Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
  • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
  • G02F1/1333—Constructional arrangements; Manufacturing methods
  • G02F1/1334—Constructional arrangements; Manufacturing methods based on polymer dispersed liquid crystals, e.g. microencapsulated liquid crystals

Definitions

• the present invention relates to a method of forming a polymer dispersed liquid crystal cell. It also relates to a cell produced by such method and to uses of such cells.
• NCAP nonlinear aligned phase
• PDLC-technique polymer-dispersed liquid crystal
• PIPS polymerization-induced phase separation
• the solubility of the liquid crystal decreases as the polymers lengthen until the liquid crystal forms droplets within a polymer network, or an interconnected liquid crystal network forms within a growing polymer network, or the polymer forms globules within a liquid crystal sea.
• the polymer starts to gel and/or crosslink it will lock the growing droplets or the interconnected liquid crystal network thereby arresting them/it in their/its state at that time.
• the droplet size and the morphology of droplets or the dimensions of the liquid crystal network are determined during the time between the droplet nucleation/initiation of network formation and the gelling of the polymer.
• PIPS Polymerisation induced phase separation
• Droplet size and morphology are determined by the rate and the duration of polymerisation, the types of liquid crystal and polymers and their proportions in the mixture, viscosity, rate of diffusion, temperature and solubility of the liquid crystal in the polymer (West, J.L., Phase-separation of liquid-crystals in polymer. Molecular Crystals and Liquid Crystals, 1988. 157: p. 427-441, Golemme, A., Zumer, S., Doane, J.W., and Neubert, M.E., Deuterium mnr of polymer dispersed liquid crystals. Physical Review a, 1988. 37(2): p. 599-569, Smith, G.W.
• Vaz, N.A. The relationship between formation kinetics and mi- crodroplet size ofepoxy based polymer-dispersed liquid-crystals. Liquid Crystals, 1988. 3(5): p. 543-571, Vaz, N.A. and Montgomery, G.P., Refractive-indexes of polymer-dispersed liquid-crystal film materials - epoxy based system. Journal Of Applied Physics, 1987. 62(8): p 3161-3172).
• UV ultraviolet light
• the rate of curing may be changed by changing the light intensity (Whitehead Jr, J.B., Gill, N.L., and Adams, C, Characterization of the phase separation of the E7 liquid crystal component mixtures in a thiol-ene based polymer. Proc. SPIE, 2000. 4107: p. 189).
• the PIPS method using free-radical polymerisation is by far the most studied, and the majority of free-radical polymerisation systems are initiated by UV light.
• the process has several advantages over other methods such as, better phase separation, uniform droplet size, and better control of the droplet size.
• the presence of dyes that absorb UV and visible radiation in the mixture prior to curing can lead to incomplete or the complete prevention of successful curing.
• the dyes may decompose upon curing.
• the phase separation is generally not fully complete and so some dyes and liquid crystal may remain trapped in the polymer after curing, the presence of such dyes in the polymer often results in a degradation in the optical performance of the films.
• TIPS thermal induced phase separation
• This technique can be used for liquid crystal materials and thermoplastic materials which are capable of forming a homogenous solution above the melt temperature of the polymer.
• the homogenous solution of liquid crystal in the thermoplastic melt is cooled below the melting point of the thermoplastic material, thereby causing a phase separation of the liquid crystal.
• the droplet size of the liquid crystal is determined by the rate of cooling and a number of other material parameters.
• Examples of TIPS-prepared composites are polymethylmethacrylate (PMMA) and polyvinylformal (PVF) with cyanobiphenyl liquid crystal.
• PMMA polymethylmethacrylate
• PVF polyvinylformal
• concentrations of liquid crystals required for TIP S-f ⁇ lm are larger in comparison to PlPS-prepared films.
• SIPS solvent- induced phase separation
• a liquid crystal and a thermoplastic material dissolved in a common solvent thereby forming a homogenous solution.
• the ensuing evaporation of the solvent results in phase separation of the liquid crystal, droplet formation and growth, and polymer gelation.
• Solvent evaporation can also be used in conjunction with thermal processing of materials which melt below their decomposition temperature.
• First of all films are formed on a suitable substrate using standard film coating techniques, e. g. doctor blading, spin coating, web coating, etc.
• the solvent is thereafter removed with no concern of droplets size or density.
