4 research outputs found
Integration of Atomistic Simulation with Experiment Using Time−Temperature Superposition for a Cross-Linked Epoxy Network
Quantitative comparison of atomistic
simulations with experiment for glass-forming materials is made difficult by
the vast mismatch between computationally and experimentally accessible timescales.
Recently, we presented results for an epoxy network showing that the
computation of specific volume vs. temperature as a function of cooling rate in
conjunction with the time–temperature superposition principle (TTSP) enables
direct quantitative comparison of simulation with experiment. Here, we
follow-up and present results for the translational dynamics of the same
material over a temperature range from the rubbery to the glassy state. Using
TTSP, we obtain results for translational dynamics out to 109 s in
TTSP reduced time – a macroscopic timescale. Further, we show that the mean
squared displacement (MSD) trends of the network atoms can be collapsed onto a
master curve at a reference temperature. The computational master curve is
compared with the experimental master curve of the creep compliance for the
same network using literature data. We find that the temporal features of the
two data sets can be quantitatively compared providing an integrated view
relating molecular level dynamics to the macroscopic thermophysical
measurement. The time-shift factors needed for the superposition also show
excellent agreement with experiment further establishing the veracity of the
approach
Quantitative Comparison of Atomistic Simulations with Experiment for a Cross-Linked Epoxy: A Specific Volume–Cooling Rate Analysis
Cross-linked
epoxy thermosets, like all glass-forming viscoelastic
materials, show both a temperature and rate dependence in their thermomechanical
properties. However, accounting for rate effects on these properties
using molecular dynamics (MD) simulations and making quantitative
comparison with experimental measurements has proven to be a difficult
task due to the extreme mismatch between experimental and computationally
accessible cooling rates. For this reason, the effect of cooling rate
on material properties in glass-forming systems (including epoxy networks)
has been mostly ignored in computational studies, making quantitative
comparison with experimental data nebulous. In this work, we investigate
a strategy for modeling rate effects in an epoxy network based on
an approach that uses theoretically informed simulation and analysis
protocols in combination with material specific time–temperature
superposition (TTSP) data obtained from experimental measurements.
To illustrate and test the strategy, we build and study an atomistic
model of a cross-linked epoxy network. Molecular dynamics simulations
are used to model the specific volume as a function of temperature
across the glass transition from the rubbery to the glassy state using
a total of five computationally accessible cooling rates. From the
trends thus identified, we pinpoint the temperatures at which the
models show rubbery and glassy behavior and use this information to
calculate the values of the glass transition temperature (<i>T</i><sub>g</sub>) for each of the different cooling rates.
Comparison with experimental data obtained from the literature (for
the identical epoxy network) shows that our computations successfully
predict the trends in specific volume in the rubbery and the glassy
regions within 0.5%. We then compare the <i>T</i><sub>g</sub> values obtained from the data analysis with those calculated using
the TTSP data obtained from the literature. Excellent agreement is
found, and the <i>T</i><sub>g</sub> values from the two
different methods are within 1.5% for all cooling rates. While our
MD simulations do not replicate the experimental cooling rates, the
agreement between these two sets of <i>T</i><sub>g</sub> values quantitatively relates the computational and experimental
data sets. This agreement indicates that atomistic simulations can
reliably capture the molecular mechanisms underlying viscoelasticity
in this cross-linked epoxy (even when cooling rates differ by orders
of magnitude) and that volume–rate analysis in conjunction
with TTSP is a reliable method to computationally characterize certain
classes of glass-forming materials. We believe that the general paradigm
and protocols developed in this work should in principle be extensible
as an efficacious means to integrate theory, computation, and experiment
for the analysis of amorphous macromolecular materials