Thermosensitive microgels

Poly(NIPAM) microgels are by far the most widely investigated and reported systems [5, 12, 66]. The first reported preparation of microgel particles of this type was by Pelton and 

Very high viscosity Low surface area Slow thermal response Slow solution response (e.g. pH sensitivity) 

Microgels have an advantage over linear polymers and macrogels, which is the speed at which the conformational change occurs. The time taken for the equilibration of microgels is reported to be approximately 1 s [67, 68] which is much faster than the time taken for polymers and macrogels in solution, which can be in the order of hours or days. The 

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Thermosensitive microgel particles (Rh = 300-500 nm) were synthesised by electron beam irradiation of dilute aqueous PVME solutions [330]. It was noted that when the irradiation of the PVME solution (4.0g I, ) was... 

Figure 19.18 Thennosensitive hybrid microgel containing gold nanoparticles. The color changes originate from nanoparticle aggregation induced by the swelling/deswelling of the thermosensitive microgel.

Thus, the results shown here demonstrate that thermosensitive microgel particles can serve as superior carriers for the adsorption of enzymes in which the activity of adsorbed enzymes are preserved. The catalytic activity of adsorbed P-D-glucosidase from almonds is increased by a factor of more than three. Moreover, the catalytic properties of immobilized enzymes can be manipulated by the volume transition of the microgel. Hence, such microgels present a novel class of active nanoreactors for biocatalysis. 

All results reviewed herein demonstrate that the microgel particles may serve as nanoreactors for the immobilization of catalytically active nanostructures, namely for metal nanoparticles and enzymes. In both cases, the resulting composites particles are stable against coagulation and can be easily handled. Moreover, the catalytic activity of metal nanoparticles can be modulated through the volume transition that takes place within the thermosensitive microgel carrier system. Similar behavior has been also observed for the temperature dependence of enzymatic activity. Thus, the microgel particles present an active carrier system for applications in catalysis. 

Catalyst metal nanoparticles embedded In thermosensitive microgels... 

Sakai, T., Takeoka, Y., Seki, T., and Yoshida, R. (2007) Organized monolayer of thermosensitive microgel heads prepared by double-template polymerization. Langmuir, 23, 8651-8654. 

GniUermo A, Cohen-Addad IP, Bazil IP, Duracher D, Elaissari A, Pichot C. Crosslink density of thermosensitive microgel particles investigated by NMR. J. Polym. Set B Polym. Phys. 2000 38(6) 889-898. 

Bmgger, B. and Richtering, W. (2007) Magnetic, thermosensitive microgels as stimuli-responsive emulsifiers allowing for remote control of separability and stability of oil in water-emulsions. Adv. Mater., 19, 2973-2978. 

Poly(N-isopropylacrylamide) (PNIPAM) is the most studied thermosensitive polymer in aqueous media. It is soluble in water at low temperatures but becomes insoluble when the temperature is increased above a certain temperature ( 32 °C) (lower critical solution temperature), which is related to the coil-to-globule transition [64, 65]. In the case of a polymer network, a volume change occurs reversibly within a narrow temperature range. The properties of such microgels can be varied to a great extent by the introduction... 

During precipitation polymerization, all ingredients are dissolved in a solvent (water) to form a homogeneous mixture in which initiation of polymerization takes place. The formed polymers are transformed into a collapsed state because the reaction temperature is far above VPTT (for example in the case of PNIPAAm) and become crosslinked by crosslinker molecules to form a colloidal polymer network or microgel. This technique has been widely used for the synthesis of thermosensitive PNIPAAm [30-35] and poly(/V-vinyl caprolactam) (PVCL) [36] microgels. 

As an example, thermosensitive PVME microgels can be used as template and stabilizer for the synthesis of composite polypyrrole (PPy) particles [32], The PVME microgels were obtained by electron beam irradiation above the phase transition temperature as described in Sect. 3.2.1 [3,4], Pyrrole (Py) was polymerized by oxidative polymerization with ferric chloride in the presence of the PVME microgels in water/ethanol mixtures as reaction medium. For comparison, the synthesis was also carried out in the presence of uncross-linked PVME. 

Keywords Catalysis Enzyme Metal nanoparticles Microgels Thermosensitive... 

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... 

Investigations by DLS measurements of composite particles indicated that the original thermosensitive properties of the PNIPA network are not suppressed by the incorporation of metal particles into the network. That is, the shrinking and re-swelling of microgel is not hampered by the incorporation of metal nanoparticles into the network. The metal composite particles show similar volume transition temperature as the carrier particle at 32°C, which is in excellent agreement with previous findings on these systems as shown in Fig. 7 [64, 65], This indicates... 

In the meantime, this phenomenon has also been observed by other groups for thermosensitive polymer-based metal nanoparticles [77, 78]. Pich et al. have used microgel particles based on the copolymer of A-vinylcaprolactam (VCL) and ace-toacetoxyethyl methacrylate (AAEM) (PVCL/PAAEM) as the carrier system for the deposition of metal nanoparticles. The microgels were first modified with poly(3,4-ethylenedioxythiophene) (PEDOT) nanorods through an in situ oxidative polymerization process. Microgels with PEDOT nanorods in the shell were then used for the... 

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]...

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... 

Wang, Q., Y. Zhao, et al. (2007). Thermosensitive phase behavior and drug release of in situ gelable poly(N-isopropylacrylamide-co-acrylamide) microgels. Colloid Polymer Science 285(5) 515-521. 

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