LOCAL CHAIN DYNAMICS OF A MODEL POLYCARBONATE NEAR GLASS-TRANSITION TEMPERATURE - A MOLECULAR-DYNAMICS SIMULATION

Constant pressure constant temperature molecular dynamics method is em ployed to investigate the atomistic scale dynamics of a model Bispheno l A polycarbonate in the vicinity of its glass transition temperature. First, the glass transition temperature and the thermal expansion coe fficients of the polymer are predicted by performing simulations at di fferent temperatures. To explore the significance of different modes o f 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 lit tle reorientation of the vectors representing these segments. Detailed analysis of trajectories and the correlation functions of the backbon e dihedrals and side methyl groups indicates that they exhibit numerou s conformational transitions. The activation energies of the conformat ional transitions obtained from the simulation are generally larger th an the potential barriers for the rotations of these dihedrals, howeve r, 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 0.7 7 MHz at 300 K. These values either fall within the range determined b y various NMR spectroscopy experiments or slightly out of the range. T he study shows that the conformational transitions between the adjacen t dihedrals are strongly correlated. Three basic cooperative modes are identified from the simulation. They are: a positive synchronous rota tion of two phenylene rings, a negative synchronous rotation of two ph enylene rings, and a carbonate group rotation. Above the glass transit ion temperature, the large scale cooperative motions become much more significant.