Low Molar Mass Liquid Crystals Research Articles

The influence of the ring system with mono/diester and imine links, and terminal unsaturated carbon chains on mesomorphism and their quantum-mechanical study

ABSTRACT Five homologous series of low molar mass liquid crystals containing mono/diester and imine-linked cores with terminal unsaturated carbon chains and/or a carboxylic group were synthesised. They were characterised with IR, UV and 1H-NMR spectroscopies. Liquid crystalline properties of all the derivatives were studied by polarised optical microscopy and their mesophase transition enthalpies and entropy changes measured by DSC. The 4-((4-n-alkoxybenzoyl) oxy) benzoic acids (n OPBA) exhibit enantiotropic smectic A and nematic mesophases, while all the 4-allyl-2-methoxyphenyl 4-n-alkoxybenzoates (n OBEg) and 4-allyl-2-methoxyphenyl (E)-3-(4-n-alkoxyphenyl) acrylates (n OCEg) do not show any liquid crystalline properties. Intermediate members of the 4-allyl-2-methoxyphenyl (E)-4-((4-n-alkoxybenzylidene) amino) benzoate series (n OPMABEg) and the 4-((4-allyl-2-methoxyphenoxy) carbonyl) phenyl 4-n-alkoxybenzoate series (n OB2Eg) exhibited only enantiotropic nematic mesophase behaviour. The optimised geometry of all the liquid crystalline compounds was calculated by density functional theory (DFT), and the effect of the twist angle ϴ of the ring system, the length of the core, HOMO-LUMO energies and electrostatic potential of the compounds on mesomorphism was examined. The effects of the linking groups, the ring systems, the length of the core and the terminal unsaturated carbon chain on mesophase behaviour were studied. The mesophase transitions of the five series were compared with each other and with structurally similar compounds to evaluate the effects of the ring systems on their mesophase behaviour.

The effect of a lateral alkyloxy chain on the ferroelectric nematic phase.

The synthesis and characterisation of two series of low molar mass liquid crystals, the 4-[(4-nitrophenoxy)carbonyl]phenyl 2-alkoxy-4-methoxybenzoates (series 5-m) and the 4-[(3-fluoro-4-nitrophenoxy)carbonyl]phenyl 2-alkoxy-4-methoxybenzoates (series 6-m) are reported in order to explore the effects of a lateral alkyloxy chain on the formation and stability of the recently discovered ferroelectric nematic phase. In both series m, the number of carbon atoms in the lateral chain, is varied from one to nine. The two series differ by the addition of a fluorine substituent in the 6-m series. 5-1 is the extensively studied ferroelectric nematogen RM734. All the members of the 5-m series exhibited both a conventional nematic, N, and ferroelectric nematic, NF, phase, whereas all the members of the 6-m series exhibit a direct NF-I transition with the exception of 6-1 that also exhibits a N phase. The replacement of a hydrogen atom by a fluorine atom reduces the nematic-isotropic transition temperature, T NI, whereas the ferroelectric nematic-nematic, or isotropic, transition temperature, T NFN/I, increases. This is interpreted in terms of the reduced structural anisotropy associated with the larger fluorine atom whereas the increase in the stability of the NF phase reflects changes in polarity and polarizability. The dependence of T NI and T NFN/I on m in both series is similar, and these initially decrease on increasing m but converge to limiting values on further increasing m. This suggests that the lateral alkyloxy chain may adopt conformations in which it lies along the major axis of the mesogenic unit.

From Single to Multi Mode Lasing: The role of materials revealed in optical simulations

ABSTRACT The comparative study of thin film Cholesteric Liquid Crystal (CLC) lasers made from different materials and optically pumped by an external solid state laser reveals a striking dependence of lasing behaviour (ranging from single mode at the edge of the selective reflection band to multimode lasing) on the morphology and microstructure of CLC films. The materials studied belong to two groups: low molar mass liquid crystals and polymers. It is shown that the orientation of individual chiral domains and fluctuations in helical pitch contribute significantly to the type of lasing displayed by the material. Different ways of preparing CLC cells that lead to predominantly one type of lasing are discussed. The importance of variations of the helical pitch and domain orientation in producing single and multimode lasing is justified by optical simulations (4x4 matrix method) of lasing in multilayered samples.

Microfluidic Preparation of Liquid Crystalline Elastomer Actuators.

