Poly(lactic acid)/Cellulose\nNanocrystal-Based Compositional\nGradient Nanocomposites with Higher Toughness: Potential for Flexible\nPackaging

The high brittleness of the PLA film restricts its wider\napplication\nin flexible packaging. Functionally graded materials, found in many\nbiological systems, demonstrate tailored material properties due to\na gradual variation in structure and composition along a specific\ndirection. Herein, we prepared compositionally graded biobased nanocomposites\nof PLA reinforced with 3-aminopropyltriethoxysilane-modified cellulose\nnanocrystals (ASCNCs). We utilized a combination of nanofiller reinforcement\nand compositional (full and half) gradient architectures to achieve\nhigher toughness and flexibility in the PLA/ASCNC graded nanocomposites.\nThe compositional gradient architecture is formed via sequential solvent\ncasting and controlled drying of different layers with varying PLA/ASCNC\nfeed ratios along the film thickness. The influence of compositional\ngradients on the microstructure and mechanical properties of the graded\nPLA/ASCNC films was evaluated. For comparison, the nongraded single-layered\nPLA/ASCNC films reinforced with different ASCNC loadings as well as\nmultilayered PLA/ASCNC laminates were fabricated. The PLA/ASCNC gradient\nnanocomposites exhibited superior toughness, strain hardening, higher\nelongation-at-break, and flexibility compared to pure PLA, PLA/ASCNC\nnongraded films, and laminates. While the PLA/ASCNC nongraded film\n(3 wt % ASCNC loading) displayed higher mechanical properties compared\nto pure PLA providing an ultimate tensile strength of 71 ± 5\nMPa, a toughness of 2.8 ± 0.7 MJ m<sup>–3</sup>, and an\nelongation-at-break of 5.5 ± 1.2%, the full gradient nanocomposite\nexhibited an excellent toughness of 57 ± 23 MJ m<sup>–3</sup> and elongation-at-break of 122 ± 40% with ultimate tensile\nstrength and stiffness comparable to the nongraded PLA/ASCNC films.\nSuch an improvement in mechanical properties in compositional gradient\nnanocomposites is primarily due to the interplay of the reinforcing\nability of ASCNCs, good interfacial interactions, gradient architecture,\nenhanced energy dissipation, controlled crack propagation, and efficient\nstress transfer through the gradient layers. In terms of functionality,\nthe gradient films displayed transparency, low water vapor permeability,\nbiodegradability, and a longer shelf life for perishable cut fruits,\nthus showing potential for flexible food packaging applications.