azido polymers-energetic binders for solid rocket propellants

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Azido Polymers—Energetic Binders for Solid RocketPropellantsBharti Gaur a , Bimlesh Lochab a , V. Choudhary a & I. K. Varma aa Centre for Polymer Science and Engineering, Indian Institute of Technology Delhi, HauzKhas, New Delhi, IndiaPublished online: 07 Feb 2007.

To cite this article: Bharti Gaur , Bimlesh Lochab , V. Choudhary & I. K. Varma (2003) Azido Polymers—Energetic Bindersfor Solid Rocket Propellants, Journal of Macromolecular Science, Part C: Polymer Reviews, 43:4, 505-545, DOI: 10.1081/MC-120025976

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©2003 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016

JOURNAL OF MACROMOLECULAR SCIENCE�

Part C—Polymer Reviews

Vol. C43, No. 4, pp. 505–545, 2003

Azido Polymers—Energetic Binders for

Solid Rocket Propellants

Bharti Gaur, Bimlesh Lochab, V. Choudhary, and I. K. Varma*

Centre for Polymer Science and Engineering,

Indian Institute of Technology Delhi,

Hauz Khas, New Delhi, India

CONTENTS

1. INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 506

2. PREPARATION OF AZIDO POLYMERS. . . . . . . . . . . . . . . . . . . . . . . 508

2.1. Polymerization of Glycidyl Azide (GA) . . . . . . . . . . . . . . . . . . . . . . 508

2.1.1. Derivatization of polyepichlorohydrin (PECH). . . . . . . . . . . 508

2.2. Direct Conversion of Epichlorohydrin to Glycidyl Azide

Polymer (GAP).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513

2.3. Simultaneous Degradation and Azidation of PECH and

Its Copolymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 514

2.4. Block Copolymers of GAP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515

2.5. Polymerization of Azidomethyl Oxetanes . . . . . . . . . . . . . . . . . . . . . 515

2.6. Poly(allyl azide) (PAA). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519

3. PHYSICAL PROPERTIES OF AZIDO POLYMERS . . . . . . . . . . . . . . . 520

4. CURING OF GAP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 522

4.1. Curing Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 522

4.2. Cure Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524

*Correspondence: I. K. Varma, Centre for Polymer Science and Engineering, Indian Institute

of Technology Delhi, New Delhi 110 016, India; Fax: 91-11-26591421; E-mail: ikvarma@

homail.com.

505

DOI: 10.1081/MC-120025976 1532-1797 (Print); 1532-9038 (Online)

Copyright & 2003 by Marcel Dekker, Inc. www.dekker.com

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4.3. Problems of GAP-Isocyanate Cure . . . . . . . . . . . . . . . . . . . . . . . . . 525

4.4. Characterization of Cured Network. . . . . . . . . . . . . . . . . . . . . . . . . 525

4.5. Curing of Azido Polymers with Dipolarophiles. . . . . . . . . . . . . . . . . 526

5. THERMAL BEHAVIOR OF AZIDO POLYMERS. . . . . . . . . . . . . . . . . 528

5.1. Uncured Azido Polymers—Linear and Branched . . . . . . . . . . . . . . . 528

5.2. Uncured BAMO And AMMO Polymer . . . . . . . . . . . . . . . . . . . . . . 532

5.3. Thermal Behavior of Cured Azido Polymers . . . . . . . . . . . . . . . . . . 533

6. PHOTODECOMPOSITION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 535

7. FORMULATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 536

7.1. Plasticizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 536

7.2. Oxidizers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537

7.3. Pyrolants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 539

7.4. Ballistic Modifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 539

8. CONCLUSION AND FUTURE OUTLOOK . . . . . . . . . . . . . . . . . . . . . 540

ACKNOWLEDGMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 540

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 540

Key Words: Energetic binders; Azide polymers; Poly(allylazide); Oxidizers;

Thermal behavior.

1. INTRODUCTION

High-energy solid rocket propellants are composite materials having abinder [hydroxy terminated polybutadiene (HTPB)], high-energy additives[e.g., ammonium perchlorate (AP)], and pyrolants (metallic powder). HTPB is aninert binder, which has been used in cast-cure propellant systems. Substitution ofHTPB by a more energetic binder may lead to an enhancement in performance ofsuch propellants.

Ammonium perchlorate, the high-energy additive, has its own inherent disad-vantages, as it produces chlorine rich combustion products (30%), which havepotential environmental implications.[1] Further, it produces white smoke trails,which are disadvantageous for specific applications demanding smokeless plume.Ammonium nitrate (AN) has long been considered a highly desirable oxidizer forsolid-fuel rocket propellants because of its extremely low cost, low sensitivity, lowsignature, and absence of halogens. However, it has a crystalline phase stabilityproblem that causes unpredictable ballistic performance in some cases and cata-strophic rocket motor failure in others. This has limited the use of AN withoutthe phase stabilizers.[2]

A new generation of propellants and explosives for missiles and space applica-tion is being developed worldwide these days. The operational trends for futureapplications of these energetic materials are to improve performance, satisfy insen-sitive munitions, comply with environmental aspects, and reduce costs.

The use of energetic additives, mainly binders and plasticizers, is consideredto be one of the practical ways to improve the energy level and other

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technical performances of solid propellants. Recent advances in propellantsresearch, therefore, are aimed at developing high energy binders which give betterperformance than the commonly used HTPB and which are compatible with the eco-friendly oxidizers ammonium nitrate (AN), hydrazinium nitroformate (HNF), orammonium dinitramide (ADN).

Azido polymers have attracted researchers’ attention for the past two decadessince the azido group contributes a positive heat of formation of 313–397 kJ/mol.This is evident from a comparison of heat of formation of ethanol (�278.5 kJ/mol)and 2-azidoethanol, N3CH2CH2OH (þ104 kJ/mol). Compatible propellants withacceptable physical properties could be formulated from azide-containing ingredi-ents.[3] These polymeric azides include poly(glycidyl azide) (GAP) and its copoly-mers, poly(bis-azidomethyl oxetane) [poly(BAMO)] and its copolymers, andpoly(azidomethyl methyloxetane) [poly(AMMO)].[4] These polymers contain azidegroups along the polymer chain as pendant groups, which make them highlyenergetic. The exothermic scission of these –N3 bond releases energy of theorder 685 kJ/mol.[5] The GAP prepolymer is cured by reacting the terminalhydroxy groups with isocyanates, as is done in the state-of-the-art hydroxy termi-nated binder.[6,7] These propellants, in addition to being fuel rich, liberate largeamounts of carbon monoxide, and hydrocarbon on their burning, and are greatlyinsensitive to impact, and provide high burn rates.

The traditional method used for propellant formulation is to mix liquidGAP polymer (Mn<3000) (binder) with curing agents (diisocyanate) and chainextender, trimethylol propane (TMP), high-energy additives, and metallicpowder. However, the energy output of such propellants will be significantlyreduced due to inert properties of diisocyanates and TMP. Another disadvantageof isocyanate cured hydroxy terminated GAP is that the propellant would notbe recyclable, as the crosslinks are irreversibly formed covalent chemical bonds,and thus recovery of energetic ingredients and binders is not possible. Therefore,an elastomeric GAP with Mn>1.4� 105 has been developed to overcome thisproblem. The synthesis and use of recyclable thermoplastic elastomer (TPE)based on azido polymers is gaining importance in the field of energetic binders.

An attractive approach to high energy, low sensitivity propellants involve theuse of energetic oxetane prepolymers. New gun propellants have been developedwhich utilize the thermoplastic elastomer (TPE) [i.e., poly(3,3-bis(azidomethyl),oxetane (BAMO)–3-azidomethyl-methyl oxetane (AMMO)] as the binder and theenergetic solid trimethylenetrinitramine (RDX). These TPE based propellantshave many desirable properties including the ability to be reused, recycled, andreclaimed, an excellent balance of flame temperature and the performance.[8]

Oxetane TPE propellants can readily be mixed and extruded in a twin-screwextruder without aid of any solvents.[9] These binders act as energy partitioningagents by allowing an energetic formulation to maintain a constant energy level ata lower solids percentage.[10]

Although considerable work has been done on high-energy azido plasticizers(molecular weight �500) and binders, a comprehensive review dealing with thestate-of-the-art of these polymers has not been published. This article reviews thesynthesis, characterization, physicochemical, thermal, and mechanical properties ofpolymeric binders containing pendant azido groups.

Azido Polymers 507

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2. PREPARATION OF AZIDO POLYMERS

2.1. Polymerization of Glycidyl Azide (GA)

The starting monomer, GA is prepared by reacting epichlorohydrin (ECH) withhydrazoic acid to give 1-azido-3-chloro-2-propanol, which is cyclized in the presenceof a base (Sch. 1).

The polymerization of glycidyl azide (GA) was then attempted to be carried outusing the carbocationic method. However, this procedure was unsuccessful as themonomer was found to be unreactive.[3]

2.1.1. Derivatization of Polyepichlorohydrin (PECH)

An alternative route, based on the polymerization of ECH to polyepichloro-hydrin (PECH), followed by the conversion of PECH to GAP by nucleophilicdisplacement of chloride by azide is more successful. Linear or branched hydroxyterminated polyepichlorohydrin has been prepared by ring opening polymerizationof the oxirane group in ECH in the presence of a diol (ethylene glycol) or a triol(1,2,3-propane triol). Azido substitution reaction has been accomplished by reactingwith sodium azide using dimethyl formamide or dimethyl sulfoxide as solvents lead-ing thereby to formation of GAP-diol or GAP-triol (Schs. 2 and 3).

