Influences of Ortho‐Fluoroazobenzenes on Liquid Crystalline Phase Stability and 2D (Planar) Actuation Properties of Liquid Crystalline Elastomers

Abstract

In general, LCEs have gained great interest as stimuli-responsive and actuator materials in the last decades. [7-16] They consist of shape anisotropic molecules (mesogens) that are incorporated in a weakly crosslinked polymeric network (an elastomer). Therefore, they show the entropy elasticity of elastomers as well as the anisotropic characteristics of liquid crystals. Mesogens are either incorporated into the polymer backbone (main-chain LCE) or linked to it via flexible spacers (side-chain LCE). [17-20] They cause the molecular selforganization in liquid crystalline (LC) phases (mesophases). In nematic phases, for example, rod shaped mesogens align with their long axis roughly parallel to a common director and thus have directional but no positional order. [21,22] Integrated in a polymeric network, they act as an anisotropic environment for the polymer chains and stretch them to some extent in the mesophase. [7,13-16,23] On the other hand, in the isotropic state the order is lost and polymers can adopt their favored isotropic coil conformation. Switching between these states results in a macroscopic shape change if the LC state is present as a monodomain within the LCE sample. Shrinkage parallel to the director and expansion in the perpendicular direction are observed while the volume is kept constant. [13-16,24] These actuation properties had already been predicted by de Gennes in 1975. [25,26] So far, actuations up to 400% [27] could be observed for main-chain LCEs while side-chain LCEs could achieve actuations of up to 70% [26,28] corresponding to the increase in length. [7] To obtain a monodomain, LCE samples need to be pretreated to align mesogens before crosslinking. This is usually done in magnetic [29,30] or electric fields, [31] with the help of photoalignment layers [32,33] or by mechanical stretching of prepolymerized samples. [7,10] After alignment, crosslinking can occur via covalent or physical netpoints. [7,8,34] To investigate actuation properties of LCEs, it is necessary to switch between the liquid crystalline and the isotropic state. This requires an external stimulus. Such stimulus can be, for example, a change of temperature, [35] irradiation with light, [36,37] a pH change, [38] or the use of electric or magnetic fields. [39,40] Most attention has been paid to temperature changes. This often requires temperatures far beyond ambient temperature and mostly the whole actuator device has to be heated which results in high energy consumption. In contrast, light as a Actuation Systems In this article, the influences of ortho-fluoroazobenzenes (o-Fbs) on liquid crystalline (LC) phase stability and actuation properties of liquid crystalline elastomers (LCEs) are presented. Such o-Fbs find interest because they allow full photoswitching with visible light (no ultraviolet (UV) radiation). Novel o-Fb monomers and crosslinkers are synthesized and characterized. Different amounts of an LC mixture are replaced with synthesized o-Fbs successively and their influences on the LC phase stability in the cis...

Visible Light‐Driven Molecular Switches and Motors: Recent Developments and Applications

“…The influence of a fluoroazobenzene derivative on the phase stability of a liquid crystal elastomer has been studied. [85] Moreover, how the presence of the visible light-driven component affects the in-plane actuation of the liquid crystal elastomer film has also been investigated. It is found that fluoroazobenzenes are less effective in photoswitching the liquid crystal phase.…”

Unlocking Entropic Elasticity of Nematic Elastomers Through Light and Dynamic Adhesion

“…A number of previously reported azobenzene-LCE systems have shown a certain light-induced reduction of Q, which induces changes in shape. [8][9][10][41][42][43][44][45][46] However, no experimental systems have been developed that show the landscape regarding the stress-strain curves through a straining cycle including viscoelasticity, thereby clearly characterizing and differentiating the basic mechanical properties of the nematic and cis-isomerinduced isotropic phases. Only a few studies on glassy or smectic LCEs have reported their modulus changes upon/ after light irradiation, by which the softening or hardening of materials was evaluated within the small strain regime.…”

LC‐Polymers and Smectic Phases with Special Substructures/Nanophase Segregation

“…[ 38–40 ] In addition, illumination to induce the shape change is often applied. [ 41–44 ] Topics are here to induce bending or to avoid it (homogeneous shrinking) and the use of visible light. [ 41–43,45 ]…”

“…[ 41–44 ] Topics are here to induce bending or to avoid it (homogeneous shrinking) and the use of visible light. [ 41–43,45 ]…”