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Synthesis of azido polymers as potential energeticpropellant binders

Y. Murali Mohan , Y. Mani & K. Mohana Raju

To cite this article: Y. Murali Mohan , Y. Mani & K. Mohana Raju (2006) Synthesis of azidopolymers as potential energetic propellant binders, Designed Monomers and Polymers, 9:3,201-236, DOI: 10.1163/156855506777351045

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Designed Monomers and Polymers, Vol. 9, No. 3, pp. 201–236 (2006) VSP 2006.Also available online - www.vsppub.com

Review

Synthesis of azido polymers as potential energeticpropellant binders

Y. MURALI MOHAN ∗, Y. MANI and K. MOHANA RAJUDepartment of Polymer Science & Engineering, Sri Krishnadevaraya University, Anantapur-515003, A.P., India

Abstract—Composite propellants are considered as major sources of chemical energy for rocketpropellants. In the preparation of composite propellant energetic formulations, the propellant binderplays a vital role. Various energetic polymeric binders were developed during last five decades tosatisfy safety, high energy and processing conditions. In this review, we discuss the various syntheticroutes for of preparation of high energetic polymeric binders, including glycidyl azide polymers(GAP)s, GAP co-polymers, oxetane polymers, oxirane polymers, azido polyesters, N,N′-bondedepoxy binders, aliphatic vinyl azide polymers and poly(allyl azide) (PAAz).

Keywords: Glycidyl azide polymer; propellant binder; poly(epichlorohydrin); azido polymers; oxiranepolymers; N,N′-bonded epoxy binders.

1. INTRODUCTION

Polymer science is recognized as a separate field of studies in physical sciencesdue to the availability of large number of polymeric materials, as well as theenormous number of commercial applications. To develop new polymeric materialsfor better performance with low cost, co-polymerization, blending, interpenetratingpolymer network formation and composites are the new methods of areas inpolymer research. These days, polymers find extensive applications in various fieldsincluding domestic, commercial, industrial, medicinal, pharmaceutical, agriculture,automobile, defence, electrical, paints, surface coatings, etc.

Telechelic low-molecular-weight polymers are used in space and propellanttechnology as propellant binders and on these lines the research has been goingon since 1950. Composite solid propellants (CSP) are the major source of chemical

∗To whom correspondence should be addressed. Present address: Department of Material Science& Engineering, Gwangju Institute of Science & Technology, Gwangju-500 712, South Korea. E-mail:[email protected]

202 Y. M. Mohan et al.

Table 1.CSP performance data containing different fuel binders with AP

Property Polyester Poly- Polyurethane Poly(butadiene)(vinyl chloride)

Process Cast Cast Cast CastDensity 1.88 1.64 1.72 1.74Chamber pressure (kg/cm2) 30–140 1–120 1–140 1–140Specific impulse (s) 180–200 225 240 250Burning rate (cal/s) 1.75 0.65–1.30 0.50–1.00 1.20Pressure exponent (n) 0.60–0.59 0.40–0.30 0.50–0.40 0.24–0.21Mechanical properties Excellent Excellent Excellent Excellent

energy for space vehicles, missiles and explosive purposes [1–5]. Composite solidpropellants are heterogeneous mixtures consisting of large proportions of oxidizer,fuel-cum-binder, curing agents, plasticizers, bonding agents, metallic fuel additivesand burning rate modifiers, etc. Among these, the oxidizer, burn rate modifiers(additives), fuel-cum-binders and plasticizers are the most influencing materialsfor better performance of any propellant [3, 5, 6]. In any conventional compositesolid propellant formulations 60–84 wt% ammonium perchlorate and 12–16 wt%polymeric binder is maintained [8]. The main counterparts of composite solidpropellants are presented in detail below.

1.1. Oxidizers

A large proportion of oxidizer, usually 60–85 wt%, is employed for making com-posite solid propellants [3]. These include ammonium perchlorate (AP), ammoniumnitrate (AN), ammonium dinitramide (ADN) and hydrazinium nitroformate (HNF).Ammonium perchlorate, the high-energy oxidizer, has its inherent drawbacks suchas production of large amount of chlorine-rich combustion products (up to 30%)and white smoke trails in the process of producing smokeless plumes as well as inenvironmental aspects. CSP performance data containing different fuel binders withammonium perchlorate as oxidizer [9] is presented in Table 1. Ammonium nitrate isalso considered a highly desirable oxidizer for composite solid propellants becauseof its low cost, low sensitivity and absence of halogens. The crystalline phase stabil-ity behaviour of AN causes unpredictable ballistic performance in some cases andcatastrophic rocket motor failure. Thus, it should be employed along with a phasestabilizer. Other oxidizers, such as ammonium dinitramide (ADN), hydrazinuimnitroformate (HNF), cyclotetramethylene tetranitramine (HMX), etc., have been inthe experimental stage for the last two decades [10].

1.2. Additives

The propellant additives are known to affect the combustion of CSP, oxidizer andpropellant binder [11–13]. Generally, additives are divided into two types [13],

