WO2007060462A1 - Particle-stabilised foams - Google Patents

Abstract

The invention relates to a particle-stabilised foam wherein the particles are derived from a polymer latex dispersion and the foam substantially retains its structure on drying.

Description

PARTICLE-STABILISED FOAMS

FIELD OF THE INVENTION

This invention relates to particle-stabilised foams, and more particularly to latex- stabilised foams and methods for their production.

BACKGROUND TO THE INVENTION

A foam is a dispersion of gas in either a liquid or solid continuous phase. Foams are of widespread importance, and are used as intermediate or end products in a diverse range of applications. These include separation of minerals by foam flotation, fire fighting, synthesis of insulating materials, food manufacturing, and preparation of lightweight construction materials. Foaming in a liquid/stabiliser mixture can occur via three main mechanisms: introduction of gas to the body of the mixture through fine holes (sparging), agitation or shaking of the mixture in the presence of gas, or in situ generation of gas bubbles by a sudden pressure drop or chemical reaction. Foam stabilisers can be broadly split into three main categories: ionic or non-ionic surfactants; colloidal stabilisers (including proteins); and solid particles.

Solid particles have been used both as foam stabilisers and de-stabilisers in surfactant- stabilised aqueous foams for many years. Hydrophobic particles are often used in antifoam formulations, and their mode of action is believed to be through a bridging- dewetting mechanism. Hydrophilic particles are believed to stabilise foams by collecting in the films and Plateau borders of a foam; consequently slowing down film drainage and kinetically increasing the foam stability. This is due to the capillary pressure caused by wetting of the particle surface by the aqueous phase. Wetting of the particle surface produces capillary forces that act to increase film thickness in the vicinity of the particles. If enough particles are present, the rate of drainage of the aqueous phase can be retarded.

Hydrophilic particles can also increase foam stability by reducing the diffusion of gas between foam bubbles (disproportionation), which eventually contributes towards the rupture of foam bubbles and collapse of the foam. There may also be structural reinforcement of a foam film due to contact, friction, or cohesion between particles.

In most of the foams studied previously, solid particles are used in combination with surfactants or colloidal stabilisers. The stability of such foams has been found to depend on the particle size, shape, concentration and hydrophilicity, in addition to the type of surfactant used.

Only a very small number of publications describe foams stabilised solely by solid particles.

Ramsden (Proceedings of the Royal Society 1903, 72, 156) first described the use of aqueous suspensions of ferric hydrate, sulfur or various pigments to form persistent bubbles.

Wilson ("A Study of Particulate Foams", DPhil Thesis, University of Bristol, Bristol, UK, 1980) explored the foam-forming behaviour of anionic charge-stabilised polystyrene (PS) latex particles with diameters ranging from 1.02 μm to 3.89 μm. Foaming could only be achieved on addition of either salt or cationic surfactant; or by making the latex strongly acidic (pH < 1). Thus the dispersions approached or surpassed the conditions required for coagulation in the bulk latex. The formation of stable wet foam depended on the particle diameter: a particle diameter of 3.89 μm required no coagulation, latexes of 2.20 μm and 2.10 μm diameter required incipient colloidal destabilisation, and latexes of 1.56 μm and 1.50 μm diameter required the latex to be partially coagulated. Stable wet foam was not obtained under any conditions using the 1.02 μm PS particles. Light diffraction studies indicated that the foam films consisted of bilayers of hexagonally packed PS particles, in which each solid-like monolayer was separated by water. No mention was made of foams surviving drying; only wet foams were discussed.

Sun and Gao (Metallurgical and Materials Transactions A - Physical Metallurgy and Materials Science 2002, 33, 3285) used 1 μm polytetrafluoroethylene (PTFE), 20 μm polyethylene (PE), or 75 μm polyvinyl chloride) (PVC) particles to produce wet foams of reasonable stability in water/ethanol mixtures. Maximum foam stabilities were obtained for the PTFE, PE, and PVC particles at liquid compositions of 40 v/v %, 10 v/v %, and 5 v/v % ethanol, respectively. Murray and co-workers (Langmuir 2004, 20, 8517) used partially hydrophobic 20 nm silica nanoparticles as the sole stabiliser for the formation of stable air bubbles. Silylation was used to increase the hydrophobicity of the originally hydrophilic silica. The authors reported that particles with 33 % and 40 % silylated surfaces formed stable bubbles in an air-saturated aqueous dispersion after a sudden pressure drop. However, these particles were not good foaming agents and only produced a dispersion of discrete bubbles. Salt was also used to fine-tune the particle hydrophobicities to further optimise the degree of stabilisation achieved.

Velev and co-workers (Langmuir 2004, 20, 10371) described the synthesis of foams prepared using polydisperse polymer microrods with an average length of 23.5 μm and an average diameter of 0.6 μm, in the absence of any surfactant. The microrods were synthesised from a bisphenol A-based epoxy resin (SU-8) using a liquid-liquid dispersion technique. Due to their rigid entangled structure and resistance to mechanical perturbations, the polymer microrod foams were very stable, even after drying.

Binks and Horazov (Angewandte Chemie-lnternational Edition 2005, 44, 3722) investigated the formation of foam stabilised with polydisperse silica nanoparticles (20- 50 nm) of intermediate hydrophobicities. The authors reported that particles with 80 % and 68 % silylated surfaces formed the most stable foams after homogenisation or after handshaking. No mention was made of foams surviving drying.

None of the above work is concerned with the production and application of stable dry foams in detail.

SUMMARY OF THE INVENTION

In a first aspect the present invention provides a particle-stabilised foam wherein the particles are derived from a polymer latex dispersion and the foam substantially retains its structure on drying.

In a second aspect the present invention provides a method for the production of a particle-stabilised foam, which comprises introducing a gas into a polymer latex dispersion to form a foam and drying the foam thus produced.

In a third aspect the present invention provides a stable dry particle-stabilised foam wherein the particles are derived from a polymer latex dispersion.

In a fourth aspect the invention provides an optical device comprising a particle- stabilised foam wherein the particles are derived from a polymer latex dispersion.

In a fifth aspect the invention provides a method of producing an optical device exhibiting moire patterns, which comprises a particle-stabilised foam derived from a polymer latex dispersion, and an optical device produced thereby.

In a sixth aspect the invention provides a method of producing an optical device exhibiting multicolour diffraction effects, which comprises a particle-stabilised foam derived from a polymer latex dispersion, and an optical device produced thereby.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a diagram of a foam column apparatus (not to scale) suitable for use in the method of the invention;

Figure 2 shows a diagram of an optical bench arrangement for carrying out the laser diffraction experiments. The distance between the translucent paper and the sample was 29.5 mm; Figure 3 shows DCP curves of the 1.62 μm (A), 1.14 μm (B) and 0.81 μm (C) diameter PNVP-stabilised PS latex particles (entries 1-3 in Table 1);

Figure 4 shows a representative SEM image of the 1.62 μm diameter PNVP-stabilised PS latex particles (entry 1 in Table 1);

Figure 5 shows a representative SEM image of the 1.14 μm diameter PNVP-stabilised PS latex particles (entry 2 in Table 1);

Figure 6 shows a representative SEM image of the 0.81 μm diameter PNVP-stabilised PS latex particles (entry 3 in Table 1);

Figure 7 shows a representative SEM image of the 0.17 μm diameter PNVP-stabilised PS latex particles (entry 5 in Table 1);

Figure 8 shows a representative SEM image of the 0.26 μm diameter PNVP-stabilised PS latex particles (entry 6 in Table 1);

Figure 9 shows digital photographs of foam column experiments after 24 h. Figures A to Figure F correspond to entries 1 to 6 in Table 4, respectively;

Figure 10 shows a cross-sectional representation of a typical latex-stabilised foam, showing how particle bilayers are formed in dry foam. As the wet foam comprising particle monolayer-stabilised nitrogen bubbles dries, the bubble surfaces are drawn together to form ordered bilayers (left to right);

