Synthesis and Self-assembly of Amphiphilic Bottlebrush Block-Copolymers
Fenyves, Ryan D.
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Using both the ‘Grafting from’ and ‘Grafting through’ approaches, well-defined bottlebrush block copolymers were synthesized. Resulting macromolecules were characterized by GPC and 1 H NMR, the self-assembled into nanostructures for further characterization. Amphiphilic bottlebrush block copolymers containing polylactide (PLA) and poly(ethylene oxide) (PEO) side chains were synthesized by a grafting-from method, and their self-assembly in aqueous environment was studied by cryogenic transmission electron microscopy. Highly uniform spherical micelles with low dispersities were obtained from bottlebrush amphiphiles with packing parameters of ∼0.3, estimated from the polymer structural data. Long cylindrical micelles and other nonspherical aggregates were observed for the first time for compositionally less asymmetric bottlebrush surfactants. Critical micelle concentration values of 1 nM, measured for PEO-rich bottlebrush amphiphiles, indicated an enhanced thermodynamic stability of the produced micelle aggregates. Further elaborating on the aqueous self-assembly of nonionic bottlebrush block copolymer surfactants containing PLA and PEO, and investigate the effect of hydrophilic backbone length on the interfacial curvature and thermodynamic stability of the formed aggregates. The micelle core size was constant across the samples and the length of hydrophilic brushes protruding from the hydrophobic cores was consistent with a highly extended bottlebrush backbones, having a relationship between backbone and shell thickness of L = 0.2* N PEO-bb nm. These results demonstrate that the length of the hydrophilic backbone plays minimal role in controlling the interfacial curvature and is not a factor in morphological prediction considerations. CMC measurements on the micelle samples suggest that the length of the hydrophilic backbone plays a very small role in the thermodynamic stability of the micelle. PS-PEO bottlebrush block copolymers were prepared by a robust grafting-from protocol. Polystyrene and polyethylene oxide macromonomers with norbornene end groups were synthesized in high purity by post-polymerization chain end coupling reactions. PS-PEO bottlebrush block copolymers underwent phase separation both in melt and in thin films to produce lamella microstructures with a domain spacing of 42-45 nm. On a silicon substrate, preferential interactions between polymer blocks and substrate/air resulted in perpendicular orientation of bottlebrush molecules at the interface.