A proton spin relaxation study of ferroelectric liquid crystals

Here we present and analyse N.M.R. measurements of the Larmor frequency dependence (dispersion) of the longitudinal proton spin relaxation time, T ₁(v), for two chiral ferroelectric mesogens (Merck IS-1912 and DOBAMBC) in the isotropic, smectic A and smectic C* phases, making use of fast field cycling techniques. Although in the low frequency range the relaxation times of IS-1912 are much shorter than those of DOBAMBC, the form of the dispersion profiles is not basically different for the two materials. This reveals contributions by smectic order fluctuations, self-diffusion and molecular rotations. The order fluctuation term, which means relaxation by collective molecular reorientations, is clearly seen by characteristic dispersion profiles in the kHz regime (T ₁ ～ v ¹ or T ₁ ～ v ^1/2), which disappear in the isotropic phase. Our results do not indicate significant dissimilarities between the main relaxation processes in the S_C and S*_C mesophases.     

Relaxation dispersion and zero-field spectroscopy of thermotropic and lyotropic liquid crystals by fast field-cycling N.M.R.

Abstract

Systematic field-cycling measurements of the T ₁ relaxation dispersion in numerous nematic liquid crystals (azoxybenzenes, Schiff's bases, biphenyls, phenyl-cyclohexanes, cyclo-cyclo-hexanes) confirm our previous observations obtained for PAA and MBBA that order fluctuations of the nematic director are a significant relaxation contribution only at low Lamor frequencies v, i.e. far below the usual megahertz range. Their significance is demonstrated most convincingly by the characteristic square-root dispersion law, T ₁ ～ v ^1/2, which occurs in the kilohertz range and which completely disappears above the nematic–isotropic phase transition. The strength of the collective relaxation mechanism varies by more than two orders of magnitude in the sequence (selection) PAA-d ₈ PAA, PAA-d ₆ PAB, OCB7, MBBA, CB7, PCH7, MBBA-d ₆ MBBA-d ₁₃ and CCH7. This finding can be understood almost quantitatively by the widely differing separations and orientations of proton pairs on the molecules, together with the different viscoelastic parameters of the nematogens. In addition, the underlying slow molecular reorientations have been observed in MBBA and PAA by intensity changes of the zero-field spectra, which are absent for high-field measurements. Similarly, smectic type order fluctuations in layered liquid crystal structures prove to be an effective relaxation mechanism only at low Lamor frequencies. This has been verified by the related linear relaxation dispersion profile, T ₁ ～ v ¹, for both thermotropic systems (TBBA, C₁₂-AA) and lamellar lyotropic mixtures (e.g. potassium laurate in water and phospholipids in water). Our results concerning the time scale of the T ₁ ～ v ^1/2 and T ₁ ~ v ¹ regime do not agree with conclusions drawn from conventional high-field techniques.     

Influence of underlying local organisations in typical nematic phase of 6CHBT and induced nematic phase of bicomponent liquid crystalline systems 4DBT–12CB – a Fast Field Cycling NMR investigation

Variable-temperature proton magnetic relaxation dispersion (PMRD) profiles are collected in the induced nematic phases of the binary liquid crystalline mixtures composed of smectic mesogens: 5-butyl-2-(4-isothiocyanatophenyl)-1,3-dioxane (4DBT) and 4′-dodecyl-4-cyanobiphenyl (12CB) and the low viscous nematogen 4-(trans-4′-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT), with a view to assess the influence of local organisations on the power spectrum of director fluctuation modes. Two distinct compositions of the binary mixture i.e. 50 mol% of 12CB (without a smectic phase) and 70 mol% of 12CB (with a smectic phase) are used. Fast Field Cycling nuclear magnetic resonance relaxometry is employed to measure proton spin-lattice relaxation rates (R₁) as a function of Larmor frequency (10 kHz–30 MHz). PMRD data analysis with the choice of a suitable model indicated nematic clusters of moderate size (~200 Å) found in the broad nematic region of 50 mol% 12CB, whose size is almost invariant with temperature. On the other hand, cybotactic clusters, i.e. local smectic organisations of relatively larger size (~2000–3000 Å), are observed near the nematic–smectic transition (T > T_AN) of 70 mol% 12CB. Interestingly, the shape geometry of such local organisations accessed from PMRD analysis is weakly anisotropic near T_AN, while being isotropic near T_NI for 70 mol% 12CB.     

