REGULAR ARTICLE
Design, synthesis and characterization of new energetic phthalateplasticizers based on imidazolium ionic liquids
REZA FAREGHI-ALAMDARI*, SEYED AMANOLLAH MOUSAVI NODOUSHAN and
NEGAR ZEKRI
Faculty of Chemistry and Chemical Engineering, Malek-ashtar University of Technology, Tehran, Iran
E-mail: [email protected]; [email protected]
MS received 12 March 2021; revised 27 May 2021; accepted 28 May 2021
Abstract. In this study, a new class of energetic phthalate plasticizers based on imidazolium ionic liquids
(ILs) were synthesized. The structure of the synthesized compounds was confirmed by 1HNMR, 13CNMR,
and FT-IR. The thermal stability of nitrocellulose (NC)/plasticizer blends was also evaluated by thermo-
gravimetric analysis (TGA). The compatibility of the NC with three synthesized energetic plasticizers was
studied by differential scanning calorimetry (DSC), scanning electron microscope (SEM), and density
functional theory (DFT) methods. The glass transition temperatures of the NC/synthesized plasticizer blends
were determined by DSC and showed desirable lowering of glass transition temperature with a single peak
and low temperature, which indicates the compatibility of NC with the synthesized IL plasticizers. The SEM
images of plasticized films show smooth surfaces which are resulted from the good compatibility of the
plasticizers with NC. The predicted relative trend of interaction energies between NC and plasticizers is well
correlated with the corresponding trend of Tg of NC/plasticizer blend. In addition, molecular electrostatic
potential (MEP) calculations were performed for all plasticizers/NC. Total electron density for the DEP/NC,
3a/NC, 3b/NC and 3c/NC samples are respectively 0.0388, 0.0942, 0.0944 and 0.0823. There is a very good
regression (R2 = 0.9126) between these values and the calculated values of the interactions energy.
Keywords. Nitrocellulose; double-base propellant; phthalate plasticizer; energetic ionic liquids;
computational study.
1. Introduction
One of the disadvantages of double-base propellants is
that they have low specific impulse and low perfor-
mance. One way to increase the specific impulse of a
double base propellant is by using energetic plasti-
cizers in its composition. The common plasticizers are
inert, so they cause to reduce the energetic properties
of explosives and propellants. Energetic plasticizers
have been used in propellant formulations in order to
improve their mechanical properties and specific
impulse.1,2
ILs have been considered for many applications
such as replacement of organic solvents,3,4 electro-
chemistry,5 enzyme-catalyzed synthesis,6 separa-
tion,7 electrolyte,8 dye-sensitized solar cells
(DSSC),9 absorption of CO210 and stationary phases
in gas chromatography.11 Numerous ILs with dif-
ferent structures and energetic properties have been
introduced as environmentally friendly explosives or
propellant fuels for a variety of energetic applica-
tions.12 In a sense, the concept of designing ILs as
energetic materials have provided a unique archi-
tectural platform for developing a new generation
of liquid energetic materials. Among various ener-
getic ILs in use, imidazolium-based ILs are known
for their chemical stability and fluid properties.13–15
The most popular plasticizers of double-based
propellants are those that contain ester groups such
as diethyl, dibutyl and dioctyl phthalates which are
a class of neutral plasticizers.16–18 Recently, ILs are
used as energetic plasticizers as their easy
synthesis, low volatility and high-temperature
stability.12
*For correspondence
Supplementary Information: The online version contains supplementary material available at https://doi.org/10.1007/s12039-021-01946-x.
