Structure analysis and thermal degradation characteristics of bio-based poly(propylene succinate)s obtained by using different catalyst amounts
Structure analysis and thermal degradation characteristics of bio-based poly(propylene succinate)s obtained by using different catalyst amounts
Linear bio-based polyester polyols were prepared with the use of succinic acid and 1.3-propanediol (both with natural origin). As a catalyst was used tetraisopropyl orthotitanate (TPT). In order to determine the effect of various catalyst content on the thermal degradation characteristics, three different TPT amounts, as a 1.3-propanediol equivalent, were used, namely 0.1 mass% (PPS-0.1), 0.2 mass% (PPS-0.2) and 0.25 mass% (PPS-0.25). The reference polyol was prepared without catalyst employment (PPS-0.0). Fourier transform infrared spectroscopy was used to confirm molecular structure of the resulted polyols. The structure was also corroborated by 1H NMR measurements, what confirmed nonsignificant catalyst amount impact on the structure of the prepared polyester polyols. Differential scanning calorimetry was carried out for glass transition temperature and melting point determination. The thermogravimetric analysis allowed to observe high thermal stability both under inert and oxidative atmosphere. This analysis affirmed also that the catalyst content did not influence significantly on the thermal degradation characteristics of the prepared polyols.
Poly(propylene succinate); Bio-based polyester polyols; Thermogravimetric analysis; DSC; Molecular structure analysis
Polyols constitute one of the major components for the
polyurethanes synthesis . They are usually liquid,
reactive substances mostly terminated by the hydroxyl or
partially amine groups , which are responsible for the
reaction with isocyanates [3, 4]. Polyester polyols represent
the second most important group (beside polyether polyols)
with around 18% of the polyols global usage . The
polyurethane materials obtained with the use of polyester
polyols are less resistant to hydrolysis compared to the
polyether polyols. However, it makes them more
favourable due to the biodegradability [6?8]. Polyurethanes based
on the polyester polyols have better thermal and fire
resistance than the polyether-based PUR and superior
solvent resistance. The greatest value of polyester polyols
application is the polyurethane elastomers (ca. 43%),
flexible foams (ca. 15?18%), adhesives, coating, etc.
Furthermore, polyesters give major possibilities to the
biorenewable PUR material obtaining .
Currently, readily accessible are the bio-components
which allow producing polyester polyols in 100%
consisting from bio-resources . One of the most important
bio-based monomers for the polyester polyols synthesis is
the succinic acid (SA). Commercially available are also
glycols, which constitute second monomer taking part in
the polycondensation reaction during polyester polyols
preparation. Bio-based glycol with the highest global
usage, the first widely available in the industry, represents
such bio-based glycol as 1.3-propanediol (PDO) (Susterra,
DuPont). Currently, these compounds are obtaining by the
biotechnological processes through the corn fermentation
by such microorganisms as a fungi, yeasts or bacteria
[10?13] or fermentation process based on the glucose,
sucrose, dextrose and biomass sugars [14, 15], respectively.
Other bio-based glycols commonly available constitute
also bio-based 1.4-butanediol (bio-BDO) and ethylene
glycol (bio-EG). It is well known that the
petrochemicalbased components have more impurities, which consisted
of the different chemical compounds. The different
contaminants can lead to the other reaction mechanisms during
the polymer synthesis; therefore, it is necessary to find the
contaminants impact on the polyester polyols synthesis
The most important information about polyesters is their
thermal stability and melting behaviour. These
characteristics give information about the promising behaviour of
the polyester during processing. Papageorgiou and Bikiaris
 carried out the comparative study about crystallisation
and melting behaviour of three polyesters based on the
succinic acid, namely poly(ethylene succinate), PES,
poly(propylene succinate), PPS and poly(butylene
succinate), PBS. All measured polymers revealed the same
molecular weight. They indicated that the slowest
crystallisation rate and the lowest crystallinity degree, among
these polyesters, exhibited poly(propylene succinate). The
results of the differential scanning calorimetry allowed to
find the equilibrium melting points of PES, PPS and PBS at
114, 58 and 133.5 C. A lower melting point confirmed
superior promising behaviour during industrial processes
for poly(propylene succinate), where the higher melting
points for PES and PBS limited their practical application.
Another thermal property with high impact on the
polyesters practical employment is glass transition temperature.