Figure 11 shows optical photomicrographs of crushed foam stabilised using the model 1.14 μm PNVP-stabilised PS latex particles (entry 2 in Table 1), illustrating moire patterns. Note the reference dot in the top left corner of each image, which is approximately equal to the diameter of a single latex particle. Thus, the observed features are much greater than the latex particles used in these foams;

Figure 12 shows a computer-generated image of an artificially created array of hexagonally close-packed spheres (A); a copy of the same computer-generated image after rotation through 24° clockwise with a reduction in opacity of 50 % (B); and direct superposition of these two images to produce a moire-type pattern (C);

Figure 13 shows representative SEM images of highly stable foams prepared using the 1.62 μm diameter PNVP-stabilised PS particles (entry 1 in Table 1; Dw/Dn = 1.008 by DCP);

Figure 14 shows representative SEM images of highly stable foams prepared using the 1.14 μm diameter PNVP-stabilised PS particles (entry 2 in Table 1; Dw/Dn = 1.011 by DCP);

Figure 15 shows representative SEM images of highly stable foams prepared using the 0.81 μm diameter PNVP-stabilised PS particles (entry 3 in Table 1; Dw/Dn = 1.025 by DCP);

Figure 16 shows representative SEM images showing clear evidence for the existence of bilayers in the highly stable foams (entries 1-3 in Table 1). Shown are dry latex foams formed from 1.62 μm (A), 1.14 μm (B), and 0.81 μm (C) diameter PNVP-stabilised PS particles. These images confirm the latex foam bilayer representation shown in figure 10;

Figure 17 shows diffraction patterns obtained from laser diffraction experiments using dried foam prepared using 1.14 μm diameter PNVP-stabilised PS particles. Typical examples of the two main diffraction patterns are shown: a clear hexagonal spot pattern (A), and a ring-like pattern comprising multiple hexagonal spot patterns (B); and

Figure 18 shows digital photographs taken in direct transmitted sunlight of the dried particulate foam stabilised using 1.62 μm PNVP-stabilised PS particles (entry 1 in Table 1), dispersed in water before sintering (A), and dispersed in water after sintering at 105 ⁰C for 10 min. (B).

DESCRIPTION OF VARIOUS EMBODIMENTS

In the present specification, a 'stable' foam is a foam that retains its structure and volume indefinitely after drying, with little or no change in volume.

In one embodiment, the foam is stabilised solely by the latex particles. Alternatively, the foam is stabilised primarily by the latex particles. By primarily is meant that the foam comprises contaminants which comprise up to 10% by weight of a polymeric stabiliser, a small molecule surfactant or a combination thereof.

In another embodiment, the particles forming the particle-stabilised foam are substantially spherical, and may, for example, have a diameter of from 0.05 μm to 10 μm, in particular from 0.1 μm to 5 μm, e.g. from 0.5 μm to 2 μm. In a particular embodiment, the foam cells are substantially polyhedral in shape. Of mention are foam cells having a diameter of from 1 μm to 50 mm.

In a further embodiment, the polymer latex dispersion is a substantially monodisperse latex dispersion of preferably substantially spherical latex particles. The polymer latex may, for example, comprise either a sterically-stabilised or a charge-stabilized dispersion, e.g. a colloidal polymer dispersion. In a particular embodiment, the polymer latex is cationically or anionically charged. Of mention are cationically charged polymer latexes comprise at least one surface amidino group. Also of mention are anionically charged polymer latexes comprising at least one surface carboxylic acid group.

In a further embodiment, the latex particles have a core/shell type structure wherein a core polymer particle is stabilised by a shell of a different polymer (the steric stabilizer) or charge. The shell can be attached by physical (e.g. adsorption) or chemical (e.g. covalent or ionic bonds) means. The shell may provide either complete, or partial, coverage of the core.

The composition of the polymer latex core is not particularly limited and can comprise any suitable polymer that can be produced in latex form. Methods well documented in the art for producing sterically-stabilised polymer latexes include suspension polymerization, emulsion polymerization (including seeded emulsion polymerisation), precipitation polymerization (including seeded precipitation polymerisation) and dispersion polymerization (including seeded dispersion polymerisation). The polymer particles are generally synthesised via a free-radical initiated mechanism, but other chemistries can also be used, e.g. anionic polymerisation, controlled radical polymerizations, Ziegler-Natta polymerisation, cationic polymerisation, chemical oxidative polymerisation or step polymerisation e.g. for the synthesis of polyurethane particles. Latex particles prepared by the dispersion of pre-formed polymers can also be used, e.g. alkyd resins. Any monomer or combination of monomers that is amenable to free radical polymerisation may be used to form the latex polymer core. Suitable monomers are typically vinyl monomers, for example styrene and its derivatives, acrylates, methacrylates, dienes, chloroprene, vinyl chloride, vinyl acetate, vinyl pyridines (eg. 2-vinyI pyridine, 4-vinyl pyridine), acrylonitrile, acrylamides, methacrylamides and olefinic monomers. Particular latex cores comprise polystyrene, poly(alkyl methacrylate) or poly(alkyl acrylate), polyolefins or polyesters, in particular polystyrene, including cross-linked polystyrene, epoxy resins, alkyd resins, polyamides, novolac resins or polyurethanes. The polymer latex core may be cross-linked during synthesis, using multivinyl monomers, for example, dϊvinylbenzene, ethylene glycol diacrylate or ethylene glycol dimethacrylate as a cross-linking agent. Cross-linking is also possible using UV radiation and heat.

The stabilizing polymer forming the shell of the latex particles can be based on a wide range of water-soluble/hydrophilic polymers, for example, polyvinylpyrrolidone), poly(acrylic acid), polyvinyl alcohol), poly(ethylene imine), water-soluble poly(meth)acrylates (e.g. poly(acrylic acid), poly[2-(dimethylamino)ethyl methacrylate]), polyvinyl alcohol), poly(methacrylic acid), poly(ethylene oxide), poly(2-vinyl pyridine), poly (4-vinyl pyridine) or poly(sodium 4-styrenesulfonate), various cellulosic derivatives (e.g. methylcellulose, ethylcellulose, hydroxypropylcellulose or carboxymethylcellulose) or a hydrophilic-hydrophobic block polymer, e.g. poly[2-(dimethylamino)ethyl methacrylate-b/oc/f-(methyl methacrylate)]. Preferably, the stabilizing polymer forming the shell of the latex particles is produced by polymerizing or copolymerising water- soluble/hydrophilic monomers.

Stimulus-responsive foam can be produced using polymer latex particles which have stimulus-responsive groups either in the latex shell or on the latex surface.

The stimulus-responsive polymer forming the shell of the latex particles can be based on a wide range of pH-, temperature- or salt-responsive polymers, for example, poly(acrylic acid) or poly[2-(dimethylamino)ethyl methacrylate].

In one embodiment, the polymer latex comprises a stimulus-responsive polymer, for example poly(acrylic acid), poly(methacrylic acid), poly[2-(dimethylamino)ethyl methacrylate], poly[2-(diethyiamino)ethyl methacrylate], poly(2-vinylpyridine), poly(4- vinylpyridine) or poly(Λf-isopropylacrylamide).

Polymer latexes stabilized with a polyacid form stable foams below the pKa value, (e.g. for poly(acrylic acid), stable foams are formed at pH values at or below its pKa of approximately 4.5) and those stabilized with a polybase form stable foams at pH values at or above the pKa value. For example, the polymer latex dispersion stabilized with poly(acrylic acid) may have a pH of less than 4, e.g. a pH ranging from 1 to 3.5.

In one embodiment, the charge on the latex particle surface comprises ionizable groups, for example vinyl pyridine groups, amine groups, carboxylic acid groups or amidino groups.

In another embodiment, the polymer latex dispersion is an aqueous dispersion, or a dispersion of polymer particles in an organic solvent, for example, a C1-6 lower alcohol, such as methanol, ethanol or 2-propanol, or tetrahydrofuran. Mixtures of water and water-miscible organic solvents can also be used.

