Synthesis, characterization, thermal and computational studies of novel tetra–azido compounds as energetic plasticizers

Article abstract of DOI:10.1016/j.molstruc.2016.09.028

In this paper, four azido compounds have been synthesized and characterized as new energetic plasticizers. Nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy, elemental analysis, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and differential thermal analysis (DTA) have been used to identify and determine the properties of the synthesized plasticizers. The plasticization effect of plasticizers on glycidyl azide polymer (GAP) has been investigated by viscosity measurements and thermal analysis of the prepolymer–plasticizer mixtures and plasticized binders. The plasticized mixtures were cured by a diisocyanate curing agent and the glass transition temperature and decomposition temperature of the cured polyurethane binders were measured. Thermal analysis of the prepolymer–plasticizer and cured polymer mixtures showed that the synthesized plasticizers are completely compatible with the GAP binder and have a very good plasticizing effect. Furthermore, equilibrium geometry and heats of formation of each of the plasticizer molecules were obtained using the thermochemical T1 recipe, which is available in wave function Spartan software. Comparing empirical heats of combustion and calculated heats of combustion by using the heats of formation showed that the suggested optimum molecular structure by the T1 recipe has a high similarity to the real molecular structure of these molecules.

Full text of DOI:10.1016/j.molstruc.2016.09.028

Accepted Manuscript
Synthesis, characterization, thermal and computational studies of novel tetra–azido
compounds as energetic plasticizers
Mohammad Hadi Baghersad, Azizollah Habibi, Akbar Heydari
PII:
S0022-2860(16)30955-3
10.1016/j.molstruc.2016.09.028
MOLSTR 22940
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 25 June 2016
Revised Date: 8 September 2016
Accepted Date: 10 September 2016
Please cite this article as: M.H. Baghersad, A. Habibi, A. Heydari, Synthesis, characterization, thermal
and computational studies of novel tetra–azido compounds as energetic plasticizers, Journal of
Molecular Structure (2016), doi: 10.1016/j.molstruc.2016.09.028.
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Graphical Abstract
Synthesis, characterization, thermal and computational studies of novel
tetra–azido compounds as energetic plasticizers
Mohammad Hadi Baghersada, Azizollah Habibi*,a, Akbar Heydarib
a Faculty of Chemistry, Kharazmi University, No. 43. P. Code 15719-14911; Mofatteh Street, Enghelab Ave.; Tehran,
Iran
b Chemistry Department, Tarbiat Modarres University, P O Box 14115-175, Tehran, Iran

ACCEPTED MANUSCRIPT
Synthesis, characterization, thermal and computational studies of novel tetra–
azido compounds as energetic plasticizers
Mohammad Hadi Baghersada, Azizollah Habibia, , Akbar Heydarib
a Faculty of Chemistry, Kharazmi University, No. 43. P. Code 15719-14911; Mofatteh Street, Enghelab Ave.; Tehran, Iran
b Chemistry Department, Tarbiat Modarres University, P O Box 14115-175, Tehran, IRAN
Keywords: Azido compounds, Energetic plasticizers, Thermal analysis, Computational study
Abstract
In this paper, four azido compounds have been synthesized and characterized as new energetic
plasticizers. Nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy, elemental analysis,
differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and differential thermal
analysis (DTA) have been used to identify and determine the properties of the synthesized plasticizers.
The plasticization effect of plasticizers on glycidyl azide polymer (GAP) has been investigated by
viscosity measurements and thermal analysis of the prepolymer–plasticizer mixtures and plasticized
binders. The plasticized mixtures were cured by a diisocyanate curing agent and the glass transition
temperature and decomposition temperature of the cured polyurethane binders were measured. Thermal
analysis of the prepolymer–plasticizer and cured polymer mixtures showed that the synthesized
plasticizers are completely compatible with the GAP binder and have a very good plasticizing effect.
Furthermore, equilibrium geometry and heats of formation of each of the plasticizer molecules were
obtained using the thermochemical T1 recipe, which is available in wave function Spartan software.
Comparing empirical heats of combustion and calculated heats of combustion by using the heats of
formation showed that the suggested optimum molecular structure by the T1 recipe has a high similarity
to the real molecular structure of these molecules.
