Manufacture of composite optical materials

Abstract

A polymer opal material comprises a three dimensionally periodic arrangement of core particles in a matrix material and exhibits structural color via Bragg reflection. IN a process for manufacturing such a material, a sandwich structure is provided, of a precursor composite material held between first and second sandwiching layers. A relative shear strain of at least 10% is imposed on the precursor composite material by curling the sandwich structure around a roller. The shear strain is cycled, in order to promote the formation of the three dimensional periodic arrangement.

Claims

1. A process for manufacturing a composite optical material comprising a three dimensionally periodic arrangement of core particles in a matrix material, including the steps: (a) providing a sandwich structure of a precursor composite material held between first and second sandwiching layers, wherein the precursor composite material comprises a dispersion of core particles in matrix material; (b) for at least part of the sandwich structure, imposing a relative shear displacement between the first and second sandwiching layers, using a curved surface of a curved support device to apply a radius of curvature to the sandwich structure, thereby providing a shear strain of at least 10% in the precursor composite material, wherein shear strain is defined as d/t.sub.c, where d is the modulus of the relative shear displacement between the first and second sandwiching layers and t.sub.c is the thickness of the precursor composite material before the shear displacement is applied to the first and second sandwiching layers; (c) after step (b), reducing the relative shear displacement between the first and second sandwiching layers in order to reduce the shear strain in the precursor composite material, wherein the process includes repeating step (b) after step (c) at least once, the repeating shear strain promoting ordering of the core particles to convert the precursor composite material to the composite optical material, and wherein the radius of curvature applied to the structure in step (b) is at least 10 m.

2. A process according to claim 1 wherein the precursor composite material comprises a population of core-shell particles, each particle comprising a core and a shell material surrounding the core, the shell material having Tg lower than that of the core.

3. A process according to claim 1 wherein the core particles have a substantially monodisperse size distribution.

4. A process according to claim 3 wherein the core particles have a mean particle diameter in the region of about 50-500 nm.

5. A process according to claim 1 wherein the temperature of the precursor composite material during step (b) is at most 300 C.

6. A process according to claim 1 wherein the material of the sandwiching layers used in the process has a higher elastic modulus than the precursor composite material, by a factor of at least 2, at the processing temperature.

7. A process according to claim 1 wherein, in step (b), the shear strain provided to the precursor composite material is at least 50%.

8. A process according to claim 1 wherein, in step (b), the shear strain provided to the precursor composite material is at most 500%.

9. A process according to claim 1 wherein steps (b) and (c) are repeated more than once, each performance of steps (b) and (c) being considered to be a shear strain cycle.

10. A process according to claim 1 wherein the radius of curvature applied to the structure in step (b) is at least 1 mm.

11. A process according to claim 1 wherein the radius of curvature applied to the structure in step (b) is at most 10 mm.

12. A process according to claim 1 wherein the shear strain is applied to the precursor composite material in the sandwich structure using a roller.

13. A process according to claim 1 wherein the precursor composite material is formed into a layer between the first and second sandwiching layers by the steps: providing a population of core-shell particles, each particle comprising a core and a shell material surrounding the core; adding a viscosity-reducing agent to the population of core-shell particles to provide a reduced-viscosity composition; and causing the reduced-viscosity composition to be formed into a layer between the first and second sandwiching layers.

14. A process according to claim 13 wherein the viscosity-reducing agent is removed from the composition after step (c) to increase the viscosity of the precursor composite material.

15. A process according to claim 14 wherein the viscosity-reducing agent is removed from the composition by evaporation.

16. A process according to claim 13 wherein the viscosity-reducing agent is modified in situ in the composition after step (c) to increase the viscosity of the precursor composite material.

17. A process according to claim 16 wherein the viscosity-reducing agent is modified by cross-linking and/or polymerisation.

18. A process according to claim 17 wherein a photoinitiator is included in the reduced-viscosity composition and the cross-linking and/or polymerisation is stimulated by UV radiation.

19. A process according to claim 16 wherein the viscosity-reducing agent is a monomer or prepolymer.

20. A process according to claim 13 wherein the material of the shell surrounding each core is grafted to the core via an interlayer, and wherein the viscosity-reducing agent is non-grafted polymer of similar composition to the material of the shell.

21. A process according to claim 20 wherein the viscosity-reducing agent has a molecular weight lower than that of the material of the shell.

22. A process according to claim 13 wherein the viscosity-reducing agent is added in an amount of 10 wt % or less based on the weight of core-shell particles.

23. A process according to claim 13 wherein the sandwich structure is formed at a temperature of 120 C. or less.

24. A process according to claim 13 wherein steps (b) and (c) are carried out at a temperature of 120 C. or less.

25. A process according to claim 13 wherein steps (b) and (c) are carried out at room temperature.

26. A process according to claim 1 wherein in at least one of the repetitions of step (b) after step (c), the direction of shear is changed.

27. A process according to claim 26 wherein an initial direction of shear corresponds to a close packing direction and a subsequent, different direction of shear corresponds to a direction which differs from the initial direction of shear by at least 5.

28. A process according to claim 27 wherein the subsequent direction of shear differs from the initial direction of shear by about 30 or more but less than 180.

29. A process according to claim 27 wherein the initial and subsequent directions of shear correspond to different close packed directions.

30. A process according to claim 27 wherein more than one subsequent direction of shear is used.

31. A process for manufacturing a composite optical material comprising a periodic arrangement of two or more components, the arrangement exhibiting structural colour, the process including the steps: (a) providing a sandwich structure of a precursor material held between first and second sandwiching layers, wherein the precursor material comprises a mixture of said two or more components; (b) for at least part of the sandwich structure, imposing a relative shear displacement between the first and second sandwiching layers, using a curved surface of a curved support device to apply a radius of curvature to the sandwich structure, thereby providing a shear strain of at least 10% in the precursor material, wherein shear strain is defined as d/t.sub.c, where d is the modulus of the relative shear displacement between the first and second sandwiching layers and t.sub.c is the thickness of the precursor composite material before the shear displacement is applied to the first and second sandwiching layers; (c) after step (b), reducing the relative shear displacement between the first and second sandwiching layers in order to reduce the shear strain in the precursor material, wherein the process includes repeating step (b) after step (c) at least once, the repeating shear strain promoting ordering of said two or more components to convert the precursor material to the composite optical material, and wherein the radius of curvature applied to the structure in step (b) is at least 10 m.

32. A process according to claim 31 wherein the periodic arrangement is a one dimensional periodic arrangement, the two or more components being arranged into alternating lamellae in the composite optical material.

33. A process according to claim 31 wherein the precursor material comprises a block copolymer, said two or more components corresponding to respective different parts of the block copolymer.

34. A process according to claim 31 wherein the material of the sandwiching layers used in the process has a higher elastic modulus than the precursor material, by a factor of at least 2, at the processing temperature.

35. A process according to claim 31 wherein, in step (b), the shear strain provided to the precursor material is at least 50%.

36. A process according to claim 31 wherein, in step (b), the shear strain provided to the precursor material is at most 500%.

37. A process according to claim 31 wherein steps (b) and (c) are repeated more than once, each performance of steps (b) and (c) being considered to be a shear strain cycle.

38. A process according to claim 31 wherein the radius of curvature applied to the structure in step (b) is at least 1 mm.

39. A process according to claim 31 wherein the radius of curvature applied to the structure in step (b) is at most 10 mm.

40. A process according to claim 31 wherein the shear strain is applied to the precursor composite material in the sandwich structure using a roller.

41. A process according to claim 31 wherein in at least one of the repetitions of step (b) after step (c), the direction of shear is changed.

42. A process according to claim 41 wherein an initial direction of shear corresponds to a close packing direction and a subsequent, different direction of shear corresponds to a direction which differs from the initial direction of shear by at least 5.

43. A process according to claim 41 wherein more than one subsequent direction of shear is used.

44. A process according to claim 1, wherein each performance of steps (b) and (c) is defined as a shear strain cycle, and wherein the process uses at least 5 shear strain cycles.

45. A process according to claim 31, wherein each performance of steps (b) and (c) is defined as a shear strain cycle, and wherein the process uses at least 5 shear strain cycles.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Preferred embodiments of the invention will be set out by way of example with reference to the drawings, in which:

(2) FIG. 1 shows a schematic view of a manufacturing process according to an embodiment of the invention, illustrating the contact angle .

(3) FIG. 2 shows a modification of FIG. 1 in which =.

(4) FIG. 3 illustrates the shear strain applied to the polymer layer by a notional linear equivalent of FIGS. 1 and 2.

(5) FIGS. 4-6 show schematic view of a manufacturing processes according to a preferred embodiment of the invention, in which the curling of the precursor composite material in the sandwich structure is reversed.

(6) FIG. 7 shows the effect of the inner radius of the polymer opal on the shear strain in the polymer opal film and the strain that would be required in the outer foil in order to relieve the shear stress in the polymer opal film.

(7) FIG. 8 shows the effect of the thickness of the polymer opal layer on the shear strain in the polymer opal film and the strain that would be required in the outer foil in order to relieve the shear stress in the polymer opal film.

(8) FIG. 9 shows the effect of the contact angle on the shear strain in the polymer opal film and the strain that would be required in the outer foil in order to relieve the shear stress in the polymer opal film.

