WO2017035291A1 - Composite exhibiting blue phase iii and electro-optical memory of a scaffold - Google Patents

Abstract

A composite includes from about 10 wt% to about 40 wt% of a polymer; a chiral dopant having a helical twisting power of at least about 100 µm-1; and a liquid crystal compound having a dielectric anisotropy of at least about +40. The composite exhibits a blue phase III.

Description

COMPOSITE EXHIBITING BLUE PHASE III AND

ELECTRO-OPTICAL MEMORY OF A SCAFFOLD

[0001] This application claims the benefit of U.S. Provisional Application No. 62/209,459 filed August 25, 2015 and titled "COMPOSITE EXHIBITING BLUE PHASE III" and U.S. Provisional Application No. 62/339, 129 filed May 20, 2016 and titled "ELECTRO-OPTICAL MEMORY OF A NANO-ENGINEERED AMORPHOUS BLUE PHASE III SCAFFOLD". U.S. Provisional Application Nos. 62/209,459 and 62/339, 129 are hereby incorporated by reference herein in their entireties.

BACKGROUND

[0002] The present disclosure relates to liquid crystal composites that exhibit blue phase III.

[0003] Liquid crystal displays (LCDs) have become increasingly popular due to high- end display performance, wide viewing angle, high resolution, and suitability for large devices (e.g., televisions). Faster switching in LCDs may be achieved by a polymer- stabilized blue phase LCD mode. Liquid crystalline blue phases include blue phase I (or BP I), blue phase II (or BP II), and blue phase III (or BP III, also known as "blue fog" or "foggy phase"). Blue phase I has a face-centered cubic structure. Blue phase II has a body-centered cubic structure. Blue phase III has an amorphous structure.

[0004] Blue phases are typically only stable in a very narrow temperature range in the vicinity of the phase transition from the cholesteric to the isotropic phase.

[0005] Polymer-stabilized blue phase compositions can exhibit sub-millisecond response times of Kerr switching and optically isotropic properties. The fast switching times and alignment-free characteristics of polymer-stabilized blue phase materials makes them useful for the development of a field sequential color LCD. However, the switching mechanism gives rise to intrinsic issues such as (1 ) an increased driving voltage due to strong anchoring of the blue phase liquid crystal at a polymer network; (2) inconsistent response under ascending and descending electric fields which causes hysteresis of transmittance with respect to an applied voltage for on-off or gray-to-gray switching; and (3) reflection of bluish light due to the Bragg reflection of the periodic cubic structure, so that the initial state is not sufficiently dark at a certain wavelength range.

[0006] One can explain the appearance of blue phases with respect to free energy by two different theoretical approaches: Landau-de Gennes and defect theories. The Landau-de Gennes theory implicitly describes the local order of blue phases in thermodynamic equilibrium, so that the local order parameter near the disclination core is gradually reduced to zero. The defect theory has been used to calculate the free energy per unit length of a single disclination, which in turn the order parameter becomes zero.

[0007] The cubic lattices of blue phase I and blue phase II stand in different orientations with respect to the surface of a substrate and crystalline growth occurs simultaneously at multiple locations. Thus, the crystalline structure of blue phase usually grows within multiple domains with different orientations and sizes. In order to suppress the hysteresis, a mono domain cubic blue phase has been proposed. However, annealing requires an extensive time period and it is thermally challenging to grow a mono domain structure over a larger area. Furthermore, the light leakage issue in the dark state of multi-domain polymer-stabilized blue phases is due to a color appearance at a certain range of wavelength, which inhibits the high contrast ratio of devices.

[0008] It would be desirable to develop blue phase III liquid crystal composites that exhibit enhanced stability, hysteresis, and dark state properties.

BRIEF DESCRIPTION

[0009] The present disclosure relates to liquid crystal composites that exhibit blue phase III. Devices containing the composites and methods for making and using the composites and devices are also disclosed.

[0010] Disclosed, in various embodiments, is a composite including from about 10 wt% to about 40 wt% of a polymer; a chiral dopant having a helical twisting power of at least about 100 pm^"1; and a liquid crystal compound having a dielectric anisotropy of at least about +40. The composite exhibits a blue phase III.

[0011] The composite may contain from about 20 wt% to about 30 wt% of the polymer. [0012] In some embodiments, the chiral dopant has a helical twisting power of at least about 1 10 μιτΓ¹.

[0013] The liquid crystal compound may have a dielectric anisotropy of at least about +50.

[0014] In some embodiments, the composite exhibits the blue phase III over a temperature range of at least about 80°C.

[0015] The composite may exhibit the blue phase III at room temperature.

[0016] In some embodiments, the polymer is formed from a mixture comprising a reactive mesogen. The reactive mesogen may be a mesogenic diacrylate monomer. Optionally, the mixture further includes a difunctional acrylate monomer.

[0017] Disclosed, in other embodiments, is a liquid crystal display including a first transparent substrate; a second transparent substrate; and a composite layer between the first transparent substrate and the second transparent substrate. The composite layer includes from about 10 wt% to about 40 wt% of a polymer; a chiral dopant having a helical twisting power of at least about 100 pm^"1 ; and a liquid crystal compound having a dielectric anisotropy of at least about +40. The composite exhibits a blue phase III.

[0018] Disclosed in further embodiments is a method for producing a composite. The method includes curing a mixture. The mixture contains from about 10 wt% to about 40 wt% of monomer units that form a polymer network during the curing; a chiral dopant having a helical twisting power of at least about 100 pm^"1; and a liquid crystal compound having a dielectric anisotropy of at least about +40. The composite exhibits a blue phase III.

[0019] Disclosed in further embodiments is a method for forming a nanostructured scaffold. The method includes forming a blue phase mixture in a desired phase (e.g., blue phase I, blue phase II, and blue phase III), stabilizing the desired phase by imprinting a polymer network, and removing the liquid crystal molecules.

[0020] These and other non-limiting characteristics are more particularly described below and in the appended materials.

BRIEF DESCRIPTION OF THE DRAWINGS [0021] The following is a brief description of the drawings, which are presented for the purposes of illustrating the exemplary embodiments disclosed herein and not for the purposes of limiting the same.

[0022] FIG. 1a includes phase diagrams for some of the compositions of Example 4. From left to right, the reactive monomer concentrations are 30 wt%, 20 wt%, and 10 wt%. The lower temperature star and the higher temperature star indicate the temperatures at which the images were taken for blue phase III and isotropic phase in FIG. 1b, respectively. FIG. 1 b includes transmission polarizing optical micrographs of blue phase III and isotropic phase (scale bar, 500 pm). The brightness of images increases by 80%. The crossed arrows represent the direction of a polarizer and an analyzer, and a denotes a deviation angle between the optic axis of the analyzer and the orthogonal direction of the polarizer, a = -3°, 0°, and 3° for left, middle, and right images, respectively. FIG. 1c includes graphs illustrating the intensity of light leakage with respect to a comparing blue phase III to isotropic phase. The curves are fitted to discrete measurement points. Error bars were obtained from the experimental standard deviation of 10 repeated measurements.

[0023] FIG. 2a includes polarizing optical micrographs of stabilized blue phase III in temperature variation (scale bar, 500 pm). Arrows represent the optic axes of a polarizer and an analyzer, and brightness of the images at a = ±3° increases by 80 % points. FIG. 2b and c illustrate reflectance of before and after stabilization, respectively, of sample 2 with respect to temperature and wavelength. The inset in FIG. 2b shows the peak wavelength of reflectance in blue phase I with respect to temperature. FIG. 2d and e illustrate optical rotatory power with respect to temperature for before and after stabilization, respectively. Error bars were obtained from the experimental standard deviation of repeated 10 measurements. FIG. 2 f-h are confocal laser scanning micrographs of stabilized blue phase III: sample 2 (inset scale bar, 5 pm), stabilized isotropic phase: sample 5, and stabilized blue phase I: sample 6 (scale bar, 15 pm), respectively.

[0024] FIG. 3a-c are confocal laser scanning micrographs of polymer molds fabricated by blue phase III: sample 2, isotropic phase: sample 5 and blue phase I: sample 6, respectively (scale bars, 15 pm). FIG. 3d includes scanning electron micrographs of polymer molds fabricated by blue phase III (scale bars, 500 nm). FIG. 3e, g, and i are polarizing optical micrographs (a = 3°; brightness of images increases by 80 % points; scale bars, 500 pm) and FIG. 3f, h, and j are light scattering images (scale bars, 1 cm) of polymer molds fabricated by blue phase III (FIG. 3e and f), isotropic phase (FIG. 3g and h) and blue phase I (FIG. 3i and j). The light scattering images were taken in a dark room with halogen light source, and polymer molds are sandwiched by glass substrates.

