Synthesis and thermal decomposition of GAP–Poly(BAMO) copolymer

Polymer Degradation and Stability 92 (2007) 1365e1377 www.elsevier.com/locate/polydegstab

Synthesis and thermal decomposition of GAPePoly(BAMO) copolymer Sreekumar Pisharath*, How Ghee Ang Energetic Materials Research Centre, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Received 11 December 2006; received in revised form 5 March 2007; accepted 13 March 2007 Available online 30 March 2007

Abstract An energetic copolymer of glycidyl azide polymer (GAP) and poly(bis(azidomethyl)oxetane (Poly(BAMO)) was synthesized using the Borontrifluorideedimethyl ether complex/diol initiator system. The synthesized copolymer exhibited the characteristics of an energetic thermoplastic elastomer (ETPE). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to study the thermal decomposition behavior and the results were compared with that of the constituent homopolymers. The main weight loss step in all the polymers coincides with the exothermic dissociation of the azido groups in the side chain. In contrast with the behavior of the homopolymers, the copolymer shows a broad exothermic shoulder peak at 298  C after the main exothermic decomposition peak at 228  C. Kinetic analysis was performed by Vyazovkin’s model-free method, which suggests that the activation energy of the main decomposition step is around 145 kJ/mol and for the second shoulder it is around 220 kJ/mol. Fourier transform infra red (FTIR) spectra of the degradation residues show that the azido groups in the copolymer decompose in two stages at different temperatures which is responsible for the double decomposition behavior. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Energetic polymer; Thermal decomposition; Model-free kinetics; Thermoplastic elastomer

1. Introduction Azido polymers are unique energetic materials which release heat by decomposition. They are used as energetic binders in propellant and explosive formulations [1]. Inert polymers such as hydroxyl-terminated polybutadiene (HTPB) are being replaced with energetic polymers such as glycidyl azide polymer (GAP) as promising energetic binders. If inert polymers are substituted by energetic polymers as binders, increased performance could be achieved at low oxidizer loading. Moreover, low oxidizer content reduces the vulnerability of formulation towards external stimuli. Thus energetic polymers as binders in propellant or explosive formulations offer the dual advantage of improved performance and reduced vulnerability.

* Corresponding author. Tel.: þ65 67906409; fax: þ65 67927173. E-mail address: [email protected] (S. Pisharath). 0141-3910/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2007.03.016

With increasing complexities in the rocket motor designs, commensurate with the performance requirements, structure and processing demands placed on the solid propellants have become more and more strenuous. These requirements have outweighed the ballistic and combustion advantages. In GAP, presence of bulky side group constituents and reduced backbone flexibility render poor low temperature properties. Hence, GAP propellants should be heavily plasticized to meet the structural requirements. GAP based propellants also exhibit low stress and strain capabilities [2]. Copolymerizing crystalline hard segments with soft amorphous GAP polymer chain is a feasible method to improve the mechanical properties of GAP polymer. It is expected that, the micro-phase separation between the hard and soft segments should provide the copolymers with good mechanical properties. Moreover, the hard segments from each polymer molecule co-crystallize with those from the other molecules to contribute to the three dimensional crosslink density of the elastomer. Therefore, they will behave as crosslinked

S. Pisharath, H.G. Ang / Polymer Degradation and Stability 92 (2007) 1365e1377

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rubber at ambient temperature. Above the melting temperature of the crystalline hard block, the physical crosslinks between the polymer chains disappear, allowing the polymer to flow like a thermoplastic. Such materials are known as thermoplastic elastomers (TPEs). The reversible physical crosslinks facilitate the recycling of TPEs, thereby avoiding the disposal problems posed by conventional crosslinked binders. One of the candidates which could be copolymerized as a crystalline hard segment with GAP is the energetic polymer poly(bis(azidomethyl)oxetane) (Poly(BAMO)), thereby forming an energetic thermoplastic elastomer (ETPE). ETPE containing formulations are in the various stages of development and are being used in missile and propellant development demonstration efforts [3]. In polymer bonded explosive (PBX) formulations, it has been observed that the decomposition characteristics of the energetic binder affect the performance properties of the oxidizer [4e7] making it important to understand the thermal decomposition behavior of the polymer binder. Also, deriving reliable kinetic data from thermal decomposition process of polymer binders will help in the modeling and prediction of combustion characteristics of formulations where the polymer binder is an integral component. In this paper, we report the synthesis and thermal decomposition kinetics of a GAPePoly (BAMO) copolymer. The thermal decomposition kinetics of the homopolymers (GAP and Poly(BAMO)) are also investigated for comparison purpose. The chemical structures of the studied polymers are presented in Fig. 1. There have been several literature reports on the studies of decomposition of GAP and Poly(BAMO) alone [8e10]. The common decomposition mechanism in these polymers is the exothermic rupture of the azide bonds to produce molecular nitrogen and energy. Secondary decomposition at higher temperatures involved rupture of polymer backbone into smaller fragments. Only limited number of research results has been published on the thermal decomposition behavior of ETPE’s. Kimura and Oyumi [11] compared thermal decomposition of energetic copolymers of BAMO such as BAMO/NIMMO (where NIMMO is 3-nitratomethyl-3-methyloxetane) and BAMO/ AMMO (where AMMO is 3-azidomethyl-3-methyloxetane).

