Liquid Crystal-isotropic Transition Research Articles

Synthesis and mesomorphic properties of novel multi-arm side-chain liquid crystal polymers with different terminal substituents

ABSTRACT In this study, two novel aromatic ester multi-arm liquid crystal monomers M 1 and M 2 with different terminal substituents centred on 3,4,5-trihydroxybenzoic acid, and a rod-shaped chiral liquid crystal monomer N were designed and synthesised. The synthesised liquid crystal monomers M and N were grafted and copolymerised with polymethylhydrosiloxane (PMHS). The structural characterisation of the synthesised liquid crystal compounds was carried out by FT-IR and 1H-NMR. The mesomorphic properties of the synthesised liquid crystal compounds were characterised by POM and DSC, and the results showed that the multi-arm liquid crystal monomers M 1 and M 2 are typical enantiotropic nematic liquid crystal compounds and N is typical thermotropic enantiotropic cholesteric liquid crystal. Polymers P 1 and P 2 are both cholesteric liquid crystal polymers. The glass transition temperatures and liquid crystal isotropic transition temperatures of polymers P 1 and P 2 series both tend to increase with the increase of multi-arm liquid crystal monomer M.

Photoinduced phase transition of N-benzylideneaniline liquid crystalline polymer and applications of photodismantlable adhesives

The liquid crystalline N-benzylideneaniline (NBA) polymer acts as an adhesive, and its bonded pieces detach upon exposure to UV light due to the trans-cis photoisomerization of the NBA moieties and subsequent phase transition of NBA from a liquid crystal polymer to an isotropic polymer. We improved the photoresponsive durability of the polymer and explored the effect of light irradiation on its thermal properties and adhesion stress. We observed that the transition points of the polymer's thermophysical properties (glass transition, liquid crystal-isotropic transition, G" slope, tan δ) shifted to lower temperatures under light irradiation.

Self-Propulsion of Nematic Drops: Novel Phase Separation Dynamics in Impurity-Doped Nematogens

We report a novel phase separation dynamics, mediated by self-propelled motion of the nucleated drops, in a mixture of a nematogen and an isotropic dopant. We show that surface flow, induced by the gradient in the concentration of the dopant expelled by the growing drops, provides the driving force for the propulsion of nematic droplets. While the liquid crystal-isotropic transition is used here to demonstrate the phenomenon, self-propulsion should be observable in many other systems in which the dynamics of a conserved order parameter is coupled to a nonconserved order parameter.

Wetting behavior above the liquid-crystal-isotropic transition in a homologous series

An ellipsometric study of the wetting behavior at the free surface above the isotropic to nematic or isotropic to smectic-A transition of nine homologous compounds with even alkyl chain lengths n in the range from four to twenty carbon atoms is presented. All compounds show a pretransitional increase of the nematic or smectic surface coverage as the bulk isotropic to liquid-crystal transition is approached from above. The behavior of the nematic compounds (n=4 to 10) can be interpreted, within the framework of a Landau model, as complete wetting. In short nematic homologs the divergence of the nematic coverage is strongly reduced by a decrease of the nematic susceptibility of the isotropic phase. The elastic coefficient L of the Landau model shows a pronounced increase with increasing n, resulting in the occurrence of a discontinuous prewetting transition in the shortest smectic homolog (n=12) that is still describable by the nematic Landau model. In the longer smectic homologs (n=14 to 20), layering steps appear in the pretransitional increase of the coverage. The results indicate probable partial wetting for the longest homolog, whereas for the other smectic compounds the distinction between complete and partial wetting is difficult on the basis of ellipsometry.

Study on binary system comprising a non-mesogen and a mesogen

Mixed liquid crystal formation has been studied in a new binary system comprising para nitroazobenzene derivatives, in which one component was a mesogen and the other was a non-mesogen. The mixtures were found to exhibit a monotropic nematic phase which was converted to an enantiotropic phase in specific ranges of temperature and concentration. The latent liquid crystal-isotropic transition temperature (LTP) of the non-mesogen was obtained by using the extrapolation method of the transition temperature-composition curve and the equal- G analysis method. The LTPs of the non-mesogen obtained by the above two methods showed good agreement with each other. The low-temperature transition of the mixtures detected by DSC was attributed to a change of the crystallite size.

