Energetic interpenetrating polymer network based on orthogonal azido-alkyne click and polyurethane for potential solid propellant

Article abstract of DOI:10.1039/c5ra10467j

High energetic propellants with synergistic mechanical strength are the prerequisites for aerospace industry and missile technology; though glycidyl azide polymer (GAP) is a renowned and a promising energetic polymer which shows poor mechanical and low-temperature properties. In order to overcome these problems, a novel energetic interpenetrating polymer network (IPN) of acyl-terminated glycidyl azide polymer (Acyl-GAP) and hydroxyl terminated polybutadiene (HTPB) is effectively synthesized and characterized via an "in situ" polymerization by triazole and urethane curing system respectively. Acyl-GAP and dimethyl 2,2-di(prop-2-ynyl)malonate (DDPM) have been synthesized and well characterized by using FT-IR, ¹H NMR, ¹³C NMR and GPC. The maximum tensile strength ～5.26 MPa and elongation 318% are achieved with HTPB-PU/Acyl-GAP triazole in 50 : 50 weight ratios. The solvent resistance properties have been investigated by the equilibrium swelling method and the glass transition temperature (T_g), morphology and thermal stability are evaluated by DSC, SEM and TGA-DTG respectively. Thus, HTPB-PU/Acyl-GAP triazole is a futuristic binder for the composite solid propellant.

Full text of DOI:10.1039/c5ra10467j

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Energetic Interpenetrating Polymer Network Based on Orthogonal
Azido-Alkyne Click and Polyurethane for Potential Solid Propellant
Abbas Tanver,a Mu-Hua Huang,*a Yunjun Luo,*a Syed Khalida and Tariq Hussainb
High energetic propellants with synergistic mechanical strength are the prerequisites for aerospace industry
and missile technology; though glycidyl azide polymer (GAP) is a renowned and a promising energetic
polymer which shows poor mechanical and low-temperature properties. In order to overcome these
problems, a novel energetic interpenetrating polymer network (IPN) of Acyl-terminated glycidyl azide
polymer (Acyl-GAP) and hydroxyl terminated polybutadiene (HTPB) is effectively synthesized and
characterized via an “in situ” polymerization by triazole and urethane curing system respectively. Acyl-GAP
and Dimethyl 2, 2-di (prop-2-ynyl) malonate (DDPM) have been synthesized and well characterized by using
1
FT-IR, H NMR, 13C NMR and GPC. The maximum tensile strength ~ 5.26 MPa and elongation 318 % are
Received 00th January 20xx,
Accepted 00th January 20xx
achieved with HTPB-PU/Acyl-GAP triazole in 50:50 weight ratios. The solvent resistance properties have been
investigated by the equilibrium swelling method and the glass transition temperature (Tg), morphology and
thermal stability are evaluated by DSC, SEM and TGA-DTG respectively. Thus, HTPB-PU/Acyl-GAP triazole is a
futuristic binder for the composite solid propellant.
DOI: 10.1039/x0xx00000x
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performances and different applications.16-20 Although GAP has
numerous advantageous like high positive heat of formation, higher
density and good compatibility with advanced oxidizers like
Introduction
Interpenetrating polymerization signifies an innovative approach to
elucidate the problem of polymer incompatibility. IPN can be
elaborated as special class of polymers, which is a combination of
two or more polymers in which one is synthesized or polymerized in
the presence of others.1,2 IPN is a useful method to develop a
product with more excellent physico-mechanical properties than
the normal polyblends. IPN is also known as the polymer alloys and
it is one of the fastest emerging areas of research in the field of
polymer blends since last two decades.3 There are two kinds of
hydrazinium nitroformate (HNF) and ammonium dinitramide, GAP
does not show excellent mechanical properties especially low
temperature properties due to its poor polymer backbone
flexibility.20 The critical temperature of GAP is 6 oC where the
polymeric binder starts to quickly lose its elastomeric properties.
