Concentration dependence of magnetic field effects on the ethyl-cyanoethyl cellulose/dichoroacetic acid cholesteric phase

The effects of an external magnetic field with intensity 9.4 T on the cholesteric phase in ethylcyanoethyl cellulose (E-CE)C]/dichloroacetic acid liquid crystalline solutions were investigated for different concentrations. It was found that the diamagnetic anisotropy of (E-CE)C is negative and the effect of the magnetic field on the orientation of the cholesteric phase is influenced by the concentration of the solution, because the liquid crystalline properties of the solutions vary with the concentration.     

Concentration dependence of magnetic field effects on the ethyl-cyanoethyl cellulose/dichoroacetic acid cholesteric phase

The effects of an external magnetic field with intensity 9.4 T on the cholesteric phase in ethylcyanoethyl cellulose (E-CE)C]/dichloroacetic acid liquid crystalline solutions were investigated for different concentrations. It was found that the diamagnetic anisotropy of (E-CE)C is negative and the effect of the magnetic field on the orientation of the cholesteric phase is influenced by the concentration of the solution, because the liquid crystalline properties of the solutions vary with the concentration.     

BAND-LIKE TEXTURE OF ETHYLCYANOETHYL CELLULOSE MESOPHASE

In the ethyl-cyanoethylcellulose ((E-CE)C)/dichloroacetic acid (DCA) cholesteric liquid crystalline solution, the hand-like texture is formed when the mesophase aggregates with the disk-like texture grow to big enough and merge with each other with increasing concentration. The band-like texture is composed of parallel equidistant bright and dark alternative strips which are about 0.2-2.0 μm in width. In the band-like texture, the layers of ordered polymer chains are perpendicular to the solution film and the axes of helicoids are parallel to it. The width of the strips is different in different zones. Under the effect of an external magnetic field, the strips in the band-like texture first become wider and then narrower gradually.Moreover, the axes of helicoids in the (E-CE) C/DCA mesomorphic solution change from the direction normal to the magnetic field to the agreement with the magnetic field direction.     

PREPARATION AND PROPERTIES OF ETHYL-CYANOETHYL CELLULOSE/POLYACRYLIC ACID COMPOSITE FILMS WITH REFLECTION COLORS*

Ethyl-cyanoethyl cellulose [(E-CE)C]/acrylic acid (AA) becomes a cholesteric liquid crystalline solution withvivid colors when the (E-CE)C concentration is 42 wt%～52 wt%. (E-CE)C/polyacrylic acid (PAA) composites withcholesteric structure were prepared by polymerzing AA in (E-CE)C/AA liquid crystalline solutions. The layers of orderedpolymer chains in the cholesteric phase were inclined during polymerization and the degree of the inclination depended onthe polymerization temperature and the concentration of the solution before polymerization. The cholesteric structure in thecomposites could not be changed when temperature was lower than 100℃. Cross-linking of the PAA in compositesimproved their water-resistance. The cholesteric order of the composites without cross-linking was destroyed when theywhere immersed in water. The color derived from the selective reflection of the cholesteric phase of the cross-linkedcomposites turned from blue to red after the composites absorbed water. The color of the composites could be returned to theoriginal one when the absorbed water was removed from the swollen composites.     

Relaxation Processes in sheared films of ethyl-cyanoethyl cellulose cholesteric liquid crystalline solutions

The relaxation processes in sheared films of ethyl-cyanoethyl cellulose [(E-CE)C]/acrylic acid (AA) cholesteric liquid crystalline (LC) solutions were studied by polarizing optical microscopy (POM) and UV-Vis spectrophotometry. Under shearing normal to the helix axis and above the critical shear rate, the planar texture arrangement of the (E-CE)C/AA cholesteric LC solution was destroyed and transformed to the nematic phase. Observed by POM, the banded texture formed quickly following the cessation of the shear, but it was unstable and disappeared after several minutes. The reflection spectrum of the sheared (E-CE)C/AA cholesteric LC solution film was recorded as a function of relaxation time. It was found that the selective reflection property was lost under the shear, but the shape of the reflection spectrum recovered quickly with cessation of the shear, and the reflection peak in the spectrum became sharper with time, returning to the original form before shearing. A proposed model of the structural transformation during the relaxation was confirmed by additional optical measurement and transmission electron microscopy.     

