Abstract:
A polymeric dispersed liquid crystal light shutter device and system and method for forming the same are disclosed. In one embodiment of the system, a catoptric form having a shape is subjacently positioned below a pre-cure or un-cured polymer dispersed liquid crystal (PDLC) light shutter device having liquid crystals dispersed in a polymer binder system between two substrates. Light is applied during curing. The liquid crystal microdroplet sizes vary according to the shape of the catoptric form such that domains of smaller liquid crystal microdroplet sizes correspond to the shape and domains of larger liquid crystal microdroplet sizes correspond to negative space relative to the shape. Upon tuning an electric field, the PDLC light shutter device changes states from presenting a surface having an image formed by non-scattering regions contrasted against opaque scattering regions, to a surface characterized by mostly or entirely clear, non-scattering light transmittance.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    This invention relates, in general, to liquid crystal display technology and, in particular, to polymer dispersed liquid crystal (PDLC) light shutter devices that include formulations of liquid crystal mixtures having nematic liquid crystals and polymer systems to provide visual effects. 
       BACKGROUND OF THE INVENTION 
       [0002]    A liquid crystal display can show an image using electro-optical characteristics of a liquid crystal, which is injected into a space defined by two substrates. The electro-optical characteristics of the liquid crystals appear when electric power is applied thereto. Such a liquid crystal display is classified as one of a variety of types including twisted nematic (TN), super twisted nematic (STN), dynamic scattering mode (DSM), and the aforemented PDLC, for example. Liquid crystal shutters are useful in various applications concerning the transmittance of light through an aperture in which it should be possible to switch the shutter between a low transmission state and a high transmission state, in response to a change in the electric influence. 
         [0003]    PDLCs consist of micron-size droplets of low-molecular weight nematic liquid crystals dispersed in a polymer binder system. A PDLC material is sandwiched between substrates having a transparent conducting electrode such as indium tin oxide, to form a shutter. Upon application of a voltage across the electrodes of the shutter, a switching occurs from an opaque, high scattering state to a clear, transparent state. PDLC materials are formed by phase separation of low-molecular weight liquid crystals from a homogeneous solution with pre-polymer or polymer. The size, shape, and density of the liquid crystal droplets depend on the techniques implemented. With existing shutters, solutions have been proposed over the years for selectively providing a tunable lens. Many of the existing devices, however, require the liquid crystal material be aligned on convex curved substrates or concave curved substrates, where it is extremely difficult to align the liquid crystal molecules on the curved substrates. Additionally, most of these devices require linearly polarized light sources in order to operate. Accordingly improvements are needed. 
       SUMMARY OF THE INVENTION 
       [0004]    It would be advantageous to provide a tunable lens in a PDLC system. It would also be desirable to enable a chemical-based solution that would mitigate the need for convex or concave substrates and the request for linearly polarized light. To better address one or more of these concerns, a polymeric dispersed liquid crystal light shutter device employing an electronically tunable image is disclosed. In one embodiment of the system, a catoptric form having a shape is subjacently positioned below a pre-cure or un-cured PDLC light shutter device having liquid crystals dispersed in a polymer binder system between two substrates. Light is applied during curing. The liquid crystal microdroplet sizes vary according to a shape of a catoptric form such that domains of smaller liquid crystal microdroplet sizes correspond to the shape of the catoptric form and domains of larger liquid crystal microdroplet sizes correspond to negative space relative to the shape. 
