Chiral Nematic Phase Research Articles

Observation of positive and negative magneto-LC effects in all-organic nitroxide radical liquid crystals by EPR spectroscopy

The generation of spin glass-like inhomogeneous magnetic interactions (the average spin–spin interaction constant: > 0 or < 0), which we refer to as positive or negative “magneto-LC effects”, respectively, was observed in the bulk liquid crystalline (LC) phases of chiral all-organic radical compounds 2 with a positive dielectric anisotropy (Δε > 0) under weak magnetic fields. The sign and magnitude of depended on the type of LC phase or superstructure; stronger positive interactions ( > 0) operated in the chiral smectic A (SmA*) phase of (2S,5S)-2b (89% ee) than in the chiral nematic (N*) phase of (2S,5S)-2a (96% ee), whereas weak negative interactions ( < 0) were observed in the achiral nematic (N) phase of (±)-2a. The origin of the positive magneto-LC effects operating in the SmA* and N* phases was interpreted in terms of the generation of ferromagnetic head-to-tail spin–spin dipole interactions, whereas antiferromagnetic interactions arising from the formation of the RS magnetic dipolar interaction were responsible for the negative magneto-LC effects in the N phase.

Chirality of an achiral bent-core nematic mesogen observed in planar and homeotropic cells under certain boundary conditions

It is shown that planar and homeotropic cells under certain boundary conditions containing an achiral bent-core C5 liquid crystalline material show chiral domains of opposite handedness. However such domains are not seen in other members of the homologous series which show cybotactic clusters of larger sizes. The stability of such chiral domains in a planar cell depends not only on temperature but also on the device parameters such as the cell gap and the angle between the rubbing directions on the substrates. The energy difference between the two states with different twist angles measured with respect to the rubbing direction is expressed in terms of the device parameters. The chirality of the material with a pitch length of 9 μm is discussed in terms of the molecular symmetry created by the formation of a pair of the two bent-core molecules, which acts as a chiral dopant. The results of the study on C5 are compared with a prediction from a theoretical model with higher order molecular interactions in the bent-core system and it is suggested that the observed phase is a tetrahedratic chiral nematic phase of D2 symmetry as recently suggested by [Longa et al., Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys., 2009, 79, 040701].

Fast Field-Cycling NMR Relaxometry Study of Chiral and Nonchiral Nematic Liquid Crystals

We present a proton NMR relaxometry study of the molecular dynamics in three liquid crystalline systems: 4'-n-pentyl-4-cyanobiphenyl (5CB), (S)-4'-(3-methylpentyl)-4-cyanobiphenyl (5CB*), and a 12% weight mixture of 5CB* in 5CB. The proton spin-lattice relaxation time (T1) was measured as a function of temperature and Larmor frequency in the isotropic, nematic, chiral nematic (N*), and smectic A phases of these liquid crystalline systems. A unified relaxation model was used to analyze the molecular dynamics, considering local molecular rotations/reorientations, translational self-diffusion, and collective motions as the relaxation mechanisms that contribute most effectively to the T1(-1) relaxation. Additionally, in the chiral nematic phase a fourth relaxation mechanism associated with the rotations induced by the translational diffusion along the helical axis (RMTD) was included in the model. All experimental results were consistently analyzed taking into account the physical parameters known for 5CB. The global analysis of the experimental results shows that the RMTDs are associated with the pitch value measured for the N* phases and that its contribution to the T1(-1) dispersion is observed at low frequencies. The T1(-1) dispersion in the smectic A phase of 5CB* is strongly dominated by the layer undulations relaxation mechanism over a broad frequency range from the low kilohertz regime to tens of megahertz. It was the first time such behavior was observed in a low molecular weight liquid crystalline system.