1. Introduction
Generally, plasticizers are organic compounds which are added to a polymer formulation with the aim of
improving its mechanical properties. Many polymer formulations need a plasticizer, both to ensure the
processability of the final compounds materials and to increase their practical property domains, e.g., for
flexibility continuation or flexibility improvement at low temperatures [1,2]. In some other uses, like in
the case of energetic materials, such as propellants, plasticizers play an important role in the energy
content of formulations [3]. Furthermore, they have been applied in the energetic propellant industry, for
oxygen balance adjustment, for glass transition temperature reduction, for propellant burning behavior
and for matrix mechanical properties improvement. An optimum energetic plasticizer has a low glass
transition temperature, a low viscosity, a low ability to migrate and a high oxygen balance. It should be
thermally stable and have a low sensitivity to physical stimuli such as friction and impact. An energetic
plasticizer should have at least one energetic functional group such as azido, nitro, nitrate, nitramine, etc
[4]. Organic azides are one of the most important energetic plasticizers used in rocket and gun propellants
[5]. Recently, azido plasticizers have been considered due to their positive heat of formation, high energy
output, burning without smoke, good thermal stability, appropriate compatibility with binders, low glass
transition temperature, low vapor pressure, high boiling point, low viscosity and good processability
[6–12]. They have shown all the above mentioned advantages. Although the presence of ester groups in a
plasticizer structure lead to an improvement in some properties such as vapor pressure, boiling point and
lubricating effect, the polarity of this group may lead to the development of strong intermolecular
interactions and increase the glass transition temperature more than it does in azido plasticizers without an
ester group [13]. In this paper, new azido compounds have been synthesized and we have investigated
Corresponding author. Tel–Fax: +98–21–88848949; e–mail: habibi@khu.ac.ir
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how the ester group elimination will advantageously affect its properties (see Fig. 1). These new
plasticizers have been designed according to the molecular structure of so called GLYN dimer plasticizer
(see Fig. 2). GLYN dimer has been recognized as one of the energetic plasticizers and is used to plasticize
polyether binders such as polyglycidyl nitrate (PGN) and poly(nitratomethyl) methyloxethanes
(polyNIMMO) [14]. The molecular structure similarity of GLYN dimer to PGN leads to an increase in its
plasticizing effect. So we first intend to synthesize a type of azido plasticizer compatible with the
glycidylazide polymer (GAP) by designing the same template but using azido groups instead of nitrate
ester groups (see compound 2c, Fig. 1). Also, compounds 1c and 3c were synthesized to investigate the
effect of increasing the carbon chain length on plasticizer properties such as glass transition temperature
(Tg), decomposition temperature (Td), viscosity and density, and also the plasticizing effect on GAP.
Additionally, compound 4c was synthesized with the aim of increasing oxygen balance. Thermal analysis
of the prepolymer–plasticizer and compatibility of the cured polymer mixtures with the GAP binder have
been examined along with the determination of equilibrium geometry and heats of formation of each
plasticizer molecules were obtained.
Fig. 1. Molecular structures of synthesized azido plasticizers.
Fig. 2. GLYN dimer plasticizer.
2. Experimental
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2.1.
Materials and Instrumentals
All materials were obtained from commercial suppliers (Fluka and Merck). Synthesized materials were
dissolved in CDCl3 and the NMR spectrum was prepared using a (300 MHz, Bruker AMX) spectrometer
with tetramethylsilane as the internal standard. An IR spectrophotometer (Shimadzu IR–460 DR–11) was
used for the preparation of the infrared spectra in the range of 400 to 4000 cm–1 wavelength numbers for
the identification of functional groups. The viscosity was measured using a Rheometer apparatus (MCR
501s, Anton Paar Physica Co.). The experiments were performed from 30 to 60 °C using a parallel plate
(diameter = 25 mm) with a gap of 1 mm at a constant shear rate of 1.0 s–1. The heating rate was 1 °C /min.
The glass transition temperature (Tg) was measured by Differential Scanning Calorimetry (DSC 204 F1
Phoenix, NETZSCH) at a heating rate of 10 °C/min from –150 to 25 °C under inert atmosphere (dry N2
gas, 50 ml/min) and the Thermo Gravimetric Analysis and Deferential Thermal Analysis (TGA/DTA)
were performed over Thermowaage L81 (Linseis Germany) with a heating rate of 10 °C/min from 25 to
600 °C under a N2 atmosphere. The heats of combustion of the synthesized plasticizers were measured
with a calorimetric oxygen bomb (IKA C 4000). The friction sensitivities of the compounds were
measured on a BAM friction apparatus.
2.2.
General procedure for synthesis of compound 1a–4a
Sodium (4.8 g., 0.21 g–atoms) was added to 20 ml. of allyl alcohol. While stirring, the desired dihalide
compound (0.1 mol) was added over a 30 minutes period. The mixture was refluxed for 3 hrs. and then
taken up in ether; the ether layer was washed with water and then the solvent was evaporated using a
rotary evaporator. The residue was dried over anhydrous sodium sulfate, and finally the residue was twice
distilled under reduced pressure to obtain the desired product.
1,3–bis(allyloxy)propane (1a):
1
Colorless liquid (yield: 65%), IR (neat): 3054, 2900, 1637, 1441, 1405, 1332, 1092, 987, 917 cm–1. H
NMR (300 MHz, CDCl3, δ ppm): 1.83 (q, J = 6.4 Hz, 2H), 3.48 (t, J = 6.4 Hz, 4H), 3.90 – 3.93 (dt, J =
5.5 Hz and 1.5 Hz, 4H), 5.09 – 5.14 (dt, J = 10.4 Hz and 1.5 Hz, 2H), 5.18– 4.26 (dq, J = 17.2 Hz and 1.5
Hz, 2H), 5.79–5.92 (m, 2H). 13C NMR (75 MHz, CDCl3, δ ppm): 29.9, 67.1, 71.7, 116.5, 134.8 ppm.
