Patent Application
20100279125
The present invention relates to a film comprising polymer dispersed liquid crystal (PDLC) which exists independently from a substrate, i.e. a substrate-free PDLC; to a fiber, a fabric, and/or a device including the same; and to methods of making the same. The invention finds particular application in conjunction with any of the following devices: light modulating device, a display device, a light shutter, a switchable window, a projection display, a direct-view display, portable electronics, high-tech fabrics and textiles, adaptive liquid crystal lenses with variable focus, curved optical devices, tunable filters, optical memory storage, bistable devices, holographically patterned substrate-free PDLC films, a stimuli responsive optoelectronic fiber/fabric, an optical sensor, substrate-free PDLC photonic materials, and a flexible LC display, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
The study of phase separated liquid crystal polymer composite structures has existed as an established field since the 1980s. Polymer dispersed liquid crystal (PDLC) systems, polymer stabilized liquid crystals, polymer wall/liquid crystal (LC) composites, anisotropic phase separated composite films, and infiltrated LCs in templates are the most common composite systems that have been used to fabricate robust LC displays.
Over years, these composite systems have proven to be a reliable solution for many kinds of LC applications, such as light shutters, switchable windows, projection displays, direct-view displays, and portable electronics. More recently, these materials have become great candidates for flexible LC display applications, as disclosed in E. A. Buyuktanir, M. Mitrokhin, B. Holter, A. Glushchenko, and J. L. West, Jpn. J. Appl. Phys. 45 (5), 4146-4151 (2006); and A. Khan, I. Shiyanovskaya, T. Schneider, N. Miller, T. Ernst et al., J. of the SID 13 (6), 469 (2005).
Stressed liquid crystals (SLCs) are another important class of LC/polymer composite displays reported and patented by West et al. in J. L. West, K. Zhang, M. Zhang, E. Büyüktanir, and A. Glushchenko, Proc. SPIE 5936, 59360L (2005). These displays present the largest phase retardation shift achievable within the shortest time interval. A device incorporating such displays was prepared by UV-polymerization of a LC/monomer mixture between two glass substrates, and later, the LC cell is stressed mechanically in one direction. The applied stress force to the cell aligns the LC domains dispersed in a polymer matrix and eliminates scattering and hysteresis at the same time. The resulting display can be used as micro-displays, diffractive optical elements, and beam steering devices.
There are many advantages of the mentioned LC/polymer composite systems when compared to other liquid crystal display (LCD) modes. For example, the advantages of droplet morphology in LC films as compared to twisted-nematic (TN) cells can be summarized as follows: ease of large-area display manufacturing; high brightness can be achieved because in most cases polarizers are not required; birefringent plastic substrates can be utilized; and the electro-optical properties of the device can be tailored according to the composition and the polymerization conditions used.
Since the polymer material usually occupies a large volume fraction of the LC/polymer composite display, it serves a number of critical functions: it provides mechanical support (ruggedness); it determines the thermomechanical stability of the composite; it protects the display functionality from the environment; it helps to distribute the applied pressure by acting as a stress transfer medium i.e., it is self-sustaining; it provides durability, interlaminar toughness and shear/compressive/transverse strengths to the system in general; and it maintains the cell gap of the display.
The formation of PDLC devices can be acquired by three main types of phase separation methods: polymerization induced phase separation (PIPS); thermally induced phase separation (TIPS); and solvent induced phase separation (SIPS). In the TIPS method, a homogeneous mixture of liquid crystal and polymer melt is prepared and the mixture is cooled at a specific rate to induce phase separation. In the SIPS method, a liquid crystal material and a liquid polymer precursor are mixed together, and the mixture is then cast into a film. The PIPS method provides homogeneity and forms a cross-linked structure of polymer that renders it insensitive to temperature changes. In addition, the PIPS method also allows better control over the morphology of the polymer structure, and enables the production of large flexible panels.
Traditionally, small size (less than 2 inches by 2 inches) PDLC cells are prepared by drop-filling the LC/prepolymer mixture (dropping the mixture onto the bottom glass substrate before covering with the top glass substrate) between two conducting layers (e.g. ITO, conducting polymers) and coated glass substrates. Finally, the cell is irradiated with UV light.
