Isotropic Transition Temperature Research Articles

Elastocaloric Response of Isotropic Liquid Crystalline Elastomers.

Liquid crystalline elastomers (LCEs) are soft materials that associate order and deformation. Upon deformation, mechanically induced changes order affect entropy and can produce a caloric output (elastocaloric). Elastocaloric effects in materials continue to be considered for functional use as solid state refrigerants. Prior elastocaloric investigations of LCEs and related materials have measured ≈2°C temperature changes upon deformation (100% strain). Here, the elastocaloric response of LCEs is explored that are prepared with a subambient nematic to isotropic transition temperature. These materials are referred as "isotropic" liquid crystalline elastomers. The LCEs are prepared by a two-step thiol-Michael/thiol-ene reaction. This polymer network chemistry enhances elastic recovery and reduces hysteresis compared to acrylate-based chemistries. The LCEs exhibit appreciable elastocaloric temperature changes upon deformation and recovery (>±3°C, total ΔT of 6°C) to deformation driven by minimal force (<<1MPa). Notably, the strong association of deformation and order and the resulting temperature change attained at low force achieves a responsivity of 14°C MPa-1 which is seven times greater than natural rubber.

Enhanced Orientation of Liquid Crystals Inside Micropores of Metal-Organic Frameworks Having Thermoresponsivity.

With the aim of controlling the orientation of liquid crystals (LCs) toward realizing external stimuli-responsive materials with tunable functionalities, we synthesized a composite of LCs and metal-organic frameworks (MOFs) by filling LCs into the pores of MOFs (LC@MOFs) for the first time. The included LCs interact with the MOFs through coordination bonds between the cyano groups of the LCs and the metal ions of the MOFs, enabling the orientation of the LC molecules inside the pores of the MOFs and the realization of birefringence of LC@MOFs. The three-dimensional nanometer interstice frameworks maintained the LC orientation even at temperatures much higher than the isotropic phase transition temperature of bulk LCs. Furthermore, the orientational state changed upon heating or cooling, inducing temperature-dependent birefringence. This study provides a new approach to the development of stimuli-responsive optical materials and stimuli-responsive MOFs.

Flexible and elastic thermal regulator for multimode intelligent temperature control

AbstractAs nonlinear thermal devices, thermal regulators can intelligently respond to temperature and control heat flow through changes in heat transfer capacities, which allows them to reduce energy consumption without external intervention. However, current thermal regulators generally based on high‐quality crystalline‐structure transitions are intrinsically rigid, which may cause structural damage and functional failure under mechanical strain; moreover, they are difficult to integrate into emerging soft electronic platforms. In this study, we develop a flexible, elastic thermal regulator based on the reversible thermally induced deformation of a liquid crystal elastomer/liquid metal (LCE/LM) composite foam. By adjusting the crosslinking densities, the LCE foam exhibits a high actuation strain of 121% with flexibility below the nematic–isotropic phase transition temperature (TNI) and hyperelasticity above TNI. The incorporation of LM results in a high thermal resistance switching ratio of 3.8 over a wide working temperature window of 60°C with good cycling stability. This feature originates from the synergistic effect of fragmentation and recombination of the internal LM network and lengthening and shortening of the bond line thickness. Furthermore, we fabricate a “grid window” utilizing photic‐thermal integrated thermal control, achieving a superior heat supply of 13.7°C at a light intensity of 180 mW/cm2 and a thermal protection of 43.4°C at 1200 mW/cm2. The proposed method meets the mechanical softness requirements of thermal regulator materials with multimode intelligent temperature control.

Study on the Memory Effect in Aerosil-Filled Nematic Liquid Crystal Doped with Magnetic Nanoparticles

A study on 5CB liquid crystal composites with SiO2 nanoparticles and an additional commixture with Fe3O4 nanoparticles using light transmission and SAW measurements is presented. The prepared liquid crystal composites exhibited an interesting memory effect characterized by the hysteresis of both light transmission and SAW attenuation responses investigated in the nematic phase. While in the case of SiO2 nanoparticles as dopants, the liquid crystal composite showed an improvement in the memory effect, the addition of Fe3O4 magnetic nanoparticles resulted in the memory effect decreasing. Additional studies showed a significant shift in both the threshold voltage and nematic–isotropic transition temperature. Measurements in the magnetic field confirmed the increasing memory effect according to that of pure 5CB. The properties of these composites could lead to a potential application for the fabrication of memory devices suitable for information storage.

