Formation of “Snowmanlike” Polystyrene/Poly(methyl methacrylate)/Toluene Droplets Dispersed in an Aqueous Solution of a Nonionic Surfactant at Thermodynamic Equilibrium

Abstract

"Snowmanlike" polystyrene (PS)/poly(methyl methacrylate) (PMMA) composite particles were prepared by evaporation of toluene from PS/PMMA/toluene droplets dispersed in an aqueous solution of polyoxyethylene nonylphenyl ether surfactant (Emulgen 911). Partitioning experiments revealed that the Emulgen 911 concentration was higher in the droplets than in the aqueous solution during toluene evaporation. As a consequence, the interfacial tensions between the polymer phases (PS and PMMA) and the aqueous phase (gammaP-T/W) were extraordinarily low (approximately 10(-1) mN/m). The interfacial tension between the PS and PMMA phases containing toluene (gammaPS-T/PMMA-T) measured by the spinning drop method was not affected by the presence of Emulgen 911. Based on minimization of the total interfacial free energy at a polymer weight fraction in the toluene droplet of 0.17, the formation of spherical droplets is expected, in agreement with experiment. The subsequent morphology change of the PS/PMMA/toluene droplets from spherical to snowmanlike during toluene evaporation under thermodynamic equilibrium is attributed to (i) the low values of gammaP-T/W, which explains the increase in the interfacial area between the droplets and the aqueous phase, and (ii) the increase in gammaPS-T/PMMA-T with increasing polymer weight fraction.