Micro-convection, dissipative structure and pattern formation in polymer blend solutions under temperature gradients

We report experimental results on time evolution of phase separation in a polymer blend solution under a temperature gradient. This study is aimed at making a self-organized structure by utilizing convection in a polymer blend solution comprising polystyrene (PS) and polybutadiene (PB) as solute with dioctylphthalate (DOP) as solvent. The polymer blend solution (PS/PB/DOP) with various compositions (PS/PB = 20/80, 50/50 or 80/20) was prepared with a total polymer concentration (PS + PB) at 3.0 wt.%. The solution was confined in a layer (0.1 mm thick), which was further subjected to a temperature gradient by controlling top and bottom temperatures (Ttop and Tbot) independently with a 10 or 20 degrees C temperature difference (Ttop < Tbot). For all compositions, micro-convection was generated due to the Soret effect, which is relevant to the fact that the temperature gradient induces a concentration gradient. An unusual phase separating process was observed as a result of dynamic coupling with convective flows. Especially, an interesting hierarchical structure was formed for a PS/PB = 50/50 solution. This structure is characterized by a doubly surrounding "sea-island". Namely, the "sea" contains large "islands" which involve tiny "islands". The large "islands" are spatially arranged and embedded in polygonal convection cells.