Ultrahigh Tough, Super Clear, and Highly Anisotropic Nanofiber-Structured Regenerated Cellulose Films

While tremendous
efforts have been dedicated to developing environmentally
friendly films made from natural polymers and renewable resources,
in particular, multifunctional films featuring extraordinary mechanical
properties, optical performance, and ordered nanostructure, challenges
still remain in achieving all these characteristics in a single material via a scalable process. Here, we designed a green route
to fabricating strong, super tough, regenerated cellulose films featuring
tightly stacked and long-range aligned cellulose nanofibers self-assembled
from cellulose solution in alkali/urea aqueous systems. The well-aligned
nanofibers were generated by directionally controlling the aggregation
of cellulose chains in the hydrogel state using a preorientation-assisted
dual cross-linking approach; i.e., a physical cross-linking
was rapidly introduced to permanently reserve the temporarily aligned
nanostructure generated by preorienting the covalent cross-linked
gels. After a structural densification in air-drying of hydrogel,
high strength was achieved, and more importantly, a record-high toughness
(41.1 MJ m–3) in anisotropic nanofibers-structured
cellulose films (ACFs) was reached. Moreover, the densely packed and
well-aligned cellulose nanofibers significantly decreased the interstices
in the films to avoid light scattering, granting ACFs with high optical
clarity (91%), low haze (<3%), and birefringence behaviors. This
facile and high-efficiency strategy might be very scalable in fabricating
high-strength, super tough, and clear cellulose films for emerging
biodegradable next-generation packaging, flexible electronic, and
optoelectronic applications.