• the film is warmed again to re-dissolve the liquid crystal in the polymer and then cooled at a rate which is chosen to give the desired droplet size and density, hi effect, the latter example is a combination of SIPS with TIPS.
• a further technique used for the construction of PDLC films is the emulsification of the liquid crystal into an aqueous solution of a film-forming polymer ("emulsion method").
• emulsion method This emulsion is coated onto a conductive substrate and allowed to dry. As the film dries, the polymer forms a solid phase which both contains and supports the dispersed liquid crystal droplets. Lamination of a second conductive substrate leads to the final PDLC film.
• emulsion method emulsion method
• This emulsion is coated onto a conductive substrate and allowed to dry. As the film dries, the polymer forms a solid phase which both contains and supports the dispersed liquid crystal droplets. Lamination of a second conductive substrate leads to the final PDLC film.
• One common fea- ture of emulsion-based systems is that the coating undergoes a significant volume change as the film dries. This shrinkage tends to deform the droplets, which are spherical in
• This shape anisotropy affects the alignment of the liquid crystal within the film cavities.
• bipolar droplets in emulsion- based films form with the droplets symmetry axis aligned in the film plane, which in turn affects the electro-optical properties of the film.
• Another problem commonly encountered with PDLC composites is the fact that additional components dissolved in the liquid crystal are sensitive to the phase separation process and are frequently damaged in the course of the polymerization and/or the formation of the polymer matrix. For example it is very difficult to include UV-sensitive dyes which survive photo- induced polymerization. Accordingly it has been a problem to produce PDLC-composites that are coloured by the inclusion of dyes.
• the polymer matrix is formed in the presence of a first material, preferably a liquid crystal material, which - after formation of the polymer matrix - is removed and replaced by a second material that is liquid crystalline.
• a first material preferably a liquid crystal material
• the method involves splitting a cell apart in order to wash out the first material remaining in the polymer matrix.
• this method is somewhat destructive to the polymer matrix because the polymer matrix is often, in effect, torn in half. Therefore the sponge like polymer dispersed liquid crystal cell fabrication (SPDLC fabrication) is not as reproducible and reliable as one would wish for.
• the splitting of the cell involves a tearing of the matrix which introduces inhomogeneities into the matrix.
• the cell is formed between two substrates, then these have to be resistant to any solvents that may be used to wash out the first material.
• the methods for forming SPDLCs as known from the prior art produces inhomogene- ous cells because at some stage of their manufacture they are split apart. Furthermore the methods known from the prior art are only applicable to certain substrates (for example glass) and cannot be applied to substrates that are sensitive to solvents (for example polymeric substrates, OTFT (organic thin film transistors).
• a further problem associated with methods according to the prior art is that one is restricted to single substrate processing, i. e. one is restricted to the substrates that were originally used for the manufacture of the SPDLC. However, it would be desirable to be able to apply commercially useful manufacturing processes such as roll-to-roll processing.
• step b) placing a third substrate on a face of said porous polymer matrix, from which face said second substrate had been lifted off in step b),
• the porous polymer matrix in step a) may also be prepared out of oligomers or oligomers in combination with monomers.
• an initiator may also be present, although such initiator is, for example, not required for PEPS with gamma-rays, TIPS and SIPS, or in a method wherein an emulsion is used ("emulsion method").
• emulsion method a method wherein an emulsion is used.
• Such mixtures comprising monomers and/or oligomers and, optionally, initiators are also herein sometimes referred to as "prepolymer".
• At least said second substrate has surface properties sufficiently dissimilar to surface properties of said porous polymer matrix, allowing said second substrate to be easily lifted off in step b).
• said second substrate has a surface layer that is soluble in a first solvent, and step b) is performed after said second substrate has been immersed in said first solvent.
• said second substrate may be of polymethylmethacrylate, and said first solvent may be methanol.
