Molecular Modeling of Polycarbonate .1. Force-Field, Static Structure,
and Mechanical Properties
FAN-CF, CAGIN-T, CHEN-ZM and SMITH-KA
Macromolecules, 27, 2383-2391(1994)
ABSTRACT
Molecular simulations of Bisphenol A polycarbonate were performed using
a modified version of the Dreiding force field. In general, the simplicity
of this generic force field was maintained. However, a few parameters were
optimized by using ab initio calculations in order to generate better
backbone torsional potentials. The validity of this modified force field
was tested on a model compound similar to polycarbonate,
4,4'-isopropylidenediphenylbis(phenyl carbonate). The crystallographic data
obtained from simulations of the model compound agreed well with the
experimental data. The modified force field was then used in simulations
of the glassy polymer. Model amorphous structures of Bisphenol A
polycarbonate were built and optimized using periodic boundary conditions.
The structure factor, S(k), was calculated from pair distribution functions
of the model structures. The peak positions in calculated S(k) compare well
with those obtained experimentally. The effects of the initial density on
chain packing were also studied. Its impacts on the final density and
energy are described. A general approach for calculating the stiffness
matrix of any shape unit cell (triclinic system) is presented. A yield
phenomenon for the model system was observed at about 10% strain. This
value is comparable with experimental results (6-8%). The yield stress
(0.25 GPa), however, was higher than experimental values, as was the
Young's modulus at small strain. These results could be partially explained
by the fact that the calculated mechanical properties represent an ideal
structure and thus provide the upper limit values for the polymer.