Glass transition temperature of GAP are much greater than those of
inert binder HTPB and this significantly limits its application in
composite solid propellants.21 Normally, the polymeric binder
features can significantly influence the structural integrity of the
propellant and can be altered effectively to adjust the mechanical
properties of the propellant for specific applications. HTPB is widely
used as polymeric binder in composite propellant technology due to
its unique physico-chemical properties.22-27 HTPB exhibits low glass
transition temperature, hydrolytic stability, high flexibility and
resistance to solvents are excellent properties for the composite
solid propellant.28,29
methodologies,
namely
sequential
and
simultaneous
polymerization. Sequential IPNs are generally prepared in which the
second component network is formed following the formation of
first component network. Simultaneous IPNs result from two
individual curing processes that occurred at the same time.4-10
There are also other types of IPNs such as semi, grafted, latex,
gradient and thermoplastic IPNs. Semi IPNs have only one
component cross-linked, whereas grafted IPNs have covalent cross-
links between both networks.11,12 IPNs with special characteristics In order to make full use of benefits of GAP and HTPB, many
have found many interesting applications, such as coating attempts have been carried out. However it is significantly inhibited
materials,13 high strength materials,11 pH-sensitive hydrogels,14 and by the poor compatibility between HTPB and GAP due to non polar
biocompatibility materials.15
nature of HTPB and polar nature of GAP.30 The cross-linked
copolymer of HTPB and GAP was synthesized showed two glass
transition temperatures at -74.03 oC and -35.84 oC, while the
decomposition pattern was similar as observed in their
homopolymers. Due to the reactivity differences of terminal
hydroxyl groups of GAP and HTPB with different isocyanates, it has
been reported that the GAP-HTPB cross-linked networks showed
improved mechanical properties over the virgin network of HTPB or
GAP.31 Single glass transition was seen with 30 % of GAP without
phase separation while beyond 50 % GAP ratio, no significant effect
seen on mechanical properties improvement due to poor
miscibility, leading to phase separation. Mathew et al. have
Recent advances in the field of propellant for high energetic
properties, GAP is one of the promising binders for propellant
industry and many researchers have reported its synthesis,
a.School of Materials Science and Engineering, Beijing Institute of Technology,
Beijing, 100081, China
b.School of Mechatronical Engineering, Beijing Institute of Technology, Beijing,
100081, China
E-mail: yjluo@bit.edu.cn; mhhuang@bit.edu.cn
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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reported maximum mechanical strength around 4.2 MPa and
elongation ˂ 200 with 30 % GAP-MDCI in GAP-HTPB cross-linked
networks.31 Bing et al. have claimed 3.83 MPa tensile strength and
593 % elongation by using GAP-HTPB blend binders with 50:50
weight ratios.32
Instead of traditional isocyanate curing system, Chinese researcher
Ding et al. adopted the triazole curing system based on HTPB and
GAP.33 Blend of propargyl-terminated polybutadiene (PTPB) with
GAP under the catalysis of cuprous chloride Cu (I) showed the
maximum mechanical strength up to 2.5 MPa and elongation
around 50 % when the molar ratios of N3 versus C≡C was 2.0. It was
a good endeavour in order to overcome the miscibility problem of
the binders and compatibility issues of the high energy ingredients
in the binder system.33 1, 3–dipolar cycloaddition between azides
and alkynes (Huisgen reaction) is one of the classical click reaction
due to the fact that it is virtually quantitative, insensitive, very
robust and orthogonal ligation reaction.
Scheme 1: Schematic representation of click chemistry reaction and
IPN structure of Acyl-GAP triazole and HTPB-PU
One of the finest ways to modify the characteristics of the polymers
in order to attain the specific properties is the chemical or physical
combination of polymers.34 Many researchers have employed HTPB
with other polymers in order to increase the mechanical strength,
thermal stability and chemical resistance. HTPB-PU/Poly (methyl
methacrylate) and HTPB-PU/Polystyrene, HTPB-PU/Poly (ethylene
oxide) based IPNs have been examined.35-40 Min et al. investigated
the block copolyurethane binder matrices of GAP/Polyethylene
glycol and GAP/Polycaprolactone for better mechanical strength.41
Experimental
Materials
GAP having molecular weight 3700 g mol-1 and hydroxyl contents
29.15 mg KOH g-1, Nitrogen contents, water contents and
functionality are 41.3 %, 0.217 % and 1.92 respectively. HTPB
molecular weight and hydroxyl contents are 3020 gmol-1 and 0.73
mmol g-1 respectively, Hydrogen peroxide (H2O2) contents, water
contents and functionality are 0.022 %, 0.015 % and 2.205
respectively. GAP and HTPB were taken from Liming Research
Institute of Chemical Industry, Henan China and was used after
In the present research, we have developed an energetic IPN which
is formed from the cross-linking of two binders system in which one
binder (GAP) is highly energetic compared with the other (HTPB).
