WO2012172084A2 - Matières optiques composites présentant une couleur structurelle - Google Patents

Matières optiques composites présentant une couleur structurelle Download PDF

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Publication number
WO2012172084A2
WO2012172084A2 PCT/EP2012/061513 EP2012061513W WO2012172084A2 WO 2012172084 A2 WO2012172084 A2 WO 2012172084A2 EP 2012061513 W EP2012061513 W EP 2012061513W WO 2012172084 A2 WO2012172084 A2 WO 2012172084A2
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arrangement
configuration
crystallographic
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composite optical
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WO2012172084A3 (fr
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Götz Peter HELLMANN
Peter Wolfgang Andreas Spahn
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DEUTSCHES KUNSTSTOFF-INSTITUT
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DEUTSCHES KUNSTSTOFF-INSTITUT
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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B1/00—Optical elements characterised by the material of which they are made; Optical coatings for optical elements
    • G02B1/002—Optical elements characterised by the material of which they are made; Optical coatings for optical elements made of materials engineered to provide properties not available in nature, e.g. metamaterials
    • G02B1/005—Optical elements characterised by the material of which they are made; Optical coatings for optical elements made of materials engineered to provide properties not available in nature, e.g. metamaterials made of photonic crystals or photonic band gap materials
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B1/00—Optical elements characterised by the material of which they are made; Optical coatings for optical elements
    • G02B1/04—Optical elements characterised by the material of which they are made; Optical coatings for optical elements made of organic materials, e.g. plastics
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/18—Diffraction gratings
    • G02B5/1828—Diffraction gratings having means for producing variable diffraction

Definitions

  • the present invention relates to composite optical materials which demonstrate structural colour characteristics and to methods of manufacturing such composite optical materials.
  • Related art
  • Natural opal is built up from domains consisting of monodisperse silica spheres of diameter 150-400 nm. These spheres are close-packed and therefore form a regular three dimensional lattice structure within each domain. The colour play of such opals is created by Bragg-like scattering of the incident light at the lattice planes of the domains.
  • US-A-4,703,020 discloses the formation of such materials by allowing silica spheres to sediment from an aqueous dispersion. This sediment is then dried and calcined at 800°C. Subsequently, a solution of zirconium alkoxide is allowed to penetrate into the interstices in the sediment and zirconium oxide is precipitated in the interstices by hydrolysis. The material is then calcined again to leave a structure in which silica spheres are arranged in a three dimensional lattice with zirconium oxide in the interstices. Forming opal-like materials in this way is exceptionally time-consuming and expensive. It is not an industrially-applicable route for the manufacture of significant quantities of materials.
  • US 2004/0253443 discloses moulded bodies formed from core-shell particles.
  • Each particle is formed of a solid core, and the solid cores have a monodisperse particle size distribution.
  • Each particle has a shell formed surrounding the core.
  • the core and shell have different refractive indices.
  • the core is formed of crosslinked polystyrene and the shell is formed of a polyacrylate such as polymethyl methacrylate (PMMA).
  • PMMA polymethyl methacrylate
  • the core has a relatively high refractive index and the shell has a relatively low refractive index.
  • a polymer interlayer may be provided between the core and shell, in order to adhere the shell to the core.
  • Granules of the core-shell particles are heated and pressed to give a film.
  • shell material is soft but the core material remains solid.
  • the cores form a three dimensional periodic lattice arrangement, and the shell material becomes a matrix material.
  • the resultant material demonstrates an optical opalescent effect.
  • US 2004/0253443 suggests mechanisms to explain the ordering of the core particles in the matrix, but these are not fully explained.
  • WO2004096894 provides similar disclosure to US 2004/0253443, and additionally proposes extruding the composite material as a sheet and subsequently rolling the material. The result is reported to be a uniform colour effect depending on the viewing angle.
  • the basic colours of opal-type colloidal crystals have been described earlier and are well understood.
  • the "structural colour” characteristics arise because of scattering and interference of light at the lattice planes of the crystal.
  • the colour depends on the set of lattice planes which scatter the light, and specifically the lattice constant of these lattice planes which in turn depends on the bead size and the angle under which the light meets the set of lattice planes.
  • the strongest colour arises from the (1 1 1 ) lattice planes which are parallel to the surface of the specimen. This colour is dependent on the viewing angle, but invariant to a rotation of the sample.
  • An advantage of the colloidal crystal films with continuous polymeric matrices is their deformability. Unlike in a suspension, deformation of a polymeric opal structure leads to a distortion of the whole lattice of the crystal. Depending on the kind and elasticity of the polymer, the deformation can be large and reversible. Strain induced colour changes have been observed and described in the following literature:
  • the present inventors consider that it is of interest to further develop composite optical materials in order to provide a composite optical material that displays structural colour that varies with rotation of the material about an axis perpendicular to the surface of the material, even when the viewing angle remains constant.
