Preparation of high strength chitosan fibers by using ionic liquid as spinning solution

Article abstract of DOI:10.1039/c2jm30555k

The morphology and mechanical properties of chitosan fibers obtained by wet-spinning using chitosan-[Gly]Cl (glycine chloride) ionic liquid as spinning dope solution are reported for the first time. The objectives were to understand both how the microstructure of the fibers could be modified and how the mechanical properties were improved by means of using [Gly]Cl ionic liquid as the spinning solution. In the new system, the main component chitosan (the degree of deacetylation was 86%, the molecular weight was about 1.5 × 10⁶) was dissolved in an aqueous [Gly]Cl ionic liquid solution; the fibers were then spun using a sodium sulfate (Na₂SO ₄)/ethanol (C₂H₅OH) aqueous solution as the coagulant, and then directly dried under freeze-drying. The fibers spun from the above mentioned system have the chitosan I crystal form, and the breaking tenacity (3.77 cN dtex^-1) is 4 times more than that (0.86 cN dtex^-1) from an acetic acid system. The orientation and crystallinity of fibers spun in [Gly]Cl solution was enhanced with an increase of spin stretch ratio, and thereby the mechanical properties of the fibers were improved. Moreover, the fibers had a smooth surface as well as a round and compact structure. More to the point, the used [Gly]Cl could be recovered by simple post processing and the chitosan fibers spun in the recycled [Gly]Cl solution also had a strong breaking tenacity. Therefore, this study verified that [Gly]Cl is a new spinning dope solution for preparing chitosan fibers with strong mechanical properties. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm30555k

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PAPER
Preparation of high strength chitosan fibers by using ionic liquid as spinning
solution
a
b
a
a
b
a
c
a
Lu Li, Bing Yuan, Shiwei Liu, Shitao Yu,* Congxia Xie, Fusheng Liu, Xianying Guo, Lijun Pei
d
and Baoquan Zhang
Received 29th January 2012, Accepted 1st March 2012
DOI: 10.1039/c2jm30555k
The morphology and mechanical properties of chitosan fibers obtained by wet-spinning using chitosan–
[
Gly]Cl (glycine chloride) ionic liquid as spinning dope solution are reported for the first time. The
objectives were to understand both how the microstructure of the fibers could be modified and how the
mechanical properties were improved by means of using [Gly]Cl ionic liquid as the spinning solution. In
the new system, the main component chitosan(the degree of deacetylation was 86%, the molecular weight
6
was about 1.5 ꢀ 10 ) was dissolved in an aqueous [Gly]Cl ionic liquid solution; the fibers were then spun
2 4 2 5
using a sodium sulfate (Na SO )/ethanol (C H OH) aqueous solution as the coagulant, and then directly
dried under freeze-drying. The fibers spun from the above mentioned system have the chitosan I crystal
ꢁ1
ꢁ1
form, and the breaking tenacity (3.77 cN dtex ) is 4 times more than that (0.86 cN dtex ) from an acetic
acid system. The orientation and crystallinity of fibers spun in [Gly]Cl solution was enhanced with an
increase of spin stretch ratio, and thereby the mechanical properties of the fibers were improved.
Moreover, the fibers had a smooth surface as well as a round and compact structure. More to the point,
the used [Gly]Cl could be recovered by simple post processing and the chitosan fibers spun in the recycled
[
Gly]Cl solution also had a strong breaking tenacity. Therefore, this study verified that [Gly]Cl is a new
spinning dope solution for preparing chitosan fibers with strong mechanical properties.
1
. Introduction
Numerous methods are available for improving the mechanical
properties of chitosan fibers. Chemical modifications of wet-spun
chitosan fibers, which are widely studied, involve the use of cross-
Chitosan is widely used in the pharmaceutical and biomedical
3
1
2
fields for its bioresorbability, biocompatibility, degradability
8,9
linking agents such as epichlorohydrin during the coagulation
10–12
step, post-treatments with glyoxal,
4
and non-toxicity. Moreover, it plays an important role in cell
11
or glutaraldehyde, or
5,6
regulation and tissue regeneration. In this context, the tech-
nology of preparing chitosan fibers is a hot topic in the field of
ecological materials.
the immersion of fibers in solutions containing phosphate and
phthalate ions, polyhedral oligomeric silsesquioxane (POSS) and
13,14
single-walled carbon nanotubes (SWNT).
In the meantime,
Chitosan is readily soluble in most aqueous acid solutions,
such as acetic acid, because of the basicity of the primary amine
groups, and several research groups have prepared wet-spun
chitosan fibers using aqueous acidic solutions with aqueous
some literature provides significant information regarding
physical treatments to improve the mechanical properties of wet-
15
16
spun chitosan fibers. The studies of Qin and Knaul demon-
strated that the Young’s modulus and tenacity could be
improved by increasing the draw ratio in the washing and
drawing baths, and possibly via the drying step, and the influence
7
alkaline coagulating baths. However, the poor tensile strength
of chitosan fibers, especially in the wet state, is a key deficiency.