(9) FIG. 10 shows darkfield intensity response in white light of samples which are identical except for their strain processing, expressed in terms of the number of curls or strain cycles to which they have been subjected.

(10) FIG. 11 shows darkfield spectra of curled samples before annealing.

(11) FIGS. 12-16 show darkfield spectra of curled samples before and after annealing.

(12) FIG. 17 shows averaged bright field spectra for various samples.

(13) FIG. 18 shows averaged bright field spectra for hot edge samples. A maximum response is seen for 60 strain cycles.

(14) FIG. 19 shows a schematic view of the shear cell 100 used in experiments to assess the effect of the shear strain on the degree of formation of the periodic arrangement in the polymer opal.

(15) FIG. 20 shows the transmission through samples depending on the strain amplitude applied in step 3 of the shear sequence.

(16) FIG. 21 shows a comparison between bright field intensity spectra in reflection for disordered (after step 1 of the shear sequence) and ordered (after a modified step 3). Sample thickness 300 m.

(17) FIG. 22 shows the corresponding transmission spectrum (only for the ordered sample) for FIG. 21. Sample thickness 300 m.

(18) FIG. 23 shows the melt flow rate for mixtures of opal polymer with different additions of butanediol diacrylate at a temperature of 90 C. The variation in flow rate with time resulted in the plot of MFR with respect to measuring point (i.e. time). The conditions of MFR testing were L/D 8 mm/2.095 mm, weight 21.6 kg.

(19) FIGS. 24 and 25 illustrate the impact of the monomer concentration on the mechanical properties of pressed opal polymer disks after UV cure via tensile testing. FIG. 24 shows tensile test results for pressed polymer opal disks cured for 20 mins using an Osram Vitalux lamp. FIG. 25 shows tensile test results for pressed polymer opal disks containing benzophenone cured for 23 mins using UVCube lamp 100 W/cm.sup.2.

(20) FIG. 26 shows a comparison of tensile tests of opal polymers with butanediol diacrylate and butanediol dimethacrylate.

(21) FIG. 27 shows the results for tensile tests of opal polymers with 5 phr of butanediol diacrylate and different mixtures of photoinitiators.

(22) FIG. 28 shows tensile test results for opal polymers with 5 phr of butanediol diacrylate, 1 phr benzophenone and 1 phr Darocur 1173 cured under different conditions of UV irradiation.

(23) FIG. 29 compares tensile tests of specimens taken from areas of different thickness for a composition of 15 phr BDDA, 1 phr benzophenone, 1 phr Darocur 1173 (cure UVCube 100 W/cm 22 mins).

(24) FIGS. 30 and 31 show tensile test results for polymer opal films with different concentrations of carbon black and different UV cures (FIG. 30: UVCube 100 W/cm, 215 s irradiation time; FIG. 31: UVCube 100 W/cm, 230s irradiation time).

(25) FIG. 32 shows the results of tensile tests of mixtures of UV cured opal polymer with 5 phr of butanediol diacrylate, 1 phr of benzophenone and 1 phr of Darocur 1173 which were stored prior to the pressing of the opal disks and the UV cure.

(26) FIG. 33 provides a schematic illustration of a small portion of a {111} close packed plane in an fcc lattice.

(27) FIG. 34 schematically illustrates a curling step in which the precursor composite material is subjected to a shear direction corresponding to the 0 direction shown in FIG. 33.

(28) FIG. 35 schematically illustrates a subsequent curling step in which the precursor composite material is subjected to a shear direction corresponding to the 30 direction shown in FIG. 33.

(29) FIG. 36 schematically illustrates a compound curling process in which the precursor composite material is subjected to sequentially different shear directions.

(30) FIG. 37 shows the effect of curling on reflection intensity (ordering of the particles in the material), when the shear direction is maintained at 0.

(31) FIG. 38 shows the effect of curling on reflection intensity (ordering of the particles in the material), when the shear direction is altered between 0 and 30.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS, FURTHER OPTIONAL FEATURES OF THE INVENTION

(32) The entire content of each the documents referred to in any section of this disclosure is hereby incorporated by reference.

(33) Suitable Materials for Polymer Opals

(34) The preferred polymer opals disclosed here are based on flexible monolithic photonic crystals formed from hard polymer spheres dispersed in a softer sticky elastomer matrix. See Pursianinen et al (2007) [O. L. J. Pursianinen, J. J. Baumberg, H. Winkler, B. Viel, P. Spahn, T. Ruhl, Optics Express 15, 9553 (2007)] and Pursianinen et al (2008) [O. L. J. Pursianinen, J. J. Baumberg, H. Winkler, B. Viel, P. Spahn, T. Ruhl, Advanced Materials 20, 1484 (2008)]. As typical for opaline photonic crystals [References 14-16], when the spheres self-assemble into an fcc lattice they can be colour-tuned by changing the size of the constituent spheres. See Vlasov et al (2000) [Y. Vlasov, V. Astratov, A. Baryshev, A, Kaplyanskii, O. Karimov, M. Limonov, Phys. Rev. E 61, 5784 (2000)], Romanov et al (2001) [S. Romanov, T. Maka, C. Sotomayor Torres, M. Muller, R. Zentel, D. Cassagne, J. Manzanares-Martinez, C. Jouanin, Phys. Rev. E 63 56603 (2001)] and McLachlan et al (2004) [M. McLachlan, N. Johnson, R. De La Rue, D. McComb, J. Mat. Chem. 14, 144 (2004)]. Using spheres of diameter about 200 nm produces Bragg peaks in the visible spectral range, whilst the elastomeric composition gives flexible films with enhanced structural control of colour. A major strength of this work is the ability to form these opals by shear-assembly which allows efficient production on industrial scales.

(35) Optical scattering spectra of different samples reported in this disclosure were quantified by mounting them on a stage and recording confocally-collected dark-field spectra on a modified microscope. Data was normalised to the scattering spectra taken under identical conditions on white diffuser plates which have a broadband Lambertian spectrum. The samples were analysed using a 5 objective lens with a numerical aperture of 0.15.

(36) Shear Ordering of Core Particles

(37) FIG. 1 shows a schematic view of a manufacturing process according to an embodiment of the invention. A polymer opal layer 10 (composite optical material or precursor composite material) is held between an outer foil 12 and an inner foil 14 (first and second sandwiching layers), to form a sandwich structure. In the drawing, it is intended that the outer foil 12 and the inner foil 14 have the same length. The sandwich structure is passed around a roller 16. The contact angle between the sandwich structure and the roller is defined as , where is the angle subtended at the centre of the roller between the locations on the roller between which the sandwich structure makes contact with the roller, (Note that, beyond , the inner foil 14 is shown in FIG. 1 as keeping in contact with the roller, but this is for illustrative purposes only.)

(38) Provided that the outer foil does not stretch relative to the inner foil, the free ends of the outer foil and the inner foil are offset by an overhang 18, because the radius of curvature of the inner foil is different to that of the outer foil. Therefore overhang 18 amounts to a shear displacement between the inner and outer foils, this giving rise to a shear stress indicated by arrow 19 on the polymer opal layer.

(39) FIG. 2 shows a similar view to FIG. 1 except that the contact angle here is radians, because the entire structure passes for a full half turn around the roller 16.

(40) In each of FIGS. 1 and 2, the schematic views show the relative displacement of the inner and outer foils being zero at one end of the structure and a maximum and the opposite end of the structure. This is illustrated schematically in FIG. 3, with the notional equivalent straight polymer opal layer 10 subjected to a shear stress 19 by relative displacement of the inner foil 14 and outer foil 12.

(41) The polymer opal layer between the inner and outer foils is in contact with and bonded to the inner and outer foils. Thus, the shear displacement between the inner and outer foils applies a shear stress on the polymer opal layer and thus a shear strain. It is possible to calculate the shear strain in the polymer opal layer as follows.

(42) Taking the thickness of the polymer opal layer to be 100 m (0.1 mm), and the inner radius of curvature of the polymer opal layer to be 10 mm (around the contact angle ), the outer radius of curvature of the polymer opal layer is 10.1 mm. Taking to be radians (3.141593 radians) (i.e. 180), the length of the outer surface of the polymer opal layer subjected to curvature around is 31.73009 mm. The contact angle of an equal length of the inner surface of the polymer opal layer is 3.173009 radians. The overhang is therefore 0.031416 radians, i.e. 0.314159 mm. The overhang corresponds to the shear displacement. The shear strain applied to the polymer opal layer is therefore (shear strain thickness), i.e. 314.159%.

(43) The calculation above assumes that the polymer opal layer does not change thickness in response to the shear stress. The calculation above also assumes that the inner and outer foils are perfectly flexible and have a Young's modulus which is very significantly greater than that of the polymer opal, so that any stretching of the outer foil can be ignored. However, it is noted that in order to relive the shear strain in the polymer opal layer, it would only be necessary for the outer foil to have a tensile strain of 0.99% in the example shown (specific to the radius used in this example. Compression in the inner foil would have a similar effect. Therefore it is important to give consideration to the mechanical properties of the foils, and to minimize any effect of the process on allowing relaxation of the shear stress on the polymer opal, other than the crystallization of the polymer opal itself.