Bikiaris et al.  indicated that the highest glass transition
temperature exhibited PES at ca. -11 C, where the PPS
and PBS exhibited this temperature at ca. -35 and -44 C,
respectively. Similar results have been obtained also by the
Qiu et al.  and Liu et al. . Chrissafis et al. 
measured thermal decomposition temperature of the
poly(propylene succinate) by thermogravimetric
measurements. In the results, PPS revealed a very high thermal
stability, which confirmed the thermal decomposition
temperature at 404 C. The same researchers indicated that
PES and PBS decomposed at 413 and 399 C, respectively
. This temperature can be compared to the degradation
temperatures of the aromatic polyesters, and they are
higher than thermal decomposition temperatures of other
In the present work, the synthesis of series of the linear
bio-based aliphatic polyester polyols is described. The
syntheses were designed to obtain the polyesters with
proposed number average molecular weight, which means
ca. 2000 g mol-1, and functionality, f = 2. The
polycondensation catalyst?tetraisopropyl orthotitanate (TPT), was
used in three different amounts. The obtained
poly(propylene succinate)s were characterised by structure
analysis with the use of Fourier transform infrared
spectroscopy and proton nuclear magnetic resonance.
Thermal degradation characteristic was determined with
the use of thermogravimetric analysis and differential
Bio-based succinic acid (SA) (solid, molecular weight
118.09 g mol-1, purity 98?100%, relative density at 20 C
0.900 g cm-3) used in this study was obtained from
BioAmber Sarnia Inc. (Ontario, Canada). Susterra
Propanediol (1.3-propanediol) (liquid, molecular weight
76.09 g mol-1, purity 99.98%, water content by Karl
Fischer 12.1 ppm, relative density at 20 C 1.053 g cm-3,
dynamic viscosity at 20 C 52 mPas) was obtained from
DuPont Tate and Lyle Corporation Bio Products (Loudon,
Tennessee, USA). Tetraisopropyl orthotitanate, Ti(O-i-Pr)4
(TPT) (liquid, molecular weight 284.22 g mol-1, purity
97%) was used as a catalyst with four different amounts.
The catalyst was purchased from TCI Chemicals (India).
All other materials and solvents used for the analytical
measurement methods for prepared bio-based polyester
polyols characterisation were of analytical grade.
Bio-based polyesters synthesis
Aliphatic bio-based polyester polyols were prepared with
the use of dicarboxylic acid, which was succinic acid, and
glycol, which was 1.3-propanediol. Both components used
were of a natural origin. Tetraisopropyl orthotitanate (TPT)
was used as a catalyst and added in three different amounts,
namely 0.1 mass% (PPS-0.1), 0.2 mass% (PPS-0.2) and
0.25 mass% (PPS-0.25). Poly(propylene succinate) was
also prepared without catalyst employment (PPS-0.0). All
linear bio-based polyester polyols were synthesised in the
bulk by two-step polycondensation method (esterification
and polycondensation). Figure 1 shows the reaction
scheme. The first step was represented by the esterification
reaction between a succinic acid (SA) and 1.3-propanediol
(PDO). Glycol was always used with an excess. The molar
ratio SA/PDO amounted to 1:1.2 which was determined
considering the final molar mass expected after full
polycondensation (approximately number average molecular
weight Mn = 2000 g mol-1 and functionality f = 2). Both
of the steps were carried out in the glass reactor, which
consisted of a three-neck flask equipped with
nitrogen/vacuum inlet, mechanical stirrer, thermometer and
condenser. The glass reactor was placed into a heating
mantle. The first step of the reaction was carried out under
a nitrogen atmosphere. Succinic acid (169 g, 1.43 mol) and
TPT cat., 160 ?C
OH reduce pressure
Fig. 1 Two-step polycondensation method for poly(propylene succinate) obtainment
1.3-propanediol (131 g, 1.72 mol) mixture was stirring at
140 C and kept for 10 h at this temperature (application
for patent in the Polish Patent Office, No. P.418808). After
the water distillation, the second step, which was the main
polycondensation reaction, was carried out. The nitrogen
was stopped, the appropriate amount of catalyst was added
to reaction mixture, and the temperature was increased up
to 160 C under reduced pressure. During
polycondensation, the acidic number was measured. After achieving the
value of the acidic number ca. or preferably below
1 mg KOH g-1, the polycondensation was finished.
