AFM investigations of a glassy cholesteric liquid crystal with hydrophobic aerosil particles

AFM investigations of a glassy heterogeneous system consisting of an oligomeric cholesteric liquid crystal and the hydrophobic aerosil R812 were carried out. With increasing aerosil concentration, a suppression of the characteristic cholesteric surface pattern was observed. Typical separated aerosil aggregates appear in the samples. Their size and form change from small lumps through bigger rod-like entities to large crystallite-like aggregates of aerosil particles. This matches with observations of light scattering of systems with low molecular mass liquid crystals and of the memory effect. The pitch of the cholesteric fingerprint pattern slightly decreases with increase in the aerosil concentration.     

Effect of high pressure on the nematic–isotropic transition in aerosil–liquid crystal composites

We report the first high pressure investigations of the nematic–isotropic transition in the composites of a liquid crystal compound with hydrophilic aerosil particles. The low concentrations of the aerosil particles used create soft gels of the composites. As expected T_N–Iso, the nematic–isotropic transition at room pressure exhibits a non-monotonic variation with increasing aerosil concentration. This non-monotonic behaviour is seen in the isobaric scans over the wide range of pressures studied, and its “magnitude” is dependent on the pressure applied. The surprising result of the present investigations on these nanocolloidal systems is that the slope of the pressure–temperature boundary also exhibits a non-monotonic dependence with the aerosil concentration, which qualitatively is similar to that of the transition temperature variation. Employing the transition enthalpy values determined at room pressure using differential scanning calorimetric scans collected at low heating rates, we calculate the transition volume dependence on the aerosil concentration. The study adds a new dimension, namely, the influence of pressure on liquid crystalline transitions in restricted geometries.     

Studies on the electro-optical properties of chiral nematic liquid crystal/aerosil particle composites

Chiral nematic liquid crystal (N*-LC)/aerosil particle (AP) composites were prepared. After the composite was sandwiched between ITO glass substrates, the inner surfaces of which had been treated for homogeneous orientation of LC molecules, the LC molecules tended to be aligned with planar texture. Upon the application of an electric filed, a focal conic texture with memory effect was induced and the composite exhibited light scattering. An electric field-induced homeotropic state was obtained after the application of a high electric field. The effects of the content of the APs and the pitch length of the N*-LC on the transmittances of the initial state and the focal conic texture, the driven voltage and the memory effect were investigated.     

Collective dynamic modes in ferroelectric liquid crystal-aerosil dispersions

The influence of the concentration of hydrophilic aerosil particles on the collective dynamic modes of the ferroelectric liquid crystal S-(-)-2-methylbutyl 4-n-nonanoyloxybiphenyl-4'-carboxylate near the SmA-SmC* phase transition is investigated by means of dielectric spectroscopy in the frequency range 10^-2-10⁷ Hz. For aerosil densities ρ_s = 0.025, 0.05, 0.08 and 0.15 g cm^-3 considerable changes in the dielectric intensities of Goldstone and soft modes are observed. The characteristic frequency of the Goldstone mode slightly increases with increasing concentration of aerosil. The frequency degeneracy occurring in the SmA-SmC* phase transition is lifted in the presence of aerosil and an increase in the frequency gap is observed. Complete disappearance of the Goldstone mode at ρ_s = 0.20 g cm^-3 occurs, along with significant broadening of the soft mode. The results are interpreted as an effect of structure, surface interactions and length scale of the helix in disordered confinement.     

Induced thermal dynamics in the melt of glycerol and aerosil dispersions

A high-resolution calorimetric spectroscopy study has been performed on pure glycerol and colloidal dispersions of an aerosil gel in glycerol covering a wide range of temperatures from 300 to 380 K, deep in the liquid phase of glycerol. The colloidal glycerol+aerosil samples with 0.07, 0.14, and 0.32 g of silica per cm3 of glycerol reveal activated energy (thermal) dynamics at temperatures well above the Tg of the pure glycerol. The onset of these dynamics appears to be due to the frustration or pinning imposed by the silica gel on the glycerol liquid and is apparently a long-range, cooperative phenomena. Since this behavior begins to manifest itself at relatively low silica densities (large mean void length compared to the size of a glycerol molecule) and speeds up with increasing density, these induced dynamics are likely due to a coupling between the flexible aerosil gel and large groups of glycerol molecules mediated by mutual hydrogen bonding. This is supported by the lack of such thermal dynamics in pure aerosil gels, pure glycerol, or aerosil gels dispersed in a non-glass-forming, non-hydrogen-bonding, liquid crystal under nearly identical experimental conditions. The study of such frustrated colloids may provide a unique avenue for illuminating the physics of glasses.     

