112]. Reproduced with the permission of John Wiley and Sons.) (c) Irradiation of gold-coated diarylethene crystals with UV and visible light enables the ON/OFF photoreversible current switching of an electric circuit. (Kitagawa and Kobatake [113]. Reproduced with the permission of Royal Society of Chemistry.) (d) Superimposed photographs of a crystal cantilever lifting a lead ball with a mass 275 times larger than that of the crystal upon irradiation with UV light from the underside of the actuator. (Morimoto and Irie [111]. Reproduced with the permission of American Chemical Society.)
Figure 2.6 (a) Photoinduced proton transfer in the
S
enantiomers of chiral salicylidenephenylethylamines upon keto-enol tautomerism. (b) Superimposed photographs of a chiral enol-(
S
)-1 crystal before and after irradiating the top of the crystal actuator with ultraviolet light. The crystalline cantilever achieved 26 nJ of work by lifting a 4.00 mg metal ring a height,
δ
, of 0.65 mm. Various photomechanical lifting works were achieved with different enantiomeric compositions within the crystal: the racemic crystal, enol-(
rac
)-1, achieved 59 nJ of work by lifting a weight with a mass 300 times larger than that of the crystal (not shown). (Koshima
et al.
[117]. Reproduced with the permission of American Chemical Society.)
Figure 2.7 Amorphous phase-mediated azobenzene isomerization and photomechanical bending in a cocrystal (1.2 mm × 90 µm × 20 µm). The cocrystal contains a 1:1 ratio of a halogen bond acceptor (
cis
-1,2-bis(4-pyridyl)ethylene) and halogen bond donor species. Bromine or iodine atoms on perfluorinated azobenzenes act as halogen bond donors, having a linear interaction with the pyridine nitrogen atoms on the halogen bond acceptor. Irradiation of the cocrystals with a 532-nm laser facilitates cis-to-trans isomerization of the halogen bond donors, via amorphous intermediates, determined by X-ray diffraction. (Bushuyev
et al.
[126]. Reproduced with the permission of Royal Society of Chemistry.)
Figure 2.8 Two-photon excitation (TPE) of a fluorophore to facilitate Förster resonance energy transfer (FRET) to photoisomerize azobenzene nanoimpellers on mesoporous silica nanocrystals and subsequent cargo release. (a) Overlap of the emission spectrum of the fluorophore and absorption spectrum of the azobenzene nanoimpeller enables FRET. (b) The chemical structure of the fluorophore. (c) Structure of the two-photon fluorophore. (d) Photoisomerization of azobenzene using two-photon (760 nm) excitation of the fluorophore. (e) Schematic of the mesoporous silica nanocrystal. (f) Transmission electron microscopy image of a single nanocrystal. Light-activated nanovalves that utilize near-infrared irradiation such as this TPE-based mechanism show promise for targeted drug delivery applications and should be further explored to extend their scope. (Croissant
et al.
[146]. Reproduced with the permission of John Wiley & Sons.)
Figure 2.9 (a) Schematic of a main-chain azobenzene-containing polymer (P1; R = C
12
H
25
) with a poly(
para
-phenylene) backbone. Irradiation with ultraviolet (UV) or visible light facilitates photoisomerization of azobenzene and conversion to the compressed and extended conformations, respectively. (Bleger
et al
. [150]. Reproduced with the permission of John Wiley & Sons.) (b) Scanning force microscopy images of P1 deposited on a modified graphite surface. The polymer crawls along the surface as it contracts upon UV irradiation. Demonstrating control over movement direction and the functionalization or tethering of the polymer strands to scaffolds may enable the macromolecules to perform work by lifting weights or transporting cargo. (Lee
et al
. [151]. Reproduced with the permission of ACS Nano.)
Figure 2.10 (a) Synthetic route and chemical structures of acrylate-type azobenzene monomers. (b) Supramolecular hydrogen-bonding interactions between main-chain polymers to facilitate physical cross-linking. (c) Photographs of a polymeric fiber fabricated by simple melt spinning. The fiber reversibly bends upon irradiation with ultraviolet and visible light. The fibers demonstrate robust photodeformation fatigue resistance and high thermal stability and show promise for applications as photomechanical actuators. (Fang
et al.
[153]. Reproduced with the permission of American Chemical Society.)
Figure 2.11 (a) Schematic of a dual-responsive micropump on a glass surface. Light or chemical stimuli may be used to induce fluid flow by a β-cyclodextrin–polyethylene glycol (β-CD-PEG) gel upon isomerization of the azobenzene moiety. (b) Schematic of the direct functionalization of glass surfaces by covalently tethering azobenzene-containing molecules. Reversible formation or disassociation of the host/guest complex with α-cyclodextrin results in fluid pumping. (c) Optical microscopy image of tracer particles in solution above a β-CD-PEG gel on a glass surface before irradiation. (d) Optical microscopy image of tracer particles accumulating at the edge of a β-CD-PEG gel after irradiation with ultraviolet light for 1 h. Scale bars, 50 µm. The reversibility of the host/guest interaction makes the design particularly appealing for rechargeable microdevices. (Patra
et al.
[158]. Reproduced with the permission of American Chemical Society.)
Figure 2.12 AFM image of a typical surface relief grating (SRG) optically inscribed into an azo polymer film. Grating amplitudes of hundreds of nanometers, on the order of the original film thickness, are easily obtained. In this image, the approximate location of the film–substrate interface has been set to
z
= 0, based on the knowledge of the film thickness. (Mahimwalla
et al.
[2]. Reproduced with the permission of Springer.)
Figure 2.13 (a) Structures and (b) photographs of the crystalline powders of azobenzenes utilized in the study. (c) The same powders after irradiation with 365 nm light for 30 min at 100 mW/cm
2
. (Norikane
et al.
[186]. Reproduced with the permission of American Chemical Society.)
Figure 2.14 (a and b) Motion of single crystals of
trans
-3,3′-dimethylazobenzene on a glass surface. (c) Schematic representation of the irradiation setup; (d–i) microscope images of translational motion of
trans
-3,3′-dimethylazobenzene after irradiation time,
t
, min 0 (d), 3 (b), 6 (f), 10 (g), 15 (h), 20 (b). Dashed white and dark gray lines represent the initial positions of crystals and droplets, respectively. (Uchida
et al.
[187]. Reproduced with the permission of Nature Publishing Group.)
Figure 2.15 (a) Isomerization of the azobenzene ligand within an MOF referred to as PCN-123. (b) Schematic illustration of CO
2
uptake in the parent MOF-5 structure and PCN-123 network in trans and cis states. (Park
et al.
[198]. Reproduced with the permission of American Chemical Society.)
