A dye, such as a fluorescent dye, is incorporated into polymer microparticles using a solvent system composed of a first solvent in which the dye and the microparticle polymer are soluble, a second solvent in which the dye and the microparticle polymer are not or only weakly soluble, and a third solvent in which the dye and the microparticle polymer are not or only weakly soluble. The first and second solvents are immiscible with each other, or at most partially miscible. The third solvent is miscible with the first and second solvents. The formulation provides substantially complete partitioning of the dye to the microparticles. The method may be used to obtain dyed polymer microparticle formed of cross-linked or non-cross-linked polymers. Libraries are provided comprising two or more sets of microparticles of different dye loadings. Fluorescent core-shell microparticles are produced from a mixture of microparticle cores incorporating one or more fluorescent dyes, a polymerization mixture comprising at least one polymerizable shell monomer, at least one free radical polymerization initiator comprising a water-insoluble oxidizing agent, and at least one water-soluble reducing agent.
1. A method of incorporating at least one dye into polymer microparticles comprising:
(a) providing:
(i) at least one first solvent in which the dye and the microparticle polymer are soluble;
(ii) at least one second solvent in which the dye and the microparticle polymer are not or only weakly soluble, said first and second solvents being immiscible or at most partially miscible;
(iii) at least one third solvent in which the dye and the microparticle polymer are not or only weakly soluble, said third solvent being miscible with the first and second solvents;
(b) suspending the polymer microparticles in a designated volume of a mixture comprising at least one second solvent and at least one third solvent;
U.S. Pat. No. 4,267,235 to Rembaum (1981) describes the synthesis of polygluteraldehyde microspheres using suspension polymerization. Cosolubilized fluorescein isothiocyanate (FITC) is used to create fluorescent microspheres. Suspension condensation polymerization of the monomer with cosolubilized dye molecules, while largely circumventing dye destruction and polymerization inhibition, generates a broad particle size distribution and hence is not a suitable route for the production of monodisperse fluorescent microspheres.
U.S. Pat. No. 5,073,498 to Schwartz et al. (1991) describes a process for making fluorescent microparticles by seeded polymerization. One or more hydrophobic fluorescent dyes are dissolved in a solution containing monomer and initiator. The solution is added to pre-swollen microparticles. The patent discloses methods permitting the introduction of three different dyes into a particle. The method suffers from the drawback of possible inhibition of polymerization by the fluorescent dye, or conversely the bleaching of the fluorescence by the polymerization process.
Multi-stage emulsion polymerization has been employed to prepare core-shell particles without surface functional groups. U.S. Pat. No. 5,952,131 to Kumaceheva et al. discloses a method for preparing stained core-shell particles. The method is based on multiple stages of semi-continuous polymerization of a mixture of two monomers (methyl methacrylate and ethylene glycol dimethacrylate) and a fluorescent dye (4-amino-7-nitrobnezo-2-oxa-1,3 diazol-labeled methyl methacrylate). The particles are then encapsulated with an outer shell by copolymerization of methyl methacrylate and butylmethacrylate in the presence of chain transfer agent, dodecyl mercaptan. Kumaceheva et al. do not prepare and do not have as an object the inclusion of surface functional group core-shell polymer product.
U.S. Pat. No. 4,613,559 to Ober et al. discloses a method for preparing colored toner by swelling. Polystyrene particles (5.5 micron) are prepared by dispersion polymerization of styrene in the presence of ethanol, poly(acrylic acid), methylcellosolve and benzoyl peroxide. Swelling is performed by dispersing the polystyrene in an aqueous solution of sodium dodecyl sulfate and acetone. Colored particles are obtained by adding an emulsified dye solution (Passaic Oil Red 2144 in methylene chloride emulsified with an aqueous solution of sodium dodecylsulfate) to the particle dispersion.
