Tough and Processable Hydrogels Based on Lignin and Hydrophilic Polyurethane

Area of Science:

• Materials Science
• Polymer Chemistry
• Biomaterials Engineering

Background:

• Lignin, a natural polymer, offers abundant polar sites for polymer cross-linking.
• Hydrophilic polyether-based polyurethane (HPU) hydrogels require improved mechanical properties and processability for wider applications.

Purpose of the Study:

• To investigate the use of lignin as a physical cross-linker for HPU hydrogels.
• To enhance the mechanical properties, processability, and adhesive capabilities of HPU hydrogels using lignin.

Main Methods:

• Incorporation of 2.5 wt% lignin into HPU hydrogel formulations.
• Mechanical testing including fracture energy and Young's modulus measurements.
• Analysis of toughening mechanisms using theoretical models (Lake-Thomas and sequential debonding theory).

Main Results:

• Lignin addition increased fracture energy (1540 to 2050 J m⁻²) and Young's modulus (1.29 to 2.62 MPa) without reducing swelling.
• Enhanced lap shear adhesiveness and immediate load recovery (95%) were observed.
• Hydrogen bonding was confirmed as the primary toughening mechanism.

Conclusions:

• Lignin effectively enhances HPU hydrogel mechanical performance and adhesion through physical cross-linking.
• The sequential debonding theory accurately models lignin's toughening effect.
• Lignin-loaded HPU hydrogels exhibit excellent processability (fiber spinning, casting, 3D printing) and biocompatibility.