INFLUENCE OF THE SOLVENT SWELLING ON MACROMOLECULAR CHOLESTERIC LIQUID CRYSTALLINE STRUCTURE*

Ethyl-cyanoethyl cellulose [(E-CE)C]/cross-linked polyacrylic acid [PAA] molecular compositeswith cholesteric order were prepared. It was found that the macromolecular cholesteric structure was changedwith the swelling of PAA in the composites. The selective reflection of the cholesteric phase shifted to thelonger wavelength and the X-ray diffraction angle shifted to the high angle direction during swelling, whichsuggested that the cholesteric pitch and the number of the layers of ordered (E-CE)C chains in the cholestericphase were increased.     

Textures and defects in lyotropic liquid crystals of ethyl-cyanoethyl cellulose

Textures and defects in ethyl-cyanoethyl cellulose [(E-CE)C]/dichloroacetic acid (DCA) cholesteric liquid crystalline solutions and in (E-CE)C/polyacrylic acid (PAA) composites were observed and studied by polarizing microscopy and electron microscopy. The existence of χ, λ and τ disclinations were observed in the mesophase with disk-like and band-like textures. Pairs of disclinations with different signs were also found in the mesophase with the band-like texture. Domain walls were observed in (E-CE)C/PAA composite films with cholesteric order by TEM. The orientation of polymer chains in the vicinity of the core of the disclinations is discussed.     

Macromolecular cholesteric order observed by electron microscopy

The macromolecular cholesteric structure in the ethyl-cyanoethyl cellulose [(E-CE)C]/acrylic acid [AA] cholesteric liquid crystalline solutions is studied by directly observing the morphology and structure of the ethyl-cyanoethyl cellulose [(E-CE)C]/polyacrylic acid [PAA] using electron microscopy. A periodical lamellar structure is observed in ultrathin slices of the composites with cholesteric order by both transmission electron microscopy (TEM) and low-voltage scanning electron microscopy (LVSEM). It is suggested that the periodical lamellar structure is induced by the twist of the molecular orientation in the cholesteric phase and reflects the structural features of the macromolecular cholesteric phase. The macromolecular cholesteric phase exhibits the twisted ring morphology in the initial stage of the formation of the liquid crystalline phase. The swelling of the ultrathin slices with cholesteric order in water is heterogeneous, which suggests the tight packing of the (E-CE)C chains in the direction of the helix axis in the macromolecular cholesteric phase. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 439–445, 1998     

磁场对乙基纤维素胆甾型液晶相的影响

高分子胆甾型液晶相是一个在高分子和液晶领域都引起广泛兴趣的课题,不仅因为自然界中的多种生物大分子,如纤维素,多肽,DNA等可以形成胆甾型液晶相;而且由于胆甾相具有特殊的螺旋结构(如图1所示),能产生一些特殊的光学性能,如强烈的旋光性,圆二色性和选择性反射光性能等;并带来相应的应用,正是由于高分子胆甾相材料的性能和应用,使得高分子胆甾相液晶的相态转变和结构变化也一直倍受关注。     

乙基氰乙基纤维素/交联聚丙烯酸复合物膜的溶胀行为

研究了乙基氰乙基纤维素 [(E CE)C] 交联聚丙烯酸 [PAA]胆甾相液晶复合物膜的厚度以及膜的组成对膜在水中的溶胀行为的影响 .复合物膜越厚则达到溶胀平衡所需要的时间越长 ,但是其最大溶胀率是相同的 .复合物膜的最大溶胀率先是随着 (E CE)C浓度的增加而增加 ,当 (E CE)C的浓度大于 5 1wt%的时候 ,复合物膜的最大溶胀率几乎不再发生变化 .复合物膜的交联密度越大 ,其最大溶胀率越小 ,溶胀速率也随着膜的交联 (点 )密度的增加而减小 .研究还发现复合物膜的交联 (点 )密度越大 ,其溶胀前后最大选择性反射光波长的位移也越小 .     

Alignment of liquid crystal molecules on oriented (E-CE)C/PAA films

A film of ethyl-cyanoethyl cellulose/polyacrylic acid, (E-CE)C/PAA, using an alignment layer of liquid crystal (5CB), was prepared by shearing and then photo-polymerization of an (E-CE)C/AA solution. The orientation of the (E-CE)C chains in the sheared film and the alignment of the 5CB molecules on the films were investigated by polarizing optical microscopy and FTIR. It was found that the (E-CE)C/PAA oriented film showed perfect alignment ability for the 5CB molecules. The director of the 5CB molecules on the oriented film does not lie along the orientation direction of the (E-CE)C main chains, but inclines to that of the (E-CE)C main chains. The direction of the 5CB molecular orientation on the (E-CE)C/PAA oriented film is influenced by the degree of orientation of the (E-CE)C chains in the oriented film.     

