Thermal and mechanical properties of chitosan nanocomposites with cellulose modified in ionic liquids
Thermal and mechanical properties of chitosan nanocomposites with cellulose modified in ionic liquids
Aleksandra Grza˛bka-Zasadzin´ ska 0 1 2
Tazdin Amietszajew 0 1 2
Sławomir Borysiak 0 1 2
0 WMG, University of Warwick , Coventry CV4 7AL , UK
1 Institute of Chemical Technology and Engineering, Poznan University of Technology , Berdychowo 4, Poznan , Poland
2 & Aleksandra Grza ̨bka-Zasadzin ́ska
In this paper, ionic liquid treatment was applied to produce nanometric cellulose particles of two polymorphic forms. A complex characterization of nanofillers including wide-angle X-ray scattering, Fourier transform infrared spectroscopy, and particle size determination was performed. The evaluated ionic liquid treatment was effective in terms of nanocrystalline cellulose production, leaving chemical and supermolecular structure of the materials intact. However, nanocrystalline cellulose II was found to be more prone to ionic liquid hydrolysis leading to formation larger amount of small particles. Each nanocrystalline cellulose was subsequently mixed with a solution of chitosan, so that composite films containing 1, 3, and 5% mass/mass of nanometric filler were obtained. Reference samples of chitosan and chitosan with micrometric celluloses were also solvent casted. Thermal, mechanical, and morphological properties of films were tested and correlated with properties of filler used. The results of both, tensile tests and thermogravimetric analysis showed a significant discrepancy between composites filled with nanocrystalline cellulose I and nanocrystalline cellulose II.
Nanocellulose; Chitosan; Ionic liquid; Thermal stability; Polymorphism; Nanocomposites
Rapidly developing technologies for manufacturing of
functional and advanced material put a growing pressure
on production of materials with specifically defined and
unique properties. Preparation of biomaterials based on
polymers of natural origin, which include cellulose,
especially nanometric cellulose, is of particular importance.
Cellulose is the most abundant natural polymer that is of
high interest among researchers. The main challenge
regarding nanometric cellulose is finding a way of its
production that would result in a product showing high
surface area, high aspect ratio, low density, and great
mechanical properties. For example, nanocrystals of
cellulose or whiskers are characterized with Young modulus
in the range from 130 to 145 GPa . Great interest in such
materials is caused by their interesting and unique
properties. Nanoparticles of cellulose undergo degradation
faster than macroscopic cellulose, whereas other important
and widely used nanoparticles such as fullerenes and
nanotubes do not undergo biodegradation at all . When
compared to micrometric cellulose higher surface area and
presence of nanopores in nanometric cellulose causes, the
increase in interactions with other substances resulting in
linkage with various nanoparticles . Advantages of
nanometric cellulose are not only connected with its
physical and chemical properties or its susceptibility to
degradation but also with its high biocompatibility and
availability, renewability of raw material, and sustainable
growth. It is used in aerogels, adhesive materials or as an
additive for shape memory segmented polyurethanes
modifying its thermal properties [4, 5]. Due to its ability to
orientate in magnetic field, cellulose is also used in liquid
crystal systems . Currently, nanometric cellulose is most
commonly produced by mechanical treatment or traditional
acidic hydrolysis that in first stage is responsible for the
disintegration of glycoside bonds in amorphous regions.
Properties of obtained biomaterials depend on morphology
and structure of nanometric cellulose what is closely
connected with hydrolysis process of cellulosic materials.
Controlling the size and shape of nanometric cellulose is
crucial for production of nanometric cellulose
characterized with unique and defined properties and therefore for
its application as functional filler for polymers. That is
possible only if controlled dissolution or controlled
enzymatic etching of cellulose takes place. Acidic hydrolysis is
mainly realized with sulfuric acid, hydrochloric acid, or
hydrogen bromide. This method has several drawbacks,
including difficulties in controlling the progress of the
reaction [7, 8] or quite intensive formation of sulfate ester
groups at the surface of nanometric cellulose that in turn
influences interactions at the nanometric cellulose/polymer
matrix interface and decreases its thermal stability . Not
without significance is the necessity to use concentrated,
toxic reagents and the subsequent separation of
nanoparticles from such solutions. Hence, ionic liquids are
suggested as an alternative hydrolyzing agent.