The details of reaction conditions used for the synthesis of these polymers aredescribed below.

H O CH2CH

CH2Cln

O CH2CH2OOCH2CH

CH2Cln

H

H O CH2CH

CH2N3n

O CH2CH2OOCH2CH

CH2N3 n

H

NaN3

HOCH2CH2OHCHCH2

O

CH2Cl +

PECH -DIOL

GAP-DIOL

n

Scheme 2. Preparation of GAP-diol.

CH2

O

CHCH2ClNaN3

HOAcN3CH2CHCH2Cl

OH

OHN3CH2CH CH2

O

-

ECH GA

Scheme 1. Synthesis of GA.

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(a) Synthesis of Polyepichlorohydrin

Polyepichlorohydrin has been synthesized by carrying out the polymerizationof ECH. The conversion depends on the nature of initiator system, monomer toinitiator ratio, solvents, reaction temperature etc. Bronsted acids (e.g., CF3SO3H)as well as Lewis acids (protogens or cationogens) have been used as initiators.[11]

Generally a monomer: initiator ratio in the range of 40� 4:1 has been used. Thepercentage conversion was low when Bronsted acids were used as initiators.Hydrocarbons (toluene), ethers (dioxane, diethyl ether), or chlorinated hydro-carbons (1,2-dichloroethane) have been used as solvents in this polymerization.The effect of catalysts (such as SnCl4, BF3-etherate, or CF3SO3H), solvents,reaction time, and reaction temperature on the percentage yield has been inves-tigated (Table 1).

The polymers thus prepared had low molecular weight and a wide molecularweight distribution. The molecular weight distribution obtained from the GPCchromatograms is given in Table 2.

As is obvious from Table 2, the product distribution is dependent upon theinitiators used. Product obtained when BF3-etherate was used as an initiator hadmolecular masses in the range of 200–2000. When SnCl4 was used as an initiator themolecular mass was considerably high as compared to CF3SO3H or BF3-etherate.When 1,2-dichloroethane or toluene was used as solvent, the reaction was rapid andthe molecular weight distribution was wide. It was noted that as much as 60% of thetetramer was obtained in the system with BF3-etherate in 1,2-dichloroethane. Withethereal solvents, the reaction was slow. High basicity of ethereal solvents is expected

n

n

CH2

O

CH HOCH2CHCH2OH

OH

H O CH2CH

CH2Cl n

OCH2CHCH2O 2CHO

O 2CHO

H

HCH

CH2Cl

CH

CH2Cl

NaN3

n

n

H O CH2CH

CH2N3 n

OCH2CHCH2O 2CHO

O 2CHO

H

HCH

CH2N3

CH

CH2N3

CH2Cl +

Propanetriol

PECH-TRIOL

Scheme 3. Preparation of GAP-triol.

Azido Polymers 509

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to suppress the propagation reaction and promote a backbiting reaction to producelow molecular weight cyclic oligomers according to reaction (Sch. 4).

As opposed to typical chain end mechanism mentioned above, the polymeriza-tion of ECH could also proceed in the presence of glycols by activated monomermechanism (AMM), which involves the concept of living polymerization that has notermination or transfer process. In AMM, an activated i.e., protonated or positivelycharged monomer molecule inserts into a growing polymer chain having an -OHgroup at the chain-end (Sch. 5).[12,13] This mechanism leads to a polymer having two-OH groups at the chain ends. Here Mn¼ [ECH]/[glycol]. Side reactions, includingcyclization, are strongly suppressed in the polymerization by AMM and well-definedlinear products are obtained. The activated monomer mechanism operates at low

Table 2. Molecular weight distribution of PECH synthesized under different reaction

conditions (sample designation same as Table 1) (data adapted from Ref.[11]).

Sample

designation n¼ 2 n¼ 3 n¼ 4 n¼ 5 n¼ 6 n¼ 7

A-1 2 0 43 14 6 35

A-2 18 1 4 22 20 35

A-3 1 0 26 12 7 54

A-4 3 2 13 22 19 41

A-5 5 0 60 31 4 0

B-1 5 0 9 11 7 68

B-2 15 0 5 6 6 68

B-3 9 0 12 12 11 56

C-1 45 0 8 21 16 10

C-2 32 0 13 17 14 24

n is the degree of polymerization.

Table 1. Effect of reaction parameters on percent yield of polyepichlorohydrin (data adapted

from Ref.[11]).

Sample

designation Initiator Solvent

Monomer/initiator

ratio

Time

(h)

Yield

(%)

A-1 BF3OEt2 1,2-Dichloroethane 44:1 4.0 98

A-2 Dioxane 42:1 4.0 11

A-3 Toluene 36:1 4.0 88

A-4 Ethyl ether 36:1 4.0 5

A-5 1,2-Dichloroethane 36:1 4.0 19

B-1 SnCl4 1,2-Dichloroethane 54:1 3 75

B-2 Dioxane 43:1 2.5 50

B-3 Toluene 44:1 23.5 73

C-1 CF3SO3H 1,2-Dichloroethane 44:1 7.0 2

C-2 Dioxane 38:1 23.0 12

Temperature �5�C (with dioxane �15�C).

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monomer concentration, which requires slow addition of monomer to the reactionmixture. The PECH-diols obtained under these conditions, had controlled molecularweight up to Mn� 2500 g/mole and with Mw/Mn<1.2. The content of cyclic oligo-mers did not exceed 0.7%. PECH of Mn� 2000 was synthesized by using borontrifluoride–ethylene glycol as catalyst while aluminium triethyl–ethylene glycolyielded PECH of Mn more than 100,000.[12]

(b) Azidation of PECH

Azidation of PECH to GAP has been carried out in an organic or an aqueoussolvent,[3,14] using an ionic azide such as lithium azide, sodium azide, or potassiumazide. A molar excess of ionic azide is needed for this purpose.[14] Complete conver-sion of polyepichlorohydrin to GAP with sodium azide in dimethyl sulphoxide(DMSO) is reported to occur at 90–95�C within 12–18 h, whereas the reaction in

R OH H O R O OH H++

++

""

Initiation:

OH H O O OH H

Propagation:

... + +... + +

""

Scheme 5. Activated monomer mechanism for synthesis of PECH.

The notation ‘‘Hþ’’ means that the proton is not located on any specific oxygenatom, but is rapidly exchanging between various sites.

OCHCH2

CH2Cl

O

CH2Cl

O

CH2Cl

:

OCHCH2-OCHCH2

CH2Cl

O

CH2ClCH2Cl

O..

CH

CH2Cl

CH2

OCH2 CH CH2Cl

CH2

CHO

CH2

CH

O

CH2Cl

ClH2C

O O

CH2Cl

CH2Cl

A

A

A-+ +

polymer

back-biting

+

propagation

+

volatile dimer

-

propagating chain

-ECH + propagating

chain

Scheme 4. Chain growth and backbiting reaction during ECH polymerization.

Azido Polymers 511

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water, in the presence of a phase transfer catalyst (methyl tricapryl ammoniumchloride) required seven days for completion at the same temperature.Nevertheless, both processes yielded GAP of a comparable quality based on elemen-tal analysis, equivalent and molecular weight determination, and thermal behavior.The conversion of PECH to GAP can be easily monitored by IR spectroscopy byfollowing the disappearance of the absorption band at 748 cm�1 (due to -CH2Clgroup) and appearance of the 2099 cm�1 band (due to azido group) (Fig. 1). Fast

Figure 1.

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neutron activation analysis of nitrogen and chlorine in the final product has alsobeen used to monitor the conversion of PECH to GAP.[15]

In the above-mentioned azidation reaction, carried out in solution, one of theproblems is the slowing of the reaction rate above �90% conversion. This decreasein rate, which more than doubles the reaction time from the initial rate, is a con-sequence of the association of the metal cation (e.g., Naþ or Liþ) of the azide saltwith the solvent medium, in which it has limited solubility. In a molten salt method,quaternary ammonium azide such as tetrabutyl ammonium azide and trace amountof water is mixed with PECH and heated at �105�C for about 0.33 h. The resultingliquid product on washing with water and drying showed 85–90% conversion ofPECH to GAP.[16]

Low molecular weight GAP (�500), which could be used mainly as an energeticplasticizer, can be prepared by a one-step method, which involves direct conversionof ECH to GAP.[17] The plasticizer helps in processing of the propellants based onGAP and enhances the stability and mechanical properties. However the hydroxyend-groups of these glycidyl azide polymers react with the isocyanate curing agentleading thereby to a loss of plasticization effect. A diazide terminated azide polymerhas, therefore, been developed to overcome this difficulty. Tosylation of hydroxyterminated polyepichlorohydrin with p-toluene sulfonyl chloride (in presence of pyr-idine) followed by replacement of chlorine and tosyl group by azide (sodium azide inDMF) is a convenient approach for the synthesis of such polymers (Sch. 6).

Azido terminated glycidyl azide polymers (GAP-A) have also been prepared bynitration of hydroxy terminated polyepichlorohydrin and subsequent reaction withsodium azide[18] (Sch. 7).

