Azide and Alkyne Terminated Polybutadiene Binders: Synthesis

Article pubs.acs.org/IECR

Azide and Alkyne Terminated Polybutadiene Binders: Synthesis, Cross-linking, and Propellant Studies S. Reshmi,*,† E. Arunan,‡ and C. P. Reghunadhan Nair† †

Polymers and Special Chemicals Group, Vikram Sarabhai Space Centre, Thiruvananthapuram 695022, Kerala, India Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, Karnataka, India

‡

S Supporting Information *

ABSTRACT: In composite solid propellants, the fuel and oxidizer are held together by a polymer binder. Among the different types of polymeric binders used in solid propellants, hydroxyl terminated polybutadiene (HTPB) is considered as the most versatile. HTPB is conventionally cured using isocyanates to form polyurethanes. However, the incompatibility of isocyanates with energetic oxidizers such as ammonium dinitramide and hydrazinium nitroformate, the short “pot life” of the propellant slurry, and undesirable side reactions with moisture are limiting factors which adversely affect the mechanical properties of HTPB based propellant. With an aim of resolving these problems, HTPB was chemically transformed to azidoethoxy carbonyl amine terminated polybutadiene and propargyloxy carbonyl amine terminated polybutadiene by adopting appropriate synthesis strategies and characterizing them by spectroscopic and chromatographic techniques. This is the first report on 1,3-dipolar addition reaction involving azide and alkyne end groups for cross-linking HTPB. The blend of these two polymers underwent curing under mild temperature (60 °C) conditions through 1,3-dipolar cycloaddition reaction resulting in triazole−triazoline networks. The curing parameters were studied using differential scanning calorimetry. The kinetic parameter, viz., activation energy, was computed to be 107.6 kJ/mol, the preexponential factor was 2.79 × 1012 s−1, and the rate constant at 60 °C was computed to be 3.64 × 10−5 s−1.The cure profile at a given temperature was predicted using the kinetic parameters. Rheological studies revealed that the gel time for curing through the 1,3-dipolar addition is 280 min compared to 120 min for curing through the urethane route. The mechanical properties of the resultant cured polybutadiene network were superior to those of polyurethanes. The cured triazoline−triazole polymer network exhibited biphasic morphology with two glass transitions (Tg) at −56 and 42 °C in contrast to the polyurethane which exhibited a single transition at −60 °C. This was corroborated by associated morphological changes observed by scanning probe microscopy. The propellant processed using this binder has the advantages of improved pot life as indicated by the end of the mix viscosity which is 165 Pa·s as compared with 352 Pa·s for the polyurethane system along with a slow build-up rate. The mechanical properties of the propellant are superior to polyurethane with an improvement of 14% in tensile strength, 22% enhancement in elongation at break, and 12% in modulus.

1. INTRODUCTION Large composite solid propellant grains, or rocket motors in particular, demand adequate mechanical properties to enable them to withstand the stresses imposed during operation, handling, transportation, and motor firing. They should also have a reasonably long “pot life” to provide a sufficient window for processing operations such as mixing and casting which makes the selection of binder with appropriate cure chemistry more challenging. In all composite solid propellants currently in use, polymers perform the role of a binder for the oxidizer, metallic fuel, and other additives. It performs the dual role of imparting dimensional stability to the composite and provides structural integrity and good mechanical properties to the propellant. Hydroxyl terminated polybutadiene (HTPB) is the most popular hydrocarbon binder used in composite solid propellants1−10 which is normally cured by reaction with diisocyanates such as tolylene diisocyanate (TDI) or isophorone diisocyanate (IPDI) to form polyurethane networks. However, this reaction is highly susceptible to spurious reaction with moisture, leading to deterioration in the properties of the propellant.10,11 In addition, the high reactivity of the isocyanate group limits the pot life of the propellant. The inherent incompatibility of isocyanates with energetic oxidizers such as © 2014 American Chemical Society

