Impact of Solvent Amount on the Structure and Actuation Properties of Liquid Crystal Elastomers

Liquid crystal elastomers (LCEs) are soft elastic networks that have liquid-crystalline ordering and rubber-elastic properties. The nematic phase, where molecules (mesogens) are aligned, depends on intermolecular interactions and temperature. This unique phase endows LCEs with high energy-dissipating properties over a wide temperature range. When the temperature rises to a specific point, LCEs transition from the nematic to the isotropic phase and change their shape. This makes LCEs useful for various fields, especially as actuators.<br/>While a lot of research has focused on modifying LCE components to alter their actuating properties, we studied how changing the solvent amount, which is removed after LCEs are polymerized, affects their actuation properties. We found that more solvent leads to higher actuation strains but lower stress, without altering the LCE composition. Notably, the actuation strain increased from 25.47% to 38.63% (almost 150% more) with temperature changes, while the actuation stress remained at 0.11 MPa. But too much solvent lowered the stress because there was less liquid crystal in the unit volume.<br/>To understand these changes, we examined the LCE structure using polarized optical microscopy and wide-angle X-ray scattering. We observed that the rice pattern size, which reflects domain size, grows with more solvent. This is due to a lower cross-linker density per unit volume, leading to increased free volume size and facilitating easier domain rotation. Additionally, the distance between mesogens increases with more solvent, allowing for greater mobility of liquid crystals. Consequently, these changes enhance the stretchability and actuation of the LCE but reduce stress resistance. This study highlights that the amount of solvent used in LCE synthesis has a significant effect on the structure and properties of the polymer, providing a way to design a LCE with desired characteristics.