Flexible PDLC films are generally produced by a lamination method, which facilitates easy fabrication of large area flexible devices. The first step is to place a sheet of plastic substrate on a smooth surface. Then the LC/prepolymer mixture is spread across the surface of the plastic substrate. Finally, the second plastic substrate is laminated onto the bottom substrate by means of a rubber roller and the cell is UV-cured.
The electro-optical characteristics of the PDLC film depend strongly on the morphology of PDLC composites, which can be modified by varying LC concentration, additives, polymer type, irradiation dose (for PIPS method), and polymerization temperature.
The operating mechanism of a typical PDLC electro-optical device is based on the manipulation of the refractive index variation by changing the orientation of LC molecules within the droplets by an external force. Generally speaking, the electrically controllable light scattering feature (or opaque state) can be realized when the field is off, where the symmetry axis of each droplet is randomly oriented. The transparent (or clear) state can be achieved when the LC molecules are aligned along the field, where the refractive index of a polymer matrix (np) matches to the ordinary refractive index no of a liquid crystal molecule. Therefore, normally incident light travels through the PDLC film without scattering and the film looks transparent.
Advantageously, the present invention provides a film comprising polymer dispersed liquid crystal (PDLC) which exists independently from a substrate, i.e. a substrate-free PDLC; a fiber, a fabric, and a device thereof; and methods thereof. The invention exhibits numerous technical merits such as improved transmittance, enhanced brightness, easier manufacturability, more flexible manufacturability, better cost-effectiveness, enhanced electro-optical performance, and improved device uniformity, among others.
One aspect of the invention provides a film comprising a polymer dispersed liquid crystal (PDLC) which exists independently from a substrate, i.e. a substrate-free PDLC.
Another aspect of the invention provides a fiber made from a film comprising substrate-free PDLC and a fabric made from such fibers.
Still another aspect of the invention provides a PDLC device including a film comprising substrate-free PDLC.
Yet another aspect of the invention provides a PDLC device including a fiber made from a film comprising substrate-free PDLC and a fabric made from such fibers.
Still another aspect of the invention provides a method of preparing a film comprising a substrate-free PDLC, which comprises:
providing a first liquid base;
providing a mixture comprising liquid crystal and monomers, wherein the liquid crystal and the monomers are substantially insoluble in the first liquid base;
spreading the mixture on the surface of the first liquid base; and
forming a layer of polymer matrix dispersed with liquid crystal domains on the surface of the first liquid base.
Still another aspect of the invention provides a method of preparing a PDLC fiber, which comprises:
providing a first liquid base;
providing a mixture comprising liquid crystal and monomers, wherein the liquid crystal and the monomers are substantially insoluble in the first liquid base;
spreading the mixture on the surface of the first liquid base;
forming a PDLC layer of polymer matrix dispersed with liquid crystal domains on the surface of the first liquid base; and
elongating the PDLC layer to form a PDLC fiber.
A further aspect of the invention provides a method of preparing a PDLC device, which comprises:
providing a first liquid base;
providing a mixture comprising liquid crystal and monomers, wherein the liquid crystal and the monomers are substantially insoluble in the first liquid base;
spreading the mixture on the surface of the first liquid base;
forming a PDLC layer with polymer matrix dispersed with liquid crystal domains on the surface of the first liquid base; and
transferring the PDLC layer onto a medium or sandwiching the PDLC layer between two mediums.