2.2. Direct Conversion of Epichlorohydrin to

Glycidyl Azide Polymer (GAP)

A single step process for preparation of hydroxy terminated GAP(Mn� 500 g/mol) has been described,[19] which is less time consuming and morecost effective. In this method sodium azide was gradually mixed with ECH (1:1molar ratio), DMF, and EG and the contents were initially stirred at 70�C. Sincethe reaction was exothermic in the initial stages, the temperature was controlled

PECH(OTs)2

NaN3

DMF

OCH2CH2 H

CH2Cl

CH2 CH O

n

H O CH CH2

CH2Cl n

O

PECH(OH)2

OCH2CH2 CH2 CH O

n

O CH CH2

n

O

CH2N3CH2N3

N3N3CH CHCH2CH2

CH2N3 CH2N3

p-toluenesulfonylchloride

pyridine

-1 -1

Scheme 6. Tosylation of PECH followed by azidation.

Azido Polymers 513

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during the first 30min. The reaction temperature was then raised to 90�C and stirringwas carried out at this temperature for 15 h. The yield was high (�90%).

2.3. Simultaneous Degradation and Azidation of PECH

and Its Copolymers

Branched GAP or copolymer of GAP (70%) with ethylene oxide (30%) (GEC)having variable molecular weight (500–40,000) has been synthesized directlythrough a simultaneous degradation and azidation process by reacting a highmolecular weight rubbery poly(epichlorohydrin) (PECH) or poly(epichlorohydrin-co-ethylene oxide) (PEEC), with the epichlorohydrin (ECH) monomer and sodiumazide (NaN3).

[20,21] Degradation and azidation of a commercial rubbery PECH orPEEC with NaN3 and lithium methoxide (basic cleaving agent) in the presence of apolyol at 120�C in polar organic solvents (DMSO, DMAc, DMF, etc.,) without theuse of ECH monomer has also been reported.[22–24] The polyols used in this synthesiswere 1,2,3-propane triol (PPT), trimethylol propane (TMP), and pentaerythritrol(PE) (Table 3). DMSO is preferred over the other solvents due to the quality ofthe final product.

OCH2CH2 H

CH2Cl

CH2 CH O

n

H O CH CH2

CH2Cl n

O

HNO3 / H2SO4

OCH2CH2

CH2Cl

CH2 CH O

n

O CH CH2

CH2Cl n

O NO2O2N

NaN3

OCH2CH2 CH2 CH O

n

O CH CH2

n

O

CH2N3CH2N3

N3N3CH CHCH2CH2

CH2N3 CH2N3

Hydroxy terminated PECH

Polyepichlorohydrin nitrate

-1 -1

Scheme 7. Nitration of PECH followed by azidation.

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The observed polydispersity index (Mw/Mn) is between 1.70 and 2.10. This typeof molecular weight distribution may be considered relatively low for polymersresulting from a degradation process. GEC had a higher viscosity than GAP witha similar molecular mass. This may be attributed to the presence of ethylene oxidegroups in the polymer chain.

2.4. Block Copolymers of GAP

A triblock copolymer poly(glycidylazide-b-butadiene-b-glycidyl azide) (GAP-PB-GAP) has been reported by Vasudevan and Sundarajan[25] (Sch. 8). In thesynthesis of this block copolymer, HTPB was used as an alcohol and boron trifluo-ride etherate (BF3-etherate) as catalyst. Epichlorohydrin was added to the catalystmixture dropwise. The polymerization was carried out at 0�C for 12 h and then for4 h at room temperature (30�C) under nitrogen atmosphere. Epichlorohydrin adds toboth the hydroxyl groups of HTPB leading to the formation of a secondary alcohol,which is able to react with epichlorohydrin by the same pathway to yield poly-(epichlorohydrin) blocks. Ring opening polymerization was followed by conversionof the -CH2Cl group into -CH2N3 group using sodium azide in DMSO at 105�Cfor 10 h.

2.5. Polymerization of Azidomethyl Oxetanes

Polymerization of substituted four membered oxetanes can produce polymerswith higher molecular weight and higher elongation as compared to GAP.[26] Thefunctionality and molecular weight of the resulting polymer may be controlled easilyas compared to epoxides. Therefore, polymers and copolymers of oxetanes contain-ing azido groups, such as 3-methyl-3-azidomethyl oxetane (AMMO) and 3,3-bis-azidomethyl oxetane (BAMO) have been investigated in the past.[26] The synthesisof poly(BAMO) has been carried out using two different procedures. In one of themethods, 3,3-bis(chloromethyl) oxetane (BCMO) was treated with sodium azide indimethyl formamide (DMF) at 85�C for 2 h. The bisazidomethyl oxetane obtainedwas then homopolymerized or copolymerized with tetrahydrofuran (THF) bycarbocationic ring opening polymerization using BF3-etherate as catalyst in the

Table 3. Solution properties of GAP and copolymers prepared by simultaneous degradation

and azidation in presence of polyols (data adapted from Ref.[20]).

Polymer Polyol [n] (dL/g) Mn Mw

GAP homopolymer PPT 0.122 10,600 18,100

TMP 0.125 11,000 18,500

PE 0.141 11,400 22,700

GEC copolymer PPT 0.185 10,400 20,900

TMP 0.216 14,700 28,500

PE 0.183 11,600 20,400

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presence of butanediol as an initiator at �5�C. The other method involved the ringopening polymerization of BCMO or copolymerization with THF usingbutanediol as initiator and BF3-etherate as catalyst at �5�C. The polymer or thecopolymer obtained was then treated with sodium azide in DMF (solvent) at 90�C(Sch. 9). The starting material, BCMO is easily obtained by ring closure of trichloroderivative of pentaerythritol.[27]

Quasi-living cationic polymerization of 3-azidomethyl-3-methyloxetane(AMMO) was achieved with a bis(chlorodimethylsilyl)benzene/Ag hexafluoroanti-monate (AgSbF6) initiating system in methylene chloride at �78�C. The averagemolecular weight (Mn) increased linearly with an increasing monomer/initiator([M]/[I]) ratio and a linear Mn vs. [M ]/[ I] ratio plot passed through the origin.[28]

Both ends of the difunctional initiator are activated.[29]

Block copolymers of BAMO with other cyclic ethers have been synthesized withan aim of developing energetic thermoplastic elastomers (TPE). The TPE thusprepared have high molecular mass, low polydispersity index, low Tg, and goodenergetic characteristics. Block and random copolymers of THF and BAMO havebeen prepared by using triflic anhydride (CF3SO2)2O as a bifunctional initiator.[30]

HO OH

OCH2Cl

BF3-etherate

O OCHCH2CH2CH

CH2ClCH2Cl

OHHO

OCH2Cl

O O CH2CH

CH2ClCH2Cl

O CH2CH

CH2Cl

OHO

CH2Cl

HO

NaN3, DMSO

O OCHCH2CH2CH

CH2N3CH2N3

O CH2CH

CH2N3

OHOCHCH2

CH2N3

HO

CHCH 2CHCH2

n

n

n

n n

n

105ºC, 10h

n

n

n

2

Scheme 8. Preparation of block copolymer poly(GAP-b-PB-b-GAP).

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Homo as well as block copolymers of low polydispersity index (1.23–1.28) could beprepared by using this initiator. In random copolymerization of THF and BAMO, r1and r2 values were found to be 0.33 and 1.12 respectively, showing thereby higherreactivity of BAMO.

Novel energetic oxetane derivatives 3-nitratomethyl-3-methyl oxetane (NMMO)and AMMO could be polymerized by triflic anhydride [(CF3SO2)2O]. The livingcationic characteristics of the polymerization confirmed via a 19FNMR technique.Novel polymers of the A-B-A type with various molecular weight (Mw¼

14,320–40,660) and low polydispersity index (PDI¼ 1.11–1.29) were obtained.[31]

Analysis of the polymer by different techniques (NMR, IR spectroscopy,and DSC) showed that the structure of the polymer was composed of a two-phasedomain structure of amorphous poly(NMMO) phases and crystalline poly(AMMO)phases.

The synthesis of triblock copolymer of BAMO, AMMO, and bis-ethoxymethyloxetane (BEMO) with center block composed of BAMO and AMMO has beenreported by Murphy et al.[32] The copolymerization required 1,4-butanediol andBF3-etherate in the ratio of 1:2. A solution of monomer (AMMO or BAMO) inmethylene chloride was added drop wise to the catalyst system at �10�C. A newmonomer (BAMO or AMMO) was added when more than 95% conversion of theprevious block is reached. The block copolymers are thus sequentially polymerizedin a ‘‘living polymer’’ manner according to activated monomer mechanism.

Synthesis of BAMO and nitratomethyl methyloxetane (NMMO) block copoly-mers has been reported by using 1,4-butanediol/BF3-etherate initiating agent.[33]

A triblock copolymer BAMO–NMMO–BAMO was prepared by using p-bis(a,a-dimethylchloromethyl)benzene ( p-DCC) with silver hexafluoroantimonate(AgSbF6) as initiator.[34] The propagating chain end is a carboxonium ion. Thepolymerization has been carried out in nitrogen atmosphere at low temperature(�70�C) using methylene chloride as solvent. The centre block of NMMO isprepared first, according to Sch. 10.

The cationic copolymerization of AMMO was initiated by a mixtureof BF3�OEt2 and diethylene glycol. The copolymerization of AMMO with3-(azidomethyl)-3-(2,5-dioxaheptyl)oxetane and 3-(azidomethyl)-3-(2,5,8-trioxadecyl)oxetane(II) was investigated in order to obtain (for solid propellants)

OCH2Cl

CH2Cl

NaN3

DMFO

CH2N3

CH2N3CH2N3

CH2N3

BF3.etherateO CH2 C

OCH2Cl

CH2Cl

BF3.etherate CH2Cl

CH2Cl

O CH2 C

CH2

CH2

NaN3

+85ºC , 2 h butanediol n

butanediol n DMF, 90ºC

Scheme 9. Synthetic route for the preparation of poly(BAMO).

