Mesogenic Groups Research Articles

The experimental evidence of multi-step δ-relaxation mode in liquid crystalline side chain polymers by time-resolved fluorescence emission spectroscopy

There has been recognized that there exist three basic relaxation modes in the molecular dynamics of liquid crystalline side chain polymers (SCLCP) – the α-, β- and δ-relaxation process, and the molecular mechanism of the δ-relaxation has been in controversy, i.e. whether it is a “flip-flop” transition or a continuous rotation. The current study provides the experimental evidence of the multiple steps relaxation by time-resolved emission spectroscopy, taking the intrinsic fluorescence from the model mesogen group of 4-cyanobiphenyl. The temperature dependence of the spectra and its dynamics provides new information on the microscopic picture of the δ-relaxation.

Block Copolyesters Containing 2,5-Furan and trans-1,4-Cyclohexane Subunits with Outstanding Gas Barrier Properties.

Biopolymers are gaining increasing importance as substitutes for plastics derived from fossil fuels, especially for packaging applications. In particular, furanoate-based polyesters appear as the most credible alternative due to their intriguing physic/mechanical and gas barrier properties. In this study, block copolyesters containing 2,5-furan and trans-1,4-cyclohexane moieties were synthesized by reactive blending, starting from the two parent homopolymers: poly(propylene furanoate) (PPF) and poly(propylene cyclohexanedicarboxylate) (PPCE). The whole range of molecular architectures, from long block to random copolymer with a fixed molar composition (1:1 of the two repeating units) was considered. Molecular, thermal, tensile, and gas barrier properties of the prepared materials were investigated and correlated to the copolymer structure. A strict dependence of the functional properties on the copolymers’ block length was found. In particular, short block copolymers, thanks to the introduction of more flexible cyclohexane-containing co-units, displayed high elongation at break and low elastic modulus, thus overcoming PPF’s intrinsic rigidity. Furthermore, the exceptionally low gas permeabilities of PPF were further improved due to the concomitant action of the two rings, both capable of acting as mesogenic groups in the presence of flexible aliphatic units, and thus responsible for the formation of 1D/2D ordered domains, which in turn impart outstanding barrier properties.

Highly efficient chirality inducers in nematic liquid crystals: synthesis of 7,7’-disubstituted 2,2’-methylenedioxy-1,1’-binaphthyls

ABSTRACTWe report the synthesis of 7,7ʹ-disubstituted 2,2ʹ-methylenedioxy-1,1ʹ-binaphthyls and demonstrate their application as chiral dopants in the nematic mixture MLC-6260 to produce highly twisted cholesteric phases. Especially mesogenic and/or polarizable groups in the 7,7ʹ-positions of the bridged binaphthyls generate unusually high helical twisting powers.

Synthesis and Photoinduced Anisotropy of Polymers Containing Nunchaku-Like Unit with an Azobenzene and a Mesogen.

A series of polymers containing nunchaku-like unit with an azo chromophore and a mesogen group was successfully prepared and photoinduced anisotropy of the obtained polymers was minutely investigated. Firstly, monomers containing nunchaku-like unit with an azo chromophore and a mesogen group linked by flexible group were synthesized. The structure of the monomers was confirmed via NMR COSY spectra. Subsequently, the obtained monomers were polymerized into corresponding polymers through RAFT polymerization. The prepared polymer samples were characterized through NMR, FTIR, gel permeation chromatography (GPC), and UV-vis testing while the thermal properties of the samples were investigated through differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA) measurements. The photoinduced isomerization of the polymers, which was researched in situ via measuring UV-vis spectra of the polymer solutions and spin-coated films under irradiation with 450 nm light or putting in darkness, demonstrated the rapid trans-cis-trans isomerization of the polymers. When irradiated with a linearly polarized light, significant photoinduced birefringence and dichroism were observed, suggesting photoinduced isomerization of azobenzene can drive orientation of mesogen in the system. This study blazes a way to design the optical materials with light-controllable birefringence and dichroism.

