Fast switching anisotropic networks obtained by in situ photopolymerization of liquid crystal molecules

Lightly cross-linked anisotropic networks with uniaxial and π/2 twisted orientations were produced by photopolymerization of monotropic mixtures containing liquid crystal mono and diacrylates. In this way the polymer backbone was immobilized and became decoupled from the motion of the mesogenic side groups. The networks showed very good reversibility and even after becoming isotropic, upon cooling, the initial orientation was recovered. In the same way, in the presence of electric fields the mesogenic groups could be reoriented in the direction of the electric field, reverting back to the initial orientation on removal of the field at a rate comparable with those observed in the monomeric state. Combining viscoelastic measurements with the dielectric behaviour of the monomeric liquid crystal and the anisotropic network, a comparison between the internal and bulk rotational viscosities was also made.     

Fast switching anisotropic networks obtained by in situ photopolymerization of liquid crystal molecules

Abstract

Lightly cross-linked anisotropic networks with uniaxial and π/2 twisted orientations were produced by photopolymerization of monotropic mixtures containing liquid crystal mono and diacrylates. In this way the polymer backbone was immobilized and became decoupled from the motion of the mesogenic side groups. The networks showed very good reversibility and even after becoming isotropic, upon cooling, the initial orientation was recovered. In the same way, in the presence of electric fields the mesogenic groups could be reoriented in the direction of the electric field, reverting back to the initial orientation on removal of the field at a rate comparable with those observed in the monomeric state. Combining viscoelastic measurements with the dielectric behaviour of the monomeric liquid crystal and the anisotropic network, a comparison between the internal and bulk rotational viscosities was also made.     

STATISTICS OF A NOVEL CLASS OF LIQUID CRYSTAL POLYMERS

A novel class of the liquid crystal polymers with mesogens laterally attached to flexible main chain backbones has been synthesized recently by us. While the flexible backbones tend to take configuration of maximum entropy the mesogenic side groups tend to orientate because of the anisotropie soft dispersion forces. A mean field theory is presented to describe the competition between these two trends which makes it possible for this class of polymers to show anisotropic (liquid crystal) phase within certain temperature range where the polymer main chains and the side chain mesogens will take approximately parallel arrangement, which is different from the normal side chain liquid crystal polymers. The temperature range of the liquid crystal phase, its ordering and phase transition all depend on the flexibility of the backbone, the strengthof the anisotropic forces of the side groups and the hinge elasticity. The results show that the liquid crystalisotropic phase transition is of the first order. The phase diagram, i. e. the dependence of the transition temperature on the structure of the polymers is also given.     

The synthesis and properties of a series of novel side-chain liquid crystal polymers based on the Baylis-Hillman reaction

A new methodology is described, based on the Baylis-Hillman reaction, by which thermally stable side-chain liquid crystal (SCLC) polymers can be prepared incorporating a variety of functional groups along the polymer backbone. This methodology also allows a polar group, especially hydroxyl, to be inserted adjacent to the point of attachment of the spacer group to the polymer backbone. The spatial arrangement of the hydroxyl and functional groups is not only conducive to the formation of liquid crystalline phases but may also influence the tacticity of the polymer backbone.     

Using switched angle spinning to simplify NMR spectra of strongly oriented samples

This contribution describes a method that manipulates the alignment director of a liquid crystalline sample to obtain anisotropic magnetic interaction parameters, such as dipolar coupling, in an oriented liquid crystalline sample. By changing the axis of rotation with respect to the applied magnetic field in a spinning liquid crystalline sample, the dipolar couplings present in a normally complex strong coupling spectrum are scaled to a simple weak coupling spectrum. This simplified weak coupling spectrum is then correlated with the isotropic chemical shift in a switched angle spinning (SAS) two-dimensional (2D) experiment. This dipolar-isotropic 2D correlation was also observed for the case where the couplings are scaled to a degree where the spectrum approaches strong coupling. The SAS 2D correlation of C(6)F(5)Cl in the nematic liquid crystal I52 was obtained by first evolving at an angle close to the magic angle (54.7 degrees ) and then directly detecting at the magic angle. The SAS method provides a 2D correlation where the weak coupling pairs are revealed as cross-peaks in the indirect dimension separated by the isotropic chemical shifts in the direct dimension. Additionally, by using a more complex SAS method which involves three changes of the spinning axis, the solidlike spinning sideband patterns were correlated with the isotropic chemical shifts in a 2D experiment. These techniques are expected to enhance the interpretation and assignment of anisotropic magnetic interactions including dipolar couplings for molecules dissolved in oriented liquid crystalline phases.     