The stable foam of the present invention can be generated, for example, by hand - shaking or the in situ generation of gas bubbles by means of a chemical reaction or a sudden pressure drop, but often a gas is blown through the latex dispersion (sparging), for example, by placing the latex dispersion in a column and blowing a suitable gas through the base of the column. The gas may be, for example, carbon dioxide, argon, oxygen, helium, air or nitrogen.

In one embodiment, the stable dry particulate foam is produced by the methods described above.

In another embodiment, the method of producing an optical device exhibiting moire patterns and the method of producing an optical device exhibiting multicolour diffraction effects comprises crushing the particle stabilised foam. In a particular embodiment, the method of producing an optical device exhibiting moire patterns and the method of producing an optical device exhibiting multicolour diffraction effects comprises dispersing the particle stabilised foam in a liquid medium. The particle stabilised foam can be dried, non-dried, sintered, non-sintered, plasticized, non-plasticized, cross-linked or non- crosslinked.

The following Examples illustrate the invention.

EXAMPLE 1

EXPERIMENTAL

Materials

Unless stated otherwise, all materials were purchased from Sigma-Aldrich and were used as received. Styrene was treated with basic alumina in order to remove inhibitor and then stored at -25 ⁰C before use. Monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA) macromonomer (Mn = 2,000; Mw/Mn = 1.10) was supplied by Sigma-Aldrich as a 50 w/w % aqueous solution. Poly(Λ/-vinylpyrrolidinone) (PNVP) (K90; Mn = 360,000) was purchased from Fluka Chemicals. Poly(acrylic acid) was used as received. Ammonium persulfate (APS), 2,2'-azobis(isobutyronitrile) (AIBN), and 2,2'- azobis(2-methylpropionamidine) dihydrochloride (AIBA) were used as free radical initiators (see Figure 5.4). HPLC grade methanol, ethanol and 2-propanol (IPA), were purchased from Fisher Scientific. Poly(vinylidene fluoride) dialysis tubing (molecular weight cut-off 500,000 Daltons) was acquired from Fisher Scientific. The water used in these experiments was deionised using an Elga EIgastat Option 3 water purification apparatus. Dry nitrogen gas (oxygen free) was supplied by BOC.

Synthesis of Polymer Particles

PNVP-Stabilised PS Latex in 2-PropanoVWater Mixtures Using AIBN Initiator

Dispersion polymerisation of styrene was performed in the presence of PNVP homopolymer as a steric stabiliser in batch mode at 70 °C using AIBN initiator. A typical synthetic procedure was as follows: PNVP (20.0 g; 10 w/w % based on styrene) was added to 2-propanol (2.0 L) in a three-necked 5 L round-bottomed flask fitted with a reflux condenser and a magnetic stirrer bar. This reaction mixture was vigorously stirred at 70 °C until the PNVP had dissolved completely, and was then degassed using a nitrogen purge. Polymerisation commenced after the addition of a mixture of styrene and AIBN initiator (2.00 g AIBN dissolved in 200 g styrene). The reaction was allowed to proceed for 24 h with continuous stirring at 300 rpm under a nitrogen atmosphere. Different diameters of PNVP-stabilised PS latexes were synthesised by following the same general method by replacing up to 20 v/v % of the 2-propanol solvent with water. Higher water contents led to smaller latexes. These latexes were purified by centrifugation (4,000-5,000 rpm for 30-90 min.).

PNVP-Stabilised PS Latex in a MethanolAA/ater Mixture Using AIBN Initiator

PNVP-stabilised PS latex particles were prepared by the dispersion polymerisation of styrene in the presence of PNVP homopolymer as a steric stabiliser at 70 °C using AIBN initiator. PNVP (1.00 g; 20 w/w % based on styrene) was added to methanol (45 ml_) and water (5.0 mL) in a one-necked 100 ml_ round-bottomed flask containing a magnetic stirrer bar. This reaction mixture was vigorously stirred at 70 ⁰C until the PNVP had dissolved completely. The reaction mixture was then degassed using five alternating cycles of evacuation and pressurisation with nitrogen (starting with evacuation). Polymerisation commenced after the injection of a mixture of AlBN and styrene (0.050 g AIBN dissolved in 5.00 g styrene) into the reaction vessel. The reaction was allowed to proceed for 24 h with continuous stirring at 500 rpm under a nitrogen atmosphere. This latex was purified by centrifugation (8,000-10,000 rpm for 90-120 min.).

PNVP-Stabilised PS Latex in Water Using APS Initiator

Emulsion polymerisation of styrene was performed in the presence of PNVP homopolymer as a steric stabiliser in batch mode at 70 °C using APS initiator. PNVP (0.500 g; 10 w/w % based on styrene) was added to water (45 mL) in a one-necked 100 mL round-bottomed flask containing a magnetic stirrer bar. This reaction mixture was vigorously stirred at 20 ⁰C until the PNVP had dissolved completely. Styrene (5.00 g) was then added to the reaction mixture, and the flask was heated to 70 ⁰C. The reaction mixture equilibrated for 30 min., and was then degassed using five alternating cycles of evacuation and pressurisation with nitrogen (starting with evacuation). Polymerisation commenced after the injection of an aqueous solution of APS (0.050 g APS dissolved in 5.0 ml_ water) into the reaction vessel. The reaction was allowed to proceed for 24 h with continuous stirring at 500 rpm under a nitrogen atmosphere. This latex was purified by centrifugation (8,000-10,000 rpm for 90-120 min.).

PNVP-Stabilised PS Latex in Water Using AIBA Initiator

Emulsion polymerisation of styrene was performed in the presence of PNVP homopolymer as a steric stabiliser in batch mode at 60 ⁰C using AIBA initiator. PNVP (0.500 g; 10 w/w % based on styrene) was added to water (45 ml_) in a one-necked 100 mL round-bottomed flask containing a magnetic stirrer bar. This reaction mixture was vigorously stirred at 20 ⁰C until the PNVP had dissolved completely. Styrene (5.00 g) was then added to the reaction mixture, and the flask was heated to 60 °C. The reaction mixture was equilibrated for 30 min., and was then degassed using five alternating cycles of evacuation and pressurisation with nitrogen (starting with evacuation). The polymerisation commenced after the injection of an aqueous solution of AIBA (0.050 g APS dissolved in 5.0 mL water) into the reaction vessel. The reaction was allowed to proceed for 24 h with continuous stirring at 500 rpm under a nitrogen atmosphere. This latex was purified by centrifugation (8,000-10,000 rpm for 90-120 min.).

Purification

The latexes were purified by either centrifugation or dialysis to ensure that non-adsorbed excess stabiliser or other surface-active materials (e.g. trace monomer) were removed.

Successive centrifugation cycles also ensured that any alcohol (co)solvent used in dispersion polymerisations was entirely replaced by water.

Centrifugation

Successive centrifugation-redispersion cycles were used to purify latex samples, with each supernatant being decanted and replaced with de-ionised water. The extent of purification for each latex was assessed by measuring the surface tension of the supernatant. Purification continued until this surface tension was close to that of water at 20 ⁰C (72.5 ± 0.5 mN m^"1). During this process, the concentration of the latex was kept approximately constant by replacing the supernatant with an equivalent amount of water.

Dialysis

Latex was loaded into sections of thoroughly washed poly(vinylidene fluoride) dialysis tubing. The sections were sealed and then immersed in water as the external solvent, which was changed daily. The surface tension of this external water was measured after at least 24 h contact with the full dialysis tubing. Purification continued until this surface tension was close to that of water at 20 ⁰C (72.5 ± 0.5 mN m^'1).

Surface Tension Measurements

Measurements of surface tension were made at 20 ⁰C using a Kruss K1OST tensiometer equipped with a standard platinum ring. Data were corrected according to Harkins and Jordan. (Journal of the American Chemical Society 1930, 52, 1751). The surface tension of distilled water was measured periodically under the same conditions and found to be 72.5 ± 0.5 mN m^"1 at 20 ⁰C.

