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>

Concentration Dependence of the Size and Symmetry of a Bottlebrush Polymer in a Good Solvent

“…The shape of the polymer can be described by the radius of gyration (R g ), which can be broken down into a long and short axis to detail structural asymmetry in the material (shown in Figure 1). Trends in the various metrics of molecular shape have been reported in recent studies in solution 14,24 and in melt 13,25 where a crossover from a spherical/ellipsoidal-like molecular shape (equivalent to star polymers) when the backbone is short relative to side chains to highly anisotropic molecular shape when the backbone becomes much longer compared to the side chain length is observed. This crossover is, however, not trivial as the molecular shape of bottlebrush polymers initially tends to become more spherical with increasing the backbone length until a critical threshold is reached and bottlebrush polymers become more anisotropic.…”

“…Compared to the vinyl class, the norbornene-based bottlebrushes have been reported more recently, and fewer investigations have been performed into their solution structure. 14,24,[29][30][31] The primary difference between norbornene-based bottlebrushes and vinyl-based bottlebrushes is that for fully grafted vinyl systems have one side chain for every two backbone carbons, while fully grafted norbornene systems have one side chain for every five backbone carbons. This intrinsically limits the grafting density of the norbornene chemistry, unless there are multiple branches from each backbone monomer.…”

“…Implicit solvent models, where the solvent is replaced by an effective interaction between the polymer segments, provides significant savings in computational time. Thus, it is not surprising that many computational studies of polymers 41,42 and in particularly bottlebrush polymers in solution 12,20,21,24 frequently rely on implicit solvent polymer models. However, a sole reliance on implicit solvent models could lead to erroneous conclusions as various solvent effects not incorporated into the model may play an important role into the phenomenon of interest.…”