Based on the pioneering work done in the HTPB and GAP system,
we wonder if an IPN structure via orthogonal curing reaction will
enhance the properties of resulting material. In order to improve
the mechanical properties of IPN for potential solid propellants, we
designed the networking polymer based on the following
considerations: 1, construct of new IPN structure. In the literature,
GAP was used directly, which will cure with isocynate, compete
with HTPB. If the terminal hydroxyl group of GAP was blocked by
O-acylation, it would not react with isocyante, and only pendant
azido group of GAP reacted with dialkyne. 2, layer by layer curation
of Acyl-GAP and HTPB with different curing agents can improve the
mechanical strength as well as thermal properties with IPN
formation.
0
vacuum dried for 3 hours at 80 C. Isophorone diisocyanate (IPDI)
having average molecular weight 222.2 and 9.0009 mmol of
NCOper gram of IPDI was taken from Beijing chemical plant. Dibutyl
tindilurate (DBTDL) 0.5 % solution in diisooctyl sebacate (DOS) was
used and taken from Beijing chemical plant also. Dimethyl malonate
and propargyl bromide (80 wt. % in toluene, ca. 9.2 molL-1) were
purchased from Aladdin Corporation, Shanghai China. Sodium
hydride (70 wt. % dispersion in mineral oil) and ammonium chloride
were also purchased from the same. Tetrahydrofuran (THF, AR),
petroleum ether, ethyl acetate (EtOAc) and an anhydrous
magnesium sulphate were purchased from Sinopharm Chemical
Reagent Co. Ltd. (China). Dichloromethane, 4-Dimethyl amino
pyridine (DMAP) and Triethylamine were purchased from the
Sigma-Aldrich and used as received.
We first synthesized the Acyl-GAP by acetylation of GAP with acetic
anhydride catalyzed by 4-(dimethyl amino) pyridine (DMAP). The
reason for end capping of terminal hydroxyl groups of GAP with
acetyl groups is to allow the highly selective curation through azido
groups with dipolarophile curing agent like DDPM. DDPM42 was also
Synthesis of Acyl-Terminated GAP
Acyl-GAP was synthesized by the reported procedure somewhere
else with some modification.43 Approximately to a solution of GAP
(2.216 g, 1.15 mmol) and acetic anhydride (0.21 ml, 2.29 mmol) in
dry CH2Cl2 (3 mL) was added triethylamine (0.32 ml, 2.29 mmol) and
1
synthesized and characterized by FT-IR, H NMR, 13C NMR and GPC.
In situ polymerization technique was adopted in order to prepare
the Acyl-GAP/HTPB IPNs. Triazole cross-linked network of Acyl-GAP
was obtained by the reaction of azido group with DDPM while
polyurethane network was prepared by the reaction of hydroxyl
group of HTPB with isocyanate group of Desmodur N-100
polyisocyante and isophorone diisocyanate (IPDI) mixed curative
system as shown in scheme 1. IPNs with different weight ratios of
Acyl-GAP and HTPB have been tried; 50:50 weight ratio results the
excellent mechanical properties. Thermal studies depict the inward
shifting of Tg and more thermally stable networks. SEM studies also
revealed the entanglements of networks in HTPB-PU/Acyl-GAP
triazole system.
o
DMAP (61 mg, 0.50 mmol) in dry CH2Cl2 (2 mL) drop wise at 0 C.
The resulting reaction mixture was stirred at room temperature
overnight. The reaction was quenched by adding water, solvent was
removed in vacuum and residue was dissolved in EtOAc (20 mL),
which was washed with water and 0.1 N HCl. The aqueous layer was
extracted with EtOAc 3 times; combined organics were dried (over
MgSO4) and concentrated to give the Acyl-GAP (94 %). Figure 1-C
shows the FTIR spectrum and Figure S1 and S2 (Supporting
Information) show the 13C NMR and 1H NMR of the Acyl-GAP
respectively.
