Application of Thermal Noise Analysis to Viscoelasticity Measurements of Single Polymer Chains using AFM with High-Tip Cantilever

Abstract

Atomic force microscope (AFM)-based single molecular force spectroscopy has been widely used to study the mechanical properties of polymers. In recent years, the development of dynamic force spectroscopy of single polymer chains has enabled us to measure the viscoelasticity of polymers. In this paper, we report the direct measurement of viscoelasticity of a single polystyrene (PS) chains in N,N-dimethyl formamide by using thermal noise analysis. This technique does not require any vibration of AFM cantilever and allows polymer chains to be measured nearequilibrium conditions. Furthermore, we evaluated the force-dependences of the elasticity and viscosity for a single PS chain.The elastic behaviors of single molecular chains were well described with the worm-like chain model in the high-stretch region. However, this model cannot provide an assessment of viscous behavior because the responses of the single polymer chain to the cantilever were so small that those were hidden by the cantilever's response to the system. To improve the measurement accuracy of the viscous behavior, we fabricated for the first time high-tip cantilever by a focused ion-beam processing, by which the hydrodynamic effect between the cantilever and substrate was reduced.