Preparation of Self-Assembled Sodium Carboxymethylcellulose/Polydopamine-Modified Cellulose Nanocrystals Ternary Nanocomposites: Implications for High-Performance UV-Shielding Films

The mechanically robust, high-performance, and functional nanocomposites have recently attracted considerable attention in both scientific and industrial research. Herein, we prepare the high-performance ternary nanocomposites using naturally occurring building blocks via a water-borne evaporation-induced self-assembly (EISA) process. Cellulose nanocrystals (CNCs) have been used as reinforcing fillers to be incorporated into the sodium carboxymethylcellulose (CMC) polymer matrix. The surface of CNCs was coated with polydopamine (PDA) to further improve the inherent properties of the CMC–CNC binary nanocomposites. The PDA acts as a bridge to enhance the interfacial interactions between CNCs and CMC. We optimized the PDA coating thickness on CNCs to tune the interfacial adhesion by varying the self-polymerization time of dopamine. The results showed that the self-polymerization of dopamine on the CNC surfaces for 24 h (DCNC-24) led to better structure formation and improved mechanical and thermal properties of the CMC–DCNC-24 ternary nanocomposites. At 50% RH, the CMC–DCNC-24 ternary nanocomposites exhibit an excellent tensile strength of 154 ± 12 MPa, Young's modulus of 10 ± 1 GPa, and toughness of 5 ± 1 MJ/m3. Due to their enhanced and stable interfacial interactions, the ternary nanocomposites demonstrate higher mechanical performances even at high relative humidity conditions compared to the pure CMC and CMC–CNC films. At 80% RH, CMC–DCNC-24 films have a strength of 36 ± 4 MPa, stiffness as high as 4.0 ± 0.5 GPa, and toughness of 4.0 ± 1.0 MJ/m3. The CMC–DCNC ternary nanocomposites have also shown enhanced thermal stability. In addition, the existence of PDA with UV-absorption ability induces UV-shielding properties to CMC–DCNC ternary films. Thus, the self-assembled CMC–DCNC ternary nanocomposites prepared in this work have implications to be applied as high-performance UV-shielding films for packaging.