Influence of hydrophilic and hydrophobic aerosil particles on the molecular modes in the liquid crystal 4-n-pentyl-4'-cyanobiphenyl

Dielectric spectroscopy in the frequency range 10⁶-10⁹ Hz was applied to investigate the influence of hydrophilic and hydrophobic aerosil particles on molecular processes in the liquid crystal 4-n-pentyl-4'-cyanobiphenyl. The dynamics of the molecular process in the isotropic phase is non-Arrhenius; it weakly depends on the aerosil density and shows critical temperature dependence of the activation energy near the isotropic-nematic phase transition. The relaxation rate of the process related to the hindered rotation of the molecule around its molecular short axis (slower process) follows an Arrhenius law over about thirty degrees in the nematic range (except close to the phase transition) with an activation energy comparable to the bulk and is almost independent of the aerosil density. The relaxation rate of the process originating from the fluctuation of the molecular long axis around the director (librational mode, faster process), however, follows the Vogel-Fulcher-Tammann law. With increasing disorder achieved by increasing the aerosil density, the relative dielectric strength of the librational mode increases in comparison with the bulk. The relaxation frequency of the slower process increases but that of the faster process decreases with increasing aerosil density. Both these effects are less pronounced for hydrophobic than for hydrophilic aerosils.     

Dynamics of the Goldstone mode near the point of polarization sign reversal in ferroelectric liquid crystal–aerosil mixtures

Dielectric spectroscopy was used to study the influence of random disorder in aerosil–ferroelectric liquid crystal dispersions on the dynamics of the Goldstone mode near the point of polarization sign reversal and the relaxation rate and dielectric strength of the collective modes. In general, the dielectric strength and the relaxation frequency of the Goldstone mode decrease, in comparison with the bulk, with increasing aerosil density near a point of polarization sign reversal. However, the characteristic frequency of the Goldstone mode varies with silica density in an opposite manner on each side of this point. This can be explained as a different sensitivity on the spatial confinement of different molecular conformers above and below the point of polarization sign reversal. The experimental results of temperature and frequency dependence of the complex dielectric constant are compared with predictions of the generalized Landau model for ferroelectric liquid crystals.     

Role of aerosil dispersion on the activated kinetics of the LC1-xSilx system

This study explores the role of aerosil dispersion on activated phase transitions of bulk octylcyanobiphenyl (8CB) liquid crystals by performing heating rate-dependent experiments. Differential scanning calorimetry (DSC) was used at various heating ramp rates in order to probe the activated phase dynamics of the system. The system, LC1-xSilx, was prepared by mixing aerosil nanoparticles (7 nm in diameter) in the bulk 8CB by the solvent dispersion method (SDM). LC represents bulk 8CB, and Sil represents aerosil nanoparticles with concentration x in percent. The concentration of the aerosil nanoparticles (x) varied from 0 to 0.2 g/cm3 in the bulk 8CB. Well-defined, endothermic peaks were found on a heating scan at melting and at the smectic-A to nematic (SmA-N) and nematic to isotropic (N-I) transitions. These peaks show a temperature shift and a change in their shapes and sizes in the presence of aerosil nanoparticles. In addition, an exothermic peak also appeared before the melting peak during the heating scan in the presence of aerosil nanoparticles. All transitions shifted significantly with different heating ramp rates, following an Arrhenius behavior, showing activated kinetics. The presence of aerosil nanoparticles caused a significant increase in the enthalpy and a decrease in the activation energy compared to the results found in bulk 8CB. This behavior can be explained by aerosil dispersion in the LC1-xSilx, inducing a disorder in the bulk 8CB. Infrared (IR) spectroscopy shows a shift to higher frequency for the broad peak at 1082 cm-1, corresponding to an Si-O bond as the density of the aerosil increases, and can be explained in terms of surface and molecular interactions between aerosil nanoparticles and 8CB liquid crystal molecules.     

DNMR study of hydrophilic and hydrophobic silica dispersions in EBBA liquid crystals

Dispersions of hydrophilic (A300) and hydrophobic (R812) silica aerosils in a Schiff-base-type liquid crystal (LC), p-ethoxy(benzylidene)-p-n-butylaniline (2O.4), EBBA, were characterized by deuterium nuclear magnetic resonance (DNMR). The formation and stability of random (RAN) versus anisotropic (AAN) aerosil networks under zero- versus in-field cooling was studied as a function of aerosil density and compared to previous studies of n-alkylcyanobiphenyl (nCB) dispersions. Whereas the LC directors of the hydrophobic R812 dispersions are almost completely annealed after in-field cooling, the hydrophilic A300 silica in EBBA gives rise to a mixture of RAN and AAN. The more complete R812 AAN partially breaks under in-field sample rotation, but the partial AAN formed by the A300 silica is stable. Weakening the aerosil network to compensate for weaker LC surface anchoring results in a complete network, but a strong LC/silica surface interaction must be combined with hydrophilic aerosils to produce AANs which are both complete and stable.     