• said second substrate has substantially hydrophobic surface properties if said polymer matrix has substantially hydrophilic surface properties and vice versa.
• said second substrate has a contact angle of a solution of monomer, or of a solution of oligomer, or of a solution of prepolymer, as defined above, in the range of from 0 to 180 degrees, preferably from 10 to 180 degrees, more preferably greater than 90 degrees, with respect to said second substrate.
• the term "contact angle of a solution of " is meant to denote the angle that a drop of a liquid composition of mono- mer/oligomer/prepolymer (i.e. a solution thereof) adopts when applied to a surface of said second substrate.
• said second substrate has a smooth surface, preferably with a surface roughness not larger than 20 ⁇ m.
• said second substrate has a low surface energy and preferably is selected from the group comprising polyethylene terephthalate (PET), , polymethylmethacrylate, polyvinyl acetate (PVA), polystyrene, acetal, ethyl vinyl acetate (EVA), polyethylene, polypropylene, polyvinylidene fluoride (PVDF, Tedlar®, polytetrafluorethylene, Teflon®), surface modified glass, e.g. silanised glass.
• PET polyethylene terephthalate
• PVA polymethylmethacrylate
• PVA polyvinyl acetate
• EVA ethyl vinyl acetate
• PVDF polyvinylidene fluoride
• Tedlar® Tedlar®
• Teflon® Teflon®
• surface modified glass e.g. silanised glass.
• said porous polymer matrix is made of a material selected from the group comprising PN393 prepolymer, polymethacrylate, polyurethane, PVA and epoxy.
• PN393 prepolymer can be obtained from Merck and FFLivasfluid GmbH, Germany and is a UV- curable acrylate-based polymer.
• said second substrate is selected from the group comprising PET, polyvinyl acetate (PVA), polystyrene, acetal, ethyl vinyl acetate (EVA), polyethylene, polypropylene, polyvinylidene fluoride (PVDF, Tedlar®, polytetrafluorethylene, Teflon®) and said porous polymer matrix is made of a material selected from the group comprising ... polymethacrylate, polyurethane, PVA and epoxy.
• PVA polyvinyl acetate
• EVA ethyl vinyl acetate
• PVDF polyvinylidene fluoride
• Tedlar® Tedlar®
• Teflon® polytetrafluorethylene
• said step d) occurs after step e) or concomitantly with step e).
• said step b) occurs after step c).
• said steps c) and e) occur concomitantly, wherein, more preferably, said steps c), d) and e) occur concomitantly.
• step c) occurs by any one or combinations of the following processes: washing out with a solvent, evaporation, sublimation, degradation, outgassing and suction, wherein, preferably, said second solvent is capable of dissolving said first material, and wherein, more preferably, said second and said first solvent are selected from the group comprising methanol, acetone, toluene, dichloromethane, tetrahydrofuran, (THF), 2-propanol, 1- propanol, water, dimethylformamide (DMF), dimethylsulfoxide (DMSO).
• a solvent evaporation, sublimation, degradation, outgassing and suction
• said second solvent is capable of dissolving said first material
• said second and said first solvent are selected from the group comprising methanol, acetone, toluene, dichloromethane, tetrahydrofuran, (THF), 2-propanol, 1- propanol, water, dimethylformamide (DMF), di
• step c) involving a solvent after step c) there follows step:
• drying preferably under vacuum, wherein, preferably, said drying occurs in a temperature range of from 40°C - 100 0 C, preferably 5O 0 C - 90 0 C, preferably around 80 0 C.
• said first substrate is resistant to dissolution in said solvent, wherein, preferably, second substrate is not resistant to dissolution in said solvent.
• At least one of said first and said third substrate is transparent to visible light.
• said first and said third substrate are electrically conductive or coated with an electrically conductive layer.
• said method includes the additional step:
• step 1) lifting off said first substrate from another face of said porous polymer matrix, which step 1) occurs at any one point selected from the following: between a) and b), concomitantly with step b), between b) and c), between c) and d), and between d) and e), concomitantly with steps b) and e), and concomitantly with steps b), c) and e), wherein, preferably, said first substrate has surface properties sufficiently dissimilar to surface properties of said porous polymer matrix allowing said first substrate to be easily lifted off in step 1).
• the term "concomitantly" when used in conjunction with more than one step, e.g.
• step 1) is concomitant with both steps b) and e
• step 1) occurs at the same time as step b) and at the same time as step e), without b) and e) themselves necessarily being concomitant with each other, although such concomitance is not excluded either.
• said first substrate has at least one feature as defined in relation to said second substrateabove.
