Unusual thickness relaxation of spin-coated polystyrene ultrathin films in the glassy state

Relaxation in the thickness of ultrathin polystyrene films (thickness < 7 nm) on two different solid substrates is investigated at various temperatures by X-ray reflectivity. A thickness relaxation (i.e., ultraslow increase in thickness) is found even at room temperature, at which point any relaxation would hardly be expected because it is lower than the bulk glass transition temperature by at least 70 degrees C. At room temperature, the thickness relaxation depends on the annealing time and annealing temperature even upon annealing in the rubbery state. The relaxation time of the films formed on a Si-OH substrate is found to be larger than those deposited on a SiO2 substrate, and decreases with increasing temperature. Whereas, the slow increase in thickness also observed at temperatures above T-g, indicates that some of the molecular chains were not in an equilibrium state, which might be due to a persistent, highly strained interfacial layer. Almost no thickness relaxation is observed at temperatures close to the glass transition point T-g, which would suggest that the T-g of ultrathin polystyrene films is determined by the competition between slow relaxation in the interfacial layer and fast relaxation originated in the free surface region. The results demonstrate that the relaxation and glass transition behavior of confined thin films are influenced by residual stress in the substrate interface region.