Symposium SM1 : Liquid Crystalline Materials---Displays and Beyond

We explore flexoelectro-optic effect in polymer stabilized cholesteric liquid crystals (CLC) with uniform lying helix (ULH) texture. A method to control the flexoelectric properties of the CLC mixture by mixing nematic liquid crystal constituents with large and small flexoelastic coefficients is demonstrated. By adding a bimesogenic LC dimer CB7CB into a low dielectric anisotropy commercial NLC mixture MLC-2048 the maximum of flexoelectrically driven in-plane rotation angle of the helical axis in a well-aligned ULH texture was increased from 3.5° to 73.2°. Prototype samples demonstrated a submillisecond flexoelectric response time both for switching between electric field on and off states, and the change of polarity of applied electric field, with typical contrast ratio of 150:1. Our findings identified quality of ULH alignment, and material miscibility challenges, and will be useful for material design of future CLC mixtures for applications in flexoelectro-optic displays.

Blue phases (BPs) have been recognized as a fast optical isotropic/anisotropic switching liquid-crystal (LC) material in recent years, and they have become a possible candidate in fast-display, three-dimensional laser and photonics applications after improving their thermal stability. For most LC materials, BPs are usually stable within a very narrow temperature range between the isotropic liquid phase and the cholesteric phase with strong chirality. I In terms of the defect theory, in BPI and BPII, DTCs have a lower elastic free energy than a uniaxially twisted director field in chiral nematic LC. Those DTCs self-assemble as three-dimensional cubic lattice structures, leading to the appearance of a disclination network. These disclination lines can be considered as cylinders filled with the isotropic liquid phase, and thus have a higher enthalpy and an additional interfacial energy, which causes the narrow temperature range of BPs. In order to extend this temperature range, the most useful way is to introduce the polymer network into the BP declination lines to help achieve a temperature of more than 60 K, due to the pinning effect. Polymer-stabilized BP (PSBP) is a key for next generation display and photonics application.
A previous study showed that the thermal stability of BP enhancement is related to the polymer component accumulating in the disclination lines, reducing the enthalpic contribution from the isotropic liquid and the interfacial energy. The sort and the concentration of reactive monomers are important for extending the BP temperature range. Another key point for achieving the stabilized effect is that the photo-polymerization process should be completed during the BP phase. It indicates that we should examine other polymerization conditions to control the morphology of the polymer network, and then discuss their influence on the thermal stability of BPI.
In this study, we would like to investigate the influences of the LC phase, the intensity of the UV irradiation, and the photo-polymerization time on the BP temperature ranges. The morphology of the polymer network formed in BPLC is studied by SEM. We prepared PSBP samples, where the polymer network is constructed in cubic BPI, amorphous BPIII and the chiral nematic phase, with the intention of comparing the temperature range of the blue phases in these samples. From these SEM photographs, the radius of the polymer chain becomes tiny when the photo-polymerization temperature and the intensity of the UV irradiation are increased. These experimental results show that the different LC phases will affect the morphology of the polymer network. Referring to the free-energy density of BP, the temperature range of BP will depend inversely on the square of the disclination diameter, as well as the polymer-chain diameter. We can obtain experimental results which agree with this theoretical prediction.

A wide-temperature liquid crystalline amorphous blue phase based on polymer network stabilization exhibits high thermodynamic stability with an extended temperature ranging from a few 1C to more than 80^oC.[1] Analyses using confocal laser scanning microscopy show that the polymer network imitates the aperiodic disclination-entangled structure of an amorphous blue phase and involves a highly intertwined interaction with liquid crystal molecules. The stabilized amorphous blue phase manifests tens of microseconds response time, a consistent achromatic dark state and it is intrinsically hysteresis-free during the application of an electric field. The topological and electro-optical features of the stabilized amorphous blue phase are further compared with the stabilized isotropic and cubic blue phases. These results not only provide a physical perspective on the electro-optical response of a liquid crystal and polymer composite but also open up a new direction for electro-optical device applications.