Chapter 3: Photomechanics: Bend, Curl, Topography, and Topology
Figure 3.1 (a) (Top line) A nematic liquid of rods with dye guests, shown with their azobenzene cores as open (
trans
) or filled (
cis
) dots. The two isomers of azobenzene. (b) A block of rubber, of unit dimensions, composed of polymer chains linked together (cross-links shown as dots). The nematic rods driving the chains to anisotropy are not shown. When the chains extend (right block, in the dark), the block suffers an extension by a factor of along the director (presaged in the isotropic state (left). One can assume that spans between reticulation points deform in proportion to the body, that is, according to the deformation gradient .
Figure 3.2 A nematic elastomer in a measuring cylinder supporting a mass. It is initially hot (first frame). Cold air is blown down the cylinder, the nematic order is restored, and the elastomer grows in length by more than a factor of 3. The process is quickly reversible upon subsequent heating. The fractional length change of the same elastomer supporting different masses responding to temperature, .
Figure 3.3 A beam of light, traveling in the -direction is incident on the surface of film of a photoactive nematic elastomer, absorption leads to bend.
Figure 3.4 (a) The decay in reduced light intensity with depth for various reduced incident intensities . (b) The reduced curvature as a function of reduced beam thickness for several values of .
Figure 3.5 Intensity versus reduced depth for (light gray) and (dark gray) at reduced times as marked. The Lambert–Beer law holds for any at .
Figure 3.6 (a) Splay–bend director conformation with varying in the -plane upon traversing the cantilever. (b) Twist director conformation with twisting in the -plane upon traversing the cantilever. (c) Nematic glass photocantilever before and 0.04, 0.2, and 0.5 s after illumination with UV light [25]. (d) A bi-rubber undergoing heating (Figure from [29], Prof. EM Terentjev). (e) A bending cantilever, with thickness and radius of curvature , the central plane being denoted by a light line, now curved, from which material positions are measured through the thickness.
Figure 3.7 A cantilever of thickness (a) before and (b) after imposed or spontaneous distortion into a saddle shape, that is, with curvatures in two directions of opposite signs. (c) A nematic solid “swimmer” supported on a pin and forming a saddle in response to illumination from above (Figure from Prof. P Palffy-Muhoray). Figure (a)–(c) from Ref. [29]. (d) A classic beam (light color, seen along its length) being forced to bend. The straight edge (black) placed transversely across it reveals the transverse curvature
Figure 3.8 (a) A circular disc on a flat sheet, with circular director lines, contracts by along the director. The ratio of perimeter to in-material radius changes from , forcing the surface to become conical with opening angle . A sphere has ratios of perimeters of circles (lines of latitude) to in-material radii that also differ from , but by amounts that depend on the size of the circles. (b) A circular disc with, for instance, radial director lines and when heated contracts by along the director and elongates by along the circumferences. The ratio of perimeter to in-material radius increases beyond , forcing the surface to become an “anticone” to dispose of the surplus perimeter. This can be seen by the trajectory of the disc's perimeter on the unit sphere or in the anticone with, here, three wavelengths in the azimuthal sense. For very large amplitudes, the shape is a ruff.
Figure 3.9 (a) The coordinates for a disclination showing at an angle the director making an angle with the reference axis. The angle of the director to the radial direction, , is hence . (b) Defects of order and . (c, d) An defect before and after heating, respectively. Slight crinkling in the almost flat state indicates the presence of the defect pattern in the sheet. Arrows show the 10-fold symmetry expected from a curvature variation given by Equation 3.26, that is, as with . (e) Two hemi-stadia director patterns joined together by a region of uniform director. Under actuation, a tent, with half-cone ends, is formed. (f) A paper model, folded to form the junction line to the uniform section and having a conical end, is decorated with reference arrows to indicate the points of the flat material in (c) corresponding to those in the risen shell.
Figure 3.10 Cartesian and circular director patterns generating Gaussian curvature. (a) Cartesian: From left to right: (i) The initially flat configuration of a circular glassy film 15 m in thickness and 7.1 mm in diameter. (ii) The positive Gaussian curvature pattern. The dashed circle indicates the boundaries of the circular film. (iii) The formation of positive Gaussian curvature in the actuated state from two distinct viewing angles. (iv) The negative curvature pattern obtained as the orthogonal dual director field. (v) The formation of negative Gaussian curvature in the actuated state from two viewing angles. Circular: (b) A logarithmic spiral nematic pattern with . (c) The director field defined by , and (d) the resulting Gaussian curvature distribution
Figure 3.11 Spherical spindles: (a) The top half of a spindle of constant Gaussian curvature and parameter . The spindle arises as a surface of revolution. (b) The director field defined by Equation 3.31 with and . The solid curve indicates the circle of radius whose length is unchanged by the pattern. (c) The director field on a disc of radius and the spherical cap of fixed boundary that is expected to form. Hyperbolic spheres: (d) A hyperbolic sphere. (e) The director field defined by Equation 3.31 with and . The solid circle indicates the circle of radius whose length is unchanged by the stimulation of the pattern and is shown in (f). (From Ref. [50].)
Figure 3.12 (a, b) folding under the area to be “lost” around an apex, which then effectively develops Gaussian curvature at its tip and forms a faceted surface (c). This is still isometric origami. (d) A sheet with a concentric square pattern of director suffers contraction along the director to become the square pyramid (e). This pyramid releases high bend energy along its edges by relaxing into a cone (f).
Figure 3.13 A vocabulary of patches of director that lead to angle changes in discrete sections of a shell as the distortion changes.
Figure 3.14 Arrays and networks of defects for topography and topology. (a) An array of pyramids rises from a flat sheet decorated with a director field consisting of concentric squares. These are discrete forms of defects, but now the square pyramids into which they rise (b) cannot relax to cones. The defects are identified by two labelled examples in red. Rising pyramids, similar to these, were used to lift heavy loads [43]. (c) Two regions of uniform director can be welded together by a grain boundary of (discrete) defects (circled in blue and red); after [52]. (d) Upon weak deformation, a transversely shrunk, still planar region in contact with a ridged region remains. (e) Upon stronger actuation, both regions are turned into parts of a faceted bottle in order to reduce the overall bend energy along the ridges. (f) An array of and defects, where neighboring s' cores are connected by slits (heavy lines) that are as yet unopened. (g) Contraction along the directors around the defects leads to the opening of slits while remaining planar–a topological rather than topographical change; taken from Ref. [44] where experimental realizations are shown.
Chapter 4: Photomechanical Effects in Amorphous and Semicrystalline Polymers
Figure 4.1 (a) Schematics of the molecular structures in both melt and solid states for semicrystalline, amorphous, and liquid crystal polymers. (b) Specific volume for totally amorphous, semicrystalline, and crystalline polymers against temperature, upon cooling from the liquid melt. As crystallinity of polymers cannot be 100%, the curve for crystalline solid illustrates the extreme limit by a totally crystalline solid.
