Equilibrium and out-of-equilibrium dynamics in a molecular layer of azopolymer floating on water studied by Interfacial Shear Rheology

Details Equilibrium and out-of-equilibrium dynamics in a molecular layer of azopolymer floating on water studied by Interfacial Shear Rheology Equilibrium and out-of-equilibrium dynamics in a molecular layer of azopolymer floating on water studied by Interfacial Shear Rheology

2010-05-18

arxiv.org/abs/1005.3156v1

Physics

Condensed Matter

Soft Condensed Matter

Proceedings of the International Discussion Meeting on Relaxation in Complex Systems, Rome, 2009 12 pages, 7 figures

Scientific paper

We report the details of the construction and calibration of an ultra sensitive surface rheometer, inspired by the setup described in [C.F. Brooks et al Langmuir 15, 2450 (1999)], which makes use of high resolution video tracking of the motion of a floating magnetized needle and is capable of measuring the viscoelastic response of a Langmuir monolayer with an accuracy of 10^-5 N/m. This instrument is then employed for the rheological characterization of a Langmuir monolayer of a photosensitive azobenzene polymer, which can be brought out of equilibrium by a suitable photoperturbation. The complex dynamic shear modulus G= G' + i G" is measured as a function of temperature and illumination power and wavelength. The reversible rheological ch anges induced in the film by photo-perturbation are monitored during time, observing a transition from a predominantly elastic (G' > G'') to a viscoelastic (G' \approx G'') regime. These results are confirmed by comparison with independent measurements performed by us using other rheological techniques. Finally a discussion is made, taking into account the results of a recent x-ray photon correlation spectroscopy experiment on the same polymer in equilibrium and out of equilibrium.

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