Controlling the Properties of Poly(3-hydroxybutyrate) through Lignin-Containing Organogels

Poly(3-hydroxybutyrate) (PHB) is a biodegradable polyester with a slow hydrolytic degradation rate linked to its high crystallinity. Here, PHB-lignin organogels were prepared using the bio-based solvent Cyrene to investigate how processing and lignin type, softwood (SKL) or hardwood (HKL) kraft lignin, affect structure and water stability. Freeze-dried organogels (up to 50 wt % lignin) were analyzed by FTIR spectroscopy, SEM, DSC, and XRD. Organogel formation reduced PHB crystallinity by about 10%, while lignin incorporation further decreased it by up to 84% (SKL) and 73% (HKL), lowering melting enthalpy and promoting cold crystallization. Although HKL caused slightly earlier crystallization suppression during cooling and smaller crystallites at low loadings, both lignin types produced similar crystallinity reductions and degradation behavior. Water exposure increased the mass loss compared with neat PHB, consistent with decreased crystallinity. Solvent extraction enabled partial lignin recovery and PHB recrystallization. Overall, a processing-based approach to tuning PHB crystallinity and degradation in lignin-containing biocomposites is demonstrated.