High Azide Content Hyperbranched Star Copolymer as Energetic

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Applied Chemistry

High Azide Content Hyperbranched Star Copolymer as Energetic Materials Guangpu Zhang, Gangfeng Chen, Jinqing Li, Shixiong Sun, Yunjun Luo, and Xiaoyu Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03596 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on October 2, 2018

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Industrial & Engineering Chemistry Research

High Azide Content Hyperbranched Star Copolymer as Energetic Materials Guangpu Zhanga,†, Gangfeng Chen a,†, Jinqing Lia, Shixiong Suna, Yunjun Luoa,b*, Xiaoyu Li a,b* a. School of Materials Science and Engineering, Beijing Institute of Technology, Beijing, 100081, China. b. Key Laboratory of High Energy Density Materials of Ministry of Education, Beijing Institute of Technology †

These authors contributed equally to this work

*Corresponding author. Tel.: +86-10-6891369; E-mail: [email protected], [email protected]

Abstract: A series of star azide copolymer (b-POBs) with hyperbranched polyether core (HBPO-c) and short linear poly (3,3’-bis-azidomethyl oxetane) arms (PBAMO-a) have been prepared. The polymers were characterized with FT-IR, 1H NMR and Quantitative

13

C NMR, gel permeation

chromatography, MALDI-TOF, and X-ray diffractometer. Due to hyperbranched structures, the crystallinity (Wc) of b-POBs were significantly decreased, and the processability was greatly improved. The enthalpy of formation, obtained by oxygen bomb calorimetric measurements, high azide content and heats of decomposition of b-POBs demonstrated their remarkable energy level. Furthermore, b-POBs had good resistance to pyrolysis up to 230 oC (T5%), and their mechanical sensitivities were also obviously lower than that of PBAMO homopolymer, showing their good safety property. Moreover, the mechanism for sensitivity reduction of b-POBs was established by analyzing the relationship between the activation course in mechanical stimulus and its crystalline structure. Keywords: azide copolymer; hyperbranched; crystallinity; low sensitivity; energetic materials 1. Introduction In the last decades, due to the demand of high energy density materials for the application of propellant and explosives, polymers with azido groups were developed, since they can release additional heat and offer a higher temperature and specific impulse on pyrolysis and combustion.1, 2 Many azide polymers have been developed, such as glycidyl azide polymers (GAP),3, poly(3-azidomethyl-3’-methyl

oxetane)

(PAMMO),5,

6

poly(3,3’-bis-azidomethyl

4

oxetane)

(PBAMO),2, 7 among which PBAMO has the highest energy output, due to its nearly 50% w.t. nitrogen content. Despite of its high energy output, because of its symmetric structure, PBAMO is a crystalline polymer with a melting point of ca 70°C, and thus PBAMO cannot be directly used as a binder or additive in the traditional curing process at 60°C.2, 8, 9 Copolymerization of BAMO with 1

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other cyclic monomers by the same cationic ring opening polymerization (CROP) is a simple and competent method to solve this problem, since the second monomer chains can disrupt the crystallization of PBAMO.2, 10. In some aspects, many of these PBAMO-based copolymers perform better than PBAMO. However, for these copolymers from BAMO and non-energetic monomers, for example tetrahydrofuran (THF)10-12 and ε-caprolactone (ε-CL)13,etc , their energy level is greatly reduced because the content of BAMO units is controlled below 60 % to achieve a decent processability. For the copolymers with BAMO and other energetic monomers (glycidyl azide (GA)14-16, AMMO17-19 and 3-nitratomethyl-3-methyloxetan (NMMO)20), energy level is relatively high, while the bulky side groups always impair the low temperature properties.10 In addition, PBAMO and some of the copolymers show high sensitivity to mechanical stimuli.14, 20-23 Therefore, from the practical point of view, it is highly desired to design a novel PBAMO-based copolymer with facile synthesis method, relatively high azide content and molecular weight, but low sensitivity and viscosity. Comparing to their linear counterparts, it has been well established that hyperbranched polymers with three-dimensional (3D) highly branched globular structure possess a large population of functional groups, lower solution or melt viscosity, no or lower chain entanglement and better solubility.24,