ammonium dinitramide (ADN) and hydrazinium nitroformate (HNF) also warrants new cure methodologies to be evolved for processing high energy propellants using HTPB as binder. Several reports exist on the modification of HTPB11−14 such as grafting of energetic groups such as poly(glycidyl azide),15 anchoring of iron pentacarbonyl, 1 6 grafting of 2(ferrocenylpropyl)dimethylsilane (FPDS), and functionalization by attaching polyazido groups through cyanuric chloride, etc.17−21 Most of these are aimed at improving the ballistic performances of HTPB based propellants. A comprehensive approach of achieving improved processability and superior mechanical properties for the propellant without compromising its ballistics is essential to meet the future requirements. 1,3-Dipolar cycloaddition reaction of an organic azide with an alkyne (Huisgen reaction) resulting in triazoles is a versatile tool in polymer chemistry for realizing cross-linked networks in good yield without any side reactions.22−28 There have been a few reports on azide−alkyne reactions for cross-linking glycidyl Received: Revised: Accepted: Published: 16612

May 25, 2014 August 7, 2014 September 14, 2014 September 15, 2014 dx.doi.org/10.1021/ie502035u | Ind. Eng. Chem. Res. 2014, 53, 16612−16620

Industrial & Engineering Chemistry Research

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Scheme 1. Synthesis Scheme for PrTPB

Scheme 2. Synthesis Scheme for AzTPB

azide polymer using different types of alkyne reagent such as bispropargyl succinate (BPS), 1,4-bis(1-hydroxypropargyl)benzene (BHPB), and bisphenol A bis(propargyl ether) (BABE), etc.,29−35 synthesis of various types of triazole binders and evaluation of their mechanical properties,36,37 a partly alkyne functionalized polymer cure system based on hyperbranched copolymers of 3-ethyl-3-(hydroxymethyl)oxetane (TMPO) and tetrahydrofuran (THF)38 and alkyne terminated HTPB.39 The poor compatibility of alkyne terminated HTPB and azide containing polymers due to the polar nature of azide groups and the nonpolar nature of the HTPB backbone has also been reported by Ding et al.,39 and hence, the phase separation aspects also need to be addressed while undertaking the functional modification of HTPB. In the present work we even report a novel approach for functionalization of HTPB to derive azidoethoxy oxy carbonyl amine terminated polybutadiene (AzTPB) and propargyloxy carbonyl amine terminated polybutadiene (PrTPB) by chemical transformation of the hydroxyl groups. The blend of the

resultant polymers, viz., ATPB and PrTPB, were subsequently cross-linked via 1,3-dipolar addition reaction without any phase separation. This work describes the synthesis, characterization, cure studies, mechanical, dynamic mechanical, and morphological characteristics of the cross-linked polymers, and propellant level studies. The novelty of this work lies in exploring the well-known 1,3-dipolar reaction for cross-linking of HTPB, resulting in propellants with improved processability and superior mechanical properties without risking the ballistic properties.

2. EXPERIMENTAL SECTION 2.1. Raw Materials. Hydroxyl terminated HTPB was synthesized by a proprietary process of Vikram Sarabhai Space Centre (VSSC). The HTPB used has a molecular weight (Mn) ∼2800 g/mol and hydroxyl value of 41.3 mg of KOH/g. This provides an average functionality of 2.2. Tolylene diisocyanate (2,4:2,6 isomer in the ratio 80:20) was procured from Bayer India Ltd. Propargyl alcohol, 2-(2-chloroethoxy)16613