This paper focuses on the microfluidic process (and its parameters) to prepare actuating particles from liquid crystalline elastomers. The preparation usually consists in the formation of droplets containing low molar mass liquid crystals at elevated temperatures. Subsequently, these particle precursors are oriented in the flow field of the capillary and solidified by a crosslinking polymerization, which produces the final actuating particles. The optimization of the process is necessary to obtain the actuating particles and the proper variation of the process parameters (temperature and flow rate) and allows variations of size and shape (from oblate to strongly prolate morphologies) as well as the magnitude of actuation. In addition, it is possible to vary the type of actuation from elongation to contraction depending on the director profile induced to the droplets during the flow in the capillary, which again depends on the microfluidic process and its parameters. Furthermore, particles of more complex shapes, like core-shell structures or Janus particles, can be prepared by adjusting the setup. By the variation of the chemical structure and the mode of crosslinking (solidification) of the liquid crystalline elastomer, it is also possible to prepare actuating particles triggered by heat or UV-vis irradiation.

Microfluidic Preparation of Liquid Crystalline Elastomer Actuators

This paper focuses on the microfluidic process (and its parameters) to prepare actuating particles from liquid crystalline elastomers. The preparation usually consists in the formation of droplets containing low molar mass liquid crystals at elevated temperatures. Subsequently, these particle precursors are oriented in the flow field of the capillary and solidified by a crosslinking polymerization, which produces the final actuating particles. The optimization of the process is necessary to obtain the actuating particles and the proper variation of the process parameters (temperature and flow rate) and allows variations of size and shape (from oblate to strongly prolate morphologies) as well as the magnitude of actuation. In addition, it is possible to vary the type of actuation from elongation to contraction depending on the director profile induced to the droplets during the flow in the capillary, which again depends on the microfluidic process and its parameters. Furthermore, particles of more complex shapes, like core-shell structures or Janus particles, can be prepared by adjusting the setup. By the variation of the chemical structure and the mode of crosslinking (solidification) of the liquid crystalline elastomer, it is also possible to prepare actuating particles triggered by heat or UV-vis irradiation.

Low molar mass thieno[3,2-b] and thieno[2,3-b]thiophenes in liquid crystal materials science: recent synthetic approaches

One of us (AJS) was fortunate enough to have known Professor George Gray during his final years of teaching at the University of Hull. In 1989, I was finishing my undergraduate studies when I sat in George's class entitled Proteins and Polypeptides. I was in awe of George, as were all students in the class, but struck by his down-to-earth demeanour, warmth, approachability and his soft Scottish accent (I still have the notes I took from his class). George was a truly inspirational teacher to everyone who knew him and I consider it a great privilege to have known him. During my time at Hull I became fascinated with thiophene-containing liquid crystals, my interest being strongly influenced by George Gray, Ken Toyne and Mike Scrowston. This manuscript is my small tribute to George and it describes our new methodological developments in the synthesis of low molar mass liquid crystals that contain the thieno[3,2-b]thiophene and thieno[2,3-b]thiophene cores.

Non-symmetric dimers: effects of varying the mesogenic linking unit and terminal substituent

Six new non-symmetric dimers belonging to the family of compounds the (R,S)-(benzylidene-4ʹ-substituted-aniline)-2ʺ-methylbutyl-4ʹʹʹ-(4ʹʹʹʹ-phenyloxy) benzoate-oxy) hexanoates have been prepared and their thermal behaviour characterised. The terminal substituents on the 4-benzylideneaniline fragment are H, CH3, C2H5, Cl, Br and I. All six dimers exhibit an enantiotropic nematic phase, and the efficiency of the terminal substituent in enhancing the nematic–isotropic transition temperature is largely consistent with that established for conventional low molar mass liquid crystals. This trend is discussed in terms of the size of the terminal substituent and its effect on the shape of the molecule. The halogen-substituted and ethyl-substituted dimers also exhibit smectic behaviour. For the halogen-substituted dimers, the driving force for smectic phase formation is considered to be, at least in part, the specific anisotropic interaction between the unlike mesogenic units. In contrast, for the ethyl-substituted dimers, the driving force is thought to be the enhanced molecular inhomogeneity arising from the longer terminal chains. The properties of these new dimers are broadly similar to those of the corresponding dimers in which the ester linkage between the spacer and mesogenic unit is an ether link. This strongly suggests that the ether- and ester-linked materials have similar average molecular shapes.