Anal. calcd. for C9H16O2 (156.22): C, 69.19%; H, 10.32%. Found: 69.32; H, 10.41%.
1,4–bis(allyloxy)butane (2a):
1
Colorless liquid (yield: 68%), IR (neat): 3175, 2915, 1646, 1444, 1417, 1344, 1099, 993, 922 cm–1. H
NMR (300 MHz, CDCl3, δ ppm): 1.50–1.56 (m, 4H), 3.27–3.29 (m, 4H), 3.79–3.81 (m, 4H), 4.97–
5.01(m, 2H), 5.07–5.14 (m, 2H), 5.68–5.81 (m, 2H). 13C NMR (75 MHz, CDCl3, δ ppm): 26.3, 32.5, 49.2,
71.5, 76.9 ppm. Anal. calcd. for C10H18O2 (170.25): C, 70.55%; H, 10.66%. Found: 70.47; H, 10.75%.
1,5–bis(allyloxy)pentane (3a):
1
Colorless liquid (yield: 72%), IR (neat): 3050, 2905, 1646, 1444, 1417, 1341, 1099, 991, 913 cm–1. H
NMR (300 MHz, CDCl3, δ ppm): 1.33–1.41 (m, 2H), 1.50 – 1.57 (m, 4H), 3.36 (t, J = 6.5 Hz, 4H), 3.87–
3.90 (dt, J = 5.5 Hz and 1.4 Hz, 4H), 5.06–5.11 (dq, J = 10.3 Hz and 1.3 Hz, 2H), 5.15–5.23 (dq, J = 17.2
Hz and 1.6 Hz, 2H). 13C NMR (75 MHz, CDCl3, δ ppm): 22.6, 29.3, 70.0, 71.5, 116.3, 134.8 ppm. Anal.
calcd. for C11H20O2 (184.28): C, 71.70%; H, 10.94%. Found: 71.57%; H, 10.82%.
Bis(allyloxy)diethylene glycol (4a):
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1
Colorless liquid (yield: 66%), IR (neat): 3050, 2850, 1636, 1448, 1414, 1337, 1101, 990, 917 cm–1. H
NMR (300 MHz, CDCl3, δ ppm): 3.54 – 3.64 (m, 8H), 4.0 – 4.03 (dt, J = 5.7 Hz and 1.3 Hz, 4H), 5.06–
13
5.11 (dq, J = 10.3 Hz and 1.3 Hz, 2H), 5.15–5.23 (dq, J = 17.2 Hz and 1.6 Hz, 2H). C NMR (75 MHz,
CDCl3, δ ppm): 69.2, 70.5, 72.0, 116.9, 134.6 ppm. Anal. calcd. for C10H18O3 (186.25): C, 64.49%; H,
9.74%. Found: 64.61%; H, 9.92%.
2.3.
General procedure for synthesis of compounds 1b–4b
A solution of bromine (2 equiv.) in chloroform (1M solution) was added dropwise to the stirred solution
of compounds 1a–4a in chloroform (1M) at 0 °C. The resultant mixture was stirred for 3 hrs. at room
temperature. The solution was washed twice with aqueous sodium thiosulfate solution and twice with
water, dried with anhydrous sodium sulfate and the solvent was evaporated under vacuum to afford the
compounds 1b–4b as colorless liquids.
1,3–bis(2,3–dibromopropoxy)propane (1b):
Colorless liquid (yield: 95%), IR (neat): 2850, 1462, 1411, 1358, 1225, 1120, 753, 571 cm–1. 1H NMR (300
MHz, CDCl3, δ ppm): 1.84 (2H, quin, J = 6.1 Hz, CH2), 3.59 (4H, t, J = 6.1 Hz, CH2O), 3.73–3.85 (8H,
13
m, CH2Br and OCH2), 4.19–4.25 (2H, m, CHBr), C NMR (75 MHz, CDCl3, δ ppm): 29.5 (CH2), 32.9
(CH2Br), 49.0 (CHBr), 67.7 (CH2O), 71.5 (OCH2) ppm. Anal. Calcd. For C9H16Br4O2 (475.84):
C, 22.72%; H, 3.39%. Found: 22.60; H, 3.51%.
1,4–bis(2,3–dibromopropoxy)butane (2b):
1
Colorless liquid (yield: 94%), IR (neat): 2900, 1439, 1355, 1223, 1107, 748, 569 cm–1. H NMR: (300
MHz, CDCl3, δ ppm): 1.69 (4H, m, CH2), 3.56 (4H, t, J = 6 Hz, CH2O), 3.76–3.88 (8H, m, CH2Br and
OCH2), 4.26 (2H, m, CHBr). 13C NMR: (75 MHz, CDCl3, δ ppm): 26.1 (CH2), 30.8 (CH2Br), 49.2
(CHBr), 70.4 (CH2O), 71.5 (OCH2) ppm. Anal. Calcd. For C10H18Br4O2 (489.86): C, 24.52%; H, 3.70%.
Found: 24.50; H, 3.95%.
