Molecular Dynamics Simulation of Oligomer Liquid and Glass. Relation to Experiment.

Cooling an atomistic model system during molecular dynamics simulation leads to a specific volume profile resembling dilatometry experiments. The characteristic change in slope over a temperature region spanning a few degrees - the glass transition - also occurs in simulations although the width of the (higher) molecular dynamics transition extends over a few tens of degrees, due to the faster simulation cooling rate, ~100 deg per 10-100 ns of simulation time vs. ~100 deg per 24-48 hours using dilatometry.

In the present study we have chosen to study the molecular dynamics Tg using selected molecules for which:

1. PVT behavior of representative small molecules is reproduced accurately.
2. The range of experimental Tgs of high MW polymer spans a broad range.
3. Quality experimental data are available for a series of oligomers of each
polymer.

In this presentation we report simulation Tgs of the three polymers polystyrene, poly(α methyl styrene), and poly(dimethyl siloxane), with a range of oligomers examined for each case. We show that the chosen PCFF+ force field gives excellent agreement of liquid densities, and then discuss how the difference between experimental and simulation Tgs varies across the range of molecular weights. Relation to known cooling rate dependence is discussed.