Surfactant-stabilized microgels

The physical or chemical crosslinking of polymers can be also realized in water-in-oil (W/O) emulsion systems. In this case, aqueous droplets of prepolymers are stabilized by oil-soluble surfactants in a continuous oil phase. Hyaluronan-based microgels were prepared by crosslinking of carboxylic units of hyaluronan with adipic dihydrazide in aqueous droplets [19]. Chitosan-based microgels were prepared by crosslinking of chitosan chains with glutaraldehyde in aqueous droplets [20-25],... 

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],... 

Gorelikov et al. [150] demonstrated a way to provokephotothermally modulated volume transitions in microgel particles in the near-IR spectral range. Gold nanorods with different aspect ratios (from 2 to 6) stabilized by cationic surfactant were inte-... 

We have compared these theoretical predictions of the low-frequency modulus to experimental measurements on compressed emulsions and concentrated dispersions of microgels [121]. The emulsions were dispersions of silicone oil (viscosity 0.5 Pas) in water stabilized by the nonionic surfactant Triton X-100 [102, 121]. The excess surfactant was carefully eliminated by successive washing operations to avoid attractive depletion interactions. The size distribution of the droplets was moderately polydisperse with a mean droplet diameter of 2pin. The interfacial energy F between oil and water was 4mJ/m. The contact modulus for these emulsions was thus F 35 kPa. The volume fraction of the dispersed phase was easily obtained from weight measurements before and after water evaporation. Concentrated emulsions have a plateau modulus that extends to the lowest accessible frequencies, from which the low-frequency modulus Gq was obtained. Figure 11 shows the variations of Gq/E"" with 0 measured for the emulsions against the values calculated in the... 

Preparation of Core-Shell Microgels. The preparation and elec-trokinetic and colloid stability behavior of styrene-iV-isopropylacrylamide and stsrrene-AA-isopropylaciylamide-co-aminoethyl methacrylate (AEM) core-shell latexes have been reported in Reference 29, where the particles having a core of polystyrene surroimded by layers of (1) poly(NIPAM) and (2) poly(NIPAM-co-AEM) were prepared. The particles were produced by first synthesizing the polystsrrene core via a surfactant-free polymerization reaction and then, when the formation of the core was 90% complete, the shell was generated by the addition of the NIPAM monomer and crosslinker N, N -methylenebisaciylamide (1). In the case of core-shell particles, NIPAM, aminoethyl methacrylate, and the... 

Emulsion polymerization is among the most popular synthetic routes to prepare vinyl-based pH-responsive polymers, especially microgel systems (Rao and Geckeler, 2011). This technique employs a radical chain polymerization methodology to form latexes of narrow particle size distributions. The emulsion polymerization systems are commonly composed of monomer(s), water, water-soluble initiator and surfactant (emulsifier). Colloidal stabilizers may be electrostatic, steric or electrosteric, or display both stabilizing mechanisms. When phase separation occurs, the formation of solid particles takes place before or after the termination of the polymerization reaction. 

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