Analysis of curing reaction of liquid-crystalline epoxy compositions by using temperature-modulated DSC TOPEM\xae

The curing behaviour of the composition of a liquid-crystalline diepoxy monomer (LCEM) with the central triaromatic mesogenic group was studied using differential scanning calorimetry, conventional DSC and temperature-modulated TMDSC and nuclear magnetic resonance 1H-NMR spectroscopy. The stoichiometric amount of primary aromatic diamine, 4,4′-diaminodiphenylmethane (DDM), was used as a curing agent. TMDSC TOPEM® allowed the separation of the thermal effects related to reversible processes (e.g. phase transitions) from irreversible processes (cross-linking reaction) and determining that irreversible curing reaction occurs at the same time as reversible transition of the LCEM from crystal to the liquid-crystalline nematic phase. Additionally, 1H-NMR analysis of the LCEM/DDM mixture preheated to a temperature from the temperature range of complex thermal changes was conducted. The obtained spectrum clearly showed the presence of partially cured products in the investigated samples and additionally proved that during the endothermic phase transition of the LCEM monomer an exothermic cross-linking reaction was occurring.

Ambipolar Discotic Liquid Crystals Built Around Platinum Diimine-Dithiolene Cores.

Platinum diimine dithiolene complexes bearing mesogenic groups on one or both ligands have been prepared through an original ligand metathesis reaction to introduce the dithiolene ligand. The neutral diimine ligands, the intermediate platinum dichloride diimine complexes, and the target compounds were characterized by a combination of electronic (electrochemistry, absorption and emission spectroscopy, DFT calculations) and structural (SAXS, DSC) tools. Several novel liquid crystalline platinum diimine-dithiolene were identified over a large temperature range, and the systems were endowed with ambipolar properties, associated with the high reversibility of both oxidation and reduction processes.

Design and phase transition behavior of siloxane-based monomeric and dimeric liquid crystals bearing cholesteryl mesogenic groups

A series of siloxane-based monomeric and dimeric liquid crystals (LCs) bearing cholesteryl mesogenic groups were newly synthesized and their phase transition behavior was investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). All the siloxane-based LCs obtained in this study showed a smectic A phase in a wide temperature range. Among the siloxane-based LCs obtained, only the dimeric LC with a shortest siloxane unit at a central part of a spacer was found to exhibit a chiral nematic phase and a blue phase in addition to the smectic A phase. The difference in the thermal properties of the siloxane-based dimeric LCs was investigated in terms of their conformations. Furthermore, the results obtained for the siloxane-based LCs were discussed with comparing with those of homologues having the normal alkyl chains and spacers.

Proton and Deuterium NMR Study of the CBC9CB Dimer System.

Using proton and deuterium NMR spectroscopy, this work provides a description of the molecular orientational order of the mesogenic groups in both the Ntb and the classical nematic phase (N) of the liquid crystal dimer CB-C9-CB. The proton NMR data were collected at high field (7 T) with the nematic domains aligned by the field. Deuterium NMR data obtained from aligned samples at 11.7 T, published by Hoffmann , A. Soft Matter 2015 , 11 , 850 , were also considered in this study. Using the first-order perturbation theory, we carried out detailed simulations of the proton spectra from the terminal mesogenic groups along with the quadrupolar splittings from the carbon-deuterium bonds in the first chain positions, which allow for the determination of the Saupe order tensor for the mesogenic groups. We show that the main mechanism that induces the change of the 1H NMR spectrum and the quadrupolar splittings at the N-Ntb phase transition is the change of the orientation of the most ordered molecular frame (eigenframe of the Saupe tensor), along with the onset of the molecular biaxiality parameter, D. This orientation change is associated with the achiral symmetry breaking at the N-Ntb phase transition.

Self-organized wrinkling of liquid crystalline polymer with plasma treatment

Abstract

Supramolecular complexes based on liquid‐crystalline polysiloxanes and copper phthalocyanine

AbstractSelf‐assembly of mesogens in one polymer system reflects the capability for integrating intermolecular interactions and functions of materials. A series of novel supramolecular complexes (SCPs) containing cholesterol derivatives, siloxane backbone and copper phthalocyanine were synthesized. The SCPs were made from sulfonic acid‐containing precursor polymers (SAPs) synthesized by the use of 4‐(allyloxy)benzenesulfonic acid, cholest‐5‐en‐3‐ol‐(3β)‐4‐(allyloxy)benzoate and polymethylhydrosiloxane. Differential scanning calorimetry, polarizing optical microscopy and X‐ray diffraction results confirmed the mesomorphic properties. The SAPs showed smectic phase, but the SCPs showed columnar phase due to more lateral copper phthalocyanine groups in the polymer systems. Moreover, because of rigid side groups and cholesteric polymer, SCPs showed greater d‐spacings between the side mesogenic groups and exhibited very good electrical conductivity. © 2018 Society of Chemical Industry

