Thermo-oxidative decomposition behavior of starch-g-poly(citronellyl methacrylate) and starch-g-poly(citronellyl acrylate) copolymers
Journal of Thermal Analysis and Calorimetry
Thermo-oxidative decomposition behavior of starch-g-poly(citronellyl methacrylate) and starch-g-poly(citronellyl acrylate) copolymers
Marta Worzakowska 0 1
Copolymers 0 1
0 Faculty of Chemistry, Department of Polymer Chemistry, Maria Curie-Skłodowska University , Gliniana 33 Street, 20-614 Lublin , Poland
1 Marta Worzakowska
The paper describes the studies on the thermo-oxidative decomposition behavior of two types of starch graft copolymers: starch-g-poly(citronellyl acrylate) copolymers with the grafting percent (%G): 21.3% ± 0.5, 35.8% ± 0.6 and 59.8% ± 0.3 and starch-g-poly(citronellyl methacrylate) copolymers with %G: 21.3% ± 0.4, 37.0% ± 0.2 and 51.8% ± 0.3 prepared applying the ''grafting from'' method. The decomposition course of copolymers by using the TG/ DTG/DSC/FTIR analysis was evaluated. As it was found, the course of TG/DTG/DSC curves was independent on the %G but it was directly dependent on the type of analyzed copolymer. It resulted in completely different decomposition course of starch-g-poly(citronellyl acrylate) copolymers under the presence of oxidative conditions as compared to starch-gpoly(citronellyl methacrylate) copolymers. Thanks to applying the FTIR analysis of the gaseous products emitted under the heating of the studied materials, the detailed decomposition course of copolymers was evaluated.
Starch; Citronellyl acrylate; Citronellyl methacrylate; Thermal behavior
Starch is an environmentally friendly, carbohydrate,
macromolecular compound which finds it applications in
many field of industry such as a food, pharmaceutical,
medicine and paper industries. According to the literature
survey, the decomposition mechanism of this natural
polymer under various environments, artificial and natural,
is widely studied and discussed. Generally, different,
simultaneous phase transformations under the heating of
starch happen. The most significant of all are melting,
evaporation, sublimation, chemical condensation,
decomposition and carbonization leading to the creation of the
mixture of volatiles [
]. Due to the availability of starch,
its low cost and the presence of hydroxyl groups in its
structure, it can be chemically modified and thus different
structure starch graft copolymers with potential
applications as fillers, stabilizers, impact resistant materials,
plastics, modifiers, matrices, excipients, etc., can be
prepared. Among them, the thermal properties of starch graft
copolymers obtained using different structure methacrylate
and acrylate monomers have been widely studied [
Recently, at the Department of Polymer Chemistry UMCS,
the intensive studies on the synthesis, physicochemical
properties and thermal behavior of novel, non-described in
the literature data, starch-g-copolymers are conducted. To
this time, we succeed in the preparation the
starch-gcopolymers based on aromatic methacrylate and acrylate
monomers such as phenyl and benzyl meth(acrylates). The
studies of their properties affirmed that the decomposition
course of those copolymers is complex and leads to the
formation of the mixture of volatile products and covered
series, simultaneous processes [
]. Due to the fact that
those copolymers were characterized by promising
physicochemical properties and thermal stability which
acted them as attractive materials for the preparation of
more environmentally friendly products which could
replace petrochemical plastics, we decided to continue our
studies on the chemical modification of starch by other
methacrylate and acrylate monomers. Lately, applying the
monomers prepared from methacryloyl or acryloyl chloride
and natural terpene alcohol: citronellol allowed obtaining
amphiphilic starch-g-copolymers with excellent chemical
resistance toward acidic, neutral and buffer environment,
high moisture resistance, which were resistant to polar and
nonpolar solvents with satisfactory thermal properties
under inert atmosphere. However, most of the materials are
processed and manufactured in the presence of air, so the
thermal properties of novel materials under oxidative
conditions are of high importance on account of their
practical utilization. And thus, the present paper describes
the thermal properties and the studies on the evaluation of
decomposition course of two types of novel materials:
starch-g-poly(citronellyl methacrylate) and
starch-gpoly(citronellyl acrylate) copolymers which differ in their
grafting percent (%G). The influence of copolymer
structure on the course of their degradation under the heating in
the presence of oxidative conditions has been evaluated.
