Lignin in Bio-Based Liquid Crystalline Network Material with Potential for Direct Ink Writing

The flow-induced supramolecular arrangement, or band texture, present in water-soluble anisotropic films prepared from blend solutions of hydroxypropyl cellulose and organosolv lignin is locked via esterification with bio-based polycarboxylic acids. Subsequent to shear casting of the blend solutions, the chemical cross-linking with citric acid-based cross-linkers and a dimerized fatty acid yields water-insoluble, anisotropic films prone to swelling in water. The liquid crystalline networks are analyzed by means of polarized optical microscopy, tensile testing, Fourier transform infrared, and swelling experiments. Depending on the cross-linker, the dry "banded" films reach up to 3.5 GPa in tensile modulus, 80 MPa in tensile strength along the shear direction, and 5 MJ/m³ toughness across the shear direction. Films are softened upon water uptake causing a reversible extinguishment of the banded texture without interfering with the specimens' anisotropy. Rheological studies point to the applicability of highly concentrated blend solutions to direct ink writing. The implementation of the findings to the additive manufacturing of cross-linked 3D structures demonstrates the potential of a resource-friendly processing of fully bio-based materials.