Synthesis of azido polymers as potential energetic propellant binders 203

(i) catalysts, which enhance the burn rate (r) value of the propellants, and (ii) in-hibitors, which decrease the burn rate value of the propellants. The effective cata-lysts, widely known to modify the decomposition and combustion rates of CSP andits constituents, are the transition metal oxides (TMO) like Fe2O3, MnO2, Ni2O3,copper chromate, ferrocene and its derivatives, etc. [12]. The concentration of acatalyst should be such that it does not alter the propellant composition and mustbe as low as possible, so that the propellant energeticy is not affected [14]. It isfound that the decomposition and burn rate value of AP/PS propellant increase upto a certain catalyst concentration beyond which the effect either levels off or de-creases [15–17]. The relative effectiveness in enhancing the burn rate in TMOfollows the order Ni2O3 ∼ Fe2O3 > Co2O3 > MnO2. As all the transition metaloxides are solids, they have certain disadvantages, such as their large particle size,surface area, etc., imparting a certain degree of inhomogeneity and ultimately re-sulting in erratic burning performance to the propellant [18]. Although the versatilecatalyst, Fe2O3, is migration resistant, it is responsible for higher viscosity buildingduring processing of the propellant [19]. To overcome some of these disadvan-tages, ferrocene and ferrocene derivatives are identified as additives for modifyingthe burn rate of CSP or AP. During combustion, ferrocene decomposes into hydro-carbons and Fe is oxidized to Fe2O3 releasing substantial amounts of energy. Inaddition to being a good energy supplier, ferrocene functions as an effective cat-alyst in enhancing the burning rate, because Fe2O3 formed in situ is of very fineparticle size compared to the species obtainable by mechanical grinding. More-over, the ferrocenyl-substituted compounds, which may be binder-soluble or mayexist as liquids or binder-cum-catalysts, possess specific functional groups to im-part improved porcessability as well as offer a multifunctional role, like burningrate modifiers, curing agents, bonding agents, etc. [20]. Ammonium salts, such asNH4F, NH4Cl, NH4Br, NH4HPO4, NH4H2PO4 and LiF, are considered as versatileinhibitor types of additive.

1.3. Polymeric binder or propellant binder

A propellant binder is a low-molecular-weight liquid pre-polymer which is acontinuous phase in CSP, mixes easily with the particulate oxidizer and also shouldcontain reactive functional groups capable of participating in chain extending orcross-linking reactions to form a solid in the binder matrix [3, 9]. A typical cross-linking reaction of a propellant binder is presented in Scheme 1.

Generally, pre-polymers employed as propellant binders should have molecularweights in the range 1000–5000. The polymer binder molecular weight andfunctionality influences the cross-linking network formation of the propellant,which in turn influences the mechanical properties of the propellant [8, 21]. Thelocation of reactive functional groups in the pre-polymers also influences themechanical strength of the propellant [22, 23]. If the function groups are randomlylocated on the pre-polymer, e.g., poly(butadiene-co-acrylic acid) curing gives rise


Scheme 1. Cross-linking or chain extension reaction of a propellant binder.

to free dangling chain ends which are not effective in producing desirable andreproducible physical properties.

Hydroxyl-terminated poly(butadiene) (HTPB) and carboxyl-terminated poly(bu-tadiene) (CTPB) are the most widely used pre-polymers in propellant binders. Eventhough the HTPB/CTPB propellant systems possess excellent physico-chemicalproperties and reduce the vulnerability of the explosive charges, at the same timethey are inert binders, i.e., these binders “dilute” the explosives, which reduces theoverall energy output and performance of the propellant composition [4]. HTPBpropellant showed high specific impulse when the ammonium perchlorate oxidizeris above 85 wt% in the propellant composition [24]. Although in the HTPB oxygenbalance is high, its enthalpy of formation value is negative (Table 2). These inertbinder systems were effectively used only in the explosive formulations for air-blastand underwater applications, but it is quite difficult to produce high performancecast-cured explosives for metal acceleration. Commercially developed HTPB/APpropellants liberate HCl gas, producing hazardous air pollutants from the propellantplumes.

In order to overcome these difficulties and also to obtain a better performance, tosatisfy insensitive munitions and environment, as well as to reduce the productioncost of propellants, high energetic propellant binders have been developed aroundthe world in the last two decades [3, 4]. The energetic propellant binders/plasticizers


Table 2.Heat of formation and oxygen balance of various binders/plasticizers

Binder/plasticizer �H 0f Oxygen balance

(kJ/kg) (g O2/100 g)

Diethylphthalate (DEP) −776 194.4Dibutylphthalate (DBP) −843 224.2Dioctylphthalate (DOP) −1122 258.1Ethylphenylurethane (EPU) −464 227.7Triacetin (TC) −1331 139.3Camphor (CM) −327 283.8Ethylcentralite (ECT) −105 256.4DBNPA (50:50) −827 57.5Butylnitratoethylnitramine (BUNENA) −140 104.31,5-diazido-3,3-nitroazapentane (DANPE) 554 79.92,4-dinitro-2,4-diaza-6-nitroazapentane (DNDANPE) −145 36.82,4-dinitro-2,4-diaza-6-azidohexane (DNDAAH) 210 62.1Glycidyl azide polymer (GAP) 1179 121.1Hydroxyl terminated poly(butadiene) (HTPB) −380 323.0Poly(isobutylene) (PIB) −1568 342.2Cellulose acetate (CA) −5518 129.6Cellulose acetate butyrate (CAB) −4523 149.2

are pre-polymers containing energetic functional groups (explosophores) such asazido, nitro (C-nitro, O-nitro and N-nitro (nitramine)) and difluoramine groupsalong the polymer backbones. These energetic binders/plasticizers not only improvethe internal energy of the formulations, but also improve the overall oxygenbalance of the propellant. This approach is successful in the development of highperformance explosives and advanced rocket propellants.

The new energetic binders, including azide functional polymers like glycidylazide polymer (GAP), poly(3,3-bis(3-azido methyl) oxetane) (poly(BAMO)), poly-((3-azido methyl)-3-methyl oxetane) (poly(AMMO)); nitrato polyethers like poly-((3-nitrato methyl)-3-methyl oxetane) (poly(NIMMO)) or poly(NMMO), poly-(glycidyl nitrate) (poly(GLYN)); poly(vinyl nitrate)s, fluoro-polymers, poly(nitroaromatic)s, N,N-bonded epoxy functional polymers and nitrated poly(butadiene)s,offer such promising characteristics to the propellant systems.