J. Chem. Sci. (2021) 133:86 � Indian Academy of Sciences
https://doi.org/10.1007/s12039-021-01946-xSadhana(0123456789().,-volV)FT3](0123456789().,-volV)
The composition of energetic ionic liquids (EILs)
reported in the literature follows a distinct trend in
which ions are employed. The cations are generally
substituted N-heteroaromatic rings or ammonium
derivatives. A majority of cations includes imida-
zolium.12 The imidazolium-based ionic liquid was
shown to be an effective plasticizer, Creates good
mechanical properties and, as well as in the decrease
of the Tg.2,19 Application of imidazolium ionic liquids
as plasticizers for poly(methyl methacrylate) (PMMA)
and poly(vinyl chloride) (PVC) has been observed.20
Many energetic ionic liquids have been reported with
several advantages over traditional energetic com-
pounds such as 2,4,6-trinitrotoluene (TNT), 1,3,5,7-
tetranitro-1,3,5,7-tetraazocane (HMX), and 1,3,5-(tris-
nitro)perhydro-1,3,5-triazine (RDX). These advan-
tages include enhanced thermal stability, higher den-
sity, negligible vapor pressure, and little or no vapor
toxicity.12
According to our studies in the field of energetic
ionic liquids, and in continuation of our efforts
towards the synthesis and application of energetic ILs
for propellants compositions,21–26 in this investigation,
design and synthesis of new energetic phthalate plas-
ticizers based on imidazolium ionic liquids have been
discussed. Herein, we have introduced a new class of
energetic phthalate plasticizers based on imidazolium
ionic liquids and their synthesis pathway. Also, the
thermal properties, SEM, and computational tech-
niques of the blends of these plasticizers and NC has
been investigated.
Calorimetric analysis was performed to prove the
energy of the synthesized compounds and DSC anal-
ysis was performed to prove the plasticizer effect on
the nitrocellulose. Finally, DFT calculations were
performed to confirm the experimental results (DSC
analysis) and the amount of interaction between the
plasticizers and NC were calculated.
2. Experimental
2.1 Material and methods
All solvents and chemicals purchased from Merck and
Fluka chemical companies. NMR spectra were recor-
ded on a Bruker DRX-500 spectrometer in DMSO and
calibrated with tetramethylsilane (TMS) as the internal
reference. Infrared (IR) spectra were recorded on a
Nicolet 800 instrument using KBr or liquid film.
Thermogravimetric analysis (TGA), isothermal TG
was performed using a PerkinElmer STA 6000
instrument and alumina pans under an argon
atmosphere with temperature-programmed rates of
10 �C�min-1, from room temperature to 400 �C. Thesample masses were 1.68-1.9 mg. The glass transition
temperature (Tg) measurements were performed using
a DSC 200F3 instrument under an N2 flow of 20.0 mL/
min and a heating rate of 10 �C�min-1, from -100 �Cto 140 �C. The Tg was computed as the midpoint of
the heat capacity increase. The viscosity of ILs was
detected using a Brookfield Viscometer at 25 �C. Thecommercial-grade with about 12.0% nitrogen content
of nitrocellulose was used in the present investigation
and diethyl phthalate (DEP) was selected for com-
parison with synthesized IL plasticizers. For all plas-
ticizers and DEP prepared a mixture of 80 mass% NC
and 20 mass% plasticizers. Quantum chemical calcu-
lations using the DFT method were therefore per-
formed by the method B3LYP with 6-31G (d,p) basis
set.
2.2 Synthesis procedures
Three types of new energetic plasticizers were syn-
thesized in three steps as seen in Scheme 1.
2.2a Synthesis of 3-(2-hydroxyethyl)-1-R-1H-imidazol-3-ium chloride (1a-1c): A 100 mL round-
bottom flask equipped with a magnetic stirrer was
charged with 2-chloroethanol (10 mmol, 0.805 g) and
N-alkyl imidazole (10 mmol) which were dissolved in
acetonitrile (10 mL). The mixture was refluxed for 3
days. The solvent was removed by a rotary evaporator.
The obtained viscous liquid was washed with ethyl
acetate (2*5 mL) to obtain a pure viscous liquid
product.
3-(2-hydroxyethyl)-1-methyl-1H-imidazol-3-iumchloride (1a): Yellow oil; yield 62%; FT-IR (t cm-1):
3333, 3150, 3095, 2957, 1575, 1166, 1072; 1H NMR
(500 MHz, DMSO-d6): d H 9.29 (s, 1H), 7.79 (t, J =
1.8 Hz, 1H), 7.75 (t, J = 1.8 Hz, 1H), 5.49 (br s, 1H),
4.24 (t, J= 5 Hz, 2H), 3.87 (s, 3H), 3.69 (m, 2H). 13C
NMR (125 MHz, DMSO-d6): dC 137.1, 130.3, 124.0,
61.7, 52.4, 46.8.