Phase behavior of lyotropic rigid-chain polymer liquid crystal studied by dissipative particle dynamics

The phase behavior of lyotropic rigid-chain liquid crystal polymer was studied by dissipative particle dynamics (DPD) with variations of the solution concentration and temperature. A chain of fused DPD particles was used to represent each mesogenic polymer backbone surrounded with the strongly interacted solvent molecules. The free solvent molecules were modeled as independent DPD particles, where each particle includes a lump of solvent molecules with the volume roughly equal to the solvated polymer segment. The simulation shows that smectic-B (S(B)), smectic-A (S(A)), nematic (N), and isotropic (I) phases exist within certain regions in the temperature and concentration parameter space. The temperature-dependent S(B)∕S(A), S(A)∕N, and N∕I phase transitions occur in the high concentration range. In the intermediate concentration range, the simulation shows coexistence of the anisotropic phases and isotropic phase, where the anisotropic phases can be the S(B), S(A), or N phases. Mole fraction and compositions of the coexisted phases are determined from the simulation, which indicates that concentration of rigid rods in isotropic phase increases as the temperature increases. By fitting the orientational distribution function of the systems, the biphasic coexistence is further confirmed. From the parameter α obtained for the simulation, the distribution of the rigid rods in the two coexistence phases is quantitatively evaluated. By using model and simulation methods developed in this work, the phase diagrams of the lyotropic rigid-chain polymer liquid crystal are obtained. Incorporating the solvent particles in the DPD simulation is critical to predict the phase coexistence and obtain the phase diagrams.     

Liquid crystalline polymers: From self-organization to functions and applications

Liquid crystals derive their unusual properties and their broad range of applications from their unique spatial and orientational order giving rise to specific symmetries, to strong anisotropies with respect to macroscopical properties and to a strong coupling to external fields. We have studied for liquid crystalline polymers modes of inducing strong modifications of the anisotropic optical properties locally in solid films by light. A storage process is described in this contribution which is based on light-induced trans-cis-trans-isomerization reactions of azobenzene chromophores attached to a polymer backbone as side groups in liquid crystalline polymers. The chromophores are able to rotate in the glassy state if subjected to linearly polarized light: the azobenzene units approach a saturation orientation which is perpendicular to the polarization direction of the light. The contribution discusses the molecular mechanism of this process as well as possible applications.     

Molecular dynamics simulations of side chain liquid crystal polymer molecules in isotropic and liquid-crystalline melts

A detailed molecular dynamics simulation study is described for a polysiloxane side chain liquid crystal polymer (SCLCP). The simulations use a coarse-grained model composed of a combination of isotropic and anisotropic interaction sites. On cooling from a fully isotropic polymer melt, we see spontaneous microphase separation into polymer-rich and mesogen-rich regions. Upon application of a small aligning potential during cooling, the structures that form on microphase separation anneal to produce a smectic-A phase in which the polymer backbone is largely confined between the smectic layers. Several independent quenches from the melt are described that vary in the strength of the aligning potential and the degree of cooling. In each quench, defects were found where the backbone chains hop from one backbone-rich region to the next by tunneling through the mesogenic layers. As expected, the number of such defects is found to depend strongly on the rate of cooling. In the vicinity of such a defect, the smectic-A structure of the mesogen-rich layers is disrupted to give nematiclike ordering. Additionally, several extensive annealing runs of approximately 40 ns duration have been carried out at the point of microphase separation. During annealing the polymer backbone is seen to be slowly excluded from the mesogenic layers and lie perpendicular to the smectic-A director. These observations agree with previous assumptions about the structure of a SCLCP and with interpretations of x-ray diffraction and small angle neutron scattering data. The flexible alkyl spacers, which link the backbone to the mesogens, are found to form sublayers around the backbone layer.     

A study on interfacial tension between flexible polymer and liquid crystal

The interfacial property in polymer-liquid crystal systems is quite different from flexible polymer-polymer mixtures due to the anisotropic properties of liquid crystals. The apparent interfacial tension between a liquid crystal and a flexible polymer was measured by deformed droplet retraction method. The deformation and recovery of a single liquid crystal droplet dispersed in a poly(dimethylsiloxane) matrix were realized by a transient shear flow and observed by polarized optical microscope. The apparent interfacial tension of polymer-liquid crystal system was found to be greatly dependent on the temperature, initial droplet deformation and liquid crystal droplet size.     