[0025] FIG. 4a and b are polarizing optical micrographs of blue phase III. FIG. 4d and e are polarizing opitical micrographs of isotropic phase. Micrographs of original stabilized phases (FIG. 4a and d) and achiral liquid crystals refilled phases (FIG. 4b and e) are included (scale bars, 500 pm) White arrows represent the optic axes of a polarizer and an analyzer, and brightness of images at a = ±3° increases by 80 % points. FIG. 4c and f illustrate optical rotatory power with respect to temperature for refilled into the polymer molds of blue phase III (FIG. 4c) and isotropic phase (FIG. 4f). Error bars were obtained from the experimental standard deviation of repeated 10 measurements. FIG. 4g-i are confocal laser scanning micrographs of the refilled phases, which are fabricated by blue phase III (FIG. 4g): sample 2, isotropic phase (FIG. 4h): sample 5 and blue phase I (FIG. 4i): sample 6 (scale bars, 15 pm).

[0026] FIG. 5a-c are confocal laser scanning microscope images of scaffolds derived from blue phase III, the isotropic phase, and blue phase I. Scale bars are 15 pm for the complete images and 3 pm for the enlarged regions. FIG. 5d and e are scanning electron microscope images of scaffolds derived from blue phase III and the isotropic phase.

DETAILED DESCRIPTION

[0027] The present disclosure may be understood more readily by reference to the following detailed description of desired embodiments included therein and the appended article, supplementary materials, and presentation slides. In the following specification and the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. In case of conflict, the present document, including definitions, will control. Preferred methods and materials are described below, although methods and materials similar or equivalent can be used in practice or testing of the present disclosure. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. The materials, methods, and articles disclosed herein are illustrative only and not intended to be limiting.

[0029] The singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

[0030] As used in the specification and in the claims, the term "comprising" may include the embodiments "consisting of" and "consisting essentially of." The terms "comprise(s)," "include(s)," "having," "has," "can," "contain(s)," and variants thereof, as used herein, are intended to be open-ended transitional phrases that require the presence of the named ingredients/steps and permit the presence of other ingredients/steps. However, such description should be construed as also describing compositions, mixtures, or processes as "consisting of" and "consisting essentially of" the enumerated ingredients/steps, which allows the presence of only the named ingredients/steps, along with any impurities that might result therefrom, and excludes other ingredients/steps.

[0031] Unless indicated to the contrary, the numerical values in the specification should be understood to include numerical values which are the same when reduced to the same number of significant figures and numerical values which differ from the stated value by less than the experimental error of the conventional measurement technique of the type used to determine the particular value.

[0032] All ranges disclosed herein are inclusive of the recited endpoint and independently combinable (for example, the range of "from 2 to 10" is inclusive of the endpoints, 2 and 10, and all the intermediate values). The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value; they are sufficiently imprecise to include values approximating these ranges and/or values.

[0033] As used herein, approximating language may be applied to modify any quantitative representation that may vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as "about" and "substantially," may not be limited to the precise value specified, in some cases. The modifier "about" should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the expression "from about 2 to about 4" also discloses the range "from 2 to 4." The term "about" may refer to plus or minus 10% of the indicated number. For example, "about 10%" may indicate a range of 9% to 1 1 %, and "about 1 " may mean from 0.9-1.1.

[0034] For the recitation of numeric ranges herein, each intervening number there between with the same degree of precision is explicitly contemplated. For example, for the range of 6-9, the numbers 7 and 8 are contemplated in addition to 6 and 9, and for the range 6.0-7.0, the number 6.0, 6.1 , 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, and 7.0 are explicitly contemplated.

[0035] The blue phase III of liquid crystals (e.g., cholesteric liquid crystals) is useful for a wide range of electro-optical applications due to its sub-millisecond switching time and no Bragg reflection in the visible spectrum. The double-twisted cylinders formed by liquid crystal molecules in the blue phase I and blue phase II cannot continuously occupy three-dimensional space and, therefore, need to be stabilized by a lattice of topological defects, known as disclinations.

[0036] The polymer-stabilized blue phase III mode devices of the present disclosure may exhibit blue phase III over a wide temperature range (e.g., greater than 60 K or greater than 80 K), enhanced dark state level (e.g., by an order of magnitude), and less hysteresis than polymer-stabilized blue phase I devices.

[0037] In some embodiments, the monomer and chiral dopant and their amounts are selected to have a chirality that approaches the length of a pitch (e.g., less than about 500 nm for cubic blue phases and less than about 200 nm for amorphous blue phase).

[0038] The polymer-stabilized blue phase III composites of the present disclosure can exhibit enhanced dark state and hysteresis-free performance relative to a polymer- stabilized blue phase I mode device. Without wishing to be bound by theory, it is believed that these benefits can be achieved due to the random structure of disclination line in blue phase III and no asymmetry in ascending and descending applied voltage when switching. The lattice structure standing in different orientation of multiple domains causes inconsistency in its electro-optical response. The periodicity of the structure in blue phase I implies selective Bragg diffraction, reflection, and/or scattering and the color appearance inhibits perfection of the dark state at a certain wavelength range when the electric field is free. The enhanced dark state of polymer-stabilized blue phase III was quantified as -1 order of magnitude than polymer-stabilized blue phase I and the reflectance of polymer-stabilized blue phase III demonstrates almost flat wavelength dependence with low level reflectivity.

[0039] The polymer-stabilized blue phase III composites of the present disclosure show significantly enhanced stability, hysteresis, and dark states properties. A cell fabricated in accordance with some embodiments of the present disclosure has been kept stable (for 2 months) and never went bad with -1 order of magnitude level of the dark state compared to polymer-stabilized blue phase I as well as almost no hysteresis. These features can be applied to fast-switching liquid crystal displays.

[0040] Typically, the temperature range of blue phases is only a few K between chiral nematic (N*) and isotropic (Iso) phases. In the case of blue phase III, texture generally cannot be observed due to the random structure that is optically isotropic.

[0041] Blue phases are optically isotropic states. An applied electric field can induce birefringence whose optic axis will be along the direction of the applied electric field. This Kerr effect can be described as Δη_ίηε_ΐ(Ε) = λΚΕ² under a constraint (Δη_ίηει(Ε)< An_sat(E_s)), with Kerr constant K, induced birefringence n_md (i.e. , birefringence induced by an applied electric field E), saturated (host liquid crystal's) birefringence An_sat, light wavelength λ, and an applied electric field E.

[0042] The normalized transmittance of polymer stabilized blue phase III may be given as

T ndAn_ind(V)

— = sin (2cp) sin ( )

1_Q A

where φ is the angle between the optic axis of induced birefringence and one of the crossed polarizers.

[0043] When the direction of the applied electric field is 45° to the crossed polarizers, the detected light intensity can be given as

. ndAn_ind

I = I₀ sin ( ) where d denotes the cell gap that light is traveling inside the cell. [0044] Hysteresis may be defined by the following equation

(Ease ^~ ^des)

H = 100% X wherein E_p denotes electric fields at peak intensity, E_asc denotes half of the peak during ascending electric field, and E_des denotes half of the peak during descending electric field.

[0045] The polymer molding in accordance with some embodiments of the present disclosure may position the location of disclinations in blue phase III, memorize the anchoring information at nano-scale, and reveal the difference in correlation length of different blue phases.

[0046] One may treat monomers in a polymer and liquid crystal composite system as a contaminant for the liquid crystalline phase behavior. Addition of monomers and photo- initiators disrupts the liquid crystal phase behavior and shifts the phase-transition temperatures. The concentration of each constituent is determined by the weight of constituent to the weight of liquid crystal in the composites. One of the most convenient ways to recognize liquid crystalline phases is texture identification using a polarizing optical microscopy.

[0047] The structure of blue phases strongly depends on the energetic favorability, mainly balancing between topological defects and elastic continuity in liquid crystals. As chirality increases and stabilizes for a double twist, it is impossible to fill out an entire 3D space while satisfying the double-twist formation. Assuming a right-handed twist persists and double-twist cylinders form a right-handed corner, singularity with the defect charge q = -1/2 cannot escape into 3D, which frustrates series of periodic cubic- type lattices with 3D disclinations in a narrow temperature range. The periodicity of cubic-type blue phases shows platelet or mosaic type textures from the Bragg scattering, and the appeared color is similar for a consistent chiral pitch.

