Molecular Motions in Flowing Semi-dilute Polymer Solutions

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Polymers are large, chain-like molecules that are the building blocks of both industrially-relevant products (i.e. plastics, coatings, and fibers) and state-of-the-art advanced materials for devices such as flexible photovoltaics and displays. For many of these applications, the polymer must be processed with an eye towards its final properties; for example, in order to create strong fibers, a "drawing" process is used to align polymer chains. Similarly, coating flows can control molecular packing (and thus charge transport) in semiconducting polymers. For both situations, polymers are manipulated in a liquid solution by applying strong flows. However, it remains challenging to understand how exactly flow influences molecular motions. This challenge arises because long polymer chains interpenetrate, so that any given molecule interacts with many others both directly and via solvent flows. The proposed research will use new computational algorithms to understand the connection between polymer motions, molecular-level fluid dynamics, and applied flows. This understanding will serve as the foundation for molecular control in polymer solution processing, by showing how concentration and strong flows can be used to tune molecular structure and correspondingly material properties. This project will also serve to train the next generation of scientists, through the interdisciplinary training and mentorship of students. In addition, this project will involve the development of outreach events that incorporate interactive experimental/computer simulation into hands-on activities designed to promote the participation of underrepresented students in STEM fields.Polymer solutions are typically processed in the ?semi-dilute? regime, where individual coils overlap significantly. Semidilute solutions are characterized by significant solvent-mediated hydrodynamic interactions (HI) and topological ?hooking?. Current theory is capable of capturing equilibrium properties (relaxation time, molecular structure) but has not been extended to flowing solutions. A new conformational-averaging procedure is capable of efficiently simulating large systems, enabling exploration of how HI and chain topology affect highly out-of-equilibrium polymer conformations in flow. This proposal will address two aims: (i) Understand how concentration and hydrodynamic interactions affect chain dynamics in flowing semi-dilute polymer solutions and (ii) capture how non-linear polymer architectures and shear flows lead to molecular ?hooking?. The first aim will characterize the distortion of hydrodynamic screening by strong flows and extend equilibrium scaling concepts to out-of-equilibrium systems. The second aim will use topological invariants to characterize hooking interactions, and how they affect the distributions of molecular conformations. The proposed work will thus provide insight important for flow-controlled solution processing of polymers, ranging from flow coating to inkjet or 3D printing.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.