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materials with less of a crystalization tendency and a lower glass transition tem-perature, (Tg) than poly(AMMO), for example, when AMMO was copolymerizedwith 15wt.% II, it was possible to obtain bifunctional oligomers with Tg �50 to�60�C, without significant loss in the nitrogen content.[35]

Linear ABA triblock and (AB)n segmented block copolymers of energetic mono-mers were synthesized. The rigid and soft blocks are prepared from AMMO andNMMO respectively. Polymerization of AMMO initiated by triethyloxonium tetra-fluoroborate and by spiro (benzoxasilole)/propanediol produced a-mono-hydroxy-poly-AMMO, and o,o0-dihydroxy-poly-AMMO of number averagemolecular weight-16,000 and 2000, respectively and a,o-dihydroxy-poly-NMMO(Mn¼ 13,000).[36] A spiro(benzoxasilole) catalyst, 3,3,30,30-tetrakis(trifluoro-methyl)-1,10-(3H,3H)-spirobis(1,2-benzoxasilole), was used to polymerize 3,3-R,R0-oxetanes: (R,R0

¼ ethoxymethyl; R¼ azidomethyl, R0¼Me; R¼ nitratomethyl,

R0¼Me; and R,R0

¼ azidomethyl) with descending rates in this order. 31PNMRspectra of polymerization mixtures quenched using Bu3P were consistent with anoxonium ion propagating species.[37]

New energetic azide group-containing polyesters have recently beenreported. These are prepared from 2,3-dibromosuccinic acid and 1,2-propanedioland 3-chloropropanediol and succinic/malonic acid. These hydroxyl terminatedpolymers had a functionality of approximately 2.0. The number average molecular

CC

CH3

CH3

CH3

CH3

Cl Cl

:OCH3

CH2NO2

AgSbF6

C

CH3

CC

CH3

CH3

CH3

CH3

CC

CH3

CH3

CH3

CH3

CC

CH3

CH3

CH3

CH3

OCH3

CH2NO2

CC

CH3

CH3

CH3

CH3

O CH2

CH2NO2

CH2O

CH3

CH2NO2

+

++

++

NMMO

++

NMMO

+

Propagating chain

Scheme 10. Preparation of triblock copolymer BAMO–NMMO–BAMO.[34]

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weight of the bromopolyester and chloropolyesters of malonic/succinic acid werefound to be 1280, 1079, and 825, respectively. These halogenated polyesters wereconverted to the corresponding azido polyesters via reaction with sodium azide. Thedecomposition temperatures of these azido polyesters were around 235, 210, and228�C respectively.[38]

Several new energetic polymer systems poly(BAMO-co-GAP) and poly(BAMO-co-glycidyl nitrate (PGN)) and poly(BAMO-co-AMMO) have been evaluated asenergetic thermoplastic elastomeric binders for composite rocket propellants.BAMO-GAP had moderately high density, high heat of formation, and high burningrate, which should make it attractive in formulations requiring high-burning-ratecompounds. BAMO-PGN had a very high density, a favorable oxygen balance,and a reasonably high heat of formation. These properties make BAMO-PGNand BAMO-GAP ideally suited as binders for high-performance energeticcompounds.[39]

2.6. Poly(Allyl Azide) (PAA)

Azidation of poly(allyl chloride) (PAC) is a convenient route for the preparationof poly(allyl azide). However allyl chloride during free radical polymerization under-goes degradative chain transfer yielding polymers of relatively low molecular masses.Poly(allyl chloride) having molecular mass in the range of 2000 g/mol could beobtained by following the cationic route. Allyl chloride (0.5mol) was polymerizedin our laboratories using Lewis acids such as anhydrous TiCl4/FeCl3/AlCl3 andaluminium powder.[40] The concentration of the catalyst was varied from 0.7 to3.2mol % of the monomer. The quantity of aluminium powder was kept constant(2 g). The reaction was carried out at low temperatures (0–20�C) and brown-coloredviscous polymer was formed.

Chlorine in poly(allyl chloride) samples was found to be in the range of 35–40%,which is lower than the calculated value of 46.05%. On the other hand, the carboncontent was higher than the expected value. A discrepancy in chlorine and carboncontent of the polymers may be explained on the basis of branching of poly (allylchloride) (Sch. 11).

1HNMR spectrum of poly(allyl chloride) is shown in Fig. 2. The chloromethylprotons in poly(allyl chloride) were observed at 4.03 ppm. The vinyl proton reso-nance signals of allyl chloride at 5.22, 5.32, and 5.94 ppm were absent in poly(allylchloride). Additional resonance signals were observed at 0.88–2.27 ppm. The inte-gration of the resonance signals at 0.88–2.27 ppm did not correspond to the numberof methyl and methylene protons in the polymer backbone for a linear-chain poly-mer. The loss of chlorine, as indicated by elemental analysis results, also suggests theformation of a branched polymer. Therefore, on the basis of elemental analysis and1HNMR it was concluded that poly(allyl chloride) was a branched polymer. Theazidation of poly(allyl chloride) was carried out using sodium azide and dimethylsulphoxide as solvent at 100�C for 12 h. The conversion of PAC to PAA was mon-itored by IR spectroscopy by following the decrease in the intensity of 736 cm�1

absorption band due to -CH2Cl and appearance of a strong band due to -CH2N3 at2095 cm�1 (Fig. 3).

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3. PHYSICAL PROPERTIES OF AZIDO POLYMERS

Depending on the backbone structure and molecular mass, the Tg of linear azidopolymers range from �20 to �70�C. The glass transition temperature of GAP wasfound to be �45�C. Tg of branched GAP and branched glycidyl azide-ethylene oxidecopolymer is in the range of �60 to �70�C.[20] The physical properties of variousglycidyl azide polymers are given in Table 4.[41,42] Since GAP is in a rubbery state at

Figure 2.

CH2 CH

CH2Cl

CH2 CH

CH2Cl

AlCl3 FeCl3 CH2 CH CH2 CH

CH2ClCH2

AlCl4FeCl4

CH2 CH

CH2

CH2 CH

CH2Cl

CH2

CH

CH2Cl

CH2 CH CH2Cl

or

+

+- -or

Scheme 11. Probable side reaction during the synthesis of poly(allyl chloride).

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temperatures above Tg, which is lower than the lowest operational temperature(�40�C) for rocket monitors, it can be considered a mechanically safe binder forcomposite propellant.[43] Tg values of GAP increased with increase in the quantityof decomposed -N3 groups. Branched GAP cured with IPDI has a Tg �23� 3�C.

Figure 3.

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The presense of ethylene oxide units in branched GAP reduced the Tg to�40� 2�C.[21]

The Tg of poly(AMMO) (molecular weight 2800–6700 g/mol) was determined byDSC and was in the range �51.5 to �45.5�C.[44] Poly(AMMO-b-NMMO-b-AMMO) had a Tm¼ 82�C and Tg¼�3�C was phase separated in the melt. Thiscopolymer decomposed at 224�C evolving 0.69 kJ/g of heat. It has excellentmechanical properties with elongation of 683% at break, 5.25MPa stress, and82% recovery. The poly(AMMO-b-NMMO)n copolymer has similar thermal prop-erties and spectroscopic characteristic, but is inferior to tri block copolymer in allrheological and mechanical properties.[36]

BAMO homopolymer has a Tg of �28�C and Tm of 76–80�C.[26] The Tg isreduced to �60�C in the copolymers of BAMO and THF.[26] The viscosity is inthe range of 500–5000 cps, depending on the molecular mass (500–5000 g/mol).The functionality of the linear polymer is 1.5–2.0 and for the branched version itvaried from 5.0–7.0. The burning rate of GAP is very sensitive to the amount of thecuring agent. The material (88.5% GAP) self-extinguishes at ambient pressure andburns at 1.96 cm/s at 6.89MPa.[3]

A maximum burn rate of 1.68 cm/s has been reported for GAP cured with TDIor isophorone diisocyanate.[6] Burning rate of BAMO polymer is less than thatof GAP.[4]

4. CURING OF GAP

4.1. Curing Agents

Hydroxy terminated azido polymers are cross-linked by polyisocyanates via aurethane forming reaction. Functionality greater than two must be used to ensurereticulation. In such systems, different mechanical properties of the binder can beobtained by adjusting the parameters of the curing reaction and the componentconcentrations, which result in varying cross-link density of the matrix. A schematic

Table 4. Characteristics of azide polymers (data adapted from Refs.[41,42]

Sample designation

Properties

GAP-DIOL

hydroxy

terminated

GAP-TRIOL

hydroxy

terminated

GAP-A

azide

terminated

�Hf (kJ/g) 1.170 1.170 2.299

�Hr (kJ/g) 1.839 1.630 —

Density (g/cc) 1.29 1.29 1.27

Appearance Light yellow

liquid

Light yellow

liquid

Light yellow

liquid

Tg (�C) �45 �45 �56 to �69

�Hf¼ heat of formation, �Hr¼ heat of reaction.

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representation of the preparation of polyurethanes by such a polyaddition procedureis shown in Sch. 12.

The choice of an appropriate curing agent is of prime importance to meet theminimum strain, stress, and hardness levels of mechanical properties, whichcrucially affect the performance of the propellant.[45] Commonly used isocyanatesare isophorone diisocyanate (IPDI), toluene diisocyanate (TDI), 4,40-diisocyanatodiphenylmethane (MDI), 4,40-dicyclohexyl methane diisocyanate (H12MDI),Desmodur N-100. The molecular formulae of some of these curing agents[7,46] aregiven in Sch. 13.

