Nematic-isotropic Transition Temperature Research Articles

Textural, thermal, optical and electrical properties of Iron nanoparticles dispersed 4′-(Hexyloxy)-4-biphenylcarbonitrile liquid crystal mixture

ABSTRACTIn this work, the effect of Iron nanoparticles (Fe NPs) dispersion in 4′-(Hexyloxy)-4-biphenylcarbonitrile (6OCB) nematic liquid crystal properties has been studied. Inclusion of Fe NPs (0.25 wt. %) in 6OCB liquid crystal (LC) on textures, isotropic–nematic transition temperature (TI–N), electro-optical and dielectric properties have been investigated in planar aligned cell. The threshold voltage (Vth) and TI–N decrease after dispersion of Fe NPs. Dielectric spectroscopy in nematic phase show that relaxation frequency (fr) also decreases after dispersion of Fe NPs in 6OCB. The observed relaxation mode is due to the flip-flop motion of LC molecules about their short axis. The band gap and AC conductivity in case of 6OCB-Fe sample increase over pure 6OCB sample. A decrease in activation energy is also noticed.

QSPRs for estimating nematic transition temperatures of pyridine-containing liquid crystalline compounds

ABSTRACTQuantitative structure–property relationships were developed to predict the nematic–isotropic transition temperatures of 92 pyridine-containing liquid crystalline compounds using molecular descriptors calculated by CODESSA software and DRAGON software. The descriptors were also analysed by using principal component analysis. Essentials accounting for a reliable model were all considered carefully during model construction and assessment process. Five variables were selected out by stepwise forward regression analysis and were used as inputs to perform the multiple linear regression, support vector machine and projection pursuit regression (PPR) study. All models were validated through two ways, that is, internal cross-validation combined with a test set. Comparatively, the PPR model performs best both in the fitness and in the prediction capacity. For the test set, it gave a predictive correlation coefficient (R) of 0.991, root mean square error of 11.799 and absolute average relative deviation of 5.456, respectively. The relationships between the descriptors and the nematic–isotropic transition temperature of compounds were also discussed. The odd–even effect in the transition temperatures of mesogens in the same homologous series was also discussed.

Fluctuation shift of the nematic–isotropic phase transition temperature

A macroscopic counterpart to the microscopic mechanism of the straightening dimer mesogens conformations, proposed recently by S.M. Saliti, M.G. Tamba, S.N. Sprunt, C. Welch, G.H. Mehl, A. Jakli, and J.T. Gleeson [Phys. Rev. Lett. 116, 217801 (2016)] to explain their experimental observation of the unprecedentedly large shift of the nematic–isotropic transition temperature is discussed. The proposed interpretation is based on singular longitudinal fluctuations of the nematic order parameter. Since these fluctuations are governed by the Goldstone director fluctuations, they exist only in the nematic state. External magnetic field suppresses the singular longitudinal fluctuations of the order parameter (similarly as is the case for the transverse director fluctuations, although with a different scaling over the magnetic field). The reduction of the fluctuations changes the equilibrium value of the magnitude of the order parameter in the nematic state. Therefore, it leads to additional (with respect to the mean field contribution) fluctuation shift of the nematic–isotropic transition temperature. Our mechanism works for any nematic liquid crystals, however the magnitude of the fluctuation shift increases with decrease in the Frank elastic moduli. Since some of these moduli supposed to be anomalously small for so-called bent-core or dimer nematic liquid crystals, just these liquid crystals are promising candidates for the observation of the predicted fluctuation shift of the phase transition temperature.

Bistable director alignments of nematic liquid crystals confined in frustrated substrates.

