Liquid Crystalline Polymer Films Research Articles

Self-organized wrinkling of liquid crystalline polymer with plasma treatment

Abstract

Random Planar Orientation in Liquid-Crystalline Block Copolymers with Azobenzene Side Chains by Surface Segregation.

Rodlike liquid-crystalline (LC) mesogens preferentially adopt a homeotropic orientation by excluded volume effects at the free surface in side-chain LC (SCLC) polymer films. The homeotropic orientation is not advantageous for in-plane LC alignment processes. Surface segregation of polymers is the phenomenon in which one component with a low surface free energy covers the surface in a mixture of two or more polymers or a block copolymer film. In SCLC block copolymer films, the surface segregation structure induces a random planar orientation due to the formation of a microphase-separated interface parallel to the substrate via the covering of one of the segregated polymer blocks. This feature article focuses on the unique, random planar orientation induced by the surface segregation of SCLC block copolymer films with the photoresponsive azobenzene (Az) mesogenic group. A transition moment of the Az mesogens is parallel to the molecular long axis, and light irradiation is conducted perpendicular to the film surface in general photoreaction processes. Therefore, the homeotropic molecular orientation in the SCLC polymer systems with Az mesogenic units inhibits efficient photoreaction reorientations in thin films. The random planar orientations by the surface segregation of a coil block in SCLC block polymers provide efficient in-plane photoreorientation and photoswitching with LC hierarchical mesostructures, such as microphase-separated structures of SCLC block copolymers and laminated LC polymer films. On the other hand, surface-segregated SCLC blocks form a high-density polymer LC brush layer with a random planar orientation by self-assembly, which exhibits efficient angular selective photoreactions. These approaches using the surface segregation of SCLC block copolymers are expected to offer new concepts for the LC photoalignment process for LC polymer devices.

Polarization modulation by means of tunable polarization gratings in an azobenzene side-chain liquid-crystalline polymer film.

Polarization modulation was achieved by means of a new type of grating recorded with two 532nm beams at varying polarization angles in an azobenzene side-chain liquid-crystalline polymer film through light regulation instead of voltage control. In contrast to conventional polarization holographic gratings, the polarization state of the ± first-order diffracted beams of the recorded gratings depended strongly on angles between polarization directions of two recording beams, while the polarization state of the second-order diffraction remained unchanged. With the polarization angle changing from 0 to 90deg, the ± first-order diffraction efficiency increased from 5.15% to 10.53%. Diffraction properties of the recorded gratings were attributed to the combination of polarization holographic gratings and amplitude gratings based on the calculation of Jones matrices and polarization holographic theory.

Two-photon actuation of crosslinked liquid-crystalline polymers utilizing energy transfer system

ABSTRACTWe prepared crosslinked azotolane liquid-crystalline polymer (LCP) films doped with a stilbene derivative (two-photon chromophore) utilizing an interpenetrating polymer network (IPN) structure. The IPN films bend toward the light source upon irradiation with femtosecond laser pulses at 600 nm, which can excite the stilbene derivative by two-photon absorption. The bending speed of the IPN films increases with the square of the laser pulse intensity, which is compelling evidence for the two-photon processes.

Calcium carbonate mineralization in chiral mesomorphic order-retaining ethyl cellulose/poly(acrylic acid) composite films

A CaCO3 mineralization was carried out using a mesomorphic network system composed of ethyl cellulose (EC) and poly(acrylic acid) (PAA), to realize a polymer–inorganic hybrid invested with a chiroptical functionality and thermal and mechanical high-performances. The EC/PAA samples were prepared in colored film form from chiral nematic liquid crystals of EC in acrylic acid (AA) by polymerization and cross-linking of the solvent monomer AA. The mineralization was allowed to proceed in a batchwise operation by immersing and swelling the liquid-crystalline polymer films in an aqueous salt solution providing Ca2+, HCO3−, and additional Mg2+ ions. By varying the ionic condition (i.e., pH and Mg/Ca ratio) of the employed salt solution, calcite (Cal), aragonite (Ara), and solely amorphous CaCO3 (ACC) were separately deposited inside the EC/PAA matrices. Irrespective of the polymorphs of the incorporated CaCO3, the mineralized EC/PAA films (weight gain, ca. 20–28%) retained the original left-handed chiral nematic arrangement and exhibited a reflective color that was shifted to the red side relative to that of the corresponding nonmineralized films. It was also confirmed that the inorganic hybridization remarkably enhanced the thermal and thermomechanical properties of the optically functionalized EC/PAA films. In thermogravimetric analysis data above 300 °C, however, a significant difference was observed between the three sorts of hybrids with Cal, Ara, and ACC, respectively.

