Effects of Crosslink Density on Photomechanical Jumping of Glassy Liquid Crystalline Polymer Networks

In nature, jumping is a key survival strategy for living animals as it allows them to rapidly navigate through rough terrains and overcome obstacles that are often several times their body size. In our previous report, we newly demonstrated a continuous and directional UV-induced photo-mechanical jumping of polymer monolith by introducing snap-through buckling of super twisted nematic (STN) structured azobenzene functionalized liquid crystal polymer networks (azo-LCNs). However, the intense light conditions of 0.3 W cm^-2 intensity and the long-term UV irradiation required for high jumping have raised concerns over the possible bond breakage and the subsequent decrease in the jumping capacity. Herein, we control the duration of UV irradiation for photopolymerization to vary the crosslink density and investigate the effect of the structure-property relationship on mild-condition jumping. Crosslink density-controlled azo-LCNs are heated to various substrate temperatures and irradiated upon different light intensities to achieve a systematic investigation of jumping heights. When the curing time is reduced from 60 min to 5 min, the storage modulus (E’) of the azo-LCN monolith is successfully lowered from 74.8 MPa to 33.1 MPa in the plateau regime, and the bending curvature is increased from 2.1 mm^-1 to 4.4 mm^-1, respectively. The reduced stiffness of the azo-LCN body increases the photomechanical strain response under actinic UV light, thereby allowing the body to store greater photo-induced potential energy. Remarkably, a maximum jumping height of 15.9 mm is reached at a milder light intensity of 0.1 W cm^-2, which is a 214% increase in height compared to the previous result obtained at identical UV light conditions.