a illustrates a step in the preparation of a substrate-free PDLC film by a polymerization-induced phase separation (PIPS) process, in which a LC/monomer mixture is stirred at a temperature above the nematic-isotropic transition temperature (TNI) of the LC material, in accord with an embodiment of the invention;
b illustrates the cross section of an experimental set-up before polymerization in accord with an embodiment of the invention;
c illustrates the cross section of an experimental set-up during UV-irradiation to induce the PIPS process, in accord with an embodiment of the invention;
a illustrates the cross section of a substrate-free PDLC film after it is drawn from the surface of a liquid in accord with an embodiment of the invention;
b shows the POM picture of interconnected polymer cavities when the polarizer is perpendicular to the analyzer, in accord with an embodiment of the invention, in which the film was prepared from nematic LC and thiol-ene based monomer and later washed with hexane;
c shows the POM picture of interconnected polymer cavities when the polarizer is parallel to the analyzer in accord with an embodiment of the invention, in which the film was prepared from nematic LC and thiol-ene based monomer and later washed with hexane;
d shows the photograph of a substrate-free PDLC film with a thickness of approximately 100 μm after the polymerization process in accord with an embodiment of the invention;
a shows the POM picture of a PDLC fiber (nematic phase) drawn from a polymerized substrate-free PDLC film prepared on the liquid surface with colors believed due to the birefringence of the LC domains dispersed in the polymer matrix in accord with an embodiment of the invention, in which the long fiber axis is placed at 45 degree between crossed polarizer and analyzer;
b shows the POM picture of a PDLC fiber above nematic-isotropic transition temperature (TNI) in an accord with embodiment of the invention, wherein the material exhibits only liquid phase, and the bright lines in the picture (with 50× microscope objective) are polymer chains;
c shows the POM picture of a PDLC fiber (at the nematic phase), which is placed at 45 degree between crossed polarizer and analyzer in accord with an embodiment of the invention, demonstrating the polarization rotation of the light propagating through the fiber;
d shows the POM picture of a PDLC fiber (at the nematic phase), which is placed along the polarizer in accord with an embodiment of the invention, demonstrating the light propagating through the fiber is blocked by aligning the long axis of the fiber along the polarizer;
Panels (a) and (b) in
Panels (c) and (d) in
All of the cited references are incorporated in the present invention in their entirety. The term “substrate-free PDLC” is herein defined as polymer dispersed liquid crystal (PDLC) which exists independently from a substrate. “LC” is the abbreviation for liquid crystal; “PET” is the abbreviation for poly(ethylene terephtalate); “ITO” is the abbreviation for Indium-tin-oxide; “OTFT” is the abbreviation for organic-thin-film-transistor; “PIPS” is the abbreviation for polymer-induced phase separation; “TIPS” is the abbreviation for thermally induced phase separation; “SIPS” is the abbreviation for solvent induced phase separation; “AC” is the abbreviation for alternating current; “POM” is the abbreviation for Polarizing Optical Microscopy; “SLC” is the abbreviation for Stressed Liquid Crystal; “TNI” is the abbreviation for nematic to isotropic phase transition temperature; and “μm” denotes micrometer.
The invention provides a film comprising a substrate-free PDLC, and examples of the substrate divorced from PDLC may be, for example, a rigid and transparent material such as glass and plastic. In various embodiments, the concentration of the liquid crystal in the film may be generally from about 10% to about 95% by weight, based on the total weight of the polymer and the liquid crystal. For example, the polymer may function as a matrix containing liquid crystal droplets.
The mesophases of the liquid crystal material may be selected from, for example, nematic, cholesteric (chiral nematic), smectic such as smectic A, smectic C, Smectic C* (chiral smectic C), ferroelectric and antiferroelectric smectic mesophases, and higher order smectics of B, E, G, H, J, and K-types; banana mesophase; lyotropic chromonic liquid crystals; and columnar mesophase.
The polymer in the film may be made from photo-polymerizable monomers selected from thiol-ene systems; monomers such as acrylates through radical polymerization; ring-opening monomers such as epoxides through cationic polymerization; photo-polymerizable blends such as nematogenic monomer and chiral dopants and polyurethane/acrylate blends; photo-polymerizable cholesteric mixtures; photo-polymerizable biomaterials; any copolymer thereof such as block copolymer; and any mixture thereof.
In some embodiments, the polymer may be made from thermal-polymerizable monomers selected from epoxy resins, polyurethanes, acrylic resins, matrices of bismaleidimide, phenolic, polyesters, polyimides, blends of polymer and monomers, and blends of monomers.
The polymer may also be made from a combination of thermal-polymerizable monomers and photo-polymerizable monomers.