[1] M. S. Kim and L.-C. Chien, Soft Matter, 11, (2015) DOI: 10.1039/c5sm01918d

Liquid crystals formed by mesogenic dimers with two rigid rod-like units connected by a flexible aliphatic bridge feature unusual structural organization, with an oblique helicoidal local director. The pitch of the oblique helicoid ranges from nanometers in the twist-bend nematic (N_tb) to micrometers in cholesterics (Ch) formed by mesogenic dimers in presence of chiral dopants and external electric field (the so-called cholesteric with oblique helicoidal structure, or Ch_oh). Both structures lead to unusual electro-optics discussed in the presentation: first order structural transition in the N_tb and electrically-controlled selective reflection of light tunable in an extraordinary broad range, from ultraviolet to visible and to infrared, in Ch_oh. The work is supported by NSF DMR-1410378 and IIP-1500204.

Organic semiconductors offer numerous advantages for electronic and opto-electronic applications such as chemical flexibility, good optical properties, low material usage, low cost processing via solution processing. However, carrier mobilities in organic semiconductors are generally orders of magnitude lower than those in inorganic semiconductors. This is a major bottle neck for increasing device efficiency. The electrical properties are additionally strongly dependent on thin film structure and morphology which is challenging to control in solution deposited films.
In this contribution we study structure-function relationships in a novel small molecule which demonstrates liquid crystal properties. The molecular films demonstrate phase changes from the crystalline to nematic to isotropic phases at temperatures of 140 C, 165 C and 250 C, respectively. By controlled film preparation we can manipulate the film crystallinity and thereby influence the electrical and optical properties. We demonstrate the influence of temperature on the structural properties of solution processed films with cross polarized microscope (CPM) and X-ray diffraction (XRD). Electrical measurements were performed on single carrier devices using current-voltage measurements and impedance spectroscopy. We found that after annealing the films to temperatures above the crystalline – nematic phase change and subsequently cooled, the mobility can be increased from (as prepared) to 10^-3 . We investigated the electric field dependence of the mobility on the dispersion of the carrier transport and demonstrated that controlled structural manipulation of the film can be used to reduce electronic disorder. These results demonstrate the potential of liquid crystal conjugated materials in organic electronic applications.

We have recently reported on dynamic electro-optic responses of the reflection band of polymer stabilized cholesteric liquid crystal (PSCLC) formulations prepared with negative dielectric anisotropy nematic liquid crystal hosts including bandwidth broadening, switchable scattering, red-shifting tuning, and now blue-shifting tuning. Here, we report our current understanding of the influence of polymer network development on the resulting electro-optic response of these PSCLCs. This understanding was developed through a recently completed systematic structure-property examination focused on the influence of the photopolymerization conditions (initiator concentration, light intensity, curing time) as well as the monomer formulation/composition. Based on the electromechanical distortion of the structurally chiral polymer stabilizing network, the kinetics of the network development as well as the distribution/uniformity of the properties are shown to be critical.

The first reported SmAP_F phase, obtained from the mesogen W586, exhibited interesting and useful electrooptics.(1) Specifically, upon application of an electric field, a significant increase in birefringence is observed without rotation of apparent optic axis. Further study demonstrates great promise for this phase in applications requiring fast phase-only electrooptic modulation.

In order to tune properties of SmAP_F materials for such applications, a search for new mesogens providing wider phase temperature ranges and improved modulation depth (birefringence change with field) alone or in mixtures, is in progress.

Several surprising and interesting discoveries have derived from this work. For example, a dramatic and unexpected odd-even effect in homologues of W586, which possesses a tricarbosilane-terminated alkoxy tail, provides insights into the nature of nanophase segregation in smectics.

In addition, new classes of SmAP_F mesogens have been prepared in an effort to produce materials for applications. Details of the characterization and EO properties of the new materials will be presented.

Another key development is the ability to obtained uniform bookshelf geometry by applying an electric field between interdigitated electrodes as the molecules cool from the SmA to SmAP_F. Uniform alignment is necessary to demonstrate the unique analog phase only modulation observed in the SmAP_F molecules. [2]

(1) a) Reddy, R. A.; Zhu, C.; Shao, R.; Korblova, E.; Gong, T.; Shen, Y.; Garcia, E.; Glaser, M. A.; Maclennan, J. E.; Walba, D. M.; Clark, N. A. "Spontaneous ferroelectric order in a bent-core smectic liquid crystal of fluid orthorhombic layers," Science 2011, 332, 72-77. b) Korblova, E.; Walba, D.; Gong, T.; Reddy, A.; Zhu, C.; Shao, R.; Maclennan, J.; Glaser, M.; Clark, N. "Design and synthesis of an achiral ferroelectric smectic liquid crystal," Proc. SPIE 2011, 8114, (1), 81140X-1 - 81140X-9.

