Effect of composition on the pressure-induced re-entrant cholesteric phase behaviour of mixtures of non-polar cholesteryl n-alkanoates

Abstract

Recently we reported a pressure-induced re-entrant cholesteric phase for ternary systems of non-polar cholesteryl n-alkanoates. In these systems one component contains a shorter n-alkyl chain than the other two. We now show that the former produces a positive excess volume in the smectic phase, which is probably responsible for the pressure-induced re-entrant phase behaviour. The maximum temperature of the cholesteric/smectic A phase boundary is found to decrease drastically with decrease of the n-alkyl chain length of this particular component.     

Pressure-induced re-entrant cholesteric phase behaviour of non-polar liquid crystals

In a previous paper we reported the existence of a pressure-induced re-entrant cholesteric phase in mixtures of non-polar liquid crystals. Now the influence of the mixing ratio on this behaviour has been studied up to 3000 bar and 190°C and the phase boundaries based on light reflection measurements have been confirmed by transmission and texture observations in a diamond anvil cell. Additional thermodynamic investigations show that when the cholesteric/smectic A phase transition line possesses a maximum temperature the pretransition enthalpy and volume disappear.     

Trimeric liquid crystals containing lateral alkyl chains

The first twelve members of the n-alkyl 2,5-bis-(4'-cyanobiphenyloxybutyloxy)-benozates have been prepared. The compounds are nematogenic; in addition the first three members exhibit a smectic A phase. The nematic-isotropic transition temperatures decrease with little or no alternation on increasing the number of methylene groups in the lateral chain; this is analogous to the behaviour found in monomeric and polymeric mesogens possessing lateral alkyl chains.     

Induction of the smectic Ad phase in polar systems II. Role of steric effects in the smectic Ad phase induction

Binary mixtures in which one component is nCBB or its chiral analogue, and the second component is nCB, nOCB, n.CN, or one of their chiral analogues are studied by thermomicroscopy. The branched compounds induce the smectic A_d phase more strongly than unbranched compounds with the same alkyl chain length. The observed behaviour is discussed from the point of view of dimer formation. In all chiral systems, the TGB_A phase appears.     

Some novel cholesteric liquid crystals

A novel series of cholesteric liquid crystals (CNTⁿ). made by quaternization of the new mesogenic unit cholesteryl isonicotinate (CN) with n-alkyl salts, C_nH_2n + ₁ (T) (n = 2 to 8, T = 4-MeC₆H₄SO₃) has been prepared. Differential scanning calorime-try and polarizing optical microscopy studies reveal that all of these salts exhibit an enantiotropic cholesteric phase but for the cholesteryl isonicotinate unit the cholesteric phase is monotropic. For all members of the series the cholesteric-isotropic transitions are not reversible as these salts begin to decompose before their cholesteric-isotropic transition point is reached.     

The SmCPA phase in five-ring bent-core compounds derived from 5-methoxyisophthalic acid

The synthesis and mesophase characterization of a homologous series of five-ring bent-core compounds derived from 5-methoxyisophthalic acid are described. Most of the compounds exhibit a polar antiferroelectric smectic C phase. Replacement of the terminal n-alkoxy chains by n-alkyl carboxylate groups, not only destabilizes the formation of mesophases but induces a calamitic mesophase. However, extension of the arms of the bent-core molecule by a phenyl moiety stabilizes the switchable phase. The mesophases were investigated using a combination of polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical methods.     

First observation of a smectic A-cholesteric phase transition in a thermotropic liquid crystal consisting of a rigid-rod helical polysilane

A smectic A-cholesteric phase transition for a rigid-rod helical polymer, poly[n-decyl-(S)-2-methylbutylsilane] (PD2MBS), with a narrow molecular weight distribution, has been observed for the first time. Polarizing optical microscopy showed that the fan-shaped texture of the smectic A phase turned into the characteristic planar texture of the cholesteric phase upon heating. The positive CD band, which corresponds to the reflection band of the cholesteric phase, gradually decreased in intensity within a range 30°C below the transition temperature on cooling, while the peak maximum shifted towards shorter wavelengths. It was concluded that the system has a very wide temperature region over which the cholesteric-smectic A phase transition occurs and in which the cholesteric pitch varies with temperature.     

Novel phase behaviour in two (smectic/cholesteric) liquid crystal mixtures

The phase behaviours of mixed liquid crystal systems having either Sm/N or Sm/Ch properties have been studied. The (smectic/nematic) binary system formed smectic phases over a wide and much enhanced range of temperature (42 C) and a broad concentration range (0-90 wt %). The ternary smectic/cholesteric system, in appropriate concentration ranges, exhibited the smectic A phase, a TGBA-like twist grain boundary A phase, the cholesteric phase and blue phases. The TGBA-like phase appeared in the cholesteric-smectic phase transition range. Three textures (chiral pitch, fan-shaped and scale-like) for the cholesteric phase of the ternary smectic/cholesteric mixtures were observed in the ranges 0-7, 7-43 and 43 wt % respectively, of cholesteric CB15, in a binary Sm/N mixture.     

