Ferroelectric Liquid Crystalline Polymers Research Articles

Achieving High Permittivity Paraelectric Behavior in Mesogen-Free Sulfonylated Chiral Polyethers with Smectic C Liquid Crystalline Self-Assembly

Ferroelectric liquid crystalline polymers (LCPs) with a high spontaneous polarization (Ps) are of great interest for the fabrication of advanced electronic devices. However, conventional ferroelectric LCPs typically exhibit low spontaneous polarization, only around 1 mC/m2. To increase the orientational polarization for ferroelectric LCPs, in this work, we designed mesogen-free comb-shaped LCPs based on isotactic polyepichlorohydrin, which contained highly dipolar sulfonyl groups (dipole moment of 4.5 Debye) in the side chains. The goal was to increase dipole density and induce ferroelectric switching. In this new series of mesogen-free LCPs, the sulfonyl groups were first moved away from the main chain to mitigate strong dipole–dipole interactions. Second, methyl-branched alkyl side chains were implemented to decrease the crystallizability and induce the Smectic C (SmC) self-assembly. Intriguingly, these SmC samples, for the first time, exhibited typical paraelectric behavior (i.e., slim S-shaped single hysteresis loops) with a high dielectric constant in the range of 20–30. Learning from the knowledge obtained from this study, we will be able to design new mesogen-free LCPs with tailor-made dipole–dipole interactions and realize ferroelectricity.

Smectic C Self-Assembly in Mesogen-Free Liquid Crystalline Chiral Polyethers with Sulfonylated Methyl-Branched Side Chains.

To realize advanced electrical applications for ferroelectric liquid crystalline polymers, high spontaneous polarization (Ps ) is highly desired. However, current ferroelectric liquid crystalline polymers usually exhibit a low Ps . In this work, mesogen-free, chiral polyethers containing sulfonylated methyl-branched alkyl side chains with a (CH2 )3 O spacer between the sulfonyl and the branched alkyl groups are designed and synthesized. In contrast to the linear n-alkyl side chains, the methyl-branched alkyl side chains induce chain tilting in the smectic layers. When double chirality exists in both the main chain and the side chains, a crystalline structure is observed after mechanical stretching. Intriguingly, when single chirality exists in either the backbone or the side chains, a liquid crystalline smectic C phase is obtained. The electric displacement-electric field study, however, does not show typical ferroelectric switching, although the dielectric constants are relatively high for these liquid crystalline polymers. This is likely because the dipole-dipole interactions among neighboring sulfonyl groups along the main chain are so strong that the ferroelectric switching is hindered in the samples. For the future work, it is desired to weaken the dipole-dipole interaction to achieve ferroelectricity in these mesogen-free liquid crystalline polymers.

Ordered hexagonal columnar liquid crystalline self-assembly of mesogen-free sulfonylated side-chain chiral polyethers and their high dielectric property

To achieve large spontaneous polarization for ferroelectric liquid crystalline polymers, we have designed a series of mesogen-free isotactic polyethers with sulfonylated side chains. Different from prior studies using linear alkyl side chains, in this work a methyl branch was implemented in the alkyl side chains of the chiral polyethers. As a result of the continuous twist from the methyl branches and strong dipole-dipole interactions among neighboring sulfonyl groups, a helical chain conformation with tilted side chains was achieved. When the branched side chains were short (e.g., C5) and double chirality existed in both the main chain and side chains, a crystalline structure was obtained. When the branched side chains were long (e.g., ≥ C8) and single chirality existed in the main chain, an ordered hexagonal columnar liquid crystalline phase was induced. Their high-field dielectric property was studied by electric displacement-electric field (D-E) loop tests; however, no ferroelectric behavior was observed up to 200 MV/m. It was considered that the dipole-dipole interaction among neighboring sulfonyl groups in the helical chains was too strong to allow easy ferroelectric flipping.