We studied in-plane bistable alignments of nematic liquid crystals confined by two frustrated surfaces by means of Monte Carlo simulations of the Lebwohl-Lasher spin model. The surfaces are prepared with orientational checkerboard patterns, on which the director field is locally anchored to be planar yet orthogonal between the neighboring blocks. We found the director field in the bulk tends to be aligned along the diagonal axes of the checkerboard pattern, as reported experimentally [J.-H. Kim et al., Appl. Phys. Lett. 78, 3055 (2001)APPLAB0003-695110.1063/1.1371246]. The energy barrier between the two stable orientations is increased, when the system is brought to the isotropic-nematic transition temperature. Based on an elastic theory, we found that the bistability is attributed to the spatial modulation of the director field near the frustrated surfaces. As the block size is increased and/or the elastic modulus is reduced, the degree of the director inhomogeneity is increased, enlarging the energy barrier. We also found that the switching rate between the stable states is decreased when the block size is comparable to the cell thickness.

Cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase

ABSTRACTThe synthesis and characterisation of several members of the 1,ω-bis(4-cyanobiphenyl-4′-yl) alkane (CBnCB) and the 1-(4-cyanobiphenyl-4′-yloxy)-ω-(4-cyanobiphenyl-4′-yl) alkane (CBnOCB) homologous series are reported. The new odd members described CB5CB, CB13CB, CB4OCB, CB8OCB and CB10OCB all exhibit twist-bend nematic and nematic phases. The members of these series already reported in literature, CB7CB, CB9CB, CB11CB and CB6OCB, were also prepared in order to allow for a direct comparison of their transitional properties. The properties of these dimers are also compared to those of the corresponding members of the 1,ω-bis(4-cyanobiphenyl-4,-yloxy) alkanes (CBOnOCB). For any given total spacer length, for odd members of these series, the nematic–isotropic transition temperatures and associated entropy changes are greatest for the CBOnOCB dimer and lowest for the CBnCB dimer. These trends are understood in terms of molecular shape. For short spacer lengths, the twist-bend nematic–nematic transition temperature (TNTBN) is higher for the CBnOCB series than for the CBnCB series but this is reversed as the spacer length increases. Of the CBOnOCB dimers, a virtual value of TNTBN was estimated for CBO3OCB and TNTBN was measured for CBO5OCB. These values are considerably lower than those observed for the corresponding members of the CBnCB or CBnOCB series. The dependence of TNTBN on molecular structure is discussed not only in terms of the molecular curvature but also in the ability of the molecules to pack efficiently. As the temperature range of the preceding nematic phase increases, so the twist-bend nematic–nematic transition entropy change decreases and the transition approaches second order for the longer spacers. For comparative purposes, the transitional behaviour of the even-membered dimers CB6CB, CB5OCB and CBO4OCB is reported and differences accounted for in terms of molecular shape.

Analysis of morphological behaviour and electro-optical properties of silica nanoparticles doped polymer dispersed liquid crystal composites

We have prepared silica nanoparticles (SNPs) doped polymer dispersed liquid crystal (PDLC) composites by using varying concentrations of SNPs. The varying concentration of SNPs has a profound impact on the morphology and electro-optical properties of PDLC. The polarizing optical microscopy (POM) studies revealed a substantial non- homogeneity/dual sized droplet morphology in the LC droplet size induced by the SNPs. The relative transmission was enhanced in the SNP-PDLC composites as compared to the undoped PDLC. The threshold voltage (Vth) of the doped composites decreased as compared to the undoped PDLC but increasing SNP concentration had a slight impact on the threshold voltage (Vth) as well as transmission. The effect of SNPs on nematic-isotropic transition temperature (TN-I) and ionic conductivity was also clarified.

Hockey-stick liquid crystalline 6-oxoverdazyl

ABSTRACTA series of hockey-stick shaped 6-oxoverdazyl radicals 1[n]–3[n], containing CH3 (a), CF3 (b) and CN (c) groups in the short ‘arm’, were prepared and their physical properties were investigated. Mesogenic behaviour was found only in the CN derivatives with the COO (1[n]c) and N=N (2[12]c) linking groups, which exhibited a nematic phase. Analysis of binary mixtures of selected compounds with bent-core nematic host 4[12] gave virtual nematic-isotropic transition temperatures, [TNI], which follow the order 1[12]b < 1[12]c < 1[12]a in one series and 1[12]c < 2[12]c < (3[12]c) in another. The observed effectiveness of the short-arm substituent, CF3 < CN < CH3, was related to the magnitude and orientation of the molecular dipole moment calculated with density functional theory methods.

