Chiral Nematic Cellulose Acetate Films; Reflectivity and Effect of Deacetylation

Cellulose acetate samples with a range of degrees of substitution (DS) were prepared by deacetylation of cellulose acetate (DS = 2.5). Chiral nematic solutions of the samples were prepared in formic acid. A handedness reversal from left to right was observed as the DS used to prepare the mesophase increased from 1.8 to 2.4. By selection of a suitably long-pitch mesophase, chiral nematic films cast from a CA (DS = 2.2)/formic acid solution showed a feint reflection of circularly polarized blue light. Deacetylation of this CA film gave a chiral nematic cellulose film with a more intense reflection band at the same wavelength. The improvement in intensity is attributed to the higher intrinsic birefringence of cellulose compared to cellulose acetate.     

Photocontrolled manipulation of a microscale object: a rotational or translational mechanism

In this paper the photocontrolled manipulation of solid materials on the surface of a liquid crystalline thin film is described. Three different types of films namely cholesteric liquid crystal (ChLC), compensated nematic liquid crystal (NLC) and nematic LC were used. The rotational and translational manipulation of the microscale solid object was induced by irradiation of light and mode of manipulation (either translational or rotational) was changed by changing the isomer of the azobenzene compound used to make the film. Rotational motion of the object was observed on the ChLC and compensated NLC films containing chirally pure azobenzene compound. The direction of rotational motion was controlled either by changing the optical isomer of the chiral azobenzene or by changing the irradiating light (from ultraviolet to visible). When racemic mixture of the chiral azobenzene compound was used, a translational motion of the object was observed. Even though the direction of the translational motion can be controlled by controlling irradiation position, more facile and precise manipulation of the objects was possible by spatially controlled irradiation of Ar(+) laser and diode UV laser.     

Structure of the B4 liquid crystal phase near a glass surface

The B4 liquid crystal phase of bent-core molecules, a smectic phase of helical nanofilaments, is one of the most complex hierarchical self-assemblies in soft materials. We describe the layer topology of the B4 phase of mesogens in the P-n-OPIMB homologous series near the liquid crystal/glass interface. Freeze-fracture transmission electron microscopy reveals that the twisted layer structure of the bulk is suppressed, the layers instead forming a structure with periodic layer undulations, with the topography depending on the distance from the glass. The surface layer structure is modeled as parabolic focal conic arrays generated by equidistant parabolas whose foci are defect lines along the glass surface. Nucleation and growth of toric focal conics near the glass substrate is also observed. Although the growth of twisted nanofilaments, the usual manifestation of structural chirality in the B4 phase, is suppressed near the surface, the smectic layers are intrinsically chiral, and the helical filaments that form on top of them grow with specific handedness.     

Structural Transition in Liquid Crystal Bubbles Generated from Fluidic Nanocellulose Colloids

The structural transition in micrometer‐sized liquid crystal bubbles (LCBs) derived from rod‐like cellulose nanocrystals (CNCs) was studied. The CNC‐based LCBs were suspended in nematic or chiral nematic liquid‐crystalline CNCs, which generated topological defects and distinct birefringent textures around them. The ordering and structure of the LCBs shifted from a nematic to chiral nematic arrangement as water evaporation progressed. These packed LCBs exhibited a specific photonic cross‐communication property that is due to a combination of Bragg reflection and bubble curvature and size.     

Coassembly of Nanorods and Nanospheres in Suspensions and in Stratified Films

The entropically driven coassembly of nanorods (cellulose nanocrystals, CNCs) and nanospheres (dye‐labeled spherical latex nanoparticles, NPs) was studied in aqueous suspensions and in solid films. In mixed CNC‐latex suspensions, phase separation into an isotropic latex‐NP‐rich and a chiral nematic CNC‐rich phase took place; the latter contained a significant amount of latex NPs. Drying the mixed suspension resulted in CNC‐latex films with planar disordered layers of latex NPs, which alternated with chiral nematic CNC‐rich regions. In addition, fluorescent latex NPs were embedded in the chiral nematic domains. The stratified morphology of the films, together with a random distribution of latex NPs in the anisotropic phase, led to the films having close‐to‐uniform fluorescence, birefringence, and circular dichroism properties.     