New side chain ferroelectric liquid crystalline polymers based on (2S, 3S)-2-chloro-3-methylpentanoic acid: synthesis and phase behaviour

The synthesis of four new chiral mesogenic monomers (M1–M4) and side chain ferroelectric liquid crystalline polymers containing (2S, 3S)-2-chloro-3-methylpentanoate is described. The chemical structures and phase behaviour of the monomers and polymers obtained in this study were characterised by Fourier transform infrared, proton nuclear magnetic resonance, polarising optical microscopy, differential scanning calorimetry, thermogravimetric analysis and X-ray diffraction. The selective reflection of light was investigated with ultraviolet/visible (UV/Vis). Their structure–mesomorphism relationships were discussed. M1 and P1 all showed a chiral smectic C (SmC*) phase. M2 and M3 revealed a SmC* phase and cholesteric phase, while their corresponding polymers P2 and P3 revealed a SmC* phase and smectic A (SmA) phase. M4 only exhibited a cholesteric phase, whereas the corresponding polymers P4 showed a SmA phase. Moreover, the selective reflection of light shifted to the long-wavelength region at the SmC* phase range and to the short-wavelength region at the cholesteric range with increasing temperature, respectively. The results seemed to demonstrate that the tendency towards melting temperature (Tm), glass transition temperature (Tg), isotropic temperature (Ti) and mesophase range for the monomers and polymers increased by increasing the mesogenic core rigidity or the number of phenyl ring. The polymerisation effect could lead to higher liquid crystalline to isotropic phase transition temperature, wider mesophase range and more ordered smectic phase formed. In addition, all the obtained polymers had a very good thermal stability and the corresponding Td increased by increasing the number of phenyl ring.

High‐Contrast Photoswitching of Nonlinear Optical Response in Crosslinked Ferroelectric Liquid‐Crystalline Polymers

A conceptually novel materials design, based on crosslinked ferroelectric liquid-crystalline polymers, is demonstrated for efficient switching of a second-order nonlinear optical (NLO) response in the solid state. By controlling the molecular alignment of the NLO moieties through two-photon isomerization of azobenzene molecules, reversible isothermal photocontrol of second-harmonic generation is achieved with contrast of up to 20.

27.4L: Late‐News Paper: A New Mode LCD with Intrinsic Continuous Grayscale Memory Using Ferroelectric Liquid Crystalline Polymer

AbstractA new mode LCD has been developed by rubbing methods with a ferroelectric liquid crystalline polymer (FLCP). It has good balanced characteristics such as a fast response time of less than 1 ms, a high contrast ratio at the same level as in‐plane switching (IPS) mode, bi‐stability, continuous grayscale, and high durability against external shock. In addition, it displays intrinsic continuous grayscale memory. This new mode LCD has significant potential as a substitute for IPS or vertically aligned (VA) mode LCDs, and also as an electronic paper by which video content can be displayed.

Synthesis and Dielectric Property of a Ferroelectric Liquid Crystalline Polythiophene Derivative

Chiral liquid crystalline substituent containing asymmetric group was introduced into the thiophene monomer. The monomer with ferroelectric liquid crystalline substituent was polymerized by the chemical reaction with the aid of Ni(0) catalyst to afford linear ferroelectric liquid crystalline conductive polymer. The ferroelectric polythiophene derivatives thus prepared were good processability because the polymer is fusible and soluble in ordinary organic solvents. Absorption band was observed in the range from 400 to 415 nm due to the electronic transition (π − π* transition) of conjugated main chain. Phase transition behavior of the polymer was examined by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction analysis. The polymers exhibited a striated fan-shaped texture under the polarizing optical microscopy observations. This is attributed to a formation of chiral smectic C (SmC*) phase with ferroelectricity. The ferroelectric behavior was also confirmed by examining the temperature dependence of dielectric constant of the polymer.

