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ABSTRACT: To exploit the mechanical potential of native
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cellulose fibrils, we report on the preparation of nanopaper with
preferred orientation of nanofibrillated cellulose (TEMPO-NFC)
by cold drawing. The preparation route is papermaking-like and
includes vacuum filtering of a TEMPO-oxidated NFC water
dispersion, drawing in wet state and drying. The orientation of the
fibrils in the nanopaper was assessed by AFM and wide-angle X-
ray diffraction analysis, and the effect on mechanical properties of
the resulting nanopaper structure was investigated by tensile tests.
At high draw ratio, the degree of orientation is as high as 82 and
89% in-the-plane and cross-sectional planes of the nanopaper, respectively, and the Young’s modulus is 33 GPa. This is much
higher than mechanical properties of isotropic nanopaper. The cold drawing method can be also applied to NFC nanocomposites
as demonstrated by preparation of TEMPO-NFC/hydroxyethyl cellulose (HEC) nanocomposites. The introduction of the soft
HEC matrix allows further tailoring of the mechanical properties.
KEYWORDS: nanopaper, cellulose nanofibers, orientation, cold drawing, nanocomposites, nanofibrillated cellulose■ INTRODUCTION
Nanofibrillated cellulose (NFC) refers to individual microfibrils
or cellulose microfibril aggregates disintegrated from the plant
cell wall of cellulose sources such as wood pulp fibers. NFC
nanofibers are few micrometers long with a typical lateral
dimension in the 5−20 nm range. NFC were first produced in
1983 by Turbak et al. They subjected a wood pulp slurry to
high mechanical forces in a homogenizer.3886
1
This led to the
disintegration of 25−100 nm nanofibrils into an aqueous
dispersion. Nowadays, disintegration of NFC is facilitated: the
use of enzymatic
2,3
or chemical pretreatment
4
lowers the
energy required for the disintegration process and leads to
smaller diameter of the nanofibrils (5−20 and 5 nm,
respectively). Furthermore, other equipments and methods
for mechanical disintegration are currently available including
homogenizer,
1
microfluidizer,
5,6
grinder,
7
blender,
8
and soni-
cator.
8,9
Even simple mechanical stirring of chemically
pretreated (TEMPO oxidized) wood pulp fibers can disinte-
grate them.
8
TEMPO-mediated oxidation of cellulosic fibers
introduces negatively charged carboxyl groups on the fibril
surface and the repulsive forces between charged fibrils facilitate
the disintegration process. Other advantages include preserva-
tion of cellulose I crystalline structure of native cellulose in the
interior of the fibrils, and the lateral dimension is typically 4−5
nm, which corresponds to the smallest fibrils present in the
plant cell wall.
NFC-based materials are from renewable resources, are
biodegradable and the cellulose crystal has high axial modulus
(∼150 GPa),
10
and NFC has strong network forming
characteristics.
11
Furthermore, pretreated wood pulp can be
disintegrated at low cost. Initially, aqueous NFC dispersions
were used as rheological modifiers for applications in food,
cosmetics and chemical products.
1
More recently, NFC from
wood pulp has become an important material building block.
NFC has been used for nanopaper,
5,12
foams
13,14
and
aerogels,
15,16
and as reinforcement in a range of polymer
matrices.
17−19
NFC has also been used as a substrate for
functional materials including electrically conductive all-
polymer batteries,
20
magnetic nanopaper,
21
and transparent