Tuning the helical twisting power of nematic liquid crystals induced by chiral 1,2-propanediol derivatives using varied substituents

Abstract In this study, a novel series of chiral 1,2-propanediol derivatives with different electron-donating and electron-withdrawing groups were synthesized and characterized by FT-IR and 1 H NMR. The helical twisting properties of all the chiral dopants were investigated by doping the chiral dopants into a nematic liquid crystal host (SLC-1717). The results indicate that the donor–acceptor electron effect have a prominent influence on helical twisting property of the chiral nematic phase induced by the chiral dopants. Introducing electron-withdrawing groups into the terminal ends of chiral 1,2-propanediol can decrease the absolute values of the helical twisting power. In addition, the helix inversion temperatures of the induced chiral nematic phase are variational with the change of terminal groups.

Reversible and fast shift in reflection band of a cubic blue phase in a vertical electric field

Two electro-optical phenomena of liquid-crystal blue phase with cubic lattice structure—(1) a fast and reversible shift in reflection band and (2) a phase transition from blue phase to chiral nematic phase—are observed in this study. By increasing the magnitude of a vertical electric field, the reflected wavelength of the blue phase can be shifted to a longer wavelength in a range of 80 nm within a few ms. This reflection band shift is induced by the reorientation of liquid-crystal molecules constructing double twist cylinders, which induces lattice distortion of the blue phase.

Non-symmetric chiral liquid crystal trimers

Two sets of non-symmetric chiral liquid crystal trimers have been synthesised and characterised. These consist of molecules containing a cholesteryl-based group, a 4-methoxyazobenzene unit and a central benzylideneaniline group, interconnected in a linear fashion via two flexible spacers. The length of the spacer connecting the 4-methoxyazobenzene moiety to the benzylideneaniline group is held constant at either five or six methylene units while the length of the second spacer is varied. All 12 trimers exhibited an enantiotropic chiral nematic phase. In addition, a smectic A phase was observed for both sets of trimers when the second spacer contained three, five or seven methylene units. The chiral nematic–isotropic transition temperatures and associated entropy changes depend critically on the parity of the flexible spacers. This behaviour is interpreted in terms of the average molecular shape and how this is controlled by the flexible spacers. The ratio of the smectic periodicity to the estimated all-trans molecular length for each of the trimers was approximately 0.3, implying a triply intercalated arrangement in which each layer contains all three mesogenic units. The driving force for the formation of this phase is considered to be electrostatic interactions between electron rich and electron deficient segments of the molecules.

Molecular Dynamics in Rod-Like Liquid Crystals Probed by Muon Spin Resonance Spectroscopy

Muoniated spin probes were produced by the addition of muonium (Mu) to two rod-like liquid crystals: N-(4-methoxybenzylidene)-4'-n-butylaniline (MBBA) and cholesteryl nonanoate (CN). Avoided level crossing muon spin resonance spectroscopy was used to characterize the muoniated spin probes and to probe dynamics at the molecular level. In MBBA Mu adds predominantly to the carbon of the bridging imine group and the muon and methylene proton hyperfine coupling constants (hfccs) of the resulting radical shift in the nematic phase due to the dipolar hyperfine coupling, the ordering of the molecules along the applied magnetic field and fluctuations about the local director. The amplitude of these fluctuations in in the nematic phase of MBBA is determined from the temperature dependence of the methylene proton hfcc. Mu adds to the double bond of the steroidal ring system of CN and the temperature dependence of the Δ(1) line width provides information about the amplitude of the fluctuations about the local director in the chiral nematic phase and the slow isotropic reorientation in the isotropic phase.

Characterization of Collective Relaxation Modes of a Ferroelectric Mixture Through Dielectric Spectroscopy

Molecular motion of a ferroelectric mixture has been studied by the dielectric spectroscopy in the frequency range of 1 Hz to 35 MHz in different mesophases viz. chiral nematic (N*), ferroelectric (SmC*) and hexatic (SmI*) phases. One soft like relaxation process in the N* phase and usual Goldstone mode in the SmC* phase have been found. A new mode has been detected at ∼50 Hz in the vicinity of SmC*- SmI* phase transition. The dielectric strength of this mode is highly temperature dependent and increases as temperature decreases. Two modes in different frequency ranges (kHz and few hundred Hz ranges) have been detected in the SmI* phase.

