Energetic interpenetrating polymer network (EIPN): enhanced thermo-mechanical properties of NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP based nanocomposite and its propellants

Abstract

A novel energetic interpenetrating polymer network (EIPN) nanocomposite was designed and tested, which was comprised of functionalized MWCNTs (fMWCNTs) covalently attached to hydroxyl terminated polybutadiene (HTPB) and glycidyl azide polymer (GAP) by a facile in situ polymerization technique. Three types of fMWCNTs (COOH-fMWCNTs, NCO-fMWCNTs and alkyne-fMWCNTs) were synthesized and well characterized by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, transmission electron microscopy (TEM), dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA). The effect of fMWCNTs on the mechanical, dispersion, and thermal properties of NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP click was investigated and synergetic properties were achieved as compared to neat HTPB and GAP PU networks. Here we develop for the first time an EIPN nanocomposite based on NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP with different weight ratios and superior tensile strength of 8.17 MPa with 312% elongation at break was achieved with thermally more stable crosslinked networks. A solid composite propellant based on NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP was also prepared and mechanical and thermal properties investigated. An extensive enhancement in the thermo-mechanical properties of the NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP EIPN based nanocomposite have been achieved which may be ascribed to good dispersion of fMWCNTs in the polymer matrix, strong interfacial bonding and entanglements of crosslinked networks during in situ polymerization. This EIPN based composite propellant with improved mechanical and thermal properties paves the way for its straightforward application as a solid fuel in advanced missile technology.

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Introduction

IPNs have gained significant attention which may mingle the properties of individuals through networks and interpenetrating polymerization to signify an innovative approach to elucidate the problem of polymer incompatibility.^1–7 Normally two kinds of routes are being used to form IPNs namely sequential and simultaneous polymerization. An IPN is an intimate combination of two polymers both in network form, at least one of which is cross-linked in the immediate presence of the other. These cross-linked polymers are held together by permanent entanglements, which are produced by homo cross-linking of two or more polymer systems.^8–11 Sequential IPNs are generally prepared in which the second polymeric component network is polymerized following the completion of polymerization of the first component network. Simultaneous IPN is a process in which both component networks are polymerized concurrently.^4–8 IPNs have achieved more and more research significance. This is because of their outstanding properties and synergetic effect introduced by the forced compatibility of the individual components, and thus the enhanced mechanical strength and resistance to thermal degradation resulting from the interpenetration and entanglements of polymer chains.^10–13

We have recently reported the preparation of energetic IPNs of azido-alkyne click and polyurethane by using acyl-GAP and HTPB with dual curing system via “simultaneous” and facile “sequential” polymerization of GAP and HTPB for composite propellants.^14,15 Here we develop for the first time EIPN nanocomposite based on NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP in order to overcome the compatibility problem of HTPB and GAP and achieving synergetic mechanical properties for the futuristic energetic binder for solid composite propellants. Since their discovery in 1991,¹⁶ multi-walled carbon nanotubes (MWCNTs) have attracted substantial interest because of their unique properties such as high mechanical strength,^17,18 high aspect ratios,¹⁹ large surface areas as well as their high thermal conductivities.²⁰ While various potential applications of MWCNTs have been investigated such as mechanical reinforcement in polymer nanocomposite, sensors, hydrogen storage media and nano-scale semiconductor devices.^21–24

Polyurethane (PU) and click chemistry are widely used for the functionalization of MWCNTs including good selectivity, facile reaction conditions, high yields and purities.^25–27 Huisgen 1,3-dipolar cycloaddition presents the possibility of incorporating triazole moieties onto the surface of MWCNTs. Numerous investigations of heterocyclic 1,2,3-triazole moieties^28,29 have been studied in energetic materials^30–32 because of their positive heat of formation of +272 kJ mol⁻¹.³³ Carbon nanotubes (CNTs) with nitrogen-rich compounds have also been reported in energetic applications.^34,35 Consequently, triazole-functionalized MWCNTs are expected to have potential in the arena of energetic nanocomposite due to their high energy of decomposition. Covalent^36–39 and non covalent functionalization^40,41 of CNTs with polymers is a feasible avenue to make the surfaces of CNTs organophilic, which enables a significant improvement in both the dispersion properties and the chemical interactions with the polymer matrix. CNTs/polymer nanocomposite have attracted the interest of many researchers worldwide due to their potential engineering applications in various areas of automobile industry, aerospace technology, energy storage and many more.^42–45

Yan-Ling Luo et al.⁴⁶ used acid functionalized MWCNTs in HTPB PU conductive polymer composite films for detection of hazardous organic solvent vapours. Chi Zhang et al.⁴⁷ incorporated modified CNT/3,3-bisazidomethyloxetane-3-azidomethyl-30-methyloxetane (BAMO-AMMO) thermoset polyurethane elastomers with different CNT-OH content, 82.5% improvement in mechanical strength achieved compared to regular trimethylolethane TME/BAMO-AMMO energetic binders with decomposition temperature 7 °C higher than that of subsequent TME/BAMO-AMMO energetic binders. Zhong Wei et al.⁴⁸ used glycidyl azide polymer as cross-linker of carbon nanotubes via an exclusive clickable one step reaction initiated by decomposition of azide groups. 10% GAP showed four times higher strength than without cross linked GAP. Wei-Hao Liao and co-workers⁴⁹ reported MWCNTs/carbon fibre/vinyl ester laminate composites. The 29.8% flexural strength of the MWCNT/CF/VE composite was greater than that of pure CF/VE composites with 1.0% acryl-MWCNTs. The coefficients of thermal expansion considerably decreased from 47.2 ppm °C⁻¹ of the pure CF/VE composites to 35.6 ppm °C⁻¹ and 19.9 °C increase in glass transition temperature (T_g) was examined with 1% acryl-MWCNTs.

Yao Chen⁵⁰ incorporated MWCNTs with polyurethane (PU) coatings using molecular layer deposition (MLD). Increase in tensile strength and modulus were achieved. Yuqi Li⁵¹ investigated MWCNTs/poly isobutylene based PU nanocomposite with 63% improvement in Young's modulus and slightly improvement in tensile strength were achieved by addition of only 0.7 wt% of MWCNTs. L. Guadagno et al.⁵² studied epoxy resin system with different weight ratios of MWCNTs and COOH-MWCNTs. Dynamic mechanical and thermal analysis illustrated that the storage modulus increased with the addition of MWCNTs. Zhang et al.⁵³ prepared MWCNTs/polyurethane nanocomposite, both the tensile strength and Young's modulus of the composites were substantially improved by about 90%, with 1 wt% of MWCNTs. The elongation-at-break of PU/carbon nanotubes composites was significantly improved by about 500% and the thermal stability of PU nanocomposite was also enhanced.

Although pristine and fMWCNTs have been incorporated in different polymers for specific applications but fMWCNTs in HTPB and GAP based composite propellant have not yet been reported so far. Here we examined the incorporation of pristine MWCNTs, COOH-fMWCNTs, NCO-fMWCNTs and alkyne-fMWCNTs in the HTPB and GAP based binder system for enhanced mechanical strength and thermally more stable networks for the potential composite propellant. Here NCO-fMWCNTs are used for polyurethane formation between OH groups of hydroxyl terminated polybutadiene (HTPB) with strong interfacial bonds whereas alkyne-fMWCNTs are incorporated in acyl-GAP for click reaction between alkynyl groups of MWCNTs and azido groups of acyl-GAP. This functionalization is expected to improve the dispersion of MWCNTs in polymer matrix. The combined effects of strong interfacial strength by covalent formation and a large increase in the interfacial area (resulting from good dispersion) would enhance the load transfer efficiency and mechanical properties of polymer nanocomposite.