Proton spin–lattice relaxation study of a partial bilayer smectic A phase

Abstract

Measurements of proton spin-lattice relaxation rates for the partial bilayer smectic A phase of 4-((4′-n-hexadecyloxybenzylidene)-amino) benzonitrile obtained at different Larmor frequencies and temperatures show that the essential relaxation mechanisms in the MHz frequency region are translational self-diffusion and local molecular reorientations similar to those in monolayer smectics. The values of the diffusion constant obtained from the fit of the theory to the experimental data show a range from 2.6 × 10^?11 m² s^?1 at 95°C to 1.7 × 10^?11 m² s^?1 at 75°C. A dynamic process specific to the partial bilayer smectic A phase seems to influence relaxation below 10 MHz. It can be associated either with the dimerization of molecules in the layers or with a higher value of the low cut-off frequency of order director fluctuations than that found in monolayer smectic A phases.     

Study of internal ring rotation and molecular reorientation in the liquid crystal 5O.7 by deuterium N.M.R. spectroscopy

Abstract

The spectral densities of motion were determined by deuterium N.M.R. relaxation measurements in the nematic, smectic A and smectic C phases of 4-n-pentyloxybenzylidene-d ₁-4′-heptylaniline and 4-n-pentyloxybenzylidene-4′-heptylaniline-2,3,5,6-d ₄. By examining two atomic sites on a 5O.7 molecule, we were able to gain information on the reorientation motion and internal rotation of the aniline ring. It was also found that director fluctuations make some contribution to the spectral density J ₁ (ω). We use the superimposed rotations model to account for the internal ring motion and the small step rotational diffusion model for the molecular reorientation. The derived rotational diffusion constants for the spinning and tumbling motions appear to give physically plausible activation energies in the mesophases of 5O.7.     

Proton NMR relaxation study of molecular dynamics of chromonic liquid crystal Edicol Sunset Yellow

Proton nuclear magnetic resonance (¹H NMR) relaxometry, over about five decades in Larmor frequency, and pulsed field gradient NMR were used to study the molecular dynamics in the chromonic nematic and isotropic phases of stacked molecules of the binary mixture composed by Edicol Sunset Yellow (ESY) and deuterated water. Our results evidence that in both phases collective motions are responsible for the spin-lattice relaxation dispersion in the Larmor frequency range below 1 MHz. In the nematic phase, the collective motion are attributed to columnar undulations within the stacked molecules, while, in the isotropic phase, the results are explained by local order fluctuations due to the formation of the stacks. The high frequency dispersion was explained by individual molecular motions like rotations around and perpendicular to the stack axis, and also self-diffusion.     

Dielectric relaxation of a short molecule in a liquid-crystalline solvent

Abstract

Dielectric relaxation measurements of 5 mole % 4-n-hexyloxycyanobenzene (I) dissolved in 4-n-pentyloxyphenyl-trans-4-n-octylcyclohexylcarboxylate (II) were carried out from 1 kHz to 10 MHz in the nematic, smectic A and smectic B phases. The relaxation frequency of I parallel to the director is about 05 MHz in the S_Bphase and increases rapidly at the transition from S_B to S_A.     

Wide-line <Superscript>1</Superscript>H NMR study of mesomorphic states of 4′-(octyloxy)-4-biphenylcarbonitrile

A sample of 4′-(octyloxy)-4-cyanobiphenyl (8OCB) was studied in the temperature range −60–80°C by wide-line ¹H NMR. The line shapes, half-widths, and second moments were determined. For the smectic phase of 8OCB, the relaxation times T ₁ and T ₂, the correlation time τ_c, and the degree of order were estimated. The ¹H NMR spectral patterns and characteristics were found to be quite different for the crystalline, smectic, and nematic phases of 8OCB, which makes it possible to reliably identify the corresponding transitions and assess the molecular dynamics and molecular order of a structure. The temperature ranges, stability conditions, and other characteristics of the liquid crystalline phases that form on heating 8OCB were determined.     

