Molecular dynamics in the nematic,SA and SC phases of 8S5 and 9S5 as studied by dielectric methods

Abstract

Two experimental techniques—time domain spectroscopy (TDS) and a steady state frequency method—have been used to study dielectric spectra for the isotropic, nematic, S_A, S_C and S_Y phases of two thioesters (4-n-pentylphenyl-4′-n-octyloxy-and 4′-n-nonyloxythiobenzoates (C _n H_2n+1 O?C₆ H₄?COS?C₆H₄?C₅H₁₁, where n = 8 and n = 9) known as 8S5 and 9S5, in the frequency range from 10 Hz to 10 GHz. In the case of 8S5 a deuteriated sample (8S5-d ₂₈) has been used to study relaxation processes in the nematic and smectic phases.     

Phase-transition studies of 5O.5 and 9O.4

As part of our detailed comparative studies of groups of liquid-crystalline compounds that belong to a homologous series, we present phase-transition studies of the compounds N-(4-n-pentyloxybenzylidene)4′-n-pentylaniline (5O.5) and N-(4-n-nonyloxybenzylidene)4′-n-butylaniline (9O.4) using different experimental techniques. The compound 5O.5 is reported to exhibit a phase sequence N, S_A, S_C, S_B and S_G, while 90.4 shows the sequence S_A, S_F and S_G. The salient features of our work on 5O.5 are (i) a new smectic F phase is found in place of the reported smectic B phase, which is confirmed by both miscibility and X-ray studies; (ii) the formation of smectic-C-like short-range order in the nematic phase well above the S_A-N transition; and (iii) a large tilt-angle variation in the smectic C phase (0–23·5°C) in a small temperature range (4·1°C). The phase changes across the S_A-I transition, and for the first time across S_F-S_A transition, are carried out by volumetric studies. The detailed inferences of these are also presented.     

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.     

A study of the induced helical pitch in a reentrant nematic mixture

Abstract

The temperature dependence of the induced helical pitch is reported for cholesteric and reentrant cholesteric phases of liquid crystal systems comprising 4-n-hexyloxy and 4-n-octyloxy-4′-cyanobiphenyl with a non-mesogenic optically active dopant. It was found that on adding small quantities of the dopant that the temperature range of the S_A phase is narrowed and subsequently disappears, while short range smectic fluctuations persist, influencing the helical twisting features. Critical index values were determined from the temperature dependence of the pitch.     

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.     

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.     

A carbon-13 study of relaxation in the mesophases of 5O·7 and the 5CB homologous series

Abstract

Carbon-13 spin-lattice relaxation times of the protonated ring carbons have been measured at 22·6 MHz in the nematic and all four smectic phases of 5O·7 (4-n-pentyloxybenzylidene-4′-n-heptylaniline). Dong has obtained the deuterium spectral densities J ₁ and J ₂ at 15·4MHz for the deuterated aniline ring of 5O·7-d ₄, and has presented and applied a theory in which the spectral densities are expressed in terms of the diffusion constants D∥ and D?. His results are used to calculate ¹³C relaxation times from the spectral densities J ₀, J ₁ and J ₂. The calculated ¹³C spin-lattice relaxation times are then compared with our experimental values to test the theory. The ¹³C spin-lattice relaxation times of all the resolved resonances in the various phases of the first four members of the 5CB homologous series have been published previously. Dong has also published an analysis of 5CB deuterium data, and we use his results for the diffusion constants D∥ and D? to calculate ¹³C relaxation times of the protonated aromatic carbons of 5CB, 6CB, 7CB and 8CB. The ¹³C relaxation times of the unprotonated aromatic carbons of the 5CB series are calculated in the manner of Wittebort et al., but using the spectral density expressions developed by Dong. The calculated ¹³C spin-lattice relaxation times of the 5CB homologous series are then compared with our experimental values to test the theory for the protonated and unprotonated ring carbons.     

Ultrasound Absorption Anisotropy in the Vicinity of Smectic A-Nematic Transition

Abstract

The results of a low frequency ultrasonic study of 4-n-butyloxybenzylidene-4'-n-butylaniline (BBBA) in the vicinity of the smectic A-nematic transition are presented. The frequency and temperature dependence of the parameters describing the anisotropic ultrasound absorption are determined experimentally and compared with the predictions of fluctuation and relaxation theories of the smectic A-nematic transition. It is shown that no single theory could explain all of the experimental data.     