Characterisation of Particles

Optical Microscopy

A drop of dilute latex was placed on a microscope slide, and optical photomicrographs obtained using a James Swift MP3502 optical microscope (Prior Scientific Instruments Ltd.) fitted with a Nikon Coolpix 4500 digital camera.

Disc Centrifuge Photosedimentometry (DCP)

Disc centrifuge photosedimentometry (DCP) measurements to determine the weight- average diameter (Dw) and polydispersity (Dw/Dn) of particles were carried out using a Brookhaven BI-DCP instrument, operating in the line start mode. Samples for DCP analysis were prepared by diluting a few drops of the aqueous latex mixture in 20 ml_ of a 1 :3 v/v % methanol/water mixture. This solution was then immersed in an ultrasonic bath for 30 seconds prior to DCP analysis. The centrifugation rate was adjusted to between 2,000 and 10,000 rpm, depending on the diameter of the particles being measured. A particle density of 1.05 g cm^"3 was assumed for the polystyrene latex particles. Thus the stabiliser layer thickness was always ignored.

Dw/Dn values less than 1.01 are generally regarded as characteristic of highly monodisperse latexes, whereas values between 1.01 and 1.05 are typical of near- monodisperse latexes, and values above 1.05 are usually associated with relatively polydisperse latexes. For comparison, 0.30 μm and 0.90 μm diameter PS latex standards (Duke Scientific Corp., USA) both have a Dw/Dn value of 1.003 as measured by DCP. Particle diameters for the larger particles in this patent (> 1 μm) were determined by DCP.

Dynamic Light Scattering (DLS) Studies

DLS studies were performed at 20 ⁰C using a Brookhaven Instruments Corporation Bl- 200SM goniometer equipped with a BI-9000AT digital correlator using a solid-state laser (125 mW, λ = 532 nm) at a fixed scattering angle of 90°. The intensity-average particle diameter (Dz) and polydispersity were calculated by cumulants analysis of the experimental correlation function using the Stokes-Einstein equation for dilute, non- interacting monodisperse spheres.

The polydispersity is a measure of the width of the particle diameter distribution. Values less than 0.01 are generally regarded as characteristic of highly monodisperse latexes, whereas values between 0.01 and 0.10 are typical of near-monodisperse latexes, and values above 0.10 are usually associated with relatively polydisperse latexes. For comparison, 0.30 μm and 0.90 μm diameter PS latex standards (Duke Scientific Corp., USA) both have polydispersity values of 0.01-0.02 as measured by DLS.

Scanning Electron Microscopy (SEM)

SEM images were obtained using a Leo Stereoscan 420 instrument operating at 20 kV and 5-10 pA. Dried samples were placed on an aluminium stub and sputter-coated with gold (to minimise sample-charging problems).

Foam Generation

By Hand

Latex was shaken for 30 seconds by hand. The shaking time, although arbitrary, was found to give reasonably reproducible results. Hand-shaking was also found to be a good indicator of the propensity of a given latex to stabilise foams. Latexes that did not produce foam upon hand-shaking did not produce foam using the foam column method either.

Foam Column

The foam column setup is shown in Figure 1. A sintered glass frit 1 of known porosity (diameter = 25 mm, depth = 3 mm) was connected to one end of a long vertical glass column 2 of similar diameter (length = 600-650 mm). The column and frit were surrounded by a water-filled jacket 3, comprising an inlet 4 and an outlet 5 for water. The space between the column and this water jacket was designed to be as small as possible, and was packed at the top with a foam seal 6 to reduce convective heat loss. The inner wall of the water jacket was constructed from thinner glass than was used for the outer wall, so as to aid conductive heat transfer. Dry nitrogen gas was connected in series to a flow rate meter, a copper coil heat-exchanger, before finally connecting to just below the sintered glass frit at inlet 7. The heat-exchanger ensured the nitrogen gas was preheated to the same temperature as the glass jacket. All glassware was thoroughly washed with THF and dried before use.

In a typical experiment, the apparatus was assembled and the water jacket temperature was set to 25 ⁰C. The apparatus temperature was allowed to equilibrate for at least 1 h before the start of the experiment. During this time nitrogen gas was passed through the frit so that the air in the column was displaced. Before addition of the latex, the nitrogen flow rate was set to 50 mL min^'1. 5 mL of the latex sample 8 was then carefully injected on top of the frit via the top of the glass column. There was a short induction time as the nitrogen below the frit reached sufficient pressure to be forced through the pores in the frit. As soon as bubbles were observed leaving the surface of the frit, the timer was started. After 5 min. the nitrogen supply was turned off, and the height of any stable foam that was formed was then measured. In some cases, the residual latex was collected for analysis. The foam was left at 25 ⁰C for at least 24 h, after which time the foam's height was re-measured and the foam recovered for further analysis.

Sintered glass frits of three porosities were evaluated in the foam column experiments: porosity values were 3 (16-40 μm pore diameter) and 4 (10-16 μm pore diameter). Unless stated otherwise, the default frit used in all the foam experiments was the porosity 4 sintered glass frit.

Characterisation of Latex Foams

Digital Photography

A Nikon Coolpix 4500 digital camera was used to collect digital photographs of foams generated using the foam column method. No further image processing was carried out on the images.

Optical Microscopy

Optical micrographs of dry foams were obtained using identical apparatus and conditions to those used for latex particles.

Scanning Electron Microscopy (SEM)

SEM images for the dried particulate foams were obtained using identical apparatus and conditions to those used for latex particles. In this case, foam samples were attached to the aluminium stub using conducting adhesive tape, prior to sputter-coating with gold.

Laser Diffraction Expeπ^'ments

The optical bench arrangement for the laser diffraction experiments is shown in Figure 2. A He-Ne laser 9 operating at 633 nm was used to illuminate a sample of dried particulate foam 10 prepared from the 1.14 μm PNVP-stabilised PS latex particles (entry 2 in Table 1) that had been sandwiched between two optical microscopy slide glasses 11. A 3 mm diameter aperture 12 was used to improve the profile of the laser beam incident on the sample. The resulting diffraction pattern was projected onto a sheet of white translucent paper 13, and recorded at beam stop 14 using a Nikon Coolpix 4500 digital camera 15.

RESULTS

Particle Syntheses

A summary of data for the syntheses of the various PS latex particles used in this patent is shown in Table 1.

Table 1. Summary of synthesis conditions and physicochemical characteristics of the PS polymer particles used.

a. measured by DCP. The number in brackets is the value for Dw/Dn. b. Dz measured by DLS. The number in brackets is the value for the polydispersity. c. Also measured by DCP, which gave a Dw of 0.22 ± 0.02 μm and a Dw/Dn of 1.000. Entries 1-6 in Table 1 were synthesised using the PNVP steric stabiliser. This water- soluble polymer adsorbs onto the surface of the PS latex particles, preventing flocculation through steric stabilisation. Some of the PNVP becomes grafted to the surface of the PS particles via radical transfer to the stabiliser. Entries 5 and 6 were additionally charge-stabilised by polymer chain-end groups resulting from the anionic APS and cationic AIBA initiators, respectively. PNVP-Stabilised PS Latex in 2-Propanol/Water Mixtures Using AIBN Initiator

Entries 1 , 2 and 3 in Table 1 were synthesised via dispersion polymerisation of styrene using the PNVP stabiliser. The largest particles were synthesised in pure 2-propanol, with a Dw of 1.62 ± 0.13 μm and a Dw/Dn of 1.008. As expected, the addition of water reduced the diameter of the particles obtained: 10 v/v % water gave particles with a Dw of 1.14 ± 0.08 μm (Dw/Dn = 1.011), and 20 v/v % water gave particles with a Dw of 0.81 ± 0.08 μm (Dw/Dn = 1.025). These particle diameters were obtained by DCP, and confirmed by optical microscopy (a representative DCP curve is shown in Figure 3). The largest diameter particles were highly monodisperse. The latex polydispersity gradually increased as the particle diameter decreased, although the smallest diameter particles remained reasonably monodisperse. Typical SEM images of these latexes are shown in Figure 4, Figure 5 and Figure 6, in which the uniform spherical morphology of these particles can be clearly observed.