In this study, NCO-fMWCNTs and alkyne-fMWCNTs were synthesized and well characterized by FTIR, XPS, TEM, FESEM, TGA–DTG and Raman spectroscopy. Pristine MWCNTs, COOH-fMWCNTs, NCO-fMWCNTs and alkyne-fMWCNTs were introduced to cure and reinforce the HTPB and acyl-GAP based polymer composites. Following the fabrication of NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP composites, dispersion, mechanical and thermal properties were also investigated. NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP click based composites exhibited molecular level dispersions of NCO-fMWCNTs and alkyne-fMWCNTs in the HTPB and acyl-GAP respectively and constructed strong, hierarchical and covalently bonded interphase structures between the NCO-fMWCNTs and OH groups of HTPB, alkyne-fMWCNTs with azido group of acyl-GAP. NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP composites exhibit a significant enhancement in mechanical and thermal properties compared to the HTPB-IPDI/N100 PU and GAP-IPDI/N100 PU based composites.

A novel energetic IPN of NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP have been synthesized via an “in situ” polymerization with different weight ratios and exhibited synergetic mechanical and thermal properties. Finally the mechanical and thermal properties of the solid composite propellants prepared and investigated by using NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP based EIPN. Novelty of this work lies in an investigating the effect of fMWCNTs in HTPB and acyl-GAP based binder system and resulting propellants with improved processability and synergetic mechanical and thermal properties without risking the ballistic properties. To the best of our knowledge, incorporation of fMWCNTs (NCO-fMWCNTs and alkyne-fMWCNTs) in HTPB and acyl-GAP based binders and composite solid propellant have never been reported so far with enhanced mechanical and thermal properties.

Experimental section

Materials

MWCNTs (trade name: CNTs # 104) with a purity of 98% were obtained from the Beijing Deke Daojin Science and Technology Co., Ltd., China. The diameter and the length of MWCNTs are 10–20 nm and 10–30 nm with ash contents < 0.5 wt% and special surface area (SSA) > 200 m² g⁻¹ respectively. Hydroxyl terminated polybutadiene (HTPB) with molecular weight 3020 g mol⁻¹, functionality 2.205, hydroxyl contents 0.73 mmol g⁻¹, hydrogen peroxide (H₂O₂) contents 0.022% and water contents 0.015% was obtained from Liming Research Institute of Chemical Industry, Henan China. HTPB was deaerated and dewatered for 3 hours at 80 °C before use. Isophorone diisocyanate (IPDI) having average molecular weight 222.2 and 9.0009 mmol of NCO per gram of IPDI was taken from Beijing chemical plant. Dibutyl tin dilaurate (DBTDL) 0.3% solution in diisooctyl sebacate (DOS) was used and taken from the same. Acetone (Aik Moh), anhydrous dimethyl formamide (DMF, Tritech Scientific), tetrahydrofuran (THF, Anhydrous, Tritech Scientific), concentrated sulfuric acid (H₂SO₄, 95–97%, Honeywell) and nitric acid (HNO₃, 69–70%, Honeywell) were used as received. Boron trifluoride triethanolamine (T313) with hydroxyl index of 13.46 mmol g⁻¹ and tris(2-methyl-1-aziridinyl) phosphine oxide (MAPO) were taken from Xi'an Hong Da New Chemical Materials Co. Ltd. AP-I and AP-III with particle size 246–350 μm and 106–165 μm respectively were purchased from Dalian North Potassium Chlorate. Co. Ltd, where as Al-III with particle size 13–15 μm was taken from Professional Fine Spherical Al. Powder Manufacturer.

Functionalization of MWCNTs

Pristine MWCNTs were charged into a round-bottomed flask equipped with a magnetic stirrer and a condenser with a mixture of H₂SO₄ and HNO₃ (with a weight ratio 3 : 2). The weight ratio of the mixed acid to MWCNTs was 200 : 1 and mixture was sonicated for 1 h and subsequently refluxed at 60 °C for 24 h with magnetic stirring.

After cooling to room temperature, the reaction mixture was diluted with 500 mL of deionised water and then vacuum-filtered through a filter paper. This washing operation was repeated until the pH reached to 7 and was followed by drying in a vacuum oven at 60 °C for 18 h. Acid treated product was assigned as COOH-fMWCNTs. Such moderate conditions led to removal of the catalysts from the carbon nanotubes and opening of the tube caps as well. The final nanotubes product fragments are whose ends and sidewalls are decorated with a various oxygen containing groups.

The NCO functionalization was carried out by a known procedure⁵⁴ with some modifications, placing COOH-fMWCNTs in a three-neck round bottom flask containing an appropriate amount of catalyst (dibutyl tin dilaurate) and acetone as a reaction medium. After sonication for 20 minutes, the isocyanate functionalization molecule (IPDI) was added to the flask, followed by heating for 7 h at 50 °C under N₂ atmosphere. After cooling to room temperature, NCO-fMWCNTs were filtered, sonicated in acetone for 1 h, and washed with acetone for several times in order to ensure the removal of excess IPDI (physically-sorbed) from the product. Finally the product was dried in vacuum oven at 60 °C for 12 hours and stored in the N₂ atmosphere until use. Reaction scheme for the functionalization of MWCNTs are shown in Scheme 1. The obtained COOH-fMWCNTs (1.0 g) was suspended in SOCl₂ (20 mL) in a 100 mL round-bottomed flask and suspension was stirred at 60 °C for 24 h. Excess SOCl₂ was removed under reduced pressure, the flask was cooled in an ice bath. A mixed solution of propargyl alcohol (10 mL, 16.9 mmol), CHCl₃ (20 mL), and anhydrous triethylamine (10 mL, 7.17 mmol) was added drop wise into the flask over a period of 30 minutes under magnetic stirring; the mixture was stirred at 0 °C for 1 h and then at room temperature for 24 h. The product was filtered through a PTFE membrane under vacuum and washed with THF and distilled water for several times. The obtained alkyne-fMWCNTs was dried under vacuum at 25 °C for 48 h.

Scheme 1 Schematic representation of click and polyurethane curation of alkyne-fMWCNTs/acyl-GAP and NCO-fMWCNTs/HTPB PU.

Preparation of NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP click nanocomposite

Pristine MWCNTs, NCO-fMWCNTs and alkyne-fMWCNTs were used to fabricate NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP nanocomposite. HTPB and acyl-GAP binders were blended with various wt% of fMWCNTs (0, 0.5, 1, 2, 3, 4, 5, 7.5 and 10). Synthesis of acyl-GAP has already been reported in our previous work.^14,15 In order to obtain a uniform dispersion of fMWCNTs in the HTPB and acyl-GAP, both were mixed in acetone, sonicated and mechanical stirred at 25 °C for 20 min each. A certain amount of catalyst (DBTDL) was added into reaction mixture (NCO-fMWCNTs/HTPB) and was stirred continuously at room temperature for 30 minutes. Finally, the reaction mixture was poured into Teflon coated mold and degassed in a vacuum oven at 30 °C in order to remove the acetone. The molds were finally cured at 70 °C for 5 days. HTPB-IPDI/N100 PU with NCO/OH ratio one was also prepared for comparison. Schematic representations of NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP click nanocomposite have been depicted in Schemes 1 & 2.

Scheme 2 Schematic preparation of the NCO-fMWCNTs/HTPB PU nanocomposite and alkyne-fMWCNTs/acyl-GAP click nanocomposite.

