2.3. Characterization and measurements
The FTIR spectra were recorded on a Nicolet AVATAR 370 DTGS
spectrometer with potassium bromide technique at 400
–
4000 cm
−
1
.
13
C NMR spectra were recorded at 50 °C in D
2
O with a Bruker Avance
400 MHz or Bruker Avance 600 MHz spectrometers. The samples ele-
mentally analyzed by a CHNS 932 Analyzer (LECO). The total DS value
was calculated from the sulfur (S%) and carbon (C%) contents ac-
cording to equation
(1)
.
=
DS
S%·72.066
C%·32.06
total
(1)
Molecular mass measurements performed by JASCO size-exclusion
chromatography system equipped with a PU-980 pump, PSS Suprema
3000 Å, and 300 Å columns (Polymer Standards Service GmbH, Mainz,
Germany), RI-930 refractive index (RI) detector (JASCO GmbH, Gross-
Umstadt, Germany). Measurements were carried out at 30 °C and 0.1 M
NaNO
3
aqueous solution was used as an eluent. The columns were ca-
librated with pullulan standards (Sigma-Aldrich Chemie GmbH).
The DP
w
for SCS samples was calculated using weight average
molecular weight (M
w
) and DS values according to equation
(2)
.
=
+
DP
M
162 102·DS
w
w
(2)
The producibility of the reaction was estimated by product yield.
The yield value was calculated from the weights of SCS obtained (m
(SCS), g) and starting cellulose (m (cellulose), g) with respect to sodium
trisulfate cellulose, according to Eq.
(3)
.
=
Yield (%)
m(SCS)·162
m(cellulose)·468
·100
(3)
3. Results and discussion
3.1. Synthesis of SCS
The reaction scheme for the homogenous sulfation of cellulose with
SO
3
/Py in DMA/LiCl is shown in
Fig. 1
. In the
fi
rst step, in
fl
uence of the
reaction conditions (temperature, time) on the molecular parameters of
the product was studied. The reactions were carried out with a cellulose
sample (DP
w
= 900) in the constant ratio of 1.0 mol sulfating reagent/
mol AGU at 40
–
90 °C for 1
–
12 h (
Table 1
). Increases in temperature
su
ffi
ciently in
fl
uenced on sulfation e
ffi
cacy of the cellulose, indicating
that the reaction occurs by the endothermic event. E
ff
ective increases of
DS values of the samples were observed in the temperature range of
40
–
80 °C and a sample with a DS value of 0.38 was obtained at 80 °C.
The DS value insigni
fi
cantly increased in an increment of the tem-
perature from 80 °C to 90 °C. Therefore, the optimal temperature was
found to be 80 °C for the sulfation reaction of cellulose. The studies
showed the temperature is a key factor in this reaction, the high tem-
perature is crucial to synthesize the samples with high DS values and
the results are consistent with previous statements
[33,39,41]
.
The DS value is one of the most important parameters, plays a
fundamental role in many behaviors of the SCS. Introducing the
charged sulfate group into the cellulose backbone imparts water solu-
bility to the cellulose sulfates, which is of practical relevant on the vi-
sion of biomedical and industrial application. On the other hand, the
amount of the functional sulfate group along the polymer chain dictates
charge density and macromolecular conformation that can directly in-
fl
uence the physicochemical and biological features. Numerous studies
revealed that chemical, physical and biological properties of SCS and
SCS based materials improve with an increase in DS values
[29,30]
.
The water solubility of the SCS samples in low DS values di
ff
ers in
heterogeneous and homogeneous reactions, regarding the distribution
of sulfate group along the polymer chain. Under the heterogeneous
conditions,
e
OH groups in amorphous regions of cellulose are mostly
involved in the reaction, which results in uneven distribution of the
sulfate group along the polysaccharide backbone. Therefore, SCS sam-
ples obtained by the heterogeneous sulfation reactions, with DS values
below 0.3 are generally insoluble or partially soluble in water. In the
homogenous routes, all
e
OH groups of the polymer can be equally
accessible to the esteri
fi
cation, therefore, sulfate groups are evenly
distributed along the polymer chain and water-soluble samples are
expectable in the DS range of 0.2
–
0.3
[32]
. In the present studies, the
samples with DS values of 0.23 and 0.28 were completely soluble in
water, indicating that sulfate group uniformly distributed along the
polymer chain.
The increase of DS values by increasing the temperature can be
explained as an increase in sulfation rate. The sulfation rate, for ex-
ample, also showed an e
ff
ective increase in the temperature range of
40
–
80 °C and achieved 0.0063 mol/min at 80 °C, insigni
fi
cantly
changed at 90 °C (
Fig. 2
a). In the reaction, an insigni
fi
cant decrease was
observed in the DP
w
with the temperature wherein the cellulose sample
showed a decrease of DP
w
value from 900 to 850 at 90 °C (
Fig. 2
b).
Producibility of the reaction was improved with the temperature and
SCS samples in the yield of 13.8
–
25.1% were obtained at 40
–
90 °C. The
results clearly revealed that temperature is a crucial to improve the DS
value of SCS and productivity of the reaction. DS and yield values of the
product were e
ff
ectively elevated in the reaction conducted at 80 °C and
insigni
fi
cantly changed at a higher temperature. Thus, further reactions
were carried out at 80 °C.
Fig. 1.
Reaction scheme for the homogenous sulfation of cellulose.
Table 1
Conditions for and results of homogenous sulfation of cellulose (DP
w
= 900)
with 1 mol SO
3
/Py per mol AGU.
Reaction conditions
Results
Temperature (°C)
Time (h)
DP
w
a
DS
total
b
Yield
c
(%)
40
1
888
0.23
13.8
50
1
882
0.28
17.5
60
1
884
0.33
20.9
70
1
871
0.36
23.3
80
1
861
0.38
24.7
90
1
850
0.39
25.1
80
2
834
0.57
32.9
80
3
821
0.68
38.5
80
4
818
0.74
42.4
80
6
811
0.78
45.5
80
8
794
0.81
46.7
80
12
765
0.84
45.8
a
Weight average degree of polymerization, the values were calculated based
on the Eq.
(2)
.
b
Total degree of substitution determined by elemental analysis method, the
values were calculated based on the Eq.
(1)
.
c
Yield of the product, the values were calculated based on the Eq.
(3)
.
B. Muhitdinov, et al.
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