1,5–bis(2,3–dibromopropoxy)pentane (3b):
Colorless liquid (yield: 96%), IR (neat): 2845, 1443, 1417, 1352, 1222, 1105, 752, 564 cm–1. 1H NMR (300
MHz, CDCl3, δ ppm): 1.39–1.46 (2H, m, CH2), 1.56–1.62 (4H, m, CH2), 1.6 (4H, quin, J = 6.2 Hz,), 3.5
(4H, t, J = 6.2 Hz, CH2O), 3.73–3.84 (8H, m, CH2Br and OCH2), 4.20–4.24 (2H, m, CHBr). 13C NMR (75
MHz, CDCl3, δ ppm): 22.4 (CH2), 29.0 (CH2), 33.1 (CH2Br), 49.1 (CHBr), 71.2 (CH2O), 71.4 (OCH2)
ppm. Anal. Calcd. For C11H20Br4O2 (503.89): C, 26.22%; H, 4.00%. Found: 26.50%; H, 4.18%.
Bis(2,3–dibromopropoxy)diethylene glycol (4b):
1
Colorless liquid (yield: 95%), IR (neat): 2865, 1452, 1337, 1225, 1113, 751, 560 cm–1. H NMR (300
MHz, CDCl3, δ ppm): 3.65–3.73 (8H, m, OCH2CH2O), 3.80–3.88 (4H, m, CH2Br), 3.89–3.96 (4H, m,
OCH2), 4.23–4.30 (2H, m, CHBr). 13C NMR (75 MHz, CDCl3, δ ppm): 33.2 (CH2Br), 49.1 (CHBr), 70.5
(CH2O), 70.9 (OCH2), 72.2 (OCH2) ppm. Anal. Calcd. For C10H18Br4O3 (505.86): C, 23.74%; H, 3.59%.
Found: 23.44%; H, 3.38%.
2.4.
General procedure for synthesis of compounds 1c–4c
A mixture of compounds 1b–4b (16.2 mmol), sodium azide (9 g, 136.4 mmol), and 200 ml of dry DMSO
was heated at 65–70 °C for 20 hrs under nitrogen atmosphere. After the solution was cooled to room
temperature, 400 ml. of water was added, and the mixture was extracted four times with 300 ml. of
petroleum ether. The organic extracts were dried over anhydrous sodium sulfate, filtered, and evaporated
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under reduced pressure and dried under high vacuum for 16 hrs. to afford compounds 1c–4c as yellow or
orange liquids.
1,3–bis(2,3–diazidopropoxy)propane (1c):
1
Orange liquid (yield: 70%), IR (neat): 2855, 2095, 1439, 1333, 1268, 1118 cm–1. H NMR (300 MHz,
CDCl3, δ ppm): 1.83 (2H, quin, J = 6.1 Hz, CH2), 3.30–3.44 (4H, m, CH2N3), 3.52–3.60 (8H, m, CH2O
and OCH2), 3.60–3.65 (2H, m, CHN3) 13C NMR: (75 MHz, CDCl3, δ ppm): 29.6 (CH2), 51.3 (CH2N3),
60.3 (CH2O), 67.8 (CHN3), 70.4 (OCH2) ppm. Anal. Calcd. For C9H16N12O2 (324.30): C, 33.33%; H,
4.97%; N, 51.83% Found: 33.10%; H, 5.13%; N, 52.11%.
1,4–bis(2,3–diazidopropoxy)butane (2c):
Yellow liquid (yield: 75%), IR (neat): 2930, 2095, 1446, 1266, 1118 cm–1. 1H NMR: (300 MHz, CDCl3, δ
ppm): 1.64–1.69 (4H, m, CH2CH2), 3.31–3.69 (14H, m) 13C NMR: (75 MHz, CDCl3, δ ppm): 26.3 (CH2),
51.6 (CH2N3), 60.4 (CH2O), 70.4 (CHN3), 71.3 (OCH2) ppm. Anal. Calcd. For C10H18N12O2 (338.33): C,
35.50%; H, 5.36%; N, 49.68% Found: 35.54%; H, 5.12%; N, 49.81%.
1,5–bis(2,3–diazidopropoxy)pentane (3c):
Orange liquid (yield: 77%), IR (neat): 1860, 2085, 1442, 1330, 1262, 1117 cm–1. 1H NMR: (300 MHz,
CDCl3, δ ppm): 1.39–1.49 (2H, m, CH2), 1.56–1.67 (4H, m, CH2), 3.37–3.49 (8H, m, CH2O and OCH2),
13
3.54–3.58 (4H, m, CH2N3), 3.63–3.65 (2H, m, CHN3). C NMR (75 MHz, CDCl3, δ): 22.5 (CH2), 29.2
(CH2), 51.5 (CH2N3), 60.4 (CHN3), 70.3 (CH2O), 71.5 (OCH2) ppm. Anal. Calcd. For C11H20N12O2
(352.36): C, 37.50%; H, 5.72%; N, 47.70% Found: 37.28%; H, 5.68%; N, 48.15%.