Memory Effect by Melt Crystallization Observed in Polymorphs of a Benzothieno-Benzothiophene Derivative.

This work provides a comprehensive illustration of a crystalline melt memory effect recorded for three solvates of the 2,7-bis(2-(2-methoxyethoxy)ethoxy)benzo[b]benzo[4,5] thieno[2,3-d]thiophene (OEG-BTBT) molecule with dichloromethane (DCM) molecules. Combined optical microscopy and X-ray diffraction measurements at different temperatures are used to get an overview of the structural and morphological properties like melting points, isotropic transition temperatures, induction times, and crystallization kinetics of the three forms. An outstanding observation is made upon annealing the three polymorphs at temperatures well above their respective melting points as well as above the optical clearance temperature. After cooling back to room temperature, recrystallization results in the formation of the initial phase present before the annealing process. This melt memory effect is observed for all three solvates. These observations can be correlated to the strong interaction between the DCM molecules and the oligoethylene glycol side chains, even in the molten state. This conclusion rationalizes the experimental observation made upon solvent vapor annealing of the crystalline sample with DCM, which unambiguously transformed the system into a disordered state.

Tunable optical, electro-optical and dielectric properties of eco-friendly graphene quantum dots-nematic liquid crystal composites

ABSTRACT The changes in physical properties obtained by dispersing hydrophilic carboxylated graphene quantum dots (GQDs, <10 nm), which are non-toxic, biocompatible, and eco-friendly, into a room-temperature nematic liquid crystal, 5CB (4-pentyl-4ˈ- biphenylcarbonitrile) are described. Nematic–isotropic transition temperatures, polarising microscopy, electro-optic, photoluminescence, and dielectric investigations are reported for two composites having concentrations of GQDs of 0.25 and 1.0 wt.% in 5CB and compared to pristine 5CB. At the lower concentration (0.25%) of GQDs there is an enhancement in the order parameter by 5.49%, the birefringence by 1.28%, and the activation energy for dielectric relaxation by 4.4% compared to pure 5CB. However, at the higher concentration (1%), aggregation of GQDs occurs. This is apparent in optical textures and resulted in the deterioration of optical and electrical properties compared to 5CB. The photoluminescence spectrum of both concentrations of GQDs are centred at the same wavelength; however, the 1% GQD-5CB mixture exhibits significant quenching and may be suitable for some optical devices. Overall, our results support that, in addition to promoting sustainable, heavy-metal-free, and environmentally safe technologies, a low concentration of GQDs in LCs produces composites that have superior electrical and optical properties; thus, making them suitable for a variety of photonics and display applications.

Synthesis and mesomorphic properties of novel multi-arm side-chain liquid crystal polymers with different terminal substituents

ABSTRACT In this study, two novel aromatic ester multi-arm liquid crystal monomers M 1 and M 2 with different terminal substituents centred on 3,4,5-trihydroxybenzoic acid, and a rod-shaped chiral liquid crystal monomer N were designed and synthesised. The synthesised liquid crystal monomers M and N were grafted and copolymerised with polymethylhydrosiloxane (PMHS). The structural characterisation of the synthesised liquid crystal compounds was carried out by FT-IR and 1H-NMR. The mesomorphic properties of the synthesised liquid crystal compounds were characterised by POM and DSC, and the results showed that the multi-arm liquid crystal monomers M 1 and M 2 are typical enantiotropic nematic liquid crystal compounds and N is typical thermotropic enantiotropic cholesteric liquid crystal. Polymers P 1 and P 2 are both cholesteric liquid crystal polymers. The glass transition temperatures and liquid crystal isotropic transition temperatures of polymers P 1 and P 2 series both tend to increase with the increase of multi-arm liquid crystal monomer M.