Influence of electrostatic interactions on the properties of cyanobiphenyl liquid crystals predicted from atomistic molecular dynamics simulations

ABSTRACTThe influence of force field details in all-atom molecular dynamics (MD) simulations on the predicted thermodynamic, structural, and dynamic properties of bulk 4-cyano-4ʹ-pentylbiphenyl (5CB) systems have been investigated in the 292–368 K temperature range. The effect of the molecular dipole moment and the details of dihedral potential for biphenyl unit were investigated using both polarisable (POL) and non-polarisable (NP) versions of the quantum chemistry-based force field. The predicted densities for the nematic and isotropic phases of bulk 5CB were found to be in excellent agreement with available experimental data. The nematic-isotropic transition temperature (TNI) showed strong sensitivity to the force field details, MD simulations with partial atomic charge distributions and molecular dipole moment corresponding to high-level quantum chemistry calculations predicted an overestimation of the TNI by about 30 K. Rescaling the charges to allow the molecular dipole to be closer to experimentally reported values of 5CB dipole in condensed phases, significantly improved the prediction of TNI as well as other thermodynamic and dynamic properties of 5CB. We also discuss how the structural, thermodynamic, and dynamic properties of bulk 5CB are affected by the flexibility of the central biphenyl dihedral and the inclusion of induced polarisation effects.

Anomalous Increase in Nematic-Isotropic Transition Temperature in Dimer Molecules Induced by a Magnetic Field.

We have determined the nematic-isotropic transition temperature as a function of an applied magnetic field in three different thermotropic liquid crystalline dimers. These molecules are comprised of two rigid calamitic moieties joined end to end by flexible spacers with odd numbers of methylene groups. They show an unprecedented magnetic field enhancement of nematic order in that the transition temperature is increased by up to 15K when subjected to a 22T magnetic field. The increase is conjectured to be caused by a magnetic-field-induced decrease of the average bend angle in the aliphatic spacers connecting the rigid mesogenic units of the dimers.

Theoretical study of closo-decaborate novel nematogen: Thermodynamic behavior and phase stability

ABSTRACTThe present article deals with the thermodynamic behavior and phase stability of closo-decaborate nematogen viz. dinitrogen-10-(4-pentyl-1-thiacyclohexyl)-closo-decaborate (DPTD) at a molecular level. The atomic net charge and dipole moment at each atomic center have been evaluated using the complete neglect differential overlap (CNDO) method. The modified Rayleigh–Schrodinger perturbation method along with multicentered–multipole expansion method has been employed to evaluate the long-range intermolecular interactions, while a ‘6-exp’ potential function has been assumed for the short-range interactions. The total interaction energy values obtained through these computations have been used to calculate the probability of each configuration at room temperature (300 K), nematic–isotropic transition temperature (435 K), and above transition temperature (500 K) using the Maxwell–Boltazman formula. Further, the entropy of each configuration has been computed during the different modes of interactions. The adopted framework provides valuable information on thermodynamic behavior, and phase stability of novel nematogen based on parameters, i.e., molecular and thermodynamic, introduced in this article.