The prepolymer (monomer) is preferably chosen from those that can be polymerized by photo-polymerization, such as photoinitiated radical polymerization (e.g., acrylates), cationic photopolymerization (e.g. ring-opening reaction of epoxides), and thiol-ene systems etc. The thiol-ene systems are preferred as they exhibit rapid reaction, low shrinkage, little or no oxygen inhibition, self initiation, and formulation latitude. In addition to a photopolymerization method, the thermal curing of prepolymers by means of heat with or without catalysts, hardeners, and modifiers may be used to produce polymer matrices for the liquid crystal/polymer composites on the liquid surface. Among them, epoxy resins, polyurethanes, acrylic resins, and matrices of bismaleidimide, phenolic, polyesters, and polyimides are common examples for thermosetting polymers that can be used for the formulation of the liquid crystal/polymer composites.
The film of the invention may further comprise one or more suitable additives, for example, dyes such as positive and negative dichroic dyes, dopants, surfactants, ferroelectric particles, ferromagnetic particles, macromolecules, piezoelectric particles, multifunctional monomers, azo dyes, biological materials, colloidal particles, mesogens, gold particles, metal particles, adhesives, chemical markers, fluorescent dyes, minerals, and quantum dots.
By introducing positive and negative dichroic dyes into the formulation, it may be possible to change the optical state of the film by stretching the film mechanically or by applying an electric field. The additional compounds such as dyes, dopants, and/or initiators, surfactants, ferroparticles, magnetic particles, and macromolecules may be added to the formulation and/or to the liquid media.
In some embodiments, the film of the invention comprises a layer of polymer dispersed liquid crystal (PDLC), and one layer of polymer free of liquid crystal on one side of the layer of polymer dispersed liquid crystal (PDLC). The film may also comprise a layer of polymer dispersed liquid crystal (PDLC), and two layers of polymer free of liquid crystal on two sides of the layer of polymer dispersed liquid crystal (PDLC) respectively.
The invention provides a PDLC device including a film comprising substrate-free PDLC. Examples of such devices include, but are not limited to, a light modulating device, a display device, a light shutter, a switchable window, a projection display, a direct-view display, portable electronics, high-tech fabrics and textiles, adaptive liquid crystal lenses with variable focus, curved optical devices, tunable filters, optical memory storage, bistable devices, holographically patterned substrate-free PDLC films, substrate-free PDLC photonic materials, and a flexible LC display. In some embodiments, the PDLC devices of the invention may further include one medium onto which the film is transferred or two mediums between which the film is sandwiched. For example, the medium may be selected from an indium-tin-oxide coated glass substrate, an indium-tin-oxide coated plastic substrate, a flexible or rigid surface coated with a conducting layer, a textile surface embedded with an organic-thin-film-transistor (OTFT), a conducting polymer, an inorganic conductor, and a hybrid organic-inorganic conductor.
In various embodiments, a fiber may be made from the film comprising the substrate-free PDLC. Such fibers may be woven into a fabric with any shape. The invention also provides a PDLC device including a fiber made from the film comprising substrate-free PDLC or a fabric thereof, for example, a stimuli responsive optoelectronic fiber/fabric, an optical sensor, and a light modulating device.
Optical responses and properties of the fibers, such as transparency, opacity, phase retardation, reflective colors, and birefringence, can be altered by constructing a liquid crystal display device consisting of a fiber between two conducting substrates and either by the application of electric and magnetic fields or by employing mechanical stress (tension or compression) directly to the PDLC fiber obtained from a substrate-free PDLC film. These fibers can also be woven as a fabric and can be used to form high-tech textiles with or without flexible or rigid substrates, and with or without conducting or nonconducting layers. If the fiber is made up of cholesteric LCs, the fiber is expected to change its reflective colors on application of external stimulus, for example temperature, stress, or electric field. These fibers, for instance, can be used to form fabrics for medical applications, such as to measure the distribution of body temperature. It is a noninvasive tool to monitor patients. PDLC fiber can also be used for photonic fiber optic applications. In addition, deformation of the PDLC fibers and films may induce flexoelectric effect under bend deformation, or conversely, the deformation of the director due to an applied electric field, in that the liquid crystal component of the PDLC fiber or films is formed from molecules with a shape asymmetry, such as banana shaped or wedge shaped liquid crystal molecules.