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Scheme 2: Schematic illustration for the reaction of HTPB with IPDI
and N100.
Synthesis of Dimethyl 2, 2-di (prop-2-ynyl) malonate (DDPM)
A solution of dimethyl malonate (2.5 mL) in 20 mL of THF was
added drop wise into the three-neck round bottom flask equipped
with reflux condenser, thermometer and magnetic stirrer already
contained solution of sodium hydride (1.71g) in 20 mL of THF. The
whole mixture reacted for one hour at room temperature and then
solution of propargyl bromide (5.1 mL) in 50 mL of THF were added
o
drop wise and allowed to reflux for 3 hours at 110 C.42 Organic
layer was extracted with the 50 mL of saturated ammonium
chloride and residual organic layer was extracted with 50 mL of
diethyl ether three times each. The combined organic were dried
over anhydrous MgSO4 and after rotary evaporation, a light yellow
solid was obtained which was recrystallized with petroleum ether/
EtOAc (volume ratio 4:1) to get the white solid purified product
(yield 82.5 %). Figure 1-D shows the FTIR spectrum, Figure S3 and S4
show the 13C NMR and 1H NMR of the DDPM respectively.
Scheme 3: Schematic illustration for the synthesis of Acyl-GAP and
reaction of Acyl-GAP with DDPM
Acyl-GAP weight proportions (90, 70, 50, 30, 10 wt. %) with respect
to HTPB. The required amount of Acyl-GAP and DDPM were
dissolved in THF, HTPB along with IPDI and N100 were also added
into the reaction mixture. Finally the DBTDL and Cu (I) were added
and the mixture was mechanically stirred under nitrogen
atmosphere at room temperature for 30 minutes. The resultant
mixture was condensed by rotating evaporation and casted into the
Teflon coated mold. All IPNs films were vacuum-dried and cured at
Preparation of single networks
o
All the reagents were dried overnight in a vacuum oven at 60 C
before use. Polyurethane network were prepared by mixing the
stoichiometric amount of HTPB, IPDI and N100 in a beaker followed
o
by degassing in a vacuum oven at 40 C and reaction flow chart is
o
shown in scheme 2. The equivalent ratio of NCO/OH was used as
1.1 and IPDI/N100 weight ratio was 1.0. The required amount of
DBTDL (0.3 %) was added as a curing catalyst. The mixture was
finally degassed in a vacuum oven before poured into a Teflon
coated mold. The mold was finally cured at 60 oC for 4 days.
60 C for 4 days. In this study, a series of experiments have been
performed in order to set the catalytic amount for triazole and
polyurethane network formation. Finally 0.3
% of DBTDL
concentration for polyurethane network and 0.1 wt. % of Cu (I) for
triazole cross-linked network were selected.
For the preparation of triazole cross-linked network, required
amount of Acyl-GAP and DDPM were dissolved in THF along with
Cu (I) 0.1 wt % which acts as a curing catalyst. Mixture was
mechanically stirred under nitrogen atmosphere at room
temperature for 30 minutes and reaction flow chart is shown in
scheme 3. The mixture was condensed in a rotary evaporator and
followed by degassing in a vacuum oven. The reaction mixture was
Characterization
Fourier transform infrared spectroscopy (FTIR) measurements were
recorded (Nicolet FTIR-8700, Thermo) in the range of 4000-500
1
cm-1, H NMR and 13C NMR spectra of Acyl-GAP and DDPM were
obtained out using a Varian mercury-plus 400 MHz nuclear
magnetic resonance (NMR) spectrometer in CDCl3 solution using
tetramethylsilane as internal standard. Gel permeation
chromatography GPC was carried out on a (GPC; LC-20A, Shimadzu)
o
casted into a Teflon mold and cured at 60 C for 2 days. Weight
ratios of DDPM from 5 to 15 % were used against Acyl-GAP. All
triazole cross-linked networks were prepared by controlling the
ratio of dipolarophile containing alkynes based on the amount of
azido groups.
o
at 40 C using THF as the mobile phase and the flow rate was 1
mL/min. Polystyrene was used as standard for calibration.
