Mesogenic Side Groups Research Articles

Comparison of the Photoinduced Orientation Structure in the Bulk and at the Near-Surface of a Photoalignable Liquid Crystalline Polymer Film

The thermally stimulated photoinduced in-plane and out-of-plane molecular reorientation behaviors of a polymethacrylate film comprised of 4-methoxycinnnamoylbiphenyl (MCB) side groups connected with a decylene spacer (PMCB10M) are compared in the bulk (>10 nm), at the inner-surface (∼10 nm), and at the near-surface (<2 nm) using polarized UV absorption and near-edge X-ray absorption fine structure (NEXAFS) spectroscopies. The biaxial reorientation characteristics in the bulk of the PMCB10 M films can be controlled by irradiating with linearly polarized (LP) UV light and subsequent annealing to generate self-organization of the MCB side groups. However, the homogeneous in-plane orientation at the near-surface, which can introduce homogeneous nematic low-molecular-weight liquid crystal mixture alignment, is observed regardless of the self-organization process. The differences in the orientation characteristics arise from long alkylene spacer and bulky mesogenic side groups.

Side chain liquid crystalline polymers with an optically active polynorbornene backbone and achiral mesogenic side groups

This work describes a series of side-on and end-on SCLCPs with an optically active polynorbornene main chain and achiral mesogens. The side-on SCLCPs tend to form achiral mesophases, while their comparative end-on analogues exhibit chiral mesophases.

Photo-isomerization effect of the azobenzene chain on the opto-mechanical behavior of nematic polymer: A molecular dynamics study

The opto-mechanical properties of a photo-responsive nematic polymer network (PRPN) are investigated using molecular dynamics simulation. For the implementation of the trans-to-cis isomerization of azo compounds, a switchable potential formalism for the N = N bond is applied to the crosslinked PRPN unit cell model. During the light switch-on and heating-up simulations at a wide range of temperatures, the scalar orientational order parameter for the mesogenic side group molecules, the effective photo-induced strain of the bulk polymer network, and the opto-mechanical properties are characterized. The correlation between the microstate which belongs to the molecular location and the macroscopically observed photostrain is identified according to the isomerization ratio of the diazene groups.

Photoinduced orientation of photoresponsive copolymers with N-benzylideneaniline and nonphotoreactive mesogenic side groups

The photoinduced reorientation behaviors of liquid crystalline (co)polymethacrylates (LCPs) comprised of N-benzylideneaniline (NBA) and 4-cyanobiphenyl or 4-methoxybiphenyl side groups are compared using linearly polarized (LP) 313 nm and LP 365 nm light. LCP films realize effective cooperative reorientation based on trans-cis-trans photoisomerization of the NBA moieties using LP 365 nm light and stabilization of the orientation structure due to copolymerization. In contrast, photochemical hydrolysis lowers the reorientation efficiency of LCP films when using 313 nm light. Changing the polarization direction of the LP 365 nm light controls the reorientation direction of the LCP films for several cycles, but the in-plane orientation order gradually decreases. Finally, LP 365 nm exposure and subsequent solvent annealing using a photo-cross-linked film achieve a repeatable write-erase-write of the orientation structure of the copolymer film.

NEXAFS Study on Photoalignment of Liquid Crystalline Polymeric Films

Polarization-dependent near-edge X-ray absorption fine structure (NEXAFS) measurement is performed to investigate orientation structures of the near-surface of the reoriented photo-cross-linkable liquid crystalline polymer (PMCB6M) films, which are applicable to liquid crystal (LC) alignment layer. The 3D orientation parameter at the near-surface of the reoriented PMCB6M film is ∼0.7, which is similar to the results obtained by a polarization absorption spectroscopy that evaluates the reorientation behavior in bulk. Furthermore, more preferable in-plane orientation is observed at the nearest surface of the film, and the phenyl rings in the reoriented mesogenic side groups show slightly perpendicular orientation to the surface normal.

Comb-shaped liquid crystalline stereoregular cyclolinear methylsiloxane copolymer containing lactic acid derivative as chiral mesogenic side group

Comb-shaped liquid crystalline (LC) stereoregular cyclolinear methylsiloxane copolymers with mesogenic chiral lactic side group were synthesized. X-ray study showed that tacticity of the main cyclolinear methylsiloxane chain influences intra- and intermolecular ordering in LC state.

Molecularly Oriented Liquid Crystalline Polymer Films Based on Axis-Selective Photo-Fries Rearrangement

Four kinds of liquid crystalline polymers (LCPs) with phenyl benzoate side groups were synthesized and their photoinduced reorientation behavior was investigated using linearly polarized (LP) UV light based on the axis-selective photo-Fries rearrangement and thermally induced self-organization of the mesogenic side groups of the materials. Due to different effect of the axis-selectively photoreacted side groups, reorientation direction and its performance depended on the chemical structure of the side groups. Both alignment ability and the molecular rotation of the photoreacted side groups played an important role in the reorientation behavior.

