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Glassy dynamics of a polymer monolayer on a heterogeneous disordered substrate
Full text linkWe 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
From cage-jump motion to macroscopic diffusion in supercooled liquids
Full text linkThe evaluation of the long term stability of a material requires the estimation of its long-time dynamics. For
amorphous materials such as structural glasses, it has proven difficult to predict the long-time dynamics
starting from static measurements. Here we consider how long one needs to monitor the dynamics of a
structural glass to predict its long-time features. We present a detailed characterization of the statistical
features of the single-particle intermittent motion, and show that single-particle jumps are the
irreversible events leading to the relaxation of the system. This allows us to evaluate the diffusion
constant on the time-scale of the jump duration, which is small and temperature independent, i.e. well
before the system enters the diffusive regime. The prediction is obtained by analyzing the particle
trajectories via a parameter-free algorithm
Spatial correlations of elementary relaxation events in glass-forming liquids
Full text linkThe dynamical facilitation scenario, by which localized relaxation events promote nearby relaxation events in
an avalanche process, has been suggested as the key mechanism connecting the microscopic and the
macroscopic dynamics of structural glasses. Here we investigate the statistical features of this process
via numerical simulations of a model structural glass. First we show that the relaxation dynamics of the
system occurs through particle jumps that are irreversible, and that cannot be decomposed in smaller
irreversible events. Then we show that each jump does actually trigger an avalanche. The characteristics
of this avalanche change upon cooling, suggesting that the relaxation dynamics crossovers from a noise
dominated regime, where jumps do not trigger other relaxation events, to a regime dominated by the
facilitation process, where a jump triggers more relaxation event
Particle jumps in structural glasses
Full text linkParticles in structural glasses rattle around temporary equilibrium positions, that seldom change through
a process which is much faster than the relaxation time, known as particle jump. Since the relaxation of
the system is due to the accumulation of many such jumps, it could be possible to connect the single
particle short time motion to the macroscopic relaxation by understanding the features of the jump
dynamics. Here we review recent results in this research direction, clarifying the features of particle
jumps that have been understood and those that are still under investigation, and examining the role of
particle jumps in different theories of the glass transition
Absence of 'fragility' and mechanical response of jammed granular materials
No full textWe perform molecular dynamic simulations of
frictional non-thermal particles driven by an externally
applied shear stress. After the system jams following a transient
flow, we probe its mechanical response in order to clarify
whether the resulting solid is ‘fragile’.We find the system
to respond elastically and isotropically to small perturbations
of the shear stress, suggesting absence of fragility. These
results are interpreted in terms of the energy landscape of
dissipative systems. For the same values of the control parameters,
we check the behaviour of the system during a stress
cycle. Increasing the maximum stress value, a crossover from
a visco-elastic to a plastic regime is observed
Dynamic phase coexistence in glass-forming liquids
Full text linkOne of the most controversial hypotheses for explaining the heterogeneous dynamics of glasses
postulates the temporary coexistence of two phases characterized by a high and by a low diffusivity.
In this scenario, two phases with different diffusivities coexist for a time of the order of the
relaxation time and mix afterwards. Unfortunately, it is difficult to measure the single-particle
diffusivities to test this hypothesis. Indeed, although the non-Gaussian shape of the van-Hove
distribution suggests the transient existence of a diffusivity distribution, it is not possible to infer
from this quantity whether two or more dynamical phases coexist. Here we provide the first direct
observation of the dynamical coexistence of two phases with different diffusivities, by showing
that in the deeply supercooled regime the distribution of the single-particle diffusivities acquires a
transient bimodal shape. We relate this distribution to the heterogeneity of the dynamics and to
the breakdown of the Stokes-Einstein relation, and we show that the coexistence of two dynamical
phases occurs up to a timescale growing faster than the relaxation time on cooling, for some of the
considered models. Our work offers a basis for rationalizing the dynamics of supercooled liquids and
for relating their structural and dynamical properties
Influence of wall heterogeneity on nanoscopically confined polymers
Full text linkWe 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
Minimal and versatile description of diffusion and swelling in polymer-solvent systems: Modeling and experimental validation
No full textThe interplay between solvent diffusion and swelling of the absorbing solid leads to a variety of possible behaviors. In the case of polymer matrices, sharp penetration and swelling fronts are commonly found, with their dynamics depending on the specific features of the polymer-solvent system. Here, at the continuum mechanical level, we introduce a model to predict both concentration profile and swelling for whatever motion of the penetration front, in the one-dimensional case. A distinctive feature of our approach is that a constitutive equation for the thermodynamics of the dry side is not required. The model is validated against paradigmatic experiments available in literature, as well as through our own data (including optical microscopy) on a water–polyvinyl alcohol system. In spite of using only a small number of parameters, the model succeeds in predicting all the tested observables of the diffusion/swelling process in those polymeric systems
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