It was observed that the heat generated by the decomposition of NIMMO group accelerates the decomposition of the BAMO unit. On the other hand, for the BAMO/AMMO copolymer, the AMMO unit doesn’t affect the thermal decomposition of BAMO. Reaction pathways or decomposition product analysis was not reported for the decomposition of the copolymer. Liu et al. [12] studied the thermal characteristics of copolymers of tetrahydrofuran (THF) with BAMO, AMMO and NIMMO. It was observed that the decomposition enthalpies were dependent on the energetic group contents of the polymers and not on copolymer types. In another detailed experimental study, Lee et al. [13] studied the pyrolysis of BAMO/ AMMO copolymer with and without titanium dioxide (TiO2) using a high power CO2 laser source. Measurement of the evolved decomposition species revealed that the products of BAMO/AMMO decomposition are similar to those reported for BAMO. Addition of TiO2 caused no significant changes in product profiles except for an increase in ammonia (NH3). The product profiles also indicated a simultaneous decomposition of backbone and side chains of the polymer. There have been no reported investigations on the thermal decomposition of GAPePoly(BAMO) copolymer which prompted us for this study. In this work, we use a modelfree or iso-conversional method suggested by Vyazovkin and Dollimore [14] to investigate the decomposition kinetics. Model-free methods allow the determination of activation energy as a function of extent of decomposition and temperature. These methods are being widely employed to study thermally stimulated processes (e.g., degradation, curing) in polymers [15]. Model-free methods, when applied to propellant formulations, help to compare the activation energy dependencies of the neat polymer binder with that of a propellant based on this polymer, allowing the elucidation of the role of the latter [16].

2. Experimental 2.1. Materials Commercially available materials were used as received unless noted otherwise. The solvents were distilled under reduced pressure over calcium hydride before use.

CH2N3 HO

CH2

C

CH2

O

H C

CH2

O

H

n

m

CH2N3

2.2.1. Synthesis of 3,3-bis(chloromethyl)oxetane (BCMO) BCMO was synthesized by the cyclization of pentaerythritol trichlorohydrin using sodium hydroxide as described in Ref. [17].

CH2N3

GAP-Poly (BAMO) Copolymer CH2N3 HO HO

CH2

C

CH2

O

H C

O

H n

CH2N3

CH2

2.2. Synthesis of monomers

H n

CH2N3

GAP

Poly (BAMO) Fig. 1. Repeating chemical units of GAP, poly(BAMO) and GAPe Poly(BAMO).

2.2.2. Synthesis of 3,3-bis(azidomethyl)oxetane (BAMO) BAMO was synthesized by the reaction of sodium azide with BCMO in alkaline medium in the presence of a phase transfer catalyst, tetrabutyl ammonium bromide as described in Ref. [18].

S. Pisharath, H.G. Ang / Polymer Degradation and Stability 92 (2007) 1365e1377

2.3. Synthesis of polymers

several times with distilled water until neutral to pH. The washed organic phase was dried over sodium sulphate, filtered, and the solvent evaporated off in vacuum at 40  C to recover 43 g of PECH (89.5% yield). GPC analysis: Mw ¼ 2340 Kg mol1; Mn ¼ 2018 Kg mol1; polydispersity index ¼ 1.16 (against polystyrene standards). FTIR (KBr): n(eOH) ¼ 3450 cm1; n(CeOeC) ¼ 1100 cm1; n(CeCl) ¼ 745 cm1.

2.3.1. Synthesis of GAP GAP polymer was synthesized in two steps. Scheme 1 outlines the synthesis of GAP. First step was the synthesis of poly(epichlorohydrin) (PECH) of adequate molecular weight. Second step was the conversion of PECH polymer to GAP. The steps involved in polymer synthesis are described below. 2.3.1.1. Step 1. 1.33 g (0.0144 mol) of butane diol (BDO) dissolved in 75 mL methylene chloride was taken in a threenecked flask fitted with a thermometer and nitrogen inlet. Seven hundred and fifty microliters of BF3edimethyl ether complex, was injected into the reaction mixture and stirred at room temperature for 60 min. After cooling the reaction vessel to 0  C using iceesalt mixture, 46.66 g (0.5044 mol) of epichlorohydrin (ECH) was added drop by drop (at the rate of w0.1 mL/min) to the reaction mixture over a period of 12 h. The mole ratio of BDO to ECH is 1:35. Rate of addition is kept as slow as possible to keep the instantaneous concentration of ECH in the reaction mixture very low. At no point of time, the temperature was allowed to go above 0  C during the reaction. After the addition of ECH, reaction was continued for another 12 h at room temperature. Thereafter, reaction was frozen by adding distilled water. The organic phase containing PECH was extracted into methylene chloride and washed HO

CH2

1367

2.3.1.2. Step 2. Twenty eight grams of vacuum dried PECH was dissolved in 100 mL Dimethyl formamide (DMF) and taken in a three-necked flask fitted with a thermometer, nitrogen inlet and water condenser. The reaction mixture was heated to 120  C, under stirring in an oil bath. Twenty five grams of sodium azide was added to the mixture and the reaction continued for another 12 h. Thereafter, reaction was frozen by adding distilled water. The unreacted azide and salted out sodium chloride were filtered off. Organic phase containing GAP was extracted into methylene chloride and washed several times with distilled water until neutral to pH. The washed organic phase was dried over sodium sulphate, filtered, and the solvent evaporated off under vacuum. GAP of 23 g was obtained (82.14% yield). GPC analysis: Mw ¼ 2490 Kg mol1; Mn ¼ 2178 Kg mol1; polydispersity index ¼ 1.15 (against polystyrene standards). FTIR (KBr): n(eOH) ¼ 3450 cm1; n(CeN3) ¼ 2100 cm1; n(CeOeC) ¼ 1100 cm1.

CH2

CH2

CH2

OH

(1,4-Butane Diol) BF3-Dimethyl ether complex 00C, CH2Cl2

O

Cl

Epichlorohydrin

HO

HC

H2C

O

(CH2)4

O

CH2

CH2Cl

HC

H2C

O

HC

OH

CH2Cl O

HO

CH

H2C

O

(CH2)4

Cl

O

CH2

CH

O

n CH2Cl

CH2

CH

OH

n

CH2Cl

CH2Cl

CH2Cl

NaN3; DMF 110°C; 10Hrs

HO

HC

H2C

O

HC

H2C

O

(CH2)4

O

CH2

CH

n CH2N3

CH2N3 Scheme 1. Synthetic scheme for GAP.