These stretchable and bendable optical fibers and films can be engineered to form materials and devices, responding to chemical changes or thermal and mechanical effects, as well as the application of electric and magnetic fields. They can be utilized in a variety of photonic applications ranging from optical sensors to light modulating devices operating in the UV-VIS to IR regions of the electromagnetic spectra. Additionally, the chemistry of the fiber or film can be changed to detect other chemicals present in the surrounding environment by observing texture, color, or shape changes. The reflective mode of cholesteric liquid crystal mesophase can also be used to form high-tech fabrics functioning as a thermo-optic device embedded or woven into clothing. In general, PDLC fibers can be used to produce stimuli responsive and/or interactive high-tech fibers/fabrics. External stimuli include electric field, magnetic field, heat, mechanical stress, chemical changes, and photonic radiation.
The method of preparing a film comprising substrate-free PDLC may comprise:
providing a first liquid base such as water, organic solvents, polymer solutions, or any combination thereof;
providing a mixture comprising liquid crystal and monomers, wherein the liquid crystal and the monomers are substantially insoluble in the first liquid base;
spreading the mixture on the surface of the first liquid base; and
forming a layer of polymer matrix dispersed with liquid crystal domains on the surface of the first liquid base. In embodiments, this step is accomplished by UV curing, thermal curing, solvent evaporation, or any combination thereof.
In spreading the mixture on the surface of the first liquid base, the density of the liquid crystal and the density of the monomers may be so selected that they are both lower than the density of the first liquid base. However, it should be understood that the density is not the only reason that the mixture floats on the surface of the liquid. It is possible to float or spread higher density material (liquid or solid) on the liquid surface by the help of surface tension of the liquid base and the interfacial tension that exists between them.
In a specific embodiment, the step of forming a layer of polymer matrix dispersed with liquid crystal domains is accomplished by polymer-induced phase separation (PIPS), which may be conducted with a UV radiation dose of from about 0.01 milliWatt per square centimeter (mW/cm2) to about 500 mW/cm2. The polymer-induced phase separation (PIPS) is conducted at a polymerization temperature of, for example, from about 0° C. to about 250° C.
In some embodiments, since a polymer matrix containing liquid crystal droplets is initially formed without substrates, it can be transferred very easily onto any media, such as onto an indium-tin-oxide coated glass substrate, plastic substrate, or any flexible or rigid surface coated with one or more conducting layers.
In an embodiment, the film preparation comprises:
providing a first liquid base;
providing a second liquid base;
providing a mixture comprising liquid crystal and monomers, wherein the liquid crystal and the monomers are substantially insoluble in the first and second liquid bases
spreading the mixture on the surface of the first liquid base to form a layer;
spreading the second liquid base on the layer of the mixture; and
forming a layer of polymer matrix dispersed with liquid crystal domains between the first liquid base and the second liquid base.
In spreading the mixture on the surface of the first liquid base to form a layer and spreading the second liquid base on the layer of the mixture, the density of the liquid crystal and the density of the monomers may be so selected that they are both lower than the density of the first liquid base but higher than the density of the second liquid base. However, it should be understood that the density is not the only reason that the mixture floats on the surface of the liquid. It is possible to float or spread higher density material (liquid or solid) on the liquid surface by the help of surface tension of the liquid base and the interfacial tension that exists between them.
The present invention also provides a method of preparing a PDLC fiber, which comprises:
In steps (iv) and (v), the PDLC layer of polymer matrix may be cured completely. Alternatively, the PDLC layer of polymer matrix may be cured incompletely and then the fiber is transferred onto another surface to be cured with UV light, heat, or a combination thereof.
In some embodiments, the fiber preparation method of the invention may further comprise the step of modifying the surface characteristics of the fiber by dipping, spraying, or coating the fiber with another layer of polymer, curable monomers, surfactants, or any combination thereof.