Synthesis of energetic interpenetrating polymer network (EIPNs)

HTPB/acyl-GAP based EIPNs were synthesized by “in situ simultaneous” polymerization technique where pre-polymers, curing agent and catalyst were mixed together (4% NCO-fMWCNTs and 3% alkyne-fMWCNTs were selected on the basis of optimum mechanical properties obtained individually). A series of HTPB/acyl-GAP based EIPNs were synthesized by varying the relative weight proportions of acyl-GAP (10, 30, 50, 70 and 90%) with respect to HTPB. The required amount of HTPB, acyl-GAP along with NCO-fMWCNTs, alkyne-fMWCNTs, IPDI/N100 (NCO/OH = 0.8) and DBTDL (0.3%) were taken in round bottom flask with magnetic stirrer and mixed for 20 minutes in order to get the homogenized suspension followed by degassing in a vacuum oven at 30 °C. The whole mixture was poured into a Teflon coated mold and degassed in a vacuum oven at 30 °C. The mold was cured at 70 °C for 5 days.

Preparation of composite solid propellants

Composite solid propellants were prepared by using NCO-fMWCNTs/HTPB PU, alkyne-fMWCNTs/acyl-GAP and NCO-fMWCNTs/HTPB PU : alkyne-fMWCNTs/acyl-GAP (50 : 50) based EIPN binder systems. For comparison, HTPB-IPDI/N100 PU and GAP-IPDI/N100 PU with NCO/OH ratio one were also prepared in order to investigate the effect of fMWCNTs on the mechanical properties of the composite solid propellants. AP-I, AP-III and Al-III were dried at 60 °C for a minimum of 72 h prior to use where as binders were vacuum dried (60 °C for 12 h) in order to remove the trapped air and moisture. Standard mixing and vacuum casting technique were applied for manufacturing these propellants. NCO-fMWCNTs and alkyne-fMWCNTs were dispersed in the anhydrous acetone, where the binders and plasticizer were homogenously mixed in the same ratio (p_l/p_ol = 1) as we have prepared the single binder systems. The filler particles like AP-I, AP-III and Al-III were dispersed in a sub mix consisting of binder, plasticizer (HA-3), crosslinking agents [T313 : MAPO (50 : 50)] and other minor components like curing catalyst T12 and curing agents i.e. NCO-fMWCNTs, alkyne-fMWCNTs and IPDI/N100 in the respective composite solid propellants. The mixture was casted under vacuum degassing and the casted propellants were cured at 60 °C for 5 days. Curing agents were added in the final step (last 20 minutes) together with the curing catalyst (T12), following the addition of all other constituents.

Composite solid propellants were prepared by using NCO-fMWCNTs/HTPB PU, alkyne-fMWCNTs/acyl-GAP and NCO-fMWCNTs/HTPB PU : alkyne-fMWCNTs/acyl-GAP (50 : 50) based EIPN binder systems with 80% solid loadings and compared with traditional isocyanate curing systems (HTPB-IPDI/N100 PU and GAP-IPDI/N100 PU). We have already examined the mechanical characteristics of NCO-fMWCNTs/HTPB PU, alkyne-fMWCNTs/acyl-GAP and NCO-fMWCNTs/HTPB PU : alkyne-fMWCNTs/acyl-GAP (50 : 50) based EIPN binder systems. Table 1S† shows the mechanical and thermal properties of the different types of composite solid propellants with various curing agents. The acyl-GAP based propellant cured with alkyne-fMWCNTs gave the tensile strength of 0.59 MPa and elongation at break of 41%, interestingly comparable with HTPB-IPDI/N100 PU based composite propellants. Dual curing systems of NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP (50 : 50) showed 0.97 MPa tensile strength and 63% elongation at break. The enhancement in mechanical properties is attributed to the well dispersion and covalently bonded fMWCNTs in to the matrix. Moreover, fMWCNTs do not effect on binder filler interactions and synergetic mechanical properties are the clear evidence of the good binder filler interaction. In future we want to investigate the thermal and ballistic properties of the various composite solid propellants. Overall, it depicts that dual curing system may be useful and advantageous practice for the development of advanced solid propellants with high mechanical strength and optimum theoretical ballistic properties as shown in Table 2S.†

Characterization and instruments

A high resolution X-ray photoelectron spectrometer (XPS) (ESCA PHI 1600, Physical Electronics, Lake Drive East, Chanhassen, MN, USA) was used to detect the presence of surface elements. Transmission electron microscope (TEM) observations were carried out using a JEM-2100 microscope (JEOL Limited, Tokyo, Japan) with 200 kV and the samples for TEM measurements were executed by one drop casting on carbon-coated copper grids followed by solvent evaporation in air at room temperature. FTIR spectra were recorded with Nicolet FTIR-8700. The Raman spectra were recorded at room temperature with a micro Raman spectrometer operating with a 514 nm laser source. Scanning electron microscopy (FESEM, JSM-7600F) was used to investigate the morphological features of composite films.

Dynamic mechanical tests were performed on a DMA 242C (Netzsch, Hanau, Germany) with a dual cantilever device at a frequency of 1 Hz. The temperature range was from −100 to 50 °C under nitrogen atmosphere with a heating rate of 3 °C min⁻¹. The dimensions of the test specimens were 30 mm × 10 mm. For FESEM sample preparation, the films were fractured in liquid nitrogen and sputtered with a thin layer of gold using a sputter coater. Thermogravimetric analyses (TGA) was performed (TGA/DSC1SF/417-2, Mettler Toledo) from 30 to 650 °C in nitrogen (N₂) at a heating rate of 10 °C min⁻¹. Instron-6022, Shimadzu Co., Ltd was used to investigate the mechanical properties including tensile strength (σ_b) and elongation at break (ε_b) of all the dumbbell-shape specimens at a constant rate of 100 mm min⁻¹ based on ASTM D638 and the results were averaged from five samples. Glass transition temperatures (T_g) were measured using differential scanning calorimetry (DSC Mettler Toledo DSC1) with heating rate of 10 °C min⁻¹ over a temperature range of −100 to 50 °C under nitrogen flow of 40 mL min⁻¹.

Results and discussion

Characterizations of functionalized MWCNTs

The FTIR spectra of pristine MWCNT, COOH-fMWCNTs, NCO-fMWCNTs and alkyne-fMWCNTs are presented in Fig. 1. For pristine MWCNTs (Fig. 1-A), a weak peak at 1620 cm⁻¹ corresponds to the C=C stretching of the MWCNTs backbone where as no any other prominent peak is observed regarding pristine MWCNTs. After acid treatment, three new characteristic peaks appear at 3425 cm⁻¹, 1745 cm⁻¹ and 1598 cm⁻¹ (Fig. 1-B) which are assigned to the hydroxyl group (OH) stretching, carboxyl group (C=O) stretching and hydroxyl group (OH) bending vibrations respectively, representing that the MWCNTs have been etched and oxidized by the acid treatment and carboxyl groups (COOH) had successfully been grafted onto the surface of the MWCNTs. One basic advantage of acid treatment of MWCNTs is purification and possibly elimination of any catalytic metallic nanoparticles. The COOH-fMWCNTs opens the door for further modification and attachment of organic moieties on the surface of MWCNTs for specific applications and covalently interaction with other functional groups. Fig. 1-C shows FTIR spectra of NCO-fMWCNTs which depicts the presence of free NCO groups in the NCO-fMWCNTs with strong absorption band at 2260 cm⁻¹ due to N=C=O stretching after treating COOH-fMWCNTs with IPDI. The grafting of IPDI hydrocarbon chain onto the MWCNTs can be vividly seen by the bands at 1387 cm⁻¹ and 1364 cm⁻¹ due to CH₃ deformation (gem-dimethyl) and in the CH stretching region between 2800 cm⁻¹ and 3000 cm⁻¹. Fig. 1-D shows FTIR spectra of alkyne-fMWCNTs representing ≡CH characteristic peak at 3289 cm⁻¹ due to propargyl group attachment onto the surface of the COOH-fMWCNTs.

