Mesogenic Units Research Articles

Self‐healing polymers for soft actuators and robots

AbstractAdvances in artificial intelligence technology bring a rising demand on stimuli responsive actuating materials and soft actuators. By the virtue of easy deformation and mechanical resilience, polymers with unique properties like shape‐memory, electro/magneto‐active, and thermal/humidity/chemical‐responsiveness have emerged as promising candidates for soft robotics. Damage tolerance or a degree of self‐healing ability improves soft robots endowed with such materials longevity, and provides safeguards for the function and potentially reduce costs. The combination of stimuli‐responsive actuation and self‐healing function involves elaborate supramolecular chemical and physical structure designs, being a challenging topic within the field of soft robotics and thus intriguing wide research interests. This review aims to discuss some highlights in convenient combination of stimuli‐responsive micro‐nanostructures and dynamic bonds chemistry within last 5 years, that further give the actuators recovery of mechanic and function, and improved comprehensive properties. Through a review of the most remarkable paradigms, the order organization of dynamic bonds with liquid crystal mesogenic units, functional micro‐nanofillers, hierarchical gradient interfaces, etc. are outlined. Finally, the perspectives and main challenges of self‐healable actuators are summarized. It is anticipated that this review will provide scientists with insights and guidance for the future innovative research in this field.

Synthesis and mesomorphic properties of bent liquid crystals containing triazole mesogenic unit with terminal flexible alkyl chain and laterally ethoxy substituted Schiff base

ABSTRACT A new series of bent-shaped liquid crystals containing a triazole ring were synthesised and characterised at ambient and elevated temperatures. The triazole-cored and bent liquid crystals possess a flexible alkyl chain at one terminal, while the other side of the core is connected by a laterally ethoxy Schiff base consisting of a CN-biphenyl moiety. The evaluation on optical and thermal properties was carried out by polarising optical microscope and differential scanning microscope which revealed the length of the alkyl chain, –CnH2n + 1, that influenced the mesophase and the related clearing temperatures (Tc). Whilst the compounds with higher number of n possess lower Tc points, the presence of SmA phase seemed to be more favourable for the compounds in which the alkyl chain length was n ≥ 12. This observation can be associated with its bending angle and charge distributions along the two arms of which the interaction between the opposite counterparts of both arms in these bent-liquid crystals would promote the formation of SmA phase. The computational calculations based on density functional theory were performed to further support the transition from non-mesomorphic to SmA phases upon the increase in alkyl chain length (n), dipole moment and polarity which contribute to its stability.

The relaxation dynamics and dielectric properties of cyanobiphenyl-based nematic tripod liquid crystals

We present a detailed investigation of the phase behaviour, molecular relaxation dynamics, rheology and dielectric properties of two cyanobiphenyl-based liquid crystal (LC) tripods, differing only in the length of their spacer units (6 or 9 carbons). These LC tripods combine properties of low molecular weight LCs and LC polymers, resulting in a range of advantageous properties including a wide nematic range (ΔT>90 K) and a large birefringence (0.3 at T−TNI=-80 K for the 6 spacer tripod). Using broadband dielectric relaxation spectroscopy, calorimetry, oscillatory and steady state shear rheology, we identified four molecular relaxation processes: the structural (α) relaxation (defining the glass transition temperature, Tg); the δ relaxation (reorientation of the mesogen unit around its short axis); the β relaxation (reorientation around its long axis); and the γ relaxation (internal tripod arm fluctuations). The β and γ relaxations follow Arrhenius temperature dependencies in the glass, whereas the α and δ relaxations merge above, but near Tg, where they follow a non-Arrhenius VFT behaviour. For higher temperatures, the two relaxations separate and for T>T⁎, where T⁎ marks a dynamic crossover, the α relaxation transitions from VFT to Arrhenius behaviour. For the two tripods, ratios of T⁎/Tg=1.14 and T⁎/Tg=1.13 were observed respectively, consistent with the ratio observed for many side-chain LC polymers (and other LC systems), and consistent with the ratio where a dynamic crossover is typically observed also for non-LC glass-formers. However, for non-LC systems, the transition to Arrhenius behaviour happens at significantly higher temperatures relative to Tg and the dynamic crossover at T⁎ is typically observed as a transition between two different VFT behaviours. We argue that for the tripods these differences arise from differences in the molecular relaxation mechanisms induced by the LC order. Finally, for temperatures where δ and α relaxations are separated, we find that ion conductivity decouples from the α relaxation, instead following the δ relaxation; this demonstrates that the ion transport properties for the tripods can be tuned by the design of the tripod mesogen arm.

