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

TL;DR

This study introduces a highly sensitive surface rheometer to analyze the viscoelastic properties of azopolymer monolayers on water, revealing photo-induced reversible transitions between elastic and viscoelastic states under different conditions.

Contribution

The paper presents a novel ultra-sensitive rheometer setup and applies it to investigate the dynamic shear response of azopolymer monolayers under light-induced perturbations, both in equilibrium and out of equilibrium.

Findings

Reversible photo-induced changes in the shear modulus of the monolayer.

Transition from elastic to viscoelastic behavior under illumination.

Confirmation of rheological behavior through independent measurements.

Abstract

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.