25

In view of these advantages, some hyperbranched energetic polymers have been

preliminarily synthesized and characterized. Hyperbranched triazole polymers with a million molecular weight were obtained by azide-alkyne cycloaddition reaction.26, 27 Graff et al.28 prepared azido-containing

hyperbranched

polymers

by

atom

transfer

radical

polymerization

of

azide-containing inimers. Another highly branched azide-capped polymers were got by the polymerization of vinyl monomers using azide radical initiator in the presence of hypervalent iodine.29 Although these hyperbranched polymers with energetic groups (azido or triazole) were successfully synthesized using some efficient methods, they were not appropriate as high-energy materials due to their relative low azide content and energy level, consequently, being common used as functional macromolecules. Azido-terminated dendritic or hyperbranched polyesters were prepared by tosylation of terminal hydroxyl groups, followed by azidation. However, their synthetic processes were very complex and time-consuming, in spite of their low sensitivity and high energetic level.30-33

In

addition,

Zhang

et

al.34

synthesized

hyperbranched

poly-3-azidomethyl-3-hydroxymethyl oxetane (PAMHMO) with good mechanical properties. 2


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However, the preparation is extremely risky due to these highly sensitive monomers (AMHMO, BAMO and AMMO),5, 35, 36 which is not suitable for industrial production. In our previous works, core-shell type hyperbranched polymers with hyperbranched polyether core and linear GAP shells were successfully prepared, and performed much better in terms of energy level, low temperature properties and processability than the linear GAP polymers.37,

38

Poly(3-ethyl-3-(hydroxymethyl)

oxetane) (HBPO), a classic hyperbranched polymer, is friendly materials for solid propellant37, 39, 40, meanwhile, it is synthesized by CROP41 corresponding to the key step of preparing PBAMO.42 Inspired by the literatures, that HBPO is introduced into the BAMO polymer might be a desirable alternative to improve the disadvantages of PBAMO homopolymer. In this study, a series of azide copolymers (b-POBs) with hyperbranched polyether core (HBPO-c) and short linear poly (3,3’-bis-azidomethyl oxetane) arms (PBAMO-a) were synthesized via the sequential CROP of 3-ethyl-3-(hydroxymethyl) oxetane (EHO) and 3,3-bis(chloromethyl) oxetane (BCMO), followed by azidation of the chlorine groups. The molar ratio between BAMO-a and HBPO-c was adjusted via the feeding ratio during CROP. The chemical and crystalline structures of b-POBs were carefully determined, and their energetic properties, rheology, thermal behavior and sensitivity were evaluated. Comparing to their linear counterparts, these b-POBs are advantageous in terms of energy level, processability and safety, making them promising compound as energetic polymeric materials. 2. Experimental methodology 2.1 Materials Boron trifluoride etherate (BF3⋅Et2O) was purchased from Sinopharm Chemical Reagent (China). 1,2-dichloroethane (DCE) was obtained from Beijing Tongguang Fine Chemicals Co. BF3⋅Et2O and DCE were dried by CaH2 and vacuum distillation before use. 3-ethyl-3-(hydroxymethyl)-oxetane (EHO, 96%, Macklin) and 3,3-bis(chloromethyl) oxetane (BCMO, 99%, Nanjing Chemlin Chemical Industry Co., China), dried by molecular sieve, were used. Sodium azide (98%, Xiya Chemical Industry Co., Shandong, China) was used as received. PBAMO homopolymer (Mn=4.3 kg mol-1) was synthesized according to literature42. The other reagents were obtained from Beijing Tongguang Fine Chemicals Co. and used as received. The glasswares used were dried oven at 150 oC before use. 2.2 Synthesis of HBPO-b-PBCMO (b-POC) copolymer The preparation of hyperbranched core (HBPO-c) was performed in a four-neck flask with 3



