Amphiphilic core-shell latexes

Details Amphiphilic core-shell latexes Amphiphilic core-shell latexes

2001-01-16

2003-06-03

Cain, Edward J. (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

Reexamination Certificate

active

06573313

ABSTRACT:

The present invention relates to amphiphilic core-shell latexes.
BACKGROUND OF INVENTION
There is an increasing demand for colloidal nanoparticles having an amphiphilic core-shell morphology because of their applications in biotechnology, coatings and adhesive, as well as solid supports. Physical adsorption of hydrophilic biopolymer or synthetic polymers is the dominant approach to prepare such microspheres. However, covalent binding techniques appear to be the most suitable with a view to ensuring irreversible fixation and better orientation of the biomolecules. Furthermore, non-specific adsorption problems for the hydrophobic particle surfaces can be avoided.
In particular, there is an increasing interest in the fabrication of composite micro- and nanoparticles that consist of hydrophobic polymer cores coated with shells of different chemical composition, see F Caruso, R A Caruso, H Möhwald,
Science,
282, 1111 (1998). In biomedical areas, there is particular interest in polymeric nanoparticles having a hydrophilic biopolymer shell, see F Caruso and H Möhwald,
J Am Chem Soc,
121, 6039-6046 (1999). Amphiphilic core-shell particles often exhibit substantially different properties than those of the templated core. For instance, they have very different surface chemical composition and hydrophilicity, and can readily be dispersed in water. Applications of such particles are very diverse. They can be used in diagnostic testing, in bioseparations of target proteins via bonding to the particle surface, and as drug reservoirs in controlled release formulations. They can also serve as a support for gene delivery and cell-growth or for a catalyst, and they can be used in coatings and composite materials. Thus, the preparation of nanoparticles having a well-defined amphiphilic core-shell morphology is extremely significant from both a scientific and a technological point of view.
The following five approaches have been used in the preparation of amphiphilic core-shell nanoparticles:
1) Step-wise deposition of polyelectrolytes from dilute solutions onto charged colloidal polystyrene latex particles. For example, multilayer shells have been formed by the alternate adsorption of oppositely charged polyelecrolytes onto positively charged particles, see G B Sukhorukov, E Donath, H Lichtenfeld, E Knippel, M Knippel, H Möhwald,
Colloids Surfaces A: Physicochem. Eng. Aspects,
137, 253 (1998) and G B Sukhorukov, E Donath, S Davis, H Lichtenfeld, F Caruso, V I Popov, H Möhwald
Polym. Adv. Tech.
9, 759 (1998)
2) Shell-crosslinked “knedel” (SCK) micelles with a core-shell nanostructure have been formed through self-assembly processes of amphiphilic block copolymers, followed by covalent crosslinking of the shells, see K L Wooley,
J. Polym. Sci. Part A: Polym. Chern,
38, 1397 (2000). The amphiphilic diblock and triblock copolymers are prepared by either living anionic or living free radical polymerisation methods, see K L Wooley,
J Polym. Sci. Part A: Polym. Chem.,
38, 1397 (2000) and V Bütün, X S Wang, M V de Paz Báñez, K L Robinson, N C Billingham, S P Armes and Z Tuzar,
Macromolecules,
33, 1 (2000).
3) Two-stage seeded emulsion copolymerisations. A seed latex is first prepared by emulsion polymerisation of a hydrophobic monomer, followed by the polymerisation of a water-soluble monomer via a seeded swelling batch or a semibatch process, see W Li, H D H Stöver,
Macromolecules,
33, 4354 (2000), or with reactive seed microspheres, see R Saito, X Ni, A Ichimura and K Ishizu,
J. Appl. Polym. Sci.,
69, 211 (1998).
4) Using reactive surfactants or macromonomer that are able to copolymerize with monomers. The resulting copolymers typically end up with a thin hydrophilic shell on the particle surface, see S Roy, P Favresse, A Laschewsky, J C de la Cal, J M Asua,
Macromolecules,
32, 5967 (1999), and O Soula, A Guyot, N Williams, J Grade, T Blease,
J. Polym. Sci. A: Polym. Chem.
