PARTICLES AND A PROCESS FOR PREPARING THE SAME

Disclosed are a plurality of particles having a narrow particle size distribution, a processes for forming the same, and films containing the same. The plurality of particles, includes one or more discrete polymer shells, wherein at least one of the polymer shells is crosslinked with at least one monomer containing two or more double bonds polymerizable by free radical means; and a core material encased in the polymer shells, wherein the plurality of particles have a polydispersity of from 1.3 to 1.0.

EXAMPLE 1 Preparation of Crosslinked Particles Using 1% ALMA Cross-Linker An emulsion was prepared by combining 369 parts of Merck E7 liquid crystal, 140 parts of monomer/initiator solution (MMA/HEMA/Sty/ALMA/t-BPO&equals;67.8/22.5/8.7/1/3.5), 92 parts of DOSS soap solution (2% solution in water), 364 parts of DI water, and 22 parts of methylated &bgr;-cyclodextrin (50.8% solution in water), Wacker Company, and then emulsifying at a very high shear for 2 minutes. A polymer emulsion latex, 170 parts, BA/Sty/Ht&equals;82/18/19 (28% polymer by weight, 0.562 micron particle size), was added to the emulsified LC/monomer mixture. The sample was subjected to mild agitation for 24 hours at room temperature and then viewed under the microscope at 500× to find particle droplets having a narrow size distribution. The above mixture, which is 50% non-aqueous, was diluted to 20% non-aqueous with an additional 163 parts of water. The reaction mixture was added to a sealed pressure tube and subjected to mild agitation in a hot water bath at 85° C. for 1 hour, then 95° C. for another hour. The sample was then dried and examined by SEM microscopy. SEM showed uniform PELC particles having a narrow particle size distribution. The resulting particles had a 1.25 micron size. The produced particles were crushed in a mortar and pestle in liquid nitrogen and the LC extracted out with heptane. SEM of this sample showed cracked hollow spheres. The presence of crosslinking was verified by adding the dried PELC particles to MDC. The particles swelled to a jelly like mass rather than being dissolved. Uncrosslinked particles readily dissolve in many solvents, especially MDC. 
 EXAMPLE 2 Preparation of Crosslinked Particles Using 100% BDDA Cross-Linker An emulsion of 369 parts of Merck E7 liquid crystal, 140 parts of monomer/initiator solution (BDDA/t-BPO&equals;100/3.5), 92 parts of DOSS soap solution (2% solution in water), 364 parts of DI water, and 22 parts of methylated &bgr;-cyclodextrin (50.8% solution in water), Wacker Company, were emulsified at very high shear for 2 minutes. A polymer emulsion latex, 170 parts, BA/Sty/Ht&equals;82/18/19 (28% polymer by weight, 0.562 micron particle size), was added to the emulsified LC/monomer mixture. The sample was then subjected to mild agitation for 24 hours at room temperature and then viewed under the microscope at 500× to find particle size droplets having a narrow size distribution. The above mixture, which is 50% non-aqueous, was diluted to 20% non-aqueous with an additional 163 parts of water. The reaction mixture was added to a sealed pressure tube and subjected to mild agitation in a hot water bath at 85° C. for 1 hour, then 95° C. for another hour. The sample was then dried and examined by SEM microscopy. SEM showed uniform PELC particles of narrow particle size distribution. The resulting particles had a 1.25 micron size. The produced particles were crushed in a mortar and pestle in liquid nitrogen and extracted with heptane. SEM of this powder sample showed cracked hollow spheres. The integrity of the particles was tested by heating the powder. Uncrosslinked PELC particles will distort, i.e., swell, flow, and permanently lose original structure at a temperature around their glass transition temperature (typically about 60-80° C. depending on the exact composition and level of plasticization by the LC). The powder was heated to 170° C. on a hot plate and under a heat lamp. No effect from the heat was observed. SEM showed the particles were unchanged. The highly crosslinked shell of the particles is unaffected by solvents. The dried powder was added to MDC and ether. No swelling or loss of particle shell integrity was observed, but the solvent extracted the LC quantitatively from the core. The extracted shells were separated from the solvent/LC solution by centrifugation of the supernatant. These particles are amenable to washing by centrifugation due to their extremely rigid nature. The liquid crystal was recovered by rotary evaporation of the supernatant. SEM showed particles which looked identical to the original PELC particles before the solvent and centrifugation treatment. The shells are perfectly hollow as evidenced by SEM of liquid N 2 fractured specimens of the recovered shell. The hollow particles readily redisperse into MDC, but float in water. 