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magnetic stir bar, thermometer, and tap funnel under nitrogen atmosphere. The system was flashed with nitrogen for 30 min before reaction. Then 0.025 mol (3.55 g) BF3·Et2O and 40 ml DCE were injected into the flask, respectively. Subsequently, the solution with EHO (0.05 mol) and DCE (40 ml) was added by the tap funnel. The mixture reacted for 48 h at 25 oC to obtain HBPO precursor. The partial precursor was taken out by syringe for characterization. Then, a certain amount of (BF3·Et2O)α (additional BF3⋅Et2O, the molar ratio of (BF3·Et2O)α to BF3·Et2O is α) was injected to the flask of HBPO precursor, and the mixture was stirred for 1 h. Subsequently, the desired amount of BCMO, diluted by DCE, was slowly dripped into the system over 8 h at 0 °C. After the introduction of BCMO, the mixture was kept to react for another 36 h at 25 °C. The polymerization was quenched by using deionized water, and the reaction mixture was poured into deionized water to precipitate the crude product. The crude product was washed with ethanol for several times. b-POC, dried in a vacuum oven at 80 °C, was obtained as white solid. By varying the Rfeed (feeding molar ratio of BCMO to EHO), b-POC sample with different BCMO to EHO were obtained (b-POC-n) The structural information of HBPO is the same to that in the literature.37 b-POC: 1H NMR (500MHz, DMSO-d6; δ, ppm): 3.60~3.67 (m, 2H, CH2Cl); 4.13-4.16 (m, 1H, -OH); 3.39~3.41 (m, 2H, -CCH2O- in PBCMO arms); 3.28~3.31 (m, 2H, -CH2OH); 3.15 (s, 2H, -CCH2O- in HBPO core); 1.28 (m, 2H, -CH2CH3); 0.8 (m, 3H, -CH3).

13

C NMR (125MHz,

DMSO-d6; δ, ppm): 71.8, 71.5 (-CCH2O- in HBPO core); 68.8, 68.0 (-CCH2O- in PBCMO arms); 62.2 (-CH2OH in HBPO core); 58.8 (-CH2OH in PBCMO arms); 45.9, 45.6 (-CH2Cl); 44.8 (-C- in PBCMO arms); 43.32, 43.26, 43.17 (-C- in HBPO core); 7.6 (-CH3). FTIR (KBr, cm-1): 3400 ν (OH), 2950~2800 ν (CH3, CH2), 1100 ν (-O-), 1050 ν (C-O), 745 ν (C-Cl). 2.3 Synthesis of HBPO-b-PBAMO (b-POB) copolymer All azidation reactions were conducted with a NaN3 to chlorine molar ratio of 1.25. In a typical procedure for b-POB-n, NaN3 (3.0 g, 3.8 g, 4.4 g, 4.8 g, and 5.0 g for b-POB-1,2,4,8 and 16) and DMSO (50ml) were added to a three-necked flask with a condenser in nitrogen at 80 °C. When NaN3 was completely dissolved under stirring, b-POB-n (5g) was directly added to the flask. The system reacted for 36 h at 120 °C, and then cooled to ambient temperature and filtered. The filtrate was poured into deionized water to precipitate the copolymer. The crude product was extracted with methylene chloride and washed with distilled water for at least three times. The organic phase was dried over sodium sulfate, filtered, and the b-POB-n (corresponding to b-POC-n) was obtained after 4


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solvent evaporation. b-POB: 1H NMR (500MHz, DMSO-d6; δ, ppm): 4.13-4.15 (m, 1H, -OH); 3.39~3.41 (m, 2H, -CCH2O- in PBAMO arms); 3.34 (m, 2H, -CH2N3); 3.28~3.31 (m, 2H, -CH2OH); 3.15 (s, 2H, -CCH2O- in HBPO core); 1.28 (m, 2H, -CH2CH3); 0.8 (m, 3H, -CH3).

13

C NMR (125MHz,

DMSO-d6; δ, ppm): 71.8, 71.5 (-CCH2O- in HBPO core); 69.5, 68.1 (-CCH2O- in PBAMO arms); 62.2 (-CH2OH in HBPO core); 59.8 (-CH2OH in PBCMO arms); 51.3~51.1 (-CH2N3); 45.0~44.7 (-C- in PBAMO arms); 43.32, 43.26, 43.17 (-C- in HBPO core); 7.6 (-CH3). FTIR (KBr, cm-1): 3400 ν (OH), 2950~2800 ν (CH3, CH2), 2100 ν (-CH2N3) 1100 ν (-O-), 1050 ν (C-O). 2.4 Characterization Fourier transform infrared Spectroscopy (FTIR). FTIR spectra were recorded at ambient temperature using FTIR spectrometer (Nicolet 8700, Thermo Nicolet Corporation, USA). Experimental conditions: the number of scans is 48 times/min; the resolution is 4 cm-1; the scanning range was 400-4000 cm-1. The samples were detected by film cast on KBr plates. NMR analysis. NMR spectra were recorded using a Bruker 500 MHz spectrometer in DMSO-d6. Quantitative

