Photo-dimerization of a chalcone-based side chain polymer for the alignment of ferroelectric liquid crystals

The alignment and optical properties of ferroelectric liquid crystal cells, having alignment films of a chalcone-based side chain polymer treated by linearly polarized UV irradiation were investigated. The long absorption band of the UV/Vis spectra gradually decreased and the FTIR spectra shifted as the irradiation times increased, indicating that cyclo-addition and isomerization reactions of the chalcone-based side chains occurred. UV dichroism demonstrated anisotropic changes in the alignment films, with a maximum at low exposure energy (0.5 J cm^-2). Liquid crystal molecules were aligned perpendicular to the polarization direction of the linearly polarized UV radiation. The azimuthal anchoring energy of liquid crystal E7 on a chalcone-based side chain polymer surface increased with exposure energy. Well aligned defect-free cells and high contrast ratio were achieved with irradiation of longer than 5 min; the geometric conditions for a stable C2 structure may be satisfied at low temperature with slowly cooling.     

Langmuir films of flexible polymers transferred to aqueous/liquid crystal interfaces induce uniform azimuthal alignment of the liquid crystal

We reported recently that amphiphilic polymers can be assembled at interfaces created between aqueous phases and thermotropic liquid crystals (LCs) in ways that: (i) couple the organization of the polymer to the order of the LC and (ii) respond to changes in the properties of aqueous phases that can be characterized as changes in the optical appearance of the LC. This investigation sought to characterize the behavior of aqueous–LC interfaces decorated with uniaxially compressed thin films of polymers transferred by Langmuir–Schaefer (LS) transfer. Here, we report physicochemical characterization of interfaces created between aqueous phases and the thermotropic LC 4-cyano-4′-pentylbiphenyl (5CB) decorated with Langmuir films of a novel amphiphilic polymer (polymer 1), synthesized by the addition of hydrophobic and hydrophilic side chains to poly(2-vinyl-4,4′-dimethylazlactone). Initial characterization of this system resulted in the unexpected observation of uniform azimuthal alignment of 5CB after LS transfer of the polymer films to aqueous–5CB interfaces. This paper describes characterization of Langmuir films of polymer 1 hosted at aqueous–5CB interfaces as well as the results of our investigations into the origins of the uniform ordering of the LC observed upon LS transfer. Our results, when combined, support the conclusion that uniform azimuthal alignment of 5CB is the result of long-range ordering of polymer chains in the Langmuir films (in a preferred direction orthogonal to the direction of compression) that is generated during uniaxial compression of the films prior to LS transfer. Although past studies of Langmuir films of polymers at aqueous–air interfaces have demonstrated that in-plane alignment of polymer backbones can be induced by uniaxial compression, these past reports have generally made use of polymers with rigid backbones. One important outcome of this current study is thus the observation of anisotropy and long-range order in Langmuir films of a novel flexible polymer. A second important outcome is the observation that the existence, extent, and dynamics of this order can be identified and characterized optically by transfer of the Langmuir film to a thin film of LC. Additional characterization of Langmuir films of two other flexible polymers [poly(methyl methacrylate) and poly(vinyl stearate)] using this method also resulted in uniform azimuthal alignment of 5CB, suggesting that the generation of long-range order in uniaxially compressed Langmuir films of polymers may also occur more generally over a broader range of polymers with flexible backbones.     

The Twist Elastic Constant and Anchoring Energy of the Nematic Liquid Crystal 4-N-Octyl-4-Cyanobiphenyl

The twist elastic constant of the nematic liquid crystal 4-n-octyl-4'-cyano-biphenyl (8CB) and the azimuthal anchoring energy at the SiO-nematic interface have been measured by using the torsion pendulum technique. The twist elastic constant of 8CB is found to be systematically larger than that measured by the Freedericksz transition technique. The azimuthal anchoring energy is found to decrease rapidly as the nematic-isotropic transition temperature is approached. This behaviour is analogous to that already reported by us for the nematogen 5CB and can be interpreted by extending the Berreman model of the anchoring energy at a grooved interface.     

