Confinement-Induced Stiffening of Thin Elastomer Films: Linear and Nonlinear Mechanics vs Local Dynamics

Abstract

Constant-pressure molecular-dynamics simulations have been carried out of a bead−rod model polymer confined between two attractive crystalline substrates. Three different substrate−substrate separations (i.e., film thicknesses) were used and two different polymer−substrate interaction strengths. The density profiles show a monomer layering close to the polymer− substrate interface. A higher density was found in this region compared to the middle bulklike layers of the films. The dependence of the film-averaged density on temperature and thickness was measured for all polymer films. Decreasing the film thickness leads to an increase of this density and of the film-averaged glass-transition temperature. Layer-resolved analysis of the segmental dynamics of the thickest films shows a gradient of the mobility upon approach of the polymer−substrate interface, while the middle-layer dynamics exhibits bulklike behavior. With decreasing film thickness, these gradients become overlapping. All polymer films were deformed uniaxially normal to the substrates beyond their linear viscoelastic regime; their elastic moduli but also their secant moduli at larger strain amplitudes were extracted. In the linear regime, the stiffness was found to increase with decreasing film thickness; this correlates well with the layer-resolved segmental dynamical behavior. A strong drop of the stiffness was observed upon increasing the deformation amplitude; this drop was more pronounced for stiffer films. It is shown that the drop in stiffness can be qualitatively explained by the drop of the relaxation times as well as by the increased heterogeneity of the dynamics in different film layers upon deformation. The thickness dependencies of the structural, dynamical, and mechanical properties become more pronounced with stronger adsorption to the substrates.

A coarse-grained molecular dynamics study of segmental structure and mobility in capped crosslinked copolymer films

“…33 A similar behavior has been observed in films with structure walls and high polymer density, 12 whereas the opposite has been reported in films of lower density. 34 Binder et al 33 performed molecular-dynamics and Monte Carlo simulations of short polymer chains confined between structureless walls and reported an acceleration of the overall segmental dynamics upon increasing the degree of confinement. A higher segmental mobility in the interfacial layers than in the middle layers of the films was observed as well.…”

Heterogeneities in Polymer Structural and Dynamic Properties in Graphene and Graphene Oxide Nanocomposites: Molecular Dynamics Simulations

“…Beside translational dynamics, one can assess the mobility of polymer chains by quantifying the orientational dynamics of them . To achieve this, one has to define a specific vector, representing the orientation of a segment or the whole polymer chain, and study the orientational changes of such vectors as a function of time.…”

“…Having calculated the autocorrelation functions for different layers, one could estimate the local characteristic relaxation times of polymer chains by fitting Kohlrausch−Williams−Watts (KWW) stretched‐exponential function where the prefactor A denotes the initial, very fast relaxation and is smaller than unity, τ is the characteristic relaxation time, and β indicates the deviation from single exponential nature of the relaxation process. Note that when β → 0, strong inhomogeneities in the dynamics exist, while β → 1 shows the homogeneous distribution of the segmental relaxation …”

“…Note that when β → 0, strong inhomoge neities in the dynamics exist, while β → 1 shows the homoge neous distribution of the segmental relaxation. [30] By using these analytical techniques, the authors presented the results of study on the equilibrium and dynamic properties of the polymers in the vicinity of the nanofiller as well as bulk region in the next sections. Note that almost all of the results presented below were the average of at least two fully independent simulation runs and the error bars in figures showed the standard error of the data.…”

Concurrent two-scale model for the viscoelastic behavior of elastomers filled with hard nanoparticles

“…Among the numerical approaches, both molecular dynamics (MD) and Monte Carlo (MC) simulation techniques have been performed on the smallest scales. For example in [14,15], MD simulations have been performed of thin films of noncross-linked polymer melts confined between substrates. In both studies, the simulations revealed a decrease in mobility close to the substrate and an increase in the thickness-averaged glass transition temperature upon decreasing the substrate spacing; these results corroborate the existence of glassy layers close to the substrate.…”