Local Chain Dynamics of a Model Polycarbonate near the
Glass Transition Temperature
C. F. FAN, T. CAGIN, W. SHI and K. A. SMITH
Macromolecular Theory and Simulations, 6, 83-102 (1997)
ABSTRACT
Constant pressure constant temperature molecular dynamics method is
employed to investigate the atomistic scale dynamics of a model Bisphenol A
polycarbonate in the vicinity of its glass transition temperature. First,
the glass transition temperature and the thermal expansion coefficients of
the polymer are predicted by performing simulations at different
temperatures. To explore the significance of different modes of motion,
various types of time correlation functions are utilized in analyzing the
trajectories. In these nanosecond scale simulations, the motion of the
chain segments is found to be highly localized with little reorientation of
the vectors representing these segments. Detailed analysis of trajectories
and the correlation functions of the backbone dihedrals and side methyl
groups indicates that they exhibit numerous conformational transitions. The
activation energies of the conformational transitions obtained from the
simulation are generally larger than the potential barriers for the
rotations of these dihedrals, however, both show the same trend. We also
have estimated the phenylene ring flip activation energy as 12.6 kcal/mol
and the flip frequency as 0.77 MHz at 300 K. These values either fall
within the range determined by various NMR spectroscopy experiments or
slightly out of the range. The study shows that the conformational
transitions between the adjacent dihedrals are strongly correlated. Three
basic cooperative modes are identified from the simulation. They are: a
positive synchronous rotation of two phenylene rings, a negative
synchronous rotation of two phenylene rings, and a carbonate group
rotation. Above the glass transition temperature, the large scale
cooperative motions become much more significant.