Residual stresses in thin polymer films

Polymeric glass forming liquid films have increasingly become materials of great interest to the evolution of a wide array of technologies ranging from membranes and adhesives to coatings and microelectronics. For many of these prospective applications, a homogeneous defect-free film topology is essential. As such, a fundamental understanding of both the mechanical properties and thermodynamic stability of these films have become important topics of research. A majority of the studies conducted on thin polymer films have shown a conclusive departure of a variety of physical attributes from normal bulk behavior, mainly for film thicknesses below 100 nm. From a broader perspective, research work on thin polymer films has effectively become a study on the relation between 2 dimensional (2D) nanoconfinement and macromolecular mobility. 2D nanoconfinement does not, however, uniquely define polymeric chain behavior. Considering the well documented effects of thermal and mechanical history on the viscoelastic response of polymer chains in the bulk, the same is considered true for thin polymer films. In particular, recently published studies highlight the effects of residual stresses on thin film stability through dewetting. Residual stresses develop during preparation of thin polymer films - the magnitudes of these stresses depend on the method of preparation and subsequent heat treatments. For the case of thin polymer films, it has been postulated by McKenna that the magnitude of these residual stresses could possibly surpass the yield stress. A survey of published literature shows that studies on the measure of residual stress magnitudes in thin polymer films are few. The direct measurement of residual stress magnitudes from the curvature of metal and ceramic polymer coated cantilevers through the bending beam method is however well documented in published literature. The bending beam method has also been used to determine the thermal stresses for thin polystyrene films coating silicon beams. The bending beam method thus presents an opportunity to directly measure the magnitude of residual stresses in thin polymer films. The objective of this thesis is to measure the magnitude of residual stress in supported thin polymer films prepared through the spin coating process.