[0048] Assuming the radius of disclination core R_c, one can explicitly describe the free energy per unit length of single disclination with defect theory:

where denoting a as a constant including Boltzmann constant, Τ_ί50 as transition temperature between blue phase and isotropic phase, T as temperature near Τ_ί50, σ as interfacial tension between isotropic core and elastic continuum, R_max as the cut-off radius of elastic continuum that is co-axial to the disclination, K as the simplified Frank elastic constant considering the splay K-n , twist K22 and bend K₃₃ distortion of the director between R_c and R_max, and K₂₄ is the saddle-splay elastic constant. The 1 st term describes the free energy cost maintaining the disclination core below Τ_ί50 at T. The 2nd term arises for an interfacial energy between the core and elastic continuum. The 3rd term is the elastic free energy of elastic continuum that is specified by the radial distance from R_c and R_max. The last term, only with negative sign, describes the interfacial elastic free energy of elastic continuum at R_c in face with disclination, which is against the other terms in F_di_Sc- Because blue phases include large interfacial area associated with disclination, so that the last term, sum of twist and saddle-splay elastic energy, cannot be neglected.

[0049] Considering the thermal equilibrium at blue phases, the non-mesogenic monomer rather exists in isotropic region due to the miscibility (entropy-driven phase separation). Thus, the polymerization of the non-mesogenic monomer mainly occurs in disclinations and RM may be polymerized closer to liquid crystals up to a certain local order parameter that still does not interfere with the existence of blue phases as Landau-de Gennes theory describes the gradual change in the local order parameter in blue phases. After stabilization, the free energy description of stabilized blue phases may need to be revised as: 1 ) the 1 st term is removed; 2) interfacial free energy in the 2nd term is replaced as between the elastic continuum and polymer mold; 3) the interfacial elastic free energy is maximized as the polymer is replaced at core so that R_c becomes R_p0i_y (R_p0i_y » R_c); 4) and added to the description of an interaction energy between polymer and elastic continuum that demonstrates anchoring of 3D morphology.

Fpoiy≡ 20poi_ynRpoiy + ¾ K\n (fl_max {p)/Rpoiy ) - π (K₂₂ + / ₂₄) + ¾lV0²(p), wherein R_p0i_y is the simplified radius of the polymer mold occupying disclinations, o_p0i_y is an interfacial tension between the confined elastic continuum and the surface of the polymer mold, and W_p and W_a are the anchoring strengths between confined elastic continuum and the polymer mold for polar and azimuthal angles, and Θ and φ are average deviation polar and azimuthal angles of director from the original 3D orientation at T. The formula is simplified, but describes phenomenological polymer stabilization of the disclinations in blue phases. The free energy after stabilization is reduced due to the absence of the 1 st term; the additional free energy cost is from the change in chiral pitch dependent upon temperature. The phase-stability may undermine where free energy becomes positive by increase in the 2nd and 4th terms.

[0050] Concerning the amount of monomer to polymer stabilizes the blue phases, a higher concentration demands a higher cost of free energy. The free energy calculation can somehow provide an estimation of the amount of monomer needed for stabilization or complete re-buildup of defects by polymer casting.

[0051] The polymer casting approach in accordance with some embodiments of the present disclosure enables wide exploration of frustrated phases [not limited to blue phases] to directly visualize the complexity of three-dimensional topology at nanoscale. The achromatic dark state of stabilized blue phase III enables a high contrast ratio and symmetric optical property from all viewing angles in response to external electric fields, thereby permitting the development of advanced electro-optical devices that have not been possible.

[0052] Also disclosed is a polymer scaffold that imitates the complex three- dimensional nanostructure of blue phase III by forming a reactive mesogen polymer network along topological defects in blue phase III. The polymer replica of blue phase III enables the first direct and conclusive morphological observation of the amorphous structure of blue phase III.

[0053] The nanostructured scaffold of blue phase III may be used as a reusable electro-optical device that retains the blue phase III ordering, chirality, optical isotropy, and sub-millisecond electro-optical switching characteristics (i.e., EO-memory) of the blue phase III even after removal of the cholesteric blue phase liquid crystal and subsequent refilling with different liquid crystals (e.g., nematic liquid crystals). Put another way, the polymer scaffold induces three-dimensional blue phase III ordering into nematic liquid crystals that cannot display blue phase III by themselves.

[0054] The EO-memory scaffolds may be formed by a process that includes forming a blue phase mixture, selecting a desired phase (e.g., blue phase I, blue phase II, blue phase III, or isotropic phase), stabilizing the phase by imprinting a polymer network (e.g., via photopolymerization of reactive mesogen monomers), and removing the liquid crystal molecules (e.g., by solvent extraction).

[0055] In some embodiments, blue phase III can be distinguished from the isotropic phase due to its optical activity which results in the observation of different colors (brownish and bluish-gray) when the analyzer is rotated slightly (e.g., 3-4°) to the left or right, that is, when the value of the deviation angle φ is positive or negative. This color difference and the bluish-gray color at negative values of φ may become more prominent in the polymer stabilized blue phase III, more so when the temperature of polymer-stabilized blue phase III is lowered closer towards room temperature. This color difference and bluish-gray color at negative values of φ can also be made more prominent and easily observable if the pitch of the cholesteric LC used in the blue phase mixture is increased by lowering the concentration of the chiral additive. The theoretical value of the pitch (P) can be estimated using the relation: P Pi nm \ = ¹⁰⁰⁰

; cxHTP

wherein c is the concentration of chiral additive HTP is the helical twisting power of the chiral additive. However, a longer pitch results in a longer response time.

[0056] The following examples are provided to illustrate the devices and methods of the present disclosure. The examples are merely illustrative and are not intended to limit the disclosure to the materials, conditions, or process parameters set forth therein.

EXAMPLES

[0057] Example 1

[0058] A blue phase liquid crystal mixture was sandwiched between glass substrates. Interdigitated electrodes were fabricated on the bottom substrate to properly form electric fields in plane for induced birefringence. Crossed polarizers were set to have one of their optic axes at 45° to the direction of the electric fields and the optic axis of induced birefringence. Polymerizations were performed by exposing ultraviolet (UV) light on prepared samples containing blue phase liquid crystal mixtures with blue phase III and blue phase I at T^* = -2.8 K and -5.3 K, respectively. The blue phases were stabilized and exhibited long-term stability for at least 2 months. An extended temperature range down to T^* -83.8 K was observed.

[0059] Example 2

[0060] The switching of polymer stabilized blue phase III was verified by macroscopic and microscopic switching images. While ramping up and down the voltage, enhanced stability was confirmed by the behavior of transmittance with respect to the applied voltage. An incident light of λ=633 nm was provided using a He-Ne laser. Almost no hysteresis existed for polymer stabilized blue phase III, unlike a comparative sample of blue phase I. Reflectance at the visible range wavelength and bluish color appeared by selective Bragg reflection for polymer stabilized blue phase I whereas polymer stabilized blue phase III implies a low level of the dark state over the entire visible wavelength range. Reflectance with respect to the wavelength was measured and compared for polymer stabilized blue phase III and polymer stabilized blue phase I. The Bragg peak was clearly observed in a range between about 410 and about 440 nm for blue phase I. The periodicity of the structure in blue phase I implied selective Bragg diffraction, reflection, or scattering, and the color appearance inhibited the perfection of the dark state at certain wavelength when the electric field was free. No such peak and a low level of reflectance over the entire visible wavelength range were observed for blue phase III. Because of the dark state whose level is less than that of polymer stabilized blue phase I (e.g., by -1 order of magnitude), polymer stabilized blue phase III can enhance electro-optic performance (e.g., contrast ratio, color generation, and viewing angle properties). The response time of electro-optic performance of the polymer stabilized blue phase III device was also found to be less than one millisecond. The measured response time is subject to switching between 10% and 90% of transmittance by applying 185 V, resulting in 0.04 ms for the rise and 0.27 ms for the decay.

[0061] Example 3

[0062] Analyses using confocal laser scanning microscopy showed that the polymer network imitated the aperiodic disclination-entangled structure of amorphous blue phase and entailed highly intertwined interaction with liquid crystal molecules. The stabilized amorphous blue phase manifested tens of microsecond response time, a consistent achromatic dark state, and intrinsically hysteresis-free during application of an electric field. The topological and electro-optical features of the stabilized amorphous blue phase were further compared with the stabilized isotropic and cubic blue phases.