Chemistry and applications of nanocrystalline cellulose and its derivatives: A nanotechnology perspective

AbstractNanocrystalline cellulose (NCC) is an emerging renewable nanomaterial that holds promise in many different applications, such as in personal care, chemicals, foods, pharmaceuticals, etc. By appropriate modification of NCC, various functional nanomaterials with outstanding properties, or significantly improved physical, chemical, biological, as well as electronic properties can be developed. The nanoparticles are stabilised in aqueous suspension by negative charges on the surface, which are produced during the acid hydrolysis process. NCC suspensions can form a chiral nematic ordered phase beyond a critical concentration, i.e. NCC suspensions transform from an isotropic to an anisotropic chiral nematic liquid crystalline phase. Due to its nanoscale dimension and intrinsic physicochemical properties, NCC is a promising renewable biomaterial that can be used as a reinforcing component in high performance nanocomposites. Many new nanocomposite materials with attractive properties were obtained by the physical incorporation of NCC into a natural or synthetic polymeric matrix. Simple chemical modification on NCC surface can improve its dispersability in different solvents and expand its utilisation in nano‐related applications, such as drug delivery, protein immobilisation, and inorganic reaction template. This review paper provides an overview on this emerging nanomaterial, focusing on the surface modification, properties and applications of NCC.

Limonene as a chiral dopant for liquid crystals: Characterization and potential applications

Abstract— The chiral molecule D(L)‐Limonene was used to induce thermotropic cholesteric‐liquid‐crystal phases. A description of the methods used to identify and characterize the chiral nematic phase and its features will be given. D‐Limonene was substituted for conventional synthesized chiral dopants in a twisted‐nematic (TN) cell and a bistable cholesteric reflective cell and showed an improvement in the response time. Further analysis showed that D(L)‐Limonene reduces the rotational viscosity of the mixture by a considerable amount. It may be considered as a “molecular lubricant.” It is surprising how many more synthesized chiral molecules have been used as the liquid‐crystal field moves forward in comparison to naturally occurring molecules. Despite the large number of synthesized molecules used as a chiral dopant, D(L)‐Limonene, can be used as a dopantfor making cholesteric phases having the advantage of being a naturally occurring substance, environmentally friendly, and with very low cost.

Effect of terminal perfluorocarbon chain containing mesogens on phase behaviors of chiral comb-like liquid crystalline polymers

A novel perfluorinated liquid crystal 4′-(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctanoyloxy)biphenyl-4-yl undec-10-enoate (PFOBU) was synthesized, which exhibited smectic C phase. Several liquid crystalline polymers (PI–PVI) were synthesized by use of poly(methylhydrogeno)siloxane, PFOBU, and cholesteryl 3-(4-allyloxy-phenyl)-acryloate. The chemical structures and liquid crystalline (LC) properties of the monomers and polymers, and some ferroelectric properties of the chiral smectic C (SC*) phase were characterized by use of various experimental techniques. The effect of perfluorocarbon chains on phase behaviors of the fluorinated LC polysiloxanes was studied as well. PI and PII showed single chiral nematic (N*) mesophase when they were heated and cooled, but PIII, PIV, PV, and PVI containing more perfluorocarbon chain units exhibited SC* phase besides N* mesophase. Introduction of perfluorocarbon chain containing mesogens to the chiral cholesteryl-containing polymer systems resulted in a SC* mesophases, indicating that the fluorophobic effect could lead to microphase segregation and modifications of smectic mesophases from the chiral nematic phase.