  • Such materials would be of interest for various applications, including security applications such as on financial instruments and documents.
  • the present invention has been devised in order to address the want of such a
  • the present invention provides a composite optical material having a three dimensional arrangement of core particles distributed in a matrix, the refractive index of the material of the core particles being different to the refractive index of the material of the matrix and the three dimensional arrangement having a periodicity such that, when a surface of the material is illuminated with white light, the composite material exhibits structural colour, wherein the three dimensional periodic arrangement is a reduced symmetry arrangement based on a crystallographic close-packed arrangement but strained therefrom to have reduced symmetry about a rotational axis perpendicular to the surface compared with the crystallographic close- packed arrangement so that Bragg reflections from planes disposed at oblique angles to the surface that would be forbidden in the crystallographic close-packed arrangement or would be subject to total internal reflection at the surface are allowed and visible from the reduced symmetry arrangement, thereby providing a variation in intensity and/or wavelength of structural colour with rotation of the material about an axis perpendicular to the surface when viewed obliquely
  • the present invention provides a method for manufacturing a composite optical material, the method including the steps:
  • the composite optical material in a first configuration of the composite optical material, providing a crystallographic close-packed arrangement of core particles distributed in a matrix, the refractive index of the material of the core particles being different to the refractive index of the material of the matrix and the arrangement having a periodicity such that, when a surface of the material is illuminated, the
  • the arrangement exhibits Bragg reflection, optionally in the visible spectrum, the crystallographic close-packed arrangement having a degree of symmetry such that Bragg reflections from at least one set of crystallographic planes are forbidden or are subject to total internal reflection at the surface;
  • the present invention provides a composite optical material obtained by, or obtainable by, a method according to the second aspect.
  • the present invention provides a composite optical material, having a first configuration and a second configuration, wherein:
  • a crystallographic close-packed arrangement of core particles is provided distributed in a matrix, the refractive index of the material of the core particles being different to the refractive index of the material of the matrix and the arrangement having a periodicity such that, when a surface of the material is illuminated, the arrangement exhibits Bragg reflection, optionally in the visible spectrum, the crystallographic close-packed arrangement having a degree of symmetry such that Bragg reflections from at least one set of crystallographic planes are forbidden or are subject to total internal reflection at the surface;
  • the crystallographic close-packed arrangement is strained to provide a reduced symmetry arrangement having reduced symmetry about a rotational axis perpendicular to the surface compared with the crystallographic close-packed arrangement so that Bragg reflections from planes disposed at oblique angles to the surface that were forbidden in the crystallographic close-packed arrangement or subject to total reflection at the surface are allowed and visible from the reduced symmetry arrangement, the composite optical material being reversibly deformable from the first configuration to the second configuration.
  • the material when sufficiently deformed, the material preferably mechanically fails before the strain of the deformation is sufficient to force the reduced symmetry arrangement to return to rotational symmetry of the crystallographic close- packed arrangement.
  • the material may fail by tearing, cracking or localised plastic yielding.
  • the reduced symmetry arrangement is substantially irreversible in the composite optical material. This is advantageous for security device applications, e.g. to reduce the risk of tampering with the security device.
  • the method further includes the step of treating the composite optical material in order to constrain the reduced symmetry arrangement from returning to the crystallographic close-packed arrangement.
  • Such treatment may include increasing the stiffness of the matrix material in the second configuration compared with the stiffness of the matrix material in the first configuration.
  • the matrix material may be treated by cross-linking treatment.
  • cross-linking treatment can be carried out by incorporation of a suitable cross-linking component in the matrix material, e.g. a UV-activatable cross-linking component.
  • the matrix material may be treated to raise the glass transition temperature (Tg) of the matrix material. This may be done by removing a softening component from the matrix (e.g. by evaporation on heating). This has a similar effect to cross-linking, explained above.
  • Tg glass transition temperature
  • the matrix may be in the first configuration above Tg of the matrix material, and then cooled to below Tg in the second configuration. This has a similar effect to cross-linking, explained above.
  • the composite optical material may be permanently bonded to a support, in order to hold the matrix material and thus prevent return of the reduced symmetry arrangement to the crystallographic close-packed arrangement.
  • the composite optical material is in the form of a film or layer and is intended to be affixed to a support, e.g. to provide an indication of the authenticity of the support, such as a banknote.
  • the core particles are disposed in the composite optical material in an arrangement based on a three dimensional crystallographic close packed lattice.
  • the core particles are disposed in the composite material in an arrangement based on a face centred cubic lattice.
  • a ⁇ 1 1 1 ⁇ plane (nominally the (1 1 1 ) plane) of the lattice is aligned substantially parallel to a major surface of the composite optical material.
