Glass Transition Dynamics and Fragility of Ultrathin Miscible Polymer Blend Films

Abstract

We use cooling-rate-dependent T g measurements (CR-T g) 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 (T g) that can be systematically shifted over 100 K simply by varying the composition of the blend. This study shows that the T g of these blends decreases below the bulk T g as the film thickness is decreased. Additionally, the degree of change in T g strongly depends on the cooling rate of the experiment. We show that the T g 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 T g 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 T g. The presence of this temperature (T*) can help explain why some experiments fail to see T g 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.

Long-range correlated dynamics in ultra-thin molecular glass films

“…It is important to note that all of these temperatures are well below bulk T g . The low apparent activation energy of ultra-thin films is consistent with previous studies of dynamics on polymeric thin films [14,15,29].…”

“…Direct measurements of properties as a function of film thickness are required for determining the correlation length for the dynamics. While to our knowledge there are no such prior studies in thin films of other organic glasses, observations in polymeric glasses [29] show a similar non-linear transition in E a as a function of film thickness, with the midpoint of transition in the 20-30 nm film thickness region. Earlier studies by Ellison and Torkelson also suggest correlated dynamics in the top and bottom layers of a polymeric film as the film thickness is reduced below 20 nm [12].…”

“…While a direct measure of absolute viscosity of thin films can not be obtained due to potential gradient in the dynamics induced by free interface, by relating the dewetting times with cooling rate-dependent T g (CR-T g ) measurements [14,29], we are able to measure the "effective viscosity" of ultra-thin films as function of film thickness and temperature. In the absence of gradients in the dynamics, the effective viscosity equals the film viscosity.…”

“…4(c) (details in SI). CR-T g measurements were used to extend the dynamical range of the bulk measurements to temperatures below bulk T g [14,29,36]. CR-T g measurements were performed on films with a thickness range of 20 nm< h <100 nm as shown in Fig 4(b) (details in SI S8 and S9).…”

Interfacial and topological effects on the glass transition in free-standing polystyrene films

“…This seems to be true for other polymer systems as well. 71 The data from Reference 70 obtained with (high) 120 K/min cooling rate for PS of M w ∼ 46 000 are normalized to the corresponding bulk T g , measured with the same cooling rate and are replotted in Fig. 7(a) for comparison.…”