A study of steric chirality: the chiral nematic phase of a system of chiral two-site HGO molecules

The liquid crystalline phase behaviour of a chiral two-site hard Gaussian overlap fluid is examined using the well-known Parsons–Lee extension of the theory of Onsager. The hard-core model is constructed such that the vector connecting the centers of two hard Gaussian segments is perpendicular to the long axes of both segments. The microscopic chirality of the particle can be controlled with the dihedral angle between the long axes of the hard Gaussian segments, the distance between the two segments, and the length-to-breath ratios of each segment. In the framework of the Parsons–Lee approach three different types of phases are considered, namely, the isotropic liquid state, and the nematic and the chiral nematic (cholesteric) liquid crystalline states. For simplicity, the orientation of the particles is restricted to the plane perpendicular to the twist axis, and the particles do not have internal freedom to rotate around their main symmetry axes. The geometric condition for the formation of a chiral nematic phase, the properties of the helical structure, and the phase boundary of the ordering transition are determined by means of a free energy minimization. It is shown that steric (shape) chirality always gives rise to a helical structure in the nematic phase, and that the low density chiral systems can undergo a transition from an isotropic liquid to a twisted nematic phase on increasing the density. Analytical expressions are obtained for the twist period (pitch) in the limit of parallel stacking of the rod-like segments in layers normal to the helical axis, which are only valid for systems characterized by weak chiral strengths. A key finding of the numerical calculations is that the pitch is very sensitive to the segment separation, but not to the density or aspect ratio. It is interesting to note that the inverse of the pitch is predicted to depend linearly on the dihedral angle in all of the cases studied.

Chiral effects of blue phase stabilisation of a binaphthyl derivative

We have recently reported a binaphthyl liquid crystal exhibiting on cooling a cubic blue phase over a wide temperature range. On heating, however, a chiral nematic phase appeared in place of the blue phase. To elucidate the origin of blue phase stabilisation we have prepared a homologous series of binaphthyl derivatives possessing two 2,3-difluoro-1,4-diphenylbenzene units and have investigated the phase transition behaviour and helical twisting power. A binaphthyl derivative exhibiting the blue phase was found to induce helical twist inversion in a chiral nematic mixture with a host liquid crystal, whereas other compounds without the blue phase induced short helical pitches independently of temperature. The blue phase material is thought to have two possible origins for its twisting power, which are able to stabilise the double-twist structure of the phase.

Chiral liquid crystal tetramers

The synthesis and characterisation of two new series of liquid crystal tetramers in which four mesogenic groups are linked via three alkyl spacers is reported. In each series the length of the outer two spacers, n, is varied between 3, 4, 5, 7, 10 and 11 methylene units while the central spacer length, m, is either 5 or 6 methylene units. The outer two mesogenic groups are cholesteryl-based, Chol, while the inner two are Schiff's bases, SB. The tetramers are referred to by the acronym Chol-n-SB-m-SB-n-Chol. All twelve tetramers exhibit an enantiotropic chiral nematic phase. The chiral nematic-isotropic transition temperatures and associated entropy changes depend critically on the combination of spacers in the tetramer and this is interpreted in terms of average molecular shapes. The tetramers with m = 5 and n = 5, 7, and m = 6 and n = 3, 4, 5, 7, show a monolayer chiral smectic C* phase, the driving force for which may be the mismatch in the cross-sectional areas of the differing mesogenic groups. The tetramers with m=6 and n = 10, 11 show a quadruply intercalated smectic A phase characterised by a smectic periodicity to estimated molecular length ratio of just 0.23. The driving force for the formation of this phase is thought to be specific electrostatic interactions between the unlike mesogenic units while the formation of the phase is strongly dependent on the length and parity of the spacers due to packing constraints. On cooling, a new phase transition is seen, possibly driven by conformational changes in the outer flexible spacers, for both these tetramers from the intercalated smectic A phase to an intercalated alternating chiral smectic C* phase in which the layers consist of either the mixed mesogenic groups or solely the cholesteryl-based moieties.