17
of the jet-stretching ratio was discussed. Although the
mechanical properties of chitosan fibers could be improved by
physical or chemical methods, the two methods still have draw-
backs. For a chemical method, some chemical assistants will be
added to the chitosan fibers, and these assistants may affect the
antibiotic property of the chitosan fibers. A physical method,
which could not change the composition of the chitosan fibers,
would need specific equipment and treatments in the process. For
a
College of Chemical Engineering, Qingdao University of Science and
Technology, 53 Zhengzhou Road, Qingdao 266042, P. R. China. E-mail:
yushitaoqust@126.com; Fax: +86 0532 84022719; Tel: +86 0532
8
b
4022864
Key Laboratory of Eco-Chemical Engineering, Ministry of Education,
College of Chemistry and Molecular Engineering, Qingdao University of
Science and Technology, 53 Zhengzhou Road, Qingdao 266042,
P. R. China
c
College of Chemistry, Chemical Engineering & Environment, Qingdao
University, 308 Ningxia Road, Qingdao 266071, P. R. China
17
jet-stretching, compared with the traditional method, an
ammonia tank would need to be added, which brings great safety
concerns to the process. At the same time, both physical methods
d
College of Materials Science and Engineering, Qingdao University of
Science and Technology, 53 Zhengzhou Road, Qingdao 266042,
P. R. China
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J. Mater. Chem., 2012, 22, 8585–8593 | 8585

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and chemical methods use chitosan–acetic acid as the spinning
dope solution. The usage of acetic acid has many drawbacks such
as toxicity, strong volatility and pollution; in addition, it is
difficult to recover the used acetic acid. Therefore, based on
previous work on the preparation of wet-spun chitosan fibers,
our work aims to develop a new strategy to improve the
mechanical properties of wet-spun chitosan fibers through opti-
mizing the spinning dope solution, not through post-treatments.
As is well known, the mechanical properties of chitosan fibers are
due to the structure of the chitosan molecular chain in the fiber.
Natural chitosan has a complex inter- and intra-molecular
hydrogen-bond network, which makes it insoluble in water and
chlorinated ILs reported to be good solvents of cellulose could
26
not dissolve chitosan at room temperature. It is well known
that 2% acetic acid solvent can dissolve chitosan well at room
temperature. Therefore, we suggest that ILs that is composed of
–COOH and –Cl can be potential solvents of chitosan. For this
reason, [Gly]Cl was designed and synthesized to test its solubility
towards chitosan. Fortunately, our study verified that [Gly]Cl
was an excellent solvent for chitosan. Moreover, to reduce the
viscosity, the above IL was diluted to give a series of aqueous
solutions with different concentrations. Among these, an IL
aqueous solvent system with stronger dissolving ability, lower
viscosity and slight degradation of chitosan was selected.
Therefore, an environmentally friendly and efficient solvent
system for wet-spun chitosan fibers was obtained. In this study,
the chitosan fibers were prepared via [Gly]Cl-aqueous solvent as
the dope in a wet spinning process and the morphology and
mechanical properties of the spun fibers were examined. A
18
leads to the chitosan molecule being curled. In the normal
process of preparing the chitosan spinning dope solution, the
solvent could cut the inter- and intra-molecular hydrogen bonds,
and with the fracture of the hydrogen bonds, the curled chitosan
molecule becomes more linear. At the same time, the chain
becomes even more regular after drawing during the process of
spinning, so the tensile strength of the chitosan fibers is
26
preliminary report of this work has been presented elsewhere.
19
increased. In other words, the more regular the linear array of
chitosan molecules, the better the mechanical properties of the
wet-spun fibers. From the above mechanism, the interaction
between the solvent and chitosan in the spinning dope solution is
the major factor which affects the structure of the wet-spun
chitosan. Therefore, to design a new green solvent that could
make the chitosan molecules tend towards a linear arrangement
in the spinning dope solution will be a strategy to improve the
mechanical properties of wet-spun chitosan fibers.
2
. Experimental
2
.1 Materials
Chitosan (the degree of deacetylation was 86% and molecular
6
weight was about 1.5 ꢀ 10 ) was received from Yuhuan Chem. Co.
Ltd and the material was directly used after drying without further
purification. All the other reagents were purchased from Aldrich.
Ionic liquids (ILs) are promising alternatives of ‘‘green
solvents’’ for their properties such as negligible vapor pressure,
broad liquid regions, high thermal stabilities, and no burning or
20
explosive point. ILs widely possess excellent solubility for
inorganic compounds, organic compounds and polymer mate-
rials, so they have been used in the fields of electrochemistry,
21
organic synthesis, separation and materials preparation. It has
been reported that ionic liquids show good solubility for cellu-
22
lose, whose structure is similar to that of chitosan. Therefore, it
was possible to discover optimal ILs for dissolving chitosan via
designing the anion and cation of ionic liquids. However, chitin
may be regarded as cellulose with hydroxyl at position C-2
23,24
replaced by an acetamido group.
