Poly(lactic acid)/Cellulose Nanocrystal-Based Compositional Gradient Nanocomposites with Higher Toughness: Potential for Flexible Packaging

The high brittleness of the PLA film restricts its wider application in flexible packaging. Functionally graded materials, found in many biological systems, demonstrate tailored material properties due to a gradual variation in structure and composition along a specific direction. Herein, we prepared compositionally graded biobased nanocomposites of PLA reinforced with 3-aminopropyltriethoxysilane-modified cellulose nanocrystals (ASCNCs). We utilized a combination of nanofiller reinforcement and compositional (full and half) gradient architectures to achieve higher toughness and flexibility in the PLA/ASCNC graded nanocomposites. The compositional gradient architecture is formed via sequential solvent casting and controlled drying of different layers with varying PLA/ASCNC feed ratios along the film thickness. The influence of compositional gradients on the microstructure and mechanical properties of the graded PLA/ASCNC films was evaluated. For comparison, the nongraded single-layered PLA/ASCNC films reinforced with different ASCNC loadings as well as multilayered PLA/ASCNC laminates were fabricated. The PLA/ASCNC gradient nanocomposites exhibited superior toughness, strain hardening, higher elongation-at-break, and flexibility compared to pure PLA, PLA/ASCNC nongraded films, and laminates. While the PLA/ASCNC nongraded film (3 wt % ASCNC loading) displayed higher mechanical properties compared to pure PLA providing an ultimate tensile strength of 71 ± 5 MPa, a toughness of 2.8 ± 0.7 MJ m–3, and an elongation-at-break of 5.5 ± 1.2%, the full gradient nanocomposite exhibited an excellent toughness of 57 ± 23 MJ m–3 and elongation-at-break of 122 ± 40% with ultimate tensile strength and stiffness comparable to the nongraded PLA/ASCNC films. Such an improvement in mechanical properties in compositional gradient nanocomposites is primarily due to the interplay of the reinforcing ability of ASCNCs, good interfacial interactions, gradient architecture, enhanced energy dissipation, controlled crack propagation, and efficient stress transfer through the gradient layers. In terms of functionality, the gradient films displayed transparency, low water vapor permeability, biodegradability, and a longer shelf life for perishable cut fruits, thus showing potential for flexible food packaging applications.