(44) FIGS. 4-6 show further considerations of the shear strain in the polymer opal layer in more detail.

(45) In FIG. 4, contrary to what is shown in FIGS. 1 and 2, the shear strain is not accommodated only at one end of the sandwich structure, but instead is allowed to be taken up at both ends of the sandwich structure. The general situation is illustrated, in which the contact angle is slightly less than radians. The angle can be considered to be an angle indicating the position of interest in the sandwich structure.

(46) FIGS. 4, 5 and 6 illustrate a complete strain cycle including both the curling (FIG. 4), straightening (FIG. 5) and reverse-curling (FIG. 6) process. The polymer opal film 10 is shown between the first foil (sandwiching layer) 12 and the second foil (sandwiching layer) 14. The thickness of the polymer opal layer is exaggerated to assist the illustration. FIG. 5 illustrates the state of the opal film after the curling process in FIG. 4. It can be seen from FIG. 5 that the ends of the polymer opal layer are distorted slightly due to the shear strain experienced in FIG. 4. The sandwich structure is then reversed and curled as shown in FIG. 6, with the second foil 14 now the outer foil and the first foil 12 now the inner foil. Then the sandwich structure is straightened again to have the form shown in FIG. 5.

(47) The sample is repeatedly curled on a stationary roller as shown in FIGS. 4-6. In order to simplify the understanding of the process, in this section we assume the (PET) foils on both sides of the polymer opal film to be perfectly bendable but not stretchable or compressible. Only in-plane shear force on the polymer opal film is considered. Therefore, shear strain of a random point on the film (see FIG. 4) can be expressed in the following equation:

(48) $=\tan \left(\right)=\{\begin{array}{c}\frac{}{}\frac{\left(2d+t\right)}{2t},\text{/}23\text{/}2\\ \frac{}{}\frac{\left(2d+t\right)}{2t},-\text{/}2<<\text{/}2\end{array}\left(0<\right)$

(49) is the shear strain, is the position angle, d is the thickness of the PET foil and t is the thickness of the opal film. This equation means shear strain is independent of the curvature of the roller, only depends on the contact angle and the thickness of the opal film as well the PET foil, and different areas of the opal film subject to different shear strain, depending on the position angle. Except the ends of the film, any point in the film will subject to a V-shape oscillatory shearing force curve with modulated amplitude at the beginning and the end of the period. Without taking into account the transition process illustrated in FIG. 5, the oscillation frequency should be approximately 1 Hz in practice, and each curling cycle contains one oscillation period.

(50) Based on these basic calculations, it is possible to predict some useful processing parameters.

(51) FIG. 7 shows the effect of the inner radius of the polymer opal on the shear strain in the polymer opal film and the strain that would be required in the outer foil in order to relieve the shear stress in the polymer opal film. In this plot, the contact angle is pi radians. It is shown that the inner radius does not affect the shear strain in the polymer film, but there is a significant effect on the required outer foil strain to relieve the stress.

(52) FIG. 8 shows the effect of the thickness of the polymer opal layer on the shear strain in the polymer opal film and the strain that would be required in the outer foil in order to relieve the shear stress in the polymer opal film. In this plot, the inner radius is 10 mm and the contact angle is pi radians. The thickness of the polymer opal layer does not affect the shear strain. The thickness of the polymer opal layer does affect the strain required in the outer foil in order to relieve the stress in the polymer opal layer, but only to a small degree.

(53) FIG. 9 shows the effect of the contact angle on the shear strain in the polymer opal film and the strain that would be required in the outer foil in order to relieve the shear stress in the polymer opal film. The contact angle is considered to be the dominating parameter for the shear strain of the polymer opal film. A suitable level is considered to be about 100. However, contact angles of about 180 are acceptable, and convenient to implement. The contact angle does not affect the strain that would be required in the outer foil in order to relieve the shear stress in the polymer opal film.

(54) The amount of shear induced in the opal is independent of radius and opal thickness and only depends on the contact angle i.e. how far around the roller the film is in contact. For an indicative optimum shear amplitude of about 170%, this means that a contact angle of about 97 should be targeted, but with any contact angle from 28-170 producing a beneficial effect. However, in other embodiments, the contact angle can be many multiples of pi (e.g. greater than 2 pi or greater than 4 pi), and good results can still be obtained.

(55) Based on these calculations, it can be concluded that contact angle is the main parameter in controlling shear strain in the polymer opal layer, and the radius of the roller is the main parameter in controlling the effect of outer foil stretching on relieving the shear stress in the polymer opal layer. The polymer opal layer thickness has no effect on the induced shear strain and only a small effect on the outer foil elasticity sensitivity. It is noted that thick polymer opal layers tend to layer when ordered because the distance from the templating effect of the interface between the polymer opal and the smooth sandwiching layer is increased. The rolling radius has no influence on the shear strain induced in the polymer opal but has a large influence on outer foil strain (and inner foil strain) below about 5 mm, therefore the foil thickness will set the lower radius limit. The outer foil elasticity is therefore very important in terms of determining the actual shear strain in the opal film. Any outer foil stretching or inner foil compression reduces the effective shear strain applied to the polymer opal film. For radii above about 10 mm, in the calculations presented here, the outer foil elasticity could completely counteract any bend induced shear. Therefore it is considered that a small roller radius is beneficial.

(56) A large number of experiments has been carried out in order to investigate the effect of curling in more detail. In what follows, a cycle of curling (a strain cycle) requires that a sandwich structure of a first sandwiching layer, a precursor composite layer and a second sandwiching layer is passed around a roller of constant radius with a maximum contact angle of about 100 with the first sandwiching layer in contact with the roller so that it has a smaller radius of curvature than the second sandwiching layer. Next, the sandwich structure is passed around the same roller but with the second sandwiching layer in contact with the roller so that it has a smaller radius of curvature than the first sandwiching layer. In this way, the direction of the shear strain applied to the precursor composite material is reversed in the second part of the strain cycle.

(57) Before considering the results of these experiments in detail, we consider FIG. 10. This shows the darkfield intensity response (10 objective) in white light of samples which are identical except for their strain processing, expressed in terms of the number of curls or strain cycles to which they have been subjected. These results show that the amount of light reflected from the samples at specific wavelengths increases as the number of strain cycles increases. There is also seen a shift in the peak reflection wavelength as the number of strain cycles increases. This red shift is a feature of much of the data presented here and appears to occur in conjunction with the ordering (crystallisation) of the polymer opal. This tends to happen particularly with thicker samples (e.g. greater than about 50 m thick) the samples presented here are typically 80-100 m thick.

(58) The results reported here are for samples formed from PEA-PMMA-PS core-shell structured polymer spheres with 0.05 wt % carbon nanoparticles. The mean diameter of the spheres was 233 nm. After extruding from an extruder, all of the samples were prepared on a roller rig for rolling (as explained above) or hot-edge process at a temperature of 150 C., otherwise stated. The thickness range for the samples was 80-100 m, unless otherwise stated. Tg for the core PS particles was about 90 C., but the degree of crosslinking of the PS was sufficient (e.g. about 10% crosslinking density) that the material could be processed at about 150 C. without substantial deformation of the core particles.

(59) FIG. 11 shows the bright field spectra of curled samples before annealing, using a 20 objective and an Ocean Optics spectrometer. The samples were treated as follows (see Table 1):

(60) TABLE-US-00001 TABLE 1 Sample Curl (times) 0 0 1 5 2 10 3 20 4 40

(61) The maximum response is shown by the samples curled 40 times.

(62) Each sample was subjected to annealing at 130 C. for 30 seconds. The results are shown in FIGS. 12-16 for pre- and post-annealed samples 0, 1, 2, 3 and 4, respectively. In general, annealing is shown to reduce the maximum reflection for each sample.

(63) Samples of a sandwich structure similar to that used for FIGS. 11-16 were treated in various ways in order to compare the effect of hot edge treatment with rolling. The sample treatment conditions are shown in Table 2. In the hot edge process, the radius of curvature of the outer sandwiching layer was 2-5 mm. The samples reported here were not subjected to annealing. Spectra were measured by using 10 objective, Optoelectronics Spectrometer, and then repeated on the same samples using 10 objective, Ocean Optics Spectrometer (considered to be more sensitive than the Optoelectronics Spectrometer.

(64) TABLE-US-00002 TABLE 2 Sample Temperature ( C.) Hot edge (H) or Roll (R) Curl times 1 150 R 0 2 150 R 5 3 150 R 10 4 150 R 20 5 150 R 30 6 150 R 40 7 150 R 60 8 150 R >=80 9 150 H 0 10 150 H 5 11 150 H 10 12 150 H 20 13 150 H 30 14 150 H 40 15 150 H 60

(65) FIG. 17 shows averaged bright field spectra for various samples 1-8 in Table 2. A maximum response is seen for 60 strain cycles.

(66) FIG. 18 shows averaged bright field spectra for hot edge samples. A maximum response is seen for 60 strain cycles.

