Bimodal AFM for Elucidation of Polyolefin Morphology

A.H. Tsou, D.G. Yablon
ExxonMobil Chemical Company, US

Keywords: AFM, materials characterization


After the development of phase imaging with tapping mode Atomic Force Microscopy (AFM) in 1994, improved cantilever sensitivity was found in some materials when the cantilever was excited at a higher eigenmode instead of the first or fundamental eigenmode. In 2004, the simultaneous excitation of the cantilever at multiple eigenmodes (first plus a higher one) was proposed by Rodriguez and Garcia1. They coined this operation method “Bimodal AFM”. Derived from theoretically computed results using only surface contact forces, they found that the second resonant mode oscillation can be sensitive to forces as low as 10 pN through the nonlinear coupling of the two oscillating modes. Experimental verification soon followed in 20062,3 demonstrating the expected enhanced contrast and force sensitivity on sexithienyl T6 molecules, graphite, and DNA. With the commercialization of Bimodal-capable AFM instruments, the ability of the Bimodal AFM to discriminate material properties in the higher order eigenmodes has been an active area of research and has been applied to polyolefins only recently. Contrast inversion was found in polyolefin blends as the amplitude used for the higher vibrational mode was varied. Through simulation of the cantilever dynamics and experimental findings using an ICP (Impact Copolymer)4, the amplitude settings for the higher eigenmodes can now be defined to prevent contrast inversion. Using proper cantilever amplitudes at higher eigenmodes with no contrast inversion, Bimodal AFM has been applied to examine semi-crystalline and amorphous polyolefin blends and compounds and has shown the technique to have impressive capabilities in discriminating almost all chemical and physical components in these materials. Additionally, Bimodal AFM was able to provide morphological explanations for the observed performance changes in these polyolefins. This presentation will discuss the instrument and operation of Bimodal AFM and their uses in elucidating polyolefin morphologies.