Both are polymers of
monosaccharides made up of b-(1-4)-2-acetamido-2-deoxy-b-d-
glucose and b-(1-4)-2-deoxy-b-d- glucopyranose units, respec-
tively (Fig. 1). Chitosan is the N-deacetylated derivative of chitin
with a typical DA (degree of acetylation) of less than 0.35
(
Fig. 1). It is, thus, a copolymer composed of glucosamine and N-
acetylglucosamine. Natural chitosan forms a more complex
inter- and intra-molecular hydrogen-bond network than cellu-
lose, due to the existence of an additional acetoamide or amino
group in its structural repeating unit. In addition, making up the
skeletons of many animals such as crustaceans, chitosan is
generally found to have larger molecular weights than cellulose
which is mainly derived from plants. As a result, it is generally
more difficult to dissolve chitosan than cellulose materials.
Previous studies on dissolving cellulose by ILs have suggested
that the solvation mainly involves the interaction of the hydroxyl
protons of the carbohydrate with the strong hydrogen bonding
ꢁ
25
and coordinating anions, in particular Cl . However, our
preliminary experiments showed that those imidazolyl
Fig. 1 Chemical structures of chitosan, chitin and cellulose.
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8
586 | J. Mater. Chem., 2012, 22, 8585–8593

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Fig. 2 The spinning unit.
Fig. 4 The XRD of native chitosan and chitosan in solution.
Fig. 3 Curves of apparent viscosity vs. shear rate about chitosan/4%
[
Gly]Cl spinning dope solution.
Table 1 Effect of composition and temperature of coagulation on the
properties of chitosan fibers (dope: 6.5% chitosan in 4% [Gly]Cl, extru-
ꢁ
1
sion rate: 13.6 m min , stretching ratio: 1.2, yarn dried with freeze
ꢃ
drying; T ¼ 20 ꢂ 2 C, water content ¼ (35 ꢂ 5)% (w/w); ‘‘—’’ Na
2 4
SO
aqueous
precipitation,
a:
solution) : v(C H OH))
volume
ratio
v(2%
Na
2
SO
4
2 5
ꢁ
1
Breaking tenacity/cN dtex
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
Fig. 5 The XRD of native chitosan and chitosan fibers.
Volume ratio
10 C
15 C
20 C
25 C
30 C
1
1
1
1
1
1
0 : 1
0 : 2
0 : 3
0 : 4
0 : 5
0 : 6
0.21
0.39
0.56
0.63
0.62
—
0.30
0.42
0.88
0.90
0.92
—
0.42
0.58
1.28
1.03
0.95
—
0.45
0.65
1.30
1.15
1.01
—
0.23
0.35
0.43
0.44
0.46
0.47
2.2 Preparation and characterization of IL
[Gly]Cl was synthesized according to the method reported in the
27
literature.
2
H O (0.55 mol) and HCl (0.08 mol, 36%) were added
into a flask (100 mL) with a stirrer, thermometer, Allihn-type
condenser and constant pressure spherical drop funnel. The
mixture was stirred while Gly (0.05 mol) was slowly added to the
Table 2 Influence of different spinning dope solutions on the mechanical properties of chitosan fibers (dope: A: 6.5% chitosan in 4% [Gly]Cl, B: 6.5%
1
ꢁ
ꢃ
chitosan in 4% acetic acid; extrusion rate: 13.6 m min , stretching ratio: 1.4, yarn dried with freeze drying; T ¼ 20 ꢂ 2 C, water content ¼ (35 ꢂ 5)%
w/w))
(
Breaking tenacity/
ꢁ
1
ꢁ1
Breaking strength/cN
Max
cN dtex
Elongation at break (%)
Initial modulus/cN dtex
Dope
Min
Max
Min
Max
Min
Max
Min
Acetic acid
[
3.72
11.13
4.47
13.9
2.89
9.63
0.86
3.77
1.04
4.83
0.67
3.21
1.58
1.91
2.28
3.47
1.06
0.47
0.19
2.30
0.32
2.87
0.04
1.53
Gly]Cl
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J. Mater. Chem., 2012, 22, 8585–8593 | 8587

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flask. After Gly was dissolved in the mixture, the flask was heated
ꢃ
ꢃ
to 60 C and the reaction was carried out at 60 C for 8 h. When the
reaction was completed, the flask was allowed to cool to room
temperature and then water was removed by vacuum to obtain
a white solid. The white product was washed by ethyl acetate. The
resulting white precipitate [Gly]Cl was isolated by filtration then
ꢃ
dried in vacuo at 60 C for 24 h. Yield: 5.08 g (91%). The structure
1
of the IL was verified by H-NMR, C-NMR and FT-IR. [Gly]Cl:
13
1
13
2
O): d 3.78 (s, 2H), 4.70 (s, 1H); C-NMR
H-NMR (500 MHz, D
(
125 MHz, D O): d 40.13, 170.08. The structure was as follows: Cl
2
+
+
[
NH –CH –COOH] IR (KBr): n 3434 (y, –NH * –CH2–
3 2 3
COOH*), 3009 (y , –CH *–COOH), 1714 (y , –C]O*), 1596 (d,
as
2
as
+
–
NH * ), 1495 (d, –CH *–CO–), 1423 (d, –COOH*), 1255 (y, –C–
3 2
O*), 1117 (y, –N–C*), 903 (u, –O–H*), 861 (d, –N–H*), 638, 496.
