Amphiphilic Bottlebrush Block Copolymers: Analysis of Aqueous Self- Assembly by Small-Angle Neutron Scattering and Surface Tension Measurements

M Alaboalirat and LQ Qi and KJ Arrington and S Qian and JK Keum and H Mei and KC Littrell and BG Sumpter and JMY Carrillo and R Verduzco and JB Matson, MACROMOLECULES, 52, 465-476 (2019).

DOI: 10.1021/acs.macromol.8b02366

A systematic series of 16 amphiphilic bottlebrush block copolymers (BCPs) containing polystyrene and poly(N-acryloylmorpholine) (PACMO) side chains were prepared by a combination of atom-transfer radical polymerization (ATRP), photoiniferter polymerization, and ring-opening metathesis polymerization (ROMP). The grafting-through method used to prepare the polymers enabled a high degree of control over backbone and side-chain molar masses for each block. Surface tension measurements on the self-assembled amphiphilic bottlebrush BCPs in water revealed an ultralow critical micelle concentration (cmc), 1-2 orders of magnitude lower than linear BCP analogues on a molar basis, even for micelles with >90% PACMO content. Combined with coarse-grained molecular dynamics simulations, fitting of small-angle neutron scattering traces (SANS) solution conformations for individual bottlebrush BCPs and micellar nanostructures for self-assembled allowed us to evaluate macromolecules. Bottlebrush BCPs showed an increase in anisotropy with increasing PACMO content in toluene-d(8), which is a good solvent for both blocks, reflecting an extended conformation for the PACMO block. SANS traces of bottlebrush BCPs assembled into micelles in D2O, a selective solvent for PACMO, were fitted to a core shell shell model, suggesting the presence of a partially hydrated inner shell. Results showed an average micelle diameter of 40 nm with combined shell diameters ranging from 16 to 18 nm. A general trend of increased stability of micelles (i.e., resistance to precipitation) was observed with increases in PACMO content. These results demonstrate the stability of bottlebrush polymer micelles, which self-assemble to form spherical micelles with ultralow (<70 nmol/L) cmc's across a broad range of compositions.

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