A Self-Healing Composite Film Made of Cellulose Nanocrystals and a Polyvinyl Acetate Copolymer

A cellulose nanocrystal (CNC)-polyvinyl acetate (PVAc) self-healing composite film was fabricated using a grafting-from approach generating polyvinyl acetate (PVAc) chains on CNC macroinitiators. These grafted-to CNCs were then mixed with bulk PVAc polymer to form a composite. Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and solid-state nuclear magnetic resonance were used to demonstrate the presence of the grafted PVAc chains on the surface of the CNCs. Transmission electron microscopy images revealed the structure of the modified CNCs, which formed closely packed clusters due to the grafted PVAc chains. The thermal properties of the CNCs and their composite films were assessed by using differential scanning calorimetry, determining the appropriate temperature for the healing of the composite film. On this basis, the film was cut into two pieces and rejoined and healed in an oven heated at 40 °C for 6 h. The healed sample was viewed under an optical microscope and electron microscopy, demonstrating the efficacy of the healing process. An array of microindentation tests across the surface of the healed specimen was conducted to quantify stiffness, revealing no detectable differences between the healed and intact regions. This healing was found to only occur for the grafted-to samples and was not evident for the composites made of PVAc and ungrafted CNCs. This work demonstrates that grafting polymer chains onto CNCs and blending these with a bulk polymer are promising approaches for fabricating composite films capable of healing macroscopic fractures.