Figure 4.2 (a) Temperature-resolved storage modulus (O, left axis) and equilibrium bending angle (Δ, right axis) for cantilever bending of photoresponsive azobenzene-functionalized polymer. (b) Photomechanical master curve for photoresponsive azobenzene-functionalized polymer with various cross-link densities. The normalized bending angle is plotted as a function of normalized temperature (
T
−
T
g
).
Figure 4.3 (a) Photomechanical response (bending angle, left axis) and crystallinity (right axis) as a function of semicrystalline PMDA concentration for a series of azobenzene-functionalized linear polyimides. Cantilevers (5 mm × 1 mm × 0.02 mm) in the inset images are obtained after 1 h of continuous irradiation at 100 mW/cm
2
intensity with
λ
= 442 nm light polarized parallel to the long axis of the cantilever. (b) Normalized peak absorbance for the semicrystalline azo-PI-PMDA and amorphous azo-PI-6FDA against time of continuous light (
λ
= 442 nm) irradiation at 100 mW/cm
2
.
Figure 4.4 Temporal observation of UV–vis absorption spectra of 4-dimehtylamino-4′-nitroazobenzene (DANAB) in methylcyclohexane solution (a) and in polyetherimide film (b) at 230 K.
Figure 4.5 (a) Polymer backbone rigidity effects on photoinduced bending response of linear azobenzene-containing polyimides upon exposure to linearly polarized 445 nm light at 120 mW/cm
2
for 1 h. The backbone rigidity is systematically varied by using different diamines (DA) including (i) PMDA, (ii) BPDA, (iii) BTDA, (iv) 6FDA, and (v) OPDA. The effect of backbone rigidity on retention or relaxation of the photomechanical response is observed by storing the cantilevers in dark for 10 days after irradiation (i′) PMDA, (ii′) BPDA, (iii′) BTDA, (iv′) 6FDA, and (v′) OPDA. Polymer backbone also significantly affected light absorption behaviors as evident in UV–vis absorption spectra of (b) PMDA and (c) OPDA samples upon the irradiation of linear polarized 445 nm light at 60 mW/cm
2
for 1 h.
Figure 4.6 (a) Optical microphotographs of hot-drawn polymers at various draw ratios under crossed polarizer (upper) and natural light (bottom) conditions. Polar plots of absorbance regarding the stretching vibration of the terminal cyano group in polymers at various draw ratios. (b) Lateral deformation in
y
-direction (
λ
y
) at positions
A
,
C
, and
E
as a function of overall stretch (
λ
x
). The arrows indicate the point where the director-rotation completes. The point for the completion of director rotation for polymers is also shown.
Figure 4.7 (a) Chemical structure of cross-linked amorphous polyimides composed of 20 mol% azobenzene cross-linker. The polar plot is prepared for the polyimide sample with the UV–vis spectrometer absorbance information at
λ
= 355 nm (
trans
-azobenzene peak) as a function of the polarization of the probe light. Before prestraining, the as-prepared polyimide film has uniform absorbance (•). After hot drawing to 70% prestrain (○), a dichroic absorbance of the azobenzene chromophores was measured due to anisotropic molecular alignment along the strained axis. (b) Photoinduced bending angle of azobenzene-containing polyimides measured in cantilever geometry (6 mm × 1 mm × 20 µm for length, width, and thickness, respectively) after 1 h of irradiation to 80 mW/cm
2
of linearly polarized
λ
= 442 nm irradiation polarized both parallel and perpendicular to the primary axis of cantilevers subjected to various prestrain values (0–70%).
Figure 4.8 (a) Schematic description of the potential energy landscape for glasses with different configuration spaces. The left image describes the rapidly quenched (RQ) sample having larger free volume. The right image illustrates the slowly quenched (SQ) sample with dense environment. (b) Polar plots of the normalized absorption value at
λ
= 355 nm for the physically aged azobenzene-containing polyimides (left, RQ; right, SQ) at different light irradiation conditions: (•) Before irradiation, () after irradiation with linearly polarized 442 nm light polarized along the
y
-direction (90−270° axis), or () along the
x
-direction (0−180° axis), and () 4 days after irradiation with linearly polarized 442 nm light polarized along the
x
-direction (0−180° axis). (c) Photomechanical bending is monitored with a cantilever geometry (5 mm × 1 mm × 20 µm) upon 100 mW/cm
2
intensity of
λ
= 442 nm light linearly polarized along the
x
-direction. Effect of different physical aging conditions is contrasted by monitoring RQ (i−iii) and SQ (iv−vi) samples. The (i, iv) inset images show cantilevers before light irradiation and after 2 h of irradiation with polarized 442 nm light (parallel to the long axis of the cantilever ) shown in (ii, v). The (iii, vi) images are captured after 72 h of dark relaxation after the light irradiation. (d) Summarized photomechanical bending response of azobenzene-functionalized polyimide cantilevers for RQ () and SQ () during 2 h of continuous irradiation 100 mW/cm
2
intensity of
λ
= 442 nm light linearly polarized to
x
-axis followed by subsequent dark relaxation. (Lee
et al.
[55]. Reproduced with the permission of American Chemical Society.)
Figure 4.9 (a) Chemical structure of freely rotating para isomer (left) and nonrotating meta isomer (right) of phenyl ether linkages within azobenzene-functionalized polyimides. (b) Loss modulus plotted against temperature for para (—) and meta (---) isomers. (c) Time-resolved monitoring of photomechanical bending angle of azobenzene-functionalized polyimide cantilever consisted of para (•) and meta (○) isomers. Linearly polarized 445 nm light is irradiated for 60 min followed by 72 h of dark relaxation. Inset images indicate bending angle at 60 min time mark for continuous light irradiation on (i) meta and (ii) para isomers. Images marked with (′) are captured after 72 h relaxation at dark.
Figure 4.10 (a) Chemical structures of compounds used in this study. (b) Stress–strain curves of 20-µm-thick films upon stretching along the director axis of azobenzene moieties. (c) Photomechanical bending behavior of films (3 mm × 1 mm × 16 µm) upon irradiation with UV (10 mW/cm
2
) and visible (40 mW/cm
2
) light.
Chapter 6: Photomechanical Effects in Polymer Nanocomposites
Figure 6.1 (a) Schematic of the photomechanical testing apparatus. (b) The X-ray image showing key scattering reflexes; the outer ring corresponding to 3.4 Å is the signal from nanotubes. The inner ring is indicative of the correlation length of mesh size, see text. The arrow shows the direction of the aligning strain. (c) The typical azimuthal intensity variation,
I
(
β
), at a scattering angle corresponding to the outer (MWCNT, 3.4 Å) ring.