The polyol reactants, coupling agents or chain extenders, which are generallyexplored in the synthesis of polyurethanes, are triethanolamine, hexanetriol,trimethylol propane,[47] glycerol, pentaerythritol , pyrogallol.[21] Their main function

CH 2NCO

NCOOCN

H 12 MD I

NCOOCNH 3 C

IPDI

NCO

N

O

O

NH

NH

NCONCO

Desmodur N-100

CH 3

NCO

NCO

CH 3

NCOOCN

80% + 20% TDI

Scheme 13. Structure of polyisocyanates used for curing azido polymers.

R OH

N C O

N C O

OH O O CN H R'R O O CN H NH C O

HO O C N

R'

NH CO O R

O R N C O

R'

N C O

n + n

n - 1

Scheme 12. Urethane network formation.

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is to make the PU well cured. They have a noticeable effect on the curing reactionof azido polymer with the isocyanate and thus affect the mechanical properties.[23]

The network structure obtained by reacting glycidyl azide polymer (GAP) withDesmodur N-100 and pentaerythritol can be schematically represented as follows(Sch. 14).

Catalysts are often required to obtain complete curing and also to speed up thereaction, although the amount and type of catalyst depends on the nature of reac-tants. The most common types of catalysts used are organometallics (e.g., organotincompounds) and tertiary amines (triethyl amine, TEA). Dibutyltin dilaurate (DBTL)is known to be the most suitable catalyst for the urethane formation between longchain diols including GAP[41] and isocyanates. The curing time of GAP is reducedfrom three weeks to 5–6 days at 60�C in the presence of DBTL (30–75 ppm).[48]

When composite catalyst, triphenyl bismuth (TPB) and DBTL are used for curingGAP-diisocyanate in the presence of trimethylol propane (TMP), curing time of 3–4days is required.[47–49] This composite catalyst suppresses CO2 formation andpromotes cross-link density.

4.2. Cure Monitoring

The curing of GAP is a slow process. The mixtures having a NCO/OH ratio of 0.7or 0.8 require a longer curing time (about three weeks) for a complete curing than dothe samples with higher NCO/OH ratio (0.9–1.2) (about two weeks). The curingreaction has been monitored by gel-time determination, viscosity and hardness mea-surements, FTIR spectroscopy, and DSC studies. Gel-time, which determines the potlife of a propellant and is related to the extent of cross-linking, affects the processing ofan uncured propellant. Propellant slurry should have a reasonably low viscosity andlong enough pot life to make it castable. Gel-time can be determined by viscositymeasurements and is strongly dependent on the amount of curing catalyst,[48] natureof isocyanate, and ratio of NCO/OH. 4,40-Diisocyanato diphenylmethane (MDI),exhibited highest rate of curing while IPDI reflected lowest rate of reaction.[21]

The hardness increases as the curing reaction proceeds and reaches a plateau ofconstant value, thereby indicating the completion of curing reaction of GAP. Theultimate hardness of the cured GAP increased with an increase in NCO/OH ratio.The propellants become harder upon solid loading. The binder with a NCO/OHratio of 0.8 had the lowest value of hardness and therefore may be considered to bemore suitable for composite propellant applications.[49] Monitoring the hardnessgives an apparent curing completion but not a true chemical completion. There

GAP

OCOHN

OCOHNO N

O

HN

HN ONHOCO

O

O

O

Scheme 14. Network structure of cured GAP using Desmodur N-100.

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might be a significant number of unreacted GAP molecules, which could notparticipate in curing due to the diffusion-controlled nature of the reaction.

The Tg of pure GAP, which is around �45� 1�C[48] increases on curing by 6–9�C and depends on the NCO/OH ratio. An increase in the extent of curing (byincrease in NCO/OH ratio) leads to an increase in Tg values. Addition of plasticizers(i.e., bis-2,2-dinitropropyl acetal and bis-2,2-dinitropropyl formal (BDNPA/F) didnot affect the curing reaction but reduced Tg to �47�C which is low enough toproduce a rubbery propellant.[48]

Kinetics of polyurethane formation between GAP and N-100 of catalyzed aswell as uncatalyzed reaction has been studied in the bulk state by quantitative FTIRspectroscopy by following a decrease in the intensity of NCO stretching band at2270 cm�1 and appearance of new band at 1726 cm�1. The enthalpy and entropy ofactivation for uncatalyzed reaction were found to be 44.1� 0.5 kJ/mol and�196�2 J/Kmol, respectively. The rate constant for the catalyzed and uncatalyzedreaction at 60�C was found to be 4.37, and 3.88� 10�6 Lmol�1s�1 respectively. Asignificant rate enhancement is observed in the presence of a catalyst at operatingconditions.[45]

4.3. Problems of GAP-Isocyanate Cure

There are certain problems, which are encountered while using isocyanatesas curing agents for hydroxy-terminated binders. The first and the most seriousproblem is the gas evolution, when GAP is cured with diisocyanates, because the-N¼C¼O group, reacts rapidly with water to release CO2 and forms numerous voidsin the cured explosive, resulting in a decrease of loading density, mechanicalstrength, performance, and safety. Hydroxyl terminated GAP, itself has inherenttrapped moisture due to hydroxyl groups, which is needed to be expelled beforecuring with isocyanates in order to prevent bubble formation. Void-free propellantsrequire de-gassing before curing. Addition of deflagerating additives eliminates thisde-gassing step.[50] A second problem is a significant decrease in the energy output ofplastic bonded explosives (PBXs) or propellants, because of the inert properties ofdiisocyanates and chain extenders (TMP). A third problem is the loss of plasticizingeffect during curing of the hydroxyl groups of GAP with isocyanates. Curing causesan increase in Tg of GAP and it increases with increasing in NCO/OH ratio.[50]

In order to avoid this problem, it is desirable either to add a plasticizer or to providea plasticizer without hydroxyl groups i.e., azide terminated GAP (GAPA). Inorder to keep both high-energy density and improved mechanical properties, useof elastomeric GAP (average molecular weight>1.4� 105) along with multi-hydroxy-functional branched GAP (B-GAP) has been reported.[47]

4.4. Characterization of Cured Network

Network structure of elastomers can be characterized by swelling, solubility, andmechanical measurements. The average molecular mass of the chain sectionsbetween cross-links, Mc, was calculated using Flory-Rehner equation from the swel-

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ling studies of GAP elastomers (Desmodur N-100, IPDI/TMP, and H12MDI/TMP)in tetrahydrofuran solvent at room temperature. The volume of the swollen networkprepared with the same ratio of NCO/OH depended on the structure of isocyanatesand was highest with IPDI and lowest with Desmodur N-100. Desmodur N-100cured elastomers for the same ratio of NCO/OH have smaller values of Mc thanthose of IPDI/TMP and H12MDI/TMP cured elastomers. A linear relationshipbetween sol fraction and Mc values was observed.

[7]

In order to assess the effect of residual DMSO in B-GAP on the properties ofcured energetic binders, small amounts of DMSO (2, 4, and 6% w/w) to that ofB-GAP were intentionally added to the curing mixtures NCO/OH ratio as 1.2using TDI as the curing agent. The cured samples containing 6% (w/w) ofDMSO, appeared less transparent and much softer than the other samples. Thishas been attributed to some degree of phase separation caused by the presence ofDMSO.[21]

NCO-terminated energetic azide can itself be used as a curing agent for OH-terminated aliphatic polymers to obtain an elastomer for solid rocket propellants.[51]

4.5. Curing of Azido Polymers with Dipolarophiles

Curing of azido polymers has also been done with multifunctional dipolaro-philes e.g., multifunctional acrylic or acetylenic esters (dimethylene glycol diacrylate,tetraethylene glycol diacrylate, ethylene glycol diacrylate, hexanediol diacrylate,pentaerythritol tri/tetraacrylate, hexane diol diacrylate) or acrylic amide molecules(hexane diamine bis-acrylamide, methylene bis-acrylamide).

Azides are 1,3-dipolar compounds and readily undergo a [3þ 2] cycloadditionwith olefinic or acetylinic esters and amides to yield a five-membered heterocyclicring (e.g., 1,2,3-triazoline or 1,2,3-triazole.[52] The kinetics and mechanism of suchcycloaddition has been reported in the literature.[53] A concerted nonsynchronusmechanism with partial charges at the transition state (charge imbalance) has beensuggested for such cycloaddition which are always cis and usually regiospecific.[54,55]

The triazoline thus synthesized is thermally labile and looses N2 to yield aziridines.Therefore, the reactions are carried out at low temperatures and may take weeks forcompletion. Rigid alkenes such as norbornene, react with azide on heating to formexo-adduct. For example, norbornene: aryl azide adducts have been prepared byrefluxing in petroleum ether (60–90�C) for 3–4 h.[55]

The use of highly functional dipolarophiles can reduce the relative amount ofcross-linking agent required to achieve a desired degree of cross-linking, therebymaintaining a higher percentage of high energy chemical moieties in the cross-linked polymer network. Further reaction between an azido polymer and a multi-functional dipolarophile is not stoichiometrically limited by the amount of hydroxy/azido groups contained in the azido polymer. An azido polymer has a relatively largeamount of pendant azido groups, and a multifunctional dipolarophile can react withsome or all of these azido groups. This allows for extensive control of the degree ofcross-linking of the azido polymer thus providing reaction products having a varietyof morphologies and/or hardness properties.[50] The typical reaction between anazido group and a multifunctional dipolarophile[56] is depicted in Sch. 15.