13

C-NMR spectra were recorded by an inversely gated and proton decoupled pulse

sequence using the same machine. Molecular weight measurement. Gel permeation chromatography (GPC) was conducted on a Waters-2695 using DMF as mobile phase (1ml/min, 35 oC). Matrix-assisted laser desorption ionization

time-of-flight

mass

spectra

(MALDI-TOF)

was

measured

on

a

Shimadzu

AXIMA-Assurance with a variable repetition rate 50Hz N2 laser, and the matrices was 2-Cyano-4-hydroxy-cinnamic acid (HCCA). X-ray diffractometer (XRD) and polarizing optical microscopy (POM). The crystallization of b-POB-n was measured with an X-ray diffractometer (X’ Pert PRO MPD, PANalyt-ical B.V.). Before measured, b-POBs were heated to 80 oC and kept for 30 min to eliminate their thermal history, and then annealed at 10°C for 72 h.10 The diffraction intensity of CuKα radiation was measured in a 2θ range between 5° and 60° at room temperature. The morphology of b-POB-n was detected by polarizing optical microscopy (POM, Leica MD2500P) equipped with a computer-controlled CCD camera. The samples were sandwiched between two clean glass slides with the gap of ca 50 µm. Before measured, b-POBs were heated to 80 oC and kept for 30 min to remove their thermal history, and then annealed at 10 °C for 72 h, then POM micrographs were captured at ambient temperature. 5



Elemental analysis. Elemental analysis was performed on Elementar Vario EL Ⅲ equipment. Thermal analysis. Glass transition temperatures were obtained by differential scanning calorimetry (DSC) using a Mettler Toledo DSC1 system in the range of -80 oC to 100 oC with heating rates of 10 K/min. Samples were heated to 100 oC to eliminate thermal history, then cooled to -80 oC by liquid nitrogen, and again heated up to 100 oC. Data reported herein were taken from the second heating cycle. Themo gravimetric analysis was carried out in TGA analyzer (TGA/DSC1SF/417-2, Mettler Toledo). Sample (1.0±0.1 mg) were placed into an alumina crucible and heated at heating rate of 10 o

C/min from 30 oC to 600 oC in nitrogen atmosphere (40 mL/min).

Rheology test. Apparent viscosity was carried out using a rotational rheometer HAAkE MARs (Vreden, Germany) fitted with a Φ=20 mm parallel plate and a gap width of approximately 0.8 mm. The operating temperature was 30°C and 60 °C and shear rate is 0.1~500 Hz. Heat of combustion test. The heat of combustion was detected by Parr 6200 oxygen bomb calorimetry (Illinois, USA), which was carried out according to Chinese Military Standards GJB 770B-2005. Mechanical sensitivity test. The impact sensitivity was tested with a CGY-3 (Nachen, Beijing) impact instrument according to the Chinese GJB772A-97 601.2. Measuring conditions: sample 30±1 mg; drop-hammer mass 2 kg. The special height (H50, 50% probability of explosion) was calculated by 25 drop tests, and the average value and standard deviation were recorded through three parallel tests. The friction sensitivity was determined with a MGY-2 (Nachen, Beijing) friction instrument according to the Chinese GJB772A-97 602.1. Measuring conditions: sample 20±1 mg; pendulum mass 1.5 kg; swaying angle 90°; pressure 3.92 MPa. The explosion probability (P, %) was calculated from 25 trials, and the average value and standard deviation were recorded through three parallel tests.43-46 3. Results and discussion 3.1 Preparation and characterization of b-POB-n Initially, we attempted to synthesize b-POC by sequential cationic ring-opening polymerizations (CROP) of 3-ethyl-3-(hydroxymethyl)-oxetane (EHO) and 3,3-bis(chloromethyl) oxetane (BCMO).37, 47

To obtain a high degree of branching of hyperbranched polyether core (HBPO-c), the

polymerization of EHO by CROP was performed using suitable molar ratio of monomers to catalysts (2:1) at 25°C.48 Then, the polymerization of BCMO directly using HBPO precursor as initiators was 6