Figure 6.2 (a) Response to IR radiation at different values of prestrain. (b) Stress versus pre-strain data from Figure 6.2. The circle represents crossover from expansion to compression. (c) The magnitude (in kilopascal) of exerted actuation stress, as a function of prestrain, for samples with increasing MWCNT loading. The right
y-
axis shows the corresponding actuation stroke: the change in natural length
L
0
upon IR irradiation.
Figure 6.3
Scheme of local and macroscopic strains and the prediction of the actuation model
. (a) Scheme illustrating how the distortion (kinking or undulation) of an individual tube, lying at an angle
θ
to the alignment axis, projects on the
z-
axis to contribute to the macroscopically uniaxial strain, Equation 6.2. (b) The result of theoretical modeling based on orientational averaging of local deformations from each nanotube, Equation 6.3; the dashed line shows the linear approximation at small prestrain
ϵ
. Nanotube contraction factor is chosen, Δ = 0.8, as suggested by the crossover strain value
ϵ
* ≈ 0.1.
Figure 6.4 (a) Normalized stress, versus time, which allows comparison of the response kinetics: The light-on response of 3 wt% composite at different values of prestrain. The right
y
-axis shows the simultaneously measured, similarly normalized, change in temperature upon irradiation. (b) The light-on response of different composites, all measured at the same 20% prestrain. (c) Illustration of the data fit, for 3 wt% composite at 20% prestrain. Experimental data is fitted by the compressed exponential (solid line) and the simple exponential (dashed line) functions to demonstrate the discrepancy. (d) The normalized stress relaxation of a 3 wt% nanocomposite illuminated at different prestrain, when the light source is switched off. (e) The light-on response of the composite with very low tube loading and also that of a sample with 3 wt% carbon black, both at 20% prestrains.
Figure 6.5
Nanotube liquid crystal elastomer composite
. (A) (1) Vacuum filtration is used to deposit carbon nanotubes (CNTs) onto an inorganic filter membrane. (2) PDMS is spin-coated on top of a glass slide. (3) The membrane consisting of LCs is pressed against the PDMS that resulted in complete transfer. (4) A second PDMS layer is spin-coated and polymerized to enable LC being part of the polymeric network resulting in “Nanotube LC Elastomer.” (B)
SEM images of LC-CNTs
: (a-1)–(a-3)): 0.01 µg/ml; (b-1)–(b-3)): 0.05 µg/ml; (c-1)–(c-3)): 0.1 µg/ml; (d-1)–(d-3)): 0.3 µg/ml; (e-1)–(e-3)): 0.5 µg/ml; Scale bars: Row 1: (a-1)–(e-1): 10 µm; Row 2: (a-2)–(e-2): 1 µm; Row 3: (a-3)–(e-3): 200 nm, (g) order parameter versus concentration and (f) magnified image of (e-3). (C)
Order parameter
: Linear correlation between spatial frequency and optical order parameter. (D)
Schlieren textures and domain size analysis
: Schlieren textures of nanotube LCs: Rotation of the polarizer by 2.5° (92.5°) resulted in enhanced contrast and better imaging of the
Schlieren
textures and domain walls suggesting long-range order. Scale bar: 2 mm. (E) Domain size measurements as a function of concentration inside the LC–polymer composites: (a) ∼0.01 µg/ml; (b) 0.05 µg/ml; (c) ∼0.1 µg/ml; (d) 0.3 µg/ml; (e) ∼0.5 µg/ml; (f) Average domain size versus CNT concentration showing almost twice the decrease in domain size with increasing concentration. Line is shown for eye guidance only.
Figure 6.6 (A)
Photomechanical responses of LC-CNT polymer composites.
Prestrains from 3% to 50% were applied before NIR excitation: (a) plain PDMS; (b) 0.01 µg/ml; (c) 0.05 µg/ml; (d) 0.1 µg/ml; (e) 0.3 µg/ml; (f) 0.5 µg/ml; (g) photomechanical response versus order parameter demonstrating increased ordering leading to improved mechanical response of the composites. (B)
Disordered versus ordered systems.
(a-1) SEM image of randomly oriented film (∼0.5 µg/ml concentration); (a-2) SEM image of LC nanotube film (∼0.5 µg/ml concentration with order parameter S=0.6); (b-1) photomechanical stress change for randomly oriented film based actuator; (c-1) photomechanical stress change for LC-film-based actuator. (C)
Kinetics of photomechanical actuation in nanotube liquid crystal elastomer.
(a) Actuation kinetics; (b) relaxation kinetics; (c) variation of stretching exponent for both actuation and relaxation as a function of concentration of nanotube liquid crystals in elastomer; (d) variation of stretching exponent with prestrains. (D)
Efficiencies of nanotube LC–polymer composites
. (a) Optomechanical conversion factor versus concentration; (b) energy conversion efficiency versus concentration at different prestrains.
Figure 6.7
Stress versus mass fraction comparisons:
Logarithmic plot of mass fractions of CNT/graphene versus stress suggesting superior performance of nanotube LC elastomers compared to previous nanotube/graphene-based nanocomposite photomechanical actuators. It should be noted that the present work has a layered composite structure unlike most previous studies, which were nanocomposites.
Figure 6.8 (A) GNP/PDMS photomechanical actuator characterization and testing: (a) SEM image of GNPs, (b) Raman spectroscopy shift of PDMS, GNP/PDMS, and CNT/PDMS. (B) Photomechanical responses and actuation kinetics of GNP/PDMS composites: (a) Typical photomechanical stress response of GNP/PDMS actuator at 3% prestrains; (b–c) actuation and relaxation of GNP/PDMS actuator, respectively. (C) Comparison of increasing photomechanically induced stress change in GNP/PDMS composites as a result of NIR illumination for increasing GNP concentrations: (a) Plain PDMS, (b) 0.1 wt% GNP, (c) 0.5 wt% GNP, (d) 1 wt% GNP, (e) 2 wt% GNP, and (f) 5 wt% GNP.
(D)
Comparison of photomechanically induced stress change in 2 wt% composites of various carbon forms: (a) Ggraphite oxide, (b) carbon nanotubes, (c) carbon black, and (d) graphene nanoplatelets.
Figure 6.9 (A) (a) Repeatability details of 2 wt% GNP/PDMS. Comparison of actuation response from three samples at 3% and 40% prestrains, and (b) average actuation response versus prestrain with ±1 standard deviation error bars; (c) increase in Young's modulus of the actuator with increase in GNP fraction. (B) Steady-state temperature measurements: Temperature decrease along the composites test samples as a result of distance from the illumination point. 0% prestrain dimension and temperature response (left side) are compared with 40% prestrain dimensions and temperature response (right side).