Synthesis of amphiphilic comb-like liquid crystalline diblock polyethers and their self-assembly in solution

We have designed and synthesized a series of novel polyether based amphiphilic liquid crystalline (LC) diblock copolymers composed of pendent hydrophobic LC mesogens and hydrophilic ethylene oxide (EO) oligomers by using sequential anionic polymerization. The majority of this comb-like diblock copolymer is constituted by EO segments except the mesogenic groups which generate the chemical dissimilarity and drive the phase segregation. We have investigated the morphology evolution of the diblock copolymers in dioxane/water co-solvent with continuous tuning of the quality of the solvent, and discovered a systematic morphological transformation from a spherical micelle to a spherical vesicle, and finally to a faceted vesicle with increase of the water content above a critical value. The formation of the structured vesicle is related to the emerging of LC order within the membranes as revealed by in situ wide-angle X-ray diffraction experiments on the polymer solution. The elastic instability imparted from collective LC interactions which is incommensurate with the global topological geometry dictates this unique behavior. This type of faceted capsules on the tens of nanometer scale are expected to be potentially useful in biotechnological applications or drug delivery.

The role of a terminal chain in promoting the twist-bend nematic phase: the synthesis and characterisation of the 1-(4-cyanobiphenyl-4′-yl)-6-(4-alkyloxyanilinebenzylidene-4′-oxy)hexanes

ABSTRACTThe syntheses and characterisation of the first 10 homologues of the 1-(4-cyanobiphenyl-4′-yl)-6-(4-alkyloxyanilinebenzylidene-4′-oxy)hexanes (CB6O.Om) are reported. All 10 homologues exhibit nematic, N, and twist-bend nematic, NTB, phases. In addition, an intercalated, anticlinic smectic C, SmCA, phase is observed for m = 3–5. A second smectic phase is seen for m = 4,5 but for which no structural information was obtained. For short chain lengths, the local packing in the nematic phases is an intercalated arrangement. As the chain length increases monolayer fluctuations are observed in both nematic phases. The NTB-N and N-isotropic transition temperatures exhibit a weak odd-even effect as m is increased in which even members show the higher values. This behaviour is accounted for in terms of the change in shape anisotropy and the dilution of the interactions between the mesogenic units on increasing m. The N-I transition temperature falls faster than the NTB-N temperature on increasing m suggesting that the dilution of the interactions between the mesogenic groups has a smaller effect on the shape-driven NTB-N transition than on N-I. Increasing the terminal chain length does not strongly promote smectic behaviour for the CB6O.Om series. This is attributed to the difficulties associated with packing these bent molecules into smectic arrangements.

Mesomorphic and structural properties of liquid crystalline side-chain polymethacrylates: from smectic C* to columnar phases

ABSTRACTThe chiral methacrylate monomers with photosensitive azobenzene group possessing the orthogonal smectic A* and tilted smectic C* (Sm-C*) phases have been synthesised and characterised. The monomers have been used as functional side chains for the design of corresponding polymethacrylates. X-ray diffraction has been applied to elucidate the structure and phase behaviour of liquid-crystalline side-chain polymethacrylates with azobenzene-containing central core, chiral fragments and aliphatic spacers and tails of different length. X-ray patterns of polymethacrylates oriented fibres impose the tilted Sm-C* order as a basic structure of these materials. This is complemented by a regular pattern of small-angle diffuse spots, which implies complex positional order on the local scale and serves as a precursor for the formation of a columnar phase. The increase of the total length of the aliphatic tail and spacer of the side-chain fragments leads to formation of the tilted columnar phase (Coltilt*) with two-dimensional monoclinic lattice. For the polymer containing 10 methylene units in both, spacer and aliphatic tail, the Coltilt* precedes the formation of the Sm-C* phase. The observed structural changes are explained as due to coupling between the smectic ordering of the mesogenic side groups and the polymer backbone conformation.