Can even-membered liquid crystal dimers exhibit the twist-bend nematic phase? The preparation and properties of disulphide and thioether linked dimers

ABSTRACTCyanobiphenyl-based liquid crystal dimers containing a disulfide link in the spacer, the 4-[4-[2-[2-[4-(4-cyanophenyl)phenyl]alkyldisulfanyl]alkyl]-phenyl]benzonitriles (CBnSSnCB), are reported. The alkyl fragment is varied from two to nine carbon atoms, and all eight homologues exhibit nematic behaviour. In addition, the dimer containing heptamethylene fragments (CB7SS7CB) exhibits a twist-bend nematic phase despite having an even-membered spacer. The values of the nematic–isotropic transition temperature, TNI, are higher for dimers containing an odd number of carbon atoms in each alkyl fragment. This surprising behaviour is interpreted in terms of the C-S-S-C torsion angle being around 88° that ensures that the molecular shape is bent irrespective of the parity of the alkyl fragments. For odd-membered fragments, however, the average shape is less bent and higher values of TNI are observed. The more uniform molecular curvature of CB7SS7CB accounts for the observation of the NTB phase. We also report the behaviour of two members of the 4-[4-[3-[2-[4-(4-cyanophenyl)phenyl]alkyldisulfanyl]-alkyl]phenyl]benzonitriles (CBnSmCB) which contain a thioether link embedded in the spacer. Both dimers exhibited nematic behaviour. The thioether link reduces the value of TNI compared to that of the dimer with an alkyl spacer. A much larger decrease in the nematic-twist-bend nematic temperature is observed and is attributed to a change in shape.

Switching Chirality in Arrays of Shape-Reconfigurable Spindle Microparticles.

The giant circular photo-galvanic effect is realized in chiral metals when illuminated by circularly polarized light. However, the structure itself is not switchable nor is the crystal chirality in the adjacent chiral domains. Here spindle-shaped liquid crystalline elastomer microparticles that can switch from prolate to spherical to oblate reversibly upon heating above the nematic to isotropic transition temperature are synthesized. When arranged in a honeycomb lattice, the continuous shape change of the microparticles leads to lattice reconfiguration, from a right-handed chiral state to an achiral one, then to a left-handed chiral state, without breaking the translational symmetry. Accordingly, the sign of rotation of the polarized light passing through the lattices changes as measured by time-domain terahertz spectroscopy. Further, it can locally alter the chirality in the adjacent domains using near-infrared light illumination. The reconfigurable chiral microarrays will allow us to explore non-trivial symmetry-protected transport modes of topological lattices at the light-matter interface. Specifically, the ability to controllably create chiral states at the boundary of the achiral/chiral domains will lead to rich structures emerging from the interplay of symmetry and topology.

Synthesis and characterization of semiconducting copper oxide nanoparticles and their impact on the physical properties of a nematic liquid crystalline material 4-pentyl-4′-cyanobiphenyl

Green synthesis of nanoparticles is a biological, eco-friendly and less expensive method which is free from chemical contaminants. In this work, copper oxide nanoparticles (CuO NPs) have been synthesized via green synthesis method. This method proved to be efficient for preparing CuO NPs at room temperature without using any inert atmosphere. The CuO NPs have been characterized using X-Ray Diffraction, Ultraviolet–Visible Spectroscopy and Fourier Transform Infrared Spectroscopy techniques which confirmed the proper synthesis of CuO NPs of the size 2–3 nm. Two different weight percents (0.2 and 0.5) of the synthesized CuO NPs have been dispersed in the nematogenic liquid crystalline material 4-pentyl-4′-cyanobiphenyl (5CB). The physical properties of pure 5CB and its nanocomposites were studied by differential scanning calorimetry, ultraviolet–visible spectroscopy and impedance spectroscopy. Thermodynamic and dielectric results show enhancement in the nematic to isotropic transition temperature of nanocomposites along with enhancement in conductivity as compared to its value for the pure 5CB. Through optical studies, the band gap has been estimated by the Tauc plots which suggest decrease in the optical band gaps of dispersed samples.

