Transient Network Theory: Bridging Molecular Mechanisms to the Viscoelasticity of Soft Polymers

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The ability to organize large populations of molecules into materials can open the door to making dynamic materials or soft machines, thus advancing the national health, prosperity, and welfare; and even securing the national defense by facilitating the emerging area of soft robotics. Such materials are often found in nature in the form of transient polymeric networks which are at the source of muscle contraction as well as self-healing and adaptation in biological tissues. Although similar molecular networks can be synthesized in the laboratory, their performance still lags far behind their biological counterparts; raising the need for a better theoretical understanding and experimental control. This project will provide a route to fundamentally understand how the organization and dynamics of such polymer networks can lead to a well-targeted emerging response. It will promote the progress of soft matter science by bridging the gap between our understanding of the behavior of a single molecule and that of an entire network, not only enabling a fundamental understanding of bio-polymers, but also in improving our ability to control synthetic materials. The project will also develop an educational program around the concept of "materials of the future and bio-inspiration" in high-schools, the enhancement of undergraduate curriculum, and dissemination of scientific knowledge through social media.From a fundamental viewpoint, this project will support the development of a transient network theory that will describe, in a statistical sense, the time evolution of a transient polymer network based on molecular processes such as chain detachment, reputation, or diffusion. Going beyond phenomenological viscoelastic models, key concepts in statistical mechanics will be used to obtain a clearer connection between transient molecular interactions between many polymer chains and the time-dependent response of the network. The project brings three key contributions: (a) a new fundamental understanding of the relation between molecular processes and rheology, elasticity and energy dissipation; (b) the ability to generate new hypotheses regarding dynamic polymers and explore their macroscopic outcome in terms of growth, fracture resistance, and self-healing, and (c) a new continuum framework to describe the extreme deformation of soft materials whose behavior lies between that of solids and fluids. A computational methodology based on finite elements will be introduced to solve the research theory and used to study and characterize the viscoelastic response of synthetic and biopolymers in terms of their inner structure.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.