Microgels isolation

Polymerization in dilute solution also allows a very straightforward isolation and purification of the microgels. After polymerization, the resulting microgels can be conveniently precipitated from the reaction solution by using suitable nonsolvents for the microgel molecules. The resulting powders can be filtered off, dried, and redispersed in suitable solvents when needed. 

The microgels could be conveniently isolated by precipitation as white powders, readily redispersable in many different organic solvents such as dialkylamides, nitriles, dichloromethane, acetone and THF. Further to this, the DMAA-based microgels exhibited a rather amphiphilic character and were also soluble in water and in alcohols such as methanol or ethanol in contrast, their counterparts based on MMA turned out to be more lipophilic and therefore insoluble in water and alcohols but soluble in organic solvents of low polarity such as toluene. 

The resulting microgel-stabilized metal nanoclusters are easily isolated, stored and further manipulated. Their remarkable catal5dic activity in technologically relevant reactions, such as C-C couplings [13a- ] and selective oxidations with molecular oxygen [13e] has been demonstrated. Extension of the applications of these nanoparticles to other areas of catalysis and materials science is currently underway. 

Light scattering and electron microscopy studies of aqueous PVME solutions and PVME microgels were carried out by Arndt et al. [329,330]. They noted that the Mw of PVME in water was always higher (up to 20 times) than its value (Mw = 46 000gmoH) determined in organic solvent (butanone), even for dilute aqueous PVME solutions well below the phase-separation temperature [330]. Moreover the molar masses of the polymer in water depended on solution preparation conditions. The authors concluded that PVME does not exist as isolated chains in water, but forms loose aggregates (Rh = 200-220 nm) which decrease in size as the solution temperature passes... 

Fig. 38. A Degradation experiments with pregel polymers isolated prior to the onset of macrogelation in 1,4-DVB polymerization [209] Variation of Mw ( ) and dz (O) with the time of ultrasonic degradation. The polymer sample was prepared at 5 g/100 mL monomer concentration and its initial Mw was 2.2 X106 g/mol. The dotted horizontal line shows Mw of zero conversion polymers ( individual microgels ). B Variation of Mw with the polymerization time t and monomer conversion x in 1,4-DVB polymerization at 5 g/100 mL monomer concentration. The region 1 in the box represents the limiting Mw reached by degradation experiments. [Reprinted with permission from Ref. 209,Copyright 1995, American Chemical Society].

In t-DVB/S copolymerization, Antonietti and Rosenauer isolated microgels slightly below the macrogelation point [221]. Using small angle neutron scattering measurements they demonstrated that these microgels exhibit fractal behavior, i.e. they are self-similar like the critically branched structures formed close to the sol-gel transition. 

Ester formation by dimethylsulfate or diazomethane is not satisfactory because the microgels become insoluble when the reaction proceeds to higher conversions. With diazomethane part of the unsaturated groups is involved in a side reaction of a 1,3-dipolar cycloaddition [132]. A more efficient method for ester formation of microgels is the reaction with 0-alkyl-N,N -bisisopropyl isoureas of the alcohols. The alkyl ureas are easily separated from solutions in methanol [294-296]. The esterified microgels were isolated by precipitation and freeze-drying. Depending on the alcohol used for ester formation, the yields of... 

Such monomer interacts with a suitable p-di ketone forming an isolable enamide which effectively mimicks the transition state of the reaction. Copolymerisation of this adduct with the crosslinker and acrylamide followed by template removal yields small microgels (around 20 nm) that are able to catalyse the aldol condensation reaction about 19 times faster than nonimprinted controls. 

In both cases the isolation of the reactive microgels from the reaction batch requires careful and elaborate series of procedures at low temperatures in the absence of oxygen (1,3). Otherwise the reactive particles will agglomerate irreversibly. 

Isolation of Microgels. The following experiments were made to isolate microgels during the process of curing and from the etchant solution ... 

Figure 10. Scanning electron micrographs of isolated secondary microgels from Epon 828-MDA sample (E-2)...

Figure 11. Transmission electron micrographs of isolated primary microgels (A) and (B) gel particles etched out by Cr Os solutions at 70°C after 4 hr (C) and (D) micro gels formed before gelation of the epoxy resins. [A and C—E-2 B and D—E-5 ]...

Hu L, Serpe MJ (2012) Color modulation of spatially isolated regions on a single poly (N-isopropylacrylamide) microgel based etalon. J Mater Chem 22 8199... 

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