Two Growth Modes of Semicrystalline\nCylindrical Poly(ε-caprolactone)‑<i>b</i>‑poly(ethylene\noxide) Micelles

Abstract

The micelles of a poly­(ε-caprolactone)-<i>b</i>-poly­(ethylene oxide) block copolymer (PCL₅₉-<i>b</i>-PEO₁₁₃) in different mixed solvents\nwere held at 53 °C\nfor 5 min, and seed solutions with different micellar morphologies\nand amounts of micellar semicrystalline seeds were prepared. The crystallinity\nof these seed micelles was identified by high-resolution transmission\nelectron microscopy (HRTEM) and selected area electron diffraction\n(SAED). It is found that mostly amorphous spherical micelles are formed\nby heating micellar solutions in H₂O/THF (5/1 v/v) and\nH₂O/dioxane (5/1 v/v) mixed solvents, a mixture of amorphous\nspherical micelles and short semicrystalline cylindrical micelles\nis yielded in H₂O/DMF (5/1 v/v), whereas mostly short semicrystalline\ncylindrical micelles are obtained in H₂O/DMSO (5/1 v/v)\nmixed solvent. The seed solutions were placed at 4 °C for micellar\ngrowth. Transmission electron microscope (TEM) shows that micellar\ngrowth driven by epitaxial crystallization of core-forming PCL chains\ntakes place and the length of grown cylindrical micelles increases\nwith time. Two growth modes are observed. One is the growth of unimers\n(or amorphous spherical micelles) on the active ends of semicrystalline\ncylindrical micelles in micellar solution in H₂O/DMF (5/1\nv/v) at the initial growth period. The other is the growth by end-to-end\ncoupling of cylindrical micelles in H₂O/DMSO (5/1 v/v).\nThe kinetics of micellar growth is strongly dependent on the growth\nmechanism. The growth of the cylindrical micelles in the H₂O/DMF (5/1 v/v) solution is much faster than that in the H₂O/DMSO (5/1 v/v) solution. On long time scale, micellar growth by\nend-to-end coupling of semicrystalline cylindrical micelles occurs\nwith slow rate in both H₂O/DMF (5/1 v/v) and H₂O/DMSO (5/1 v/v) solutions, and the growth rate in H₂O/DMF\n(5/1 v/v) solution is even slower than that in H₂O/DMSO\n(5/1 v/v).