Polymerization methods have been employed to prepare core-shell particles containing surface functional groups. U.S. Pat. No. 5,395,688 to Wang et al. discloses magnetically-responsive fluorescent polymer particles comprising a polymeric core coated with a layer of polymer containing magnetically-responsive metal oxide. The final polymer shell is synthesized with a functional monomer to facilitate covalent coupling with biological materials. The procedure of Wang et al. is based on three steps: (1) preparation of fluorescent core particles; (2) encapsulation of metal oxide in a polystyrene shell formed over the fluorescent core by free radical polymerization in the absence of emulsifier but with an excess of initiator; and (3) coating of the magnetic fluorescent particles with a layer of functional polymer. The functional polymer has carboxyl, amino, hydroxy or sulfonic groups. Wang et al. do not describe a method for obtaining the colored core and also does not address the problem of destruction of dye during the free radical polymerization process.
U.S. Pat. No. 4,829,101 to Kraemer et al. discloses two-micron fluorescent particles obtained by core-shell polymerization. The core is obtained at 80 C by polymerizing a mixture of isobutyl methacrylate, methyl methacrylate and ethylene glycol dimethacrylate via ammonium persulfate initiation. A shell is synthesized over the core by semi-continuously adding, in a first step, a mixture of the same monomers containing a fluorescent dye (fluoro-green-gold). Through the end of the reaction, two different monomer mixtures are added over a one hour period: a first mixture containing methyl methacrylate, ethylene glycol-bis-(methacrylate) and glycidyl methacrylate, and a second mixture containing methacrylamide and initiator. The polymerization is initiated with 4,4′-azobis-(cyanovaleric acid).
Okubo et al.,
Colloid Polym. Sci. 269:222-226 (1991), Yamashita, et al.,
Colloids and Surfaces A., 153:153-159 (1999), and U.S. Pat. No. 4,996,265 describe production of micron-sized monodispersed polymer particles by seeded dispersion polymerization. Polymer seed particles are pre-swelled with large amounts of monomer prior to seeded polymerization. The swelling is carried out by slow, continuous, drop-wise addition of water to an ethanol-water mixture containing the seed particles, monomers, stabilizer and initiator. The addition of water decreases the solubility of the monomer in the continuous phase, leading to precipitation and subsequent absorption of monomer onto or into the seed polymer particles. The monomer absorbed into the seed polymer particle is then polymerized to produce large monodispersed polymer particles.
B. Partitioning of Water-Soluble or Oil-Soluble Dyes
Fluorescent particles can be produced by permitting dye molecules to partition into pre-swollen microparticles according to a technique originally described by L. B. Bangs (
Uniform Latex Particles; Seragen Diagnostics Inc., 1984, p40). The process involves dissolution of a dye molecule or mixture of dye molecules in a solvent or solvent mixture of choice containing polymer microparticles. Absorption of the solvent by the microparticles leads to swelling, permitting the microparticles to absorb a portion of the dye present in the solvent mixture. The staining process is usually terminated by removing the solvent. The level of dye partitioning is controlled by adjusting the dye concentration, and in the case of a plurality of dyes, the relative abundance of individual dyes. Microparticles stained in this manner are quite stable and uniform. However, in many cases, depending on the choice of solvent system, a large dye excess is required to attain the desired partitioning, leading to significant loss of expensive dye material.
U.S. Pat. No. 5,723,218 to Haugland et al. (1998); U.S. Pat. No. 5,786,219 to Zhang et al. (1998); U.S. Pat. No. 5,326,692 to Brinkley et al. (1994); and U.S. Pat. No. 5,573,909 to Singer et al. (1996) describe protocols for producing various fluorescently-colored particles by swelling and dye partitioning in organic solvent and organic solvent mixtures. Various types of fluorescent particles, for example, fluorescent particles containing multiple dyes, particles exhibiting controllable and enhanced Stokes shifts, and particles displaying spherical zones of fluorescence, are described.