Recent advances in the propellant technology is aimed to develop new highenergetic binders/plasticizers to perform better than the conventional HTPB binderand should also be compatible, as well as be eco-friendly with high energeticoxidizers, such as ammonium nitrate (AN), ammonium nitronitramide (ADN),hydrazinuim nitroformate (HNF) and cyclotetramethylene tetranitramine. Since theenergetic polymer binders are important constituents in the propellant compositionin the field of explosives and space technology, the present review focuses onexploring various synthetic ways for the preparation of different energetic propellantbinders.


2. VARIOUS SYNTHETIC ROUTES FOR PREPARATION OF ENERGETICPOLYMERIC PROPELLANT BINDERS

Glycidyl azide polymers and their related energetic polymer binders are consideredas promising candidates for propellant binders in the rocket propellant technology.This review provides the complete details of different methods of preparation ofvarious energetic propellant binders or plasticizers. The present review deals with anumber of synthetic routes for the preparation of glycidyl azide polymers (GAP) andGAP co-polymers, oxetane polymers, oxirane polymers, nitrato polyethers, fluoro-polymers, poly(nitroaromatic)s, N,N-bonded epoxy polymers, azido polyesters,poly(allyl azide), aliphatic poly(vinyl azide)s and so on.

2.1. Glycidyl azide polymers and its co-polymers

2.1.1. Glycidyl azide polymers. The starting compound, glycidyl azide monomer(GA) [3], was prepared by reacting epichlorohydrin (ECH) with hydrazoic acid fol-lowed by cyclization as shown in Scheme 2. The cationic ring-opening polymer-ization of glycidyl azide did not yield glycidyl azide polymer (GAP) due to theun-reactiveness of GA.

Due to inability to get GAP from GA monomer, alternative methods were fol-lowed to prepare GAPs. The methods include: direct conversion of epichlorohydrin(ECH) to GAP, derivation form poly(epichlorohydrin) (PECH) to GAP and simul-taneous degradation and azidation of poly(epichlorohydrin)s and co-polymers.

2.1.1.1. Direct conversion of ECH to GAP. A suitable method was adopted forthe single-step preparation of GAP from epichlorohydrin [25]. This approach wassuitable for the preparation of low-molecular-weight (i.e., M̄w = 540 and M̄n =400) hydroxyl-terminated GAP having 1.3 g/ml density with a glass transitiontemperature (Tg) of −70◦C. Sodium azide was added to a mixture containing ECH(1:1 mol ratio), dimethylformamide (DMF) and ethylene glycol (EG). In order tocontrol the exothermic reaction in the initial stages, the contents were stirred at 70◦Cfor about 30 min and then the temperature was increased to 90◦C under continued

Scheme 2. Preparative method of GA.


Scheme 3. Direct synthetic route of GAP formation.

stirring at this temperature for 15 h to give 90% yield. This process was identifiedas cost effective and less time-consuming process.

The synthesis of GAP (M̄n = 390) was also reported by another approach ina single-step process [26]. In this process 100 g ECH was added to a mixturecontaining 100 g sodium azide and 100 g EG in 200 ml 1,1,1-trichloroethane (TCE)at 80◦C. In order to control the exothermicity of the reaction, the reaction mixturewas diluted with TCE.

Murali and Raju [27] proposed a facile route for the synthesis of low-molecular-weight hydroxyl-terminated glycidyl azide polymers containing different diol unitsin the polymeric chains. A schematic preparation route is presented in Scheme 3.In this investigation few diols were employed as initiators and found that all theGAPs containing initiated diol units had molecular weights between 590 and 710containing 5–7 GAP repeating units. The GPC profile of GAPs is presented inFig. 1. The formation of GAPs containing different diol units in the polymer chainwas confirmed by spectral, thermal and elemental analysis. The GAPs reported inthis investigation showed similar thermal behaviour compared to pure GAP withslight variation in their decomposition behaviour due to the presence of differentdiol units in the polymer chains. However, the glycidyl azide polymers showedvery low glass transition temperatures of −70 to −72◦C.

2.1.1.2. Derivatization of GAP from poly(epichlorohydrin) (PECH). In thepreparative methods of GAP, a better and successful approach was based on thepolymerization of ECH to PECH followed by nucleophilic displacement of chlorineatoms by azide groups in the azidation reaction of PECH with sodium azide. Thesynthetic details of the preparation of GAP are presented in Scheme 4.


Figure 1. Gel-permeation chromatographs of GAPs (a) GAPEG; (b) GAPPG; (c) GAPDEG;(d) GAPMPD and (e) GAPRS.

The complete details of the polymerization conditions of ECH and azidation ofPECH with sodium azide are discussed below.

(i) Preparation of PECH. Epoxides are usually polymerized by anionic andcationic initiators due to high degree of strain involved in the 3-membered ring(oxirane). PECH was prepared by the ring-opening cationic polymerization of ECH[27–31]. The physical properties of PECH depend on the microstructure of the


Scheme 4. Synthetic scheme of GAP.

polymer. The atactic PECH was amorphous with a glass transition temperature of−20 to −25◦C, whereas isotactic PECH was semi-crystalline and had a meltingpoint of 125◦C. The different micro-structural features can be obtained by usingdifferent initiating systems and polymerization conditions. Cationic initiators, suchas Lewis acids or tertiary oxonium salts, often complexed with water, alcoholor ether, lead to an atactic hydroxyl-terminated low-molecular-weight polymer(<4000) [33, 34]. The high-molecular-weight PECH (15 × 103) was obtainedby using 1,4-butanediyl ditriflate as initiator [35]. Organometallic initiators wereeffectively employed in a Vanderberg process to produce elastomeric PECH [36].