Molecular structure and the twist-bend nematic phase: the role of spacer length in liquid crystal dimers

ABSTRACT The liquid crystal dimers, the 1-(4-substitutedazobenzene-4′-yloxy)-4-(4-cyanobiphenyl-4′-yl)butanes (CB4OABX), are reported in which the terminal substituent is either a methyl, methoxy, butyl, butyloxy, cyano or nitro group. The butyloxy spacer endows these dimers with the required molecular curvature to exhibit the twist-bend nematic phase in addition to showing the conventional nematic phase. Their transitional properties are compared to those of the corresponding dimers with either a pentyloxy or hexyloxy spacer. As expected, the even-membered pentyloxy-based dimers show the highest nematic–isotropic transition temperature, TNI, and exhibit smectic behaviour. These observations are attributed to their linear molecular shapes. The values of both the twist-bend nematic–nematic transition temperature, TN TB N, and TNI increase on passing from the butyloxy to hexyloxy spacer, but the change in TNI is greater than that in TN TB N. Thus, the ratio TN TB N is greater for the shorter spacer reinforcing the view that molecular curvature drives the formation of the NTB phase relative to the N phase. By comparison, the melting point decreases on passing from the butyloxy to hexyloxy spacer. Thus, increasing molecular curvature simultaneously increases both the melting point and NTB phase stability and this highlights the design challenge in obtaining dimers that exhibit enantiotropic NTB–I transitions.

Metal oxide alumina nanowire-induced polymer-dispersed liquid crystal composites for low power consumption smart windows

Herein, polymer dispersed liquid crystals (PDLCs) based smart windows are proposed using a host thiol-ene-based UV curable photopolymer and nematic liquid crystals (E7). The proposed PDLC was prepared by polymerization-induced phase separation technique. PDLCs were doped with alumina nanowires (ANWs) in different concentrations and investigated their dispersion effect on various physical properties. An increase in nematic to isotropic transition temperature was observed in ANWs doped PDLC nanocomposite. Reduction in threshold and saturation voltages by 62.5% (1.5 V) and 56.57% (16.5 V), at 1.0 and 0.25 wt.% of doped ANWs respectively in PDLC was noticed. This reduction could be attributed to an increased electric field caused by lowering medium resistivity and increased cell capacitance. A decrease in dielectric anisotropy with temperature and ANWs doping in all samples was also observed. The maximum absorbance for 0.5 wt.% ANWs doping in PDLC at a wavelength of 511 nm were observed. The electric field-induced results indicate that the proposed ANWs doped PDLC composite film can operate at a much lower voltage (19.5 V) and show improved transparency. These findings may assist to develop fast-switching smart windows and other energy-saving devices.

Phase transition behavior of pentylcyanobiphenyl liquid crystal containing n-dodecyl-2-hydroxy-1-naphthaldimine

The present work focuses on the phase transition behavior of a thermotropic liquid crystal incorporating a Schiff base having a terminal alkyl chain of 12 carbons, such as, n-dodecyl-2-hydroxy-1-naphthaldimine (SB). Given its interesting molecular structure, the SB was chosen as doping agent for a liquid crystal of cyano-biphenyl type, such as, 4-cyano-4′-n-pentyl-biphenyl (5CB). The study of the guest–host systems, at different concentrations of the solute, was performed using differential scanning calorimetry and UV–visible spectroscopy, equipped with the hot compartment. The fusion and the nematic – isotropic transition temperatures as well as the enthalpies ΔH 0 and entropies ΔS 0 variations of the 5CB-based systems were determined from their spectral data. The thermal properties and orientational order parameters were also determined for the pure and SB doped 5CB, as well as the activation energy for the guest system. The filamentary texture was observed by polarized light optical microscopy for both the pure and Schiff base-doped 5CB.

