Curvature-Coupled Hydration of Semicrystalline Polymer Amphiphiles Yields flexible Worm Micelles but Favors Rigid Vesicles: Polycaprolactone-Based Block Copolymers

Crystallization processes are in general sensitive to detailed conditions, but the present understanding of underlying mechanisms is insufficient. A crystallizable chain within a diblock copolymer assembly, for example, is expected to couple curvature to crystallization and thereby impact rigidity as well as preferred morphology, and yet the effects on dispersed phases have remained unclear. The hydrophobic polymer polycaprolactone (PCL) is semicrystalline in bulk (Tm = 60 °C) and is shown here to generate flexible worm micelles or rigid vesicles in water from several dozen poly(ethylene oxide)-based diblocks (PEO−PCL). Despite the fact that “worms” have a mean curvature between that of vesicles and spherical micelles, “worms” are seen only within a narrow, process-dependent wedge of morphological phase space that is deep within the vesicle phase. Fluorescence imaging shows worms are predominantly in one of two states − either entirely flexible with dynamic thermal undulations or fully rigid; only a few w...