O

CH2

CH

n CH2N3

CH2N3

OH

S. Pisharath, H.G. Ang / Polymer Degradation and Stability 92 (2007) 1365e1377

1368 1

H NMR (CDCl3): d ¼ 3.7 ppm (3H, eCH2eCHe); 3.59 ppm (2H, eCH2N3).

copolymer to GAPePoly(BAMO) copolymer by conversion of chloro to azido groups. The steps involved in polymer synthesis are described below.

2.3.2. Synthesis of Poly(BAMO) Scheme 2 outlines the synthesis of Poly(BAMO). 0.125 g (0.00144 mol) of (BDO) in 15 mL methylene chloride was taken in a three-necked flask fitted with a thermometer and nitrogen inlet. Five hundred microlitres of BF3edimethyl ether complex, was injected into the reaction mixture and stirred at room temperature for 60 min. After cooling the reaction vessel to 0  C using iceesalt mixture, 4.65 g (0.0277 mol) of BAMO taken in 9 g of methylene chloride was added to the reaction mixture over a period of 4 h. The solution was allowed to come to room temperature and left to react for 24 h. The reaction was quenched by the addition of saturated brine solution (50 mL). The organic phase was separated and washed with 10% sodium bicarbonate solution and the solvent was removed by vacuum evaporation. The polymer was precipitated by the addition of methanol and dried under vacuum at 30  C. Yield was 80%. GPC analysis: Mw ¼ 2179 Kg mol1; Mn ¼ 1653 Kg mol1; polydispersity index ¼ 1.31 (against polystyrene standards). FTIR (KBr): n(eOH) ¼ 3417 cm1; n(CeN3) ¼ 2106 cm1; n(CeOeC) ¼ 1103.2 cm1. 1 H NMR (CDCl3): d ¼ 3.36 ppm (4H, (eOCH2)2); 3.30 ppm (4H, (eCH2N3)2).

2.3.3.1. Step 1. Eight grams of pre-synthesized vacuum dried PECH diol was dissolved 30 mL of distilled methylene chloride was taken in a 250 mL three-necked reaction flask equipped with a nitrogen inlet, magnetic stirrer, and thermometer. Five hundred microlitres of BF3edimethyl ether complex was injected into the reaction flask and left to react for 1 h at room temperature. Thereafter, the reaction flask was cooled to 0  C using iceesalt mixture and calculated quantity of freshly distilled BCMO was added drop by drop through a dropping funnel over a period of 6 h. After the complete addition, the reaction mixture was left to react for 24 h at room temperature. The reaction mixture was poured into excess of methanol to precipitate out the polymer in 70% yield. The polymer was dried in vacuum at 30  C. Yield ¼ 65%. 2.3.3.2. Step 2. Three grams of PECHePoly(BAMO) copolymer was reacted with molar excess of sodium azide in 20 mL of DMF in a three-necked flask. The reaction mixture was heated to 120  C, under stirring in an oil bath. After 24 h, the reaction was frozen by the addition of distilled water. The organic phase containing the copolymer was extracted into methylene chloride and washed with 10% sodium bicarbonate solution until neutral to pH. The solution was poured into excess of methanol to precipitate out the copolymer. The copolymer was dried at 40  C under vacuum for 72 h. Yield was 80%.

2.3.3. Synthesis of GAPePoly(BAMO) copolymer Synthesis of GAPePoly(BAMO) copolymer was achieved in two stages as outlined in Scheme 3. First step was the synthesis of PECHepoly(BCMO) copolymer of adequate molecular weight. Second step is the conversion of PECHepoly(BCMO) HO

CH2

CH2

CH2

CH2

OH

(1,4,-Butane Diol) N3 N3

BF3-Dimethyl ether complex 0C,

0

CH2Cl2 BAMO

O CH2N3 HO

H2C

C

CH2N3 H2C

O

(CH2)4

CH2N3

O

CH2

C

CH2

OH

CH2N3

N3 N3

O CH2N3 H

O

H2C

C

H2C

CH2N3

CH2N3 O

(CH2)4

O

CH2

m

Scheme 2. Synthetic scheme for poly(BAMO).

C

CH2

O

H m

CH2N3

S. Pisharath, H.G. Ang / Polymer Degradation and Stability 92 (2007) 1365e1377

H

O

CH

CH2

O m/2

R O R=(CH2)4

CH2

CH

O

1369

H m/2

CH2Cl

CH2Cl

blk PECH Block Cl Cl

BF3.OMe2,O0C CH2Cl2

BCMO

O

CH2Cl

CH2Cl H m/2

O

CH2

C

CH2

CH2

O

PECH

O

C

CH2

O

n

H m/2

CH2Cl

CH2Cl blk

blk

BCMO Block

BCMO Block NaN3, DMF, 110°C

CH2N3

CH2N3 H m/2

O

CH2

C

CH2

O

CH2

O

GAP

C

CH2

n

O

H m/2

CH2N3

CH2N3 blk

blk

BAMO Block

BAMO Block

Scheme 3. Synthetic scheme for GAPePoly(BAMO) copolymer.