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unsuccessful due to the fact that the yield of b-POC was only ca 20%. As suggested by the NMR results (Figure S1), HBPO precursors could not directly initiate the polymerization of BCMO. Some researchers found that the concentration of catalyst directly affected the polymerization of oxetane by CROP, and it was very difficult to generate polymerization of 3,3-bis(azidomethyl)-oxetane in relative low concentration of BF3·Et2O catalyst.49, 50 Accordingly, we tried to introduce a certain amount of additional BF3·Et2O ((BF3·Et2O)α) to HBPO precursor system for increasing the concentration of catalyst. Fortunately, in Figure S2, there is an increasing tendency for the yield of b-POC-2 (as a sample) as α increases (when α≥1.5, reached ca 90%). Furthermore, according to the NMR results in Figure S1, the ring-opening polymerization of BCMO using HBPO as initiator are conducted by the activated monomer mechanism.51 In order to better investigate the performance characteristics of hyperbranched star copolymer, b-POB-n with different arm-core ratio was obtained by adjusting the mole ratio of monomers. The synthetic route to b-POB copolymer are shown in Figure 1.

Figure 1 Synthetic route of b-POB The chemical structures of copolymers were characterized by FTIR spectroscopies (taking n=2 as an example). Figure 2 shows the FTIR spectra of HBPO, b-POC-2 and b-POB-2. There were some hydroxyl groups in these polymers due to the 3400 cm-1 peaks. A new peak appearing at 2100 cm-1 in the spectrum of b-POB-2, which is attributed to -N3 groups, and the disappearance of the peak at 742 cm-1, characteristic absorption of C-Cl, demonstrated that the azidation of b-POC-2 was completed. For b-POB-n with different Rfeed, the ratio of azide absorbance and hydroxyl absorbance (A) changes accordingly in Figure S3, implying that the compounds with different azide content were obtained.

7



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Figure 2 FTIR spectra of HBPO, b-POC-2 and b-POB-2 NMR results further confirmed the chemical structures of b-POC and b-POB polymers. Figure 3 shows the 1H NMR and 13C NMR spectra of HBPO, b-POC-2 and b-POB-2. Compared with that of b-POC-2, the 1H NMR and 13C NMR spectra of b-POB-2 showed new peak signals at 3.34 (-CH2N3) and 51.3 ppm (-CH2N3) while the chemical shifts attributed to chloromethyl disappeared completely, suggesting a successful azidation.

Figure 3 The spectra of 1H (a) and 13C (b) NMR for HBPO, b-POC-2 and b-POB-2

These polymers were characterized via 1H NMR and Quantitative

13

C NMR. Degree of

branching (DB) of HBPO-c for b-POBs was determined by the equation DB=(D+T)/(D+T+L)52 (where D, L and T are integration of the areas of Dendritic, Linear and Terminal unit in Quantitative 13

C NMR spectrum (Figure S4) ), and the results are shown in Table 1. In the 1H NMR spectra of

b-POB-n (Figure 4a) the peak intensity of PBAMO (-CH2-) increases gradually as Rfeed increases. The PBAMO-a grafted to HBPO-c ratio (Ra/c) was obtained by 1H NMR and listed in Table 1. It was 8


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found that when n≥8, the values of Ra/c were much smaller than Rfeed, which was different from that of our previous report37, and the most likely reason was that when PBCMO-a chains reaches a certain length, some b-POC could crystallize out for the reaction solution during copolymerization process and limit the chain growth further to some extent. In Figure 4b, with an increase of Rfeed, the intensity of the peaks from PBAMO increased whereas that of HBPO and terminal unit T decreased or even disappeared, demonstrating that PBAMO was covalently grafted to HBPO-c. The degree of hydroxyl initiation in HBPO-c (J, Table 1) was obtained by these peaks changes in the Quantitative 13

C NMR of b-POB-n.