dx.doi.org/10.1021/ie502035u | Ind. Eng. Chem. Res. 2014, 53, 16612−16620


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0.4 mm, and the tests were performed at a temperature of 30 ± 2 °C at a cross-head speed of 50 mm/min. Rheological analysis was done using a Bohlin Gemini 2 rheometer using a 20 mm parallel plate assembly in oscillation mode at a frequency of 1 Hz and controlled strain of 1%. The gap between the plates was maintained as 0.5 mm. The isothermal experiments were done by measuring the storage (G′) and loss modulus (G″) at different time intervals at 60 °C. Dynamic mechanical analysis (DMA; TA Instruments, Q800) was carried out a heating rate of 3 °C/min over a temperature range of −100 to 70 °C at fixed frequency of 1 Hz using rectangular specimens of dimension of 5 × 3 × 2 mm3. The morphological studies of the sample were carried out using an Agilent 5500 scanning probe microscope with a scan size of 90 × 90 μm2 and a scan speed of 2.0 lines/s in contact and tapping mode. 2.5. Methods. 2.5.1. Determination of Cross-Link Density. To evaluate the cross-link density, the cured polymer samples were cut into pieces of approximately 5 × 5 × 5 mm3 sizes and soaked in toluene for 72 h. The soaked sample was weighed after 72 h after the solvent was gently wiped off. From the swell ratio, the cross-link density was calculated using the Flory Rehner equation.43 2.5.2. Propellant Processing. Propellant batches were processed in a 1 kg scale in a Guitard horizontal mixing system at 40 °C and with an average mixing time of 3 h. The typical solid propellant formulation consisting of PrTPB− AzTPB as binder (azide:alkyne ratio of 0.85:1) in THF solvent and aluminum as metallic fuel (2% by weight) and ammonium perchlorate as an oxidizer content of 77% by weight was chosen. For comparison, HTPB with TDI as curing agent (isocyanate:hydroxyl ratio of 0.85:1) was also processed in the same manner. End of mixing (EOM) viscosity and build-up values were measured using a Brookfield viscometer. The samples were cured at 60 °C for 5 days. The cured propellant samples were tested for mechanical properties. The burn rates were measured using acoustic emission technique at an operating pressure of 6.9 MPa.

ethanol, sodium azide, propargyl bromide, and dibutyltin diluarate (DBTDL) were obtained from M/s Aldrich. Methanol, toluene, and tetrahydrofuran of high purity (AR grade) from M/s Merck were used. 2.2. Synthesis. 2.2.1. Synthesis of Isocyanate Terminated HTPB. The isocyanate terminated prepolymer of HTPB (ITPB) was synthesized based on a reported procedure40 by reacting HTPB with excess TDI. In a typical reaction, 10 g (0.004 mol) of HTPB containing catalyst (a few drops of DBTDL) was added dropwise under nitrogen purging to 1.21g (0.007 mol) of TDI (isocyanate−hydroxyl molar ratio, 2:1) at 40 °C and was kept stirred for 5 h. The isocyanate content of 2.7% in the polymer41 corresponds to the theoretical value. 2.2.2. Synthesis of Propargyloxy Carbonyl Amine Terminated Polybutadiene. The PrTPB was synthesized by reacting ITPB with propargyl alcohol (Scheme 1). About 6 mL (0.107 mol) of propargyl alcohol was added drop by drop to 10 g (0.004 mol) of ITPB. The reaction was carried out in bulk in the presence of DBTDL as catalyst at 80 °C for 4 h (in nitrogen atmosphere) under magnetic stirring. The product was dissolved in THF and precipitated into excess methanol. The product was washed with methanol and dried under reduced pressure at 60 °C for 3 h. Yield ∼ 89%. 2.2.3. Synthesis of Azidoethoxy Carbonyl Amine Terminated Polybutadiene. AzTPB was obtained by reacting ITPB with 2-(2-azidoethoxy)ethanol (Scheme 2). For this, 2-(2azidoethoxy)ethanol was synthesized as per a reported procedure.42 For the synthesis of AzTPB, about 6 g (0.046 mol) of 2-(2-azidoethoxy)ethanol was dissolved in 20 mL of toluene and was added drop by drop to the previously prepared ITPB (10 g, 0.004 mol). The reaction was carried out at 60 °C for 5 h in the presence of DBTDL as the catalyst. The mixture was dissolved in THF, and the product was isolated by precipitating in excess methanol, washing with methanol, and drying under reduced pressure at 60 °C for 3 h. Yield ∼ 84%. 2.3. Curing of the Polymers. The polymers, viz., PrTPB and AzTPB, were cured at various azide:propargyl molar ratios of 0.75:1, 0.85:1, and 1:1 without any catalyst. The mixtures were then cast into sheets in aluminum molds, and the cure reaction was carried out at 60 °C for a period of 5 days. For comparison, HTPB−TDI urethane (molar ratios of 0.75:1, 0.85:1, and 1:1 with respect to isocyanate to hydroxyl concentration) was also prepared. 2.4. Instrumentation. Fourier transform infrared (FTIR) spectra were recorded on a PerkinElmer Spectra GXA FTIR spectrometer in the range of 650−4000 cm−1 using NaCl plates. 1H NMR spectra were recorded using a Bruker Avance (300 MHz) spectrometer in CDCl3 solvent. The molecular weight distribution was determined using gel permeation chromatograph (GPC, Waters model 600) in conjunction with a differential refractive index detector, Waters HR3 and HR4 microstyragel columns, and THF as the solvent. Curing reaction was monitored using TA Instruments Q20, differential scanning calorimeter (DSC) Mechanical properties, viz., tensile strength, elongation at break, and modulus, of the cured polymers and propellants were evaluated using Universal Testing Machine (INSTRON Model 4469) at a cross-head speed of 500 mm/min at 27 ± 2 °C. Dumbbell specimens conforming to ASTM-D-412 (equivalent to IS3600) were used for these tests. The sample size was an overall length of 110 mm, width (at ends) of 25 ± 1 mm, the length of the narrow parallel portion of 33 ± 2, grips at 60 ± 2 mm, and the width of the narrow parallel portion of 6 ±