International patent application WO 99/19515 of Chandler et al. (1997) describes an improved method for the production of a series of ratiometrically-encoded microspheres with two dyes. A protocol for the production of 64 different encoded microspheres is reported. A swelling bath composition using a mixture of an organic solvent and alcohol (under anhydrous conditions) also is disclosed.
U.S. Pat. No. 5,266,497 to Matsudo et al. (1993) describes a method for generating a dye-labeled polymer particle which uses a hydrophobic dye dissolved in an organic solvent emulsified in water. The dyed particles were used for immuno-chromatographic purposes.
U.S. Pat. No. 4,613,559 to Ober et al. (1986) describes the synthesis of colored polymer particles using oil-soluble dyes. The disclosed method uses an emulsion of a dichloromethane dye solution in a water and acetone mixture for coloring the particles.
C. Functionalization of Internal or External Microparticle Surfaces
Production of fluorescent particles by surface functionalization involves the covalent attachment of one or more dyes to reactive groups on the surface of a preformed microparticle. This leaves the dye molecules exposed to the environment, which can hasten the decomposition of the dye. In addition, surface functionalization often renders a particle surface very hydrophobic, inviting undesirable non-specific adsorption and, in some cases, loss of activity of biomolecules placed on or near the particle surface. These problems can be circumvented by attaching a stained small particle, in lieu of a dye molecule, to the surface of a carrier particle. The efficacy of this method in generating large sets of encoded particles from a small number of dyes (ratio encoding) is unclear.
U.S. Pat. No. 4,487,855 to Shih (1984); U.S. Pat. No. 5,194,300 to Cheung (1993); and U.S. Pat. No. 4,774,189 to Schwartz (1988) disclose methods for preparation of colored or fluorescent microspheres by covalent attachment of either one or a plurality of dyes to reactive groups on the preformed particle surface. Battersby et al., "Toward Larger Chemical Libraries: Encoding with Fluorescent Colloids in Combinatorial Chemistry"
J. Am. Chem. Soc. 2000, 122, 2138-2139; Grondahl et al., "Encoding Combinatorial Libraries: A Novel Application of Fluorescent Silica Colloids",
Langmuir 2000, 16, 9709-9715; and U.S. Pat. No. 6,268,222 to Chandler et al. (2001) describe a method of producing fluorescent microspheres by attaching to the surface of a carrier microparticle a set of smaller polymeric particles that are stained.
D. Encapsulation Methods
Formation of fluorescent particles by encapsulation utilizes a solution of a preformed polymer and one or more dyes. In one approach, the solution is dispensed in the form of a droplet using a vibrating nozzle or jet, and the solvent is removed to produce polymer particles encapsulating the dye. This process requires specialized process equipment and displays only limited throughput. Alternatively, a polymer dye mixture is emulsified in a high-boiling solvent and the solution is evaporated to yield polymer-encapsulated dye particles. This process often generates non-spherical particles with broad size distribution.
U.S. Pat. No. 3,790,492 to Fulwyler et al. (1974) discloses a method to produce uniform fluorescent microspheres from a pre-dissolved polymer and dye solution using a jet. U.S. Pat. No. 4,717,655 to Fulwyler et al. (1988) discloses a process which includes two dyes in pre-designated ratios in a polymer microparticle to produce five distinguishable two-color particles.
The various prior art methods of producing fluorescent microparticles suffer from certain disadvantages. Where strong swelling solvents are used, the microparticles must be cross-linked to prevent them from disintegrating and deforming in the dye solution. This constraint represents a severe limitation since the majority of dyes require for their dissolution at any reasonable concentration solvent systems in which most polymer particles of interest, notably polystyrene particles, also will dissolve. These considerations have restricted the application of solvent swelling in the prior art to chemically stabilized ("cross-linked") microparticles. This restriction introduces additional difficulty and cost in microparticle synthesis; highly cross-linked particles are often very difficult to synthesize. Also, restriction to cross-linked particles limits the degree of microparticle swelling and thus the degree of dye incorporation. Specifically, the application of solvent swelling protocols of the prior art conducted on cross-linked microparticles generally limits penetration of the dye to the outer layer of the microparticle, thereby precluding uniform staining of the entire interior volume of individual particles and generally also precluding the realization of high levels of dye incorporation. What is needed is a staining process that can utilize non-cross-linked, as well as cross-linked, particles. What is needed is a method that will provide dye-loaded non-cross-linked polymer microparticles, which may be used, for example, to prepare libraries of dyed microparticles having containing different dyes and/or different dye amounts.
