A comparative study of photoinduced deformation in azobenzene containing polymer films

Abstract

In this paper two groups supporting different views on the mechanism of light induced polymer deformation argue about the respective underlying theoretical conceptions, in order to bring this interesting debate to the attention of the scientific community. The group of Prof. Nicolae Hurduc supports the model claiming that the cyclic isomerization of azobenzenes may cause an athermal transition of the glassy azobenzene containing polymer into a fluid state, the so-called photo-fluidization concept. This concept is quite convenient for an intuitive understanding of the deformation process as an anisotropic flow of the polymer material. The group of Prof. Svetlana Santer supports the re-orientational model where the mass-transport of the polymer material accomplished during polymer deformation is stated to be generated by the light-induced re-orientation of the azobenzene side chains and as a consequence of the polymer backbone that in turn results in local mechanical stress, which is enough to irreversibly deform an azobenzene containing material even in the glassy state. For the debate we chose three polymers differing in the glass transition temperature, 32 °C, 87 °C and 95 °C, representing extreme cases of flexible and rigid materials. Polymer film deformation occurring during irradiation with different interference patterns is recorded using a homemade set-up combining an optical part for the generation of interference patterns and an atomic force microscope for acquiring the kinetics of film deformation. We also demonstrated the unique behaviour of azobenzene containing polymeric films to switch the topography in situ and reversibly by changing the irradiation conditions. We discuss the results of reversible deformation of three polymers induced by irradiation with intensity (IIP) and polarization (PIP) interference patterns, and the light of homogeneous intensity in terms of two approaches: the re-orientational and the photo-fluidization concepts. Both agree in that the formation of opto-mechanically induced stresses is a necessary prerequisite for the process of deformation. Using this argument, the deformation process can be characterized either as a flow or mass transport.

Light-Triggered Formation of Surface Topographies in Azo Polymers

“…The driving mechanism for the creation of topographies in both linear azo polymers and glassy LCN coatings remains controversial. For linear azo polymers, multiple mechanisms have been proposed [41]. Whereas with the azo polymer networks (azo-LCN), free volume generation in a glassy material seems to fit most of experimental data, this solution comes under question in light of the recent findings showing that also mechanical properties of the polymer network change upon illumination [110].…”

“…The resulting alignment of the trans-isomer will therefore be perpendicular to the direction of the electrical component of polarized light (Figure 2c). The realignment process is either powerful enough to move the polymer chains and/or fluidize the polymer, leading to the formation of an anisotropic fluid state [41]. It is known that during the light absorption of the azobenzene, localized heat is generated which can enable additional mobility [42].…”

Driving Cells with Light‐Controlled Topographies

“…[18] Azobenzenes are among the most popular molecular photoswitches because of their ability to undergo a conformational change between two isomeric states when irradiated with UV or visible light. [25][26][27][28][29] When dealing with 2D light-responsive substrates, topographies can be instructed by a direct laser writing exposure of azopolymeric films previously spun on planar glass substrates. [25][26][27][28][29] When dealing with 2D light-responsive substrates, topographies can be instructed by a direct laser writing exposure of azopolymeric films previously spun on planar glass substrates.…”

Light-induced dynamic shaping and self-division of multipodal polyelectrolyte-surfactant microarchitectures via azobenzene photomechanics

“…Photochromic materials based on azobenzene derivatives are particularly attractive for the implementation of photo-mechanical responses owing to their robust and reversible trans-cis light-induced isomerisation that affects both the molecular volume and orientation of the transition dipole moment34567. Shape deformations involving bending, twisting, expansion-contraction, swelling, melting or oscillations have been observed in crystalline molecular assemblies89101112, microgel particles13, azobenzene-containing polymer films14151617 and liquid crystal azobenzene-containing elastomeric polymers181920212223. Repeatable deformations are usually achieved in these materials by the sequential use of ultraviolet (UV) and blue light irradiations to induce alternative trans - cis followed by reverse cis - trans isomerisation of the azobenzene moieties.…”

“…In contrast, attempts to design artificial systems responding to continuous light excitation under non-equilibrium conditions have only recently been achieved. For example: directional crawling of azobenzene crystals on a glass surface has been achieved in response to irradiation with spatially oriented UV and blue light sources via localized melting and crystallization at the rear and front edges of the crystals, respectively10; a biomimetic peristaltic-like motion has been implemented in a liquid crystalline elastomer microswimmer via photo-isomerisation-induced local contraction-expansion of the soft material under structured dynamic light fields23; and surface relief gratings have been spontaneously produced in azobenzene-containing polymer thin films subjected to polarized and structured light sources16242526.…”