Enhancing the combustion of nAl with AlF3 coating: gas-solid reaction mechanism for reducing combustion agglomeration of Al powder.

The combustion agglomeration of nano-aluminum (nAl) powder leads to incomplete combustion, which seriously hinders its application as metal fuel. In this work, nAl@AlF3 composites were produced by coating nAl with AlF3via a facile chemical deposition method. TEM and SEM analyses indicated that the AlF3 layer was evenly coated on the surface of nAl with a thickness of 4.6-9.1 nm, thereby varying the quantity of AlF3 applied. Experimental results from combustion indicated that the prepared nAl@AlF3 composites exhibit superior combustion efficiency, a higher combustion rate, and reduced combustion agglomeration as compared to raw nAl. Contrary to the widely accepted explanation that volatilization of AlF3 hinders Al combustion agglomeration, we proved that the gas-solid reaction between nAl and AlF3 plays an important role in inhibiting the sintering of nAl particles produced. The gaseous intermediate (i.e., AlOF and HF) released from the hydrolysis of AlF3 could reduce the diffusion barrier of Al2O3 to facilitate the reaction of Al core, which enhances the combustion reaction kinetics. More importantly, these gaseous products actively participate in the reaction cycle to continuously exert their catalytic effects.

Convoluted micellar morphological transitions driven by tailorable mesogenic ordering effect from discotic mesogen-containing block copolymer.

Solution-state self-assemblies of block copolymers to form nanostructures are tremendously attractive for their tailorable morphologies and functionalities. While incorporating moieties with strong ordering effects may introduce highly orientational control over the molecular packing and dictate assembly behaviors, subtle and delicate driving forces can yield slower kinetics to reveal manifold metastable morphologies. Herein, we report the unusually convoluted self-assembly behaviors of a liquid crystalline block copolymer bearing triphenylene discotic mesogens. They undergo unusual multiple morphological transitions spontaneously, driven by their intrinsic subtle liquid crystalline ordering effect. Meanwhile, liquid crystalline orderedness can also be built very quickly by doping the mesogens with small-molecule dopants, and the morphological transitions are dramatically accelerated and various exotic micelles are produced. Surprisingly, with high doping levels, the self-assembly mechanism of this block copolymer is completely changed from intramolecular chain shuffling and rearrangement to nucleation-growth mode, based on which self-seeding experiments can be conducted to produce highly uniform fibrils.

Research on the Thermal Aging Performance of a GAP-Based Polyurethane Elastomer.

Glycidyl azide polymer (GAP)-based polyurethane is an ideal elastomeric matrix for high-energy, low-smoke, and insensitive solid propellants. As the skeleton structure of GAP propellants, changes in the structure and properties of GAP elastomers during aging lead to the deterioration of propellant performance (especially in relation to mechanical properties), which causes safety risks. A high-temperature-accelerated aging experiment (70 °C) on a GAP elastomer was conducted. The evolution of the microstructure of the GAP elastomer system was analyzed by Fourier-transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR), and variations in the macroscopic properties were analyzed by the hardness test and the uniaxial tensile test. The experimental results showed that thermal aging of the GAP elastomer is a coupled process of multiple chemical reactions. The azide groups, urethane groups, and ether bonds were the weak links in the network structure, breaking during the aging process, and the crosslinking density rose and then decreased. Macroscopic properties also showed segmented changes. The aging process was divided into three stages: post-curing (stage one); when the crosslinked network began to break (stage two), and when the crosslinked network was destroyed (stage three). Changes in the microstructure and macroscopic properties were consistent. This work is of great significance for exploring the aging mechanism of GAP propellants and extending their storage life.

A Novel Method of Improving the Mechanical Properties of Propellant Using Energetic Thermoplastic Elastomers with Bonding Groups.