[0063] Liquid crystals (HCCH, HTG135200-100, Δη=0.204 @ 589 nm, 20°C, Δε=+57.2 @ 1 kHz, 25°C) were mixed with a chirai dopant (HCCH, R501 1 , helical twisting power ~ 1 10 pm^~1 @25 °C) and reactive monomer composite consisting of a mesogenic diacrylate (Merck, RM257) and difunctional acrylate monomer (Aldrich, bisphenol A dimethacrylate). Prior to curing, the polymer and liquid crystal composite was characterized by mapping the phase diagram with respect to temperature, chirai fraction, and chirality. In order to accelerate photopolymerization, a small amount of photoinitiator (Aldrich, Irgacure 651 ) was added to the composite. The relative amounts (wt%) and photopolymerization temperatures for samples of blue phase III, Iso, and blue phase I are provided in the table below.

[0064] A glass substrate was deposited by indium-tin-oxide and patterned to have interdigitated electrodes to apply an in-plane electric field, and then assembled to a plane glass substrate using ultraviolet curable adhesive (Noland, NOA65), which was mixed with 10 m spherical spacers for a uniform cell gap. The cell, capillary-injected by polymer and liquid crystal composite, was loaded on to a hot stage with a calibrated temperature controller (Instec, Inc., precision of ±0.1 °C). The reactive monomers in the composite were photopolymerized at designated temperatures using collimated ultraviolet light (Uvata, spot UV LED) with irradiance of 0.8 mW/cm² at 365 nm while the phases were monitored by a polarizing optical microscope (Nikon Corp.). Photographs of the cells were taken with a digital camera. [0065] Photomicrographs were automatically acquired while temperature was ramped up and down for verification of the phase sequence. Cubic blue phases were examined by textures. Blue phase III was distinguished from isotropic phase by observing optical activity in blue phase III. The optical activity in blue phase III was confirmed by photomicrographs taken at a few degrees of positive and negative deviation angles between an analyzer and a polarizer. The reflection spectra were measured by a spectrometer (Ocean Optics, USB2000+) from a collimated tungsten halogen light source (Ocean Optics, LS-1 ) after photopolymerization.

[0066] Cells with the stabilized phases were placed between crossed polarized and maintained the directions of electric fields at 45° to the optic axes of the crossed polarizers. A photodiode detector was used to detect the transmitted light from laser light sources (He-Ne: λ=533 nm, Ar-ion: 514, 460 nm), while an electric field was applied to the interdigitated electrode in the cells. Both input and output signals were controlled by homemade software via a terminal block (National Instruments, Corp., BNC-2090).

[0067] In order to acquire the images with high magnification, a short wavelength laser light (λ=408 nm) was equipped in a confocal laser scanning microscope (Olympus, LEXT OLS3100) with minimized optical aberrations. For preparation of the samples, a thin top glass substrates (< 100 pm) was assembled onto the substrate with the interdigitated electrodes to satisfy the working distance of an objective lens with magnification.

[0068] Photomicrographs before photopolymerization were taken for blue phase III at 83 °C, isotropic phase at 87 °C, and blue phase I at 82.5 °C. At deviation angles between an analyzer and a polarizer β= ±3°, the color or images became bluish and yellowish for negative and positive β, owing to the optical activity in blue phase III which is distinguished from the isotropic phase. The color difference at the deviation angles in blue phase III was caused due to optical activity. The optic axis of plane-polarized light after the polarizer became rotated when traveling through the blue phase III layer, so that the optic axis of polarization deviates from the optic axis of the analyzer. The deviation at positive and negative angles contributes differently to corresponding wavelength ranges. [0069] After polymer-stabilization, the stabilized phases were well maintained over 80 °C (the range of the stabilized blue phase III was from below 0 °C to 87 °C). The photos of the cells and the micrographs showed similar dark states for blue phase III and the isotropic phase, whereas blue phase I exhibited light leakage corresponding to Bragg diffraction. The reflection spectra of the stabilized blue phase III and isotropic phase revealed similar behaviors but slightly higher reflectance was observed for the stabilized blue phase III than that of the isotropic phase near short-wavelength range, whereas a peak or seemingly overlapped peaks were measured in the stabilized blue phase I.

[0070] The blue phase III cell was switched from the dark state to the bright state and back to the dark state. The corresponding photomicrographs of the switching behavior showed excellent optical contrast between the field-off and field-on states. The on-state photomicrograph showed a large field-induced birefringence in between the electrodes and the stripes indicating that no electric field contributed to the induced birefringence on the center of the electrodes. The electro-optical behaviors of the stabilized cells were investigated with transmittance versus applied electric fields curves with red (e.g., 633 nm), green (e.g., 514 nm), and blue (e.g., 460 nm) light sources for blue phase III, isotropic phase, and blue phase I. The measured hysteresis H with RGB light sources was minimum for the stabilized blue phase III (R: 0.5%, G: 0.5%, B: 2.2%) but significant for the stabilized isotropic phase (R: 6.0%, G: 8.8%, B: 1 1 .3%) and relatively less significant for the stabilized blue phase I (R: 5.9%, G: 3.7%, B: 4.2%).

[0071] Photomicrographs were taken before, shortly after, and several tens of minutes after applying an electric field. The stabilized blue phase III showed consistent dark state before and after applying an electric field. After several tens of minutes, the residual light leakage still remained for the stabilized isotropic phase but disappeared for blue phase I. Without wishing to be bound by theory, it is believed that the electrostriction effect in the stabilized blue phase I responds for several minutes and the switching mechanism of the stabilized isotropic phase is apparently distinguished form the stabilized blue phase III.

[0072] To identify the differences in the electro-optical switching in detail, direct observation and thorough investigation of topology at a focal plane near the electrodes in stabilized cells was done by confocal laser scanning microscopy (CLSM). The CLSM images manifest stronger reflection on the electrodes. The stabilized blue phase III reveals consistent isotropy in topology before and after the applied electric field, while it showed spatially different structural deformation between electrodes under the applied electric field (E=18.5 V/pm). The deformation was most severe upon the edge of the electrode due to the stabilized blue phase III and isotropic phase, the reflected intensity in short-range was quite non-uniform in blue phase III, unlike in the isotropic phase. In blue phase III, reactive monomers are more miscible in disclinations. The polymer network actually imitates the disclinations of blue phase III. Consequently, polymerization occurs around disclinations in blue phase III. In isotropic phase, the short-range reflection is relatively uniform although the overall topology is similar to that of the stabilized blue phase III. During photopolymerization, phase separation occurs in the isotropic phase. The entropy-driven randomness guides the polymer network growing bulky, and liquid crystals are confined in a pore-type space such as the polymer-dispersed liquid crystal. In the center region of the pore, liquid crystal molecules are not anchored strongly enough to memorize their original orientations due to relatively low anchoring strength. The analysis supports the idea that residual light leakage is generated in the isotropic phase. In the stabilized blue phase I, the topology revealed grains with boundaries and each grain has a periodic lattice structure. The periodic lattices were deformed under the applied electric field (i.e., electrostriction effect took place). After applying the electric field, the topological deformation in the periodic undulations remained.

[0073] Electro-optical response times of the stabilized cells were measured. Both rise and decay times were measured between 10% and 90% transmittance changes. In the stabilized blue phase III, the response time was less than 100 s for both rise and decay. In the case of the stabilized isotropic phase, the low anchoring strength gave rise to slow decay time with respect to the interaction between liquid crystal molecules and the polymer network. Although the response time of blue phase I was still in the sub-millisecond range, decay time was much slower than that of blue phase III at every RGD light source. Several properties of the stabilized phases are provided in the Table below.

Phase (saturation (hysteresis (RGB

electric field) at average

wavelength decay time)

of 514 nm)

Blue phase -18.5 0.49 0.07 > 83.0 Isotropic III

Isotropic -1 1 .0 8.78 5.70 > 87.0 Isotropic phase

Blue phase 1 -17.0 3.74 0.61 > 82.5 Grains with boundaries with periodic undulations

[0074] Under a polarizing microscope, the isotropic phase showed no color due to the symmetry. Cubic blue phases also implied an optically isotropic state owing to the cubic symmetry, but colorful platelet and/or mosaic type textures were observed because of Bragg reflection. The symmetry of blue phase III was similar to that of the isotropic phase and had neither long-range order nor structural periodicity. In the CLSM images of the stabilized blue phase III and isotropic phase observed at the focal plane, the different localized reflection may result from surface roughness and may imply a structural and topological difference between the stabilized blue phase III and the isotropic phase.

