Liquid Crystalline Films Research Articles

Vitrified Chiral−Nematic Liquid Crystalline Films for Selective Reflection and Circular Polarization

Nematic and left-handed chiral−nematic liquid crystals comprising methoxybiphenylbenzoate and (S)−(−)-1-phenylethylamine pendants to a cyclohexane core were synthesized and characterized. Although pristine samples were found to be polycrystalline, thermal quenching following heating to and annealing at elevated temperatures permitted the molecular orders characteristic of liquid crystalline mesomorphism to be frozen in the glassy state. Left at room temperature for 6 months, the vitrified liquid crystalline films showed no evidence of recrystallization. An orientational order parameter of 0.65 was determined with linear dichroism of a vitrified nematic film doped with Exalite 428 at a mole fraction of 0.0025. Birefringence dispersion of a blank vitrified nematic film was determined using a phase-difference method complemented by Abbe refractometry. A series of vitrified chiral−nematic films were prepared to demonstrate selective reflection and circular polarization with a spectral region tunable from blue...

The application of the phase transition in nematic liquid crystals for the optical detection of volatile organic compounds

It is demonstrated that optical transducers can take advantage from the outstanding optical properties of nematic liquid crystals. The measurements point out that the birefringence and the phase transition of a liquid crystal can be exploited for the threshold sensing. For that purpose a device consisting of an orientated liquid crystalline film between two crossed polarizers was used. The nematic liquid crystal was also used as a sensitive coating material on an integrated optical Mach-Zehnder interferometer. The calibration experiments were carried out with volatile organic solvents and different calibration curves were measured for meta-xylene and para-xylene.

High-performance laminates through chemical welding of thin liquid crystalline polymer films

A systematic study of the welding (healing) process of polymers with different chemical compositions, structures and properties was carried out. It was shown that welding can be due to a number of physical and/or chemical factors. Diffusion alone led to a relatively weak welding under the selected conditions. A jump was observed in the temperature dependence of the shear stress of polyimides. The abrupt increase in the activation energy of welding can be explained by assuming a change in the nature of the created bonds from physical to chemical ones. For the first time the chemical reactions taking place at the contacting surfaces were directly experimentally proved by electron spin resonance. In some cases, for the first time, the healing process was carried out at temperatures below the glass transition temperature. In this way, the contribution of the diffusion process to the healing may be distinguished from that of the chemical reactions. On the basis of a poly(ethylene terephthalate) copolyester containing 60 mol% p-hydroxybenzoic acid thin liquid crystalline films (160 µm thick) are obtained by melting the polymer at 300°C and chilling at 0°C. The undrawn films have a high degree of orientation as shown by X-ray measurements. These films have excellent mechanical properties because of the molecular orientation. In order to avoid losses in the mechanical strength due to increase in their thickness, laminates were prepared using thin liquid crystalline films. Lamination was carried out by annealing under pressure at 170°C for 6 hr, resulting in samples with excellent mechanical properties regardless of their thickness. A method is proposed that makes it possible to combine the unique mechanical properties of thin films of liquid crystalline polymers with a lamination process in order to obtain thick and very strong materials. Copyright © 1999 John Wiley & Sons, Ltd.

Nanoscale Fibrils and Grids: Aggregated Structures from Rigid-Rod Conjugated Polymers

Langmuir−Blodgett (LB) molecular processing of conjugated polymers [poly(phenylene ethynylenes)] into highly aligned films has revealed conditions for the formation of liquid crystalline monolayer films that structurally evolve into fibril aggregates. The structural requirements for poly(phenylene ethynylene)s to display liquid crystalline phases capable or alignment by LB methods were determined. The reconstruction of monolayers into fibril structures was found to require a low glass-transition temperature (Tg), weak surface anchoring, and a monolayer with a high energy that can be stabilized by reorganization. This assembly of polymers into aggregated structures produces rigid structural units analogous to naturally occurring fibrous proteins such as collagen and elastin. These oriented, shape-persistent nanoscale structures create new possibilities for the construction of complex supramolecular structures, and this capability has been demonstrated by the formation of a nanoscale grid.

Photoinduced alignment control of photoreactive side-chain polymer liquid crystal by linearly polarized ultraviolet light

Alignment control of mesogenic groups in a photocrosslinkable polymer liquid crystalline film was achieved by linearly polarized (LP) ultraviolet (UV) irradiation followed by heating. The alignment direction was parallel to the electric vector of the incident LPUV light and the tilt angle of the mesogenic groups could be controlled by a one-step slantwise LPUV irradiation. The photoinduced birefringence dn was 0.07 and the order parameter of the mesogenic group was 0.28.

