Glass Transition Dynamics and Fragility of Ultrathin Miscible Polymer Blend Films

Abstract

We use cooling-rate-dependent Tg measurements (CR-Tg) to indirectly probe the relaxation dynamics and fragility of thin films of polystyrene (PS)/poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) blends. Thin films of this miscible blend have a single glass transition temperature (Tg) that can be systematically shifted over 100 K simply by varying the composition of the blend. This study shows that the Tg of these blends decreases below the bulk Tg as the film thickness is decreased. Additionally, the degree of change in Tg strongly depends on the cooling rate of the experiment. We show that the Tg of 16 nm films of a 50% PS blend is 15 K lower than that of bulk at a cooling rate of 1 K/min but decreases only 4 K at a cooling rate of 120 K/min. By analyzing the cooling rate dependence of the Tg for various thicknesses of the 50% PS blend, we demonstrate that the fragility of these blends decreases with film thickness. This behavior is similar to what is observed in ultrathin films of polystyrene, which suggests that the deviations from bulk dynamics in PS/PPO blends are due to enhanced mobility near the free surface. Similar to pure PS, if extrapolated to higher temperatures, the dynamics of thin films intersect the bulk dynamics at a temperature a few degrees above bulk Tg. The presence of this temperature (T*) can help explain why some experiments fail to see Tg depression in thin films of these blends. Lastly, we show that while the fragility of the bulk blend changes due to differences in the fragility of the homopolymers, ultrathin films (h = 16 nm) have identical average dynamics and fragilities regardless of the blend composition at the same temperature relative to Tg. This result implies that enhanced mobility near the free surface affects the dynamics of these blend thin films similarly.