From Cellulose Nanospheres, Nanorods to Nanofibers: Various Aspect Ratio Induced Nucleation/Reinforcing Effects on Polylactic Acid for Robust-Barrier Food Packaging

Abstract

The traditional approach toward improving the crystallization rate as well as the mechanical and barrier properties of poly(lactic acid) (PLA) is the incorporation of nanocelluloses (NCs). Unfortunately, little study has been focused on the influence of the differences in NC morphology and dimensions on the PLA property enhancement. Here, by HCOOH/HCl hydrolysis of lyocell fibers, microcrystalline cellulose (MCC), and ginger fibers, we unveil the preparation of cellulose nanospheres (CNS), rod-like cellulose nanocrystals (CNC), and cellulose nanofibers (CNF) with different aspect ratios, respectively. All the NC surfaces were chemically modified by Fischer esterification with hydrophobic formate groups to improve the NC dispersion in the PLA matrix. This study systematically compared CNS, CNC, and CNF as reinforcing agents to induce different kinds of heterogeneous nucleation and reinforce the effects on the properties of PLA. The incorporation of three NCs can greatly improve the PLA crystallization ability, thermal stability, and mechanical strength of nanocomposites. At the same NC loading level, the PLA/CNS showed the highest crystallinity (19.8 ± 0.4%) with a smaller spherulite size (33 ± 1.5 μm), indicating that CNS, with its high specific surface area, can induce a stronger heterogeneous nucleation effect on the PLA crystallization than CNC or CNF. Instead, compared to PLA, the PLA/CNF nanocomposites gave the largest Young's modulus increase of 350 %, due to the larger aspect ratio/rigidity of CNF and their interlocking or percolation network caused by filler-matrix interfacial bonds. Furthermore, taking these factors of hydrogen bonding interaction, increased crystallinity, and interfacial tortuosity into account, the PLA/CNC nanocomposite films showed the best barrier property against water vapor and lowest migration levels in two liquid food simulates (well below 60 mg kg for required overall migration in packaging) than CNS- and CNF-based films. This comparative study was very beneficial for selecting reasonable nanocelluloses as nucleation/reinforcing agents in robust-barrier packaging biomaterials with outstanding mechanical and thermal performance.

Ring-opening polymerization-induced crystallization-driven self-assembly of poly-L-lactide-block-polyethylene glycol block copolymers (ROPI-CDSA)

“…17). The distinction between nanorods and nanofibers is typically related to their aspect ratios, where nanorods typically have ratios of 3-15 and fibers typically are >> 15 56 . Here, the fibers appear to act as nucleation sites for the spheres and rods, directing their formation.…”

Reinforcing capability of cellulose nanocrystals obtained from pine cones in a biodegradable poly(3-hydroxybutyrate)/poly(ε-caprolactone) (PHB/PCL) thermoplastic blend

“…This effect could be due to the heterogeneous nucleating effect of CNCs on PHB which, give rise to the development of PHB small crystals with different morphology compared to typical PHB crystals [45,46]. Therefore, the presence of multiple PHB melting peaks may be related to the formation of a different crystal structures [47]. The melt peak temperature and crystallinity decreases with increasing CNCs content in the films, indicating imperfect crystals leads an easier and faster melting of the PHB as the content of CNCs increases [48].…”

Crystalline nanocellulose/thermoplastic polyester composites prepared by in situ polymerization

“…However, it is remarkable that increases of 20°C or higher were achieved at very low concentrations of CNC. By contrast the increase observed in PESu in the presence of 5 wt% SiO 2 particles was only about 5°C . The enhanced effectiveness in nucleation may be attributed to the higher aspect ratio of CNC platelets, which allows a much larger specific surface area for interaction with the polymer chains, compared with the low‐aspect ratio fillers, such as SiO 2 .…”