Photo-controlled patterned wrinkling of liquid crystalline polymer films on compliant substrates

Photo-chromic liquid crystalline polymer (LCP) is a type of smart materials which are sensitive to light. Here we harness its photo-mechanical response to flexibly control surface patterning, through modeling a film involving homeotropic nematic liquid crystals with director perpendicular to the polymer film attached on a compliant substrate. Theoretical and numerical analyses were conducted to explore the surface instability of such film/substrate systems under both uniform and non-uniform illuminations by ultraviolet (UV) light, respectively. By minimizing energy, the film can buckle into checkerboard patterns at the critical photo load. Fourier spectral method is applied to study the post-buckling evolution of various 3D wrinkling patterns including wavy shaped, ring-like, checkerboard, stripe and herringbone patterns. Besides, non-uniform illumination is investigated through square and circular patterned lights, respectively, where the wrinkles can be ordered and controlled remotely and flexibly. Furthermore, the geometric and size effects of local illumination are discussed and characterized by some critical dimensionless parameters. Phase diagrams are provided and agree with experimental observations, which could be used to guide the photo design of wrinkling morphogenesis that is particularly attractive for remote control applications.

Easily Processable Temperature-Responsive Infrared-Reflective Polymer Coatings

A temperature-responsive near-infrared reflective coating was fabricated based on a side-chain liquid crystal siloxane polymer using a simple wired-bar method. The cholesteric liquid crystalline polymer film showed a blue shift of the reflection band of ∼1000 nm in the IR region upon heating. The temperature-responsive change of the reflection band was reversible. Compared to that of the same mixture system in an alignment cell, the coating showed a significantly faster response. This research demonstrates an easy way to prepare a temperature-responsive IR-reflective coating that shifts its reflection to a shorter wavelength upon heating. As IR radiation of shorter wavelengths is more strongly represented in sunlight than longer wavelengths, this coating could be used to selectively reduce heating of an indoor space when the temperature is high. This is promising for the future application of smart climate control.

Birefringent Pattern Formation in Photoinactive Liquid Crystalline Polymer Films Based on a Photoalignment Technique with Top-Coating of Cinnamic Acid Derivatives via H-Bonds.

The application of a top-coating of 4-methoxy cinnamic acid (MCA) onto a photoinactive liquid crystalline polymeric film containing benzoic acid (BA) side groups (P6BAM) is shown to enable thermally stimulated, photoinduced reorientation of the polymer structure. Annealing the MCA-coated P6BAM films leads to H-bond formation between BA and MCA, which also effectively smooths the film surface. Exposure to linearly polarized (LP) UV light initiates axis-selective photoreaction of the MCA groups; subsequent thermal treatment in the LC temperature range of P6BAM amplifies molecular reorientation of the BA side groups, while simultaneously eliminating the MCA molecules. Selective inkjet coating of MCA provides a facile route for the fabrication of patterned, oriented, and rewritable P6BAM films with multiple controlled alignment directions.

