Most polymers solidify into a glassy amorphous state, accompanied by a rapid increase in the viscosity when cooled below the glass transition temperature (T(g)). There is an ongoing debate on whether the T(g) changes with decreasing polymer film thickness and on the origin of the changes. We measured the viscosity of unentangled, short-chain polystyrene films on silicon at different temperatures and found that the transition temperature for the viscosity decreases with decreasing film thickness, consistent with the changes in the T(g) of the films observed before. By applying the hydrodynamic equations to the films, the data can be explained by the presence of a highly mobile surface liquid layer, which follows an Arrhenius dynamic and is able to dominate the flow in the thinnest films studied.

Qualitatively different thickness dependences have been observed in the glass transition temperature, T g , of polystyrene (PS) films supported by hydrogen-passivated silicon (H-Si). It has been suggested that upon annealing at high temperatures in air, the polymer/substrate interface of these films (i.e., PS/Si), though buried underneath the PS layer, might be oxidized, rendering the films a different polymer/ substrate interface (i.e., PS/SiO x -Si), which may account for the different thickness dependences of the T g observed. In this experiment, we examine if the buried substrate interface of PS/H-Si films can indeed be oxidized by annealing the films at 150 °C in air. Our result shows that a residual film does form on top of the H-Si surface, but it is a bound layer of PS. X-ray photoelectron spectroscopic (XPS) analyses and independence of the residual film on the initial PS thickness evidence that the H-Si substrate buried underneath a PS film is not oxidized by annealing. We discuss a possible explanation to how the different thickness dependences may be observed in the T g of these films.

No abstract

There has been continuing effort to understand the cause for the thickness dependence observed in the glass transition dynamics of polymer films. In a previous experiment, we showed that a two-layer model, assuming the films to contain a high-mobility surface layer residing on top of a bulklike inner layer, can explain the thickness dependence found in the viscosity of unentangled polystyrene films. Here, we examine the validity of this model in polystyrene films that are entangled. Unlike the unentangled films, the entangled ones are initially out-of-equilibrium, exhibiting a plateau modulus ∼1/10 times the bulk value. Upon annealing, the viscosity typically grows with time and eventually saturates. For the films with thickness above 20 nm, the saturated viscosity is the same as the bulk and takes ∼5–10 reptation times to reach. We find that the saturated viscosity is fully explainable by the two-layer model. A straightforward interpretation would imply that the surface mobile layer exists at equilibrium and modifies the dynamics of unentangled and entangled polymer films in a similar way.

This paper describes the concept of a new, efficient high-temperature oxygen sorption process based on a perovskite-type ceramic sorbent for oxygen removal and air separation. The new sorption process takes advantage of the unique properties of certain perovskite-type ceramics that can adsorb a large quantity of oxygen, but not other gases, at high temperatures (300-800 °C). The essential principle of this new sorption process is based on the changing of oxygen nonstoichiometry of the perovskite-type ceramics with temperature and oxygen partial pressure. Two highly oxygen-deficient perovskite oxides, La 0.1 Sr 0.9 Co 0.5 Fe 0.5 O 3-δ , and La 0.1 Sr 0.9 Co 0.9 Fe 0.1 O 3-δ , were examined as candidate materials for the oxygen sorption process. Oxygen sorption equilibrium properties were studied by thermogravimetric analysis (TGA) at 500 and 600 °C and oxygen pressures ranging from 1.3 × 10 -4 to 1 atm. The oxygen removal performance at 500 and 600 °C was also investigated in a fixed-bed adsorption column. An infinitely large selectivity, a relatively high oxygen sorption capacity, and fast sorption kinetics are the main characteristics of this new type of sorbent. The process can be used to remove trace oxygen from other gases or to produce high-purity nitrogen and ultrapure oxygen from air.

Spin-coating is a common method of making thin polymer films. Recent experiments show that polymer films produced by this method are highly nonequilibrated. By monitoring the temporal evolution of the surface structure of freshly spin-cast polystyrene films on Si with molecular weights, 2.3 ≤ M w ≤ 393 kg/mol, we find that the relaxations can be fully accounted for by thermal excitations of surface capillary waves on the film surface. Modeling of the data based on this relaxation scheme leads to excellent agreement between the viscosity of the films and that of the bulk polymers. Our results provide compelling evidence that thickness uniformity is the major cause of the nonequilibration of the films.

No abstract

No abstract