To the present studies, the series of copolymers which
differ in their grafting percent (%G) were chosen, as shown
in Table 1. The copolymers were obtained according to the
methodology described in Refs. [
24, 26, 27
]. Generally, the
graft copolymerization conditions are placed in Table 1.
Copolymer 1—13C/CP MAS NMR (75.5 MHz, d ppm) 174.8
(C=O), 81.1, 93.5, 101.8 (CH–O), 72.6 (C–O), 61.5 (CH2–
O), 20.0, 25.8, 30.3, 37.5 (CH3, CH2, CH); FTIR (thin film,
cm-1): 3336 (m OH), 2866, 2922, 2945 (m C–H), 1728 (m
C=O), 1375, 1448 (d C–H), 1018, 1076, 1150 (m C–O),
758, 839, 928 (m=C–H).
Copolymer 2—13C/CP MAS NMR (75.5 MHz, d ppm) 174.9
(C=O), 81.2, 93.5, 101.7 (CH–O), 72.6 (C–O), 61.6 (CH2–
O), 20.1, 25.8, 30.1, 37.7 (CH3, CH2, CH); FTIR (thin film,
cm-1): 3338 (m OH), 2867, 2920, 2947 (m C–H), 1728 (m
C=O), 1377, 1448 (d C–H), 1018, 1074, 1150 (m C–O),
760, 838, 929 (m=C–H).
Copolymer 3—13C/CP MAS NMR (75.5 MHz, d
ppm) :174.8 (C=O), 81.1, 93.7, 101.5 (CH–O), 72.5 (C–
O), 61.6 (CH2–O), 20.1, 25.6, 30.0, 37.8 (CH3, CH2, CH);
FTIR (thin film, cm-1): 3338 (m OH), 2865, 2921, 2947 (m
C–H), 1728 (m C=O), 1378, 1445 (d C–H), 1018, 1074,
1152 (m C–O), 758, 838, 930 (m=C–H).
Copolymer 4—13C/CP MAS NMR (75.5 MHz, d ppm) 176.8
(C=O), 81.3, 93.3, 102.0 (CH–O), 72.6 (C–O), 61.3 (CH2–
O), 19.9, 25.7, 30.4, 37.5, 45.1 (CH3, CH2, CH); FTIR (thin
film, cm-1): 3320 (m OH), 2865, 2950 (m C–H), 1726 (m
C=O), 1370, 1445 (d C–H), 997, 1014, 1076, 1145 (m C–
O), 748, 840, 927 (m=C–H).
Copolymer 5—13C/CP MAS NMR (75.5 MHz, d
ppm) :176.8 (C=O), 81.2, 93.3, 102.1 (CH–O), 72.6 (C–
O), 61.4 (CH2–O), 19.9, 25.8, 30.5, 37.5, 45.0 (CH3, CH2,
CH); FTIR (thin film, cm-1): 3322 (m OH), 2867, 2950 (m
C–H), 1726 (m C=O), 1368, 1445 (d C–H), 998, 1015,
1076, 1147 (m C–O), 748, 842, 927 (m=C–H).
Copolymer 6—13C/CP MAS NMR (75.5 MHz, d ppm) 176.6
(C=O), 81.3, 93.1, 101.9 (CH–O), 72.6 (C–O), 61.1 (CH2–
O), 19.8, 25.5, 30.4, 37.2, 45.1 (CH3, CH2, CH); FTIR (thin
film, cm-1): 3324 (m OH), 2865, 2952 (m C–H), 1726 (m
C=O), 1370, 1447 (d C–H), 998, 1014, 10767, 1147 (m C–
O), 749, 840, 925 (m=C–H).
Reaction temp./ C Reaction time/ min
21.3 ± 0.5
35.8 ± 0.6
59.8 ± 0.3
21.3 ± 0.4
37.0 ± 0.2
51.8 ± 0.3
The structure of the obtained materials was confirmed based on spectroscopic analyses
Thermo-oxidative behavior of starch-g-poly(citronellyl
acrylate) and starch-g-poly(citronellyl methacrylate)
copolymers was studied using a STA 449 F1 Jupiter
Netzsch instrument (Germany) linked online with a FTIR gas
analyzer Bruker Tensor 27, TGA 585 (Germany).