- and β-cyclodextrin complexes with n-alkyl carboxylic acids and n-alkyl p-hydroxy benzoates. A molecular mechanics study of 1:1 and 1:2 associations

Molecular mechanics (MM) methods were employed to evaluate stabilization upon formation of inclusion compounds between two different guest molecules and - and β-cyclodextrins (CDs) for two different stoichiometries 1:1 and 1:2. The two guest molecules studied were n-alkyl carboxylic acids and n-alkyl p-hydroxy benzoates with variety of chain lengths. The computed stability for the inclusion compounds between -CDs and n-alkyl carboxylic acids reproduced experimental data reported in the literature. The transition between 1:1/1:2 complexes occurred at an alkyl chain length of n_C=9. It was previously demonstrated by diffusion coefficients measures that a stable 1:2 stoichiometry inclusion compound could be formed between n-alkyl p-hydroxy benzoates and -CD for the chain length n_C>4. The computed results reproduced the experimental ones. The combination between OPLS and GB/SA resulted in better agreements with experiments than those obtained with MM2 and MM3.     

The preparation and properties of low molar mass liquid crystals possessing lateral alkyl chains

The first twelve members of three new mesogenic homologous series have been characterized, each of which are composed of molecules possessing lateral alkyl chains. The n-alkyl 2,5-bis-(4-n-hexyloxy-benzoyloxy)-benzoates are purely nematic whereas both nematic and smectic behaviour is observed for the 4-cyano-4'-biphenylyl 3'-n-alkyloxybenzoates and the 4-cyano-4'-biphenylyl 3',4'-n-alkyloxybenzoates. The transitional properties of these series are similar to those of the analogous conventional mesogens and hence, can be rationalized without making any special assumptions concerning the conformational distribution of the lateral alkyl chain.     

The scope and limitations of liquid-crystalline behaviour in monosaccharide amphiphiles Comparison of the thermal behaviour of several homologous series of D-glucose derived compounds with an amino-linked alkyl chain

A short review of the scope and limitations of liquid-crystalline behaviour in carbohydrate derivatives is presented. In order to investigate the influence of structure variations on the thermal behaviour of monosaccharide amphiphiles, six homologous series of D-glucose and 2-deoxy-D-glucose derivatives with an aminolinked n-alkyl chain were prepared. The observed thermal behaviour could be readily explained with the aid of a refinement of the qualitative model that was presented earlier [1]. All compounds were found to be mesogenic, and the observed mesophase was smectic A_d (i.e. a partially overlapping bilayer structure) in all cases.     

Twisted smectic A phases in side chain liquid crystal polymers

The syntheses of two side chain liquid crystal polymers, a polyacrylate and a polymethacrylate, are reported. In each of the polymers the liquid-crystalline side group carries an asymmetric carbon atom, thereby making some of the liquid crystal phases formed by the polymers optically active and chiral. For the chiral polyacrylate smectic A and chiral ferroelectric smectic C phases are observed, however for the chiral polymethacrylate a cholesteric phase is detected above the smectic A phase. It is found that the smectic A to cholesteric phase transition is mediated by the formation of an intermediary twisted smectic A phase. This intermediary phase is a liquid-crystalline analogue of the Abrikosov flux phase found in Type II superconductors.     

Twisted smectic A phases in side chain liquid crystal polymers

Abstract

The syntheses of two side chain liquid crystal polymers, a polyacrylate and a polymethacrylate, are reported. In each of the polymers the liquid-crystalline side group carries an asymmetric carbon atom, thereby making some of the liquid crystal phases formed by the polymers optically active and chiral. For the chiral polyacrylate smectic A and chiral ferroelectric smectic C phases are observed, however for the chiral polymethacrylate a cholesteric phase is detected above the smectic A phase. It is found that the smectic A to cholesteric phase transition is mediated by the formation of an intermediary twisted smectic A phase. This intermediary phase is a liquid-crystalline analogue of the Abrikosov flux phase found in Type II superconductors.     

Structure and dynamics of rodlike polyester with long n-alkyl side chains over a wide range of temperatures by solid state C NMR

Solid state ¹³C NMR experiments on poly(p-biphenylene terephthalate) with long n-dodecyl side chains have been carried out over a wide range of temperatures, in order to elucidate conformational and dynamical behavior of the polyester in the crystalline state and thermotropic liquid crystalline state. From these experimental results, it is found that at temperatures from room temperature to 80 °C the n-alkyl side chains take both of the immobile and the mobile regions, and at temperatures above 120 °C take only the mobile region. In the immobile region the n-alkyl side chains are in the all-trans zigzag conformation and in the mobile state are undergoing fast exchange between the trans and gauche conformations. On the other hand, the terephthalate moiety of the main chain undergoes rotational motion after the melt of n-alkyl side chains as the temperature is increased.     

