Measurement of the loss tangent of a thin polymeric film using the atomic force microscope
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D.J. Yarusso 3M Center, Commercial Graphics Division, St. Paul, Minnesota 55144 (Received 16 June 2003; accepted 3 November 2003)
An atomic force microscope was used to measure the loss tangent, tan ␦, of a pressure-sensitive adhesive transfer tape as a function of frequency (0.01 to 10 Hz). For the measurement, the sample was oscillated normal to the surface and the response of the cantilever resting on the polymer surface (as measured via the photodiode) was monitored. Both oscillation amplitude and phase were recorded as a function of frequency. The atomic force microscopy measurement gave the same frequency dependence of tan ␦ as that measured by a dynamic shear rheometer on a film 20 times thicker. The results demonstrate that the atomic force microscope technique can quantitatively measure rheological properties of soft thin polymeric films.
I. INTRODUCTION
The mechanical1 properties of thin films have become increasingly important in recent years due in large part to the development of novel coatings, magnetic storage drives, optoelectronics, and microelectromechanical systems (MEMS).1 Many of these technologies consist of a coating material deposited onto a substrate of another material, and the value of the mechanical properties often reflect not only the sample of interest but also the underlying substrate. To understand and predict the performance of these systems, we must be able to measure the mechanical properties of thin films and multilayers. Mechanical properties such as hardness, elasticity, and creep can be obtained using nanoindentation.2–7 This measurement involves recording the displacement of a probe (indenter) as it indents a sample as a function of the applied force. The area of contact can be calculated from knowledge of the probe geometry or the indent can directly be imaged via microscopy. Analyses of force– distance curves and knowledge of the contact dimensions (or probe geometry) enable the mechanical properties of the film to be determined. Atomic force microscopy (AFM) can also be used to indent thin films and obtain force–distance curves for mechanical measurements.8–11 For the mechanical properties to be measured, the sample must be deformed. For this to occur, the indenter spring stiffness must be comparable to the contact stiffness. For quantitative measurements, the contact mechanics of the deformation and a)
Address all correspondence to this author. [email protected] J. Mater. Res., Vol. 19, No. 1, Jan 2004
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the spring (cantilever) constant should be known. In general, the spring stiffness of the cantilever is difficult to measure and the contact area must be inferred, making quantitative mechanical measurements by AFM difficult. Indentation measurements alone are generally inadequate for many polymeric samples. Polymeric materials are mostly viscoelastic (i.e., have both a viscous component as well as an elastic component).12 Thus, instead of describing the polymer film in terms of a single elastic modulus
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