Nanotechnology and its applications in lignocellulosic composites, a ...

Kamel – eXPRESS Polymer Letters Vol.1, No.9 (2007) 546–575
Resultant films and coatings show long-life stability
as well as self-healing characteristics.
Structured layer-by-layer films have potential
applications as antireflective coatings [100], waveguides,
bio/optical sensors [101], separation technologies
and drug delivery systems [102]. Conventionally,
layer-by-layer assembly has employed
solution-dipping (or dip-coating) in beakers of various
sizes containing dilute aqueous polymer solutions.
This inexpensive method works for most substrates
independent of shape but has not always
resulted in adequately homogeneous films. Alternatively,
spin-coating is the most widely used technique
for obtaining uniform films in lithography
and other micromachining applications. The spincoating
process involves the acceleration of a liquid
solution on a rotating substrate and is characterized
by a balance of centrifugal forces (spin speed) and
viscous forces (solution viscosity). Films created
by this way have been found to be consistent and
reproducible in thickness [103]. Nanocrystalline
cellulose is amenable to sequential film growth by
layer-by-layer assembly, as presented schematically
in Figure 8.
Thin multilayered films incorporating polyelectrolyte
layers such as poly (allylamine hydrochloride)
and nanocrystalline cellulose layers were
prepared by the electrostatic layer-by-layer methodology,
as well as by a spin coating variant. Both
techniques gave rise to smooth and stable thin
films, as confirmed by atomic force microscopy
surface morphology measurements as well as scanning
electron microscopy investigations. Films prepared
by spin-coating were substantially thicker
than solution-dipped films. Thus both techniques
are viable for producing structured nanocomposites,
where the large aspect ratio cellulose units
may serve to strengthen the elastic polymer matrix
[93].
Figure 8. Schematic representation of the build-up of electrostatically
adsorbed multilayered films. The
polyelectrolyte, cationic poly(allylamine
hydrochloride), is shown by the curved line and
colloidal cellulose nanocrystals are represented
by straight rods; counterions have been omitted
for clarity
3. Characterization
Characterization of nanofibers and nanocomposites
can be performed using different techniques such
as transmission electron microscopy (TEM), X-ray
and neutron diffraction, dynamic infrared spectroscopy,
atomic force microscopy (AFM), differential
scanning calorimetry (DSC), small angle
neutron scattering (SANS), etc.
3.1. Nanoindentation techniques
Mechanical properties of materials have been commonly
characterized using indentation techniques.
Properties that are measured by indentation
describe the deformation of the volume of material
beneath the indenter (interaction volume). Deformation
can be by several modes: elasticity, viscoealasticity,
plasticity, creep, and fracture. These
deformation modes are described by the following
properties, respectively: elastic modulus, relaxation
modulus, hardness, creep rate, and fracture toughness
[2].
3.2. Microscopy characterization
Scanning electron microscopy (SEM) as well as
atomic force microscopy (AFM) can be used for
structure and morphologies determination of cellulose
whiskers and their nanocomposites. From conventional
bright-field transmission electron
microscopy (TEM) it was possible to identify individual
whiskers, which enabled determination of
their sizes and shape. AFM overestimated the width
of the whiskers due to the tip-broadening effect.
Field emission SEM allowed for a quick examination
giving an overview of the sample; however,
the resolution was considered insufficient for
detailed information.
By comparing TEM, Field emission scanning electron
microscopy (FESEM) and AFM analysis of
cellulose whiskers as in Figure 9, it is shown that;
a) the whiskers did not differ significantly in contrast
from the carbon film, b) it was difficult to
clearly discern individual whiskers from agglomerated
structures, and therefore an estimate of the
width of the whiskers was not obtained while in, c)
the structures differed from the needlelike shape as
observed in TEM. The whiskers appeared significantly
broader having a rounded shape. AFM could
therefore be a powerful alternative to conventional
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