Sustainable PLA Bio-Nanocomposites: Integration of TPU Nanofibrils and CNC for Enhanced Crystallization, Toughness, Stiffness, Transparency, and Oxygen Barrier Properties

This study explores the interplay between elastomeric nanofibrillar thermoplastic polyurethane (TPU) and cellulose nanocrystals (CNCs) to expand the applications of poly(lactic acid) (PLA) composites via optimization of the nanofibril TPU/CNC ratio. Advanced analytical methods reveal the contrasting effects of CNC stiffening and TPU toughening capabilities. Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) confirm isophorone diisocyanate (IPDI) as a cross-linker and chain extender, altering the polymer network, while X-ray photoelectron spectroscopy (XPS) suggests hydrogen bonding between CNC and TPU. Scanning electron microscopy (SEM) shows that CNC reduces TPU spherical domain sizes from 270–350 to 200–270 nm and alters the nanofibril TPU diameter from 90–280 to 100–320 nm. CNC accelerates PLA crystallization, reducing the crystallization half-time from 21 to 0.87 min, and optimizes crystallinity at 100 °C. Higher annealing temperatures reduce oxygen transmission rates from 66 to 16 cc/(m2·day) with 1 wt % CNC at 130 °C due to denser α-crystal formation. Transparency studies show minimal impact on PLA clarity up to 0.6 wt % CNC, with fibrillar TPU maintaining superior transparency. Mechanical tests reveal significant increases in tensile toughness, from 1.9 MPa in neat PLA to 30.9 and 38.2 MPa with 3 and 6 wt % TPU, respectively. CNC further enhances these properties at lower TPU concentrations, improving tensile strain up to 3900 times that of neat PLA while maintaining tensile strength and Young's modulus. Morphological analysis reveals detailed toughening mechanisms, where integrating fibril TPU with CNC refines void structures and enhances fibril formations, leading to ductile cup-and-cone fracture behaviors. This configuration significantly improves ductility, promoting plastic deformation and forming microvoids and crazes. These findings highlight the potential of optimized CNC and TPU ratios to broaden the functional scope of PLA composites, suggesting promising strategies for advanced material design toward an eco-friendlier industry.