Room temperature ionic liquids are a unique group of
organic salts consisting of organic cation and
disproportionate organic or nonorganic anion, with melting
temperature below 100 C . They show an ability to
dissolve both organic and nonorganic matter, and thus they
are often labeled as new generation or ‘‘green solvents’’
. Despite the fact that the mechanism of cellulose
dissolution with ionic liquid remains disputable, some ionic
liquids, favorably imidazole-based, are used to produce
nanometric cellulose. Moreover, it was proved that
aqueous ionic liquid is as well or even more effective in terms
of nanometric cellulose production than dehydrated ones
[12, 13]. Time and temperature of ionic liquid treatment
were also found to be of critical importance . Even
though ionic liquids offer a great opportunity for
production of nanocrystalline cellulose (higher degree of
homogeneity, less aggressive reaction medium, medium
recovery, etc.), still only few papers regarding application
of, e.g., 1-butyl-3-methylimidazolium chloride or
1-butyl3-methylimidazolium hydrogen sulfate for that purpose
were published [14–16]. The reason is that currently the
main aim of most of the research is to find new ionic
liquids that enhance the effectiveness of dissolution of
cellulose coming from lignocellulose materials [17–23]
rather than the preparation of nanometric cellulose with
ionic liquids. Ionic liquids are also relatively expensive, but
they are recyclable, what can make the production of
nanocrystalline cellulose more economically feasible and
less environmentally adverse .
When aiming to produce nanometric cellulose, it should
be also taken into consideration that starting material,
cellulose, may exist in two polymorphic forms. This forms
called cellulose I and cellulose II have a different
arrangement of polysaccharide chains and different sizes of
elementary cell . It is also important that cellulose II is
characterized with much lower degree of crystallinity than
cellulose I. There are some papers in which the influence of
cellulose type on nucleation and crystallization process of
semicrystalline polymer matrices was reported as
significant [25, 26]. That diversity of crystalline structures of both
celluloses may also have a great influence on hydrolysis of
cellulose and effectiveness of nanometric cellulose
production. Likewise, divergent structure of hydrogen bonds
in cellulose I and cellulose II is responsible for differences
in thermal stability of these two crystalline forms . It
was already shown that in case of the acid hydrolysis of
cellulose its polymorphic form has an influence on the
particles sizes of produced nanometric celluloses. The
higher yield of nanocrystalline cellulose II was ascribed to
different crystallographic structures of starting celluloses
and lower crystallinity of cellulose II, which was more
susceptible to acid hydrolysis than cellulose I . Still,
the influence of polymorphic forms of cellulose on
formation of cellulose nanocrystals using ionic liquids has not
been reported yet.
Attractive mechanical properties of nanometric cellulose
make it ideal filler for polymers. What is more,
incorporation of nanometric cellulose particles in natural polymer
is an attempt to mimic the nature. Combining nanometric
cellulose with chitosan seems to be particularly interesting.
Chitosan is a compound of marine origin, obtained by
partial deacetylation of chitin. It consists of N-acetyl
glucosamine and D-glucosamine units which are a source of
amine and hydroxyl groups. Such chitosan/nanometric
cellulose composite merges the properties of chitosan
(biodegradability, antibacterial properties, transparency,
antimicrobial activity) and nanometric cellulose (high
surface area, very good barrier, and mechanical properties)
[29–32] resulting in obtainment of composite materials that
can be successfully applied in packaging industry (film for
food, paper coatings), in chemical industry (catalysts,
adsorbents), and in biomedicine (carrier of active
substances, filaments) [33–36]. Despite of these valuable
advantages, there are also compelling problems hindering
wider application of such chitosan-based composites.
Chitosan exhibits a high sensitivity to numerous types of
degradation, including thermodegradation. Thermal
analysis showed that this biopolymer cannot withstand
temperature higher than 200–220 C . This poor thermal
stability of chitosan-based systems often limits its wider
application, but it can be overcome by addition of
compounds that act as stabilizing agent. There are some papers
which report an improvement of thermal stability of the
chitosan with hydroxyapatite, nanoclay , or calcium
carbonate . Nanofibrillated cellulose produced using
enzymes was also successfully used to enhance the thermal
stability of the chitosan films .
