Facilitation of interfacial dynamics in entangled polymer films

In this article, we use cooling-rate dependent T_g measurements (CR-T_g) to indirectly probe the relaxation dynamics of supported polystyrene thin films of various molecular weights, all chosen to be above the entanglement molecular weight. We show that the dynamics in these films deviate from bulk dynamics below a temperature T^* = T_g + 6 K = 380  K ± 1  K. We show that T^* for films of all thicknesses and molecular weights is the same as the temperature at which the free surface dynamics deviate from the bulk dynamics. The apparent activation barrier of the glass transition in thin films decreases towards that of the free surface as the film thickness decreases. This provides strong evidence that thin film dynamics are facilitated by the enhanced mobility at the free surface. The observation of T^* can help resolve some seemingly contradictory data by suggesting that studies performed at higher temperatures (T > T^*), or which probe shorter relaxation times (τ < τ^* ∼ 1 s) would not observe properties that deviate from bulk values. We also demonstrate that the relaxation dynamics of supported entangled polystyrene films slow down slightly as the molecular weight of polystyrene increases. An eight nanometer film of M_w =2240 kg/mol polystyrene shows a T_g reduction of 27 K at a cooling rate of 1 K/min, while a film of the same thickness made of M_w =45.8 kg/mol polystyrene has a 36 K reduction of T_g compared to the bulk film at the same cooling rate. We hypothesize this is either due to the density of a dynamically “dead” layer near the substrate increasing with molecular weight, or partial anchoring of long chains, which could hinder segmental diffusion near the free surface.