3. RESULTS AND DISCUSSION 3.1. Characterization of the Polymers. Azide and propargyl end functional polybutadiene were synthesized by the chemical transformation of terminal hydroxyl groups of HTPB and were characterized. FTIR analysis of ITPB confirmed the presence of NCO group due to the absorption at 2270 cm−1 (Figure 1c). The other characteristics absorptions are N−H (stretching) of the urethane group at 3368 cm−1and -CO peaks of urethane, at 1740 cm−1. The characteristic peaks of polybutadiene at 966 cm−1 for 1,4-trans, 911 cm−1 for 1,2-vinyl, and 724 cm−1 for 1,4-cis are also observed. In the case of PrTPB (Figure 1a) a sharp peak centered at 3305 cm−1 corresponds to the C−H (stretching) of the propargyl group (-CC−C−H) and disappearance of NCO absorption at 2270 cm−1, as described in Figure 1c, confirmed the anchoring of the alkyne groups. In AzTPB, the peak at 2108 cm−1 indicated the presence of azide groups in addition to the disappearance of NCO absorption (Figure 1b). The characteristic peaks of polybutadiene remained unchanged in both polymers. The FTIR data confirm the complete conversion of ITPB to PrTPB and AzTPB. GPC traces of HTPB, PrTPB, and AzTPB are given in Figure 2. The calculated Mn corrected for hydrodynamic volume are 3450, 6330, and 7460. While, weight average molecular weights (Mw) are 8530, 20060, and 24720, and the polydispersity 16614



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AzTPB to higher molecular weights in comparison to HTPB is due to partial chain extension during TDI coupling. 3.2. Cure Optimization. 3.2.1. DSC Analysis. PrTPB reacts with AzTPB to yield triazoles (Scheme 3). Since HTPB is Scheme 3. Curing of Mixture of PrTPB with AzTPB

known to contain polyhydroxyl functional groups,44,45 its derivatives are also expected to contain polyfunctional groups (propargyl and azide), accounting for higher cross-linking. The cure reaction was monitored by nonisothermal DSC analysis at a heating rate of 10° C/min in nitrogen atmosphere. DSC shows that the cure reactions of PrTPB with AzTPB occur in the temperature range of 70−165 °C with an enthalpy change of 75 ± 2 J/g. This is followed by decomposition of the residual azide in the temperature range of 167−215 °C with an apparent enthalpy change of ∼6 J/g (Figure 3). The reaction of azide group with olefinic unsaturation is also known.46 Accordingly in this study, it is observed that AzTPB undergoes a self-curing reaction through the addition of the azide groups on to the double bond of polybutadiene, yielding triazoline. This reaction is confirmed by the DSC analysis of AzTPB, where an exotherm corresponding to the azide−ethylenic unsaturation reaction was observed in the temperature regime of 88−153 °C with an enthalpy change of ∼23 J/g. DSC also shows decomposition of azide in the temperature range of 153−215 °C with an enthalpy change of ∼91 J/g (Figure 4). The addition reaction is feasible under ambient condition as evidenced by the insolubility of the AzTPB polymer and further confirmed by the FTIR analysis of cured AzTPB. The FTIR studies indicated the disappearance of peaks due to azide groups at 2108 cm−1 and change in the appearance of peak in the range of 1599−1639 cm−1 (Supporting Information, SI). In order to avoid the addition of azide to the unstaturation, AzTPB is stored in refrigerated condition.