• said method comprises the additional step
• step m) placing a fourth substrate on said another face of said porous polymer matrix which step m) occurs after step 1), wherein, preferably, said another face of said polymer matrix is opposite to said face where said third substrate is placed in step d).
• said second material which is liquid crystalline, is dye doped.
• said first material is a liquid crystal material, wherein, preferably, said first and said second liquid crystal materials are different.
• a polymer dispersed liquid crystal cell produced by the method according to the present invention, wherein, preferably, said porous polymer matrix only adheres to one substrate, e.g. said first substrate despite being in contact with said first and said third substrate, or it adheres to no substrate, despite being in contact with said third and said fourth substrate.
• the objects of the present invention are solved by the use of a polymer dispersed liquid crystal cell according to the present invention in a display, a smart window, a membrane, an optical valve, a Bragg grating, an optically sensitive memory, an infrared shutter, a gas flow sensor, an optical wavefront sensor, an optical wavefront corrector, a pressure sensor and/or a polarizer.
• the inventors have surprisingly found that by using an additional substrate to be used temporarily only which is then lifted off the assembly, one can achieve sponge-like polymer dispersed liquid crystal cells (SPDLCs) which are much more homogenous in terms of pore size and structure and which are much easier to reproduce. Furthermore, a greater versatility of substrates is achieved to the extent that also substrates can be used in the resulting SPDLC which, if they were used from the start of the manufacturing process, would be destroyed by the same.
• SPDLCs sponge-like polymer dispersed liquid crystal cells
• FIG 1 shows one embodiment of the method according to the present invention, wherein an substrate (S') is used, which is lifted-off from the SPDLC,
• FIG. 2 shows how the on-state transmittance (TlOO) and off-state transmittance (TO) vary with dye concentration in a dye doped sponge like polymer dispersed liquid crystal cell (D- SPDLC) according to the present invention
• FIG. 3 shows how the driving voltage changes with dye concentration in dye doped sponge like polymer dispersed liquid crystal cells (D-SPDLC) according to the present invention.
• Figure 4 shows the response time characteristics of a flexible D-SPDLC according to the present invention.
• Figure 5 shows the difference between prior art splitting method and the lift-off method according to the present invention. Not much change can be observed macroscopically (apart from irregular inhomogeneity), but microscopically, destruction of polymer network can be observed with the prior art method, while the lift-off method according to the present invention shows none or less destruction on the polymer matrix formed.
• Example 1 uses an additional lift-off substrate (S') to which the polymer matrix does not adhere, as shown in Figure 1.
• This method allows for the fabrication of polymer matrix on one substrate (Sl). LC washout and drying can be done on the Sl that is resistant to the solvent.
• Another substrate (S2) can be placed on Sl to form a cell with polymer matrix, and then the cell can be refilled with a desirable liquid crystal (LC).
• TL213 LC Merck
• PN393 UV-curable polymer FLforensicsfluid
• TL213 is a chlorinated nematic LC mixture suitable for an active matrix display.
• PN393 is a blend of alkyl acrylates with a refractive index of 1.47, and it is cured (polymerized and solidified) by UV light of wavelengths between 350-360nm.
• the cells were irradiated by UV light (360 nm, Spectroline, Model EN-180L/F, 230V, 50Hz, 0.17A) from a distance of 14 mm for 10 minutes at a room temperature of 24°C. As the curing of the poly- mer progressed, the LC became insoluble in the polymer. This phase separation process leads to the formation of a PDLC film or matrix.
• the PET substrate S' was lifted off gently, and the LC was fully removed from of the polymer matrix by washing the opened cell with a solvent (in this report methanol) such that the LC was dissolved in the solvent and removed from the matrix.
• a solvent in this report methanol
• the residual solvent was removed from the polymer matrix by placing the cell in a vacuum oven at 8O 0 C at 20mBar for 3 hours.
• the end result was an open porosity sponge consisting of a polymer matrix with cavities (voids) or pores.
• an ITO coated PET film (S 2) on top of the matrix, on top of Sl, and re-bonding cells together with a suitable edge adhesive a flexible polymer sandwich cell with the porous polymer matrix of voids was created.
