Self-assembly process of different poly(oxystyrene)-poly(oxyethylene) block copolymers: Spontaneous formation of vesicular structures and elongated micelles

In the present work, we investigated the micellization, gelation, and structure of the aggregates of three poly (ethylene oxide)-polystyrene oxide block copolymers (E₁₂S₁₀, E₁₀S ₁₀E₁₀, and E₁₃₇S₁₈E₁₃₇, where E denotes ethylene oxide and S styrene oxide and the subscripts the block length) in solution. Two of them have similar block lengths but different structures (E₁₂S₁₀ and E₁₀S₁₀E ₁₀) and the other has longer blocks (E₁₃₇S ₁₈E₁₃₇)- For the first time, the spontaneous formation of vesicles by a poly(oxystyrene)-poly(oxyethylene) block copolymer is reported. These vesicular structures are present when copolymer E₁₂S ₁₀ self-assembles in aqueous solution in coexistence with spherical micelles, as confirmed by the size distribution obtained by dynamic light scattering and pictures obtained by polarized optical microscopy, and transmission and cryo-scanning electron microscopies. Vesicle sizes vary between 60 and 500 nm. On the other hand, for copolymers E₁₀S ₁₀E₁₀ and E₁₃₇S₁₈E₁₃₇, only one species is found in solution, which is assigned to elongated and spherical micelles, respectively. If we compare the high aggregation number derived by static light scattering for the triblock block copolymer micelles, with the maximum theoretical micellar dimensions compatible with a spherical geometry, we can see that the micellar geometry cannot be spherical but must be elongated. This is corroborated by transmission electron microscopy images. On the other hand, tube inversion was used to define the mobile-immobile (soft-hard gel) phase boundaries. To refine the phase diagram and observe the existence of additional phases, rheological measurements of copolymer E₁₃₇S ₁₈E₁₃₇ were done. The results are in good agreement with previous values published for other polystyrene oxide-poly(ethylene oxide) block copolymers. In contrast, copolymers E₁₂S₁₀ and E ₁₀S₁₀E₁₀ did not gel in the concentration range analyzed. Thus, only certain concentrations of copolymer E₁₀S ₁₀E₁₀ were analyzed by rheometry, for which an upturn in the low-frequency range of the stress moduli was observed, denoting an evidence of an emerging slow process, which we assign to the first stages of formation of an elastic network.