The NMR spectra of the ionic liquid were recorded with a 500
3
MHz Bruker spectrometer in DMSO or CDCl and calibrated
with tetramethylsilane (TMS)as the internal reference. FT-IR was
recorded with a Nicolet-510P. The spectra were collected over the
ꢁ1
range of 4000–400 cm .
2
.3 Preparation of spinning dope solution
A typical spinning dope solution was prepared by dissolving
chitosan in 400 mL of 4% acetic acid (w/w) or 4% [Gly]Cl (w/w)
Fig. 6 Schematic illustration of transformations of curly chitosan to
linear chitosan during the process of fiber preparation by dissolving
chitosan in [Gly]Cl.
ꢃ
aqueous solution at 30 C. The chitosan powder was slowly
added to the solution to make sure the chitosan could be
completely dissolved. The dissolving process was observed by
a polarizing microscope. When the chitosan was completely
dissolved in the acetic acid or IL, a black field could be observed
through a polarizing microscope. Different concentrations
(
w/w %) of chitosan/4% acetic acid or chitosan/4% [Gly]Cl were
prepared: 5% (21 g), 6% (25.5 g), 6.5% (27.8 g) and 7% (30.1 g).
The concentrationa of the spinning dope solutions of chitosan/
4
2
% [Gly]Cl were determined by rheological experiment (Section
.7). When the effect of urea on the properties of the fibers was
studied, urea was added to the chitosan/4% [Gly]Cl solution, and
the amount of urea was w(urea) : w(chitosan) ¼ 1 : 100.
2
.4 Wet spinning of chitosan fibers
The spinning dope solution was filtered through a candle filter
system before adding it to the reservoir of the spinning system,
where it was degassed under vacuum. A laboratory scale extru-
sion unit was used (Fig. 2), comprised of a reservoir, a metering
ꢁ1
Fig. 7 Curves of shear stress vs. shear rate of the different systems. A:
pump (1 mL rev ), and a spinneret (20 holes, 80 pm diameter).
The filaments were coagulated in a dilute Na SO /C H OH bath,
6.5% chitosan/4% acetic acid and B: 6.5% chitosan/4% [Gly]Cl.
2
4
2
5
with an immersion length of ca. 100 cm. The take-up rollers,
drawing system, drying rollers, and the winding up procedure
were described previously. The filaments were washed and dried
by freeze-drying.
2
.5 Regeneration and recycling of [Gly]Cl
After spinning, the solution in the coagulation bath and washing
bath was gathered into a flask, and then was distilled. During the
distillation, the solution became thicker and Na SO salt was
2
4
precipitated. At this time, the distillation was terminated and the
2 4
flask was allowed to cool to room temperature. The Na SO was
ꢃ
Fig. 8 The morphology of cross-sections of chitosan fibers from the
isolated by filtration and then dried in vacuo at 60 C for 24 h; the
different spinning dopes.
recovery rate of Na
2 4
SO was above 95%. The filtrate was
8
588 | J. Mater. Chem., 2012, 22, 8585–8593
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Table 3 Influence of different stretching ratios on the mechanical properties of chitosan fibers (dope: 6.5% chitosan in 4% [Gly]Cl, extrusion rate: 13.6
ꢁ
1
ꢃ
m min , yarn dried with freeze drying; T ¼ 20 ꢂ 2 C, water content ¼ (35 ꢂ 5)% (w/w))
Breaking tenacity/
ꢁ
1
ꢁ1
Breaking strength/cN
Max
cN dtex
Elongation at break (%)
Initial modulus/cN dtex
Stretching
ratio
Min
3.40
3.78
12.45
9.63
Max
Min
Max
Min
Max
Min
1.1
1.2
1.3
1.4
5.35
5.45
6.62
6.69
0.85
1.27
2.90
3.77
1.10
1.53
3.25
4.83
0.54
0.95
2.67
3.21
0.83
0.94
1.68
1.91
1.38
1.45
1.89
3.47
0.22
0.37
0.96
0.47
0.23
0.24
1.82
2.30
0.38
0.42
1.99
2.87
0.10
0.15
1.58
1.53
13.32
11.13
14.25
13.9
Fig. 10 SEM showing the effect of urea on the morphology of chitosan
fibers.
model 1122, with load cell-type 1105. The gauge length was
ꢁ
1
2
0 mm and the extension rate was 20 mm min . The test was
ꢃ
performed at 20 C, 65% R.H. The figures quoted are the aver-
ages of 10 tests; the standard deviation was generally less than
5
%. When required, tests were also performed on yarns previ-
ously immersed in water at room temperature, to obtain wet
strength values.
