Influence of Confinement on the Fragility of Antiplasticized and Pure Polymer Films

We investigate by molecular dynamics simulation how thin film confinement modifies the fragility of a model glass-forming liquid characterized previously in the bulk. Film confinement is found to reduce the relative fragility of the polymer fluid, leading to effects similar to simulations of the addition of an antiplasticizer additive. A reduction of fragility is not observed for the antiplasticized polymer film. These effects are interpreted in terms of variation in the string (cooperatively moving segments) concentration with film confinement and the addition of antiplasticizing additives.

Figure 1

Relaxation times plotted against $T$. The lines are fits to Eq. (1). The red circles (○) are for the AP system, the black diamonds (⋄) are for the PP system, solid lines are used for the bulk configurations, and dashed lines for the thin films. The inset shows fits to Eq. (2) for the PP and AP film systems. All figures available online in color.

Figure 2

Average string length as a function of $T/T_c$ for the AP (○) and PP (⋄) in the bulk (solid lines) and thin films (dashed lines). The inset shows the mole fraction of antiplasticizer particles in the cooperative strings ($L_s\ge 2$) in the AP bulk (solid) and thin film (dashed) systems at various $T$’s.

Figure 3

String density within PP and AP polymer films at $T=1.23T_c$ (solid lines) and $T=1.51T_c$ (dashed lines). The inset shows the local composition (mole fraction polymer) of the AP films at the same two $T$’s.