UV irradiation gradient induced banded texture in photo-polymerized ethyl-cyanoethyl cellulose/poly(acrylic acid) cholesteric liquid crystalline films and patterns fabrication thereof

The photo-polymerization-induced banded texture of ethyl-cyanoethyl cellulose/acrylic acid/copper acrylate ((E-CE)C/AA/CuAA₂) cholesteric liquid crystalline (CLC) solutions were investigated. The results indicate that the CLC phase can be fixed by the photo-polymerization. Banded texture was obtained in the photo-polymerized CLC films. The orientation of the banded texture induced by the photo-polymerization depends on the gradient of UV irradiations on the surface of the (E-CE)C/AA/CuAA₂ CLC solutions. CLC films with different patterns can be obtained by introducing the UV irradiation gradient on the (E-CE)C/AA/CuAA₂ CLC solutions surface by using masks with different patterns. The simple approach for the fabrication of CLC films with different patterns may have the potential application in the fields of holographic image storage.     

Investigation of molecular order in liquid-crystalline solutions of cellulose triacetate using PMR spectroscopy

Liquid-crystalline solutions of cellulose triacetate (CTA) in trifluoroacetic acid (TFA)–CH₂Cl₂, TFA–1.2-dichloroethane (1,2-DCE) solvent mixtures were examined by means of PMR spectroscopy. CTA forms both cholesteric and nematic phases in these solvents depending on the CTA concentration. In cholesteric solutions the CH₂Cl₂ signal is initially a singlet and then splits into a doublet. The time dependence of the splitting and the effect of CTA concentration are reported. The results suggest that the cholesteric phase slowly changes into a nematic phase in the magnetic field. The splitting of the CH₂Cl₂ proton signal into a doublet and the 1,2-DCE signal into a quartet are due to direct magnetic dipole-dipole interactions. Rotation of the sample in the magnetic field results in the disappearance of the doublet or quartet and suggests that the solvent molecules are originally oriented in the direction of the magnetic field. In the biphasic region, immediate splitting of the CH₂Cl₂ proton signal suggests that the anisotropic phase is nematic.     

THE MORPHOLOGY AND STRUCTURE OF ETHYLCYANOETHYL CELLULOSE IN STYRENE AND POLYSTYRENE

Ethyl-cyanoethyl cellulose ((E-CE)C)/styrene solution could form anisotropic system when the concentration was high enough. The (E-CE)C/polystyrene(PS)multiphase polymer could be obtained by radical polymerization of the styrene in the solution. The (E-CE)C/PS multiphase polymer maintained both the crystalline structure of the (E-CE)C and the amorphous structure of the PS. In the multiphase polymer produced from the isotropic solution, however, the (E-CE)C formed spherulites and spread in the PS amorphous phase. While, in the multiphase polymer produced from the anisotropic solution, the (E-CE)formed cylinderic crystalline aggregates. Moreover, the ordered lamellar texture was also observed in the multiphase polymer produced from the anisotropic solution.     

Effects of pretilt angle and anchoring energy on alignment of uniformly lying helix mode

We investigated the effects of pretilt angle and anchoring energy on the formation of a uniformly lying helix (ULH) texture of cholesteric liquid crystals (CLCs). Pretilt angle was controlled by the thickness of a vertical alignment layer coated onto a planar alignment layer. In the given pretilt angle, the anchoring energy was enhanced by introducing reactive mesogen to the vertical alignment material. To characterise quantitatively the formation of the ULH texture we introduced reflectance, governed by areas of the ULH region and the planar-aligned CLC region. We found that the ULH texture was more widely formed under the condition of higher pretilt angle and weaker anchoring energy. Also, a more uniform alignment of the ULH texture was achieved with the higher pretilt angle even under the same anchoring energy condition.     

Molecular dynamics simulation of polymer dispersed liquid crystal droplets under competing boundary conditions