Mesogen-jacketed liquid crystalline polymers and elastomers bearing polynorbornene backbone

This manuscript describes a series of mesogen-jacketed liquid crystalline polymers (MJLCPs) and elastomers (MJLCEs) bearing a novel polynorbornene backbone, onto which the liquid crystalline mesogen is laterally attached by a single covalent bond without spacers. The side-on mesogenic monomers are synthesized via palladium-catalyzed exo-hydroarylation of norbornadiene, and then polymerized using a Grubbs' catalyst initiated ring opening metathesis polymerization (ROMP) method. MJLCEs are prepared by mixing the MJLCPs with bifunctional benzophenone cross-linkers, pulling fibers to achieve a fine alignment, and crosslinking under UV illumination. Their mesomorphic, thermo-actuation and mechanical properties are studied in detail by a combination of 1H NMR, gel permeation chromatography, thermogravimetric analysis, differential scanning calorimetry, polarized optical microscopy, X-ray diffraction and dynamic mechanical analysis. These newly designed polymers can decouple the dynamics of the very bulky and rigid polynorbornene main chain and the mesogenic side groups to achieve liquid crystallinity, only if the length of alkoxy terminal chains of the mesogens is long enough (the number of the carbons in the alkoxy group, n > 9). Moreover, interesting external force induced reversible thermal-actuation behaviors of smectic MJLCEs are observed and analyzed as well.

Photoinduced Reorientation of a Liquid Crystalline Polymer with Phenyl Benzoate Mesogenic Side Groups on the Basis of an Axis-Selective Photo-Fries Rearrangement

Many types of photoreactive polymers have been investigated to regulate their molecularly orientated structures for polarization-controlled optical and display applications. Herein, we demonstrate the axis-selective photoreaction of thin films and their molecular reorientation behaviors using four types liquid crystalline poly(meth)acrylates with phenyl benzoate side groups and linearly polarized (LP) 248 nm light. These simple liquid crystalline polymers (LCPs) exhibit sufficient photoinduced molecular reorientations. Photoinduced molecular reorientation based on an axis-selective photo-Fries rearrangement is simultaneously generated with an in-plane order parameter (S) up to 0.33 when the irradiation temperature is in the lower part of the LC temperature range of the material, but multidomain formation of the mesogenic groups inhibits reorientation. To prevent thermally generated multidomain formation, an axis-selective photoreaction of an amorphous film followed by thermally generated self-organization...

Effect of Field Strength on Electrooptical Properties of Low-Molecular Weight and Polymer Liquid Crystals

The influence of a strong electric field on isotropic melts of 6-cyanobiphenyl and comb-shaped polyacrylate with side mesogenic groups was investigated. It was established that the electric field induces the isotropic—nematic phase transition in melts of these compounds. A relaxation process was discovered that destroys the nematic ordering induced in substances by electric fields. It was found that the relaxation time τ of the orientational order parameter is proportional to the square of electric field strength. For comb-shaped polymer liquid crystals, the time of transition from the nematic to the isotropic phase is several orders longer then that for low-molecular liquid crystal.

Comb-shaped liquid crystalline stereoregular cyclolinear methylsiloxane copolymers: synthesis, behaviour in bulk and behaviour in monolayers

Stereoregular cyclolinear organosiloxane copolymers containing vinyl groups with diorganosiloxane spacers as well as cyclohexasiloxane fragments were synthesised. Comb-shaped liquid crystalline (LC) cyclolinear polymethylsiloxanes with mesogenic side groups were prepared by hydrosilylation of stereoregular cyclolinear organosiloxane copolymers with (4′-cyanobiphenyl-4-yl)-11-(1,1,3,3-tetramethyldisiloxyl)undecanoate in the presence of the Karstedt catalyst. The chemical structure of the copolymers synthesised was confirmed by proton and silicon nuclear magnetic resonance spectroscopies and infrared spectroscopy. Their phase behaviour was characterised by differential scanning calorimetry, X-ray analysis and optical polarising microscopy. It was shown that the appearance of the LC state, the temperature range of its existence and polymesomorphism are determined by the molecular mass of the copolymer and by the content of the mesogenic groups. The introduction of three mesogenic groups per monomer unit results in polymesomorphic transitions. The tacticity of the backbone influences the interchain distances in the comb-shaped LC cyclolinear methylsiloxane copolymers. Studying the Langmuir films of the stereoregular LC methylsiloxane copolymers revealed that different types of molecular arrangements could be observed depending on the number of mesogenic groups attached. LC copolymers having three mesogens per monomer unit demonstrate a two-stage monolayer collapse.