Simultaneous, TG/DTG/DSC studies in Al2O3 crucibles using
sensor thermocouple type S TG-DSC, at the temperature
range of 40–700 C with sample mass ca. 10 mg and with a
heating rate of 10 C min-1 were performed. The synthetic
air with a flow rate of 100 mL min-1 was applied as a
furnace atmosphere. The gaseous FTIR spectra in the range
from 600 to 4000 cm-1 with a resolution of 4 cm-1 were
collected over the whole analysis time.
Results and discussion
Starch-g-poly(citronellyl acrylate) copolymers
Figure 1 presents the TG, DTG and DSC curves achieved
under the heating of starch-g-poly(citronellyl acrylate)
copolymers. As it is clearly visible, the course of the
presented curves is very similar and it is independent on %G
of copolymers. Generally, on TG/DTG curves, three main
temperature ranges where the changes in mass loss are
indicated. The first mass loss (Dm1) from 2.5 to 4.7% at the
temperatures up to ca. 170 C and with Tmax1 ca. 89–96 C
was observed, as shown in Table 2. In addition, when one
have a look at the DSC curves, one can see only one
endothermic peak at this temperature range. Analyzing the
gaseous FTIR results, gathered at Tmax1, the presence of the
absorption signals responsible for the water vapor (bands at
the wavelength of approx. 1400–1800 and
3500–3900 cm-1) was indicated, as shown in Fig. 2. So,
the results confirmed that this stage was not connected with
the degradation of copolymers but only with the physical
transformation (water evaporation from the samples which
was absorbed under the synthesis of copolymers and the
The second mass loss (Dm2) happens between the
temperatures ca. 170–440 C. It is worth noticing that this
stage is composed at least with two, non-well-separated
stages. It indicates on the complex course of the studied
samples. Commonly, the mass loss in this stage is
comparable for all the studied copolymers and amounts ca.
65.6–66.5%. In addition, the first one stage was described
by high intensity DTG peak with the rate of the
transformation ca. 11.9–12.7% min-1 and DTG peak temperature
at ca. 284–286 C, as shown in Table 2. However, the
second one is characterized by low-intensity DTG peak
with the rate of the transformation ca. 1.5–2.5% min-1 and
similar Tmax2a (399–405 C). Comparing the DSC curves,
its similar course was also observed at these temperature
ranges. Only, exothermic signals which were likely due to
the oxidation decomposition processes of the studied
materials were appeared. The FTIR spectrum of the gases
extracted at Tmax2 is a typical gases spectrum for starch
decomposition products in oxidative conditions, which is in
agreement with the literature data [
]. However, on the
FTIR spectrum gathered at Tmax2a, one can see mainly the
emission of CO2 (bands at the wavelength of approx. 670
and 2330–2365 cm-1), CO (bands at the wavelength of
approx. 2000 and 2200 cm-1), H2O (bands at the
wavelength of approx. 3500–3900 cm-1), CH4 (band at the
wavelength of approx. 3014 cm-1) and small amounts of
other organic gaseous products. Among them, it can be
suspected the occurrence of some species having aliphatic
structure which is confirmed by the presence of the bands
at the wavelengths at the range of 2860–2960 cm-1
corresponding to the stretching vibrations of C–H and
lowintensity bands at the wavelengths of 1380–1460 cm-1
responsible for the deformation vibrations of C–H. Besides
those signals, on the FTIR spectrum, small-intensity
absorption signal at the wavelength of approx. 3050 cm-1
connected with the stretching vibrations of =C–H and the
signals at 900–980 cm-1 which are due to the out-of-plane
deformation vibrations could testify to the emission of
some alkene species. Unfortunately, the stretching bands of
C=C were unseen from the spectra. It could be due to the
emission of water vapor which masks the C=C bands. (As
it is visible, the water bands give jagged spectra.) Under
this decomposition stage, also the absorption bands
responsible for the stretching vibrations of C–O
(1060–1168 cm-1), for the stretching vibrations of C=O
(ca. 1172 cm-1) and the stretching vibrations of C–H in
aldehyde group (ca. 2720 cm-1) are clearly appeared
]. It indicates on the emission of some organics
having oxygen in their structure. Taking into account the
structure of obtained copolymers and the FTIR results it
can be concluded that at temperatures above 300 C, the
decomposition of copolymers assumes a number of
simultaneous reactions. Among them, main and side chains
scissors, decarboxylation, dehydration and oxidation
processes were the most expected. It led to the formation of
alkanes, alkenes and aldehydes as main volatile products,
as it was shown in Scheme 1.