When still dispersed in their original 2-propanol or 2-propanol/water mixtures, these latexes did not stabilise foams. However, as the purification by successive centrifugation cycles progressed, the quantity and stability of the foam that was generated gradually increased. After 3-5 centrifugation cycles the quantity of foam began to decline, although the foam remained very stable. This is probably due to the removal of excess PNVP during the centrifugation-redispersion cycles. Surface tension measurements of the supernatant solutions decanted after each centrifugation step support this hypothesis.

PNVP-Stabilised PS Latex in a MethanolAA/ater Mixture Using AIBN Initiator .

Entry 4 in Table 1 was also synthesised via dispersion polymerisation of styrene using the PNVP stabiliser. However, a more polar 9:1 v/v % methanol/water mixture was used, thus smaller particles were obtained (Dz = 0.68 μm and the polydispersity was 0.167 as measured by DLS).

PNVP-Stabilised PS Latex in Water Using APS Initiator

Emulsion polymerisation of styrene using the PNVP stabiliser in aqueous solution gave particles with a Dz of 0.17 μm and a polydispersity of 0.176 as measured by DLS (entry 5 in Table 1). This polymerisation was initiated using APS₁ resulting in anionic sulfate end groups on the polymer chains that are expected to contribute to the stability of the particles through charge stabilization as well as steric stabilisation. Typical SEM images for these particles are shown in Figure 7, in which the uniform spherical morphology of these particles can be observed.

PNVP-Stabilised PS Latex in Water Using AIBA Initiator

A second aqueous emulsion polymerisation of styrene using the PNVP stabiliser gave particles with a Dz of 0.26 μm and a polydispersity of 0.114 as measured by DLS (entry

6 in Table 1). However, this time the polymerisation was initiated by AIBA, which results in cationic end groups on the polymer chains. These are also expected to contribute to particle stability through charge stabilization as well as steric stabilisation. Representative SEM images for these particles are shown in Figure 8, in which the uniform spherical morphology of these particles can be clearly observed.

Foam Generation

Experimental Design

Although simple hand-shaking was sufficient for the generation of foam, a more reproducible and quantifiable method was desired. Thus, a foam column was used to determine relative foam-forming abilities under controlled experimental conditions. Dry nitrogen was used as the sparging gas. A diagram of the foam column is shown in Figure 1. In all cases where stable foam was generated in sufficient quantity to cover the entire diameter of the foam column, the observed foam heights were not limiting: if the nitrogen flow was not terminated after 5 min., the foam continued to rise up the column. More detailed analysis showed that the rate of change of foam height with time was constant. This is due to the relatively rigid nature of the highly stable foams: the foam acts as a plug in the column, and is simply pushed up by inflowing gas underneath it. In principle, with a continuous flow of nitrogen combined with a method for the delivery of fresh latex to the sintered glass frit, highly stable particulate foam could be generated indefinitely. Measurements of the approximate bubble diameter at the top, middle and bottom of the foam after its generation provided a further indication of the foam quality. All foam column experiments were carried out using a foam column equilibrated at 25 ⁰C, with 5.0 ml_ latex sample and a constant nitrogen flow rate of 50 mL min^"1 for 5 min. This should give a maximum theoretical foam height of approximately 550 mm.

Exploration of Foam Formation Using Model Latex

The 1.14 μm diameter PNVP-stabilised PS latex (entry 2 in Table 1) was used as a model latex in initial experiments that explored some of the physical parameters that influence foam formation. Table 2 shows data from reproducibility.

Table 2. Results obtained from foam column reproducibility experiments using the 1.14 μm diameter PNVP-stabilised PS particles (entry 2 in Table 1. 5 mL of 7.5 w/v % aqueous latex was sparged with nitrogen gas at a flow rate of 50 mL min^"1, through a porosity 4 sintered glass fit (10-16 μm pore diameter) for 5 min. at 25 ⁰C.

The foam column experiments demonstrated excellent reproducibility with regard to the initial bubble diameter ranges, which showed no significant difference between the experiments.

Table 3 shows a summary of the effect of varying several physical parameters that were expected to influence foam formation. In addition to the initial foam height obtained after the foam column experiment, the approximate bubble diameter range for the foam at the top, middle, and bottom of the column are also reported. Table 3. Effect of varying the physical parameters on foam formation using the model 1.14 μm PNVP-stabilised PS particles (entry 2 in Table 1) using the foam column. 5 ml_ of aqueous latex was sparged with nitrogen gas at a flow rate of 50 ml. miri^"1, through a sintered glass frit of known porosity for 5 min. at 25 ^CC.

a. In all cases the bubble diameter ranges after 24 h were identical to the initial values. b. This section of the foam column contained large cells/voids instead of true foam.

The foam height obtained for the foam prepared using the latex at 1.0 w/v % (250 mm) is significantly lower than for the other foams reported in Table 3. This can be explained by the incomplete spanning of the foam column diameter by a small quantity of relatively fragile foam. As the foam moved up the column, it broke apart and allowed the gas to escape, halting the rise of the foam.

The bubble diameter ranges measured during the various experiments are more informative. As the latex concentration was increased from 1.0 w/v % to 7.5 w/v %, the volume of foam-free voids column decreased: all three experiments using 7.5 w/v % latex generated foam for the entire 5 min. The average bubble diameters also decreased in line with the increasing latex concentration. This confirms that the quantity of foam is dependent on the concentration of latex particles available, and shows that the bubble diameter increases as the latex concentration is reduced. The latex concentration also drops during a given foam column experiment, as the latex becomes increasingly depleted. There was no significant change in either the foam height or the bubble diameter after 24 h in the foam column, compared to initial measurements taken 5 min. after the flow of nitrogen had ceased. Indeed, these foams appeared to be indefinitely stable, surviving for at least six months after their preparation in either wet or dry states. Foams generated using aqueous solutions of SDS (a common industrial surfactant) or PNVP (the colloidal stabiliser used for the synthesis of the model latex) do not display the same rigidity, and collapse upon drying at 25 ⁰C.

After the reproducibility experiments, the resulting depleted latex dispersions were analysed for their remaining latex content and were found to contain an average of 0.7 w/w % solids, compared to an original concentration of 7.5 w/w % solids. This means that at least 90 % of the particles in the original latex were used to form highly stable foam.

Exploration of Foam Formation Using Other Latexes

The other latexes described in Table 1 were also assessed for their foam-forming ability. Table 4 shows the results obtained from these particles during experiments using the foam column.

Table 4. Results obtained from foam column experiments investigating the foam-forming ability of the PS particles described in Table 1. 5 mL of aqueous latex was sparged with nitrogen gas at a flow rate of 50 mL min^"\ through a porosity 4 sintered glass fit (10-16 μm pore diameter) for 5 min. at 25 ⁰C.

a. Sample ID refers to the samples described in Table 1. b. This section of foam column contained large cells/voids instead of true foam (see comments). c. The bubble diameter ranges after 24 h were identical to the initial values. d. Although the nominal foam height inside the column had not decreased after 24 h, a significant proportion of the foam had collapsed internally. Entries 1-4 in Table 4 were all stabilised using PNVP and had essentially equal latex weight concentrations, so can therefore be compared to determine the effect of varying surface area. The largest PS particles (1.62 μm diameter) generated the least foam, which was evident from the absence of foam in the lower 230 mm of the column. This is consistent with these particles possessing the lowest specific surface area, meaning that the latex is depleted faster for a given nitrogen flow rate. Bubbles created in the early stages of the experiment (top of the column) are also larger, which may be related to a lower preferred degree of curvature for the larger diameter particles. Particle diameters of 1.14 μm and 0.81 μm produced smaller bubbles in all stages of the experiment as the particle diameter decreases, which is consistent with the increasing specific surface area of these latexes. The particles with a mean diameter of 0.68 μm had the largest specific surface area among the particles produced by dispersion polymerisation, yet the foam stabilised with these particles had relatively poor stability.