The morphological structures of the fiber were observed by
a HITACHI S-2600HS scanning electron microscope (SEM)
with a 15 kV accelerated voltage. Samples were gold coated by
ion sputtering with a JEOL JFC-1100-E and current 10 mA for
Fig. 9 SEM of chitosan fibers of different stretching ratios formed by
wet spinning.
a mixture of [Gly]Cl and water, and here it was assumed that the
9
0 s before observation.
[
Gly]Cl was not lost during the process of recuperation. Some
water was then added to the mixture to give 400 mL solution
reservoir volume is 500 mL), and chitosan was added to the
X-ray powder diffraction patterns of the samples were obtained
on a XB-3A instrument using monochromatic Cu-Ka radiation
(
(
l ¼ 0.15418 nm). It was operated at 40 kV and 100 mA. The
solution to prepare a spinning dope solution according to the
method in section 2.3. The amount of chitosan was #27.8 g,
because the spinning dope solution of 6.5% chitosan/4% [Gly]Cl
has 27.8 g chitosan.
ꢃ
experimental conditions correspond to a step width of 0.02 and
ꢁ1
ꢃ
scan speed of 2q min and diffraction region 2q ¼ 10–60 .
The average diameter of fibers was evaluated using a trans-
mission optical microscope (Leitz), from 10 values measured on
different points of the monofilament.
The denier (D ¼ weight (g) per 9000 m of monofilament) of
each chitosan fiber sample was determined by weighing at least
20 m of chitosan monofilament. The length (L) of the mono-
filament was determined by two methods. The first method
measured the length of several fibers to achieve a sufficient total
2
.6 Characterization of chitosan or chitosan fibers
The mechanical properties of the chitosan yarn samples, such as
the breaking strength, breaking tenacity, elongation at break,
and initial modulus, were measured on an Instron tensile tester,
Table 4 Influence of textile assistant (urea) on the mechanical properties of chitosan fibers (dope: w(urea) : w(chitosan) ¼ 1 : 100, 6.5% chitosan in 4%
ꢁ
1
ꢃ
[
Gly]Cl, extrusion rate: 13.6 m min , stretching ratio: 1.3, yarn dried with freeze drying; T ¼ 20 ꢂ 2 C, water content ¼ (35 ꢂ 5)% (w/w))
Breaking tenacity/
1
cN dtex
ꢁ
ꢁ1
Breaking strength/cN
Max
Elongation at break (%)
Initial modulus/cN dtex
Textile assistant
Min
Max
Min
Max
Min
Max
Min
Urea
No
15.63
13.32
18.35
14.25
13.16
12.45
1.82
2.90
2.19
3.25
1.50
2.67
1.06
1.68
2.12
1.89
0.28
0.96
0.91
1.82
1.00
1.99
0.82
1.58
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containing 4% acetic acid or 4% [Gly]Cl at room temperature.
The chitosan solution was then filtered and subsequently soni-
cated to remove trapped air bubbles. The final product (chitosan/
4
% acetic acid solution or chitosan/4% [Gly]Cl solution) had
a light yellow color and a pH of 5–6. The rheological charac-
terization of the hydrogel samples was performed in a C-VOR
rheometer from Bohlin Instruments, Inc., USA, fitted with
ꢃ
a cone-and-plate geometry (cone angle of 4 and diameter of
2
0 mm or 40 mm) and a circulating environmental system for
temperature control. The rheometer used is a shear stress
controlled instrument that consists of applying a torque (force)
and measuring the resultant displacement (movement). Torque
and displacement are converted to ‘‘rheological format’’ by
means of the measuring system constants. To prevent drying of
the samples during experiments, a steel ring of 60 mm was placed
around the measuring geometry, the annulus was filled with
water, and the sample holding region was sealed with an insu-
lated cover. As soon as the sample was introduced onto the plate,
the data were collected. The test methods employed were oscil-
latory temperature, time, stress, and frequency sweeps at
ꢃ
a constant temperature of 25 C and a pH of 5–6.
3. Results and discussion
3
.1 Concentration of spinning dope solution
The parameter of viscosity was regarded as the primary criterion
for the selection of a suitable concentration of chitosan/4% [Gly]
Cl spinning solution. Fig. 3 shows the flow curves of the chitosan/
4
% [Gly]Cl solutions of various concentrations. It can be seen
Fig. 11 The effect of urea on the structure of chitosan in [Gly]Cl solvent.
from Fig. 3 that the chitosan/4% [Gly]Cl solution is a shear-
thinning fluid that belongs to the class of non-Newtonian fluids.