Bis(2,3–diazidopropoxy)diethylene glycol (4c):
Orange liquid (yield: 70%), IR (neat): 2845, 2085, 1438, 1334, 1265, 1117 cm–1. 1H NMR: (300 MHz,
CDCl3, δ ppm): 3.34–3.41(2H, dd, J = 12.6 Hz and 6.6 Hz, CH2N3), 3.44–3.50 (2H, dd, J = 12.6 Hz and
4.4 Hz, CH2N3), 3.62–3.72 (m, 14H). 13C NMR (75 MHz, CDCl3, δ ppm): 51.6 (CH2N3), 60.4 (CHN3),
70.6 (CH2O), 70.9 (CH2O), 71.0 (OCH2). Anal. Calcd. For C10H18N12O3 (354.33): C, 33.90%; H, 5.12%;
N, 47.44% Found: 34.05%; H, 5.15%; N, 47.79%.
Caution! As azido compounds are classified as sensitive energetic materials, safety considerations must
be taken into account at all times. Although we experienced no difficulties in handling the synthesized
compounds, small–scale use is always preferred.
2.5.
Preparation of GAP polyurethane binders with energetic plasticizer
The GAP polyurethane binder was plasticized with 10 wt% of an energetic plasticizer with a mixed TPB
catalyst at an OH/NCO equivalent ratio of 2. The GAP prepolymer (Mn = 1815, OHn = 0.927 mmol/g)
and required amount of an energetic plasticizer were premixed at 60 °C under vacuum for 1 hr. and
cooled to 25 °C for 0.5 hr. HMDI (Hexamethylene diisocyanate) was then added into the mixture of
GAP–energetic plasticizer. After stirring for 0.5 h, TPB (Triphenylbismuth) catalyst (5 wt%) were added
to the mixture at 25 °C and stirred for another 0.5 hr. under vacuum. After pouring into a mold, the
mixture was finally degassed in a vacuum oven at 30 °C for 3 hr., followed by curing in an oven at 60 °C
for 7 days.
3. Results and discussion
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3.1.
Synthesis
The synthesis of the azido plasticizers 1c–4c involved three steps (Scheme 1): (a) the synthesis of
compounds 1a–4a from the corresponding dibromo compounds and allyl alcohol; (b) the bromination of
the double bonds of compounds 1a–4a and (c) the substitution of the bromo groups by azido groups
(Scheme 1). In the first step, Williamson ether synthesis was used as an easy and well–known method for
synthesizing of ethers from alcohols and alkyl halides. As the synthesis of 1,4–bis(allyloxy)butane (2a)
using allyl alcohol and 1,4–dibromobutane has been previously reported [15], we used this same method.
The bromination of compounds 1a–4a was carried out by the addition of a solution of bromine in
chloroform at –5 °C. The azido groups were then introduced into the molecular structure by a
nucleophilic substitution reaction of halides by azide anion. The tetra bromo product (1b–4b) obtained
from the second step, was dissolved in DMSO and then sodium azide was added by stirring. The mixture
was heated under N2 atmosphere overnight at 65–70 °C to obtain compounds 1c–4c.
Scheme 1. Synthesis of azido plasticizers.
3.2.
Characterization
All of the synthesized compounds were detected completely by using analytical methods including
infrared (IR) spectroscopy, nuclear magnetic resonance (1H–NMR and 13C–NMR) spectroscopy and
elemental analysis (see ESI for details).
The presence of stretching vibrations in the infrared spectrum of compounds 1a–4a is attributed to
=C–H bond at the 3000–3100 cm–1 region and carbon–carbon double bond at the 1620–1650 cm–1 region
indicates the presence of double bond. The elimination of the stretching vibration related to the =C–H
bond and the carbon–carbon double bond and the observation of a stretching vibration related to the C–Br
bond at the 500–800 cm–1 region indicates the complete bromination of the double bonds and the
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formation of the compounds 1b–4b. Also, complete elimination of the C–Br bond stretching vibration
and the emergence of the strong stretching vibration of the –N3 group near 2100 cm–1 indicates the
complete substitution of bromo groups by azide anions.
Obvious peaks are observed for the plasticizer compounds 1c–3c in their 1H–NMR spectra at the 1–2 ppm
region. These peaks are related to protons of methylene groups which are not connected to oxygen in the
middle chain of the molecules. One quintet peak for the compound 1c with integration factor of 2 related
to a methylene group, one multiplet peak for the plasticizer compound 2c, integrating for 4 protons, is
related to two methylene groups, and two multiplet peaks for compound 3c with integrations of 2 and 4
are related to three methylene groups are observed. The other proton peaks are observed in the 3.3–3.6
ppm region. Integration of these peaks accurately shows the presence of 12 protons for 1c and 14 protons
1
for 2c and 3c. The H–NMR spectrum of compound 4c includes two signals with integration of 2 in the
form of doublet of doublet in at the 3–4 ppm region which is related to the methylene protons of the
middle chain of the molecule. The signals related for the other protons are observed at the 3.6–3.8 ppm
region integrating for 14 protons in the form of a complex pattern. Given that all synthesized compounds
have two chiral centers, two protons of each methylene group are chemically nonequivalent but this
nonequivalence is observed only in the 1H–NMR spectrum of the methylene groups in the middle chain of
compound 4c in the form of a doublet of doublet peak. Because the other protons have a very close
chemical shift, this peak is not observed specifically and often has a complex pattern. Carbon species
counting in 13C–NMR confirms their molecular structure well. Given the symmetry of the molecules, 5
peaks are observed for compound 1c, 5 peaks for compound 2c, 6 peaks for compound 3c and 5 peaks for
compound 4c.
