Unique two-way free-standing thermo- and photo-responsive shape memory azobenzene-containing polyurethane liquid crystal network

In this work, azobenzene-containing polyurethane liquid crystal networks (PULCN(AZO)s) were synthesized using a one-pot strategy to demonstrate excellent two-way free-standing thermo-/photo-responsive shape memory effects. Based on the step-growth nature of hydroxyls and isocyanates, the architectures of the networks were adjusted by controlling the stoichiometries of chemical materials. A uniform monodomain sample was prepared by external stress relaxation via a reversible addition reaction of a dynamic carbamate bond. Two independent transition temperatures assigned to glass transition temperature/melting temperature and liquid crystal phase transition temperature were employed to thermally trigger triple shape memory effects and two-way autonomous actuation. In addition, taking advantage of the trans-cis photoisomerization of azobenzene, the programmed network showed a reversible bending and unbending shape change upon irradiation by visible light at 450 and 550 nm, respectively. Coupling the autonomously thermo-induced contraction/extension actuation and reversible photo-induced bending/unbending behaviors of PULCN(AZO)s in one system will expand their potential applications in emerging multifunctional devices.