The relatively poor mechanical properties of extruded modified double base (EMDB) propellants limit their range of applications. To overcome these drawbacks, a novel method was proposed to introduce glycidyl azide polymer-based energetic thermoplastic elastomers (GAP-ETPE) with bonding groups into the propellant adhesive. The influence of the molecular structure of three kinds of elastomers on the mechanical properties of the resultant propellant was analyzed. It was found that the mechanical properties of the propellant with 3% CBA-ETPE (a type of GAP-ETPE that features chain extensions using N-(2-Cyanoethyl) diethanolamine and 1,4-butanediol) were improved at both 50 °C and -40 °C compared to a control propellant without GAP-ETPE. The elongation and impact strength of the propellant at -40 °C were 7.49% and 6.58 MPa, respectively, while the impact strength and maximum tensile strength of the propellant at 50 °C reached 21.1 MPa and 1.19 MPa, respectively. In addition, all three types of GAP-ETPE improved the safety of EMDB propellants. The friction sensitivity of the propellant with 3% CBA-ETPE was found to be 0%, and its characteristic drop height H50 was found to be 39.0 cm; 126% higher than the traditional EMDB propellant. These results provide guidance for studies aiming to optimize the performance of EMDB propellants.

An 'active site anchoring' strategy for the preparation of PBO fiber derived carbon catalyst towards an efficient oxygen reduction reaction and zinc-air batteries.

In order to promote the wide application of clean energy-fuel cells, it is urgent to develop transition metal-based high-efficiency oxygen reduction reaction (ORR) catalytic materials with a low cost and available rich raw material resources to replace the currently used precious metal platinum-based catalytic materials. Herein, a novel 'active-site-anchoring' strategy was developed to synthesize highly-activated carbon-based ORR catalysts. Firstly, poly(p-phenylene benzobisoxazole) (PBO) fiber with a stable chemical structure was selected as the main precursor, and iron was complexed on its surface, and then poly-dopamine (PDA) was coated on the surface of PBO-Fe to form a PBO-Fe-PDA composite structure. Therefore, carbon-based catalyst PBO-Fe-PDA-900 with abundant Fe2O3 active sites was prepared by anchoring iron sites by PDA after pyrolysis. As a result, the PBO-Fe-PDA-900 catalyst displayed a 30 mV higher half-wave potential (0.86 V) than that of a commercial Pt/C electrocatalyst. Finally, PBO-Fe-PDA-900 was used as a cathode material for zinc-air batteries, showing a high peak power density superior to Pt/C. This work offers new prospects for the design of efficient, non-precious metal-based materials in zinc-air batteries.

Azide Ionic Liquids for Safe, Green, and Highly-Efficient Azidation Reactions to Produce Azide Polymers.

Azide compounds are widely used and especially, polymers bearing pendant azide groups are highly desired in numerous fields. However, harsh reaction conditions are always mandatory to achieve full azidation, causing severe side reactions and degradation of the polymers. Herein, we report the design and preparation of two azide ionic liquids (AILs) with azide anion and triethylene glycol (E3 )-containing cation, [P444E3 ][N3 ] and [MIME3 ][N3 ]. Compared with the traditional sodium azide (NaN3 ) approach, both AILs showed much higher reaction rates and functional-group tolerance. More importantly, they could act as both reagents and solvents for the quantitative azidation of various polymeric precursors under mild conditions. Theoretical simulations suggested that the outstanding performance of AILs originated from the existence of ion pairs during the reaction, and the E3 moieties played a crucial role. Lastly, after the reaction, the AILs could be easily regenerated, presenting a safer, greener, and highly efficient synthesis route for azide polymers.

Preparation and Properties of a Novel High-Toughness Solid Propellant Adhesive System Based on Glycidyl Azide Polymer-Energetic Thermoplastic Elastomer/Nitrocellulose/Butyl Nitrate Ethyl Nitramine.