Ultrathin film coatings of aligned cellulose nanocrystals from a convective-shear assembly system and their surface mechanical properties

Ultrathin films of aligned cellulose nanocrystals (CNCs) were deposited on solid supports by using convective and shear forces. Compared to previous systems involving high electric or magnetic fields to control the orientation of these rod-like natural nanoparticles, the proposed process of alignment was very simple, inexpensive and with potential for scale up. The effect of concentration of CNC in aqueous suspensions, type of solid support, relative humidity and rates of withdrawal of the deposition plate were determined by using atomic force microscopy (AFM) and ellipsometry. The degree of orientation was quantified from the number density of CNCs in leading angles by using image analyses. Also, the contribution of shear and capillary forces on alignment parallel and normal to the withdrawal direction was elucidated. The best alignment of CNCs in the withdrawal direction, favored by shear effects, was achieved with gold and silica supports with a pre-adsorbed cationic polyelectrolyte layer and at a CNC suspension concentration above 2.5% (w/w), below the critical concentration for chiral nematic phase separation. Compared to the bare solid support, nanoindentation of the obtained coatings of ultrathin films of oriented CNCs provided enhanced surface mechanical strength and wear resistance. A transverse Young's modulus, hardness and coefficient of friction of 8.3 ± 0.9 GPa, 0.38 ± 0.03 GPa and 0.51 ± 0.23 GPa, respectively, were measured. Notably, the transverse Young's modulus was found to be in agreement with reported values predicted by molecular modeling and measured for single CNCs by using atomic force microscopy.

Synthesis and phase behaviour of new cholesteric monomers and side chain smectic polymers based on cholesterol

The synthesis of five new cholesteryl-based monomers (M-1−M-5) and the corresponding smectic comb-like polymers containing cholesteryl groups (P-1−P-5) is presented. The chemical structures were characterised by FT-IR, 1H NMR and elemental analyses. The specific optical rotations were evaluated with a polarimeter. The phase behaviour was investigated by polarising optical microscopy, differential scanning calorimetry, thermogravimetric analysis, and X-ray diffraction. The specific optical rotation values of these monomers and polymers with the same number of phenyl rings and terminal groups were nearly equal; however, they decreased with increasing the aryl numbers in the mesogenic core. The monomers M-1−M-5 showed oily streak and focal conic optical textures, or finger print textures characteristic of the chiral nematic phase. The polymers P-1−P-5 showed the smectic A phase. The melting, clearing, and glass transition temperatures increased, and the mesophase temperature ranges widened with increasing the aryl number in the mesogenic core. However, although the molecular structures of M-4 and M-5 were similar to those of M-3, namely their mesogenic cores contained three phenyl rings, their phase behaviour differed considerably, and T m and T i of M-4 and M-5 were less than those of M-3. In addition, M-4 and M-5 showed a clear glass transition similar to the polymer. Furthermore, the ester linkage bond and aryl arrangement in the mesogenic core also affected the phase behaviour.

Right-handed double-helix ultrashort DNA yields chiral nematic phases with both right- and left-handed director twist

Concentrated solutions of duplex-forming DNA oligomers organize into various mesophases among which is the nematic (N(∗)), which exhibits a macroscopic chiral helical precession of molecular orientation because of the chirality of the DNA molecule. Using a quantitative analysis of the transmission spectra in polarized optical microscopy, we have determined the handedness and pitch of this chiral nematic helix for a large number of sequences ranging from 8 to 20 bases. The B-DNA molecule exhibits a right-handed molecular double-helix structure that, for long molecules, always yields N(∗) phases with left-handed pitch in the μm range. We report here that ultrashort oligomeric duplexes show an extremely diverse behavior, with both left- and right-handed N(∗) helices and pitches ranging from macroscopic down to 0.3μm. The behavior depends on the length and the sequence of the oligomers, and on the nature of the end-to-end interactions between helices. In particular, the N(∗) handedness strongly correlates with the oligomer length and concentration. Right-handed phases are found only for oligomers shorter than 14 base pairs, and for the sequences having the transition to the N(∗) phase at concentration larger than 620mg/mL. Our findings indicate that in short DNA, the intermolecular double-helical interactions switch the preferred liquid crystal handedness when the columns of stacked duplexes are forced at high concentrations to separations comparable to the DNA double-helix pitch, a regime still to be theoretically described.

