Mesomorphic properties of side-chain cholesteric liquid-crystalline elastomers

New monomer cholesteryl 4-(10-undecylen-1-yloxybenzoyloxy)-4′-ethoxybenzoate (M₁), crosslinking agent biphenyl 4,4′-bis(10-undecylen-1-yloxybenzoyloxy-p-ethoxybenzoate) (M₂) and a series of side-chain cholesteric elastomers were prepared. The chemical structures of the monomers and elastomers obtained were confirmed by element analyses, FT-IR, and ¹H NMR. The mesomorphic properties and thermal stability were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X-ray diffraction measurements. The influence of the content of the crosslinking unit on the phase behavior of the elastomers was examined. M ₁ showed cholesteric phase, and M ₂ displayed nematic phase. The elastomers containing less than 12 mol% of the crosslinking units revealed reversible mesomorphic phase transition, wide mesophase temperature ranges, and high thermal stability.     

Non-isothermal pyrolysis of used lubricating oil and the catalytic effect of carbon-based nanomaterials on the process performance

Pyrolysis is a commonly used method for the recovery of used lubricating oil (ULO), which should be kinetically improved by a catalyst, due to its high level of energy consumption. In this research, the catalytic effects of carbon nanotube (CNT) and graphene nanoplatelets on the pyrolysis of ULO were studied through thermogravimetric analysis. First, the kinetic parameters of ULO pyrolysis including activation energy were calculated to be 170.12 and 167.01 kJ mol^?1 by FWO and KAS methods, respectively. Then, the catalytic effects of CNT and graphene nanoplatelets on pyrolysis kinetics were studied. While CNT had a negligible effect on the pyrolysis process, graphene nanoplatelets significantly reduced the temperature of maximum conversion during pyrolysis from 400 to 350 °C, due to high thermal conductivity and homogenous heat transfer in the pyrolysis process. On the other hand, graphene nanoplatelets maximized the rate of conversion of highly volatile components at lower temperatures (<?100 °C), which was mainly due to the high affinity of these components toward graphene nanoplatelets and also the effect of nanoplatelets’ edges which have free tails and can bond with other molecules. Moreover, graphene nanoplatelets decreased the activation energy of the conversion to 154.48 and 152.13 kJ mol^?1 by FWO and KAS methods, respectively.

     

Kinetic analysis of reversible thermal decomposition of solids

The isoconversional method was used to elucidate the kinetics of reversible solid-state reactions occurring under nonisothermal linear heating. The characteristic dependencies of the effective activation energy (E) on the extent of conversion (W) were established for two model processes: a reversible first-order reaction and a reversible reaction followed by an irreversible one. For the first process, E is almost independent of W and varies between the activation energy of the direct and inverse reaction. For the second, process with an endothermic reversible step, the dependence of E on W is of decreasing shape. The effective activation energy is limited by the sum of the activation energy of the irreversible reaction and the enthalpy of the reversible reaction, at low conversions, and by the activation energy of the irreversible reaction at high conversions. Analyses of the kinetic data for the dehydration of crystalhydrates, as well as other processes proceeding through a reversible step, show the dependencies of E on W characteristic of a reversible reaction followed by an irreversible one. © 1995 John Wiley & Sons, Inc.     

Synthesis of photochromic liquid-crystalline triblock copolymers by pseudoliving reversible addition-fragmentation chain-transfer polymerization

Symmetric photosensitive fully liquid-crystalline triblock copolymers are synthesized by pseudo-living reversible addition-fragmentation chain-transfer radical polymerization for the first time. The polymerization of 3-[methyl(phenyl)amino]propyl acrylate mediated by three different symmetric trithiocarbonates with various leaving groups is studied. It is shown that reversible addition-fragmentation chain-transfer agents make it possible to synthesize narrowly dispersed homopolymers with controlled molecular masses. Poly[(3-[methyl(phenyl)amino]propyl acrylate) trithiocarbonates] are used as polymeric reversible addition-fragmentation chain-transfer agents in the block copolymerization of the phenyl benzoate acrylic monomer. The chemical modification of block copolymers yields desirable photosensitive triblock copolymers containing azobenzene groups. The effect of the molecular structure of triblock copolymers on their phase behavior and thermal properties is examined.     

Measurement of the Kerr effect in cholesteric blue phases

The electric field induced birefringence within the blue phases and the isotropic phase of several different liquid crystals has been measured. The temperature dependences within the blue phases are found to be strong, characteristically different for each phase, and unlike what is predicted by simple theoretical arguments.     

Hydrostatic pressure effects on polymer cholesteric liquid-crystalline structure. I. Equilibrium structure of hydroxypropyl cellulose aqueous solution

The effect of pressure on the polymer cholesteric liquid-crystalline structure of hydroxypropyl cellulose aqueous solutions was studied using reflection spectra measurements. Pressures applied to the polymer liquid crystals ranged from 1 to 2000 bar. The equilibrium reflection spectrum of the cholesteric structure shifted to longer wavelengths, showing that the cholesteric pitch of the liquid-crystalline structure increases as the applied pressure increases. At pressures higher than 200 bar the maximum wavelength of reflection shifted linearly with the increase in applied pressure. At lower pressures, the cholesteric structure was influenced by the surface plane of the quartz window.     