Differential scanning calorimetric (DSC) analysis was carried out on
Mettler Toledo DSC1 over a temperature range of -100 to 50 C
with heating rate of 10 C/min under nitrogen flow of 40 mL/min.
2-3 mg of each sample was taken in small aluminum cell. Thermo
gravimetric analysis was performed using TGA analyzer
(TGA/DSC1SF/417-2, Mettler Toledo) with heating rates of
10 oC/min under nitrogen flow of (40 mL/min) from room
temperature to 600 oC. The mechanical parameters, including
o
Synthesis of interpenetrating polymer network (IPNs)
o
The Acyl-GAP/HTPB IPNs were synthesized by an in situ
polymerization method where all the reactants, curing agents and
catalysts were mixed together (same proportions for each network
as indicated in a single network preparation). A series of Acyl-
GAP/HTPB IPNs were prepared by changing the relative amount of
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tensile strength (σb) and elongation at break (εb) of all the dumb
bell shape samples were measured on universal testing machine
(Instron-6022, Shimadzu Co., Ltd.) with a constant rate of 100
mm/min and mean values of five samples were taken. Scanning
electron microscope (SEM) S-4800 (Hitachi) was used in order to
study the surface morphology. All samples were fractured with
liquid nitrogen and after gold sputtering, microphotographs were
taken.
The number average molecular weight (Mn), weight average
molecular weight (Mw) and poly dispersity index values
(PDI= Mw/Mn) of the GAP and Acyl-GAP are shown in table S1. The
Mn of GAP and Acyl-GAP was obtained 4783 and 4825 g/mol
respectively while the Mw were 7193 and 7331 g/mol respectively.
A PDI value of GAP and Acyl-GAP was achieved 1.504 and 1.5193
respectively. Increase in average molecular weight was due to
partial extension in chain due to acetylation of GAP.
The impact sensitivity as well as the friction sensitivity of Acyl-GAP,
PU, triazole and IPN’s were measured. The impact sensitivity was
measured with a 5 kg drop weight using a BAM drop weight
apparatus. The results are based on tests on both sides of the 50 %
probability level using the up-and-down method. The friction
sensitivity was measured with a BAM friction apparatus by applying
a thin film to a ceramic tile and lowering the friction apparatus arm.
The weight was placed on the outer groove of the arm, by pressing
the button, dragging ceramic pellet across the sample. The sample
was observed for any sign of ignition, spark or smoke and repeated
six times by changing the mass. The results of these tests are given
in Table S4 (Supporting information).
Figure 1: FTIR spectra of (A) HTPB, (B) GAP, (C) Acyl-GAP, (D) DDPM,
(E) Acyl-GAP/DDPM, (F) Acyl-GAP/HTPB cured film.
In situ FT-IR kinetic studies of Acyl-GAP/DDPM and HTPB/IPDI-
N100
In order to get comprehensive information regarding reaction
mechanism, in situ FTIR kinetic study was carried out. Figure S5
(Supporting information) shows the reaction kinetics of
polyurethane and triazole network formation in terms of
characteristic peaks conversion with respect to time, HTPB network
formation kinetics was followed by monitoring the change in
intensity of absorption band of NCO stretching at 2258 cm-1 and CO
stretching at 1730 cm-1. Variations in the height of peaks were
calculated with Omnic software. NCO stretching band decreases
and finally disappears upon completion of reaction. New absorption
peaks at 1731cm-1 and 1508 cm-1 assigned to CO and NH stretching
appeared due to polyurethane formation. While triazole cross-
linked network kinetics was followed by observing the change in
intensity of ≡CH stretching of alkynyl groups at 3291 cm-1 and
appearance of new peaks around 1620 cm-1 and 1555 cm-1 due to
–N=N and –C=N respectively,42 however the peak at 2102 cm-1 due
to –N3 group remains in the spectrum due to high concentration.
