Reduction of the glass transition temperature in polymer films: A molecular-dynamics study

Varnik, Fathollah; Baschnagel, J.; Binder, Kurt

doi:10.1103/physreve.65.021507

Cited by 211 publications

(233 citation statements)

References 53 publications

(112 reference statements)

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“…Lower local densities should of course accelerate the dynamics and vice versa. That such oscillation in the density do indeed occur is a result of many computer simulations of confined liquids [46,47,48,49,50,51,52,53,54]. Depending on the investigated liquid and the surface interaction, the amplitude of such oscillations can be very large [47,48,50], but there are also situations where the density is almost constant [55,56,57,58].…”

Section: Introductionmentioning

confidence: 99%

The Relaxation Dynamics of a Supercooled Liquid Confined by Rough Walls

2004

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We present the results of molecular dynamics computer simulations of a binary Lennard-Jones liquid confined between two parallel rough walls. These walls are realized by frozen amorphous configurations of the same liquid and therefore the structural properties of the confined fluid are identical to the ones of the bulk system. Hence this setup allows us to study how the relaxation dynamics is affected by the pure effect of confinement, i.e. if structural changes are completely avoided. We find that the local relaxation dynamics is a strong function of z, the distance of the particles from the wall, and that close to the surface the typical relaxation times are orders of magnitude larger than the ones in the bulk. Because of the cooperative nature of the particle dynamics, the slow dynamics also affects the dynamics of the particles for large values of z. Using various empirical laws, we are able to parameterize accurately the z−dependence of the generalized incoherent intermediate scattering function Fs(q, z, t) and also the spatial dependence of structural relaxation times. These laws allow us to determine various dynamical length scales and we find that their temperature dependence is compatible with an Arrhenius law. Furthermore, we find that at low temperatures time and space dependent correlation function fulfill a generalized factorization property similar to the one predicted by mode-coupling theory for bulk systems. For thin films and/or at sufficiently low temperatures, we find that the relaxation dynamics is influenced by the two walls in a strongly non-linear way in that the slowing down is much stronger than the one expected from the presence of only one confining wall. Finally we study the average dynamics of all liquid particles and find that the data can be described very well by a superposition of two relaxation processes that have clearly separated time scales. Since this is in contrast with the result of our analysis of the local dynamics, we argue that a correct interpretation of experimental data can be rather difficult.

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Section: Introductionmentioning

confidence: 99%

The Relaxation Dynamics of a Supercooled Liquid Confined by Rough Walls

2004

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“…In order to explain the dynamics of polymers under nanoscale confinement, Varnik and co-workers 50 suggested that close to T g the dynamics should be affected by the emergence of cooperatively rearranging regions (CRRs) of many particles together, as introduced by Adam and Gibbs. 51 According to their argument, at some temperature, the maximum cluster size ξ of the CRR's equals the film thickness and the maximum relaxation time is τ cl ∼ ξ v .…”

Section: Introductionmentioning

confidence: 99%

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

2014

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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.

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“…29 An unentangled polymer melt confined between two repulsive walls was studied using MD simulations, and the reduction in T g upon decreasing film thickness was explained by the faster chain dynamics due to the presence of the smooth walls. [30][31][32] Yoshimoto et al 33 employed nonequilibrium MD simulations using a coarse grained polymer model to show that mechanically soft layers are formed near the free surfaces of glassy thin films and that T g also decreased as the film thickness decreased.…”

Section: Introductionmentioning

confidence: 99%

Molecular Dynamics Simulation of Size-Dependent Structural and Thermal Properties of Polymer Nanofibers

2007

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We present the results of molecular dynamics (MD) simulations of amorphous polymer nanofibers to study their size-dependent properties. The fibers consist of chains that mimic the prototypical polymer polyethylene, with chain lengths ranging between 50 and 300 carbons (C50 to C300). These nanofibers have diameters in the range 1.9 to 23.0 nm. We analyzed these nanofibers for signatures of emergent behavior in their structural and thermal properties as a function of diameter. The mass density at the center of all fibers is constant and comparable to that of the bulk polymer. The surface layer thickness ranges from 0.78 to 1.39 nm for all fibers and increases slightly with fiber size. The calculated interfacial excess energy is 0.022 ( 0.002 J/m 2 for all of the nanofibers simulated. The chains at the surface are more confined compared to the chains at the center of the nanofiber; the latter acquire unperturbed dimensions in sufficiently large nanofibers. Consistent with experiments and simulations of amorphous polymer films of nanoscale thickness, the glass transition temperature of these amorphous nanofibers decreases with decreasing fiber diameter, and is independent of molecular weight over the range considered.

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Reduction of the glass transition temperature in polymer films: A molecular-dynamics study

Cited by 211 publications

References 53 publications

The Relaxation Dynamics of a Supercooled Liquid Confined by Rough Walls

The Relaxation Dynamics of a Supercooled Liquid Confined by Rough Walls

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

Molecular Dynamics Simulation of Size-Dependent Structural and Thermal Properties of Polymer Nanofibers

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