Figure 6.10 (A) Schematic: (a) photothermal actuation schematic. Three thin GNP/PDMS composite strips are mounted with their upper ends fixed to a rigid plate. The free end of sample [1] is unrestricted, while samples [2] and [3] have a weight attached, thus inducing prestrain into the composite actuator. [3] Illumination via an IR LED results in energy transduction to the polymeric chains, causing a contraction in the actuator (and thus usable work). (b) Simplified single-axis nanopositioner layout. A laser displacement sensor is used to measure stage position. Independently controlled diodes on either side of the stage allow for differential positive- or negative-axis stage motion. (B) (a) SEM of plain GNP powder (not mixed with PDMS). (b) XPS data. (c) Changes in composite opacity as GNP loading is increased from 0 to 2 wt%. In all five slides, spin time is constant at 15 s. (d) Series of four slides with identical GNP loading (0.5 wt%), but decreasing spin times. (e) GNP/PDMS composite thicknesses are shown as a function of GNP wt% loading. Curves for a 15 s spin time as well as 90 s spin time are displayed. (f) Composite thickness as a function of spin time is shown. Composite samples were fabricated with spin casting times between 15 and 90 s. (g) Sample GNP composite actuator (2 wt%) used in our two-axis nanopositioners. The first inset is a SEM image showing a corner of the composite strip, and the second inset is a detail of a GNP stack sticking out of the polymer.
Figure 6.11 (A)
Nanopositioner
. (a) isometric of fully assembled nanopositioner. (b) Nanopositioner with cover plate removed, showing positioning diodes and dual GNP/PDMS composite actuators. (c) Top view of the nanopositioner with the actuators removed, clearly showing diode layout. (d) Simplified control schematic for a single positioning diode. (B)
Nanopositioner kinetics
. (a) maximum stage displacement as a function of GNP wt% loading for coarse-adjust and fine-adjust positioning diodes. (b) Displacement kinetics as a function of composite actuator thickness for a 1 wt% GNP sample. Useful displacement limit and time are also indicated. (C)
Resolution.
Ordered and actual (a)
x
-axis and (b)
y
-axis positions. Stage was ordered to sequentially move to positions +10, +20, +30 µm, and then −10, −10, −10 µm. Annotations [1] and [2] indicate position commands for 0 µm and −10 µm, respectively. (c) Sample positioning error (
x
actual
−
x
ordered
) with micrometer scale. Spikes are due to mismatch between actual and ordered positions when the control loop receives a new position request. (d) Sample positioning error (
x
actual
−
x
ordered
) with nanometer scale. (e) Detail of position response and subsequent stage oscillations. Positive and negative oscillation bounding curves are indicated by annotations [3] and [4], respectively.
Chapter 7: Photomechanical Effects in Photochromic Crystals
Figure 7.1 Examples of reversible photochromic reactions that have been used to drive photomechanical motion in crystals. (a) trans–cis isomerization of azobenzene; (b) ring formation and cleavage reaction of diarylethene derivatives; (c) ring formation and cleavage isomerization of furylfulgide; (d) intramolecular hydrogen transfer reaction of salicyldienoanilines; (e) intramolecular linkage isomerization of a nitropentaaminecobalt (III) complex; (f) [2 + 2] cycloaddition reaction of cinnamic acid; and (g) [4 + 4] cycloaddition reaction of anthracene derivatives.
Figure 7.2 (a) AFM image of a single-crystal
9TBAE
nanorod before illumination and (b) after illumination with 365 nm. Scale bar is 6 µm. Note that the diameter of the rod in the
xy-
plane appears greater than 200 nm due to its convolution with the broad AFM tip.
Figure 7.3 Open- and closed-ring chemical structures and reaction scheme for a diarylethene derivative, 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene. The images illustrate the reversible deformation of a single crystal that can be switched back and forth using ultraviolet (365 nm) and visible (500 nm) light. A square single crystal of (1) with corner angles of 88° and 92° reversibly changes to a shape with corner angles of 82° and 98°. The crystal thickness was 570 nm.
Figure 7.4 Movement of a gold microparticle by a rod-like diarylethene crystal after irradiation with ultraviolet (365 nm) light. The gold microparticle is 90 times heavier than the rod-like crystal (250 × 5 × 5 µm) and appears in the images as a black spot. The ultraviolet-light-induced bending of the crystal could push the gold microparticle as far as 30 µm. The exposure time of each frame was 500 µs (2000 frames per second), and the numbers above the images are frame numbers.
Figure 7.5 (a) Attenuation of exciting light leads to a gradient of reacted and unreacted molecules. This forms a bimorph-type structure where the motion is driven by strain between the different phases. (b) Complete reaction of the crystal leads to a reconstruction to accommodate new packing arrangements of the product molecules and an overall shape change of the crystal.
Figure 7.6 Bidirectional bending of a thin crystal composed of a
cis
-azobenzene derivative using 457 nm light. The arrow at the top of the Figure indicates the direction of irradiation.
Figure 7.7 A nanowire composed of a azobenzene derivative (left side) and a polystyrene nanowire (right side) are both attached to the tip of a glass microcapillary tube. UV irradiation of the azobenzene nanowire causes it to bend toward the polystyrene nanowire, clamping a microsphere between them. Scale bar, 20 µm.
Figure 7.8 Various kinematic effects observed after UV irradiation of coordination metal complex crystals. (a) Rolling or flipping; (b) separation of a small fraction of the crystal, which propels the remaining portion of the crystal; (c) explosion or splitting of a crystal.
Figure 7.9 SEM images and histogram analyses of (a)
9MA
microneedles and (b)
9MA
microribbons. The inset of (a) shows an enlarged histogram with a narrower bin size to show the aspect ratio of the
9MA
microneedles in more detail. Changing the crystal growth solvent changes the crystal aspect ratio in a controlled way.
Figure 7.10 Optical microscope images of bending and unbending of a
9MA
microneedle (a) and twisting and untwisting of a
9MA
microribbon (b) during 365 nm UV irradiation. Scale bars: 20 µm.
Figure 7.11 Plot of curvature versus dimer fraction for the three 9-methylanthracene microneedles shown as squares (a), circles (b); and triangles (c). The curvature peaks at 40–60% reaction, then decreases as the needles become predominantly dimer.
Figure 7.12 Single 200-nm-diameter nanorod of
9AC
(∼60 µm long) briefly exposed to 365 nm light in a 50% solution of phosphoric acid in water. The dotted circle shows the illuminated region (35 µm in diameter). The nanorod repeatedly flexes back and forth (after UV illumination = panels b, d, f, h; after dark period = panels a, c, e, g). The time required to revert back is around 5 min at room temperature. Scale bar = 20.7 µm.