Chiral ferronematic liquid crystals: a physico-chemical analysis of phase transitions and induced helical twisting

ABSTRACT A possibility was assessed to modify physico-chemical properties of ferronematic mixtures by introducing additional mesogenic and non-mesogenic components. The mixture, consisting of RM734 and DIO at the 70:30 weight ratio of the components, was doped with nematic 5CB and its chiral isomer CB15. This allowed a substantial lowering of the ferronematic temperature range, as well as induction of helical twisting in both nematic (N) and ferronematic (NF) phases. The effects of 5CB and CB15 on the NF to N phase transition were nearly identical, but the isotropic transition temperature decreased more strongly upon addition of CB15. Optical transmission vs. temperature measurements showed lower transmittance in the NF phase, probably due to scattering on the domain boundaries. In the vicinity of the NF → N transition, a noticeable range of anomalously low transmission was noted both on heating and cooling, suggesting a complex character of molecular reorientation features. In the chiral NF (N*F) system, helical twisting was recorded using Grandjean-Cano wedge, with the helical pitch decreasing upon cooling in both N* and N*F phases. In the cooling mode, transient undulations emerged in the N*F phase. The complex picture of the ferronematic transition is supported by POM images taken at different temperatures.

Self‐assembly, mesomorphic behavior, and ionic conductivity of polymerized ionic liquid crystals with a threshold switching characteristic

AbstractA series of pyridinium‐based polymerized ionic liquid crystals (PILCs) with different anions were synthesized, including poly(pyridinium bromide)s and poly(pyridinium trifluoroacetate)s, and their chemical structure, liquid‐crystalline properties and conductivity properties were investigated by many experimental techniques. At low temperature, some PILCs containing less cholesteryl mesogens presented smectic A phases; while the PILCs containing more cholesteryl mesogens displayed bicontinuous cubic phases. At high temperature, all the PILCs showed nematic phases. Compared with the corresponding poly(pyridinium bromide)s based on identical polymer cations, the poly(pyridinium trifluoroacetate)s showed lower glass transition and isotropic transition temperature. The PILCs displayed excellent temperature‐dependent ionic conductivity, making it attractive for applications in temperature‐controlled electronic switches. In cubic and smectic A phases, the PILCs presented Arrhenius‐like ionic conductivity behavior, and they exhibited Vogel–Fulcher–Tammann‐like behavior in nematic phases.

Programmable Shape Change in Semicrystalline Liquid Crystal Elastomers.

Liquid crystal elastomers (LCEs) are stimuli-responsive materials capable of reversible and programmable shape change in response to an environmental stimulus. Despite the highly responsive nature of these materials, the modest elastic modulus and blocking stress exhibited by these actuating materials can be limiting in some engineering applications. Here, we engineer a semicrystalline LCE, where the incorporation of semicrystallinity in a lightly cross-linked liquid crystalline network yields tough and highly responsive materials. Directed self-assembly can be employed to program director profiles through the thickness of the semicrystalline LCE. In short, we use the alignment of a liquid crystal monomer phase to pattern the anisotropy of a semicrystalline polymer network. Both the semicrystalline-liquid crystalline and liquid crystalline-isotropic phase transition temperatures provide controllable shape transformations. A planarly aligned sample's normalized dimension parallel to the nematic director decreases from 1 at room temperature to 0.42 at 250 °C. The introduction of the semicrystalline nature also enhances the mechanical properties exhibited by the semicrystalline LCE. Semicrystalline LCEs have a storage modulus of 390 MPa at room temperature, and monodomain samples are capable of generating a contractile stress of 2.7 MPa on heating from 25 to 50 °C, far below the nematic to isotropic transition temperature. The robust mechanical properties of this material combined with the high actuation strain can be leveraged for applications such as soft robotics and actuators capable of doing significant work.

Amphiphilic Liquid Crystalline Polymer Micelles That Exhibit a Phase Transition at Body Temperature