In the light of the above considerations, it is interesting
to study whether the incorporation of two different
polymorphic forms of nanocrystalline cellulose produced only
with ionic liquid can improve thermal and mechanical
properties of chitosan-based composites. Although the
literature contains some examples of the use of
nanocrystalline cellulose as a reinforcing agent for chitosan
composites, there are no reports on influence of
polymorphic variety of cellulose produced by means of ionic liquid
hydrolysis on properties of chitosan-based composites at
the time of writing this article. The knowledge of this
subject seems to be essential for designing new
biomaterials with enhanced properties and wider potential
applications. For that reason, the main purpose of this research
was to evaluate the influence of crystallographic form of
nanocrystalline cellulose produced with ionic liquid on
thermal and mechanical properties of chitosan-based
Cellulose Avicel PH-101 with average particle size of
50 lm and high molecular weight chitosan from crab shells
(degree of deacetylation 75–85%) was purchased from
Sigma-Aldrich. Pure sodium hydroxide (Chempur) was
used for preparation of 16% solution and subsequently used
as mercerizing agent. Acetic acid 80% purchased from
POCH S.A. was diluted to 2% (v/v) solution and used for
composites formation. Ionic liquid
1-butyl-3-methylimidazolium hydrogen sulfate (bmimHSO4) with water
content B1.0% was also purchased from POCH S.A.
Cellulose pulp mercerization
Cellulose (C I) pulp was treated with 16% NaOH at room
temperature. After 5 min of continuous stirring, an excess
of water was added to stop the mercerization process. The
suspension was centrifuged at 10.000 rpm for 10 min.
Finally, the produced cellulose II (C II) was washed with
the excess of distilled water to remove NaOH and then
dried in the air at 70 C for 6 h.
36 h. After that time, water was added so that ionic liquid–
water ratio was 80:20 mass/mass. As the temperature of
90 C was reached, the reaction was continued for next
12 h under constant stirring at 400 rpm. Subsequently, the
hydrolysis was stopped by adding water. The suspension
was neutralized with significant amounts of water until
pH & 7 was reached, then sedimented, centrifuged, and
filtered. Produced CNC I was dried at 70 C for 6 h.
Nanocrystalline II preparation
Nanocrystalline cellulose II (CNC II) was prepared in the
same manner as CNC I with that difference that C II was
used as a starting material.
Chitosan/nanocrystalline cellulose composites were
produced by solvent casting method. Firstly, chitosan was
dissolved in 2% (v/v) CH3COOH. Secondly,
nanocrystalline celluloses at different concentrations were added to
chitosan so that mixtures containing 1, 3, and 5% mass/
mass of CNC (in relation to dry mass of chitosan) were
obtained. Also, as comparison, composites with 5% of
cellulose I and cellulose II were prepared. All mixtures
were homogenized by ultrasonication for 20 min, applied
on Petri dishes, and dried for 12 h at 35 C. The samples
were named according to the following convention: CHT
stands for chitosan, number stands for percentage addition
of filler, and C I, C II, CNC I, or CNC II stands for filler
type. For example, name CHT/5 CNC II means chitosan
with 5% loading of nanocrystalline cellulose II.
Fourier transform infrared spectroscopy (FTIR)
FTIR spectra confirming the composition of samples were
recorded on an ATI Mattson Infinity Series FTIR
spectrometer equipped with a deuterated triglycine sulfate
detector in range from 500 to 4000 cm-1.
Wide-angle X-ray scattering (WAXS)
Structures of nanocrystalline celluloses were analyzed by
means of wide-angle X-ray scattering (WAXS) using
CuKa radiation at 30 kV and 25 mA anode excitation. The
X-ray diffraction patterns were recorded for the angle
range of from 5 to 30 in the step of 0.04 /3 s.
Nanocrystalline cellulose I preparation
Particle size distribution
Nanocrystalline cellulose I (CNC I) was obtained through
controlled ionic liquid hydrolysis of cellulose. Cellulose I
was immersed in tenfold weight excess of bmimHSO4 for
Mastersizer 2000 (Malvern Instruments Ltd.) and Zetasizer
Nano ZS (Malvern Instruments Ltd.) employing the laser
diffraction technique in the range of 0.2–2000 lm and
0.6–6000 nm, respectively, were applied to determine
particle size and the dispersive properties of (nanometric)
Thermogravimetric analysis (TG)
A thermogravimetric analyzer (Jupiter STA 449F3,
Netzsch) was used to investigate the influence of filler
type on thermal stability of the composites.
Measurements were taken in the atmosphere of nitrogen (flow
rate 20 cm3 min-1) at a heating rate of 10 K min-1 over
a temperature range of 30–1100 C, with an initial
sample weight of approximately 5 mg.
