Bending Dynamics and Directionality Reversal in Liquid Crystal Network Photoactuators Casper L. van Oosten,* ,† Daniel Corbett, ‡ Dylan Davies, † Mark Warner, ‡ Cees W. M. Bastiaansen, † and Dirk J. Broer †,§ EindhoVen UniVersity of Technology, Post Office Box 513, NL-5600 MB EindhoVen, The Netherlands, CaVendish Laboratory, Madingley Road, Cambridge, CB3 0HE, United Kingdom, and Philips Research Laboratories, High Tech Campus 4, NL-5656AE EindhoVen, The Netherlands ReceiVed August 7, 2008; ReVised Manuscript ReceiVed September 11, 2008 ABSTRACT: Liquid crystalline photoactuators typically bend toward the light source, driven by the isomerization of azobenzene. In samples with a relatively large thickness and high azobenzene loading such as LC photoactuators, intense optical beams are seen to be absorbed in spatially nonexponential ways. Here we show that the dynamics of the related mechanical behavior is also strongly nonlinear, where the actuator reaches a maximum bend before unbending again to its equilibrium deformed state. The effect is amplified when combined with actuators with an internal composition gradient, leading to a reversal of the bending direction away from the light source. Introduction The dynamics of the trans-cis isomerization of azobenzene in a polymer network has attracted attention for both funda- mental studies of polymer behavior and practical applications. The isomerization causes a change in the absorbance spectrum of the azobenzene unit, and through the dynamics of this optical change, the azobenzene unit is able to report on the local rigidity of its surroundings. 1,2 In an ordered liquid crystalline network, the trans-cis isomerization of azobenzene leads to an order reduction, causing a macroscopic contraction along the molec- ular director and an expansion perpendicular to it. 3 In this way, the polymer functions as a photomechanical actuator. The classical case of the liquid crystal network or liquid crystal elastomer based photoactuator is that of a film with a planar uniaxial molecular director, where the gradient in light intensity through the thickness of the film causes bending. 4-10 Recently we showed that the transmission of light through these actuators follows a nonexponential behavior in time, strongly deviating from those cases where the absorption is low. 11 In these high- absorbing actuators, the isomerization gradient forms the basis of the photomechanical response and drives the directionality of the shape deformation. It is the dynamics and directionality of this shape deformation that is the focus of this work. Here, we shall first use a theoretical approach to describe the dynamics of the photo bending. We start with the dynamics of the optical response, which will be expanded to predict the mechanical behavior. Two cases will be considered: the classic case of the LC photoactuator with planar uniaxial molecular director and a new type of photoactuator with internal composition gradient. In both cases, we will observe a complex, nonexponential behavior in time. Using experiments, we will show that this theory matches the experiments. In the last part of this work, we will show that the systems with the internal composition gradient show a reversal in bending direction upon prolonged exposure. Under normal room conditions, azobenzene dyes are mostly in their elongated trans-state. A typical azobenzene molecule is A3MA, as shown in Figure 1, which has an absorption maximum in the UV at 356 nm. Upon exposure to UV light, the molecule undergoes an isomerization to the bent cis-state. The sample changes color as the absorption of UV light decreases and the absorption of visible light increases. For A3MA, the cis-state has absorption peaks in the visible at about 450 nm and in the near UV at about 310 nm, with a significant reduction of the absorption at 356 nm (Figure 1b). Relaxation back to the trans-state is driven thermally and by exposure to light at the peak wavelengths of the cis-state. The photosta- tionary, UV-illuminated state is therefore an equilibrium state between the forward trans-cis isomerization rate and the thermal relaxation rate. One convenient way to optically characterize a film is by its ratio of the Beer absorption length of the trans-state d t to the film thickness w. 12 The Beer absorption length is a measure for the absorptivity of the material and depends on material parameters such as dye concentration. For films with moderate to high dye loads (w/d t > 1), the transmission of intense beams of light through the film follows a nonexponential profile both in space and time. 11,13 We will first adopt the analysis of the dynamics of the optical absorption from previous work 11 and then introduce the analysis of the effected mechanical behavior. Assuming the back relaxation only happens thermally, the dynamics of the isomerization can be written as * To whom correspondence should be addressed. E-mail: c.l.v.oosten@ tue.nl. † Eindhoven University of Technology. ‡ Cavendish Laboratory. § Philips Research Laboratories. Figure 1. Molecular structures (a) and absorption spectra of A3MA for the dark (trans-dominated) and UV-exposed (cis-dominated) states (b). A Macromolecules XXXX, xx, 000 10.1021/ma801802d CCC: $40.75  XXXX American Chemical Society Published on Web 10/23/2008