Stretching Dynamics of Single Comb Polymers in Extensional Flow

Molecular architecture plays a key role in determining the physical properties and emergent functional properties of polymeric materials. Despite recent progress in the synthesis of structurally defined polymers, we still lack a complete understanding of how the emergent properties of topologically complex polymers arise from molecular-scale phenomena. In this work, we study the nonequilibrium dynamics of DNA-based comb polymers in extensional flow using a combination of single molecule fluorescence microscopy and Brownian dynamics (BD) simulations. In this way, we directly observe the stretching dynamics of single DNA comb polymers in planar extensional flow. Transient stretching dynamics of isolated comb polymers is studied as a function of branch density and location, branch molecular weight, and flow strength. High-fidelity BD simulations are used to provide a direct complement to single molecule experiments, providing key insights into the molecular stretching mechanisms for single combs in flow. Our results show that comb polymers stretch through fundamentally different conformational pathways compared to linear polymers. In particular, comb polymers exhibit hindered transient stretching in extensional flow, which arises due to nonlinear chain topologies. From a broad perspective, this work provides a molecular-based understanding of topologically complex polymers in flow, which could aid in the modeling and processing of advanced polymeric materials with nonlinear topologies. ■ INTRODUCTION In recent years, branched polymers have been used in a wide array of commercial applications ranging from organic electronics to membrane separations. Despite the increasing prevalence of topologically complex polymers in designing and building functional materials, we lack a complete understanding of the role of molecular architecture on the emergent physical properties of these materials. Comb-shaped polymers and brush polymers consist of a main polymer backbone with grafted side chains known as branches or arms. This molecular architecture enables chemical and functional versatility for numerous applications, including drug delivery, optical devices, antifouling coatings, renewable energy devices, and desalination membranes. Comb polymers are known to exhibit more complex flow behavior compared to their linear counterparts, including intricate linear viscoelastic signatures, distinct stress overshoots in the startup of shear flow, enhanced strain hardening in extensional flow, and nonlinear stress relaxation. Interestingly, comb polymers exhibit seemingly disparate macroscopic properties in flow, such as shear thinning (corresponding to a decrease in viscosity with increasing flow rate in shear flow) together with strain hardening (corresponding to an increase in viscosity upon increasing strain in extensional flow) for the same material. Nevertheless, these combined behaviors result in favorable processing properties for comb polymers compared to their linear analogues: shearthinning materials can be easily extruded with lower viscosities at high shear rates, whereas strain-hardening materials provide stabilized films and fibers during extensional flow processing. Given the importance of comb polymers as versatile materials in modern society, there is a clear need to understand the molecular origins of complex flow behavior and bulk-scale phenomena. Recent advances in comb polymer synthesis and purification have enabled detailed studies of the bulk rheological properties of well-defined branched polymers. These studies have shown dramatic effects of molecular-scale architectural defects on bulk rheological properties, such that minor changes in local chain connectivity vastly impact global material response. For example, the onset of nonlinear phenomena occurs at lower flow rates for branched polymers compared to linear polymers of similar molecular weight, with strain hardening becoming strikingly more pronounced as the branch molecular weight is increased in comb polymer melts. Although recent rheological measurements have uncovered interesting flow properties of these materials, bulk-level techniques tend to measure average properties while only indirectly inferring molecular-scale