(67) FIGS. 17 and 18 are plotted on identical scales. Therefore it is possible directly to compare the averaged bright field spectra for rolled and hot edge samples subjected to the same number of strain cycles. Thus, it is possible to compare samples 1 and 9, 2 and 10, 3 and 11, 4 and 12, 5 and 13, 6 and 14, and 7 and 15, respectively. The results show that at low numbers of strain cycles, the reflections from the hot edge samples have greater intensity than the reflections from the rolled samples. However, at about 20 strain cycles and higher, the reflections from the rolled samples have greater intensity than the reflections from the hot edge samples.

(68) Further work was carried out to collect bright field, dark field and transmission spectra for samples 1-8 and samples 9-15 using the Ocean Optics Spectrometer.

(69) The results showed that between 0-60 shear strain cycles, intensities as well as the peak wavelengths of bright field and dark field spectra increase with total number of strain cycles. Intensities of the bright field and dark field spectra of the rolled samples reach their highest values at 60 strain cycles times. However, although the peak wavelengths continue to increase in some cases very slowly with strain cycles above 60, the intensity of the bright field reflection peak decreases significantly, and at the same time the full width at half maximum (FWHM) increases.

(70) The inventors have carried out further experiments to study the effect of the shear strain on the degree of formation of the periodic arrangement in the polymer opal.

(71) FIG. 19 shows a schematic view of the shear cell 100 used in these experiments. The shear cell used was a Linkam CS450. A sample of the precursor composite material is placed in a gap between top plate 102 and bottom plate 104. Bottom plate 104 is moveable with respect to the top plate in order to generate shear strain in the precursor composite material. Viewing windows 106, 108 are provided in the top and bottom plates, respectively. Here, the strain is defined as positive or negative from a central zero position. At the zero position the sample is not deformed. Bottom plate 104 is moved until the required oscillatory strain is reached. The plate then moves back in the opposite direction, passing through zero, where the sample is undeformed, and then onwards until the strain is achieved in the opposite direction (e.g. +50%, through 0% and on to 50%).

(72) The samples were treated as shown in Table 3.

(73) TABLE-US-00003 TABLE 3 Steady (S) Shear or Shear Shear Frequency sequence Oscillating Gap strain rate (per of oscillation Time step (O) (m) (%) second) (Hz) Direction (s) 1 S 300 na 2 na Clockwise 10 2 S 300 na 0.1 na Clockwise 10 3 O 300 200% na 1 hz Clockwise 10 4 O 300 50% na 2 hz Clockwise 10

(74) Step 1 of the shear sequence step in Table 3 is a pre-shear to randomise the precursor composite material this provides the base condition for subsequent steps.

(75) Step 2 is a slow shear to relax elastic forces.

(76) Step 3 is a large oscillation shear in order to promote crystallisation, i.e. ordering of the particles. The direction here is the direction of the first part of the oscillation cycle.

(77) Step 4 is a small oscillation shear to negate phase dependence.

(78) A sequence of transmitted spectra was collected through the windows 106, 108 before the start of the shear sequence. The spectra overlayed each other over a 2 second period.

(79) Next, a sequence of transmitted spectra was collected through the windows 106, 108 during a 3 second time period within step 1 of the shear sequence. The effect of step 1 was shown to increase the transmission of light between 400-580 nm. Also, the stop band 580-630 nm became narrower. It is considered that a side effect of crystallisation (increased ordering) in the polymer opal is less scatter in the polymer opal layer and wavelengths in outside the stop band experience enhanced transmission.

(80) Two transmission spectra were collected during step 2 of the shear sequence. One was taken at the beginning of step 2 and one was taken at the end of step 2 (10 seconds later). This demonstrated a relaxation effect of the slow shear of step 2.

(81) Next, a sequence of transmitted spectra was collected through the windows 106, 108 during a 10 second time period within step 3 of the shear sequence. The progression showed the earlier spectra having relatively low transmission compared with later spectra in the series. The effect is to allow greater transmission at some frequencies during step 3.

(82) Repeating the shear sequence of step 3 enhances the effect seen.

(83) A sequence of transmitted spectra were collected through the windows 106, 108 during a 10 second time period within step 3 of the shear sequence, but with the strain amplitude reduced to 25%. The overall effect of the shear was reduced compared with 200% strain amplitude.

(84) FIG. 20 shows the transmission through samples depending on the strain amplitude applied in step 3 of the shear sequence. A maximum effect can be seen at between 100% and 200% shear strain.

(85) The inventors also investigated the effects of further increasing the periodicity of the particles in the composite, by using 20 seconds oscillation at 200% amplitude and 1 Hz followed by 20 seconds oscillation at 100% amplitude and 2 Hz. FIG. 21 shows a comparison between bright field intensity spectra for disordered (after step 1 of the shear sequence) and ordered (after step 3 modified as explained above). The ordering process results in a sharp intensity peak which is relatively selective in terms of wavelength. FIG. 22 shows the corresponding transmission spectrum for the ordered sample only.

(86) Manufacture of Core-Shell ParticlesExperimental Detail

(87) Preparation of Monodisperse Core-Interlayer-Shell Polymer Beads

(88) The beads produced here were similar to those described in US 2004/0253443.

(89) A 10 L reactor with stirrer, condenser, argon inlet and heating mantle was heated to 75 C. and flushed with argon.

(90) 2.750 g sodium dodecylsulfate

(91) 2800.000 g demineralised water

(92) 36.000 g styrene

(93) 4.000 g butane dioldiacrylate

(94) were premixed and fed into the reactor. The stirrer was adjusted to 250 rpm. The temperature of the mixture was monitored. At 65 C., three freshly prepared solutions were subsequently added:

(95) 0.360 g sodium disulfite in 5 g demineralised water

(96) 5.180 g sodium persulfate in 20 g demineralised water

(97) 0.360 g sodium disulfite in 5 g demineralised water

(98) Cloudiness was observed after 10 min. After an additional 10 min, an emulsion consisting of:

(99) 2.300 g sodium dodecylsulfate

(100) 4.000 g potassium hydroxide

(101) 2.200 g Dowfax2A1 (Dow Chemicals)

(102) 900.000 g demineralised water

(103) 700.000 g styrene

(104) 70.000 g butane dioldiacrylate

(105) was fed dropwise at 10 mL/min. 30 min after the addition was finished, a freshly prepared solution of

(106) 0.250 g sodium persulfate in 5 g demineralised water

(107) was added. After 15 min, a second emulsion consisting of

(108) 0.500 g sodium dodecylsulfate

(109) 2.100 g Dowfax 2A1

(110) 320.000 g demineralised water

(111) 250.000 g methyl methacrylate

(112) 30.000 g ally) methacrylate

(113) was fed dropwise at 14 mL/min. 20 min after the addition was finished, a third emulsion consisting of

(114) 4.000 g sodium dodecylsulfate

(115) 2,000 g potassium hydroxide

(116) 1600.000 g demineralised water

(117) 1400.000 g ethylacrylate

(118) 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 m sieve and added dropwise into a mixture of 17 L methanol and 100 mL 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 100 micron sieve. The filter cake was dried for three days at 45 C. in a convective oven.

(119) Preparation of Monodisperse Core-Interlayer-Shell Polymer Beads with Higher T.sub.q and OH-Functionality

(120) A 10 L reactor with stirrer, condenser, argon inlet and heating mantle was heated to 75 C. and flushed with argon.

(121) 2.750 g sodium dodecylsulfate

(122) 2800.000 g demineralised water

(123) 36.000 g styrene

(124) 4.000 g butane dioldiacrylate

(125) were premixed and fed into the reactor. The stirrer was adjusted to 250 rpm. The temperature of the mixture was monitored. At 65 C., three freshly prepared solutions were subsequently added:

(126) 0.360 g sodium disulfite in 5 g demineralised water,

(127) 5.180 g sodium persulfate in 20 g demineralised water

(128) 0.360 g sodium disulfite in 5 g demineralised water

(129) Cloudiness was observed after 10 min. After an additional 10 min, an emulsion consisting of:

(130) 2.300 g sodium dodecylsulfate

(131) 4.000 g potassium hydroxide

(132) 2.200 g Dowfax2A1

(133) 900,000 g demineralised water

(134) 700.000 g styrene

(135) 70.000 g butane dioldiacrylate

(136) was fed dropwise at 10 mL/min. 30 min after the addition was finished, a freshly prepared solution of:

(137) 0.250 g sodium persulfate in 5 g demineralised water

(138) was added. After 15 min, a second emulsion consisting of:

(139) 0.500 g sodium dodecylsulfate

(140) 2.100 g Dowfax 2A1

(141) 320.000 g demineralised water

(142) 250.000 g ethylacrylate

(143) 30.000 g allyl methacrylate

(144) was fed dropwise at 14 mL/min. 20 min after the addition was finished, a third emulsion consisting of:

(145) 4.000 g sodium dodecylsulfate

(146) 2.000 g potassium hydroxide

(147) 1600.000 g demineralised water

(148) 404.7 g ethylacrylate

(149) 603.3 g isobutyl methacrylate

(150) 42 g hydroxyethyl methacrylate

(151) 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 m sieve and added dropwise into a mixture of 17 L methanol and 100 mL 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 100 micron sieve. The filter cake was dried for three days at 45 C. in a convective oven.