GPC analysis: results are provided in Table 1. FTIR (KBr): n(eOH) ¼ 3436 cm1; n(CeN3) ¼ 2102.3 cm1; n(CeOeC) ¼ 1114.8 cm1. 1 H NMR (CDCl3): d ¼ 3.74 ppm (3H, eCH2eCHe); 3.63 ppm (2H, eCH2N3) (GAP block); 3.50 ppm (4H, (eOCH2)2); 3.43 ppm (4H, (eCH2N3)2) (PBAMO block). 2.4. Measurements GPC analyses of polymers was carried out in a Waters instrument fitted with a 2414 differential refractive index detector against polystyrene standards using tetrahydrofuran (THF) as eluent at 1 mL/min. Table 1 Sample

Mw (g/mol)

Mn (g/mol)

Polydispersity

PECH diol Copolymer 1 Copolymer 2

2706 3553 5440

2016 2499 3777

1.34 1.4 1.44

1

H NMR spectra were recorded in a Bruker 400 MHz instrument using CDCl3 solvent. FTIR spectra were recorded in a Shimadzu FTIR-8300. Weight loss measurements were carried out in a Shimadzu TGA-50 thermogravimetric analyzer equipped with thermal analysis software for data analysis. Approximately 5 mg of samples were used for each run. The samples were heated from room temperature to 350  C at a variety of heating rates ranging from 0.1  C/min to 20  C/min in aluminum crucibles under constant flow of nitrogen (50 mL/min). From a typical TGA experiment at a given heating rate, the fractional weight loss or conversion (a) is computed as ððW0  WT Þ=ðW0  WN ÞÞ, where W0 is the initial weight of the sample, WT is the weight at temperature T, and WN is the final weight of the sample. DSC measurements were carried out in TA DSC 2010 equipment containing thermal analysis software for data interpretation. In representative runs, 2e3 mg of samples in sealed aluminum hermetic pans were ramped from room temperature to 350  C at the rate of 2  C/min in a steady flow of nitrogen (50 mL/min).

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2.5. Kinetic analysis of TGA data

3. Results

The most straightforward way to evaluate the effective activation energy as a function of decomposition is the Friedman’s method [19]. In this method, no approximations are introduced. However, this is a differential method which produces significant numerical instability and noise interference. Hence, integral iso-conversional methods were proposed. For non-isothermal conditions, integral isoconversional methods were proposed by Ozawa and Flynn and Wall commonly referred to as the OFW method [20]. This method uses an oversimplified approximation of the temperature integral thereby limiting the accuracy of the predictions. To avoid such inaccuracies, Vyazovkin proposed an alternative non-linear iso-conversional method [14]. According to this method, for a set of n experiments carried out at different heating rates (b), the activation energy at any particular value of extent of decomposition (a) can be obtained by minimizing the function 4 (Ea):

3.1. Synthesis of polymers

n X n X IðEa ; Tai Þbj   i¼1 js1 I Ea ; Taj bi

ð1Þ

In Eq. (1), the temperature integral is defined as:

IðE; Ta Þ ¼

ZTa exp

  E dT RT

ð2Þ

TaDa

The temperature integral in Eq. (2), will account for stronger variation of activation energy with temperature, where Da is the difference in consecutive values of a chosen for analysis. MATLAB program was developed for the numerical integration of Eq. (2) and minimization of Eq. (1). A function ‘quadv’ which uses recursive adaptive Simpson algorithm was used for numerical integration. Minimization was done using the ‘fminunc’ function for unconstrained problem which uses a quasi Newton line search algorithm.

HO

R Diol

The important step in the synthesis of GAP polymer is the synthesis of PECH diol of adequate molecular weight. Usually, for propellant applications, the molecular weight of the prepolymer used as binders is restricted between 2000 g/mol and 3000 g/mol. This is due to the fact that, higher molecular weight of the prepolymer will lead to higher viscosity incurring high energy costs in processing operations. Synthesis of PECH takes place by the cationic ring opening polymerization of ECH in the presence of BF3edimethyl ether complex as initiator and BDO as the co-initiator. In cationic ring opening polymerizations, the initiation step is the protonation of the monomer by the initiator. Thereafter, protonated monomer could either react with BDO or with the non-protonated monomer. The reaction proceeds through two propagation mechanisms depending on whether the protonated monomer reacts with BDO (activated monomer mechanism (AMM)) or non-protonated monomer (activated chain end mechanism (ACE)). The quality of the polymer obtained depends on the dominance of one mechanism over the other. The operation of AMM will lead to the formation of the preferred linear diol polymers and the operation of ACE leads to unnecessary low molecular weight oligomers. These low molecular weight oligomers are mainly constituted of cyclic compounds. Scheme 4 presents the details of the mechanisms of ACE and AMM. From the involved kinetics, in order to enhance the contribution of AMM, the instantaneous concentration of the non-protonated monomer should be kept as low as possible [21]. We have clearly demonstrated this aspect in one of our earlier papers [22]. By exercising good control over the rate of monomer addition, linear PECH diols with number average molecular weight (Mn) of 2040 g/mol and weight average molecular weight (Mw) of 2481 g/mol and dispersity ¼ 1.21 were synthesized. Like oxiranes, oxetanes such as BCMO and BAMO polymerize through the cationic polymerization route in the

CH2Cl

OH

AMM O

RO

CH2

OH

Linear diol polymer

Cl CH2Cl

H

Protonated Epichlorohydrin ACE

O

CH

HO

CH2

CH

Cyclic oligomers O

Cl

Cl

Epichlorohydrin

Scheme 4. Illustrations of AMM and ACE mechanisms involved in cationic ring opening polymerizations. AMM produces linear polymers. ACE produces cyclic oligomers by backbiting reactions.

