Topological Interactions in Polymer Gels

University Of North Carolina At Chapel Hill, Chapel Hill NC

Investigators

Abstract

TECHNICAL SUMMARY This award supports theoretical research and education in the area of polymer networks and entangled gels. The elastic properties of polymer networks and gels, used in a wide range of applications from hard rubber boots to soft gel replacements for eye lenses, are determined by chemical cross-links as well as by topological entanglements between network strands. Existing theories of gels treat entanglements in qualitatively the same way as crosslinks. However, experiments suggest that their relative role changes upon network swelling and deformation. Thus a microscopic theory is needed that provides a qualitative explanation of this phenomenon as well as a quantitative description of macroscopic properties of deformed and swollen entangled gels. The PI aims to develop a theory that will allow the calculation of the elastic modulus and equilibrium swelling of entangled gels along with stress-strain dependence for their uniaxial and biaxial deformations. The theory will be extended to explore novel networks with unique properties such as high deformability and low elastic modulus. The effect of trapped entanglements in swollen and deswollen gels will be modeled, emphasizing their qualitative difference from temporary entanglements in polymeric liquids. The dependence of topological interactions on network deformations will be calculated, and used to understand why the strength of these interactions becomes weaker in elongation directions and stronger in compression directions. New numerical methods will be developed to determine the dependence of entanglement parameters, such as confining tube diameter and persistence length, on deformation of polymeric systems with fixed topology. These methods will be used to test the assumptions and predictions of different theories of entangled networks. This project will provide graduate students and postdoctoral fellows with an excellent opportunity for training in valuable analytical and numerical techniques. Some of the results of this research will be used to develop material for a textbook. The proposed project will stimulate the interest of high school students in modern scientific methods by engaging them in active research. Examples of elastic gels will be used in the design of the updated "Zoom In" exhibit at the Morehead Planetarium and Science Center as well as in lectures by PI to high school students at the "Science Spectrum" and "Science at the Edge" series. NONTECHNICAL SUMMARY This award supports theoretical research and education on networks of long chain-like molecules, including interpenetrating chains that are swollen by a solvent, like water. The unique interplay of solid-like properties on large length scales and liquid-like properties on small length scales makes these polymer networks and gels the world?s most deformable elastic materials. Their elastic properties, used in a wide range of applications from hard rubber boots to soft gel replacements for eye lenses, are determined by chemical interactions between the chain-molecules as well as by entanglements among chain-molecules in the network. Most theories for the elastic and mechanical properties of these kinds of materials treat the effects of entanglements in polymer networks qualitatively the same way as chemical bonds creating hard links between polymer strands. Experiments suggest, however, that the relative role of the two physical effects changes upon network swelling and deformation caused by external conditions. The PI aims to fill the need for a microscopic theory that can explain this phenomenon and describe the properties of these materials. The theory has the potential to uncover new routes for designing soft materials with a desired set of properties. The project will provide undergraduate and graduate students as well as postdoctoral fellows with an excellent opportunity of training in valuable analytical and numerical techniques. Some of the results of this research will be used in the developing materials for a textbook. The proposed project will stimulate the interest of high school students in modern scientific methods by engaging them in active research. Examples of elastic gels will be used in the design of the updated "Zoom In" exhibit at the Morehead Planetarium and Science Center as well as in lectures by PI to high school students at the "Science Spectrum" and "Science at the Edge" series.