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ABSTRACT Liquid crystal (LC) concentrations in stratified holographic polymer-dispersed liquid crystal (PDLC) films have been quantified from depth profiles obtained by time-of-flight secondary ion mass spectrometry (SIMS). The volatile... more
ABSTRACT Liquid crystal (LC) concentrations in stratified holographic polymer-dispersed liquid crystal (PDLC) films have been quantified from depth profiles obtained by time-of-flight secondary ion mass spectrometry (SIMS). The volatile nature of the LCs was hindered during SIMS analysis by capping the PDLC samples with poly(vinyl alcohol) and cooling to cryogenic temperatures. It remains difficult to gain quantitative results from SIMS analysis due to matrix effects yielding complex SIMS fingerprint spectra. With the multivariate statistical method discriminant function analysis the LC contents in the stratified holographic PDLC films were quantified. The concentration of the LC-rich layers was determined to be 32.9 ± 3.4 wt % LC, and for the polymer-rich layers it was 28.8 ± 2.7 wt % LC. The low concentration difference was supported by imaging analysis and modeling results.
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In light-driven liquid-crystal network (LCN) actuators, large performance improvements are obtained by varying the orientation of the molecular director through the thickness of the film actuator. Experiments show that sub-millimeter... more
In light-driven liquid-crystal network (LCN) actuators, large performance improvements are obtained by varying the orientation of the molecular director through the thickness of the film actuator. Experiments show that sub-millimeter bending radii are achieved using a splayed molecular orientation. Systems with a splayed or twisted nematic (TN) director profile drive greater amplitude and faster bending than uniaxial planar systems with the same chemical composition. The bending radii of these systems are predicted using a simple model including effects of light intensity, material composition and actuator thickness.
Research Interests: Materials Science, Mechanics, Liquid Crystals, Photochemistry, Soft Matter, and 15 moreMicroscale Chemistry, Medicine, Elasticity, Computer Simulation, Mathematical Sciences, Photometry, Physical sciences, Phase transition, Bending, Actuator, Liquid Crystal, Miniaturization, Equipment Design, Equipment Failure Analysis, and Planar
Controlled shape changes of polymerized liquid crystalline coatings is often achieved via prepatterning the molecular orientation of liquid crystal (LC) monomers at the stage of preparation. In this work, using the so-called hybrid... more
Controlled shape changes of polymerized liquid crystalline coatings is often achieved via prepatterning the molecular orientation of liquid crystal (LC) monomers at the stage of preparation. In this work, using the so-called hybrid alignment of the LC, we produce surface structures of positive Gaussian curvature of coatings without complex techniques such as photoalignment. A mixture of LC monomers coated onto a glass plate with planar alignment of the director is exposed to air, which promotes vertical alignment. The competing planar and homeotropic boundary conditions result in a) thickness dependent director and b) spontaneous formation of spindle-like regions, limited by disclination loops, that are called the reverse tilt domains (RTDs). The disclination separates different director configurations inside and outside the RTD. The RTDs produce relatively big protrusions (100 − 600 nm) of the LC network coating. Actuation of the coating by heat increases the amplitude of RTD protrusions.
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UV-curable polyetherurethane acrylates and poly- (dimethyl-co-methylphenyl siloxane) acrylates have been investigated for the primary buffer coating of optical fibers. Curing behavior, Emodulus, glass transition temperature and refractive... more
UV-curable polyetherurethane acrylates and poly- (dimethyl-co-methylphenyl siloxane) acrylates have been investigated for the primary buffer coating of optical fibers. Curing behavior, Emodulus, glass transition temperature and refractive index have been studied in relation to the molecular structure of both acrylate types. The benefits of the appropriate composition with respect to fiber drawing speed and performance are ultrafast curing, low operation
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ABSTRACT Inkjet printing and spin-coating have been used to prepare patterns using a silver-containing metallo-organic decomposition ink. The patterned ink was reduced to silver by exposure to UV light and subsequent treatment with... more
ABSTRACT Inkjet printing and spin-coating have been used to prepare patterns using a silver-containing metallo-organic decomposition ink. The patterned ink was reduced to silver by exposure to UV light and subsequent treatment with hydroquinone solution. This process, which took less than a minute, was performed at room temperature, which allowed low glass transition temperature polymeric substrates, such as PET, to be used. The conductivity of the silver patterns was found to be 10% that of bulk silver. The mechanical stability was also measured, with a linear increase in resistance seen for increasing strain, and no significant change in resistance seen after 12 000 cyclic deformations.