(152) Preparation of Polymer Compounds with Additives for the Melt-Processing

(153) 100 g of polymer was mixed with 1 g of Licolub FA1 (Clariant) and 0.05 g Special Black 4 (Evonik) at 140 C. and 100 rpm in an twin-screw DSM Xplore 5 microextruder. The material was passed 4 times through the extruder.

(154) Preparation of a Compound of CS330 for the Melt Processing with Photoinitiator for Additional Photocrosslinkinq

(155) 100 g of polymer was mixed with 1% Licolub FA1 (Clariant), 2% benzophenone (Sigma-Aldrich) and 0.05 g Special Black 4 (Evonik) in an twin-screw DSM Xplore 5 microextruder at 120 C. and 100 rpm. The material was passed 4 times through the extruder.

(156) Preparation of Opal Disks by Pressing

(157) 6 g of polymer compound was heated on a hotplate set to 150 C. The softened polymer mass was placed between two PET foils and two polished, high-gloss ironless steel sheets and pressed in a Collin press at 150 C. and 130 bar hydraulic pressure for 3 minutes.

(158) Improvements to Processability of Composite Optical Material

(159) The present inventors have realised that it would be advantageous for the precursor composite material to be more easily processable. Typical materials disclosed above are processed via melt-extrusion in which the temperature of the core-shell particles is increased above Tg of the shell material, typically at about 150 C. However, even at this temperature, the material is extremely viscous. It is therefore difficult to extrude films that are both thin and wide. It is possible to extrude a ribbon and subsequently roll the ribbon out to form a film, but this requires additional processing steps and may result in unwanted crinkling of the sandwiching layers and consequential non-uniformity of the optical material.

(160) The present inventors have carried out detailed work to address this problem. Several approaches have been considered in order to improve the processability without adversely affecting the properties of the final composite optical material, which is referred to in parts of this disclosure as the polymer opal.

(161) One approach was to extrude films of thickness less than 40 m directly through a slit die. The intention was that subsequent rolling could then be used to induce the formation of structural colour but this rolling was not intended for reducing the film thickness. However, the melt of precursor composite material was too viscous for the extrusion of thin films through a die. An attempt was made to use plasticisers in order to reduce the melt viscosity to promote extrusion of thin films. Suitable plasticisers (at suitable addition levels) were found to reduce the viscosity of the polymer melt significantly without adversely affecting the formation of structural colour. However, the result of the use of plasticisers was that the melt and the films were very sticky. Thin, free-standing films were impossible to manufacture because the material was mechanically too weak.

(162) The use of plasticisers in the manufacture of polymer opals is disclosed in U.S. Pat. No. 6,337,131. In the view of the inventors, this would result in the same problems as identified above.

(163) Preferred core-shell particles for use in the present invention are as disclosed in EP-A-1425322. These particles are more properly designated as core-interlayer-shell particles, in which the polymer of the shell is grafted to the core by graftingthis is the function of the interlayer. It is therefore preferred that the shell is grafted to a significant degree. This can be obtained by an interlayer. Otherwise the grafting agent can be added to the core which means that more grafting agent has to be used, but in that case the interlayer can be omitted.

(164) The particles typically have diameter 180-350 nm (depending on the required structural colour behaviour of the final composite optical material) and typically consist of: Core: Polystyrene, crosslinked with 10 wt % butanediol diacrylate Interlayer: Methylmethacrylate, crosslinked with 10 wt % allyl methacrylate which also is necessary for the grafting of the shell polymer Shell: Uncrosslinked polymer chains of poly ethyl acrylate, main fraction grafted via the interlayer with allylmethacrylate onto the core

(165) Grafting via the interlayer allows for crystallisation of the particles in the melt under shear as it prevents the shell polymer from floating off the cores. Opal films from core-shell beads without grafting are typically formed under conditions of low shear e.g. by drying from dispersions. Such techniques are much less appropriate for large scale production than the techniques disclosed here.

(166) It is found that when pressing techniques are used to form the composite optical material, the presence of a grafting interlayer is an important component in ensuring that structural colour behaviour is obtained.

(167) Studies on model beads with silica cores which could be removed from the shell polymer by etching with hydrofluoric acid have revealed that the typical grafting efficiency in opal polymers for the melt processing can be as high as about 70%. (Spahn, Peter Kolloidale Kristalle aus monodispersen Silika-Polymer Hybridpartikeln TU Darmstadt [Dissertation], (2008) http://tuprints.ulb.tu-darmstadt.de/id/eprint/1010). Determination of the molecular mass distribution of the shell polymer showed that it contains a significant fraction of branched polymer (get) with Mw10.sup.6 g/mol. Such opal polymers have a very high melt viscosity due to the high fractions of cores (which do not melt in the processing), gelled and grafted shell polymer. This makes processing into thin opal films difficult. The high viscosity of the melt particularly limits the maximum width and minimum thickness of polymer opal film achievable by melt processing with typical equipment. Thus it becomes necessary to use rollers with a high line force this makes the processing apparatus complex and expensive. Furthermore, the high viscosity limits the line speed of the foil production and therefore the throughput.

(168) As discussed above, the use of high-shear processes after film formation can significantly improve the structural colour behaviour of polymer opal films. This has been found to be the case particularly for films of thickness 100 m or less. Such processes would significantly benefit from progress in easing the manufacture of the film from the melt.

(169) The understanding in the field at the time of making the present invention was that grafting to the largest extent was necessary in order to promote ordering of the cores during shear processing. Thus, any contamination of the shell polymer with low molecular weight substances (and clearly therefore also any deliberate addition of low molecular weight substances) must be avoided, because such low molecular weight substances would not be grafted onto the cores.

(170) EP-A-1425322 discloses the addition of additives including crosslinkers, in the field, it is well understood that these substances should only be used in small concentrations as they reduce the fraction of grafted material and thereby impair the formation of structural colour during shear of the melt.

(171) Crosslinking of the opal films is known in order to achieve suitable elasticity, toughness and durability. Without crosslinking, polymer opal films tend to flow under load. Two strategies for the crosslinking of opal films had been explored: thermal crosslinking using blocked polyisocyanates and UV-cure.

(172) Thermal crosslinking needs latent crosslinkers which are non-reactive under the conditions of polymer processing into opal films. The crosslinkers are activated by heat after the formation of the opal film and the generation of the structural colour is finished. Using heat for the crosslinking carries with it the danger of losing colour brilliance by diffusion or relaxation of the cores forming the crystalline lattice.

(173) Crosslinking by UV irradiation is gentler because it can be achieved at ambient temperature. Unfortunately the shell polymer of the particles, which is typically synthesized by free-radical polymerisation in emulsion, does not have acrylic or methacrylic moieties left which could be used for efficient UV curing. Without such reactive groups, UV cure is slow and inefficient. In an example conducted for reference, benzophenone was used as a photoinitiator to induce crosslinking via H-abstraction from the polymer chains and subsequent polymer radical recombination (Benjamin Viel Vernetzte Kunstopalfolien aus Latices, Diplomica Verlag Hamburg 2003). An irradiation of 20 min with an Osram Vitalux lamp was applied. This, however, achieved only a moderate increase in mechanical strength.

(174) Thus, this type of UV curing approach requires uneconomically long irradiation times and yet still leads to insufficient curing and unsatisfying mechanical durability. Furthermore, the surface of the polymer opal films remains sticky. The addition of reactive monomers capable of crosslinking under UV irradiation has not been attempted in the past because firstly the monomers were considered disadvantageous for the formation of structural colour (because they are not grafted to the cores) and secondly it was considered that such monomers would not react with the shell polymer significantly to improve the mechanical properties of the polymer opal films.

(175) Surprisingly it has now been found that the addition of some lower molecular mass materials can improve processability without impairing the formation of the structural colour under shear. Particularly advantageous is the addition of monomers and reactants capable of UV cure for subsequent crosslinking.

(176) Two approaches have been investigated. The first can be considered to be temporary modification of the material by the addition of a volatile solvent. The second can be considered to be permanent modification, by the addition of low volatile solvents, monomers or other reactants which are liquid under the processing conditions, or by reducing the molar mass of the shell polymer.

(177) The first approach allows a stepwise process where the formation of the polymer opal film, supported by the solvent, can be done in a processing step which establishes the shape of the body of composite precursor material. The solvent can then be removed by easy evaporation. The development of the structural colour by shear can then be made in a subsequent step, thereby avoiding any potential negative influence of the solvent. This approach allows use of any solvents in any concentration without the risk of impairing the development of structural colour under shear. It gives the freedom to produce very thin polymer opal films even with low-force equipment.

(178) The second approach uses permanent modifications which remain present during the shear processing. Therefore more attention for a proper choice of the additives is necessary to ensure that the development of the structural colour by shear is not impaired. It has been confirmed that it is possible to improve the flowability of the opal polymer melt without impairing the development of structural colour by several different techniques.

(179) In some embodiments of this second approach, the molecular weight of the shell polymer can be reduced during the synthesis using chain transfer agents.

(180) In other embodiments of this second approach, non-grafted shell polymer can be added, preferably shell polymer of low molecular mass. This can be added to the melt. This is significant because it was previously considered that full grafting of the shell polymer was of high importance.

(181) In other embodiments of this second approach, low-molecular liquids such as low volatile solvents, plasticisers, monomers or other reactants can be added into the melt.