Induced Homeotropic Alignment of Nematic Liquid Crystals by Doping Side-on Carbosilane-based Oligomers

In this study, carbosilane-based oligomers with tetraallylsilane in the core whose periphery was modified with side-on type mesogenic groups were synthesized with systematic change in the spacer le...

The effect of the shape of a mesogenic group on the structure of supramolecular aggregates based on wedge-shaped and cone-shaped dendrons

This review addresses the key principles underlying the formation of liquid crystalline phases based on wedge-shaped and cone-shaped dendrons of different chemical nature. Despite rich phase diversity of the above systems, the development of a mesophase can be reliably predicted by the geometric model, which compares the shape of a wedge dendron with the radial density distribution in the relevant Voronoi polyhedra. 2D columnar phases formed by chiral [7]-heterohelicene molecules with long aliphatic side chains are described in detail. The as-formed columnar aggregates are shown to possess a helical 132 symmetry and are composed of thirteen blocks, and each block involves six molecules. In this case, the internal structure of the first-level (lower) supramolecular aggregate appears to be different from that of the helical supramolecular structure.

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.

(Invited) Stimuli Responsive Liquid Crystal Elastomers/Gels

Liquid crystal elastomers (LCEs) are rubbery polymer networks containing the mesogenic groups in their main and/or side chains, and liquid crystal gels (LCGs) are the LCEs swollen by LC solvents. A unique feature of LCEs and LCGs is the strong correlation of the macroscopic shape (deformation) and the LC alignment. This feature enables us to actuate the LCEs and LCGs by various types of external field which change the orientation order of LCs such as temperature change, electric field, solvent uptake etc. Another important feature of LCEs and LCGs is that one can control the mode of induced deformation by programming the type of the alignment of the “director” (the vector representing the sectoral average molecular orientation of LC) in the stage of cross-linking. In addition, LCEs and LCGs exhibit interesting mechanical properties as a result of the coupling of imposed strain with the director realignment. Thus LCEs and LCGs show a rich variety of intriguing stimulus response behavior due to the combination of LCs and elastomers/gels, and they are a promising material for soft actuators and sensors. The cholesteric LCEs and LCGs (Ch-LCEs and Ch-LCGs) possessing helical director configuration with regular pitch behave as “photonic elastomers”, because the cholesteric LCs exhibit unique optical property, i.e., selective reflection for the incident circularly polarized light: They selectively reflects the incident light with circular polarization whose handedness is the same as that of helical director configuration, and whose wavelength corresponds to the helical pitch (Bragg reflection). We demonstrate that the reflection band of Ch-LCEs can be varied by imposed compressive strain [1] as well as temperature change [2] with maintaining the selectivity. Ch-LCGs exhibit pronounced electro-opto-mechanical effect, i.e, finite deformation together with significant change in reflection spectra.[3] Further, the Ch-LCE film with the helical axis normal to the thickness show periodical surface undulation which corresponds to the helical director configuration in response to temperature variation.[4] The nematic LCEs with twist or hybrid director alignment exhibit twist or bending deformation in response to temperature change.[5,6] When these LCEs are allowed to swell in solvents, the temperature change drives the twist or bending deformation coupled to volume change. The total variation and the T-sensitivity of twist pitch or curvature of these LCGs significantly depend on whether the solvent has nematicity or does not.[7] [1] Varanytsia, A., Nagai, H., Urayama, K., Pallfy-Muhoray, P., Sci. Rep., 5, 17739 (2015). [2] Nagai, H., Urayama, K., Phys. Rev. E, 92, 022501 (2015). [3] Fuchigami, Y., Takigawa, T., Urayama, K., ACS Macro Lett., 3, 813 (2014). [4] Nagai, H., Liang, X., Nishikawa, Y., Nakajima, K., Urayama, K., Macromolecules, 49, 9561 (2016). [5] Sawa, Y., Fangfu, Y., Urayama, K., Takigawa, T., Gimenez-Pinto, V., Selinger, R., Selinger, J., PNAS, 108, 6364 (2011). [6] Sawa, Y., Urayama, K., Takigawa, T., DeSimone, A., Teresi, L., Macromolecules, 43, 4362 (2010). [7] Doi, H., Urayama, K., Soft Matter, 13, 4341 (2017).