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A review with 35 refs. on various possibilities of making structured, oriented polymer films by in situ photopolymn. of pre-ordered liq.-cryst. monomers. Formation of uniaxially oriented films and their basic process parameters and the... more
A review with 35 refs. on various possibilities of making structured, oriented polymer films by in situ photopolymn. of pre-ordered liq.-cryst. monomers. Formation of uniaxially oriented films and their basic process parameters and the formation of superstructures with high degrees of director control are discussed. Photoinitiated polymn. of pre-oriented mesogenic polyfunctional monomers yields films of densely crosslinked polymers with a monodomain liq.-cryst. mol. order. The morphol. of the monomer mesogenic phase is fixed by the rapid photocrosslinking, which enables the formation of either nematic or one of the various smectic structures, depending on the phase behavior of the monomers. The desired macroscopic mol. order in the monomeric state can be accomplished by using techniques such as external fields or surface-induced orientation. Surface structures are also produced by replication polymn., and combinations of these techniques enable the modulation of the orientational director in the plane of the films and also into the third dimension perpendicular to the film surface. Photocrosslinking enables lithog. techniques to fix the director selectively by local polymn., before permanently fixing the total structure. The introduction of chiral centers in the liq.-cryst. monomers produces helicoidally ordered networks, whose pitch of the mol. helix can be adjusted accurately by compn. or by polymn. temp. This fine tuning of 3-D mol. orientation within thin polymeric films makes this process very unique in the world of oriented polymers.
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Controlling sophisticated motion by molecular motors is a major goal on the road to future actuators and soft robotics. Taking inspiration from biological motility and mechanical functions common to artificial machines, responsive small... more
Controlling sophisticated motion by molecular motors is a major goal on the road to future actuators and soft robotics. Taking inspiration from biological motility and mechanical functions common to artificial machines, responsive small molecules have been used to achieve macroscopic effects, however, translating molecular movement along length scales to precisely defined linear, twisting and rotary motions remain particularly challenging. Here, we present the design, synthesis and functioning of liquid‐crystal network (LCN) materials with intrinsic rotary motors that allow the conversion of light energy into reversible helical motion. In this responsive system the photochemical‐driven molecular motor has a dual function operating both as chiral dopant and unidirectional rotor amplifying molecular motion into a controlled and reversible left‐ or right‐handed macroscopic twisting movement. By exploiting the dynamic chirality, directionality of motion and shape change of a single motor embedded in an LC‐network, complex mechanical motions including bending, walking and helical motion, in soft polymer materials are achieved which offers fascinating opportunities toward inherently photo‐responsive materials.
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Polymers with a controlledmolecular organization in all three dimensions are of interest because of their unusual, but very accurately adjustable and addressable optical, electrical, and mechanical properties. An established method to... more
Polymers with a controlledmolecular organization in all three dimensions are of interest because of their unusual, but very accurately adjustable and addressable optical, electrical, and mechanical properties. An established method to produce 3-D ordered polymers is the photoinitiated polymerization of liquid crystal (LC) monomers [1-4]. The variety in possible LC phases of low-molar-mass reactive mesogens provides diversity in the choice of the type of the molecular order, all being accessible to be fixed by the polymerization process [5-8]. Known techniques to establish monolithic molecular order in LCs, such as rubbed surfaces, surfactant-treated surfaces, external electric, or magnetic fields or flow, can be applied or even can be combined with each other to create films of even more complex molecular architectures [9]. The molecular structure of LC monomers can be tailored, for example, to optimize on the mechanical and optical properties of the films. Furthermore, blends of monomers can be made to adjust the properties in the monomeric state, such as the LC transition temperatures and the flow viscosity, and in the polymeric state, such as the elastic modulus, the glass transition (Tg) temperature, and the refractive indices.