Molecular and collective relaxations of ferroelectric side chain liquid crystalline polysiloxanes

The influences of mesogenic group chemical structures on dielectric relaxation behavior were investigated for ferroelectric side chain liquid crystalline polymers (FLCPs). The relaxation time and activation energies of the Goldstone mode, α-, and β-relaxations decrease with increasing spacer length because of the plasticizer effect of the spacer. Moreover, the relaxation intensity increases with increasing spacer length for FLCPs. An FLCP with a longer spacer length exhibits a higher mesogenic group mobility, and subsequently leads to easier reorientation toward the alternating electrical field. An increase in mesogenic core rigidity results in an increase in the relaxation time and activation energies, and a decrease in the relaxation intensities for the Goldstone mode, α-, and β-relaxations. Moreover, the β-relaxation is suppressed and cannot be observed in the glassy state for FLCPs containing naphthyl biphenylcarboxylate as the mesogenic group. Shorter relaxation time, smaller activation energies, and higher intensity of the α-, and β-relaxations were obtained for FLCPs containing chiral moiety with a flexible heptyl alkyl chain. However, the relaxation intensity of the Goldstone mode for FLCPs containing this chiral moiety was smaller than that for FLCPs containing the chiral moiety with a butyl alkyl chain. For FLCPs containing a chiral moiety with two asymmetrical centers, their Goldstone mode relaxation showed larger amplitude. The α- and β-relaxations are suppressed for these FLCPs because of the dense packing and memory effect of the smectic phase. The relationship between the chemical structure of the mesogenic group and dielectric relaxations is discussed in great detail. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2035–2049, 2006

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).

Soft mode and related behaviour in the SmA and SmC* phases of a ferroelectric liquid crystalline polymer by dielectric spectroscopy

Anomalous dielectric relaxation behaviour is observed in the ferroelectric liquid crystalline polymer (viz. ferroelectric copolysiloxane (R)-COPS 11-10) around the ferroelectric SmC* to paraelectric SmA phase transition. Measurements have been performed on sample of thickness ∼10 μm in indium-tin-oxide coated cell in the frequency range 10 Hz to 13 MHz. With increase of temperature, a gradual shift of the soft mode frequency towards the higher frequency side was observed, while a decrease in the relaxation strength was seen with the corresponding increase in temperature. The shifts of the soft modes in the SmC* and SmA phase are considered to be due to change in the viscosity of the polymer, as an increase in viscosity increases fluctuations of the coupling between the dipoles in the network even far from the paraelectric-ferroelectric phase transition. Application of a bias field causes a shift of the critical frequency towards the higher frequency side, while the dielectric strength (δε) decreases under the bias field. The Cole-Cole fitting parameters obtained from the best fit of the dielectric constant data are found to be consistent with other similar materials. Another relaxation mode (molecular mode) was also observed which comes into play in both the smectic phases (SmC% and SmA) and contributes to the dielectric permittivity.

Transient Light scattering of Ferroelectric Liquid Crystals Enhanced by Adding Ferroelectric Liquid Crystalline Polymers

A side-chain type ferroelectric liquid crystalline polymer (FLCP) was mixed with a low-molecular-weight ferroelectric liquid crystal (LMWFLC) to improve the light switching contrast for the transient light-scattering mode (TSM). Optical transmittance through an optically homogeneous chiral smectic C phase in a FLCP/LMWFLC composite system was observed by a polarizing optical microscope. The transmittance of the transparent state of the composite system upon the application of a d.c. electric field was almost the same in comparison with that of LMWFLC. On the other hand, the light-scattering intensity for the composite system upon the application of an a.c. electric field was somewhat stronger than that for LMWFLC. The enhancing mechanism of the transient light scattering induced by an addition of FLCP to LMWFLC has been discussed through the light switching measurements for free-standing FLC thin films.

Synthesis and characterization of halogen-containing ferroelectric liquid crystals and side chain liquid crystalline polymers

A new series of ferroelectric liquid crystals and side chain liquid crystalline polymers based on halogen-containing chiral centres has been synthesized. Chemical structures were analysed by NMR. Liquid crystal phases were characterized by differential scanning calorimetry, optical polarizing microscopy, and X-ray diffractometry. The behaviour of the liquid crystalline phases was investigated as a function of spacer units and differing terminal asymmetric moieties. It was found that phase transition temperatures decreased with increasing length of the oligooxyethylene spacer unit. Differing terminal asymmetric moieties led to differing mesophase phenomena. Furthermore, a wide temperature range (including room temperature) of a chiral smectic C phase was achieved.