  • major surface it is intended that the composite optical material is in the form of a sheet, film or layer.
  • the step of transforming the first configuration of the composite optical material to the second configuration includes the step of straining the crystallographic close-packed arrangement along a direction that is not parallel to a close packed direction in the crystallographic close-packed arrangement.
  • close packed direction in the crystallographic close-packed arrangement it is intended to refer to the lines of close packing that occur in crystallographic close-packed arrangements.
  • the close packed directions contained in the (1 1 1 ) plane are ⁇ 1 10> type directions.
  • the step of transforming the first configuration of the composite optical material to the second configuration includes the step of straining the crystallographic close-packed arrangement along a direction that is substantially perpendicular to a close packed direction in the crystallographic close-packed arrangement.
  • the directions perpendicular to close packed directions are ⁇ 221 > type directions.
  • the close packed direction of interest is [10-1 ]
  • the perpendicular directions are [1 -21 and [-12-1 ].
  • the strain applied to the composite optical material is tensile strain.
  • the strain applied during the step of transformation from the first configuration to the second configuration is at least 40%. More preferably, the strain applied during the step of transformation from the first configuration to the second configuration is at least 50%, still more preferably at least 60%. As will be appreciated, this amount of strain is extremely high. The present inventors have found that even under such high strain, the strain is taken up substantially homogeneously by the composite optical material, allowing similar crystalline transformations and/or crystalline reorientation to occur across the composite optical material and thereby ensuring that the structural colour exhibited is substantially uniform across the composite optical material.
  • the composite optical material is typically formed (as explained elsewhere in this disclosure) by shear processing of a precursor composite material. It has been found that suitable ordering of the core particles in the matrix can be obtained by repeated shearing back and forth along a shear processing direction. Such processing tends to produce a composite optical material in which a close packed direction is parallel to the shear processing direction.
  • the strain applied during the step of transformation from the first configuration to the second configuration is not parallel to the shear processing direction. More preferably, the strain applied during the step of transformation from the first configuration to the second configuration is substantially perpendicular to the shear processing direction.
  • the primary reflections from the material in the unstrained configuration are typically (1 1 1 ) reflections for an fee lattice.
  • the reflection from these planes which are typically parallel to the surface, are strongest when the viewing angle and the illumination angle are the same.
  • the gloss and the remission of the surface are strongest in the same direction. Therefore, the (1 1 1 ) colour is usually impaired by background light, typically white background light. This reduces the perceived intensity of the (1 1 1 ) structural colour.
  • the structural colour appears on a darker background, because the gloss and remission of the surface typically are not strong in the directions in which the structural colour from those oblique planes are strongest.
  • the intensity of the reflections from the reduced symmetry arrangement i.e. the second configuration
  • that of the close packed arrangement i.e. the first configuration
  • the intensity of at the reflections from the reduced symmetry arrangement in at least one oblique viewing direction is at least 0.1 , more preferably at least 0.2.
  • the intensity of at the reflections from the reduced symmetry arrangement in at least one oblique viewing direction is substantially the same as the (1 1 1 ) reflections from the same reduced symmetry arrangement.
  • the reflections from the oblique planes visible in the reduced symmetry arrangement are seen in a range of rotational angles of about ⁇ 25° from a maximum intensity of the reflections.
  • the reflections from the oblique planes visible in the reduced symmetry arrangement are seen in a range of tilt angles of about ⁇ 13 ° from a maximum intensity of the reflections. Rotational and tilt angles are explained in Fig. 3.
  • the second configuration exhibits Bragg reflection from at least one set of planes that would be forbidden in an fee structure.
  • the inventors speculate that the crystallographic structure of the composite optical material in the second configuration is a monoclinic structure.
  • the core particles have a difference in refractive index compared with the matrix material, of at least 0.001 , more preferably at least 0.01 , still more preferably at least 0.1 .
  • a population of core-shell particles in order to form the composite optical material, there is provided a population of core-shell particles.
  • Each core-shell particle preferably comprises a core and a shell material surrounding the core.
  • the population may take the form of granules.
  • the population is heated to a temperature at which the shell material is flexible and soft.
  • the population is then preferably subjected to the action of a mechanical force to initiate three dimensionally periodic arrangement of the core particles in a matrix of the shell material. This mechanical force is preferably provided by an extrusion process.
  • the result of the extrusion process is typically a ribbon of precursor composite material.
  • the ribbon of precursor composite material is captured and held between first and second sandwiching layers.
  • the resulting structure may then be rolled (or calendered or otherwise pressed) in order to cause the precursor composite material to flow further.
  • suitable structural colour can be achieved at this point in the process and further colour enhancement steps may not be necessary.