The Twist Elastic Constant and Anchoring Energy of the Nematic Liquid Crystal 4-N-Octyl-4-Cyanobiphenyl

Abstract

The twist elastic constant of the nematic liquid crystal 4-n-octyl-4′-cyano-biphenyl (8CB) and the azimuthal anchoring energy at the SiO-nematic interface have been measured by using the torsion pendulum technique. The twist elastic constant of 8CB is found to be systematically larger than that measured by the Freedericksz transition technique. The azimuthal anchoring energy is found to decrease rapidly as the nematic-isotropic transition temperature is approached. This behaviour is analogous to that already reported by us for the nematogen 5CB and can be interpreted by extending the Berreman model of the anchoring energy at a grooved interface.     

Washing effects on the anchoring energy and surface order parameter on rubbed polymer surfaces containing the trifluoromethyl moiety

The washing effects on liquid crystal (LC) alignment capability in a NLC, 4-n-pentyl-4'-cyanobiphenyl (5CB), on a rubbed polyimide (PI) surface containing the trifluoromethyl moiety were studied. The extrapolation length d_e of 5CB on the rubbed PI surface decreases with the rubbing strength RS. Also, the large extrapolation length d_e of 5CB for washing with IPA was measured at RS=114 mm. The polar anchoring energy of 5CB on the rubbed PI surface was decreased by the washing process; it increased with the rubbing strength RS on the PI surface. The surface order parameter S_S of 5CB for all the washing processes is smaller than for the non-washing process. Consequently, the polar anchoring energy and surface order parameter S_s in 5CB are largely attributed to washing effects.     

Electrical conductivity and dielectric constant measurements of liquid crystal-gold nanoparticle composites

The behaviour of the anisotropic electrical conductivity of liquid crystal-gold nanoparticle (LC-GNP) composites consisting of a commercially available room temperature nematic compound doped with alkylthiol-capped GNPs has been investigated. The nematic-isotropic transition of the composite decreases nearly linearly with increasing X, the concentration of GNP (in weight %) at a rate of about 1°C /weight %. The inclusion of GNPs increases the electrical conductivity of the system with the value increasing by more than two orders of magnitude for X = 5%. However, the anisotropy in conductivity, defined as the ratio of the conductivity along (σ_∥) and orthogonal (σ_⊥) to the director shows a much smaller but definite decrease as X increases.     

Measurement of the azimuthal anchoring energy of liquid crystals in contact with oligo(ethylene glycol)-terminated self-assembled monolayers supported on obliquely deposited gold films

We report measurements of the orientations and azimuthal anchoring energies of the nematic liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) on polycrystalline gold films that are deposited from a vapor at an oblique angle of incidence and subsequently decorated with organized monolayers of oligomers of ethylene glycol. Whereas the gold films covered with monolayers presenting tetra(ethylene glycol) (EG4) lead to orientations of 5CB that are perpendicular to the plane of incidence of the gold, monolayers presenting tri(ethylene glycol) (EG3) direct 5CB to orient parallel to the plane of incidence of the gold during deposition of the gold film. We also measure the azimuthal anchoring energy of the 5CB to be smaller on the surfaces presenting EG3 (3.2 +/- 0.8 microJ/m2) as compared to EG4 (5.5 +/- 0.9 microJ/m2). These measurements, when combined with other results presented in this paper, are consistent with a physical model in which the orientation and anchoring energies of LCs on these surfaces are influenced by both (i) short-range interactions of 5CB with organized oligomers of ethylene glycol at these surfaces and (ii) long-range interactions of 5CB with the nanometer-scale topography of the obliquely deposited films. For surfaces presenting EG3, these short- and long-range interactions oppose each other, leading to small net values of anchoring energies that we predict are dependent on the level of order in the EG3 SAM. These measurements provide insights into the balance of interactions that control the orientational response of LCs to biological species (proteins, viruses, cells) on these surfaces.     

Experimental measurement of the azimuthal anchoring energy function at a SiO-nematic interface

The functional form of the azimuthal anchoring energy, i.e. the anisotropic part of the interfacial free energy, at the interface between the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl and an obliquely evaporated SiO substrate is measured for the first time by using a reflectometric method. The anchoring energy function is obtained by measuring the director rotation on the interface caused by an external magnetic field ranging from 0 to 2·3 T. The dependence of the anchoring energy on the director azimuthal angle is found to be well fitted by the function W_a(ϕ) = W_asin² ϕ in agreement with the predictions of the Berreman model for the anchoring at a grooved interface.     

