Single molecule studies of branched polymer dynamics in non-dilute solutions

The macroscopic properties of polymers are dictated by their molecular architecture and microstructure. Branched polymers exhibit a multitude of distinct rheological phenomena, such as shear thinning and enhanced strain hardening, compared to linear polymers as a direct consequence of their complex, non-linear chain architecture. Owing to their unique physical properties, branched polymers are widely used in advanced materials. Although branched polymers are increasingly used in emerging technological applications, our current understanding of the dynamic behavior of topologically complex polymers is limited and largely based on bulk rheological and scattering experiments. Despite recent advances in polymer synthesis, characterization, and modeling, we lack a complete understanding of how molecular-scale processes give rise to the macroscopic properties of branched polymers.\n\nSingle molecule techniques provide a powerful approach to directly observe the dynamics of individual polymer chains, thereby bridging the gap in understanding between molecular and bulk-scale properties. Recent single molecule studies have explored the dynamics of linear and ring polymers in dilute and non-dilute solutions, and well-defined comb polymers in ultra-dilute solutions (Chapter 1). However, single molecule studies have not yet been extended to non-dilute solutions of branched polymers. In this Ph.D. dissertation, I report the direct observation of branched polymer dynamics in non-dilute solutions using single molecule techniques. \n\nWe begin by studying the dynamics of comb-shaped polymers in semi-dilute solutions using single molecule fluorescence microscopy. In particular, the relaxation dynamics of fluorescently labeled short-chain branched comb polymers are studied in a background of linear unentangled semi-dilute linear polymers (Chapter 2). Dual-color fluorescence imaging allows for direct visualization of branches and backbones simultaneously but separately. Using this approach, we characterize the relaxation times of comb polymers as a function of branching density. In all cases, experimental results for branched polymers are compared to those for linear polymers. Interestingly, we find that comb polymer relaxation follows a non-monotonic trend in semi-dilute solution. Unexpectedly, combs with low branching density relax faster than their linear counterparts, whereas increasing the number of branches ultimately slows down relaxation. Single molecule experiments are complemented by Brownian Dynamics (BD) simulations with and without intra- and intermolecular hydrodynamic interactions (HI). Our results show that this non-monotonic dependence in the longest polymer relaxation time arises due to a subtle yet important interplay between hydrodynamic shielding and polymer architecture, and this effect is exaggerated upon increasing branch length of the comb polymers.\n\nWe further probe the transient stretching dynamics of comb polymers in a background of semi-dilute unentangled linear polymers in extensional flow (Chapter 3). Comb polymer dynamics in semi-dilute solutions are compared to those in dilute solutions and also to linear polymers in semi-dilute solutions. Our results show that comb polymers in semi-dilute solutions exhibit complex stretching conformations and broad distributions of polymer stretching pathways in transient flows due to intermolecular interactions and the presence of branch points. We show direct visual evidence that the prevalence of polymer folds and kinks during the unfolding of comb polymers in flow is a consequence of folds and kinks forming at branch points of the comb polymer, leading to hindered stretching pathways. We further show visual evidence of flow-induced hooking events in extensional flow even in unentangled polymer solutions.\n\nGiven the key role of branch position and branch molecular weight on polymer dynamics, we developed new synthesis techniques allowing us to extend single polymer studies beyond combs with short-chain branches (Chapter 4). These architectures include long-chain branched polymers, symmetric 3-arm star polymers, and structurally controlled comb polymers with branches confined to desired sections of the backbone. We show proof-of-concept for the building blocks for these syntheses and discuss possible architectures that can be synthesized using these methods. We further probe the single molecule fluorescence techniques to directly study the dynamics of symmetric 3-arm polymers in dilute solutions in extensional flow (Chapter 5). Here, we study the relaxation and transient and steady-state stretching dynamics of symmetric 3-arm polymers and compare results to the behavior of linear chain counterparts. We find that transient stretching dynamics of star polymers are rendered more complex than linear polymers due to the presence of multiple free ends. Furthermore, 3-arm star polymers were found to exhibit a delayed coil-stretch transition compared to linear polymers, which had been predicted in prior simulation-based studies of polymer dynamics. Moreover, our results show that the steady-state stretched extensions of star polymers show a much larger distribution compared to their linear analogs. Finally, in collaboration with a research group at McGill University, we study the equilibrium stretching dynamics of single comb polymers under confinement using a technique known as convex lens-induced confinement (Chapter 6). We further use this approach to understand the impact of additives such as fluorescent dyes on the mechanical properties of DNA. \n\nOverall, this work extends single molecule studies of branched polymer dynamics to non-dilute solutions using model comb polymers. Single molecule studies allow for the direct observation of molecular scale processes, allowing us to bridge the gap in our understanding of polymer physics and bulk rheological properties for architecturally complex polymers. Moving forward, the methods developed in this dissertation can be extended to more concentrated polymer solutions, increasingly complex polymer chain architectures, and different flow fields. In this way, our work provides new avenues for answering fundamental questions regarding the impact of polymer chain topology and concentration on macrosopic properties of polymers.

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