1D05 高分子SmC*液晶フィルムの特性とその応用

1D05 高分子SmC*液晶フィルムの特性とその応用

3c01 分子配向性光配向膜表面のNEXAFSによる配向挙動評価(化学・材料,口頭発表,2013年日本液晶学会討論会)

3c01 分子配向性光配向膜表面のNEXAFSによる配向挙動評価(化学・材料,口頭発表,2013年日本液晶学会討論会)

2a05 オリゴチオフェンドープ型液晶高分子フィルムの光応答性(フォトニクス・光デバイス,口頭発表,2012年日本液晶学会討論会)

2a05 オリゴチオフェンドープ型液晶高分子フィルムの光応答性(フォトニクス・光デバイス,口頭発表,2012年日本液晶学会討論会)

Competitive concurrence of surface wrinkling and dewetting of liquid crystalline polymer films on non-wettable substrates

Polymeric thin films coated on non-wettable substrates undergo film-instabilities, which are usually manifested as surface deformation in the form of dewetting or wrinkling. The former takes place in fluidic films, whereas the latter occurs in solid films. Therefore, there have rarely been reports of systems involving simultaneous deformations of dewetting and wrinkling. In this study, we propose polymeric thin films of liquid crystalline (LC) mesogens prepared on a non-wettable Si substrate and apply a treatment of plasma irradiation to form a thin polymerized layer at the surface. The resulting compressive stress generated in the surface region drives the formation of wrinkles, while at the same time, dipolar attraction between LC molecules induces competitive cohesive dewetting. Intriguing surface structures were obtained whereby dewetting-like hole arrays are nested inside the randomly propagated wrinkles. The structural features are readily controlled by the degree of surface cross-linking, hydrophilicity of the substrates, and the LC film thickness. In particular, dewetting of LC mesogens is observed to be restricted to occur at the trough regions of wrinkles, exhibiting the typical behavior of geometrically confined dewetting. Finally, wrinkling-dewetting mixed structures are separated from the substrate in the form of free standing films to demonstrate the potential applicability as membranes.

A chaotic self-oscillating sunlight-driven polymer actuator.

Nature provides much inspiration for the design of materials capable of motion upon exposure to external stimuli, and many examples of such active systems have been created in the laboratory. However, to achieve continuous motion driven by an unchanging, constant stimulus has proven extremely challenging. Here we describe a liquid crystalline polymer film doped with a visible light responsive fluorinated azobenzene capable of continuous chaotic oscillatory motion when exposed to ambient sunlight in air. The presence of simultaneous illumination by blue and green light is necessary for the oscillating behaviour to occur, suggesting that the dynamics of continuous forward and backward switching are causing the observed effect. Our work constitutes an important step towards the realization of autonomous, persistently self-propelling machines and self-cleaning surfaces powered by sunlight.

Orientation Direction Control in Liquid Crystalline Photoalignable Polymeric Films by Adjusting the Free-Surface Condition.

Adjusting the free-surface condition facilely controls the in-plane and out-of-plane orientations in liquid crystalline polymer (LCP) films. Top coating with aromatic molecules onto LC polymethacrylate films with N-benzylideneaniline (NBA) or 4-methoxybiphenyl (MB) side groups (PNBAM or PMBM) and subsequent annealing generate a random planar orientation while simultaneously removing the coated aromatic molecules, whereas annealing noncoated films induces a homeotropic orientation of the mesogenic side groups. Additionally, irradiating a top-coated PNBAM film with linearly polarized (LP) 365 nm light induces an in-plane molecular reorientation of the NBA side groups without changing the orientation in the homeotropically oriented region. Changes in the surface topology of the LCP films due to the reorientation processes are investigated in detail. Inkjet coating with aromatic molecules and LP 365 nm light exposure precisely controls the in-plane and out-of-plane alignment pattern in a PNBAM film.

Influence of film thickness on the reorientation structure of photoalignable liquid crystalline polymer films

Polarized UV absorption and near-edge X-ray absorption fine structure (NEXAFS) spectroscopies are used to investigate the influence of film thickness on the thermally stimulated photoinduced reorientation of a polymethacrylate film comprised of 4-methoxycinnnamoylbiphenyl (MCB) side groups connected with a decylene spacer (PMCB10M) in the bulk (>10 nm), at the inner-surface (∼10 nm), and at the near-surface (<2 nm). Irradiation with linearly polarized (LP) UV light and subsequent annealing generate a biaxial molecular reorientation in the bulk of thick PMCB10M films, while the out-of-plane orientation is restricted in films less than 55-nm thick. In contrast, the in-plane reorientation is dominant at the near-surface regardless of the film thickness. Consequently, the roles of the rigid mesogenic MCB moieties and the flexible long decylene spacer depend on the film thickness.

