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CP: Chemische Physik

CP 16: Neue Methoden

CP 16.4: Vortrag

Dienstag, 23. März 1999, 17:00–17:15, Gg

Dynamic viscoelastic profiles of soft polymer interfaces via noise analysis of AFM cantilevers — •Diethelm Johannsmann, Martin Gelbert, Andreas Roters, Martin Schimmel, and Jürgen Rühe — Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz

The dynamic properties of the environment of an AFM tip can be probed by observing the cantilever’s mechanical noise. Within the "simple harmonic oscillator" approximation the power spectral density (PSD) is given by Lorentzians with the friction coefficient ξ, the effective mass m, and the effective spring constant κ as fit parameters. In this way microrheological parameters close to interfaces can be measured.
The viscoelastic profiles obtained from soft polymer brushes show an elastic interaction extending to a tip-sample distance equal to about the brush height. There is a strong increase in friction (lubrication) at tip-surface distances much larger than the brush thickness which is attributed to breathing modes. For tips deeply immersed into a brush the noise spectra are no longer described by resonance curves. There is an excess noise at low frequency, which we attribute to viscoelastic dispersion. By the fluctuation dissipation theorem the viscoelastic spectrum ξ(ω) can be explicitly derived. We compare data taken on different brushes and in bulk polymers.
[1] A. Roters, M. Gelbert, M. Schimmel, J. Rühe, D. Johannsmann, Phys. Rev. E 56, 3256 (1997)