Modeling Viscoelasticity and Dynamic Nematic Order of Exchangeable Liquid Crystal Elastomers.

Exchangeable liquid crystal elastomers (XLCEs), an emerging class of recyclable polymer materials, consist of liquid crystalline polymers which are dynamically crosslinked. We develop a macroscopic continuum model by incorporating the microscopic dynamic features of the cross-links, which can be utilized to understand the viscoelasticity of the materials together with the dynamic nematic order. As applications of the model, we study the rheological responses of XLCEs in three cases: stress relaxation, strain ramp, and creep compliance, where the materials show interesting rheology as an interplay between the dynamic nematic order of the mesogenic units, the elasticity from the network structure, and the dissipation due to chain exchange reactions. Not only being useful in understanding the physical mechanism underlying the fascinating characteristics of XLCEs, this work can also guide their future fabrications with desired rheological properties.

High Thermal Conductivity and Low Dielectric Constant of Polyesters Based on Fluorine and Mesogenic Unit

AbstractHigh thermal conductivity polymeric dielectrics with advanced dielectric properties are urgently demanded in various applications. In this study, a series of thermoplastic polyesters containing different content of fluorine and mesogenic unit in main chains are synthesized utilizing polycondensation. The preferred orientation of liquid crystal can effectively inhibit phonon scattering, and the introduction of fluorine can reduce the polarizability of the polymer and increase the free volume. By introducing the biphenyl mesogenic unit and fluorine into the main chain, its thermal conductivity has been increased to 0.425 Wm−1k−1 from 0.183 Wm−1k−1(the polymer without liquid crystal element), which is 132% higher than that of the liquid crystal polymer without fluorine. In addition, the dielectric constant of polyester decreased to 2.97, and the dielectric loss drops to 8.74 × 10−4. The influence mechanism of two ways on its thermal and dielectric properties is explored. Good thermal conductivity and excellent dielectric properties make potential in the thermal management system and flexible electronics.

Programming Positive Mechanofluorescence in Liquid Crystalline Elastomers

Programming Positive Mechanofluorescence in Liquid Crystalline Elastomers

Synthesis and Mesomorphic Properties of New Non-symmetric Liquid Crystal Dimers

Abstract: New non-symmetric liquid crystal dimers, N-(4-(n-(4-(benzothiazol-2-yl)phenoxy) alkyloxy) benzylidene)-2,4-difluoroaniline (DSFCn), containing two mesogenic core units of benzothiazole and benzylideneaniline were reported. The core system was connected by a flexible alkyl spacer, -(CH2)n-(where n =2-6, 8, 10, and 12). The spectroscopic techniques were employed to confirm the molecular structure of the title compounds. Differential scanning calorimetry and polarizing optical microscopy were used to study the mesomorphic properties of the synthesized compounds. Observation under crossed polarized light reveals that compounds (n = 4, 5, 6, 8, 10, 12) are purely nematogens. The nematic phase in the dimers are caused by the presence of the lateral fluorine atoms at the benzylideneaniline core. Mesophase thermal stability has been suppressed significantly by the fluorine atom at the lateral position which increased the molecular breadth and subsequently, results in weaker overall lateral intermolecular attraction. The structure-property relationships of the dimers were inferred through comparison of thermal properties of the present dimers with those of the earlier reported analogous that do not possess any lateral substituents.

Switchable Phase Helicity Independent of the Absolute Configuration of the Stereocenter: Anomalous Induction between Sergeants and Soldiers in Chiral Liquid-Crystalline Polymers.

Controlling the secondary phase in chiral liquid-crystalline (LC) polymers is of great importance since it transfers and amplifies molecular information to the macroscopic properties. However, the chiral superstructures of the LC phase are determined exclusively by the inherent configuration of the parent chiral source. Here, we report the switchable supramolecular chirality of heteronuclear structures by the untraditional command between common chiral "sergeant" units and various achiral "soldier" units. Different chiral induction pathways between sergeants and soldiers were observed for copolymer assemblies with mesogenic and non-mesogenic soldier units, demonstrating the formation of a helical phase independent of the absolute configuration of the stereocenter. In the presence of non-mesogenic soldier units, the classical SaS (Sergeants and Soldiers) effect in the amorphous phase was observed; whereas in a full LC system, bidirectional command of sergeants was activated in response to the phase transition. Meanwhile, a full spectrum of morphological phase diagrams including spherical micelles, worms, nanowires, spindles, tadpoles, anisotropic ellipsoidal vesicles, and isotropic spherical vesicles were successfully achieved. Such spindles, tadpoles, and anisotropic ellipsoidal vesicles have rarely been obtained previously from chiral polymer systems.