Extended conjugated mesogens: synthesis and mesomorphic properties of H-shaped mesogens based on 3,3ʹ,5,5ʹ-tetrasubstituted 2,2ʹ-bithiophene with oligo(1,4-phenyleneethynylene) arms

ABSTRACTA new class of extended conjugated mesogens, namely H-shaped mesogens based on 3,3ʹ,5,5ʹ-tetrasubstituted 2,2ʹ-bithiophene with oligo(1,4-phenyleneethynylene) arms, have been synthesised, and the relationships between molecular structures and mesomorphic properties investigated. Tetraalkyl, tetraalkoxy and dialkyldialkoxy derivatives, [R1C6H4CCC6H2(C2H5)2CC]2[R2C6H4CCC6H2(C2H5)2CC]2C8H2S2 where R1 and R2 = alkyl and alkoxy chains of different lengths, exhibit nematic phases. The length, number and position of the terminal chains strongly affect the mesomorphic properties. The tetraalkyl derivatives in which R1 = R2 = pentyl to heptyl exhibit enantiotropic mesophases, whereas the derivatives with octyl or nonyl chains exhibit monotropic mesophases. The tetraalkoxy derivatives in which R1 = R2 = pentyloxy to nonyloxy all exhibit enantiotropic nematic phases. The mesophase range increases with increasing alkoxy chain length, except that the octyloxy and nonyloxy derivatives have almost the same temperature range. The dialkyldialkoxy derivatives in which R1 = alkyl; R2 = alkoxy and in which R1 = alkoxy; R2 = alkyl (R1 and R2 = heptyl, nonyl, hexyloxy or nonyloxy) exhibit enantiotropic mesophases. The derivatives in which R1 = alkoxy have a significantly lower crystal–nematic transition temperature than the corresponding derivatives (R2 = alkoxy), although the two types of derivatives have a similar nematic–isotropic transition temperature.

Screening out the non-Arrhenius behaviour of nematic-isotropic transition by room temperature ionic liquid.

Differential Scanning Calorimetry (DSC) and optical polarization microscopy of a mixture of the liquid crystalline material (N-(4-methoxybenzylidene)-4-butylaniline, MBBA) and a Fe-based room temperature ionic liquid 1-ethyl-3-methylimidazolium tetrachloroferrate ([Emim](+) [FeCl4](-), EMIF) indicate a decrease in the nematic-isotropic (N-I) phase transition temperature (T(NI)) with an increase in EMIF concentration, explained by a proposed model of Coulomb "screening" of MBBA quadrupoles by the EMIF ions along with ionic "self screening." DSC studies of EMIF-MBBA and pure EMIF and comparison with pure MBBA results show that the major transitions in pure EMIF have Arrhenius behaviour, but more importantly the previously found convex Arrhenius behaviour of the pristine MBBA [K. Dan et al., Europhys. Lett. 108, 36007 (2014)] becomes Arrhenius in the mixture, indicating a conversion of the entropic N-I activation barrier to an enthalpic one. In presence of EMIF, a drastic decrease in the intensity of out-of-plane distortions of benzene rings in MBBA is found from Fourier transform infrared spectroscopy, consistent with significant reduction in the conformational states of MBBA. This suppression of large amplitude motion is again consistent with a Coulomb screening and gives a molecular basis for the entropic-to-enthalpic conversion of the N-I activation barrier.

Entropic screening preserves non-equilibrium nature of nematic phase while enthalpic screening destroys it.

The present manuscript describes the role of entropic and enthalpic forces mediated by organic non-polar (hexane) and polar (methanol) solvents on the bulk and microscopic phase transition of a well known nematic liquid crystalline material MBBA (N-(4-methoxybenzylidene)-4-butylaniline) through Differential Scanning calorimetry (DSC), UV-Visible (UV-Vis), and Fourier Transform Infrared (FTIR) spectroscopy. DSC study indicates continuous linear decreases in both nematic-isotropic (N-I) phase transition temperature and enthalpy of MBBA in presence of hexane while both these parameters show a saturation after an initial decay in methanol. These distinct transitional behaviours were explained in terms of the "depletion force" model for entropic screening in hexane and "screening-self-screening" model for methanol. Heating rate dependent DSC studies find that non-Arrhenius behaviour, characteristic of pristine MBBA and a manifestation of non-equilibrium nature [Dan et al., J. Chem. Phys. 143, 094501 (2015)], is preserved in presence of entropic screening in the hexane solution, while it changes to Arrhenius behaviour (signifying equilibrium behaviour) in presence of enthalpic screening in methanol solution. FTIR spectra show similar dependence on the solvent induced screening in the intensities of the imine (-C = N) stretch and the out-of-plane distortion vibrations of the benzene rings of MBBA with hexane and methanol as in DSC, further establishing our entropic and enthalpic screening models. UV-Vis spectra of the electronic transitions in MBBA as a function of temperature also exhibit different dependences of intensities on the solvent induced screening, and an exponential decrease is observed in presence of hexane while methanol completely changes the nature of interaction to follow a linear dependence.

