Preparation and Formation Mechanism of Covalent–Noncovalent Forces Stabilizing Lignin Nanospheres and Their Application in Superhydrophobic and Carbon Materials

Self-assembled lignin nanospheres (LNS) have attracted much attention due to the new opportunities provided for the preparation of value-added products derived from lignin. However, the internal connections of the LNS generally depend on weak intermolecular forces, leading to low solubility resistance and thermostability. In this study, we present a simple method for the fabrication of covalent–noncovalent forces stabilizing lignin nanospheres (HT-LNS) through utilizing the natural characteristic that lignin molecules undergo irreversible condensation under high-temperature stimulation. Experiments demonstrated that the action of temperature resulted in the fracture of β-O-4 ether and Cα–Cβ bonds, as well as hydroxyl and −OCH3 lignin molecule groups, leading to the formation of free radicals in the LNS. In addition, a large number of adjacent intramolecular and intermolecular radicals almost simultaneously generated chemical cross-linking via α-5, β-5, β–β′ bonds, and so forth. The amount of lignin molecules participating in the cross-linking reaction increased with temperature, which gradually reduced the HT-LNS diameter from 597 to 477 nm and enhanced the maximum decomposition peak from 367.7 to 395.1 °C. The solubility of nanospheres in ethanol and tetrahydrofuran (THF) decreased from 93.92 to 10.39% and from 98.09 to 22.45% with increasing treatment temperature, respectively. The HT-LNS can be employed in the preparation of superhydrophobic coatings, replacing non-environmentally friendly silica nanoparticles. The water contact and slide angles were determined as 151.9 ± 1.4 and 9.4 ± 0.5°, respectively. Moreover, the application of HT-LNS for the preparation of lignin-based carbon nanospheres maintained a perfect spherical structure with tiny graphitic area and the content of carbon atoms reached up to 94.99%. This study provides a simple and effective technology platform for the development of green materials.