Polyelectrolyte brushes: Stabilization through Controlled Charge Placement

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In this project funded by the Macromolecular, Supramolecular and Nanochemistry Program in the Division of Chemistry, Professor Christopher K. Ober at Cornell University is developing and studying polyelectrolyte brushes with controlled charged placement. Polymer brushes are special, large molecular structures with polymer chains densely tethered to a surface. They have unique and useful properties including wormlike conformation, compact molecular dimensions, and notable polymer chain end effects due to their compact and confined structures. These particular polyelectrolyte brushes can interact with a biological environment actively or passively depending on the nature of a living system. As such, the brushes are promising materials for biomedical devices, chemical sensors and drug-delivery systems. In this research, synthetic methods are chosen to construct charged polyelectrolyte brushes, allowing systematic investigations of how charge placements changs the swelling, shape, mobility, and stability of the synthesized brushes. Characterization is carried out using sophisticated chemical measurement techniques and also includes the effect of solution acidity and metal ions on brush behavior. This research is interdisciplinary in nature and includes international collaborations with researchers in Germany and the United Kingdom. The research program exposes graduate students and associated researchers to international science perspectives. Graduate students serve as mentors and role models to undergraduate students, while at the same time gaini supervisory skills. Undergraduates from Cornell University take part in the research during the academic year while students from other schools with an emphasis on underrepresented communities participate in research during the summer months. Outreach activities include high school students and teachers through organized workshops. This research is focused on the preparation and study of a series of model poly(electrolyte)(PEL) brushes with carefully tailored placement of cationic and anionic groups used either alone or in combination in a single strand and bottle brush architecture based on acrylamide and styrene-maleimide polymers. Characterization includes study of the effect of pH and metal ions on brush behavior of these new architecture PEL brushes. Neutron and X-ray reflectometry studies of swollen polymer films are used as tools for understanding brush behavior. Neutron reflectivity using labeled polymer chains to separately gather information about the different components (brush, sidearm, ions) of the polymer brush is also employed. Brushes are grown from surfaces by means of controlled radical polymerization of charged or protected monomers. Ionic groups are attached to acrylamide or maleimide backbones through tailored amine functional segments including peptoid units of selected lengths and ionic group content. Longer peptoid side chains on a polymer backbone form bottle brush-like architectures while controlled charge spacing and location selectively add coulombic stress near to or away from the polymer backbone. In addition to conventional vertically grown brushes, the research team is capitalizing on new lateral brush growth geometries through demonstrated microfabrication capabilities that enable parallel optical and X-ray or optical and neutron characterization of brush swelling under a range of wetting and electrolyte conditions. This research addresses important questions on the conformation of dense polyelectrolyte brushes, including the effects of chain confinement and charge spacing. The results have the potential to further advance fundamental understanding of the factors affecting the stability and stimulus-responsive nature of surface-grafted bottlebrush polymers.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.