Nanomechanical properties and morphology of thick polyurethane films under contact pressure and stretching

The surface nanomechanical properties and morphology of ∼500-μm-thick polyurethane films subjected to normal loads in the range of 3–35 nN and longitudinal strains up to 150% were investigated with an atomic force microscope. Nanoindentation experiments performed at very low loads and small penetration depths demonstrated that the elastic behavior of the polyurethane surface is remarkably different from that of the bulk material. The distinctly different mechanical response to applied pressure between surface and near-surface (bulk) regions of the polymer is attributed to packing of the polymer chains at the contact region. The polyurethane copolymer consists of two immiscible hard and soft segments. It is suggested that upon stretching, randomly oriented hard segments migrate to the surface, leading to an increase of the hard-to-soft segment concentration and a decrease of the surface roughness. A general description of the polyurethane surface and bulk behavior under stretching and compression is presented in the context of the obtained results.