Confined Flow in Polymer Films at Interfaces

Measurements of nanomechanical properties of unannealed homopolymers show that the spin coating process in the presence of an interactive interface can be the source for molecular constraints in the interfacial boundary regime of elastomeric films. The surface mechanical response of poly(ethylene−propylene) (PEP) films of various thicknesses spin cast on hydrogen passivated silicon surfaces was studied using scanning force microscopy (SFM). At least two distinct rheological regimes can be observed in thicker films: high-entropy viscous plug flow at low load and low-entropy shear sliding at high load. The transition point from high-entropy to low-entropy shear has been found to shift to lower loads for decreasing film thickness. The transition point was found to disappear, and only shear sliding was observed at a critical film thickness, which was comparable to the radius of gyration of the sample polymer. These results were interpreted in a model wherein a highly distorted anisotropic layer induced by high-speed spinning and solvent evaporation is formed near the silicon interface. Pinning interactions prevent the layer from annealing, and a two-fluid-type response occurs. The surface free energy, determined by SFM lateral force−Dupré adhesion measurements, was found to be film thickness independent and in good agreement with previous macroscopic measurements.