Decoupling of the Chemical and Mechanical Surface Contributions in a Force Curve Measurement with AFM
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Decoupling of the chemical and mechanical surface contributions in a force curve measurement with AFM
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 Atomic Force Microscope (AFM) was used to perform surface force measurements in contact mode to investigate surface properties of model systems at the nanoscale. Model systems were considered and compared. The first one was related to systems of controlled chemical surface properties with identical mechanical properties (chemically modified silicon substrates with hydroxyl, amine, methyl and ester functional groups). The second one deals with model polymer networks (Cross-linked polydimethylsiloxane or PDMS) of controlled mechanical properties and identical surface chemistry. The third system consists in a model polymer network, whose surface is chemically controlled with the same groups as before with silicon substrates. The results show that the viscoelastic contribution is dominating in the adhesion force measurement. Finally, we propose a relationship (derived from the Gent and Schultz’s one), which expresses the AFM adhesion force as a function of mechanical energy dissipated in the contact and the surface properties of the material. INTRODUCTION The Atomic Force Microscope (AFM) is a promising device for the investigation of material surface properties at the nanoscale. Precise analysis of adhesive and mechanical properties, and in particular, of surface polymer model can be achieved with a nanometer probe. The purpose of this study is to quantify the different contributions (adhesive and mechanical) included in a surface force measurement with AFM in order to establish relationships between viscoelastic properties of the bulk, of the surface and the adhesive forces. In particular, we were interested in the measurements of local attractive or adhesive forces of controlled chemical and mechanical model substrates in contact mode. These are performed with a silicon nitride tip (Si3N4), at ambient temperature, in the air. MATERIALS Si(100) silicon wafers (supplied by MAT TECHNOLOGY France) polished on one side were used as substrate for SAM film grafting. In this paper, “as received silicon (Sias received)” refers to a silicon wafer previously cleaned with ethanol in an ultrasonic bath. That means that a contaminated layer still remains on the surface. Four organosilane grafts (supplied by ABCR Karlsruhe-Germany) were used for the elaboration of homogeneous model surfaces on the substrate. Two hydrophobic model surfaces were prepared by using hexadecyltrichlorosilane (C16H42O3Si or SiCH3) and 1H,1H,2H,2H-perfluorodecylmethyldichlorosilane (C11H7Cl2F17Si or SiCF3) and two hydrophilic model surfaces by using (6-aminohexyl)-
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