This paper is devoted to the molecular dynamics simulation of structural organization inside a polydispersed liquid crystal (LC) droplet under competing boundary conditions. The droplet is assumed to be placed at the liquid crystal interface between two different regions of the solid polymer matrix, which accordingly separates the droplet into two hemispheres: the first of these is under radial boundary conditions; the second hemisphere is under bipolar boundary conditions. The droplet is considered as a jagged sphere filled with LC molecules, modelled as classical spins (unit vectors), whose centres of mass are associated with sites of a cubic lattice inside the cavity. The orienting action of the polymer matrix, and hence the resulting boundary conditions, are modelled by the interaction between the internal LC molecules (possessing only orientational degrees of freedom), and those of a delimiting surface layer (a jagged spherical shell), whose orientations are fixed, radial or bipolar, respectively. All interactions are modelled by the short range McMillan pair potential. The molecular orientation inside the LC droplet has been determined for various anchoring strengths of the interaction between internal spins and boundary layers. We have investigated the structure of the spherical defect resulting in the central region of the droplet, as well as of the boojum - like defects existing near the poles of the droplet. It has been found that a change of relative radial and bipolar anchoring strengths can affect both central and boojum - like defects. The effect of an external field on the molecular orientation inside the droplet has also been investigated. It has been found that a sufficiently strong external field increases the radius of the spherical defect placed in the central region of the droplet.     

Molecular dynamics simulation of polymer dispersed liquid crystal droplets under competing boundary conditions

This paper is devoted to the molecular dynamics simulation of structural organization inside a polydispersed liquid crystal (LC) droplet under competing boundary conditions. The droplet is assumed to be placed at the liquid crystal interface between two different regions of the solid polymer matrix, which accordingly separates the droplet into two hemispheres: the first of these is under radial boundary conditions; the second hemisphere is under bipolar boundary conditions. The droplet is considered as a jagged sphere filled with LC molecules, modelled as classical spins (unit vectors), whose centres of mass are associated with sites of a cubic lattice inside the cavity. The orienting action of the polymer matrix, and hence the resulting boundary conditions, are modelled by the interaction between the internal LC molecules (possessing only orientational degrees of freedom), and those of a delimiting surface layer (a jagged spherical shell), whose orientations are fixed, radial or bipolar, respectively. All interactions are modelled by the short range McMillan pair potential. The molecular orientation inside the LC droplet has been determined for various anchoring strengths of the interaction between internal spins and boundary layers. We have investigated the structure of the spherical defect resulting in the central region of the droplet, as well as of the boojum ‐ like defects existing near the poles of the droplet. It has been found that a change of relative radial and bipolar anchoring strengths can affect both central and boojum ‐ like defects. The effect of an external field on the molecular orientation inside the droplet has also been investigated. It has been found that a sufficiently strong external field increases the radius of the spherical defect placed in the central region of the droplet.     

The induced magnetic field in cyclic molecules

The response of a molecule to an applied external magnetic field can be evaluated by a graphical representation of the induced magnetic field. We have applied this technique to four representative, cyclic organic molecules, that is, to aromatic (C(6)H(6), D(6h)), anti-aromatic (C(4)H(4), D(2h)) and non-aromatic (C(4)H(8), D(4h), and C(6)H(12), D(3d)) molecules. The results show that molecules that contain a pi system possess a long-range magnetic response, while the induced magnetic field is short-range for molecules without pi systems. The induced magnetic field of aromatic molecules shields the external field. In contrast, the anti-aromatic molecules increase the applied field inside the ring. Aromatic, anti-aromatic, and non-aromatic molecules can be characterized by the appearance of the magnetic response. We also show that the magnetic response is directly connected to nucleus-independent chemical shifts (NICS).     

Orientational transition in the cholesteric layer induced by electrically controlled ionic modification of the surface anchoring

A reorientation of cholesteric liquid crystal with a large helix pitch induced by the electrically controlled ionic modification of the surface anchoring has been studied. In initial state, the cholesteric helix is untwisted completely owing to the normal surface anchoring specified by the cations adsorbed at the substrates. As a result, the homeotropic director configuration is observed within the cell. Under the action of DC electric field, one of the substrates becomes free from the layer of surface active cations, therefore, setting the planar surface anchoring. The latter, in turn, leads to the formation of the hybrid chiral structure. The threshold value and dynamic parameters have been estimated for this process as well as the range of control voltages, which do not allow the electrohydrodynamic instabilities. The twisted hybrid director configuration observed in the experiment has been analysed by means of the simulation of polarisation change of light propagating through the cholesteric layer with asymmetric (planar and homeotropic) surface anchoring on the cell substrates.     

Raman spectra of magnetic field induced cholesteric → nematic phase transition in doped p-octoxybenzoic acid

Raman spectroscopy is employed to investigate helical twist formation in nematic and smectic C phases of p-n-octoxybenzoic acid (OBA) doped with a small amount of Cholesteric Nonanoate (CN). A cholesteric→ nematic phase transition in OBA/CN is induced by an external magnetic field in the temperature range 135–148°C. The threshold field (H_c) is equal to 8.0 kGauss for 0.33 % by weight CN/OBA, and no hysteresis is observed.