Microgels, core-shell

Nayak, S. Lyon, L. A., Ligand functionalized core/shell microgels with permselective shells, Angew. Chem. Int. Ed. 2004, 43, 6706 6709... 

Precipitation polymerization allows preparation of microgel particles with a coreshell structure. This can be achieved by using monomers of different reactivity or hydrophilicity in a batch polymerization process. Alternatively, core-shell microgels can be prepared by seed polymerization techniques or by stepwise addition of co-monomers to the reaction mixture. 

Li et al. [100] synthesized core-shell microgels with temperature-sensitive PNIPAAm core and pH-sensitive poly(4-vinylpyridine) (P4VP) shell. Narrowly distributed microgel particles with core diameter of 95 nm and shell thickness of approximately 30 nm were obtained. 

Temperature- and pH-sensitive core-shell microgels consisting of a PNIPAAm core crosslinked with BIS and a polyvinylamine (PVAm) shell were synthesized by graft copolymerization in the absence of surfactant and stabilizer [106] The core-shell morphology of the microgels was confirmed by TEM and zeta-potential measurements. Other examples of core-shell microgel systems are PNIPAAm-g-P(NIPAM-co-styrene) colloids [107] or PS(core)-g-PNIPAAm (shell) particles [108],... 

Fig. 11 Hydrodynamic radius vs. temperature of core-shell microgels with different shell thickness at shell crosslinker content of 9.0 mol%. The parent PNIPAAm core is shown for comparison. Reprinted from [102] with permission. Copyright 2003 American Chemical Society...

Fig. 12 Radial density

Recently, core-shell type microgels, which contain a hydrophobic core and a hydrophilic thermosensitive shell, have become attractive for scientists because such systems can combine the properties characteristic of both the core and the shell [53], We have prepared core-shell microgel particles consisting of a poly(styrene) core onto which a shell of polyCA-isopropylacrylamide) (PS-PNIPA) has been affixed in a seeded emulsion polymerization [54-56], In this case, the ends of the crosslinked PNIPA chains are fixed to a solid core, which defines a solid boundary of the network. In this respect, these core-shell latex particles present crosslinked polymer brushes on defined spherical surfaces. The solvent quality can be changed from good solvent conditions at room temperature to poor solvent conditions at a temperature... 

Recently, we have successfully used these thermosensitive core-shell microgel particles as templates for the deposition of metal nanoparticles (Ag, Au, Pd, Pt, and Rh) [29, 59, 60], The reduction to metallic nanoparticles in the presence of microgel particles was done at room temperature via the addition of NaBPL and could be followed optically by the color change of the suspensions, as shown in Fig. 3. The immobilization of metal nanoparticles might be due to the strong localization of... 

Metal nanoparticles embedded in thermosensitive core-shell microgel particles can also work efficiently as catalyst for this reaction. Figure 13 shows the oxidation reaction of benzyl alcohol to benzaldehyde in aqueous media by using microgel-metal nanocomposite particles as catalyst. All reactions were carried out at room temperature using aerobic conditions. It is worth noting that the reaction conditions are very mild and no phase transfer catalyst is needed. It has been found that microgel-metal nanocomposites efficiently catalyze the aerobic oxidation of benzyl alcohol at room temperature. No byproducts have been detected by GC after the reaction, and water is the only product formed besides the aldehyde. 

Fig. 14 Catalytic oxidation of benzyl alcohol in the presence of metal nanoparticles immobilized in thermosensitive core-shell microgels at different temperatures. At lower temperatures (T < 32°C) the microgel network is hydrophilic and swollen in water, whereas at high temperatures (T > 32°C), the network shrinks and becomes hydrophobic. Thus, microgel particles embedding the metal catalyst will move to the oil phase, which will be favorable for the uptake of hydrophobic benzyl alcohol into the metal-microgel composite. Therefore, the catalytic activity of the metal-microgel composites will be affected both by the volume transition and the polarity change of the microgel [29]...

Fig. 19 Reactions used for testing the activity of enzymes (P-D-glucosidase) immobilized in the thermo sensitive core-shell microgel template at different temperatures. Enzymatic hydrolysis of the substrate oNPG produces d-glucose and o-nitrophenol. The concentration of the resulting o-nitrophenol can be monitored photometrically...

Mei Y, Lu Y, Polzer F, Ballauff M, Drechsler M (2007) Catalytic activity of palladium nanoparticles encapsulated in spherical polyelectrolyte brushes and core-shell microgels. Chem Mater 19 1062-1069... 

Lu Y, Proch S, Schrinner M, Drechsler M, Kempe R, Ballauff M (2009) Thermosensitive core-shell microgel as a nanoreactor for catalytic active metal nanoparticles. J Mater Chem 19 3955-3961... 

Hellweg T, Dewhurst CD, Eimer W, Kratz K (2004) PNIPAM-co-polystyrene core-shell microgels structure, swelling behaviour and crystallisation. Langmuir 20 4330-4335... 

Gan D, Lyon LA (2002) Synthesis and protein adsorption resistance of PEG-modified poly(/V-isopropylacrylamide) core/shell microgels. Macromolecules 35 9634—9639... 

Core—shell microgels can be prepared utilizing CCT macromonomers496 and self-stabilized cross-linked latexes.497-501... 

Jones, C.D. Lyon, L.A. Synthesis and characterization of multiresponsive core-shell microgels. Macromolecules 2000, 33, 8301-8306. 

Chi, C., T. Cai, et al. (2009). OUgo(ethylene glycol)-based thermoresponsive core-shell microgels. Langmuir 25(6) 3814-3819. 

In this chapter, we review recent work on two special types of polymerie earrier systems, namely, the spherical polyelectrolyte bmshes (SPBs) and thermosensitive core-shell microgels, which have been used successfully for the immobilization of metal nanopartieles. Figure 1.1a gives a schematic representation of the SPB particles Long linear polyelectrolyte chains are grafted densely to a colloidal core particle. The term brush implies that the grafting of the chains is sufficiently... 

Figure 1.1. (a) Stmeture of the spherical polyelectrolyte brushes having cationic polyelectrolyte chains on their surface. The core consists of poly(styrene) and has diameters of approximately 100 nm. The chains are densely grafted to the surface of these cores by a grafting-from technique ( photoemulsion polymerization, cf. Ref. 24). (b) The core-shell microgel particles shown in a schematic fashion The core consists of poly(styrene) (PS) whereas the network consists of poly(iV-isopropylacrylamide) (PNIPA) crosslinked by JVdV -methylenebisacrylamide (BIS). 

Recently, we have successfully used thermosensitive core-shell microgel particles as a template for the deposition of metal nanoparticles (Ag, Au, Pd, Pt, and Rh).25, These microgel particles consist of a PS core onto which a shell... 

Here we have reviewed our recent studies on metallic nanoparticles encapsulated in spherical polyelectrolyte brushes and thermosensitive core-shell microgels, respectively. Both polymeric particles present excellent carrier systems for applications in catalysis. The composite systems of metallic nanoparticles and polymeric carrier particles allow us to do green chemistry and conduct chemical reactions in a very efficient way. Moreover, in the case of using microgels as the carrier system, the reactivity of composite particles can be adjusted by the volume transition within the thermosensitive networks. Hence, the present chapter gives clear indications on how carrier systems for metallic nanoparticles should be designed to adjust their catalytic activity. 

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