As the concentration increases, the shape of the curve changes
gradually from near Newtonian behavior, at a solution concen-
tration of 5%, to non-Newtonian shear thinning at higher
concentrations. The solution viscosity was low when the chitosan
content was below 6%. Dramatic increases in solution viscosity
occurred at higher chitosan concentrations, with the 7% solution
showing a viscosity of more than 340 poises. The 6% solution
showed a solution viscosity of about 87.6 poises, but 100 poises is
typical of a wet spinning solution. A 6.5% chitosan solution was,
length over 20 m. The results of the pilot method controlled the
second one. Thus, the length was the result of the multiplication
ꢁ
1
2
of the time of spinning (min) by the roller 2 speed (v in m min ).
ꢁ3
In both cases, the precision was 10 g. An average denier was
calculated from the two methods.
After measuring the diameter and the denier of each sample of
28
fiber, we could deduce their density by the following equation:
sffiffiffiffiffiffiffiffiffiffiffiffiffi
sffiffiffiffiffiffi
W
10D=9
d ¼ 20
or d ¼ 20
pr
pr
15
therefore, used as the spinning dope.
where d is the diameter (mm), W the weight of 10000 m of
ꢁ3
monofilament (dtex), r the density (g cm ) and D the denier.
3
.2 Composition and temperature of coagulation bath
The spinning dope solution of chitosan was spurted out from the
spinning jet into the coagulation bath one and two, and in the
coagulation bath the thread of chitosan and the coagulator were
interdiffused to form the chitosan fiber.
2
.7 Rheological experiment on chitosan solvent
A solution (100 mL) containing a certain amount of chitosan was
prepared by dissolving the chitosan powder in distilled water
Table 5 The mechanical properties of chitosan fibers from regeneration of [Gly]Cl as the solution (dope: 6.5 or 5.9% chitosan in 4% [Gly]Cl, extrusion
ꢁ
1
ꢃ
rate: 13.6 m min , stretching ratio: 1.3, yarn dried with freeze drying; T ¼ 20 ꢂ 2 C, water content ¼ (35 ꢂ 5)% (w/w))
Breaking tenacity/
ꢁ
Elongation at
break (%)
Initial modulus/
ꢁ1
1
Breaking strength/cN
cN dtex
cN dtex
No.
Amount of chitosan/g
Max
Min
Max
Min
Max
Min
Max
Min
1
2
3
26.0
26.0
23.6
13.32
12.85
11.37
14.25
13.45
13.21
12.45
11.32
10.22
2.90
2.44
1.80
3.25
3.05
2.89
2.67
2.57
1.43
1.68
0.58
0.63
1.89
0.83
0.87
0.96
0.24
0.27
1.82
1.52
0.93
1.99
1.78
1.23
1.58
1.02
0.75
8
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In the traditional wet-spinning method, acetic acid is often
used as the solution, and the coagulator is a mixture of NaOH
Fig. 5. From Fig. 4, it can be seen that chitosan/IL and chito-
san/acetic acid show a slight, broad diffraction peak centered
14,29,30
ꢃ
and C H OH aqueous solution.
NaOH can neutralize
acetic acid interdiffused from the spinning dope solution and
OH can accelerate interdiffusion between the spinning dope
near 2q ¼ 28 , which can not be eliminated by decreasing the
2
5
chitosan concentration or prolonging the time of dissolution, and
no diffraction peak centered near 2q ¼ 12ꢃ compared with
crystalline chitosan. This result implies that the crystalline
domains in the chitosan particles are incompletely disrupted by
[Gly]Cl or acetic acid during the dissolving process. Anyway, we
can conceive that such a treatment should make chitosan more
2 5
C H
solution and coagulator and improve the form of the chitosan
fibers. When [Gly]Cl was used as the solution, a mixture of
2 5
NaOH and C H OH aqueous solution was also used as the
coagulator to prepare chitosan fibers with excellent properties.
However, NaOH can react with the carboxylic group of [Gly]Cl
to form the sodium salt so the [Gly]Cl is difficult to recover.
Therefore, it is important to find a new coagulator that can
promote the form of the chitosan fibers and be beneficial for the
regeneration of [Gly]Cl.
32
flexible than crystalline chitosan. For Fig. 5, the chitosan fibers
from [Gly]Cl are still type I configuration, but the chitosan fibers
from acetic acid are changed to amorphous because the diffrac-
ꢃ
tion peak centered near 2q ¼ 12 disappears and the diffraction
ꢃ
peak near 2q ¼ 20 is weak and broad. It is well known that
Na
2
SO
4
aqueous solution can promote dehydration of muci-
chitosan type I has stronger molecular forces than amorphous
33,34
15,31
lage glue to form fibers,
which is a good inspiration for the
can improve the thread of chitosan to keep
their micelle and form the fibers by the effect of static electricity
between macromolecule chains. Here a mixture of 2% Na SO
aqueous solution and C OH at different ratios was investi-
chitosan,
so the chitosan fibers from [Gly]Cl has stronger
2
ꢁ
present work. SO
4
mechanical properties than those from acetic acid. Therefore, we
design the schematic illustration of the transformation of curly
chitosan to linear chitosan during the process of fiber prepara-
tion by dissolving chitosan in [Gly]Cl. In a solution of chitosan/
2
4
2 5
H
+
ꢁ
gated to determine a suitable ratio mixture as coagulator, and the
ꢃ
[Gly]Cl, [Gly]Cl is a strong electrolyte, and [Gly] and Cl
results can be seen in Table 1. At 20–25 C, when the ratio of 2%
uniformly distribute in water in ionic form. Even more impor-
+
tantly, [Gly] can easily enter the network structure of chitosan to
Na SO /C H OH in the aqueous solution was too high, the
2
4
2
5
surface of the chitosan fiber was quickly solidified, leaving the
gel-like middle part of the fiber, and thus the fiber was badly
prepared with poor morphological and mechanical properties.