Figure 1. FTIR spectra of (a) PrTPB, (b) AzTPB, and (c) ITPB.

Figure 2. GPC chromatograms of (a) PrTPB, (b) AzTPB, and (c) HTPB.

indices (PDI) are 2.5, 3.2, and 3.3 respectively for HTPB, PrTPB, and AzTPB. The shift in chromatogram in PrTPB and 16615



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E computed by Kissinger method is 107.6 kJ/mol. A = 2.79 × 1012 s−1, and the rate constant at a temperature of 60 °C computed by the relation k = Ae−E/RT is 3.64 × 10−5 s−1. 3.2.3. Prediction of Isothermal Cure Time. The experimentally determined kinetic parameters including activation energy, preexponential factor, and rate constant are useful for predicting the isothermal cure profile of the system for a given temperature. This enables optimization of the cure temperature and duration for the propellant. This can be done using eq 2, relating time (t), temperature (T), and fractional conversion (α). Typically, a time−conversion prediction of a second order reaction (for isothermal cure at 60 °C) is shown in Figure 5. As

Figure 3. DSC traces of curing of PrTPB with AzTPB and decomposition of the azide group (heating rate, 10 °C/min).

Figure 5. Prediction of isothermal cure profile (at 60 °C), n = 2.

per this figure, a conversion of 95% is achieved in 70 h. This was further confirmed by FTIR, as observed by the disappearance of the azide peak at 2108 cm−1 and appearance of a peak at 1637 cm−1, due to CC of triazole, which is absent in the prepolymers, viz., PrTPB and AzTPB (SI).

Figure 4. DSC traces of self-curing of AzTPB and decomposition of the azide group (heating rate, 10 °C/min).

α = 1 − {1 − A(1 − n)t e−E / RT }1/(1 − n)

The propensity of the azide groups for addition to the double bond is lower when comapred to that for ethynyl groups.46 However, the relative concentration of double bonds is almost 100 times that of triple bonds. Hence, there is good probability for addition of azide to the olefinic unsaturation of polybutadiene resulting in cross-linking. The enthalpy calculated from the exotherm in DSC for the AzTPB−PrTPB blend corresponds to the overall enthalpy of formation of triazoline and triazole groups. Since the system is uncatalyzed, the decomposition of the azide groups initiates before the completion of the cure reaction with an exotherm appearing with a peak reaction temperature (Tm) at 190 °C. 3.2.2. Cure Kinetics. The cure kinetics was followed by the nonisothermal DSC method based on varying heating rates (Φ) of 5, 7, and 10 °C/min. The peak reaction temperatures (Tm) obtained are 137, 141, and 147 °C for heating rates of 5, 7, and 10 °C/min, respectively. The kinetics of the cure reaction was evaluated by the variable heating rate method of the Kissinger method.47 The activation energy (E) is obtained from the slope of the plot of log (ϕ/Tm2) against 1/Tm. The pre-exponential factor (A) was calculated using the relation given in eq 1. A = ϕEe E / RTm /RTm 2

(2)

where E = activation enery, A = preexponetial factor, n = order of reaction, t = time for conversion, and α = fractional conversion. 3.3. Rheological Characterization. The rheological behavior of the blend of PrTPB and AzTPB was investigated at 60 °C, and the results were compared with that of the HTPB−TDI system at the same temperature. The evolution of the storage modulus (G′) and loss modulus (G″) with reaction time is given in Figure 6 (at 60 °C). The modulii (storage and loss) increases as a result of increased cross-linking. The crossover point of loss modulus and storage modulus is considered as the gel point. For the PrTPB−AzTPB system, the gel point occurs after 280 min and for the HTPB−TDI system, it occurs after 120 min at 60 °C (cf. SI) indicating a faster rate of curing for the urethane reaction. Thus, the cure reaction invoking the isocyanate and hydroxyl group has a shorter pot life. 3.4. Evaluation of Cross-link Density. The cross-link density of the cured samples was calculated using the Flory Rehner equation, Ve = −[ln(1 − V2) + V2 + χV2 2]/V5(V21/3 − V2/2)