Cholesteric Liquid Crystalline Copolymers for Gas Chromatographic Separation of Polycyclic Aromatic Compounds

A novel series of side-chain liquid crystalline copolysiloxanes containing [S]-1-(2-naphthyl) ethyl 6-[4-(10-undecen-1-yloxy) biphenyl-<svg style="vertical-align:-0.0pt;width:12.4375px;" id="M1" height="17.7125" version="1.1" viewBox="0 0 12.4375 17.7125" width="12.4375" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(.017,-0,0,-.017,.062,17.65)"><path id="x34" d="M456 178h-96v-72q0 -51 12.5 -62.5t72.5 -16.5v-27h-256v27q65 5 78 17t13 62v72h-260v28q182 271 300 426h40v-407h96v-47zM280 225v295h-2q-107 -148 -196 -295h198z" /></g> <g transform="matrix(.012,-0,0,-.012,8.225,9.488)"><path id="x2032" d="M227 744l-123 -338l-31 15l73 368q12 3 41.5 -8t36.5 -20z" /></g> </svg>-carbonyloxy]-2-naphthoate mesogenic and 4-biphenyl <svg style="vertical-align:-0.0pt;width:12.4375px;" id="M2" height="17.7125" version="1.1" viewBox="0 0 12.4375 17.7125" width="12.4375" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(.017,-0,0,-.017,.062,17.65)"><use xlink:href="#x34"/></g> <g transform="matrix(.012,-0,0,-.012,8.225,9.488)"><use xlink:href="#x2032"/></g> </svg>-allyloxybenzoate mesogenic side groups in the backbone and side chains liquid crystalline copolymers were prepared and evaluated as possible stationary phases for gas chromatography capillary columns. All copolymers display enantiotropic cholesteric phases. These mesomorphic polysiloxanes specimens with the widest temperature range were used as the stationary phase in a gas chromatography capillary column, and it showed good thermal and physical stability, excellent chemical inertness, and unique separation properties for polycyclic aromatic compounds. These cholesteric LC copolysiloxane stationary phases show much better separation effect for the polycyclic aromatic compound than those of the nematic and smectic LC copolysiloxanes.

Synthesis and Performance of a Novel Coumarin-Containing Side-Chain Liquid Crystalline Polymer

A new type of able liquid crystalline polymer with a coumarin-containing mesogenic side group that could be photo-crosslinked was synthesized in this paper. The structure of the intermediates, monomers and polymers were characterized with FTIR and 1HNMR measurement. DSC, TG, and hot stage polarized optical microscopy were employed to study the phase transition temperature, mesophase texture, and thermal stability of the liquid crystalline polymers. The results indicated that both the monomer and polymer exhibited liquid-crystalline features over a wide temperature range.

Photoresponsive Liquid-Crystalline Polymers Containing a Block Mesogenic Side Chain: Systematic Studies on Structural Parameters for Nanophase-Separated Structures

A family of liquid-crystalline polymers (LCPs) containing a block mesogenic side group was synthesized by varying the hydrophilic moiety (oligooxyethylene) ratio. As the oligooxyethylene content increased, the range of the LC phase narrowed or vanished. The polymers were subjected to annealing and exposed on the surface of films, and their morphologies were observed using an atomic force microscope. While the annealed films prepared from the LCPs showed nanophase separation on the surface, the film of the amorphous polymer exhibited no phase separation. Under the optimized conditions, two types of irradiation procedures were adopted: (i) UV (365 nm) light illumination for a phase transition induced by trans–cis photoisomerization and (ii) linearly polarized visible (>436 nm) illumination for a photoinduced alignment change. These results revealed that the photoresponsive behavior of the nanostructure is strongly coupled to the molecular alignments induced by annealing.

Generation of Anisotropic Emission by Light-Induced Orientation of Liquid Crystalline Polymers

Functionalized polymers containing mesogenic, photosensitive, and fluorescent side groups were synthesized for all-in-one films with anisotropic absorption and emission. Aligned films were prepared by a two-step bulk-alignment procedure combining a photochemical step at room temperature and a thermal aligning step at temperatures of the mesophases. The irradiation with linearly polarized light results in small values of anisotropy via angular-selective photoreactions of cinnamates, stilbenes, and anthracene moieties. The photoinduced anisotropy was significantly amplified by annealing above the glass transition temperature of the liquid crystalline polymers, resulting in a cooperative alignment of all side groups leading to dichroism values up to 0.7 (dichroic ratio up to 6.7) and ratios of anisotropic fluorescence up to 8. The irradiation conditions for the induction of anisotropy were optimized concerning wavelength and dose with respect to the absorption of photosensitive groups and the limited photostability of the dyes. Depending on the distribution of the photoproducts, alignment parallel or perpendicular to the electric field vector of the incident light was induced in dependence on the polymer composition and irradiation conditions.