  • the composite material is then preferably allowed to cool to a temperature at which the shell material is no longer soft.
  • the resulting sandwich structure can then be subjected to further processing in order to provide the required degree of periodicity of the core particles in the matrix.
  • Such a subsequent colour enhancement step is considered to be suitable for thin opal films (e.g. of thickness less than 200 ⁇ ) which are preferred in some embodiments of this invention.
  • Other processes can be used in place of extrusion.
  • the action of mechanical force may take place via one or more of: uniaxial pressing (e.g.
  • the precursor composite material is preferably in the form of a film or layer. Suitable films or layers can preferably also be produced by calendering, film blowing or flat-film extrusion.
  • the demoulding When the precursor composite material is produced by injection moulding, it is particularly preferred for the demoulding not to take place until after the mould with moulding inside has cooled.
  • the mould may advantageously be heated before the injection operation.
  • the processes set out above rely on mechanical shearing of the film in order to produce the required periodicity in the film.
  • a dispersion e.g. an aqueous dispersion
  • the core-shell particles can be dried in order to form the required film. Because of the absence of shear, the orientation of the lattice is not so easy to obtain as in the processes discussed above.
  • the (1 1 1 ) planes form the surface of the film, but for further orientation (closely packed strips of particles a discussed below for the shear processes) a directional, vertical drying is carried out as described in Jiang et al (1999) [Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. "Single-Crystal Colloidal Multilayers of Controlled Thickness” Chem. Mater. 1999, 1 1 , 2132-2140] and Wohlleben (2007) [Wendel Wohlleben, Frank W. Bartels, Stephan Altmann, and Reinhold J.
  • the film can be formed of polystyrene- polyethylacrylate core shell particles.
  • the core particles have a substantially monodisperse size distribution.
  • the size of the core particles depends on the intended wavelength(s) at which the composite optical material should provide the required optical effect(s). For example, it may be desirable for the core particles to have a mean particle diameter in the range from about 5 nm to about 2000 nm. More preferably, the core particles have a mean particle diameter in the region of about 50-500 nm, more preferably 100-500 nm. Still more preferably, the core particles have a mean particle diameter of at least 150 nm. The core particles may have a mean particle diameter of at most 400 nm, or at most 300 nm, or at most 250 nm.
  • the material of the core particles remains substantially rigid and substantially undeformed during the process. This can be achieved by: using a high crosslinking density in the core particles; and/or by using processing temperatures below the glass transition temperature (Tg) of the core material.
  • Tg glass transition temperature
  • a suitable degree of crosslinking may be, for example, 1 % crosslinking density or higher. More preferably, the degree of crosslinking is 2% or more, more preferably about 10% crosslinking density.
  • inorganic core materials may be used.
  • the shell of the core-shell particles is bonded to the core via an interlayer.
  • Suitable composite optical materials may be manufactured by suitable shearing of the precursor composite material, typically between first and second sandwiching layers.
  • suitable shearing of the precursor composite material typically between first and second sandwiching layers.
  • one suitable approach is to deform the precursor composite material progressively and repeatedly over a hot edge. This is disclosed, for example, in WO
  • the ordering of the core particles in the precursor composite material begins preferentially at the interfaces between the precursor composite material and the sandwiching layers. During the process, it is considered that the ordering then extends inwards into the precursor composite material. Therefore, at large thickness values, precise ordering of the material at the centre of the structure may not be achievable. However, for such large thickness values, this may not be a problem because there will be very many ordered layers nearer the interfaces with the sandwiching layers.
  • the thickness of the composite optical material is at most 1 mm. More preferably, the thickness of the composite optical material is at most 0.5 mm, or at most 0.4 mm, or at most 0.3 mm.
  • the thickness of the composite optical material is preferably at least 10 ⁇ , since thinner structures may not have sufficient mechanical integrity for practical uses and may not provide sufficiently strong reflections in order to exhibit a significant structural colour effect. More preferably, the thickness of the composite optical material is at least 20 ⁇ , or at least 30 ⁇ , or at least 40 ⁇ , or at least 50 ⁇ , or at least 60 ⁇ , or at least 70 ⁇ , or at least 80 ⁇ . A thickness of about 100 ⁇ has been found to be suitable, for example.
  • the composite optical material and/or the precursor composite material may comprise auxiliaries and/or additives. These can serve in order to provide desired properties of the body. Examples of auxiliaries and/or additives of this type are antioxidants, UV light, or ultraviolet light, or ultraviolet light, or ultraviolet light, or ultraviolet light, or ultraviolet light, or ultraviolet
  • stabilisers biocides, plasticisers, film-formation auxiliaries, flow-control agents, fillers, melting assistants, adhesives, release agents, application auxiliaries, demoulding auxiliaries and viscosity modifiers, for example thickeners, pigments and fillers.