The suitable range of molecular weight of PECHs as precursors in the preparationof GAPs in the propellant technology is from 1000 to 4000 [3]. The nature ofinitiating system, monomer to initiator ratio, type of solvent, reaction temperature,etc., used in the polymerization of ECH influences the molecular weight ofthe resulting polymer [32]. Lewis and Brønsted acids were generally used asinitiators and the useful range of monomer to initiator ratio was 40 ± 4:1 [37].Table 3 illustrates the influence of various reaction parameters, including solvent,


Table 3.Effect of reaction parameters on% of yield of PECH

Designation Initiator Solvent Monomer to Time Yieldinitiator ratio (h) (%)

I-1 BF3-etherate 1,2-dichloromethane 44:1 4.0 98I-2 Dioxane 42:1 4.0 11I-3 Toluene 36:1 4.0 88I-4 Ethyl ether 36:1 4.0 5I-5 1,2-dichloromethane 36:1 4.0 19II-1 SnCl4 1,2-dichloromethane 54:1 3.0 75II-2 Dioxane 43:1 2.5 50II-3 CF3SO3H Toluene 44:1 23.5 73III-1 1,2-dichloromethane 44:1 7.0 2III-2 Dioxane 38:1 23.0 12

Polymerization temperature = 5◦C (dioxane = 15◦C).

initiator and monomer to initiator ratio, on the conversion of ECH to PECH.The molecular weight distribution of PECH depends on the initiator employed inthe polymerization. The SnCl4 initiator leads to give a polymer with somewhatnarrow molecular weight distribution, whereas the borontrifluoride etherate (BF3-etherate)-initiated polymer shows a broad molecular weight distribution with higheryield than any other initiators (up to 98% in 1,2-dichloroethane). Okamoto [38]reported the cationic ring-opening polymerization of ECH using triethyloxoniumhexafluorophosphate (TEOP) and BF3 as initiators in the presence of ethyleneglycol, butane-1,4-diol or water. The results indicated that the BF3-etherate-initiatedpolymer gave polymers having higher secondary hydroxyl-terminal end groups.

In the presence of ether solvent medium, the polymerizations suppress thepropagation step and promote the backbiting reactions, which are responsible forthe formation of low-molecular-weight chains [39, 40]. Francis et al. [41] reportedpreparation of PECHs with different molecular weights.

(ii) Azidation of PECH. The nucleophilic displacement of chlorine atoms inPECH by azide groups was achieved by the reaction of PECH with slight excess ofionic azide, such as sodium azide, lithium azide, or potassium azide in an organic oraqueous solvent [3, 4, 42]. The complete conversion of PECH to GAP with sodiumazide in dimethylsulphoxide (DMSO) took 12–18 h [43, 44]. The same reaction,if carried out in water using a phase-transfer catalyst, took 7 days for completeconversion [45]. It was also possible to convert all the chlorine atoms by azidegroups at 100◦C in DMF solvent [46]. It was recognized that after 90% completionof the azidation reaction in a solvent medium, the rate of reaction decreases due tothe association of the metal ions of the azide salt with the solvent medium leading tolimited solubility, thereby increasing the time required for complete conversion [47].In this process, a quaternary ammonium azide salt, namely tetrabutyl ammoniumazide, was employed as a reagent for the conversion of chlorine atoms to azidegroups. The conversion reaction was carried out with a trace amount of water for


Table 4.Elemental analysis of glycidyl azide polymer by FNA method

Polymer Reaction time Nitrogen Chlorine Oxygen(h) (%) (%) (%)

PECH I — 0.0 40.0 21.0GAP I a 7 13.0 28.0 18.0GAP I b 8 25.0 16.0 20.0GAP I c 9 28.0 13.0 18.0PECH II — 0.0 42.0 17.0GAP II a 10 39.0 5.0 16.0GAP II b 11 40.15 3.3 16.0

PECH and the reaction mixture was heated to 105◦C for about 20 min. Afterpurifying the product by washing with water and then drying showed 85–90%conversion of chlorine atoms to azide groups.

Panda et al. [48, 49] reported a fast neutron activation (FNA) method to monitorthe azidation reaction of PECH with sodium azide. This study suggests that evenafter completion of 12 h of the azidation reaction in DMSO at 100◦C, 3.3 wt%chlorine was still present in the polymer. A complete elemental analysis spectrumof nitrogen, chlorine and oxygen of GAP is presented in Table 4.

Murali et al. [50] prepared GAPs containing different diol units in a two-stepprocess involving the preparation of PECHs and their conversion into GAPs byreacting with sodium azide. GAP formation, as well as the presence of different diolunits in the polymeric chains, was confirmed by UV, IR, 1H-NMR and 13C-NMRspectral analysis. The conversion of PECH to GAP is confirmed by observing thecomplete disappearance of the CH2Cl peak at 746 cm−1 in IR spectra of glycidylazide polymers. IR spectra of PECH and GAPs containing diol units are shownin Fig. 2A and 2B, respectively. 1H-NMR spectra of GAPs containing diolunits depicted in Fig. 3 clearly demonstrate the complete conversion of pendentchloromethyl groups into azidomethyl groups. The effect of diol units on thepolymerization of ECH was studied in detail. The diol units present in the GAPchains have showed significant influence on the thermal decomposition behaviouras well as on the glass transition temperature of GAPs. Complete data of the thermalproperties of these GAPs are tabulated in Table 5.

In contrast to the available methods, a facile approach was followed to improvethe azide content in GAP by employing a new diol, namely 2,2-bis(bromomethyl)-1,3-propane diol (BMPD), in the PECH synthesis. This investigation describesthe synthesis of glycidyl azide polymer having high azide content (42.54–43.80%nitrogen content) [51]. The improved azide content in the GAP was confirmed byelemental and DSC analysis. A representative DSC thermogram of GAP containingAMPD units is illustrated in Fig. 4. A detailed preparative method is shown inScheme 5. Thermal analysis data of these polymers are presented in Table 6.


(A)

(B)

Figure 2. IR spectra of (A) PECH and (B) GAP containing different diol units.