Acid and hydroxyl number
Carboxyl end group value measurements were taken in
accordance with the Polish Standard PN-86/C45051.
Samples about 1 g of the prepared polyesters were
dissolved in ca. 30 cm3 of acetone at room temperature.
Thereafter, the solutions were titrated with the use of a
standard solution of potassium hydroxide KOH in distilled
water (0.1 mol dm-3) and phenolphthalein as indicator.
Hydroxyl group determination was performed with the
use of sample about ca. 0.5 g of polyester. The sample was
dissolved in 5 cm3 of the acetic anhydride solution
prepared in accordance with the Polish Standard
PN-88/C89082. The solution was refluxed for 30 min. After that,
1 cm3 of pyridine was added and heating for 10 min.
Thereafter, 50 cm3 of distilled water was added, and the
mixture was cooled to room temperature and titrated with
the use of a standard solution of potassium hydroxide KOH
in distilled water (0.5 mol dm-3) and phenolphthalein as
Fourier transform infrared spectroscopy
Fourier transform infrared spectroscopy was used to obtain
the spectra of the samples of pure components used in this
study (1.3-propanediol and succinic acid) and prepared,
without catalyst employment, bio-based polyester polyol
(PPS-0). The measurements were taken using a Nicolet
8700 FTIR spectrometer (Thermo Electron Corporation,
USA) with the use of ATR technique. The resolution was
4 cm-1. Sixty-four scans in the wavenumber range from
4500 to 500 cm-1 were taken.
Nuclear magnetic resonance (1H NMR)
Proton nuclear magnetic resonance (1H NMR) spectra of
the prepared bio-based polyester polyols were obtained
with the use of Bruker spectrometer. Operating frequency
was 400 MHz for protons. The ca. 10% w v-1 solutions of
the poly(propylene succinate) polyesters were prepared in a
CDCl3 solvent at ambient temperature. The simulation and
iteration of spectra were carried out using Bruker software.
Differential scanning calorimetry
The glass transition temperature and melting point
determination of the prepared bio-based polyester polyols were
characterised by using a DSC 204 F1 Phoenix Analyzer,
equipped with a cooling system. In the first cycle, the
samples were heated at a rate of 20 C min-1 where the
temperature ranged from -80 to 100 C for erasing of the
polyols thermal history. Then, they were cooled down to
-80 C at a cooling rate of 10 C min-1. In the third
cycle, the samples were heated up to 100 C at a rate of
5 C min-1. All the measurements were taken under
nitrogen atmosphere. DSC curves for linear bio-based
polyester polyols prepared without catalyst employment
and with the highest amount of the catalyst were plotted as
the results of the DSC analysis.
The thermogravimetric analysis allowed to characterise the
thermal stability of the prepared bio-based polyester
polyols. The measurements were taken with the use of an
NETZSCH TG 209F3 analyser (Netzsch, Germany). The
specimens, weighing in the range from ca. 6?8 mg each,
were analysed under an inert (nitrogen) and oxidative (air)
atmosphere. The used temperature ranged from 35 to
600 C at a heating rate of 20 C min-1.
Thermogravimetric (TG) and differential thermogravimetric (DTG)
curves for prepared linear bio-based polyester polyols were
plotted as the results of the TG analysis.