  • one or more species of nanoparticles is included in the matrix material, in addition to the cores of the core-shell particles.
  • These particles are selected with respect to their particle size in such a way that they fit into the cavities of the packing (e.g. sphere packing) of the core particles and thus cause only little change in the arrangement of the core particles.
  • the particle size Through specific selection of corresponding materials and/or the particle size, it is firstly possible to modify the optical effects of the composite optical material, for example to increase its intensity.
  • suitable "quantum dots”, to functionalise the matrix are inorganic nanoparticles, in particular carbon nanoparticles (e.g. carbon nanotubes), nanoparticles of metals or of ll-VI or lll-V semiconductors or of materials which influence the
  • nanoparticles are noble metals, such as silver, gold and platinum, semiconductors or insulators, such as zinc chalcogenides and cadmium chalcogenides, oxides, such as haematite, magnetite or perovskite, or metal nitrides, for example gallium nitride, or mixed phases of these materials.
  • the matrix material can include one or more dyes.
  • a suitable dye may be fluorescent.
  • the nanoparticles have an average particle size of 50 nm or less.
  • the nanoparticles may have an average particle size of at least 5 nm. An average particle size in the range 10-50 nm (e.g.
  • the proportion by weight of the nanoparticles in the composite is less than 1 %, more preferably less than 0.5%, less than 0.1 % and still more preferably less than 0.01 %.
  • the nanoparticles preferably are distributed uniformly in the matrix material.
  • the interlayer is a layer of crosslinked or at least partially crosslinked polymers.
  • the crosslinking of the interlayer here can take place via free radicals, for example induced by UV irradiation, or preferably via di- or oligofunctional monomers.
  • the crosslinked or partially crosslinked interlayer provides reactive functions for the grafting of polymer chains of the shell polymer. It is preferred to use the same functional groups for crosslinking of the interlayer and for the grafting of the shell polymer.
  • Preferred interlayers in this embodiment comprise from 0.01 to 100% by weight, particularly preferably from 0.25 to 10% by weight, of di- or oligofunctional monomers. Grafting can also be obtained by using di- or oligofunctional monomers in the core, but this is not preferred as more of the di- or oligofunctional monomer is needed. Suitable di- or oligofunctional monomers are, in particular, isoprene and allyl methacrylate (ALMA).
  • the interlayer preferably has a thickness in the range from 10 to 20 nm. Thicker interlayer materials may be possible.
  • the shell is formed of a thermoplastic or elastomeric polymer. Since the shell essentially determines the material properties and processing conditions of the core-shell particles, the person skilled in the art will select the shell material in accordance with the usual considerations in polymer technology, but with particular attention to the
  • the core particles are preferably spherical, or substantially spherical, in shape.
  • the distribution of the diameter of the core particles is substantially
  • monodisperse e.g. with a standard deviation of 20% or less, more preferably 10% or less, more preferably 5% or less, still more preferably 3% or less.
  • the core : shell volume ratio can be in the range from 2:1 to 1 :5, preferably in the range from 3:2 to 1 :3 and particularly preferably in the region below 1 .2:1 .
  • the volume of core and interlayer present in the material is less than 50 vol%, e.g. about 45 vol%.
  • Figs. 1 and 2 illustrate an optical effect exhibited by a composite optical material film according to a preferred embodiment of the invention.
  • Fig. 3 illustrates the viewing arrangement for composite optical material films treated according to preferred embodiments of the invention.
  • Figs. 4-7 illustrate a simple representation of the optical effect of three composite optical material films according to an embodiment of the invention.
  • Figs. 8A-1 OA show the evaluation of a series of UV/visible spectra taken at different rotation angles for a composite optical material film according to an embodiment of the invention.
  • Figs. 8B-10B show the evaluation of a series of UV/visible spectra taken at different tilt angles for a composite optical material film according to an embodiment of the invention.
  • Figs. 1 1 A and 1 1 B illustrate two principal axes of the composite optical material considered in the preferred embodiments.
  • Fig. 12 shows stress-strain curves for strip of composite optical materials according to embodiments of the invention stretched along the ⁇ ⁇ and ⁇ ⁇ directions, at rates of 10 and 200 ⁇ "1 .
  • Fig. 13 shown Poisson's ratio for the same strips as in Fig, 12, in-plane (y) and perpendicular to the plane (z).
  • Figs. 14A and B shows dark-field Bragg scattering spectra as uniaxial strain increases for composite optical materials according to embodiments of the invention, using contours ⁇ along the (a) ⁇ ⁇ and (b) ⁇ ⁇ directions.
  • FIG. 14B shows selected dark-field spectra as strain increases.
  • Fig. 14C shows peak scattering wavelength and
  • Fig. 14D shows extracted linewidths, for strain in each direction.