[0075] The degree of hysteresis upon the applied electric fields in the stabilized blue phase III and isotropic phase tends to be higher at short wavelengths, which is consistent with the Kerr effect. However, the opposite tendency was found for the stabilized blue phase I. Without wishing to be bound by theory, it is believed that the electrostriction effect and/or complex topology contribute to the electric switching as well. As the lattice is elongated along the electric field, which is perpendicular to the direction of light traveling, the wavelength range corresponding to transmittance shifts to long wavelength range.

[0076] The saturation electric field of the stabilized blue phase III was approximately 1 .68 and 1 .09 times greater than that of the stabilized isotropic phase and blue phase I, respectively. However, the stabilized blue phase III achieved excellent achromatic dark state, 17.9 and 7.6 times less hysteresis at a wavelength of 514 nm, and 81.4 and 8.7 times faster response time than that of the stabilized isotropic phase and blue phase I, respectively.

[0077] Example 4

[0078] In order to thoroughly investigate the phase behavior and polymer mold fabrication in blue phases by photo-polymerization, 8 samples were prepared by varying three variables: the concentration of reactive-mesogenic diacrylate monomer (RM), chiral dopant, and temperature of photo-polymerization. FIG. 1a shows the phase diagrams for variations of RM concentration with respect to the chiral dopant concentration. Because of mesogenic characteristics of RM, the similar chiral dopant concentration exhibited different chirality depending on the amount of RM, which makes the entire diagram shift along the axis of chiral fraction. For example, blue phase III appears at a relatively lower chiral fraction as well as at a smaller amount of RM in the composite as shown in FIG. 1a. Here, chirality is denoted by κ and relative chirality is denoted by κ' without and with counting RM for mesogenic contribution, respectively.

[0079] As a consequence of increase in temperature, blue phase III begins to appear at temperature ranges of 353.7 - 356.8 K, 348.9 - 352.5 K, and 343.2 - 348.8 K for the sample 2, 3 and 4, respectively. And those chiral dopant concentrations of three samples, by counting the amount of RM as a mesogen can be transferred to reveal ~ 4.5 wt%, that gives rise to κ' approximately 5.1 pm^~1. In such a high chirality, the periodicity of cubic-type blue phases turning into a 3D aperiodic structure as temperature increases, i.e., randomly entangled disclinations of blue phase III.

[0080] The liquid crystal material (HTG 135200-100, HCCH) was mixed with a chiral dopant (R501 1 , HCCH), and reactive monomers mixture consisting of a mesogenic diacrylate monomer (RM257, Merck) and Bisphenol A dimethacylate (Aldrich) to verify the phase sequence for the phase diagram in various concentrations. To accelerate photo-polymerization, a small quantity of photo-initiator, 2,2-Dimethoxy-1 ,2- diphenylethan-1 -one (Irgacure 651 , Aldrich) was mixed in the reactive monomer and liquid crystal solution. [0081] Cells for phase identification were prepared from two glass-substrates and assembled using ultraviolet curable adhesive (NOA65, Noland), which was mixed with 10 mm spherical spacers for maintaining a uniform gap between substrates. With no surface treatment layer, plane indium tin oxide coated glasses were used. After fabricating the cells (cell gap d = 10.2 mm), polymer and liquid crystal composite was capillary-injected into the cells above isotropic transition temperature.

[0082] A polarizing optical microscope (Nikon) was used to acquire polarizing optical micrographs for observing textures of chiral nematic phase and blue phases with a hot stage and calibrated temperature controller (Instec) in an accuracy of ± 0.1 K. The optical rotatory power was measured by spatial mean brightness (gray-scale) of polarizing optical micrographs at various deviation angles of analyzer a with a halogen lamp as a light source. The spatial mean brightness was batch-measured by open software (Image J).

[0083] The reflection spectrum was measured by a spectrometer (USB2000+, Ocean Optics) with a halogen lamp as the light source. The spectrometer is calibrated with standard bright (mirror) and dark (velvet cloth) states.

[0084] The samples were exposed to ultraviolet light (spot UV LED, Uvata) with irradiance of 0.8 mW/cm², 365 nm at designated phases. The dosage of ultraviolet exposure was carefully retained at sufficiently low intensity as not to disturb or destroy the equilibrium state while observing the textures by a polarizing optical microscope in situ.

[0085] The stabilized samples were submerged in a solution (Hexane:Dichloromethane = 70:30). An achiral (nematic) liquid crystal was injected into the polymer mold driven by capillary reaction in a vacuum chamber.

[0086] A thin top glass substrate (~ 100 pm) was assembled to refer to the distance between an objective lens to blue phase layer for the observation by a confocal laser scanning microscope (LEXT OLS3100, Olympus).

[0087] After washout the liquid crystal mixture, cells were carefully disassembled. The remaining polymer molds of samples on one of the substrates (ultraviolet irradiation faced-side) were deposited by gold particles to observe the polymer mold by a scanning electron microscope (Quanta 450, FEI). [0088] After evacuating chiral liquid crystals, the scattering experiment uses a collimated halogen light to illuminate the polymer molds, which are sandwiched by glass substrates.

[0089] A summary table including information about the chemical compositions, photo- polymerization temperatures, and extended temperature ranges of the stabilized phases for 8 samples is provided below.

[0090] The weight percentages refer to the weight of each constituent to the weight of the liquid crystal. T_photo and T_ext denote photo-polymerization temperature and extended temperature range of the stabilized phase, respectively. For κ, chirality was estimated by assuming a helical twisting power of 1 15 pm^"1 for R501 1 and 50 pm^"1 for ISO- (60BA)₂ at fixed temperature without counting RM257 as a mesogen. Samples 1 and 8 were prepared using chiral dopant ISO-(60BA)₂.

[0091 ] In the blue phase III sample, it was hard to observe and recognize the phase using a polarizing optical microscopy due to such a perfect dark state showing no birefringence and minimal light scattering. However, because of high chirality, blue phase III reveals optical activity that gives rise to the rotation the optic axis of a plane polarized light. The description of optical activity is appropriately represented by the local order parameter with anisotropic part of dielectric tensor ε¾ as a linear combination of five independent structural modes m: planar, conical spiral and non-chiral modes (m = ±2, ±1 and 0, respectively). In the limit of long wavelength λ (wave vector of light k « K, i.e., chiral pitch p « λ), the optical activity can be derived as

where

Α - (ει² - ε²ι )

and

Α ^ [₃(ε! - ε-² ₂) + (ε^ - ε-²,)]

with constant a and b. The 1 st term in equation is associated with the mode m = ±1 , whereas the 2nd term is in the opposite sign of the 1 st term, which contributes to both modes m = ±1 and ±2. The two modes are in competition, which leads to the sign and magnitude of optical rotatory power. In FIG. 1 b, polarizing optical micrographs of blue phase III and isotropic phase are taken under the deviated crossed polarizers with a deviation angle a = 3° between an analyzer and the orthogonal direction to the polarizer. The different colors of blue phase III at positive and negative a demonstrated that blue phase III layer rotates polarization, thus the direction of polarization deviates from that of an analyzer. FIG. 1c shows the intensity variation as a function of a. Here, the sign of a at the minimum intensity, where the slope of curves is zero, is positive for blue phase III in opposition to isotropic phase, which manifests the appearance of blue phase III below the isotropic phase.

[0092] Sample 2 at the temperature of blue phase III demonstrated the achromatic dark state with no textures at a = 0° (FIG. 1b), and after careful photo-polymerization using exposure of ultraviolet (Methods), the temperature range extended > 80 K (FIG. 2a). In FIG. 2b and 2c, reflectance of sample 2 demonstrated the difference in phase- behavior of before and after photo-polymerization. The selective reflectance peak shows the temperature range of blue phase I, and the inset in FIG. 2b specifies the peak range approximately from 424 to 456 nm. However, the reflectance of blue phase III demonstrates no verification even though the reflectance after photo-polymerization shows disappearance of selective peaks in the phase-behavior as shown in FIG. 2c.

[0093] FIG. 2d and 2e compare the optical rotary power before and after photo- polymerization, respectively. The magnitude of optical rotatory power jumps at the transition from blue phase III to blue phase I due to the transition in the structure, but gradually changes between isotropic phase and blue phase III. In FIG. 2d, the magnitude of optical rotatory power in negative sign becomes maximum near the transition region between isotropic phase and blue phase III because the mode m = ±2 becomes necessary to consider in addition to the mode m = ±1 . In this region, the transition is associated with the coexistence of isotropic phase and blue phase III. However, this phenomenon is not observed for the stabilized blue phase III seemingly because of the interaction between the polymer mold and liquid crystal molecules (FIG. 2e). The optical rotatory power of stabilized blue phase III shows almost linear dependence on temperature below the abrupt transition from isotropic phase. The slopes of linear fit for both before and after stabilization are approximately -4.07 and - 2.13, respectively, which implies that the optical rotatory power after stabilization is in association of not only liquid crystals but also the coexistence of a polymer mold. The behavior of optical rotation below the temperature range of blue phase III may be in different regimes with the polymer mold because of the interaction of liquid crystals and polymer, i.e., the dependence of chiral pitch with respect to temperature can be interfered by the interaction.