Fig. 1 FTIR spectra of (A) pristine MWCNTs, (B) COOH-fMWCNTs, (C) NCO-fMWCNTs, (D) alkyne-fMWCNTs.

Raman spectroscopic studies of MWCNTs provide comprehensive information regarding degree of structural ordering and the bonding states of carbons in the MWCNTs structure. Fig. 1S† depicts Raman spectra of pristine, COOH-fMWCNTs, NCO-fMWCNTs and alkyne-fMWCNTs. Two characteristic bands are seen at 1576 cm⁻¹ (G band) which is due to sp²-hybridization corresponds to tangential mode and 1348 cm⁻¹ (D band) corresponds to disorder mode.⁵⁵ The intensity ratio (I_D/I_G) of the D and G-band determines the degree of functionalization of MWCNTs.⁵⁶ The intensity ratios (I_D/I_G) of COOH-fMWCNTs, NCO-fMWCNTs and alkyne-fMWCNTs are 1.11, 0.92 and 0.90 respectively, whereas pristine MWCNTs I_D/I_G ratio is approximately 1.24. This can be attributed to the covalent functionalization of organic moieties on the surface of pristine MWCNTs. The change in the band intensity ratio (I_D/I_G) of the fMWCNTs reflects the change of the hybridization of C atoms on the nanotubes from sp² to sp³.

TGA is a useful technique for determining the extent of organic moieties grafted on pristine MWCNTs. Fig. 2 shows the distinctive TGA thermograms which indicate the weight losses of pristine MWCNTs, COOH-fMWCNTs, NCO-fMWCNTs and alkyne-fMWCNTs. Almost no weight loss occurs for pristine MWCNTs; however graphitized structure starts to oxidize at temperature around 800 °C.⁵⁷ The COOH-fMWCNTs shows extensively different weight loss trend as compared to the pristine MWCNTs. A considerable decomposition starts from 125 °C and total 2.6% weight loss takes place up to 600 °C which is due to decomposition of organic substance grafted on the MWCNTs after acid functionalization. The NCO-fMWCNTs shows an apparent decomposition between 150 and 450 °C, which is attributed to the thermal degradation of the grafted isocyanate part on the surface of COOH-fMWCNTs. Alkyne-fMWCNTs shows noticeable decomposition between 110 and 550 °C, residue remained after decomposition of NCO-fMWCNTs and alkyne-fMWCNTs are around 93.75% and 92.1% respectively. Thermogravimetric curves of COOH-fMWCNTs, NCO-fMWCNTs and alkyne-fMWCNTs depict that organic moieties grafted after acid and isocyanate functionalization were estimated to be 2.6 and 6.25% and 7.9% respectively which confirmed the successful functionalization of pristine MWCNTs. Fig. 2S† shows the thermal degradation of the different fMWCNTs including pristine MWCNTs up to 900 °C, exhibiting the same degradation behaviour but the change in residue weight is due to the organic moieties grafted on MWCNTs. Paulina et al. and Qifei Jing et al.^58,59 investigated the TGA pattern of pristine and functionalized MWCNTs in ethylene diamine-functionalized multi-walled carbon nanotubes showing the same pattern as we have performed.

Fig. 2 TGA curves of pristine MWCNTs, COOH-fMWCNTs, NCO-fMWCNTs and alkyne-MWCNTs.

XPS analysis can be used to elucidate the surface composition of the MWCNTs for supplementary characterizing their change and to recognize the functional groups on the MWCNTs. Surveys scan verifies the elemental composition of the surface of MWCNTs. The XPS survey spectra of pristine, COOH, NCO and alkyne-fMWCNTs are shown in Fig. 3S.† Fig. 4 & 5S† present the main peaks of C 1s of the pristine MWCNTs appeared at 284.5 eV (C=C), 285.2 eV (C–C) due to sp² and sp³-hybridzed C respectively while 290.5 eV (π–π*) which is ascribed to their graphitic structure.^60,61 COOH-fMWCNTs have a imperative degree of oxidation such as the O–C=O (288.9 eV) and C–OH (286.5 eV) as a result of acid treatment.^61,62 Oxygen-containing functional moieties on the MWCNTs present significant possibilities for additional chemical modifications. The XPS results reveal that another peak appeared at 290.1 eV for NCO-fMWCNTs, which derives from the N=C=O bonds.⁶³ The N=C=O peak depicts that the IPDI successively covalently bonded onto the COOH-fMWCNTs which further modifies the surface characteristics of the COOH-fMWCNTs. Fig. 5S† also shows XPS spectra of the distinctive peaks of O 1s and N 1s of the COOH-fMWCNTs and NCO-fMWCNTs respectively. Figures show O 1s and N 1s spectra of COOH-fMWCNTs and NCO-fMWCNTs where peaks around 531.1 eV and 533.3 eV correspond to C=O (carbonyl) and C–O (hydroxyl) respectively where as the peak of N 1s appears at 400 eV, and its high resolution spectrum reveals two peaks at 399.4 and 400.8 eV, which are ascribed to –CONH– and –NCO respectively.

TEM images of the pristine MWCNTs, COOH, NCO and alkyne-fMWCNTs are depicted in Fig. 3. For pristine MWCNTs (Fig. 3-A), the outer diameter is about 16 nm, which is in accordance with the specifications provided by the manufacturer (diameter: 10–20 nm). The morphology of the pristine MWCNTs consists of a layer-by-layer graphitic structure with close distances between the interspaces of walls. The COOH-fMWCNTs (Fig. 3-B) shows an imperfect structure in the outermost layers caused by disordered and amorphous carbons.⁶² Many graphitic layers of the outer wall surface are oxidized and etched by the acid treatment. However, basic graphitic structures and tubular shapes remained unchanged, suggesting that the acid oxidation does not destroy the structural integrity of MWCNTs. After further interaction with IPDI, SOCl₂ and propargyl alcohol, the morphology of the NCO-fMWCNTs and alkyne-fMWCNTs (Fig. 3-C & D) exhibit an obvious rough coating on the MWCNTs surface due to the IPDI and propargyl moieties being grafted onto the tube walls. This thin layer is the direct evidence that specifies the successful and effective grafting of definite functional groups onto the surface of MWCNTs.

Fig. 3 TEM images of (A) pristine MWCNTs, (B) COOH-fMWCNTs, (C) NCO-fMWCNTs and (D) alkyne-fMWCNTs.

The results from the XPS, TGA, FTIR, Raman and TEM clearly specify that the MWCNTs were successfully functionalized by the acid treatment, NCO and alkyne functionalization processes. This study illustrates an effective way for altering the surface characteristics of MWCNTs. The effective dispersion of fMWCNTs in the HTPB and acyl-GAP polymeric matrix is expected due to covalently bonded with hydroxyl and azido groups of binder systems which provide a strong interphase for improving the reinforcement in NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP click nanocomposite as compared to pristine MWCNTs.

