Lubrication in polymer-brush bilayers in the weak interpenetration regime: Molecular dynamics simulations and scaling theories

Desai, Parth Rakesh; Das, Siddhartha

doi:10.1103/physreve.98.022503

Cited by 7 publications

(7 citation statements)

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“…The centers of mass of the grafted polymer beads are rigidly located at the corresponding grafting positions with either z = 0 (denoted bottom grafting plane) or z = d g (top grafting plane). Instead of using a particle-based description for the wall, 58 a particle-less smooth wall 68 is employed to more conveniently consider the wall confinement on the chains. In order to ensure that monomers do not cross the wall boundaries, one smooth wall is placed below the bottom grafting plane (z = −σ) and the other is above the top grafting plane (z = d g + σ).…”

Section: Models and Simulation Methodsmentioning

confidence: 99%

Structural and Dynamical Coupling in Solvent-Free Polymer Brushes Elucidated by Molecular Dynamics Simulations

Chang

2021

Langmuir

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We investigate the chain configuration and segmental dynamics in interacting solvent-free polymer brushes using molecular dynamics simulations. The brush systems are designed to mimic the interstitial space between a pair of neighboring polymer-grafted nanoparticles in solvent-free nanoparticle–organic hybrid materials. Each brush consists of uniformly grafted chains formed by a given number of monomer beads. In monodisperse systems, two opposing brushes have the same chain length and grafting density. In mixed conditions, we consider binary systems with two surfaces being separately grafted with polymers of distinct chain lengths at different grafting densities as well as bidisperse systems with polymers of two different lengths being tethered to the surfaces at a fixed grafting density. We demonstrate that the brush configuration and interpenetration are both governed by the need that monomer beads have to uniformly fill the space. For systems with longer chain lengths and/or higher grafting densities, the larger interwall separation yields more stretched brush conformations and reduced extents of interbrush mixing. As a result, the polymer configurational entropy is generally decreased and the segment-to-segment relaxation dynamics is slowed down accordingly. The grafting of chains at a high density not only makes the relaxation dynamics deviate from the standard Rouse prediction but also leads to distinct relaxation times for the free and tethered segments. The more slowly relaxing tethered segments play a more important role in determining the overall end-to-end fluctuations. Moreover, the two distinct relaxation processes are consistent with the two-stage decay in the Rouse mode fluctuation autocorrelation function. In the presence of brush bidispersity, the collaboration between polymers of different lengths is evidently observed in the brush profiles. The variations of the chain configuration for the two polymers are complementary, and the associated relaxation dynamics of the two species are significantly coupled.

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Section: Models and Simulation Methodsmentioning

confidence: 99%

Structural and Dynamical Coupling in Solvent-Free Polymer Brushes Elucidated by Molecular Dynamics Simulations

Chang

2021

Langmuir

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“…Polymer brushes with one end each chain grafted on a surface are ranked among the most promising systems in polymer science, − in particular for the modification of surfaces to, for example, tune wettability, adsorption, , stabilization, − and lubrication. − In nanomedicine and material science, polymer brushes can be combined with stimuli-responsive components, such as active minority chains or reactive end-groups to create responsive polymer brushes, which become more interesting for drug delivery, biosensors, and smart surfaces. − Such a stimuli-responsive behavior usually relies on the phase transitions of the active component in the brush, and therefore, understanding their transition properties is previously crucial for the design of switch materials on demand.…”

Section: Introductionmentioning

confidence: 99%

Adsorption Active Diblock Copolymers as Universal Agents for Unusual Barrier-Free Transitions in Stimuli-Responsive Brushes

Klushin

Skvortsov

et al. 2021

Macromolecules

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We reconsider a recently proposed design for smart responsive brushes, which is based on a conformational transition in very dilutely embedded block copolymers with a surface active block (Qi et al., Macromolecules 53, 5326, 2020). Under certain conditions, the transition acquires an unusual character: it remains very sharp, but the barrier separating the adsorbed and desorbed states disappears completely. We show that these features are very robust with respect to changing almost all system parameters: the lengths of the inert and active blocks of the minority chain, the brush length, its density, and its polydispersity. The only relevant condition is that the inert block of the minority chain is long enough to extend outside the brush density profile. We develop an analytical theory that predicts the relevant characteristics of the transition and verify it with Monte Carlo simulations. We also show that the surprising universality of the transition properties is rooted in an underlying connection to the force-induced desorption transition, which is known to combine the features of the first- and second-order transitions with a pretransition fluctuation growth accompanied by phase coexistence.