Ferroelectric Polarization Hysteresis in a Liquid Crystal Polymer by Means of LIMM

We present pyroelectric measurements of the electric field hysteresis in a ferroelectric liquid crystalline polymer. Different electrode arrangements were used to study the pyroelectric response for different directions of the applied electric field with respect to the film geometry and the direction of the incident heat wave. The results demonstrate that in a sandwich cell electrode/foil/FLCP/foii/electrode charge accumulation in the polyester foils leads to a reverse hysteresis loop while conventional hysteresis behavior is observed when the electrodes are in direct contact with the FLCP. In addition we have measured the pyroelectric current in the plane of the film and observed ferroelectric switching and hysteresis in this configuration as well. The LIMM-spectrum shifts significantly toward lower frequencies in this configuration, which is assumed to be mainly due to a heat flow perpendicular to the Laser beam.

Atomic force microscopic studies of the influence of stretching on thin oriented films of ferroelectric liquid crystalline elastomers

To analyze the impact of the molecular structure on the network formation in ferroelectric liquid crystalline polymers, two copolymers are studied, which are identical except for the molecular position of the cross-linkable group: (i) Polymer P1 with cross-linkable groups attached to the backbone via a short spacer and (ii) Polymer P2 with the cross-linkable group in terminal position of a mesogenic side group. When mechanical stress is imposed on thin films in homeotropic orientation through stretching, the two elastomers react differently to the deformation, as seen by AFM imaging of the surface topology: For Polymer P1, “intra-layer” cross-linking results in two dimensional networks in each backbone layer, separated by liquid-like FLC sidegroup layers. As there are practically no vertical connections in this intralayer network, no vertical distortions occur. In Polymer P2 a three dimensional, “inter-layer” network is formed; the system reacts with a distortion of the smectic layering.

Aggregation and electro-optical effects in a ferroelectric (side-chain liquid crystalline polymer/low molecular weight liquid crystal) composite system

Aggregation and electro-optical effects in the binary mixture containing ferroelectric side chain liquid crystalline polymer (FLCP) and low molecular weight liquid crystal (FLC) have been investigated. The FLCP is miscible with FLC having a spiral texture. The binary mixture exhibits a smectic A, a smectic C, and a chiral smectic C phase. The pitch length in the chiral smectic C phase of the binary mixture increases with increasing weight fraction of FLC. The d-spacing of the smectic layer decreases slightly with increasing weight fraction of FLC. A reversible transparent-opaque (light scattering) change was observed in the chiral smectic C state upon application of positive and negative DC electric fields, respectively. Transparent and opaque states are stable when the applied electric field is removed.

Structure and mobility in ferroelectric liquid crystalline elastomers

Time-resolved Fourier transform infrared (FTIR) spectroscopy with polarized light was employed to study the structure and mobility of a homologous series of ferroelectric liquid crystalline polymers (FLCPs) and ferroelectric liquid crystalline elastomers (FLCEs) in response to an external electric field. The chemical composition of the samples, besides the cross-linking units, is similar. For the elastomers, two different cross-linking architectures are realized: “intralayer” cross-linking leads to the formation of two-dimensional networks, whereas “interlayer” cross-linking forms three-dimensional networks. Due to its specificity, FTIR spectroscopy enables analysis of the reorientational dynamics for the different molecular moieties in detail, thus revealing information about reorientation times, angular excursion, and the phase relationship in the rearrangement of the various molecular groups. In comparison to the un-cross-linked FLCP, both elastomeric samples exhibited smaller reorientation angles and an increase of the reorientation times. In the case of the interlayer cross-linked FLCE, an elastic memory effect was observed: For the reversal from negative to positive field polarity, the reorientation times were longer than for those in the opposite direction. For the intralayer cross-linked sample, it was shown that the backbone molecules reorient slower than the other molecular units (“locomotive effect”). For the un-cross-linked FLCP and the two FLCE samples, different coupling mechanisms between the network and the mesogenic parts are derived from the measurements.