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

In 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 (T_I–N), electro-optical and dielectric properties have been investigated in planar aligned cell. The threshold voltage (V_th) and T_I–N decrease after dispersion of Fe NPs. Dielectric spectroscopy in nematic phase show that relaxation frequency (f_r) 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.     

Optically-active comb-branch liquid crystalline polymers having the chiral center in the flexible spacer unit. III. Copolymers of (S)-2-(4′-cyano-4-biphenylyloxy)-1-methylethoxy ethyl methacrylate and di[6-(4-methoxy-4′-oxybiphenyl) hexyl]-2-methylene butane-1,4-dioate

A new series of copolymers with optically-active liquid crystalline side chain units has been synthesized from the comonomers (S)-2-[2-(4′-cyano-4-biphenylyloxy-1-methylethoxy]ethyl methacrylate ( 1 ) and di[6-(4-methoxy-4′-oxybiphenyl)hexyl]-2-methylene butane-1,4-dioate ( 4 ). Chiral nematic phases were exhibited by two members of the series, rich in monomer 1 , while a smectic phase was exhibited in copolymers rich in 4 . While it was thought possible that ordered chiral liquid crystalline phases may be induced by copolymerizing chiral mesogenic monomers with mesogenic derivatives of itaconic acid where the high side chain density encourages greater ordering in the system, no evidence of smectic C^* phases could be found in the present systems. © 1993 John Wiley & Sons, Inc.     

Molecular engineering of liquid crystal polymers by living polymerization. II. Living cationic polymerization of 11-[(4-cyano-4′-biphenyl) oxy] undecanyl vinyl ether and the mesomorphic behavior of the resulting polymers

The synthesis and living cationic polymerization of 11-[(4-cyano-4′-biphenyl)oxy]-undecanyl vinyl ether ( 6 – 11 ) are described. The mesomorphic phase behavior of poly( 6 – 11 ) with different degrees of polymerization was compared to that of 6 – 11 and of 11-[(4-cyano-4′-biphenyl) oxy] undecanyl ethyl ether ( 8 – 11 ) which is the model compound of the monomeric structural unit of poly( 6 – 11 ). 6 – 11 displays a monotropic S_A and a monotropic nematic mesophase while 8 – 11 an enantiotropic S_A mesophase. Poly( 6 – 11 ) with low degrees of polymerization exhibits an enantiotropic S_A mesophase. Poly( 6 – 8 ) with high degrees of polymerization displays an enantiotropic S_X (i. e., an unidentified smectic phase) and an enantiotropic S_C mesophase. These results demonstrate that the transformation of the nematic mesophase of the monomer into a smectic mesophase after polymerization, occurs at the level of monomeric structural unit.     

X-ray study of the nematic and smectic A phases of the cyano-substituted pyridines

We report an X-ray study of lamellar ordering in the nematic and smectic A phases of a homologous series of polar liquid crystals, the 2-(4-alkylphenyl)-5-cyanopyridines (nCP). Experiments were carried out using a diffractometer with a linear position sensitive detector. In the nematic and smectic A phases of the nCP and their mixtures with non-polar 4-n-butyl-4′-methoxyazoxybenzene two types of layering were found. One corresponds to the fluctuations of the smectic density wave with a monolayer wavevector q ₁, and the other is due to the partial bilayer fluctuations with the incommensurate wavevector q ₂ ≠ q ₁/2. The temperature dependences of the X-ray scattering intensity and the longitudinal correlation length for both types of layering in the nematic phase are presented. The critical behaviour in the vicinity of the smectic A-nematic phase transition occurs for a fluctuation mode, either q ₁ or q ₂, depending on the position on the liquid crystal phase diagram. The influence of the molecular structure of cyano-substituted pyridines on the formation of layered structures of different types is also discussed.     

