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: reza_fareghi@yahoo.com; reareghialamdari@mut.ac.ir

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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