Glycidyl azide polymer (GAP)-energetic thermoplastic elastomer (GAP-ETPE) propellants have high development prospects as green solid propellants, but the preparation of GAP-ETPEs with excellent performance is still a challenge. Improving the performance of the adhesive system in a propellant by introducing a plasticizer is an effective approach to increasing the energy and toughness of the propellant. Herein, a novel high-strength solid propellant adhesive system was proposed with GAP-ETPEs as the adhesive skeleton, butyl nitrate ethyl nitramine (Bu-NENA) as the energetic plasticizer, and nitrocellulose (NC) as the reinforcing agent. The effects of the structural factors on its properties were studied. The results showed that the binder system would give the propellant better mechanical and safety properties. The results can provide a reference for the structure design, forming process, and parameter selection of high-performance GAP-based green solid propellants.

Controllable One-Step Assembly of Uniform Liquid Crystalline Block Copolymer Cylindrical Micelles by a Tailored Nucleation-Growth Process and Their Application as Tougheners.

The fabrication of uniform cylindrical nanoobjects from soft materials has attracted tremendous research attention from both fundamental research and practical application points of view but has also posed outstanding challenges in terms of their preparation. Herein, we report a one-step method to assemble cylindrical micelles (CMs) with highly controllable lengths from a single liquid crystalline block copolymer by an in situ nucleation-growth strategy. By adjusting the assembly conditions, the lengths of the CMs are controlled from hundreds of nanometers to micrometers. Several influencing factors are systematically investigated to comprehensively understand the process. Particularly, the solvent quality is found determinative in either enhancing or suppressing the nucleation process to produce shorter and longer CMs, respectively. Taking advantage of this strategy, the lengths of CMs can be nicely controlled over a wide concentration range of four orders of magnitude. Lastly, CMs are produced on decent scales and applied as additives to dramatically toughen glassy plastic matrix, revealing an unprecedented length-dependent toughening effect.

Assembly of Supramolecular Nanoplatelets with Tailorable Geometrical Shapes and Dimensions.

The craving for controllable assembly of geometrical nanostructures from artificial building motifs, which is routinely achieved in naturally occurring systems, has been a perpetual and outstanding challenge in the field of chemistry and materials science. In particular, the assembly of nanostructures with different geometries and controllable dimensions is crucial for their functionalities and is usually achieved with distinct assembling subunits via convoluted assembly strategies. Herein, we report that with the same building subunits of α-cyclodextrin (α-CD)/block copolymer inclusion complex (IC), geometrical nanoplatelets with hexagonal, square, and circular shapes could be produced by simply controlling the solvent conditions via one-step assembly procedure, driven by the crystallization of IC. Interestingly, these nanoplatelets with different shapes shared the same crystalline lattice and could therefore be interconverted to each other by merely tuning the solvent compositions. Moreover, the dimensions of these platelets could be decently controlled by tuning the overall concentrations.

Tribological and Mechanical Applications of Liquid-Crystal-Polymer-Modified Carbon-Fiber-Reinforced Polyamide-Polyurethane Composites.

An aromatic copolyester liquid crystal polymer (LCP) was introduced into carbon-fiber-reinforced polyamide-polyurethane (CF/PA-PU) composites through melt blending to improve the tribological properties of the composites. The effects of LCP on the mechanical, processing, and thermal properties of CF/PA-PU composites were compared to those of commonly-used graphite (Gr). The results showed that at 5 wt.% LCP content, the coefficient of friction (COF) was decreased by 16.06%, and the wear rate by 32.22% in the LCP/CF/PA-PU composite compared to the CF/PA-PU composite. Furthermore, using LCP instead of Gr showed significantly improved mechanical properties and reduced processing viscosity. The tensile strength of 5%LCP/CF/PA-PU composite could reach 99.08 MPa, while the equilibrium torque was reduced, being 26.85% higher and 18.37% lower than those of CF/PA-PU composite, respectively. The thermal stability of LCP/CF/PA-PU composites was also enhanced. The addition of 5 wt.% LCP to CF/PA-PU composite increased the initial decomposition temperature by 14.19% compared to CF/PA-PU. In sharp contrast, the addition of Gr increased equilibrium torque and actual processing temperature leading to processing difficulties and instability. This approach offers a novel strategy for tribological applications and tackles the problem of high viscosity in CF/PA-PU composites.