一种新型侧链液晶聚合物的喇曼光谱研究

The Raman scattering on a novel liquid crystal side chain polymer, poly-2,5-bis (p-meth-oxybenzoyloxy) styrene, has been performed. We observed that the characteristic Raman sp-ctra of the vinyl group in the monomer changed much deeply in the polymer, especially for the Raman line correponding to the C=C of the vinyl group which disappeared completly in the polymer. Contrast with that, the Raman spetra of the rest group did not present any substantial difference between the monomer and the polymer. These experimental facts show-ed that the vinyl group of the monomer was tranfered to the polymeric backbone while the chemical structure of the mesogenic unit in the monomer remained after the polymerization,and also conformed that it was successful in the synthesis of a liquid crystal side chain plymer without a flexible spacer between the mesomorphic unit and the molecular main chain.     

一种新型侧链液晶聚合物的喇曼光谱研究

根据传统的侧链型液晶聚合物理论,在液晶基元和主链之间必须有柔性链相连。最近我们设计并合成了单体为2,3-双(对烷氧基苯甲酰氧基)苯乙烯的侧链液晶聚合物,该聚合物突破了文献[1]的理论,把液晶基元从重心位置横挂于主链之上,其间无柔性链段,只以单键相接。我们用喇曼光谱研究了该新型侧链液晶聚合物,表明它存在上述结构特点。     

Controllable alignment of nematics by nanostructured polymeric layers

A method for a continuous control of the pretilt angle of the easy axis in the range 0–90° degrees and of the anchoring strength by using nanostructured polymers as alignment layers is described. The nanostructured polymers are blends of two different side-chain polymers each of them promoting planar and homeotropic alignment, respectively. A model to interpret the alignment of a nematic liquid crystal induced by such polymer layers is proposed. We show that in this case the anisotropic part of the surface tension can be approximated by a simple extension of the Rapini–Papoular expression. The predicted trend of the pretilt of the easy axis versus the concentration of the side-chain polymer promoting the planar alignment, for instance, is in good agreement with the experimental data. We also show that the effective anchoring strength of the system depends on the concentration of the side-chain polymer promoting planar alignment, and exhibits a minimum for a well-defined value of this quantity. The results obtained in this work seems to be of importance for liquid crystal displays technology since the control of the pretilt and the anchoring strength strongly affect the performance of liquid crystal displays.     

Viscoelastic properties of liquid crystal polymers: Collective motions and nuclear spin relaxation

Transverse deuterium (²H) nuclear spin relaxation experiments have been performed on a ²H labelled main chain liquid crystal polymer. Relaxation rates are determined as a function of temperature and pulse frequency using a modified Carr-Purcell-Meiboom-Gill pulse train. The results are analysed in terms of a hydrodynamic model for fluctuations of the liquid crystal director. Analytic expressions are employed which relate the transverse spin relaxation rate to the anisotropic viscoelastic parameters of the polymer and allow estimates to be obtained for the effective viscosity and average elastic constant of the polymer. The molecular weight dependence of the viscoelastic parameters has been investigated and is found to be consistent with theoretical predictions for highly extended liquid crystal polymers.     

Direct observation of the main-chain conformation of a combined liquid-crystal polymer

The conformation of the main-chain (backbone) of a combined main-chain/side-chain liquid crystalline polymer has been qualitatively determined by small angle neutron scattering in the oriented nematic, the smectic A and the smectic C phases. The polymer backbone presents only a weak anisotropy, of prolate shape, in the nematic and the smectic C phases. A stronger reorientation of the backbones in the direction of the applied magnetic field is measured for the S_A phase. However, this anisotropy remains small compared to the stretching of a main-chain liquid crystal polymer and the smectic structure results apparently from side-chain ordering. On the other hand, hydrodynamic measurements show that the combined polymer, in solvent, is as flexible as a polystyrene chain. This result is compatible with an explanation for the weak observed anisotropy.     

Utility of a liquid crystalline diacrylate for bistable switching cholesteric gel displays

We investigated cholesteric liquid crystals containing an anisotropic polymer network created by in situ photo-polymerization of a mesogenic diacrylate. In contrast to previous studies, we used a liquid crystalline diacrylate to achieve bistable switching between the reflecting Grandjean texture and the diffuse scattering focal conic texture. The switching behavior depends considerably on the orientation of the liquid crystal during the polymerization. For polymerization in the Grandjean texture, the response times for switching to the reflecting state were lowered compared to the pure liquid crystal. The chromaticity coordinates indicate that this new type of display is very promising for additive color mixing.     

