Directed 2D nanosheet assemblies of amphiphilic lignin derivatives: Formation of hollow spheres with tunable porous structure

Abstract

Synthesis of polymeric hollow spheres with porous structures has attracted significant attention in material applications. In the present work, without requiring any template and core removal, hollow spheres with tunable porous structures from amphiphilic lignin polymers were synthesized by directing the different length of hydrophobic alkyl chains. Remarkably, the results from scanning electron microscope (SEM) showed that lignin hollow sphere possessed only one hole by acetylation, while it became porous structures with the increase of alkyl chain length by acylation of propionylation, butyrylation and valerylation. Amazingly, all the LHSs exhibited strong emission peaks in visible spectral range as excitated with wavelength from 260 to 340 nm. In addition, as revealed by SEM and transmission electron microscope (TEM) images, the formation mechanism for the porous hollow spheres was proposed. The presence of hydrophobic and π–π interactions between the aromatic groups resulted in the lignin molecules gathering mutually to form the bilayers (nanosheets) with some imperfection sites. Then, the 2D patches intended to reduce their total energy via curling, which favored the formation of initial sphere structures, and other 2D patches kept continuously stacking on the surface of spheres, resulting in formation of porous lignin hollow spheres. Our study established a new method for fabricating porous hollow spheres from amphiphilic lignin polymers.