Two Growth Modes of Semicrystalline Cylindrical Poly(ε-caprolactone)-b-poly(ethylene oxide) Micelles

Abstract

The micelles of a poly(ε-caprolactone)-b-poly(ethylene oxide) block copolymer (PCL59-b-PEO113) 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 H2O/THF (5/1 v/v) and H2O/dioxane (5/1 v/v) mixed solvents, a mixture of amorphous spherical micelles and short semicrystalline cylindrical micelles is yielded in H2O/DMF (5/1 v/v), whereas mostly short semicrystalline cylindrical micelles are obtained in H2O/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 H2O/DMF (5/1 v/v) at the initial growth period. The other is the growth by end-to-end coupling of cylindrical micelles in H2O/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 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 end-to-end coupling of semicrystalline cylindrical micelles occurs with 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 (5/1 v/v) solution is even slower than that in H2O/DMSO (5/1 v/v).