Scanning electron microscopy (SEM)
The morphology of chitosan-based samples was studied
using scanning electron microscopy (Hitachi TM3030Plus
SEM) at acceleration voltage of 15 kV, using backscatter
and secondary electron detectors. The specimens were not
Tensile properties of produced composite films were
defined using Zwick and Roell Allround-Line Z020 TEW
testing machine. Samples of 10 mm width and thickness
ca. 0.1 mm were tested with speed 5 mm/min and initial
force 0.2 N in accordance with standard ISO 527-3. The
arithmetic mean of seven replicate determinations was
taken into consideration in each case.
Results and discussion
Supermolecular and chemical structure of fillers
The effect of mercerization on supermolecular structures of
produced nanocrystalline celluloses was examined by
means of WAXS technique (Fig. 1). Also, starting
materials—C I and C II—are shown in Fig. 1.
The diffraction patterns of CNC I showed peaks at
approximately 2H = 15 , 17 , 22.7 which are
characteristic for structure of cellulose I. The crystalline structure of
the mercerized cellulose was assigned to cellulose II
because peaks at 2H = 12 , 20 , 22 were recorded. Since
the WAXS pattern for CNC II exhibits not peaks coming
from native cellulose, it is apparent that alkali treatment
was successful and 100% of C I was transformed into C II.
It is consistent with the literature that reports that NaOH
concentrations higher than 10% are responsible for
mercerization of cellulose I and therefore its transformation
into cellulose II . It is known that mercerization
Fig. 1 WAXS patterns for celluloses and nanocrystalline celluloses
process causes an intermolecular rearrangement between
the individual chains of cellulose and in result its
crystallinity degree decreases. On the other hand, intensive
ionic liquid pretreatment reduces the degree of cellulose
crystallinity . Both of these statements are confirmed
with findings of WAXS analysis.
FTIR spectra of nanocrystalline cellulose fillers (Fig. 2)
were recorded in order to confirm their chemical structures.
Table 1 presents assignments of signals to the vibrations.
FTIR analysis showed main characteristic peaks of
cellulose in both samples. At the same time, no traces of
imidazole ring coming from ionic liquid were present
proving that produced celluloses were sufficiently washed
and that ionic liquid did not change the chemical structure
of materials. Moreover, the lack of bands at 1731 cm-1 and
approximately 1513 and 1239 cm-1 implies that no lignin,
pectin, or hemicelluloses were present in the samples.
Fig. 2 FTIR spectra of nanocrystalline celluloses
Table 1 Vibrational frequencies wavenumber/cm-1 attributed to
Dispersive and morphological properties of fillers
So far, in this paper, starting materials were called
celluloses, while produced cellulosic materials were named
nanocrystalline celluloses. Now, we would like to present
the results proving the correctness of the terms mentioned
above. Hence, particle size distributions of celluloses are
presented in Fig. 3, and the Zetasizer Nano ZS results
describing particles sizes of the obtained fillers are shown
in Fig. 4.
Particle size distributions of celluloses (Fig. 3) show
that mercerization process caused the decrease in particle
sizes from *48 lm for C I up to *40 lm for C II. Despite
this fact, produced particles were still in micrometric range,
and thus further treatment leading to the production of
nanometric cellulose was necessary.
Ionic liquid treatment of micrometric celluloses caused
the formation of both nanometric particles and micrometric
agglomerates (Fig. 4).
Sample of CNC I was almost equally divided into two
fractions, while sample of CNC II was less homogenous,
but in general four diameter ranges can be distinguished.
When considering diameters B122 nm, it can be noticed
Fig. 3 Particle size distribution for a) C I and b) C II
that 42% of CNC I particles were in this size range, while
for CNC II it was 28.2%. In the second range, from 142 up
to 220 nm, particles of CNC II dominate (almost 22% of
the samples, twice as much as for CNC I). If we move to
the third range, 255–1900 nm, here no particles of CNC I
were observed, but over 23% volume of the CNC II sample
was this size. In the last, fourth range (B2300 nm), only
26.1% of CNC II and 47.2% of CNC I, was found.