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The above-mentioned reaction is insensitive to moisture and hence there is noneed to avoid moisture in reactants (e.g., drying of fillers etc.). Dipolarophiles haveadvantages in producing cross-linked high-energy materials. Cross-linking at azidogroups with these dipolarophiles changes the azido group to a triazoline or a triazolegroup, which can ensure stability[56] and enhances the burn rate of the cross-linkedpolymer.[51] Azido polymers with improved burn rate have been prepared by usingazido polymers, an acrylic and an acetylenic ester or amide groups.[56]

The curing of poly(allyl azide) (PAA) using ethylene glycol dimethacrylate(EGDMA) has also been reported.[40] The reaction can be depicted according tothe reaction (Sch. 16).

N3 HC C C

O

O R CC

O

O CH

C

O

O R

NN

N

C

O

OH

NN

N

H

Polymer +

Polymer Polymer

2

Scheme 15. Reaction scheme for cross-linking of azido polymers by difunctional

dipolarophiles.

CH2 CH2O OC CC

CH3CH3

CH2CH2

O O

C

CH2 CH2O OC C C

CH3CH3

CH2CH2

O O

C

N

NN N N

N

N3+Polym er

Polym erPolym er

Scheme 16. Curing of PAA using EGDMA.

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Preferred amounts of the multifunctional dipolarophile are in the range of20–100 pph (parts by weight of an ingredient/100 parts by weight of the azido poly-mer). Such an additional reaction can be easily carried out at ambient conditions inabout 1–24 h. The reaction can be carried out in the temperature range of 0�C toabout 90�C. However, it is preferable to use temperatures between 25–40�C. Thereaction rate is typically hindered by the presence of solvent.[50]

Azides are known to react readily with both electron-deficient as well aselectron-withdrawing dipolarophiles. Therefore, studies on highly electron-deficientdipolarophiles such as bismaleimides, bisitaconimides, or endo-5-norbornene-2,3-dicarboximide (bisnadimides) with allyl azide and glycidyl azide were carried outby Varma et al.[57] The curing of azide could be carried out at 40–60�C to yieldflexible products with improved thermal properties.

5. THERMAL BEHAVIOR OF AZIDO POLYMERS

Thermal stability and degradation of uncured and cured azido polymers hasbeen investigated by DSC and thermogravimetric analysis in nitrogen atmosphere.Analysis of gases generated by the decomposition has been done by gas chromatog-raphy and IR spectroscopy.

5.1. Uncured Azido Polymers—Linear and Branched

In the DSC scans of GAP a single exothermic peak is observed in the tempera-ture range of 186–273�C with exothermic peak temperature 247� 5�C due to elim-ination of nitrogen from the azide group.[57] The energy liberated at this stage isaround 1.9 kJ/g.[58,59] Sahu et al.[59] however have reported a bimodal exotherm inGAP polymers of molecular weight 1150 and 2150 g/mol, with exothermic peaktemperatures at 218, 242, and 222�C, 262�C, respectively. The DSC scan (Fig. 4)of poly(allyl azide) showed an exothermic transition in the temperature range of155–274�C, with the exothermic peak temperature at 231�C. The energy liberatedwas 1.099 kJ/g.[40]

The thermal decomposition of polyglycidyl azide (GAP) and bis(azidomethyl)-oxetane-tetrahydrofuran copolymer (BAMO/THF) showed overall first-orderkinetics. Additonal azide groups at the terminal positions in GAP decomposedindependently and increased the rate of decomposition. However, the decompositonkinetics was less affected by the additional azide groups in the main chain. Thedecompositon properties of aged polymer samples were the same as those ofunaged ones.[60]

The DSC traces of azido polymers have also been recorded using different ramprates (0.2–10�C/min). A ramp rate higher than 5�C/min causes violent detonation inthe DSC cell (Fig. 5). Such explosions are also observed when more than 5mg ofsample is used.[40]

In the TG trace of GAP recorded in nitrogen atmosphere, two mass loss stepswere observed. The first decomposition corresponds to nearly 36% mass loss inthe temperature range of 210–270�C. The second stage is completed around

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410�C. The temperature of the maximum rate of mass loss in the first step ofdecomposition was 250�C, which coincides with the exothermic peak temperaturesin DSC scans. Therefore, the first step of mass loss in TG can be attributed toelimination of N2, which theoretically should have been around 30% in GAP.

Figure 5.

Figure 4.

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However, a mass loss of 36% was observed and may be due to breakdown ofpolymer backbone at higher temperature.

The thermal decomposition of poly(allyl azide) also proceeded in two steps.[40]

The first step of the decomposition occurred in the temperature range of 160–322�C.This decomposition corresponded to 28.3% mass loss (Fig. 6). The exothermictransition (observed in DSC) and accompanied by a mass loss (�28%) in the TGtrace can be attributed to the breakdown of azido group. Theoretically the decom-position of the azido group in a linear PAA should have resulted in a mass loss of33.7%. The poly(allyl azide) thus had lower azido content, which may be attributedto the branched structure of PAA (Sch. 11).

Major mass loss in GAP and PAA was observed above 400�C (�59%) andwas due to breakdown of the polymer backbone leading to the formation ofhydrogen, carbon monoxide, carbon dioxide, methane, ammonia, hydrogencyanide gas, and other higher hydrocarbons.[61] Small molecules are formed byBAMO and GAP, but some large fragments are evolved by AMMO.[61] A charresidue of 15% at 600�C and 7% at 800�C was obtained and was attributed tothe formation of cross-linked structure by imine intermediate (inter- as well asintra-molecular) (Sch. 17).

The N3 group elimination in GAP has been followed by IR spectroscopy. Adecrease in the intensity of N3 groups at 2100 and 1280 cm�1 was observed onheating to 170�C and new peaks appeared at 1529 (NH bending), 1677(C¼N-C-H stretch), 1725 (C¼N-C-C stretch), and 3350 cm�1 (NH stretching).The intensity of the C-O-C (ether) at 1410 cm�1 remained unchanged duringthe initial stages of decomposition supporting the view that in the initial stagesof decomposition only the side chain breakdown was the main process. Intra- andintermolecular bridge formation (Sch. 17) has been proposed to account for the

Figure 6.

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insolubility of partially degraded samples and for the appearance of -C¼N-C-type of structures in IR.[58]

After the elimination of N2 from azido group of GAP further reaction of nitreneintermediate proceed according to Sch. 18.[61]

The combustion characteristics (burning rates, temperature profiles) and kineticparameters (order of reaction, activation energy, pre-exponential factor of rate con-stants) for the thermal decomposition of GAP together with the composition of theproducts of both the combustion and decomposition of uncrosslinked GAP (with amolecular weight of 350 or 2000) and cured GAP were studied. The final tempera-ture of a flame of GAP was measured at 1000 to 1100K. However, approximately47% of the mass of the combustion products was volatile gases N2, H2, CO, CO2,CH4, C2H4, C2H6, NH3, H2O, acetonitrile, acrylonitrile, and furane, as obtained bymass-spectrometry by using freezing/thawing in a liquid nitrogen trap.[62]

HO (CH2 CH

CH2

N3

O CH2 CH

CH2

N3

O) H HO (CH2 CH O CH2 CH O) H

C-H

N-H

C-H

N-H

N2

CH2

CH

O CH2 CHCH

NN

O

CH

CH2CH

N

O OCH

CH

CH2

CH2

CH2CH O OCHCH2

N

CH

CH2

n n + 2n

IntramolecularcrosslinkingIntermolecular

crosslinking

Scheme 17. Intra- and inter-molecular reactions of the imino groups.

OH CH CH2

CH2N

CH CH2

CH2N

O CH CH2

CH2N

O CH CH2

CH2N

O

HCN C2H4

HCN CO CH4

NH3 CO CH2CO

NH3 CH2O HCCOO + +

+

+ +

+ +

(I)

(II)

(III)

(IV)

:

:

:

:

Scheme 18. Decomposition patterns of the nitrene backbone after N2 has been liberated

from GAP.

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5.2. Uncured BAMO and AMMO Polymer

A mass spectroscopic study on the thermal decomposition of BAMO andother azide polymers was made by Faber and Srivastava.[63] During isothermalexperiments, the melting temperature of BAMO was found to be 75�C. Slightdecomposition began at 130�C, and at that temperature the release of N2 fromazide groups was observed to be the primary decomposition path, where the rateof N2 release increased as the temperature is increased. The three-carbon backboneof the polymer remains intact until a higher temperature, approximately 160�C,when the mass spectrometer (MS) showed fragments of HCN, H2, CH2, CH3, O,OH, H2O, H2CO, CH2OH, and NO2 in addition to N2. These species showed theinitiation of secondary decomposition. BAMO monomer and polymer were investi-gated by Brill and his coworkers[64,65] with the rapid scanning Fourier infrared(RSFTIR) technique to characterize the slow (5K/min) and rapid (50–255K/min)thermolysis of these compounds and with the simultaneous mass and temperaturechange (SMATCH)/FTIR spectroscopy to determine the kinetics of mass-lossprocess of the compounds.