37, 4205 (1999], and A Búcsi, J Forcada, S Gibanel, V Héroguez, M Fontanille, Y Gnanou,
Macromolecules,
31, 2087 (1998).
5) Graft copolymerisations of water-soluble monomers onto a functionalised core particle surface. For example, Ce(IV)-initiated grafting of N-(2-methoxyethyl acrylamide) onto poly(styrene-co-2-hydroxyethyl acrylate) particles has been reported, see D Hritcu, W Muller and D E Brooks,
Macromolecules,
32, 565 (1999).
In spite of the success of these approaches in the preparation of amphiphilic core-shell nanoparticles, there are still some major drawbacks to these systems. For example:
In the first approach, the deposition procedure is very complicated and time-consuming. After each adsorption step, the free polyelectrolytes need to be removed by repeated centrifugation and washing cycles. In addition, the polyelectrolyes are physically adsorbed on the particle surface via charge interactions. Thus, the shell layer is very sensitive to pH changes.
Tedious multiple step syntheses are required for the preparation of amphiphilic block copolymers, reactive surfactants, macromonomers and the functionalised latex particles used in the second to fourth approaches.
In the third and fifth approaches, the hydrophilic monomers usually have higher reactivity than the matrix monomers, thus resulting in low surface incorporation and formation of a large amount of water-soluble polymers. Furthermore, highly oxidative conditions are required for the grafting processes that prevent the use of biological molecules.
OBJECT OF THE INVENTION
Thus a new technique for making amphiphilic core-shell nanoparticles is extremely desirable from both a scientific and a technological point of view.
SUMMARY OF INVENTION
The present invention provides amphiphilic core-shell latex nanoparticles. The core is composed of homopolymer of a hydrophobic vinylic monomer, and grafted copolymer of the hydrophobic vinylic monomer. The shell to which the polymer is grafted is hydrophilic, nitrogen-containing polymer.
Thus, we have developed a facile route to prepare a variety of well-defined amphiphilic core-shell latex nanoparticles with covalent linkages. In our approach, a graft copolymerisation of a vinylic monomer onto an nitrogen-containing, water-soluble polymer is conducted in water or other aqueous systems.
PREFERRED EMBODIMENTS
In a preferred process, radicals are first generated on the nitrogen atoms of the hydrophilic polymer through interaction with alkyl hydroperoxide or by other means, and then initiate the free-radical polymerisation of vinylic monomer. The hydrophobic side chains of vinylic polymer generated during the reaction phase separate to form latexes of monodisperse core-shell particles with the hydrophobic polymer as the core and the hydrophilic polymer as the shell.
For example, poly(ethyleneimine) (PEI) is a commercially available water-soluble polymer. It contains 25% primary, 50% secondary and 25% tertiary amino groups. It was discovered that the graft copolymerisation of methyl methacrylate (MMA) onto PEI could be readily achieved in water in the presence of a trace amount of an alkyl hydroperoxide (ROOH) at 80° C. A nearly quantitative conversion of MMA is obtained in 2 h, giving a stable white emulsion with mean particle sizes ranging from 120 to 135 nm (diameter) and a very narrow size distribution (~1.1). TEM micrographs clearly reveal that the nanoparticles have core-shell morphology with the PMMA as the core and PEI as the shell. The presence of PEI in the shell layer has been further confirmed with Zeta potential measurements.
DETAILED DESCRIPTION
The nitrogen-containing hydrophilic polymer can be natural or synthetic. The nitrogen is preferably present as an amine group. Primary amine (—NH
2
), secondary amine (—NRH), and tertiary amine (—NR
2
) are the preferred functional groups for this reaction. Structurally, the amino containing polymers may be in the form of linear or cyclic aliphatic or aromatic amine. The amino function may be located in the polymer main chain or in the side chains. Less preferred functional groups are amides including unsubstituted amide (—CONH
2
), mono-substituted

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