The geometry and intermolecular binding of HCN…BrC1 probed by rotational spectroscopy

Ground-state rotational spectra of several isotopomers of a linear complex formed between HCN and BrCl were observed by pulsed-nozzle, F-T microwave spectroscopy and analysed to give the spectroscopic constants B₀, D_J, _χ(Br), _χ(Cl), and M_bb(Br). The order HCN…BrCl of the nuclei was established from the changes in the B₀ values on isotopic substitution. The small fractional electronic charge (δ ≈ 0.05) transferred from Br to Cl on complex formation and the intermolecular stretching force constant (k_σ ≈ 11 N m⁻¹) estimated from the spectroscopic constants indicates a weak interaction of HCN and BrCl, with little perturbation of the charge distribution of BrCl.     

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.     

Synthesis and properties of azo dye aligning layers for liquid crystal cells

The synthesis and properties of azo dyes that can be used for photoaligning liquid crystals (LCs) have been investigated. The structures and the synthetic procedure for the azo dyes are presented. The photoaligning of azo dyes takes place purely due to the reorientation of the molecular absorption oscillators perpendicular to the UV light polarization. The qualitative model for the phenomenon in terms of the rotational diffusion of the azo dye molecules in the field of the polarized light is discussed. The order parameters S = -0.4 (80% of the maximum absolute value S_m = -0.5) were measured from the polarized absorption spectra at the wavelength 372 nm. A temperature stable pretilt angle of 5.3° was obtained by a two-step exposure of the azo dye film using normally incident polarized light followed by oblique non-polarized light. The azimuthal anchoring energy of the photoaligned substrate was A_ϕ ≈10^-4 J m^-2, which is the same as the anchoring of the rubbed polyimide (PI) layer. The voltage holding ratio value of a photoaligned LC cell was found to be even higher than for a rubbed PI layer, which enables the applications of azo dyes as aligning layers in active matrix liquid crystal displays. The thermal stability of the photoaligned azo dye layers is sufficiently high, but UV stability has to be improved, e.g. by polymerization. A new LCD aligning technology based on polymerized azo dye layers is envisaged.     

Investigation of surface anchoring strength and pretilt angle in NLC on rubbed polythiophene surfaces with alkyl chain lengths

A high pretilt angle for the nematic liquid crystal, 4-n-pentyl-4'-cyanobiphenyl (5CB), was observed on rubbed polythiophene (PTP) surfaces having alkyl chains with more than ten carbon atoms. We consider that this is due to a surface-excluded volume effect caused by the long alkyl chains between the LC molecules and the PTP surfaces. The polar anchoring strength in 5CB on rubbed PTP surfaces with long alkyl chains has also been successfully evaluated. The extrapolation length d_e of 5CB increases with increasing alkyl chain lengths above the seven carbons of alkyl chain R7; that is, it may be attributed to the high pretilt angle. An extrapolation length of 5 nm is observed in 5CB for the seven carbons alkyl chain R7 on the PTP surface; this indicates high anchoring strength.     

High pressure studies of the static permittivity tensor components in the nematic phase of 6CB

The dielectric permittivity tensor components, ε_II and ε_⊥, in the nematic phase of 6CB (4-n-hexyl-4'-cyanobiphenyl) were measured in the pressure range 0.1-130 MPa and the temperature range 12-58°C. The dielectric anisotropy, Δε(p, V, T) = ε_II - ε_⊥, was analysed in isothermal, isobaric and isochoric conditions taking into account the pVT data and the well known Maier and Meier equation. On that basis the nematic order parameter S(p, V, T) was determined. This was used to calculate the parameter Γ relating the interaction potential with the volume (density). Its value Γ = 4.1 agrees very well with other estimates.     

Photolytic dechlorination of 4-chlorophenol in O2-saturated aqueous solutions in the absence of photocatalysts or additives using ArF (193 nm) and KrF (248 nm) excimer lasers

One of the toxic products from chlorine bleaching of wood pulp is 4-chlorophenol (4-CP). Detoxification of such compounds usually requires their dechlorination. The present study involves a fairly detailed comparison of the attempt and success of the photolytic dechlorination of 4-CP in O₂-saturated aqueous solutions using ArF^* (193 nm) and KrF^* (248 nm) excimer laser radiation at higher (1.1 × 10⁻² M) and lower (4.5 × 10⁻⁴ M) starting substrate concentrations and comparisons of quantum yield (φ), product distributions, etc.