Study on GAP Adhesive-Based Polymer Films, Energetic Polymer Composites and Application.

To lay the foundation for environmentally friendly energetic polymer composites, GAP (glycidyl azide polymer) adhesive-based polymer films with different curing parameter R (mol ratio of hydroxyl/isocyanate) and energetic polymer composites with different RDX contents were studied. GAP/TDI (toluene diisocyanate)/GLY(glycerol) was selected as the adhesive system. The tensile strength and elongation at the break of the polymer film with R = 2.2, was 14.34 MPa and 176.86%, respectively, as observed by an AGS-J electronic universal testing machine. A relatively complete cross-linking network and high hydrogen bonding interaction were observed by LF-NMR (low-field nuclear magnetic resonance, where the cross-linking density was 11.06 × 10-4 mol/cm3) and FT-IR (fourier transform infrared spectroscopy, where the carbonyl bonding ratio was 64.84%). Forty percent RDX(hexogen) was added into the adhesive system. The tensile strength was 4.65 MPa, and the elongation at the break was 78.49%; meanwhile, the heat of the explosive was 2.87 MJ/kg, and the residue carbon rate was only 2.47%. The tensile cross-sections of energetic polymer composites were observed by SEM (Scanning electron microscopy).

Synthesis and characterization of hydroxyl-terminated polybutadiene modified low temperature adaptive self-matting waterborne polyurethane.

Hydroxyl-terminated polybutadiene (HTPB) is a flexible telechelic compound with a main chain containing a slightly cross-linked activated carbon-carbon double bond and a hydroxyl group at the end. Therefore, in this paper, HTPB was used as a terminal diol prepolymer, and sulfonate AAS and carboxylic acid DMPA were used as hydrophilic chain extenders to prepare low-temperature adaptive self-matting waterborne polyurethane (WPU). Due to the fact that the non-polar butene chain in the HTPB prepolymer cannot form a hydrogen bond with the urethane group, and the solubility parameter difference between the hard segment formed by the urethane group is large, the gap of T g between the soft and hard segments of the WPU increases by nearly 10 °C, with more obvious microphase separation. At the same time, by adjusting the HTPB content, WPU emulsions with different particle sizes can be obtained, thereby obtaining WPU emulsions with good extinction properties and mechanical properties. The results show that HTPB-based WPU with a certain degree of microphase separation and roughness obtained by introducing a large number of non-polar carbon chains has good extinction ability, and the 60° glossiness can be as low as 0.4 GU. Meanwhile, the introduction of HTPB can improve the mechanical properties and low temperature flexibility of WPU. The T g,s (the glass transition temperature of soft segment) of WPU modified by the HTPB block decreased by 5.82 °C, and the ΔT g increased by 21.04 °C, indicating that the degree of microphase separation increased. At -50 °C, the elongation at break and tensile strength of WPU modified by HTPB can still maintain 785.2% and 76.7 MPa, which are 1.82 times and 2.91 times those of WPU with only PTMG as soft segment, respectively. The self-matting WPU coating prepared in this paper can meet the requirements of severe cold weather and has potential application prospects in the field of finishing.

In situ Nucleation-Growth Strategy for Controllable Heterogeneous Supramolecular Polymerization of Liquid Crystalline Block Copolymers and Their Hierarchical Assembly.