Symmetric and non-symmetric chiral liquid crystal dimers

The synthesis and characterisation of three sets of non-symmetric liquid crystal dimers consisting of a cholesteryl-based unit and either 4-methoxybiphenyl, 4-cyanobiphenyl or (S)-2-methylbutyl 4′oxybiphenyl-4-carboxylate are described. The transitional properties of these non-symmetric dimers are compared to those of the corresponding parent symmetric dimers. The symmetric dimers exhibit only chiral nematic or nematic phase behaviour. By contrast, members of the non-symmetric dimer series containing either 4-cyanobiphenyl or (S)-2-methylbutyl 4′-oxybiphenyl-4-carboxylate groups exhibit smectic behaviour. The former series show an interdigitated smectic A phase while for the latter the structure of the smectic A phase depends on the length of the flexible spacer. Specifically, for short spacer lengths a monolayer arrangement is seen while for long spacers an intercalated smectic A phase is formed. For an intermediate spacer length, the small-angle X-ray scattering pattern for the intercalated smectic A phase reveals short-range incommensurate structural fluctuations associated with the monolayer smectic A phase. The driving force for the formation of the intercalated phase may be an electrostatic interaction between the electron rich carbonyl linking group attached to the cholesteryl unit and the electron deficient aromatic unit while the monolayer arrangement may be stabilised by the van der Waals interactions between the cholesteryl unit and the alkyl chains. Blue phases are observed only for a small number of these non-symmetric dimers and these all contain an odd-membered spacer. This is in accord with the rather general observation that blue phases are observed for odd-membered non-symmetric dimers and not their even-membered counterparts.

Surface Grafting of Cellulose Nanocrystals with Poly(ethylene oxide) in Aqueous Media

Aqueous suspensions of poly(ethylene oxide)-grafted nanocrystalline cellulose (PEO-grafted NCC) were prepared in order to achieve steric instead of electrostatic stabilization. A two-step process was employed: in the first step NCC suspensions prepared by sulfuric acid hydrolysis were desulfated with sodium hydroxide, and in the second step the surfaces of the crystals were functionalized with epoxy-terminated poly(ethylene oxide) (PEO epoxide) under alkaline conditions. The PEO-grafted samples were analyzed by conductometric titration, ATR-IR, solid-state NMR, MALDI-TOF MS, SEC MALLS, and AFM. The covalent nature of the linkage was confirmed by weight increase and MALDI-TOF analysis. The PEO-grafted cellulose nanocrystals (CNCs) formed a stable colloidal suspension that remained well dispersed, while the desulfated nanoparticles aggregated and precipitated. Upon concentration of the PEO-grafted aqueous NCC suspension, a chiral nematic phase was observed.

Charged fibrous viruses (fd) in external electric fields: dynamics and orientational order

We recently found a number of phases and dynamical states that are induced in a concentrated suspension of charged, colloidal rods (fd-viruses) by an alternating external electric field (Kang and Dhont 2008 Eur. Phys. Lett.84 14005; 2010 Soft Matter6 273). The various phases and dynamical states are the result of interactions between the charged rods through their polarized electric double layers, polarized layers of condensed ions and/or electro-osmotic flow. At a relatively high frequency, a homogeneous, homeotropically aligned phase is induced (the H-phase). We present a dynamic light-scattering study of the microscopic dynamics of the rods, varying the frequency and field amplitudes along different pathways within this phase. Scattering experiments are performed at very small scattering angles with a home-made vertically mounted dynamic light-scattering setup, where Brownian motion perpendicular to the direction of alignment is probed. The orientational order is measured by means of birefringence experiments. The remarkable finding is that relaxation times and the degree of alignment are independent of the frequency and the amplitude of the applied electric field throughout the entire H-phase. Only within a small region in the neighborhood of the transition line, where the H-phase transforms to an inhomogeneous chiral-nematic phase, is there a frequency and amplitude dependence of relaxation times, which are shown to be the result of the appearance of transient, pre-transitional domains. We also recently identified a non-equilibrium critical point, where a time- and length-scale connected to a dynamical state are shown to diverge (Kang and Dhont 2009 Eur. Phys. J. E 30 333). Approaching this critical point from the side of the H-phase, we find that the light-scattering correlation functions develop a very slowly decaying mode, the origin of which requires further investigation.