At the higher starting substrate concentration (1.1 × 10⁻² M) the average initial quantum yields, i.e. early in the reaction process, for the disappearance of 4-CP (φ ≈2 0.30) and for the generation of chloride ions (φ ≈2 0.25) were about the same for both 193 and 248 nm radiation. However, when the number of photons absorbed (n) became greater than about 3 × 10²⁰, more chloride ions (higher φ) were generated with the 193 nm radiation than with the 248 nm radiation. Oligomers were the major products for both wavelengths of radiation, but the quantity of oligomers generated was greater using the 193 nm radiation. At 248 nm a significant amount of hydroquinone was generated, whereas hydroquinone was not detected with the 193 nm radiation. In addition, a significant amount of 4-chlorocatechol was generated during the direct photolysis of 4-CP using either wavelength of radiation. This is a new result for the photolysis of 4-CP at a wavelength longer than 193 nm in the absence of added H₂O₂.

At the lower starting substrate concentration (4.5 × 10⁻⁴ M) the average initial quantum yields for the disappearance of 4-CP (φ ≈2 0.55) and for the generation of chloride ions (φ ≈2 0.45) were both much greater using the 193 nm radiation than with the 248 nm radiation and 0.10 respectively). At 193 nm oligomers were still the major products generated, but the fraction of oligomeric products generated was less than the fraction at the higher substrate concentration. This is consistent with the fact that at the lower substrate concentration a significant amount of hydroquinone was generated at 193 nm, whereas it was not detected at this wavelength for the higher substrate concentration. During the 248 nm photolysis of 4-CP at this lower starting substrate concentration, hydroquinone was the only major product generated in these experiments. Oligomers were not observed at this lower substrate concentration using 248 nm radiation, whereas oligomers were generated at the higher substrate concentration.

The results demonstrate the utility of using an excimer laser for the photolytic dechlorination of 4-CP without added photocatalysts or additives. It is also possible to suggest a number of explanations, given in the text, which are consistent with our findings.     

Electro-optical effects of anchoring transition in polymer dispersed liquid crystal films

Two groups of polymer dispersed liquid crystal films have been studied, one with a fixed cell thickness but varying liquid crystal (LC) concentrations and the other with a fixed LC concentration but varying cell thicknesses. A sudden decrease in transmittance with increasing temperature was observed for films whose LC domain sizes were comparable to their cell thicknesses. In particular, spontaneous alignment of LC directors was observed below ∼20°C when LC domains were formed spanning the space between upper and lower indium tin oxide-coated glass substrates. With increasing temperature, this axially aligned homeotropic configuration changed gradually into homogeneous configurations. We believe that the sudden decrease in transmittance originated from the anchoring transition at the glass substrates and polymer walls. In addition, it has been found that the intensity of ultra violet irradiation has a strong effect on the director configuration of LC domains, and that the cure temperature affects the anchoring transition temperature significantly.     

Supported carbon molecular sieve membranes based on a phenolic resin

The preparation of a composite carbon membrane for separation of gas mixtures is described. The membrane is formed by a thin microporous carbon layer (thickness, 2 μm) obtained by pyrolysis of a phenolic resin film supported over a macroporous carbon substrate (pore size, 1 μm; porosity, 30%). The microporous carbon layer exhibits molecular sieving properties and it allows the separation of gases depending on their molecular size. The micropore size was estimated to be around 4.2 Å. Single and mixed gas permeation experiments were performed at different temperatures between 25°C and 150°C, and pressures between 1 and 3.5 bar. The carbon membrane shows high selectivities for the separation of permanent gases like O₂/N₂ system (selectivity≈10 at 25°C). Gas mixtures like CO₂/N₂ and CO₂/CH₄ are successfully separated by means of prepared membranes. For example, the membrane prepared by carbonization at 700°C shows at 25°C the following separation factors: CO₂/N₂≈45 and CO₂/CH₄≈160.     