Investigating the Cusp Between Dendritic and Supermolecular Architectures in Self‐Organizing Liquid Crystals

AbstractLiquid Crystals are thought of as being complex fluids, underpinned by self‐assembly and self‐organisation within partially ordered fluids, and which sit between solid and liquid states of matter. The realm of the field stretches from low molar mass to polymeric materials, from organic to inorganic substances, and from being functional materials to biological organelles. In this article, we explore the cusp of the change over from nano‐ to macro‐sciences through the formation of ‘supermolecular’ systems, and we debate if such materials are in fact oligomers or materials having giant molecules possessing discrete chemical constitutions? We performed this investigation by synthesising mesogenic materials based upon difluoroterphenyls, terminally substituted with a chiral motif based on (S)‐2‐octanol, and which could be appended onto a variety of siloxane scaffolds. We studied the structures and properties of six mesomorphic materials in which we systematically increased the number of mesogenic units attached to the scaffold.

Bis cinnamate substituted λ-shaped azo liquid crystal: effect of molecular structure on mesomorphic properties

A new class of λ-shaped azo materials with a cinnamate linkage group has been designed, synthesized, and well-characterized. Further, the liquid crystalline property has been studied by using POM and DSC analysis. Out of six derivatives, five compounds displayed liquid crystalline properties. The liquid crystalline property is observed in an enantiotropic manner. The structure–property relationship is further studied for the understanding of the exhibition of liquid crystalline properties. The results showed that their liquid crystalline phase behavior was closely related to the number of mesogenic units, rigidity, and flexibility of the molecular systems.

Mesogenic Units Containing Polymer Electrolytes for Light and Safe Batteries

As the core component of solid-state lithium-ion batteries (SSLIBs), the bottleneck of solid-state electrolyte is to achieve fast lithium-ion transport, high electrochemical stability, and mechanical flexibility. Polymer electrolytes offer the possibility of constructing solid-state electrolytes with the above features due to their excellent molecular designability. This preview highlights novel mesogenic (or liquid crystal)-containing polymer electrolytes (MPEs) exhibiting a combination of high ionic conductivity, high electrochemical stability, and mechanical flexibility. Insights into such MPEs enabling light and safe SSLIBs are also discussed.

One-flow multi-step synthesis of a monomer as a precursor of thermal-conductive semiconductor packaging polymer via multi-phasic separation

As semiconductor technology becomes rapidly advanced, semiconductor packaging materials demand critical characteristics in various aspects. Besides, the development of these new epoxy mold compounds (EMC) by batch reaction process requires time-consuming experimentation and the multi-step chemistry further reduces the efficiency of the synthetic process. We report a one-flow multi-step process for the synthesis of newly developed Np-C4-Np, which comprises two mesogenic units connected with a flexible spacer, as a monomeric precursor of semiconductor packaging material. Graphical convolution neural network (GCNN), a deep learning model, predicts a common solvent for three-step reactions, thereby enabling serial esterification-deprotection-epoxidation integrated with in-situ multi-phasic separations that were accomplished within 76 min in flow over the long batch process (>20 h excluding separation step). Eventually, the Np-C4-Np is synthesized rapidly with higher space–time yield compared to the batch system, thus confirming benefits in terms of productivity. Moreover, we prevent the deterioration of electronic circuits during the semiconductor packaging process by lowering the molding temperature (126 °C) of EMC and quickly dissipating the heat generated from semiconductor chip by increasing the thermal conductivity (0.34 W ∙ m−1∙ K−1) of EMC.

Conformational degrees of freedom and stability of splay-bend ordering in the limit of a very strong planar anchoring.

