Microgel emulsifier

As shown in Fig. 6, the amount of polymerized monomer droplets strongly depends on the emulsifier concentration. With increasing emulsifier concentration, the amount of monomer initially present in the monomer droplets decreases in favor of monomer solubilized in micelles. Concurrently the fraction of polymerized monomer droplets decreases and more microgels are formed. Above a certain emulsifier concentration which is about 0.8 mol/1 in thermal initiation, the monomer is completely solubilized prior to polymerization and no polymerized monomer droplets are formed. 

Microgels by Emulsion Copolymerization of Self-Emulsifying Unsaturated Polyesters and Comonomers... 

By emulsion copolymerization (ECP) of self-emulsifying unsaturated polyesters (EUP) and bifunctional monomers, such as styrene (S), microgels may be prepared which have a rather uniform diameter [109]. This uniformity of size is due to a special mechanism of particle formation involved in using EUP as comonomers. 

The [nri] of microgel solutions decreases with increasing degree of neutralization of the carboxyl acid groups of the EUP (Fig. 19) because the emulsifier concentration increases and, accordingly, the micelles or microemulsion droplets become smaller. In this case an external emulsifier poly(oxymethylene) octylphenyl ether was added to insure complete solubilization over the whole range of neutralization. 

Fig. 20. Dependence of the yield and composition of microgels on pH of the emulsified reaction mixture [116]. (EUP/S black and white circles 0.33, black and white triangles 1.5 other data see Figure 19).

As the EUP is an emulsifier, an increase of the EUP/comonomer ratio not only causes an increase of the number of micelles and microemulsion droplets respectively but also of the number of microgels and, correspondingly, a decrease of their molar mass [110,126] and their diameter [127]... 

The molar mass of microgels obtained by ECP of EUP and comonomers ranges from below 106 to more than 107. Similar to the decrease of the particle size with increasing concentration of other emulsifiers, an increase of the EUP-fraction in the monomer mixture decreases the Mw of the microgels (Fig. 26). 

The parameters which influence the particle size of microgels have been studied during self-emulsifying, seeded emulsion copolymerization of an unsaturated polyester and butyl acrylate [134]. 

Fig. 57. Diameters of microgels prepared with different emulsifier concentrations (SDS). Composition (mol %) NjN -tetramethylenebismethacrylamide (10%),N-n-hexylmethacrylamide, propenic acid amide-N-(4-methyl-2-butyl-l,3 dioxolane (50%)...

A microgel of a dz = 76 nm which is suitable for coupling with proteins, can be prepared by emulsion terpolymerization of NjAT -tetramethylene bisacrylamide, n-hexylmethacrylamide and propene acid amide-N-(4-methyl-2-butyl-1,3-diox-olane) [291 ]. The diameter of these microgels may be varied by the concentration of the emulsifier (Fig. 57) and is rather uniform. As the CMC of this system is about 2.5 X10"3 mol SDS/1, it may be assumed that below this value the copolymerization essentially takes place in the monomer droplets, whereas at higher concentrations of SDS preferentially the monomers in micelles are polymerized. 

Microgels which have been prepared in emulsions or microemulsion have a more compact structure than those obtained by polymerization in solution. For microemulsion copolymerization, preferentially self-emulsifying comonomers, such as unsaturated polyesters, are used as polymerizable surfactants, because no emulsifier must be removed after the reaction. By choosing suitable monomer combinations the composition, size and structure of microgels can be widely varied, thus adjusting these macromolecules to special applications. 

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