2.1.1.3. Simultaneous degradation and azidation of PECH and co-polymers.Simultaneous degradation and azidation process was developed for the preparationof branched GAP or glycidyl azide-ethylene oxide co-polymer (GEC) havingmolecular weights ranging from 500 to 40 × 103 [52, 53]. The polymers wereobtained by reacting the high-molecular-weight rubbery PECH or its counterpart,


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−55.

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249.

5738

0.62

245.

625

4.0

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−71.

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1537

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Figure 4. DSC thermogram of GAP containing AMPD units.

Table 6.Thermal analysis data of PECH and GAPs

Polymer code TGA DSC

T0(◦C) Tmax (◦C) Tmax (◦C) Heat of

decomposition (J/g)

PECH 1 242 285 319 —GAP-AMPD 1 207 229 242 1841.42GAP-AMPD 2 208 234 243 1863.50GAP-AMPD 3 225 241 244 1895.95

T0, onset decomposition temperature; Tmax, peak decomposition temperature or maximum decom-position temperature.

poly(epichlorohydrin-co-ethylene glycol) co-polymer (PEEC) with sodium azide inorganic solvent. Ahad [55, 56] patented a novel and improved degradation processfor the preparation of branched GAP or GEC with controlled molecular weightranging from 1000 to 2 × 106. This process involves reaction of high-molecular-weight solid rubbery PECH or its counterpart, PEEC, with sodium azide in thepresence of a suitable organic solvent at an elevated temperature. Bui et al. [56]reported the preparation of long chain polyurethanes with branched GAPs or GECswith different isocyanates in order to study their mechanical properties such aselongation at break and tensile strength. Recently, the same group also reportedthe synthesis and characterization of some branched GAPs and GECs for curing


Scheme 5. Glycidyl azide polymer formation.

reactions [57]. This study also includes the determination of viscosity, molecularweight and thermal properties. Table 7 demonstrates the polydispersity index(PI = M̄w/M̄n) of the polyurethanes having the values in between 1.70 and 2.10.The molecular weight distributions of GAPs and GECs are considered to be lowwhen compared to PECHs or PEECs, due to the result of the degradation process.Further, the GECs have higher viscosity than GAPs with similar molecular weight.This behaviour is attributed to the presence of ethylene oxide repeating units in theco-polymer chain.

2.1.2. GAP co-polymers. Cao and Zhang [58] reported the preparation of a GAP-THF co-polymer. This co-polymer showed better low temperature properties, lowerhazard sensitivity and thermal behaviour similar to neat GAP. Recently, co-polymersof GAP and THF were reported having monomer weights in the ratio of 50:50 and75:25 [59]. These co-polymers were found to have molecular weights of 1764 and2100 and a Tg of −68 and −63◦C, respectively.


Table 7.Molecular weight, polydispersity index and viscosity data of GAPs and GECs

Polymer Sample code Polyol M̄w M̄n M̄w/M̄n [η](dl/g)

GAP homo-polymer HG-1 GOL 18 100 10 600 1.7 0.122HG-2 GOL 28 800 15 200 1.90 0.163HT-1 TMP 18 500 11 000 1.68 0.125HT-2 TMP 22 500 14 000 1.60 0.140HP-1 PE 22 700 11 400 2.00 0.141

GEC co-polymer CG-1 GOL 20 900 10 400 2.01 0.185CG-2 GOL 20 000 9500 2.10 0.181CT-1 TMP 28 500 14 700 1.94 0.216CT-2 TMP 24 700 13 600 1.82 0.201CP-1 PE 21 200 10 800 1.96 0.186CP-2 PE 20 400 11 600 1.76 0.183

GOL, glycerol; TMP, trimethylol propane; PE, penthaerythritol; The concentration of solutions1.700 × 10−2 g/ml to 0.400 × 10−2 g/ml; PI, polydispersity index; [η], intrinsic viscosity.

Scheme 6. Synthetic scheme of GAP-THF co-polymer formation.

Murali et al. [60] investigated the synthesis and thermal properties of GAP-THF in detail. Scheme 6 shows the synthetic route for the preparation of GAP-THF co-polymers. Various analytical tools such as UV, IR, 1H-NMR and 13C-NMR were used to confirm their co-polymer formation. VPO was employed todetermine the molecular weights of co-polymers. Thermal properties were alsorevealed in detail. TGA thermogram of GAP-THF co-polymers showed two weight-loss regions. These are due to the nitrogen elimination from the azide group of theGAP units in the co-polymers and the degradation of the polyether main chain ofthe GAP and PECH repeating units, respectively. Figure 5 shows a representativeTGA of GAP-THF co-polymer 1.

Tri-block co-polymers of GAP and PEG were synthesized by using variousPEGs as diols in the cationic ring-opening polymerization of epichlorohydrin using


Figure 5. TG-DTA thermogram of GAP-THF 1 co-polymer.

Table 8.Thermal properties of GAP-b-PEG-b-GAP co-polymers

Co-polymera DSC results TG-DTG results DTA results

Tg Decomposition Energy (Td, ◦C) (Td, ◦C)(◦C) temperature (◦C) (J/g)

T0 Texo Td Tf

GAP co-polymer 1 −63 162 208 252 284 1510 239.2 244.9GAP co-polymer 2 −68 164 209 263 293 1230 243.5 249.5GAP co-polymer 3 −72 167 209 271 294 1054 247.6 256.5

a GAP co-polymers 1, 2 and 3 are containing PEG 200, PEG 400 and PEG 600, respectively. T0,initial decomposition temperature; Texo, onset decomposition temperature; Td, peak decompositiontemperature; Tf , final decomposition temperature.

borontrifluoride etherate as initiator and sub-sequent azidation of the co-polymerwith sodium azide [61]. They were fully characterized by spectral methods suchas UV, IR, 1H-NMR and 13C-NMR. The formation of co-polymer was confirmedfrom 13C-NMR spectra of GAP-PEG-GAP co-polymer, as shown in Fig. 6. Figure7 further confirms the formation of azide group on the polymeric chains. Thermalhistory of these co-polymers was also investigated. Table 8 gives details of thermalcharacteristics of GAP-PEG-GAP co-polymers. It is observed from Fig. 8 that theTg of the GAP-PEG600-GAP co-polymer is −72◦C.