1-ethyl-3-(2-hydroxyethyl)-1H-imidazol-3-iumchloride (1b): Yellow oil; yield 71%; FT-IR (t cm-1):
3280, 1652, 1555, 1379, 1071; 1H NMR (500 MHz,
DMSO-d6): d H 9.11 (s, 1H), 7.81 (d, J = 4.5 Hz, 1H),
7.76 (d, J = 4.5 Hz, 1H), 5.71 (br s, 1H), 4.30 (t, J= 5
Hz, 2H), 3.78 (q, J= 5 Hz, 2H), 3.65 (t, J= 5 Hz, 2H). ).13C NMR (125 MHz, DMSO-d6): dC 137.4, 130.3,
124.0, 61.9, 59.3, 52.4, 46.8.
3-(2-hydroxyethyl)-1-vinyl-1H-imidazol-3-iumchloride (1c): Yellow oil; yield 63%; FT-IR (t cm-1):
86 Page 2 of 12 J. Chem. Sci. (2021) 133:86
3246, 3080, 1651, 1551, 1170, 1072; 1H NMR (500
MHz, DMSO-d6): d H 9.85 (s, 1H), 8.37 (s, 1H), 7.99
(s, 1H), 7.39-7.44 (m, 1H), 6.06 (d, J= 1.5 Hz, 1H),
5.57 (br s, 1H), 5.37 (d, J= 1 Hz, 1H), 4.32 (d, J= 0.5
Hz, 2H), 3.76 (d, J= 0.5 Hz, 2H); 13C NMR (125 MHz,
DMSO-d6): d C 136.0, 129.2, 124.0, 119.4, 108.9,
59.5, 52.4.
2.2b Synthesis of 3,30-((phthaloyl bis(oxy)) bis(ethane-2,1-diyl)) bis (1-R-1H-imidazol-3-ium)dichloride (2a-2c): A 250 mL round-bottom flask
equipped with a magnetic stirrer and condenser was
charged with a mixture of phthalic anhydride (5 mmol)
and 1a-c (20 mmol) that dissolved in acetonitrile (25
mL). Then 10 mol% of p-toluene sulfonic acid was
added. The mixture was refluxed for 24 h. The solvent
was removed by rotary evaporator. The obtained
viscous liquid was extracted with ethyl acetate (2*10
mL) and deionized water (10 mL). The organic phase
dried on Na2SO4. The solvent was removed by rotary
evaporator. The viscous brown liquid was washed with
diethyl ether, to obtain the pure bis imidazolium
dichloride 2a-c.
3,30-((phthaloyl bis(oxy)) bis(ethane-2,1-diyl)) bis(1-metyl-1H-imidazol-3-ium) dichloride (2a): Yel-
low oil; yield 41%; 1H NMR (500 MHz, DMSO-d6): dH 9.26 (t, J= 2.5 Hz, 2H), 7.53-7.86 (m, 8H), 4.24 (s,
4H), 3.85 (s, 6H), 3.70 (s, 4H); 13C NMR (125 MHz,
DMSO-d6): dC 169.2, 137.2, 133.9, 131.2, 130.6,
123.7, 123.0, 59.8, 51.9, 36.1.
3,30-((phthaloyl bis(oxy))bis (ethane-2,1-diyl)) bis(1-etyl-1H-imidazol-3-ium) dichloride (2b): Yellowoil; yield 47%; FT-IR (t cm-1): 3080, 2962, 1654,
1552, 1174, 1070; 1H NMR (500 MHz, DMSO-d6): dH 9.10 (s, 2H), 7.80 (d, J= 6 Hz, 2H), 7.73 (t, J= 5 Hz,
4H), 7.55-7.57 (m, 2H), 4.23 (t, J= 5 Hz, 4H), 3.97 (q,
J= 5 Hz, 4H), 3.67 (t, J= 6 Hz, 4H), 1.51 (t, J= 5 Hz,
6H). 13C NMR (125 MHz, DMSO-d6): dC 171.9,
140.5, 136.9, 133.9, 132.2, 125.5, 124.8, 61.3, 51.6.