Rebirth of Liquid Crystals for Sensoric Applications: Environmental and Gas Sensors

Films and droplets of liquid crystals may soon become an essential part of sensitive environmental sensors and detectors of volatile organic compounds (VOCs) in the air. In this paper a short overview of recent progress in the area of sensors based on liquid crystals is presented, along with the studies of low molar mass liquid crystals as gas sensors. The detection of VOCs in the air may rely on each of the following effects sequentially observed one after the other: (i) slight changes in orientation and order parameter of liquid crystal, (ii) formation of bubbles on the top of the liquid crystalline droplet, and (iii) complete isotropisation of the liquid crystal. These three stages can be easily monitored by a photo camera and/or optical microscopy. Detection limits corresponding to the first stage are typically lower by a factor of at least 3–6 than detection limits corresponding to isotropisation. The qualitative model taking into account the reorientation of liquid crystals is presented to account for the observed changes.

Thermal, Mechanical and Magneto-Mechanical Characterization of Liquid Crystalline Elastomers Loaded with Iron Oxide Nanoparticles

ABSTRACTLiquid crystalline elastomers (LCEs) are materials that reveal unusual mechanical, optical and thermal properties due to their molecular orientability characteristic of low molar mass liquid crystals while maintaining the mechanical elasticity distinctive of rubbers. As such, they are considered smart shape-changing responsive systems. In this work, we report on the preparation of magnetic sensitized nematic LCEs using iron oxide nanoparticles with loadings of up to 0.7 wt%. The resultant thermal and mechanical properties were characterized by differential scanning calorimetry, expansion/contraction experiments and extensional tests. The magnetic actuation ability was also evaluated for the neat elastomer and the composite with 0.5 wt% magnetic content, finding reversible contractions of up to 23% with the application of alternating magnetic fields (AMFs) of up to 48 kA/m at 300 kHz. Thus, we were able to demonstrate that the inclusion of magnetic nanoparticles yields LCEs with adjustable properties that can be tailored by changing the amount of particles embedded in the elastomeric matrix, which can be suitable for applications in actuation, sensing, or as smart substrates.

Star-like polymer click-functionalized with small capping molecules: an initial investigation into properties and improving solubility in liquid crystals

A novel class of polymer, star-like poly(tert-butyl acrylate) (PtBA) capped with 4-isocyano-4′-(prop-2-yn-1-yloxy)biphenyl (CNBP), with a well-defined size was synthesized via atom transfer radical polymerization (ATRP) and click reaction (i.e., azide–alkyne cyclization). The star-like architecture was composed of 21 separate arms connected to a high-functionality β-cyclodextrin (β-CD) core. The inner star-like PtBA blocks were hydrolyzed into poly(acrylic acid) (PAA), yielding star-like CNBP-capped PAA (i.e., star-like PAA-CNBP). The starlike polymers were characterized by gel permeation chromatography (GPC) and dynamic light scattering (DLS) to confirm narrow size distribution, as well as being examined by proton nuclear magnetic resonance (1H-NMR), Fourier transform infrared spectroscopy (FTIR), Raman, and atomic force microscopy (AFM) to verify successful attachment of the click-functionalized capping agent (i.e., CNBP) with structural similarity to low molar mass liquid crystal, 4-cyano-4′-pentylbiphenyl (5CB). Structure and phase behavior were evaluated by AFM and optical imaging. Star-like PAA-CNBP was found to possess improved solubility in a liquid crystal host and interesting surface structures.

Understanding the fast thermal isomerisation of azophenols in glassy and liquid-crystalline polymers

The good solubility of azophenols in low molar mass liquid crystals together with the ability of their related polymers to form homogeneous nematic and glassy thin films make such azoderivatives valuable chromophores to get a great variety of photoactivatable systems with fast switching speeds under ambient conditions. In fact, the final applicability of these systems is mainly determined by the thermal cis-to-trans isomerisation rate of the photoactive azophenol used, in other words, by the intimate mechanism the reaction goes through. The kinetico-mechanistic study reported herein shows that the rate of the thermal back reaction for azophenols is very sensitive to the local environment where the azo chromophore is located, mainly to its capability to establish hydrogen bonding with its surroundings. With a proper design, azophenol-based polymers can exhibit thermal isomerisation rates as fast as those of the monomers in solution even without the presence of any solvent.