  • the dotted line in Fig. 14D shows yield strain.
  • the inset in Fig. 14C shows reduction in scattering efficiency with strain.
  • the inset in Fig. 14D shows movement of (1 1 1 ) plane spheres for different strain directions.
  • Fig. 15 shows a schematic hard-sphere model, showing how extension along [-101 ] (arrows along face of sample film) leads to shear along (-1 1 -1 ) (arrows along plane inclined to face of sample film and solid arrows in inset), which pulls close-packed planes apart enough to allow slip (open arrow in inset).
  • the rubber-like opal films disclosed in US-B-6, 337,131 (equivalent to EP-A-955323) and US 2004/0253443 (equivalent to WO03025035 and EP-B-1425322) were found to be particularly applicable. They consist of monodisperse polymer core particles embedded and crystallized inside a matrix of soft polymer which renders the so-called "opal films” or "rubbery opals" easily deformable. Their deformation under strain is uniform and can propagate through the whole film (particularly so when the whole film consists of a single domain).
  • the melt-processing technique disclosed in US 2004/0253443 is especially suited to yield large area samples with well-known orientation of the colloidal crystal lattice.
  • this melt-processing is carried out by pressing a molten mass of core-interlayer-shell particles between two flat metal sheets covered with a protective foil to prevent sticking.
  • the melt flows outwards and the particles crystallize forming a colloidal crystalline opal film disc while the polymer shells coalesce to form a continuous polymer matrix.
  • the synthesis of the core-interlayer-shell beads and the preparation of the opal disks has been described in detail in Ruhl and Hellmann (2001 ) and Ruhl et al (2003) [T. Ruhl, P. Spahn, G. P. Hellmann "Artificial opals prepared by melt
  • the shell polymer of the beads which forms the matrix of the opal films should have a Glass Transition Temperature Tg which is lower than the temperature of the film during the deformation.
  • Tg Glass Transition Temperature
  • the temperature during the deformation should be adjusted appropriately. If the temperature during the deformation is fixed, e.g. ambient temperature (typically taken to be 23 °C), a suitable composition of the shell polymer can be chosen to adjust Tg.
  • the adjustment of Tg by a variation of the polymer composition is well known to the specialist and industrial standard. For emulsion polymerization, it is described e.g. in "WaBrige Polymerdispersionen : Synthese, compassion,
  • Emulsion polymerization is a suitable technique for the formation of the core-shell particles.
  • Many acrylic, methacrylic, styrenic, vinyl and other monomers are available for the emulsion polymerization.
  • the present inventors have found that comonomers of ethyl acrylate and iso-butyl methacrylate are especially appropriate as shell polymers. Both react quickly and completely during the emulsion polymerization.
  • the ethylacrylate lowers the Tg and yields soft, elastic polymers.
  • the iso-butyl methacrylate increases the Tg while the refractive index of the copolymer remains low.
  • Poly-iso-butylmethacrylate has a refractrive index of 1 .45.
  • Copolymers of ethylacrylate and iso-butylmethacrylate can be varied between soft and sticky, very elastic through tough and leatherlike to brittle. The crystal lattice orientation inside the opal discs was determined by Pursiainen et al (2008) [O. L. J. Pursiainen, J. J. Baumberg, H. Winkler, B. Dahl, P. Spahn, T. Ruhl "Shear-Induced Organization in Flexible Polymer Opals" Adv. Mater.
  • the present inventors have found that where a subsequent strain of sufficient magnitude is applied either parallel or perpendicular to the close packed directions, different effects are seen. Specifically, an effect is seen when the strain is applied perpendicular to the close packed direction that is not seen when the strain is applied parallel to the close packed direction.
  • the inventors found that a novel kind of structural colour, dependent on viewing angle and on rotation of the sample appears when strain is applied normal to the dense packed lines of beads.
  • FIG. 1 illustrates images taken in plan view (normal to the surface of the film) showing schematically (using black and white shading) the shift of the reflection colour to shorter wavelengths with increasing strain.
  • the same sample film is illustrated in each view, with the strain progressively increasing from left to right across Fig. 1 .
  • Fig. 3 illustrates the viewing arrangement for composite optical material films treated according to preferred embodiments of the invention.
  • Composite optical material (polymer opal) film 10 has a substantially planar upper major surface. The film 10 is illuminated with white light. A small area of the film is observed via optical fibre 12.
  • Optical fibre 12 is oriented so as to define a plane with the axis N normal to the planar upper surface of film 10.
  • the angle in this plane between the axis N and the optical fibre is the viewing angle V.
  • viewing angle V (represented by a square symbol) is 50°.
  • the optical fibre can be rotated about the axis N. This defines the angle of rotation R of the optical fibre.
  • R is variable between 0 and 180 °.
