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 beta-O-4 ether and C-alpha-C-beta 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 alpha-5, beta-5, beta-beta' 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 degrees 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 degrees, 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.

Automated summary

This study presents a method for the fabrication of stable lignin nanospheres (HT-LNS) through high-temperature stimulation to improve solubility and thermal stability. Results showed that as temperature increased, HT-LNS diameter decreased, maximum decomposition peak increased, and solubility decreased. These nanospheres can be used for preparing superhydrophobic coatings and carbon nanospheres.