Electroclinic effect in a chiral paranematic liquid-crystal layer above the bulk nematic-to-isotropic transition temperature.

Electroclinic measurements are reported for two chiral liquid crystals above their bulk chiral isotropic-nematic phase transition temperatures. It is found that an applied electric field E induces a rotation θ [∝Ε] of the director in the very thin paranematic layers that are induced by the cell's two planar-aligning substrates. The magnitude of the electroclinic coefficient dθ/dE close to the transition temperature is comparable to that of a bulk chiral nematic, as well as to that of a parasmectic region above a bulk isotropic-to-chiral smectic-A phase. However, dθ/dE in the paranematic layer varies much more slowly with temperature than in the parasmectic phase, and its relaxation time is slower by more than three orders of magnitude than that of the bulk chiral nematic electroclinic effect.

Non-symmetric chiral liquid crystal dimers. Preparation and characterisation of the (S)-(benzylidene-4′-substitutedaniline)-2″-methylbutyl-4‴-(4″″-phenyloxy)-benzoateoxy)hexanoates

The properties of six chiral non-symmetric dimers belonging to the family of (S)-(benzylidene-4′-substitutedaniline)-2″-methylbutyl-4‴-(4″″-phenyloxy)benzoate-oxy)hexanoates are described. The terminal substituents on the 4-benzylidene-4′-substituted fragment are H, Cl, Br, I, CH3 and C2H5. All six dimers exhibit an enantiotropic chiral nematic phase. The relationship between the chiral nematic-isotropic transition temperature and the terminal substituent is discussed in terms of the size of the substituent and its effect of the molecular shape, and its ability to interact with mesogenic group to which it is attached. The C2H5-substituted dimer also shows an enantiotropic monolayer chiral smectic C phase, the formation of which is attributed to the enhanced molecular inhomogeneity arising from the longer alkyl chain. The weak smectic tendencies seen for the halogen-substituted dimers are attributed to potentially repulsive interactions between the ester links that would occur in an intercalated smectic arrangement which would be expected for dimers of this type.

Mesomorphism dependence on lateral substitution of functional groups and their position in series: 4(4′-n-alkoxy benzoyloxy)-3-chloro phenyl azo-2″-methoxy benzenes

ABSTRACTEleven homologues of the title series were synthesized. The methyl-to-pentyl derivatives are nonmesomorphic. The nematic mesophase commences from the sixth member of the series without any smectic phase. An odd–even effect in the nematic–isotropic transition curve is not observed. The nematic mesophase appeared as a threaded or a Schlieren-type texture as observed through a hot-stage polarizing microscope. The nematic–isotropic transition temperatures are between 89°C and 127°C with the mesomorphic range varying from 9°C to 44°C at the hexyl and tetradecyl derivative of the series, respectively. The nematic–isotropic transition curve initially rises and then falls in a normal manner as the series is ascended, but it abnormally rises beyond the 10th homologue. The series is enantiotropic nematic with a middle-ordered melting type. Analytical data support the structures of the molecules. The thermal stability and some other mesomorphic characteristics are compared with structurally similar homologous series. The average nematic–isotropic thermal stability is 105.4°C.

