Dilute Solution Structure of Bottlebrush Polymers

Abstract

<div>Bottlebrush polymers are a class of macromolecules that has recently found use</div><div>in a wide variety of materials, ranging from lubricating brushes and</div><div>nanostructured coatings to elastomeric gels that exhibit structural color. These</div><div>polymers are characterized by dense branches extending from a central backbone,</div><div>and thus have properties distinct from linear polymers. It remains a challenge</div><div>to specifically understand conformational properties of these molecules, due to</div><div>the wide range of architectural parameters that can be present in a system, and</div><div>thus there is a need to accurately characterize and model these molecules. In</div><div>this paper, we use a combination of viscometry, light scattering, and computer</div><div>simulations to gain insight into the conformational properties of dilute</div><div>bottlebrush polymers. We focus on a series of model bottlebrushes consisting of</div><div>a poly(norbornene) (PNB) backbone with poly(lactic acid) (PLA) side chains. We</div><div>demonstrate that intrinsic viscosity and hydrodynamic radius are experimental</div><div>observations \emph{sensitive} to molecular architecture, exhibiting distinct</div><div>differences with different choices of branch and backbone lengths. Informed by</div><div>the atomistic structure of this PNB-PLA system, we rationalize a coarse-grained</div><div>simulation model that we evaluate using a combination of Brownian Dynamics and</div><div>Monte Carlo simulations. We show that this exhibits quantitative matching to</div><div>experimental results, enabling us to characterize the overall shape of the</div><div>bottlebrush via a number of metrics that can be extended to more general</div><div>bottlebrush architectures.</div>

Bridging Simulation Length Scales of Bottlebrush Polymers Using a Wormlike Cylinder Model

“…We use a simulation model of bottlebrush polymers developed in our prior work, 52 which includes explicit side chains and can be quantitatively compared to the experimental results for intrinsic viscosity and hydrodynamic radius. 52 This model represents the bottlebrush architecture via a coarse-grained bead-spring model, with the backbone consisting of N bb beads and each side chain consisting of N sc beads. The grafting density, f, determines the number of side chains attached to a backbone bead.…”

“…It is possible to map the parameters N sc and N bb to synthetic bottlebrush degrees of polymerization, specifically for poly(lactic acid) side chains grafted to a poly(norbornene) backbone, for the special case that f = 1. 52 This bottlebrush model considers an overall energy, U = U s + U ev + U b , that includes a spring potential U s , an excluded volume potential U ev , and a bending potential U b . For the spring potential, we use finitely extensible nonlinear elastic (FENE) springs to maintain the connectivity between the beads:…”

“…Efforts include Monte Carlo (MC) simulations (both lattice-based 22 and off-lattice 23−33 ), bond fluctuation models, 34−36 cellular automaton models, 37 molecular dynamics (MD) models, 15,38−51 and Brownian dynamics (BD) models. 52 In addition, numerical theory such as self-consistent field theory 53,54 and polymer reference interaction site model (PRISM) theory 55 have also been used to capture the conformational attributes of bottlebrush polymers. The focus of most of these studies is to understand the key questions of bottlebrush conformation, namely, how do densely grafted side chains stiffen the backbone, and how do we quantify the stiffness of the bottlebrush via a Kuhn length or persistence length?…”

“…The latter molecular model is based on a hybrid BD/MC simulation model developed by the authors, with properties that arise from resolving the side chain structure and in quantitative agreement with the experiment. 52 We focus on four quantities end-to-end distance ⟨R 2 ⟩, radius of gyration ⟨S 2 ⟩, hydrodynamic radius R H , and intrinsic viscosity [η]to show that the wormlike cylinder model can accurately capture the results of the underlying fine-grained model. We use a parameterization procedure that systematically shows that the WLCy model is consistent among a wide range of conformational and dynamic properties and for a large number of polymer architectures (e.g., side chain length, backbone length, and grafting density).…”

Unraveling the Conformations of Backbone and Side Chains in Thermosensitive Bottlebrush Polymers

“…In addition to the length of side chains, the backbone length is also known to affect BBP's shape and conformations of backbone and side chains. 1,3,49 Therefore, to investigate the effect of the ratio of N bb to N sc on the conformations of backbone and side chains, we designed the BBPs with N bb = 36 and 9 beads. They were grafted with 30-mer or 10-mer PNIPAM side chains with d = 3 or 1, resulting in BBPs with ∼33 or 100% grafting densities, respectively.…”