Viscosity measurements of PEO solutions by AFM with long nanowires

Long, constant diameter nanowires on the end of atomic force microscope (AFM) probes are expected to provide clean force distance (F-D) measurements that are easily interpretable. For measurements of the viscosity of low molecular weight liquids this proves to be true. Comparable values are found for glycerol when the nanowire is drawn from the liquid at a constant velocity over microns in scan length and when the nanowire insertion depth changes no more than that due to thermal Brownian vibration fluctuations, as is used for the Q damping measurement method - which also matches macroscopic viscometry methods. However, for measurements of the high molecular weight random chain polymer poly(ethylene oxide) (PEO) the differences are dramatic, with the Q damping method giving values that are orders of magnitude smaller than the linear drag force measurement. Since the deflection amplitude is sub-nanometer, the unusually low viscosity might be due to displacements that are smaller than the entanglement length of the polymer. Additional preliminary investigations are reported that do show somewhat increased viscosity with 10× increased vibration amplitude. Also time dependence of the measurements appears to be related to adsorption of monolayer coatings of PEO on the needle.