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Surfaces, Interfaces, and Applications

Tuning the Reactivity of Metastable Intermixed Composite n-Al/PTFE by Polydopamine Interfacial Control Wei He, Pei-jin Liu, Feiyan Gong, Bowen Tao, Jian Gu, Zhijian Yang, and Qi-Long Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10197 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 29, 2018

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ACS Applied Materials & Interfaces

Tuning the Reactivity of Metastable Intermixed Composite n-Al/PTFE by Polydopamine Interfacial Control Wei He1, Pei-Jin Liu*1, Feiyan Gong2, Bowen Tao3, Jian Gu3, Zhijian Yang2, Qi-Long Yan*1 1, Science and Technology on Combustion, Internal Flow and Thermo-structure Laboratory, Northwestern Polytechnical University, Xi'an 710072, China; 2, Institute of Chemical Materials, CAEP, Mianyang, 621900, China; 3, Science and Technology on Aerospace Chemical Power Laboratory, Xiangyang 441003, China

Abstract: The metastable intermixed composite (MIC) is one of the most popular research topics in the field of energetic materials (EMs). The goal is to invent EMs with tunable reactivity and desired energy content. However, it is very difficult to tune the reactivity of MIC due to its high reactivity and sensitivity caused by enlarged specific surface area and intimate contact between the oxidizers and fuels. Herein, we demonstrated a facile fabrication method that can be used to control the reactivity between the nano-aluminum (n-Al) and polytetrafluoroethylene (PTFE) using an in-situ synthesized polydopamine (PDA) binding layer. It was found that PDA can adhere to both n-Al and PTFE particles, resulting in integrated n-Al@PDA/PTFE MICs. In comparison to traditional n-Al/PTFE MICs, the n-Al@PDA/PTFE showed an increased energy release and reduced sensitivity, and more importantly tunable reactivity. By regulating the experimental conditions of coating, the thickness of PDA could be well controlled, which makes the tunable reactivity of n-Al@PDA/PTFE possible. The PDA interfacial layer may increase the pre-ignition reaction (PIR) heat of Al2O3/PTFE and therefore the overall reaction heat of n-Al/PTFE. It also reveals that the PDA interfacial layer postponed the PIR, leading to an increase in onset thermal decomposition temperature (To). As To increased, a more complete reaction between PTFE and Al nanoparticles could be achieved. Keywords: Polydopamine; Reactivity; Sensitivity; n-Al/PTFE; MICs.

1. Introduction Solid propellants find important applications in launching vehicles and missiles, where high specific impulse (Isp) is essentially required.1,2 Historically, one of the most effective methods for improvement of the combustion performances is to use micron-scale metallic fuels due to their high heat of combustion.3 However, combustion of metallic fuel at this scale often produces large agglomerated droplets, leading to long residence times and incomplete combustion for the propellant.4 Recent works have shown that the performance of solid propellants can be significantly improved by introducing nanoscale metallic fuels in the formulations.5-7 For instance, a significant increase of burning rate and lower ignition time and temperature can be achieved by changing the particle size of Al from 3 µm to 30 nm for a typically composite solid propellant.5 The application of nanoscale metallic fuels also increases the Isp of the rocket motor through reduction in both the two-phase flow loss to thrust and the coarse particle size of products.1,8 For the metallic components of composite solid propellants, n-Al has been the most frequently used one, as it improves the specific impulse and meantime suppresses the high frequency combustion instability.9 Nevertheless, * Corresponding author. Tel.: +86(029) 88492781; Fax: +86(029) 88492781; Email: [email protected] (Q.-L. Yan) [email protected] (P.-J. Liu)