[0094] Because the magnitude of the optical rotatory power for stabilized blue phase III becomes high as chiral pitch reduces by decrease in temperature, the light leakage caused bluish color appearance while keeping a = 0° as shown in FIG. 2a. The achromatic dark state was maintained at a for minimum brightness.

[0095] CLSM can provide topological surface information rather than polarizing optical information. The CLSM observation gives direct imaging of the surface of stabilized sample 2, 5 and 6 for blue phase III, isotropic phase and blue phase I as shown in FIG. 2f, FIG. 2g, and FIG. 2h, respectively, blue phase I shows mosaic texture at room temperature. Depending on the orientation of lattice sitting on a substrate, the periodicity of blue phase I is clearly visualized at some domains in FIG. 2h. Even though blue phase III is amorphous and random structure, the surface of blue phase III shows localized tendency of periodic undulations from chirality as shown in insets of FIG. 2f. Further observations of CLSM images are shown in FIG. 3a-c for polymer mold after evacuating chiral liquid crystal mixture, and in FIG. 4g-i for the surface of stabilized phases after refilling achiral liquid crystals.

[0096] During photo-polymerization, the amount of monomers needs to increase for proper stabilization of the phase. However, sample 1 showed no appearance of blue phases even at 3 wt% monomer concentration, which gives rise to the need of RM for high concentration of monomers. Samples 3 and 4 demonstrated that about 10 - 20 wt% monomer concentrations were not enough to stabilize blue phase III. Consequently, the contributions of RM being polymerized near disclinations may be limited due to almost equal preference of RM to being not only in isotropic disclination core but also in the elastic medium.

[0097] For sample 7, blue phase I is stabilized with about 10 wt% monomer concentration, but is unlikely to stabilize the blue phase III. This result implies that the amount of RM polymerized within the distance R_p0i_y_BPi is smaller than that within Rpoiy_BPiii - In order to prove the implication, stabilized samples 2, 5 and 6 with high concentration of polymer were thoroughly washed out to evacuate chiral liquid crystal mixture and leave the polymer mold framework. The surfaces of each sample are observed by CLSM as shown in FIG. 3a-c (1 st, 2nd and 3rd columns represent blue phase III, isotropic phase and blue phase I, respectively), and detailed morphology of blue phase III polymer molds are observed by SEM as shown in FIG. 3d. The SEM images reveal that polymer molds trace the disclinations of blue phase III and blue phase I as shown in FIG. 3d. For isotropic phase, the polymer mold was bulky with closed pores due to the polymerization at isotropic phase. The correlation length of polymer molds, which implies the average length of the area that liquid crystals occupy in the stabilized system, is measured and it demonstrates different density of disclinations in blue phase III and blue phase I. In the SEM images, measurements of correlation length D_p0i_y in blue phase III and blue phase I turn out to be D_p0iy_BPiii = 40.1 ± 15.3 nm and D_p0iy_BPi = 81 .5 ± 36.1 nm. The shorter correlation length in blue phase III implies that the shorter pitch in blue phase III gives rise to the amorphous entanglement of disclinations, which ends up with melting into the isotropic phase. Based on the observation of visualized disclinations, the lattice in blue phase I seems to locally twist and squeeze so that the series of singularity, i.e., disclination line may be curved and entangled, as it is in a melted state. The SEM images of the polymer molds successfully visualize the 3D entanglement of disclinations in blue phase III, and the visualized result of SEM images are in good agreement with the visualization of blue phase III by a theoretical model.

[0098] In addition, polarizing optical micrographs and light scattering properties of the polymer molds are observed as shown in FIG. 3e,g,i and FIG. 3f,h,j, respectively. Based on averaged dimensionless size parameter x ~ (D_p0iyjso + D_p0iy_BPiii) / (2λ) ~ 0.44, assuming λ = 550 nm, the light scattering in the Rayleigh regime may be considered. The scattering property for the polymer mold of isotopic phase shows the diffused ring pattern with reddish color, the polymer mold of blue phase III displays similar ring pattern but less intensity with bluish color, and the polymer mold of blue phase I shows almost no scattering as shown in FIG. 3h, FIG. 3f, and FIG. 3j, respectively. The size of polymer mold is highly associated with scattering intensity, which is proportional to the λ^"4 in the Rayleigh regime.

[0099] The comparison of blue phase III (FIG. 4a-c) and isotropic phase (FIG. 4d-f) demonstrates that the role of each polymer mold interacting with refilled achiral liquid crystals. The polarizing optical micrograph of refilled blue phase III (FIG. 4b) shows similar colors, at a = ±3°, compared to that of the original stabilized blue phase III (FIG. 4a). Without wishing to be bound by theory, it is believed that the increased brightness is due to the higher magnitude of optical rotatory power as one may compare in FIG. 2e and FIG. 4c. The behaviors of optical rotation for the original stabilized blue phase III and refilled phase are similar to the slopes of optical rotatory power approximately 2.13 and 2.36 even if the isotropic transition temperature of the refilled phase is higher than the original (FIG. 4c) due to the achirality of the refilled liquid crystals. This result validates that the polymer mold is capable of inducing chirality for achiral liquid crystals by imitating 3D entangled disclinations in blue phase III.

[0100] In order to support evidence of the role of polymer molds, similar experimental steps were addressed with the stabilization of isotropic phase and refilling achiral liquid crystals (FIG. 4d-f). Although the photo-polymerization performed at isotropic phase (sample 5), the images at room temperature demonstrate similar color behavior as stabilized blue phase III, which leads to optical activity in stabilized isotropic phase. That is presumably because of optical activity from the structural level instead of molecular level. However, after carefully evacuating chiral liquid crystal mixture (FIG. 4d) and refilling achiral liquid crystals (FIG. 4e) into the cell, no linear increase in the optical rotation with respect to temperature is observed as shown in FIG. 4f. The results strongly support that, unlike the isotropic phase polymer mold, the blue phase III polymer mold can be used as an isotropic-like chirality-induction agent.

[0101] CLSM images after refilling achiral liquid crystals show the surface of rebuilt phases in FIG. 4g,h,i for blue phase III (sample 2), isotropic phase (sample 5) and blue phase I (sample 6), respectively. In FIG. 4g, no local undulation is observed unlike in FIG. 4f due to achirality of host liquid crystals. FIG. 4i shows the domain of blue phase I with the texture as well as in FIG. 2h.

[0102] Example 5

[0103] A blue phase mixture was formed. The mixture contained 4.41 wt% of a chiral additive (R501 1 , Merck), 19.88 wt% of a mesogenic diacrylate monomer (RM257, Merck), 2.09 wt% of a difunctional acrylate monomer (bisphenol A dimethacrylate, Sigma-Aldrich), 0.08 wt% of a photoinitiator (Irgacure 651 , Sigma-Aldrich), and 73.54 wt% of a nematic mixture (HTG135200-100, HCCH).

[0104] In-plane switching cells were made by depositing indium-tin-oxide on a glass substrate in an interdigitated electrode (indium tin oxide) pattern. The patterning was performed using photolithography in a cleanroom environment. The patterned substrate was assembled onto a plain glass substrate using UV-curable adhesive (NOA 68, Norland) mixed with 5 pm spherical spacers for a uniform cell gap.

[0105] Three in-plane switching cells including two parallel glass substrates were capillary-injected with the blue phase mixture. For observation of the phase transition sequence from the cholesteric to blue phase I, blue phase III, and finally isotropic phase, a polarizing optical microscope (POM) (Nikon) was used as the temperature of the cells was ramped at a rate of 0.2 °C/min using a temperature-controlled stage (Instec, Inc.). The cells were exposed to UV light from a collimated UV light emitting diode (Uvata Precision Optoelectronics) with an intensity of 1 .5 mW/cm² for 30 minutes at 80.4 °C for polymer-stabilized blue phase I, 82.6 °C for polymer-stabilized blue phase III, and 91 °C for polymer-stabilized isotropic phase in order to polymerize the reactive mesogens. The cells were immersed in a 70:30 v/v solution of hexane and dichloromethane for 48 hours to remove the cholesteric liquid crystal. The solvent was refreshed after 24 hours.

