Synthesis and characterization of a novel fluorine-containing triblock copolymer as a potential binder
Graphical abstract
Introduction
Solid propellants are a kind of composite containing a large number of high-energy materials, oxidizers, metallic additives, polymeric binder and so on [1], [2], [3]. Among them, the polymeric binder plays a key role in forming these components into an overall structure [4], [5], [6], [7].
Hydroxy-terminated Polyether (HTPE) is the most commonly used polymeric binder in propellants, which has low viscosity, high solid loading and good chemical properties of isocyanate curing [8], [9]. The polymeric binder makes solid materials wet well and reduce voids in the entire system, so it can improve the mechanical and safety properties of explosives and propellants [10], [11]. HPTE binder is a hydroxyl-terminated polyether binder made by block copolymerization of polyethylene glycol and polytetrahydrofuran [12]. It shows good mechanical properties and ballistic properties which can provide the propellant with insensitive performance under dangerous conditions such as heating, shock waves, mechanical impacts [13], [14]. HTPE polyurethane elastomer possesses not only comprehensive mechanical properties, but also a relatively low glass transition temperature(Tg), good thermal stability, to meet the desirable requirements of solid propellants for binder [15], [16], [17], [18]. However, the energy level of the solid propellant is not high due to without energy in HTPE binder. Besides, the hydrogen bonds in HTPE binder become weaker in high temperature environments, thus the activity of HTPE molecular chains will be reduced in low temperature situation [19], [20]. It means that the HTPE binder cannot meet the well mechanical properties under a wide temperature range. Therefore, it is very meaningful to improve the HTPE structure to achieve the satisfyingly mechanical properties under high and low temperature.
Fluorinated polymers have been widely used as binders in high-energy materials over the past two decades due to their long-term chemical stability, low friction coefficient, high density, compatibility with energetic materials, and broad range of working temperature [21], [22], [23], [24]. Moreover, fluoropolymer-metal composition develops an excellent property which it can provide higher reaction energy to effect the material characterizations. For example, the addition of aluminum can effectively improve the specific impulse (Isp) performance by at least 15 % [25], [26], [27]. Besides, fluorinated compounds have also been used in high-energy oxidants because fluorine has a higher exotherm than oxygen [28], [29], [30]. In addition, the sublimed temperature of AlF3 is at 1277 ℃ (1 atm), and the boiled temperature of Al2O3 is at 3000 ℃. Therefore, the fluorination of aluminum results in a reduction process in condensed phase products (and may reduce two-phase flow loss). Because the sublimation of AlF3 produced by the reaction in the solid propellant, the particles may be broken leading to reduced agglomeration [31], [32], [33], [34]. The introduction of fluorine can improve the oxygen balance of solid propellant, and the oxidizer content can be reduced in the solid propellant when other energetic additives are increased to improve the energy level in the solid propellant [35]. Thus, synthesis of fluorinated HTPE via copolymerization of fluorinated monomers may be a promising method to improve its application properties. As far as I know, such designed synthetic polymeric binders have been rarely reported.
In this work, through the cationic ring-opening polymerization of 2-(2,2,3,3-Tetrafluoro-propoxymethyl)-oxirane (FPO) and epichlorohydrin (ECH) with Polytetramethylene Ether Glycol (PTMG) respectively, a novel fluorine-containing triblock copolymer PFPO-PTMG-PFPO and a fluorine-free triblock copolymer PECH-PTMG-PECH were obtained. PTMG was used as initiator and boron trifluoride diethyl ether (BF3·OEt2) was used as catalyst. The chemical structure, molecular weight of polymer was characterized by Fourier transform infrared (FTIR), 1HNMR and gel permeation chromatography (GPC), and the thermal properties were characterized by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) respectively.
Section snippets
Materials
The 1,2-epichlorohydrin, cyclohexane, sodium hydroxide and methylenechloride were purchased from China Medicine, Shanghai Chemical Reagent Corporation. A boron trifluoride diethyl ether (BF3·OEt2) was purchased from J&K scientific Ltd. 3,3,2,2-tetrafluoropanol was purchased from Zhengzhou JACS chemical reagent Corporation. Polytetramethylene Ether Glycol (PTMG) (Mn = 1000) was purchased from Shanghai Aladdin chemical reagent Corporation. The methylene chloride was distilled over CaH2 and
Synthesis and mechanism of PFPO-PTMG-PFPO
The PFO-PTMG-PFPO was synthesized by cationic ring opening copolymerization. PTMG was used as macroinitiator, and BF3·OEt2 was served as catalyst. There were two types of initiators: macroinitiator and small-molecule initiators. Small-molecule initiators included 1,4-butanediol, ethylene glycol and so on. Macroinitiators contained polycaprolactone (PCL)[36], poly(ethylene glycol)[37], HTPB[38], [39], [40] and PTMG to synthesize triblock HTPE copolymer with ethylene oxide. In this work, PTMG was
Conclusion
A novel fluorine-containing triblock copolymer PFPO-PTMG-PFPO was obtained by the cationic ring-opening polymerization of 2-(2,2,3,3-Tetrafluoro-propoxymethyl)-oxirane (FPO) with Polytetramethylene Ether Glycol (PTMG). The results of 1H NMR and FTIR indicated that PFPO-PTMG-PFPO had been prepared successfully. The three molecular weights of PFPO-PTMG-PFPO with fluorine-containing side chains had a higher glass transition temperature. With the increase of the fluorine-containing side chains, the
Funding
This work was supported by the Nature Science Associate Foundation (NSAF) of China (Grant numbers: U1830127).
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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