S. Pisharath, H.G. Ang / Polymer Degradation and Stability 92 (2007) 1365e1377

presence of Lewis acid catalysts. Both AMM and ACE mechanisms are also applicable to the ring opening of oxetanes too. There have been published reports on the comparison of reactivities of BCMO and BAMO to undergo ring opening polymerization. It was observed that BAMO and BCMO polymerize through different mechanisms and AMM is more favored with BAMO [23]. Hence, we used BAMO monomer directly to synthesize Poly(BAMO). Poly(BAMO) with number average molecular weight (Mn) of 1653 g/mol and weight average molecular weight (Mw) of 2179 g/mol and dispersity ¼ 1.31 was synthesized. In the synthesis of copolymers of PECH and BCMO, we used PECH diol as the macro-initiator along with BF3e dimethyl complex as the catalyst. By this route, the PECH molecule will form the central block and the ring opened BCMO units will be attached to the both ends of PECH moiety. We carried out the synthesis of 2 batches of the PECHe Poly(BCMO) copolymers namely copolymer 1 and copolymer 2 with varying amount of BCMO. An overlay of the GPC chromatograms of the polymers is presented in Fig. 2. The result indicates that, with increasing amount of BCMO, the GPC peak of PECH diol shifts to lower retention times (higher molecular weights), meaning that the BCMO units are getting anchored to the PECH diol block. Results of the GPC analyses are presented in Table 1. It could be observed that the weight average molecular weight of PECH diol increases from 2706 g/mol to 5440 g/mol with increasing amount of BCMO. Fig. 3 compares the FTIR spectra of BCMO monomer with the PECHepoly(BCMO) copolymer. The FTIR peak at 980 cm1 due to the oxetane ring present in the monomer is absent in the copolymer. It is evident that BCMO undergoes ring opening during the polymerization process. We converted copolymer 2 to the azido derivative and used it for the thermal decomposition studies. Conversion of chloro groups to azido groups is a nucleophilic substitution reaction. As presented in Fig. 4, the reaction could be easily monitored by FTIR spectroscopy. The

1371 BCMO PECH-Poly(BCMO)

C-Cl stretch at 740 cm-1

Oxetane ring at 980 cm-1 3500

3000

2500

2000

1500

1000

Wavenumber (cm-1) Fig. 3. Comparison of FTIR spectra of BCMO monomer and PECHepoly (BCMO) copolymer. The peak due to the oxetane ring at 980 cm1 is absent in the copolymer indicating that the oxetane undergoes ring opening in the presence of PECH diol.

disappearance of the CeCl stretching band in FTIR spectra at 748 cm1 and appearance of a new stretching band for CeN3 at 2100 cm1 confirm the substitution reaction. The GAPePoly(BAMO) copolymer exhibited a weight average molecular weight of 5791 g/mol, number average molecular weight of 4136 g/mol and a polydispersity index of 1.4. Fig. 5 compares the 1H NMR spectra of GAP with that of GAPePoly(BAMO). For GAP (Fig. 5a), the spectra show signals due to protons of methylene and methine groups in the polymer backbone at d 3.6 and that due to the pendant azidomethyl unit at d 3.68. NMR spectra of GAPePoly(BAMO) copolymer presented in Fig. 5b show two other peaks at d 3.49 and d 3.44 corresponding to the Poly(BAMO) unit along

GAP-Poly(BAMO) PECH-Poly(BCMO) PECH Copolymer 1 Copolymer 2 C-Cl stretch at 745 cm-1

Increasing BCMO content C-N3 stretch at 2100 cm-1

20

21

22

23

24

25

Retention time (mins) Fig. 2. GPC curves of PECH diol and its copolymers with BCMO. Feed amounts [PECH diol]:[BCMO], copolymer-1:80:20, copolymer-2:75:25. The curves shift to lower retention times (higher molecular weights) with increasing BCMO content.

3500

3000

2500

2000

1500

1000

Wavenumber (cm-1) Fig. 4. Comparison of FTIR spectra of PECHepoly(BCMO) copolymer and GAPePoly(BAMO) copolymer. Appearance of new peak at 2100 cm1 for GAPePoly(BAMO) copolymer indicates the substitution of chloro groups in PECHepoly(BCMO) copolymer with the azido groups.

S. Pisharath, H.G. Ang / Polymer Degradation and Stability 92 (2007) 1365e1377

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d a

a

a

CH2

H C

a HO

(a)

O

H

(b)

a

H

O

a

CH2N3 c

c

H CH2 C

OCH2

b

b

CH2

OH

CH2N3

CH2N3

CH2N3

C

d

b b C d

4.5

4.0

3.5

3.0

4.5

Chemical Shift (ppm)

4.0

3.5

3.0

2.5

Chemical Shift (ppm)

Fig. 5. Comparison of 1H NMR spectra of (a) GAP and (b) GAPePoly(BAMO) copolymer. The new peaks at d 3.49 and d 3.44 in (b) correspond to the poly (BAMO) unit which has copolymerized with GAP.

with the peaks from GAP unit. Copolymer consists of 72% ECH units and 28% BAMO units as calculated from NMR spectra. It is clear that cationic ring opening polymerization is an efficient route to synthesize polymers with low polydispersity index. However, this route is limited to the synthesis of low molecular weight polymers. Hence, for the synthesis of copolymers of molecular weight greater than 10,000 g/mol, a block linking approach is adopted, in which, the end capped energetic polymer blocks are linked to each other by oligomeric diol molecules such as polyethylene glycol (PEG) [3]. 3.2. Thermal decomposition behavior For new propellant formulations, rigorous analysis of its thermal decomposition behavior is essential for its safety and performance prediction. Energetic polymers find application as polymer binders in propellant formulations. As a critical component, decomposition of polymer binder also plays a significant role in controlling the thermal decomposition process of the whole formulation. Therefore, we rigorously investigated the thermal decomposition aspects of energetic polymers. Figs. 6a, b, and c presents the overlay of TGA and DSC curves for GAP, Poly(BAMO) and GAPePoly(BAMO) copolymer. TGA results are presented as conversionetemperature plots. Conversion is another measure for degree of decomposition. In all the curves, the main weight loss step in TGA coincides with the exothermic decomposition peak in the DSC curve. The weight loss is due to the exothermic scission of azido groups to release nitrogen, which is the main decomposition mechanism of azido based energetic polymers. The magnitudes of weight loss and decomposition enthalpy vary from one polymer to another. As shown in Fig. 6a and b, exothermic decomposition temperatures of GAP and poly (BAMO) are similar at 231  C. Poly(BAMO) decomposes with a higher enthalpy of decomposition of 1970 J/g when

