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

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 and trans forms are observed through measuring the related clearing temperatures via polarized optical microscopy. LCE precursors containing 0, 1, 3, 5, 7, and 10 mol% o-Fb monomer or crosslinker are aligned in a magnetic field and polymerized as films in the LC phase. Thermal and visible light-controlled photochemical 2D (planar) actuation properties of LCE samples are investigated and compared to LCEs containing a liquid crystalline non-fluorinated azobenzene monomer as reference. It is shown that, while the favorable photochemistry is obtained, the integration of o-Fbs is less effective for photoswitching of the LC phase, because i) less o-Fbs can be integrated without destroying the LC phase and ii) the trans-cis isomerization is less effective in destroying the LC phase.