Dielectric and Mechanical Properties of Azobenzene Polymer Layers under Visible and Ultraviolet Irradiation

Abstract

Photoinduced changes in the mechanical and dielectric properties of azobenzene polymer films were measured utilizing the method of electromechanical spectroscopy. The measurements revealed a strong correlation between the time-dependent behavior of the plate compliance and the dielectric constant under irradiation. Actinic light causes a light softening of the film that also manifests itself in the increase of the dielectric constant, whereas ultraviolet irradiation results in an initial plasticization of the film followed by its hardening. The latter is accompanied by decrease of the dielectric constant. A semiquantitative model based on the kinetics of the photoisomerization process in azobenzene polymers is proposed. We assume that both visible and ultraviolet irradiation increase the free volume in the layer due to photoisomerization. Additionally, ultraviolet light increases the modulus of the polymer matrix due to the presence of a high density of azobenzene moieties in the cis state. These assumptions allowed us to reproduce the time-dependent behavior of the bulk compliance as well as the dielectric constant at different irradiation intensities, for both visible and ultraviolet light, with only two adjustable parameters.

Reversible Photo‐Mechanical Switching Behavior of Azobenzene‐Containing Semi‐Interpenetrating Network under UV and Visible Light Irradiation

“…Recently, several studies have revealed that the frequent photoisomerization could induce the softening of polymer films due to the light-induced free volume formation under UV and visible irradiation. [13,14] These studies support the above results and discussion. A similar effect has been observed for the light-induced surface relief grating (SRG) of the polymer thin layer containing azobenzene derivatives.…”

“…A similar effect has been observed for the light-induced surface relief grating (SRG) of the polymer thin layer containing azobenzene derivatives. [13][14][15][16][17][18][19] In the SRG phenomenon, softening of the polymer layers was detected by atomic force microscopy. [15,18] …”

Volume generation towards dynamic surface morphing in liquid crystal polymer networks

“…So it is postulated that the reduction of molecular order is accompanied with the density decrease (and corresponding volume increase), which is independent from the molecular alignment configurations [28]. Free volume formation in azobenzene side-chain polymers by photoactuation have been published earlier [45,46]. Barrett and co-workers reported even a volume increase of 17% at room temperature, measurements that were supported by neutron reflectometry.…”

Theory of light-induced deformations in azobenzene polymers: structure-property relationship

“…[9] According to this concept, illumination of an azobenzene polymer layer with visible light induces a transition into a macroscopic low-viscosity melt and such a "molten" polymer can be then irreversibly stretched upon application of a very weak light-induced stress. However, it was shown recently with the help of three different experimental techniques [15][16][17][18] that illumination with a visible light does not affect material properties of an azobenzene polymer such as bulk compliance, Young modulus and viscosity. Therefore, one can certainly claim that an azobenzene polymer under light irradiation remains in a glassy state and, hence, the theories which need a concept of photo-induced softening are not able to describe correctly the phenomenon.…”