3,30-((phthaloyl bis(oxy)) bis (ethane-2,1-diyl)) bis(1-vinyl-1H-imidazol-3-ium) dichloride (2c): Yellowoil; yield 49%; FT-IR (t cm-1): 2900, 1715, 1647,
1573, 1415, 1211, 1035; 1H NMR (500 MHz, DMSO-
d6): d H 9.26 (s, 2H), 7.84-7.86 (m, 2H), 7.77 (d, J=1.5 Hz, 2H), 7.73 (s, 2H), 7.53 (q, J= 3 Hz, 2H), 7.41
(q, J= 3.5 Hz, 2H), 6.10 (d, J = 10 Hz, 2H), 5.41 (d, J=8 Hz, 2H), 4.24 (t, J= 5 Hz, 4H), 3.70 (t, J= 5 Hz, 4H),13C NMR (125 MHz, DMSO-d6): dC 170.9, 140.9,
137.3, 134.1, 131.7, 129.3, 126.3, 125.1, 104.2, 61.2,
53.2.
2.2c Synthesis of 3,30-((phthaloyl bis (oxy)) bis(ethane-2,1-diyl)) bis (1-R-1H-imidazol-3-ium)dicyanamide (3a-3c): Silver dicyanamide was
prepared by mixing equal molar amounts of silver
nitrate and sodium dicyanamide in aqueous solutions
followed by filtration.24
A mixture of bis imidazolium dichloride 2a-c (10
mmol) and silver dicyanamide (20 mmol) were dis-
solved in deionized water (20 mL). Then the mixture
was stirred at 40 �C for 24 h. The resulting precipitate
was separated by filtration and washed with diethyl
ether. The resultant organic phase was dried with
Na2SO4, and the solvent was removed by rotary
evaporator to afford bis imidazolium dicianamide 3a-3c as brown viscous oil as final. 1H NMR spectra of
energetic synthesized phthalate plasticizers based on
imidazolium ionic liquids (3a-3c) are shown in
Figure 1.
3,30-((phthaloyl bis(oxy)) bis (ethane-2,1-diyl)) bis(1-metyl-1H-imidazol-3-ium) dicyanamide (3a):Yellow oil; yield 4.42 g (92%); FT-IR (t cm-1): 3019,
2987, 1718, 1579, 1422, 1221, 1123; 1H NMR (500
MHz, DMSO-d6): d H 9.10 (s, 2H), 7.70 (d, J= 2.5 Hz,
Scheme 1. Multistep synthesis route of new energetic phthalate plasticizers based on imidazolium ionic liquids 3a)R=methyl, 3b) R=ethyl, 3c) R=vinyl.
J. Chem. Sci. (2021) 133:86 Page 3 of 12 86
Figure 1. 1H NMR spectra of synthesized energetic phthalate plasticizers based on imidazolium ionic liquids (3a-3c).
86 Page 4 of 12 J. Chem. Sci. (2021) 133:86
2H), 7.66 (d, J= 5 Hz, 2H), 7.42 (d, J= 5 Hz, 2H), 7.38
(d, J= 5 Hz, 2H), 4.21 (t, J= 5 Hz, 4H), 3.86 (s, 6H),
3.72 (t, J= 5 Hz, 4H); 13C NMR (125 MHz, DMSO-
d6): d C 170.2, 139.0, 136.2, 133.2, 131.0, 125.2,
125.0, 60.7, 52.9, 37.2; Viscosity (cP) 921.
