Soft-Segment Containing Lignin-Based Polyhydroxyurethanes: Controllable Flexibility Through PDMS Integration

Lignin, a biomass-derived polymer, is rich in aromatic groups composed of phenylpropane units, providing rigidity, thermal stability, and mechanical strength ideal for structural applications. However, its inherent stiffness limits flexibility. To address this, lignin is copolymerized with bis(3-aminopropyl)-terminated poly(dimethylsiloxane) (PDMS-NH₂), introducing tunable elasticity and improved mechanical properties. Lignin is first modified with CO₂ to produce cyclic carbonate-functionalized lignin (CCL). The CCL reacts with PDMS-NH₂ amines via the ring-opening of cyclic carbonates, forming soft-hard polyhydroxyurethane copolymers with adjustable properties (CCL-PDMS-PHUs). Structural characterization confirms urethane bond formation, with peaks at 1680 cm⁻¹ (FT-IR) and chemical shifts at 161.33 ppm (¹³C NMR) and 7.99 ppm (¹H NMR). Thermal analysis reveals 5% decomposition temperatures of 246-265 °C and glass transition temperatures (Tg) ranging from 44 to 66 °C, indicating tunable thermal stability. Mechanical testing shows that CCL-PDMS-PHU60 (60% PDMS-NH₂) exhibited greater softness, while CCL-PDMS-PHU40 (40% PDMS-NH₂) displayed higher stiffness, highlighting PDMS-NH₂'s effect on flexibility. These results demonstrate that CCL-PDMS-PHUs offer customizable mechanical and thermal properties, making them promising materials for applications requiring tailored elasticity and thermal performance.