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MRS Advances © 2020 Materials Research Society DOI: 10.1557/adv.2020.9

Comparison of neutral and charged polyelectrolyte bottlebrush polymers in dilute salt-free conditions Alexandros Chremos1 and Ferenc Horkay1 1

Section on Quantitative Imaging and Tissue Sciences, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA

Abstract: We investigate the structure of neutral and charged bottlebrush polymers in salt-free solutions at different polymer concentrations. In particular, we use molecular dynamics simulations by utilizing a coarse-grained bead-spring model that includes an explicit solvent and complementary experiments made by small angle neutron scattering (SANS). We find that the charged groups along the side chains exert significant repulsive forces, resulting in polymer swelling and backbone stretching. In addition to the primary polyelectrolyte peak, we find that bottlebrush polymers exhibit an additional peak in the form and static structure factors, a feature that is absent in neutral polymers. We show that this additional peak describes the intra-molecular correlations between the charged side chains.

INTRODUCTION Bottlebrush polymers are highly branched macromolecules composed of tightly spaced side chains tethered along a polymer backbone chain. The packing of side chains results in strong excluded volume effects, leading to extended conformations that typically precludes the development of intermolecular entanglements and results in materials with Rouse-like relaxation dynamics, i.e., no rubbery plateau [1]. This type of polymers have gained considerable interest in recent years because of their wide range of applications, including rheological modifiers, nanoporous materials, supersoft elastomers, and photonic bandgap materials [1-6]. Moreover, bottlebrush polymers play an important role in certain biological systems. For example, the main cartilage proteoglycan aggrecan exhibits a bottlebrush structure and is important in the proper functioning of articular cartilage. It forms large microgel-like complexes (via its interaction with hyaluronan and link protein) that endows cartilage with load-bearing properties [7-10]. Degradation of aggregan due to aging, injury, or diseases can lead to loss of mechanical properties and severe chronic pain. However, the design and creation of synthetic aggrecan remains a challenge because it is poorly understood how the molecular characteristics of the bottlebrush architecture contribute to the macroscopic properties and function of cartilage.

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The challenge in modeling bottlebrush polymers is that their molecular architecture has multiple molecular parameters, such as grafting density, side chain length, and backbone length, which influence the polyme