disrupt the inter- and intra-molecular hydrogen bonds (Fig. 6).
In addition, chitosan is a cationic polyelectrolyte in dilute acid
+
solution, and [Gly] increases the repulsion between cations in
When the ratio of 2% Na
2
SO /C
4
2
H
5
OH in the aqueous solution
the chitosan molecules, which further stretches the chitosan
molecular chain in solution and prompts curly chitosan mole-
cules to become linear; at the same time, the chitosan molecules
are rearranged to increase the strength of the fibers in the
was too low, the fiber was formed slowly, also causing unsatis-
factory mechanical properties. For a balanced ratio, a mixture of
v(2% Na
2
SO
4
) : v(C
2
H
5
OH) ¼ 10 : 3 was a suitable coagulator.
35
At the same time, the effect of the coagulation temperature was
investigated. From Table 1 it can be seen that the chitosan fibers
drawing force.
Furthermore, in acetic acid solvent, the extensibility of chi-
tosan molecules is poor and the breaking tenacity of the fibers
is also poor. From Fig. 7, it can be seen that at low shearing
rates regions of the flow curve, the initial slope of 6.5% chi-
tosan/4% acetic acid is steeper than that of 6.5% chitosan/4%
[Gly]Cl. It is suggested that the degree of tangling of chitosan
molecular chains is high in acetic acid and the high aggregative
state strengthens the force of hydrogen bonding between the
ꢃ
had the best mechanical properties at 20–25 C. When the
ꢃ
temperature was 30 C, the fiber was rapidly formed but some-
times broken due to the intensive molecular movement. When
ꢃ
the temperature was below 20 C, the fiber was slowly formed
and the solubility of Na
In conclusion, the mixture of v(2% Na
0 : 3 was selected as coagulator and the process of coagulation
2
SO
4
in the water and ethanol decreased.
2
SO ) : v(C
4
2 5
H
OH) ¼
1
ꢃ
36
was carried out at 20–25 C.
chitosan molecules. Besides the force of the hydrogen bonds,
van der Waals’ forces between the chitosan molecules form
a physical cross-linking point, and the active power decreases at
room temperature, so that the extensibility of the fibers from
acetic acid is weaker than that of those from [Gly]Cl, in
3
.3 Preparation of chitosan fibers using chitosan–[Gly]Cl as
spinning dope solution
36
The traditional wet spinning for preparing the chitosan fibers is
as follows: a solution of 6.5% chitosan/4% acetic acid as dope, 2%
6
NaOH as coagulating bath. According to the above test, we first
agreement with the results of the mechanical properties. On
the other side, SEM was be used to observe cross-sections of
fibers from acetic acid and [Gly]Cl (Fig. 8). From the images, it
can be seen that the cross-sectional area of the fibers is
approximately circular and hence the fibers could be cylin-
drical. The fracture surface of the fiber from acetic acid is
rough, uneven and inadequate (Fig. 8); the fracture surface of
the fiber from [Gly]Cl is uniform, smooth and adequate, and
the results prove the above deduction. It is well known that the
intensity of the fiber is due to the orientation of chitosan
molecule and the modulus is due to the crystallinity. With the
increasing degree of orientation, the degree of order and crys-
tallinity was enhanced, and furthermore, the strength and
modulus of the fibers was amplified and the breaking elonga-
tion lessened. The results are in agreement with the XRD.
carried out wet spinning for preparing chitosan fibers using
a 6.5% chitosan/4% [Gly]Cl solution as dope and a mixture of
v(2% Na SO
2
4
) : v(C
H
2 5
OH) ¼ 10 : 3 as coagulating bath. At the
same time, the mechanical properties of the fibers prepared by
the new path were investigated, and the results can be found in
Tables 2 and 3.
The mechanical properties of fibers using chitosan/[Gly]Cl as
the dope are better than those fibers using chitosan/acetic acid as
the dope, as the breaking tenacity increased from 0.86 to 3.77
(
Table 2). This result commendably confirms our opinion. The
XRD profiles of chitosan in different solutions, chitosan fibers
from different dopes and native chitosan are shown in Fig. 4 and
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When the dope was ejected from the spinneret orifice, the fiber
received the force of extrusion and friction from the ambient
spinneret orifice, and the force caused orderly macromolecular
avoided, so the solubility of [Gly]Cl for chitosan decreased
resulting in the properties of the chitosan fibers decreased.