Figure 4 The spectra of 1H NMR (a) and Quantitative 13C NMR (b) for b-POBs

Because of highly branched feature and polydispersity of hyperbranched polymers, accurate determination of their molecular weights is relatively difficult37, 41, and thus MALDI-TOF, GPC and NMR analysis were used. The GPC results of b-POB-n are shown in Table 1 and Figure 5a. The molecular weight of b-POB-n increases slowly as Ra/c increased. The results of MALDI-TOF of HBPO-c for b-POBs are listed in Table 1 and Figure 5b, which agree with those from GPC. The calculated molecular weights of b-POB-n (Mn,NMR, Table 1) were obtained by 1H NMR. Although the data of Mn,NMR also showed a similar trend of increasing, they were much higher than the corresponding Mn,GPC due to the fact that the hydrodynamic volumes of hyperbranched polymers are less than that of linear calibrated polymer (such as polystyrene) in GPC analysis.41 From the molecular weight of HBPO-c and b-POBs, the average number (Q) of arms and the average number (U) of repeating unit (BAMO) of arms can be roughly estimated by the following equation:

, ,

, ,

9



where MwEHO and MwBAMO are the formula weight of EHO and BAMO. These results (Table 1) indicated that the increase of molecular weight of b-POB-n was mainly because of the increase of Q in n≤4, whereas in n>4 that was mainly due to the increase of U, which could also explain the small increase of molecular weight of b-POBs from GPC. These structural features suggested that the molecular weight of hyperbranched star polymer from GPC was far less than its actual molecular weight, so the molecular weight of b-POB from NMR was used. Meanwhile, all b-POBs showed relatively narrow PDIs (≤1.63), agreeing with the kinetic theory of narrow molecular weight distribution for hyperbranched copolymer with a multifunctional core moiety.47, 53 In addition, when n≤4, the hydroxyls of HBPO partially initiated polymerization of the second monomers to form linear arms (J≤82.1%, Q16.7), almost all of hydroxyls of HBPO-c took part in the polymerization of arms, and obtained relative complete star copolymers, which might properly reduce the distribution (PDI≈1.3). The same phenomena were reported for hyperbranched star copolymers in these literatures.37, 47 This narrow PDIs also excluded the presence of a large number of homopolymers. It can be seen that b-POBs with different molecular weights could be successfully prepared by properly adjusting Rfeed.

Figure 5 The GPC (a) and MOLDI-TOF-MS (b) curves of different polymers

10


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Table 1. The structural characteristics of b-POBs ωBAMOc

Jd

Rfeeda

Ra/cb

b-POB-1

1/1

0.9/1

47.4

47.7%

b-POB-2

2/1

2.1/1

67.7

b-POB-4

4/1

3.1/1

b-POB-8

8/1

5.6/1

b-POB-16

16/1

9.7/1

Polymer

DBe

Mn-core (kg mol-1)

Mn (kg mol-1)

PDI

Qg

Uh

5.1

1.63

9.1

1.9

8.7

1.60

13.1

2.9

5.3

11.6

1.60

15.0

3.7

7.2

18.4

1.30

16.7

5.9

8.2

31.7

1.28

18.2

9.7

GPC

MALDI

GPC

NMRf

0.41

2.1

2.2

4.3

71.3%

0.42

2.1

2.2

4.9

75.6

82.1%

0.42

2.0

2.1

84.8

95.8%

0.41

2.0

2.0

90.6

100%

0.40

2.0

2.1

(%mol)

a

Feeding mole ratio of BCMO to EHO;

b

PBAMO-a : HBPO-c molar ratio, obtained by Ra/c=(3×S3.4)/(4×S0.8), where S0.8 and S3.4 are the values of

area integral for peaks at 0.8 and 3.4 ppm in 1H NMR of b-POB-n; c

BAMO content in b-POBs, obtained by Ra/c;

d

Degree of hydroxyl initiation in HBPO-c, calculated by J=S59.8/(S59.8+S62.2), where S59.8 and S62.2 are integral

values of peak area at 59.8 and 62.2 ppm in Quantitative 13C NMR of b-POB; e

Degree of branched of HBPO-c, obtained by Quantitative 13C NMR;

f

Mn,NMR=Mn-core,MALDI+Ra/c×Mn-core,MALDI×MwBAMO/MwEHO;

g

The average number of PBAMO-a in b-POBs;

h

The average number of repeating unit of PBAMO-a in b-POBs.

