Encapsulation with Polymer

Encapsulation with preformed polymers or in-situ formed polymers can also promote the surface hydrophobicity of inorganic fine particles. In this case, the surface feature of the treated particulates is different from that of surfactant-treated versions because both the surface interaction and the morphology of the encapsulating polymer are more complex.

Hyperdispersant, which has been successfully used to treat inorganic dyestuffs in the paint industry (some of which fall into the category of nanoparticles), is a macromolecular dispersant [69]. Similar to surfactants, polymeric dispersants consist of two major components. One is a functional group, like -OH, -NH2, -NR3+, -COOH, -COO-, -SO3H, -SO3-, or -PO2-, which helps to anchor dispersants to the particle surface through hydroxyl and electrostatic bonds. The other is a soluble macromolecular chain, like polyolefine, polyester, polyacrylate, or polyether, which is appropriate for dispersion in different media of low to high polarity. In comparison with traditional surfactants, hyperdispersants have the following advantages: (i) they are anchored to the particles' surface more strongly than are surfactants and hence can hardly be desorbed; (ii) the long polymeric chains

OCH3 CH3 CH3 O ch3 OH + H3CO-Si-CH2CH2—(Si-O)n-Si-(CH2)—C-O-Si-CH3

och3 Ch3 Ch3 Ch3

^H^O^ is adsorbed strongly onto the UFP surface

Solvent ^

^H^O^ is adsorbed strongly onto the UFP surface

CH3 -Si-CH3

Figure 5. Grafting polymers onto surface hydroxyl groups of UFP. Reprinted with permission from [68], T. Yoshihara, Int. J. Adhes. Adhes. 19, 353 (1999). © 1999, Elsevier Science.

can interfere with the reagglomeration of the particles more effectively; (iii) their specific molecular structures, characterized by an A-B or B-A-B block copolymer type, ensure the efficiency of particle isolation and avoid bridging between particles through an identical soluble chain. When nano-particles are treated, however, it should be noted that hyper-dispersants can only encapsulate nanoparticle agglomerates and can hardly penetrate the agglomerates, because of their long macromolecular structure.

In the presence of silica colloid, Yoshinaga and co-workers utilized radical polymerization of vinyl monomers, such as styrene, methyl methacrylate, and 2-hydroxyethyl methacrylate (HEMA), initiated by 2,2'-azobis (2-amidinopropane) dihydrochloride (AAP), to produce SiO2/polymer composites with a retained particle size of 470 nm [70]. Since the surface of silica colloid particles possesses negative charges on the electric double layer, the initiator AAP can be concentrated, resulting in free radicals on the silica surface. It is thus expected that an efficient polymerization of vinyl monomer would take place.

The binding between the in-situ formed polymer and the silica surface is driven by an electrostatic attraction. It was found that the solvent used during polymerization was an important factor influencing the percentage of the attached polymer. At an initial stage, polymerization of vinyl monomers occurred first on the silica surface, leading to a certain degree of hydrophobicity. This simultaneously promoted aggregation of the polymer chains to be formed in the ethanolic bulk solution because of the poor solubility of the polymer in an ethanol-rich solvent. In this context, a solvent with lower miscibility for the polymer should facilitate the attachment of the macromolecular chains to the particles.

Grafting of Hydrophobic Polymer onto the UFP

Grafting of Hydrophobic Polymer onto the UFP

Figure 6. Grafting of hydrophobic polymer. Reprinted with permission from [68], T. Yoshihara, Int. J. Adhes. Adhes. 19, 353 (1999). © 1999, Elsevier Science.

Figure 4. Influence of H2O on UFP dispersal system. Reprinted with permission from [68], T. Yoshihara, Int. J. Adhes. Adhes. 19, 353 (1999). © 1999, Elsevier Science.

Figure 6. Grafting of hydrophobic polymer. Reprinted with permission from [68], T. Yoshihara, Int. J. Adhes. Adhes. 19, 353 (1999). © 1999, Elsevier Science.

