Ultrasound-assisted preparation of nanocomposites based on fibrous clay minerals and nanocellulose from microcrystalline cellulose

In the current work, ultrasound irradiation is used to obtain nanocomposite materials by assembling fibrous clay minerals (sepiolite or palygorskite) and nanocellulose (cellulose nanofibers, CNF), which can be directly produced from microcrystalline cellulose (MCC) through high-shear mechanical and sonomechanical treatments in the presence of the silicate. In this process, the ultrasound energy, evaluated from 1 to 50 kJ, produces the fibrillation from the outer layers of MCC together with partial disintegration of the amorphous regions, leading to high aspect ratio cellulose nanofibers with diameter below 100 nm and variable length around 1 mu m. In the presence of sepiolite, a stable gel is formed due to the homogeneous dispersion of both organic and inorganic components, even at very low applied energies like 1 kJ. The generated biohybrid gels can be processed by solvent casting as self-standing films, which show an increase in the Young's modulus with respect to films of sonicated MCC alone, with the best results of 3.0 GPa for sepiolite loadings around 20-35% w/w. The ultrasound treatment produced also a strong decrease in the hydrophilicity of MCC when the applied energy was higher than 20 kJ, as determined from the corresponding water sorption isotherms. However, the applied energy does not affect the water sorption capacity of the nanocomposites containing sepiolite, showing similar isotherms for a given composition independently of the applied energy. The main difference among those nanocomposites is due to the sepiolite content, and thus, the isotherms corresponding to compositions with 20% w/w of sepiolite are closer to that of MCC alone. Those samples with low sepiolite loadings and prepared at applied energies of 20 kJ or higher also showed better water vapor transmission rates (WVTR). These results confirm that an optimal loading of 20% w/w sepiolite provided the lowest value of WVTR (286 g/m(2) day) in comparison to nanocomposites with higher sepiolite content, while keeping good mechanical properties (Young's modulus of 2.8 GPa). This is an easy methodology for preparation of nanocellulose/fibrous clay minerals composite materials that allows the further incorporation of other functional nanoparticles such as carbon nanotubes, to obtain in this case biohybrid materials provided with electrical conductivity.