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.