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The light and energy-efficiency of classical liquid crystal displays is notoriously poor due to the use of absorption-based linear polarisers and colour filters. For instance, the light efficiency of PVAL polarisers is typically between... more
The light and energy-efficiency of classical liquid crystal displays is notoriously poor due to the use of absorption-based linear polarisers and colour filters. For instance, the light efficiency of PVAL polarisers is typically between 40 and 45 % and the colour filters have a typical efficiency below 35 % which results in a total light and energy-efficiency of the display below 10%. In the past, a variety of polarizers were developed with an enhanced efficiency in generating linearly polarized light. Typically, these polarizers are based on the polarisation-selective reflection, scattering or refraction of light: i.e. one polarisation direction of light is directly transmitted to the LCD/viewer and the other polarization direction of light is depolarised and recycled which results in a typical efficiency for generating linearly polarized light of 70-85 %. Also, special colour filters have been proposed based on chiral-nematic reactive mesogens which increase the efficiency of generating colour. Despite the enormous progress in this field, a need persists for improved methods for generating polarized light and colour based on low cost optical components with a high efficiency. Here, the use of holographic phase gratings is reported for the generation of polarized light and colour. The phase grating are recorded in a photopolymer which is coated onto a back-or frontlight for LCDs. Typically the recording is performed in the transmisson mode or in the waveguiding mode and slanted phase gratings are generated with their refractive index modulation at an angle between 20° and 45° with the normal of the substrate. It is shown that phase gratings with a high refractive index modulation and a high efficiency can be generated by a proper selection of the photopolymer and illumination conditions. These phase gratings couple-out linearly polarized light with a high contrast (> 100) and the light is directed directly to the LCD/viewer without the need for redirection foils. Dependent on the type of phase grating, the different colours are coupled-out at a slightly different angle which potentially increases the efficiency of classical colour filters. Moreover, the phase gratings are completely transparent in direct view which opens the possibility to use them in frontlights for LCDs. Holographic polarization gratings posses a periodic pattern in the polarization state of light (and not in the intensity of light). A periodic pattern in the polarization direction of linearly polarized light is obtained upon interference of two circularly polarized laser beams. In the second part of the lecture, it is shown that these periodic polarization patterns can be recorded in a linear photo-polymerizable polymer (LPP) and that such an alignment layer induces a period rotation in the director of (reactive and non-reactive) liquid crystals. By a proper design, optical components can be produced with only first order diffraction and with a very high efficiency (> 0.98). It is shown that these diffraction gratings are potentially useful in projection displays with a high brightness and energy efficiency.
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TiO2 nanoparticle–photopolymer composites have been employed for volume holographic recording, as reported by Sanchez and co-workers on p. 1623. Photoinduced segregation of the high refractive index, grafted nanoparticles between... more
TiO2 nanoparticle–photopolymer composites have been employed for volume holographic recording, as reported by Sanchez and co-workers on p. 1623. Photoinduced segregation of the high refractive index, grafted nanoparticles between polymer-rich areas leads to improved refractive-index modulation amplitudes with respect to the base material without nanoparticles. The cover schematically shows a holographic grating registered in this nanocomposite material. These nanocomposite materials should enable the production of holographic optical elements to efficiently control light with angle and wavelength selectivity. This could be used, for example, in liquid-crystal display technology. A new and efficient photopolymer for the recording of volume holograms is presented. The material comprises a mixture of UV-sensitive acrylates and grafted titanium dioxide nanoparticles with an average size of 4 nm. We report the formation of holographic gratings with refractive-index modulation amplitudes of up to 15.5 × 10–3—an improvement of more than a factor of four over the base material without nanoparticles—while maintaining a low level of scattering and a high transparency in the visible-wavelength range. The influence of the composition of the acrylate system on the final properties of the holographic material is also investigated and discussed. The presence of multifunctional monomers favors the compositional segregation of the different components, while the addition of monofunctional acrylate, highly compatible with the grafting of the nanoparticles, favors the dilution of these nanoparticles.
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ABSTRACT Liquid crystalline polymethacrylates with benzanilide and photochromic azobenzene side groups and a related terpolymer containing an additional side group with a phenylethynyl substituted anthracene chromophore were oriented by... more
ABSTRACT Liquid crystalline polymethacrylates with benzanilide and photochromic azobenzene side groups and a related terpolymer containing an additional side group with a phenylethynyl substituted anthracene chromophore were oriented by the irradiation with linearly polarized light. The orientation of both polymers were compared irradiating with polarized visible or, alternatively, UV light. Caused by the co-operativity of the photoorientation process, the light-induced orientation of the azobenzene groups is connected to the alignment of the non-photochromic side groups below the glass transition temperature. The light-induced order generated in the glassy state was significantly amplified by the subsequent annealing of the irradiated films at temperatures in the mesophases. Factors of amplification of about 30 were found in the case of both polymers. The photo-induction process and its amplification by thermotropic self-organization were investigated in dependence on the polymer composition, the irradiation dose and the wavelength of the incident light with respect to the absorption of the dye and its limited photo-stability. The required dose or the irradiation time, respectively, were significantly reduced by the optimization of the light-induced and thermal processing. In this way, dichroic films of co- and terpolymers were created. However, the green fluorescence of the anthracene chromophore is effectively quenched by the azobenzene side group within the film.