Figure 7.13 Series of optical microscopy images of (a)
9AC
and (b)
4F-9AC
microcrystals after a 1 s exposure to 405 nm light causes them to deform. The
4F-9AC
crystal completely untwists in 25 s, while the
9AC
crystal requires 420 s to unbend. Both scale bars are 50 µm. (c) Fluorescence recovery curves of
4F-9AC
(fastest, left),
2F-9AC
(middle), and
9AC
(slowest, right), showing the different photodimer dissociation rates for the different crystals.
Figure 7.14 Ultrafast electron microscopy images of [Cu(TCNQ)] charge-transfer crystal in the “off” structure (a and c; no pulsed-laser irradiation) and in the “on” structure (b and d; pulsed-laser irradiation) (scale bar = 500 nm). (e) Higher-resolution image of the crystal in the “off” structure illustrating the gap that is closed by laser irradiation. (f) Plot showing the results of a sequence of “on” and “off” cycles. The channel width changed from 0 (pulsed-laser irradiation) to 140 ± 5 nm (no pulsed-laser irradiation).
Chapter 8: Photomechanical Effects in Piezoelectric Ceramics
Figure 8.1 Illumination responses of photovoltaic current for 1.5 mol% MnO
2
-doped 0.895 PbTiO
3
-0.105 La(Zn
2/3
Nb
1/3
)O
3
ceramic.
Figure 8.2 Experimental setup for measuring photovoltaic and photostrictive effects.
Figure 8.3 Photocurrent measured as a function of applied voltage under illumination.
Figure 8.4 Energy band-gap model of excited electron transition from deep donor-impurity level in PLZT.
Figure 8.5 Wavelength dependence of photovoltaic current in 0.895PT–0.105LZN and PLZT (3/52/48).
Figure 8.6 Short-circuit current
J
ph
(a) and open-circuit electric field
E
ph
(b) as a function of illumination intensity
I
for pure and MnO
2
-doped 0.895PT–0.105LZN.
Figure 8.7 Dependence of (a) photoconductivity, (b) photovoltage, and (c) photocurrent on illumination intensity in a PLZT 3/52/48 sample with 1 mm in thickness; (d) the result for a sample with 140 µm in thickness.
Figure 8.8 (a) Measuring system of the dependence of photovoltaic effect on light polarization direction. (b) Photovoltaic voltage and current as a function of the rotation angle.
Figure 8.9 Interrelation of photovoltaic current with remanent polarization in PLZT family.
Figure 8.10 Photovoltaic response of PLZT (3/52/48) for various impurity dopants (illumination intensity: 4 mW/cm
2
).
Figure 8.11 Photovoltatic current, voltage, power, and tip displacement of a bimorph specimen as a function of dopant concentration in WO
3
-doped PLZT (3/52/48).
Figure 8.12 Contour maps of (a) photovoltatic voltage
E
ph
, (b) photocurrent
I
ph
, and (c) piezoelectric constant
d
33
in the PLZT (
x
/
y
/1 −
y
) system.
Figure 8.13 Grain size dependence of photostrictive characteristics in PLZT (3/52/48).
Figure 8.14 Comparison of measured and computed normalized photocurrents with photovoltaic coefficient (
i
m
/
k
) of 0.5 at.% WO
3
-doped PLZT (3/52/48).
Figure 8.15 Model to compute the dependence of photocurrent on sample thickness. The sample was modeled as thin slices along the thickness direction and the corresponding circuit diagrams are also shown.
Figure 8.16 Variation of photocurrent with surface roughness of 0.5 at.% WO
3
-doped PLZT. Comparison with the normalized computed photocurrent for the two surface profiles is also made.
Figure 8.17 Structure of the photo-driven bimorph and its driving principle.
Figure 8.18 Tip deflection of the bimorph device made from WO
3
0.5 at.% doped PLZT under a dual-beam control (illumination intensity: 10 mW/cm
2
).
Figure 8.19 Structure of the photo-driven relay.
Figure 8.20 Photo-driven micro-walking machine made of two photostrictive bimorphs. Alternating irradiation provides a walking motion.
Figure 8.21 Tip deflection of the bimorph device made from WO
3
0.5 at.% doped PLZT under a dual-beam control (illumination intensity: 10 mW/cm
2
).
Figure 8.22 (a) Schematic diagram of an arch-shaped photoactuating film device and (b) its triangular top shape.
Figure 8.23 Response speed improvement of the photostrictive bulk ceramic and of the device in the sequence of year and the key technology development.
Chapter 9: Switching Surface Topographies Based on Liquid Crystal Network Coatings
Figure 9.1 (a) Schematic representation of the formation of a liquid crystal network. (b) Besides uniaxial alignment, the LCs can be ordered in twisted, splayed, or chiral-nematic configurations, for example, by using surface techniques or chiral additives. Some examples of (c) LC diacrylates and (d) monoacrylates often used in polymerizable LC formulations.
Figure 9.2 Photoinduced shape changes. (a) A copolymerized monomer with an azobenzene moiety undergoes a trans-to-cis conversion when exposed to UV light. (b) When embedded in a splayed LC network, it induces contraction at one side of the film and expansion at the opposite side. (c and d) LC monomers containing azobenzene monomers can be inkjet-printed on a pattern of sacrificial polyvinyl alcohol, which after polymerization of the LC monomers can be removed, yielding partly freestanding cilia. (e) When these cilia are exposed to UV light, they bend to a curved state. When the light is switched off, they bend back to close to flat.
Figure 9.3 The principle of surface actuation where the initially highly ordered state of an LC network is disturbed and the film surface protrudes by changes of the packing of the rod-like moieties at less ordered locations.
Figure 9.4 (a) Mask exposure of a chiral-nematic coating results in local deformation of the film. (b) The addition of 5 wt% chiral LC monomer to the monomer mixture induces the formation of the molecular helices perpendicular to the film surface, and the addition of 2 wt% azobenzene monomer makes the film photosensitive. (c) The height of the resulting protrusions is analyzed by interference microscope and is around 10% of the initial film thickness. (d) The three-dimensional image of the surface shows the correspondence with the mask that is used for patterned exposure.
Figure 9.5 (a) Interference microscopy measurements show 3D image of (b) surface topographies formed in isotropic films containing azobenzene and its surface profile. (c) Comparison of the absorbance of azobenzene and stable Tinuvin dye added to exchange the azobenzene to determine heating effects. (d) Interference microscopy measurements show the 3D image and (e) the corresponding surface profile when the Tinuvin containing sample is UV-exposed similarly to the azobenzene-containing coatings.