 EXAMPLE 3 Preparation of Crosslinked Particles Using 100% TMPTA Cross-Linker Added After Droplet Formation An emulsion was prepared by combining 362 parts of Merck TL205 liquid crystal, 725 parts of ethyl ether. 188 parts of DOSS solution (2% solution in water), 508 parts of DI water, and 45 parts of methylated &bgr;-cyclodextrin (50.8% solution in water), Wacker Company, and then emulsifying at very high shear for 4 minutes. A polymer emulsion latex, 500 parts, BA/Sty/Ht&equals;82/18/19 (10.00% polymer by weight, 0.562 micron particle size), was added to the emulsified LC mixture. The sample was then subjected to mild agitation for 120 minutes at 85° C. The sample was then viewed under the microscope at 500× to find droplets that having a narrow droplet size distribution. The ethyl ether was removed by heating and bubbling nitrogen through the sample. A monomer solution, 138 parts, (100 parts TMPTA and 3.5 parts of t-BPO) was emulsified with 23 parts of DOSS solution (2% solution in water), and 110 parts of DI Water. This mixture was added to the LC emulsion from above. In several minutes the monomer was incorporated into the LC droplets. The above mixture was diluted to 20% non-aqueous with 90 parts of water. The reaction mixture was added to a sealed pressure tube and was subjected to mild agitation in a hot water bath at 85° C. for 1 hour and then an additional 1 hour at 95° C. The sample was then dried and examined by SEM microscopy. SEM showed particles of narrow particle size distribution. The resulting particles had a 1.25 micron size. The produced particles were crushed in a mortar and pestle in liquid nitrogen and extracted with heptane. SEM of this sample shows cracked hollow spheres. The 20% particle emulsion was mixed at 25° C. with a film forming binder polymer latex 2-EHA/Sty/HEA/MAA/MA&equals;50.6/23.9/20/0.5/5 which had been presaturated with 25% TL205 LC. The ratio of dried particles to dry binder was 1:1. Seven (7) grams of IRN-150, Rohm and Haas mixed bed deionizing ion exchange beads were then added. The IRN beads remove ionics, including the soap sodium dioctyl sulfosuccinate, which was present at about 1000 ppm. The deionization is monitored with a conductivity probe. Initially the reading was off scale (>200 ppm). However, after about 30 minutes the reading was about 100 micromhos, corresponding to <100 ppm total ionics. The sample was then filtered through a burette packed with fine glass wool, and then through a 20 micron stainless steel screen. The sample was degassed under vacuum. The sample emulsion which was about 28% non-volatile was drawn down on a 36 centimeter×13 centimeter×1.1 millimeter ITO coated glass substrate using a 10 centimeter doctor blade (Gardner type), at a gap setting of 0.051 millimeters, which was set with a feeler gauge. Owing to a clean glass substrate and the correct viscosity of the mixture, wetting of the mixture was good. The sample was placed in a vacuum oven overnight at 25° C. The dry film weight was calculated to be 20 microns by dividing the weight of the dry film by the area of the dry film in the appropriate units (wt in grams×10000/area in centimeters). The density of the film was about 1 g/cm3. The next day, after further storage in a vacuum, 5 centimeter×7.5 centimeter samples were cut for polymer dispersed liquid crystal (PDLC) device fabrication. ITO coated Mylar was used as the top substrate. The top piece is laminated using a laminator. The device is then sealed all around the perimeter of the device using melted paraffin wax (mp&equals;56° C.). The sample was then tested for switching voltage using a HeNe laser and detection device. The beam was normalized to read 100% transmission through air. The electro-optical performance was typical of a polymer dispersed liquid crystal containing PELC. 