         [0005]    Upon tuning an electric field, the PDLC light shutter device changes states from presenting a surface having an image formed by non-scattering regions contrasted against opaque scattering regions, to a surface characterized by mostly or entirely clear, non-scattering light transmittance. A corresponding PDLC light shutter device and method for forming the same are additionally disclosed. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
           [0007]      FIG. 1  is a diagrammatic view of one embodiment of a liquid crystal shutter being utilized to provide each of an opaque, high scattering state; a translucent, image state; and a clear, transparent state; 
           [0008]      FIG. 2  is a diagrammatic view of another embodiment of a liquid crystal shutter being utilized to provide each of an opaque, high scattering state; a translucent, image state; and a clear, transparent state; 
           [0009]      FIG. 3  is a diagrammatic view of one embodiment of the liquid crystal shutter depicted in  FIG. 1  in a high scattering opaque state; 
           [0010]      FIG. 4  is a diagrammatic view of one embodiment of the liquid crystal shutter depicted in  FIG. 1  in a translucent, image state; 
           [0011]      FIG. 5  is a diagrammatic view of one embodiment of the liquid crystal shutter depicted in  FIG. 1  in a low scattering transparent state; and 
           [0012]      FIG. 6  is a diagrammatic view of one embodiment of a methodology and process for forming polymeric dispersed liquid crystal light shutter device according to the teachings presented herein. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts, which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the present invention. 
         [0014]    Referring initially to  FIG. 1 , therein is depicted a PDLC light shutter device or, more succinctly, a liquid crystal shutter that is schematically illustrated and generally designated  10 . Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid, and those of a solid crystal. For instance, a liquid crystal may flow like a liquid, but have the molecules in the liquid arranged and/or oriented in a crystal-like way. One type of liquid crystal, in the aforementioned polymer dispersed liquid crystal or PDLC, comprises micro-size droplets of low-molecular weight nematic liquid crystals dispersed in a polymer binder system. The liquid crystal shutter  10  includes a PDLC material interposed between substrates having transparent conducting electrodes. In  FIG. 1 , the liquid crystal shutter  10  is being utilized as a window  12 , behind which, an individual  14  is standing. Moreover, as shown, the liquid crystal shutter  10 , in response to electronic tuning, includes an image  16 , which may be a logo, a graphic mark, an emblem, a symbol, or words, for example. 
         [0015]    Upon application of a voltage across the electrodes of the liquid crystal shutter  10 , as shown by arrow  18 , the liquid crystal shutter  10 , switches from an opaque, high scattering state, labeled as an “ OFF  State” to a translucent, image state, labeled “ IMAGE  State,” wherein the image  16  is visible. Upon removal of the voltage (V), as shown by arrow  20 , the liquid crystal shutter  10  switches from the translucent, image state to the opaque, high scattering state. Continuing with the discussion of the “ IMAGE  State,” upon the application of a further voltage across the electrodes of the liquid crystal shutter  10 , as shown by arrow  22 , the liquid crystal shutter  10  switches from the translucent, image state to a clear, transparent state, labeled as the “ ON  State,” wherein the individual  14  can be seen standing behind the window  12 . As shown by arrow  24 , by the reduction in voltage, the “ ON  State” returns to the “ IMAGE  State.” It should be appreciated that although the liquid crystal shutter is presented as a window, the teachings presented herein extend to any type of aperture including apertures for looking through, apertures having a need for clear and opaque states, switchable glass, privacy glass, smart windows, smart glasses. Further the light crystal shutter may be glass, plexiglass, polycarbonate or other material as will be discussed hereinbelow. 
         [0016]      FIG. 2  is a diagrammatic view of another embodiment of a liquid crystal shutter  10  being utilized to provide each of an opaque, high scattering state, a translucent, image state, and a clear, transparent state. As shown, by electronically tuning the PDLC shutter device with the application and reduction of voltage as shown by arrows  18 ,  20 ,  22 , and  24 , the PDLC shutter device is selectively tunable among the “ OFF  State,” “IMAGE State,” and “ ON  State.” Depending on the curing parameters, in one embodiment, the image  16  may be partially visible in either the “ OFF  State,” the “ ON  State,” or both. Or alternatively, by way of example, the image  16  may be partially visible in the “ OFF  State,” the “ IMAGE  State”, and the “ ON  State.” Further still, depending on the curing parameters, in one implementation, the “ OFF  State,” the “ IMAGE  State”, and the “ ON  State” may have varying degrees of opacity. 