3.3.
Sensitivity
Investigation of the sensitivities of the synthesized azido compounds using BAM friction apparatus
showed that they haven’t any sensitivity to friction up to 360 Newtons and there was no ignition or
explosion. Unfortunately, due to lack of access to suitable apparatus, an impact sensitivity test could not
be performed for these compounds.
3.4.
Thermal Properties
The thermal stabilities of the plasticizers have an important role in the burning rate of the propellants and
the explosion of explosive materials. So it's necessary to test the decomposition behavior of plasticizers.
All synthesized plasticizers have been tested by TGA and DTA. TGA was used to investigate the
decomposition quality and the amount of sample mass reduction by increasing temperature, and DTA was
used to measure the temperature and enthalpy of decomposition for each plasticizer.
The TGA thermograms of compounds 1c–4c showed that the synthesized compounds decompose in two
stages by reaching their decomposition temperatures (see Fig. 3). These two stages were as follows: 57 to
59 % weight loss from 120 ºC to 260 ºC and 11 to 17 % weight loss from 260 ºC to 550 ºC, so that up to
the temperature of 550 ºC totally 68 to 74 % weight loss was observed. It seems that in the first stage, all
azido groups decompose and emerge in the form of nitrogen gas and in the second stage ether bonds
decompose [16]. More investigations using DTA (see Fig. 4) from 25 ºC to 600 ºC showed decomposition
of all these compounds during an exothermic process by increasing from an onset decomposition
temperature (Td, onset) of 166 to 170 ºC and a maximum decomposition temperature (Td,max) of 230 to
235 ºC. The obtained thermal analysis data for the synthesized plasticizers are summarized in Table 1.
According to this table, the maximum decomposition temperatures of the synthesized plasticizers are not
very different from each other, but the amount of released energy in the decomposition of compound 2c is
more than in the others.
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The glass transition temperature (Tg) in polymers and macromolecules is related to their entropy, free
volume and mobility. Tg can be a suitable criterion to determine the polymer chain’s mobility. So the
investigation of Tg in the synthesized molecules is necessary for practical purposes such as plasticization
[17–19]. The Tg value of each plasticizer is measured using DSC from –120 ºC to 25 ºC (see Fig. 5).
Among the synthesized plasticizers, compound 1c has a lower Tg than the others. It seems that
introducing an oxygen atom into the molecular structure leads to an increase in the polarity and thus
increases the intermolecular interactions and Tg in compound 4c.
dM-rel [%]
1c
10
0
2c
3c
4c
-10
-20
-30
-40
-50
-60
-70
-80
T [°C]
0
100
200
300
400
500
600
Fig. 3. TGA curves of compounds 1c–4c
Heat Flow
[mJ/s]
1c
2c
3c
4c
140
120
100
80
60
40
20
0
-20
0
T [°C]
600
100
200
300
400
500
Fig. 4. DTA curves of compounds 1c–4c
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DSC
[mW/mg]
1c
2c
3c
4c
0.00
-0.05
-0.10
-0.15
-0.20
-0.25
T [°C]
-0.30
-120
-100
-80
-60
Fig. 5. DSC curves of compounds 1c–4c
Table 1
Thermal properties of compounds 1c–4c.
Total
weight
loss (%)
Tg
(ºC)
Td, onset Td, max ∆Hdecomposition
Compound
Weight loss steps
(ºC)
(ºC)
(J/g)
122.2–252.8 ºC, 58.3 %
252.8–550.0 ºC, 11.3 %
121.1–251.3 ºC, 57.4 %
251.3–550.0 ºC, 13.5 %
121.1–253.6 ºC, 57.3 %
253.6–550.0 ºC, 16.2 %
119.5–259.0 ºC, 57.4 %
259.0–550.0 ºC, 11.3 %
–101.3
–92.3
–96.4
–86.0
169.8
166.8
169.6
167.6
230.4
232.3
232.1
234.9
1382.4
1617.2
1616.6
1585.6
69.6
70.9
73.5
68.7
1c
2c
3c
4c
3.5.
Plasticization effect of 1c–4c
Plasticizer–GAP prepolymer mixtures were prepared to study the efficiency and plasticizing effect of
synthesized plasticizers with weight fractions of 50:50 and 65:35 at room temperature. The prepared
mixtures didn't show any phase separation during a long period of time (even up to 2 months or more)
which indicates the compatibility of the synthesized plasticizers with GAP prepolymer.