Entries 5 and 6 in Table 4 also initially produced foam for the entire duration of the foam column experiment, and then experienced significant collapse as the foam dried. The particles synthesised using the anionic APS initiator (entry 5 in Table 5.4) suffered collapse to a greater extent compared to neutral AIBN-initiated latex particles (entry 4 in Table 4); whereas the particles synthesised using a cationic AIBA initiator (entry 6 in Table 4) collapse to a lesser extent.

The fact that the sterically-stabilised anionic particles (entry 5) foam at ail without prior destabilisation is surprising, given the negative results reported by Wilson supra. However, the long-term foam instability is not unexpected, since these charged particles are unlikely to pack as well as their neutral counterparts. Immediately after forming, the entry 5 foam had a slight blue tint to it, and was much more transparent than the foams prepared using the larger latexes. There was also very little foam in contact with the sides of the foam column, which was probably due to the anionic character of the glass surface. As the foam dried, it became whiter, more opaque, and almost entirely collapsed.

The sterically-stabilised cationic particles (entry 6) were expected to form more stable foams due to their initial attraction to the anionic nitrogen/water interface. However, once adsorbed to the interface the particles should experience mutual repulsion. Consequently, the packing efficiency of these particles was expected to be inferior compared to the neutral particles. The sterically-stabilised cationic foam suffered much less collapse than for the sterically-stabilised anionic particle foam (entry 5) and, surprisingly, also less collapse than the smallest neutral PNVP-stabilised particles (entry 4). This latter result is interesting given that the neutral particles are significantly larger, and should also pack more closely to form a more robust foam.

Digital photographs of the stable foams (entries 1-6 in Table 4) recorded after 24 h are shown in Figure 5.

Highly monodisperse fluorescent 1.57 μm diameter PNVP-stabilised PS latex was used to prepare highly stable foam by hand-shaking. The latex was synthesised using a similar protocol to the 1.62 μm diameter PNVP-stabilised PS latex (entry 1 in Table 1), except that 1 w/w % of the styrene was replaced with 1-pyrenylmethyI methacrylate, a fluorescent monomer. Well-ordered hexagonally close-packed arrays of particles were observed by optical microscopy and SEM. Confocal laser scanning microscopy was used to obtain photomicrographs of cross-sectional areas of individual foam bubbles. These microscopy studies confirmed that the particle monolayers were adsorbed at the air/water interface of the bubbles.

Highly monodisperse 0.73 μm and 1.20 μm diameter cross-linked poly(methyl methacrylate) (PMMA) particles stabilised with PNVP (donated by AGFA, Belgium) also generated highly stable foam upon shaking by hand.

Highly monodisperse PEGMA-stabilised poly(2-vinylpyridine) latex particles prepared by emulsion polymerization using the PEGMA stabilizer (A. Loxley, B. Vincent, Colloid Polym. Sci. 275, 1108 (1997)) with a diameter of 0.37 μm were also used to prepare highly stable foam at pH 7.5 that retained its shape and volume after drying. Multicolour diffraction effects were observed during optical microscopy experiments, resulting from Bragg diffraction by colloidal crystals consisting of ordered microgel particle arrays.

Foam Structure

Foam generated using the larger PNVP-stabilised PS latex particles (entries 1-3 in Table 1) are highly stable in both wet and dry states. The foam structure was evaluated using optical and confocal laser microscopy, SEM, and laser diffraction. Optical microscopy studies after applying pressure and SEM studies after deliberating breaking the dried foam provided convincing evidence that these highly stable foams consist primarily of particle bilayers comprising two well-ordered hexagonally close-packed monolayers. The bilayers are formed when the surfaces of adjacent nitrogen bubbles, stabilised by a monolayer of particles, are drawn together as the foam dries (see Figure 10).

Moire Patterns

Moire patterns were observed in optical photomicrographs of dry foams. Moire patterns are produced when two (or more) arrays of regular hexagonally-packed PS particles, formed during the foam column experiments. Figure 11 shows an optical micrograph of dried foam stabilised with 1.14 μm PNVP-stabilised PS particles exhibiting moire patterns. The particular moire patterns shown in these micrographs can only occur with well-ordered arrays of particles.

Note the red reference dot in the top left corner of each image, which is approximately equal to the diameter of a single latex particle. Thus, the observed features are much greater than the latex particles used in these foams.

The highly ordered nature of the PS particle arrays in the stable latex foams is apparent from the SEM images (see Figure 13, Figure 14 and Figure 15). The degree of ordering is very high for the largest particles, and decreases as the mean particle diameter decreases. The numbers of particles contained. within individual colloidal crystal domains were determined by analysis of SEM images (not shown). The largest typical domains observed in foam prepared using the 0.81 μm diameter PNVP-stabilised PS latex particles contained 90-110 particles per domain. In the case of the foam generated using the 1.14 μm diameter PNVP-stabilised PS latex particles, a typical large domain contained 2,100 particles. A typical large colloidal crystal domain for foam prepared using the 1.62 μm diameter PNVP-stabilised PS latex particles covered a larger area than was possible to image using SEM (while maintaining sufficient resolution for particle counting). Therefore the 9,600 particles observed in a typical colloidal crystal domain for this foam can be regarded as a lower limit. The significant reduction in the colloidal crystal domain size correlates with the particle diameter and the increase in polydispersity as the latex particle diameter decreases, since slightly different diameters necessarily pack less efficiently. Particle bilayers are also clearly evident in some of the higher magnification images (see Figure 16).

Laser Diffraction Experiments

The highly ordered nature of the particles in the latex foams prepared using the 1.14 μm diameter PNVP-stabilised PS particles was investigated using laser diffraction. Samples of dried particulate foam obtained from foam column experiments were illuminated using a He-Ne laser operating at 633 nm (see Figure 2). Typical photographs of the diffraction patterns are shown in Figure 17. There are two main types of diffraction pattern: a clear hexagonal spot pattern (Figure 17A) and a ring-like pattern comprising multiple hexagonal spot patterns (Figure 17B).

The pattern observed in Figure 17A is undoubtedly Bragg diffraction from a single hexagonally close-packed colloidal crystal of PS particles. The pattern observed in Figure 17B is most likely Bragg diffraction resulting from multiple smaller colloidal crystals at different relative orientations. The angle of diffraction (θ) from a two- dimensional array where a laser beam of wavelength λ is incident perpendicularly to the plane of the array depends on the lattice spacing (d) of the particles, according to the following equation:

cf = nλ sin θ

Allowing for a ± 0.5 mm degree of error in the measurement of both the sample to translucent paper distance, and the beam to diffraction spot distance, a value for θ of 33.67 ± 1.12° is obtained. Equation (1) then gives a lattice spacing of 1.14 ± 0.03 μm, which is in excellent agreement with the number-average particle diameter measured by DCP (Dn = 1.13 ± 0.06 μm).

Optical Effects Multicolour Diffraction Effects

Striking multicolour diffraction effects were observed on viewing some of the dried particulate foams using bright transmitted light such as direct sunlight (although weaker effects were observed under artificial light). Possible interaction between the glass walls of the foam column and the PS particles was ruled out as a possible explanation for this iridescence, since foam extracted from the centre of the foam column also produced the same multicolour effects. The colours appear to be generated as a result of Bragg diffraction from the PS lattice, with the exact colour depending on the angle of incidence and lattice orientation of the randomly oriented colloidal crystal domains.

The foam prepared using the larger 1.62 μm diameter PNVP-stabilised PS particles (entry 1 in Table 1) displayed the most intense multicolour diffraction effects, while foam comprising smaller 0.81 μm diameter PNVP-stabilised PS particles (entry 3 in Table 5.1) displayed reduced intensity compared to the 1.14 μm diameter particles. This colour intensity variation correlates with the significant difference in the size of the colloidal crystal domains observed by SEM and the PS particle diameter.