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Control over main‐chain motion of chiral π‐conjugated polymers can lead to unexpected new functionalities. Here, it is shown that by combining photoswitchable azobenzene units in conjugation with chiral fluorene comonomers and appropriate... more
Control over main‐chain motion of chiral π‐conjugated polymers can lead to unexpected new functionalities. Here, it is shown that by combining photoswitchable azobenzene units in conjugation with chiral fluorene comonomers and appropriate plasticizers, the polymer organization and chiroptical properties of these alternating copolymers steered by light and its state of polarization can be dynamically controlled. The configuration of the stereogenic centers in the side chains of the fluorene units determines the handedness of the cholesteric organization in thermally annealed films, indicating cooperative behavior. The polymer alignment and helicity of the supramolecular arrangement can be switched by irradiating with linearly and circularly polarized light, respectively. Intriguingly, when switching the handedness of thermally induced cholesteric organizations by illuminating with circularly polarized light that is opposite to the handedness of the cholesteric phases, a nematic‐like intermediate state is observed during helix interconversion. By the sequence of irradiation with left and right circularly polarized light followed by thermal annealing, an asymmetric motion, reminiscent of that seen in molecular motors is observed. These findings suggest that functional conjugated polymers can exhibit emergent properties at mesoscopic scale.
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Preface Editors Contributors Densely Cross-Linked Systems Densely Cross-Linked Liquid Crystal Networks by Controlled Photopolymerization of Ordered Liquid Crystal Monomers: Properties and Applications, Johan Lub and Dirk J. Broer... more
Preface Editors Contributors Densely Cross-Linked Systems Densely Cross-Linked Liquid Crystal Networks by Controlled Photopolymerization of Ordered Liquid Crystal Monomers: Properties and Applications, Johan Lub and Dirk J. Broer Spatially Ordered Polymers Self-Assembled in Ordered Liquid Crystal Templates, Liang-Chy Chien, Carmen Otilia Catanescu, and Lanfang Li Responsive Reactive Mesogen Microstructure, Darran R. Cairns, Matthew E. Sousa, and Gregory P. Crawford Viewing Angle Compensation Films for LCD Using Reactive Mesogens, Ichiro Amimori and Tokuju Oikawa Interferometric Applications Using Liquid Crystalline Networks, Toralf Scharf and Gerben Boer Anisotropic Emitting Cross-Linked Polymers Based on Liquid Crystals, L. Oriol, M. Pinol, and Jose Luis Serrano Photomechanical Effects of Cross-Linked Liquid-Crystalline Polymers, Jun-ichi Mamiya, Yanlei Yu, and Tomiki Ikeda Photoreactive Processes for Flexible Displays and Optical Devices, Sin-Doo Lee and Jae-Hoon Kim Polymer MEMS, Casper L. van Oosten, Cees W.M. Bastiaansen, and Dirk J. Broer Polymerizable Liquid Crystal Networks for Semiconductor Applications, Maxim N. Shkunov, Iain McCulloch, and Theo Kreouzis Reactive Mesogens in Organic Light-Emitting Devices, Peter Strohriegl Weakly Cross-Linked Systems: Liquid Crystal Elastomers Physical Properties of Liquid Crystalline Elastomers, Eugene M. Terentjev Lagrange Elasticity Theory of Liquid Crystal Elastomers, Tom C. Lubensky and Olaf Stenull Orientational Order and Paranematic-Nematic Phase Transition in Liquid Single Crystal Elastomers: Nuclear Magnetic Resonance and Calorimetric Studies, Bostjan Zalar, Zdravko Kutnjak, Slobodan Zumer,and Heino Finkelmann Computer Simulations of Liquid Crystal Polymeric Networks and Elastomer, G. Skacej and Claudio Zannoni Electromechanical Effects in Swollen Nematic Elastomers, Kenji Urayama and Toshikazu Takigawa Smectic Elastomers, Mark Warner Physical Properties of Magnetic Gels, Helmut R. Brand, Philippe Martinoty, and Harald Pleiner Side-On Nematic Liquid-Crystalline Elastomers for Artificial Muscle Applications, Min-Hui Li and Patrick Keller Index
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Numerous established and newly emerging technologies are taking advantage of the novel properties of liquid crystalline (LC) materials. The unique optical, electro-optical and mechanical properties of LC fluids, polymers and composites... more
Numerous established and newly emerging technologies are taking advantage of the novel properties of liquid crystalline (LC) materials. The unique optical, electro-optical and mechanical properties of LC fluids, polymers and composites enable applicability and significant improvements in a number of advanced technologies including displays, telecommunication, information storage and mechanical actuation. Advances in both low-molar mass and polymeric systems, ranging from synthesis to device physics, enable the continued integration of this class of materials into real-world applications. This book spans a diverse set of LC-related topics, from reports on the latest hot research areas, including flexoelectro-optics, V-shaped switching, chiral discotics, and banana phases, to developments in LC-polymer composite systems and LC gels. Much of the work has potential applicability, ranging from sophisticated photonics and beam-steering to creation of 'electronic paper'. Topics include: smectic and chiral liquid crystals and gels; photo-polymerization and liquid crystals; emerging concepts in liquid crystals; rheology of liquid crystals and main chain/lyotropic systems.