3,30-((phthaloyl bis(oxy))bis(ethane-2,1-diyl))-bis(1-etyl-1H-imidazol-3-ium) dicyanamide (3b):Yellow oil; yield 5.09 g (95%); 1H NMR (500 MHz,
DMSO-d6): d H 9.17 (s, 2H), 7.92 (d, J= 1 Hz, 2H),
7.76 (t, J= 5 Hz, 4H), 7.61-7.65 (m, 2H), 4.31 (t, J= 5
Hz, 4H), 4.10 (t, J= 5 Hz, 4H), 3.76 (t, J= 5 Hz, 4H),
1.65 (t, J= 5 Hz, 6H); 13C NMR (125 MHz, DMSO-
d6): d C 171.3, 140.1, 136.8, 133.9, 132.0, 125.5,
124.9, 61.0, 51.7, 36.7, 20.7; Viscosity (cP) 905.
3,30-((phthaloyl bis(oxy))bis(ethane-2,1-diyl))-bis(1-vinyl-1H-imidazol-3-ium) dicyanamide (3c):Yellow oil; yield 4.95 g (93%); 1H NMR (500 MHz,
DMSO-d6): d H 9.24 (s, 2H), 7.88 (d, J= 1 Hz, 2H),
7.82 (t, J= 5 Hz, 2H), 7.77 (d, J= 5 Hz, 2H), 7.59 (t, J=5 Hz, 2H), 7.44-7.46 (m, 2H), 6.18 (d, J= 10 Hz, 2H),
5.50 (d, J= 12.5 Hz, 2H), 4.31 (t, J= 6 Hz, 4H), 3.77 (t,
J= 6 Hz, 4H); 13C NMR (125 MHz, DMSO-d6): dC170.7, 140.7, 137.1, 134.2, 131.9, 129.0, 126.1, 125.4,
104,2, 61.5, 53.7; Viscosity (cP) 932.
2.3 Samples preparation
To evaluate the effect of plasticizers on the thermal
properties of NC and their compatibility, NC/plasticizer
blends were prepared by the solvent method. For the
sample preparation, NC was mixed with 5 mL of ace-
tone at room temperature. Then, the plasticizer was
added to the mixture of NC and acetone solvent. The
prepared samples were placed in the vacuum oven at
50 �C for 5 h. Finally, acetone-free blendswere prepared.
DEP is a common plasticizer for use in double
based solid propellants. Therefore, DEP was selected
for comparison with synthesized IL plasticizers.
Eventually, blends of 80% NC and 20% plasticizer
were prepared.
3. Results and Discussion
3.1 Synthetic way of IL plasticizers
Scheme 1 shows the synthesis pathway of energetic
phthalate plasticizers based on imidazolium ionic liq-
uids. First the reaction of N-alkyl imidazole and
2-chloroethanol leading to imidazolium ionic liquid 1.
Then, phthalic anhydride reacted with 1 in the pres-
ence of PTSA as a catalyst to obtain IL phthalate
plasticizers 2. Finally, the anion exchange was done by
silver dicyanamide for increasing the energy of plas-
ticizers to gain energetic ILs 3.
3.2 Thermal properties of the synthesized ILplasticizers
The thermal stability of the ILs were measured by TG
analysis. TG and the differential thermogravimetric
(DTG) curves are shown in Figure 2. According to the
results of thermal analyses, all synthesized ILs have
high thermal stability. The results of thermal analyses
show that IL plasticizer 3b has the best thermal sta-
bility in the synthesized ILs. Derivative thermo-
gravimetry plots change in mass with temperature, dm/
dt, and resolves changes more clearly (Figure 2b).
3.3 Thermal studies of plasticizer/NC blends
To understand the plasticizing effect of IL plasticizers
on NC, thermal decomposition temperature and glass
transition temperatures of the plasticizer/NC blends
were determined. Values of the glass transition tem-
perature of Plasticizer/NC samples with heating rate
10 �C.min-1 under nitrogen atmosphere have been
shown in Figure 3a.