Results and discussion
Synthesis and characterisation of the obtained
All prepared polyester polyols were synthesised with the
use of well-known two-step polycondensation method. The
first step was the esterification reaction, which was
conducted for 10 h for all of the prepared polyester polyols
without using catalyst. After minimum 60% of water
removal, the catalyst was added. The second step, which
was the main polycondensation reaction, was carried out by
individual time for all synthesised polyesters until
achievement of the acid number ca. or preferably lower
than 1 mg KOH g-1. Justification of the end point of the
polycondensation reaction choice was the carboxyl end
group value due to the acid number occurring in some
synthetic polyester polyols commonly used in the
Table 1 Properties of the prepared polyester polyols
Bio-based polyester polyol
polyurethane industry. Table 1 shows properties of the
The addition of the catalyst influenced the time of the
second step of the reaction. Bio-based polyester polyol
prepared without tetraisopropyl orthotitanate employment
exhibited the longest reaction time. For the acid number
obtainment at the lowest level, the reaction time totalled
180 h. Increasing catalyst employment resulted in decrease
in the polycondensation time. Poly(propylene succinate)
prepared with the use of the highest catalyst amount
(0.25 mass% as the glycol equivalent) revealed the lowest
time for achieving the acid number value at
0.83 mg KOH g-1. The lowest hydroxyl number disclosed
the PPS-0.0 sample in 44.60 mg KOH g-1 where the
highest value exhibited PPS-0.2 (72.40 mg KOH g-1). All
measured hydroxyl number values are similar with the
commercially available synthetic polyols used in the
Chemical structure analysis
The structure analysis was performed using the FTIR and
1H NMR measurements. The Fourier transform infrared
spectra of the poly(propylene succinate), prepared without
catalyst used, and pure component used for polyesters
synthesis (1.3-propanediol and succinic acid) are shown in
Fig. 2. The wide peak characteristic for the 1.3-propanediol
spectrum in the wavelength range between 3570 and
3170 cm-1 was attributable to the stretching vibrations of
hydrogen-bonded hydroxyl groups. For the succinic acid
spectrum, the peak assigned to the hydrogen-bonded
carboxyl groups stretching vibration appeared as the broad
peak centred at 3300?2500 cm-1. The peaks at
3000?2850 cm-1 were assigned to the methylene groups
which are visible for glycol and polyester spectra. The
methylene group stretching vibration from succinic acid is
shifted to wavenumber at 2500?2250 cm-1. Two intensive
peaks visible for poly(propylene succinate) spectrum at
1725 and 1150 cm-1 indicated ester groups . The peak
at 1725 cm-1 is assigned to carbonyl group stretching
vibration related to the ester groups from poly(propylene
succinate) . The absorption at 1690 cm-1 visible on the
End group value/mg KOH/g
1725 1690 1150 1030
3000 2500 2000
Fig. 2 FTIR spectra of the used 1.3-propanediol, succinic acid and
synthesised, without catalyst employment, poly(propylene succinate)
succinic acid spectrum also indicated the ?C=O stretching
vibration but assigned to the carboxyl group. The bond in
the wavenumber at 1150 cm-1 is assigned to C?O?C
stretching vibration and at the 1030 cm-1 indicated C?C?
O stretching vibration from homopolyester . The peaks
at 1170 cm-1 on the glycol spectrum and at 1200 cm-1 on
the succinic acid spectrum were also attributed to the
stretching C?C?O group vibration but originating from
hydroxyl and carboxyl groups, respectively .
1H NMR spectra were used to study the structure of the
synthesised polyesters (Fig. 3). Based on the spectrum of
the poly(propylene succinate) prepared with the 0.2 mass%
catalyst employment, the chemical shift of the protons was
investigated. The characteristic intensive single peak at
Fig. 3 1H NMR spectrum of
the PPS-0.2 sample
2.63 ppm attributed to methylene protons ?a? from succinic
acid (?CH2?C(O)?) [17, 25]. Peaks named ?b? (?CH2?O?)
and ?c? (?CH2?) at 4.20 and 2.00 ppm, respectively,
attributed to triple and multiple peaks corresponding to
methylene protons from propylene glycol
(1.3-propanediol) [20, 21]. The intensive single peak at 7.26 ppm
attributed to used solvent, which was CDCl3 and the little
single peak at 1.56 ppm attributed to the water content in
the sample [22, 26, 27]. At the PPS-0.2 sample spectrum,
other peaks are also visible in lower intensity which can
indicate the end groups of oligomers. The little triple peak
at 3.65 ppm named ?x? attributed to methylene protons
from hydroxyl terminated ends (?CH2?OH) of polyester
macromolecules . The peak at 4.35 ppm named ?z?
attributed to the triple peak corresponding to methylene
protons (?CH2?O?) from glycol terminated ends group.
Peak named ?y? at ca. 1.90 ppm attributed to methylene
protons also from glycol terminated end group (?CH2?).