         [0017]    As previously mentioned, liquid crystal lens have been proposed over the years for selectively controlling the index of refraction of light passing through the lens such that a gradient-index liquid crystal lens with a tunable focal length is provided. In one embodiment of the liquid crystal shutter  10 , a flat profile is provided such that an alignment on convex curved substrates or concave curved substrates is not necessary. Further, a linearly polarized light source is not necessary in order to operate and, for example, view the individual  14 . In one implementation, the polymer binder system may include light curable adhesives selected from the group consisting of acrylates, methacrylates, thiolene-based polyurethanes, and mercapto-esters with a photoinitiator. 
         [0018]      FIG. 3  depicts one embodiment of the liquid crystal shutter  10  wherein encapsulated liquid crystal microdroplets  30  are distributed uniformly in a polymer binder system  32 , which may have the form of a plastic matrix, to create the PDLC material that is then sandwiched between two transparent substrates  34 ,  36 . In one implementation, the substrates  34 ,  36  are positioned parallel or substantially parallel to each other and include a transparent body having a transparent conducing layer therewith. The transparent body may be selected from materials consisting of glasses and plastics, for example. Moreover, the transparent body may include a refractive index from about 1.51 to about 1.52. The polymer binder system  32  may also have a refractive index from about 1.51 to about 1.52. It should be appreciated that the refractive indexes of the transparent substitutes  34 ,  36  and the polymer binder system  32  are matched as close as possible to improve transparency. The transparent conducting layer may comprise an indium-tin-oxide conducting layer or other suitable conducting layer, for example. 
         [0019]    As illustrated, the liquid crystal shutter  10  includes inhomogeneous droplet size distributions of the liquid crystal microdroplets  30  in the polymer binder system  32 , which as will be discussed in  FIG. 6 , are formed by exposing UV light to the LC/monomer mixture using a catoptric form, for example. The inhomogeneous droplet size distribution includes a large domain  50  and a small domain  52 . In the brighter region corresponding to presence of the catoptric form, the polymerization rate is faster resulting in smaller liquid crystal microdroplets  30  at the small domain  52 . In the weaker UV exposure regions corresponding to the absence of the catoptric form, the liquid crystal microdroplets  30  are larger at the large domain  50 . It should be appreciated that the gradient of liquid crystal microdroplet sizes can vary from approximately a few nanometers to micrometers, depending on formation characteristics. 
         [0020]      FIG. 3 , which is an “ OFF  State” shows a light scattering state of the liquid crystal microdroplets  30  in the polymer binder system  32 , in the absence of an applied electric field. Within each of the liquid crystal microdroplets  30 , liquid crystals have tangential wall alignment; however, there is a two dimension random orientation of molecules in comparing various liquid crystal microdroplets  30 . In terms of optical properties, this corresponds to a highly light scattering state. 
         [0021]    That is, in the absence of an applied electric field ({right arrow over (E)}=0), the optic axes of the liquid crystal microdroplets  30  have no preferred direction in which to point in the plane, so that incident light encounters a mismatch between the refraction index (n p ) of the matrix and the effective refraction index (˜n eff ) of the liquid crystal microdroplets. The result of the mismatch is that the light is scattered and the liquid crystal shutter  10  appears opaque. On the other hand, if an electrical field ({right arrow over (E)}) is applied as shown in  FIG. 3  and  FIG. 4 , the orientations of molecules among various microdroplets is completely aligned. The applied electric field ({right arrow over (E)}) aligns the directors within the droplets to a partial or completely transparent state such that n eff →n o =n p  when → is {right arrow over (E)} at high field. 
         [0022]    With reference to the light scattering state of  FIG. 3 , the optic axis of the droplets is indicated by n e . If the ordinary refractive index of the liquid crystal, n o , matches that of the polymer binder system  32 , n p , then light scatters according to the value and orientation distribution of n e . In a low field, if n eff  is close to n e  and nematic directors are randomly oriented ( OFF  state), light is strongly scattered. If n e  is reoriented to be parallel to the direction of normally incident light, as in the case under an applied electric field, then, in principle, no light is scattered (n o ˜n p ). It should be appreciated that the liquid crystal microdroplets may be fabricated as spherical or elliptical shapes and that the shape of the liquid crystal microdroplets may change shape and size due to other factors when the liquid crystal shutter  10  is assembled, as the liquid crystal microdroplets are compressed in combination before curing has begun. 
