Ferroelectric Liquid Crystal Polymers Research Articles

Ferroelectric switching of electrically commanded alignment layers for liquid crystal displays

Optical second harmonic generation (SHG) was used to probe the structural and dynamic properties of electrically commanded alignment layers for liquid crystal displays. The layers were 200nm thick and were made of siloxane-based side chain ferroelectric liquid crystal polymer (FLCP). The effect of an external dc electric field in the range of 0–2V∕μm on the SHG signal was probed in an empty sandwich cell and in a cell filled with an isotropic liquid (hexadecane). In both cases the nonlinear induced polarization shows characteristics of the “V-shaped” switching, which is associated with a relatively minor reorientational perturbation of the film. This signifies a presence of domain structure, which is strongly pinned to the substrates. The values of switching times deduced from the SHG response are two orders of magnitude larger from the values obtained by optical birefringence measurements. This observation suggests that in the top surface layer of the FLCP, which is mainly responsible for the birefringence and also for the so-called electrically commanded surface effect, the reorientation of the polymer side chains takes place much faster than in the regions close to the substrate.

Switching of Electrically Commanded Alignment Layers Probed by Optical Second Harmonic Generation

Optical second harmonic generation (SHG) was used to probe the response of a 200 nm thick electrically commanded alignment layer made of ferroelectric liquid crystal polymer (FLCP) film to an external DC electric field. The results show that for the field magnitudes in the range 0–2 V/μm the field induced reorientation effects represent only a relatively minor structural perturbation of the film. The switching process is monostable and shows characteristics of the “V-shaped” switching. The response time of the SHG signal is relatively longer than what is measured for the bulk FLCP. Several mechanisms are considered as possible reasons for that.

Photorefractive effect in composites of ferroelectric liquid crystal and photoconductive polymer

The photorefractive effect in composites of a ferroelectric liquid crystal (FLC) and several photoconductive polymers was investigated. The photorefractivity of mixtures of photoconductive polymers and an FLC (polymer/FLC), as well as that of photoconductive‐polymer‐stabilized ferroelectric liquid crystals (PPS‐FLCs) was examined. The polymer/FLC samples exhibited two‐beam coupling gain coefficients of about 6∼12 cm−1 in a 5 µm gap cell. The photopolymerization of a methacrylate monomer in the FLC medium established a polymer‐stabilized state in which the alignment of FLC molecules was mechanically stabilized. The noise in a two‐beam coupling signal was reduced significantly in the PPS‐FLC samples.

Electrically commanded surfaces for nematic liquid crystal displays

Electrically commanded surfaces (ECS) is a liquid crystal display concept whereby the switching of the alignment layer, which is driven by an electric field applied across the layer, is further transferred to the bulk liquid crystal material via elastic forces. This work presents the electro-optic response of a sandwich cell with alignment layer made of siloxane-based ferroelectric liquid crystal polymer, representing the ECS. The bulk liquid crystal material of choice was an in-house nematic mixture comprising fluorinated liquid crystalline compounds with negative dielectric anisotropy (Δε<0). We report a distinct linear electro-optic response, arising from the field-induced in-plane switching of the nematic which in turn is mediated by the ECS.

Photorefractivity of Mixtures of a Ferroelectric Liquid Crystal and Photoconductive Polymers

Abstract The photorefractive properties of mixtures of a ferroelectric liquid crystal and photoconductive polymers were investigated by two-beam coupling experiments. A photorefractive effect based on reorientation of spontaneous polarization was induced in these mixtures. Asymmetric energy exchange was observed in the Bragg diffraction condition and gain coefficients of 10–15 cm−1 were obtained. It was found that the internal electric field was efficiently formed in polymer/FLC mixtures.

Main-Chain Ferroelectric Liquid Crystal Polymers for Electronic Nonlinear Optics Applications 1

We report the synthesis of organic polymer optical fibers with the following interesting supermolecular stereochemistry: 1) A thermodynamically stable macroscopic polar axis oriented approximately normal to the fiber axis; and 2) A nonlinear optical (NLO) chromophore oriented along the polar axis. Such a polymer fiber has been created using a main-chain ferroelectric liquid crystal polymer (FLCP) frozen in the glassy state at room temperature. The material is composed of “side-by-side twin” mesogens wherein the DR-1 NLO chromophore is oriented normal to the director. The polymer, which possesses excellent liquid crystal and glass transition temperatures for the present study, was prepared by acyclic diene metathesis polymerization of a liquid crystal diene. Details of the layer and director structure in the FLCP glasses are under investigation.

Electrically commanded surfaces: A new liquid‐crystal‐display concept

Abstract— Fast in‐plane switching of the optic axis was realized in liquid‐crystal displays (LCDs) based on the concept of Electrically Commanded Surfaces (ECS). According to this concept, the liquid‐crystal layer in such a display is aligned by means of thin ferroelectric liquid‐crystal‐polymer (FLCP) film deposited onto the inner side of the display substrates. An electric field, applied normal to the substrates, switches the molecules of the ferroelectric film, representing the commanded surface that, via elastic forces, further transfers to the liquid‐crystal layer. The concept of electrically commanded surfaces opens the door to a new generation of advanced LCDs exhibiting extraordinary performance such as fast in‐plane switching.