compared to GAP, which shows a value of 1696 J/g. The higher value of decomposition enthalpy for Poly(BAMO) is due to the extra number of azide groups in Poly(BAMO) than in GAP. DSC curve of Poly(BAMO) (Fig. 6b) also shows the endothermic double melting peaks at 47  C and 54  C illustrating the strong crystallization tendency of the polymer [24]. Evaluating the TGA curves, 54% of Poly(BAMO) has decomposed at 231  C compared to only 42% of GAP. This higher weight loss of Poly(BAMO) could also be attributed to the extra azide groups attached to the polymer backbone. For both GAP and Poly(BAMO), the degree of mass loss shows a steep increase (10e80%) at the main decomposition temperature region of 200e250  C after which the curve achieves 100% mass loss gradually. Decomposition behavior of the GAPePoly(BAMO) copolymer presented in Fig. 6c is different from that of homopolymers of GAP and Poly(BAMO). DSC curve shows an exothermic decomposition maximum at 228  C with a decomposition enthalpy of 1400 J/g. After the main decomposition peak, another broad shoulder peak is also observable with a peak maximum temperature at 298  C. This shoulder peak accounts for a decomposition enthalpy of 50 J/g. From the TGA curves, the degree of mass loss increases from 5% to 55% during the main decomposition temperature region, levels off at 250  C, increases further until 100% mass loss is achieved at 350  C. The broad shoulder peak observed in DSC curve is due to the second mass loss region between 250  C and 350  C. The DSC curve of the copolymer also shows a broad endothermic melting peak at 66  C with an enthalpy of 10 J/g. This observation indicates that copolymerization of Poly(BAMO) units with GAP has induced crystalline nature into an otherwise amorphous GAP polymer. A low temperature DSC scan of the copolymer showed a glass transition temperature (Tg) of 35  C. The combined presence of a low temperature Tg and melting transition shows that the synthesized

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8 Exo

Conversion

0.4 2

20

0.2

0.6

-0.2

15

-0.4 -0.6

47°C 54°C

-0.8

0.4

10

25 30 35 40 45 50 55 60 65 70 75

Temperature (°C)

5

0.2 0

0.0 50

150

100

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250

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0

0.0

350

50

Temperature (°C)

Heat Flow (mW)

4

0.8

Heat Flow (mW)

0.6

Exo

0.0

6

0.8

231°C

(b) Heat Flow (mW)

(a)

25 1.0

231°C

Conversion

1.0

100

150

200

250

300

350

Temperature (°C) 14

1.0

(c)

228°C

Exo

12

0.6

10 8

-0.1

66°C 35 40

0.4

45

50

55 60

65 70

6

75

Temperature (°C)

298°C

4

0.2

Heat Flow (mW)

Conversion

0.8

Heat Flow (mW)

0.0

2 0.0

0 50

100

150

200

250

300

350

Temperature (°C) Fig. 6. Overlay of TGA and DSC curves for (a) GAP, (b) poly(BAMO) and (c) GAPePoly(BAMO) copolymer. The melting transitions observed in poly(BAMO) and GAPePoly(BAMO) copolymer are shown as insets in respective plots.

copolymer is a TPE. However, it is interesting to observe that the GAPePoly(BAMO) copolymer is having a higher melting point compared to the homopolymer. A similar behavior has been observed for a triblock copolymer of BCMO and THF [25] in which the triblock copolymer exhibited a higher melting point than the poly(THF). 3.3. Kinetics For energetic materials, activation energy of thermal decomposition could be correlated with important performance parameters such as heat of explosion, detonation velocity, ChapmaneJouguet pressure, and detonation energy [26]. Hence it is important to obtain activation energy data out of thermal decomposition experiments. Model-fitting methods provide an easy framework of deducing activation energy by forcefully fitting variety of reaction models to a set of decomposition data. Experiments run at a single heat rate will suffice for these methods. However, the predictions obtained from model-fitting methods lead to erroneous predictions, especially when the decomposition processes involve multiple steps. The International Confederation for Thermal Analysis and Calorimetry (ICTAC) recommend adopting model-free

methods for analysis which employ kinetic curves obtained at multiple heating rates. Model-free methods allow evaluation of kinetic parameters without implicit assumption of a reaction model. These methods provide the effective activation energy as a function of extent of conversion. Many reviews are available explaining the computational aspects of various modelfree methods and its application to thermal analysis of variety of materials [27,28]. We used the non-linear model-free method proposed by Vyazovkin to analyze thermal decomposition kinetics of GAP, Poly(BAMO) and GAPePoly(BAMO) copolymer. The experimental conversionetemperature curves at different heating rates for the decomposition of GAP, Poly(BAMO) and GAPePoly(BAMO) copolymer are presented in Fig. 7. Both GAP and Poly(BAMO) exhibit a single-step variation of conversion with temperature (Fig. 7a and b). The copolymer exhibits a double-step conversion temperature profile with a plateau at 250  C dividing the two steps (Fig. 7c). Also, for the copolymer, the decomposition process extends to 350  C, compared to 260  C for GAP and Poly(BAMO). Activation energies for different extent of conversions (Ea) were calculated from the conversionetemperature profiles by applying Eq. (1). The activation energy dependence with

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1374

1.0

(a)

0.8

0.20°C/min 0.50°C/min 0.80°C/min

1.0

(b)

0.8

Conversion

Conversion

2.50°C/min

0.6

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5.0°C/min 7.50°C/min

0.6

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270

280

Temperature (°C)

1.0

170

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190

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250

Temperature (°C)

(c) 1.0°C/min 4.0°C/min 5.0°C/min

Conversion

0.8

0.6

0.4

0.2

0.0 180

200

220

240

260

280

300

320

340

360

Temperature (°C) Fig. 7. Experimental conversionetemperature profiles at various heat rates obtained from TGA results for (a) GAP, (b) poly(BAMO) and (c) GAPePoly(BAMO) copolymer.