(182) Improvement of the flowability has been found by addition of additives, in the form of plasticisers (high boiling solvents) and polyethylacrylate (lower molecular weight, ungrafted shell polymer). The UVvis spectra of pressed opal disks using various additives showed that the generation of structural colour was not impaired by the addition of the additives.

(183) It was found that when the precursor composite material is formed with core-shell particles using a poly ethylacrylate shell polymer of reduced molar mass (synthesised in emulsion by addition of a thiol as a chain transfer agent), the resultant material was extremely soft and sticky, and therefore not of use for free-standing films. Pressed opal disks showed strong structural colour. However, this colour was not persistent and disappeared during storage of the disks.

(184) It is therefore preferred to use reactive additives, since these could avoid the drawback identified above. After curing, such additives become a chemically bonded permanent constituent of the opal films. Another significant advantage is that the emission of VOC and the migration of substances of low molecular mass is reduced or avoided altogether, which helps to meet regulatory requirements concerning environmental protection and consumer protection.

(185) Latent reactive additives are preferred. They are latent in the sense that they are non-reactive during the polymer processing into polymer opal films but activated by stimuli after the formation of the films and the generation of structural colour are finished. Suitable stimuli are heat, radiation, pressure, humidity, etc. UV radiation is most preferred as it allows clean processes with low emissions and it requires only low investment. Furthermore, many proven monomers, reactants and additives are commercially available allowing wide adjustment of the properties of the cured product.

(186) Surprisingly it has been found that using monomers to improve processability of the opal polymers and subsequent UV cure based on a free radical polymerisation of a (meth)acrylate containing composition is particularly advantageous. Compared to UV cure of the opal polymer without added monomers, the curing reaction proceeds much more rapidly and a higher mechanical strength of the polymer opal films is easily obtained. Also, a significant reduction of the surface stickiness is observed allowing easy removal of the protective (PET) foil.

(187) Polymers based on (meth)acrylates are known as an additive for UV-curable lacquers (e.g. Degalan available from Evonik Rhm GmbH). We note that there is no apparent discussion in the literature available from the supplier in relation to chemical reaction between polymer and the monomers.

(188) UV curing is an established industrial technique for curing of coatings, prints, adhesives, dental cements and other materials. A wealth of monomers and reactants known to the person skilled in the art is commercially available.

(189) Several chemical approaches for the UV cure are well known, the most established being free radical polymerisation and cationic cure. Furthermore, some special reactions like UV-initiated [2+2] cycloaddition have been published.

(190) UV cure using free radical polymerisation is mostly based on reactants bearing ethylenically unsaturated moieties. Any monomer or oligomer capable of free radical polymerization may be used. Preferred are monomers or oligomers (or mixtures thereof) with vinylic, acrylic or methacrylic groups. Polymerisable mixtures of monomers or oligomers with different degrees of functionality (mono-, di-, tri- and higher) may be used. The mixture may comprise multi-functional monomers or multi-functional oligomers with the same degree of functionality. Examples for suitable monomers and oligomers are listed, for example, in EP-A-1911814 A, U.S. Pat. No. 6,310,115, EP-A-2305762. Still further examples are described in the academic literature e.g. R. Schwalm UV Coatings Elsevier B. V. 2007, ISBN-10: 0-444-52979-9; R. S. Davidson Report 136: Radiation Curing in RAPRA Review Reports, Volume 12, No 4, 2001, RAPRA technology Ltd ISSN: 0889-3144. Another source of information are the product lists and brochures of suppliers specialising in monomers and oligomers for UV-curable formulations e.g. Sartomer (now part of Arkema).

(191) The curing reaction is initiated by radicals generated from light sensitive initiators (photoinitiators) either by decomposition (Norrish-type I) or H-abstraction (Norrish-type II). Examples for Norrish-type I photoinitiators are derivatives of benzoin ethers, benzyl ketales or alkoxyacetophenones. Examples for Norrish-type II photoinitiators are derivatives of benzophenone and isothioxanthone. Other suitable photoinitiators are described in the patents and literature mentioned above and in the product brochures of specialised suppliers e.g. BASF SE (also offering the former products of CIBA), Lamberti and Spectra Group Ltd Inc.

(192) The curing proceeds by the polymerisation of the unsaturated moieties after the initiation by the radicals. The reaction is sensitive to oxygen. The protective foil, which is present in the case that the structural colour is to be developed by the shear processing techniques described above and below, is particularly advantageous for UV cure by the free-radical polymerisation because it keeps the oxygen away.

(193) Additional additives may be are combined with the monomers and the photoinitiators. Sensitizers which allow using longer-wavelength radiation for the activation of the photoinitiators are particularly advantageous for the curing of pigmented systems. Even curing with visible light can often be achieved by a proper combination of photoinitiators and sensitizers. Co-initiators, often amines, are used to increase curing speed and reduce surface tackiness. They are also advantageous as a hydrogen source for the Norrish-type II photoinitiators.

(194) Cationic cure typically based on reactants bearing epoxy-groups which can polymerise by ring-opening-polymerisation. Various co-reactants like oxethanes, carbonates or polyalcohols can be co-polymerised. This advantage could be used to chemically bond the opal polymer to the polymer chains growing during the cationic cure. Suitable moieties can easily be incorporated into the polymer chains of the opal polymer as already shown for hydroxylic groups.

(195) Typically cationic UV curing is initiated by cations originating from a photosensitive sulfonium- or iodonium salt (photoinitiator). Additional additives (e.g. sensitizers) may be used to increase the curing speed or curing efficiency at longer wavelength. Special reactions such as photoinitiated [2+2] cycloaddition based on coumarines or maleimides may offer the possibility of reversible curing.

(196) For the choice of monomers and reactants, their impact on safety and processability should be considered. Advantageous properties are low volatility to enable compounding and processing of the opal polymers at elevated temperatures; low toxicity (most monomers are at least irritant to skin, eyes and the respiratory system); good miscibility with the opal polymer; significant decrease of the viscosity of the opal polymer thereby improving the process ability and the generation of structural colour by shear; high reactivity for short irradiation and curing times; good commercial availability and low price.

(197) Furthermore the impact on the properties of the cured opal films has to be considered. Mechanical properties, chemical resistance, durability, adhesion on foil and other properties are influenced both by the monomers and reactants used for the UV cure and the kind of opal polymer. To achieve a desired combination of properties both the choice of monomers and reactants of the UV cure and the composition of the opal polymer should be optimized.

(198) Materials Used in Examples

(199) Several combinations of opal polymers with UV curing monomers (free radical polymerisation) have been tested. The composition of the particles was:

(200) TABLE-US-00004 32.5 wt % Core poly styrene, crosslinked with 10% butane diol diacrylate 11 wt % Interlayer poly ethylacrylate crosslinked with 10% of allyl methacrylate 56.5 wt % Shell copolymer of 72 wt % ethylacrylate, 25 wt % i-butyl methacrylate, 3 wt % hydroxyethylmethacrylate

(201) The temperature of the glass transition of the shell polymer forming the matrix of the opal films was determined by DCS Tg=0-4 C.

(202) Acrylic, bifunctional:

(203) 1,4 Butanediol diacrylate

(204) ##STR00001##

(205) CAS 1070-70-8

(206) Hazard code: Corrosive (C);

(207) Harmful in contact with skin, Causes burns, May cause sensitization by skin contact

(208) Boiling point 83 C./0.3 mmHg (lit.)

(209) Methacrylic, bifunctional:

(210) 1,4 Butanediol dimethacrylate

(211) ##STR00002##

(212) CAS 2082-81-7

(213) Hazard code: Irritant (Xi);

(214) Irritating to eyes, respiratory system and skin, May cause sensitization by skin contact

(215) Boiling point 132-134 C./4 mmHg (lit.)

(216) Ethylenglycol Dimethacrylate

(217) ##STR00003##

(218) CAS 97-90-5

(219) Hazard code: Irritant (Xi)

(220) Irritating to the respiratory system, May cause sensitization by skin contact

(221) Boiling point 98-100 C./5 mmHg (lit.)

(222) Allylic, bifunctional:

(223) Diallyl Phthalate

(224) ##STR00004##

(225) CAS 131-17-9

(226) Hazard code: Harmful Xn, N

(227) Harmful if swallowed, Very Toxic to aquatic organisms, may cause long-term adverse effects

(228) in the aquatic environment

(229) Boiling point 158 C.

(230) Other suitable monomers are: Isobornyl acrylate CAS 5888-33-5 Isobornyl methacrylate CAS 7534-94-3 Sartomer SR256 2(2-Ethoxyethoxy) ethylacrylate CAS 7328-17-8 Sartomer SR259 (Polyethyleneglycol (200)) diacrylate CAS 26570-48-9 Sartomer SR454 ethoxylated (3 mol) trimethylolpropane triacrylate Sartomer SR 210 (Polyethylene glycol (200)) dimethacrylate CAS 25852-47-5 Sartomer SR 252 (Polyethylene glycol (600)) dimethacrylate CAS 25852-47-5 Glycerol propoxylate triacrylate CAS 52408-84-1 Tripropylene glycol diacrylate CAS 42978-66-5

(231) Monomers having the isobornyl group yield a very low viscosity of the melt. The (polyglycol)-based monomers are very low volatile and nearly odourless.

