Functionalization of cellulose nanocrystal powder by non-thermal atmospheric-pressure plasmas

Despite promising characteristics such as the biodegradability and the environmentally benign nature of cellulose nanocrystal (CNC) based composites, their poor dispersion and agglomeration in thermoplastic matrix during the melting process is a “bottleneck” in the development of these composites. In this work, a cylindrical atmospheric pressure dielectric barrier discharge was employed to functionalize CNCs to reduce their surface hydrophilicity and improve their dispersion in polar organic solvents. Three different gas mixtures were used for plasma treatment, argon/methane, argon/silane and an argon/methane followed by argon/silane. In all cases, the plasma treatment was conducted below 90 °C as determined from optical emission spectroscopy analysis. The X-ray diffraction analysis of both raw and plasma-treated CNC powder confirmed that the CNC crystallographic properties remained unchanged after plasma treatment. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analysis revealed the presence of hydrophobic C–H_x moieties on the CNC granular surface after argon/methane plasma treatment whereas SiH_x, Si–O–Si, SiC bonds were formed after argon/silane plasma treatment. Under these experimental conditions, water wettability tests revealed some significant water repellency for the treated cellulosic material. Moreover, the SiHx moieties formed in silane-treated CNCs clearly enhanced the hydrophobicity of the CNC powder. Contrariwise, the sole CHx moieties synthesized by argon/methane plasma did not yield such enhancement of the CNC wettability. High-resolution scanning electron microscopy images showed the presence of agglomerated granules with 5–10 µm diameters in size. The surface functionalities of CNC powder enhanced its dispersibility in polar solvents. Overall, this study emphasizes that atmospheric pressure dielectric barrier discharge is suitable to process thermo-sensitive CNCs.