UV isomerisation in nematic elastomers as a route to photo-mechanical transducer

Abstract

The macroscopic shape of liquid-crystalline elastomers strongly depends on the order parameter of the mesogenic groups. This order can be manipulated if photo-isomerisable groups, e.g. containing N=N bonds, are introduced into the material. We have explored the large photo-mechanical response of such an azobenzene-containing nematic elastomer at different temperatures, using force and optical birefringence measurements, and focusing on fundamental aspects of population dynamics and the related speed and repeatability of the response. The characteristic time of "on" and "off" regimes strongly depends on temperature, but is generally found to be very long. We were able to verify that the macroscopic relaxation of the elastomer is determined by the nematic order dynamics and not, for instance, by the polymer network relaxation.

Bleaching and stimulated recovery of dyes and of photocantilevers

“…A higher cis absorption arises if the incident light frequency is shifted to be closer to the cis absorption line. One can rearrange eqn (6) by evaluating it at x = w and noting that −w/d t = ln(I B ), where B denotes the Beer, t = 0 reduced intensity. Thus denoting by R the ratio I w /I B , one has: ln(R) = 1 − α/(ηβ)…”

“…Increasing α leads to deeper penetration, with the initial decay being essentially linear. However close inspection of the α = 10 curve reveals a slight upwards curvature, a result of the higher order corrections in β that take us from (8) to (6). Eventually I(x) becomes exponential at penetration depths x ∼ d t α significantly greater than d t .…”

“…Smaller changes arise in glasses, where subtle arrangements of director have been employed to achieve more complex mechanical responses [2]. Since light and heat have analogous effects, one expects and indeed finds that optical absorption leads to analogous mechanical strains, both in photo-elastomers [5,6] and in photo-glasses [7]. Such a mechanism of mechanical change offers the possibility of micro-opto-mechanical systems (MOMS) where elements can be optically-induced to bend as elastomeric photo-swimmers [8] or as glassy cantilevers [7,9,10].…”

Polysiloxane Side-Chain Azobenzene-Containing Liquid Single Crystal Elastomers for Photo-Active Artificial Muscle-Like Actuators

“…So, azobenzene-containing LSCEs are very attractive materials for the fabrication of light-controlled artificial muscle-like actuators. [37][38] During the last decades, both the preparation and properties modulation of light-driven artificial muscle-like actuators based on azobenzene-containing LSCEs are topics of intensive research. Thus, in this chapter we will describe the last investigations that have been performed in our group towards not only to enhance the mechanical efficiency of such materials but also to optimise the response time of the final artificial muscle.…”

Photomechanical Response of Composite Structures Built from Azobenzene Liquid Crystal Polymer Networks

“…Photomechanical responses in azo-LCN materials have been most often observed as uniaxial contraction of a film (typically elastomers-large strain) [6,[17][18][19] or bending of a cantilever (typically glasses-lower strain) [7][8][9][10][11][12][13][14][15][16]. The magnitude or directionality of the photomechanical response of azo-LCN materials has repeatedly been controlled by the alignment of the linear polarization of the actinic source and the sample geometry [8,9,11,12,[14][15][16].…”