         [0023]    By way of illustration, the “effective” refractive index, which may be the “average,” approaches the “extraordinary” at a maximum. By way of example, if “ordinary” or no is 1.52, the polymer or n p  is 1.52, and the “extraordinary” or n e  is 1.56, the “effective” or n eff  is 1.52 i.e. equals the ordinary in a strong electric field, but can move to 1.56 as incident light encounters the n e  in the  OFF  or no-electric field scattering state. 
         [0024]    As mentioned, in  FIG. 4 , in response to an application of an electric field ({right arrow over (E)}=↑) across the transparent substrates  34 ,  36 , the liquid crystal shutter  10  provides for the partial transmission of light to cause an “ IMAGE  State”. The electric field causes the optic axes of the liquid crystal microdroplets  30  in the large domain  50  to align parallel to the field and normal to the surfaces of the transparent substrates  34 ,  36 . The optic axes of the liquid crystal microdroplets  30  in the small domain  52 , on the other hand, have no preferred direction in which to point in the plane, so that incident light encounters a mismatch between the refraction index n p  of the matrix and the average refraction index (˜n e ) of the liquid crystal microdroplets. The result of the mismatch is that the light is scattered and, with respect to the small domain  52 , the liquid crystal shutter  10  appears opaque. 
         [0025]    In this transmission state, which may be an “ ON  STATE,” incident light  40  detects no mismatch between average refractive index of the liquid crystal droplets (˜n 0 ) with respect to the large domain and the polymer binder system  32  (n p ) and light is transmitted so that the image  16  within the liquid crystal shutter  10  appears clear. 
         [0026]    As mentioned, in  FIG. 5 , in response to an application of an electric field ({right arrow over (E)}=↑) across the transparent substrates  34 ,  36 , the liquid crystal shutter  10  provides transmission of light. The electric field causes the optic axes of the liquid crystal microdroplets  30  in both the large domain  50  and the small domain  52  to align parallel to the field and normal to the surfaces of the transparent substrates  34 ,  36 . In this transmission state, incident light  40  detects no mismatch between average refractive index of the liquid crystal droplet (˜n 0 ) and the polymer binder system  32  (n p ) and light  46  is transmitted so that the liquid crystal shutter  10  appears clear. As mentioned, by the application and removal of the driving voltage the liquid crystal shutter  10  may be alternated between the light scattering state of  FIG. 3 , the partial scattering of  FIG. 4  and the light transmission state of  FIG. 5 . 
         [0027]    With reference to  FIGS. 3 through 5 , in the voltage  OFF  state of  FIG. 3 , the liquid crystal shutter  10  is non-transparent and light scattering is observed as oriented directions of the liquid crystal microdroplets  30  are random. This is because the droplet sizes are much smaller than the wavelength. As the voltage is applied to the liquid crystal shutter  10  as shown in  FIG. 4  and  FIG. 5 , the liquid crystal microdroplets are reoriented along the electric field direction. The turn  ON  voltage of such liquid crystal microdroplets  30  depends on the droplet sizes: the smaller the droplet, the higher the threshold voltage. As a result, the gradient refractive index profile is generated that initially permits incident light  40  to be partially scattered as scattered light  44  by the small domains  52  and transmitted by the large domains  50 , giving rise to transmitted light  46  and the appearance of the image  16 . In one embodiment, as the voltage increases, all of the domains, including the large domain  50  and the small domain  52 , are aligned such that incident light  40  is transmitted as transmitted light  46 . 
         [0028]    With respect to the driving voltage characteristics of  FIGS. 4 and 5 , a lower voltage for image distinction/visualization, for example, from about 14 V to about 50 V AC range, may be utilized in  FIG. 4 . For full transmission or a clear, plain window,  ON  at higher ranges in  FIG. 5 , for example, may be from about 65 V to about 110 V AC. Voltage correlates to liquid crystal domain size. The catoptric form includes reflective shapes, including patterns, that are objects like a mold that give shape to a formless subject to create the image  16 . Depending on the reflectivity of the catoptric form, the liquid crystal domain size adjacent to the catoptric form will vary with the liquid crystal domain size being smaller with the increased reflectivity of the catoptric form. In one implementation, the catoptric form includes a UV-reflective material and in one particular implementation, the catoptric form may be aluminum foil. 