[0106] Upon exposure to the UV light, the mesogenic monomers in the blue phase III mixture polymerized along entangled disclinations, thereby tracing and memorizing the structural information of blue phase III at the nanoscale. The bulkier, matrix-pore type morphology of the scaffold based on the isotropic phase indicated a distinctly less uniform structure with a lower short-range order than blue phase III. This structure is similar to that of polymer dispersed liquid crystals (PDLC), where liquid crystal molecules are confined in several pore-like regions in a polymer matrix. The CLSM image of the scaffold derived from blue phase I made it apparent that the polymer network imitated the periodic arrangement of blue phase I disclinations.

[0107] The scaffolds were refilled by capillary injecting a nematic mixture (HTG135200-100, HCCH), which was washed out using the above procedure and later refilled with 4-cyano-4'-pentylbiphenyl (5CB, HCCH). During refilling, the cells were kept at an elevated temperature (100 °C for refilling HTG and 50 °C for refilling 5CB) on a hot stage overnight. After refilling, the cells were kept at room temperature for at least 24 hours before characterization.

[0108] The reflection spectra of the original stabilized phases and the refilled scaffolds were measured using a spectrometer (USB2000+, Ocean Optics) with a tungsten- halogen light source (LS-1 , Ocean Optics). All measurements were performed at 20 °C.

[0109] Cells were placed between crossed polarizers (a fixed polarizer and a freely rotating analyzer) such that the direction of the applied electric field was maintained at an angle of 45° to the optical axes of the polarizers. A photodiode detector and a He- Ne laser light source (λ=633 nm) was used to measure the transmitted light intensity through the cells when an AC electric field was applied. The transmitted light intensity was recorded as a function of applied electric field (frequency = 1 kHz) by ramping the filed from 0 V/pm to 40 V/pm. To measure the switch-on and switch-off times, the cells were driven by an AC electric field (rectangular pulses) with amplitude 40 V/pm and frequency 1 kHz. The transmitted light intensity was recorded as a function of time during the switch-on and switch-off processes and the characteristic response time was defined as the time taken to switch between 10% and 90% of the maximum transmittance. All measurements were performed at 20 °C. Photographic images of cells in the field-off and field-on state were taken using the digital camera of a mobile phone, while the cells were placed between crossed polarizers against a white backlight. Photomicrographs of the cells in the field-off and field-on state were taken using a polarizing optical microscope (Nikon).

[0110] blue phase I was easy to identify due to its characteristic blue platelet texture and selective Bragg reflection. On the other hand, the amorphous structure of blue phase III exhibited an optically isotropic symmetry with no Bragg reflection in the visible spectrum. This means that no birefringent textures can be observed in POM and it is difficult to distinguish blue phase III from the isotropic phase between crossed polarizers. Blue phases are optically active and rotate the optic axis of linearly polarized light as it passes through them. This attribute of blue phases allows blue phase III to be distinguished from the isotropic phase. When the deviation angle (φ) between the transmission axis of the polarizer and the direction orthogonal to the transmission axis of the analyzer is 0°, that is, between crossed polarizers, polymer-stabilized blue phase III shows a perfect dark state with no birefringence and appears identical to the isotropic phase. When φ=±4°, the color of the polymer-stabilized blue phase III texture appears brownish and bluish-gray (foggy blue) at positive and negative values of φ, respectively. This is because of the rotation of the optic axis of linearly polarized light by polymer- stabilized blue phase III, which results in an asymmetric contribution to the optic axis of the analyzer. The deviation at positive and negative values of φ contributes differently to corresponding wavelength ranges. The polymer-stabilized isotropic phase, however, lacks optical activity and hence, shows identically colored brownish textures at positive and negative values of φ.

[0111] EO-memory scaffolds nano-engineered from blue phase III, the isotropic phase, and blue phase I were investigated using short wavelength laser light (λ = 408 nm) in a confocal laser scanning microscope (Olympus, LEXT OLS3100).

[0112] The glass substrates of the cells containing the scaffolds nano-engineered from blue phase III and the isotropic phase were carefully disassembled after immersing in liquid nitrogen to ensure minimum damage to the structure of the scaffold. Gold nanoparticles were sputtered on the scaffolds to increase conductivity. An FEI Quanta 450 scanning electron microscope was used for morphology investigations.

[0113] The optical rotary power was measured by the spatial mean brightness (a gray intensity in pixels) of POM images. The spatial mean brightness was batch measured using the Jay-based image processing program ImageJ.

[0114] For demonstrating double-twisted helical self-assembly in blue phases, a blue phase liquid crystal film was prepared on a copper grid using a mixture containing 55 wt% of a nematic liquid crystal (LC BL 006, Merck) and 45 wt% of a chiral dopant (R81 1 , Merck). A FEI Tecnai F20 transmission electron microscope was used for imaging the film as it cooled from the isotropic phase.

[0115] FIG. 5a-c are confocal laser scanning microscope images of scaffolds derived from blue phase III, the isotropic phase, and blue phase I. Scale bars are 15 pm for the complete images and 3 pm for the enlarged regions. FIG. 5d and e are scanning electron microscope images of scaffolds derived from blue phase III and the isotropic phase. Scale bars are 500 nm. Due to the brittle nature of the blue phase I scaffold, it is difficult to perform SEM imaging without damaging the sample. Thus, only CLSM was used.

[0116] The cholesteric liquid crystals were washed out to obtain the porous polymer scaffolds. The scaffolds were refilled with HTG without chiral dopant. The HTG nematic mixture had a clearing temperature of 97 °C, is achiral, and does not form blue phases by itself. The HTG-refilled scaffolds were subjected to the same POM and EO characterization as the original stabilized phases. To confirm that the EO-memory scaffolds are reusable, they were again washed out and refilled with a different liquid crystal, 4-cyano-4'-pentylbuphenyl (5CB), followed by the same POM and EO characterization as before.

[0117] POM and spectral investigations revealed that once an achiral nematic LC is inside a scaffold, it shows a texture and reflectance spectrum that is very similar to the original stabilized phase from which the scaffold is derived. For example, when HTG and 5CB were injected in a scaffold derived from blue phase III, both LCs show polymer-stabilized blue phase Ill-like brownish and bluish-gray textures at positive and negative deviation angles respectively. This asymmetric contribution by refilled scaffolds to the optic axis of the analyzer suggests that the polymer nanostructure of the blue phase III scaffold memorizes the chiral nature of the local molecular arrangement of the original polymer-stabilized blue phase III and induces optical activity in refilled achiral nematic LCs. A comparison of the the optical rotatory power (ORP) of the HTG-refilled scaffold and original stabilized phase of blue phase III at various temperatures supported this interpretation. The slopes of the linear fits of the ORP of the HTG-refilled blue phase III scaffold (m = ~-2.36) and originalized polymer-stabilized blue phase III (m = ~-2.13) were very similar. These results strongly suggested that polymer scaffolds nano-engineered from blue phase III may also be as chirality-induction agents. The refilled blue phase Ill-mimicking scaffolds displayed a lack of Bragg reflection peaks in the visible spectrum and hence, an achromatic dark state similar to the original polymer- stabilized blue phase III. This characteristic is particularly useful in display applications, where the device should be clear in the visible spectral region. When nematic LCs were refilled in scaffolds derived from the isotropic phase, they also display textures similar to the original polymer-stabilized isotropic phase, that is, perfectly dark between crossed polarizers and brownish at both positive and negative deviation angles, with no indication of optical activity. This is verified by the zero ORP values of the refilled scaffolds measured at various temperatures. When nematic LCs were refilled in the scaffold derived from blue phase I, they exhibited textures and reflection spectra that were almost identical to the original polymer-stabilized blue phase I. Essentially, the scaffolds can induce blue phase III, blue phase I or the isotropic phase into LCs that are inherently achiral nematics.