Finally, third main decomposition stage between the
temperatures from ca. 440 C to ca. 680 C with Tmax3 at
502–520 C and similar mass loss (26–28%) was appeared.
The DTG peak with intermediate decomposition rate
(3.0–3.2% min-1) was detected. At this temperature range,
one, broad exothermic signal was visible from DSC curves
which suggested the oxidation processes of formed
residues. It was confirmed based on the FTIR results where
only the emission of inorganic species such as CO2, CO
and H2O was detected (Table 2).
Scheme 1 The main
decomposition products of
acrylate) copolymers at Tmax2a
On the thermogravimetric curves, four main temperature
ranges where the mass loss of the copolymers under the
heating in oxidative atmosphere were observed, as shown
in Fig. 3. The first stage was connected with the physical
transformation: evaporation of moisture from the
copolymers, as it was confirmed based on the gaseous FTIR
spectra recorded at Tmax1, as shown in Fig. 4.
Between the temperatures of 150–210 C and Tmax2
from 172 to 186 C, the copolymers start to decompose,
Table 3. The mass loss is small, and it does not exceed
10%. In addition, on DTG curves low-intensity peaks with
the rate of the transformation ca. 1.2–2.0% min-1 were
appeared. The presence of this signal was unexpected since
the decomposition of starch and poly(citronellyl
methacrylate) polymer happened at relatively higher
6, 27, 31
]. However, analyzing the gaseous FTIR
spectra gathered at Tmax2, the appearance of the bands
characteristic for the stretching vibrations of C–H at
2827–2977 cm-1, the stretching vibrations of =C–H at
3068 cm-1, the stretching vibrations of C=O at
1700–1790 cm-1, the deformation vibrations of C–H at
1344–1470 cm-1, the stretching vibrations of C–O at
1066–1203 cm-1 and the deformation vibrations of =C–H
below 900 cm-1 [
] may indicate on the emission of
some fragments from poly(citronellyl methacrylate) as a
result of the depolymerization process initiated at the end
polymer chains and further oxidation of the emitted
species. What is interesting, on the DSC curves, the
exothermic signals with Tmax ca. 166–172 C are observed which
is the confirmation that under this decomposition step some
of the oxidation processes happen.
The further heating of the copolymers caused their
subsequent decomposition and the creation of some other
volatiles. The next decomposition stage was visible from
the temperature of ca. 210 C up to the temperature of ca.
430–450 C, and it was composed of at least three stages
named as Tmax3, Tmax3a Tmax3b. The mass loss was similar
to all the studied methacrylate copolymers and amounted
ca. 60–68%. Dividing this decomposition stage onto two
stages: first from the temperature of ca. 210 C to 315 C
(Tmax3) and the second from 315 C to 430–450 C (Tmax3a
and Tmax3b) allowed us more accurately evaluating the
decomposition course of the copolymers. The mass loss
between the temperatures of 210–315 C was from 32 to
38%. On the gaseous FTIR spectra collected at this
temperature range, the presence of the following absorption
bands: for water vapor above 3500 cm-1, for CO2 at
670 cm-1 and at 2300–2352 cm-1, for CO at
2000–2200 cm-1, for aldehydes (the stretching vibrations
of C–H at 2720 and at 2800 cm-1, the stretching vibrations
for C=O at 1735–1743 cm-1), for acids (the stretching
vibrations of C–O at 1250–1300 cm-1 and the stretching
vibrations of C=O at 1770–1795 cm-1), for furanes (the
stretching vibrations of C=C at 1510–1540 cm-1 and the
out-of-plane deformation vibrations of C=C at
820–970 cm-1) and for aliphatic species (the stretching
Fig. 4 Gaseous FTIR spectra for the studied starch-g-poly(citronellyl
methacrylate) copolymers in air
vibrations of C–H at 2875–2974 cm-1 and the deformation
vibrations of C–H below 900 cm-1) was indicated [
The type of the emitted gaseous species proved the
decomposition of starch from the copolymers at this stage
and the partial oxidation processes of some gaseous
decomposition products created under the heating of starch
1–6, 30, 31
]. The oxidation processes were confirmed
based on DSC analysis where only the exothermic signals
between the temperatures of 210–315 C were observed.
