Broadband, High Frequency Viscoelastic Property Characterization of Polymers with Atomic Force Microscopy

A. Deolia, S.P. Carter, R.B. Wagner
Purdue University,
United States

Keywords: atomic force microscopy, photothermal excitation, viscoelasticity, polymers


Pyrotechnic shock loading of space systems, hypervelocity debris strikes on satellites, and high-frequency resonators and switches based on microelectromechanical systems expose polymer and polymer composites to forces with characteristic frequencies greater than 20 kHz. The atomic force microscope (AFM) is well suited for material property characterization at these high frequencies. The AFM microcantilever has a stiffness on the order of a few N/m and mass on the order of a few ng giving it a fundamental resonance frequency on the order of hundreds of kHz. The smallest AFM cantilevers can push their fundamental resonance frequency up to a few MHz. Quasistatic AFM methods based on force-displacement curves or on force modulation can measure mechanical properties across a large fraction of the cantilever’s fundamental resonance frequency. Currently, practical issues related to forcing and calibration limit our ability to perform viscoelastic property characterization across a broad frequency range with AFM. Specifically, piezoelectric actuators tend to have complicated frequency-dependent dynamic responses because of the mechanical structure of the actuator and surrounding environment. Direct forcing mechanisms, such as photothermal and magnetic excitation, bypass this problem. However, in the quasistatic frequency regime these direct mechanisms simultaneously modify the shape in which the cantilever vibrates, invalidating the commonly used optical lever calibration procedure. In this work, we investigate photothermally excited AFM force curves as a method of large frequency bandwidth material property characterization. Based on a combination of finite element modeling, one-dimensional beam mechanics models, and experimental observations we develop and test a broadly applicable calibration procedure for photothermally driven force curves. We use this calibration procedure to characterize polymer samples with well-understood properties and cross-compare our results with dynamic mechanical analysis. This is a key step in enabling broad bandwidth mechanical property characterization into the MHz frequency regime with AFM.