Figure 9.6 Line-patterned coatings with locally different director profiles. (a) A coating with alternating stripes chiral-nematic order next to isotropic order deforms from a flat state (b) to a deformed state 9c) under exposure to UV light. (b, c) Interference microscopic images taken before and during exposure, which show a modulation depth of around 10% relative to the initial coating thickness. (d) A coating with alternating stripes of planar chiral-nematic order next to homeotropic order. (e) The cross section of the coating measured by interference microscope measure prior to UV exposure. The small corrugations, enlarged in the inset, originate from an imprinted ITO pattern. (f) The same film under exposure to UV light. The planar chiral-nematic area expands relative to the homeotropic area with a modulation depth of around 20%.
Figure 9.7 Randomly patterned coatings with locally different director patterns. (a) A fingerprint pattern, which forms in a chiral-nematic coating with the helix axes aligned parallel to the substrate surface. (b) The fingerprint expands at the locations where the rod-like molecular units are oriented parallel to the surface and shrink at the positions where they are aligned perpendicular, resulting in a modulation depth of around 20% of the initial coating thickness. (c) An illustration of a coating with a polydomain pattern. (d) Here also, the domains with (close to) planar orientation expand, whereas the domains with (close to) homeotropic orientation shrink.
Figure 9.8 Density change in a chiral-nematic film containing azobenzene and Tinuvin; (a) before UV exposure, both the Tinuvin and azobenzene film are at the bottom of flask; (b) snapshot of films during exposure showing the azobenzene film to float and the Tinuvin film to remain at the bottom; and (c−e) after removal of UV light, the azobenzene film starts shrinking to reach its initial position at the bottom. The Tinuvin-modified film remained at the bottom.
Figure 9.9 Mechanical response measured as a height change of the thin film versus time upon actuation by 365 nm light of intensity 78 mW/cm
2
and a subsequent relaxation in dark. The film thickness is 4 µm, and the corresponding density decrease is 1.3%.
Figure 9.10 Modulation depth measured at a mask exposed chiral-nematic polymer film under various illumination conditions. The 365 nm LED light intensity was chosen to be 100, 200, and 300 mW/cm
2
. The 455 nm LED light was added in different intensities in ratios varying between 0 (455 nm LED switched off) and 1 (455 nm intensity equal to 365 nm intensity). The insets show the actual interference microscopy structures measured with and without 455 nm light.
Chapter 10: Photoinduced Shape Programming
Figure 10.1 Shape memory polymers are thermally responsive materials that have been programmed into a metastable state; this state can be fixed indefinitely by cooling. Upon heating, entropic elasticity drives recovery from the metastable state to the globally state (A). (Lendlein and Kelch [10]. Reproduced with the permission of John Wiley and Sons.) The shape memory cycle, both programming and recovery, is compared to the behavior of an elastomer. An elastomer subjected to similar programming conditions is shown for comparison (b); note the lack of shape fixing upon cooling in the elastomer (B). (Liu
et al.
[6]. Reproduced with the permission of Royal Society of Chemistry.) Shape memory polymers can be incorporated into composite structures and used to generate complex shape change (C). (Felton
et al.
[13]. Reproduced with the permission of Royal Society of Chemistry.)
Figure 10.2 The shape memory effect in polymers can be triggered using a photothermal stimulus. This remote triggering of this shape change has been proposed for two-way shape memory in a variety of biomedical applications, including a shape memory polymer foam designed to fill an embolism. Time-lapse photographs of a foam undergoing laser-triggered recovery in a model embolism (a). (Maitland
et al.
[17]. Reproduced with the permission of SPIE.) Photothermal heating of an SMP has also been proposed as a thrombectomy device capable of being inserted in a small form factor and undergoing subsequent shape change (b). (Small
et al.
[16]. Reproduced with the permission of Optical Society of America.) Photothermal triggering of shape memory behavior in carbon-nanotube-filled elastomers leads to complex shape recovery due to anisotropic heating (c). (Koerner
et al.
[18]. Reproduced with the permission of Nature Publishing Group.)
Figure 10.3 The photothermally triggered shape memory effect can be used to create objects that morph from flat to complex 3D shapes by localizing light absorption. By printing IR-absorbing black ink on prestretched polystyrene, mountain and valley folds can be introduced (a–c). By making arrays of these folds, origami-inspired shape memory can be obtained (d–f). (Liu
et al.
[32]. Reproduced with the permission of Royal Society of Chemistry.) Complex arrays can be generated, leading to complex 3D structures such as an icosahedron (g–i). Time-lapse images of photothermally triggered recover of an icosahedron is shown in (j). (Creative Commons License http://creativecommons.org/licenses/by/4.0/legalcode). (Lee
et al.
[33]. Reproduced with the permission of Royal Society of Chemistry.)
Figure 10.4 Photochemical modifications of polymer structure can be used to design light-responsive shape memory polymers. By exposure to long-wave UV light, a cinnamyl-containing polymer, cross-links are formed fixing a distinct shape (b). Upon exposure to short-wave UV light, these cross-links are broken and the original shape (a) is recovered. These shapes include those resulting from tensile deformation (A) and complex bending deformation (B). The reversible formation of these cross-links within the polymer network is shown in (C). (Lendlein
et al.
[40]. Reproduced with the permission of Nature Publishing Group.) Photo-origami can be accomplished through the process shown in (D). This process relies on a photoinduced fixing of strain through reconfigurable cross-links. A six-sided box was demonstrated (E) and a model developed that matched experimental observations (F–H). (Ryu
et al.
[41]. Reproduced with the permission of AIP Publishing LLC.)
Figure 10.5 Isomerization of azobenzene can be used to generate all-optical shape memory effect in liquid-crystalline polymers. The isomerization of azobenzene from the trans isomer to the cis isomer can be accomplished with UV light and can be reversed with visible light or heat (a). Azobenzene-containing monomers can be polymerized with nonresponsive liquid-crystalline monomers to generate glassy liquid crystal polymer networks (b). The resulting materials exhibit light-activated shape memory where (i) permanent shape, (ii) mechanical deformation, (iii) photo fixing, and (iv) shape retention (in the absence of light) are observed. Exposure to right-handed circularly polarized light unlocks the photo-fixed state (v), allowing for the recovery of the permanent shape (vi) (c). (Lee
et al.
[61]. Reproduced with the permission of Royal Society of Chemistry.)
Figure 10.6 Photothermal two-way shape memory of thermoresponsive gels produces durable and potentially complex two-way shape memory materials. Carbon nanotube/NIPAM composites exhibit two-way shape memory in response to an NIR laser. This two-way shape memory can be repeated at least 1200 times (a). (Fujigaya
et al.