Mechanistic insight into circularly polarized photoluminescence from a chiral-nematic film

Circularly polarized photoluminescence (CPPL) was characterized for rod-like molecules of Exalite 428 helically arranged in a chiral-nematic liquid crystalline film. With an unpolarized excitation at 370nm, CPPL intensities measured at 428nm, and a selective reflection wavelength ranging from 7.2 to 72.6mum, the observed dissymmetry factor, g, was found to be in excellent agreement with theoretical prediction, without resorting to adjustable parameters. As a result, new insight into liquid crystal-induced CPPL has emerged. Specifically, it was found that circular dichroism and circular polarization of the excitation beam prior to inducing linearly polarized photoluminescence (LPPL) at quasi-nematic layers play an insignificant role. The relatively large g value in the spectral region far removed from the selective reflection band was attributed to the circular polarization of LPPL emanating from all quasi-nematic layers comprising the chiral-nematic film. In the absence of a fluorescent dye, the propagation of unpolarized light through the chiral-nematic film under otherwise identical conditions results in no circular polarization. The present study has provided a foundation for generating circularly polarized light by way of photo-excitation with unpolarized light of a chiral-nematic film containing a fluorescent dye. e e

Incoherent-to-coherent optical conversion by means of azobenzene liquid-crystalline films

Incoherent-to-coherent optical conversion by means of azobenzene liquid-crystalline films

X-ray scattering from smectic films on a substrate

The combined effect of thermal fluctuations and substrate roughness replication in smectic liquid-crystalline films on a substrate is described. For that purpose a theory is developed which is an extension of existing continuum models. Both thermal fluctuations and roughness replication can be quantified in terms of the surface tension and the elastic constants for compression and bending of the smectic material. Model calculations show the effect of the various parameters. It is shown that the growth of roughness due to thermal fluctuations largely cancels the diminishment due to roughness replication. This explains why previous x-ray scattering measurements could be satisfactorily described in terms of fractal correlation functions.

In-Situ Observation of Thermal Amplification of Photogenerated Optical Anisotropy of Films of Azobenzene Polymers

ABSTRACTThin films of polymers containing p-cyanoazobenzene side chains were studied to reveal its applicability for optical storage media, surface grating optics and aligning films for liquid crystal (LC) display devices. We followed the generation of optical anisotropy of LPLirradiated films of three polymethacrylates with azobenzene side chains exhibiting amorphous, crystalline and liquid-crystalline natures during heat treatment by mean of an ellipsometry and polarized UV-visible spectroscopy. Whereas photogenerated optical anisotropy of amorphous polymer films disappeared above glass transition temperature, the photo-induced anisotropy of films of liquid crystalline and crystalline polymer films was significantly amplified by heat treatment. In case of crystalline polymers amplified optical anisotropy was maintained even at 300°C whereas partial decomposition was observed.

A Study of Liquid Crystalline Elastomers as Piezoelectric Devices

AbstractFerroelectric liquid crystalline polymer films exhibit a number of unique physical properties and have been extensively studied during the last few years. Ferroelectric polymer films exhibiting a high level of piezoelectric activity would provide a new route to piezo- and pyro-electric sensors.We report the fabrication of free-standing ferroelectric liquid crystal films which exhibit piezoelectric coefficients up to 4.4pC/N. We also report the investigation of the physical properties of these films by dynamic mechanical analysis.

Holographic Gratings in Azobenzene Side-Chain Polymethacrylates

Optical storage properties of thin unoriented liquid crystalline and amorphous side-chain azobenzene polymethacrylate films are examined by polarization holographic measurements. The investigated materials are free radical copolymers derived from two photochromic monomers, 6-(4-oxy-4‘-cyanoazobenzene)hex-1-yl methacrylate and 8-(4-oxy-4‘-cyanoazobenzene)oct-1-yl methacrylate, and a nonphotochromic optically active comonomer, (−)-menthyl methacrylate. The thermal behavior and phase transitions of the polymers have been investigated in detail through differential scanning calorimetry and polarizing optical microscopy. Atomic force microscopy investigations have been carried out on the polarization holographic gratings recorded in the polymethacrylate films. A surface relief grating has been found to appear in all films after irradiation. The copolymers with 50−75% dye content exhibit the largest surface relief. The stored information seems to be stable up to approximately 70 °C, except in the case of polyme...