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the application of n-Al has various problems, which include limitation in content of active Al and mechanical sensitivity. In particular, the addition of the energetic materials can chemically interact with the n-Al during combustion, resulting in high pressure exponents. In comparison, MICs that contain n-Al as the main component also provide the benefits of reduced reactants and increased reactivity by using nanoscale fuels and oxidizers.10 Those features make n-Al based MICs an excellent alternative of n-Al. By application of n-Al/fluoropolymers MICs, aluminum fluoride could be formed instead of aluminum oxide with gaseous nature under relatively low temperature. Thus, enhanced combustion and avoided sintering can be expected. Meantime, the MICs based on n-Al using fluoropolymers as oxidizers would excite the surface exothermic reaction between the Al2O3 shell and fluorine.11-13 The surface exothermic reaction is critical, since it determines ignition sensitivity and exothermicity, which therefore produces enough heat to quickly decompose fluoropolymers, resulting in increased reactivity of Al.14 However, the application of n-Al/fluoropolymers MICs is still limited in spite of excellent performances. Main challenge is that the high reactivity caused by promoted interfacial contacts between nanometer scale components makes it difficult to tune their performances with controlled safety. Tuning the reactivity of n-Al/fluoropolymers MICs is the key to extend the application of such materials. As mentioned above, the surface exothermic reaction has a significant effect on the overall reaction, and it can be considered as an effective way of tuning the reactivity of n-Al/fluoropolymers MICs. By controlling this reaction, main reaction rate of n-Al/fluoropolymers MICs can be improved due to a large amount of heat generated by the exothermic reactions on the interface and the followed catalytic effects of the corresponding products. That is to say, tunable reactivity can be realized by change of the reaction mechanisms on the interfaces. Fabrication of n-Al/fluoropolymers MICs with controlled surface reaction mechanism is challenging due to the absence of applicable interface controlling materials and fabrication approaches. There are various approaches to fabricate MICs based on Al and fluoropolymers, but it is difficult to get the MICs with tunable reactivity and low sensitivity by using simple mechanical mixing method. There are some other alternative conventional approaches, which include electrospinning, electrophoretic deposition or vapor deposition.10 Those methods are always expensive and time-consuming. Recent researches on the biomaterials benefit the preparation of novel advanced MICs. For example, biomaterials such as DNA,15 protein cages,16 aloe vera,17 protamine and hematin18 have been applied in the preparation of highly energetic MICs as the coating atents.19 Dopamine is another attractive biomaterial that could be used for this purpose due to its robust adhension capability. Initial efforts indicated that dopamine

can

be

oxidized

and

spontaneously

self-polymerize

under

alkaline

conditions.20-24

The

self-polymerization reaction can be achieved without any complicated instrumentation or harsh reaction conditions forming polydopamine film. Thus, the core-shell MICs can be fabricated simply by self-polymerization of dopamine on the surface of the component nanoparticles. More importantly, the thickness can be finely controlled by change of the self-polymerization time and monomers concentration, 20, 25, 26 which ensures the tunable reactivity of the resulting MICs. Later researches also revealed some important advantages, of which, enhanced mechanical 2


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property and thermal conductivity of the composites have been reported.27-29 Above all, the functional groups such as amine and catechol can serve as the starting points for covalent modification or anchors for metal ions, simultaneously strong reducing capability towards mental ions prevent the oxidization of n-Al during the preparation and storage of PDA controlled MICs. With abovementioned advantages, the objective of this research is to prepare interfacial controlled n-Al/PTFE MICs by using polydopamine as an interfacial layer. The n-Al@PDA/PTFE was obtained and fundamentally characterized in terms of its thermal and initiation reactivity. The function of the thickness of PDA interfacial layer was also investigated by thermal analysis and combustion performances evaluation.

2. Experimental 2.1. Materials The n-Al particles with average diameter of 80 nm were supplied by Novacentrix Company, which was used as the fuel particles as they were in the MICs. These n-Al particles were covered with alumina (Al2O3) with a thickness of about 2.5 nm, passivating the fuel from spontaneous reaction with surrounding oxygen. The PTFE produced

by

DuPont

was

used

as

the

oxidizer.

