Glass Transition and Structure of Organic Polymers from All-Atom Molecular Simulations

Molecular dynamics
simulations of polymer samples with all-atom
resolution provide important insight into the relationship between
the atom-level structure and macroscopic properties of polymers. The
computational setup of molecular simulations in such a case deserves
to be validated, paying attention not to overlook various aspects
or inferior settings or postprocessing analyses that have the potential
to distort the simulation outcome or at least to make the simulated
ensemble too incompatible with its experimental counterparts, such
as their polydispersity, initial conformation, or thermal history
of the samples. The accuracy of the simulation results obtained from
existing all-atom nonpolarizable force fields for three selected polymers
is independently benchmarked with respect to the polymer densities
and glass transition temperatures. Errors of structural or thermodynamic
properties arising due to insufficient sample equilibration or inadequate
simulation setup are quantified. Special attention is paid to the
selection of reference literature data for polymer systems that are
well characterized and as similar as possible to the computationally
treated samples. Very different performances of predictions of the
glass transition temperatures occur among the individual target polymers,
with both their sampling uncertainty and errors from reference experimental
data ranging from acceptable below 10 K to highly unsatisfactory 100
K in individual cases. The accuracy of the predicted glass transition
temperature is found to be higher for polymers exhibiting faster internal
dynamics and distinct trend shifts between the glass and the liquid.
On the contrary, when the glass transition occurs gradually over a
wider temperature range, it becomes very difficult to capture the
adequate transition temperature within molecular simulations, regardless
of the evaluation protocol used. Bulk density proves to be the most
reliable observable for subsequent trend shift analyses, which typically
yield similar results regardless of performing equilibrium or nonequilibrium
simulations and adopting the bilinear or hyperbolic regression analyses.