Controlled production of patterns in iridescent solid films of cellulose nanocrystals

A method to produce predefined patterns in solid iridescent films of cellulose nanocrystals (CNCs) by differential heating of aqueous CNC suspensions during film casting has been discovered. Placing materials of different temperatures beneath an evaporating CNC suspension results in watermark-like patterns of different reflection wavelength incorporated within the final film structure. The patterned areas are of different thickness and different chiral nematic pitch than the surrounding film; heating results in thicker areas of longer pitch. Thermal pattern creation in CNC films is proposed to be caused by differences in evaporation rates and thermal motion in the areas of the CNC suspension corresponding to the pattern-producing object and the surrounding, unperturbed suspension. Pattern formation was found to occur during the final stages of drying during film casting, once the chiral nematic structure is kinetically trapped in the gel state. It is thus possible to control the reflection wavelength of CNC films by an external process in the absence of additives.     

Alignment of liquid crystals using Langmuir‒Blodgett films of unsymmetrical bent-core liquid crystals

ABSTRACT

The properties of the thin films of liquid crystal (LC) molecules can be governed easily by external fields. The anisotropic structure of the LC molecules has a large impact on the electrical and optical properties of the film. The Langmuir monolayer (LM) of LC molecules at the air–water interface is known to exhibit a variety of surface phases which can be transferred onto a solid substrate using the Langmuir?Blodgett (LB) technique. Here, we have studied the LM and LB films of asymmetrically substituted bent-core LC molecules. The morphology of LB film of the molecules is found to be a controlling parameter for aligning bulk LC in the nematic phase. It was found that the LB films of the bent-core molecules possessing defects favour the planar orientation of nematic LC, whereas the LB films with fewer defects show homeotropic alignment. The defect in LB films may introduce splay or bend distortions in the nematic near the alignment layer which can govern the planar alignment of the bulk LC. The uniform layer of LB film facilitates the molecules of nematic to anchor vertically due to a strong van der Waals interaction between the aliphatic chains leading to a homeotropic alignment.     

手征性侧链液晶高分子取向结构的研究

用偏光显微镜,红外二色性和Ｘ 射线衍射研究了一种手征性侧链液晶高分子的相态织构和弛豫行为．偏光显微镜观察这种侧链液晶高分子冻结取向液晶态薄膜时,可观察到与剪切方向垂直的明暗相间的条带织构．红外二向色性的结果表明,取向态中侧链上的介晶基元倾向于与剪切方向垂直排列．取向和非取向膜的Ｘ射线衍射揭示了该侧链液晶高分子具有反铁电性液晶的两套反相螺旋结构．取向薄膜在液晶态的弛豫行为表明,取向作用能促进侧链高分子近晶相层状结构的生长,而且介晶基元的取向在弛豫过程中能保持下来．     

Electrically induced and thermally erased properties of side-chain liquid crystalline polymer/liquid crystal/chiral dopant composites

The properties of synthesized side-chain liquid crystalline polymer (SCLCP)/liquid crystal (LC)/chiral dopant composites having a chiral nematic (N*) phase at room temperature were investigated by polarized optical microscopy (POM) and a UV/VIS/NIR spectrophotometer. The composite exhibited a planar texture after it was filled into cells under homogeneous boundary conditions and it was transparent. When an electric field was applied to the composite, a focal conic texture was formed and the composite became light scattering. After the electric field was turned off, the light-scattering state remained stable for some time, i.e. the light-scattering state exhibited a memory effect. The focal conic texture changed into the planar texture when the composite was heated and the composite became transparent again. Therefore, the composite had electrically induced and thermally erased properties. The SCLCP had some influence on the memory effect and on the thermo-electro-optical properties of the composite.     