Mechanical properties of NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP click nanocomposite

This study compares the tensile strengths and elongation of NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP click nanocomposite. NCO-fMWCNTs and alkyne-fMWCNTs were incorporated at various ratios to investigate their reinforcement effects. Fig. 6S & 7S† show the tensile strength and elongation at break of the NCO-fMWCNTs/HTPB PU with different mass fractions of NCO-fMWCNTs (0, 0.5, 1, 2, 3, 4, 5, 7.5 and 10 phr). The results indicate that the tensile strength of the NCO-fMWCNTs/HTPB PU nanocomposite increased with increasing NCO-fMWCNTs mass fraction. Tensile strength of NCO-fMWCNTs/HTPB PU nanocomposite increases from 1.84 to 7.73 MPa (0.5 to 10% NCO-fMWCNTs) and elongation at break from 453 to 137%. Without functionalized MWCNTs, HTPB-IPDI/N100 PU depicts 1.43 MPa tensile strength and 477% elongation at break (NCO/OH = 1). NCO moieties grafted on MWCNTs are used in the PU formation along with IPDI/N100. For better studies, curing ratio IPDI/N100 (NCO/OH = 0.8) was selected for optimum PU linkage through NCO-fMWCNTs and achieved synergetic mechanical properties.

Tensile strength increases from 1.62 to 8.93 MPa (0.5 to 10% NCO-fMWCNTs) and elongation at break from 581 to 221% (Fig. 6S†). NCO-fMWCNTs are being used in crosslinking between OH groups of HTPB and NCO groups grafted on MWCNTs. Therefore, the weight ratio of NCO-fMWCNTs substantially affected the mechanical properties of the nanocomposite. Fig. 8S† depicts elongation at break of the alkyne-fMWCNTs/acyl-GAP click with different mass fractions of alkyne-fMWCNTs (0, 0.5, 1, 2, 3, 4 and 5 phr). For comparison, GAP was cured with isocyanate curing system with IPDI/N100 (NCO/OH = 1). The results indicate that the tensile strength of the alkyne-fMWCNTs/acyl-GAP nanocomposite increased with increasing alkyne-fMWCNTs mass fraction. Tensile strength of alkyne-fMWCNTs/acyl-GAP nanocomposite increases from 0.67 to 1.65 MPa (0.5 to 5% alkyne-fMWCNTs) and elongation at break decreases from 188 to 73%. Without alkyne-fMWCNTs, GAP-IPDI/N100 PU depicts 0.56 MPa tensile strength and 197% elongation at break (NCO/OH = 1).

Mechanical properties of NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP click based EIPN nanocomposite

As we have discussed earlier that alkyne-fMWCNTs is used in acyl-GAP click nanocomposite and we have selected an in situ simultaneous polymerization technique for EIPN with NCO-fMWCNTs/HTPB PU. Both PU and click curation took place simultaneously between NCO-fMWCNTs/HTPB and alkyne-fMWCNTs/acyl-GAP for EIPN formation. Fig. 4 shows the tensile strength and elongation at break of the NCO-fMWCNTs/HTPB PU with different mass fractions of alkyne-fMWCNTs/acyl-GAP (0, 10, 30, 50, 70, 90 and 100%). The results indicate that the tensile strength of the NCO-fMWCNTs/HTPB PU nanocomposite increased with increasing alkyne-fMWCNTs/acyl-GAP weight ratio. The optimum tensile strength 8.17 MPa and 312% elongation at break was achieved with 50 : 50% (NCO-fMWCNTs/HTPB : alkyne-fMWCNTs/acyl-GAP). These enhanced mechanical properties may be due to the entanglement and interpenetration of the networks during in situ polymerization. Due to catenation, chain flexibility is somewhat restricted and elongation at break is significantly reduced. Beyond 50% acyl-GAP, the elasticity of acyl-GAP is inhibited by the azido groups and tensile strength and elongation at break frequently drops down to 1.18 MPa and 114% respectively with 100% alkyne-fMWCNTs/acyl-GAP network.

Fig. 4 Effect of % of acyl-GAP on the (σ) and (ε_b) of NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP EIPNs.

Dispersion test

As the pristine MWCNTs tend to agglomerate due to van der Waals forces, functionalization of MWCNTs is thought to be useful approach to improve the dispersion of CNTs in polymer matrix and the mechanical characteristics of MWCNTs-reinforced nanocomposite. In order to estimate the degree of dispersion, pristine, COOH-fMWCNTs, NCO-fMWCNTs and alkyne-fMWCNTs were dispersed in anhydrous acetone and its dispersity was investigated. Few mg of each sample was taken in a vial with 25 mL of acetone and ultra-sonicated for 30 minutes and the respective observations were recorded after 12 h. Fig. 5 depicts that pristine MWCNTs settled down and agglomerate due to strong van der Waals forces whereas COOH-fMWCNTs, NCO-fMWCNTs and alkyne-fMWCNTs showed a better dispersibility and homogenously dispersed in acetone due to better solubility and grafting of organic moieties on the walls of CNTs.

Fig. 5 Dispersion tests of (A) (1) pristine MWCNTs, (2) COOH-fMWCNTs, (3) NCO-fMWCNTs and (4) alkyne-fMWCNTs and FESEM images of the cross-sectional fracture surface of (B) HTPB-IPDI/N100 PU with pristine MWCNTs and (C) NCO-fMWCNTs/HTPB : alkyne-fMWCNTs/acyl-GAP.

Fig. 5-C shows FESEM images of the fractured surfaces of the NCO-fMWCNTs/HTPB : alkyne-fMWCNTs/acyl-GAP (50 : 50) based EIPN nanocomposite. The NCO-fMWCNTs and alkyne-fMWCNTs are uniformly dispersed, which is due to the covalent functionalization of NCO-fMWCNTs and alkyne-fMWCNTs with hydroxyl groups of HTPB and azido groups of acyl-GAP respectively. Effective dispersion of MWCNTs at the nanoscale level is prerequisite for enhancement of mechanical properties. Here, we have succeeded in fabricating the EIPN which have higher tensile strength and breaking strength than pure HTPB and acyl-GAP PU system. This synergetic mechanical strength is due to the spectacular reinforcement effect of the covalent bonding between the fMWCNTs and acyl-GAP and HTPB nanocomposite. For a comparative study of mechanical properties of nanocomposite, pristine MWCNTs were also incorporated into HTPB binder system (Fig. 5-B), MWCNTs were agglomerated due to strong van der Waals forces, as a result of that no significant enhancement of mechanical strength seen.

Thermal studies

Dynamic mechanical analysis

Dynamic mechanical analysis (DMA) was performed to observe NCO-fMWCNTs/HTPB : alkyne-fMWCNTs/acyl-GAP based EIPN nanocomposite stiffness behavior as a function of temperature and to analyze the effect of fMWCNTs on thermo-mechanical performance of EIPNs. Fig. 6 shows the loss tangent and Fig. 9, 10S† show the variation in dynamic measurements of storage and loss modulus as a function of temperature obtained from DMA. Dynamic mechanical properties of EIPNs nanocomposite with acyl-GAP contents 0, 10, 30, 50 and 70% were characterized. Samples with 90 and 100% alkyne-fMWCNTs/acyl-GAP contents could not be analyzed at specific frequency because samples were broken down from terminal in the sample holder due to low mechanical strength. Fig. 6 depicts the variation in tan δ and glass transition temperatures (T_g) with weight ratios of acyl-GAP and HTPB. Figure also shows that inward shift of glass transition temperature took place from −71 °C to −68 °C with a single glass transition temperature, where as with NCO-fMWCNTs/HTPB : alkyne-fMWCNTs/acyl-GAP (50 : 50) based EIPN nanocomposite, two glass transitions were observed at −71 °C and −44 °C. The results from DMA analysis clearly illustrates that EIPNs with acyl-GAP up to 30% did not show phase separation with a single broad transition and maximum tan δ (1.05). Beyond 30% of acyl-GAP, the two glass transition temperatures might be due to phase separation. But surprisingly synergistic mechanical strength was observed at 50 : 50 NCO-fMWCNTs/HTPB : alkyne-fMWCNTs/acyl-GAP.

