Pulling Polymers on Energetically Disordered Surfaces: Molecular Dynamics Tests of Linear and Non-linear Response

Influence of wall heterogeneity on nanoscopically confined polymers

We investigate via molecular dynamics simulations the behaviour of a polymer melt confined between surfaces with increasing spatial correlation (patchiness) of weakly and strongly interacting sites. Beyond a critical patchiness, we find a dramatic dynamic decoupling, characterized by a steep growth of the longest relaxation time and a constant diffusion coefficient. This arises from dynamic heterogeneities induced by the walls in the adjacent polymer layers, leading to the coexistence of fast and slow chain populations. Structural variations are also present, but they are not easy to detect. Our work opens the way to a better understanding of adhesion, friction, rubber reinforcement by fillers, and many other open issues involving the dynamics of polymeric materials on rough, chemically heterogeneous and possibly "dirty" surfaces.

Effect of Uniformly Applied Force and Molecular Characteristics of a Polymer Chain on Its Adhesion to Graphene Substrates

The force-induced desorption of a polymer chain from a graphene substrate is studied with molecular dynamics (MD). A critical force needs to be exceeded before detachment of the polymer from the substrate. It is found that for a chain to exhibit good adhesive properties the chain configuration should consist of fibrils-elongated, aligned sections of polymers and cavities which dissipate the applied energy. A fibrillation index is defined to quantify the quality of fibrils. We focus on the molecular properties of the polymer chain, which can lead to large amounts of fibrillation, and find that both strong attraction between the polymer and substrate and good solvency conditions are important conditions for this. We also vary the stiffness of the chain and find that for less stiff chains a plateau in the stress-strain curve gives rise to good adhesion however for very stiff chains there is limited elongation of the chain but the chain can still exhibit good fibrillation by a lamella-like rearrangement. Finally, it is found that the detachment time, t, of a polymer from the adsorbed substrate is inversely proportional to force, F (i.e., t ∝ F(-γ)), where exponent γ depends on the solvent quality, polymer-substrate attraction, and chain stiffness.

Glassy dynamics of a polymer monolayer on a heterogeneous disordered substrate

We present molecular dynamics simulations of a polymer monolayer on randomly functionalized surfaces that are characterized by different fractions of weakly and strongly attractive sites. We show that the dynamics slow-down upon cooling resembles that of a strong glass-forming liquid. Indeed, the mean-square displacements show an increasingly lasting subdiffusive behaviour before the diffusive regime, with signs of Fickian yet not Gaussian diffusion, and the dynamic correlation functions exhibit a stretched exponential decay. The glassy dynamics of this relatively dilute system is dominated by the interaction of the polymer with the substrate and becomes more marked when the substrate composition is heterogeneous. Accordingly, the estimated glass transition temperature shows a non-monotic dependence on surface composition, in agreement with previous results for the activation energy and with an analysis of the potential energy landscape experienced by the polymer beads. Our findings are relevant to the description of polymer-surface adhesion and friction and the development of polymer nanocomposites with tailored structural and mechanical properties.

Impact of Interaction Strength and Surface Heterogeneity on the Dynamics of Adsorbed Polymers

We present molecular dynamics simulations of bead-and-spring polymer chains on chemically heterogeneous, energetically disordered surfaces at near-monolayer coverages. The surfaces consist of random mixtures of weakly (W) and strongly (S) attractive sites. We explore systematically the effect of surface composition on the diffusive dynamics of the chains. The polymer diffusion coefficients have a near-Arrhenius temperature dependence, with activation energies which have a nonmonotonic dependence on the fraction of S sites. In other words, we see a nonmonotonic dependence of the interfacial polymer dynamics on its affinity with the surface, when the latter involves some heterogeneity. The maximum activation energy belongs to the surface containing 75% S and 25% W sites, which combines near-maximum average polymer-surface interactions with near-maximum spread or disorder in these interactions. Our results have interesting implications for polymer adhesion and friction and structure-property relationships in polymer nanocomposites.

Dynamics of a polymer adsorbed to an attractive homogeneous flat surface

Polymer conformation and statistical sizes change with the surface contact number during the adsorption.