The morphologies of b-POBs were observed with polarized optical microscopy (POM), as shown in Figure 6. It was clear for samples with n≤4, there are a few separated Maltese bright patches with a diameter of ca 10µm in b-POB matrix, indicating that a small amount of chain segments was orderly arrayed in a micro-domain and formed spherulites. When n≥8, however, the diameters of b-POBs spherulites significantly increased, reaching ca 50µm, and the density also simultaneously increased. These phenomena were different from that of linear homo-PBAMO (lots of overlapping spherulites), implying that hyperbranched HBPO segments could significantly influence the crystallization of PBAMO segments.

11



Figure 6 The POM snapshots of b-POB-n

Furthermore, in order to determine the relationship between the crystal characteristics and molecular structure of azide copolymer, these polymers have been investigated by XRD. From the XRD profiles in Figure 7, we could observe that HBPO is amorphous but b-POBs are semi-crystalline. By comparing the spectra, it could be concluded that the spherulites in b-POBs matrix are mostly from BAMO segments. As a result, based on the obtained diffraction patterns, we confirmed the degree of crystallinity, crystallite size and lattice strain.

Figure 7 XRD curves of different polymers

The degree of crystallinity was calculated from the XRD peak intensities, meanwhile, the crystallite size and lattice strain were obtained by fitting the peak with Williamson-Hall equation.54 The detailed fitting processes are shown in Figure S5, and the results are presented in Figure 8. As can be seen from the data, the degree of crystallinity of b-POBs slowly increases from 9.6 % (n=1) to 12


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16% (n=4). It suddenly jumped to 42 % at n=8 and then 52 % at n=16, but still lower than that of homo-PBAMO (78.5%). Meanwhile, the average crystallite size of b-POB-n barely changes when n≤4, but sharply decreases from 338Å-176Å at n>4 and then slowly close to that of homo-PBAMO probably because that the number of generated crystal nucleus and crystallization rate at n>4 are much higher. As for the lattice strain, an almost opposite change was observed, indicating that crystal defects of b-POBs increases with high content of PBAMO. When Ra/c is relatively small (n≤4), hyperbranched structure with large free volume55 accounted for a dominant position in molecular spatial distributions of b-POBs because of unreacted hydroxyl of HBPO-c (J≤82.1%) and the less and shorter linear PBAMO-a chains (Q≤15, U≤3.7). They destroyed the regularity of intra- and inter-molecular spatial arrangement of BAMO units to a great extent, but not eliminated the existence of a few regular micro-block BAMO units56, 57, resulting in that a limited number of crystal nucleus growing slowly in b-POB matrix were formed (a few spherulites), which tended to generate a small amount of large-sized, less-defective crystals.58 Therefore, these b-POBs still presented low crystallinity (Wc95%) and produced more and longer linear PBAMO-a chains (Q≥16.7, U≥5.9). Thus, a large number of crystal nucleus were quickly formed, which were apt to gain many small-sized, more-defective crystals58, leading to a remarkable increase in the degree of crystallinity of b-POB (41.9%~52.2%). However, it cannot reach the value of linear PBAMO (77.6%) due to amorphous HBPO-c. As will be discussed later, these special structural characteristics of b-POB-n directly influence its physicochemical property as energetic materials.

Figure 8 Crystalline parameters of b-POB-n 13



3.2 Properties of b-POB-n as energetic materials Rheology Apparent viscosity is one of the important parameters for the processability of energetic polymers. However, the viscosity of linear PBAMO is hardly measured, since it is a solid at room temperature. Interestingly in our study, b-POB-n (n=1,2,4) are liquid at room temperature (but b-POB-8 and 16 are still solid), implying that they can be used for energetic binders in propellant systems. Thus, the apparent viscosity of these samples was measured at 30 °C and 60 °C, which are popular technological temperatures of composite solid propellants. These results are listed in Table 2 and Figure S6. The apparent viscosity from b-POB-1 to b-POB-4 gradually decreases at 30 °C, and the same tendency appearing at 60 °C. The viscosity decreases markedly with increasing molecular weight of b-POB, which is totally contradictory to that of linear polymer. According to the structural characteristic of b-POB-n, when n≥8, b-POB is a solid or semi-solid mass due to high crystallinity (Wc>41 %), when n≤4, the primary influence factor of viscosity change is hyperbranched structure, not entirely crystallinity because of relatively low crystalline degree (Wc

High Azide Content Hyperbranched Star Copolymer as Energetic

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