Nanomechanical Probing of Layered Nanoscale Polymer Films With Atomic Force Microscopy
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H. Shulha and M. Lemieux Department of Materials Science & Engineering, Iowa State University, Ames, Iowa 50011
N. Myshkin Metal-Polymer Institute, National Academy of Sciences, 246050 Gomel, Belarus
V.V. Tsukruka) Department of Materials Science & Engineering, Iowa State University, Ames, Iowa 50011 (Received 2 August 2003; accepted 9 October 2003)
The approach developed for the microindentation of layered elastic solids was adapted to analyze atomic force microscopy probing of ultrathin (1–100 nm-thick) polymer films on a solid substrate. The model for analyzing microindentation of layered solids was extended to construct two- and tri-step graded functions with the transition zones accounting for a variable gradient between layers. This “graded” approach offered a transparent consideration of the gradient of the mechanical properties between layers. Several examples of recent applications of this model to nanoscale polymer layers were presented. We considered polymer layers with elastic moduli ranging from 0.05 to 3000 MPa with different architecture in a dry state and in a solvated state. The most sophisticated case of a tri-layered polymer film with thickness of 20–50 nm was also successfully treated within this approach. In all cases, a complex shape of corresponding loading curves and elastic modulus depth profiles obtained from experimental data were fitted by the graded functions with nanomechanical parameters (elastic moduli and transition zone widths) close to independently determined microstructural parameters (thickness and composition of layers) of the layered materials.
I. INTRODUCTION
The ability to probe surface mechanical properties with nanometer-scale lateral and vertical resolutions is critical for many emerging applications involving nanoscale (1–100 nm) compliant coatings for microelectromechanical and microfluidic devices where nanoscale details of surface deformations and shearing play a critical role in overall performance.1–6 Usually, a nanomechanical probing experiment exploits either atomic force microscopy (AFM) or microindentation techniques.7,8 The utilization of conventional microindentation techniques for systems consisting of highly compliant (elastic modulus ranging below 3000 MPa) and ultrathin (thickness below 100 nm) polymer coatings faces critical challenges associated with uncontrolled initial indentation, which can easily span the entire thickness of compliant nanoscale coatings and limits lateral resolution to hundreds a)
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J. Mater. Res., Vol. 19, No. 3, Mar 2004 Downloaded: 16 Mar 2015
of nanometers. Despite numerous technical issues associated with the AFM nanoprobing (e.g., nonaxial loading, jump-into contact, high local pressure, and topographical contributions), a number of successful applications have recently been demonstrated including nanomechanical probing of spin-coated and cast polymer films, organic lubricants, self-assembled monolayers, polymer br
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