Comparison with miniemulsion polymerization

The presence of a small amount of polymer (PSt or PMMA) inside the homogenized monomer droplets reduces the sensitivity of Np to changes in [initiator]51 and the decrease of the dependency is more pronounced for the miniemulsion polymerization with PMMA (Table 2) [19, 79, 93]. Under these circumstances, the added polymer increases the lifetime of monomer droplets and the probability of monomer droplet nucleation. The ratio Np>f/Nm>i was found to be very close to 1 for the mini-emulsion polymerization with PMMA, but it is much above 1 for the run with PSt. The interaction between the polymer particle surface and emulsifier increases with increasing hydrophobicity of polymer and, thence, PSt should promote the formation of more stable monomer droplets in the preparation of the mini-emulsion. However, the reverse seems to be true for the highly diluted polymer particles with the predissolved PSt. PSt mainly locates in the monomer droplet core, whereas the more hydrophilic PMMA tends to diffuse closer to the droplet surface layer and interact with emulsifier therein. Thus, the stronger interaction of PMMA with the droplet surface in comparison with PSt makes PMMA a more efficient hydrophobe. 

Batch miniemulsion polymerizations aiVAc/BA were investigated using SO/50 and 25/75 molar ratios of the two monomers, SHS/HD as the surfactant/ cosurfactant system and anunonium persulfate as initiator [6]. The polymeriza-ticxis were conducted at 60°C with 25% solids fiamulations. The miniemulsion droplets were created using ultrasonificatioiL The polymerizations were c iarac-terized their reaction kinetics, copolyma compositions and propoties. and final particle size distributions. Parallel conventional emulsion polymmzations (i.e. no cosurlactant, no high shear) were conducted for comparison. 

In batch experiments, the solids were varied from 35 to 75% [10]. The primary surfactant was Aerosol A103 (disodium ethoxylated nonyl phenol half ester of sulfosuccinic acid) with HD as the cosurfactant. These were used in concentrations of 1 and 4 wt% on monomer, respectively. Two KPS concentrations, 1 and 2 wt% on water, were tried. The miniemulsions were produced by ultra-sonification. Parallel conventional emulsion polymerizations were conducted for comparison to the miniemulsion polymerizations (75 °Q. Coagulum-free latexes resulted from miniemulsion polymerizations up to 60% solids, while only 50% solids could be achieved for the cxmventional process. These differences were attributed to the resulting particle size distributions where the miniemulsion polymerizations produced latexes with larger particles, broader distributions and lower viscosities than their conventional counterparts. As in other studies, this difference in PSDs was explained by differing nucleation mechanisms. However, as in other studies, it was not possible to determine whether the nucleation in the miniemulsion systems was predominantly by radical entry into chxjplets. 

A typical styrene miniemulsion polymerization process using cetyl alcohol or hexadecane as the costabilizer does not show a constant reaction rate period and it can be divided into four major regions based on the polymerization rate versus monomer conversion curve [11, 47], as shown schematically in Figure 5.4b. For comparison, the polymerization rate versus conversion profiles for conventional emulsion polymerization (Figure 5.4a) and microemulsion polymerization (Figure 5.4c) are also included in this figure. First, the rate of miniemulsion polymerization increases rapidly to a primary maximum and then decreases with increasing monomer conversion. This is followed by the increase of polymerization rate to a secondary maximum. After the secondary maximum is achieved, the rate of polymerization then decreases rapidly toward the end of polymerization [47]. 

In general, conventional emulsion polymerization is faster in comparison with the miniemulsion polymerization because more latex particles (reaction loci) are nucleated and the rate of polymerization is linearly proportional to the number of latex particles per unit volume of water. Nevertherless, the rate of polymerization per particle is larger for miniemulsion polymerization, as evidenced by the higher concentrations of free radicals and monomer in the latex particles. 

The hydrophobic monomers styrene and MMA were copolymerized with the sodium salt of vinylbenzylsulfosuccinic acid as a polymerizable surfactant grafting of the surfactant onto the particles was estimated to be about 50-75% [51]. A polymerizable surfactant was formed by the esterification of hydroxypropylmethacrylate or hydroxyethyhnethacrylate with succinic anhydride [53]. However, in addition to the surfmer, sodium dodecyl sulfate (SDS) was employed to provide a sufficient stability to the latexes. A mono-fluorooctyl maleate surfactant was used to stabilize the polymerization of styrene in miniemulsion [55]. Although the polymerizable moiety was not fixed at the end of the fluorinated chain (the hydrophobe part), the surfactant could be copolymerized with the styrene monomer. Subsequently, on comparison of the infrared (IR) spectra (vibration of -CF2 and -CFj) before and after dialysis, it was estimated that 92% of the surfactant had remained grafted post-dialysis. 

For inverse miniemulsions the surfactant is used in a very efficient manner, at least in comparison to inverse microemulsions [11,12] or inverse suspensions [13], which are used for subsequent polymerization processes. Again, surface coverage of the inverse miniemulsion droplets with surfactant is incomplete and empty inverse micelles are absent, but this is important if any interpretation of the reaction mechanism is required. 

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