Fig. 6 Variation of tan δ with temperatures on (A) 0% acyl-GAP, (B) 10%, (C) 30% (D) 50%, (E) 70% acyl-GAP in NCO-fMWCNTs/HTPB : alkyne-fMWCNTs/acyl-GAP EIPNs.

Again with 70% acyl-GAP, single glass transition achieved and inward shifting of T_g and enhanced mechanical strength were also observed. All these may be attributed to interfacial interaction between fMWCNTs and polymer systems, interpenetration and entanglements of acyl-GAP and HTPB networks during simultaneous polymerization. Interfacial interaction reduces the mobility of the polymeric matrix around the MWCNTs and leads to increase in thermal stability and tensile strength.

DMA is used here to provide proof on the interfacial phenomena for the nanocomposite. From Fig. 9S,† it is obvious that the storage modulus of EIPNs nanocomposite (D) based on NCO-fMWCNTs/HTPB : alkyne-fMWCNTs/acyl-GAP (50 : 50) is much higher than that of other EIPNs which indicates that the crosslinked networks has been effectively strengthened by entanglement and interfacial interactions. At 50% acyl-GAP (D) contents, loss modulus (Fig. 10S†) shows two transitions indicating phase separation where as single transition seen in loss modulus of other EIPNs. This shifting of the loss modulus and loss tangent peaks to higher temperatures for NCO-fMWCNTs/HTPB : alkyne-fMWCNTs/acyl-GAP nanocomposite is attributed to the decrease in the effective polymer chain mobility of the interface region. The load transfer can only be performed between the outer most layers of multiwall nanotubes and the polymeric matrix, an improved interaction between the nanotubes and the polymeric matrix should lead to a stronger shift of the glass transition temperature.

This is consistent with the observations reported by Vlasveld et al. that covalent bond between amino-functionalized CNTs and epoxy improve the efficiency of load transfer from matrix to fillers resulting in an increase in loss modulus due to more energy dissipation in composites.⁶⁴ Shaffer et al.⁶⁵ described this effect for semi crystalline poly(vinyl alcohol)/nanotubes composites. They showed a moderate influence in the case of PVA/MWCNT-nanocomposite containing up to 60 wt% carbon nanotubes, but predicted a strong effect for amorphous matrices, as observed in our case. Wei-Hao Liao, et al.⁶⁶ investigated interfacial interaction between MWCNTs and the VE matrix to promote the transfer of stress from the VE to CF. Similar phenomenon has been studied by Luyi Sun using DMA and investigated the epoxy/single-walled carbon nanotubes (epoxy/SWCNT) composites.⁶⁷ Kenan Song et al. investigated the effect of carbon nanochips concentration on understanding structure property relationships in polymer composites with low nanocarbon loadings.⁶⁸

Previous researches have reported that the resistance to heat flow caused by polymer–CNT interface is responsible for the low thermal conductivities of polymer-based CNT composites. The interface thermal resistance also called the Kapitza resistance (KR), is expected to be more important in the composites filled with nano-structured fillers because these inclusions are small in size and their surface to volume ratios are high.⁶⁹

DSC analysis

Fig. 7 depicts the DSC thermograms of the pure GAP PU, HTPB PU and NCO-fMWCNTs/HTPB : alkyne-fMWCNTs/acyl-GAP based EIPN nanocomposite with different weight ratios. Glass transition temperature (T_g) of the polyurethane based HTPB (A) and GAP (G) networks was found to be −77 °C and −36 °C respectively. With the increase in weight ratio of alkyne-fMWCNTs/acyl-GAP up to 30% (B & C) in NCO-fMWCNTs/HTPB based EIPN nanocomposite, single T_g was observed and also somewhat increased from −77 °C to −73 °C. Although small change in glass transition has been observed but single T_g in (B) and (C) may be ascribed to entanglements, catenation and interphase interaction of alkyne-fMWCNTs/acyl-GAP : NCO-fMWCNTs/HTPB nanocomposite and shortening of space during in situ simultaneous crosslinking. DSC thermogram (D) with alkyne-fMWCNTs/acyl-GAP and NCO-fMWCNTs/HTPB (50 w : 50 w%) nanocomposite depicts two glass transition temperatures at −74.8 °C and −36.5 °C, which may be due to phase separation but auspiciously enhancement in mechanical strength was observed which we have already discussed in mechanical section of the manuscript. Single T_g was observed with a weight ratio of acyl-GAP from 70 to 90% (E & F) in alkyne-fMWCNTs/acyl-GAP : NCO-fMWCNTs/HTPB based EIPNs nanocomposite and also somewhat increased from −77 °C to −73 °C.

Fig. 7 DSC thermograms with (A) 0% acyl-GAP, (B) 10%, (C) 30%, (D) 50%, (E) 70%, (F) 90% and (G) 100% acyl-GAP in NCO-fMWCNTs/HTPB : alkyne-fMWCNTs/acyl-GAP-HTPB EIPNs.

Impact and friction sensitivity of the binder systems and propellants

The impact, friction and electrostatic discharge sensitivity tests of single networks of polyurethane based GAP, HTPB and alkyne-fMWCNTs/acyl-GAP : NCO-fMWCNTs/HTPB based EIPNs nanocomposite were performed using standard procedure.⁷⁰ Results clearly depict that single networks of GAP, HTPB and alkyne-fMWCNTs/acyl-GAP : NCO-fMWCNTs/HTPB based EIPNs nanocomposite were insensitive to impact, friction and ESD with >40 joules, >360 newton and having no ignition at 5.0 joules respectively. We have measured the impact and friction sensitivity of the propellants based on HTPB PU and NCO-fMWCNTs/HTPB in the range of 3–4 N m and friction sensitivities in the range of 40–50 N. Whereas GAP PU, alkyne-fMWCNTs/acyl-GAP and alkyne-fMWCNTs/acyl-GAP : NCO-fMWCNTs/HTPB based propellants are in the range of 4.5 N m and 210 N impact and friction sensitivity respectively.

TGA/DTG analysis

Besides the mechanical and dispersion properties, the effects of fMWCNTs on the thermal properties of the HTPB and acyl-GAP nanocomposite were also investigated. It is interesting to report that not only mechanical performance of NCO-fMWCNTs/HTPB and alkyne-fMWCNTs/acyl-GAP EIPN increased; the thermal stability of these nanocomposites was also significantly enhanced. TGA results for pure HTPB, GAP PU and NCO-fMWCNTs/HTPB : alkyne-fMWCNTs/acyl-GAP nanocomposite with different weight ratios of HTPB and acyl-GAP based EIPN with fMWCNTs are shown in Fig. 8. Both pure HTPB and GAP based PU and its EIPN nanocomposite decompose in a two-stages; the first stage decomposition of GAP took place due to release of nitrogen,^71,72 while the second stage decomposition involves the degradation of polyether main polymeric chain of GAP. HTPB cross-linked network decomposition also occurs in two stages with indefinite division. The first stage decomposition took place as a result of depolymerisation and incomplete decomposition of cyclised products. Second stage decomposition corresponds to dehydrogenation and decomposition of the remaining cyclised products.^73–76

Fig. 8 TGA curves of (HTPB) 0% acyl-GAP, (EIPN-1) 10%, (EIPN-2) 30%, (EIPN-3) 50%, (EIPN-4) 70%, (EIPN-5) 90% and (GAP) 100% acyl-GAP in NCO-fMWCNTs/HTPB : alkyne-fMWCNTs/acyl-GAP-HTPB based EIPNs.

