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

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, T_g, 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 T_g of the film occurred at the same density (or packing fraction) as the bulk system at the bulk glass transition temperature T_g^B. By equating these densities at their respective glass transition temperatures, as suggested by the simulations, we deduced that T_g/T_g^B 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.