Figure 4 illustrates differences between polyesters
prepared with the use of a catalyst and without catalyst
content. It can be seen that PPS-0.2 sample spectrum indicates
the little shift at the 1.90, 3.68 and 4.35 ppm when the
PPS0.0 indicates this peak with lower intensity. Chrissafis et al.
 explained that these peak intensities can correspond to
the lower molecular weight of the synthesised polyester
Differential scanning calorimetry
The polyols thermal properties are related to their structure
and molecular weight. The polyols, which the methyl
groups hanging from the main chain resulting in a greater
Fig. 4 1H NMR spectra of the
PPS-0.0 and PPS-0.2 samples
Tm = 51.5 ?C
Tm = 47.5 ?C
Tg = ?26.8 ?C
Tg = ?30.0 ?C
Tg = ?32.3 ?C
Tg = ?34.5 ?C
free volume reveal lower glass transition temperature.
Moreover, the increasing molecular weight of polyol
increases the degree of crystallisation. In consequence, the
mobility of the amorphous chains would be restricted by
neighbouring crystals. In the sequel, the glass transition
temperature will be lower and the melting point will be
higher [29, 30]. Figure 5 shows the results of the
differential scanning calorimetry measurements of two of the
prepared bio-based polyester polyols. The graph presented
the curve courses of the poly(propylene succinate)s
prepared without catalyst employment (PP-0.0) and with the
highest amount of the catalyst (PPS-0.25). The first run was
carried out with the heating rate of 20 C min-1, and the
second run was carried out with the heating rate of
5 C min-1 after quenching under a nitrogen atmosphere
with the rate of 10 C min-1. The addition of the catalyst
influenced the melting point and glass transition
temperatures of the synthesised bio-based polyester polyols. The
catalyst impact on these properties is also related to the
effect on the polyol molecular weight. PPS-0.0 sample
reached the melting point at 51.5 C, where the polyol
PPS-0.25 revealed this temperature at 47.5 C. The same
tendency is also visible in the case of the glass transition
temperature. With the catalyst employment, this
temperature decreases from -26.8 to -30.0 C for
PPS0.0 and PPS-0.25, respectively. The erasing of the polyols
thermal history allowed to observe significant changes in
the curve courses during the second run. The glass
transition temperatures were shifted from -26.8 to -32.3 C for
PPS-0.0 and from -30.0 to -34.5 C for PPS-0.25. It can
be described due to the chain flexibility enhancement after
bio-based polyester polyols thermal history erasing. The
results indicated also that this history erasing affected the
prepared polyols crystallisation. It can be corroborated by
the lack of the crystallisation peak at the cooling curves
and melting point at the second run curves. Tsai et al. 
concluded that the odd number of the carbon atoms in the
copolyesters backbone relevantly inhibits their
crystallisation rate. After thermal history erasing, the prepared
biobased poly(propylene succinate) polyols revealed the lack
of the melting point. The same results have been obtained
by Papageorgiou and Bikiaris . Moreover, the
researchers indicate that the melting temperatures of the
semi-crystalline PPS polyesters depended on the thermal
history of the samples and it is found to range from 42 to
50 C or even more. After melt quenching, the
poly(propylene succinate) can adopt totally amorphous
Thermogravimetric measurements allowed to obtain the
thermal decomposition characteristics all of the prepared
bio-based polyester polyols. The resulting curves were
plotted on the differential thermogravimetric (DTG;
Figs. 6, 7) and thermogravimetric (TG; Figs. 8, 9) graphs
at heating rate 20 C min-1 under nitrogen and air
atmosphere. The thermal decomposition characteristics are
summarised in Table 2. The obtained results allow
confirming that oxidative atmosphere influences the thermal
degradation temperatures on slightly lower values than
inert atmosphere. The similar results were obtained also by
Wang et al.  and Ciecierska et al. .
From the DTG curve courses, it can be seen that
increasing catalyst employment during PPS syntheses
caused an increase in the speed of the mass loss in the case
of inert atmosphere. The highest speed of mass loss
revealed polyol prepared with the use of 0.25 mass% of the
catalyst. Differences in the DTG curves intensity are more
visible in the case of measurements taken under oxidative
atmosphere. The catalyst usage caused the increase at the
speed of mass loss, but with increasing catalyst amount, the
speed falls (in air). The specimen PPS-0.25 revealed the
most similar value to reference sample?the lowest speed
of mass loss in comparison with other catalysed specimens.
Table 2 shows that the prepared bio-based polyester
polyols samples revealed the similarity at the thermal
decomposition temperatures in the case of oxidative and
inert atmosphere. In Figs. 6 and 7, the small peaks are
visible for all samples at the temperature ca. 360 C.