Surface and interface designs in side-chain liquid crystalline polymer systems for photoalignment

In side-chain liquid-crystal (SCLC) polymer systems, the liquid crystalline (LC) mesogenic groups preferentially orient normal to the substrate plane due to the excluded volume effect (homeotropic alignment). Photoresponsive azobenzene (Az) mesogens have a transition moment parallel to the molecular long axis. Light irradiation for photoreactions is generally applied perpendicular to the film surface. Therefore, a homeotropic orientation inhibits efficient photoreactions and photoalignments in Az SCLC polymer systems. This review focuses on new approaches to induce a random planar orientation in Az SCLC polymer systems by interface and surface molecular design. The mesogens in a high-density SCLC polymer brush formed by surface-initiated living radical polymerization adopt a random planar orientation. In the film of an SCLC block copolymer with an amorphous block, a random planar orientation is induced via surface segregation of either of the blocks. The random planar orientations of SCLC polymer systems are thermally stable and offer efficient in-plane photoalignment and photoswitching with hierarchical LC molecular architectures, forming, e.g., microphase-separated (MPS) SCLC block copolymers and layered polymer LC systems. These surface and interface molecular designs are expected to provide new concepts and possibilities for LC polymer devices. The liquid crystalline (LC) rod-like mesogens preferentially orient normal to the substrate plane due to the excluded volume effect (homeotropic alignment) in side-chain liquid-crystal (SCLC) polymers free-standing films. For in-plane alignment, a homeotropic orientation is unsuitable because the mesogens are in a direction opposite to the in-plane directions. This review focuses on new approaches to induce a random planar orientation in SCLC polymer systems by interface and surface molecular design utilizing a high-density polymer brush structure and surface segregation structures of block copolymers.

Non-symmetric chiral nematic liquid crystal dimers containing trifluoromethyl and 1,2-propanediol

ABSTRACTA series of non-symmetric liquid crystal (LC) dimers with the same chiral core 1,2-propanediol (PD) have been synthesised, termed as ABBA-PD-TFBA, PBBA-PD-TFBA, ABA-PD-TFBA, PBA-PD-TFBA and AA-PD-TFBA, respectively, in which one of the two mesogenic groups, the fluorinated mesogenic unit, was kept fix and the other arm was different. The intermediate compounds and LC dimers were characterised by FTIR, 1H NMR, differential scanning calorimetry, thermogravimetric analysis, polarised optical microscopy and X-ray diffractometer (XRD). The results of the measurements indicated that ABBA-PD-TFBA, PBBA-PD-TFBA and ABA-PD-TFBA displayed optical activity and enantiotropic chiral nematic phase, and PBA-PD-TFBA was an enantiotropic nematic LC while AA-PD-TFBA was a monotropic LC, displaying both nematic phase and smectic A phase on cooling. The results indicated that PD was able to induce the chiral nematic phase, nevertheless, the rigidity of the mesogenic arm, the flexibility of the terminal group and even the type of the terminal chemical bond played an important effect on the thermal properties of the dimers, and even on the formation of the chiral nematic phase. It is also worth noting that C=C at the terminal helped to stabilise the LC phase.

Self-Assembly of Supramolecular Aggregates Based on Sector- and Cone-Shaped Dendrons and Bolaamphiphiles

Using a number of classes of such sector-shaped macromolecules as derivatives of 2,3,4- and 3,4,5- tri(dodecyloxy)benzenesulfonic acid and dendrimers based on gallic acid as an example, the main stages in the formation of supramolecular ensembles are considered: the formation of individual supramolecular aggregates due to the weak noncovalent interactions of mesogenic groups, and the subsequent ordering within these aggregates, which lowers the free energy of a system. Supramolecular aggregates are in turn organized into two- or three-dimensional supramolecular lattices. It is shown that the shape of the supramolecular aggregates and its change along with temperature are functions of the chemical structure of the mesogenic group (resulting in the controlled design of complex self-organizing systems with a given response to external stimuli).