By: Ellen Kay R.Cacatian Introduction Polymer Stabilized ( NEMATIC) Liquid Crystals Polymer Stabilized (CHOLESTERIC) Liquid Crystals Applications Displays & their associated technology have become an important part of everyday life, but development is not standing still. Lower power-consumption displays are increasingly in demand, as power demands from conventional display technologies still limit their usage. October 2010 SHARP Microelectronics of the America PLCs - Polymer Liquid Crystals PDLCs- Polymer Dispersed Liquid Crystals PSLCs Polymer Stabilized Liquid Crystalline WHAT is DIFFERENCE between the three? PDLCs - deals with higher polymer concentration range above 20%wt. - in display application presence of hazy images is a problem. LOWERING the polymer concentration by 10% wt. can improve electro-optical performance. WHY??? Because it has the ability to form POLYMER NETWORKS that stabilize liquid crystals. 1. Polymer Stabilized NEMATIC Liquid Crystals Polymer Stabilized CHOLESTERIC Liquid Crystals 2. TN- twisted Nematic a significant reduction in operational voltage is achieved in low polymer concentration. STN- Super Twisted Nematic undesirable stripping texture can be eliminated & reduction in driving voltage is achieved. The most common LCD that is used for everyday items like watches & calculators. consists of a nematic liquid crystal sandwiched between two plates of glass. Require less power and are less expensive to manufacture However, STN displays typically suffer from lower image quality and slower response time for mainstream laptops, some inexpensive mobile phones In early 90's they had been used in some portable computers such as Amstrad's PPC512 and PPC640 Polymer Network Controlling Factors 1. 2. 3. 4. 5. 6. morphology liquid crystal texture monomer concentration Photo polymerization temperature UV intensity exposure time How is PSLC is prepared? 1. Dissolve & photo polymerize the monomers ( < 5%wt.) in liquid crystal matrix. Formation of a polymer network occurs. 2. The homogenous alignment is achieved through: a. Pretreatment b. Coating c. Rubbing On the glass cell faces is provided. 1 Combining a small quantity of monomer & photo initiator to: 2 Nematic or Cholesteric liquid crystal but in Cholesteric ,addition of Chiral Dopant is needed. 3 After desired texture is achieved, field is applied U.V light is used to Photo polymerize the sample SEM for Morphology analysis The solution contains: (monomer + photo initiator) 3% wt. of either (Diacrylate monomer) BAB or BAB6 with 0.3% wt. photo initiator BME dissolved in E48. After being sandwiched between glass cell faces, the solution is photo polymerized under U.V light source. Then polymer phase separation and network formation take place. Finally the cell was carefully split open to permit study of polymer network using SEM. SEM micrograph: Low network density areas appear near the bottom and again near the top of the image with much higher density in between. The molecular director of the nematic solvent during polymerization was in the vertical direction. A high magnification SEM: image showing the fine scale structure. Note the nodular clusters that compose the aggregate network structure. This structure is unoriented but other polymerization conditions can yield oriented fibrils. A high magnification SEM: image showing the fibrous network morphology FOUR PHASE-CONTRAST OPTICAL MICROGRAPH IMAGES OF THE SAME REGION AT VARIOUS STAGES OF POLYMERIZATION OF A 2 WT% BAB IN TOLUENE AFTER : A) B) C) D) 8s 26s 28s 46s Of UV irradiation, respectively from top to bottom. The formation of the network from a uniform solution (i.e. without an initiator concentration gradient) can be examined in situ as a function of UV photo polymerization time, Optical micrographs below illustrate the results that the structure becomes finer as either the initiator concentration or the UV intensity increases. Because the concentration of free radical species increases. This, in turn, increases the number of polymer aggregates (at fixed monomer concentration) and therefore reduces the size of the aggregate at the point of gelation. Cholesteric Liquid Crystals- have many application as electro-optic materials in thin film devices in as much as nematic liquid crystals do. The presence of polymer network formed at low concentrations provide similar advantages in enhancing the stability of structure & return of crystals director to desired stable configuration. Reducing the switching time, and helping to determine and maintain the poly-domain size. Refers to the orientation of liquid crystals molecules in the vicinity of surface 1.Planar texture - if messogens are confined between closely space plates oriented with rubbing directions parallel to the liquid crystal sample 2. Homeotropic texture if messogens are oriented normal to the surface with the presence of electric field 3. Focal conic Texture - If small electric field is apply normal to plates the messogens experience TORQUE no filed apply Axis of helix formed by directors lies normal to the plates Parallel to the rubbing direction In the planar texture, they reflect circularly polarized light If electric field is increased above threshold value Strong filed Helical structure of focal conic structure is untwisted and it becomes perpendicular to the plates If small electric field is apply normal to plates