Phase stability and ordering of nematogen at molecular level: A computer-aided modeling

ABSTRACTThe present article deals with the phase stability and ordering of nematogen, viz., 6-octyloxy-2-naphthylyl-4-octoxybenzoate (ONOB) at molecular level. A comparative picture has been given between molecular charge distribution, and phase stability based on AM1, PM3, CNDO, and MNDO methods. The modified Rayleigh–Schrodinger perturbation method along with multicentered-multipole expansion method has been employed to evaluate the long-range intermolecular interactions, while a “6-exp” potential function has been assumed for the short-range interactions. The total interaction energy values obtained through these computations have been used to calculate the probability of each configuration at room temperature, nematic-isotropic transition temperature, and above transition temperature using the Maxwell–Boltzmann formula. Further, the entropy of each configuration has been computed during the different modes of interactions. An effort has been made to develop a computational model at molecular level based on configurational entropy, and translational rigidity parameters introduced in this article.

Front Propagation at the Nematic-Isotropic Transition Temperature

We study the gradient flow model for the Landau--de Gennes energy functional for nematic liquid crystals at the nematic-isotropic transition temperature on prototype geometries. We study the dynamic model on a three-dimensional droplet and on a disc with Dirichlet boundary conditions and different types of initial conditions. In the case of a droplet with radial boundary conditions, a large class of physically relevant initial conditions generate dynamic solutions with a well-defined nematic-isotropic interface which propagates according to mean curvature for small times. On a disc, we make a distinction between “planar” and “nonplanar” initial conditions, and “minimal” and “nonminimal” Dirichlet boundary conditions. Planar initial conditions generate solutions with an isotropic core for all times, whereas nonplanar initial conditions generate solutions which escape into the third dimension. Nonminimal boundary conditions generate solutions with boundary layers, and these solutions can either have a largely ordered interior profile or an almost entirely disordered isotropic interior profile. Our examples suggest that while critical points of the Landau--de Gennes energy typically have highly localized disordered-ordered interfaces, the transient dynamics exhibits observable interfaces of potential experimental relevance.

Ordering and Nematic Phase Behavior of a Naphthyl Ester: A Computational Model Based on Semiempirical Approach

The present article deals with the ordering and nematic phase behavior of naphthyl-ester, viz., 4-octylphenyl-6-octyloxy-2-naphthoate based on semiempirical approach at a molecular level. The atomic net charge and dipole moment at each atomic centre has been evaluated using the complete neglect differential overlap method. The modified Rayleigh–Schrodinger perturbation method along with multicentered-multipole expansion method has been employed to evaluate the long-range intermolecular interactions, while a “6-exp” potential function has been assumed for the short-range interactions. The total interaction energy values obtained through these computations have been used to calculate the probability of each configuration at room temperature, nematic-isotropic transition temperature, and above transition temperature using the Maxwell–Boltzmann formula. Further, the entropy of each configuration has been computed during the different modes of interactions. An effort has been made to develop a computational model at molecular level based on thermodynamic parameter introduced in this article.

Synthesis and Characterization of Novel Thermotropic Liquid Crystals with a Lateral Chloro Group

Twelve novel homologues having azolinkage of the series 4-Carbethoxy-[3′chloro-4′ (4″-n-alkoxy benzoyloxy)] azobenzenes has been synthesized. The methyl to n-hexyl homologues exhibit only a nematic mesophase, while the n-heptyl to n-dodecyl homologues exhibit both smectic and nematic mesophases. The n-tetradecyl and n-hexadecyl homologues exhibit only smectic mesophases. The plot of transition temperatures versus number of carbon atoms in alkoxy chain exhibits an odd-even effect for the nematic-isotropic transition temperatures. The mesogenic behavior of present series is explained by comparing its homologue with other homologue of related mesogenic series. The synthesized compounds were characterized by combination of elemental analysis and standard spectroscopic methods. For the exhibition of mesomorphic property the role of ester and azo linkages has been discussed. The impact of lateral chloro group on mesomorphism is also discussed.