Anchoring and electro-optical dynamics of thin liquid crystalline films in a polyimide cell: experiment and theory

The surface-dependent anchoring and electro-optical (EO) dynamics of thin liquid crystalline films have been examined using Fourier transform infrared spectroscopy. A simple nematic liquid crystal, 4-n-pentyl-4'-cyanobiphenyl (5CB), is confined as 40, 50, and 390 nm thick films in nanocavities defined by gold interdigitated electrode arrays (IDEAs) patterned on polyimide-coated zinc selenide (ZnSe) substrates [Noble et al., J. Am. Chem. Soc. 124, 15020 (2002)]. New strategies for controlling the anchoring interactions and EO dynamics are explored based on coating a ZnSe surface with an organic polyimide layer in order to both planarize the substrate and induce a planar alignment of the liquid crystalline film. The polyimide layer can be further treated so as to induce a strong alignment of the nematic director along a direction parallel to the electrode digits of the IDEA. Step-scan time-resolved spectroscopy measurements were made to determine the rate constants for the electric-field-induced orientation and thermal relaxation of the 5CB films. In an alternate set of experiments, uncoated ZnSe substrates were polished unidirectionally to produce a grooved surface presenting nanometer-scale corrugations. The dynamical rate constants measured for several nanoscale film thicknesses and equilibrium organizations of the director in these planar alignments show marked sensitivities. The orientation rates are found to vary strongly with both the magnitude of the applied potential and the initial anisotropy of the alignment of the director within the IDEA. The relaxation rates do not vary in this same way. The marked variations seen in EO dynamics can be accounted for by a simple coarse-grained dynamical model.     

Dielectric studies of liquid crystals under high pressure II. Low frequency relaxation process in 4-n-pentyl-4'-cyanobiphenyl in relation to theories of the nematic state

The results of high pressure dielectric studies of 4-n-pentyl-4'-cyanobiphenyl (5CB) are analysed in terms of theories of the nematic state. The retardation factor g∥ = τ∥/τ₀ and the effective, single-particle potential of mean torque were calculated at the nematic-isotropic transition temperature T_NI and along the isothermal, isobaric and isochoric paths within the nematic phase of 5CB. The potential of mean torque is compared with the order parameter known for the same conditions. The values of parameter γ relating the potential to the volume is discussed.     

Broadband dielectric studies of weakly polar and non-polar liquid crystals

Complex dielectric permittivities, for two orientations of the director n, parallel (E || n) and perpendicular (E ⊥ n) to the probing electric field E, of the weakly polar liquid crystals (LCs) 4,4'-dihexylazoxybenzene (D6AOB) and 4,4'-diheptylazoxybenzene (D7AOB) as well as the non-polar LC diheptylazobenzene (D7AB) have been measured in the frequency range 75 kHz to 1 GHz. The measurements were performed in the nematic, smectic and isotropic phases of the LCs. The dielectric anisotropies Δε (=ε_||-ε_⊥) obtained from the values of dielectric permittivities at 100 kHz in the nematic phase were found to increase with decreasing temperature. However, for the DnAOBs, the Δε values are somewhat smaller than that for D7AB which does not have a permanent dipole moment. In the nematic phase two molecular relaxation processes were observed for both DnAOBs in each of the orientations—parallel and perpendicular. The four processes merge into two separate processes in the isotropic phase. For D7AB no orientational relaxations were observed in the experimental frequency range.     

Photochemistry of dicyanopyridines

The photochemistry of a variety of dicyanopyridines (2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-dicyanopyridine) in solution at room temperature was investigated. Pulsed UV (308 nm) laser irradiation in deoxygenated acetonitrile yields the triplet state with lifetimes between 4 and 10 μs and absorption bands in the 400 and 320 nm regions. In the presence of added HCl an air-insensitive transient (τ ≈ 10–12 μs, λ_max ≈ 360–380 nm) was observed, suggesting the formation of a protonated excited state.

Irradiation in the presence of amines resulted in the production of the pyridyl radical anion (τ ≈ 40–80 μs, air sensitive, λ_max ≈ 360–380 nm) formed by electron transfer from the amine to the pyridine triplet excited state. Stern-Volmer analysis gave electron transfer rate constants in the range (1–8) × 10⁻⁸ M⁻¹ s⁻¹.

In methanol solvent, irradiation yielded an air-insensitive transient assigned as the neutral pyridyl radical (τ ≈ 30–200 μs, λ_max ≈ 370–385 nm). The formation of these transients is discussed in the context of previous photochemical electron spin resonance and product studies.