(2) Shen, L. Goodhew, R. Shao, M. Moran, E. Korblova, D.M. Walba, N.A. Clark, J.E. Maclennan, and P. Rudquist. “Field alignment of bent-core smectic liquid crystals for analog optical phase modulation,” Appl. Phys. Lett., 2015, 106, 191191

Three blue phases, BPI, BPII and BPIII, exist between the chiral nematic phase and the liquid phase. Compared with the electro-optical properties of BPI and BPII, BPIII is a fast response photonic device with no residual birefringence, less hysteresis effect and low driving voltage when an in-plane electric field is applied [5]. However, the thermal stability of BPs is usually low. In terms of the free energy of the blue phase, the thermal stability of the blue phase is decided by the balance between the free energies of the disclinations and the double-twisted cylinders. Of the three BPs, BPIII usually occupies a narrow temperature range of ~0.1K just below the isotropic phase when the chirality is high enough. There have been many suggestions for widening the BP temperature range. Among those, the most effective way to extend the temperature range of BPI or BPII is polymer stabilization. However, the polymer network cannot obviously stabilize the amorphous BPIII in a wide temperature range. Transflective devices that incorporate both transmissive and reflective functions in the one device are attractive for outdoor image readability in portable displays. Their power consumption is lower than transmission display, because they use the sunlight as the light source in outdoor applications. Many methods have been proposes to produce a transflective nematic liquid-crystal displays. Polymer-stabilized blue phase I (PSBPI) has been suggested as a replacement for NLC materials due to its fast response, optical isotropy, insensitivity to the cell gap with the application of an in-plane switching field, and the fact that no alignment layers are needed. Thus, a single-cell-gap PSBP with bumpy reflectors on one of the glass substrates is made as a transflective display.
In this paper, our purpose is to demonstrate the BPIII can be a tranflective device. In our BPIII material, we do not need to use a polymer network or add any nanoparticles to widen the temperature range, because the temperature range of BPIII in our sample is over 10°C and covers room temperature. Due to the structure of BPIII, a perfect optically isotropic dark state can be obtained in a null field. An in-plane electric field is applied to the BPIII cell through interdigital transparent electrodes on one of the substrates. Here, no reflectors are used in our cell. Measuring the light efficiencies of transmission and reflection, they are almost equal. Moreover, the transmitting and reflecting light are independent of the incident wavelength and the total response time of the IPS-BPIII cell is less than 2 ms. These experimental results exhibit that BPIII can be a potential candidate for use in transflective devices, even for use on a transparent display.

The investigation of electric-field induced reorientation of dispersions containing reflective particles is motivated by the desire to produce low-power reflective displays, transflective displays, color tunable/switchable mirrors/windows for privacy control, and tunable/switchable optical filters. Cholesteric liquid crystal (CLC) microplatelets have been shown to reorient under application of electric field when dispersed in an isotropic, moderately conductive propylene carbonate host.^[1,2] This work evaluates and contrasts the stimuli-responsive behavior of inorganic and organic microplatelets dispersed in both propylene carbontate and nematic liquid crystal hosts. In isotropic hosts, such reorientation can be explained by the Maxwell-Wagner polarization effect via charge accumulation at the host fluid / microplatelet interface. However, changing from isotropic to nematic liquid crystal hosts adds complexity to the forces (e.g., torque) acting on particles both with and without the presence of electric field.^[3] Understanding the mechanisms of reorientation for miroplatelets exhibiting diverse chemical and topological surface properties should elucidate distinct mechanisms that enable dynamic optical elements.