13
association of the fibers to arrange along the axis of the orifice.
4
. Conclusions
However, the orientation of the fibers is unstable, and a swelling
effect can be produced during the process of fiber extrusion, and
meanwhile the effect leads to a macromolecule of newly disor-
dered astatic state after being ejected. Therefore, the only way to
enhance the strength of the fibers is by drawing. In this part, the
mechanical properties and morphology of the chitosan fibers are
investigated after drawing to stretching ratios of 1.1, 1.2, 1.3 and
In this paper, we reported a new solvent, that is [Gly]Cl, for
chitosan materials. The results from XRD experiments suggested
that the chitosan fibers from [Gly]Cl have a crystal structure
close to type I, the same as the native chitosan, and the chitosan
fibers from acetic acid have a crystal structure close to type II,
implying that a transition of crystal form may occur during the
dissolving and the recovery processes. On the basis of these
results, it is possible to establish a new biomass-ionic liquid
platform (BILP) for processing and homogeneous chemical
modifications of chitosan. Such a BILP can be also used as
a pretreatment technique for many heterogeneous chemical
reactions and enzyme degradations in chitosan-based poly-
saccharides. We then demonstrated that chitosan fibers exhibited
1
.4 (Table 3). From the data of Table 3, with the increase of
drawing times, the fibers are obviously strengthened and tenacity
strength hugely enhanced. In addition, the toughness and
strength of the fibers are improved; it is agreed to use chitosan/
acetic acid as the dope. It can also be seen from the SEM (Fig. 9);
the fiber is smoother and denser with the increasing of drawing
time. It is suggested that with the increasing of drawing times,
macromolecular fibers are more uniformly arranged along the
good mechanical properties by using 6.5% chitosan (Mw ¼ 1.5 ꢀ
37
direction of the force.
6
0 )/4% [Gly]Cl as a spinning dope solution, without any agents
1
or post-treatments commonly used in the literature. The increase
ꢁ1
3
.4 Effect of addition agent on the mechanical properties of
of breaking tenacity (from 0.86 to 3.77 cN dex ) and initial
modulus (from 0.19 to 2.3) of the stretched fibers could be
explained considering the evolution of the morphological prop-
erties. Moreover, [Gly]Cl has excellent stability as the chitosan
fibers have good properties using recycled [Gly]Cl as a solution.
It is important to underline that the results, in term of the
mechanical properties, of the chitosan fibers obtained by using
chitosan/[Gly]Cl as a dope from our gel-spinning process, are
better than in previous studies. Thus, the improved mechanical
properties of the wet-spun chitosan fibers prepared in this study
should contribute to their use in biomedical applications as
hernioplasty, suturation or wrinkle filling. They are also partic-
ularly useful for making materials such as yarns and woven, non-
woven or knitted fabrics.
chitosan fibers
So far, the effect of an addition agent on the structure of the
chitosan molecule in acid solvent has been widely studied by
35
38,39
researchers. Chen
studied the effect of urea on the confor-
mational change of chitosan. The results showed that urea made
the conformation of the chitosan molecules extend, and the
molecular conformation even changed when the amount of urea
added was enough. In this part, urea was added to the dope of
chitosan/[Gly]Cl to improve the strength of the chitosan fibers.
However, from the mechanical properties of the chitosan fibers,
the properties of the fibers without urea are better than of those
with urea (Table 4). It was also observed that the surface of the
fibers was rough from SEM (Fig. 10). The reason may be that
+
Gly] can easily enter the network structure of chitosan to
[
disrupt the inter- and intra-molecular hydrogen bonds (Fig. 11).
In addition, chitosan is a cationic polyelectrolyte in dilute acid
+
solution, and [Gly] increases the repulsion between cations in
Acknowledgements
The authors thank Program of 863 (2009AA03Z426), National
Natural Science Foundation of China (No.20876080 and
the chitosan molecules, which causes further stretching of the
chitosan molecular chain in solution and prompts curly chitosan
2
1106074), High-Tech Innovation Project Specialized Plan of
Shandong Province (2007ZCB01369) and the Specialized
Research Fund for the Doctoral Program of Higher Education
of China (20093719110002), Application foundation projects of
Qingdao (10-3-4-4-11-jch), Shandong outstanding young and
middle-aged scientist research award funded projects (BS2009
CL033) for financial support.
2
molecules to become linear. When urea was added, –NH could
competitively form new hydrogen bonds with chitosan, which
shields the positive charge of the chitosan molecule. As a result,
the intermolecular repulsion decreased, the linear structure
recovered to the curly structure, and finally the viscosity of the
40
dope was low and the properties of the fibers dropped.
3
.5 Regeneration of [Gly]Cl as the solution
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was not obviously decreased and the chitosan fibers also had
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