In an embodiment, fiber material characteristics, as well as elastic and mechanical properties, are changed as a result of changing the formulation and processing parameters of PDLC films.
The present invention provides a method of preparing a PDLC device, which comprises:
providing a first liquid base;
providing a mixture comprising liquid crystal and monomers, wherein the liquid crystal and the monomers are substantially insoluble in the first liquid base;
spreading the mixture on the surface of the first liquid base;
forming a PDLC layer with polymer matrix dispersed with liquid crystal domains on the surface of the first liquid base; and
transferring the PDLC layer onto a medium or sandwiching the PDLC layer between two mediums. Examples of the medium include, but are not limited to, an indium-tin-oxide coated glass substrate, a plastic substrate, a flexible or rigid surface coated with a conducting layer, a textile surface embedded with an organic-thin-film-transistor (OTFT), a conducting polymer, an inorganic conductor, and a hybrid organic-inorganic conductor.
In a variety of exemplary embodiments, the present invention provides light modulating devices utilizing electrically switchable polymer dispersed liquid crystal (PDLC) films and fibers, and a fabrication method thereof. Particularly, the present invention provides a substrate-free PDLC display component in which the PDLC film is formed without top and bottom substrates and the fabrication thereof. The PDLC fibers are drawn from thin substrate-free PDLC films prepared on the water surface. These substrate-free PDLCs films and fibers offer novel functionalities, flexibility, and enhanced electro-optical performance over glass based LC/polymer composites when subjected to external stimulus, such as electromagnetic fields and mechanical deformation. In addition, these films and fibers can be stretched in any direction to modify optical characteristics and electro-optical responses. Mechanically stretched thin LC films and fibers can be switched electrically, and therefore, can potentially be used as high-tech fabrics, fiber optics, and light modulating devices.
In a variety of exemplary embodiments, the invention provides a substrate-free flexible PDLC component of the device that was polymerized on the liquid surface by using the polymer-induced phase separation (PIPS) method. Since the polymer matrix containing LC droplets was initially formed without substrates, PDLC film and fiber can be transferred very easily onto any media, such as on indium-tin-oxide (ITO) coated glass or plastic substrates, or textile surfaces embedded with organic-thin-film-transistor (OTFT) to switch the display. Alternatively, conducting polymers can be directly printed or coated onto the film surface. Conducting fibers (melt spun or electrospun) can also be used to switch the films. These fiber and film structures may be embedded with conducting wires, or incorporated into a thin film and/or core-shell structure embedded with conducting organic, inorganic or polymeric materials, wires, circuit, or metal electrodes.
The substrate-free films and fibers drawn from these films can also be easily sandwiched between ITO-coated plastic or glass substrates in order to form a switchable PDLC film. The intensity of the transmitted light can be varied by changing the orientation of the LC molecules inside the droplets via an application of an electric field or by stretching. Conducting polymers, inorganic conductors, and/or hybrid organic-inorganic conductors can be applied to the top and bottom of the substrate-free PDLC films and fibers in order to obtain electro-optical responses. The substrate-free PDLC film production can be adapted in the roll-to-roll display manufacturing process. This display can be used as switchable windows, displays to be used as advertisement or information boards, and high-tech fabric displays that can be switched between light scattering opaque and transparent states by applying an alternating current (AC) electric field. The PDLC fibers can be used to produce electrically controlled polarizers. The operating principle of a PDLC fiber polarizer is based on anisotropic light scattering of PDLC fibers resulting from unidirectionally oriented LC domains dispersed in a stretched polymer structure. These polarizers will provide light amplitude and phase modulations.
In a variety of exemplary embodiments, the method of the invention may reduce the PDLC window production cost dramatically by eliminating the lamination of the film between two ITO-coated plastic substrates. Instead, the substrate-free and very flexible PDLC film can be sandwiched directly between two ITO-coated glass substrates or the like. It is expected that once the plastic substrate film and the lamination method are eliminated, the cost of the manufacturing of the PDLC display may decrease. The uniformity of the device may also increase substantially.