Phase equilibria and photopolymerisation-induced phase transitions of mesogenic diacrylate monomer and low molecular mass liquid crystal mixture

Experimental phase diagrams of binary mesogenic mixtures of reactive mesogenic diacrylate (RM257) monomer and low molar mass liquid crystals (E7) were determined by means of differential scanning calorimetry and optical microscopy. The combined free energy densities of Flory–Huggins for liquid–liquid demixing, Maier–Saupe for nematic ordering, and phase field free energy for crystal solidification was proposed to describe the phase diagrams of the starting E7/RM257 mixtures. The phase diagram thus constructed is an ideal mixing type, exhibiting a narrow loop of isotropic + nematic (I + N) coexistence region followed by the crystal + nematic (Cr1 + N) region in descending order of temperature. Of particular interest is the permanent fixation of the mesophase structures upon photopolymerisation of neat RM257 in the corresponding nematic and crystalline phases. Upon photopolymerisation of a low RM257 content mixture in both isotropic and nematic states, the nematic–isotropic transition of E7 was found to persist. The permanent structural anchoring is seen upon photo-curing of the 90/10 RM257/E7 mixture in the crystalline state.

Mechanics of Homeotropically Ordered Smectic‐A Elastomers with Global Oblate Chain Conformation

Smectic-A elastomers combine one-dimensional translational order of rod-like segments with the rubber elasticity of a polymer network. In recent years, detailed investigations were carried out on elastomers showing a global prolate chain conformation. In this communication, the first experiments on fluorinated SA elastomers exhibiting a global oblate chain conformation are presented, where the polymer chains are on average compressed along the layer normal of the lamellar phase structure. The mechanical anisotropy is studied by means of thermoelastic experiments and stress-strain measurements. For the first time, the layer compression modulus B of smectic elastomers is directly measured. B is significantly larger as compared to conventional low molar mass liquid crystals and decreases significantly with increasing local disorder introduced by the isotropic crosslinker.

Deformable Viscoelastic Cholesteric Films with Nanoparticles

Large spectral shifts of the selective reflection band and color changes are achieved in highly viscous mixture of cholesteric polymers and low molar mass liquid crystals filled with nanoparticles and subject to mechanical deformations. The color of the material changes instantaneously during deformation; the time for the color to be completely restored increased with the viscosity of the polymer mixture. The viscosity increases with increasing concentration of polymer or nanoparticles. These properties of the material were explored in building a highly sensitive mechanical sensor.

Increasing the Isomerisation Kinetics of Azo Dyes by Chemical Bonding to Liquid‐Crystalline Polymers

It is well known that the proper substitution of the azobenzene core allows tuning the thermal cis-to-trans isomerisation kinetics of azo dyes. The thermal isomerisation process of nitro-substituted azobenzenes is accelerated up to 13 times with respect to that in the common isotropic solvents when they are doped in nematic low molar mass liquid crystals. This kinetic acceleration is even stronger when these azo dyes are covalently linked to a nematic siloxane polymer. In this environment, the isomerisation process is accelerated more than 10(3) times. This effect is presented herein for the first time. The possible application of the networks obtained as possible photo-actuators has been also considered.

Mechanism of colour changes in stretchable cholesteric films

Large shifts of the selective reflection band and colour changes are achieved in highly viscous mixtures of cholesteric polymers and low molar mass liquid crystals subject to mechanical deformation. The liquid crystal mixture was sandwiched between two silicone strips and stretched. The colour of the material changed instantaneously during stretching, and the time for the colour to be completely restored increased with the viscosity of the polymer mixture. A quantitative model considering the material as an incompressible viscoelastic fluid is proposed. The model accurately describes colour changes (shift of the selective reflection band) and its relaxation back to the original spectral location.

The crystallinity and miscibility of syndiotactic polystyrene blends after mixing with low molar mass liquid crystal or commercial lubricant

