Mapping Coexistence Phase Diagrams of Block Copolymer Micelles and Free Unimer Chains

Abstract

Block copolymer micelles can carry molecular cargo in a nanoscopic package that is tunable via polymer structure in combination with cargo and/or solution properties. We use NMR spectroscopy and diffusometry to investigate spherical micelles that self-assemble from diblock poly(ethylene oxide)-b-(ε-caprolactone) (PEO–PCL) at 1% w/v in D2O–THF-d8 mixed solvents. For two series of micelles with block masses of 2 and 3 kg/mol (PEO and PCL, respectively) and 5 and 8 kg/mol, we quantify diffusion coefficients and relative populations of micelles and free unimers over a range of temperatures (21–50 °C) and solvent compositions (10–100 vol % THF-d8). Micelles and free unimers coexist over surprisingly large areas of these micelle–unimer phase diagrams and with a large variation of relative populations. This study provides a new avenue for understanding how inter- and intramolecular interactions fundamentally affect unimer exchange, cargo release, the thermodynamics of chain partitioning, and the tuning of micelle structure and dynamics.