The degree of particle swelling in prior art solvent swelling-based methods of dye incorporation determines the rate of dye transport into the particles. Diffusion barriers lead to non-uniform dye distribution in the microparticles. For this reason, intense micro-mixing (brought about by either efficient mechanical mixing or by sonication) is required in order to produce uniformly stained populations of microparticles. These vigorous mixing procedures, while effective for laboratory scale preparation, are not easily adapted to larger scales. For example, sonication often requires specialized equipment such as probe sonicators, and limits the parallel completion of multiple staining reactions. What is needed is a dyed particle manufacturing process that requires less vigorous mixing or no mixing.
Microparticles stained by prior art swelling methods are vulnerable to subsequent exposure to solvents that may cause substantial loss of dye and may preclude the implementation of protocols providing for multiple sequential dye incorporation steps.
In the prior art methods, high levels of dye partitioning frequently are not attainable because of the limited solubility of the dye in the bath. Even when solubility is not an issue, the low partition coefficients of many dyes requires a large excess of dye in solution with deleterious effects on subsequent bioanalytical assays. In fact, when carboxylate-modified beads are prepared by prior art solvent-swelling methods, the carboxyl function may become inoperative, and may be no longer available for functionalization by covalent coupling to other chemical groups. In addition, valuable dye material is lost in significant quantities. What is needed is a process for preparing fluorescent microparticles that achieves substantially complete dye incorporation even from poorly soluble dye/solvent formulations.
Further, what is needed is a reproducible method for preparing a plurality of distinguishable fluorescently encoded (colored) core-shell particles without destroying the dye.
SUMMARY OF THE INVENTION
A method of incorporating at least one dye into polymer microparticles is provided, comprising:
a) providing:
- (i) at least one first solvent in which the dye and the microparticle polymer are soluble;
- (ii) at least one second solvent in which the dye and the microparticle polymer are not or only weakly soluble, said first and second solvents being immiscible or at most partially miscible;
- (iii) at least one third solvent in which the dye and the microparticle polymer are not or only weakly soluble, said third solvent being miscible with the first and second solvents;
(b) suspending the polymer microparticles in a designated volume of a mixture comprising at least one second solvent and at least one third solvent;
(c) adding to said polymer microparticle suspension a dye solution comprising at least one first solvent and at least one dye dissolved therein, the amount of dye corresponding to the desired final state of dye incorporation in the microparticles; and
(d) adding to said polymer microparticle suspension at least one second solvent to induce substantially complete partitioning of the dye from the suspension liquid phase to the microparticles.
In another embodiment, the invention is a method of preparing a library of dyed polymer microparticles comprising two or more sets of microparticles of different dye loadings. The method comprises: forming a first set of microparticles according to the above dye incorporation method, the microparticles of the first set characterized by a first dye loading; and forming a second set of microparticles according to the above dye incorporation method, the microparticles of the second set being characterized by a second dye loading different from the first dye loading. The difference in dye loading between the microparticles of the first set and the microparticles of the second set may comprise a difference in at least one of dye concentration and dye identity.
The particles produced by the dye incorporation method of the invention may be used as the core for the formation of core-shell particles. The dye incorporation method thus optionally further comprises the step of forming polymer shells around the microparticles to provide core-shell fluorescent polymer microparticles.