 EXAMPLE 4 Preparation of Crosslinked Particles Using 10% TMPTA Cross-Linker An emulsion was prepared by combining 369 parts of Merck E7 liquid crystal, 140 parts of monomer/initiator solution (MMA/HEMA/Sty/TMPTA/t-BPO&equals;58.8/22.5/8.7/10/3.5), 92 parts of DOSS soap solution (2% solution in water), 364 parts of DI water, and 22 parts of methylated &bgr;-cyclodextrin (50.8% solution in water), Wacker Company, and then emulsifying at very high shear for 2 minutes. A polymer emulsion latex, 170 parts, BA/Sty/Ht&equals;82/18/19 (28% polymer by weight, 0.562 micron particle size), was added to the emulsified LC/monomer mixture. The sample was then subjected to mild agitation for 24 hours at room temperature and then viewed under the microscope at 500× to find droplets having a narrow size distribution. The above mixture, which was 50% non-aqueous, was diluted to 20% non-aqueous with an additional 163 parts of water. The reaction mixture was added to a sealed pressure tube and subjected to mild agitation in a hot water bath at 85° C. for 1 hour, then 95° C. for another hour. The sample was then dried and examined by SEM microscopy. SEM showed PELC particles having a narrow particle size distribution. The resulting particles have a 1.25 micron size. The produced particles were crushed in a mortar and pestle in liquid nitrogen and extracted with heptane. SEM of this sample showed cracked hollow spheres. A PDLC film was made directly from the particles. A sample of the particle dispersion was dried under a heat lamp at 150° C. The resulting film was fractured under liquid nitrogen and the LC was extracted with heptane. SEM showed a PDLC film wherein the LC had been removed and the resulting voids were perfectly uniform with a cavity size precisely the same as the LC core of the original PELC. Another PDLC film was prepared from pure particles at 150° C. for electro-optical response as in Example 3. The sample performed as a typical PDLC containing PELC. 
 EXAMPLES 5-9 Particles were prepared as in Examples 1, 2 and 4, except that the crosslinking monomer component was as shown in Table 1. The PD of each sample was determined using BI-DCP. The results are shown in Table 1. 2 TABLE 1 Examples Crosslinker Crosslinker % PD 5 ALMA 1% 1.017 6 TMPTA 1% 1.019 7 ALMA 10% 1.015 8 TMPTA 100% 1.007 9 TMPTA 10% 1.013 The resultant PDs illustrate that particles are produced having a narrow particle size distribution which is substantially monodisperse, i.e., a PD of 1.0. 
 EXAMPLE 10 Preparation of Hollow Particles Using 100% TMPTA Cross-Linker An emulsion was prepared by combining 125 parts of dioctyl phthalate (DOP), 50 parts of TMPTA, 1.75 parts of t-BPO, 33 parts of DOSS solution (2% solution in water), 112 parts of DI water, and 8 parts of methylated &bgr;-cyclodextrin (50.8% solution in water, Wacker Company), and then emulsifying at a high shear rate for 2 minutes. A polymer emulsion latex, 84 parts, BA/Sty/Ht&equals;82/18/19 (10.00% polymer by weight, 0.306 micron particle size), was added to the emulsified mixture. The sample was then subjected to mild agitation for 8 hours at room temperature. The above mixture was diluted to 20% non-aqueous with 587 parts of water. The reaction mixture was added to a sealed pressure tube and was subjected to mild agitation in a hot water bath at 85° C. for 1 hour and then an additional 1 hour at 95° C. The sample was then vacuum-dried and examined by SEM microscopy. SEM showed uniform spherical particles of narrow particle distribution. The resulting particles had a 600 nanometer diameter. SEM of the sample showed cracked hollow spheres when the particles were cryofractured and extracted with heptane. Both DOP-filled and intact hollow spheres were dried for use as opacifying agents in a vinyl resin blend. Removal of the dioctyl phthalate core material was accomplished using solvent extraction and washing of the particles as described in Example 2. The DOP-filled and hollow cross-linked particles were each dry blended with a clear polyvinyl chloride formulation. The mixtures comprised 5 parts of cross-linked particles to 100 parts of PVC resin (Borden Chemical Products, BCP-57). The blends were mixed using a Haake Rheocord (50 rpm) at 175° C. for 10 minutes. Plaques (3.8 cm×5.1 cm) were prepared using a Reliable Press operating at a temperature of 190° C. The pressing operation applied 9 metric tons for 3 minutes, followed by 63 metric tons for 2 minutes and a 5-minute cooling period at 63 metric tons. Light scattering properties of the resulting plaques (0.3 cm thick) were measured using a Hunter Lab optical sensor. A control plaque using the same PVC formulation but containing no cross-linked particles was also prepared. Table 2, below, shows differences in the optical properties of the matrix polymer with and without cross-linked additives. The ratio of diffusely transmitted (scattered) light to total transmittance, T d /T t , in each sample of the same thickness gives the percent haze values. Plaques containing the hollow sphere additives produced in this example exhibited the highest scattering properties. 3 TABLE 2 Total Diffuse Transmittance Transmittance Sample Haze (T t ) (T d ) Control (BCP-57) 12.4 57.80 7.15 BCP-57/DOP-filled 46.2 57.51 26.55 TMPTA particles (100:5) BCP-57/Hollow 75.0 44.23 33.18 TMPTA particles (100:5)