The substituent effect on mesomorphic properties of 4-octyloxyphenyl 4-(4-R-3-nitrobenzoyloxy)benzoates and 4-(4-octyloxybenzoyloxy)phenyl 4-R-3-nitrobenzoates

Abstract

The thermal properties of 4-octyloxyphenyl 4-(4-R-3-nitrobenzoyloxy) benzo-ates (1) and 4-(4-octyloxybenzoyloxy)phenyl 4-R-3-nitrobenzoates (2) have been examined, where R = hydrogen, halogens, alkyl and alkoxy groups. The derivatives of compound 1 incorporating hydrogen, halogens, methoxy and nitro groups show a smectic A phase having a bilayer arrangement, and the others with a long alkoxy group show the S_A phase with the monolayer arrangement. The derivatives of compound 2 incorporating halogens, and the nitro group show the S_A phase with the monolayer arrangement. The alkoxy derivatives show a smectic C phase as well as the nematic phase. The nitro group at the lateral position tends to increase the ratio of the S_A-N transition temperature to the N-I. The effect of the nitro group on the smectic properties has been discussed in terms of the structural and electrostatic nature of the nitro group.     

Broadband dielectric relaxation study on a diluted solution of a ferroelectric liquid crystal by time domain reflectometry

Abstract

In this work, complex permittivity measurements on a diluted solution of the ferroelectric liquid crystal, 4-[(S,S)-2,3-epoxyhexyloxy]-phenyl 4-(decyloxy)-benzoate, which shows ferroelectric smectic C phase (S*_C), have been performed. Using time domain reflectometry, at frequencies between 10 MHz and 10 GHz, in the temperature range from 50°C down to 10°C for every 10°C, at 5, 10 and 30 wt.% in benzene we observe two independent relaxation processes around 150 MHz and 2 GHz. It is shown that the high frequency process is due to the internal molecular reorientations while the low frequency one is due to molecular orientation around the long molecular axis. It is concluded that the reorientation around the long axis is hindered and leads to a resultant macroscopic polarization.     

E.S.R. and D.S.C. investigations of phase transitions in polymorphic 4-n-alkoxybenzylidene-4′-n-alkylanilines

Abstract

It is shown that the McMillan parameter M = T _SAN/T _N1 (where T _SAN and T _NI are respectively the temperatures of the smectic A to nematic (SAN) and the nematic to isotropic (NI) phase transitions) is useful in analysing the crossover between second and first order behaviour of the S_NN transition in the nO.m homologous liquid crystal series (the 4-n-alkoxybenzylidene-4′-n-alkylanilines). Using a phase diagram of orientational ordering versus M for this series, as obtained in this work (from E.S.R. and D.S.C), a symmetric tricritical point with mean field exponent β₂ = 1 is demonstrated. In a preliminary study of E.S.R. linewidth parameters B and C of nitroxide spin probes dissolved in members of the nO.m series exhibiting a first order S_AN transition, critical-type divergences are observed near this transition. In the case where M is closer to 0.959 (the value at the tricritical point), these divergences appear similar to those previously observed in related nO.m members with a second order S_AN transition; however, they are considerably enhanced for an M value closer to unity (i.e. more removed from the tricritical point). This indicates the importance of coupling between orientational and positional order parameters in the observed critical-type divergences.     

A convenient synthesis and the mesomorphic properties of new chiral ferroelectric thiobenzoates

A secondary chiral (R)-(?)-2-alcohol underwent the Mitsunobu reaction with triphenylphosphine, diethyl azodicarboxylate and ethyl 4-hydroxybiphenylcarboxylate, resulting in the desired (S)-(+)-product with high enantiomeric purity (>99% ee), with the chiral branched chain attached to the biphenyl. This method is operationally simple and provides the very important chiral precursor in good yields (62% in dry THF and 72% in dry Et₂O). The condensation of the (S)-(+)-acid chloride from this material and a suitable 4-n-alkylthiophenol in toluene in the presence of pyridine or triethylamine furnishes the chiral (S)-(+)-thiobenzoate liquid crystals in good yields (80–83% in pyridine and 65–68% in Et₃N). (S)-(+)-4-(1-Methylheptyloxy)biphenyl 4-alkylthiobenzoates are abbreviated (S)-MHOBSn , where n varies from 4 to 10 and denotes the number of carbon atoms in the alkyl chain. DSC, polarizing microscopy and X-ray diffraction showed that the (S)-MHOBSn series possesses a rich phase polymorphism: two highly ordered tilted phases CrG* and SmI*, as well as the ferroelectric smectic C (SmC*) and chiral nematic (N*) phase. In this series, the seldom observed transition between the chiral phases SmI*–SmC* is seen. All the compounds possess stable enantiotropic SmC* and N* phases. The existence of weak intermolecular hydrogen-bonding in (S)-MHOBSn was confirmed by FTIR spectroscopy.     

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.     

The helical structure of highly ordered smectic phases

Optical studies of smectic phases have been performed in homogeneously oriented samples of chiral 4-(2′-methylbutyl) phenyl-4′-n-octylbiphenyl-4-carboxylate (CE8). The helix structure has been found in smectic phases C, I and J, but not in the smectic G phase. Two chiral phases have been found between S_I* and S_G phases. Up to now one of them has not been observed. The pitch of the helix has been measured in all of the twisted smectic phases, including the S_J* phase. The existence of the helix in this phase suggests that the correlations between smectic layers are not very strong.     