Polyhedral oligomeric silsesquioxane (POSS) nanoparticles have been considered as useful giant molecules for the formation of vertical alignment (VA) of liquid crystal (LC). However, due to their poor compatibilities and weak interactions with LC host molecules, the pristine POSS nanoparticles were highly aggregated themselves in the LC media and formed the macroscopic particles, resulting in severe light scattering. To overcome this problem, we successfully synthesized the cyanobiphenyl monosubstituted POSS giant molecule (POSS-CBP1), which showed an excellent dispersion in nematic (N) LC media and formed the perfect VA of LC without using conventional polymer-based VA layers. On the basis of the systematic experiments and careful analysis, we revealed that the cyanobiphenyl part chemically attached to the pristine POSS with an alkyl chain can play a key role in improving the initial solubility and interaction with LC media. The finely tuned POSS-CBP1 was gradually diffused onto the substrate for the formation of VA layer without forming the macroscopic aggregations.
Therefore, the novel POSS-CBP1 VA layer can allow us to develop electro-optical applications and to cut the manufacturing cost. This work was supported by the BK21 Plus, the BRL 2015042417, and MOTIE/KDRC 10051334.

Optically isotropic liquid crystal (LC) media were prepared by doping the star-shaped cyclic oligosiloxane LC molecular surfactants (SiLC) into the rod-shaped twin LC host molecules (DiLC). The phase diagram was constructed by differential scanning calorimetry (DSC). The phase diagram of the SiLC/DiLC mixtures showed the broad coexistence regions such as smectic A + crystal (SmA1 + Cr2), liquid + crystal (L1 + Cr2) and liquid + nematic (L1 + N2) at the intermediate compositions along with the narrow single phase crystal (Cr2), smectic (SmA1) and nematic (N2) regions. The morphologies and structures of these coexistence regions were investigated by polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXD). At the 80/20 SiLC/DiLC composition, the optical anisotropy was induced under an alternating current (AC) electric field above its isotropization temperature. The fabricated optically isotropic LC medium in mixtures of the SiLC molecular surfactants and nematic LC host opens new doors for the electro-optical devices. This work was supported by the BK21 Plus, the BRL 2015042417, and MOTIE/KDRC 10051334.

Without the conventional polymer-based liquid crystal (LC) alignment process, a newly synthesized dual photo-functionalized amphiphile (ADMA₁) was successfully applied as a robust photo-reversible LC alignment layer by self-assembly and photo-polymerization. The LC alignment layer constructed by directly adding dual photo-functionalized amphiphiles into LC media can cut the manufacturing cost as well as open new doors for the fabrication of novel electro-optical devices. The chemical structure of ADMA₁ was determined by ¹H nuclear magnetic resonance spectroscopes (NMR), ¹³C NMR, MALDI-ToF, and elemental analysis. The dual photochemical behavior of ADMA₁ was identified by ultraviolet-visible spectroscopy. The homeotropic alignment of nematic LC was investigated by both orthoscopic and conoscopic polarized optical microscopy. The self-assembled ADMA₁ monolayer was investigated by atomic force microscopy to analyze the surface morphological changes of the ADMA₁ layer according to light irradiation. From the combined the experimental results, it was realized that ADMA₁ gradually diffused onto the substrate of LC cell and formed monolayered ADMA₁ protrusions by lateral self-assembly. This work was supported by the BK21 Plus, the BRL 2015042417">2015042417, and MOTIE/KDRC 10051334.

A photochromic liquid crystalline (LC) polymer {4-butoxy-2’-(4-hex-5-enyloxy)-4’-(4-hex-5-enyl-oxybenzyl-oxy)azobenzene, abbreviated as PLCP} was newly synthesized via the acrylic diene metathesis polymerization. Utilizing the combined techniques of differential scanning calorimetry, cross-polarized optical microscopy, and wide-angle X-ray diffraction (WAXD), it was realized that PLCP formed a nematic (N) phase below the isotropization temperature and the N phase turned to the glassy N phase below the glass transition temperature. From the two-dimensional (2D) WAXD fiber pattern, it was realized that PLCP forms the N LC ordered phase containing the synclinically tilted smectic C (SmC)-type cybotactic clusters, in which the molecular packing structure is related to the head-to-side connected chemical structure of PLCP. By using light and heat, we successfully demonstrated the rewritable UV-patterned PLCP film as well as the reversible mechanical deformation of the uniaxially oriented PLCP microfiber. Due to the wireless remote controllability of reversible actuating and patterning behaviors of PLCP polymer, PLCP can be applied as a key material in optoelectronic and bio-mimetic devices. This work was supported by the BK21 Plus, the BRL 2015042417, and MOTIE/KDRC 10051334.