Optical and surface morphological properties of polarizing films fabricated from a chromonic dye by the photoalignment technique

The inherent chromonic lyotropic liquid crystalline properties of a dye have been manipulated to fabricate multi-axial micropolarizing thin films by means of the photoalignment technique. The dye aqueous solution is deposited on a photopatterned polymer film to result in the macroscopic alignment of the dye molecules, followed by drying at ambient temperature. The solid polarizing dye layers thus produced exhibit very a high contrast ratio and degree of polarization in the region of visible light. Addition of a small amount of surfactant to the dye solution is a prerequisite for the generation of a nematic chromonic phase and for the formation of homogeneous thin dye layers on the polymer film. The correlation between the optical and surface morphological properties of the dye layers is discussed.     

Electrically induced and thermally erased properties of side‐chain liquid crystalline polymer/liquid crystal/chiral dopant composites

The properties of synthesized side‐chain liquid crystalline polymer (SCLCP)/liquid crystal (LC)/chiral dopant composites having a chiral nematic (N*) phase at room temperature were investigated by polarized optical microscopy (POM) and a UV/VIS/NIR spectrophotometer. The composite exhibited a planar texture after it was filled into cells under homogeneous boundary conditions and it was transparent. When an electric field was applied to the composite, a focal conic texture was formed and the composite became light scattering. After the electric field was turned off, the light‐scattering state remained stable for some time, i.e. the light‐scattering state exhibited a memory effect. The focal conic texture changed into the planar texture when the composite was heated and the composite became transparent again. Therefore, the composite had electrically induced and thermally erased properties. The SCLCP had some influence on the memory effect and on the thermo‐electro‐optical properties of the composite.     

Defect formation in N* LCs via photovoltaic fields: impact of surface treatment

ABSTRACT

Field-induced defect creation was investigated in chiral nematic liquid crystal test cells, where a Fe-doped lithium niobate thin-cut was applied as field generating substrate. These photo responsive test cells were exposed to focused continuous wave laser light, which resulted in the formation of localised defects. The covering indium tin oxide coated glass of the test cells was treated with a polyvinyl alcohol alignment layer. This alignment layer was either applied with a doctor blade technique or by spin casting of a polyvinyl solution, which was thermally annealed, subsequently. The surface roughness of the alignment layers was investigated with atomic force microscopy, which was interesting to understand how defects and locally lying helices were supported or suppressed in various samples. Well-confined circular defects were formed in test cells with spin-casted alignment layers and the appearance and unwanted interaction with oily streaks could be efficiently reduced in these samples. The results are promising. Samples with an oily streak-free homogenous standing helix orientation of the chiral nematic liquid crystal are an ideal canvas for rewritable pattern formation in a chiral nematic LC: Such samples have high potential as reconfigurable diffractive elements such as rewritable diffraction gratings.     

Preparation of Poly(3,4‐ethylenedioxythiophene) in a Chiral Nematic Liquid‐Crystal Field

Summary: The 3,4‐ethylenedioxythiophene (EDOT) monomer in a chiral nematic liquid‐crystal electrolyte was polymerized by application of a voltage to yield a thin film. Circular dichroism measurements indicated a Cotton effect for the film. Optical texture suggests that the polymer shows a finger‐print texture and a spiral texture similar to that of the chiral nematic phase. This simple method provides a new technique for preparing chiral conducting films in a thermotropic chiral liquid‐crystal field.

Optical micrograph of (R)‐PEDOT* (no polarizer).     

Induced Circular Dichroism of Chiral Nematic Cellulose Films

Colloidal chiral nematic suspensions of cellulose were prepared from dissolving grade wood pulp. On evaporation of the water, the suspension dries down to give iridescent cellulose films. The optical properties of the films may be characterized by incorporating dyes in the films, and following the ordering of the dye molecules by measurement of induced circular dichroism (ICD). Structural changes to the films, such as decreasing the chiral nematic pitch by increasing the salt content and increasing the chiral nematic order through magnetic alignment, can be monitored by measuring the changes in ICD.     