Scheme 2 The main
decomposition products of
methacrylate) copolymers at
Tmax3a and Tmax3b
In turn, the mass loss in the next temperature range
where double, non-well-separated DTG peaks were
observed was from 22.5 to 33%. Moreover, at this
temperature range, the presence of exothermic signals on the
DSC curves was confirmed, as shown in Fig. 3. What is
interesting, the FTIR spectra extracted at Tmax3a and Tmax3b
have a similar course. It may testify to the creation of
similar volatile products and thus on the decomposition of
the same part of copolymer. Looking at the FTIR spectra
collected at Tmax3a and Tmax3b, one can see the appearance
of the signals responsible for the stretching vibrations of
=C–H (3083 cm-1), the stretching vibrations of C–H
(2873–2990 cm-1), the stretching vibrations of C=O
(1735–1790 cm-1), the deformation vibrations of C–H
(1360–1450 cm-1), the stretching vibrations of C–O
(1039–1290 cm-1), the out-of-plane deformation
vibrations of =C–H (800–908 cm-1) and the bands
characteristic for CO2, CO and H2O. Additionally, at Tmax3b, the
emission of some CH4 was confirmed based on the
occurrence of the absorption bands at 3014 cm-1.
Although absorption signals at similar wavelengths on the
FTIR spectra for starch-g-poly(citronellyl methacrylate) at
Tmax3a and Tmax3b, as shown in Fig. 4, and for
starch-gpoly(citronellyl acrylate) at Tmax2a, as shown in Fig. 3,
were visible, the appearance of the FTIR spectra for
starchg-poly(citronellyl methacrylate) copolymers was quite
different. It could indicate on the creation of completely
different structure decomposition species under the heating
of copolymers 4–6. Also, the exothermic effect at this
temperature range is characterized by lower intensity for
starch-g-poly(citronellyl methacrylate) copolymers as
compared with starch-g-poly(citronellyl acrylate)
copolymers, which could be due to the creation of volatiles
which are resistant to oxidation, as shown in Figs. 1, 3.
Analyzing the obtained results, we could suspect the
depolymerization process of poly(citronellyl methacrylate)
chains and the creation of citronellyl methacrylate
monomer as a main decomposition product at temperatures
above 315–320 C, besides the formation of inorganic
species which are due to the decarboxylation and oxidation
processes, Scheme 2.
The last decomposition stage above the temperature of
430–450 C with Tmax4 533–555 C and the mass loss
ranges from 29.5 to 21.2% was observed. DTG curves
indicated on low-intensity peaks with the rate of the
transformation ca. 3.1–3.3% min-1. Moreover, huge,
exothermic signals on DSC curves at Tmax ca. 524–556 C
were visible. Under this decomposition stage, the emission
of CO2, CO and H2O as main volatile products was the
confirmation of the oxidation processes of the formed
Generally, based on the presented results, the course of
thermogravimetric and calorimetrical curves and the
position and type of absorption bands responsible for the
emission of gaseous decomposition products had a similar
appearance for copolymers independently on their %G.
However, the thermal resistance, decomposition course and
the type of volatiles were directly dependent on the type of
the studied copolymers.
Comparing the TG/DTG/DSC/FTIR results, it was
found completely different decomposition course of
starchg-poly(citronellyl acrylate) copolymers than
starch-gpoly(citronellyl methacrylate) copolymers under air
Starch-g-poly(citronellyl acrylate) copolymers started to
decompose at ca. 200–210 C which was evaluated based
on the 5% of mass loss. Its decomposition runs in two main
stages visible at the temperature ranges from ca.
170–440 C to ca. 440–680 C. According to the gaseous
FTIR results, main decomposition volatiles, besides the
gaseous products characteristic for the oxidative
decomposition of starch, were CO2, CO, H2O, alkane, alkene and
aldehyde fragments. It indicated on the random main and
side chains scissors, decarboxylation and oxidation of
poly(citronellyl acrylate) chains and oxidation of formed
The beginning of the decomposition of
starch-gpoly(citronellyl methacrylate) copolymers at ca.
160–170 C was observed. Its decomposition by three
main stages at the temperature ranges of ca. 150–210 C,
ca. 210–430–450 C and ca. 440–680 C was
characterized. As it was confirmed based on spectroscopic data,
main decomposition products, besides the volatiles created
from starch, were citronellyl methacrylate, CO2, CO and
H2O which were due to the depolymerization of
poly(citronellyl methacrylate), decarboxylation and
oxidation of formed species and residues.
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