MEASURING THE ELASTIC PROPERTIES OF THIN POLYMER-FILMS WITH THE ATOMIC-FORCE MICROSCOPE

The elastic properties of thin gelatin films were investigated with th e atomic force microscope (AFM). The degree of swelling and thus the s oftness of the gelatin can be tuned by immersing it in mixtures of pro panol and water. Therefore, we have chosen gelatin films as a model sy stem to characterize the measurement of elasticity of thin and soft sa mples. The major aim of this study was to investigate the influence of the film thickness on the apparent elastic (Young's) modulus. Thus, w e prepared wedge-shaped samples with a well-defined thickness of up to 1 mu m. The Young's modulus of our samples was between 1 MPa and 20 k Pa depending on the degree of swelling. The elasticity was calculated by analyzing the recorded force curves with the help of the Hertz mode l. We show that the calculated Young's modulus is dependent on the loc al film thickness and the applied loading force of the AFM tip. Thus, the influence of the hard substrate on the calculated softness of the film can be characterized as a function of indentation. It was possibl e to determine the elastic properties of gelatin films with a thicknes s down to 50 nm and a Young's modulus of similar to 20 kPa.