Chrissafis et al.  indicated that these little peaks are due
to oligomer degradation. For PPS-0.0 and PPS-0.1, peaks at
391 and 395 C (Fig. 6), respectively, are correlated with
the decomposition temperature of the polyesters that
exhibited some smaller molecular chain. These polyesters
revealed higher polydispersity, and molecules with lower
molecular weight decomposed at a lower temperature. The
highest temperature of the maximum speed of mass loss
Fig. 6 DTG measurement
results obtained under the
Fig. 7 DTG measurement
results obtained under the air
Fig. 8 TG measurement results
obtained under the nitrogen
275 300 325 350 375 400
under the nitrogen revealed PPS-0.2, where for PPS-0.25,
this temperature significantly decreased (407.7 and 405.1,
respectively). For measurements prepared in the air, the
highest temperature of the maximum speed of mass loss
revealed reference sample and then PPS-0.1 and PPS-0.25
(406.5, 400.1 and 399.3, respectively).
Figures 8 and 9 show the TG curve courses where the
slight differences are visible. Samples PPS-0.1 and
PPS0.25 revealed faster mass loss at the temperature range
from 100 to 360 C than sample PPS-0.2. In the term of
oxidative atmosphere, the PPS-0.1 decomposed first,
which is correlated with the data in Table 2. After
achieving the temperature 360 C, the curve courses in
Figs. 8 and 9 were intensively dropped which indicated
quick decomposition. As can be seen for all measured
bio-based polyester polyols after achieving ca. 450 C, no
char residues were found for all samples and both
atmospheres. Chrissafis et al.  also indicated that the
PPS decomposed at the almost whole mass (about
Fig. 9 TG measurement results
obtained under the air
300 325 350 375 400
T5% is a temperature of 5% mass loss, T50% is a temperature of 50% mass loss, T90% is a temperature of 90% mass loss, and Tmax is a temperature
of the maximum rate of mass loss
Bio-based polyester polyol
The series of the linear bio-based polyester polyols were
synthesised with planned molecular structure and
functionality. The chosen properties were created in accordance
with the requirements of the thermoplastic polyurethane
industries. The Fourier transform infrared spectra
confirmed the ester bonds formation, which are visible at the
most intensive peaks at 1725 and 1150 cm-1 on the
poly(propylene succinate)s spectra. Also, 1H NMR analysis
corroborated the linear polyester polyols obtaining.
Increasing catalyst amount influenced the reaction time
reduction. Catalyst employment resulted in decrease in the
total reaction time from ca. 180 h, for PPS-0.0, to ca. 19 h,
for PPS-0.25, respectively. Differential scanning
calorimetry results confirmed the impact of the catalyst
usage on the melting point and glass transition temperature
of the synthesised bio-based polyester polyols. Catalyst
employment decreased the melting point from 51.5 C for
PPS-0.0 to 47.5 C for PPS-0.25. Furthermore, the glass
transition temperatures also decreased with the catalyst
employment, exactly from -32.3 C, for PPS-0.0, to
-34.5 C, for PPS-0.25 (at the second heating rate, after
the thermal history erasing). The differential
thermogravimetric analysis showed that the speed of mass loss of the
prepared bio-based polyester polyols increases with the
growing catalyst amount. The results indicated that the
growing catalyst amount led to polyols, which exibited the
shift of the onset decomposition temperature (T5%) from
361.5 to 297.4 C, under the nitrogen and from 351.5 to
300.0 C, under oxidative atmosphere, for PPS-0.0 and
PPS-0.25, respectively. Nevertheless, the other
temperatures confirmed nonsignificant differences in the thermal
decomposition characteristics in the case of all samples
measured in the both gasses.
Acknowledgements The authors gratefully acknowledge BioAmber
Sarnia Inc. (Canadian corporation) for giving the samples of succinic
acid used in this study. The sincere acknowledgements are also
directed to the DuPont Tate&Lyle Corporation for supplying the
glycol (1.3-propanediol) samples used in this study. Thanks are also
due to Mr Dr Micha? Strankowski and Mr Dr ?ukasz Piszczyk from
the Department of Polymers Technology in the Gdansk University of
Technology for the differential scanning calorimetry and
thermogravimetric measurements preparation of the bio-based polyester
polyols described in this work.
Open Access This article is distributed under the terms of the
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tivecommons.org/licenses/by/4.0/), which permits unrestricted use,
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