Thermo-Responsive Shape-Memory Polyurethane Foams from Renewable Lignin Resources with Tunable Structures–Properties and Enhanced Temperature Resistance

Polyurethane foams (PUFs) with renewable technical lignin additives have received both academic and industrial interest, justifying the need for further elucidation of their structure–property relationships. In this study, four types of lignin were mixed with polyester polyols at different substitution levels (10–25% wt). After thorough characterization of the lignin using NMR and GPC, viscosity measurements of the polyol and lignin mixtures indicated a positive correlation between the functional groups of lignin (COOH and aliphatic OH) and their polyol miscibility. The resulting lignin-based PUFs, after mixing the substituted polyol with polymethylenediphenyl diisocyanate, had a cellular structure with a density of 0.08–0.09 g/cm³ and a compressive modulus that ranged from 0.3 to 0.82 MPa. These properties were related to the amount of aromatic OH and COOH in lignin due to their low reaction rate with isocyanate groups relative to that with aliphatic hydroxyl groups. In terms of their thermo-responsive shape-memory properties, all PUFs showed increased glass-transition temperatures (T_g, 120–160 °C) and excellent stretch fixity (100%) and stretch recovery (96–80%) ratios that resulted in materials with promising applications related to shape-programmable PUFs with high-temperature resistance.