Ionic conductivity of a novel smectic polymer electrolyte

Polymers having the following general formula have been synthesised ▪ where R is a long chain alkyl substituent such as -C 16H 33. The number of skeletal oxygen atoms n = 5 or 6 (coded C16O5 or C16O6) so that the helix formed by association of neighbouring side groups R, has 18 or 21 skeletal bonds per rotation and so imitates the PEOLi + or PEONa + 2 1 helix. Complexes with LiBF 4 in stoichiometries of 1 mol salt per structural (helical) repeat unit and 0.5 mol per repeat have been prepared. Small-angle- and wide-angle X-ray diffraction, thermal analysis and hot stage polarised light microscopy have been used to show that the complexes form smectic systems in the uncomplexed and complexed forms. In the complexes, cations are enclosed within the helices and the anions lie between planes of helices and the alkyl side chains so that the overall d spacing is 42.9 Å. The -C 16H 33 side chains melt at ca. 40 °C in the complexed material, to give a liquid crystal phase. The liquid crystal-isotropic transition varies slightly according to polymer and stoichiometry and is 89 °C for C16O5:LiBF 4 (1:1). The ionic conductivity-temperature relationship shows well-defined transitions at the phase changes and the complex impedance behaviour also reflects the several states of the system. The conductivity of the (1:1) complex is higher than the (1:0.5) complex and is comparable to P(EO) 6:LiBF 4 in the crystal and istropic phases, but is somewhat lower over the temperature range of the liquid crystal phase.

Nematic and smectic mesophases in a new series of Cu(II) organometallic complexes

Abstract The synthesis and the phase behaviour of a homologous series of copper, bis[N-[[4-[4-(alkoxy)benzoyloxy],2-hydroxyphenyl]methylene]-methanamino] complexes is reported. They all exhibit thermotropic mesomorphism. The thermal stability range of the mesophase decreases slightly with increasing length of the alkoxy terminals. With the onset of smectic mesomorphism, for alkoxy groups containing ten carbon atoms, the stability interval of the nematic phase decreases progressively so that the compound containing 14 carbon atoms in the alkoxy terminal chains exhibits only smectic mesomorphism. Optical texture analysis suggests the smectic C nature of the phase in all cases. The enthalpies for the liquid crystal-isotropic transition have been measured and they are comparatively low. Remarkably low values have also been measured for the melting enthalpies of the smectogenic members of the series. These are associated with solid phase polymorphism which is discussed briefly.

Nematic Mesophases in 9-Methyl and 9-Bromo-2-Fluorenyl 4-Alkoxybenzoates

Abstract The esters, 9-methyl-2-fluorenyl 4-alkoxybenzoates with C5, C7, G9 C11, and C14 chain lengths were synthesized in order to determine the effect of the 9-substituent in the fluorene rings on their mesomorphic properties. Their structures were determined by spectroscopic methods including 1H-NMR, 13C-NMR, and 2D-COSY-NMR. All these compounds showed a nematic phase with a nematic-isotropic transition temperature between 74–100°C. The nematic phase in these materials could be supercooled to room temperature. The nematic phase also appeared in the corresponding 9-bromo esters but at higher temperatures and was enantiotropic only in one homolog, C9+. Comparisons were also made with the 7-bromo-2-fluorenyl 4-alkoxybenzoates with C7, C9 and C14 chain lengths. These compounds all yielded the smectic A phase with the nematic phase only appearing in the C7 homolog. The presence of bromine in the 7-position of the fluorenyl esters increased the liquid crystal isotropic transition temperature to about 200°C.

Depression of the liquid crystal-isotropic transition temperature by a magnetic field

A reduction in the temperature of the transition to the biphasic and clearing points by the application of a magnetic field in excess of the critical unwinding field has been observed in a cholesteric liquid crystal. This observation qualitatively is in accordance with the phenomenological expansion of the excess free energy in the order parameter S. Inclusion of a twist term in the gradients of S leads to a rescaling of T* which is inversely proportional to the equilibrium pitch P0. When the pitch of the cholesteric liquid crystal is increased or removed by a magnetic field, a reduction in the transition temperature is expected. The observed effect is apparently insignificant in thermotropic liquid crystals but may be more pronounced in the lyotropic liquid crystal of this study due to its very small latent heat.