pdlc1

NEMATEL__________________________________________________________________________________________________________________________________Polymer Dispersed Liquid Crystals (PDLC) * What is PDLC? Conventional polymer dispersed liquid crystal (PDLC) films consist of a thin (10- 25µm) film of polymer that contains micron-sized droplets of a nematic liquid crystal (Figure 1). In the undriven state the liquid crystal director in the nematic droplets has no preferred orientation with respect to the plane of the film, and a difference between the refractive index of the polymer and the average refractive index of the liquid crystal results in scattering of incident light. For this to occur efficiently the droplet diameter should be of the same order of magnitude as the wavelength of visible light. The film therefore appears opaque. When an electric field is applied to the film, the nematic liquid crystal in the droplets reorients so that the director is parallel to the field, and therefore perpendicular to the plane of the film. If the ordinary refractive index of the liquid crystal (no) is matched to the refractive index of the polymer, then light incident normal to the film does not encounter any variation in refractive index, and passes through the film without being scattered. Such films are therefore opaque in the off state but become clear when a voltage is applied. On removal of the electric field the anchoring forces between the liquid crystal and the polymer droplet walls act to restore the liquid crystal molecules to their original orientation, and the film once again becomes scattering. However, for angles of incidence other than normal, the effective refractive index of the liquid crystal also contains a component of ne, and is not therefore exactly matched to the polymer refractive index. As a result, a small amount of scattering occurs, increasing with angle. These films therefore appear slightly hazy when viewed away from the normal (Figure 2).Uses of PDLCThe electrically controlled scattering behaviour enables the PDLC film to act as a light shutter, and as such finds potential use not only as a display technology, but also in other non-display applications. PDLC films, which can be laminated between a variety of transparent, conducting coated substrates (e.g. glass or polyester sheet), have been suggested, demonstrated and commercialised in a variety of novel products. The best known can be classified as vision products, i.e. windows and internal partitions, but other applications such as projection screens and lampshades have also been suggested. In some cases dyes have been incorporated and simple reflective displays have been demonstrated. In the future more applications for PDLC may become reality, including the use in complex displays for projection television where silicon active matrix is used as one substrate.*With kind permission of Merck KGaA, Darmstadta) PDLC film in the off-state, no applied voltage. There is no overall orientation of the liquid crystal molecules within the droplets. Light scattering occurs at the interface between the droplets and the polymer and the polymer matrix due to the refractive index mis-match and the film appears opaqueb) PDLC film in the on-state. The liquid crystal molecules within each droplet are aligned with the applied electric field. Normally incident light passes through unaffected and the film appears clear. At wider angles (bottom ray) slight scattering can occur giving rise to off-axis hazeMethods of Preparation PDLC films were first demonstrated by Ferguson in 1981, who made the film by emulsifying a polymer, water and a liquid crystal mixture -this method of preparation led to a so-called NCAP (Nematic Curvilinear Aligned Phase) film. Other methods of preparing films having similar electro-optical properties were later described by Doane et al., the most important of these methods being the polymerisation of a homogeneous solution of liquid crystal and pre-polymer. As the resultant polymer forms it causes the liquid crystal to 'phase separate', ideally in the form of discrete droplets. This technique is usually referred to as Polymerisation Induced Phase Separation (PIPS). The polymerisation process may be initiated by heat (e.g. an epoxy resin/curing agent) or by UV light (e.g. an acrylate or thiol-ene system). Other methods of film preparation have also been demonstrated where the phase separation process is controlled by temperature change (TIPS) or solvent evaporation (SIPS). Technical Considerations 1. Off-State Opacity The opacity of a PDLC film is dependent upon: • The birefringence of the liquid crystal. A high birefringence increases the difference between the refractive index of the polymer (no) and the average refractive index of the liquid crystal, and therefore leads to more scattering. The