Ferroelectric liquid crystalline polymers based on mesogens with halogen-containing terminal groups

A new series of halogen-containing side chain ferroelectric liquid crystal polymers was synthesized. Mesophases were characterized by differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction and molecular simulation. The behaviour of the liquid crystalline phase was investigated with variation of chiral centres, spacer units and grafted ratios. It was found that the thermal stability and temperature range of the chiral smectic C phase decreased with increasing length of the oligo-oxyethylene spacer, and decreasing mesogenic group content. The bulky substituent attached to the chiral centre reduces molecular packing in smectic liquid crystal phases, which disturbs the orientation of the side chain liquid crystal polymer. Furthermore, the influence of molecular structure on electrooptical properties of FLCPs has been studied by broad band dielectric spectroscopy (from 0.1 to 1 × 106 Hz).

An all-organic composite actuator material with a high dielectric constant.

Electroactive polymers (EAPs) can behave as actuators, changing their shape in response to electrical stimulation. EAPs that are controlled by external electric fields--referred to here as field-type EAPs--include ferroelectric polymers, electrostrictive polymers, dielectric elastomers and liquid crystal polymers. Field-type EAPs can exhibit fast response speeds, low hysteresis and strain levels far above those of traditional piezoelectric materials, with elastic energy densities even higher than those of piezoceramics. However, these polymers also require a high field (>70 V micro m(-1)) to generate such high elastic energy densities (>0.1 J cm(-3); refs 4, 5, 9, 10). Here we report a new class of all-organic field-type EAP composites, which can exhibit high elastic energy densities induced by an electric field of only 13 V micro m(-1). The composites are fabricated from an organic filler material possessing very high dielectric constant dispersed in an electrostrictive polymer matrix. The composites can exhibit high net dielectric constants while retaining the flexibility of the matrix. These all-organic actuators could find applications as artificial muscles, 'smart skins' for drag reduction, and in microfluidic systems for drug delivery.

Polymeric ferroelectric liquid crystals with a 2-methylalkanoyl group-effect of substitution in the core benzene ring

Polymeric ferroelectric liquid crystals with a 2-methylalkanoyl group in their mesogenic side chains exhibit large spontaneous polarization and are expected to respond quickly to an electric field. We prepared and examined novel polymeric liquid crystals. They contained a 2-methylbutanoyl or a 2-methyloctanoyl group, in which the ortho position to the alkanoyl group on the core was replaced by a fluorine atom, a hydroxy group, or a methyl group. We studied the effect of the substitutions on their properties and found that they lower the clearing points. The low spontaneous polarizaion of the polymer that underwent the fluorine substitution can be explained by the electronegativity of the fluorine atom. IR analysis verified that the substituted hydroxy group formed an intramolecular hydrogen bond with the adjacent alkanoyl carbonyl group. The steric hindrance introduced by the methyl substitution increased the tilt angle and slightly increased the intermolecular distance between the mesogens.

BPM Analysis of an Integrated Optical Switch using Polymeric Optical Waveguides and SSFLC at 1.55 µm

An optical switch using polymeric waveguides and ferroelectric liquid crystals (FLC) has been modelled by using a 3D vectorial beam propagation method (BPM) at the wavelength of 1.55 µm used in optical communications. The switch structure is a vertical directional coupler consisting of a surface stabilised FLC (SSFLC) embedded between two polymeric optical waveguides and ITO layers as electrodes. A polymeric buffer layer has been introduced to reduce losses due to ITO absorption. Polymeric waveguides and buffers used in our analysis can be obtained by different compositions of P(PFS-GMA) (poly(pentafluorostyrene-co-glycidyl methacrylate)). Several devices have been modelled with different values of the refractive indexes of the individual layers. Simulations show the benefits in terms of loss reduction due to the presence of the buffer. An optimised device presents an extinction ratio of more than 50 dB and losses lower than 1 dB with a coupling length of only 174 µm.

Synthesis and characterization of ferroelectric side chain liquid crystal polymers bearing banana-shaped chiral mesogens

A series of ferroelectric liquid crystal polymers having banana-shaped side chain mesogens were synthesized through photo-polymerization of epoxide moiety. 2,5-disubstituted-thiophene sub-unit was used in synthesizing the banana-shaped monomers. These liquid crystal compounds were characterized by NMR, differential scanning calorimetry (DSC) and optical polarized microscopy (POM). Mesomorphism was investigated as a function of spacer units. All the synthesized low molar mass banana-shaped compounds exhibit smectic blue phase, but chiral smectic C phase could be observed only on compounds having longer spacer. The clearing temperature of low molar mass compounds fluctuated a little when spacer length varies. All polymers exhibit cholesteric mesophase and an observable glass transition. These liquid crystalline compounds reveal strong photoluminescence at visible region (λmax = 475 nm for M9EPX) and have potential use in polarized organic light emitting diode materials.