Free-standing smectic films at high temperature

Optical reflectivity studies on free-standing liquid crystal films above the bulk smectic temperature range have revealed different melting phenomena. Our measurements are performed on tilted smectic phases (smectic C*, smectic C) using optical microscopy in polarized light in order to visualize the changes of the film structure. We observe the formation of twodimensional defect structures from string-like lines in very thick (about 1000 layers) as well as in thin (about 20 layers) films. In thick films these structures nucleate around the temperature of the bulk smectic-cholesteric phase transition, while in thin films the formation of the defects occurs well above this temperature and just before the thinning transitions. In thick and intermediate thickness films, cholesteric or nematic droplets and a ‘quasi-smectic’ structure are observed. The films exhibiting the ‘quasi-smectic’ structure definitely exist at higher temperatures than the smectic films with the same thickness.     

Smectic layer structure of ferroelectric liquid crystal formed between fine polymer fibres

This paper describes the alignment of ferroelectric liquid crystal (FLC) structures formed between aligned polymer fibres, where the FLC smectic layers are determined by polarising microscopy and X-ray diffraction. The FLC/polymer composite films were formed from a nematic phase FLC/monomer solution using a photopolymerisation-induced phase separation method. It was found that bending of the FLC smectic layers was induced in both the film plane and the cross-sectional plane at the phase transition from smectic A to chiral smectic C of the FLC material. The light transmittance properties of the composite film between crossed polarizers was analysed by light propagation simulation in several optical anisotropic media, based on the evaluated smectic layer model.     

Free-standing smectic films at high temperature

Optical reflectivity studies on free-standing liquid crystal films above the bulk smectic temperature range have revealed different melting phenomena. Our measurements are performed on tilted smectic phases (smectic C*, smectic C) using optical microscopy in polarized light in order to visualize the changes of the film structure. We observe the formation of twodimensional defect structures from string-like lines in very thick (about 1000 layers) as well as in thin (about 20 layers) films. In thick films these structures nucleate around the temperature of the bulk smectic-cholesteric phase transition, while in thin films the formation of the defects occurs well above this temperature and just before the thinning transitions. In thick and intermediate thickness films, cholesteric or nematic droplets and a 'quasi-smectic' structure are observed. The films exhibiting the 'quasi-smectic' structure definitely exist at higher temperatures than the smectic films with the same thickness.     

Director fields of disclinations in the nematic mesophase decorated by focal‐conic texture for side‐chain liquid crystalline polymers

A new decoration method termed as focal‐conic texture decoration has been established to map the director field of disclinations of nematic mesophases for a side‐chain liquid crystalline polymer (SCLCP) based on its transition from nematic to smectic phase with focal‐conic texture observed under a polarizing optical microscope. FT‐IR microscopy was used to ascertain the relationship between the mesogenic orientation direction and the orientation direction of focal‐conic texture, which exhibits consequently the director field of a disclination.     

Smectic layer structure of ferroelectric liquid crystal formed between fine polymer fibres

This paper describes the alignment of ferroelectric liquid crystal (FLC) structures formed between aligned polymer fibres, where the FLC smectic layers are determined by polarising microscopy and X-ray diffraction. The FLC/polymer composite films were formed from a nematic phase FLC/monomer solution using a photopolymerisation-induced phase separation method. It was found that bending of the FLC smectic layers was induced in both the film plane and the cross-sectional plane at the phase transition from smectic A to chiral smectic C of the FLC material. The light transmittance properties of the composite film between crossed polarizers was analysed by light propagation simulation in several optical anisotropic media, based on the evaluated smectic layer model.     

Alignment and switching of nematic liquid crystals embedded in porous chiral thin films

Porous thin films with engineered microstructures have been fabricated using glancing angle deposition (GLAD). GLAD films with chiral microstructures have been previously shown to exhibit unique chiral optical response. The pores of these films were embedded with (non-chiral) nematic liquid crystals (LCs) to produce a new composite optical material wherein the GLAD film induces chiral nematic-like LC orientation. We demonstrate here reversible electro-optic switching of the LC component of these hybrid films. Unaddressed, cells of GLAD/LC hybrid films show enhanced chiral optic response compared with the unfilled GLAD film. When addressed, the chiral optic response vanishes.