Revisiting the Morphology Development of Solvent-Swollen Composite Polymer Particles at Thermodynamic Equilibrium^†

Morphology of polystyrene (PS)/poly(methyl methacrylate) (PMMA)/toluene droplets, in which phase separation proceeds, dispersed in SDS aqueous solution was examined. It changed from ex-centered PS-core/PMMA-shell to hemisphere with increasing SDS concentration. At low polymer weight fraction (w_p), PS and PMMA phases contained non-negligible amount of PMMA and PS, respectively. The small amount of PS and PMMA in PMMA and PS phases, respectively, affected significantly the interfacial tension between polymer/toluene and aqueous solutions. Interfacial tension between PS and PMMA phases at low w_p was measured by the spinning drop method, showing a quite low value (≈10^-2 mN/m). Predicted morphology obtained from calculation of minimum total interfacial free energy of the droplets using the interfacial tensions agreed well with the experimental observation.