COMMENTS ON THE USE OF THE FORCE MODE IN ATOMIC-FORCE MICROSCOPY FOR POLYMER-FILMS

Atomic force microscopy (A.F.M.) was first described as a powerful tec hnique for studying insulating, hard surfaces. Since then, it has also been considered as an appropriate technique for investigating, at a s ubmicromic scale, elastic and viscoelastic properties of soft material s as polymer films. An attempt is made to show how macroscopic models can be fruitfully employed in order to interpret the force curves obta ined in AFM on polymer films. Through an analysis of the slope variati on and the way the instability occurs at the tip-sample contact, it is shown that a macroscopic approach is a useful way to explain most of the features of the force curves. Furthermore the importance is underl ined of the initial conditions. It is shown that for polymer samples w hich have a stiffness within the range of the probe one, drastic chang es of the force curve shape can occur when the initial conditions vary . Finally, this approach should allow one to clarify the conditions fo r which the macroscopic approach fails.