Bioinspired Bouligand-Structured Cellulose Nanocrystals/Poly(vinyl alcohol) Composite Hydrogel for Enhanced Impact Resistance

Impact-protective materials are gaining
importance because
of the
widespread occurrence of impact damage. Hydrogels have emerged as
promising candidates owing to their lightweight and flexible nature.
However, achieving soft impact-resistant hydrogels with exceptional
stiffness, strength, and toughness remains a challenge. Inspired by
the Bouligand structure found in the smasher dactyl club of stomatopods,
we propose a straightforward multiscale hierarchical structural design
strategy. This strategy integrates self-assembly and salting-out techniques
to enhance the impact resistance of soft hydrogels. Rigid cellulose
nanocrystals (CNCs) self-assemble into Bouligand-like structures within
soft poly(vinyl alcohol) (PVA) matrix via supramolecular interactions.
This rational structural design combines the CNC Bouligand structure
with a cross-linked network of soft PVA crystalline domains, resulting
in a composite hydrogel with impressive mechanical properties: high
tensile fracture strength (30.2 MPa), elastic modulus (62.7 MPa),
and fracture energy (75.6 kJ m–2), surpassing those
of other tough hydrogels. Moreover, the multiscale hierarchical structure
facilitates various energy dissipation mechanisms, including crack
twisting, tortuous crack paths, and PVA chain orientation, resulting
in notable force attenuation (80.4%) in the composite hydrogel. This
biomimetic design strategy opens new avenues for developing soft and
lightweight impact-resistant materials.