Atomic Force Microscopy (AFM) is a method of visualising surfaces by using the forces between atoms. It is a relatively new technique, being reported for the first time in 1986. It has been employed on rubber compounds for a wide range of purposes, such as: Producing a force-extension curve for a single polymer chain 1; Observation of filler distribution in rubber 2,3; Investigation of morphology of rubber blends 3,4; and characterisation of elastomers using nanoindentation 5. However, it is not widely used compared to techniques such as electron microscopy, and there is still great potential for new ideas for investigating the properties of rubber using AFM.
The simplest form to both understand and perform is Contact AFM, which can be compared to studying a surface by moving a finger backwards and forwards along it to detect variations in height. The atomic-scale equivalent involves a probe with an extremely sharp tip (radius of curvature 5-10nm) passing over a surface, so that a relatively small number of atoms in the tip are interacting with a similarly small number of atoms on the surface. As the probe scans over the surface, the inter-atomic forces between the tip and the surface constantly change. The probe is mounted at the end of a flexible cantilever, which is bent as the force increases or decreases. This bending is measured using a laser reflected off the back of the cantilever, and a feedback system causes the position of the probe to be adjusted up or down, in order to maintain the desired force. The changes in height required to preserve the constant force are recorded to build up first a profile, and then a 3D image of the surface.
A more advanced form of AFM is Intermittent-Contact AFM. To continue the finger analogy, in this mode the image built up by tapping the finger on the space at regular intervals instead of dragging it along the surface. The advantage is that the tip is abraded much more slowly than in Contact AFM. This is important because the sharper the tip, the smaller the features that can be observed are, because the tip is interacting with a smaller area of the sample surface. Also, between each “tap” the cantilever is vibrated at (or, more commonly, slightly below) its natural resonant frequency. Because the vibration of the cantilever is affected by its proximity to the sample, information about the surface can be obtained regarding its physical characteristics. This is collected in two ways, known as phase imaging (because it looks at the effect of the surface on the frequency of the cantilever’s oscillations) and amplitude imaging (because it looks at the magnitude of the cantilever’s oscillations).
The AFM acquired by RC is an MFP-3D model, produced by Asylum Research of Santa Barbara, California. In addition to the standard AFM visualisation modes, the MFP-3D offers the ability to produce images by vibrating the cantilever not only at its resonant frequency but also at its first harmonic frequency. This capability is known as Dual AC imaging, and provides an extra level of detail about the structure and properties of the sample surface.
For more information do not hesitate to contact David Lowe.
(1. K. Nakajima, H. Watabe, and T. Nishi. Single polymer chain rubber elasticity investigated by atomic force microscopy. Polymer 47, 2006, 2505–2510. 2. c. c. w ang and s. h. w u. Microdispersion of carbon blacks in rubber, part I: some quantitative aspects by Afm image analysis. Rubber Chemistry and Technology 79, 2006, 783-789. 3. I. H. J eon ,* H. K im , and S. G. K im. Characterization of rubber micro-morphology by atomic force microscopy (Afm). Rubber Chemistry and Technology 76, 2003, 1-11. 4. E. Radovanovic , E. Carone Jr., and M. C. Goncalves. Comparative AFM and TEM investigation of the morphology of nylon 6-rubber blends Polymer Testing 23, 2004, 231–237. 5. M.Mareanukroh, R. K. Eby, R. J. Scavuzzo, and G. R. Hamed. Use of Atomic Force Microscope as a nanoindenter to characterize elastomers. Rubber Chemistry and Technology 73, 2000, 912-925.)