The need for spatial resolution and compositional sensitivity on soft and multiphase materials has driven the evolution of Atomic Force Microscopy (AFM).  AFM technique moved from contact mode AFM (monitoring the static deflection of the cantilever) in the 1980’s to dynamic AFM (excitation and detection of cantilever vibration) in the early 1990’s, most specifically, to AM-AFM (Amplitude Modulated AFM) or tapping-mode.  Phase imaging obtained by operating AM-AFM in the repulsive tip-sample interaction regime has been broadly applied to generate composition-sensitive maps of heterogeneous surfaces, including morphologies of cryo-faced polyolefins, polyolefin blends and compounds.  More recently, the AFM is experiencing further development from single to multi-frequency excitation and detection of the cantilever-tip motion.  Bimodal AFM, where the cantilever-tip ensemble is simultaneously excited at two eigenmodes, was first proposed by Rodriguez and Garcia based on numerical simulations in 2004.  The theory of phase imaging in Bimodal AFM has been further refined and verified experimentally since.  The amplitude of the first mode in Bimodal AFM is modulated the same as in AM-AFM.  As a result of the coupling of the vibrational modes, the force sensitivity of the second mode has been shown to reach 0.2 pN and the second-mode phase shift sensitivity has been demonstrated to be raised by one order of magnitude.  In applying Bimodal AFM to polyolefins, 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), 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.

Bimodal AFM for elucidation of polyolefin morphology

249th National Meeting (2015)

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