Foam generated using the 0.26 μm diameter PNVP-stabilised particles synthesised using the cationic AIBA initiator (entry 6 in Table 4) displays only weak colour effects in transmitted light. It appears yellow/green in reflected light, although the effect is much less intense than for the diffraction colours observed due to transmitted light for the previous foam examples (entries 1-3 in Table 4).

Foam Dispersion in Liquids

If dried particulate foam stabilised with the 1.62 μm, 1.14 μm or 0.81 μm PNVP- stabilised PS particles (entries 1-3 in Table 1) is mixed with water, then the particles partially redisperse and a cloudy mixture is formed. This behaviour is expected, due to the presence of water-soluble PNVP at the surface of the PS particles. However, if the dried foams are mixed with non-solvents for the PNVP and PS, then dispersions of particulate foam fragments can be formed. These dispersions retain the multicolour effects of the original dried foam. These larger particles were chosen as they gave the most intense multicolour effects after drying. The right-hand photograph in each pair shows the dispersed foam fragments in each of the oils, while the left-hand photograph shows an image of the pure oil taken under the same conditions for comparison. The same foam was then sintered by placing a sample in an 105 °C oven, for 10 min. This was designed to briefly heat the particulate foam to just above the glass transition temperature of PS (100 ^CC), thus allowing the surfaces of the latex particles to fuse with each other and hence stabilising the latex foam fragments. This heat treatment was sufficient to allow subsequent dispersion of the foam in water without any of the particle redispersion problems observed earlier. Figure 18 shows digital photographs of this foam dispersed in water, before (A) and after (B) sintering. The particles in the sintered foams proved resistant to redispersal in the water phase for at least several months.

EXAMPLE 2

Charge-Stabilised PS Latex Synthesized in Methanol Using AIBN Initiator

Charge-stabilised PS particles (stabilized purely by the cationic AIBA initiator fragments; i.e. no polymeric stabiliser was used) were prepared by the dispersion polymerization of styrene at 60 ⁰C using AIBA initiator. Styrene (5.0 mL) and methanol (50 mL) were added to a round-bottomed flask containing a magnetic stirrer bar. AIBA initiator (46.0 mg) was added and the mixture was degassed with nitrogen. The reaction mixture was heated to 60 ⁰C under a steady flow of nitrogen and then stirred for 24 h. The resulting milky-white colloidal dispersion was purified by several centrifugation-redispersion cycles (2,000 rpm for 15 min.), with each successive supernatant being carefully decanted and replaced, gradually changing from pure alcohol to water via alcohol/water mixtures.

Cationic charge-stabilised PS latex particles with a bimodal particle diameter distribution were used to generate highly stable foams upon hand-shaking. The latex comprised mainly 1 μm diameter particles with a minor population of submicrometer-diameter particles. The particles were stabilised by cationic polymer chain-end groups resulting from the cationic AIBA initiator. Partial flocculation of this latex in the bulk solution suggested that these particles were not particularly colloidally stable after transferring from methanol to an aqueous medium. Again, hexagonally close-packed particles were observed on the surface of the foam bubbles by optical microscopy.

EXAMPLE 3

PDMA-stabilized PS.

Dispersion polymerization of styrene was performed in the presence of PDMA-6-PMMA diblock copolymer as a colloidal stabilizer, in batch mode at 60 ⁰C using AIBN initiator. A typical synthetic procedure was as follows: PDMA-fc-PMMA (0.5 g; 10 w/w % based on styrene) was added to methanol (50 mL) in a three-necked 100 ml_ flask fitted with a reflux condenser and a magnetic stirrer bar. This reaction mixture was vigorously stirred at 60 ⁰C until the PDMA-b-PMMA had dissolved completely, and was then degassed using a nitrogen purge. The polymerization commenced after the injection of a mixture of styrene and AIBN (0.05 g AIBN in 5 g styrene) into the reaction vessel, and was allowed to proceed for 24 h with continuous stirring at 250 rpm under a nitrogen atmosphere. Conversion of styrene was almost 100 %, as determined by a gravimetrical method.

Near-monodisperse 0.98 μm diameter PS latex particles stabilised with a poly[2- (dimethylamino)ethyl methacryIate]-b-PMMA diblock copolymer formed highly stable foams upon hand-shaking. The 2-(dimethylamino)ethyl methacrylate residues are expected to be only partially protonated at the solution pH of 7.3. Hexagonally close- packed PS particles were observed on the surface of the air bubbles by optical microscopy. . .

EXAMPLE 4

PAA-Stabilised PS Latex in Methanol Using AIBN Initiator

Dispersion polymerization of styrene was performed in the presence of poly(acrylic acid) (PAA) homopolymer as a colloidal stabilizer in batch mode at 70 ⁰C using AIBN initiator. A typical synthetic procedure was as follows: PAA (1.2 g; Mn = 250,000; 12. w/w % based on styrene) was added to an ethanol/water mixture (68.5 g ethanol and 20 g water) in a three-necked 100 mL flask fitted with a reflux condenser and a magnetic stirrer bar. This reaction mixture was vigorously stirred at 70 ⁰C until the PAA had dissolved completely, and was then degassed using a nitrogen purge. The polymerization commenced after the injection of a mixture of styrene and AIBN (0.168 g AIBN in 10 g styrene) into the reaction vessel, and was allowed to proceed for 48 h with continuous stirring at 250 rpm under a nitrogen atmosphere. Conversion of styrene was almost 100 %, as determined by a gravimetric method.

Relatively monodisperse pH-responsive 1 μm diameter PS latex particles stabilised with poly(acrylic acid) (PAA) homopolymer were also used to prepare highly stable foams at a solution pH of 3 or less. At this pH the PAA chains are in a desolvated, collapsed conformation due to protonation of the acid groups. Only unstable foam was formed at pH 4.7, whereas at pH 6.4 or above no foam could be generated.

The minimum PS particle diameter that can be successfully used to form stable foam has been reduced significantly compared to prior work by Wilson supra. This is an important result, since smaller latexes are readily synthesised by one-shot aqueous emulsion polymerisation. Emulsion polymerisation is an easier and cheaper industrial process compared to alcoholic dispersion polymerisation.

The latex foams prepared using PNVP-stabilised PS particles were stable to drying with little or no change in volume, and interesting optical effects were observed such as moire patterns and multicolour diffraction effects. Although rather brittle, such foams were stable with respect to collapse. These foams can be dispersed in various non-aqueous solvents that are non-solvents for both PNVP and PS. Moreover, these particulate foams are sufficiently robust after sintering to enable their dispersion in water without significant reduction in optical effects.

The relatively straightforward synthesis of the particulate foams of the invention using simply latex, gas and water suggests a potentially cheap large-scale synthesis of two- dimensional colloidal crystals. There are various possible industrial applications for these unique materials, including their use in plastic jewellery, as a component in decorative ornaments or glass treatments, or for forming a critical constituent of security inks. If optical effects are paramount, then it is usually important for use near-monodisperse polymer particles in the production of these colloidal crystals, since the packing efficiency is important to the formation of regular arrays of particles. Disordered arrays would lead to diffuse scattering and the reduction of colour effects.

It appears that the nature of the latex core does not significantly influence the foam- forming ability of the latex (only true if latex remains non-solvated). Rather, it seems that the nature of the particle stabiliser, the mean latex diameter, and perhaps the latex polydispersity play roles in determining whether highly stable foam is generated by a particular latex.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A particle-stabilised foam wherein the particles are derived from a polymer latex dispersion and the foam substantially retains its structure on drying.

2. A particle-stabilised foam according to claim 1, wherein the foam is stabilised solely by the latex particles.

3. A particle-stabilised foam according to claim 1 or 2, wherein the particles forming the particle-stabilised foam are substantially spherical.

4. A particle-stabilised foam according to claim 3, wherein the particles have a diameter of from 0.05 μm to 10 μm, for example 0.1 μm to 5.0 μm, in particular 0.50 μm to 2.0 μm.

5. A particle stabilised foam according to any one of the preceding claims, wherein the dried foam cells are substantially polyhedral in shape.