Physical properties of diglycerol esters in relation to rheology and stability of protein-stabilised emulsions

The surface activity and lyotropic phase behaviour of concentrated diglycerol-esters of fatty acids with chain length of C14, C16, C18 and C18:1 (cis-oleic acid) are investigated. Diglycerol-esters show a much stronger reduction in the interfacial tension at a low concentration (0.01–0.1%) than corresponding monoglycerides. The diglycerol-esters form lamellar mesophases above their Krafft point, and no other types of mesophases are found in the temperature region examined (0–80°C). The lamellar phases show a limited swelling capacity, corresponding to a water layer thickness of ≈24 Å, which is found when the ratio of diglycerol-ester to water is 60:40, or lower. At high water concentrations (>90%) multi-lamellar liposomes are formed. The diglycerol-monooleate form lamellar phases in water in the temperature region from zero to 80°C. This is in strong contrast to the corresponding glycerol-monooleate, which forms cubic and reversed hexagonal mesophases in water. Oil in water emulsions are stabilised by diglycerol-esters by formation of liquid crystalline interfacial films around the oil droplets, which can be seen in polarised light microscopy. In presence of milk proteins in the aqueous phase the emulsion stability is depending on the protein to emulsifier ratio. At 40°C a mixed interfacial film of diglycerol-monooleate (DIGMO) and protein is present at the oil–water interface, but when cooled to 5°C, the proteins are displaced by DIGMO. This behaviour affects the stability and rheological properties of emulsions stored at low temperatures.

Time-Resolved Investigation of Photoinduced Birefringence in Azobenzene Side-Chain Polyester Films

We present time-resolved measurements of photoinduced birefringence in liquid crystalline polyester films. After excitation with a single nanosecond pulse, the induced birefringence is monitored on time scales ranging from nanoseconds up to minutes. The maximum birefringence is attained in a dark reaction after excitation. Subsequently, most of the induced birefringence is lost due to relaxation processes involving up to four different time constants. We observed that at high pulse energies the relaxation characteristics are strongly influenced by thermal effects.

Comparison of bulk and thin film structures of the liquid crystal 28OBC: measurements and simulations

Freely-suspended liquid crystalline films of ethyl 4'-n-octyloxybiphenyl-4-carboxylate (28OBC) were prepared and transferred onto different substrates which enable detailed structural characterization. The structures of these thin film assemblies, which are only accessible in this way, were determined and compared with the crystal and molecular structure of 28OBC as formed by crystallization from toluene solution. The compound crystallizes in the monoclinic space group P21/c, a = 11.168(1) A, b = 7.595(2) A, c = 49.106(1) A, beta = 94.01(1) degree, Z = 8. The two symmetrically independent molecules of the asymmetric unit have been used as starting geometries for semi-empirical MO calculations. The difference between the experimentally observed and the optimized molecular structures is interpreted as the influence of the crystal field. The structures of the crystalline and E film phases have been investigated by SAXR and TED and the former has been found to be different from the bulk structure. The structural relationships between the different phases are discussed and a suggestion for the crystalline film structure is given as deduced from simulations of electron diffraction patterns.

Grand canonical ensemble Monte Carlo simulations of confined `nematic' Gay–Berne films

In grand canonical ensemble Monte Carlo (GCEMC) simulations, we investigated the microscopic structure of liquid-crystalline films confined between two plane parallel solid surfaces (i.e. walls) consisting of N s discrete, rigidly fixed atoms distributed across the plane of a wall according to the (100) structure of the face-centered cubic lattice. Parameters of the film–wall interaction potential are chosen such that a homeotropic alignment of film molecules is favored. In the simulations the thermodynamic state of the film is determined by the temperature T, the chemical potential μ, the distance between the walls s z , and the film–wall interfacial area A. Thermodynamic states of the film are chosen such that a corresponding bulk liquid crystal is nematic. These films are referred to as `nematic' to emphasize the physical nature of the bulk phase in thermodynamic equilibrium with the film. To simulate nematic phases in GCEMC we modified the classic Gay–Berne potential for the interaction between a pair of film molecules, so that the isotropic–nematic phase transition in the bulk occurs at sufficiently low densities. Variations of the microscopic structure with increasing s z are correlated with the normal component of the stress tensor T zz ( s z ). Our results show that molecules in inner portions of the film undergo a reorganization from an originally planar orientation of their symmetry axes to a perpendicular one with respect to the plane of a wall. This orientational change is manifested as a periodic sequence of shoulders in T zz ( s z ) where the periodicity length Δ s z is close to the larger diameter of the ellipsoidal film molecules.

UV laser induced refractive index modulation of liquid crystalline polymer films deposited by linearly polarized laser CVD

The precursor of polyimide was spin coated on the substrate and cured at 180 or 240 °C for 1 h. Then, exposure of a linearly polarized KrF excimer laser to a polyimide layer was performed through a photo-mask. In both exposed and non-exposed areas, the monomers were sublimated to deposit liquid crystalline polymer films under the vacuum upon KrF excimer laser irradiation. Optical birefringence and IR dichroism indicated the molecular orientation of the film. In both exposed and non-exposed areas, the molecular orientation was higher in the case of the lower imidized surface. The molecular orientation became lower by exposure either for excess dose energy or excess pulse energy. The deposited films fabricated with periodic mask-patterns exhibited the optical diffraction due to periodic change of refractive index.