(1) 16616

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with the unsaturation on the backbone of polybutadiene yielding triazole−triazoline networks. Thus, the cross-links are constituted by both triazole and triazolines. 3.5. Evaluation of Mechanical Properties. The mechanical properties, viz., tensile strength (TS), elongation at break, and stress at 100%, for the cured polymers were compared with those for HTPB based polyurethanes. The tensile strength of the triazole−triazoline system increased from 0.99 to 1.52 MPa, while the elongation at break increased from 480 to 660%, and the stress at 100% elongation varied from 0.31 to 0.50 MPa as the alkyne to azide stoichiometry evolved from 1:0.7 to 1:1. The properties of the PrTPB−AzTPB systems are superior to those of HTPB−TDI (Table 2). At comparable stoichiometry, the triazole−triazoline cross-linked system provided better tensile strength and elongation (at comparable modulus) vis-avis the polyurethanes. The fracture energy49 of the cured polymers, PrTPB−AzTPB (alkyne:azide = 1:1) and HTPB− TDI (isocyanate:hydroxyl = 1:1) was computed from the stress−strain graph. The fracture energy for the PrTPB− AzTPB system is 3.33 J/cm2 which is higher than that for the HTPB−TDI system (3.03 J/cm2). 3.6. Dynamic Mechanical Characterization. DMA (Figure 7) of triazoline−triazole-network polymers for an

Figure 6. Rheogram depicting evolution of storage and loss modulii with time at 60 °C (1:1 blend of AzTPB−PrTPB).

where V2 is the volume fraction of the polymer in the swollen specimen, Vs is the molar volume of the solvent, and χ is the polymer−solvent interaction parameter. V2 is computed from V2 =

(w2|ρ2 ) {(w2|ρ2 )|(w1|ρ1)}

(4)

where ρ1 and ρ2 are the densities of solvent and polymer, w2 is weight fraction of polymer, and w1 is weight fraction of the solvent. A value of 0.43 is taken48 for χ (for HTPB−toluene interaction). The effect of triazole groups on cross-link density was neglected. In fact, the spacings between functional groups in the case of AzTPB and PrTPB are more than that of HTPB due to chain extension occurring during their synthesis. The theoretical cross-link density of the PrTPB−AzTPB cured system for a molar equivalence of 1:1 ( with respect to azide:propargyl) is 5.01 × 10−5 mol/cm3 which is lower than that of HTPB−TDI urethane (1.15 × 10−4 mol/cm3). However, a higher experimental cross-link density (Table 1) of the polybuta-

Figure 7. Tan δ and storage modulus vs temperature plot for cured PrTPB−AzTPB.

Table 1. Cross-link Densities of Cured HTPB−TDI and PrTPB−AzTPB Systems

alkyne to azide equivalence of 1:1 shows a biphasic transition with two glass transitions (Tg). The one at −56 °C corresponds to butadiene−polyurethane backbone and that at 42 °C may be due to the triazole−triazoline phase (Figure 8). The storage modulus of triazole−triazoline networks is higher than that of cured HTPB−TDI polyurethane (cf. SI). This is expected in view of the higher cross-link density of the triazole−triazoline network arising from additional reaction of azide with olefinic unstaturation. The biphasic behavior observed in DMA is corroborated by morphological changes observed in PrTPB−

cross-link density (mol/cm3) equivalence ratio (azide:alkyne or isocyanate:hydroxyl)

cured PrTPB−AzTPB cured HTPB−TDI

0.7:1 0.85:1 1:1

1.20 × 10−8 3.43 × 10−6 2.86 × 10−4

0.65 × 10−8 0.27 × 10−6 0.62 × 10−4

diene−triazole (2.86 × 10−4 mol/cm3) confirms that additional cross-linking takes place through reaction of the azide group

Table 2. Mechanical Properties of Cured HTPB−TDI and PrTPB−AzTPB Systems mechanical properties cured HTPB−TDI

cured PrTPB−AzTPB

equivalence ratio (azide:alkyne or isocyanate:hydroxyl)

tensile strength (MPa)

elongation at break (%)

stress at 100% elongation (MPa)

tensile strength (MPa)

elongation at break (%)

stress at 100% elongation (MPa)

0.7:1.0 0.85:1.0 1:1

0.45 0.64 0.86

920 460 240

0.12 0.28 0.52

0.99 1.29 1.52

480 520 660

0.31 0.42 0.50

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Figure 8. SPM Images of morphological changes during heating of the cured network from 40 to 50 °C.

indicates that functional modification does not deteriorate the ballistics.