Accordingly, it can be concluded that sample of CNC I
consisted of slightly larger amount of particles up to
220 nm, but it was also characterized with large volume of
micrometric fraction. In the study on acidic hydrolysis of
cellulose I and cellulose II , it was found that even
though fraction of CNC II nanometric particles was higher
than of CNC I, the micrometric particles of CNC I were
significantly bigger than those of CNC II (max. 3580 and
1990 nm for CNC I and CNC II, respectively). Similarly,
ionic liquid treatment led to formation of CNC I particles
with diameter of 4800 nm, whereas the biggest particles of
CNC II had only 3090 nm. In case of both, ionic liquid and
acidic hydrolysis, it can be stated that native cellulose is
more prone to form agglomerates. It seems that the main
resemblance in the ionic liquid and acid treatment is the
formation of large micrometric fraction of CNC I.
The key to understanding the differences in sizes of
CNC I and CNC II derived from ionic liquid treatment is
the mechanism of cellulose dissolution (formation of
nanometric cellulose is a controlled process of dissolution)
with reference to crystalline structures of starting
celluloses. The dissolution mechanism of cellulose in ionic
liquids was long believed to be based on hydrogen bonds
interactions between the anion of the ionic liquid and the
hydroxyl groups of cellulose . Lately, it was shown that
also cation can play an important role in the dissolution
process. This mechanism involves the synergistic
interaction of cation and anion of ionic liquid and can be
explained as follows: Firstly, anions bind to the hydroxyl
groups on the surface of the cellulose, causing the
weakening of the hydrogen bonds between cellulose chains.
Secondly, the separation of cellulose chains is promoted by
the intercalation of cations that are attracted to anions
because of their opposite charge. It is also known, that the
intercalation of the cation into cellulose network is easier if
the molecule of anion is big (HSO4- [ Cl-) . If we
take a closer look at the processes taking place during
mercerization, it turns out that during this treatment
conformation of the cellulose chains changes. In result,
loosening of the structure along with decrystallization occurs.
Consequently, it can be assumed that, due to the loosening
the structure and development of the amorphous regions
which absorb chemicals easier than highly compacted
crystalline regions , mercerization facilitates the
intercalation of cation coming from ionic liquid. That
Fig. 4 Particle size distribution of CNC I and CNC II
explains why sample of CNC II consisted of smaller
amount of micrometric particles than CNC I. Yet, the
aspect of hydrogen bonds present in cellulose II cannot be
omitted. The antiparallel chain model of C II is responsible
for formation not only interchain, but also of interplane
hydrogen bonds, making its hydrogen bonding system
more complex compared to that of C I . Still, it is
suspected that hydrogen bonds between ionic liquid and C
II are much stronger than interplane hydrogen bonds of C
II, enabling successful separation of cellulose chains.
Thermal stability of composites
The TG curves of chitosan and its composites with
microand nanometric celluloses are given in Fig. 5, while the
corresponding tabulated values obtained from these
measurements are provided in Table 2.
All the TG curves are smooth, with only two mass loss
steps which indicates that the thermal degradation of
chitosan and chitosan-based composites in nitrogen
atmosphere is simple and, apart from water loss, is a one-step
reaction. The first loss around 100 C is associated with the
evaporation of a residual acetic acid as well as water that is
physically absorbed and strongly hydrogen bonded to
chitosan and cellulosic fillers. As shown in Table 2, 5%
mass loss for pristine chitosan was noted at significantly
higher temperature than for its composites. It suggests that
the evaluated composites contained more bonded water.
The reason for that was the presence of cellulosic filler and
cellulose is a hydrophilic material which is well known for
being able to absorb high amounts of water. In general, it is
believed that, because of the contrasting sorption
mechanisms (bulk for C II and surface for C I), less crystalline C
II can adsorb more water than C I [47, 48]. This relation
can be seen for samples of CHT/5 C I and CHT/5 CII, but
in our case it is no longer noticeable for composites with
nanometric filler. The second stage (170–400 C) of
thermal decomposition is assigned to the degradation of
chitosan, caused by the rupture of the glycosidic linkages
between the glucosamine and N-acetylglucosamine rings
[49, 50]. The comparison of data presented in Table 2
clearly shows that addition of filler to chitosan matrix
caused lowering of thermal stability of samples. For CHT/5
C II, the 10% mass loss took place at temperature 18 C
lower than for CHT and the other two samples containing
CNC I and C I behaved similarly. However, at 10% mass
loss, there was almost no difference between chitosan
sample (237 C) and CHT/5 CNC II (236 C). The
tendency that composites filled with CNC II were
characterized with lowest impairment of thermal stability was
maintained throughout further measurement range. At 50%
mass loss, it is obvious that discrepancy between pristine
chitosan and its composites is definitely vaster but CHT/5
CNC II film stands out again. Temperature at which 50%
mass loss was reached for composite with CNC II was
554 C (672 C for unmodified chitosan), while an average
temperature for other films was in range of 424–450 C.