Results and Discussion
Acyl-GAP was synthesized by the acetylation of GAP with DMAP,
triethylamine and acetic anhydride in CH2Cl2. Figure 1 shows the
FTIR spectrum of GAP, Acyl-GAP, Acyl-GAP/DDPM cured film and
Acyl-GAP/HTPB cured film. Figure 1-B depicts that the main
characteristic peaks at about 1283 and 2102 cm-1 corresponds to
the -N3 of GAP, asymmetric and symmetric -CH2 and -CH stretching
peaks between 2926 and 2874 cm-1 while broad intense stretching
vibrations of OH and C-O-C of GAP at about 3300-3500 cm-1 and
1120 cm-1 respectively. Figure 1-C shows the FTIR spectrum of Acyl-
GAP, compared with the spectrum of GAP 1-B, besides the main
peaks, a new strong band at 1740 cm-1 clearly specifies the C=O
stretching frequency and disappearance of broad OH peak after
acetylation of GAP.
As shown in Figure 1-D and E, the peak located at 3291 cm-1 which
was assigned to ≡CH stretching vibraꢀon of alkynyl group of DDPM
disappeared in the Acyl-GAP/DDPM cured film, however the peak at
2102 cm-1 as a result of azido groups remains in the spectrum due
to excess in nature. FTIR spectrum of Figure 1-A and F shows HTPB
and Acyl-GAP/HTPB cured film. As shown in Figure S2 (Supporting
In situ FTIR kinetic studies show that almost 80 % triazole and
polyurethane network formation takes place within 20 hours and
Figure S5 demonstrates the reaction conversion-time profile. Rate
of conversion is very fast and after 80 % conversion, polyurethane
reaction slows down and rate of diffusion is greatly reduced and
almost 100 % conversion completed in four days. While triazole
reaction is very fast as compared to the urethane and 100 %
conversion took place in less than 50 hours. This may be due to
catalytic amount of Cu (I) and excess of –N3 groups which favours
the completion of reaction in short time. The observations as a
result of in situ kinetic studies of PU and triazole network formation
are very important for defining the IPN conditions.
1
information), H NMR spectrum of Acyl-GAP has a peak at around
3.8 ppm due to protons of polyether main chain of GAP attributed
to -CH2, -CH and 3.4 ppm ascribed to the -CH2N3. New peak
observed after acetylation at around 2.11 ppm is attributed to CH3.
Figure S1 (Supporting information), depicts 13C NMR spectrum of
Acyl-GAP, appearance of new peak at 163 ppm is ascribed to the
carbonyl carbon and peak at 22 ppm is attributed to the –CH3 of the
1
acetyl group. H NMR spectrum of DDPM presented three peaks
Cross-linking Densities and Swelling Behaviours of the Acyl-
GAP/DDPM Materials
located at 2.1 ppm (2H, c), 3.02 ppm (4H, b) and 3.76 ppm assigned
to –CH, -CH2 and –CH3, while 13C NMR spectrum depicts five
characteristics peaks at (c) 23.1ppm, (a) 53.6 ppm, (d) 72.07 ppm,
(e) 77.46 ppm and (b) 169.4 ppm.
Cross-linking density and the swelling behaviour are important
factors to determine the mechanical performance of the material.
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Figure 2: Effect of weight ratio of DDPM vs Acyl-GAP on the tensile
Figure 3: Effect of weight ratio of % Acyl-GAP vs HTPB on the tensile
strebgth (σ) and breaking elongation (εb)
strebgth (σ) and breaking elongation (εb)
By changing the weight ratio of Acyl-GAP to DDPM, cross-linking
densities were examined. Figure S6 (Supporting information)
depicts the effects of weight ratio of Acyl-GAP to DDPM on the
swelling degree and cross-linking densities of the as prepared Acyl-
GAP/DDPM films. Generally increase in cross-linking points inhibits
the swelling behaviour. With the increase in weight ratio of Acyl-
GAP to DDPM from 5 to 15 %, cross-linking density increased from
0.42 to the maximum value of 0.78 while the swelling degree
decreased from 37 % to the minimum value of 4 %. The weight ratio
effect of Acyl-GAP to HTPB on cross-linking density is shown in
figure S7. With increase in weight ratio, cross-linking density
increase from 0.42 to 0.80 and then decreased. At 50 % of Acyl-
GAP, cross-linking density is maximum, beyond this; cross-linking
density goes down and reaches up to 0.57 due to maximum steric
hindrance. Generally higher cross-linking density results in higher
tensile strength and lower elongation at break.33
breaking elongation during in situ polymerization has been
observed. It is interesting to report that with 50 % of Acyl-GAP in
Acyl-GAP/HTPB IPNs, highest tensile strength of 5.26 MPa with 318
% elongation achieved and this may be ascribed due to the
permanent entanglements (catenation) of both cross-linked
networks. Due to enhanced interlocking of urethane and triazole
networks, chain flexibility is highly restricted and elongation at
break is significantly reduced. Beyond 50 % Acyl-GAP, the flexibility
of Acyl- GAP is inhibited by the –N3 groups of the side chain and
tensile strength and elongation at break progressively drops up to
2.69 MPa and 86 % respectively in pure triazole network. These
might be the possible reasons for increase in tensile strength and
decrease of elongation at break with the increase of Acyl-GAP. IPNs
with more cross-linked structure and stiffness possess higher tensile
strength than that of PU and triazole networks individually.
