Document details

Moisture-responsive mechanisms are widespread in nature, particularly in plants exhibiting hygroscopic motion. Inspired by these systems, recent research has focused on designing materials that replicate humidity-driven movements for various applications. Cellulose-based films can reversibly deform in response to humidity changes, enabling anisotropic movements like those observed in Erodium awns. These movements serve as inspiration to create polymeric bio-based humidity-driven soft actuators and water harvesting collectors that can be produced when cellulose-based films are prepared using appropriate techniques. In this study, hydroxypropyl cellulose (HPC) films were crosslinked with citric acid (CA) and processed by shear casting followed by thermal treatment. Eight samples were produced: two pure HPC films dried at room temperature and at 150 °C, respectively, and six crosslinked films with variable CA content, all treated at 150 °C. Structural and mechanical characterization studies using Small Angle Light Scattering (SALS), Polarized Optical Microscopy (POM), Scanning Electron Microscopy (SEM), and tensile testing revealed that temperature and CA concentration strongly influence the films’ anisotropy. Optimal anisotropic properties were observed at 7.5 wt% CA, where POM also identified topological defects, indicating a phase transition from nematic to a cholesteric ordering. Films with this CA content also showed the most distinct periodic bands, suggesting enhanced domain formation and increased rigidity, in agreement with tensile test results. Upon exposure to moisture, the films exhibited reversible helicoidal twisting closely resembling Erodium awn motion. These findings demonstrate that citric acid crosslinking effectively modulates the hierarchical structure and moisture responsiveness of HPC films, paving the way for their application as humidity sensors and water-harvesting materials.