Synthetic routes to ferroelectric liquid crystal polymers. FLC copolyacrylates

For polymerization 11-acryloyloxy(4-(2-methylheptyloxy)-4′-biphenyl)-undecanoate monomer was synthesized by reacting acrylic acid with 11-bromo(4-(2-methylheptyloxy)-4′-biphenyl)-undecanoate under phase transfer conditions. The monomer melts at 55 °C and has no mesomorphic transition. Polymerization of this monomer at 80 °C in toluene by using AIBN as initiator did not result a polymer. Anionic copolymerization of the monomer with methyl acrylate, methyl methacrylate and butyl methacrylate at -80 °C in toluene and in the presence of n-butyl lithium as catalyst did not result a copolymer. For polymeranalog reaction (S)-4′-(2-methyl-butyloxy-carbonyl)-phenyl-4-(11-bromounde-canoyloxy)-benzoate has been synthesized and characterized by TLC, FTIR-CNMR-spectroscopy and DSC analysis. Copolymers of acrylic acid and methyl methacrylate have been synthesized via radical copolymerization. Copolymers containing less than 50 % acrylic acid proved to be suitable for phase transfer reaction with the mesogenic group. By using FLC side groups for polymeranalog reaction FLC polymers are expected.

Ferroelectric liquid crystals for second order nonlinear optics

Abstract

Pyroelectric investigations of polarization and charge distributions in sandwich cells containing a ferroelectric liquid crystalline polymer

With the laser intensity modulation method (LIMM), polarization and charge profiles in sandwich cells containing the ferroelectric liquid crystal side group polymer LCP1 have been investigated. In a sample with a thickness larger than the helical pitch, contributions to the polarization profile due to surface anchoring were found. Additional contributions inside the cell occurred. The validity of the results, concerning numerical deviations in the deconvolution process of the pyrospectra, has been tested by simulations. The influence of space charges for measurements with a static external electrical field is discussed, giving contributions to the profile in addition to those arising from polarization.

Static and dynamic properties of ferroelectric liquid crystal and their novel applications

The properties of ferroelectric and polymeric ferroelectric liquid crystals (FLC and PFLC) have been studied as functions of molecular structure, bias voltage and boundary conditions. Anomalous rotation of the smectic layer has been observed in the ferroelectric and antiferroelectric phases. The direction of the rotation is reversible with the polarity reversal of the applied voltage. The transition temperature (Tc) from smectic A to smectic C* becomes higher with decreasing thickness in free-standing FLC films. On the other hand, Tc in the free-standing PFLC film is similar to that of thick sandwich cell. Electro-optic effects in FLC and PFLC in spin-coated films, free-standing films and in sandwich cells have also been clarified.

Nonlinear optical properties of ferroelectric liquid crystal

Nonlinear optical phenomena such as second harmonic generation (SHG) have been studied in various ferroelectric liquid crystals. SHG efficiency depends on the molecular structure of ferroelectric liquid crystal and can be controlled by applied voltage. Phase matching of SHG has been well realized in these ferroelectric liquid crystals. SHG in polymeric ferroelectric liquid crystals, ferroelectric liquid crystal-conducting polymer composites, thin ferroelectric liquid crystal films and liquid crystals which exhibit flexoelectric effect have also been discussed.

Behavior and properties of side chain thermotropic liquid crystal polymers

The phase behavior, ferroelectric and electrooptical properties and potential applications of ferroelectric liquid crystal polymers are reviewed. The molecular structure, mechanical properties and non linear optical properties of side chain liquid crystalline networks are also discussed

Segmental orientation and mobility of ferroelectric liquid crystal polymers

IR spectroscopy was used to study the orientation and mobility of different molecular segments in a side chain ferroelectric liquid crystalline polymer (FLCP) in the book-shelf geometry. It was directly shown that the tilt angles for the mesogenic units and the spacers are different. The data obtained allowed us to construct a detailed model of segmental orientation in the SC phase for this FLCP. This model is consistent with the ‘zigzag’ model for tilted smectic phases. The rotational bias of carbonyl bonds is also confirmed and a possible orientation function for the carbonyl group is discussed. Time-resolved step-scan FTIR spectroscopy enabled us to follow the intra- and inter-molecular response of the FLCP to an external electric field with a time resolution of 5 mus. It was detected that mesogenic moiety, spacer and backbone take part in the reorientation process. The time responses of different molecular segments are similar on the time scale of a few hundred microseconds.

Surface alignment of ferroelectric liquid crystals using side chain ferroelectric liquid crystal polymer

New surface alignment material for surface stablized ferroelectric liquid crystal displays (SSFLCDs) is investigated. Side chain ferroelectric liquid crystal polymer (SCFLCP) such as Poly{methyl 4-[[[S-(-)-2-methyl-1-butoxy]carbonyl] phenyl ]4′-hexyloxybenzoyl}Siloxane was used as the alignment layer which resulted in good uniform alignment of FLC molecules without zig-zag defects. However, RN-715 polyimide alignment layer showed random alignments with some zig-zag defects in the SSFLC cells. After the field stablization of±20 V AC, the typical stripe-shaped patterns appeared. Good electro-optical performance was observed by using SCFLCP.