Styrene droplets

The colloidal stability of polymer dispersion prepared by the emulsion copolymerization of R-(EO)n-MA was observed to increase with increasing EO number in the macromonomer [42, 96]. Thus C12-(EO)9-MA did not produce stable polymer latexes, i.e., the coagulum was observed during polymerization. This monomer, however, was efficient in the emulsion copolymerization with BzMA (see below). The C12-(EO)20-MA, however, appears to have the most suitable hydrophilic-hydrophobic balance to make stable emulsions. The relative reactivity of macromonomer slightly decreases with increasing EO number in macromonomer. The most hydrophilic macromonomer with co-methyl terminal, Cr(EO)39-MA, could not disperse the monomer so that the styrene droplets coexisted during polymerization. The maximum rate of polymerization was observed at low conversions and decreased with increasing conversion. The decrease in the rate may be attributed to the decrease of monomer content in the particles (Table 2). In the Cr(EO)39-MA/St system the macromonomer is soluble in water and styrene is located in the monomer droplets. Under such conditions the polymerization in St monomer droplets may contribute to the increase in r2 values. 

Prior to 1962, droplets below 1 pm were considered too unstable to participate in the nucleation process. In 1962, Higuchi and Misra [47] proposed that the addition of a water insoluble compound to the monomer will enhance the stabiUty of small droplets by prohibiting diffusion. In 1973, Ugelstad et al. [48] showed how submicron styrene droplets could be made stable enough to participate in the nucleation processes by adding small amounts of cetyl alcohol. Later, Ugelstad [48] used Eq. 2 to explain these experimental observations. 

The mechanism of this mini-emulsion emulsification process was investigated by the conductometric titration of aqueous hexadecyltrimethylammonium bromide-cetyl alcohol mixtures with benzene or styrene combined with transmission electron microscopic examination of the morphology of the mixed emulsifiers and the styrene droplets formed (2,8). Figure 1 shows that the titration curves with and without cetyl alcohol are quite different that for... 

Measurement of the styrene droplet sizes showed that the initial droplet sizes were relatively large (ca. 0.5ym) but decreased greatly to ca. O.lym as the conductance decreased to the inflection point V., then increased by coalescence to the inflection point V2 and increased slowly thereafter, principally by growth. The rodlike particles, which were observed before the titration, began to disappear during the decrease in conductance to the inflection point and disappeared completely before the inflection point V was reached. 

The second slowly decreasing leg is interpreted as the. particle growth process in which the number of styrene droplets is approximately constant and the added styrene diffuses through the aqueous phase and swells these droplets further. 

The results of experiments run with both styrene and toluene in a 1 1 SLS/lauryl alcohol solution showed that identical "tails" formed on the styrene and toluene droplets, indicating that this phenomenon was not the result of thermally initiated polymerization of the styrene. Also, no "tails were observed on air drops in the same mixed emulsifier solution or on styrene droplets in an aqueous solution of SLS alone. 

Figure 1. Spinning drop photographs of a styrene droplet in 1 1 SLS/cetyl alcohol solution, based on lOiriM SLS, at 65°C.

Several efforts were made to prepare anisotropic hybrid particles. Lu et al. [128] formulated a miniemulsion with a dispersed phase containing tetraethylorthosilicate (TEOS), styrene, and MPS, stabilized by CTAB. After initiation of the styrene polymerization, a copolymer from styrene and MPS was formed. Addition of ammonia induces hydrolysis and condensation of TEOS to silica. The processes induce phase separation to a styrene droplet with the growing PS, and a TEOS droplet with the growing silica. The droplets are bridged by the PS-PMPS copolymer. Conducting the reaction without MPS generates separate silica and PS particles. 

Mixed bmshes of hydrophobic and hydrophilic polymers grafted onto Au-NPs were also used as stabilizers for styrene droplets. Subsequent suspension polymerization of styrene led to PS particles with Au-NPs on their surface. Such raspberry particles might open new perspectives for superhydrophobic coatings with dual-size roughness. " ... 

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