A glass transition temperature was observed for all
of the synthesized plasticizer/NC blend. The glass
transition temperatures of the DEP/NC, 3a/NC, 3b/NC
and 3c/NC blends are respectively 66.12, 55.22, 45.90,
and 64.76 �C.One of the most important properties of the poly-
mers is the glass transition temperature (Tg), which
measures chain mobility. This is a temperature at
which a polymer transforms from hard, glassy material
to a soft, rubbery material. Plasticizers lower the glass
transition temperature of the polymers.27,28
Plasticizers reduce the glass transition temperature
of polymers through a process of plasticization which
can be explained by the principle of free volume.2,16
Plasticizers increased the distance between NC chains
and reduces intermolecular force, and then generates
free volume between NC and plasticizer.
It should be noted that the thermal stability of the
synthesized IL plasticizers refers to the stability of
their molecular skeletons. In general, the bond energy
of C-N (in IL plasticizers) is lower than that of the C-C
(in DEP plasticizer). Therefore, the stability of IL/NC
blends is lower than that of DEP/NC. In the plasti-
cizer/NC blends, we observed the lowest glass transi-
tion temperature for the 3b plasticizer. This is because
these side chains (ethyl) increase the free volume.
J. Chem. Sci. (2021) 133:86 Page 5 of 12 86
The decomposition peak profiles of TGA analysis
have been shown in Figure 2b. The results showed that
all synthesized IL plasticizers/NC blends have lower
decomposition temperatures than DEP/NC blend.
3.4 Enthalpy of combustion of the synthesized ILplasticizers
Combustion enthalpy is an important parameter of a
drivetrain, which eventually links to the energy con-
tent.29,30 Heat of combustion analysis was performed
to evaluate the energetic synthesized IL plasticizers.
Enthalpy of combustion in the atmosphere of oxygen
at 30 bar pressure for synthesized IL plasticizers 3a,3b, 3c and DEP are respectively 6764, 7269, 7211 and
5468 cal.g-1±1%.
This analysis showed the enthalpy of combustion of
the synthesized IL plasticizers is higher than DEP
which is a common double-base propellant plasticizer.
The presence of nitrogen and oxygen in the structure
of synthesized ILs causes their enthalpy of combustion
to be higher than DEP and other neutral plasticizers.
The specific impulse of solid propellants depends on
the energy content of their components. The use of
these plasticizers with higher enthalpy of combustion
in the formulation of solid propellants, due to the
increase the energy content, ultimately increase the
specific impulse of propellants.
3.5 Density functional theory studies
The compatibility of NC with the synthesized IL
energetic plasticizers was studied using DSC and DFT
methods. It is crucial to understand intermolecular
interactions between NC and plasticizers to predict the
factors responsible for the physical compatibility of
NC/plasticizer blends. Plasticizer interacts with the
polymer chains (NC) on the molecular level to speed
up its viscoelastic response (or increase chain mobil-
ity) and ultimately increases the free volume and thus
decreases the Tg.2 Calculating the interaction between
the plasticizer and the polymer allows us to have
accurate information about the compatibility of the
plasticizer and polymer. Interaction of plasticizer and
0102030405060708090
100
50 100 150 200 250 300 350 400 450 500 550
Wei
ght (
%)
Temperature (°C)
3c plasticizer
3b plasticizer
3a plasticizer
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
50 150 250 350 450
Hea
t Flo
w (W
/g)
Temperature (°C)
3c plasticizer3b plasticizer3a plasticizer
endo
a
b
Figure 2. a: TG curves of the synthesized IL plasticizers obtained at 10 �C.min-1 b: DTG of the synthesized ILplasticizers.
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
-60 -10 40 90
Hea
t Flo
w (W
/g)
Temperature (°C)
DEP-NC3a-NC3b-NC3c-NC
Tg=45. Tg=64.76
Tg=66.12
Tg=55.22
endo
0102030405060708090
100
25 75 125 175 225 275 325 375
Wei
ght (
%)
Temperature (°C)
DEP-NC3a-NC3b-NC3c-NC
a b
Figure 3. a) DSC thermograms of the blends of NC (80 mass%) and plasticizer (20 mass%) with a heating rate at10 �C.min-1 b) The decomposition peak profiles of TGA analysis of the blends of DEP/NC, 3a/NC, 3b/NC and 3c/NCsamples with a heating rate at 10 �C.min-1.