Influence of the smectic A-nematic transitional order on the formation of the smectic phases of 4-n-hexyloxybenzylidene-4′-n-butyIaniline and 4-n-butyloxybenzylidene-4′-n-octyIaniline from an electrically deformed nematic phase

Abstract

The crucial role of the smectic A-nematic transitional order for the formation of the smectic A, B and G phases from an electrically deformed nematic phase of the liquid crystal 4-n-hexyloxy-benzylidene-4′-n-butylaniline (6O.4) with a typical smectic A-nematic first order transition and the formation of the smectic A and B phases from an electrically deformed nematic phase of the liquid crystal (4-n-butyloxy-benzylidene-4′-n-octylaniline (40.8) with a smectic A-nematic second order transition has been demonstrated. The nematic phase was deformed by an AC voltage of 2U,_th 5U _th and 10U _th, where U _th is the threshold voltage which causes the appearance of the Fréedericksz transition in the homeotropic nematic layer. The smectic textures have been observed on free cooling of the nematic phase or after the use of an oven. The smectic A phase of the liquid crystal 60.4 was observed with the formation of a clear smectic A-nematic phase boundary while the smectic A phase of the liquid crystal 40.8 has been formed from intermediate pretransitional stripes, observed by Cladis and Torza [1]. The homeotropic anchoring of the direction was crucial for the formation of the smectic phases of the liquid crystal 40.8 but not significant for the liquid crystal 60.4.     

Frequency spectra of disordered metals: especially liquid Rb

The phonon frequency spectrum g(ω) of a crystal, such as body centred cubic (bcc) Rb, is known to be characterized by the Van Hove singularities at ω?≠?0. However, for a liquid metal like Rb, g(ω) has a single, hydrodynamic-like singularity, namely a cusp ∝ ω ^(1/2), at ω?=?0. Here, we note first that computer simulation on liquid Rb near freezing has revealed a rather well-defined Debye frequency ω_D. Therefore, we propose here a zeroth-order model g ₀ (ω ) of g(ω) for Rb, which combines the Debye model with the ‘hydrodynamic’ ω ^(1/2) cusp. The corresponding velocity autocorrelation function 〈 v (t)·v (0)〉 has correctly a long-time tail ∝ t ^-(3/2). The terms from g ₀ (ω ) involving ω_D are then damped by weak exponential factors exp (-α _i t), and the resulting first-order approximation, g ₁ (ω ) say, to the frequency spectrum is found to have features in common with the molecular dynamics (MD) simulation form. Thus ω_D is fixed, as well as transport coefficients for the known thermodynamic state. The article concludes with a more qualitative discussion on supercooled liquids, and on metallic glasses such as Fe, for which MD simulations exist.     

Proton magnetic relaxation study of pretransitional phenomena in the isotropic phase of a nematic liquid crystal I. Dynamics of local order fluctuations

Nuclear magnetic relaxation was investigated in a broad temperature region above the clearing point (T_c) of a nematic liquid crystal. Dependence of spin-spin relaxation time on the pulse interval observed in the Curr-Purcell-Meiboom-Gill (CPMG) experiment indicates an exchange of nuclei between the states differing in local magnetic fields. By fitting of the Luz-Meiboom equation to the CPMG results, the mean lifetime of sites and modulation frequency δω were determined. The rather slow exchange (lifetimes changing with temperature in the range 20-90 ms) is suggested as manifesting the local order fluctuations in the pretransitional zone of the LC. A simple two-site model of a pretransitional zone was considered (cluster ? isotropic surrounding). Dipole-dipole interactions in clusters are unaveraged due to the local ordering, whereas in the isotropic subphase local magnetic fields are motionally averaged. Therefore, local order fluctuations are accompanied by the exchange observed in the CPMG sequence. Correspondence of the temperature dependence of δω to the Curie-Weiss law was established: δω^-2 ∝ T - T*, (T* = T_c - 1), thus providing proof of our interpretation.     

Fractal-like scaling in liquid crystals. Odd–even effect of scaling exponents in the 4,4′-di-n-alkyloxyazoxybenzene homologous series

Abstract

A power law behaviour of dielectric loss versus frequency has been found in members of the 4,4′-di-n-alkyloxyazoxybenzene series. For each member the scaling of the data with respect to v/v _max is possible in the liquid-crystalline temperature range; this suggests a fractal-like geometry in nematic and smectic phases. The values of the scaling exponents, indicating a cooperativity of molecular reorientation around the short axes, exhibit an odd–even effect.     