We study the self-organization in a monolayer (a two-dimensional system) of flexible planar trimer particles. The molecules are made up of two mesogenic units linked by a spacer, all of which are modeled as hard needles of the same length. Each molecule can dynamically adopt two conformational states: an achiral bent-shaped (cis-) and a chiral zigzag (trans-) one. Using constant pressure Monte Carlo simulations and Onsager-type density functional theory (DFT), we show that the system consisting of these molecules exhibits a rich spectrum of liquid crystalline phases. The most interesting observation is the identification of stable smectic splay-bend (S_{SB}) and chiral smectic-A (S_{A}^{*}) phases. The S_{SB} phase is also stable in the limit, where only cis- conformers are allowed. The second phase that occupies a considerable portion of the phase diagram is S_{A}^{*} with chiral layers, where the chirality of the neighboring layers is of opposite sign. The study of the average fractions of the trans- and cis- conformers in various phases shows that while in the isotropic phase all fractions are equally populated, the S_{A}^{*} phase is dominated by chiral conformers (zigzag), but the achiral conformers win in the smectic splay-bend phase. To clarify the possibility of stabilization of the nematic splay-bend (N_{SB}) phase for trimers, the free energy of the N_{SB} and S_{SB} phases is calculated within DFT for the cis- conformers, for densities where simulations show stable S_{SB}. It turns out that the N_{SB} phase is unstable away from the phase transition to the nematic phase, and its free energy is always higher than that of S_{SB}, down to the transition to the nematic phase, although the difference in free energies becomes extremely small when approaching the transition.

Imine based Four-Ring Chalcone-Ester liquid Crystals: Synthesis, Characterization, mesomorphic behaviour and DFT approach

In this study, new imine-based chalcone-ester liquid crystals with symmetric and unsymmetrical alkoxy chains were synthesized and their mesomorphic behaviour was examined. The FT-IR, 1H NMR, 13C NMR, and mass spectral analysis have all been used to characterize new mesogens. From the study of mesophase behaviour, the compounds of CHE-(6–18) and 12-CHE-(6–18) series with different alkoxy chains at both the ends exhibited stable enantiotropic smectic C in addition to nematic mesophase for compounds with longer alkoxy chains. Whereas the three compounds of CHAE series were found to be non mesogenic. By comparing new compounds with structurally similar compounds, the impact of inclusion/replacement of various mesogenic units have been discussed. The theoretical calculations were performed to illustrate the mesomorphic behaviour of the studied compounds in terms of aromaticity and π–π stacking. Also, the experimental findings of the individual compounds were correlated with the calculated polarizability, dipole moment, and aspect ratio. Moreover, a projection of the molecular electrostatic potential and the frontier molecular orbitals has been considered to demonstrate the influence of the polarity of the terminal chains and the mesogenic cores on the energy gap of the FMOs and the distribution of electrostatic charges on the investigated compounds.

Synthesis and molecular structure of V-shaped liquid crystalline compounds: Spectroscopic, mesomorphic, DFT investigations, electrochemical and fluorescence studies

A series of V-shaped compounds consisting of a central phenyl core connected by two identical mesogenic units which possess the imine (C=N) and ester (COO) bonds along with the terminal alkoxy chain (-OCnH2n+1) where n = 6-12. The ultimate compounds were synthesised and characterized by CHN elemental analysis, Fourier-Transform Infrared (FT-IR), nuclear magnetic resonance (1H-NMR), and UV-Visible (UV-Vis) spectroscopic techniques. The mesomorphic properties, thermal behaviour and conformation of compounds were studied by using polarised optical microscopy (POM), and differential scanning calorimetry (DSC). It can be generalized from present study that the V-shaped compounds with odd n carbon number in alkoxy chains were non-mesogenic. Whilst the compounds with shorter alkoxy chains of n = 6 and 8 were monotropic nematic, the compounds which possess longer alkoxy chains (n = 10 and 12) exhibited enantiotropic nematic phase. The computational method Chem3D Pro suggests the molecular structures of all the compounds with even n carbon number in the alkoxy chain. The Gaussian calculation with B3LYP/6-31G+(d,p) condition was employed to further demonstrate the chemical reactivity for all target compounds based on HOMO-LUMO analysis, the energy difference between HOMO-LUMO and surface charges. The target molecules of which the member containing the even n parity along the mesogenic arms have shown the same trend of electrocatalytic behaviour particularly toward the detection of hydrogen peroxide via the cyclic voltammetry study. Furthermore, the member with n=12 has been validated as a remarkable fluorogenic probe for the detection of Zn2+ ion.

Performance of Liquid Crystalline Elastomers on Biological Cell Response: A Review

Dynamic interplay between extracellular matrix with anisotropic structure and biological cells has directed the scaffold path of development toward the utilization of actuation-responsive biomaterials. Among stimuli-responsive polymers, liquid crystalline elastomers (LCEs), owing to attachment of highly anisotropic mesogenic units to the amorphous elastomeric chains, are able to reveal reversible orientational order. Dynamic order–disorder phase transition and stimuli-responsiveness are the main factors to consider them as rationally smart mimickers of biological cell topology. This Review explains briefly the structures and synthesis methods of LCEs as well as their materiobiology with an emphasis on biochemical and biomechanical features of cells from molecular view. In this regard, some indispensable factors such as quasi-liquid crystalline behavior of cells, degree of orientational order in different cells and liquid crystalline induced topographical signals as growth enhancing agent for cells are investigated. Moreover, the interactions of cell-liquid crystalline substrate and the performance of LCE scaffolds on various cell lines response are given as an overview.

