Greener transformation of lignin into ultralight multifunctional materials

Abstract

Shape recoverable and ultralight materials have received significant attention for multifunctional applications, and emulate the performance found in natural materials. Nature utilizes fiber motifs to obtain structural properties and simultaneously serve as scaffolds for functionally graded materials. These structural matrices have dimensions that can be mimicked through electrospinning of polymeric materials into submicron to nanoscale fibrous scaffolds. This study explains how extremely brittle biopolymer materials composed of 99 wt% softwood kraft lignin were transformed into three-dimensional (3-D) flexible fibrous materials without using any external chemical cross-linking agents. This work reports a new design concept to form advanced lignin-based materials by exploiting the inherent thermal instability of technical lignin, which is heat treated in order to create a purely elastic material. The in situ rheological tests indicated that lignin fibers undergo simultaneous softening starting at 175 °C and cross-linking above 225 °C that allows the formation of a connected 3-D network. The resulting ultralight materials were composed of short sub-micron fibers that showed significant elastic resilience with shape recovery after deformation. Further, the samples were carbonized with minimal shrinkage resulting in carbon based fibrous materials. These materials show multiple functionalities for both thermally stabilized and carbonized samples including flexibility and shape recovery across a 350 °C temperature range, significant oil/solvent absorption from water, fire resistance, formation of slippery surfaces, and electrical conductivity as a function of deformation.