Measurement of splay elastic constant and rotational viscosity of an induced nematic binary system

We investigated the temperature dependence of the splay elastic constant (K₁₁) and rotational viscosity (γ₁) of a binary liquid crystal system comprising 5-trans-n-butyl-2-(4-isothiocyanatophenyl)-1,3-dioxane (4DBT) and 4-cyano-4′-n-undecyloxy-biphenyl (11OCB), exhibiting induced nematic phase. Both the splay elastic constant and rotational viscosity increased following lowering of the temperature. In the vicinity of smectic A-nematic (SmA-N) transition, both the relaxation time and rotational viscosity exhibited a strong pretransitional behaviour. The critical exponent (ν) presented here led to valuable qualitative information about the pre-transitional behaviour of the rotational viscosity data near the SmA-N phase transition. The extracted ν values ranging between 0.336 and 0.352 are in fair agreement with those predicted in the de Gennes model as compared to that by the mean-field model.     

A comparison of the structure,flexibility and mesogenic properties of 4-methoxy-4′-cyanobiphenyl and the α,α,α-trifluorinated derivative

Abstract

The compound 4-cyano-4′-(α,α,α-trifluoromethoxy)biphenyl (1OCBF₃) has been synthesized. Unlike the fully protonated analogue, 4-cyano-4′-methoxybiphenyl (1OCB), it does not show a liquid crystalline phase on cooling from the melting point (51°C) to room temperature. The transition temperature to a monotropic nematic phase was obtained as approximately 0°C by determining the transition temperatures of mixtures with 1OCB. The structures, conformational properties and orientational ordering of both 1OCB and 1OCBF₃ as solutes in a nematic solvent ZLI 1132 have been investigated via the 17 dipolar couplings obtained by analysing the proton and fluorine NMR spectra of these solutions. It is concluded that the major difference between the two molecules lies in the potential, V(φ₂), governing rotation about the ring–oxygen bonds. In 1OCB the potential has the same form as in anisole, with a minimum when the C–O bond is in the plane of the attached ring (φ₂ = 0°), and a maximum of about 15 kJ mol^?1 when φ₂ is 90°. In 1OCBF₃ the barrier to rotation about the ring–O bond decreases substantially to being near zero.     

DTA,X-ray and dielectric relaxation studies of 14CB at atmospheric and elevated pressures

The pressure-temperature phase diagram of 4′-tetradecyl-4-cyanobiphenyl (14CB) up to 220 MPa (2.2 kbar) and between 320–400 K was established using DTA. The temperature range of the smectic A (SmA) phase slightly increases with pressure. The layer spacing d at 1 atm was determined as a function of temperature using X-ray diffraction. It was related to the molecular length l by the ratio d/l ? 1.4. The dielectric relaxation measurements in the isotropic and smectic A_d phases of 14CB at 1 atm were performed in the frequency range 10 kHz-3 GHz. Contributions from both principal rotational motions, i.e. around the short and long molecular axes, were separated. The relaxation measurements under high pressure in the SmA phase covered the low frequency process. The longitudinal relaxation time τ₁, characterizing the molecular reorientations around the short axis, was analysed with respect to the pressure and temperature dependences, giving activation volumes, Δ^# V = RT (? ln τ₁ / ?p)_T, and activation enthalpies, Δ^# H = RT(? ln τ₁ / ?T ^-1)_p, respectively. Surprisingly, all the activation quantities characterizing the rotational motions of 14CB molecules under different conditions are nearly the same as those determined recently for the much shorter homologue, 8CB. This indicates that the 14CB molecule is in fact relatively short due to conformational motions of the alkyl tail.     

Sulphur containing mesogens. The mesophasic behaviour of di(4-alkyloxybenzoates) of 4,4′-dimercaptobiphenyl

Abstract

A homologous series of di(4-alkyloxybenzoates) of 4,4′-dimercaptobiphenyl: CH₃(CH₂) _n-1O?C₆H₄?COS?C₆H₄?C₆H₄?SOC?C₆H₄?O(CH₂) _n-1CH₃,n=1–7, has been synthesized and the thermotropic liquid-crystalline behaviour investigated. All compounds exhibit enantiotropic mesomorphism over a remarkable temperature range. While the mesophase thermal stability is moderately higher than that found for the corresponding oxygenated analogues, the smectic stability is definitely lower. In fact, all the compounds are nematic but smectic mesomorphism (S_C) is observed for n = 7. Compounds with n = 6 or 7 exhibit enantiotropic highly ordered smectic (or disordered crystal) phases, probably S_G in type.