Confinement effects on thin polymer films

Abstract

We present the results of four projects investigating the effects of confinement on polymeric systems. The first study dealt with polymer blends that are quenched using a spincoating technique rather than a temperature quench. The mass fraction of two blends was varied to determine the effect of the substrate-blend interface on the thin film phase separation morphology. Quantitative measurements of the morphology on three different substrates revealed significant differences in the phase separation morphology as a result of the different wetting properties of the polymer blend on the substrates.

The second project dealt with the effect of mechanical confinement on the phase separation of polymer blend thin films. We measured the phase separation morphology of polystyrene/poly (methyl methacrylate) (PS/PMMA) blend films of thickness h on a silicon oxide (SiO_x) substrate with a SiO_x capping layer. A novel phase separation morphology was observed for small capping layer thicknesses L as well as a transition from lateral to lamellar morphology as L is increased. A simple model is presented which explains the observed lateral morphology, and the morphology transition, in terms of a balance between the free energy increase associated with forming the interfaces between PS-rich and PMMA-rich domains, and the free energy increase associated with the elastic bending of the SiO_x capping layer. Direct control of the amplitude and period of the deformation is achieved by varying h and L. Reasonable agreement is obtained between the predicted amplitude of the rippling of the film surface and that measured directly using atomic force microscopy.

For temperatures greater than the glass transition temperature T_g, thin freely-standing polymer films are unstable to the formation of holes. In the third project, we have studied the formation and growth of two types of holes: those which form spontaneously when the films are heated above T_g, and those purposely nucleated using a heated scanning tunneling microscope tip. For both types of holes, we observe exponential growth of the hole radius, corresponding to the viscous regime of hole formation, and a decrease in the film viscosity with decreasing film thickness h for h < 250 nm.

In the last project the thermal stability of freely-standing films was enhanced by symmetrically confining the films between thin layers of silicon oxide to form SiO_x/PS/SiO_x trilayer films. Aggressive annealing of the films produced a novel morphology consisting of long, parallel domains with a well-defined periodicity. A simple model is presented which describes the scaling behavior of the morphology. We discuss the direct control of the morphology through manipulation of the individual film thicknesses and the long-range Van der Waals or dispersion interactions.