The quantities of the crystallinity of syndiotactic polystyrene (SPS) blended with another polymer in the group of poly(α-methyl styrene), poly(n-butyl methacrylate) or poly(cyclohexyl acrylate) with or without the additives were measured by X-ray diffraction and calculated by Ruland’s method. The SPS was synthesized by using metallocene catalyst and modified-methylaluminoxane as cocatalyst. The additive of low molar mass liquid crystal chemical (cyclohexyl-biphenyl-cyclohexane (CBC33)) or lubricant (glycerol monostearate (GMS)) was individually added to the blends of SPS in order to investigate the effects on the crystallinity of the blended SPS. From the experimental results, it was found that the percent crystallinities of the blends decreased with decreasing the percent of SPS in the blend because of the dilution of SPS. The depression of the percent crystallinity was in the order of PaMS > PCHA > PBMA according to the compatibility with SPS. The addition of GMS or CBC33 slightly decreased the percent crystallinity of the pure SPS. The addition of GMS impeded the depression of the SPS crystallinity in the blends, because their percent depression from pure SPS is similar (at around 25%) regardless to the components of the blends. The blends with added CBC33 have the similar depression of crystallinity as the pure blends because of the low concentration of CBC33 and the good compatibility of CBC33 with the SPS.

Why do non-symmetric dimers intercalate? The synthesis and characterisation of the α-(4-benzylidene-substituted-aniline-4′-oxy)-ω-(2-methylbutyl-4′-(4″-phenyl)benzoateoxy)alkanes

Ten new non-symmetric liquid crystal dimers belonging to the family of compounds α-(4-benzylidene-substituted-aniline-4′-oxy)-ω-(2-methylbutyl-4′-(4″-phenyl)benzoateoxy)-alkanes have been synthesised and their transitional properties characterised. The dimers contain either a hexamethylene or octamethylene spacer, while the terminal substituents on the 4-benzylideneaniline fragment are H, CH3, F, Cl and Br. The unsubstituted dimers are not liquid crystalline, while the remaining compounds exhibit enantiotropic nematic behaviour. The trends in the clearing temperatures, according to the chemical nature of the terminal substituent, are largely consistent with those established for conventional low molar mass liquid crystals. Three of the dimers also exhibit an intercalated smectic A phase, specifically the two bromo-substituted dimers and the chloro-substituted dimer containing a hexamethylene spacer. The driving force for the formation of this phase is considered to be, at least in part, the specific anisotropic interaction between the unlike mesogenic units. The absence of smectic behaviour for the isosteric methyl-substituted dimers reveals that steric factors alone cannot stabilise the intercalated smectic A phase.

Liquid Crystals in Tribology

Two decades ago, the literature dealing with the possible applications of low molar mass liquid crystals, also called monomer liquid crystals (MLCs), only included about 50 references. Today, thousands of papers, conference reports, books or book chapters and patents refer to the study and applications of MLCs as lubricants and lubricant additives and efforts are made to develop new commercial applications. The development of more efficient lubricants is of paramount technological and economic relevance as it is estimated that half the energy consumption is dissipated as friction. MLCs have shown their ability to form ordered boundary layers with good load-carrying capacity and to lower the friction coefficients, wear rates and contact temperature of sliding surfaces, thus contributing to increase the components service life and to save energy. This review includes the use of MLCs in lubrication, and dispersions of MLCs in conventional polymers (PDMLCs). Finally, new lubricating system composed of MLC blends with surfactants, ionic liquids or nanophases are considered.

Effect of low molar mass liquid crystal and lubricant on miscibility and thermal properties of syndiotactic polystyrene blends

AbstractThis research concerned with the miscibility and thermal properties of syndiotactic polystyrene (SPS) blended with several polymers such as poly (α‐methyl styrene)—PaMS, poly(ethyl methacrylate)—PEMA, poly(n‐butyl methacrylate)—PBMA, poly(cyclohexyl acrylate)—PCHA, and poly(cis‐isoprene)—PIP. The SPS synthesized by using metallocene catalyst and modified‐methylaluminoxane (MMAO) as cocatalyst. From the experimental results, it was found that the SPS can be miscible with PaMS, PEMA, PBMA, PCHA, and PIP. Furthermore, the effects of the addition of low molar mass liquid crystal (cyclohexyl‐biphenyl‐cyclohexane, CBC33) and lubricant (glycerol monostearate, GMS) on thermal properties of the blended polymers with SPS are concerned. Both CBC33 and GMS can lower the crystallization temperature (Tc) of the blend and can also slightly lower the melting temperature (Tm) of the blends. This might be because of the higher mobility of SPS molecules induced the separation from the crystal both in the case of the depression of Tc or Tm. The higher mobility of the blends happened together with the further reduction of the melt viscosity of the blends according to CBC33 that can be proven by Motong et al. in 2008 (Motong et al., J Appl Polym Sci 2008, 107, 1108). © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009