Thermal Studies
Mechanical Properties
Figure 4 depicts the DSC thermo grams of pure PU, triazole and
different weight ratios of Acyl-GAP/PU while table 1S (Supporting
information) summarized the glass transition temperatures (Tg).
The weight ratio effects of Acyl-GAP to DDPM on tensile strength
(σb) and breaking elongation (εb) of the Acyl-GAP/DDPM films are
shown in Figure 2. Increase in weight ratio of DDPM to Acyl-GAP,
tensile strength (σb) gradually increased from 0.75 to 4.54 MPa
while the breaking elongation initially increased up to 86 % and
then decreased. This decrease in breaking elongation is due to the
higher cross-linked density.42 As the weight ratio of the DDPM to
Acyl-GAP increases from 5 % to 15 %, more and more alkynyl
groups took part in triazole linkages and this increase of cross-
linking points restricts the deformation, as a result of that tensile
strength increased with an expense of breaking elongation. Figure 2
shows that optimum mechanical properties achieved by using 10 %
of the DDPM and beyond this ratio although the tensile strength
increases but the elongation decreases dramatically due to stiffer
network associated to azido side linkage.
o
The Tg of PU and triazole networks was found to be -77.5 C and
o
-34.5 C respectively. With the increase in weight ratio of Acyl-GAP
up to 30 %, only one Tg is observed and also slightly increased from
-77.5 oC to -72.5 oC, this may be due to interlocking of polymer
networks and shortening of space among cross-linked points.
Although thermo gram with 50:50 % Acyl-GAP/HTPB weight ratio,
shows two glass transition temperatures at -75.3 oC and -36.9 oC
Figure 3 depicts the dependence of the mechanical parameters,
including tensile strength (σb) and elongation at break (εb) of the
Acyl-GAP/HTPB IPNs. Pure urethane network shows the tensile
strength (σb) and breaking elongation (εb) 1.28 MPa and 464 %
respectively, while triazole cross-linked network shows the tensile
strength (σb) and breaking elongation (εb) 2.69 MPa and 86 %
respectively. Figure 3 also indicates that by increasing the weight
ratio of the Acyl-GAP, tensile strength first increased from 1.28 MPa
to 5.26 MPa and then steadily decreased up to 2.69 MPa, but the
breaking elongation gradually decreased from 464 % to 86 %, a
substantial increase in the tensile strength and decrease in the
Figure 4: DSC thermo grams of PU, (A) 10 %, (B) 30 %, (C) 50 %, (D)
70 %, (E) 90 % Acyl-GAP and Triazole
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this may be due to phase separation but auspiciously having
synergistic mechanical properties. Same trend of inward shift of Tg
can be seen from 70 % Acyl-GAP to triazole network.
