All-Atom Molecular Dynamics Simulations of Uncharged Linear Polymer Bottlebrushes: Effect of the Brush Sizes and the Number of Side-Chain Monomers

Abstract

Bottlebrush polymers (BBPs), characterized by grafted polymer side chains on linear backbone polymer chain, have emerged as a unique and versatile class of macromolecules with extensive applications in the fields of material science, electronics, battery materials, self-healing technology, etc. In this paper, we employ all-atom molecular dynamics (MD) simulations to present a comprehensive study of poly(methyl methacrylate)-g-poly(2-ethyl-2-oxazoline) (PMMA-g-PEtOx) BBP and its structural and hydration properties for varying number of backbone monomers (N_BB) and side chain monomers (N_SC), as well as properties of water molecules supported by the BBP. We find that the radius of gyration follows a scaling of R_g ∼N_SC^0.36 for smaller grafts and R_g ∼ N_SC^0.52-0.58 for longer grafts. We also find that the overall shape of the bottlebrush goes from a rod to sphere-like shape with the increase in N_SC. Both the hydration per side chain monomer and hydrogen bonds (HBs) per oxygen and nitrogen of the side chain monomer reduce with an increase in N_SC, caused by a corresponding enhancement in localization of the side chain monomers in the interior of the BBP. Furthermore, steric influences ensure the number of water-oxygen HBs is much more than the number of water-nitrogen HBs (with oxygen and nitrogen atoms belonging to the monomer side chains). Also, the BBP-supported water molecules demonstrate two distinctly ordered domains with one more structured and one less structured. The more structured domain disappears with an increase in N_SC that causes more side chain monomers to localize in the interior of the BBPs. Finally, we observe that despite the highly negative partial charges of the oxygen and nitrogen atoms (of the side chain monomers), the dipole orientation distributions of water molecules around these atoms exhibit the presence of a neutral environment rather than an anionic environment. Overall, we anticipate that our study will generate significant interest in probing the various BBP systems in greater atomistic detail.