Measuring the Elastic Properties of Thin Polymer Films with the Atomic Force Microscope

TLDR

Gelatin softness can be tuned by swelling in propanol–water mixtures. The study aims to characterize elasticity measurements of thin gelatin films and to investigate how film thickness affects the apparent Young's modulus. Thin gelatin films were examined with AFM, using wedge-shaped samples up to 1 µm thick, and their elasticity was derived from force curves via the Hertz model. The measured Young’s modulus ranged from 1 MPa to 20 kPa, decreasing with swelling, and was found to depend on local film thickness, loading force, and substrate influence, allowing elastic characterization of films as thin as 50 nm with a modulus of ~20 kPa.

Abstract

The elastic properties of thin gelatin films were investigated with the atomic force microscope (AFM). The degree of swelling and thus the softness of the gelatin can be tuned by immersing it in mixtures of propanol and water. Therefore, we have chosen gelatin films as a model system to characterize the measurement of elasticity of thin and soft samples. The major aim of this study was to investigate the influence of the film thickness on the apparent elastic (Young's) modulus. Thus, we prepared wedge-shaped samples with a well-defined thickness of up to 1 μm. The Young's modulus of our samples was between 1 MPa and 20 kPa depending on the degree of swelling. The elasticity was calculated by analyzing the recorded force curves with the help of the Hertz model. We show that the calculated Young's modulus is dependent on the local 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 possible to determine the elastic properties of gelatin films with a thickness down to 50 nm and a Young's modulus of ∼20 kPa.