The glass transition temperature of thin films: A molecular dynamics study for a bead-spring model.

Abstract

Molecular dynamics simulations were carried out on free-standing liquid films of different thicknesses h using a bead-spring model of 10 beads per chain. The glass transition temperatures, Tg, of the various films were determined from plots of the internal energy versus temperature. We used these simulations to test the validity of our earlier conjecture that the glass transition of a confined liquid could be approximated by pre-averaging over the non-uniform density profile of the film. Using the density profiles from our simulations, we computed the average density of the free-standing films as a function of temperature. In all our film simulations we found, within the error of the simulation, that Tg of the film occurred at the same density (or packing fraction) as the bulk system at the bulk glass transition temperature TgB. By equating these densities at their respective glass transition temperatures, as suggested by the simulations, we deduced that Tg/TgB is proportional to h0/h. This is consistent with previous simulations and experimental data. Moreover, the parameter h0 is determinable in our model from the density profile of the films.