Continuum force microscopy study of the elastic modulus, hardness and friction of polyethylene and polypropylene surfaces

The atomic force microscope (AFM) has been modified to a continuum force microscope (CFM) by using a tip of a large radius of curvature (1000 nm), in an attempt to reduce the pressure in the contact region. The elastic modulus, hardness, and friction of the surfaces of low density polyethylene, high density polyethylene, isotactic polypropylene, and atactic polypropylene have been quantitatively measured at low loads (10(-8)-10(-6) N) with the CFM. The effect of pressure applied by the tip has been observed in the measurements, resulting in an increase in the elastic modulus and the shear strength of the polymer surface with increasing pressure. The effect of pressure increases with decreasing density of the polymer. The higher values of elastic modulus and hardness of the polymer surface correlate well with the higher crystallinity of the polymers. Frictional properties of the polymers show characteristics that can be explained by the JKR model; the relative frictional behavior of the polyolefin surfaces however is controlled by the deformation component of the friction, i.e., the yield strength, the elastic modulus and the pressure dependence of the shear strength.