Equilibrium Pathway of Ultrathin Polymer Films as Revealed by Their Surface Dynamics

Abstract

The majority of thin polymer films are prepared by spin-coating. In spin-coating, a drop of polymer solution is spread into a film which then dries into a solid in less than a minute. Abruptness of this process is believed to leave the polymer chains insufficient time to equilibrate, causing them to be kinetically trapped in an intermediate, transitory state. Indeed, several out-of-equilibrium attributes have been unequivocally identified, including residual stress, reduced chain entanglement and on-going polymer adsorption to the substrate surface. Upon annealing above the glass transition temperature, T _g, these attributes evolve toward equilibrium. Another important, though less discussed out-of-equilibrium attribute is the smoother-than-equilibrium surface structure of as-cast spin-coated polymer films. Because of this, these films usually roughen upon heating. Importantly, the roughening process, especially for nanometer films, often results in the formation of deep holes and thereby ultimate rupture of the films. A question then naturally arises as to whether evolutions in the residual stress, chain entanglement and polymer adsorption may interfere with the roughening process, whereby modify the film stability. In recent years, our group has developed methods to monitor and analyze the surface roughening process. By applying these methods, important physical properties such as the effective viscosity and shear modulus of the films have been deduced. In this chapter, we shall briefly review these methods and the respectively model analyses. On the basis of these discussions, we then deliberate whether or not evolutions in the chain entanglement density, residual stress and adsorption phenomenon in the films may have influenced the roughening process measured, and thereby project their impacts on the film stability.