According to another embodiment, a method for producing fluorescent core-shell microparticles is provided. The method comprises: (a) providing microparticle cores incorporating one or more fluorescent dyes; (b) contacting the microparticle cores with a polymerization mixture comprising at least one polymerizable shell monomer, at least one free radical polymerization initiator comprising a water-insoluble oxidizing agent, and at least one water-soluble reducing agent; and (c) polymerizing the shell monomer to form a polymer shell around the microparticle cores. The polymerization mixture preferably comprises an oil-in-water emulsion.
In one embodiment of the method for producing fluorescent core-shell microparticles, contacting the microparticle cores with the polymerization mixture comprises the steps of swelling the cores with a first mixture comprising the shell monomer and water-insoluble oxidizing agent to provide uptake of said first mixture into the cores, and then contacting the swelled microparticle cores with the water-soluble reducing agent. The polymerization mixture further optionally comprises at least one water-soluble free radical scavenger.
According to another embodiment of the invention, dyed polymer microparticle are provided comprising a non-cross-linked polymer, and at least one dye incorporated in the microparticle.
DESCRIPTION OF THE FIGURES
FIG. 1(
a) is a schematic representation of a ternary solvent solution for use in the present invention.
FIG. 1(
b) is a diagram of the sequence of steps of an embodiment of the invention.
FIG. 2 is a plot of the fluorescence of the collection of particles prepared according to Example 1, below.
FIG. 3 is a plot of the fluorescence of the collection of particles prepared according to Example 18, below.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, a method of producing stained polymer microparticles is provided. The method takes advantage of the fact that at ordinary temperature and pressures many organic liquids are immiscible or partially miscible in other organic liquids and in water. A third component, soluble in both immiscible liquids, will distribute itself between the two liquids. According to the present invention, these principles are utilized to obtain incorporation of dye into polymer microparticles. As illustrated below, the method is capable of producing a library of distinguishable dye-stained microparticles with reproducible dye encoding and minimal intra-sample variation of dye content. The dye encoding results from varying the dye loading of the particles. By "loading" with respect to the dye contained in a microparticle is meant the amount and/or character of the dye incorporated into the microparticle. The loading can thus vary by at least one property selected from the (i) amount of incorporated dye and (ii) the identity of incorporated dye. Encoding may thus take the form of varying the amount of a single dye as between different sets of microparticles, varying the chemical nature of the dye (using different dyes, or different combinations of dyes), or both.
A homogenous ternary solvent mixture according to the present invention for the preparation of dyed microparticles, particularly fluorescently-dyed microparticles, is schematically illustrated in FIG. 1(
a). Solvent #1 is a strong solvent for both the dye and the polymer from which the microparticle is formed. Solvent #2 is a weak solvent or non-solvent for the dye and the polymer. In a preferred embodiment, Solvent #2 is an aqueous solvent, preferably water. Solvents #1 and #2 are either immiscible or partially miscible with respect to each other. A third solvent, Solvent #3, is a weak solvent or non-solvent for the dye and polymer, but serves as a co-solvent with respect to Solvents #1 and #2 in that it is miscible with both Solvents #1 and #2. In a preferred embodiment, Solvent #3 is an alcohol.
The prior art "swelling" methods of microparticle dye incorporation are limited by the narrow range of choices of available solvents for dyes of interest, requiring the use of cross-linked particles. These prior art methods involve identifying a solvent of choice in which the dye is soluble over a range of concentrations, and preparing a dye solution of desired concentration. Then, the dye solution is contacted with the polymer microparticles for a period of time so as to permit the dye to penetrate into in the microparticles.
Prior art swelling methods of fluorescent particle production suffer from limited dye solubility in the dye bath. Even when dye solubility is not an issue, the low partition coefficient of many dyes for the polymer requires a large excess of valuable fluorescent dye, which is lost. In contrast, the present invention produces microparticles of very high dye content, approaching 100% dye incorporation even from poorly soluble dye/solvent formulations. No dye is wasted by remaining in the solvent bath.
