The goal of this experimental project is to tease apart the intertwined connections between charge and chain conformation of well-defined polyelectrolyte (PE) systems. This will be accomplished by investigating the molecular-level structure of "weak" and "strong" PE brushes as a function of size and tethering density, and parameters that set the charge state of the system pH and monovalent salt concentration. As the nanoscale structure dictates the range and strength of interactions across interfaces, knowledge of how PE brushes adjust their molecular-level, solvated structure, both normal to and laterally along the tethering surface, in response to changes in their solution environment is necessary for the creating PE-modified surfaces with tailored properties. Results from the research will impact many applications for PEs, including adhesive, anti-fouling, or lubrication-like coatings, drug-delivery systems, and biomaterial surfaces. This research is enabled by the development of a robust method to directly synthesize well-defined PE brushes of poly(methacrylic acid) (PMAA) and sodium-poly(styrene sulfonate) (PSS) and access to world-class facilities for neutron science at Oak Ridge National Laboratory (ORNL). The molecular-level structure of these weak (PMAA) and strong (PSS) PE brushes will be studied using specular and off-specular neutron reflectivity measurements, complemented by atomic force microscopy imaging of the topology and multi-angle ellipsometry measurements of the average layer thickness. The research effort is enhanced by collaboration with Dr. John Ankner of the Spallation Neutron Source at ORNL, and Prof. Jimmy Mays of the University of Tennessee and ORNL.
Intellectual Merits of the Proposed Activity. The proposed systematic study of the effect of surface density, size, pH, and salt type and concentration on the molecular-level structure and swelling behavior of strong and weak PE brushes will provide a framework for understanding the complex connections between conformation and charge in PE brush systems. The novel synthetic strategy based on atom-transfer radical polymerization overcomes limitations associated with alternative approaches for making PE brushes. In addition to providing a nanoscale view of how weak and strong PE brushes adjust their structure normal to and along the surface, results will be used to arbitrate predictions from theory. The proposed work breaks new ground by examining ion specific effects, including interactions with a biomolecule key to thromobotic events, and how the internal salt concentration in PE brushes depends upon tethering density and pH. The collaborations with ORNL scientists, which build on an established track record, will help ensure successful outcomes.
Broader Impacts of the Proposed Activity. Understanding the structure of PE brushes is important for a wide range of applications, including adhesive or anti-fouling coatings, colloid stabilization, and the development of biomaterial coatings or drug delivery vehicles, sensors and electrorheological switches. Results from the research may stimulate new ideas about how to engineer biomaterial or membrane coatings with tailored properties, actuating layers for sensors or microfluidic devices, or high-affinity separation agents. This project will enhance interactions between Clemson University and ORNL and through the collaborations, foster the professional and personal growth of graduate and undergraduate students. As a result of being some of the first users of the Spallation Neutron Source, researchers will work with ORNL to contextualize and communicate to broader audiences the significance of their research and neutron science in the study of material interfaces. In addition to providing a training ground for the development of young engineers from underrepresented groups, a new effort aimed at providing an authentic, discovery-based research experience for aspiring secondary science education majors will be launched. This effort is aimed at helping soon to be teachers become better equipped to incorporate discovery and scientific inquiry experiences into their classrooms, thereby fueling the interests of future generations of students.