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“…Most theoretical and simulation studies of polymer brush friction focus on parallel plate geometries [17][18][19][20][21][22][23][24][25][26][27][28][29]31,32]. However, the contact geometries might be different in applications where brushes are applied.…”

Section: Introductionmentioning

confidence: 99%

“…In fact, interdigitation between brushes can be switched using asymmetric brushes [15] and this can be employed to control the friction between surfaces [16].Molecular simulations have been employed extensively to obtain a microscopic picture of polymer brush friction [17][18][19][20][21][22][23][24][25][26]. With help of these simulations, friction-velocity relations have been derived [27][28][29][30][31][32]. For bilayer brush-systems, shear stresses act in the overlap zone (pink central region in Figure 1a) where polymers from the opposing brushes interact.…”

mentioning

confidence: 99%

“…Therefore, they tilt and stretch (see Figure 1a), which reduces the overlap length l. Due to the reduction in overlap with increasing velocity, the friction force f increases sub-linearly with velocity v at high shear rates, i.e., f ∝ v κ . The shear-thinning exponent κ depends on the compression as well as the nature of the polymers and varies between 0 and almost 1 [27][28][29][30][31][32]. For symmetric, solvated, neutral, linear bilayer brushes, the shear-thinning exponent is expected to be κ = 0.52-0.58 [31].…”

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confidence: 99%

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Polymer Brush Friction in Cylindrical Geometries

2019

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Polymer brushes are outstanding lubricants that can strongly reduce wear and friction between surfaces in sliding motion. In recent decades, many researchers have put great effort in obtaining a clear understanding of the origin of the lubricating performance of these brushes. In particular, molecular dynamics simulations have been a key technique in this scientific journey. They have given us a microscopic interpretation of the tribo-mechanical response of brushes and have led to the prediction of their shear-thinning behavior, which has been shown to agree with experimental observations. However, most studies so far have focused on parallel plate geometries, while the brush-covered surfaces might be highly curved in many applications. Here, we present molecular dynamics simulations that are set up to study the friction for brushes grafted on the exterior of cylinders that are moving inside larger cylinders that bear brushes on their interior. Our simulations show that the density distributions for brushes on the interior or exterior of these cylinders are qualitatively different from the density profiles of brushes on flat surfaces. In agreement with theoretical predictions, we find that brushes on the exterior of cylinders display a more gradual decay, while brushes on the interior of cylinders becomes denser compared to flat substrates. When motion is imposed, the density profiles for cylinder-grafted brushes adapt qualitatively differently to the shear motion than observed for the parallel plate geometry: the zone where brushes overlap moves away from its equilibrium position. Surprisingly, and despite all these differences, we observe that the effective viscosity is independent of the radius of the brush-grafted cylinders. The reason for this is that the viscosity is determined by the overlap between the brushes, which turns out to be insensitive to the exact density profiles. Our results provide a microscopic interpretation of the friction mechanism for polymer brushes in cylindrical geometries and will aid the design of effective lubricants for these systems.

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Lubrication in polymer-brush bilayers in the weak interpenetration regime: Molecular dynamics simulations and scaling theories

Abstract: We conduct molecular dynamics (MD) simulations and develop scaling laws to quantify the lubrication behavior of weakly interpenetrated polymer brush bilayers in the presence of an external shear force. The weakly interpenetrated regime is characterized by 1 Show more

Cited by 7 publications

References 71 publications

Structural and Dynamical Coupling in Solvent-Free Polymer Brushes Elucidated by Molecular Dynamics Simulations

Structural and Dynamical Coupling in Solvent-Free Polymer Brushes Elucidated by Molecular Dynamics Simulations

Adsorption Active Diblock Copolymers as Universal Agents for Unusual Barrier-Free Transitions in Stimuli-Responsive Brushes

Polymer Brush Friction in Cylindrical Geometries

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