Synthesis and characterization of new ferroelectric liquid crystals containing a (2S)-2-[6-(4-hydroxybiphenyl-4'-carbonyloxy)-2'- naphthyl] propionate mesogenic group and oligo(oxyethylene) spacers

Three series of ferroelectric liquid crystals containing a (2S)-2-(6-(4-hydroxybiphenyl-4'-carbonyloxy)-2'-naphthyl)propionate mesogenic group and oligo(oxyethylene) spacers were synthesized. These obtained liquid crystal compounds were characterized by NMR, differential scanning calorimetry (DSC), optical polarized microscopy (POM), and X-ray powder diffraction measurements. Some of these materials containing four phenyl rings of ester cores (i.e. -Ph-Ph-COO-naph-) and chiral heptyl tail exhibited a rich mesomorphic behaviour, a blue phase (BP), a cholesteric phase (Ch), a smectic A (S_A), a twist grain boundary A (TGB_A), and a chiral smectic C (S_C*) phase. Another series containing four phenyl rings of ester cores and chiral butyl and pentyl chain tails revealed only a S_A phase and a S_C* phase. Moreover, a crystal E phase was observed in the short spacer chain (n = 0 or 1) homologues of three series of compounds. Also, the mesomorphism properties were discussed as a function of spacer units, numbers of aromatic rings of core, and different terminal asymmetric moieties.     

Effect of chirality on phase transitions in re-entrant liquid crystals

The phase diagrams for enantiomers and their racemic counterparts have been compared in order to determine the influence of chirality on re-entrant phenomena. It has been found that chirality affects the location of the partially bilayer SmAd and monolayer smectic SmA1 phase areas, while the position of the smectic X phase remains nearly unchanged. This result shows that the N-SmA phase transition is influenced by a molecular chiral discrimination effect. Moreover, in enantiomeric systems, a N* re1-N* re2 phase transition has been detected by DSC within the re-entrant nematic gap which is limited by the smectic Ad and the SmX phases. Based on preliminary data, the SmX phase seems to be either a weakly tilted SmC antiphase or an incommensurate orthogonal mesophase.     

Behaviour of the pitch of the cholesteric and chiral smectic C helix near and at the cholesteric-smectic A-chiral smectic C multicritical point

A cholesteric-smectic A-chiral smectic C multicritical point was established in binary mixtures of 4-n-hexyloxyphenyl-4'-(2'-methylbutyl)biphenyl-4-carboxylate with cholesteryl myristate or cholesteryl benzoate. At this point the cholesteric phase, smectic A phase, and chiral smectic C phase become indistinguishable. Whereas the pitch of the cholesteric helix at the S_A-Ch phase transition is infinite already in the vicinity of the multicritical point, the pitch of the cholesteric helix at the S*_C-Ch transition becomes infinite only at this point. In accord with the theory of Beresnev the pitch of the chiral smectic C helix remains finite at the multicritical point. Additional high pressure experiments support the results obtained at atmospheric pressure.     

High pressure study of the cholesteric/smectic C* phase transition of 4-n-hexyloxyphenyl-4'-(2'-methylbutyl)biphenyl-4-carboxylate (CE3)

The cholesteric (Ch)/smectic C* (S_c*) phase transition of CE3 has been studied up to 2kbar and 115°C by a light reflection method. In contrast to the cholesteric/smectic A phase transition, which can cross over from first to second order at elevated pressure, the Ch/S_c* transition of CE3 was found to remain first order. This result is in agreement with most theoretical predictions. The pretransformation on the cholesteric side of the Ch/S_c* transition is influenced only weakly by increasing pressure. Because of the high viscosity of the smectic C* phase corresponding results could not be obtained on the smectic side of the transition.     

Pyridinium salt liquid crystals Effect of mesogen extension and alkyl chain length

The thermotropism of 1-n-alkyl-(4-methyl and 4-tolyl)pyridinium bromides were compared for alkyl chain lengths ranging from n = 12 to 22 carbons. A smectic A mesophase is present in both series for the longer chain compounds, n ≥ 16, with the clearing temperature being similar for both series but increasing rapidly with chain length. The series with the elongated mesogen also possesses an ordered mesophase identified as smectic G. The transition between this mesophase and the S_A or isotropic phase in the 4-tolyl series, and the transition to and from the crystalline phase in both series, are affected relatively little by the alkyl chain length. It seems that the S_A mesophase is governed primarily by the amphiphilic character of the substances, whereas elongation of the ionic head group is responsible for the appearance of a more ordered mesophase at intermediate temperatures.     

A nematic phase created in mixtures of polar smectics The effect of smectic layer properties

Phase diagrams of binary mixtures composed of compounds with the NCS terminal group (n-DBT, n-PBT, n-TPB (smectic A₁) or n-BT (smectic E₁)) and n-OCB, n-CB (smectics A_d) are presented. It is shown that the width of the nematic gap that separates the A₁ or E₁ phase region from the smectic A_d phase is related to the interaction energy of the molecules in the smectic layers and to the difference in the smectic layer spacings.