Shaping Liquid Crystal Rubbers by Chemical Potential Gradient

Techniques for shaping materials have constantly evolved from the making of hand axes in the Stone Age to today's 3D and 4D printing. One of the 4D printing techniques is utilizing chemical potential gradient where uniform swelling of gels with a structural gradient leads to preprogrammed shapes. This work investigates the inverse effect: swelling a uniform structure by a diffusion gradient. First, the recently observed temporal shape changes of liquid crystal elastomer (LCE) films based on diffusion of low molecular weight liquid crystal drops into the LCE are simulated, showing excellent agreement with the experiments. Second, ways of locking the desired LCE shape by UV-induced polymerization or by crystallization of a reactive low molecular weight LC additive are investigated. It is shown that the crystallization of the diffusing low molecular weight LC provides the fastest and most complete locking of the shape. It is envisioned that using the gradient of the chemical potential either by pre-patterning the director structure of the LCE film or by appropriate spatial distribution of LC droplets, any complex shapes of LC rubbers can be engineered. This may provide a new and effective way of shaping rubbers for applications in various fields, such as sealants, soft robotics and bioimplants.