The

dopamine

hydrochloride

(98%)

and

(hydroxymethyl)aminomethane (Tris, 99%) were purchased from Sigma-Aldrich, and used without further purification. 2.2. Sample preparations Dopamine hydrochloride (2 g/mL) was dissolved in 10 mM Tris-HCl (pH 8.5) and stirred for 10 min. The Al nanoparticles (500 mg) were added to this dopamine solution (300 mL) followed by stirring at room temperature (25 ˚C) for 2h, 6h, 12h, 18h and 36h, respectively. Then the particles were filtered and washed by distilled water for several times and added to 100 mL ethanol solvent. Finally, the commercially available PTFE nanoparticles were further added with stirring at 25˚C for 10h. 2.3. Characterization The microstructures of PDA-coated Al nanoparticles and PTFE nanoparticles were examined by means of scanning electron microscopy (SEM). The SEM analysis was performed on a ZEISS ΣIGMA with a working distance of 8.5 mm at 5 kV or 10 kV accelerating voltage. Transmission electron microscopy (TEM) was taken on a JEM1200EX microscope. X-ray photoelectron spectroscopy (XPS) data was obtained using a PHI Quantera II SXM (Ulvac-PhiInc., Japan) at 25W under a vacuum lower than10-6 Pa. The thermal behavior was tested and characterized by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). DSC and TGA were performed on a simultaneous TGA/DSC Instrument. The measurements were taken under 50ml/min N2 ambient purge in the temperature range of 25–600 ˚C. The attempted heating rates were 5, 10, 15, 20 ˚C min-1. For these measurements, the n-Al/PTFE and n-Al@PDA/PTFE were placed uniformly in an alumina sample pan rather 3



than the conventional platinum pan. The TA Universal Analysis 2000 graphical software was used to determine the onset (To) and peak (Tp) temperatures of exothermic events as well as integrated enthalpies. It was also used to determine decomposition temperatures (Td) and mass loss data. The impact sensitivity was analyzed by characteristic height H50 using BAM standard with a drop hammer of 2 kg. The infrared thermic device (Image IR7325) was used to study the burning temperature and burning time.

Figure 1. Schematic description of the fabrication of coated Al nanoparticles with in-situ synthesized PDA.

3. Results and Discussion 3.1 The coated n-Al with in-situ synthesized PDA The self-polymerization of dopamine is a result of the combination of covalent polymerization and noncovalent self-assembly. The schematic illustration on coating of n-Al with in-situ synthesized PDA is shown in Figure 1.20, 30 During the self-polymerization process, dopamine is oxidized first to form dopaminequinone, and 4


