Particles and a process for preparing the same

Details Particles and a process for preparing the same Particles and a process for preparing the same

2002-09-24

2004-04-13

Sergent, Rabon (Department: 1711)

Plastic and nonmetallic article shaping or treating: processes

Encapsulating normally liquid material

Liquid encapsulation utilizing an emulsion or dispersion to...

C264S004100, C264S004330, C264S004600, C427S213330, C427S213340, C523S201000, C525S902000

Reexamination Certificate

active

06719932

ABSTRACT:

The present invention relates to a plurality of particles and a process for preparing the same. In particular, the present invention relates to a plurality of particles having a core material encased in a crosslinked polymer shell and a process for preparing the same.
At times it is helpful, to restrict a core material, for instance a liquid material, to a specific domain. Restriction of the core material controls the fluid and volatile nature of the core material, thus allowing easier management and use of the material. For example, liquid crystal domains are generally much easier to form into a uniform layer for film preparation when restricted to discrete domains.
One such means for containing a core material domain has been to disperse the core material within a polymer wherein the core material is partially or completely surrounded by the polymer. The particles so formed can then be utilized for various purposes. For instance, such particles may be dried to form a powder or film which includes the core material. Alternatively, a film may be produced from an aqueous dispersion of the particles by spreading the dispersion across a surface to form an aqueous film and removing water and any other volatiles. Also, polymer in the dispersion may be cured to form a layer of material wherein the core material is dispersed within the cured polymer.
For many applications, it is advantageous for such particles to have a narrow particle size distribution. However, conventional phase separation techniques, whereby particles are formed, generally result in particles having broad distributions in the shapes and sizes of the particles formed. In U.S. application Ser. No. 08/704,316 filed Sep. 19, 1996, a polymer encased liquid crystal particle is formed by swelling a seed particle with a suitable liquid material and polymerizable monomers to form particles of uniform size. The monomers are polymerized to form a polymer which phase separates from the core material resulting in a polymer shell which encapsulates the liquid material. Such liquid crystal particles, having a narrow particle size distribution, and films made therefrom are shown to have improved electro-optical properties.
There are several problems associated with the use of particles formed of polymers encasing core materials. When used, such particles may be exposed to certain conditions, for example, heat or solvents, which may adversely affect the particles. Generally, the stability of such particles is a function of the stability of the polymer shell to heat, solvents, etc., as well as the stability and volatility of the core material. For instance, use of polymers having a lower glass transition temperature (T
g
) may provide beneficial properties to the polymer. However, low T
g
polymers by their nature are less heat stable and will distort and eventually morphologically deteriorate at temperatures above the T
g
. Also, the core material may decompose or volatilize in response to heat or solvents.
Crosslinking or curing the polymer used to restrict or disperse the core material has been utilized to overcome problems of particle stability. In the majority of methods disclosed in the prior art the polymer shell is either crosslinked after polymerizing the monomers or suitable polymers or prepolymers are provided and then cured by various means. In particular, the aforementioned co-pending application Ser. No. 08/704,316 teaches that the particles of narrow particle size distribution formed therein may be crosslinked after formation of the particles. However, such crosslinking would require additional steps after formation of the particle.
The addition of monomers capable of crosslinking in situ during polymerization to form the polymer shell was thought to be problematic. Specifically, it was believed that the crosslinking monomer(s) would interact with the core material during polymerization leading to improper phase separation and poor or incomplete shell formation. Such interaction could also broaden the particle size distribution. Consequently, there is a need for a process for preparing crosslinked particles of narrow size distribution having discrete polymer shell(s) wherein the particles are formed without additional crosslinking steps after formation of the particle.
The present inventors have now prepared crosslinked particles having discrete polymer shell(s), a narrow particle size distribution and an improved stability to heat and solvents, even when low T
g
polymers are used. Moreover, with these particles it is possible to form films without use of adhesive binders. While not wishing to be bound by theory, it is believed that by crosslinking in situ during polymerization of the polymer shell, using the process of the present invention, a polymer shell is obtained which is more efficiently crosslinked than the crosslinking effected after formation of the polymer shell or shells. Moreover, crosslinking occurs without effect from unfavorable crosslinking interactions with the core material and without the requirement of extra steps after particle formation.
In a first aspect of the present invention, there is provided a process for forming a plurality of particles, including the steps of: (A) providing a first emulsion comprising a core material and one or more monomers in an aqueous phase, wherein at least one of the monomers is a monomer containing two or more double bonds and a second emulsion including seed particles; (B) combining the first emulsion and the second emulsion; (C) forming discrete domains, in the aqueous phase, wherein the domains include the core material and the one or more monomers, and (D) polymerizing the one or more monomers to form a plurality of particles, each particle including a discrete crosslinked polymer shell encasing the core material, wherein the plurality of particles have a polydispersity of from 1.3 to 1.0.
In another aspect of the present invention, there is provided a process for forming a plurality of particles, including the steps of: (A) providing a first emulsion comprising a core material in an aqueous phase and a second emulsion comprising seed particles; (B) combining the first emulsion and the second emulsion; (C) forming discrete domains, in the aqueous phase, wherein the domains include the core material; (D) adding one or more monomers to the discrete domains, wherein at least one of the monomers is a monomer containing two or more double bonds; (E) polymerizing the one or more monomers to form a plurality of particles, each particle including a discrete crosslinked polymer shell encasing the core material, wherein the plurality of particles have a polydispersity of from 1.3 to 1.0.
In a further aspect of the present invention, there is provided a plurality of particles, each particle comprising: (A) 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; and (B) a core material encased in the polymer shells, wherein the plurality of particles have a polydispersity of from 1.3 to 1.0.
In a further aspect of the present invention is provided a polymer particle including (A) 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; and (B) a core material encased in the polymer shells, wherein a plurality of the particles have a polydispersity of from 1.3 to 1.0.
In an additional aspect of the present invention is a process for forming a plurality of particles, including the steps of (A) providing (i) a first emulsion including a core material in an aqueous phase, (ii) a second emulsion including seed particles and (iii) one or more monomers, wherein at least one of the monomers is a monomer containing two or more double bonds; (B) combining the first emulsion, the second emulsion and the one or more monomers; (C) forming discrete domains, in the aqueous phase, wherein the domains include the core material; (D) polymerizing the one or more monomers to form a plur

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