Synthesis, characterization and biological activity of Schiff bases based on chitosan and arylpyrazole moiety

4.1.3. Thermal analysis 
Fig. 3 shows the TG thermograms and their corresponding derivative weight loss curves (DTG) of 
chitosan and 
ChBs 
under nitrogen atmosphere and the detailed data are summarized in Table 2. 
Differential thermal analysis (DTA) which can provide further information on the thermal transition 
of the investigated samples is shown in Fig. 4. It is seen from Fig. 3 that all samples exhibit an 
initial small drop between 50 and 150
o
C, which corresponds to absorbed moisture, with a weight 
loss of approximately 7%. This weight loss appears as a faint endothermic peak around 60
o
C in 
DTA thermogram. The initial degradation of chitosan appears within the range of 170-369°C with 
ca. 45% weight loss. The maximum degradation rate temperature occurs at ca. 305°C which is also 
confirmed by the appearance of an exothermic peak in the DTA thermogram (Fig. 4). This 
degradation complex process involves the dehydration of the saccharide rings, depolymerization 
and decomposition of the acetylated and deacetylated units of chitosan [52-54]. The strong 
exothermic peak, which appears at 534°C in the DTA curve within the degradation temperature 
range of 369-599°C connected with ca. 45% weight loss, corresponds to the residual cross-linked 
degradation of chitosan [55]. 
From TG-DTG thermograms of the ChBS,
it is obvious that all 
derivatives exhibit three stages of weight loss. The first weight loss (7.4-9.0 %) is assigned to the 
loss of adsorbed water molecules, the second weight loss (30.2-47.5 %) corresponds to the 

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decomposition of free chitosan unit, and the third one (43.7-54.4 %) may be attributed to the 
decomposition of condensed chitosan backbone and the attached arylpyrazole moieties. 
Considering 
the temperature at which the thermal degradation starts as a criterion of the thermal stability of the 
prepared ChBs, 
it is seen that ChBs exhibit lower thermal stability than chitosan. This may be 
attributed to the decrease in the number of primary amino groups of chitosan as a result of Schiff 
base modification, which in turn leads to the decrease in crystallinity and the cleavage of inter-
molecular hydrogen bond between the chitosan molecules. It is found that, 
the thermal stability 
increases according to the following order: Ch 
>
Ch-H 
>
Ch-NO
2 
≈ Ch-Cl > Ch-CH
3
> Ch-OCH
3
. 
From Table 2, it is shown that
, 
the maximum degradation rate temperature (T
max
)
of the first main 
degradation step of the ChBs shifts to lower temperature compared with chitosan. This degradation 
step appears as exothermic peak around 305
o
C and slightly affected by substituent group of the aryl 
moiety.
On the other hand, the 
T
max 
of the second degradation step for Ch-CH
3
and Ch-OCH
3
derivatives,
with electron donating substituent of aromatic group
, appears at 492 and 524
o
C, 
respectively, which is lower than that of chitosan (535
o
C), while for Ch-Cl. Ch-NO
2
derivatives 
with electron withdrawing substituent of aromatic group and un-substituted Ch-H derivatives
, it 
appears at 558, 609 and 629
o
C, respectively, which are higher than that of chitosan. 
From DTA data 
(Fig. 4 and Table 2)
, 
it is shown that the exothermic peak associated with the decomposition of this 
step is dependent on the type of substituent in the aromatic ring. It 
appear at 633, 493, 529, 573 and 
611
o
C for Ch-H, Ch-CH
3
, Ch-OCH
3
, Ch-Cl and Ch-NO
2
, respectively, 
are
consistent with the 
maximum decomposition temperature in TG thermograms. 
The apparent activation energy for the thermal degradation of chitosan and its Schiff base 
derivatives were determined from the TG curves using Coats and Redfern model [56]. To calculate 
and understand the nature of the decomposition, the complete thermogram was divided into distinct 
sections according to their degradation steps. The Coats and Redfern model is expressed as follows:
ln "#
$%&∝
(
)  ln *
+,
-
.
/
0 #
-
.
,(
(3) 
where 
1 is the fraction of sample decomposed at temperature T, R is the general gas constant, 2
3
is 
the activation energy, A is the frequency factor, and β is the heating rate. 
1 can be calculated as 
follow: 
∝  
4
5
4
6
4
5
 4
7
 (4) 
 
where, 
8
o
is the initial weight of the sample, 
8
9 
is the weight of the sample at the particular 
temperature T, and 
8
e
is the weight at the end of degradation step. The activation energies E
a
 can 
be calculated from the slope of the linear fitted line between ln [-ln (1- α) /T
2
] vs. 1/T as shown in 

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Fig 5. The obtained activation energies and the correlation coefficients (r
2
) for chitosan and chitosan 
Schiff bases are listed in Table 2. It can be observed that the values of E
a 
for the first degradation 
step (second weight loss step in TGA thermogram ) for the Ch-H, Ch-CH
3
and Ch-NO
2
samples are 
123, 123 and 117 
kJ mol
-1
, respectively which are comparable with the E
a 
value of chitosan (119 
KJ 
mol
-1
)
. Lower E
a
values are observed for Ch-OCH
3
and Ch-Cl (92 and 96 
kJ mol
-1
, respectively) 
compared to that of chitosan. For the second degradation step (third weight loss step), the value of 
activation energy for Ch-H sample is close to that of chitosan, while for Ch-Cl and Ch-NO
2 
are 
lower than that of chitosan. This indicates that the presence of electron withdrawing groups (―Cl 
and ―NO
2
) in aryl moiety has an accelerating effect on the second decomposition step of ChBs. On 
the other hand, higher E
a
values were observed for Ch-CH
3
and Ch-OCH
3
compared with chitosan, 
indicating that the presence of electron-donating groups (―CH
3
and ―OCH
3
) has a delaying effect 
on the decomposition of ChBs. 
Fig. 3.
Fig. 4.
Fig.5.
Table 2.