We demonstrate a bistable and photoswitchable diffraction grating based on self-assembled bubble domain (BD) texture of cholesteric liquid crystals (CLC). The BD texture is generated in CLCs confined into a cell with homeotropic surface anchoring when equilibrium helical pitch p is approximately equal to the cell gap thickness d, d/p~1.0. An applied electric field is used to switch the CLC between transparent homeotropic and light scattering BD textures. The optical density of scattering BD texture is controlled in a wide range by introducing a light-sensitive azo-benzene type chiral dopant and a dichroic dye into the CLC mixture. Light illumination induced reversible trans-cis isomerization of the chiral dopant enables fine tuning of the CLC pitch, providing photoswitching with a bistability between transparent HO and opaque BD optical states, and a spatial photopatterning using a shadow mask. Dichroic dye dopant creates a bistable guest-host effect based on the BD and homeotropic textures. All optical states are stable in rest after applied electric field or light excitations are removed and the switching is completed. Due to the ability to modulate a broad range of the wavelength of light from UV to near-IR a bistable BD texture dye-doped systems provide promising applications for the wide range of diffractive photonic devices, optical light shutters or light extractors.

Due to the unique structural properties and chemical multi-functionalities of dendritic molecules, dendrimers have been considered to be promising materials for the advanced applications in nano- and bio-technologies. We newly designed and synthesized a ribbon-shaped chiral liquid crystal (LC) dendrimer containing photochromic azobenznes and an isosorbide chiral center (abbreviated as AZ₃DLC). Based on the diffraction experiments combined with thermal and morphological results, it was realized that AZ₃DLC formed a low-ordered chiral smectic LC phase at a high-temperature phase region. The pitch length of the spontaneously formed helical superstructure can be precisely controlled by tunning the wavelength of light. This work was supported by the BK21 Plus, the BRL 2015042417, and MOTIE/KDRC 10051334.

Liquid crystal elastomers are very interesting soft materials due to the large changes in their microscopic and macroscopic properties at the isotropic-nematic phase transition. The possibility to have a controlled, tunable and reversible shape variation induced by thermal energy has been deeply investigated both on the theoretical and experimental point of view [1]. In order to induce a similar shape variation by applying an external stimulus, such as an electric or magnetic field, several attempts have been recently adopted [2,3].
In this report, we focus on a particular bi-layered system, composed by a Liquid Single Crystal Elastomers (LSCE) film covered by a thin layer of gold [4]. As previously reported for similar systems [3], the thickness of the film can produce either the formation of micron-sized wrinkles or macroscopic bending actuation via direct Joule heating of the composite material.
In this work, we present an experimental study of the thermo-mechanic, thermo-elastic and the Joule heating actuation behavior. Our findings have been supported by continuum elasticity modeling based on finite elements. The simulation takes into account the temperature map throughout the sample and the stress induced by polymer alignment. The change in the elastic properties have been computed by first-principle calculations.

Acknowledgements:
V.Domenici, G. Romano and A. Kolpak thank the MIT-UNIPI project for collaboration between University of Pisa (Italy) and MIT (Boston), project titled: "Energy Storage Devices and Actuators based on Composites of Liquid Crystal Elastomers".

References:
[1] Warner, M. & Terentjev, E. M. Liquid crystal elastomers. (Oxford Univ. Press, 2007).
[2] Agrawal, A., Yun, T.-H., Pesek, S. L., Chapman, W. G. & Verduzco, R. "Shape-responsive liquid crystal elastomer bilayers". Soft Matter 10, 1411-1415 (2014).
[3] Greco, F., Domenici,V., Mattoli, V., Desii, A., Sinibaldi, E., Mazzolai, M., Zupancic, B, Zalar, B. “Liquid single crystal elastomer/conducting polymer bilayer composite actuator: Modelling and experiments”. Soft Matter, 9, 11405-11416 (2013).
[4] Zupancic, B., Zalar, B, Remskar, M., Domenici, V. “Actuation of Gold-Coated Liquid Crystal Elastomers”. Appl. Phys. Express, 6, 021701 (2013).