(232) These commercially available photoinitiators were used:

(233) Norrish Type II:

(234) Benzophenone

(235) CAS 119-61-9

(236) Hazard code: Irritant (Xi); N

(237) Irritating to eyes, respiratory system and skin, May cause sensitization by skin contact; Very toxic to aquatic organisms, may cause long-term adverse effects in the aquatic environment

(238) Boiling point 305 C., Melting Point 49 C.

(239) Norrish Type I:

(240) Darocure 1173 (Ciba, now part of BASF SE)

(241) ##STR00005##

(242) 2-Hydroxy-2-methyl-1-phenyl-1-propanon

(243) CAS 7473-98-5

(244) Hazard code: Harmful (Xn)

(245) Harmful if swallowed

(246) Boiling point 102-103 C./4 mmHg (lit)

(247) Irgacure 819 (Ciba, now part of BASF SE)

(248) ##STR00006##

(249) Bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide

(250) CAS 162881-26-7

(251) Hazard code: Irritant (Xi)

(252) May cause sensitization by skin contact; May cause long-term adverse effects in the aquatic environment

(253) Melting Point 131-135 C.

(254) Irgacure 184 (Ciba, now part of BASF SE)

(255) ##STR00007##

(256) 1-Hydroxycyclohexyl phenyl ketone

(257) CAS 947-19-3

(258) Hazard code: Irritant (Xi)

(259) Irritating to the eyes

(260) Melting Point 45-49 C.

(261) Compounding and Extrusion of Examples

(262) In the following, phr refers to parts per hundred resin. Thus, as an example, for addition of component B to resin A, 15 parts B added to 100 parts A would represent 15 phr B.

(263) The photoinitiators, 1 phr wax Licolub FA1, 0.05 phr carbon black Special Black 4 and the monomers were mixed into the melt of the opal polymer with a lab microextruder 5, DSM Xplore, operated at 90 C. The extruded strands were collected, mixed and extruded again. In total three passes were used to achieve homogeneous mixing. Mixtures with 10% or more monomer were very sticky and paste-like. The temperature of the extruder was adjusted due to the decrease in melt viscosity with increasing concentration of monomers. Table 4 below gives an overview. The lower process temperatures are indicative of the improved flowability.

(264) TABLE-US-00005 TABLE 4 Processing temperature of the microextruder for the compounding of the opal polymers with monomers: Addition of monomer/phr Processing temperature 0 120 C. 5 90 C. 10 65-70 C. 15 50-60 C. 30 RT-40 C.

(265) Melt Flow Rate Measurement of Examples

(266) The flowability of the melt was measured with melt indexer Schmelzindex Prfgert MI-2, Gttfert. This instrument measures the flow of a polymer melt under pressure through a die. The measurement is in step with the practical process where the opal polymer is pressed through a slit die forming the film. Due to the stickiness of the mixtures with monomers, no reliable number for the melt flow rate was obtained. Instead of approaching a constant value, the flow rate increased until the reservoir of the instrument was empty. Nevertheless significant differences in flow rates of polymer with different concentrations of monomer could be easily be distinguished. The measurements were made at 90 C. under a load of 21.6 kg and with a die of geometry L/D=8 mm/2.095 mm,

(267) Forming Opal Disks of Examples

(268) Opal films were pressed with a Dr Collins hydraulic press. 5 g of opal polymer or the mixtures with monomers were placed between sheets of protective foil (PET Mylar A 75, DuPont) and high-gloss steel plates. After warming to 90 C., the press was closed with a hydraulic pressure of 150 bar and kept closed for 3 min.

(269) UV Cure of Examples

(270) Instead of the Osram Ultra Vitalux 300 lamp (artificial sunlight) mentioned above, which emits low UV but much heat, an industrial-type mercury lamp in cold mirror configuration, UV Cube 2000 was used. The radiation source is a common type for UV cure of coatings, The output power of the UV Cube can be switched between 100 W/cm and 200 W/cm. The arc length of the mercury lamp was 10 cm. The PET-covered opal disks were irradiated at a distance of 4 cm. The warming during irradiation was moderate. The opal disks became warm to the touch after 3 min of irradiation with 200 W/cm.

(271) Tensile Testing of Examples

(272) Test specimens were stamped out from the cured opal films sandwiched by the PET foils. After subsequent removal of the PET foils, tensile testing was carried out with a tensile tester zwickiline, Zwick GmbH, at a speed of 1 mm/min at ambient temperature.

(273) Melt Flow Rate Results

(274) FIG. 23 shows the flow rate for mixtures of opal polymer with different additions of butanediol diacrylate at a temperature of 90 C. The flowability was enormously improved by the addition of the monomer. The opal polymer without any monomer had an extremely low melt flow rate (MFR) of about 0.1 mL/min. Such a low MFR is common for very high viscous, high molecular weight polymer used only for extrusion. 10% of monomer increased the MFR to 3-4 mL/min which is typical for industrial polymers used in many processes, including injection moulding. The MFR of the mixture with 15% of monomer is on average as high as 22 mL/min, similar to the behaviour of very low viscous commercial polymers. This result is very significant: The opal polymer can be tuned by the addition of up to 10% of monomer matching the range of viscosities of standard polymers for common industrial processes.

(275) Mechanical Properties Results

(276) FIGS. 24 and 25 illustrate the impact of the monomer concentration on the mechanical properties of pressed opal polymer disks after UV cure via tensile testing. The results show the enormous increase in mechanical strength by UV cure with butanediol diacrylate added. Similar results are obtained both with the Osram Vitalux (FIG. 2420 mins irradiation) and the UVCube (FIG. 25100 W/cm.sup.2 23 mins irradiation) as sources of radiation but different irradiation times.

(277) Impact of the Monomer Type on the Mechanical Properties

(278) From the monomers tested butanediol diacrylate, butanediol dimethacrylate, ethylenglycol dimethacrylate and diallyl phthalate the first two were the most promising. The ethylenglycol dimethacrylate polymerised during feeding into the melt at 60 C. With butanediol diacrylate and butanediol dimethacrylate no such premature polymerisation in the melt occurred although the UV cure of the pressed films was surprisingly fast. Even with 32 phr of diallylphthalate, no cure of the sticky polymer mixture was observed even after prolonged irradiation with the UVCube.

(279) Although diallylphthalate and ethylenglycol dimethacrylate are not ideally suited to the particular processing conditions described here, this does not signify that these particular monomer cannot be used in the methods of the invention. These monomers may find used under in other methods, under different process conditions, optionally with other particles.

(280) FIG. 26 shows a comparison of tensile tests of pressed opal polymer disks with butanediol diacrylate and butanediol dimethacrylate, each sample containing 2 phr benzophenone as photoinitiator. The UV cure was with the UVCube (100 W/cm.sup.2 22 mins). As expected, butanediol dimethacrylate hardens the opal polymer even more than the acrylate. Because of the better elasticity, butanediol diacrylate (BDDA) was made standard for further investigations.

(281) Comparison of Different Photoinitiators

(282) FIG. 27 shows the results for tensile tests of opal polymers with 5 phr of butanediol diacrylate and different mixtures of photoinitiators. The cure was with the UVCube (100 W/cm.sup.2 215 secs). Benzophenone, which was used for the UV cure of opal films without added monomers, has the disadvantage of unsatisfactory miscibility. In concentrations exceeding 2 wt %, benzophenone tends to separate from the opal polymer. Therefore other photoinitiators of better miscibility were added. Three combinations of less reactive photoinitiators, with broader and longer wavelength absorption for through-cure, and reactive photoinitiators, with absorption at lower wavelengths for surface cure, were identified for further testing. The results are shown in FIG. 27. The pressed opal disks were irradiated for only 15 s on both sides. The mixture of benzophenone and Irgacure 184 is slightly more active than the other mixtures. As also the volatility and temperature resistance of Irgacure 184 is very low compared to Darocur 1173, this mixture seems to be the most appropriate for large scale production. The liquid Darocur 1173 was preferred in combination with the low-melting benzophenone for the small scale investigations because it is easier to feed into the microextruder.

(283) Impact of Irradiation Time

(284) FIG. 28 shows tensile test results for opal polymers with 5 phr of butanediol diacrylate, 1 phr benzophenone and 1 phr Darocur 1173 cured under different conditions of UV irradiation. A significant increase in mechanical strength indicative for the degree of cure is observed with an irradiation time of at least 15 s on both sides of the opal film. The difference between opal disks irradiated for only 15 s and longer times, up to 2 min on each side, is less pronounced.

(285) Impact of Film Thickness

(286) The thickness of pressed opal disks is typically higher in the central area and lower towards the edges. FIG. 29 compares tensile tests of specimens taken from areas of different thickness for a composition of 15 phr BDDA, 1 phr benzophenone, 1 phr

(287) Darocur 1173 (cure UVCube 100 W/cm 22 mins). The results confirm that UV cure is particularly advantageous for thinner films. The UV radiation is absorbed passing through the film. The exponential decrease of the radiation intensity with film depth makes the through-cure of thick films difficult. As expected, the thicker film shows slightly lower mechanical strength indicating weaker cure as the fraction of less cured material in the depth of the film is larger.