Free Surface Command Layer for Photoswitchable Out-of-Plane Alignment Control in Liquid Crystalline Polymer Films.

To date, reversible alignment controls of liquid crystalline materials have widely been achieved by photoreactive layers on solid substrates. In contrast, this work demonstrates the reversible out-of-plane photocontrols of liquid crystalline polymer films by using a photoresponsive skin layer existing at the free surface. A polymethacrylate containing a cyanobiphenyl side-chain mesogen adopts the planar orientation. Upon blending a small amount of azobenzene-containing side-chain polymer followed by successive annealing, segregation of the azobenzene polymer at the free surface occurs and induces a planar to homeotropic orientation transition of cyanobiphenyl mesogens underneath. By irradiation with UV light, the mesogen orientation turns into the planar orientation. The orientation reverts to the homeotropic state upon visible light irradiation or thermally, and such cyclic processes can be repeated many times. On the basis of this principle, erasable optical patterning is performed by irradiating UV light through a photomask.

Photoresponsive Liquid-Crystalline Polymer Films Bilayered with an Inverse Opal Structure

Photoresponsive Liquid-Crystalline Polymer Films Bilayered with an Inverse Opal Structure

Alignment structures and diffraction properties of chiral nematic liquid crystal cells with periodically patterned photoalignment films

Liquid crystal (LC) cells with periodic alignment distributions were fabricated using chiral nematic LCs (N*LCs), which were prepared using mixtures of a nematic LC and a chiral dopant, along with photoreactive liquid crystalline polymer (PLCP) films. Periodic structures were formed by polarization holographic recording in the PLCP films. The director distribution in each cell depended on the ratio of chiral dopant present, i.e., the inherent helical pitch of the N*LCs. These periodic alignment structures with line defects in the LC grating cells were well explained on the basis of the elastic continuum theory of the N*LCs and the photoalignment effect of the PLCP films. The diffraction properties of the grating LC cells were also investigated using a polarized visible laser. The observed intensity and polarization states of the diffracted beams were consistent with theoretical ones calculated using the director distribution models. Our results clarify that the diffraction properties of the grating LC cells can be controlled by the helical pitch of the N*LCs.

High-performance liquid-crystalline polymer films for monolithic “composites”

Highly oriented, free-standing films of a thermotropic liquid-crystalline polymer (LCP) were prepared using a new processing technique, referred to as “foil-spintrusion”. These films underwent solid-state fusion at elevated temperatures to yield homogeneous, self-reinforced monolithic “composites” without the use of additional adhesives. In this study, unidirectional LCP monoliths were evaluated for their mechanical performance. The monoliths exhibited highly anisotropic behavior, with stiffness and tensile strength along the direction of orientation of around 65GPa and 1.7GPa, respectively, combined with excellent damping properties in the viscoelastic regime. In addition, experiments showed that the material has a high tensile strength (>2.2GPa) at high strain rates, indicating promising behavior under high impact conditions. As expected for such highly oriented polymer systems, the monoliths were found to exhibit poor compressive and shear strengths due to low internal coherence of the material itself. However, it was shown that the LCP monoliths could be useful for applications requiring high specific stiffness, for example as facings for composite sandwich panels.

Photoalignment of Liquid Crystalline Polymer Films Containing Mesostructures and a Free Surface Command Layer

Investigations on the photoaglinment processes of liquid crystalline materials started around 25 years ago, and currently great developments have been made in both academic and industrial areas. However, there remain many issues to be explored in this field. This short review introduces our recent advances regarding the photoalignment and realignment process using azobenzene-containing liquid crystalline polymer materials. Two topics are involved here: the photoalignment of microphase separation structures of block copolymer films, and photoalignment controls from the free surface of a liquid crystalline polymer film.