Polarized Light Emission in a Liquid Crystalline Copolymer Film Comprised of Fluorenevinylene and Photo-Cross-Linkable Mesogenic Side Groups

A polymethacrylate copolymer comprised of light-emissive cyano-substituted dihexylfluorenevinylene (CN-DHFV) side groups and photo-cross-linkable 4-(4-methoxycinnamoyloxy)biphenyl (MCB) mesogenic side groups was synthesized and characterized. Irradiating a thin copolymer film with linearly polarized ultraviolet (LPUV) light and subsequent annealing generated a cooperative molecular reorientation of both side groups parallel to the polarization (E) of LPUV light. The resulting reoriented film showed a polarized light emission of the CN-DHFV groups with a polarization ratio of 5.6.

Dynamics of the Kerr effect in the isotropic phase of comb-shaped nematic polymers

The equilibrium Kerr effect and its dynamics are investigated in isotropic melts of the comb-shaped LC polymer poly(4-[(4′-cyano-1,1′-biphenyl-4-yl)oxy]butyl-2-acrylate) and its copolymers with acrylic acid or methyl methacrylate and isotropic melts of the LC ionomer containing rubidium ions in a wide temperature range. A sharp change in the temperature dependence of relaxation times for the induced macroscopic orientational order is found for the LC copolymers and the LC ionomer in the temperature range 10–15 K higher than the temperature of the nematic-isotropic phase transition. In contrast, the phenomena caused by the small-scale dynamics, such as electric conductivity and dipole relaxation, are approximated by the conventional Arrhenius type dependence, and the equilibrium electrooptical properties of the copolymers are well characterized via the Landau-De Gennes model. The nonclassical behavior of macroscopic relaxation times may be associated with temperature variations in the contribution of main polymer chains to the orientation macroscopic dynamics of the melt, in which the decisive role is played by side mesogenic groups.

Novel mechanical properties in lamellar phases of liquid-crystalline diblock copolymers

Structural properties of flexible nematic diblock copolymers in the lamellar phase are investigated using a mean-field model. We address two complementary questions on the mechanics of the system: 1) How does the nematic order affect the elasticity of the one-dimensional solid? 2) What effect does the block copolymer microstructure has on the orientation of the nematic director? In the limit when the microstructure does not influence the nematic director orientation we predict a soft lamellar compression mode. When the microstructure does influence the nematic director orientation, small compressions lead to conventional elasticity, until a critical strain is reached, where there is then a transition to a softer response. On the other hand, we show that an identifiable lamellar symmetry provides a direction along which the nematic director prefers to align. Our model provides avenues to explore nonlinear properties of flexible diblock copolymers in which the monomers on both sides have mesogenic side groups.

Cyclic Azobenzene-Containing Side-Chain Liquid Crystalline Polymers: Synthesis and Topological Effect on Mesophase Transition, Order, and Photoinduced Birefringence

A cyclic side-chain liquid crystalline polymer (SCLCP) bearing azobenzene mesogens, namely, poly{6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate} (PAzoMA), was successfully synthesized by using “click” cyclization of the linear polymer precursor with alkyne and azide end groups. Samples of cyclic-PAzoMA of various molecular weights were prepared, characterized, and studied in comparison with their linear counterparts. The results show that the topological constraint arising from the tortuosity of the ring structure and the absence of chain ends in cyclic-SCLCPs affects profoundly the liquid crystalline (LC) phase transitions (temperature, enthalpy, and entropy) of mesogenic side groups and that this topological effect is more prominent for smaller SCLCP rings. Moreover, the photoinduced anisotropy in films as a result of the trans−cis photoisomerization of azobenzene mesogens was investigated, and cyclic-PAzoMA was found to behave differently from linear-PAzoMA. On the one hand, the cyclic polymer exh...

Photoinduced Microphase Separation in Block Copolymers: Exploring Shape Incompatibility of Mesogenic Side Groups

Photoinduced microphase separation in block copolymers (BCP) was achieved for the first time, using a rationally designed diblock copolymer composed of two side-chain liquid crystalline polymers (SCLCP). The miscibility of the two blocks was promoted by the miscibility between the two types of mesognic side groups, while upon UV exposure inducing the trans-cis isomerization of azobenzene mesogens on one SCLCP, the shape incompatibility of bent cis isomers with an ordered liquid crystalline phase drove the separation of the two blocks resulting in a microphase separated morphology. This result shows the perspective of using light to process and organize BCP morphology and related nanostructures in a lithography-free manner.