Formation and Properties of Vesicles from Cyclic Amphiphilic PS-PEO Block Copolymers

Linear polystyrene-poly(ethylene oxide)-polystyrene (PS-PEO-PS) block copolymers and corresponding cyclized PS-PEO counterparts with three different PS molecular weights were synthesized and self-assembled to investigate the effects arising from the topology. Linear PS₅-PEO₄₅-PS₅ (L1) and cyclic PS₁₀-PEO₄₅ (C1) formed micelles. As previously reported for poly(n-butyl acrylate) and PEO block copolymers, the micelles from C1 showed more than 30 °C higher phase transition temperature (cloud point, T_c) than those from L1. Linear PS₁₀-PEO₄₅-PS₁₀ (L2) and cyclic PS₂₀-PEO₄₅ (C2) resulted in the formation of a structure called large compound micelles. Self-assembly of linear PS₄₀-PEO₄₈-PS₄₀ (L3) and cyclic PS₈₆-PEO₄₈ (C3) lead to the formation of vesicles. The vesicles were characterized by TEM, DLS, and SLS. Remarkably, the vesicles from L3 (T_c = 69, 59, and 48 °C in the presence of 1, 5, and 10 wt % of NaCl, respectively) were found to be somewhat more thermally stable than those from C3 (T_c = 62, 52, and 43 °C in the presence of 1, 5, and 10 wt % of NaCl, respectively). This trend of the thermal stability was counterintuitively opposed to the case of the micelles. Moreover, T_c of the vesicles was controlled by the ratio of L3 and C3.