6. A particle stabilised foam according to any one of the preceding claims, wherein the foam cells have a diameter of from 1 μm to 50 mm.

7. A particle stabilised foam according to any one of the preceding claims, wherein the polymer latex dispersion is a substantially monodisperse latex dispersion.

8. A particle stabilised foam according to any one of the preceding claims, wherein the polymer latex comprises a sterically-stabilised or a charge-stabilized dispersion.

9. A particle stabilised foam according to claim 8, wherein the polymer latex is cationically charged.

10. A particle stabilised foam according to claim 9, wherein the cationically charged latex comprises at least one surface amidino group.

11. A particle stabilised foam according to claim 8, wherein the polymer latex is anionically charged.

12. A particle stabilised foam according to claim 11, wherein the anionically charged latex comprises at least one surface carboxylic acid group.

13. A particle stabilised foam according to any one of the preceding claims, wherein the particles of the polymer latex dispersion have a core/shell structure wherein a core polymer particle is stabilised by a shell of a different polymer or charge.

14. A particle stabilised foam according to claim 13, wherein the polymer latex core comprises polymerised vinyl monomers or olefinic monomers.

15. A particle stabilised foam according to claim 14, wherein the vinyl monomers comprise styrene, styrene derivatives, acrylates, methacrylates, dienes, chloroprene, vinyl chloride, vinyl acetate, vinyl pyridine, acrylonitrile, acrylamides or methacrylamides.

16. A particle stabilised foam according to claim 14, wherein the polymer latex core comprises polystyrene, poly(alkyl methacrylate), poly(alkyl acrylate), polyolefins, polyesters, epoxy resins, alkyd resins, polyamides, novolac resins or polyurethanes.

17. A particle stabilised foam according to any one of the preceding claims, in which the polymer latex core is cross-linked.

18. A particle stabilised foam according to any one of claims 13 to 17, wherein the stabilizing polymer forming the shell of the latex particles is hydrophilic polymer.

19. A particle stabilised foam according to claim 18, wherein the stabilizing polymer forming the shell of the latex particles is poly(/V-vinylpyrrolidinone), poly(acrylic acid), polyvinyl alcohol), poly(ethylene imine), poly(methacrylic acid), poly(ethylene oxide), poly (2-vinyl pyridine), poly (4-vinyl pyridine), or poly(sodium 4-styrenesulfonate), a cellulosic derivative, or a hydrophilic-hydrophobic block polymer.

20. A particle stabilised foam according to claim 19, wherein the cellulosic derivative is methylcellulose, ethylcellulose, carboxymethylcellulose or hydroxypropylcellulose

21. A particle stabilised foam according to claim 19, wherein the hydrophilic- hydrophobic block polymer is poly[(2-dimethylamino)ethyl methacrylate]Hb/oc/c- poly(methyl methacrylate).

22. A particle stabilised foam according to any one of claims 13 to 21, wherein the shell of the latex particles comprises ionisable groups.

23. A particle stabilised foam according to claim 22, wherein the ionisable groups comprise vinyl pyridine groups, amine groups, carboxylic acid groups or amidino groups.

24. A particle stabilised foam according to any one of the preceding claims, wherein the polymer latex dispersion is an aqueous dispersion, a dispersion of polymer particles in an organic solvent, or a dispersion of polymer particles in a mixture of water and an organic solvent.

25. A particle stabilised foam according to any one of the preceding claims, wherein the polymer latex dispersion comprises a stimulus-responsive polymer.

26. A particle stabilised foam according to claim 25, wherein the stimulus-responsive polymer is poly(acrylic acid), poly(methacrylic acid), poly[2-(dimethylamino)ethyl methacrylate], poly[2-(diethylamino)ethyl methacrylate], poly(2-vinylpyridine), poly(4- vinylpyridine) or poly(N-isopropylacrylamide).

27. A method for the production of a particle-stabilised foam, which comprises introducing a gas into a polymer latex dispersion to form a foam and drying the foam thus produced.

28. A method according to claim 27, wherein the foam is stabilised solely by the latex particles.

29. A method according to claim 27 or 28, wherein the latex particles forming the particle-stabilised foam are substantially spherical.

30. A method according to any one of claims 27 to 29, wherein the latex particles have a mean diameter from 0.05 μm to 10 μm, for example 0.1 μm to 5.0 μm, in particular 0.50 μm to 2.0 μm.

31. A method according to any one of claims 27 to 30, wherein the dried foam cells are substantially polyhedral in shape.

32. A method according to any one of claims 27 to 31 , wherein the foam cells have a diameter of from 1 μm to 50 mm.

33. A method according to any one of claims 27 to 32, wherein the polymer latex dispersion is a substantially monodisperse latex dispersion.

34. A method according to any one of claims 27 to 33, wherein the polymer latex comprises a sterically-stabilised or charge-stabilized dispersion.

35. A method according to any one of claims 27 to 34, wherein. the particles of the polymer latex dispersion have a core/shell structure wherein a core polymer particle is stabilised by a shell of a different polymer or charge.

36. A method according to any one of claims 27 to 35, wherein the polymer latex is produced by suspension polymerization, precipitation polymerization, emulsion polymerization or dispersion polymerization.

37. A method according to any one of claims 27 to 36, wherein the polymer particles are synthesised via a free-radical initiated mechanism, controlled radical polymerizations, Ziegler-Natta polymerisation, anionic polymerisation, cationic polymerisation, chemical oxidative polymerisation or step polymerisation.

38. A method according to any one of claims 35 to 37, wherein the polymer latex core is produced by polymerizing a vinyl monomer or an olefinic monomer.

39. A method according to claim 38, wherein the vinyl monomer is styrene, styrene derivatives, acrylates, methacrylates, dienes, chloroprene, vinyl chloride, vinyl acetate, acrylonitrile, acrylamide or methacrylamides.

40. A method according to claim 38, wherein the polymer latex core is cross-linked

41. A method according to claim 40, wherein cross-linkage is promoted by a cross- linking agent comprising multivinyl monomers.

42. A method according to claim 41, wherein the multivinyl monomers comprise divinylbenzene, ethylene glycol dimethacrylate or ethylene glycol diacrylate.

43. A method according to any one of claims 35 to 42, wherein the stabilizing polymer forming the shell of the latex particles is produced by polymerizing or copolymerising hydrophilic monomers.

44. A method according to any one of claims 27 to 43, wherein the polymer latex dispersion is an aqueous dispersion, a dispersion of polymer particles in an organic solvent, or a dispersion of polymer particles in a mixture of water and an organic solvent.

45. A method according to any one of claims 27 to 44, wherein the polymer latex dispersion has a pH lower than the pKa of the polyacid stabiliser or above the pKa of the polybase stabiliser.

46. A method according to any one of claims 27 to 44, wherein the polymer latex dispersion has a pH of from 1 to 3.5.

47. A method according to any one of claims 27 to 46, wherein the foam is generated by sparging, shaking or in situ gas generation.

48. A method according to claim 47, wherein the foam is generated by blowing a gas through the latex dispersion by placing the latex dispersion in a column and blowing a suitable gas through the base of the column.

49.. A method according to claim 48, wherein the gas is carbon dioxide, argon, oxygen, helium, air or nitrogen.

50. A stable dry particle-stabilised foam wherein the particles are derived from a polymer latex dispersion.

51. A stable dry particulate foam according to claim 50, produced by a method according to any one of claims 27 to 49.

52. An optical device comprising a particle stabilised foam wherein the particles are derived from a polymer latex dispersion.

53. A method of producing an optical device exhibiting moire patterns, which contains a particle-stabilised foam derived from a polymer latex dispersion.

54. An optical device produced by a method according to claim 53.

55. A method of producing an optical device exhibiting multi-colour diffraction effects, which contains a particle-stabilised foam derived from a polymer latex dispersion.

56. An optical device produced by a method according to claim 55.

57. A method according to claim 53 or 55, wherein the particle stabilised foam is dispersed in a liquid medium.

58. An optical device produced by a method according to claim 57.