Solid-liquid phase boundaries in polyelectrolyte complexes and multilayers

Texas A&M Engineering Experiment Station, College Station TX

Investigators

Abstract

PART 1: NON-TECHNICAL SUMMARY Polymeric materials with ionic groups stand to make important impacts in applications ranging from therapeutics to water purification and reprocessable plastics. However, because they are extraordinarily sensitive to changes in pH, salt, and temperature, it remains challenging to control the formation, performance, and properties of these materials. This project examines polymeric materials that contain positive and negative charges, which assemble into larger structures called complexes. One of the defining features of these complexes is their ability to separate into solid and liquid phases during the assembly phases. There is an important knowledge gap in understanding how these solid-liquid boundaries respond to temperature, other types of molecular interactions (hydrogen bonding), and polymers with mixed charges. This project seeks to answer what molecular factors control the phase boundary and why does this boundary exist; how the molecular motions of the polymers in the material vary with assembly conditions; and how these materials can be translated to analogous coatings. Because complexation is influenced by a multitude of factors intrinsic to the polymers themselves, model systems are selected to isolate the effects of these factors. The technological broader impact will be the expanded knowledge of charged polymer materials’ characteristics for assembly, processing and stability. The project will include outreach to the general public through TAMU’s Chemistry Open House, 4-H youth development of the Brazos Valley, and other venues. PART 2: TECHNICAL SUMMARY This project will experimentally investigate and compare the phase behavior of polyelectrolyte complexes (PECs) and polyelectrolyte multilayers (PEMs) using different polyelectrolyte systems that have tunable hydrogen bonding or zwitterionic charge to systematically isolate the contributions of these features. Better understanding of the factors that influence the phase behavior of polyelectrolyte complexes and their similarities with polyelectrolyte multilayers is important for the future design of these assemblies for more widespread applications. Most experimental and theoretical investigations have focused on electrostatic factors with varied success, but there is a knowledge gap in exploring non-covalent interactions and temperature effects. To address this knowledge gap, the project is organized into three objectives, centering around complexes and multilayers made from synthetic polycations, polyanions, and polyzwitterions. The first objective is to probe the diffusion of polyelectrolyte chains in solid and liquid complexes using fluorescence recovery after photobleaching technique. The second objective is to observe solid-liquid-solution phase boundaries and their respective upper critical and lower critical solution temperatures using optical microscopy, turbidity measurements, and isothermal titration calorimetry. The third objective is to compare polyelectrolyte multilayers and complexes in solid and liquid regions of the phase map by monitoring multilayer growth and deconstruction at a variety of conditions including pH, ionic strength, and temperature. Swelling of the multilayer under different conditions post-assembly will also be examined. Altogether this work aims to reveal the nature of the liquid-solid phase boundary in complexes at varying temperatures and if that boundary corresponds to the nature of multilayer growth. . 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.