Minora et al. [62] reported co-polymers based on GAP and BAMO. Their methodinvolved the polymerization using borontrifluoride etherate/alcohol initiating sys-tem in benzene and its subsequent azidation. GAP-PGN co-polymer was prepared


Figure 6. 13C-NMR spectra of GAP-b-PEG(200)-b-GAP co-polymer.

Figure 7. UV spectra of PECH-b-PEG-b-PECH and GAP-b-PEG-b-GAP co-polymers.


Figure 8. DSC glass transition curve of GAP-b-PEG(600)-b-GAP co-polymer.

Scheme 7. Synthesis of (A) GAP-BAMO co-polymer and (B) GAP-PGN co-polymer.

by reacting poly(glycidyl nitrate) (PGN) with metallic azide in organic solvent inthe presence of a phase transfer catalyst to replace nitrate groups by azide groups.The preparative schemes of GAP-BAMO and GAP-PGN co-polymers are presentedin Scheme 7.

Subramaniam [63] reported the GAP-b-HTPB-b-GAP tri-block co-polymer forpotential propellant binder applications. The synthesis of a tri-block co-polymerwas similar to the preparation of GAP by a two-step process. In the preparationof HTPB-b-GAP co-polymer, HTPB was employed as initiator in place of low-molecular-weight diol and BF3-etherate as catalyst [63–65]. The synthesis of


Figure 9. GPC curves of cross-linked (HTPB-GAP 1) and graft (HTPB-GAP 4) co-polymers.

Table 9.Results and conditions of the polymerization of HTPB with GAPMI

Sample GAPMI HTPB Reaction HTPB-GAP Molecular(g) (g) time (h) co-polymer weight (GPC)

Yield (%) Type M̄n M̄w

HTPB-GAP 1 1.00 0.80 5 54.20 Cross-linked 8952 41 865HTPB-GAP 2 1.00 0.40 5 48.42 Cross-linked 8721 35 301HTPB-GAP 3 1.00 0.20 5 42.65 Cross-linked 9461 28 183HTPB-GAP 4 1.00 0.40 3 35.25 Graft 3442 4418

Yield of co-polymer = (weight of GAPMI + weight of HTPB)/weight of co-polymer.Temperature = 110◦C for reactions 1–3; 90◦C for reaction 4.

HTPB-g-GAP co-polymer [66] was done by graft co-polymerization of HTPB withGAP macroinitiator, which in turn was obtained by the reaction of GAP with 4,4-azobis-4-cyanopentanoyl chloride (ACPC) in the presence of triethyl amine (TEA).The yields were found to be between 22.61 and 31.89%.


Sche

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8.G

raft

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ss-l

inki

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nH

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Figure 10. TGA curve of cross-linked HTPB-GAP co-polymer.

Murali and Raju [67] successfully prepared cross-linked and graft co-polymersof HTPB and GAP using a facile method. Scheme 8 gives the complete schematicdiagram of the preparation of GAP azoinitiator and graft/cross-linking reaction ofHTPB and GAP. The difference between graft and cross-linked HTPB-GAP co-polymer could easily distinguished by GPC analysis. The GPC profile of the co-polymers is presented in Fig. 9. Synthetic data of these co-polymers are tabulated inTable 9. The cross-linked HTPB-GAP or HTPB-g-GAP co-polymers were obtainedby fractional precipitation at γ values of 0.9–1.1 and 1.6–2.0, respectively. The un-reacted HTPB and GAP were obtained at γ values of <0.8 and >2.0. Here γ isthe ratio of solvent (chloroform) to non-solvent (methanol). Thermal study couldhelp in identifying the presence of both polymeric segments in the co-polymer. Itis clear from TGA curve of co-polymer, as shown in Fig. 10. Further DSC analysis(Fig. 11) supports the presence of HTPB and GAP blocks in the co-polymer. TheHTPB-GAP cross-linked co-polymer showed two glass transition temperatures oneat −74.03◦C and the other at −35.84◦C, due to incompatibility of the HTPB andGAP polymeric chains in the co-polymer. Further incompatibility of HTPB andglycidyl azide polymers was also investigated [68]. Chee [69] and Sun et al. [70]found interaction parameters values of HTPB and the GAP blend system to be<0, indicating immiscible nature. Incompatibility of HTPB and GAP blend (Fig.12) was confirmed by the non-linear behaviour of the physical properties, such asultrasonic velocity, refractive index and density with HTPB/GAP blend in toluenefollowing the method of Rajulu et al. [71].


Figure 11. DSC curves of HTPB and HTPB-GAP cross-linked co-polymers.

Eroglu et al. [72] reported a synthesis route for the preparation of block co-polymers of GAP containing either poly(styrene) or poly(vinyl acetate) units.The co-polymerization process involves the preparation of GAP macroazonitrile,which produces free-radicals by heating at 70◦C. The styrene or vinyl acetate waspolymerized in the presence of GAP macroaznitrile under nitrogen atmosphere.The block co-polymerization scheme is presented in Scheme 9. The block co-polymer of GAP and PMMA was prepared by redox polymerization method usinga ceric ion/HNO3 initiating system [73]. The polymerization starts by formingfree radicals by interaction with terminal hydroxyl groups of GAP and these freeradicals attack MMA monomer and propagate the polymerization reaction to formas block co-polymer of GAP-b-PMMA. This facile method may result in a co-polymer having hydroxyl functional groups, in contrast to conventional free-radicalpolymerizations. The redox-polymerization of GAP and MMA is depicted inScheme 10.