[92]. Reproduced with the permission of John Wiley and Sons.) By localizing the irradiation source, an elastin composite hydrogel two-way shape memory material can be used to generate complex and controllable motion, such as a hand with individually addressable fingers (b) or a crawler (c). (Wang
et al
. [93]. Reproduced with permission from American Chemical Society.)
Figure 10.7 Spatially patterned hydrogel photothermal two-way shape memory materials can undergo complex mechanical responses to a photothermal stimulus. Layered nanocomposite hydrogels can be activated into complex shapes by limiting two-way shape memory to a subset of the layers in the gel. This is achieved through the use of nanoparticles with different absorption spectra (a–f). (Zhu
et al.
[100]. Reproduced with the permission of American Chemical Society.) Hydrogels bound to a pattern surface undergo a reversible creasing instability that can be triggered photothermally (g). The resulting surfaces can reversibly display or hide chemical functionalities in predetermined patterns (h, i). Within a sample, subsets of these patterns can be activated using localized irradiation (j). (Yoon
et al.
[101]. Reproduced with the permission of John Wiley and Sons.)
Figure 10.8 Liquid crystal polymer networks can be triggered reversibly using photothermal stimulus to form complex two-way shape memory materials. Using photoalignment, the molecular order of liquid-crystalline polymer networks can be patterned. When an IR-absorbing dye is included in the network, complex photothermal two-way shape memory is observed (a). For instance, a defect pattern can be introduced, which reversible morphs from flat to conical in nature. The opening angle of the cone depends on the intensity of the photothermal stimulus (b) [114]. Liquid crystal elastomer – silicone rubber bilayers that undergo reversible bending (c) can be fabricated. By incorporating carbon nanotubes, photothermal stimulus can be applied, and the resulting composite can be formed into a crawler (d) capable of using a notched surface (e) to generate locomotion (f) [113]. (Broer and Mol [115]. Reproduced with the permission of John Wiley and Sons.)
Figure 10.9 Liquid crystal polymer networks are capable of photochemical two-way shape memory when polymerized with azobenzene-containing monomers. Printed cilia-like two-way shape memory materials (a) exhibit photoinduced two-way shape memory in response to sequential exposure to UV and visible light (b). (van Oosten
et al
. [131]. Reproduced with permission from Nature Publishing Group.) A continuous circular film can be used to power a light-activated motor in response to continuous UV and visible light (c and d). (Yamada
et al
. [132]. Reproduced with permission from John Wiley and Sons.) Azobenzene-doped liquid crystal elastomers were also demonstrated to swim across a surface in response to continuous irradiation, until the sample left the illuminated area. (Camacho-Lopez
et al.
[133]. Reproduced with the permission of Nature Publishing Group.)
Chapter 11: Photomechanical Effects to Enable Devices
Figure 11.1 Effect of intensity of 445 nm laser light on the bending of twisted nematic ALCN when the polarization is set parallel to the long axis of the cantilever. The orientation of the direction of polarization with respect to the nematic director determines if the cantilever bends toward the light (0 to 90) or away from it (90 to 0) sample. is the surface that is irradiated, and B is the nonirradiated back. The hatched line illustrates the orientation of the nematic director. Vertical direction is the long axis of the cantilever.
Figure 11.2 A light-driven robotic manipulator.
Figure 11.3 Apparatus for irradiating arches created from photoactive materials from the top and bottom using a blue-green laser with polarization set parallel to the long axis of the arch.
Figure 11.4 Photoinduced snap-through in azobenzene-functionalized polymers. (a) Geometry before irradiation. (b) The edge of instability. (c) Inversion of the geometry following ultrafast snap-through. The ruler in the Figure is in millimeter. (d) High-speed image sequence showing the progress of snap-through. The scale bar is 3 mm.
Figure 11.5 Bidirectional snap-through in monodomain ALCN sample. (a) Original geometry that is illuminated from the bottom; (b) geometry at the limit point; (c) onset of snap-through upward; and (d) the final geometry. (e) Illumination of the sample from the top; (f) the geometry at the limit point; (g) the onset of downward snap-through; and (h) the final geometry after the to-and-fro snap-through.
Figure 11.6 Modulating the onset of snap-through via mechanical design.
Figure 11.7 Priming the sample for photomechanical snap-through using photo fixing with 445 nm irradiation where (a) the original geometry is irradiated for a short time to create (b) an intermediate primed state, which is closer to limit point compared to the prior arch. Then, when needed, the primed state will be irradiated, and it will reach (c) the limit point quickly, and (d) snap-through can ensue.
Figure 11.8 (a) A silvered arch that can be triggered with 445 nm light to undergo ultrafast snap-through and switch between reflective (1) and nonreflective states (0). Triggering light must also have a polarization that is set parallel to the long axis of beam for this to occur. (b) An array of such phototriggered micromirrors can be used to project beams from those incident on them. (c) A potential unit cell of a morphing structure that is composed of a “tripod” made using photomechanical materials that can undergo snap-through. The annular table atop the tripod can adopt various configurations depending on whether/which of the arch-shaped legs are snapped through. The reorientation of the table when one of the arches undergoes snap-through is also shown.
Figure 11.9 Engineering tristable mechanisms using bistable elements. (Chen and Du [24]. Reproduced with the permission of ASME.) Reproduced with permission from Figure 11 of Journal of Mechanisms and Robotics, 2013, 5, 011007 and (Chen
et al.
[25]. Figure 2 of Journal of Mechanisms and Robotics, 2010, 2, 014501.)
Figure 11.10 Stable positions of a rotary device created with individual arch-shaped elements. Dark gray lines illustrate rigid elements, while light gray lines illustrate flexible elements. Replacing the flexible elements with a photoresponsive polymer will allow for the development of a light-driven stepper motor.
Figure 11.11 Multistability of lattices constructed using intersecting helices that mimics the tail sheath of the bacteriophage T4.
Figure 11.12 (a) Transformation of a flat ring into a range of geometries as a function of the overcurvature (). (b) Utilization of the underlying mechanics of overcurvature in consumer products. (c) Nematic director field that can emulate these mechanics in liquid-crystalline polymers. Reproduced with permission from Figure 1 and 2 of [28]. (d) Progressive supercoiling of a ring that is subjected to twist. (e) Creating hinges by bending a metallic measuring tape that is characterized by a preset curvature along the short axis. Bending such a tape will create localized folds, which can be manipulated by modifying the fixturing at the ends.
Chapter 12: Photomechanical Effects in Materials, Composites, and Systems: Outlook and Future Challenges
Figure 12.1 “Ashby plot” relating maximum output force (in compression, N) to maximum stroke (m) for a range of materials.