use of lower birefringence liquid crystal mixtures leads to a lower opacity. but a wider viewing angle can be reached before the film starts to look hazy. The film thickness Increasing the film thickness increases the amount of scattering The liquid crystal droplet diameter For optimum scattering of white light the droplet diameter should be 1-2µm. At smaller diameters the shorter wavelengths become preferentially scattered and the film takes on a reddish hue. The droplet diameter can be controlled by the rate of phase separation as the film is produced. The proportion of liquid crystal phase separated out of the polymer matrix• • • ••2. On-state clarity The clarity of the film in the driven state is optimised by matching the refractive index of the polymer (no) and the ordinary refractive of the liquid crystal (no). In practice, some liquid crystal does not separate, but remains dissolved in the polymer, thus increasing (no). The recommended Merck pre-polymer/liquid crystal combinations have been designed to taking this factor into account.As the viewing angle of a PDLC film increases the clarity tends to drop due to off- axis haze. This results from the incident light encountering a higher effective refractive index in the liquid crystal due to an increasing contribution from the extraordinary refractive index (ne). Off-axis haze can be reduced by using a lower birefringence mixture, but at the expense of the opacity in the off state. 3. Switching voltage The operating voltage is typically in the range 20-80 volts, but very low voltage films for active matrix use are also possible. The voltage required to switch a PDLC film is dependent upon many different factors, including: • • • • • film thickness dielectric anisotropy of the liquid crystal (∆ε) resistive and dielectric properties of the film liquid crystal droplet size and shape -smaller droplets require higher voltages for the liquid crystals to be aligned by an electric field configuration of the nematic director within the droplets4. Response times These are principally governed by droplet size and shape, viscosity, elastic constants, anchoring forces and drive voltage. The total response time (ton + toff) is typically in the range of tens of milliseconds.References Much information is available in the scientific literature on the principles, preparation and uses of PDLC devices. Details of a selective number of these given below: General reviews of PDLC 1. J W Doane, "Polymer Dispersed Liquid Crystal Displays" in IlLiquid Crystals, Applications and Uses", Editor B Bahadur, World Scientific (1991) 2. P S Drzaic, "Liquid Crystal Dispersions", World Scientific, (1995) 3. D Coates, J.Mat.Chem., 5(12),2063-2072, (1995) Merck PDLC Publications 1. "Liquid Crystal Mixtures tor Polymer Matrix Displays", D Coates, S Greentield, Sage and G Smith, SPIE Proceedings, 1257, 39 (1990) 2 "Reverse Mode PDLC Display incorporating a Dual Frequency Addressable Liquid Crystal", P No/an and D Coates, Mol. Cryst.Liq. Cryst.Letters, 8(4), 75. (1991)3. "Liquid Crystal Microdroplet Composition in a UV Cured PDLC film", Mol. Cryst.Liq.Cryst.Letters, 8(6), 129, (1992) 4. "High On-state Clarity Polymer Dispersed Liquid Crystal Film", P Nolan, M Tillin and D Coates, Liquid Crystals, 14(2), 339, (1993) 5. 6. 7. 8. "Recent Develapments in Materials tar TFT/PDLC Devices", D Caates, S Greenfjeld, M Gaulding, E Brawn and P Nalan, SPIE Proceedings, 1911, 2 (1993) "Optimised PDLC for Active Matrix Addressed Light Valves in Projection Systems", E Ginter, E Leuder, T Kalifass, S Huttelmaier, M Dobler, V Hochholzer, D Coates and M Tillin, Eurodisplay, 105, (1993) "Reflective Mode PDLC Displays -Paperwhite Display", P Nolan, M Tillin, D Coates, E. Ginter, E. Lueder and T. Kalifass, Eurodisplay, 397 (1993) "Normal and Reverse Mode PDLC Devices", D Coates, Displays, 14(2), 94, (1993)Merck Licrilite® Materials for PDLC Merck has developed a range of liquid crystal mixtures for PDLC applications, as well as a number of pre-polymer mixtures that have been optimised for use with the liquid crystal. Materials for Vision products The BL range of liquid crystal mixtures is designed for preparation of PDLC films by photoinduced phase separation from a mixture with a pre-polymer, such as MXM035 or SAM 114. The liquid crystal is typically used in a 50:50 ratio with the pre-polymer , so that a freestanding coherent film is formed, suitable for lamination to other substrates. Materials for TFT/PDLC By using an active matrix backplane to drive the PDLC film, complex displays can be made. The absence of polarisers, and the efficient scattering achieved by the use of a high birefringence liquid crystal, makes these devices attractive far use in projection displays. The TL range of mixtures use components which provide a high birefringence while maintaining high stability, and