In contrast to prior art solvent swelling based methods, the dye incorporation method of the present invention may be used with equal efficacy for the dyeing of non-cross-linked as well as cross-linked particles. By "cross-linked" as describing a polymer comprising a microparticle is meant a polymer in which chains are joined together to form a three-dimensional network structure. Cross-linking can be carried out during the polymerization process by use of a cross-linking agent, that is, an agent that has two or more groups capable of reacting with functional groups on the polymer chain. Cross-linked polymers may also be prepared by the polymerization of monomers with an average functionality greater than two.
The invention thus provides, for the first time, dye-loaded microparticles that are composed of a non-cross-linked polymer. This is a significant improvement because highly cross-linked particles are often very difficult to synthesize. Furthermore, unlike many prior art particle dyeing methods which rely on intense mixing to achieve uniformity in dye staining of the microparticles, the present method requires only mild agitation. The mild agitation is required merely to keep the particles suspended. This is a significant improvement over prior art methods because the intense mixing of those methods requires specialized equipment and is difficult to scale up.
Polymer cross-linking generally restrains swelling of microparticles formed from cross-linked polymers, and also prevents penetration of the dye into the particle. As a result, the dye is restricted to a thin outer layer of the microparticle, and limits the dye loading. The ability to utilize non-cross-linked polymers as the microparticle material allows, for the first time, the production of dyed polymer microparticles that are characterized by a substantially uniform dye distribution throughout the volume of the microparticle. By "substantially uniform" is meant that the stained particle produces a symmetric and unimodal fluorescent intensity profile under conditions of fluorescent imaging. In contrast, a surface-stained particle (where the fluorescent agent is confined to the surface, or, in a shallow region close to the surface) produces a symmetric but bimodal fluorescent intensity profile.
Dyeing of functional group-modified microparticles by prior art selling methods may adversely affect the integrity of the functional group. As demonstrated by Example 28, below, functional group-modified particles may be dyed according to the practice of the present invention without loss of functional group integrity.
It will also be apparent from the description of the process of the invention that any polymer may be used to provide the polymer particles provided a stable dispersion of the polymer particles is available or can be made. The material may comprise a homopolymer or copolymer, the latter term meant to include not only polymers formed of two monomer units, but also polymers formed of three or more monomer units, sometimes termed "terpolymers". Hydrophobic polymers are preferred. Polymers comprising monomers of the vinyl class, that is, monomers containing the vinyl group, are particularly preferred, most particularly the styrene group. One group of preferred polymers includes polystyrene or polystyrene copolymers containing from about 50% to about 100% by weight styrene monomer units. The polymer optionally may be cross-linked or uncross-linked. In one embodiment, the microparticle is formed of polystyrene cross-linked with 1% divinylbenzene, based on the weight of the microparticle. In another embodiment, the microparticle comprises styrene/methacrylic acid copolymer containing from about 0.6 to about 1% methacrylic acid, based on the weight of the microparticle.
Suitable polymeric materials include, by way of example and not by way of limitation, polymers of the following monomers:
acrylic acid, or any ester thereof, such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, 2-ethyl hexyl acrylate or glycidyl acrylate;
methacrylic acid, or any ester thereof, such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, lauryl mathacrylate, cetyl methacrylate, stearyl mathacrylate, ethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, glycidyl methacrylate or N,N-(methacryloxy hydroxy propyl)-(hydroxy alkyl) amino ethyl amidazolidinone;
allyl esters such as allyl methacrylate;
itaconic acid, or ester thereof;
crotonic acid, or ester thereof;
maleic acid, or ester thereof, such as dibutyl maleate, dioctyl maleate, dioctyl maleate or diethyl maleate;
styrene, or substituted derivatives thereof such as ethyl styrene, butyl styrene or divinyl benzene;
monomer units which include an amine functionality, such as dimethyl amino ethyl methacrylate or butyl amino ethyl methacrylate;
monomer units which include an amide functionality, such as acrylamide or methacrylamide;
vinyl-containing monomers such as vinyl ethers; vinyl thioethers; vinyl alcohols; vinyl ketones; vinyl halides, such as vinyl chlorides; vinyl esters, such as vinyl acetate or vinyl versatate; vinyl nitriles, such as acrylonitrile or methacrylonitrile;
- vinylidene halides, such as vinylidene chloride and vinylidene fluoride;
- tetrafluoroethylene;
- diene monomers, such as butadiene and isoprene; and
- allyl ethers, such as allyl glycidyl ether.
