Tuning Biocompatible Block Copolymer Micelles by Varying Solvent Composition: Dynamics and Populations of Micelles and Unimers

Optimization of micellar molecular encapsulation systems, such as drug delivery vehicles, can be achieved through fundamental understanding of block copolymer micelle structure and dynamics. Herein, we present a study of PEO–PCL block copolymer spherical micelles that self-assemble at 1% wt/vol in D2O–THF-d 8 mixtures. Varying solvent composition as a function of cosolvent THF-d 8 at constant polymer concentration (1% wt/vol) allows sensitive study of how small molecule additives influence micelle structure and dynamics. We conduct nuclear magnetic resonance spectroscopy and diffusometry on two block copolymer (2k series: PEO2k–PCL3k; 5k series: PEO5k–PCL8k) spherical micelles that show drastically different behaviors. Unimers and micelles coexist in solution from 10–60 vol % THF-d 8 for the 2k series but only coexist at 60 vol % THF-d 8 for the 5k series. At ≥ 60 vol % THF-d 8 micelles disassemble into free unimers for both series. We observe relatively flat micelle diffusion coefficients (∼1 × 10–10 m2/s) with increasing THF-d 8 below 60 vol % for both 2k and 5k series, with only small changes in micelle hydrodynamic radius (≈14 nm) over this range. We compare these results to a detailed SANS and microscopy study described in a companion paper. These fundamental molecular dynamics, unimer population, and diffusion results, as a function of polymer composition and solution environment, provide critical fodder for controlled design of block copolymer self-assembly.