Real-Time Polarized Photo-Rheo-IR Investigation of Shear-Induced Alignment in Liquid Crystal Elastomers

Materials with dynamic responses are pivotal for advances in soft robotics, flexible electronics, and additive manufacturing. Liquid crystal elastomers (LCEs) are especially promising due to their reversible shape changes and anisotropic mechanical properties, which depend on the molecular alignment induced during their processing and their capability to retain it afterward. While techniques such as hot-melt extrusion (HME) in 3D printing have been used to induce shear alignment of liquid crystalline domains, current approaches to reproduce, capture and characterize in real-time the evolution of this behavior are scarce. Here, we use a RM82-based oligomer liquid crystal photopolymerizable ink (LC ink) as a model system to monitor simultaneously the evolution of its order parameter ⟨P2⟩ and its rheological properties employing a rheo-IR equipment that integrates a strain-controlled rheometer with polarized attenuated total reflection infrared spectroscopy to reproduce the conditions experienced by the LC ink upon its HME. To preserve the shear-induced alignment in the RM82-based LC ink, a photopolymerization step is applied during HME, and a polarized photo-rheo-IR setup is employed to mimic the irradiation process upon extrusion. These experiments bridge the molecular and macroscopic scale, enabling us to gain valuable insights on the behavior of the LC ink upon its extrusion. This integrated approach provides direct, time-resolved insight into the interplay between processing conditions and resulting microstructure and properties, offering a promising pathway for rational processing guidelines to modulate and optimize the LCE performance, notably in actuators and soft robotics.