Creation of a nematic phase in mixtures of smectic A1 phases

Abstract

Phase diagrams were determined for binary mixtures consisting of two 5-n-alkyl-2-(4'-isothiocyanatophenyl)-1,3-dioxane compounds (n-DBT) or 4'-isothio-cyanatophenyl 4-(trans-4'-n-decylcyclohexyl)benzoate and n-DBT. All compounds investigated have monolayer smectic A phases. A nematic phase in the upper temperature range and a nematic gap between two smectic regions also were observed, with the smectic layer spacing ratio, d/d', of 1.23 and 1.87 respectively. The variation of the enthalpy of transition with mixture composition in relation to changes of layer spacing ratio are also discussed for these systems.     

Possible smectic A ordering in a droplet of 4-n-octyloxybenzoic acid liquid crystal

Abstract

Our microtextural as well as depolarized light scattering analyses of 4-n-octyloxybenzoic acid liquid crystal droplets provide evidence for a possible smectic A ordering induced by the free surface. This appears near the bulk smectic C-nematic transition at T _SCN. A simple mean field analysis, based on a Landau-de Gennes-Benguigui free energy functional with an additional surface parameter K _s, allows, in principle, the possibility for a smectic A surface phase at temperatures slightly above T _SCN.     

Optical activity and light scattering in the isotropic and smectic A* phases of a highly chiral smectic liquid crystal

Abstract

The results of optical activity measurements on the smectic A* phase of 1-methylheptyl 4′-[(4-n-tetradecyloxyphenyl)proprioloyloxy]biphenyl-4-carboxylate (14P1M7) and the chiral nematic phase of a chiral–racemic mixture of S-4-(2-methylbutyl)phenyl 4-decyloxybenzoate (CE6) are shown to be extremely similar. This is in full agreement with the proposed model of the A* phase as a twistgrain-boundary (TGB) phase. In addition, new light scattering measurements using circularly polarized light in a back-scattering geometry yield information on the fluctuations in the isotropic phase. Unlike in chiral nematics where only one structural mode is affected, the data show a strong deviation from the normal temperature dependence near the isotropic–smectic A* transition for two structural modes. Possible reasons for this behaviour in highly chiral smectic liquid crystals are discussed.     

Density studies in terephthalylidene-bis-p-n-alkylanilines

Abstract

The temperature dependence of density in terephthalylidene-bis-p-n-alkylanilines (TBAA5 and 6) is studied to investigate the phase transitions, associated volume jumps, order of the transitions, estimated pressure dependence of transition temperatures, and pretransitional effects. The compounds exhibit nematic, smectic A, smectic C, smectic F, smectic G and smectic H phases with higher clearing temperatures. The smectic A to smectic C transition, which is a fluctuation induced first order transition in TBAA5, is found to be a second order transition in TBAA5 and 6. The results are discussed in the light of other experimental reports. The estimated pressure dependence of transition temperatures along with the reported experimental P[sbnd]T data are discussed. The N[sbnd]S_A transition is first order in TBAA5 and 6. The studies across other transitions are also discussed.     

The influence of double bonds in the terminal chain of 2-ring compounds on the physical properties of S*c mixtures

Abstract

We present the properties of S*_c mixtures containing new 2-ring 5-n-alkyl-2-(4-n-alkenyloxyphenyl)pyridines and pyrimidines with systematically varying positions and configurations of the double bond Trans configurations at odd positions (counting the number of atoms from the core including the oxygen and the first carbon atom of the double bond) suppress the S_A phase, increase the S*_c tilt angle, Θ, and the spontaneous polarization, P _s, and lead to long switching times τ. Cis configurations at even positions suppress the nematic phase in favour of smectic phases, decrease Θ and P _s, and shorten τ. Other positional configurational combinations strongly reduce the clearing point. Furthermore, our results indicate that the preferred conformation of the alkenyloxy chain consists of alternating cis and trans units.     

A study of homologation and the occurrence of an SA-SC-SA sequence of phases in the 4-n-alkoxy-3-fluorophenyl 4-(5-n-alkyl-2-thienyl)benzoates

Abstract

Seventeen esters, derived from 4-n-alkoxy-3-fluorophenols and 4-(5-n-alkyl-2-thienyl)benzoic acids, have been prepared and their liquid crystal transition temperatures determined by thermal optical microscopy. On cooling the isotropic liquid, the S_A-S_C-S_A sequence of phases reported for the octyloxy-octyl and octyloxy-nonyl esters has been observed for certain other homologues, principally members of the 4-(5-n-nonyl-2-thienyl)benzoates. For these compounds, the temperature range of occurrence of the intermediate S_C phase decreases as the length of the alkoxy chain increases (for the hexyloxy-nonyl to dodecyloxy-nonyl esters) and the S_c phase is absent for the tetradecyloxy-nonyl homologue.