Valorization of water hyacinth: Efficient extraction of nanofibrillated crystals using ultrasound-assisted TEMPO oxidation and their application in flexible piezoelectric films

This study presents an efficient extraction of nanocellulose crystals (CNCs) and nanofibers (CNFs) through ultrasound-assisted TEMPO oxidation method using deep-processed heavy metal-containing calabash as raw material. The extraction method, integrating TEMPO-mediated oxidation and ultrasound techniques, enhanced the yield of CNCs and CNFs. The results demonstrate that CNCs yielded 63 %, with a carboxyl content of 1.27 mmol/g, while CNFs yielded 31 %, with a carboxyl content of 1.21 mmol/g. CNCs and CNFs exhibited excellent mechanical properties, with CNCs having an average length of 214.4 nm, average diameter of 2.72 nm, and a length-to-diameter ratio of 78.8. CNFs had an average length of 437.8 nm, average diameter of 5.7 nm, and a length-to-diameter ratio of 76.8. X-ray diffraction (XRD) results showed crystallinity of 87.1 % for CNCs and 81.2 % for CNFs. The thermal stability of the fibers was confirmed up to 230℃. The film prepared by combining 10 % PVA with 90 % CNC exhibited a tensile strength of 32.11 MPa, Young's modulus of 2027.1 MPa, and could generate a maximum voltage of 0.83 V. This study provides an effective pathway for the value-added utilization of deep-processed calabash and highlights the broad application prospects of the composite film in the field of flexible sensors.