Measuring the glass-transition temperature in thin atactic polystyrene films: molecular dynamics approach

摘要

We have performed molecular-dynamics simulations to explore the influence of confinement on the glass-transition temperature (T_g) for supported atactic-polystyrene thin films of different thicknesses, (1 ÷ 10)nm, and different strengths of attraction to the substrate, (0.1 ÷ 3.0) kcal/mol. For each film we define three different (substrate, middle and surface) layers using the simulated density. We calculate the T_g by measuring the density variation with a temperature both for each film and corresponding layers. We find that for the whole film the T_g remains almost constant for films down to 2 nm where the middle layer vanishes, and drops down for thinner films. We claim that the redistribution of mass in the three film layers may explain the change with film thickness of the average T_g, if the latter is determined from linear fits of the average glass and melt densities.