Effects of Mesogenic Cycloaliphatic Units on the Microstructure and Properties of Thermotropic Liquid Crystalline Polyesters

AbstractWe report the synthesis and structure‐property relationship of new thermotropic liquid crystalline polyesters (TLCPs) with different mesogenic cycloaliphatic unit content in the mainchain. For this purpose, a series of TLCPs composed of three different repeating units based on 4‐hydroxybenzoic acid (HBA, 75.0 mol%), 2‐hydroxy‐6‐naphthoic acid (HNA, 25.0–15.0 mol%), and terephthalic acid/1,4‐cyclohexanedimethanol (TPA/CHDM, 0.0–10.0 mol%) were synthesized via two‐step bulk polymerization. The structure, thermal and mechanical properties of the TLCPs were investigated by considering the effects of size and content of TPA/CHDM‐derived repeating units, in comparison to HNA‐based repeating units. Infrared spectroscopic data showed that TPA/CHDM‐derived repeating units were successfully incorporated into HBA/HNA‐based TLCPs. The polarized optical and electron microscopic images revealed the existence of thermotropic liquid crystalline features and associated fibrillar structures for all TLCPs. The melting temperatures of the TLCPs decreased with increasing the TPA/CHDM‐based repeating units, whereas the glass transition temperature increased, which is due to the trade‐off effect of the segmental mobility restriction in the amorphous phase and the destruction of the thermotropic liquid crystalline phase by the introduction of TPA/CHDM‐derived repeating units in the TLCP mainchain. In addition, the maximum elastic storage modulus was attained for the TLCP with 2.5 mol% TPA/CHDM‐derived repeating units.

Observation of helical self-assembly in cyclic triphosphazene-based columnar liquid crystals bearing chiral mesogenic units

Cyclotriphosphazene-based discotic liquid crystals are reported exhibiting helicoildal columnar arrangement and fluorescence turn-on phenomenon.

Shape Morphing Directed by Spatially Encoded, Dually Responsive Liquid Crystalline Elastomer Micro-Actuators.

Liquid crystalline elastomers (LCEs) with intrinsic molecular anisotropy can be programmed to morph shapes under external stimuli. However, it is difficult to program the position and orientation of individual mesogenic units separately and locally, whether in-plane or out-of-plane, since each mesogen is linked to adjacent ones through the covalently bonded polymer chains. Here, dually responsive, spindle-shaped micro-actuators are synthesized from LCE composites, which can reorient under a magnetic field and change the shape upon heating. When the discrete micro-actuators are embedded in a conventional and nonresponsive elastomer with programmed height distribution and in-plane orientation in local regions, robust and complex shape morphing induced by the cooperative actuations of the locally distributed micro-actuators, which corroborates with finite element analysis, are shown. The spatial encoding of discrete micro-actuators in a nonresponsive matrix allows to decouple the actuators and the matrix, broadening the material palette to program local and global responses to stimuli for applications including soft robotics, smart wearables, and sensors.

Synthesis and phase behavior of Y-shaped Schiff base liquid crystal oligomers

ABSTRACT In this paper, we designed and synthesised three Schiff base mesogenic units X-YP and six Y-shaped liquid crystal oligomers X-AYP and X-BYP (×= -CH3O, -CH3, -Cl). Molecular structures of the prepared compounds were confirmed via FT-IR, 1H NMR and elemental analysis. The mesomorphic properties were investigated by differential scanning calorimetry (DSC) and polarised light microscopy (POM). The photochemical properties of Y-shaped liquid crystal oligomers were preliminarily studied by UV-Vis spectrum. The results indicated that X-YP containing OCH3 and Cl substituents exhibited mesophase, while X-YP with CH3 substituent was non-mesogenic. The synthesized Y-shaped liquid crystals X-AYP and X-BYP both had liquid crystal properties and exhibited nematic phase during heating and cooling, which belonged to thermotropic enantiotropic nematic liquid crystals. The polarity of the terminal groups and the number of mesogenic units had a significant effect on the mesophase behaviour of Y-shaped liquid crystals. The absorption intensities of X-AYP and X-BYP in the UV spectra were affected by the terminal substituents.