TGA and DTG curves of the nanocomposite were shifted toward higher temperature when compared to that HTPB and GAP based PU. As shown in Fig. 9, the onset temperature of degradation for the nanocomposite was about 22 °C higher in EIPN-4 where as similar shifting of temperatures towards higher side were seen in case of EIPN-1 to EIPN-5. The improvement in thermal stability may be attributed to high thermal conductivity of MWCNTs. The interfacial bonding between fMWCNTs and polymer matrix may further endorse the heat dissipation, thus delaying the decomposition of the NCO-fMWCNTs/HTPB and alkyne-fMWCNTs/acyl-GAP EIPN based nanocomposite more efficiently.

Fig. 9 DTG curves of (HTPB) 0% acyl-GAP, (EIPN-1) 10%, (EIPN-2) 30%, (EIPN-3) 50%, (EIPN-4) 70%, (EIPN-5) 90% and (GAP) 100% acyl-GAP in NCO-fMWCNTs/HTPB : alkyne-fMWCNTs/acyl-GAP-HTPB based EIPNs.

Moreover; due to in situ polymerization between PU and click reaction, entanglement and interpenetration may takes place among crosslinked networks, peak decomposition temperature shifted to higher side as can be seen in Fig. 9. These results specify that the thermal stability of EIPN based nanocomposite is improved due to well dispersion, catenation and covalently bonding of MWCNTs with the binders. Higher the cross-linked network, more energy is required for the decomposition,⁷⁷ this trend clearly depicts the confirmation for thermal stability and compact network formation due to interpenetration and physical entanglement of networks.

Conclusions

We presented here a comprehensive assay on the covalent functionalization of MWCNTs and versatile approach of incorporation of NCO and alkyne-fMWCNTs in HTPB and acyl-GAP based EIPNs nanocomposite by in situ polymerization. NCO-fMWCNTs/HTPB PU : alkyne-fMWCNTs/acyl-GAP (50 : 50 weight ratios) based EIPN nanocomposite exhibited superior tensile strength and elongation at break with 8.17 MPa and 312% respectively. TGA–DTG studies depict that decomposition temperature shifted to higher side (almost 22 °C) with thermally more stable crosslinked networks, furthermore; inward shifting of T_g has been observed due to entanglement and homogeneous dispersion of fMWCNTs in the nanocomposite matrix with strong interfacial interaction between them. Solid composite propellant based on NCO-fMWCNTs/HTPB PU and alkyne-fMWCNTs/acyl-GAP showed optimum tensile strength 0.97 MPa with 63% elongation at break, and it could be the futuristic solid fuel for advanced missile technology.

