Probing cellulose hydrogel dehydration with Brillouin spectroscopy: Insights into mechanical properties

Hydrogels are three-dimensional networks of hydrophilic polymers that retain large amounts of water and can be tailored for medicine, agriculture, electronics, and cosmetics. Their softness and tunable density complicate contact-based mechanical testing. Here, we explore the mechanics of a cellulose-based hydrogel using Brillouin light scattering (BLS) spectroscopy, a non-contact optical probe. We study micro-fibrillated cellulose hydrogels prepared via two drying routes—a dense, ambient-dried film and a foam-like, freeze-dried aerogel—pre-characterized by tensile testing. BLS is then used to extract longitudinal sound velocity and stiffness in three scattering geometries: reflective backscattering 180a and 90r and transmissive 90a. As expected, the structural arrangement imposed by drying (i.e., material density) dominates the stiffness, resulting in a stiffness of 3 GPa for the aerogel and 19 GPa for the hydrogel film. We further track moisture effects by changing the relative humidity (RH) level (40% RH and 75% RH), which leads to a decrease in frequency shift and a broadening of the Brillouin peaks with increasing RH, and a drop in stiffness by factor two. Time-resolved BLS tracks dehydration kinetics: fully wetting the hydrogel film and merely changing RH produce different Brillouin frequency shift dynamics. These results explore BLS as non-contact method for in situ measurement of mechanical properties during conditioning, with further potential applications during processing of technologically relevant soft and polymeric materials.