Effect of Stress Relief and Ionic Charge on Polyelectrolyte Brush Behavior

Cornell University, Ithaca NY

Investigators

Abstract

Polymers are large molecules formed by linking together hundreds of small molecular sub-units called monomers into long chains. Depending upon on the chemical structure of the monomer, the polymer can either be slippery (like Teflon) or sticky (like tape). By changing the monomer, one can create surfaces that prevent adhesion of cells, making them better for medical uses. Nature accomplishes this by attaching charged polymer segments to surfaces, forming structures that resemble brushes. While these brush polymers offer unique properties that could have substantive societal impacts, they also pose significant challenges. The dense packing of the charged groups can stretch the chemical bonds and, in extreme cases, the bonds can break. With the support of the Macromolecular, Supramolecular and Nanochemistry program of the Chemistry Division, Professor Ober at Cornell University is creating and studying new polymer brushes that are effective coatings but also have long lifetimes (i.e., do not have bonds that easily break). Collaborators, Professor Ryan (University of Sheffield) and Professor Ruehe (University of Freiburg), are helping to characterize the polymer structures. In addition to its scientific impact, the project benefits society by educating undergraduate and graduate students to become globally aware, interdisciplinary scientists. Surface-bound bottlebrush polymers are potentially synthetic mimics for shock absorbing components in the human body, thus, the broader scientific impact of this project outside of macromolecular chemistry is potentially quite large. Insights into the polyelectrolyte brush behavior may also help in the understanding the attachment of biomolecules, cells and microorganisms to surfaces and thus be used to control into unwanted adhesions on ships for the marine industry. The research team is creating and studying a series of stable model polymer brushes with carefully tailored placement of ionic groups in both single strand and bottlebrush architectures. Physical phenomena associated with charge, binding and attachment of soluble species, and the rather unusual case of mechanochemistry that leads to brush breakage are being investigated. Professor Ober and his collaborators are preparing specially tailored polyelectrolyte brushes with selected vertical and horizontal brush architectures and studying their physical properties. Ionic groups are attached directly to the acrylate or methacrylate polymer backbone or through tailored sequences of peptoid units of precise ionic group spacing and location. Confinement is controlled by the use of nanopatterning in which the pattern sizes are comparable to brush height (tens of nanometers). The effect of cation charge is studied using a range of mono- and multivalent cations. Characterization activities include: X-ray scattering studies of brush swelling and shrinkage under a range of wetting and electrolyte conditions and neutron reflectivity studies using labeled polymer chains to separately gather information about the different components (backbone, sidearm, ions) to develop a detailed picture of the charged polymer brush. Graduate students serve as mentors and role models to undergraduate students, while at the same time gaining supervisory skills. Undergraduates from Cornell University take part during the academic year while REU students from other schools with an emphasis on underrepresented students participate in research during the summer months. Lessons learned from these research programs are integrated into lessons for high school teachers in a Research Experiences for Teachers (RET) program and in teachers' workshops.