Dynamics of the Goldstone mode near the point of polarization sign reversal in ferroelectric liquid crystal-aerosil mixtures

Dielectric spectroscopy was used to study the influence of random disorder in aerosil-ferroelectric liquid crystal dispersions on the dynamics of the Goldstone mode near the point of polarization sign reversal and the relaxation rate and dielectric strength of the collective modes. In general, the dielectric strength and the relaxation frequency of the Goldstone mode decrease, in comparison with the bulk, with increasing aerosil density near a point of polarization sign reversal. However, the characteristic frequency of the Goldstone mode varies with silica density in an opposite manner on each side of this point. This can be explained as a different sensitivity on the spatial confinement of different molecular conformers above and below the point of polarization sign reversal. The experimental results of temperature and frequency dependence of the complex dielectric constant are compared with predictions of the generalized Landau model for ferroelectric liquid crystals.     

The effect of molecular weight of polymer matrix on properties of polymer-dispersed liquid crystals

The electro-optical properties and memory effects are important characters of polymer-dispersed liquid crystal (PDLC) devices. Molecular weight of polymer matrix influences the morphology of liquid crystal droplets in PDLC film and the performance of PDLC devices. In this letter, PDLC films were doped with a small amount of chain transfer agent (CTA), in order to control the molecular weight of polymer matrix. It is observed that the addition of CTA induces a reduction of threshold voltage. In addition, the effect of molecular weight on memory effects of PDLC devices was also discussed. It is found that the entanglement between polymer and liquid crystal molecule increased with the molecular weight, which reduces the memory effect.     

A novel polymer dispersed liquid crystal film prepared by reversible addition fragmentation chain transfer polymerization

Polymer as an important component of polymer dispersed liquid crystal (PDLC) has a great influence on electro-optical properties. In this letter, the effect of molecular weight of polymer matrix on the electro-optical properties of PDLC films was investigated with reversible addition fragmentation transfer (RAFT) polymerization. It was found that the saturation voltage and memory effect were apparently influenced by molecular weight of polymer which can be regulated efficiently by irradiation time, while the morphology of liquid crystal droplets kept unaltered. It was estimated that the increase of molecular weight of polymer enhanced entanglement between polymer and liquid crystal, which induced the different surface interaction and electro-optical properties.     

Calorimetric study of the isotropic to nematic phase transition in an aligned liquid crystal nano‐colloidal gel

A high‐resolution calorimetric study of the specific heat (C_p ) has been carried out for the isotropic to nematic phase transition in an aligned liquid crystal (octylcyanobiphenyl ‐ 8CB) and aerosil nano‐colloid gel. A stable alignment was achieved by repeated thermal cycling of the samples in the presence of a strong uniform magnetic field, which introduces anisotropy to the quenched random disorder of the silica gel. In general, the specific heat features of the I?N transition in aligned (anisotropic) gel samples are consistent with those seen in random (isotropic) gel samples, namely the observance of two C_p peaks and non‐monotonic transition temperature shifts with increasing silica concentration. However, larger transition temperature shifts with silica density, modification of the phase conversion process in the two‐phase coexistence region, and a larger effective transition enthalpy are observed for the aligned samples. The lower‐temperature aligned C_p peak is larger and broader while exhibiting less dispersion than the equivalent peak for the random gel. This may be a consequence of the alignment altering the evolution from random‐dilution‐dominated to random‐field‐dominated effects. The exact origin of the larger transition temperature shifts is uncertain but the larger enthalpy suggests that the nematic state is different in the aligned system than in random gels. The general non‐monotonic behaviour of the transition temperature is interpreted using dimensional analysis as a combination of an effective elastic stiffening of the liquid crystal combined with a liquid crystal and aerosil surface interaction energy.     

Electrooptic properties of liquid crystals filled with silica nanoparticles of different sorts

In this work we investigate electrooptic properties of nematic liquid crystal (LC) E7 from Merck filled with two types of silica nanoparticles (NPs). The particles of these types are of similar size and shape, however, they are made by different methods. The NPs of the first type, aerosil (A), are prepared by a high-temperature hydrolysis of chlorosilanes. The second type of NPs, based on hydrolysis and controlled precipitation of tetraethylorthosilicate in alcohol, yields monodispersed NP (MNPs) of silica. Transmittance versus applied voltage curves of the suspension layers are measured and analyzed. Our results show a strong dependence of the electrooptic properties on the particle preparation method. The contrast ratio (CR) 61:1 of LC-A suspension is achieved at relatively low A content (17 wt %). Unlike it, the value of the CR of LC-MNP suspensions is low (about 2:1) even at 30 wt % of MNPs in the mixture. Both types of suspensions show electrooptic memory. The maximum of the memory efficiency of LC-A suspensions is detected at 4 wt % of A, while for LC-MNP suspensions the maximum is achieved at 15 wt % of MNPs. The difference in the results for the LC-A and LC-MNP series of composites is caused by much stronger aggregation of MNPs compared with A in the LC matrix. Moreover, reactive particle fusion and formation of silica microcrystallites substantially enhances the effect caused by MNP aggregation. These processes deteriorating electrooptic performance may be stabilized by introduction of a polymer network in LC-MNP suspensions.     