conversion for GAP, Poly(BAMO) and GAPePoly(BAMO) copolymer are presented in Fig. 8. TGA curves of the samples collected at a heat rate of 5  C/min are also plotted to show the correlation of Ea with temperature. For both the homopolymers, activation energy dependence is an increasing function of conversion (Fig. 8a and b). Two decomposition pathways could be clearly evidenced for GAP and Poly(BAMO). From the initial stages of decomposition until a ¼ 0.75, Ea remains practically constant at w170 kJ/mol. This value is consistent with the reported values of activation energy of azido polymers measured by various experimental methods [29,30]. Towards the end of the decomposition process, Ea increases to w280 kJ/mol for both the polymers. GAP exhibits an abrupt increase, whereas Poly(BAMO) shows a more gradual variation. For the copolymer (Fig. 8c), 4 decomposition stages of varying Ea could be identified. During the first mass loss step (a ¼ 0.03e0.36), Ea remains constant at 145 kJ/mol. For the second mass loss step Ea increases from w145 kJ/ mol at a ¼ 0.36 to w220 kJ/mol at a ¼ 0.62. Hereafter, Ea remains constant until a ¼ 0.78 and further increases to w260 kJ/mol until a ¼ 0.98.

Activation energy parameters derived from the model-free method support the experimental TGA and DSC results. Ea remains practically constant at w170 kJ/mol, for GAP and Poly(BAMO) exhibiting single-step decomposition until it increases to w280 kJ/mol towards the end of decomposition process. For the copolymer, double decomposition step in TGA and DSC results could be identified as two regions with different activation energies in the Ea-dependence plot. First region is between a ¼ 0.03 and a ¼ 0.36 with Ea of w145 kJ/mol and the second region with Ea of w220 kJ/ mol between a ¼ 0.62 and a ¼ 0.8. Thus application of model-free techniques to thermal decomposition of materials could successfully identify the Ea dependence of steps involved. 3.4. Mechanistic considerations The shape of activation energy dependence helps to shed light on the kinetic scheme involved in the decomposition process. An increasing function of Ea dependence as observed in our studies generally denotes the operation of competitive

S. Pisharath, H.G. Ang / Polymer Degradation and Stability 92 (2007) 1365e1377 230

300

400 Activation Energy Temperature

350

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(b)

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250 230

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Activation Energy Temperature

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Activation Energy/kJ mol-1

(a) Activation Energy/kJ mol-1

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0.2

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Conversion

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Activation Energy Temperature

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300 280

Region 1 Region 2

260 240

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Temperature (°C)

Activation Energy/kJ mol-1

0.6

Conversion

300

200

0.4

200 180

50 0.0

0.2

0.4

0.6

0.8

1.0

Conversion Fig. 8. Dependence of activation energy on the extent of conversion and the dependence of the extent of conversion on temperature at the heating rate of 5  C/min for (a) GAP, (b) poly(BAMO) and (c) GAPePoly(BAMO) copolymer.

reactions during the decomposition [31]. Increasing Ea dependence was observed for degradation of linear polymers in the absence of oxygen. The result was interpreted as competition between the decomposition of individual macromolecules and intermolecular associates formed during the course of the reaction [32]. Mechanism of decomposition of azide polymers has been investigated by employing a variety of experimental techniques. Here we attempt to correlate the Ea dependence with the suggested decomposition mechanism of azido polymers. The established mechanism is the elimination of nitrogen from the pendant azide groups of the polymer to form an imine. It has also been proposed that the imine intermediate undergoes intermolecular cyclizations to form intermolecular associates (Scheme 5) [33,34]. We believe that the increasing function of Ea dependence is due to the competition between the decomposition of linear GAP polymer and intermolecular associate formed during the reaction. In order to gain more insights into the mechanism of decomposition, we took FTIR spectrum of the decomposition residues of the polymers at 250  C (Fig. 9) and compared with the spectrum of GAPePoly(BAMO) copolymer before decomposition. This temperature was chosen because the

conversionetemperature profile of the copolymer exhibits a plateau at this temperature. This will enable us to account for the energetic factor responsible for the second stage degradation observed in the copolymer. Comparing the spectra, it could be observed that for the degraded copolymer the intensity of the characteristic azide band at 2100 cm1 and 1270 cm1 is reduced considerably after thermal decomposition. These bands are completely absent for the degraded Poly(BAMO) and GAP. Also, after the pyrolysis, a new band appears at 1650 cm1 for the degraded polymers. This band could be readily assigned to the C]N vibration of the imine intermediate formed during the elimination of nitrogen from the azide group as shown in Scheme 5. It is noteworthy that the band associated with CeH stretching vibrations of the polymer doesn’t change with pyrolysis suggesting that the polymer backbone is stable at 250  C. This observation confirms that the azide group thermally decomposes before the backbone does. FTIR study presented above indicates that some residual azido groups are remaining in the copolymer after the first stage decomposition, whereas for degraded Poly(BAMO) and GAP all the azido groups have completely decomposed at 250  C itself. These residual azido groups in the copolymer

S. Pisharath, H.G. Ang / Polymer Degradation and Stability 92 (2007) 1365e1377

1376

O

H C

H2C

O

R

O

CH2

CH2N3

CH

O

CH2N3 -N2

O

H C

H2C

CH

O

R

O

NH

H C

H2C

CH

HN

CH

O

Imine

Imine

O

CH2

O

R

O

CH2

4. Conclusions

CH

CH

CH

N

N

O

Scheme 5. Generalized mechanism of thermal decomposition of azido polymers.

are responsible for its double decomposition behavior observed in the TGA/DSC results. Also, as clear from the model-free analysis, the second stage decomposition takes place at higher activation energy than the first step. Previous studies on thermal characteristics of azido copolymers using model-fitting methods have suggested that the activation energy of azide decomposition is unaffected by copolymerization [11]. By adopting a more rigorous model-free approach,

2100 cm-1 1650 cm-1 1270 cm-1

2927 cm-1

(a)

(b) (c) (d)

3300

3000

2700

2400

2100

1800

1500

we could prove that activation energy of decomposition of the azido groups could vary with the copolymerization. Although the general mechanism of decomposition of azido polymers remains the same, we believe the splitting of activation energy of azide decomposition into two stages in the copolymer is due to a complex time dependent intramolecular interaction between its constituent blocks. This aspect warrants a more detailed investigation. Finally, we hope that the results presented in this paper could add up to existing knowledge base of the GAPePoly(BAMO) literature and facilitate its realization as a high performance energetic binder.