The self-assembly morphologies of subunits are largely governed by thermodynamics, which plays a less important role in dimensional control. Particularly for one-dimensional assemblies from block copolymers (BCPs), the negligible energy difference between short and long ones imposes great challenges in length control. Herein, we report that by incorporating additional polymers to induce in situ nucleation and trigger the subsequent growth, controllable supramolecular polymerization driven by mesogenic ordering effect could be realized from liquid crystalline BCPs. The length of the resultant fibrillar supramolecular polymers (SP) is controlled by tuning the ratio between nucleating and growing components. Depending on the choice of BCPs, the SPs can be homopolymer-like, heterogeneous triblock, and even pentablock copolymer-like. More interestingly, with insoluble BCP as a nucleating component, amphiphilic SPs are fabricated, which can undergo spontaneous hierarchical assembly.

Influence of Solid Filler on the Rheological Properties of Propellants Based on Energetic Thermoplastic Elastomer.

Glycidyl azide polymer-energetic thermoplastic elastomer propellant (GAP-ETPE) has high development prospects as a green solid propellant, although the preparation of GAP-ETPE with excellent performance is still a challenge. Focusing on the demand of high-strength solid propellants for free-loading rocket motors, a GAP-ETPE model propellant with excellent overall performance was prepared in this work, and the influence of adhesive structure characteristics on its fluidity was studied. Furthermore, the influence of filler on the rheological properties of the model propellant was investigated by introducing hexogen (RDX) and Al, and a corresponding two-phase model was established. The results may provide a reference for the structural design, molding process, and parameter selection of high-performance GAP-based green solid propellants.

Poly(glycidyl azide) as Photo-Crosslinker for Polymers.

Crosslinking polymers to form networks is a universal and routinely applied strategy to improve their stability and endow them with solvent resistance, adhesion properties, etc. However, the chemical crosslinking of common commercial polymers, especially for those without functional groups, cannot be achieved readily. In this study, we utilized low-molecular weight poly(glycidyl azide) (GAP) as polymeric crosslinkers to crosslink various commercial polymers via simple ultraviolet light irradiation. The azide groups were shown to decompose upon photo-irradiation and be converted to highly reactive nitrene species, which are able to insert into carbon-hydrogen bonds and thus crosslink the polymeric matrices. This strategy was demonstrated successfully in several commercial polymers. In particular, it was found that the crosslinking is highly localized, which could endow the polymeric matrices with a decent degree of crosslinking without significantly influencing other properties, suggesting a novel and robust method to crosslink polymeric materials.

Preparation of different morphology Cu/GO nanocomposites and their catalytic performance for thermal decomposition of ammonium perchlorate.

Cu nanoparticles are more active catalytically than CuO nanoparticles, which have been widely studied as catalysts for organic synthesis, electrochemistry, and optics. However, Cu nanoparticles are easily agglomerated and oxidized in air. In this research, columnar, flower-like, bubble-like and teardrop-shaped Cu/GO nanocomposites were fabricated via a water-solvent thermal method and high temperature calcination technique using deionized water (H2O), methanol (CH3OH), ethanol (CH3CH2OH) and ethylene glycol (EG) as the solvent, respectively. The structures, the morphology and the catalytic performance and catalytic mechanism for thermal decomposition of ammonium perchlorate (AP) of the Cu/GO nanocomposites have been studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), nitrogen adsorption tests (BET), simultaneous thermogravimetry-differential scanning calorimetry (TGA/DSC) and thermogravimetric couplet with Fourier transform infrared spectroscopy (TGA-FTIR), respectively. The experimental results show that the morphology of the Cu/GO nanocomposites has a significant effect on the surface area and the teardrop-shaped Cu/GO nanocomposites have the largest specific surface area and the best catalytic performance among them. When 5 wt% of the Cu/GO nanocomposites was added, the decomposition temperature of AP decreased from 426.3 °C to 345.5 °C and the exothermic heat released from the decomposition of AP increased from 410.4 J g-1 to 4159.4 J g-1. In addition, the four morphological Cu/GO nanocomposites exhibited good stability, their catalytic performance for thermal decomposition of AP remained stable after 1 month in air. Excellent catalytic performance and stability were attributed to the strong catalytic activity of pure metal nanoparticles, and GO can accelerate electron movement and inhibit the agglomeration of nanoparticles, as well as the multiple effects of inhibiting the oxidation of Cu nanoparticles in air. Therefore, it has important application potential in high-energy solid propellant.

