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Mobility of Polymer-Tethered Nanoparticles in Unentangled Polymer Melts

Ge, Ting, Rubinstein, Michael
Macromolecules 2019 v.52 no.4 pp. 1536-1545
friction, hydrodynamics, melting, nanoparticles, polymers
A scaling theory is developed for the motion of a polymer-tethered nanoparticle (NP) in an unentangled polymer melt. We identify two types of scaling regimes depending on the NP diameter d and the size of a grafted polymer chain (tail) Rₜₐᵢₗ. In one type of regime, the tethered NP motion is dominated by the bare NP, as the friction coefficient of the tails is lower than that of the less mobile particle. The time dependence of the mean square displacement (MSD) of the tethered NP ⟨Δr²(t)⟩ in the particle-dominated regime can be approximated by ⟨Δr²(t)⟩bₐᵣₑ for the bare NP. In the other type of regimes, the tethered NP motion is dominated by the tails when the friction coefficient of the tails surpasses that of the particle at times longer than the crossover time τ*. In a tail-dominated regime, the MSD ⟨Δr²(t)⟩ ≈ ⟨Δr²(t)⟩bₐᵣₑ only for t < τ*. ⟨Δr²(t)⟩ of a single-tail NP for t > τ* is approximated as the MSD ⟨Δr²(t)⟩ₜₐᵢₗ of monomers in a free tail, whereas ⟨Δr²(t)⟩ of a multitail NP for t > τ* is approximated as the MSD ⟨Δr²(t)⟩ₛₜₐᵣ of the branch point of a star polymer. The time dependence of ⟨Δr²(t)⟩ in a tail-dominated regime exhibits two qualitatively different subdiffusive regimes. The first subdiffusive regime for t < τ* arises from the dynamical coupling between the particle and the melt chains. The second subdiffusive regime for t > τ* occurs as the particle participates in the dynamics of the tails. For NPs with loosely grafted chains, there is a Gaussian brush region surrounding the NP, where the chain strands in Gaussian conformations undergo Rouse dynamics with no hydrodynamic coupling. The crossover time τ* for loosely grafted multitail NPs in a tail-dominated regime decreases as the number of tails increases. For NPs with densely grafted chains, the tails are hydrodynamically coupled to each other. The hydrodynamic radii for the diffusion of densely grafted multitail NPs are approximated by the sum of the particle and tail sizes.