  • FIG. 4-7 A simple representation of the effect of an embodiment of the invention is shown in Figs. 4-7, in which three polymer opal samples A, B and C are strained along their length, perpendicular to a close packed direction, and affixed to a support (in this case the samples are fixed to a paper substrate using sticky tape).
  • the samples are not changed. The only thing that is changed is the direction of viewing of the samples. The viewing angle is maintained substantially constant and the rotation angle is progressed from about 0° to about 90 °.
  • each one of the samples A, B and C does not exhibit structural colour in the viewing direction of Fig. 4.
  • Fig. 5 is rotated compared with Fig. 4.
  • samples A and C do not exhibit structural colour in the viewing direction of Fig. 5
  • sample B shows a red structural colour.
  • Fig. 6 is rotated compared with Fig. 5. Although samples A and B do not exhibit structural colour in the viewing direction of Fig. 6, sample C shows a strong red structural colour. Fig. 7 is rotated compared with Fig. 6. Although sample B does not exhibit structural colour in the viewing direction of Fig. 7, sample A shows a strong red structural colour and sample C shows a weaker red structural colour than in Fig. 6.
  • Figs. 8A-1 OA show the evaluation of a series of UV/visible spectra taken at different rotation angles.
  • Fig. 8A shows a contour map of the intensity of the structural colour effect and the peak wavelength of the structural colour effect with rotation.
  • Fig. 10A shows a line plot of the intensity of the structural colour effect with rotation.
  • Fig. 9A shows a line plot of the peak wavelength of the structural colour effect with rotation.
  • Fig. 8B shows a contour map of the intensity of the structural colour effect with tilt of the sample. This measurement shows at about 0° the reflection of (1 1 1 ) plus the surface gloss effect (mirror image of light source) and at about 50 ° tilt the backscattering reflection from oblique planes. The intensities of these reflections are similar, and confirmed by Fig. 10B.
  • Fig. 9B shows a line plot of the peak wavelength of the structural colour effect with tilt.
  • a fixation of the second configuration (the strained state) is advantageous, in order that the user does not need to strain the composite optical material in order to reveal the new structural colour effect.
  • the new type of viewing angle-dependent and rotation-angle dependent structural colour has many applications.
  • the structural colour effect is strong especially as it appears as a kind of backscattering.
  • These composite optical material films are suitable as security features for banknotes, passports, credit cards, brand labels and other applications or as lettering on signs which are readable only from a certain direction.
  • the present inventors have further investigated the likely mechanism behind the novel effect exhibited by the preferred embodiments.
  • both the mechanical and optical properties of the material in the second configuration are found to be anisotropic, depending on the specific stretch direction relative to the close-packed particles in the close-packed (1 1 1 ) planes.
  • the present inventors find that a sphere-packing model predicts the low-strain behaviour ⁇ 40% strain. This model is unable to account for the phase transitions observed > 40% strains but predicts the findings for low strain accurately.
  • Shear-assembled fee lattices of 200-nm polymer spheres within single-domain elastomeric films were distorted.
  • the inventors consider, without wishing to be bound by theory, that suitable distortion induces phase transitions to an anisotropic monoclinic state at strains ⁇ 40%.
  • the transition to a lower-symmetry state breaks the traditional diffraction rules of fee lattices, "turning on" unusual structural colour scattering in specific orientations.
  • Polymer opal crystalline films are produced by shear assembly of hard-core or soft-shell nanoparticles resulting in the growth of highly ordered (1 1 1 ) planes from a film surface. Compression, extrusion, and rolling can yield films of high order, and these techniques have been optimized to produce the current films.
  • the shear processing direction referred to here is the shear processing that is used to control the fee self-assembly.
  • the algorithm locates the positions of impurities and imperfections observed in real-time microscope images of the polymer film and tracks their relative coordinates as they move. Hence, together with the spectroscopy which measures the planar spacing vertically, the full strain distortions could be directly tracked in real time.
  • Fig. 12 shows stress-strain curves for strips stretched along the ⁇ ⁇ and ⁇ ⁇ directions, at rates of 10 and 200 ⁇ " .
  • Fig. 13 shown Poisson's ratio for the same strips as in Fig, 13a, in-plane (y) and perpendicular to the plane (z).
  • the stress-strain curve would be a vertical line on the scale shown in Fig. 12, while the soft sticky shell material on its own follows a near-horizontal line.
  • the ordered particle composite thus acquires distinctly new anisotropic properties compared to its component materials.
  • the material exhibits a soft elasticity with significant anisotropy and shows more strain hardening for ⁇ >70% along the ⁇ ⁇ direction.
  • the extracted Poisson ratio (see Fig. 13) is also anisotropic, with larger contraction along the y direction when stretched along ⁇ ⁇ and faster z compression of the layers when stretched along ⁇ ⁇ .
