Surface Mechanical Property Determination of Soft Materials Through an AFM Nanoindentation Experiment
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Surface mechanical property determination of soft materials through an AFM nanoindentation experiment
Olivier NOEL, Maurice BROGLY, Gilles CASTELEIN and Jacques SCHULTZ Université de Haute Alsace (UHA) Institut de Chimie des Surfaces et Interfaces (ICSI)-CNRS UPR 9069 BP 2488-MULHOUSE CEDEX-FRANCE. ABSTRACT In this paper, we have studied the capability of the AFM to perform nanoindentation experiments. We mainly focused on the importance of a rigorous experimental procedure to get quantitative and reproducible data with the Atomic Force Microscopy (AFM). Systematic calibration procedure of AFM measurements, as well as a complete description of the mechanical behavior of the soft material is necessary before producing reliable quantitative data. In particular, the influences of the creep and of the strain rate have been studied. Then, this technique was used to probe model cross-linked polydimethylsiloxane (PDMS) and to extract their surface mechanical properties at the nanoscale. Young modulus of each sample was calculated by comparing different contact mechanics theories (Hertz and JKR theories and a power law expression). In conclusion, the contact mechanics relationships have to be redefined and adapted to soft materials. In particular, it appears necessary to consider the specific mechanical contribution of the polymers. INTRODUCTION AFM is a promising technique to get insights of surface properties such as adhesion, friction …at the nanoscale [1]. Thus, recent studies [2] have used this tool to determine mechanical properties of biological cells or thin polymer films on the basis of nanoindentation experiments. However, in addition with a lack of systematic calibration of the apparatus, relationships of contact mechanics between two elastic bodies are fluently used to get quantitative data. In the present study, we propose a systematic calibration of the AFM to get quantitative measurements. We will also discuss the relevance to apply contact mechanics relationships (mainly derived from elastic contact mechanics) to soft materials (for example, PDMS), for which the viscoelastic and adhesive contributions should not be neglected. In particular, interpretation of force-indentation depth curves (F-I curves) was carefully studied before applying properly the contact mechanics models. MATERIALS PDMS samples were cross-linked under nitrogen in a glove box using tetrakis(dimethylsiloxy)silane as a cross-linker and a platinum-based catalyst. All the chemicals were supplied by ABCR (Karlsruhe-Germany). The classification of PDMS substrates refers to the length of the chains before crosslinking (Figure 1). Then, PDMS 1.5k is the stiffest substrate, whereas 53k refers to the more compliant one.
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Deformation at break (%)
PDMS
Elasticity Domains (%)
1.5k
40
196
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