The method of the invention enables the single-step photo-polymerization of the LC/prepolymer composite films and fibers, where a light modulating LC material formed of a birefringent liquid crystal is interspersed as spherical or randomly distorted droplets in an isotropic polymer. It also eliminates other costly and complex methods, such as carrier-substrates and lift-off processes, for manufacturing a fully flexible liquid crystal display component.
This example shows the improvement of device brightness by eliminating the plastic substrates completely from the production of large-area PDLC units. A commercially available ITO-coated poly(ethylene terephtalate) (PET) substrate (127 micrometer thick) from Southwall Technologies Inc. (Palo Alto, Calif., USA) was purchased, and its transmittance % was measured in the visible region (450-700 nm) of the electromagnetic spectrum. A commercially available ITO-coated glass substrate (1.1 millimeter thick) from Corning Incorporation with Code 7059 was prepared for comparison, and its transmittance % was similarly measured. Samples of size 3 cm×3 cm were cut and placed inside the spectrometer. Light transmittance values were measured for both samples using a Lambda 19 Perkin Elmer UV/VIS/NIR Spectrometer at room temperature in the visible region.
a illustrates a step in the preparation of a substrate-free PDLC film by the polymerization-induced phase separation (PIPS) process. With reference to
b illustrates the cross section of an experimental set-up before the polymerization. With reference to
c illustrates the cross section of an experimental set-up during UV-irradiation to induce the PIPS process. With reference to
It is possible to change the size and the shape of the droplet structure as well as completely alter the morphology of the final film by varying the experimental parameters, such as UV light intensity, temperature, formulation of the mixture, the weight ratio of monomer to LC in the mixture, type and temperature of the liquid. In addition, external or internal masks can be used to change the morphology and topology of the polymerized film and fibers. For instance, it is possible to obtain substrate-free PDLC lenses by using specially design masks to pattern the film.
The side of the LC/prepolymer mixture in contact with the liquid is called the liquid-side, and the other side is called the air-side. The smooth surface of the liquid creates an almost perfectly flat bottom surface for the LC/prepolymer mixture. Initially, deionized water is chosen as the liquid substrate because water is nontoxic, nonflammable, cheap, readily available, does not leave unremovable residue, can be heated up to 100° C., and does not mix with thermotropic LC or the prepolymer. Water also does not dissolve the polymer after the polymerization process. Additionally, it does not wash out the LC from the PDLC film. The main disadvantage of water as a liquid substrate is its boiling point (about 212 degrees Fahrenheit (100° C.) at sea level). Since some of the LC/prepolymer mixtures may form homogeneous solutions at above 100° C., the aqueous medium can be altered or replaced with other liquid phase organic material, inorganic materials, and polymer solutions for higher temperature applications. Additionally, the topology of the PDLC films can be changed during photopolymerization via a templating process by using systems such as particle arrays, ions, linear polymer chains, vesicles, or liquid crystalline mesophases in the liquid phase, as disclosed in J. D. Clapper, L. Sievens-Figueroa, and C. A. Guymon, Chem. Mater. 20, 768-781 (2008).
After the UV-curing process in Example 2, the PDLC film was drawn from liquid base 205 surface and dried at room temperature. In order to observe the morphology of the polymer matrix after the polymerization process, the polymerized PDLC film was kept in hexane over night to wash out LC material trapped inside the film.
In fabricating a substrate-free polymer liquid crystal device, several other photo-polymerizable supporting layers can be dispensed on the liquid base as a first layer. For instance, for very low concentrations of LC (<30 wt %), the prepolymer can be dispensed onto the liquid surface first. Later, the LC/prepolymer mixture can be dropped on top of the floating prepolymer material. Another layer of prepolymer can also be added as a third layer over the LC/prepolymer layer. Finally, the prepolymer and the LC/prepolymer layers are polymerized altogether to obtain a substrate-free PDLC film, which can be used in a PDLC device as illustrated in
PDLC fibers were drawn from the surface of the polymerized thin substrate-free film and dried. The height and width of the PDLC fibers can be tuned between 1 micrometer to 100 micrometers.