Particularly preferred homopolymers and copolymers comprising vinyl-containing monomers include polystyrene, poly(methyl methacrylate), polyacrylamide, poly(ethylene glycol), poly(hydroxyethylmethacrylate), poly(vinyltoluene) and poly(divinylbenzene).
Suitable polymeric materials may include, by way of example and not by way of limitation the following polymers: polyoxides, such as poly(ethylene oxide) and poly(propylene oxide); polyesters, such as poly(ethylene terepthalate); polyurethane; polysulfonate; polysiloxanes, such as poly(dimethyl siloxane); polysulfide; polyacetylene; polysulfone; polysulfonamide; polyamides such as polycaprolactam and poly(hexamethylene adipamide); polyimine; polyurea; heterocyclic polymers such as polyvinyl pyridine and polyvinyl pyrrolidinone; naturally occurring polymers such as natural rubber, gelatin, cellulose; polycarbonate; polyanhydride; and polyalkenes such as polyethylene, polypropylene and ethylene-propylene copolymer.
The polymeric material may contain functional groups such as carboxylates, esters, amines, aldehydes, alcohols, or halides that provide sites for the attachment of chemical or biological moieties desirable to enhance the utility of the particles in chemical or biological analyses. Methods for preparing microparticles from such polymers are well known in the art. Representative procedures for preparing microparticles as well as cross-linked microparticles are set forth in the Preparative Examples, below.
The methods of the present invention may also be applied to the staining of core-shell microparticles. Core-shell microparticles comprise a central core of one or more core polymers and a shell of one or more shell polymers containing the core. The polymer shell may be formed by any polymer-coating technique. Core-shell morphology is thermodynamically favored if the shell-forming polymer exhibits higher polarity, or lower interfacial tension than does the core-forming polymer. Core-shell morphology also is favored if the volume fraction of the shell-forming polymer is greater than that of the core-forming polymer. Thus, synthesis of core-shell particles is performed at a shell/core weight ratio greater than 1. In certain embodiments, the core polymer is hydrophobic and the shell polymer is relatively hydrophilic and carries functional groups of interest.
Copolymers of styrene and a monomer more hydrophilic than styrene (e.g., methacrylic acid) are preferred for the core polymer over polystyrene homopolymer. The comonomer serves to decrease the hydrophobicity of the core and to render it more compatible with the hydrophilic shell polymerization compositions.
Within these constraints, any monomer or combination of monomers may be selected as the shell polymer. A mixture of vinyl monomers is preferred. According to one embodiment of the invention, a monomer mixture of methyl methacrylate as the major constituent, and hydroxyethyl methacrylate and methacrylic acid as minor constituents, is used to form a shell over a polystyrene or modified polystyrene core. One such monomer mixture is composed of by weight about 6% hydroxyethyl methacrylate, from about 5% to about 20% methacrylic acid, the balance being methyl methacrylate. These monomers are more hydrophilic than polystyrene.
Microparticle size may be chosen appropriately for the intended end use. Typically, particles will range in size from about 0.1 to about 100 microns in diameter, more typically from about 0.5 to about 50 microns, even more typically from about 2 to about 10 microns. Preferably, the microparticles are "monodisperse", that is, microparticles in a set have a narrow size range, preferably displaying a coefficient of variation of the mean diameter ("CV") of no more than about 5%.
Microparticles may be rendered magnetically responsive by incorporation of an appropriate magnetic mater