The TGA data revealed that the first stage mass loss occurred rapidly at about187�C and was completed at about 247�C with the approximately 35% mass loss.This stage was predominantly related to the elimination of N3 bond in the infrared(IR) spectra of the sample. The second stage mass loss process occurred slowlywithout heat liberation and was caused by the decomposition of remaining frag-ments as detected by the strong IR absorption of C¼N, C-H, C-O bonds. The rate ofgradual weight loss was compared under isothermal and nonisothermal conditionsfor AMMO, BAMO, and GAP by TGA during the initial 20–40% of mass losswhere decomposition of the azide group dominates.[65] Isothermal rate constants, k,obtained for different azide polymers are given in Table 5. The following order ofrates of isothermal decomposition was observed: GAP>BAMO>AMMO. BAMOpolymer and copolymer with tetrahydrofuran (THF) were studied by Miyazaki andKubota.[66]

Thermal stability of aged THF copolymers with AMMO or NMMO decreasedin the order poly(THF)>poly(AMMO-THF-AMMO,) poly(NMMO-THF-NMMO.[67] DSC studies revealed that the copolymers of THF with BAMO,AMMO, NMMO have low Tg and large decomposition enthalpies which werebrought about by attached azido groups.[68] NMMO based polymers exhibitedlower thermal stability and relatively higher decomposition enthalpies. Thermal

Table 5. Isothermal TGA rate constants (s�1) for azide polymers (data adapted from

Ref.[65]).

Polymer

180�C

(x� 106)

185�C

(x� 106)

190�C

(x� 106)

195�C

(x� 106)

200�C

(x� 106)

205�C

(x� 106)

210�C

(x� 106)

AMMO — 4.85 8.21 13.00 19.27 33.36 50.20

BAMO — 12.76 24.13 38.65 64.88 85.62 138.10

GAP 12.98 22.88 35.18 59.50 82.45 133.10 —

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stability of these polymers was further improved by curing with isocyanateand pentaerythritol. Poly(BAMO) and poly(BAMO-THF) degrade exothermicallywith Tmax of 237�C and 241�C respectively; and the activation energy of 39 and40 kcal/mol.[29]

5.3. Thermal Behavior of Cured Azido Polymers

The TGA showed a two-stage decomposition pattern in the case of poly (allylazide).[40] A mass loss of 20–25% was observed in the temperature range of161–324�C and may be due to the elimination of nitrogen from the azido group.The second stage decomposition was in the temperature range of 313–525�Caccounting for the mass loss of 52–61%. This is due to the thermal degradation ofthe main chain along with the 1,2,3-triazoline group formed by the cross-linkingreaction of poly(allyl azide) with ethylene glycol dimethacrylate (EGDMA)(5–45 phr). The TG trace of poly(allyl azide) cured with EGDMA is given inFig. 7. Violent detonation was observed when a high heating rate and samplemass of more than �5mg was used (Fig. 8).

The combustion wave structure and thermal decomposition process of GAPpropellent, cured with hexamethylene diisocyanate and crosslinked with trimethylol-propane to formulate the propellant were studied to determine the parameters thatcontrol the burning rate. The burning rate of the propellant is high even though theadiabatic flame temperature is less than that of conventional solid propellants. Theenergy released at the burning surface of this propellant is caused by the scission ofN–N2 bond which produces gaseous N2. The heat flux transferred back from the gasphase to the burning surface is very small compared with the heat generated at theburning surface. The activation energy of the decomposition of the burning surfaceof this propellant was 87 kJ/mol.[69]

Figure 7.

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The Arrhenius activation energies (Ea) of decomposition of uncrosslinked andcured GAP are mostly clustered in the 150.5–183.9 kJ/mol range, which is similar tothe RN–N2 bond strength of 146.3–171.4 kJ/mol (Table 6).

An attempt has been made to address several of the discrepancies in thechemistry of GAP. There is broad consistency among the global values of Ea fordecomposition of GAP and general agreement that the rate is dominated by thedecomposition of the azide group, discrepancies exist in the occurrence of later stageproducts. For example, the decomposition of the alkane imine or nitrene after therelease of N2 is found to produce NH3 and HCN in significant amounts in severalstudies. It is found that NH3 content increases with the -OH content of GAP, whichsuggests that end-chain azide groups mostly form NH3. The HCN/NH3 ratioincreases with increasing temperature. However, NH3 is also formed in isocyanatecured GAP and GAP plasticizer, which possess no -OH groups. Some differences areexpected from the use of different types of samples, e.g., different MW, terminalgroups, or cure. The activation energy obtained from DSC and TGA for branchedGAP (B-GAP) is in the range of 138–145 kJ/mol, which is lower than that for GAP-TRIOL (160–178 kJ/mol), indicating that B-GAP decomposes more easily ascompared to GAP-TRIOL.[74]

The burning rate and thermochemical measurements have been conducted toobtain information on the effects of pressure, initial temperature, and catalyst onammonium perchlorate/3-azidomethyl-3-methyloxetane polymer (AP/AMMO) andammonium perchlorate/hydroxy-terminated polybutadiene propellants. Pressureinsensitive burning (i.e., plateau burning) was observed when the HTPB binderwas replaced with the AMMO binder for the AP composite propellants. The

Figure 8.

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temperature sensitivity of both AP/AMMO and AP/HTPB propellants increases aspressure increases. The addition of ferric oxide accelerates the burning rate of bothAP/AMMO and AP/HTPB propellants. The temperature sensitivity of the gas-phasereaction is increased significantly when the HTPB binder is replaced with theAMMO binder.[70]

Measurements of gaseous species and temperature profiles for RDX/BAMOpseudo-propellants were performed by Lee et al.[76] The propellants were madefrom a physical mixture of RDX and BAMO in weight ratio of 80:20.Experiments were conducted at atmospheric pressure in argon with heat fluxes of100 and 400 W/cm2 delivered by a CO2 laser. The products of each ingredient, RDXand BAMO, were found to exist simultaneously throughout the gas phase; however,primary reaction chemistry in the gas phase was dominated by RDX.

The accelerated aging of BAMO-HMX propellent was conducted at 347K forseveral weeks. BAMO-HMX propellants for a very low cross-linking ratio formed acavity between HMX and BAMO binder by evolution of N2, CO2, and H2O duringaccelerated aging. An exotherm, generated by the decomposition of azide binder,initiated and accelerated the thermal decomposition of HMX. The burning rate ofBAMO-HMX propellant was larger than those of BAMO binder and HMX, respec-tively. However, the propellant could not maintain the combustion at low pressure,at which its burning rate was equal to that of BAMO binder.[77]

6. PHOTODECOMPOSITION

Photosensitivity of polymers with pendant azide groups is well known. Suchpolymers are used as negative photoresists mixed with an unsaturated hydrocar-bon polymer under intense UV radiation.[59] The azide group undergoes decom-position, producing a nitrene radical, which quenches by abstracting H fromanother polymer, reacting with itself forming an azo group or by reacting witha double bond of another polymer forming an aziridine linkage. In photoresistapplications, the advantage of aziridine formation by nitrene is generally realizedleading to cross-linking and photo insolubility. Scheme 19 describes the photo-decomposition of the azide group and the reactions of the nitrene radical thusformed.

Table 6. Literature Arrhenius kinetics for the decomposition of GAP (data adapted from

Ref.[61]).

Method Ea (kJ/mole) A (s�1) Temp. range (K) Comments Ref. no.

DSC 166–177 7� 1015–

8.9� 1015520–540 uncured 43, 70&71

DSC 152.1 2� 1013 520–535 cured 70

DSC 178 5.0� 1017 — cured 43

TGA 154–164 — 400–473 — 65, 72&73

A is the frequency factor.

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Acceleration of the photoinsolubility of GAPs can be achieved by the additionof an unsaturated hydrocarbon polymer [e.g., poly(ethylene-propylene-ethylene-norbornene)], where the nitrene radical of GAPs reacted with >C¼C< resultingin cross linking through the formation of an aziridine ring.

7. FORMULATIONS

As mentioned earlier, a typical propellent formulation comprises of severalingredients. For example, a solid rocket motor propellant consists of 70–85wt%solids chosen from energetic solids and oxidizers and 15–30wt% of a binder suchas polyoxetane (poly(BAMO/AMMO)) or GAP and an isocyanate curing agent. Thedetails of these ingredients are described in the subsequent text. A plasticizer, such astriethylene glycol dinitrate (TEGDN), N-butyl-2-nitratoethyl nitramine (BuNENA),trimethylolethane trinitrate (TMETN), and butanetriol trinitrate (BTTN), is usedwith the polyoxetane binder, preferably at a 0–3.0:1 plasticizer–polymer weight ratio.The energetic solids and oxidizers can include such material as particulate NH4ClO4,Al, HMX, etc. A propellant formulation with acceptable mechanical propertiescontained BAMO-AMMO-Desmodur N 100 copolymer 12.5, TEGDN 12.5,ammonium perchlorate 48.0, HMX 5.0, and Al 22.0wt%.[78]

7.1. Plasticizers

The curing and solid loading increases the Tg of the polymers. Therefore, forbeing used as binder in composite propellants, the Tg of GAP should be lowered tocompensate the reverse effect of curing and solid loading on the Tg. The Tg valuesof gum stocks upon curing can be compensated by using a plasticizer. Variousplasticizers which are used in order to improve the poor mechanical properties ofGAP at low temperature are iso-decyl pelargonate (IDP), dioctyl adipate (DOA),

R N3 R N N2

R N R N N R

R N H C R NH C

CH CR N R NH

R N R N

+

+

:

:2

:

++:

: +

.

Scheme 19. Photodecomposition of azido polymer under UV irradiation.

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dimethylene glycol dibenzoate (DMGDB),[21] dioctyl azelate (DOZ), dioctyl seba-cate (DOS), diethyl phthalate (DEP), 1,2,4-butanetriol trinitrate (BTTN), trimethyl-olethane trinitrate (TMETN), polyethylene glycol (PEG), bis-2,2-dinitropropylacetal and bis-2,2-dinitropropyl formal (BDNA/F).[42] The plasticizer had no effecton the decomposition characteristics of GAP. The addition of a plasticizer affects themechanical properties (e.g., ultimate hardness) of the gum stocks significantly.

