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

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.