86 Page 6 of 12 J. Chem. Sci. (2021) 133:86
matrix polymer as being changed by a polarization of
charges in molecular fragments, hydrogen bonding or
Lewis acid-base interactions.2
One of the methods commonly used to study the
interaction between two molecules is DFT calculation.
Thus DFT calculations were performed to examine
more precisely the interactions between the NC and
plasticizers.
To perform the calculations, first, the structure of all
plasticizer and NC (before interaction) were designed
using GaussView software and optimized by Gaussian.
Optimized structures of these molecules are shown in
Figure 4.
Molecular electrostatic potential (MEP), used for
identifying the electrostatic interaction between
molecules.31 MEP can successfully describe the
charge distribution around a molecule generated by its
nuclei and electrons.32,33
To understand the reaction sites, MEP calculations
were performed. The results of these calculations are
shown in Figure 5.
In Figure 5, the blue locals indicate electron defi-
ciency and the red locals indicate electron-rich areas.
Thus, the blue regions act as electrophiles and the red
regions act as nucleophiles.
For the DEP molecule, carbonyl groups are red and
indicate the accumulation of electron density around
the carbonyl groups. For the synthesized IL plasticiz-
ers, the anions have the highest charge accumulation
and the cations are generally green. Also in the case of
nitrocellulose, the oxygen atoms are yellow, indicating
a slight accumulation of charge. In the next step,
plasticizers and NC were placed next to each other and
structural optimization was performed on them to
obtain the most stable complex. Optimized structures
of these molecules are shown in Figure 6. In order to
compare the compatibility of the synthesized IL
plasticizers and NC, the interaction energy (I.E) of
these structures and DEP with NC has been calculated.
Table 1 presents the computed energies of the plasti-
cizers and NC. Also, interaction energies and energies
of the blends have been gathered. The energy inter-
action model is a suitable method for understanding
bimolecular reactivity. The interaction energy (EIn-
teraction) is equal to EBlend-(ENC?EPlasticizer).35,36
The plasticizer in between the polymeric layers of
NC causes weakening of the intermolecular interac-
tions of NC chains. The predicted relative trend of
interaction energies between plasticizer and NC is
well-correlated with the corresponding trend of com-
patibility of plasticizer/NC blends. The interaction
energy of the plasticizer/NC blends is in the increasing
order of 3b/NC[3a/NC[3c/NC[DEP/NC.
Figure 4. Optimized structures for a) DEP b) 3a c) 3b d) 3c e) NC.
J. Chem. Sci. (2021) 133:86 Page 7 of 12 86
Figure 5. MEP of a) DEP b) 3a c) 3b d) 3c e) NC molecules. The values of the MEPs varies between -6e-2 (red) and?6e-2 (blue) au.
Figure 6. Optimized structures obtained after DFT calculations for NC/plasticizer blends: a) NC/DEP b) NC/3b c) NC/3ad) NC/3c.
86 Page 8 of 12 J. Chem. Sci. (2021) 133:86
Table 1. Calculated energies of compounds and interaction energy.
Name Plasticizer
Energy (Hartree)
NCILs
plasticizer
NC-Plasticizer
blendsI.E
DEP –2449.4801 –766.6525 –3216.1440 –0.0114
3a –2449.4801 –1777.3666 –4226.8665 –0.0198
3b –2449.4801 –1856.0056 –4305.5108 –0.0251
3c –2449.4801 –1853.5230 –4303.0212 –0.0181
Figure 7. Molecular graphs generated by AIM calculations for NC/plasticizer blends: a) NC/DEP, b) NC/3a, c) NC/3band d) NC/3c.
J. Chem. Sci. (2021) 133:86 Page 9 of 12 86
More negative value of such binding energy indi-
cates the tightly holding of NC suggesting relatively
lower Tg of resultant NC/plasticizer blend and more
compatibility. Similarly, a lower negative binding
energy value suggested a relatively higher Tg of the
resultant NC/plasticizer blend and less compatibility.
To better understand the reaction sites, calculation
of atoms in molecules (AIM) were also performed.
The results of these calculations are shown in Figure 7.
