Nonconventional emulsion polymerization of styrene with mixed anionic and nonionic emulsifiers

Emulsion polymerization of styrene (St) in the presence of mixed anionic/no nionic emulsifiers (SDS/NP 40) has been investigated. The polymerization ra te (R-p) vs. conversion (X) curves show a relatively constant R-p region (S mith-Ewart Interval 2) in the range of X=20-40% and a shoulder at X=ca. 75% . R-p is independent of the emulsifier charge composition except the run wi th SDS or NP 40 as the sole emulsifier. The latex particle size (d(w)) incr eases with increasing X and the degree of the increased d(w) is more pronou nced at low X. The number of latex particles strongly increases up to ca. 2 0% conversion and then remains relatively constant toward the end of polyme rization. The particle size distribution of latex particles is quite narrow . The average number of free radicals per particle ((n) over bar) is below or close to 0.5 for the polymerization system with SDS alone and mixed SDS/ NP 40 emulsifiers before the system experiences the gel effect at high X. O n the contrary, for the system with NP 40 alone, (n) over bar increases sig nificantly with increasing X and it is well above 0.5 throughout the reacti on. The colloidal and kinetic data obtained from this work are discussed in terms of the hairy or close-packed particle (droplet) surface layer, the d ecreased entry and exit rates of radicals, the relatively high oil-solubili ty of NP 40, accumulation of polystyrene and NP 40 within the monomer dropl ets, preservation of monomer droplets during polymerization and formation o f double emulsion droplets. The increased uniformity and stability of the i nitial emulsion may promote the monomer-starved condition even at a relativ ely low level of X.