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then it is followed by a nucleophilic reaction and a rearrangement reaction, producing 5,6-dihydroxyindole (DHI). As covalent polymerization products, the DHI-DHI dimer and dopamine-DHI-DHI trimeric conjugated polymer are generated by covalent oxidative polymerization of DHI or DHI with dopamine, respectively (path 1). Physical self-assembly (noncovalent self-assembly) of dopamine and DHI will form a (dopamine)2/DHI physical trimer, which can further interact with the covalently polymerized products to generate polydopamine (PDA) as the final product (path 2). The functional groups, such as planar indole units, amino group, catechol functions and indolic/catecholic π-system, enable the robust adhesion capability of PDA. Thus, the polydopamine film synthesized on the surface of the n-Al particles, and it gets thicker as reaction time increases. The micrographs of n-Al nanoparticles after coating by PDA are shown in Figure 2. As shown in Figure 2a, the PDA film was well formed and uniformly coated on the n-Al particles after 2h’s reaction. It can be observed the thickness of PDA increases with the time without change the structure of n-Al (Figure 2b to 2e). In particular, the oxide cap of n-Al with a constant thickness of about 2.5 nm regardless of the coating time is visible as the dark zone between the active Al and the PDA layer through TEM (Figs. 2 a-e). The TEM images of the pristine n-Al nanoparticles with determined oxide layer thickness are shown in Figs. S1a and S1b. Note that PDA began to form hemispherical agglomerates after 36h’s coating on the n-Al particle surfaces, which implies that a certain amount of free catechols does not participate in surface adhesion. The SEM images of n-Al@PDA particles are shown as Fig. 2f, where uniformly n-Al particles with rough surfaces could be observed. After coating, the n-Al/PTFE and n-Al@PDA/PTFE were prepared by mechanically mixing in the solvent (see details in experimental section). The SEM images of mechanical mixture n-Al/PTFE showed closely stacked particles, where the PTFE tended to aggregate due to the weak interactions between the PTFE and n-Al particles (Fig. 2g). In the case of n-Al@PDA/PTFE, it is clear that the particles were mixed with intimate contact interfaces and higher homogeneity (Fig. 2h). More experimental details are summarized in Table S1. It can be seen that the density of n-Al@PDA particles decreases from 2.76 to 2.45 g cm-3 when coating time increases from 2h to 36h. This result indicates the increase of PDA contents as the density of PDA is much lower than that of n-Al. Accordingly, the content of PDA calculated by TG experiments (Fig. S2) was increased to 8.7 wt% as coating time reaches to 36h. It is impractical to measure the thickness of n-Al@PDA by simply using the TEM images as a first glance. The PDA contents calculated by density tests and TG experiments could be more precise and statistical. By all means, these results indicate the identical grow law of PDA layer as function of reaction time.

5



Figure 2. The micrographs of Al nanoparticles (n-Al) after coating by PDA, and compared with the corresponding MICs: TEM pictures of n-Al particles after 2h’s (a), 6h’s (b), 12h’s (c), 18h’s (d) and 36h’s (e) coating, (f) SEM micrographs of n-Al after coating, (g) n-Al/PTFE MICs prepared by mechanical mixing, (h) n-Al@PDA /PTFE MICs prepared by mechanical mixing.

Fig. 3a shows the X-ray diffraction (XRD) of the n-Al particles before and after PDA coating. In agreement with the SEM and TEM analysis, n-Al@PDA gives nearly identical XRD spectra of 2Ɵ degree from 10° to 80°. This result confirms that the crystal structure of n-Al does not change after coating. The presence of PDA on the n-Al surface is determined by XPS (X-ray photoelectron spectroscopy) analysis (Figure S3). All the main peaks of the sample can be indexed to Al 2p, Al 2s, C 1s, N 1s and O 1s electrons exclusively, confirmed successful coating of PDA on the surface of n-Al. As expected, the result is in good agreement with the findings reported in the literature. 20, 30 In particular, the peaks of O 1s are observed in n-Al@PDA and n-Al@PDA/PTFE, which reveals the interfacial contact between n-Al@PDA and PTFE, due to an increased binding energy in n-Al@PDA/PTFE (Figure 3c and 3d). Besides, the increased C-O and C=O binding energies should arise from the interaction between PDA and PTFE, in good agreement with the literature 32. This result indicates that PDA coated on the surfaces of n-Al can also adhere to the surface of the PTFE, forming H-bonds between the catechol groups of PDA with the fluorine atoms in PTFE molecules (Figs. 3b).

6


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Figure 3. (a) XRD spectra of n-Al before and after PDA coating, (b) Proposed binding system of n-Al@PDA/PTFE and SEM picture of n-Al@PDA/PTFE showing the binding between n-Al@PDA and PTFE, (c) High-resolution XPS spectra of O 1s peaks for n-Al@PDA, (d) and n-Al@PDA/PTFE. Deconvolution of the O 1s signal in n-Al@PDA sample shows the presence of Al2O3 (∼530.4 eV), C−O (hydroxyl, ∼531.0 eV) and C=O (carbonyl, ∼531.4 eV) groups. After mixing with PTFE, the binding energies associated with C-O and C=O groups are enhanced to∼531.9 eV and ∼533.2 eV, respectively.