Figure 12. The variation of (A) ultrasonic velocity, (B) refractive index and (C) density withcomposition of HTPB/GAP blend in toluene at 30◦C.


Scheme 9. Preparation scheme of GAP-b-PS or GAP-b-PVAc co-polymers.

Scheme 10. Synthesis of PMMA-b-GAP-b-PMMA co-polymer.

2.2. Oxetane polymers and co-polymers

The cationic ring-opening polymerization of four-membered substituted oxetanesproduces high-molecular-weight polymers [74]. Both the molecular weight and


Scheme 11. Synthesis of poly(BAMO).

the functionality of these polymers can be controlled well, in contrast to thegeneral polyepoxides [4, 42]. Because of these advantages the polymers orco-polymers of oxetanes containing azido groups [74], such as poly(AMMO),poly(BAMO) and poly(NMMO), have been reported in detail in the past. Manser[75] synthesized the energetic oxetane polymers from monomers such as 3,3-bis(azido methyl)oxetane (BAMO), 3-azidomethyl-3-methyl oxetane (AMMO) and3-nitrato methyl-3-methyl oxetane (NIMMO).

Poly(BAMO) is one of the most energetic pre-polymers having two azido methylgroups [3]. Poly(BAMO) is a solid at room temperature and shows very poormechanical properties. In order to improve the mechanical properties, thesemonomers were co-polymerized with flexible monomers like THF or NMMO.The monomer BCMO was obtained by ring closure of the trichloro derivativeof pentaerythritol [76]. The synthesis of poly(BAMO) was carried out in twodifferent ways [42, 77, 78]. Scheme 11 presents the two different preparativeroutes of poly(BAMO). In the first method, 3,3-bis(chloro methyl) oxetane wastreated with sodium azide in dimethyl formamide at 85◦C for 2 h and thenhomo-polymerized or co-polymerized with THF by ring-opening polymerizationin the presence of butane diol as initiator at −5◦C using BF3-etherate as catalyst.Alternatively, poly(BCMO) or its co-polymers were prepared following ring-opening polymerization and subsequent azidation with sodium azide in DMFsolvent at 90◦C to obtain poly(BAMO) and BAMO co-polymers.

A new Lewis acid initiator, 3,3,31,31-tetrakis(trifluoromethyl)-(3H,3H1)-spirobis-(1,2-benzoxasilole) (SBS), was employed for the polymerization of BAMO indichloromethane solution at 20◦C [79]. A diethylene glycol (DEG)/BF3-etherateinitiating system gave good results in terms of molecular weight, safety, yield andfunctionality. The block and random co-polymers of BAMO were prepared by usingdifunctional initiator namely, triflic anhydride (CF3SO2)2O and these polymers havea low polydispersity index (1.23–1.28) [80]. In these co-polymerization reactionsthe reactivity ratios of BAMO with THF were found as 1.12 and 0.33, respectively,indicating that BAMO has higher reactivity than THF. Telechelic BAMO/NMMOand BAMO/NMMO/PE co-polymers were synthesized in dichloromethane using1,4-butane diol with borontrifluoride etherate and polyester (PE)/borontrifluoride


etherate initiating systems, respectively [81]. The co-polymers obtained showedhigh molecular weight, low glass transition temperature and good energetic charac-teristics.

Poly(AMMO) is a stable polymer, which was prepared by polymerization ofAMMO in the presence of borontrifluoride etherate/anhydrous methanol for 24 h[82]. The resulting polymer had a molecular weight in the range of (8–10) ×103. Quasi-living cationic polymerization of AMMO was reported in methylenechloride using p-bis(α,α-dimethyl chloromethyl) benzene (p-DCC)/silver hexa-fluoroantimonate (AgSbF6) as initiating system at −78◦C to produce poly(AMMO)[83, 84]. This new approach provided the possibility to tailor the molecularweight of polymers and was attained by just changing the monomer/initiator ra-tio. It was identified that with increasing the monomer/initiator ratio the molec-ular weight of poly(AMMO) increases. Murphy et al. [85] reported the synthe-sis of a tri-block co-polymer of poly(BAMO-AMMO-BAMO). The polymeriza-tion of BAMO and AMMO was conducted in the presence of 1,4-butane diol/BF3-etherate initiating system (1:2 ratio) at −10◦C. In the polymerization process, anew monomer was introduced only after 95% completion of the polymerization.A BAMO and NMMO block co-polymer was also reported with the same initiat-ing system. The same tri-block co-polymer, poly(BAMO-AMMO-BAMO), was re-ported with a p-DCC/AgSbF6 initiating system at −70◦C under nitrogen atmospherein dichloromethane solvent [77].

To receive less crystallization tendency as well as to obtain lower glass transi-tion temperature to the propellant binder AMMO was co-polymerized with 3-(azidomethyl)-3-(2,5-dioxaheptyl) oxetane and 3-(azidomethyl)-3-(2,5,8-trioxadecyl) ox-etane [78]. It was found that the AMMO oligomer having a Tg of −50 to−60◦C was obtained when AMMO was co-polymerized with 15 wt% 3-(azidomethyl)-3-(2,5,8-trioxadecyl) oxetane. The triethyloxonium tetrafluoroborate andspiro(benzoxasilole)/propane diol initiating system was employed in the co-poly-merization of AMMO with NMMO [86]. The SBS [87] initiator was widely em-ployed to polymerize several oxetanes (BAMO, AMMO and NMMO) in the tem-perature range of −10 to + 20◦C. Poly(BAMO-co-GAP) and poly(BAMO-PGN)were also reported as promising materials to be used as novel propellant bindershaving high density, high heat of formation and burn rate characteristics.