the molecules experience TORQUE Due to interactions with adjacent molecules & with anchoring effects near the plates this texture formed they scatter light in forward directions. For planar texture The Cholesteric Liquid crystals display selective reflection The maximum reflection occurring at a wavelength equal to the pitch which may be pre-selected by choices of materials. Focal conic texture it scatters the light of all frequencies It is formed during the initial stage of film preparation Processes is identical to Nematic but small amount of CHIRAL DOPANT is added To produce the desired Cholesteric PITCH It mimics the texture of ordinary Cholesteric mesophase are sometimes referred to by the acronym PSCT (Polymer Stabilize Cholesteric texture) A liquid crystal material that is broken up into small domains Usually do not necessarily coincide with the boundaries between regions of common helicity in the focal conic texture Even in the planar texture, polydomains have slightly different orientations relative to each other important in many factor of Display purposes Reflective Cholesteric Liquid Crystals Light Shutter HOW?? If pitch is at visible wavelength it obeys SELECTIVE REFLECTION At Planar state incident light = bright At Focal state incident light = true color Planar and Focal state are @ E=O, means locked in and will remain intact until upon acted. Switching from Planar state to focal state requires low voltage, BUT return to focal to planar state requires HIGH voltage pulse. HOW is Bistable Reflective Cholesteric cell prepared? Same as with the formation of (PSLCs) to have short picth & photopolymerize in initial planar state. . ADVANTAGES: 1.Employing this would relatively lowers the power requirement 2.Does not require back lighting & polarizer like traditional LCD 3. Offers high multiplexed passive display and mechanical stability Need to lengthen the Cholesteric pitch To the order of infrared wavelength and bistable effect at E=O is avoided Instead choice of either planar or focal conic stable is determined by time of polymer network formation The starting material is the same as with bistable but concentration of chiral agent is need to, lengthen the PITCH. WILL FOCUS ON: 1. 2. 3. Reverse mode Shutter Normal mode Shutter Illustration of characterization A sol n of longer pitch w/ few percent of reactive monomer This is place in two glasses plate whose inner surface have been coated w/ transparent conducting medium ex. ITO Formation of planar texture Photopolymerization is achieved w/o applying field yields polymer network, w/ a stabilizing the planar between. NOTE: Applying very high electric field will switch to an untwisted texture. It will also deform the polymer network and no longer return to its original planar texture. FIGURE (A) A reverse mode cell with no applied electric field (off state). FIGURE (B) A reverse mode cell with an applied electric field (on state). It is polymerized in homeotropic texture produce by strong field If Electric field is removed the cell settle into focal conic structure In the absence of an electric field the cell scatters light and is opaque. With the application of an electric field the cell is once again transparent With regard to electro-optic properties normal and reverse mode cells depend on voltage applied to cell NOTE: As the VOLTAGE across the NORMAL mode cell increases the TRANSMITTANCE increases A lower voltage yield Low transmittance High voltage produces Homeotropic texture w/ high transmittance FIGURE (C) A normal cell with an Electric field applied (on state) FIGURE (D) A normal cell without an electric field (off-state) normal mode shutter which is opaque (light scattering) in the field-OFF state, transparent in the field-ON state. reverse-mode shutter with the opposite field conditions. The transparent state of both cells is haze-free for all viewing angles making the material attractive for window applications and direct view displays. Transmittance of reverse mode cell increases as the voltage decreases Maximum transmittance occurs at voltage = zero In reverse mode cell s minimum transmittance occurs at highvoltage LIGHT SHUTTER application: The choice of mode depends upon on the desired operations Polymer Stabilized Cholesteric Texture (PSCTs) HOW IS IT FORMED??? Photo curable monomers dispersed in a cholesteric liquid crystal mixture to form it Initially low concentration of monomer WHY LOW concentration of Polymer? 1. 2. 3. If not it could yield dense polymer networks This would result to a significant light scattering in focal conic state, but in planar state color and brightness is reduced by scattering of light. However high concentration of interest for its structural benefit, it provides self-adhering & self-sustaining structure necessary for flexible devices on large area of polymer. 1st method: involves irradiating only selectives areas of cell containing ultra-violet (UV) curable monomers and light crystals with UV light through a photo mask. 2nd method: uses the same materials but the polymer is attracted to the interpixel region by a patterned electric field. This is done by etching a cross pattern of indium tin oxide (ITO) on The substrate. The temperature is then decreased to phase separate the sample while an electric field is applied by means of the ITO.
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