Collaborative Research: Robust General Methods for Determination of Polyelectrolyte Molecular Weight and Polydispersity

With the support of the Macromolecular, Supramolecular, and Nanochemistry (MSN) Program in the Division of Chemistry, Professors Ralph H. Colby from Pennsylvania State University and Louis A. Madsen from Virginia Polytechnic Institute and State University are developing methods for measuring the molecular weight of polyelectrolytes. Polyelectrolytes are long chain macromolecules whose repeating units contain an electrolyte group. This group dissociates in water, making the polymer charged. Similar to salts, polyelectrolyte solutions are electrically conductive. Many biological and artificial macromolecules are polyelectrolytes and find application in biomedical, technological and industrial science. Some notable examples include implant coatings, controlled drug delivery, water-purification systems, food coatings and cosmetics. While there are many developed techniques used to accurately determine the molecular weight of neutral polymers, methodologies for ionic or charged polymers are comparatively scarce. The principal reason is the lack of adequate theoretical approaches to describe polyelectrolyte properties and very complex behavior in solution. This research aims to provide a basis for molecular weight determination for ionic polymers having a single type of charge (polyanions or polycations), including both natural and synthetic systems. The methods being developed here, if successful, could significantly accelerate the design and synthesis of polyelectrolytes. Additionally, the development and validation of the proposed theoretical and experimental methods is expected to further impact the physical underpinnings of polymer science and enable new understanding of ionic polymers. This work also promises to facilitate the characterization of molecular weight for many biopolymers such as hyaluronic acid, heparin, DNA and RNA. Training opportunities and exposure to polymer chemistry concepts and techniques will be provided to graduate and undergraduate students at both institutions. Research activities will also be utilized to recruit students into macromolecular programs and prepare them for careers in polymer science. Outreach activities at Virginia Tech will include presentations that highlight concepts in ionic polymer gels, macromolecular chain motions and energy storage. This hands-on K-12 outreach program seeks to broaden thinking and encourage elementary and high school students to consider pursuing science and technology careers. The ongoing REU (research experiences for undergraduates) program at Pennsylvania State University will be leveraged to include recruitment of students from underrepresented groups. Polyelectrolytes are increasingly used in alternative energy technologies, engineered human tissues, drug delivery, and a host of other applications. A critical barrier to wider development and application of polyelectrolytes lies in the extreme effort required to characterize their absolute molecular weight. Furthermore, the balance between strong electrostatic repulsions along each chain with the entropic penalty for stretching the chain is delicate and not yet understood in detail, making simple models only work under specific circumstances. This project will focus on a detailed scaling theory for polyelectrolyte behaviors in solution that underlies four distinct methods (using measured terminal modulus, relaxation time, chain diffusion, correlation length, and viscosity) for measuring number-average molecular weight (Mn). These four methods will employ measurements by solution rheology, X-ray scattering, and NMR diffusometry. A strong emphasis will be placed on developing a toolbox that establishes well-defined operating strategies and parameter spaces for practical applications of these methods in the semi-dilute unentangled regime. The scope of measurements will include a wide array of polyelectrolyte chemistries. Furthermore, a technique to reliably quantify polydispersity for polyelectrolytes using the diffusion-based molecular weight determination method by NMR spectroscopy is under development. Lastly, the utility of the methods for analyzing neutral polymers will also be explored. If successful, this research will not only provide a toolbox of experimental techniques that others can use, but also a theoretical framework for scaling relationships to describe data that translate into molecular weight and polydispersity, parameters of critical importance in characterizing charged 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.