Untwisting of the helical superstructure in the cholesteric and chiral smectic C*phases of cross-linked liquid-crystalline polymers by strain

It is possible to untwist reversibly the helical superstructure of elastomers with cholesteric and chiral smectic C*phases by using strain. In that way a cholesteric structure can be transformed into a nematic structure and a chiral smectic C*into a smectic C structure. The latter case is especially interesting because a structure without a macroscopic polarization (chiral smectic C*) is transformed into one with a macroscopic polarization (smectic C like arrangement).     

Hydrostatic pressure effects on polymer cholesteric liquid-crystalline structure. I. Equilibrium structure of hydroxypropyl cellulose aqueous solution

Abstract

The effect of pressure on the polymer cholesteric liquid-crystalline structure of hydroxypropyl cellulose aqueous solutions was studied using reflection spectra measurements. Pressures applied to the polymer liquid crystals ranged from 1 to 2000 bar. The equilibrium reflection spectrum of the cholesteric structure shifted to longer wavelengths, showing that the cholesteric pitch of the liquid-crystalline structure increases as the applied pressure increases. At pressures higher than 200 bar the maximum wavelength of reflection shifted linearly with the increase in applied pressure. At lower pressures, the cholesteric structure was influenced by the surface plane of the quartz window.     

Electromechanical effect in cholesteric mixtures with a compensation temperature

Abstract

An electromechanical effect arises in cholesteric (N*) liquid crystals due to a cross-coupling between fluxes and forces specific to chiral systems. We recently described an experiment to study this effect on samples with fixed boundary conditions. Here, we have derived the basic theoretical expressions relevant to this experiment. We have now used this technique to record the electromechanical signal across the compensation temperature of cholesteric mixtures consisting of cholesteryl chloride, 4′-heptyl-4-cyanobiphenyl and 2-cyano-4-heptylphenyl 4′-pentylbiphenyl-4-carboxylate. The sign of the electromechanical signal changes at the compensation point, clearly demonstrating the macroscopic origin of this effect.     

Laser addressed thermo-optic effect in a novel dyed liquid-crystalline polysiloxane

Initial experimental evaluation of a novel liquid-crystalline polysiloxane for thermo-optical recording is presented. A versatile system using a single laser source has been used to gather information on the dynamics of laser addressing. To ensure the most stable, highest-contrast pixel the polymer must be heated through its biphasic region and some way into the isotropic phase. It was found that at 60°C using laser pulses of 300μs or less, write-in and selective erasure times were on the submillisecond timescale, and that bulk erasure required 1-2 min. The use of the polymer in an analogue optical store has been demonstrated and an unoptimized sensitivity of 12nJ/μm² for 632°8 nm light was found at 24°C.     

Characterisation and effect of polymer network deformation in reverse-mode polymer-stabilised cholesteric texture

In reverse-mode polymer-stabilised cholesteric texture (PSCT), the dynamic response is derived from local director reorientation governed by dielectric coupling effect/self-assembly and polymer network deformation. A double exponential rise/decay model is proposed to investigate the underlying physical mechanisms. Through simulation of the transient rise and decay processes, the polymer network deformation in PSCT can be quantitatively evaluated. Less deformation and faster restoration speed of the polymer network can suppress hysteresis. These results provide useful guidelines for future PSCT fabrication and performance optimisation.     

Biasing effect of non-conductive solids in x-ray photoelectron spectra

The effect of biasing the sample holder was investigated in a range of ?300 to +300 V, in order to study the surface charging of non-conductive samples. Shifts in kinetic energies of photoelectron and peak broadening were compared to the dielectric constants and the volume resistivity of the solid.     

Helix reversals as bad neighbors to liquid crystal organizations in cholesteric states and thermally reversible gels of poly(ALKYL isocyanates)

While the temperature dependence of the lyotropic cholesteric pitch of the single helical sense poly ((R)-2,6-dimethylheptyl isocyanate) is in line with theory, comparable data on this state produced by chiral doping of the lyotropic nematic state of poly(n-hexyl isocyanate) could suggest an interplay between the supramolecular chirality of the liquid crystal and the dynamic equilibrium of the left and right hand helical blocks in this otherwise racemic polyisocyanate. The exclusion of helix reversals, as undersireable kinks in the liquid crystal organization, could play a role in this effect. Such helix reversal exclusions can also explain the peculiar chiral optical changes associated with the thermally reversible gelation of poly(n-hexyl isocyanate) copolymers in hydrocarbon solvents. These gels likely arise by entering the broad biphasic region of the Flory phase diagram leading to the formation of liquid crystal aggregates.