Citric Acid-Modified Cellulose-Based Tough and Self-Healable Composite Formed by Two Kinds of Noncovalent Bonding

The development of tough, self-healing materials is a notable accomplishment. Previously, self-healable materials were soft materials with a tensile stress less than 10 MPa. Herein, we describe citric acid-modified cellulose (CAC)-based composites that are tough, stretchable, and self-healing. These properties are due to two kinds of noncovalent bonds from host–guest interactions between cyclodextrin derivatives that act as host units and alkyl derivatives with carboxyl ended groups that act as guest units in which hydrogen bonding (H-bonding) occurs with the carboxyl or hydroxyl groups in CAC. The host–guest interactions and H-bonding from the CAC-based composites material show a high energy dissipation that results in a high tensile stress (21 MPa) and high fracture energy (151 MJ·m–3) which are comparable to those in commercially available low-density polyethylene (LDPE). The reinforcement increases the mechanical properties by 10 times, as compared to those of the polymer without CAC. In addition to the high fracture energy, the CAC-based composites also show high stretchability and reach a strain of 1000%, which is usually reported in the 10% scale for composite materials. These two kinds of bonds also show interesting self-healing properties for the CAC-based composites. The self-healing ratio for the CAC-based composites is 56% at room temperature and 84% at 80 °C. This study successfully demonstrates that CAC-based composites using host–guest interactions and H-bonding show toughness, stretchability, and a self-healing capability.