大分子引发剂分子量对接枝聚合物基体的PDLC电光性能的影响

以可逆加成-断裂链转移(RAFT)、引发转移终止(iniferter)活性自由基聚合相结合的方法,用一步法制备了不同分子量的大分子引发剂RAFT-PS-co-PCMSI(MI),并通过紫外光聚合诱导相分离法制备了以接枝聚合物为基体的聚合物分散液晶(PDLC)膜.研究了不同分子量的MI对PDLC的微观形貌,关闭状态透光率,阈值电压,饱和电压以及记忆效应等方面的影响.研究表明,降低PDLC中MI的分子量,会使得液晶微滴粒径增大,阈值电压(Vth)、饱和电压(Vsat)减小,记忆效应、关闭状态透光率升高.     

Electrically induced and thermally erased properties of side-chain liquid crystalline polymer/liquid crystal/chiral dopant composites

The properties of synthesized side-chain liquid crystalline polymer (SCLCP)/liquid crystal (LC)/chiral dopant composites having a chiral nematic (N*) phase at room temperature were investigated by polarized optical microscopy (POM) and a UV/VIS/NIR spectrophotometer. The composite exhibited a planar texture after it was filled into cells under homogeneous boundary conditions and it was transparent. When an electric field was applied to the composite, a focal conic texture was formed and the composite became light scattering. After the electric field was turned off, the light-scattering state remained stable for some time, i.e. the light-scattering state exhibited a memory effect. The focal conic texture changed into the planar texture when the composite was heated and the composite became transparent again. Therefore, the composite had electrically induced and thermally erased properties. The SCLCP had some influence on the memory effect and on the thermo-electro-optical properties of the composite.     

Electrically induced and thermally erased properties of side‐chain liquid crystalline polymer/liquid crystal/chiral dopant composites

The properties of synthesized side‐chain liquid crystalline polymer (SCLCP)/liquid crystal (LC)/chiral dopant composites having a chiral nematic (N*) phase at room temperature were investigated by polarized optical microscopy (POM) and a UV/VIS/NIR spectrophotometer. The composite exhibited a planar texture after it was filled into cells under homogeneous boundary conditions and it was transparent. When an electric field was applied to the composite, a focal conic texture was formed and the composite became light scattering. After the electric field was turned off, the light‐scattering state remained stable for some time, i.e. the light‐scattering state exhibited a memory effect. The focal conic texture changed into the planar texture when the composite was heated and the composite became transparent again. Therefore, the composite had electrically induced and thermally erased properties. The SCLCP had some influence on the memory effect and on the thermo‐electro‐optical properties of the composite.     

Study of dielectric parameters of liquid crystal elastomer

Liquid crystal elastomers exhibit several interesting phenomena like elastic response, shape memory effect and mechanical actuation triggered by external stimuli and have got much importance among all types of actuated materials. In the present study, we synthesised nematic liquid crystal elastomers (NLCEs) by using Finkelmann procedure. The chemical structure of elastomers is confirmed by Fourier transform infrared spectroscopy. The dielectric response of NLCEs and the monomer used in its synthesis has been studied in the frequency range 20 Hz to 1 MHz. The effect of bias on dielectric response of NLCE and its comparison with its liquid crystal analogue is also reported.     

Memory effect in thin samples of chiral smectic C liquid crystals

On applying an appropriate electric field to a thin sample of ferroelectric liquid crystal a texture of long domains appears perpendicular to the smectic layers. We present observations showing that such treatment of the cell leads to its perfect bistability (i.e. a memory effect). The analysis of the equilibrium equations describing smectic liquid crystal elasticity shows how the director orientation is fixed both by a bulk deformation caused by the electric field treatment and by surface anchoring.     

The influence of thixotropic additives on the properties of sealants based on oligoester urethane thiol

The influence of thixotropic additives on the viscosity and thixotropy of sealants based on oligoester urethane thiol was examined. It was found that the fumed A-175 silica aerosil is the most efficient as a thixotropic additive for the sealants based on oligoester urethane thiol. The introduction of glycols along with the aerosil or hydrated silicon dioxide makes it possible to vary widely the viscosity and thixotropy of the sealant compositions based on oligoester urethane thiol.