More detailed look at TG results leaves no doubt that the
thermal properties of composite films are highly dependent
on the characteristics of (nano)metric fillers. Thus, below
we will try and correlate them.
The TG results clearly show that sample containing CNC
II was characterized with better thermal stability than film
with CNC I. But then, composites with CNC II turned out to
be more thermally stable than those with C II. Similar
tendency was noted for composites based on (nano)cellulose I,
but only if high mass loss is considered. It seems that not
only polymorphic variety but also size of filler is important
in terms of thermal stability. Naturally, a question for the
reason behind this observation arises. It is known that
during mercerization parallel chain arrangement of the cellulose
I changes into more stable, antiparallel arrangement of
Fig. 5 TG curves for chitosan
and its composites
Table 2 Tabulated TG values for tested films
CHT/5 CNC II
cellulose II and more interplane hydrogen bonds are being
formed. As said before, these bonds are important in terms
of successful ionic liquid treatment, but additionally, they
play an important role during the thermal degradation of the
sample. In comparison with interchain bonds in C I, more
energy is required to disrupt inter- and intrachain bonds in C
II and therefore start the thermal decomposition process
. That is concluded to be the cause of better thermal
stability of composites based on mercerized (nano)cellulose.
The answer to why CNC II underwent degradation at higher
temperatures than C II is not straightforward. It is widely
accepted that small particles with large number of free-end
chains present in the sample induce decomposition at lower
temperature [51, 52]. According to that theory, TG results
obtained for CHT/5 C II sample should be better than for
CHT/5 CNC II. On the contrary, thermal stability of film
with CNC II was the least impaired, and there was a strong
resemblance between TG curve of CHT/5 CNC II and CHT
sample. Probably, nanometric particles of CNC II were
better incorporated in chitosan matrix and surrounded by it
more tightly. The CNC I filler consisted of larger amount of
micrometric particles (4.4% of the CNC II and 36.8% of
CNC I with diameters C3 lm) and therefore could not have
been so good wetted by chitosan. Consequently, they were
less susceptible to thermal decomposition than particles of
CNC I. Even if nanofiller had more free-end chains, at this
level of loading it was not a leading issue. This idea is
supported by work of Crews et al.  who related poor
dispersion of filler in matrix with the lack of the hydrolysis
being performed. The ionic liquid treatment of cellulose
causes the disruption of some hydrogen bonds that are
partially responsible for nonhomogenous structure of
composites. Although C II particles had bigger diameters,
presumably they were poorer bonded with chitosan matrix than
CNC II. It is likely that improvement of thermal stability
could be noticed if the addition of CNC to matrix would be
higher. That was reported in paper  where the thermal
stability of the chitosan-based films increased substantially
with the incorporation of the nanometric cellulose.
Some published works support thesis that nanometric
cellulose can effectively improve thermal stability of
polymers [54, 55], but there are also studies that report no
impact or even reduction in thermal stability . In this
paper, we did not succeeded to produce CNC-based
composites with enhanced thermal stability, but we were able
to correlate the type of cellulosic filler with thermal
stability of the composites.
Majority of scientific publications regarding nanometric
cellulose points out its unique mechanical properties. The
influence of nanometric cellulose on tensile properties of
composites can be obtained even at its low concentrations.
Our findings strongly support this hypothesis.
Table 3 Tensile properties of chitosan and its composites (average
value ± standard deviation)
Table 3 presents tensile strength (TS), elongation at
break (EB), and Young’s modulus (YM) parameters
obtained for tested films.
Nanometric cellulose is well known for enhancement of
mechanical properties of composites, even at very low
loadings. Its unique mechanical properties were observed
also in our composites. Addition of 1 and 5% of CNC I
caused the value of YM to increase almost four times in
relation to chitosan. For chitosan loaded with 3% of CNC I,
this change was even more considerable—YM increased
almost eight times, reaching the value of 1253 MPa. It is
known that as the amount of the filler in polymer matrix
increases, the gradual increase in Young’s modulus is
observed. However, after specific optimal concentration
was reached, YM decreased. It is related to the
agglomeration of small particles and formation of bigger
agglomerates that are no longer effective in terms of stress
transfer. Therefore, for composites containing 5% CNC I,
decrease in the YM value was observed. Similar
relationship was reported for chitosan filled with unzipped
multiwalled carbon nanotube oxides and agar/nanocrystalline
cellulose composites [57, 58]. Accordingly, it seems that in
case of CNC I the optimal amount of filler was not 1, but
3%. Though, when compared to composites filled with
CNC II, these values become less impressive. It is so
because addition of only 1% of CNC I was responsible for
the increase in YM up to 3316 MPa. When the amount of
CNC II in chitosan matrix increased, the value of YM
decreased, but still were higher than for neat chitosan and
CHT/CNC I composites.