References

1. S. Ajithkumar, N. K. Patel and S. S. Kansara, Eur. Polym. J., 2000, 36, 2387–2393.
2. S. Chen, Q. Wang and T. Wang, Polym. Test., 2011, 30, 726–731.
3. P. Pissis, G. Georgoussis, V. A. Bershtein, E. Neagu and A. M. Fainleib, J. Non-Cryst. Solids, 2002, 305, 150–158.
4. I. Gitsov and C. Zhu, J. Am. Chem. Soc., 2003, 125, 11228–11234; B. J. P. Jansen, S. Rastogi, H. E. H. Meijer and P. Lemstra, J. Macromol. Sci., Part A: Pure Appl.Chem., 1999, 32, 6290–6297.
5. H. Nakanishi, M. Satoh, T. Norisuye and Q. Tran-Cong-Miyata, Macromolecules, 2004, 37, 8495–8498.
6. J. S. Turner and Y. L. Cheng, Macromolecules, 2000, 33, 3714–3718.
7. D. Gao and M. Jia, J. Appl. Polym. Sci., 2013, 128, 3619–3630.
8. Y.-Y. Hu, J. Zhang, Q.-C. Fang, D.-M. Jiang, C.-C. Lin, Y. Zeng and J.-S. Jiang, J. Appl. Polym. Sci., 2015, 132, 41537.
9. S. Chaudhary, S. Parthasarathy, D. Kumar, C. Rajagopal and P. K. Roy, J. Appl. Polym. Sci., 2014, 131, 40490.
10. J.-J. Wang, M.-B. Wu, T. Xiang, R. Wang, S.-D. Sun and C. S. Zhao, J. Appl. Polym. Sci., 2015, 132, 41585.
11. F. Abbasi, H. Mirzadeh and A. A. Katbab, J. Appl. Polym. Sci., 2002, 86, 3480–3485.
12. O. Fichet, F. Vidal, J. Laskar and D. Teyssie, Polymer, 2005, 46, 37–47.
13. M. M. Jayasuriya and D. J. Hourston, J. Appl. Polym. Sci., 2012, 124, 3558–3564.
14. A. Tanver, M. Huang, Y. Luo, S. Khalid and T. Hussain, RSC Adv., 2015, 5, 64478–64485.
15. A. Tanver, F. Rehman, A. Wazir, S. Khalid, S. Ma, X. Li, Y. Luo and M. Huang, RSC Adv., 2016, 6, 11032–11039.
16. S. Iijima, Helical microtubules of graphitic carbon, Nature., 1991, 354(6348), 56–58.
17. J. P. Salvetat, J. M. Bonard, N. H. Thomson, A. J. Kulik, L. Forro, W. Benoit and L. Zuppiroli, Appl. Phys. A, 1999, 69, 255–260.
18. Y. Breton, G. Desarmot, J. P. Salvetat, S. Delpeux, C. Sinturel, F. Beguin and S. Bonnamy, Carbon, 2004, 42, 1027–1030.
19. R. Andrews and M. C. Weisenberger, Curr. Opin. Solid State Mater. Sci., 2004, 8, 31–37.
20. J. T. Abrahamson, N. Nair and M. S. Strano, Nanotechnology, 2008, 19, 195701.
21. Y. C. Jung, N. G. Sahoo and J. W. Cho, Macromol. Rapid Commun., 2006, 27, 126–131.
22. T. L. Wang and C. G. Tseng, J. Appl. Polym. Sci., 2007, 105, 1642–1650.
23. H. C. Kuan, C. C. M. Ma, W. P. Chang, S. M. Yuen, H. H. Wu and T. M. Lee, Compos. Sci. Technol., 2005, 65, 1703–1710.
24. X. Jiang, J. Gu, L. Lin and Y. Zhang, Pigm. Resin Technol., 2011, 40, 240–246.
25. N. G. Sahoo, S. Rana, J. W. Cho, L. Li and S. H. Chan, Prog. Polym. Sci., 2010, 837–867.
26. H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem., Int. Ed., 2001, 40, 2004–2021.
27. X. Meng, X. Xu, T. Gao and B. Chen, Eur. J. Org. Chem., 2010, 5409–5414.
28. V. E. Matulis, O. A. Ivashkevich, P. N. Gaponik, P. D. Elkind, G. T. Sukhanov, A. B. Bazyleva and D. H. Zaitsau, J. Mol. Struct.: THEOCHEM, 2008, 854, 18–25.
29. K. E. Gutowski, R. D. Rogers and D. A. Dixon, J. Phys. Chem. B, 2007, 111, 4788–4800.
30. C. Dubois, S. Desilets, G. Nadeau and N. Gagnon, Propellants, Explos., Pyrotech., 2003, 28, 107–113.
31. J. H. Jung, Y. G. Lim, K. H. Lee and B. T. Koo, Tetrahedron Lett., 2007, 48, 6442–6448.
32. C. Hu, X. Guo, Y. Jing, J. Chen, C. Zhang and J. Huang, J. Appl. Polym. Sci., 2014, 131, 40007.
33. G. Drake, T. Hawkins, A. Brand, L. Hall and M. Mckay, Propellants, Explos., Pyrotech., 2003, 28, 174–180.
34. Y. Zhong, M. Jaidann, Y. Zhang, G. Zhang, H. Liu, M. Ioan Ionescu, R. Li, X. Sun, H. Abou-Rachid and L. S. Lussier, J. Phys. Chem. Solids, 2010, 71, 134–139.
35. H. Abou-Rachid, A. Hu, V. Timoshevskii, Y. Song and L.-S. Lussier, Phys. Rev. Lett., 2008, 100, 1–4.
36. Y. Hu, L. Wu, J. Shen and M. Ye, J. Appl. Polym. Sci., 2008, 110(2), 701–705.
37. L. Sun, G. L. Warren, J. Y. O'Reilly, W. N. Everett, S. M. Lee and D. Davis, et al., Carbon, 2008, 46(2), 320–328.
38. S. Wang, R. Liang, B. Wang and C. D. Zhang, Carbon, 2009, 47(1), 53–57.
39. N. G. Sahoo, S. Rana, J. W. Cho, L. Li and S. H. Chan, Prog. Polym. Sci., 2010, 35(7), 837–867.
40. S. A. Ntim, O. Sae-Khow, F. A. Witzmann and S. Mitra, J. Colloid Interface Sci., 2011, 355(2), 383–388.
41. A. Nish, J. Y. Hwang, J. Doig and R. J. Nicholas, Nat. Nanotechnol., 2007, 2(10), 640–646.
42. A. A. Muataz, G. Nazlia, M. El-Sadig, T. Hairani, T. G. Chuah, F. A. Maan, A. Fakhru’l-Razi and R. Dayang, Fullerenes, Nanotubes, Carbon Nanostruct., 2006, 14(4), 641–649.
43. G. Nazlia, A. Fakhru’l-Razi, A. R. Suraya and A. A. Muataz, Fullerenes, Nanotubes, Carbon Nanostruct., 2007, 15(3), 207–214.
44. B. Kannan and M. Burghard, Small, 2005, 1, 180–192.
45. L. Abdelghani, G. Vivet, B. B. Nouet, C. Doudou, J. Poilâne and J. Chen, Mater. Lett., 2008, 62, 394–439.
46. L. Yan-Ling, M. Yan, X. Feng and Y. Yong, Polym.-Plast. Technol. Eng., 2012, 51, 290–297.
47. C. Zhang, J. Li, Y. J. Luo and B. Zhai, J. Energ. Mater., 2015, 33, 305–314.
48. Z. Wei, L. Du and L. Wang, J. Nanosci. Nanotechnol., 2012, 12, 787–792.
49. W.-H. Liao, H.-W. Tien, S.-T. Hsiao, S.-M. Li, Yu-S. Wang, Y.-L. Huang, S.-Y. Yang, C.-C. M. Ma and Y.-F. Wut, ACS Appl. Mater. Interfaces, 2013, 5, 3975–3982.
50. C. Yao, Z. Bin, G. Zhe, C. Chaoqiu, Z. Shichao and Q. Yong, Carbon, 2015, 82, 470–478.
51. Y. Li, K. F. Hu, H. Jiao, X. Liu, Q. Wang, G. Pan, X. Zhang and T. Wang, Polym. Compos., 2015, 36, 198–203.
52. L. Guadagno, B. De Vivo, A. Di Bartolomeo, P. Lamberti, A. Sorrentino, V. Tucci, L. Vertuccio and V. Vittoria, Carbon, 2011, 49, 1919–1930.
53. C. Zhang, J. Li, Y. J. Luo and B. Zhai, J. Energ. Mater., 2015, 33, 305–314.
54. Z. Chungui, J. Lijun, L. Huiju, H. Guangjun, Z. Shimin, Y. Mingshu and Y. Zhenzhong, J. Solid State Chem., 2004, 177, 4394–4398.
55. X. H. Chen, H. F. Wang, W. B. Zhong, T. Feng, X. G. Yang and J. H. Chen, Macromol. Chem. Phys., 2008, 209(8), 846–853.
56. B. K. Price, J. L. Hudson and J. M. Tour, J. Am. Chem. Soc., 2005, 127(42), 14867–14870.
57. T. I. T. Okpalugo, P. Papakonstantinou, H. Murphy, J. McLaughlin and N. M. D. Brown, Carbon, 2005, 43(1), 153–161.
58. P. Canete-Rosales and A. A.-L. Soledad Bollo, Sens. Actuators, B, 2014, 191, 688–694.
59. Q. Jing, J. Y. Law, L. P. Tan, V. V. Silberschmidt, L. Li and Z. L. Dong, Composites, Part A, 2015, 70, 8–15.
60. S.-Y. Yang, C.-C. M. Ma, C.-C. Teng, Y.-W. Huang, S.-H. Liao, Y.-L. Huang, H.-W. Tien, T.-M. Lee and K.-C. Chiou, Carbon, 2010, 48(3), 592–603.
61. T. I. T. Okpalugo, P. Papakonstantinou, H. Murphy, J. McLaughlin and N. M. D. Brown, Carbon, 2005, 43(1), 153–161.
62. H.-Z. Geng, K. K. Kim, K. P. So, Y. S. Lee, Y. Chang and Y. H. Lee, J. Am. Chem. Soc., 2007, 129(25), 7758–7759.
63. K. Yang, M. Gu, Y. Guo, X. Pan and G. Mu, Carbon, 2009, 47(7), 1723–1737.
64. D. P. N. Vlasveld, W. Daud, H. E. N. Bersee and S. J. Picken, Composites, Part A, 2007, 38, 730–738.
65. M. S. P. Shaffer and A. H. Windle, Adv. Mater., 1999, 11, 937–941.
66. W.-H. Liao, H.-W. Tien, S.-T. Hsiao, S.-M. Li, Y.-S. Wang, Y.-L. Huang, S.-Y. Yang, C.-C. M. Ma and Y.-F. Wu, ACS Appl. Mater. Interfaces, 2013, 5, 3975–3982.
67. L. Suna, G. L. Warrena, J. Y. O'Reillya, W. N. Everetta, S. M. Leea, D. Davisb, D. Lagoudasb and H.-J. Suea, Carbon, 2008, 46, 320–328.
68. K. Song, Y. Zhang and M. L. Minus, Macromol. Chem. Phys., 2015, 216, 1313–1320.
69. C. W. Nan, G. Liu, Y. Lin and M. Li, Appl. Phys. Lett., 2004, 85, 3549.
70. K. Selim, S. Özkar and L. Yilmaz, J. Appl. Polym. Sci., 2000, 77, 538–546.
71. Y. M. Mohan and K. M. Raju, Des. Monomers Polym., 2005, 8, 159.
72. K. N. Ninan, K. Krishnan, R. Rajeev and G. Viswanathan, Propellants, Explos., Pyrotech., 1996, 21, 199–202.
73. J. K. Chen and T. B. Brill, Combust. Flame, 1991, 87, 217–232.
74. J. Simon and A. Bajpai, Eur. Polym. J., 2003, 39, 2077–2089.
75. Y. Ding, C. Hu, X. Guo, Y. Che and J. Huang, J. Appl. Polym. Sci., 2014, 131, 40007.
76. E. Landsem, T. L. Jensen, T. E. Kristensen, F. K. Hansen, T. Benneche and E. Unneberg, Propellants, Explos., Pyrotech., 2013, 38(1), 75–86.
77. E. Landsem, T. L. Jensen, F. K. Hansen, E. Unneberg and T. E. Kristensen, Propellants, Explos., Pyrotech., 2012, 37, 581–591.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07742k

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