One of the most commonly used methods for
studying the interaction between two molecules is the
AIM. In these calculations, there are critical locals
between the two interacting molecules, and according
to the amount of electron density (qb) in these critical
points, the amount of interaction energy can be cal-
culated.36,37 Using laplacian of electron density
(r2qb), kinetic energy density (Gb) and potential
energy (Vb), the type of interaction can be identified.
The dotted lines depict regions of local charge
concentration (local interaction between the NC
molecules). The total electron density for all critical
points of the system was also calculated. Total elec-
tron density for the DEP/NC, 3a/NC, 3b/NC and 3c/
NC samples are respectively 0.0388, 0.0942, 0.0944
and 0.0823. There is a very good regression (R2 =
0.9126) between these values and the calculated and
the interactions energy.
According to the results of the AIM calculations, the
plasticizer with the more critical points has a higher
electron density. The more electron density was
indicative of the greater the interaction between the
plasticizer and the nitrocellulose and as a result more
compatibility.
The result of DFT calculations completely confirms
the experimental results (DSC analysis) and the same
change in the amount of Tg is seen in the results of
DFT calculations.
3.6 SEM
Considering that synthesized IL plasticizer 3b was
more compatible with NC than other synthesized
plasticizers, SEM photomicrographs were prepared for
a more detailed examination of IL 3b. The surface andcross-section of 3b/NC blend were investigated with a
scanning electron microscope. The SEM photomicro-
graphs of NC and IL (3b) /NC blend are shown in
Figure 8.
The SEM photomicrographs for IL (3b)/NC blend
detected meaningful changes in the surface morphol-
ogy of NC. SEM photomicrographs of plasticized
films showed smooth surfaces that are resulted from
the good compatibility of synthesized IL plasticizer
with NC.
Figure 8. SEM photomicrographs of NC (a, b, c) and IL (3b)/NC blend (d, e, f) at different magnifications.
86 Page 10 of 12 J. Chem. Sci. (2021) 133:86
4. Conclusions
In this investigation, design and synthesis of three new
energetic phthalate plasticizers based on imidazolium
ionic liquids were performed. The heat of combustion
analysis showed, the enthalpy of combustion of the
synthesized IL plasticizers are higher than DEP, which
is a common double-base propellant plasticizer. To
understand the compatibility of NC and synthesized IL
plasticizers, NC/plasticizer blends were prepared and
their thermal analyses (DSC and TGA) and SEM were
studied. Also, some computational techniques were
used to study them more preciously.
The results of the thermal analysis showed that all
synthesized IL plasticizer-NC blends have lower
glass transition temperatures than DEP/NC blend.
Among the synthesized plasticizer/NC blends, IL
(2c)/NC had the lowest Tg (45 �C). The relative
ability of plasticizers to reduce the Tg of the
resultant NC/plasticizer blend is in the order of 3b/
NC[3a/NC[3c/NC[DEP/NC; so the synthesized ILs
have better performance in reducing the Tg of the
NC than the commonly used plasticizer (DEP).
Decomposition temperatures of the synthesized
plasticizers were lower than DEP. The lower bond
energy of the synthesized IL plasticizers caused
them to decompose in the lower temperature. SEM
images of plasticized films by synthesis plasticizers
showed smooth surfaces that will result in a
homogeneous matrix of the NC and plasticizer and
plasticizer compatibility.
The predicted relative trend of interaction energies
between plasticizer and NC by quantum chemical
calculations showed that DEP has lower interaction
than all synthesized IL plasticizers. As a result, all of
the synthesized IL plasticizers were compatible with
nitrocellulose. The result of DFT calculations com-
pletely confirms the DSC analysis and the same
change in the amount of Tg are seen in the results of
DFT calculations.
Supplementary Information (SI)
All additional information pertaining to the characterization
of the compounds using 1H NMR, 13C NMR, FT-IR spectra
and AIM calculations are given in the supporting informa-
tion. Supplementary information is available at www.ias.ac.
in/chemsci.
Acknowledgement
The authors are grateful to the Malek Ashtar University of
Technology for supporting this work.
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