Ring sliding dynamics of single- and double-threaded cyclodextrin-polyethylene glycol polyrotaxane slide-ring polymers under tension

Slide-ring materials comprising polyrotaxanes are an emerging class of soft materials composed of mechanically interlocked polymer strands threaded by macrocyclic rings. Double-threaded γ-cyclodextrin-based slide-rings in which two polymer chains are threaded through the same ring give rise to materials with exceptional extensibility and toughness that can overcome the elasticity-toughness trade-off inherent to traditional polymer architectures. However, a fundamental molecular-level understanding of how tension, ring chemistry, and threading geometry govern ring mobility remains lacking. In this work, we employ all-atom molecular dynamics simulations to model the thermodynamics and sliding dynamics of single and pairs of native and permethylated α- and γ-cyclodextrin rings on single- and double-threaded polyethylene glycol (PEG) polyrotaxanes as a function of applied tension. Double-threaded configurations, permethylated rings, and rings with stronger binding energies show consistently lower diffusivities than their single-threaded, native, and weakly bound counterparts. The 3D diffusivity of single- and double-threaded native and permethylated cyclodextrin monomers and dimers monotonically decreases with applied tension due to a reduction in the amplitude of the thermal fluctuations. Similar monotonic trends are generally preserved for 1D diffusivity along the PEG chain contour, but less sterically-constrained single-threaded native cyclodextrins in both the monomer and dimer state can exhibit non-monotonic trends due to the interplay of tension-dependent thermal fluctuations that promote mobility and the presence of backbone dihedral gauche defects that serve as a barrier to ring sliding.