1200

900

Wavenumber (cm-1) Fig. 9. Comparison of FTIR spectra of (a) GAPePoly(BAMO) copolymer with thermally degraded residues of (b) GAPePoly(BAMO) copolymer, (c) poly(BAMO) and (d) GAP. The degraded residues were collected at 250  C.

A copolymer of GAP and Poly(BAMO) was synthesized using the BF3edimethyl ether complex/diol initiator system. The synthesized copolymer exhibited the characteristics of an ETPE. Application of model-free treatment to thermal degradation kinetics of the copolymer showed that activation energy of decomposition of azide groups is affected by copolymerization. In homopolymers, all the azide groups decompose in a single step. However in the copolymer, azide group decomposition is split into two stages of different activation energies. References [1] Mohan YM, Mani Y, Raju KM. Synthesis of azido polymers as potential energetic propellant binders. Des Monomers Polym 2006;9:201e36. [2] Stacer RG, Husband DM. Molecular structure of ideal solid propellant binder. Propell Explos Pyrotech 1991;16:167e76. [3] Sanderson AJ, Edwards W. Synthesis of energetic thermoplastic elastomers containing oligomeric urethane linkages. US Patent 6,815,522; 2004. [4] Singh G, Felix SP, Soni P. Studies on energetic compounds, part 31: thermolysis and kinetics of RDX and some of its plastic bonded explosives. Thermochim Acta 2005;426:131e9. [5] Ger MD, Hwu WH, Huang CC. A study on the thermal decomposition of mixtures containing an energetic binder and a nitramine. Thermochim Acta 1993;224:127e40. [6] Oyumi Y, Inokami K, Yamazaki K, Matsumoto K. Burning rate augmentation of BAMO based propellants. Propell Explos Pyrotech 1994;19:180e6. [7] Bazaki H, Kubota N. Effect of binders on the burning rate of AP composite propellants. Propell Explos Pyrotech 2000;25:312e6. [8] Korobeinichev OP, Kuibida LV, Volkov EN, Shmakov AG. Mass spectrometric study of combustion and thermal decomposition of GAP. Combust Flame 2002;129:136e50. [9] Lee YJ, Tang CJ, Kudva G, Litzinger TA. Thermal decomposition of 3,30 -bis-azidomethyl-oxetane. J Propul Power 1998;14:37e44. [10] Sahu SK, Panda SP, Sadafule DS, Kumbhar CG, Kulkarni SG, Thakur JV. Thermal and photodegradation of glycidyl azide polymers. Polym Degrad Stab 1998;62:495e500. [11] Kimura E, Oyumi Y. Thermal decomposition of BAMO copolymers. Propell Explos Pyrotech 1995;20:322e6. [12] Liu YL, Hsiue GH, Chiu YS. Thermal characteristics of energetic polymers based on tetrahydrofuran and oxetane derivatives. J Appl Polym Sci 1995;58:579e86. [13] Lee YJ, Litzinger TA. Thermal decomposition of BAMO/AMMO with and without TiO2. Thermochim Acta 2002;384:121e35. [14] Vyazovkin S, Dollimore D. Linear and nonlinear procedures in isoconversional computations of activation energy of nonisothermal reactions in solids. J Chem Inf Comput Sci 1996;36:42e5.

S. Pisharath, H.G. Ang / Polymer Degradation and Stability 92 (2007) 1365e1377 [15] Vyazovkin S, Sbirrazzouli N. Isoconversional kinetic analysis of thermally stimulated processes in polymers. Macromol Rapid Commun 2006;27:1515e32. [16] Sell T, Vyazovkin S, Wight CA. Thermal decomposition kinetics of PBAN binder and composite solid propellants. Combust Flame 1999;119:174e81. [17] Stockel RF. Method of preparing 3,3-bis(chloromethyl)oxetane. US Patent 4,031,110; 1977. [18] Malik AA, Manser GE, Carson RP, Archibald TG. Solvent free process for the synthesis of energetic oxetane monomers. US Patent 5,523,424; 1996. [19] Friedman HL. Kinetics of thermal degradation of char forming plastics from thermogravimetry application to a phenolic resin. J Polym Sci Part C 1965;6:183e5. [20] Flynn JH, Wall LA. A quick direct method for the determination of activation energy from thermogravimetric data. Polym Lett 1966;4:323e8. [21] Kubisa P, Penczek S. Cationic activated monomer polymerization of heterocyclic monomers. Prog Polym Sci 1999;24:1409e37. [22] Pisharath S, Ang HG. Thermal decomposition kinetics of a mixture of energetic polymer and nitramine oxidizer. Thermochim Acta, in press, doi:10.1016/j.tca.2007.03.017. [23] Cheradame H, Andreolety JP, Rousset E. Homopolymerization of 3,3-bis(azidomethyl)oxetane and its copolymerization with 3-chloromethyl-3(2,5,8-trioxadecyl) oxetane. Makromol Chem 1991;192:901e18. [24] Hardenstine KE, Henderson GVS, Sperling LH, Murphy CJ, Manser GE. Crystallization behavior of poly(3,3-bisethoxymethyl oxetane) and

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