  • Figs. 14A and 14B shows dark-field Bragg scattering spectra as uniaxial strain increases, using contours ⁇ along the (a) ⁇ ⁇ and (b) ⁇ ⁇ directions.
  • the inset in Fig. 14B shows selected dark-field spectra as strain increases.
  • Fig. 14C shows peak scattering wavelength and Fig. 14D shows extracted linewidths, for strain in each direction.
  • the dotted line in Fig. 14D shows yield strain.
  • the inset in Fig. 14C shows reduction in scattering efficiency with strain.
  • the inset in Fig. 14D shows movement of (1 1 1 ) plane spheres for different strain directions.
  • the reader is referred to Fig. 3 of A. Kontogeorgos et al (2010) [A. Kontogeorgos, D.R.E. Snoswell, C.E. Finlayson, J.J. Baumberg, P.
  • Fig. 4b of A. Kontogeorgos et al (2010) which plots separation of (1 1 1 ) planes showing blueshifts of (1 1 1 ) for strain in the ⁇ ⁇ and ⁇ ⁇ directions. Free plane gliding occurs for 6% separation of close-packed planes (horizontal dashed line), which occurs for (-1 1 -1 ) planes (inclined dashed line). Strength of diffraction is shown as the intensity of each point.
  • Fig. 4c of A. Kontogeorgos et al (2010) which shows the angle of reflection as measured from the normal to film surface as in Fig. 3.
  • the hard-sphere model predicts the expansion of this (-1 1 - 1 ) layer spacing for different stretch directions (inclined dashed lines in Fig. 4b of A.
  • a 10 L reactor with stirrer, condenser, argon inlet and heating mantle was heated to 75 °C and flushed with argon.
  • 603.3g isobutyl methacrylate 42g hydroxyethyl methacrylate was added dropwise at 18 mL/min. The synthesis was terminated 60 min after the last addition was finished.
  • the latex was filtered through a 100 ⁇ sieve and added dropwise into a mixture of 17 L methanol and 100ml_ of concentrated aqueous solution of sodium chloride under stirring. The polymer coagulated and formed a precipitate which settled after the stirring was terminated. The clear supernant was decanted, the precipitate was mixed with 5 L demineralised water and subsequently filtered through a l OOmicron sieve. The filter cake was dried for three days at 45 °C in a convective oven.
  • a 10 L reactor with stirrer, condenser, argon inlet and heating mantle was heated to 75 °C and flushed with argon.
  • microextruder at 120 °C and 100 rpm. The material was passed 4 times through the extruder.

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  • Chemical & Material Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Engineering & Computer Science (AREA)
  • Nanotechnology (AREA)
  • Optical Integrated Circuits (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Biophysics (AREA)
  • Manufacture Of Macromolecular Shaped Articles (AREA)

Abstract

L'invention concerne une matière optique composite telle qu'une opale en polymère, ayant une disposition tridimensionnelle des particules centrales réparties en matrice. L'indice de réfraction de la matière des particules centrales est différent de l'indice de réfraction de la matière de la matrice et la disposition tridimensionnelle a une périodicité telle que, quand une surface de la matière est éclairée par une lumière blanche, la matière composite présente une couleur structurelle. La disposition périodique tridimensionnelle est une disposition à symétrie réduite basée sur une disposition cristallographique compacte mais déformée par rapport à celle-ci pour avoir une symétrie réduite comparativement à la disposition cristallographique compacte. Des réflexions de Bragg provenant de plans qui seraient interdits dans la disposition cristallographique compacte sont ainsi autorisés et visibles dans la disposition à symétrie réduite. Ceci produit une variation d'intensité et/ou de longueur d'onde de la couleur structurelle en fonction de la rotation de la matière autour d'un axe perpendiculaire à la surface observée en oblique tandis que l'angle de vision reste sensiblement constant. L'invention présente également des procédés de fabrication d'une telle matière.
PCT/EP2012/061513 2011-06-15 2012-06-15 Matières optiques composites présentant une couleur structurelle Ceased WO2012172084A2 (fr)

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CN112965268A (zh) * 2021-02-05 2021-06-15 中山蓝宏科技有限公司 具有多角度光致变色效果的柔性光子晶体及其制备方法

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9561615B2 (en) 2011-01-12 2017-02-07 Cambridge Enterprise Limited Manufacture of composite optical materials
CN112965268A (zh) * 2021-02-05 2021-06-15 中山蓝宏科技有限公司 具有多角度光致变色效果的柔性光子晶体及其制备方法
CN112965268B (zh) * 2021-02-05 2022-07-05 中山蓝宏科技有限公司 具有多角度光致变色效果的柔性光子晶体及其制备方法

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