TGA/DTG Analysis
One of the most important tools in the propellant industry is
thermal stability of the polymer products. Thermal decomposition is
directly related with important performance parameters like heat
of explosion, detonation energy and detonation velocity.44,45 Figure
5 depicts the TGA thermo gram of polyurethane, triazole and
PU/Triazole networks. TGA scan of triazole network shows weight
loss in two stages. The first stage decomposition of triazole takes
o
place in the temperature range of 190
38
stagedecomposition involves the degradation of polyether main
̶
270 C with a weight loss of
%
due to liberation of nitrogen,17,21 while the second
o
chain of Acyl-GAP in the temperature range of 270 ̶ 464 C. The
residue left behind after the decomposition is around 29 %.31
Figure 6: DTG curves of PU, Triazole and different ratios of Acyl-
HTPB cross-linked network decomposition undergoes in two stages
with indistinct separation. The first stage decomposition is in the
GAP/HTPB
o
range of 210
depolymerization, cyclization and partial decomposition of cyclized
products. Second stage decomposition is in the range of 415
490 oC
̶
415 oC with a mass loss of 21 % due to
between TGA analyzer and FTIR were heated to 220 C. Primary
evolved gases are due to the decompositions of triazole and
polyurethane cross-linked networks. The gas products at 250 oC
were identified as CO2 (2380 and 668 cm-1), N2O around (2240 cm-1)
and HCN (712 cm-1). In addition, the emissions of CH2O, C2H4O or
̶
due to the dehydrogenation and decomposition of the remaining
cyclized products.46-48 Residue left behind after the complete
decomposition is almost 1 %. From thermo gravimetric curves, a
better thermal stability can be seen with increasing the weight ratio
of Acyl-GAP in PU/Triazole networks. Peak decomposition
temperatures of Acyl-GAP, PU, triazole and IPN’s are shown in table
S3. It can be observed from DTG curves (Fig. 6) that during the first
o
their mixtures (2750-3000 cm-1) were also identified at 460 C. FTIR
spectroscopy does not detect the N2 and H2 due to symmetric
nature of these gaseous products.
Morphological Studies
and second stage, peak decomposition temperatures shifted from Figure 7 depicts the morphologies of fractured surfaces of PU,
o
248 C to 253 oC and from 458 oC to 462 oC respectively. Higher the triazole and HTPB-PU/Acyl-GAP triazole cross-linked networks.
cross-linked network, more energy is required for the Morphological feathers of polyurethane and triazole cross-linked
decomposition,33 this trend clearly shows the evidence for thermal networks are entirely different. Wrinkles and ravines of the
stability and compact network formation due to interpenetration fractured surface are prominent in triazole network. SEM
micrograph 7-A clearly shows the smooth and glassy
microstructures because it was brittle material due to the rigid
cross-linked triazole network whereas the micrograph 7-B of the
polyurethane exhibits the rough microstructures as compared to
the triazole. From the cross sectional surfaces of HTPB-PU/Acyl-GAP
Triazole (7-C and 7-D), formation of IPN is vividly seen by the
existence of intricate entanglements, wherein, the polymer chains
appear to penetrate inward and outward over one another in the
polymer matrix showing good compatibility.49 We also observed
micro to nanometer thick long fibers which are crossed and
entangled with each other. The inwardly shifting of Tg in DSC and
superior mechanical properties of HTPB-PU/Acyl-GAP triazole
networks as described earlier is the manifestation of entanglements
of both networks.50
and physical entanglement of networks.
TG-FTIR Spectroscopic Studies
To elucidate the thermal decomposition pattern of PU/Triazole
cross-linked networks, TG-FTIR is used to identify the constituents
of gas products. The 3-D TG-FTIR of the decomposition products are
shown in the Figure S8 (Supporting information), while FTIR spectra
of the respected gases are presented in the Figure S9. The gas lines
Conclusion
A novel approach has been adopted in order to fabricate the
energetic binder with improved mechanical and thermal properties.
Acyl-GAP and DDPM were efficiently synthesized and characterized
for triazole curing system and different weight ratios used to get
the adequate mechanical characteristics. A series of HTPB-PU/Acyl-
GAP triazole cross-linked networks containing 90, 70, 50, 30 and 10
wt. % of Acyl-GAP were synthesized via an “in situ” process in which
all ingredients were mixed together and the networks were formed
independently. The optimum mechanical strength and elongation
were achieved with 50:50 HTPB-PU/Acyl-GAP triazole weight ratios.
Figure 5: TGA curves of PU, Triazole and different ratios of Acyl-
GAP/HTPB
6 | J. Name., 2012, 00, 1-3
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