3.2 Thermal reaction behaviors A comprehensive thermal analysis is performed by DSC. Figure 4a and 4b shows the heat flow properties as a function of temperature for the mechanically mixed n-Al/PTFE and the PDA controlled MIC n-Al@PDA/PTFE (6 h). For both mechanically mixed n-Al/PTFE and interfacial-controlled one, two obvious exothermic peaks are shown even though the heat release is very different. It can be clearly seen that there is a small exothermic change at the temperature range of 340-400˚C shown in the DSC curve of mechanically mixed n-Al/PTFE. This is due to a surface reaction (pre-ignition reaction, PIR) caused by fluorination of the alumina shell of the n-Al, which was found to play a critical role in the interface solid-state oxidation-redox reaction mechanisms of this type of MICs. 12,13

The initial temperature of PIR (TPIR) for mechanically mixed n-Al/PFPE is about 365 ℃ (at 10 ℃ min-1), lower

than that of the corresponding polydopamine interfacial controlled ones. As the coating time increases, the TPIR would be postponed to even higher temperature, which is above 420℃ in the case of n-Al@PDA/PTFE after 6 h’s coating (Table 1). It is true that PIR has a significant effect on the reaction of n-Al and PTFE by changing their initial stage of the reaction, which partially provides the heat to start the following main reaction. The PDA-coated n-Al suggests a new way of manipulating the PIR, which actually is a way to tune the reactivity of conventional MICs of any kinds. Figure 4c shows the dependence of the maximum mass loss rate and onset temperature (To) of first exotherms on the coating time. It can be observed that, the maximum mass loss rate rapidly increases as the coating time increases from 0h to 18h, which indicates the dependence of reactivity on the coating time (thickness). 7



The increased maximum mass loss rate is caused by the intimate contact interfaces and uniformly dispersions of reactants. However, the To was delayed dramatically with the increase of coating time in the range of 0-18 h, whereas the To was only increased 9 °C if the coating time was further increased from 18 h to 36 h. It means that there is a certain upper limit for the improvement of To. The increased To is caused by the postpone of the reaction between PTFE and Al2O3, since this reaction is limited by the reactivity between PDA and PTFE. Another evidence is the apparent activation energy (Ea, calculated by Kissinger method31), as shown in Figure 4d (to compare these two exothermic peaks of the DSC curves, the overlapped exothermic reaction peaks of these MICs have been separated as shown in Figure S4). The Ea of the first exotherm increased from 163.6 kJ mol-1 to 335.1 kJ mol-1 as the coating time increased from 0 h to 12 h. However, the Ea of the final exothermic peak (main reaction) seems to be much less dependent on the coating time, in comparison to that of the initial exothermic process. It may tell us the two following facts: a) the PDA interfacial layer hindered the PIR; b) with the coating time increased, increased thickness of PDA interfacial layer has a negative effect on the PIR, but the thickness has little influence on the PIR if it reaches to a certain level, where the critical thickness is after coating of 18 hours. When the coating time reaches to 18 h, the reaction process between PTFE and Al becomes quite different. As shown in Figure 4e, the reaction regions of the PIR and the final reaction step overlapped for the n-Al@PDA/PTFE with coating of 18 h, especially under higher heating rates. This phenomenon indicates that reaction mechanism between PTFE and Al is largely dependent on the heating rate if the coating of PDA is thick enough. It is simply because the thickness of PDA plays a key role in reaction mechanism, whereas a higher heating rate corresponds to a higher temperature gradient. The PDA has much lower heat conductivity than that of Al particles, resulting in higher dependence of heat transfer on the heating rate. Figure 4f shows the heat of reaction for different samples at 20 ℃ min-1. Heat of reaction of the first exothermic peak was increased with coating time (

PTFE

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