The influence of ink preparation on the properties of 3D-printed liquid crystal elastomers

Liquid crystal elastomers (LCEs) are excellent soft active materials for applications such as artificial muscles and soft robotics due to their capacity for large, reversible, and stimuli-responsive shape changes. This actuation depends on the alignment of their internal rigid units (mesogens). Direct ink write (DIW) is a method for creating aligned LCEs, utilizing shear and extensional forces to align the LCE oligomer ink before in situ curing. Conventionally, ink preparation is treated as a passive step with the prevailing assumption that the initial chemical formulations dictate the final material behavior. Here, we demonstrate that the preparation method alters both the underlying polymer network and the mechanical properties independent of any changes to the starting materials or feed ratios. LCE oligomer inks were synthesized using three standardizations of literature techniques: heating using an oil bath (OB), heating with a heat gun (HG), and solvent-assisted heating (SOL). Despite maintaining all material components, feed ratios, reaction times, and temperatures constant, the preparation method alone produced oligomeric inks with differences in viscosity and degrees of polymerization. When printed via hot DIW, this resulted in differences in birefringence, alignment, tensile mechanical properties, and thermal actuation in the printed filament. Furthermore, the impact of the preparation method became increasingly pronounced as a function of the print draw ratio (DR). Our findings emphasize that LCE ink processing methodology is critical for controlling the properties of DIW-printed LCEs.