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Crosslinked Liquid Crystal Elastomers (CrLCEs) are at the forefront of the development of optoelectronics and photonics. Over the years, a materials toolset has been developed ranging from bulk synthesised nematic monomers to dedicated... more
Crosslinked Liquid Crystal Elastomers (CrLCEs) are at the forefront of the development of optoelectronics and photonics. Over the years, a materials toolset has been developed ranging from bulk synthesised nematic monomers to dedicated smectic and chiral monomers with advanced applications such as polarisation optics, iridescent coatings, and photo responsive mechanics. CrLCEs are often prepared through a two-step approach. First, conventional reactive liquid crystal monomers are chain-extended into (short) oligomer chains which are then crosslinked to form the elastomer network. The advantage of this approach is that it gives access to a wide range of processing methods, including conventional techniques suited for liquid crystal monomers, but also emerging processing techniques such as 3D- or roll-to-roll printing. Adding chiral molecules to a nematic oligomer forms the chiral nematic or cholesteric liquid crystal phase, well-known for its characteristic helicoidal ordering of the calamitic molecules. Currently we are interested in the effect of the position of the chiral component in relation to the elastomer network: either as part of the oligomer main chain or as a pendant group to it. In this contribution we found that the dopant position may influence the way in which the cholesteric material aligns. The main chain dopant follows behaviour previously reported in similar systems, while the side chain dopant adds more process-related complexity to the optical properties. With this work we aim to add a new design consideration to the already versatile platform of CrLCEs.
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This chapter contains sections titled: Introduction Materials and their Photoresponsive Behavior Single-exposure Photoembossing Dual-exposure Photoembossing Complex Surface Structures from Interfering UV Laser Beams Surface Structure... more
This chapter contains sections titled: Introduction Materials and their Photoresponsive Behavior Single-exposure Photoembossing Dual-exposure Photoembossing Complex Surface Structures from Interfering UV Laser Beams Surface Structure Development under Fluids Conclusion
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ABSTRACT
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Polymers with a controlledmolecular organization in all three dimensions are of interest because of their unusual, but very accurately adjustable and addressable optical, electrical, and mechanical properties. An established method to... more
Polymers with a controlledmolecular organization in all three dimensions are of interest because of their unusual, but very accurately adjustable and addressable optical, electrical, and mechanical properties. An established method to produce 3-D ordered polymers is the photoinitiated polymerization of liquid crystal (LC) monomers [1-4]. The variety in possible LC phases of low-molar-mass reactive mesogens provides diversity in the choice of the type of the molecular order, all being accessible to be fixed by the polymerization process [5-8]. Known techniques to establish monolithic molecular order in LCs, such as rubbed surfaces, surfactant-treated surfaces, external electric, or magnetic fields or flow, can be applied or even can be combined with each other to create films of even more complex molecular architectures [9]. The molecular structure of LC monomers can be tailored, for example, to optimize on the mechanical and optical properties of the films. Furthermore, blends of monomers can be made to adjust the properties in the monomeric state, such as the LC transition temperatures and the flow viscosity, and in the polymeric state, such as the elastic modulus, the glass transition (Tg) temperature, and the refractive indices.