Surface versus Volume Properties on the Nanoscale: Elastomeric Polypropylene

The difference between the mechanical properties of a material at the surface and in the bulk is an open issue in polymer science. We studied the mechanical surface properties of polypropylene using atomic force microscopy in peak-force tapping mode. The bulk properties were obtained from layer-by-layer measurements of elasticity, adhesion, and dissipation, with the successive layers removed via wet-chemical ablation. The original sample surface revealed nearly similar mechanical properties for the amorphous and crystalline regions due to a thin (∼22 nm) amorphous top layer. However, in the bulk material, the elastic modulus of crystalline regions was greater than that of amorphous regions. We observed nanoscale crystalline inhomogeneities caused by phase separation that can affect the mechanical stability of polypropylene on the macroscopic scale. The combination of force–volume analysis together with successive ablation of the sample layers form the basis of quantitative nanomechanical tomography.