Atomic force microscopic studies of thin oriented films of ferroelectric liquid crystalline elastomers with varying network architecture

Cross-linked ferroelectric liquid crystalline (FLC) polymers were studied by atomic force microscopy (AFM). Polysiloxane copolymers were synthesized with mesogenic and photo-cross-linkable side groups, the latter connected either directly to the backbone via a short spacer or as a terminal group on a part of the mesogens. These elastomers were prepared as thin, freely suspended films in homeotropic orientation. Topographic measurements depict plateaus separated by steps of characteristic height corresponding to the surfaces and edges of smectic layers. From the temperature behavior, as well as from the reaction to mechanical deformation, a model for the network architectures is proposed. In the first case (“intralayer cross-linking”), a predominantly two-dimensional network is formed within the backbone layers separating the smectic layers; in the second case (“interlayer cross-linking”), a three-dimensional network is established that is dependent on the mesophase present during cross-linking.

Temperature Dependent AFM on Ultrathin Oriented Films of FLC Elastomers

Crosslinked ferroelectric liquid crystalline polymers are studied by atomic force microscopy. Polysiloxane copolymers have been synthesized with mesogenic and photo-crosslinkable side groups, the latter connected either directly to the backbone via a short spacer, or as terminal group on a part of the mesogens. These elastomers are prepared as thin freely suspended films in homeotropic orientation. The topography consists of plateaus separated by steps of characteristic height, corresponding to the surfaces and edges of smectic layers. From the temperature behavior as well as from the reaction to mechanical deformation, a model for the network architectures is proposed: In the first case (“intra-layer crosslinking”), a predominantly two dimensional network is formed within the backbone layers separating the smectic layers, in the second case (“inter-layer crosslinking”) a primarily three dimensional network is established which is dependent on the mesophase of crosslinking.

Electric-Field-Induced Segmental Reorientation in Ferroelectric Liquid Crystalline Polymers and Elastomers

Macroscopically oriented ferroelectric liquid crystalline elastomers were prepared by UV-induced cross-linking of the corresponding polymer. Time-resolved FT-IR spectroscopy is employed to analyze structure and mobility in these materials on the level of the different molecular moieties. The influence of cross-linking is analyzed in detail in its effect on the molecular motion and its time dependence in response to an external electric field. The angular excursion of the motion of the spacer in response to an external electric field is much smaller than that of the mesogen. This is in pronounced contrast to low molecular FLC. The angle between the mesogen and the spacer changes continuously during the reorientation in the Sm A phase, suggesting continuous changes in the spacer conformation. It is shown that the ferroelectric order is frozen by the cross-linking reaction. A new method is described to detect the phase transition Sm C*− Sm A in ferroelectric liquid crystalline elastomers. It is based on a di...

Mechanical Deformation Behavior in Highly Anisotropic Elastomers Made from Ferroelectric Liquid Crystalline Polymers

Cross-linked ferroelectric liquid crystalline polymers are studied by atomic force microscopy. Polysiloxane copolymers have been synthesized with mesogenic and photo-cross-linkable side groups, the latter connected either directly to the backbone via a short spacer or as terminal groups on a part of the mesogens. Although the polymers are otherwise identical, the restrictions imposed on the network formation process by the anisotropy of the smectic mesophase are different for the two positions of the cross-linkable group: In the first case (“intralayer cross-linking”), a predominantly two-dimensional network is formed in the backbone layers separating the smectic layers; in the second case (“interlayer cross-linking”), a primarily three-dimensional network is established which is dependent on the mesophase of cross-linking. These elastomers are prepared as thin freely suspended films in homeotropic orientation. The topography consists of plateaus separated by steps of characteristic height, corresponding to the surfaces and edges of smectic layers. If a film of “intralayer cross-linked” elastomer (network formation in the microphase-separated backbone layers) is elongated, the layers can slide on one another, showing occasional tears but no surface roughening (roughness 0.5 nm at 30% elongation). In an “interlayer cross-linked” film (network formation via the mesogens), the three-dimensional network introduces forces perpendicular to the direction of the mechanical deformation, leading to a characteristic depression pattern on the surface which indicates a distortion of the smectic order.