The Tg of GAP is decreased from �45 to �65�C with bis-2,2-dinitropropylacetal/formal (BDNPA/F) and to �48.4�C with dioctyl adipate (DOA) at a 25/75plasticizer/polymer weight ratio. The decomposition properties remain practicallyunchanged, while the Tg increased to �36.8�C upon curing.[43]

Nitroplasticizer (BDNA/F) has been found to be more compatible with ener-getic binder GAP than PEG.[79] Nitrate plasticizer, TMETN, played an importantrole in improvement of not only combustion properties but also insensitivities ofGAP/AN propellants.[80]

Low molecular mass GAPs can be exploited as an energetic plasticizer.[18] GAPwith a molecular mass of 500 would be useful as a plasticizer in composites andpropellant compositions.[17] An azide-terminated azido polymer (GAP-A) acts asplasticizer for propellants.[18] The replacement of nitroglycerin plasticizer by GAPin composite-modified double-base propellants results in greater safety in handling,reduced detonation sensitivity, higher burning rate and specific impulse, andimproved mechanical properties.[81]

Cycloaddition of azido group containing polyoxyalkylenes by reaction withdiethenyl benzene to form triazole crosslinked, energetic plasticizers for compositespropellants has been reported recently. Thus, repeating units of GAP, (AMMO),(BAMO-NMMO), and (BAMO-AMMO) are effective as energetic plasiticizerswhen cross-linked with 1,4-bis(ethynyl)benzene, 1,4-bis(ethynylcarbonyl)benzene,or 1,3-bis(cyanoethynyl) benzene.[82]

7.2. Oxidizers

The primary function of an oxidizer in a propellant is to provide oxygen forthe combustion of hydrocarbon fuel species. Higher loading of oxidizer increases thespecific impulse (Isp) of a propellant but simultaneously it adversely affects themechanical properties.[1] Positive oxygen balance, high heat formation, high density,and high thermal stability are the other important criteria for an energeticoxidizer. Most commonly used oxidizers are crystalline fine particles of ammoniumperchlorate (AP), ammonium dinitramide (ADN), glycerol trinitrate (NG), cyclote-tramethylene tetranitramine (HMX), cyclotrimethylene trinitramine (RDX),(TAGN), hydrazinium nitroformate (HNF), hexanitro hexaaza iso-wurzitane, orHNIW (CL-20). Sorguyl, bicyclo HMX, and tetranitro-bicyclononanone (K56) aresome examples of nitraza polycycles, which are gaining importance. Sorguyl andK56 are hydrolytically unstable whereas bicyclo HMX is extremely difficult tosynthesize.

AP and HMX are two solid ingredients often used in modern solid propellants,either composite propellent or composite modified double base propellants. Duringthe past decade, the search for more energetic propellants has led to the development

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of CL-20, ADN, HNF, etc. Formulation of BAMO/AMMO with CL20, RDX andthe combination of the two leads to potentially attractive high energy TPE gunpropellants.

Structure and characteristics of some of these oxidizers are given in Sch. 20 andTable 7, respectively.

ONO2

ONO2

ONO2

NG

N N

N

NO2

NO2O2N

RDX

N

NN

N

NO2

NO2O2N

O2N

HMX

N

N N

N

N

N

O2N NO2

NO2O2N

CL-20

NO2

NO2

ADN

NH4+ N-

NH2NH3+ C

NO2

NO2

NO2

HNF

N

N N

N

NO2NO2

NO2NO2

Bicyclo HMX

N

N N

NO O

NO2NO2

NO2NO2

Sorguyl

N

N N

NO

NO2NO2

NO2NO2

K56

Scheme 20. Representative structure of oxidizers.[83,84]

Table 7. Characteristics of oxidizers (data adapted from Refs.[83,85])

Oxidizer Formula

Mol. wt.

(g/mol)

Density

(g/cm3)

�Hf

(kJ/mol)

AP ClH4NO4 117.5 1.95 �296

AN H4N2O3 80 1.72 �365

ADN H4N4O4 124 1.81 �150

HNF CH5N7O6 183 1.86–1.93 �71 to �72

RDX C3H6N6O6 222 1.805 þ70

HMX C4H8N8O8 296 1.91 þ84

Bicyclo HMX C4H6N8O8 294 1.87 þ125

Sorguyl C4H2N8O8 322 2.035 �292

K56 C5H6N8O9 322 1.975 �144

CL-20 C6H6N12O12 438 1.92–2.04 þ339 (g,a,b)þ372 (")

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Ammonium perchlorate is the most widely used oxidizer in composite and com-posite modified double base (CMDB) rocket propellants. AP has two inherent dis-advantages, (1) it produces chlorine rich combustion products (30%), posingenvironmental hazards such as ozone depletion and acid rain and (2) it produceswhite smoke trail during inclemental weather, which is detrimental for specific appli-cations demanding smokeless plume. Among other oxidizers, AN and ADN offerschlorine free formulations. However, AN has drawbacks of hygroscopic nature andmultiple transition phases in the temperature range of practical importance andcomparatively low energy. ADN and HNF have attracted global attention. CL-20is also being evaluated as a rocket propellant component.[41,83] HNF has certainadvantages over ADN such as its simple method of synthesis, nonhygroscopicnature, higher density, and melting point. The addition of crystalline particlessuch as AP and nitramine particles decreases the burning rate of GAP significantlyeven though the combustion temperature increases.[86] The particle size of the oxidi-zer also plays an important factor in determining the performance of the propel-lants.[83,85,87] The energetic composites of GAP/nitroplasticizer (BDNA/F) andpolyethylene glycol with HMX in various particle sizes have been studied andcompared with HTPB composites.[79]

The flame structure of GAP propellants is altered by the nature of the oxidizer.The flame is highly heterogeneous in GAP/AP propellants but homogeneous inGAP/HMX and GAP/TAGN propellants.[85]

7.3. Pyrolants

The addition of metal particles i.e., Al, Mg, Ti, B, Zr, etc., to GAPforms energetic materials (GAP/metal pyrolants). These are also called fuel materialparticulates. The metal particles mixed within GAP react with the nitrogen, C, COproduced by the thermal decomposition of GAP. The most probable chemical reac-tions, which are likely to take place are formation of aluminium or magnesiumnitride (AlN, Mg3N2) and titanium carbide (TiC). GAP-coated amorphous-boron-based fuel-rich propellants exhibit more vigorous phenomena, high burning rates,and a lesser extent of residue agglomeration than the uncoated baseline propellant.Moreover, reaction mechanisms were proposed to elucidate the combustion pro-ducts.[33] However, the reaction process is highly dependent on the type of metalparticles mixed. Thus the burning rates and thermal decomposition vary with thetype of metals mixed. Though GAP contains no oxidizer fragments in its products,addition of metal particles increases the energy of GAP.[89]

The flame temperatures of GAP/metal pyrolants increases with increase in con-centration of pyrolants. The addition of B or Al generates higher flame temperaturecompared with Mg, Ti, or Zr.

7.4. Ballistic Modifiers

In general, ballistic modifiers bring down the maximum decomposition tempera-ture. They are also known as burning rate modifiers. Their main function is to

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catalyze the burn rates. Commonly exploited ballistic modifiers are copper chromite(Cu(CrO2)2), ferric oxide, butyl ferrocene, catocene (C27H32Fe2). The mixture ofcatocene and copper chromite in BAMO/NMMO copolymers augmented the burn-ing rate 1.6 times with decreasing temperature sensitivity.[33] The combustion rate ofGAP-HMX propellant was increased by addition of 2% lead citrate and 0.6% C.New molybdenum oxide/vanadium oxide burning modifiers, with pure AN(atomized from melt) has resulted in improved combustion behavior and stabilityof GAP/AN-propellants.[84]

8. CONCLUSION AND FUTURE OUTLOOK

Energetic fuel binders based on cyclic ethers (oxirane, oxetane derivatives) andhaving pendant azido group have been investigated in the past two decades for gunand rocket propellants. These polymers (e.g., GAP, BAMO, AMMO) are promisingcandidates for energetic binders in future composite propellants having minimumsmoke, reduced pollution, and sensitivity. Such propellants in addition to being fuelrich, liberate large amounts of H2, N2, CO, and gaseous hydrocarbons on burning inthe primary chamber, are insensitive to impact and provide high burn rates.

Inspite of all the advantages mentioned above, there are certain limitations ofthese azide binders. The decomposition products of cured GAP contain HCN, whichis highly toxic. Attempts have to be made to suppress the HCN evolution. The Tg ofGAP increases on curing and by presence of oxidizers, pyrolants, etc. There is a needto develop energetic binders with lower Tg.

Energetic binders based on hydrocarbon polymers containing pendant azidegroup may provide an alternative approach for achieving this goal.

This review focused on the synthesis, characterization, physical properties, andthermal behavior of these energetic azido binders. Preliminary studies on poly(allylazide) have also been included. It is obvious that one can tailor the properties ofthese polymers by changing their backbone structure and by copolymerization.More studies are needed for developing energetic binders using structuralmodifications.

ACKNOWLEDGMENT

The Armament Research Board, Government of India, Ministry of Defenceis gratefully acknowledged for providing the financial assistance for carrying outthis work.

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azido polymers-energetic binders for solid rocket propellants

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