Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
2003-05-27
2004-12-07
Acquah, Samuel A. (Department: 1711)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C264S004100, C264S004300, C264S004320, C264S004330, C264S004700
Reexamination Certificate
active
06828025
ABSTRACT:
FIELD OF THE INVENTION
This invention provides a process for encapsulating a polar organic solvent which comprises subjecting one or more C
1-12
alkyl acrylates or C
1-12
alkyl methacrylates to living polymerization in the presence of the polar solvent.
BACKGROUND OF THE INVENTION
Polymeric capsules and hollow particles can be prepared both from monomeric starting materials as well as from oligomers and pre-formed polymers. (Arshady, R., Microspheres, Microcapsules Liposomes 1999, 1, 1461-1732). In most cases, the process involves a disperse oil phase in an aqueous continuous phase, and the precipitation of polymeric material at the oil-water interface causing each oil droplet to be enclosed within a polymeric shell. Interfacial polycondensation is used to prepare poly(urea) (Beestman, G. B., et. al., U.S. Pat. No. 4,417,916, Nov. 29, 1983), poly(amide), or poly(ester) capsules (Arshady, R. J., Microencapsulation 1989, Vol. 6, No. 1, 13-28), for instance, by reaction between an oil soluble monomer and a water soluble monomer at the oil-water interface. On the other hand, vinyl polymers such as poly(styrene), acrylates and methacrylates prepared by free radical polymerization under suspension (Kasai et. al., U.S. Pat. No. 4,908,271 (Mar. 13, 1990)) or emulsion polymerization (McDonald et. al., U.S. Pat. No. 4,973,670 (Nov. 27, 1990)), (McDonald, C. J. et. al., Macromolecules 2000, 33, 1593-1605) conditions have been used to prepare hollow or capsular polymer particles. In this approach, the dispersed oil phase usually serves as the polymerization solvent. The oil phase is chosen so as to be a good solvent for the monomeric starting materials but a non-solvent for the product polymer. Therefore, upon polymerization the system is comprised of three mutually immiscible phases. Over the past three decades, several groups have studied the factors governing the morphologies that two immiscible phases can adopt when they are brought together in a third immiscible phase by means of a velocity gradient or otherwise. Their findings have led to an understanding of the morphologies that result when, for instance, two immiscible polymers are brought together in a non-solvent for either. This occurs in a seeded emulsion polymerization when the formed polymer and seed polymer are in mutual contact, and are dispersed in an aqueous phase. The same fundamental principles govern the morphology of composite particles that result when a polymer, and a non-solvent organic oil are brought together in an aqueous dispersion, as is the case during the encapsulation of an organic oil.
Torza and Mason (Torza, S. et. al., Journal of Colloid and Interface Science 1970, Vol. 33, No. 1, 67) studied the phase behavior of low viscosity, immiscible organic liquids dispersed in an aqueous phase as the drops were subjected to varying shear and electric fields. They defined the spreading coefficient, S
i
=&ggr;
jk
−(&ggr;
ij
+&ggr;
ik
), where i, j, and k represent the three immiscible phases and &ggr;, the interfacial surface tension. For the premise that, &ggr;
12
>&ggr;
23
, it follows that S
1
<0. The definition of S
i
, leads to only three possible sets of values of S
i
:
S
1
<0
, S
2
<0
, S
3
>0; [1]
S
1
<0
, S
2
<0
, S
3
<0; [2]
S
1
<0
, S
2
>0
, S
3
>0; [3]
It was shown that for interfacial conditions of equation [1] the core-shell morphology is preferred, while for equation [2] the hemispherical morphology is preferred. Good agreement was found between the theoretical predictions and experimental results. It is noteworthy, that Torza and Mason used low viscosity oils that are able to diffuse rapidly and assume the lowest interfacial energy morphology within the time frame of the experiment. Hence, their results may not extend to cases when one or more of the components is a high molecular weight polymer, since diffusional resistance may prevent equilibrium morphology from being realized during the experimental time frame.
Sundberg et. al. (Sundberg, D. C. et. al., Journal of Applied Polymer Science 1990, 41, 1425) published a theoretical model based on the Gibbs free energy change of the process of morphology development. Starting with three immiscible phases; oil, polymer and water, they showed that the Gibbs free energy change per unit area for the process leading to a core shell morphology (with oil encapsulated within the polymer phase), is given by:
&Dgr;
G=&ggr;
op
+&ggr;
pw
(1−&phgr;
p
)
−2/3
−&ggr;
ow
[4]
Where &ggr;
op
, &ggr;
pw
, and &ggr;
ow
are the oil-polymer, polymer-water and oil-water interfacial tensions and &phgr;
p
is the volume fraction of the polymer (in polymer plus oil “combined phase”). In the limit as &phgr;
p
tends to zero, equation [4] reduces to,
&Dgr;
G
=(&ggr;
op
+&ggr;
pw
)−&ggr;
ow
[5]
Thus, when &ggr;
ow
>(&ggr;
op
+&ggr;
pw
) [6], the core shell morphology with the core oil being engulfed by the polymer is the thermodynamically stable morphology. Analogous expressions were derived for the hemispherical, inverse core shell and distinct particle morphologies. Using these expressions the authors were able to predict the expected morphologies for a given set of interfacial conditions. The predictions were checked and confirmed by experiment.
In an earlier work, Berg et. al. (Berg, J. et. al., Microencapsulation 1989, Vol. 6, No. 3, 327) showed the above analysis is equally valid when the polymer is synthesized in situ by free radical polymerization. Poly(methyl methacrylate) was prepared via free radical polymerization by dispersing n-decane or hexadecane, methyl methacrylate and an oil soluble initiator in water containing a surfactant or stabilizer. It was shown that the resultant morphology was critically dependent on the type of emulsifier used. The authors concluded that this observation appeared to be related to the minimization of interfacial energy for the particles as they are dispersed in water. Thus, the particle morphology that results from in situ polymer synthesis in suspension/emulsion polymerization conditions is predominantly driven by interfacial energy criteria. It must be stated that the above model assumes thermodynamic equilibrium and therefore predicts “final” equilibrium particle morphology. The fact that it correctly predicts the particle morphology when polymer is synthesized in situ implies that phase separation kinetics competes favorably with polymerization kinetics (under the experimental conditions used by Berg et. al.).
The work of the four major research groups in the area, i.e., Kasai et. al., Okubo et. al., McDonald et. al., and Sundberg et. al., shows that present techniques allow only the encapsulation of relatively hydrophobic solvents. McDonald et. al. and Sundberg et. al. have encapsulated highly non-polar core oils such as decane and octane. McDonald et. al. (McDonald, C. J. et. al.,
Macromolecules
2000, 33, 1593-1605), (McDonald et. al., U.S. Pat. No. 4,973,610, Nov. 27, 1990) have also reported the preparation of hollow latex particles by conventional and seeded emulsion polymerization. The conventional emulsion system consisted of an oil phase dispersed in water with the aid of surfactant. The oil phase contained monomer that is essentially non-water soluble and a hydrocarbon oil that dissolves the monomer but is non-solvent for the formed polymer. The aqueous phase consisted of a water soluble initiator and a water miscible alcohol.
The seeded emulsion polymerization differed from the above system in that the seed latex particles were swollen by the oil phase prior to initiation of the polymerization. The encapsulation process occurred in two stages. First, a low molecular weight polymer was made (e.g., 8000 Da using chain transfer agents). The polymer being insoluble in the hydrocarbon solvent begins to phase separate and concentrate at the oil water interface.
Ali Mir Mukkaram
Stöver Harald D. H.
Acquah Samuel A.
McMaster University
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