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The self-assembly of poly(ε-caprolactone)-b-poly(ethylene oxide) block copolymers (PCL n PEO44 and PCL n PEO113) 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 n PEO44 the micellar morphology changes from spherical, rodlike, wormlike, to lamellar, as the length of the PCL block increases. In contrast, most of PCL n PEO113 (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.

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The micelles of a poly(ε-caprolactone)-b-poly(ethylene oxide) block copolymer (PCL 59 -b-PEO 113 ) in different mixed solvents were held at 53 °C for 5 min, and seed solutions with different micellar morphologies and amounts of micellar semicrystalline seeds were prepared. The crystallinity of these seed micelles was identified by high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). It is found that mostly amorphous spherical micelles are formed by heating micellar solutions in H 2 O/THF (5/1 v/v) and H 2 O/dioxane (5/1 v/v) mixed solvents, a mixture of amorphous spherical micelles and short semicrystalline cylindrical micelles is yielded in H 2 O/DMF (5/1 v/v), whereas mostly short semicrystalline cylindrical micelles are obtained in H 2 O/DMSO (5/1 v/v) mixed solvent. The seed solutions were placed at 4 °C for micellar growth. Transmission electron microscope (TEM) shows that micellar growth driven by epitaxial crystallization of core-forming PCL chains takes place and the length of grown cylindrical micelles increases with time. Two growth modes are observed. One is the growth of unimers (or amorphous spherical micelles) on the active ends of semicrystalline cylindrical micelles in micellar solution in H 2 O/DMF (5/1 v/v) at the initial growth period. The other is the growth by end-to-end coupling of cylindrical micelles in H 2 O/DMSO (5/1 v/v). The kinetics of micellar growth is strongly dependent on the growth mechanism. The growth of the cylindrical micelles in the H 2 O/DMF (5/1 v/v) solution is much faster than that in the H 2 O/DMSO (5/1 v/v) solution. On long time scale, micellar growth by end-to-end coupling of semicrystalline cylindrical micelles occurs with slow rate in both H 2 O/DMF (5/1 v/v) and H 2 O/DMSO (5/1 v/v) solutions, and the growth rate in H 2 O/DMF (5/1 v/v) solution is even slower than that in H 2 O/DMSO (5/1 v/v).