The research in the field of developing nanomaterials-Ferroelectric Liquid Crystals (FLCs) based composites is rapidly growing with the notion of tailoring their physical properties and possibility of entering in to novel multidisciplinary applications of Liquid Crystal materials. Current study deals with the in-depth investigation of physical properties of CeO₂ nanoparticles (NPs)-FLC composites with varied Ceria NPs concentration. In previous studies on nanomaterials based LC composites, physical property changes were understood considering the core properties of guest and host materials. However in our present study, the mechanism behind significant changes in dielectric behavior and electro-optic parameters of nanocomposites will be discussed in correlation with structural deformation of functional group present in FLC and lattice straining induced by dispersed NPs using infrared transmission spectroscopy. The temperature and frequency dependence of dielectric permittivity exhibits a gradual increase dielectric constant with CeO₂ NPs addition. The increase in saturated spontaneous polarization (Ps) and switching at a lower bias was also observed in these nanocomposites. IR transmission peaks pertaining to uC=O and uC-O stretching modes of FLC’s ester functional group showed shift in peak positions and appearance of new weak intensity C=O bonded chiral group vibrational mode at 1718 cm^-1 after Ceria dispersion. These variations are attributed to stress induced and coordinated bond formation between planar polar ester C=O group and Ce³⁺ ion through lone pairs of oxygen by CeO₂ NPs present in the interstitial sites of FLC’s polar component. Apart from this, the mechanisms involved in the changes of the Photoluminescence spectrum of FLC–ceria nanocomposites with varying ceria concentration and excitation wavelengths are studied in detail. These studies will provide important information in designing and tailoring properties of FLCs for luminescent display device application.

We propose a simple way to fabricate highly transparent nanoscale polymer dispersed liquid crystal (nano-PDLC) films between glass substrates and investigate their incident angle dependent optical transmittance properties with both collimated and Lambertian intensity distribution light sources. We also demonstrate that doping nano-PDLC films with 0.1 % InP/ZnS core/shell quantum dots (QD) results in a higher optical transmittance. This work lays the foundation for such nanostructured composites to potentially serve as roll-to-roll coatable light extraction or brightness enhancement films in emissive display applications, superior to complex nanocorrugation techniques proposed in the past.

New photoresponsive polymers containing photoisomerizable azo and high birefringence diphenyl diacetylen mesogenic units in the side chain have been designed and synthesized to achieve enhanced photosensitivity such as modulation of refractive index and diffraction efficiency. Newly synthesized polymers were studied by spectral methods (NMR, UV-Vis, IR) and thermal analysis (DSC, POM, TGA). The photochromic behavior of the polymer films was evaluated by observing trans-cis isomerization process of azo-groups. Refractive-index modulation of the new photochromic polymers for developing rewritable hologram media will be discussed.

The incorporation of anisotropic nanomaterials such as gold nanorods in liquid crystal media provides avenues towards enhancing current liquid crystal properties as well as the creation of novel hybrid material systems sensitive to external fields, temperature, and other stimuli. In this presentation, the delicate relationship between nanoinclusion parameters, photoresponsive materials, and liquid crystalline media will be discussed.

Liquid crystal elastomers have attracted recent interest for their capacity to undergo complex and reversible mechanical responses. The topology inputs to generate these mechanical responses are similar or identical to those used to fabricate high-performance optical components, such as diffractive waveplates. In this contribution, we assimilate the mechanical and optical functionalities into a responsive optical element capabile of being reconfigured by actuation of the material. The versatility of this approach is illustrated with various diffractive optical elements, and their performance is characterized.

Photoalignment and photopatterning of photosensitive materials have become a subject of intensive fundamental and application studies. The issues associated with azobenzene-based photoalignment layers (PALs) are the vulnerability to thermal stress and exposure to short wavelength ultraviolet light. Furthermore, weak surface anchoring of PAL is also an obstacle for practical applications. In this work, an in-situ protection formation on PALs has been demonstrated. We studied both the structure-property and concentration effects of reactive monomers (RMs) on the morphology of the protection on PAL. The morphological studies show that both the molecular structure and concentration of RM play important roles in the protection achievements. We will present the enhanced stability of PAL, increased anchoring strength and improved electro-optical properties of the photoaligned liquid crystal cells.