Nanolignin by Ultrasonication: Tuning the Process for Tailored Materials Characteristics

In this work, a Design-of-Experiments (DoE) methodology was adopted to study and optimize the ultrasonication process of lignin in water to obtain lignin nanoparticles (LNPs) so as to establish qualitative and quantitative cause-effect relationships between the operating variables (i.e., lignin concentration, processing time, and sonication amplitude) and the distinctive features of the so-obtained nanostructured materials. Monitoring the evolution of the chemical, physical, morphological, and thermal characteristics of lignin upon ultrasonic irradiation enabled the identification of the most relevant process parameters (viz., factors) affecting the average particle size and shape, concentration of hydroxyl/carboxyl groups, and suspended fraction of particles, which were taken as the target materials characteristics (viz., systems responses). Interestingly, the selection of suitable process conditions allowed to obtain exclusively spherical LNPs with perfectly circular shape profile, thus providing the first demonstration of the formation of lignin nanospheres through a size-reduction method not relying on the use of solvents or other chemicals. Furthermore, highly predictive fitting models were generated and validated to accurately and reliably estimate the system responses at any experimental point within the investigation range. As a validation of such process optimization, poly-(vinyl alcohol) (PVA) films incorporating up to 20% (w/w) of either pristine or ultrasound-treated lignin were prepared so as to evaluate the effect of tailored lignin (nano)-particle characteristics on the chemical, morphological, thermal, and mechanical properties of the resulting (nano)-composite systems. The nanometric size of LNPs, together with their spherical morphology and lower molecular weight vs pristine systems was found to foster stronger matrix-filler interactions and improved distributive mixing within the PVA matrix, ultimately leading to higher-performance nanocomposite materials. This work provides new insights into the production of LNPs via ultrasonication, demonstrating that precise control over their size, morphology, and chemical functionalities can be predictively attained by the optimal tuning of relevant process parameters.