[0118] EO merit parameters, such as the driving electric field and switch-on (τ_οη) and switch-off (T₀ff) response times of the original stabilized phases, were compared with those of the EO-memory scaffolds after refilling with HTG and 5CB. The transmitted light intensity of cells was plotted as a function of the applied electric field to estimate the driving electric field of the original stabilized phases and refilled scaffolds. The EO response times for the rise (switch-on) and decay (switch-off) processes were determined by using a photodiode detector to measure the electric field-induced change in transmitted light intensity through cells placed between crossed polarizers. The response time is defined as the time taken by the cell to switch between 10% and 90 % of the maximum transmittance. Photographic images and POM images of the cells were taken in the field-off and field-on states for visual observation of EO switching. The EO behavior of the original stabilized phases and the refilled scaffolds follows the Kerr effect, which is a non-linear, second-order EO effect wherein, an applied electric field induces a birefringence in a medium that is optically isotropic in zero electric field. The driving electric field was ~40 V/pm for all original phases and their respective refilled scaffolds. The transmittance-electric field curves of the original polymer-stabilized blue phase III and its corresponding scaffold after refilling with HTG and 5CB are almost overlapping, underlining the EO-memory effect of the scaffolds. Both switch-on and switch-off times of polymer-stabilized blue phase III, polymer-stabilized isotropic phase and polymer-stabilized blue phase I are shorter than 0.2 ms and remain in the sub- millisecond range (τ_οη < 0.1 ms, T₀ff < 0.5 ms) after refilling the corresponding scaffolds with HTG and 5CB. Thus, in addition to the overlapping transmittance-electric field curves, the comparable sub-millisecond switching times of the original stabilized phases and their respective refilled scaffolds successfully demonstrate the EO-memory effect of the scaffolds.

[0119] The switch-on times of the refilled scaffolds were much shorter than those of the original stabilized phases from which they are nano-engineered and follow the order: T_on(5CB-refilled scaffold) < T_on(HTG-refilled scaffold) < T_on(original stabilized phase). In particular, the 5CB-refilled scaffolds show switch-on times of only a few tens of microseconds, which is shorter than the response times of previously reported in- plane switching-driven polymer-stabilized blue phases (~0.5 ms) and blue phase- templated nematics (~0.1 ms). The res onse time is given by

wherein λι is the rotational viscosity of the system, P is the pitch length of the cholesteric liquid crystal, and k is the elastic constant. The rotational viscosity is dependent upon the viscosity of the host LC, which in this case was HTG mixed with chiral dopant for the original stabilized phases and either HTG or 5CB without chiral dopant for the refilled scaffolds. HTG is a high dielectric anisotropy (Δε = +57.2) nematic mixture including liquid crystals with several bulky fluorinated functional groups. As a result, the viscosity of HTG is relatively high (0.183 Pa s). The viscosity of HTG in the original stabilized phases is even higher since it is mixed with a chiral dopant. The viscosity of 5CB (0.082 Pa s) is an order of magnitude lower than that of HTG, since 5CB is a smaller molecule with no bulky, polar groups. While the balance of several properties is required for optimum EO performance, a low viscosity LC is usually favorable from the response time viewpoint. As a result, the shortest switch-on times are achieved in scaffolds containing the lowest viscosity LC.

[0120] For the blue phase mixture used in this study, the theoretical pitch estimated using is ~204 nm. For such low pitch blue phase mixtures in the unpolymerized state, it may be difficult for the untrained eye to quickly discern the difference in colors of blue phase III at positive and negative deviation angles, since they appear very much alike (brownish for positive values of φ and brownish-orange for negative values of φ). Hence, the easiest way to identify blue phase III in the unpolymerized state is to identify the exact blue phase lll-isotropic transition temperature. Once this transition temperature is accurately determined, one can safely assume (for the blue phase mixture) that the system will be in the blue phase III phase for at least a 1 -1 .5 °C below the identified blue phase lll-isotropic transition temperature. The mixture can then be photopolymerized at a temperature that is about 1 °C below the identified blue phase lll- isotropic transition temperature.

[0121] In some embodiments, as the temperature increases, the blue phase I phase completely disappears at 82.6 °C giving a texture that appears brownish-orange at φ=- 4° and perfectly dark between crossed polarizers. A first-time observer may incorrectly identify this as the isotropic phase. In fact, this is the blue phase III phase. As the temperature increases further, a disturbance appears in the texture at 83.6 °C. This disturbance can be described as whitish ripples or flashes (for lack of a better word) against the brownish-orange background. These ripples/flashes indicate the blue phase III to isotropic transition. The ripples/flashes grow till 84.6 °C and then start receding. They disappear at 86.8 °C, indicating a complete transition from the blue phase III to the isotropic phase. Thus, from this study, one can conclude that at 82.6 °C, the system is in the blue phase III phase and starts transitioning to the isotropic phase at 83.6 °C. Hence, to obtain polymer-stabilized blue phase III, the chosen photopolymerization temperature is 82.6 °C.

[0122] For visual observation of EO switching in the original stabilized phases and refilled scaffolds, photographic images of cells in the field-off and field-on state were taken using the digital camera of a mobile phone, with the cells placed between crossed polarizers against a white backlight. The applied in-plane electric field had a frequency of 1 KHz and an amplitude 40 V/pm. The switching was repeatable. This effectively illustrated the "re-use" potential of the EO-memory scaffolds, which remain switchable even after washing out and after refilling with two different liquid crystals and repeatedly switching the cells several times. The blue phase I systems show a characteristic bluish field-off state (dark state), while the polymer-stabilized blue phase III and isotropic systems show an achromatic dark state.

[0123] It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.

Claims

CLAIMS:

1 . A composite comprising:

from about 10 wt% to about 40 wt% of a polymer;

a chiral dopant having a helical twisting power of at least about 100 pm^"1; and

a liquid crystal compound having a dielectric anisotropy of at least about

+40;

wherein the composite exhibits a blue phase III.

2. The composite of claim 1 , wherein the composite comprises from about 20 wt% to about 30 wt% of the polymer.

3. The composite of claim 1 , wherein the chiral dopant has a helical twisting power of at least about 1 10 pm^"1.

4. The composite of claim 1 , wherein the liquid crystal compound has a dielectric anisotropy of at least about +50.

5. The composite of claim 1 , wherein the composite comprises from about 20 wt% to about 30 wt% of the polymer; wherein the chiral dopant has a helical twisting power of at least about 1 10 pm^"1; and wherein the liquid crystal compound has a dielectric anisotropy of at least about +50.

6. The composite of claim 1 , wherein the composite exhibits the blue phase III over a temperature range of at least about 80°C.

7. The composite of claim 1 , wherein the composite exhibits the blue phase III at room temperature.

8. The composite of claim 1 , wherein the polymer is formed from a mixture comprising a reactive mesogen.

9. The composite of claim 8, wherein the reactive mesogen is a mesogenic diacrylate monomer; and wherein the mixture further comprises a difunctional acrylate monomer.

10. A liquid crystal display comprising:

a first transparent substrate;

a second transparent substrate; and

a composite layer between the first transparent substrate and the second transparent substrate;

wherein the composite layer comprises:

from about 10 wt% to about 40 wt% of a polymer;

a chiral dopant having a helical twisting power of at least about 100 pm^~1; and

a liquid crystal compound having a dielectric anisotropy of at least about +40; and

wherein the composite exhibits a blue phase III.

1 1 . The liquid crystal display of claim 1 1 , wherein the composite comprises from about 20 wt% to about 30 wt% of the polymer.

12. The liquid crystal display of claim 1 1 , wherein the chiral dopant has a helical twisting power of at least about 1 10 pm^"1.

13. The liquid crystal display of claim 1 1 , wherein the liquid crystal compound has a dielectric anisotropy of at least about +50.

14. The liquid crystal display of claim 1 1 , wherein the composite comprises from about 20 wt% to about 30 wt% of the polymer; wherein the chiral dopant has a helical twisting power of at least about 1 10 pm^"1; and wherein the liquid crystal compound has a dielectric anisotropy of at least about +50.

15. The liquid crystal display of claim 1 1 , wherein the composite exhibits the blue phase III over a temperature range of at least about 80°C.

16. The liquid crystal display of claim 1 1 , wherein the composite exhibits the blue phase III at room temperature.

17. The liquid crystal display of claim 1 1 , wherein the polymer is formed from a mixture comprising a reactive mesogen.

18. The liquid crystal display of claim 17, wherein the reactive mesogen is a mesogenic diacrylate monomer; and wherein the mixture further comprises a difunctional acrylate monomer.

19. A method for producing a composite comprising:

curing a mixture, wherein the mixture comprises:

from about 10 wt% to about 40 wt% of monomer units;

a chiral dopant having a helical twisting power of at least about 100 pm^"1; and

a liquid crystal compound having a dielectric anisotropy of at least about

+40;

wherein the composite exhibits a blue phase III.

20. The method of claim 19, wherein the composite comprises from about 20 wt% to about 30 wt% of the polymer; wherein the chiral dopant has a helical twisting power of at least about 1 10 pm^"1; and wherein the liquid crystal compound has a dielectric anisotropy of at least about +50.