Solvation forces in thin films confined between macroscopically curved substrates

The microscopic structure of molecularly thin fluid films confined between solid substrates with macroscopically curved surfaces is investigated by means of grand canonical ensemble Monte Carlo (GCEMC) simulations, in which the thermodynamic state of the film is determined by its chemical potential μ and temperature T. This situation is akin to experiments involving the surface forces apparatus (SFA) in which the film is confined between two crossed cylinders of macroscopic radius R. The key quantity measured directly in SFA experiments is the “force per radius R,” F(h)/R, exerted by the film on the curved surfaces. This “solvation force” can be related to the local stress Tzz(h) normal to the locally planar surfaces, where h is the shortest distance between them. Because Tzz(h) and the microscopic structure of the confined film can be computed by GCEMC, the relation between Tzz and the macroscopically defined quantity F/R can be employed to interpret the dependence of the latter in terms of variations of the film’s local microscopic structure with h. For a liquid-crystalline film it is shown that reorientational effects are distinctly manifest in Tzz(h) but are reflected only weakly in F(h)/R and are therefore likely to be missed if the interpretation of SFA experiments is based solely upon the latter. Pseudo-experimental F(h)/R curves generated by GCEMC for a nematic liquid-crystalline film are in qualitative agreement with recent SFA data [M. Ruths, S. Steinberg, and J. N. Israelachvili, Langmuir 12, 6637 (1996)], which suggests that one may gain deeper insight into the microscopic structure of confined films through a decomposition of experimentally determined solvation forces F(h)/R into Tzz(h) by inverting the integral relation linking the two.

Preparation and characterization of side-chain liquid crystalline polymer thin films aligned on PTFE friction-transferred layers

Annealing of a side-chain LC polymer (SC-LCP) and a fluorescent dye containing copolymer deposited on friction-transferred PTFE substrates is found to yield macroscopically aligned glassy films. The results obtained by electron diffraction, u.v.-visible and FTi.r. spectroscopy clearly demonstrate that the mesogenic side-groups as well as the fluorescence dye orient with their long axes parallel to the PTFE chain axis. Thus, the SC-LCP aligned thin films adopt a near perfect bookshelf geometry in which the smectic layers are oriented edge-on and perpendicular to the alignment direction. The order parameter P2 increases from 0.60 to as high as 0.85 when the annealing temperature in the smectic mesophase approaches the smectic to nematic phase transition temperature.

Substrate-induced order in confined nematic liquid-crystal films

In orientationally biased grand canonical ensemble Monte Carlo (GCEMC) simulations we investigated the microscopic structure of liquid-crystalline films confined between two plane parallel solid surfaces (i.e., walls) consisting of Ns discrete, rigidly fixed atoms. These wall atoms are distributed across the plane of a wall according to the (100) structure of the face-centered cubic lattice. Parameters of the film–wall interaction potential are chosen such that a homeotropic alignment of film molecules is favored. In the simulations the thermodynamic state of the film is determined by the temperature T, the chemical potential μ, the distance between the walls sz, and the film–wall interfacial area A. Thermodynamic states of the film are chosen such that a corresponding bulk liquid crystal is nematic. To simulate nematic phases in the GCEMC we modified the classic Gay–Berne potential for the interaction between a pair of film molecules so that the isotropic–nematic phase transition in the bulk occurs at sufficiently low densities. Reliability of the GCEMC method under these conditions is illustrated by a self-consistent comparison between Monte Carlo simulations in the canonical and grand canonical ensembles. In the bulk the nematic nature of the modified Gay–Berne fluid is established by computing the Mayer–Saupe order parameter S and suitably defined pair correlation functions which show that the bulk phase is not smectic even though S is fairly large. For a single temperature we investigate the isotropic–nematic phase transition in the modified Gay–Berne fluid which turns out to be a first-order phase transition. In the corresponding confined film variations of the microscopic structure with increasing sz are correlated with the normal component of the stress tensor Tzz(sz). Our results show that molecules in inner portions of the film undergo a reorganization from an originally planar orientation of their symmetry axes to a perpendicular one with respect to the plane of a wall. This orientational change is manifested as a periodic sequence of shoulders in Tzz(sz) where the periodicity length Δsz is close to the larger diameter of the ellipsoidal film molecules.

Photo-induced alignment of a photo-crosslinkable liquid-crystalline copolymer film by linearly polarized UV light

The anisotropic photoreaction of a liquid-crystalline (LC( copolymer comprising one co-monomer being anisotropically photo-crosslinkable by linearly polarized (LP( -UV light and the other being insensitive to UV-irradiation is presented. Polarized UV and IR studies suggested a photo-induced alignment of the mesogenic side groups during the LP-UV photoreaction at the LC temperature range of the copolymer. The direction of the aligned mesogenic groups is parallel to the electric vector of the incident LP-UV light and the induced birefringence is 0.07.