Hasegawa and co-workers proposed an encapsulation process capable of uniformly covering fine inorganic powders of submicron size with a filmy polymer [71]. The encapsulation of barium sulfate (0.6 /m) and calcium carbonate (0.4 /m) was attempted by a soapless emulsion polymerization of methyl methacrylate in water (in the presence of the inorganic powders). The encapsulation status of the particles with the polymer formed varied considerably with the species of the initiator and the reaction atmosphere. When potassium persulfate (acting as an initiator) and nitrogen were used, the fine powder surface was partially covered by polymer particles that were the same size as the powder particles. In addition, many polymer chains did not adhere to the powder surface, meaning that the monomers were not used effectively to cover the powders. In the case of a redox initiator under an air atmosphere, however, the powder surface was well encapsulated with a film-like polymer layer. It was believed that the formation of a large number of oligomers with short alkyl chains at an earlier stage of reaction should be responsible for the above difference. On the basis of this mechanism, the authors suggested that the addition of an extremely small quantity of a certain surfactant to the reaction system prior to the polymerization would facilitate the polymerization on the powder surface.

Du et al. [72] demonstrated that when a seed emulsion polymerization was carried out in the presence of nano-particles, a core-shell structure can be obtained, in which the inorganic particles stay in the middle layer while the polymer is located at inner and outer layers. For the system containing Fe2O3 nanoparticles (3-5 nm) and styrene/acrylic acid/butyl acrylate (St/AA/BA) latex particles, for example, the size of the resultant composite microspheres is 80 nm. Changes in the infrared absorption band of carbonyl groups indicated a strong interaction between Fe2O3 nanoparticles and the surface groups of the seed latex particles.

Nanoparticles covered by a polymer layer can also be obtained during the manufacture of nanoparticles. For instance, nanocrystalline titania coated with poly (methacrylic acid) (PMAA) was prepared by microwave-induced plasma [73]. Such an in-situ treatment is characterized by high effectiveness and homogeneity (Fig. 7). Transmission electron microscopy (TEM) observation showed that the titania particles (rutile in majority with a few anatase) were 10-25 nm in diameter, with a 7-nm-thick PMAA layer.

During the synthesis of metallic and semiconductive particles, polymeric materials are frequently employed as particle

TiCI4

METHACRYLIC ACID

RESONANT CAVITY

COOLING & COLLECTING

MICROWAVE SOURCE (1000W, 2450MHz)

VACUUM

VACUUM

Figure 7. Synthesis of nanocrystalline titania with PMAA coating by microwave plasma. Reprinted with permission from [73], H. B. Liu, Chemistry 10, 44 (1997). © 1997, Chinese Society for Chemistry.

stabilizers to prevent agglomeration and growth of the particles. In the present case, however, existing nanopar-ticle agglomerations must be separated in advance, that is, before encapsulation with a polymer in solution can take place. This means that, under these circumstances, particle deagglomeration, electrostatic stabilization of the solvent, and the interpenetration plus electrostatic stabilization of the polymer play an important role. Gonsalves and Chen illustrated the above points when preparing highly loaded AlN/polyimide nanocomposites (Fig. 8) [74]. In their work, N-methylpyrrolidinone (NMP) was used as a solvent, functioning as a medium for the deagglomeration of the nanoparticles and their stabilization through electrostatic interactions. With the addition of poly(amide acid) (PAA), the nanoparticles were further deagglomerated and stabilized through both steric and electrostatic stabilization. Acting as a polyelectrolyte, PAA tends (a) to diffuse into the pores of the secondary structures of the AlN nano-particles and (b) to be absorbed by the particle surfaces. To prevent any possible destabilization of the homogeneous suspension system, a rapid precipitation method making use of different nonsolvents and suspension/nonsolvent ratios should be used. For the AlN/PAA system, triethylamine was considered the best choice because it completely precipitated out of the composite phase of the suspension.

Ultrasonic +

High speed mechanical agitation

Ultrasonic +

High speed mechanical agitation

Addition of Poly(amic acid)

Deagglomeration and Stabilization through Interactions with NMP Solvent Molecules

Adsorption and diffusion of the polymer Into the pores of the secondary structures of the AIN nanoparticulates

Further deagglomeration and stabilization by the following mechanisms

Steric Stabilization

Steric Stabilization

Figure 8. Deagglomeration and stabilization of AlN nanoparticles in a polyelectrolyte solution. Reprinted with permission from [74], K. E. Gonsalves et al., J. Mater. Res. 12, 1274 (1997). © 1997, Materials Research Society.

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