The DSC thermograms of the mixtures indicate a break and therefore a glass transition temperature.
Increasing the percentage of plasticizer leads to a reduction in Tg of the prepolymer–plasticizer mixtures
compared to pure prepolymer. The obtained data are summarized in Table 2. We can explain glass
transition temperatures which have been measured experimentally by mathematical equations. These
equations make relations between the glass transition temperature of the polymer–plasticizer mixture and
the glass transition temperature of the pure components.
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Table 2
Effect of synthesized plasticizers on Tg of GAP prepolymer.
Averaged interaction parameter
Glass transition
temperature Tg (ºC)
–53.7
S. No.
Composition
̅
(I)
–
1
2
GAP prepolymer
GAP + 35% 1c
GAP + 50% 1c
GAP + 35% 2c
GAP + 50% 2c
GAP + 35% 3c
GAP + 50% 3c
GAP + 35% 4c
GAP + 50% 4c
–75.3
–86.5
–69.6
–78.3
–74.3
–79.6
–69.4
–74.3
–28.86
3
4
5
–15.85
–21.53
–18.56
To investigate the plasticizing effect of a plasticizer in prepolymer–plasticizer mixtures many equations
are used, one of which is the Fox equation [20]. However, the problem with the Fox equation is that it
doesn't consider the interaction between the prepolymer and the plasticizer molecules, and therefore
Jenckel and Hensch suggested Eq. 1 which is more compatible with experimental data [21]:
ꢀ ꢁ
1
Tg= Tg1w1+ Tg2w2+ Iw1w2
In this equation, “w” is the weight fraction, subtitle “1” is related to the polymer, subtitle “2” is related to
the plasticizer, and “I” is the interaction parameter which is related to chain stiffness, free volume and
molar cohesive energy. This parameter indicates the plasticizer molecular flexibility and depends on the
plasticizer’s molecular structure and represents the ability of a plasticizer in the reduction of Tg. “I” value
can be obtained by Eq. 2:
Tg-Tg1w1-Tg2w2
ꢀ ꢁ
2
I =
w1w2
̅
If averaged “I” values I are higher and more negative, it shows that the plasticizer moves more easily in
the polymer’s matrix, resulting in more flexibility and therefore a greater reduction in the glass transition
temperature of the formulation. So, the interaction parameter for each binary mixture was calculated by
̅
̅
using equation (2) and then the obtained values were averaged I (Table 2). According to I values, the
mobility and plasticizing effect of compound 1c on GAP prepolymer was more than that of the others.
Polyurethane rubbers containing 10% of each plasticizer were produced to achieve more practical data.
DSC method was used to determine Tg and DTA method was used to determine Td to investigate the
synthesized plasticizers effect on these rubbers. Table 3 shows the data obtained from analysis of GAP
polyurethane rubbers using DSC and DTA methods. The Tg of the plasticized GAP polymers has
decreased compared to GAP polymer without plasticizer. The presence of a break (one Tg) indicates the
complete compatibility of the synthesized azido plasticizers with the GAP binder. Introducing 10% (by
weight) of the synthesized azido plasticizers into the GAP polyurethane binder formulation, decreases on
average the Tg by 10 °C and provides a very negligible reduction in the decomposition temperature of
plasticized polymers compared to GAP polymer (Table 3).
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Table 3
Effect of synthesized plasticizers on thermal properties of GAP polyurethane binder.
Glass transition
temperature (ºC)
–33.8
Decomposition
temperature (ºC)
223.8
S. No.
Composition
1
2
3
4
5
GAP polyurethane
GAP + 10% 1c
GAP + 10% 2c
GAP + 10% 3c
GAP + 10% 4c
–43.8
–43.5
–42.6
–42.4
222.0
221.6
221.3
221.5
An important plasticizing parameter is viscosity reduction which is achieved by adding plasticizer to the
binder formulation. The solid loading amount can be increased by decreasing viscosity in the propellant
and explosive compounds. Since the mixture temperature is different in various propellants, the viscosity
of the prepolymer–plasticizer mixtures was measured from 30 ºC to 60 ºC and plotted in a viscosity–
temperature curve. Viscosity measurement results for GAP–plasticizer mixtures (50:50) from 30 to 60 ºC
are plotted as a curve in Fig. 6. According to this figure, compounds 1c–4c cause a reduction in the
viscosity of mixtures compared to GAP prepolymer. The viscosity reduction gradients for the mixtures
are lower than that of the GAP prepolymer. GAP prepolymer viscosity is more than the viscosity of the
mixtures at 30 ºC. This viscosity difference is reduced by increasing the temperature to 60 ºC but the
mixing action of the formulation components becomes better in plasticized mixtures due to their lower
viscosity. The highest amount of viscosity reduction is developed in the presence of compound 3c.
viscosity
[Pa.s]
GAP prepolymer
GAP + 1c
GAP + 2c
GAP + 3c
GAP + 4c
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
T [°C]
60
30
40
50
Fig. 6. Viscosity variation of GAP prepolymer and mixtures of GAP prepolymer/ synthesized plasticizers
(50/50, w/w) measured as a function of temperature.