Phase behavior of liquid crystalline elastomers based on a tri-vinyl mesogenic cross-linker

A series of cross-linked liquid crystalline polymers are prepared by graft copolymerization, and their liquid crystalline properties are characterized by DSC and POM. The results show that low levels of cross-linking do not obviously affect the phase behavior of the network polymers; in contrast, high levels of cross-linking may have more drastic influences, and liquid crystalline phases may lose, and more marked variation in phase transition will occur in materials with more direct coupling through a shorter or stiffer coupling chain between mesogenic side units and polymer backbone. At the same time, the coupling between the polymer chain and sidegroups results in stress-induced orientation in LCEs.     

Anisometric,non-mesogenic,tailor-made monomer for reverse-mode shutters

Reverse-mode operation films can be achieved with polymer dispersed liquid crystals by many methods. One method consists in the use of either rough or polyimide treated supports in order to induce homeotropic alignment of the initial mixture that will separate in liquid crystal and polymer rich phases. The major disadvantage is the need for mesogenic monomers to keep a good alignment of the liquid crystal after the polymerization process. In this paper we present results concerning the use of an anisometric, non-mesogenic, tailor-made monomer for the preparation of excellent reverse-mode polymer dispersed liquid crystal shutters cast on rough surfaces. The combined use of rough surfaces and non-mesogenic monomers allows us to exceed earlier limits in physical and chemical properties, cost, and device dimensions by the use of a wider class of monomer molecules.     

A new anisotropic soft-core model for the simulation of liquid crystal mesophases

A new anisotropic soft-core model is presented, which is suitable for the rapid simulation of liquid crystal mesophases. The potential is based on a soft spherocylinder, which can be easily tuned to favor different liquid crystal mesophases. The soft-core nature of the potential makes it suitable for long-time step molecular dynamics or dissipative particle dynamics simulations, particularly as a reference model for mesogens or as an anisotropic solvent for use in combination with atomistic models. Results are presented for two variants of the new potential, which show different mesophase behaviors. Variants of the potential can also be linked together to produce more complicated molecular structures. Here, as an example, results are provided for a model multipedal liquid crystal, which has eight liquid crystalline groups linked to a central core via semiflexible chains. Here, despite the complexity of molecular structure, the model succeeds in showing the spontaneous formation of a liquid crystal phase. The results also demonstrate that there is a very strong coupling between the internal structure of the multipedal mesogen and the molecular order of the phase, with the mesogen spontaneously undergoing major structural rearrangement at the transition to the liquid crystal phase.     

Langmuir-Blodgett films of preformed polymers containing biphenyl groups

Polymers containing liquid crystal groups have been studied previously as waveguides, and they have been deposited as spacer materials with various chromophores to form alternating films However, only a few members of this group of materials have been studied in any detail, and very little structural information has been obtained so far. Therefore, a more detailed examination of these materials as mono- and multilayers was undertaken. A new group of materials including the same mesogenic group, polymeric sulfones, was also studied. The polymers gave steep isotherms with high collapse pressures, indicating good packing of the monolayers, and could be deposited to form thick multilayers. X-ray diffraction showed that an ordered multilayer was formed, and the effects of the polymers chemical nature on the structure of the LB films are discussed. It appears that the dominant factor in monolayer structure is the nature of the polymer backbone rather than that of the liquid crystal side chains, which play a secondary role.     

Anisotropic cellulose-derived matrix for dispersed liquid crystals

New cellulose-derived dispersed liquid crystal free-standing thin films were prepared by a shear-casting technique from anisotropic and isotropic solutions of thermotropic (acetoxypropyl)cellulose in N,N-dimethylacetamide with different amounts of the nematic E7. For films prepared from anisotropic solutions, dispersed nematic E7 droplets of micron and submicron size were found to coexist, with the band texture characteristic of polymer liquid crystals after shearing. Mechanical properties including Young's modulus and stress-strain curves were determined along and perpendicular to the shear direction, and revealed the anisotropic behaviour of films prepared from lyotropic solutions. The nematic E7 component appears to promote chain mobility and rearrangement of the polymer matrix and to modify the viscoelastic properties of the matrix.