Macromolecules
1996,28,
6365-6367
6365
Polymer
Nanocomposites Reinforced
by
Cellulose
Whiskers
V.
Favier,
H.
Chanzy,*
and
J.
Y.
Cavaille*
Centre de Recherches sur les Macromolkcules Vkgktales,
CNRS and Universitk Joseph Fourier,
BP
53,
38041 Grenoble Cedex 09, France
Received April
7,
1995
Revised Manuscript Received June
20,
1995
There are numerous examples where animals or
plants synthesize extracellular high-performance skel-
etal biocomposites consisting of a matrix reinforced by
fibrous biopolymers.
lw3
Cellulose is a classical example
of these reinforcing elements which occur as whiskerlike
microfibrils that are biosynthesized and deposited in a
continuous fa~hion.~-~ In many cases, this mode of
biogenesis leads
to
crystalline microfibrils that are
almost defect-free, with the consequence of axial physi-
cal properties approaching those of perfect crystals. In
the present study, we have attempted
to
mimic biocom-
posites by blending cellulose whiskers from the mantles
of tunicates with synthetic polymer latices. The films
cast from such mixtures had a nanocomposite organiza-
tion whose structure and mechanical properties are
described in the present paper.
A batch of edible-grade tunicate
Microcosmus
fulca-
tus,
from the Mediterranean, was obtained from a local
fish shop. These sea animals had an overall diameter
between
5
and 10 cm with a
1
cm thick cellulose tunic.
After anesthetizing with chloroform, the animals were
gutted and their tunic was cut into small fragments that
were deproteinized by three successive bleaching treat-
ments, following the method of Wise
et
a1.6
The
bleached mantles (the "tunicin") were then disintegrated
in water, first with a Waring blender (at a concentration
of
5%
by weight) and then with
15
passes through a
Gaulin laboratory homogenizer operated at 400 bar (at
a concentration
of
1%
by weight). The resulting aqueous
tunicin suspension was mixed with HzSO4 to reach a
final acidwater concentration
of
55%
weight fraction.
Hydrolysis conditions were
60
"C
for 20 min under
strong stirring. A dispersion of cellulose whiskers
resulted. After sonication, the suspension was neutral-
ized and washed by dialysis. It did not sediment
or
flocculate as a consequence of surface sulfate groups
created during the sulfuric acid treatment.' When
concentrated by evaporation, the suspensions displayed
typical liquid crystal characteristics.8
The suspensions
of
cellulose whiskers were homoge-
neously mixed with polymer latices
(Tg
=
0
"C) resulting
from the copolymerization of styrene
(35%
wlw), butyl
acrylate
(65%
w/w), and a small amount
of
acrylic acid.
The suspensions were poured into poly(tetrafluor0eth-
ylene) molds and allowed to dry slowly for
1
month at
room temperature. Homogeneous and bubble-free 2
mm
thick films resulted. These films, cut into strips
35
mm
long and
6
mm wide, were analyzed with a DMTA
Metravib SA Mecanalyseur operating with a forced
oscillation pendulum. The experiments were achieved
at a fixed frequency
of
0.1 Hz and in a temperature
range
200-500
K.
Electron microscopy and electron diffraction analysis
on cellulose whiskers was achieved with a Philips
EM400T operated at 120 kV.
For
this, cellulose whisker
suspensions were deposited on carbon-coated grids and
observed under low-dose conditions, using a 20 pm
0024-9297/95/2228-6365$09.00/0
objective aperture that gave bright-field diffraction
contrast images. Microdiffraction on individual cel-
lulose whiskers was performed by the diffraction method
of Riecke and Ruska9 with
a
probe of
50
nm that
required a C2 condenser aperture of
5
pm. Observations
on the reinforced films were made on ultrathin sections
obtained by cryosectioning. These sections were ob-
served in bright-field diffraction contrast with a Philips
CM200 CRY0 operated at 200 kV and equipped with a
15 pm objective aperture. The images obtained under
low-dose conditions revealed (in black) the cellulose
crystallites within the clear polymer matrix.
An
image
of the latex particles was made under low-dose condi-
tions by depositing drops of latex suspensions on carbon-
coated grids.
A typical preparation of tunicin crystals is shown in
Figure 1A. This sample consists of parallelepiped rods
with lengths ranging from 100'nm
to
several microme-
ters for widths on the order of 10-20 nm. Upon testing
by a microelectron diffraction technique (inset), each
element gave a spot difiactogram that corresponded
to
a section of the reciprocal lattice of cellulose
I@
(here
the
a*c*
section) and persisted when the electron probe
was scanned along a given rod. As the diffractogram
indicated that the cellulose chain axis is along the long
dimension of the rods, each rod
is
therefore a whisker-
like tunicin crystal with no apparent defect. Figure
1B
is a low-dose image of the latex particles that were used
in this study. Each particle has a diameter of around
150 nm. Some
of
these particles are individual and have
kept their initial spherical shapes. Most
of
them,
however, are coalesced together due
to
the conditions
of
sample preparation, viz., room temperature which is
20
K
above the
Tg
of the latex.
Figure 2 corresponds
to
an
ultrathin cry0 cross section
of
a nanocomposite film containing
6%
(w/w) of tunicin
within the latex. In this image, recorded under low-
dose and diffraction contrast conditions, the cellulose
crystals are in black within the clear polymer matrix.
This image reveals that the cellulose whiskers are
distributed throughout the structure, without segrega-
tion or association.
When reinforced by a small percentage of tunicin
whiskers, the polymer films showed improved mechan-
ical properties which were particularly striking when
the films were heated above the glass transition
of
the
polymer. This is illustrated in Figure
3,
where part A
shows a plot of the shear modulus
G
as a function of
temperature for various whiskerflatex compositions
ranging from
0
to 14% (wlw). The curve corresponding
to the pure matrix is typical for a thermoplastic: below
Tg,
G
remained constant at around
1
GPa and dropped
rapidly to
1
MPa during the crossing of the glass-
rubber transition temperature. Above this temperature,
it
behaved as a viscous liquid, with
G
decreasing rapidly
with increasing temperature. The films that contained
cellulose whiskers had a slight increase in their
G
value
below
Tg,
but the drop in
G
value above
Tg
was
dramatically reduced: only from
1
to
0.1 GPa for a film
reinforced by
6%
(wlw) whiskers. Above
Tg,
the rein-
forced films behaved
as
rubbers as their
G
value stayed
constant over a wide temperature range. This is il-
lustrated in Figure 3B that corresponds
to
a film
reinforced by 6% (w/w) whiskers for which
G
keeps a
value of 0.1 GPa all the way
to
500
K,
a temperature at
which cellulose starts to decompose.
The variation of the shear modulus
G,
taken at 325
K
(Le.,
50
K
above
Tg)
as a function of the whisker
0
1995 American Chemical Society