         [0029]      FIG. 6  depicts one embodiment of a methodology and process for forming a PDLC light shutter device employing an electronically tunable lens. In the illustrated embodiment, a catoptric form comprising a shape of aluminum foil is utilized. More specifically, at step  62 , a test run is conducted wherein a PDLC regular cure substrate  70  and a reflection gradient  72  are subjected to a UV light treatment. In one embodiment, the reflection gradient  72  may be a gradient with set, increasing levels of reflectivity, for example, that help determine correct reflectivity of a catoptric form  74 . By way of example and not by way of limitation, this may be a strip of aluminum with a greyscale—increasing darkness-gradient imprinted thereon or a metal-based gradient. At step  64 , the catoptric form  74  having a shape  80  is provided. The exposure gradient  72  allows for the selection of appropriate exposure or exposures. At step  66 , a pre-cure or un-cured light shutter device  76  having a flat profile is provided, which may include two substrates disposed substantially parallel and a polymer binder system interposed between the substrates with a plurality of liquid crystals dispersed in the polymer binder system. 
         [0030]    Continuing with the description of step  66 , the catoptric form is placed in selectively temporarily close contact with the second substrate of the pre-cure or un-cured light shutter device  76  such that the catoptric form  76  is subjacent to the second substrate. Then, light is applied in the range of about 300 nm to about 700 nm to cure the liquid crystal microdroplet sizes. In one embodiment, a narrower range of UV-light may be applied. The liquid crystals include inhomogeneous liquid crystal microdroplet sizes corresponding to the shape  80  of the catoptric form  74 . More particularly, the liquid crystal microdroplet sizes vary according to the shape of the catoptric form such that domains of smaller liquid crystal microdroplet sizes correspond to the shape  80  and another domain of larger liquid crystal microdroplet sizes correspond to negative space relative to the shape. 
         [0031]    At step  68 , the catoptric form  16  is removed from the substrate and the light shutter device  78  is cured, thereby providing the liquid crystal light shutter device  12  having a flat profile. The image  16  is a sufficiently cured region  82 , corresponding to the shape  80 , while the negative space relative to the image is a fully cured region  78 , corresponding to the negative space  84  relative to the shape  80 . It should be appreciated that with the catoptric form-technology presented herein smaller, faster-cured liquid crystal domains versus larger, slower-cured liquid crystal domains are present as the catoptric form reflects UV-light to create faster-cured, smaller domain regions. Once formed, the light shutter device  12  provides, in absence of an application of an electric field across the substrates, optic axes of the liquid crystal microdroplets of the larger and smaller domains with no preferred direction and light is scattered, whereby the liquid crystal shutter  10  appears opaque. 
         [0032]    The light shutter device  12  provides, in response to an application of a first electric field across the first and second substrates, the optic axes of the liquid crystal microdroplets of the various or first domains have an aligned direction and light is transmitted therethrough, the optic axes of the liquid crystal microdroplets of the second domains have no preferred direction and light is scattered, whereby the liquid crystal shutter  10  appears opaque with an image thereon. The light shutter device also provides, in response to an application of a second electric field across the first and second substrates, the optic axes of the liquid crystal microdroplets of the first and second domains have an aligned direction and light is transmitted therethrough, whereby the liquid crystal shutter  10  appears transparent. 
         [0033]    The order of execution or performance of the methods and process flows illustrated and described herein is not essential, unless otherwise specified. That is, elements of the methods and process flows may be performed in any order, unless otherwise specified, and that the methods may include more or less elements than those disclosed herein. For example, it is contemplated that executing or performing a particular element before, contemporaneously with, or after another element are all possible sequences of execution. 
         [0034]    While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.