b shows the POM picture of a PDLC fiber above nematic-isotropic transition temperature (TNI). With reference to
a and 6b are the same portion of the fiber. In
c and 6d show the POM images of a PDLC fiber demonstrating light polarization sensitivity of the fibers. In
The LC domains in the fiber can be electrically switched, and therefore, can be used as light modulating LC display. In addition, the size and shape of the drawn PDLC fiber can be changed by external situmulus, such as by stretching. Unidirectionally oriented LC domains dispersed in a stretched polymer structure can be used to produce electrically controlled devices by utilizing anisotropic light scattering properties of the LC domains. Highly oriented polymer structure coupled with oriented and birefringent LC domains can provide light amplitude and phase modulations.
Similar to Example 2,
In Examples 8 and 9, hexane and tetrahyrofuran were purchased from Sigma-Aldrich (St. Louis, Mo.). Commercially available 4-pentyl-4′-cyanobiphenyl (5CB) and E7 liquid crystals were obtained from Merck Licristal EM Chemicals. The morphology of the polymer matrix was examined by using a JEOL JSM-6300V SEM. Samples for SEM were coated for 30s with Au—Pd (gold-palladium) using Ar (argon) inert gas plasma. After UV curing process, the PDLC films and fibers were immersed in hexane/tetrahyrofuran (2:1 v/v) solution over night to wash out LC material.
Similar to previous examples, the substrate-free PDLC films and fibers were composed of nematic LC (E7, Merck) and thiol-ene based monomer (N0A65) (50/50 wt %), and fabricated on an aqueous surface. To observe the morphology of the polymer matrix after the polymerization process, the polymerized PDLC film was kept in THF/hexane over night to wash out LC material trapped inside the film.
PDLC fibers were similarly drawn from the polymerized thin substrate-free PDLC films of Example 8. The polarized optical microscopy (POM) images of the PDLC fiber are shown in panels (a), (b), (c) and (d) in
For optical and electro-optical characterization, the PDLC fibers of Example 9 were sandwiched between two indium-tin-oxide (ITO) coated glass (Corning, 1.1 mm thick) substrates. A square wave electric field was supplied by a function generator (30 MHz Standford Research Systems) and a wideband amplifier (Krohn-Hite Corp.). E7 with positive dielectric anisotropy (Δ∈=14) has the nematic-isotropic phase transition temperature, TNI, at 58° C. All the materials were used without further purification. The LC fiber cell corners (˜30 μm thick) were fixed with epoxy-based glue. An electric field was applied to the cells via ITO electrodes. A square wave alternating current (AC) electric field was supplied by a function generator (Hewlett Packard 33120A, 15 MHz) and a wideband amplifier (FLC electronics F20 AD). 1 kHz square waveform was produced repetitively in a burst mode with the function generator. The pulse duration was 100 ms, i.e. the function generator sends a voltage pulse with a frequency of 1 kHz for a period of 100 ms and switches off for a period of 100 ms. The amplitude of the voltage pulse (Vp) was varied from 0 to 160 V. The intensity of transmitted light through the cell was measured using a photodiode placed at an eyepiece of an Olympus microscope by keeping the illumination and the magnification constant. The 10× microscope objective was used to collect the transmitted light intensity. The optical response characteristics of the fibers and the voltage pulses were monitored by a computer-based oscilloscope and PC software from Pico Technology Ltd. Finally, all the results were digitally saved to the computer, and results were plotted using OriginPro 7 Software (OriginLab Corp.).
The fibers can be electrically switched upon application of an electric field, and the relaxation time of the fiber is faster than traditional PDLC films. The electro-optical characterizations of the fibers are shown in
The composite fibers can be electrically switched upon application of an AC-electric field. The invention provides the first electrically switchable and light modulating LC/polymer fibers for use in stimuli responsive optical fibers, textiles, and optoelectronic devices.
The invention has been described with reference to the embodiments presented. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the embodiments provided be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.
This application claims priority based on the U.S. Provisional Application 61/173,804 filed on Apr. 29, 2009, which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61173804 | Apr 2009 | US |
