Micellar Morphologies of Poly(ε-caprolactone)-b-poly(ethylene oxide) Block Copolymers in Water with a Crystalline Core

The self-assembly of poly(ε-caprolactone)-b-poly(ethylene oxide) block copolymers (PCL_nPEO₄₄ and PCL_nPEO₁₁₃) with narrow polydispersity in aqueous medium was studied using transmission electron microscopy. In this system, the formed micelles are composed of a crystalline PCL core and a soluble PEO corona. We demonstrated that the PCL-b-PEO block copolymers can form micelles with abundant morphologies, depending on the lengths of the blocks and composition. It is observed that for PCL_nPEO₄₄ the micellar morphology changes from spherical, rodlike, wormlike, to lamellar, as the length of the PCL block increases. In contrast, most of PCL_nPEO₁₁₃ (n = 21−147) block copolymers form spherical micelles, and only PCL₂₃₂PEO₁₁₃ exhibits mixed spherical and lamellar micellar morphologies. The effect of microstructure on micellar morphology was semiquantitatively interpreted in terms of reduced tethering density (σ). It is found that lamellar micelles are formed when σ is smaller than a critical value of between 3.0 and 4.8. A larger σ indicates crowding of the tethered chain, and spherical micelles tend to be formed.