good resistivity and voltage holding ratio (VHR) characteristics. A minimum specification of 1 x 1012 Ω⋅cm is set for the bulk resistivity and a VHR > 98% is obtainable. Low voltage PDLC films can be made using about 80% TL liquid crystal in combination with a suitable polymer such as PN393. This is a UV-curable pre- polymer mixture which, when cured with low power light at a controlled temperature (e.g. Philips "Blacklight", 4-6 mW/cm² at 25-30°C ) provides films with a switching voltage (V90) of 6-8V in a 10µm cell. The high birefringence value of the liquid crystal allow high contrast ratios in projection systems; values of > 40:1 on screen are possible.Merck Licrilite® Materials for PDLC PDLC film preparation The liquid crystal and the pre-polymer are mixed in the desired ratio by stirring at room temperature until homogeneous, and then filled into a suitable cell (ITO-coated glass, cell gap typically 10-20µm) by capillary action. Vacuum filling should not be used as this may lead to evaporation of the more volatile components in the pre- polymer. Alternatively the liquid crystal/pre-polymer mixture may be coated on to a substrate (e.g. ITO-coated glass or plastic) by a technique such as bar or doctor blade coating. The mixture is cured by exposure to UV light at a wavelength of 350- 360nm. Suitable light sources include low power fluorescent tubes (e.g. Philips "Blacklight", 4-6 mW/cm²) or higher power mercury or metal halide lamps (10-100 mW/cm²). A typical curing schedule is: Lamp intensity at substrate : Exposure time: 4 -14mW/cm² 1 - 2 minsIt is important to cure the material under conditions of controlled temperature and UV power. A temperature in the range 22-30°C is usually optimal. A post cure, at a lower lamp power, is advisable to give full cure. This process is complete when no further change in the N-I of the film or the operating voltage (V90) is observed. In the case of a coating on to an open substrate, the curing must take place in an inert atmosphere to prevent inhibition of the polymerisation by atmospheric oxygen. A second substrate with electrode may then be laminated to the cured film. It is advisable to experiment with lamp power, cure time, LC concentration and temperature in order to optimise the PDLC film properties.LICRILITE® Polymers for PDLC ApplicationsSAM114 This pre-polymer system is designed for use with BL mixtures, in particular BL036 and BL038. It is designed for coating onto glass or flexible substrates, and is typically used as a 50:50 mixture with the liquid crystal. The pre-polymer contains acrylate materials, which are classified as irritants. Properties of the pre-polymer Viscosity of SAM 114: Viscosity of 50% SAM 114 + 50wt/wt % BL036: Maximum solubility of BL036 in SAM114: Properties of the polymer Refractive Index of Polymer: Refractive Index of Polymer + 20wt/wt % BLO36: Tg of Polymer: Tg of Polymer in PDLC film: 1.514 (20°C,589 nm) 1.533 (20°C, 589 nm) -22°C -38°C 150 cSt (20°C) 157 cSt (20°C) = 55% (20°C)LICRILITE® Polymers for PDLC Applications MXMO35 This is an improved version of SAM114 providing a wider operating temperature range and lower operation voltage. It is intended for use with BL035 and for coating techniques, and is typically used in an approximately 50:50 ratio with the liquid crystal. The pre-polymer contains acrylate materials, which are classified as irritants. Properties of the pre-polymer Viscosity of the pre-polymer: Viscosity of the pre-polymer + 55wtlwt% BLO35: Maximum solubility of BLO35 in MXMO35: Properties of the polymer Refractive Index of the polymer: 1.482 Refractive Index of the polymer + 20wt/wt% BLO35: 1.507 Tg of the pure polymer: Tg of the polymer in a PDLC film: Typical electro-optical properties The electric-optic curve of a 20 ~m thick PDLC film containing 55wt/wt % BL035 in 45wt/wt% MXM035, measured at ambient temperature, is shown below. A film of this kind will operate satisfactorily from -10°C to +50°C. -37°C -53°C 36cSt (20°C) 65cSt (20°C) <60% (20°C)LICRILITE® Polymers for PDLC ApplicationsPN393This pre-polymer system is specifically formulated far use with Merck TL mixtures, and is designed to be used over active matrix substrates. Use of a high ratio (up to 80%) of the TL Liquid Crystal mixture allows a low operating voltage to be achieved. If other liquid crystals, such as BL mixtures, are used the wrong morphology (reverse morphology) may result.The pre-polymer contains acrylate materials, which are classified as irritants.Properties of the Pre-polymerViscosity of PN393 : 3cSt (20°C)Viscosity of 80 wt/wt % TL205 + PN393: 36cSt (20°C)Maximum solubility of TL205 in PN393: 81 % (20°C)Properties of the PolymerRefractive Index of Polymer: 1.473 (20°C, 589nm)Refractive Index of Polymer + 20wt/wt % TL205: 1.499 (20°C, 589nm)Tg of Polymer: -27°CTg of Polymer in PDLC film: -32°C。