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3.6.
Computational studies of 1c–4c
Due to lack of accurate experimental data, at the present time much effort has to be made by using
calculations to arrive at properties such as heat of formation. However, these calculations are very
complex and time consuming for medium–sized organic molecules (molecular weights lower than 400 to
500). Recipes based on Gaussian-n theories (GX recipe) are the most successful computational chemistry
methods in terms of accuracy which use several models of various quantum calculations to achieve
estimations close to real values [22]. The simplest GX recipe is G3(MP2) which includes several huge
calculative stages such as MP2/6–31G* geometric calculations, HF/6–31G* frequency calculations and
QCISD(T)/6–31G* energy calculations. However, the G3(MP2) model is practically applicable for
molecules with molecular weights lower than 150, but it’s very time consuming in the case of large
molecules with several degrees of freedom. Another recipe which is used in this field, is called the
thermochemical T1 that is applicable by using Wavefunction Spartan (14V 1.1.2) software. This recipe is
equal to the G3(MP2) model in terms of accuracy but has a significantly lower calculation volume. The
HF/6–31G* model is used instead of the MP2/6–31G*in the geometric calculations of this recipe. HF/6–
31G* frequency calculations and QCISD(T)/6–31G* energy calculations have been removed and MP2
energy, which has a high volume, have been replaced with the RI–MP2 model with similar calculations
and a lower volume. These changes reduce calculation time by 2 to 3 times rendering the T1 recipe
convenient and practical for molecules with a molecular weight of 400 to 500 [23]. GX instruction, due to
its very high volume of calculations is, in practice, restricted to molecules with one or two conformational
freedom degrees, but T1 instruction can be employed normally for molecules with several conformational
freedom degrees and this makes it possible to measure the energy difference between conformers for
compounds such as 1c–4c which have many conformers. Given that all synthesized plasticizers have two
chiral centers, the formation of three kinds of stereoisomers in the form of S,S, R,R and R,S or S,R during
the synthesis process of these compounds is possible, but in practice only one product was obtained.
However, the equilibrium geometry of molecule and heat of formation for each stereoisomer was
obtained using T1 recipe. Theoretically, the probability of the formation of isomers S,S, R,R and R,S or
S,R, is 25, 25 and 50 percent, respectively. So to compare the theoretical and experimental values of the
heats of formation, the weighted average values were used (see ESI for details). Obtained data from
calculations is summarized in Table 4. Experimental heat of combustion for each plasticizer was obtained
using a calorimetric oxygen bomb. These values along with calculated values by heat of formation
(enthalpy of the hypothetical reaction of complete burning in oxygen) for each plasticizer are also
summarized in Table 4. By comparing the experimental heats of combustion with the calculated values it
can be concluded that the structures of each plasticizer is very close to the equilibrium molecular structure
suggested by the T1 recipe and the heats of formation and combustion of these new plasticizers lie within
the range of those for known energetic plasticizers [24–26].
Table 4
Calculated ∆Hf using T1 recipe, calculated ∆Hcombn. and experimental ∆Hcombn. using a calorimetric
oxygen bomb for compounds 1c–4c.
Calculated ∆Hf
(kJ/mol)
848.08
Calculated ∆Hcombn.
(kJ/mol)
Experimental ∆Hcombn
(kJ/mol)
S. No.
Compound
1
2
3
4
–6675.98
–7341.78
–7990.69
–7205.39
–6858.98
–7588.89
–8269.20
–7334.01
1c
2c
3c
4c
834.58
804.19
698.19
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4. Conclusions
Four novel azido plasticizers have been synthesized from low-cost starting materials and their molecular
structures were confirmed by spectral data and elemental analysis. The equilibrium geometry of
molecules and heat of formation for each plasticizer was obtained using T1 recipe. Comparison of the
calculated heats of combustion for each plasticizer with experimental values indicates that there is a good
agreement between these data, showing that the optimized structures by thermochemical T1 recipe have a
high similarity to the real molecular structure. High heat release capability, insensitivity to friction, and
very low Tg are the main characteristics of the compounds. The DSC analysis of plasticizer–prepolymer
mixtures for determining Tg showed that the synthesized plasticizers are completely compatible with GAP
prepolymer. Evaluation of the synthesized plasticizers in the prepolymer mixtures and binders showed
that they have good plasticizing effects on GAP prepolymer and binder in terms of their thermal and
rheological properties. These compounds can be introduced as potential energetic plasticizers for
advanced explosive and propellant formulations with good process ability and higher energy output.
Acknowledgement
We thank Mr. Sayed Yaser Sadat, for elegant designing of graphical abstract.
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•
•
•
•
•
Tetra novel azido plasticizers are synthesized from readily available reagents.
All the compounds are insensitive to friction up to 360 Newtons.
The synthesized plasticizers are completely compatible with the GAP binder.
There is a good agreement between the calculated heats of combustion and experimental values.
The optimized molecular structures of the plasticizers by thermochemical T1 recipe have a high similarity to the real
molecular structure of these molecules.