• the S2 substrate can be different from the Sl substrate, and it is not limited to PET. Both Sl and S2 are not limited to being a transparent material, although in the case of display one of them should be transparent to visible light, so that one can use the cell as display. Both Sl and S2 must be conductive or have conductive layer such as ITO or FTO, in order to switch the LC display.
• Sl substrate is not limited to PET, i.e. it can e.g. be an ITO coated glass. In this embodiment, Sl substrate has to be resistant to the solvent used, but S2 substrate does not have to be solvent resistant. For example, one can use organic TFT (OTFT) (organic thin film transistors) as S2. hi general OTFT is not resistant against solvent nor is it transparent.
• OTFT organic TFT
• the inventors also envision a process such as roll to roll, for fabricating SPDLCs, wherein a plurality of substrates that exhibit lesser or greater degree of adhesion to the film, is used.
• adhesion of a substrate to the matrix might be controlled by the effect of polymerization or by the addition of solvent and is dependent on the interplay between matrix and substrate.
• SPDLC Sudden PDLC
• Black-4 Black-4 (B4) from Mitsubishi Chemical Japan, which consists of six different azo and anthraquinone dyes mixed together.
• the LC used for the refilling was different from the initial TL213 LC.
• TL203 a nematic LC mixture obtainable from Merck was used for the refilling and it has a nematic to isotropic temperature (T NI ) of 77 0 C, with an n e of 1.73, an n 0 of 1.53, ⁇ n of 0.20 (589 nm at 2O 0 C), and a ⁇ of 11.
• the cell was heated to HO 0 C for 20 seconds and left until it cooled to room temperature.
• This annealing process helps to reduce flow alignment defects in the LC introduced during the filling.
• the temperature can be lower if the LC with low nematic to isotropic temperature is used. This is the only heat applied to substrate S2.
• the electro-optic response properties of flexible D-SPDLCs, as produced in Example 1, with various 0 wt% and 3 wt% dye concentrations were measured.
• the test cells were driven using an amplified DAQ PCMCIA card (6024E, National Instruments) controlled using Lab Windows CVI software (programmed in-house) running on a laptop.
• the response of the cells was measured using an optical microscope (DMRX-HC, Leica) fitted with a photodiode (Edmund Optics NT54035). 6 cells were measured in total; 3 cells refilled of 0 wt% B4 (un- doped) TL203, 3 cells refilled of 3 wt% B4 (undoped) TL203.
• Each of the data points represents the average of the measurements taken of the 3 cells.
• the flexible D-SPDLC switched as normal D-SPDLC with glass substrates.
• Figure 2 shows how the on- state transmittance (TlOO) and off-state transmittance (TO) vary with dye concentration.
• TlOO on- state transmittance
• TO off-state transmittance
• FIG 3 shows how driving voltage change with dye concentration.
• ElO is the electric field required to achieve 10% transmittance of TlOO
• E90 is the electric field required to achieve 90% transmittance of TlOO.
• Figure 4 shows the response time characteristic of the flexible D-SPDLC.
• Rise time is the time taken from when the voltage was applied, to when the cell transmittance reaches 90% of the maximum transmittance (TlOO).
• Decay time is the time taken from when the applied voltage is turned off, to when the cell transmittance reaches 10% of the maximum transmittance (TlOO).
• the rise time stays under 50 ms and shows slight decrease, while the decay time increases with the increase of dye concentration.
• the present invention allows for a greater versatility of substrates that are used in sponge like polymer dispersed liquid crystal cells. It furthermore provides for a greater homogeneity, processability and reproducibility of such cells.

Landscapes

• Physics & Mathematics (AREA)
• Chemical & Material Sciences (AREA)
• Nonlinear Science (AREA)
• Dispersion Chemistry (AREA)
• Mathematical Physics (AREA)
• Crystallography & Structural Chemistry (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Liquid Crystal (AREA)

Priority Applications (2)

Applications Claiming Priority (2)

Publications (1)

Family

ID=35079125

Family Applications (1)

Country Status (5)

Cited By (1)

Families Citing this family (16)

Citations (3)

Family Cites Families (7)

• 2005
  • 2005-02-16 EP EP05003283A patent/EP1693698A1/en not_active Withdrawn
• 2006
  • 2006-01-31 CN CN2012102867290A patent/CN102866525A/zh active Pending
  • 2006-01-31 CN CNA2006800051647A patent/CN101137929A/zh active Pending
  • 2006-01-31 JP JP2007555485A patent/JP2008530618A/ja active Pending
  • 2006-01-31 US US11/816,329 patent/US7649595B2/en not_active Expired - Fee Related
  • 2006-01-31 WO PCT/EP2006/000822 patent/WO2006087095A1/en not_active Ceased

Patent Citations (3)

Non-Patent Citations (2)

Also Published As

Similar Documents

Legal Events