In terms of TS enhancement 1% filler loading turned out
to be the most effective. When compared to chitosan
(25 MPa), the biggest increase in this parameter was
measured for CHT/1 CNC II (80 MPa) and then for CHT/1
CNC I (47 MPa). As the amount of filler reached 3%, the
TS for CHT/CNC I (27 MPa) was comparable to neat
chitosan, whereas for CHT/CNC II it was still definitely
higher (39 MPa). The lowest values of TS were noted for
the highest, 5% addition of filler, and, taking into
consideration standard deviation, these numbers were similar to
the TS of chitosan.
The differences in the values of the EB parameter of the
evaluated samples were quite vast. As expected, film of
pristine chitosan was found to be the most elastic. In
general, addition of filler causes the decrease in this
parameter, and this thesis was also compliant with our
results. Surprisingly, while all the other composites were
characterized with reduced elongation, value obtained for
CHT/1 CNC II was analogous to unmodified chitosan (36
and 42%, respectively).
SEM microphotographs of composites films presented in
Fig. 6 show that even at the highest loading of filler
(Fig. 6b, d) the homogeneity of samples was quite good.
Fig. 6 SEM microphotographs
for chitosan films with: a 1%
CNC I, b 5% CNC I, c 1% CNC
II, and d 5% CNC II
Considering samples with the lowest amount of filler
(Fig. 6a, c), it is obvious that in CHT/1 CNC II film more
particles of filler were observed. It can be also noticed that
apart from some agglomeration of filler taking place,
nanometric particles were still present in the analyzed
Atef et al.  prepared agar films with addition of 2.5,
5, and 10% of CNC. Composites with 2.5% of CNC were
characterized with improved values of YM and TS, but at
higher concentration of filler, an impairment was observed.
At 2.5% CNC loading, EB parameter was lower than for
unmodified agar. The increase in these tensile properties
(YM, TS) was ascribed to the similar structure of CNC and
chitosan which both have polysaccharide structure.
Alternatively, lowering the mechanical properties with
increasing loading of filler was a consequence of agglomeration of
CNC and heterogeneous size distribution. Noted decrease
in EB is characteristic for nanocomposites and is a result of
rigid nature of the filler and strong interactions between
components of composite, responsible for restriction of the
matrix motion [59, 60]. Not only Atef et al. but other
research groups [61, 62] too showed that due to good
dispersion, low content of nanofiller (CNC) in polymer
matrix (polylactide) is the best in terms of enhancement of
mechanical properties. This hypothesis is also supported by
this and our previous work [28, 40], but stays in opposite to
Khan et al.  who claimed that 5% of CNC is optimal
for chitosan. It comes as no surprise that aspect ratio of a
filler, its interaction with polymer matrix, and uniform
dispersion of filler are the main factors limiting mechanical
properties of composites . However, in this research, a
polymorphic form of nanofiller seems to be the key
As previously stated, the greatest enhancement of
tensile properties was obtained for composite with 1% of
CNC II as a filler. Decrease in EB parameter was also
definitely smaller for this sample than for any other. The
reason for such differences between CNC I and CNC II is
explained below. During the mercerization of C I,
formation of more uniform and also smoother particles of C
II takes place. In terms of cellulosic fibers, mercerization
process helps to relieve some residual stresses, what
makes the fibers stronger . The positive role of NaOH
treatment of cellulosic materials used in composites was
confirmed by Ichazo et al.  and Borysiak .
