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    ABSTRACT: To exploit the mechanical potential of native
    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
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