2.3. Oxirane polymers

Oxirane polymers show similar properties to that of oxetane polymers [88], butthe difference between them is the methylene groups in the repeating units.Poly(glycidyl nitrate) (poly(GLYN)) is the most important pre-polymer, possess-ing a higher density (1.46) and higher heat of formation (2661 kJ/kg) valuesthan GAP and poly(NMMO). Hydroxyl-terminated poly(GYN) was prepared fromGYN, which itself was obtained by the nitration of glycidol. The polymerizationswere conducted using tetrafluoroboric acid etherate initiator to give normal difunc-tional polymers. The mechanism of polymerization followed the activated monomer


Scheme 12. Preparation of poly(glycidyl nitrate).

mechanism as shown in Scheme 12, which is similar to other ring-opening polymer-izations.

2.4. Azido polyesters

Keicher and Wasmann [89, 90] reported telechelic polyesters containing pendantazide groups to use as pre-polymeric castable binders in explosive systems. Thesepolymers were prepared from 2,2-bis(bromomethyl)-1,3-propane diol with differentdiacids, including malonic acid, succinic acid, glutaric acid and adipic acid,resulting in bromo-polyesters. These bromo-polyesters were converted into azidopolyesters by reacting with sodium azide in DMSO for 10–100 h at 80◦C, dependingon the polyester. A complete spectrum of their molecular weights, glass transitiontemperatures, viscosities and functionalities was revealed in these studies. Theschematic representation of the preparation of polyesters containing azido methyl


Scheme 13. Preparation of polyesters containing azido methyl groups.

groups is presented in Scheme 13. Similarly, polyesters containing azido methylgroups were prepared by the condensation polymerization of 1,2-propane diolwith 2,3-dibromosuccinic acid using p-toluene suphonic acid as catalyst in toluenesolvent at 115◦C for 20 h and then converting these bromo polyesters into azidopolyesters by reacting them with sodium azide in DMSO for 24 h at 80◦C [91].This investigation revealed the nitrogen content, viscosity as well as decompositionbehaviour of polyesters.

2.5. N,N′-bonded epoxy binders

Oommen et al. [92] have reported the synthesis of series of hydrazones based onviscous epoxy resins having N N bonds in their backbones and then developed


Scheme 14. Chemical structure of N,N′-bonded epoxy binder.

Scheme 15. Poly(allyl chloride) and poly(allyl azide) chemical structures.

them as energetic binders for composite solid propellants. These materials arebased on α,ω-epoxy functional compounds having N,N′ bonds in the backbones[92–95]. Jain et al. [96] reported such compounds by the epoxidation of bis-dicarbonylhydrazones of adipic, azelaic and sebacic dihydrazides. The chemicalstructure of the N,N′-bonded epoxy binders is presented in Scheme 14.

2.6. Poly(allyl azide) (PAA)

PAA is a brown-coloured resin which can be obtained from the azidation ofpoly(allyl chloride) [42]. Free radical polymerization of allyl chloride results invery-low-molecular-weight products due to the degradative chain transfer reactions.Poly(allyl chloride) having a molecular weight of 2000 was obtained by cationicpolymerization [97]. In the polymerization of allyl chloride various Lewis acidswere employed, including TiCl4/FeCl3/AlCl3 and aluminum powder at low tem-


Scheme 16. Stepwise preparative route of poly(DAIPM).

peratures (0–20◦C). The effect of concentration of catalyst with respect to monomerwas also investigated. PAA was prepared by azidation of poly(allyl chloride) havingan azide content of about 35–40%, which is comparatively less than the calculatedvalue of 46.05%. This is due to the formation of branches in the polymer insteadof linear. Linear and branched poly(allyl chloride) and PAA structures are shown inScheme 15.

2.7. Aliphatic vinyl azide polymers

James et al. [98] investigated the aliphatic vinyl azide polymers and co-polymers forenergetic applications. It showed considerable attention as a surface network photo-cross-linker for plasticized PVC to reduce the plasticizer migration from plasticizedPVC. This report involves the synthesis of vinyl monomer namely, 1,3-diazido iso-propyl methacrylate (DAIPM) by the reaction between epichlorohydrin and sodiumazide to obtain 1,3-diazidopropane-2-ol and subsequent reaction with methacrylicacid in the presence of dicyclohexyl carbodiimide (DCC)/dimethylamino pyridine(DMAP). The obtained DAIMP monomer was employed for homo-polymerizationor co-polymerization with 0.1 wt% methyl methacrylate (MMA) at 75◦C for 5 h.The homo and co-polymers were characterized by spectral, GPC, thermal studies.The stepwise route of preparation of the homo-polymer is shown in Scheme 16.


Figure 13. Distribution of patents and published work on various aspects of GAP.

2.8. Scope for future investigations on polymeric propellant binders

The solid propellant field has been established during last five decades, althoughthere are many unresolved problems of significant relevance as far as new appli-cations are concerned. Figure 13 shows the distribution of patented and publishedworks on various aspects of GAP. This diagram shows that very little work was car-ried out on co-polymers of GAP and there is a lot of scope for carrying out furtherinvestigations on modification GAP, as well as preparation of co-polymers based onGAP with a view to obtain (i) high energy output, (ii) easy handling and (iii) easyprocessing. The utility of GAP is found to be greater as propellant binder in propel-lant technology. There are many reports on GAP and its propellants, but at the sametime a few drawbacks are noticed, including incompatibility with horse power pro-pellant binder HTPB, poor mechanical properties at low temperatures, higher glasstransition temperature, poor processing temperatures, etc. [3, 4, 42]. Consequently,it is necessity to develop new high-energetic polymeric binders based on GAPs.

3. CONCLUSIONS

A number of synthetic routes for the preparation of energetic polymeric propellantbinders such as glycidyl azide polymers (GAP)s, GAP co-polymers, oxetane poly-mers, oxirane polymers, azido polyesters, N,N′-bonded epoxy binders, aliphaticvinyl azide polymers, poly(allyl azide) (PAA) have been discussed in detail. Fur-ther, this review suggests that there is a lot of space to study on GAP co-polymerspropellant binders and its propellant formulations.


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