Green cross-linked coir cellulose nanocrystals/poly (vinyl alcohol) composite films with enhanced water resistance, mechanical properties, and thermal stability

Poly(vinyl alcohol) (PVA) based materials have received enormous attention ascribed to the merits of good mechanical properties, biodegradability, and environmental friendliness, but its widespread applications are severely hindered by poor water resistance. In the current study, self-made coir cellulose nanocrystal (CCNC) was applied as a reinforcing filler together with a cost-effective and green cross-linker (oxalic acid, OA) for preparation of CCNC/OA/PVA composite film via solution casting. After the thermal treatment of 120°C for 2 h, the ultimate product exhibited remarkable improvements in tensile strength (132.4 MPa), Young's modulus (4427.1 MPa), water resistance (degree of swelling: 33.6%; contact angle: 84.7°; solubility: 0.9%), and thermal stability relative to the cleavage of the polymer chains (depolymerization). A reinforced synergetic effect of physical interaction (hydrogen bond) and hydrophobic ester linkages has been suggested according to the comprehensive analyses of functional groups, crystal structure, thermal behavior and stability, and fracture surface morphology.