AzTPB samples by scanning probe microscopy (SPM). When the sample is heated in the temperature range of 40−50 °C, morphological change occurs which is may be due to the thermal transition of the triazole−triazoline groups in the polymer (Figure 8). 3.7. Propellant Processability, Mechanical Properties, and Burn Rate. The propellant level studies were conducted using a blend of AzTPB with PrTPB (azide−alkyne molar stoichiometry of 0.85:1) as binder, with ammonium perchlorate as oxidizer and aluminum powder as metallic fuel. For comparison, the properties of the propellant processed using HTPB−TDI polyurethanes (isocyanate:hydroxyl molar stoichiometry of 0.85:1) as binder were also evaluated. The processability, mechanical properties at ambient temperature, and burn rate of the two propellants are given in Table 3.

4. CONCLUSION Azide and alkyne terminated polybutadienes were synthesized from HTPB, and cross-linking was effected through 1,3-dipolar cycloaddition. The curing of the two polymer systems was effected to form a triazole−triazoline network, and the curing was monitored by DSC. The related kinetic parameters were useful for predicting the cure profile of the system. The rheological characterization of the system indicated a slower cure rate for PrTPB−AzTPB than for the HTPB−TDI polyurethane system, and hence, the former system has a longer pot life. The cross-link density, mechanical properties, and fracture energy of triazole−triazolines based on PrTPB and AzTPB were superior to those of polyurethanes. DMA studies indicated a biphasic transition and a higher storage modulus for the triazole−triazoline networks in comparison to the polyurethane system. The biphasic characteristics observed in DMA were further corroborated by morphological changes identified by SPM analysis. The propellant based on the novel binder offers the advantage of enhanced pot life with superior mechanical properties (14−22%) than polyurethanes without affecting the burn rate.

Table 3. Propellant Properties propellant viscosity (Pa·s) at 40 °C end of mix after 3 h mechanical properties tensile strength (MPa) elongation (%) modulus (MPa) burn rate (at 6.93 MPa; mm/s)

PrTPB−AzTPB

HTPB−TDI

165 312

352 864

1.28 81 4.0 16.29 ± 0.03

1.10 63 3.50 16.31 ± 0.01

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ASSOCIATED CONTENT

S Supporting Information *

The studies indicate that the mechanical properties of propellant based on AzTPB−PrTPB are superior than those based on polyurethanes. The tensile strength of the AzTPB− PrTPB propellant is 1.28 MPa, elongation at break is 81%, and the Young’s modulus value is 4.0 MPa, which are higher than those for urethane based propellant (Table 3). The enhancement in tensile strength and modulus is due to the higher crosslink density of the polymer network (as discussed in section 3.4). Surprisingly the elongation properties are also higher which may be attributed to the fact that molecular weight between cross-links is higher in PrTPB and AzTPB than in HTPB. The propellant tends to exhibit easy flow characteristics with the low end of the mix viscosity of 165 Pa·s at 40 °C in comparison to the urethane based system which has 352 Pa·s. The build-up rate of viscosity is also lower compared to the urethane based propellant which is advantageous for processing. The burn rate of the two propellants is comparable which

Figure.S1 showing 1H NMR spectrum of (a) PrTPB and (b) AzTPB (in CDCl3), Figure S2 showing FTIR spectra of selfcured AzTPB polymer, Figure S3 showing FTIR spectra of cured PrTPB−AzTPB polymer, Figure S4 showing a rheogram of the HTPB−TDI system (at 60 °C), and Figure S5 showing the tan δ and storage modulus vs temperature plot for cured HTPB−TDI. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +914712565558. Fax: +91471-2564203. Notes

The authors declare no competing financial interest. 16618



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ACKNOWLEDGMENTS We thank the Director, Vikram Sarabhai Space Centre, for permission to publish this work. Analytical support from the Analytical and Spectroscopy Division and characterisation support from CTSS, Propellant Engineering Division, VSSC are also acknowledged.

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