Similarly, composites of poly(ethylene oxide) with two
polymorphic forms of cellulose nanocrystals (CNC) showed
that the enhancement of tensile properties for composites
with CNC II was greater than for CNC I . It was
caused by good interaction between polymer matrix and
cellulose crystals which was determined by the hydrogen
bonding. The same mechanism can be found in our
composites, where CNC II with larger amount of
interand intraplane bonds was better bonded with chitosan
matrix than CNC I. Apart from the optimal concentration
of the filler, findings of this research stay in opposite to
the mechanical results obtained for chitosan filled with
acid-derived CNC. Tensile properties of
chitosan/acidderived CNC composites  indicate that addition of
CNC I is more effective than CNC II. The reason for that
is probably the fact that the acid-derived CNC I had rather
fibrillar structure, while the sample of acid-derived CNC
II consisted of spherical or irregularly shaped particles. In
case of composites with fibrillar fillers, the stress transfer
mechanism is different from this occurring in composites
with particulate fillers , thus differences in
mechanical properties of such materials are understandable.
However, in this case it seems that the amount of
micrometric particles present in filler was the factor
influencing the mechanical properties of composites the
most. Analysis of particle size distribution clearly shows
that even the sample of CNC I consisted of larger number
of smaller particles than CNC II, the difference in total
volume of particles B220 nm is not so distinct
(approximately 53% for CNC I and 50% for CNC II). Definitely
bigger divergence can be observed when the micrometric
size range is considered. In this range, there is more small
particles of CNC II than those of CNC I. They are still in
the size of microns, but in contrast to CNC I the
overwhelming amount of particles had diameters below 3 lm
(only 4.4% of the CNC II sample and 36.8% of CNC I had
diameters over 3 lm). Authors of paper on reinforcing
properties of silica in polypropylene composites 
showed that the increase in particle size entail enhanced
debonding of the filler from polymer matrix. In that
research, filling polypropylene with 1 lm silica gave
better results than in case of 3 lm particles, but it has to
be mentioned that of course, the best results were obtained
for the smallest particles. Additionally, it is due to the fact
that for smaller particles with higher total surface area, the
mechanism of stress transfer is more efficient. As a result,
in samples containing small silica particles formation of
holes and voids on the fracture surface of the composite
did not appear.
Concluding, three main factors limiting the mechanical
properties of our composites were: (1) polymorphism of
cellulose that affects formation of hydrogen bonds between
matrix and filler, (2) the content of the filler in composites,
and (3) size of the filler. At high loading rates, a loss of
mobility of the polymer chains occurs making it harder to
adapt to deforming forces. In order to withstand them,
particles of filler undergo debonding, leading to formation
of holes, presence of which decreases the mechanical
properties of the composite. This undesirable pulling out of
filler escalates even more if particles of filler are large.
Here, composites of CHT/1 CNC II containing the smallest
amount of filler with the lowest micrometric fraction were
found to be the most mechanically resistant.
In this research, cellulose I and cellulose II were subjected to
ionic liquid treatment leading to production of nanometric
celluloses. They were subsequently used for forming
chitosan-based composites. Films of chitosan with
nanocrystalline cellulose I and nanocrystalline cellulose II were
characterized in terms of mechanical and thermal properties.
This research shows that enhancement of properties of
chitosan/CNC composites is a very complex issue and not only
the amount of micro- and nanometric fraction of the filler, but
also its polymorphic form has a great influence on final
properties of such biocomposites. More detailed findings of
this work can be shortly summarized as follows:
Nanometric celluloses were successfully obtained with
use of a commercially available ionic liquid. However,
mercerization preceding ionic liquid treatment was
found to facilitate the intercalation of cation coming
from ionic liquid. As a consequence, sample of CNC II
consisted of smaller amount of micrometric particles
than CNC I.
TG results show that incorporation of micro- and
nanometric cellulose of each polymorphic form
resulted in lowering the thermal stability of composites
when compared with unmodified chitosan.
Interestingly, thermal stability of composites containing CNC
II was higher than film with CNC I. This was related to
poorer dispersion of CNC I filler in the chitosan matrix.
Film with CNC II was also more thermally stable than
analogous composite with C II, which was probably
caused by less durable bonding of big C II particles
with the matrix.
In terms of enhancing mechanical properties, sample
with 1% loading of CNC II was definitely the most
effective. It was due to a low addition of the filler that
did not restrain adaptation to deforming forces and
small particles of CNC II which were well hydrogen
bonded with the matrix and at the same time were not
prone to debonding from the chitosan matrix.
Acknowledgements This research was supported by the grant of
Poznan University of Technology 03/32/DSPB/0703.
Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://crea
tivecommons.org/licenses/by/4.0/), which permits unrestricted use,
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appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were
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