Nonconventional emulsion polymerization of styrene with mixed anionic and nonionic emulsifiers

Abstract

Emulsion polymerization of styrene (St) in the presence of mixed anionic/nonionic emulsifiers (SDS/NP 40) has been investigated. The polymerization rate (Rp) vs. conversion (X) curves show a relatively constant Rp region (Smith-Ewart Interval 2) in the range of X = 20-40% and a shoulder at X = ca. 75%. Rp is independent of the emulsifier charge composition except the run with SDS or NP 40 as the sole emulsifier. The latex particle size (dw) increases with increasing X and the degree of the increased dw is more pronounced at low X. The number of latex particles strongly increases up to ca. 20% conversion and then remains relatively constant toward the end of polymerization. The particle size distribution of latex particles is quite narrow. The average number of free radicals per particle (n̄) 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. On the contrary, for the system with NP 40 alone, n̄ increases significantly with increasing X and it is well above 0.5 throughout the reaction. The colloidal and kinetic data obtained from this work are discussed in terms of the hairy or close-packed particle (droplet) surface layer, the decreased entry and exit rates of radicals, the relatively high oil-solubility of NP 40, accumulation of polystyrene and NP 40 within the monomer droplets, preservation of monomer droplets during polymerization and formation of double emulsion droplets. The increased uniformity and stability of the initial emulsion may promote the monomer-starved condition even at a relatively low level of X.