Liquid crystalline polymers (LCPs), which exhibit unique structures and properties intermediate between those of liquids and solids, are widely utilized as functional and advanced materials for fabricating optical devices and high-performance fibers. This utility stems from their ability to abruptly change their organized structures and mobilities at their liquid crystalline-isotropic phase transition temperatures, similar to the properties of biological membranes. Despite these numerous potential applications of LCPs, no study on their use in medical applications such as drug delivery has been reported. In the present study, we synthesized amphiphilic side-chain LCPs (LCP-g-OEGs, where OEG is oligo(ethylene glycol)) for medical applications, where the LCP-g-OEGs undergo a nematic-isotropic phase transition at body temperature. The LCP-g-OEGs formed micelles with a diameter of approximately 130 nm in aqueous media. The micelles were stable and did not dissociate in aqueous media even when the temperature exceeded the nematic-isotropic phase transition temperature (TNI). Although the release of a dye as a model drug from micelles was suppressed at temperatures lower than TNI, their dye release was drastically enhanced at temperatures higher than TNI. The LCP-g-OEG micelles regulated dye release reversibly in accordance with stepwise changes in temperature, without undergoing dissociation, differing from the behavior of standard temperature-responsive micelles. The temperature-responsive dye release behavior is induced by dramatic changes in their well-organized and dynamic structures as a result of the nematic-isotropic phase transition. These results demonstrate that the LCP-g-OEG micelles have a lot of medical applications as reversibly stimuli-responsive drug carriers.

Toward In Silico Design of Highly Tunable Liquid Crystal Elastomers

In this work, a two-component acrylate liquid crystal elastomer, with varying composition and templating phase, is synthesized in the laboratory and investigated in parallel using atomistic molecular dynamics simulations. The anisotropic nature of both the mono- and bifunctional acrylates used in this study enables a large tunability in the compositional range while still retaining liquid crystalline properties in the final elastomer. The use of simulations allows important evaluation and comparison of physical properties such as glass transition temperature, nematic to isotropic phase transition temperature, and order parameter. The dependence of physical properties (glass transition, nematic to isotropic transition, order parameter, coefficient of thermal expansion, and mechanical properties) is established as a function of chemical composition, showing a high degree of tunability. Interestingly, the templating phase (nematic or isotropic) is also shown to impact the subsequent elastomer properties, with excellent agreement shown here between experiments and simulations. The in silico approach to polymerization, coupled with excellent comparison with the experimental system, represents a new methodology for the targeted design of liquid crystal elastomers with specific physical properties.

Thermally induced self-rupture of a constrained liquid crystal elastomer

Liquid crystal elastomer (LCE) is a promising soft material which exhibits large actuation and can respond to different external fields with proper modifications. In the case of thermally actuated LCEs, the fracture properties are crucial to ensure that LCEs can robustly function under different loading and heating conditions, without unexpected failure. In this work, we systematically characterize the fracture energy of LCE at different temperatures and loading rates. We found that LCE behaves like a tough elastomer at low temperatures and high loading rates, but the fracture energy dramatically drops once LCE is heated beyond the nematic to isotropic transition temperature (TNI). At high temperatures, a constrained precut LCE sample can undergo self-rupture due to the large actuation stress. We have also used the measured fracture energy to successfully predict the critical temperature of self-rupture for different sample geometries.

Thermodynamic and spectroscopic characterization of a weakly polar liquid crystalline compound dispersed with polyvinyl pyrrolidone capped gold nanoparticles

In the present study a deliberate attempt has been made to examine the consequences of dispersion of gold nanoparticles in an azoxybenzene-based nematic liquid crystal, namely 4,4′- di pentyl azoxy benzene. Investigations were made on gold nanoparticles doped nematic liquid crystals analysing differential scanning calorimetry, polarized light microscopy, ultraviolet–visible absorption spectroscopy, photoluminescence and Fourier-transform infrared spectroscopy (FTIR). Gold nanoparticles of different concentrations 0.25, 0.5, and 1.0 wt% were dispersed in pure 4,4′- di pentyl azoxy benzene, and it was observed in the experiment that no significant change occurs in the crystal to nematic phase (Cr-Ne) transition temperature with the increase in gold nanoparticles concentration. However, the nematic to isotropic (Ne-Iso) transition temperature decreases significantly with the increase in gold nanoparticles concentration. This study reports around 76% increase in ultraviolet–visible absorbance and 46% enhancement of photoluminescence intensity with the suitable amount of gold nanoparticles dispersion in 4,4′- di pentyl azoxy benzene. The surface plasmonic resonance (SPR) and re-excitation of 4,4′- di pentyl azoxy benzene molecule due to collision with gold nanoparticles has been accredited to the escalation in photoluminescence intensities.