Cracking the Mystery of Polyelectrolyte Coacervate Structure and Dynamics

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PART 1: NON-TECHNICAL SUMMARYPolyelectrolyte complexes are a mixture of two kinds of electrically charged, long polymer molecules, of opposite charge. Their opposite charge causes the two kinds of polyelectrolytes, which can be very different from each other, to nevertheless attract each other and form an intimate mixture, or complex. An important example of a polyelectrolyte complex is the chromosome, which contains DNA, which is a negatively charged polymer and hence repels itself. Nature therefore uses positively charged proteins to bind to the DNA and bundle it into the compact shape of the chromosome. Among the many other examples of polyelectrolyte complexes are biological membranes, underwater adhesives, drug delivery vehicles, and food processing agents. Despite their importance and the growing interest in them for advanced applications, basic understanding of how to control their structure and mechanical properties remains undeveloped. The proposed work focuses on measurements of the mechanical properties of these complexes, such as their viscosity and stiffness, and computer simulations to determine how these properties are controlled by the composition, including the polyelectrolytes, salts, and pH. Understanding the relationship between the composition and mechanical properties will provide deep insight into why complexes have the properties they do, and how to design these properties for future applications. There may also be connections to biological function, which are affected by the micro-mechanical properties of these complexes. The research will be integrated with education of graduate and undergraduate students, outreach, and creation of software.PART 2: TECHNICAL SUMMARYThe PI and his group will measure the linear rheology of polyelectrolyte coacervates of widely varying composition and containing different salts, and seek to test time-temperature, time-salt, time-hydration, and time-pH superpositions that allow data to be collapsed onto approximate “master curves.” Specifically, they will measure the linear rheological properties of polycations poly(N,N-dimethylaminoethyl methacrylate) and poly(diallyldimethylammonium) with polyanions poly(acrylic acid) and poly(styrene sulfonate), and with salt ions Na+ or K+, and Cl- or Br-. They will explore the surprising differences and anomalies among coacervates, depending on the polyelectrolytes, salts, and the chain lengths used, including an asymmetry in relaxation time produced by changing the molecular weight of polyanion vs. polycation, anomalous dependence of viscosity on degree of polymerization, and a low-frequency plateau modulus for some, but not all, coacervates. The systematic approach will provide a comprehensive understanding of the key determinants of rheological behaviour and overcome the limitations of existing data sets. They will also carry out molecular dynamics simulations to determine the nature of the local interactions among polyelectrolytes and salt ions that determine the rates of relaxation. This will be done in part through development of novel time correlation functions to determine whether local monomer diffusion is governed by ion-pairing dynamics, or by collective “glassy” dynamics, or some mixture of the two..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.