Deep eutectic solvent-programmed interphase for tunable strength, stiffness-damping balance, and moisture durability in bamboo fiber/polyhydroxyalkanoate composites.

Insufficient interfacial compatibility and inefficient stress transfer limit the structural performance of natural fiber reinforced polyhydroxyalkanoate (PHA) biocomposites. In this work, the solid-to-liquid ratio of a choline chloride lactic acid (ChCl-LA) deep eutectic solvent (DES) was used as a programmable parameter to regulate the bamboo fiber (BF)/PHA interphase without external coupling agents. Van Soest analysis confirmed that DES pretreatment decreased hemicellulose and lignin contents from 22.9% and 23.0% to 9.9% and 8.7%, respectively, while increasing the relative cellulose fraction from 43.1% to 60.5%. These compositional changes, together with FTIR, XPS, and XRD results, indicate partial reduction or redistribution of non-cellulosic components, fiber surface reconstruction, and an increase in crystallinity index from 58.74% to 67.73%. The optimized interphase improved load transfer, giving D5-BF/PHA a 45.49% higher tensile strength than the untreated control. D15-BF/PHA provided a balanced stiffness-damping response with a tan δ peak of 0.27 and showed the lowest long-term water uptake. DES pretreatment also increased Tonset and Tmax by up to 10.75 °C and 12.65 °C, respectively, and raised the PHA crystallization temperature from 98.08 °C to 100.55 °C. These findings demonstrate that DES dosage programming offers an effective, scalable, and coupling-agent-free route to tune interfacial efficiency and balance mechanical, thermal, and moisture durability performance in fully biobased BF/PHA composites.