Multistable Conventional Azobenzene Liquid Crystal Actuators Using Only Visible Light: The Decisive Role of Small Amounts of Unpolymerized Monomers

Abstract

Azobenzene liquid crystal polymers offer the potential to fabricate autonomously operated actuators remotely controlled by light. Usually, in fully polymerized actuators, only three different states can be obtained: the initial state, the metastable light-actuated state, and the recovered state, where the first and last states are identical and stable. Here, we show that conventional azobenzene liquid crystal polymers that retain a small amount of unpolymerized monomers can exhibit multistable deformation states after manual bending and upon irradiation. This nonbonded fraction of monomers migrates under the influence of local stress gradients, such as those resulting from bending, enabling the actuator to adapt its shape to counteract the stresses induced manually or upon irradiation, resulting in stable recovery states that differ from the initial shape. Oscillatory movement of the azobenzenes and photosoftening facilitate monomer migration and thus allow multiple stable shapes using only lower-energy blue and green lights. Such materials have the potential for biomedical and microfluidic applications where light-induced, multistable states are desired and harmful UV-light needs to be avoided.

Photomodulation of the Mechanical Properties and Photo‐Actuation of Chitosan‐Based Thin Films Modified with an Azobenzene‐Derivative

“…Several studies indicate that when irradiated with UV‐light, the actuation of liquid crystalline elastomers (LCE) tends to bend toward the light source. [ 29,30 ] Additionally, the illumination lead to a photoinduced softening of the LCE samples. In contrast, TEGABS/chitosan films exhibit an opposite effect, where irradiation causes an increase in stiffness (photostiffening) and a concurrent bending movement away from the light source.…”

Straightforward Access to Block and Gradient Light‐Responsive Poly(2‐Oxazoline)S via Cationic Ring Opening Polymerization Of 2‐Azobenzenyl‐2‐Oxazoline

“…Although the harmfulness of UV light limits the usage of azobenzene-containing polymers in bio-related applications, it is still highly attractive for other fields. [12][13][14] The majority of to-date described polymers with azobenzene units in the side chain are based on poly(meth)acrylates obtained via controlled radical polymerization of the respective monomers. Only a few examples of other classes of polymers containing azobenzene are known.…”

“…Although the harmfulness of UV light limits the usage of azobenzene‐containing polymers in bio‐related applications, it is still highly attractive for other fields. [ 12–14 ]…”

A Photomechanical Film in which Liquid Crystal Design Shifts the Absorption into the Visible Light Range

“…[ 23 ] Film thicknesses vary greatly between 2 [ 24 ] and 48 µm, [ 25 ] but thicknesses ≈20 to 30 µm are most common. [ 26 ] This is a limitation, because although thinner films bend more easily, they are more fragile and less useful in actuators.…”

“…For photomechanical polymers, certain basic robotic movements have been described. [ 26 , 27 ] However, the photo sensitive molecules used in this first generation of smart material require harmful UV‐light or blue light (blue light can also be harmful for organisms. See for example ref.…”