(288) Impact of the Concentration of Carbon Black

(289) Carbon black is detrimental to the UV cure as it strongly absorbs the radiation needed for the activation of the photoinitiator. Unsurprisingly the mechanical strength, which is indicative of the degree of cure, is best for the sample with the lowest concentration of carbon black. This is shown in FIGS. 30 and 31. It is clear to see that every increase in carbon black concentration directly lowers the degree of cure for given conditions of irradiation. Doubling the irradiation time from 215 s (FIG. 30) to 230 s (FIG. 31) cannot compensate for the decrease in crosslinking efficiency caused by doubling the concentration of carbon black. The composition used in FIGS. 30 and 31 is 5 phr BDDA, 1 phr benzophenone, 1 phr Darocur 1173 and various concentrations of carbon black (Special Black 4).

(290) Storage Stability of Opal Polymer with Added Monomer

(291) FIG. 32 shows the results of tensile tests of mixtures of UV cured opal polymer with 5 phr of butanediol diacrylate, 1 phr of benzophenone and 1 phr of Darocur 1173 which were stored prior to the pressing of the opal disks and the UV cure. The UV cure was 22 mins using the UVCube at 100 W/cm. These results show that there is an increase in mechanical strength the longer the mixtures were stored. The reason of the change is unknown at the time of writing. Despite the increase in hardness pressing of the opal polymer and generation of structural colour was not impaired. This observation is important, as it indicates the possibility of preparing mixtures of opal polymer with monomers and photoinitiator in advance of the production of the films.

(292) Impact of the Monomers on the Development of Structural Colour

(293) Pressed opal disks show strong structural colour after UV cure even with 32 phr monomer added.

(294) As the monomer adds to the volume of the matrix of the opal films, the lattice of the colloidal crystal is expanded which leads to a red-shift of the structural colour. The red-shift depends directly on the amount of monomer added. This effect is well understood. The brilliance of the structural colour depends on the order of the colloidal crystal and on the strength of the scattering of the cores. It had been observed earlier and was recently confirmed that an optimum of the volume fraction of the cores exists. So far the monomers have been added without considering the impact on volume fraction of the cores. Therefore it is advantageous to change the core shell ratio of the opal polymer beads accounting for the monomer added later (for its improvement of the processability). Of particular interest is the potential impact of the monomer addition on the maximum processing speed for generating the structural colour with the shear techniques described above and below. Since the viscosity of the opal polymer is enormously reduced by the monomer addition, the ordering of the cores can occur at lower forces and with fewer shear repetitions than without the monomer.

(295) The reduction in viscosity of the composite precursor material allows for forming processes other than extrusion and/or rolling to be considered. For example, suitable films may be formed by printing processes, or other film forming processes such as roller coating, bar coating, doctor blading, etc. Using a composite precursor material with an additional 32 phr BDDA provides a material which is very viscous at room temperature, but formable via bar coating into a 40 m film in which structural colour could be developed. Subsequently UV irradiation allowed the strength of the film to be increased, forming a self-supporting film.

(296) Improvements to Ordering of Composite Optical Material

(297) During the shearing of the precursor composite material, the ordering of the core particles gradually improves. It is typical in polymer opal materials for the core particles to self-arrange into a close packed arrangement. Typically this is an fcc arrangement. A close packed plane (a {111} plane for an fcc arrangement) is typically formed parallel to the sandwiching layers. Furthermore, the direction of shear (i.e. the direction along which the first and second sandwiching layers are displaced relative to each other) typically corresponds to a close packed direction (a <110> direction for an fcc arrangement) in the close packed arrangement.

(298) When the direction of shear is substantially the same from shear cycle to shear cycle for the entire ordering process, there is a risk of development and growth of crystal defects and/or structural defects in the periodic arrangement of core particles. Without wishing to be bound by theory, these are thought to be due to the consistent shear direction allowing minor crystalline defects (e.g. dislocations) to grow and/or accumulate with the result that the final product may include an observable optical defect. Furthermore, it is found that using a consistent shear direction results in a requirement for a relatively large number of shear cycles in order to develop a suitably periodic arrangement to exhibit high quality structural colour effects.

(299) In a preferred embodiment of the invention, the shear ordering of the precursor composition material is altered from one shear cycle to another. This improves the ordering of the core particles and reduces of the size and/or number of observable defects in the resultant composite optical material.

(300) FIG. 33 provides a schematic illustration of a small portion of a {111} close packed plane in an fcc lattice. A close packed direction (a <110> type direction) is indicated in the drawing at 0. Angularly offset from this close packed direction is a direction at 30. This direction is itself not a close packed direction, but instead bisects two close packed directions (which are angularly offset from each other by 60 in the (111) plane).

(301) FIG. 34 schematically illustrates a curling step in which the precursor composite material is subjected to a shear direction corresponding to the 0 direction shown in FIG. 33. FIG. 35 schematically illustrates a subsequent curling step in which the precursor composite material is subjected to a shear direction corresponding to the 30 direction shown in FIG. 33.

(302) In FIG. 34, precursor composite material 200 is held between first and second sandwiching layers (not shown) formed of PET. The material is passed around roller 202 which is rotated at a speed corresponding to the speed of the material 200 in order to minimise frictional interaction between the material 200 and the roller 202. The material 200 is curled around the roller so that the resultant shear direction is parallel to the edges of the material 200.

(303) In the subsequent rolling step shown in FIG. 35, the same precursor composite material 200 is passed around roller 204 which is rotated at a speed corresponding to the speed of the material 200 in order to minimise frictional interaction between the material 200 and the roller 204. In contrast to the curling step shown in FIG. 34, the material 200 is curled around roller 204 so that the resultant shear direction is not parallel to the edges of the material 200, but instead is offset from the shear direction used in FIG. 34 by 30.

(304) The material 200 may be passed forwards and backwards around the rollers 202, 204 several times in order to promote ordering.

(305) As will be apparent to the skilled person, it is possible to construct arrangements of rollers that are angularly offset from each other in order to provide a continuous (or semi-continuous) process in which the precursor composite material is subjected to angularly varying shear directions. A simple example is shown in FIG. 36, in which precursor composite material 200 is first curled around first roller 210 to give a 0 shear direction and then curled around second roller 212 in order to give a 30 shear direction. It will be noted that the angular offset between the rotational axes of the rollers 210, 212 is also 30.

(306) FIG. 37 shows the effect of curling on ordering of the particles in the material, when the shear direction is maintained at 0. The results show the reflection intensity from the composite optical material. The results show that unidirectional shear provides some improvement of ordering up to about 20 passes, after which the ordering starts to deteriorate.

(307) FIG. 38 shows the effect of curling on ordering of the particles in the material, when the shear direction is altered between 0 and 30. The results again show the reflection intensity from the composite optical material. The results show that bidirectional shear provides very significant improvement of ordering up to 40 passes, the ordering being very much better than in the unidirectional shear example.

(308) In some embodiments, more than one subsequent direction of shear may be used, in order to provide further improvement of ordering of the particles.

(309) The variation in shear direction is also found to provide surprisingly advantageous results when combined with the use of a viscosity-reducing agent as set out above.

(310) Use of Block Copolymers in Place of Core-Shell Particles

(311) The disclosure above relates primarily to composite optical materials in which a three dimensional ordered arrangement is provided by self-assembly of core-shell particles the so-called polymer opal materials. The present inventors have realised that aspects of the invention may be applied to other materials than polymer opal materials.

(312) An example of a different type of material that exhibits structural colour are composite optical materials formed using block copolymers. Parnell et al [A. J. Parnell et al Continuously tuneable optical filters from self-assembled block copolymer blends, Soft Matter, 2011, 7, 3721] disclose the blending of two symmetric high molecular weight diblock copolymers (poly(styrene-b-isoprene) (PS-b-PI)) of different molecular weights. The precursor material is shear aligned to form a one dimensional Bragg reflector. Similarly, Chan et al [E. P. Chan et al Block copolymer photonic gel for mechanochromic sensing Advanced Materials, Volume 23, Issue 40, pp. 4702-4706 (2011)] disclose a symmetric polystyrene-b-poly-2-vinylpyridine (PS-b-P2VP) diblock copolymer self-assembled into a one dimensional lamellar stack.

(313) The use of shear-assisted self-assembly of the ordered lamellae is disclosed in Parnell et al. This can be improved using the curling or edge techniques disclosed above. In particular, curling or edging to a defined radius of curvature provides a much more repeatable shearing than manual oscillation. Therefore the ordering results in the diblock copolymer materials is correspondingly more repeatable.

(314) In the preferred embodiments, the shear strain magnitude and the number of repetitions is as set out above for the polymer opal materials.

(315) It is also preferred that the shear strain direction is altered, as for the polymer opal materials. However, one important difference is that the block copolymer materials do not have a close packing direction in the sense that polymer opals do. Therefore the angle between the different shear strain directions is of lesser importance in relation to block copolymer materials.

(316) The embodiments set out above have been described by way of example. On reading this disclosure, modifications of these embodiments, further embodiments and modifications thereof will be apparent to the skilled person and as such are within the scope of the present invention.