Research on Mechanical Properties and Sensitivity of a Novel Modified Double-Base Rocket Propellant Plasticized by Bu-NENA.

The research and development of rocket propellants with a high solid content and superior mechanical and security performance is urgently needed. In this paper, a novel extruded modified double-base (EMDB) rocket propellant plasticized by N-butyl-N-nitratoethyl nitramine (Bu-NENA) was prepared to overcome this challenge. The results indicated that Bu-NENA decreased the mechanical sensitivity successfully, contributing to the mechanical properties against traditional nitroglycerin (NG) based EMDB propellants, while hexogen (RDX), which is beneficial to propellant energy, was not conducive to the elongation and sensitivity of the propellants. By contrast with the blank group (NG-based EMDB propellant, R0), the elongation of the optimized propellant at -40 °C was promoted by 100% from 3.54% to 7.09%. Moreover, the ß-transition temperature decreased from -33.8 °C to -38.1 °C due to superior plasticization by Bu-NENA, which represents a better toughness. The friction sensitivity dropped by 100% from 46% to 0%. Simultaneously, the height for 50% probability of explosion (H50) increased by 87.2% from 17.2 cm to 32.2 cm. The results of this research could be used to predict a potential prospect in tactical weapons.

Thermal decomposition of nano Al-based energetic composites with fluorinated energetic polyurethane binders: experimental and theoretical understandings for enhanced combustion and energetic performance.

Energetic composites composed of polymeric binders and metallic fuels are widely used in industrial and military fields, and their performance is largely dependent on the combustion process. Fluorinated energetic polymeric binders can facilitate the combustion of metallic fuels such as aluminum particles and enhance the energetic level of the energetic composites. In this report, fluorinated energetic polyurethanes (FPUs) were applied as binders for energetic composites with aluminum nanoparticles (AlNPs). The fluorinated components in the energetic binder could be a uniform dispersion inside the composites, endowing the composites with decent mechanical properties and high combustion rate. Most significantly, compared with the composites without fluorine, FPU/AlNP energetic composites not only showed a remarkably improved combustion efficiency, but also, surprisingly, a dramatic enhancement in the heat of explosion by 91.2%, despite the low content of fluorine. By analyzing the combustion products together with kinetic simulations derived from chemical reaction neural network (CRNN) modelling, a detailed mechanistic understanding of the combustion process was provided, suggesting the importance of synergistic effects brought by the fluorinated and energetic components.

Halogen-free instinct flame-retardant waterborne polyurethanes: composition, performance, and application.

Ideal halogen-free instinct flame-retardant waterborne polyurethanes have high flame-retardant efficiency, environmental friendliness, fine compatibility, and good thermostability. Phosphorus flame-retardants are currently widely used in halogen-free instinct flame-retardant waterborne polyurethanes (HIFWPU), especially those with phosphorous-nitrogen co-structures. Phosphorous-nitrogen HIFWPU have become a hotspot because their co-structures provide higher flame-retardance as compared to waterborne polyurethanes. This review introduces three main types of HIFWPU based on composition, performance and application. HIFWPU not only have improved flame-retardance but also satisfy the various requirements for functionality. HIFWPU have been widely developed in textile, furniture, automobile, and aerospace applications.

Effects of liquid crystal polymer (LCP) on the structure and performance of PEEK/CF composites.

Carbon fiber reinforced polyether ether ketone (PEEK/CF) composites feature diverse advantages and have been applied in various fields. However, the high melt viscosity of PEEK leads to their poor processing performance and affects their practical applications. Here a liquid crystal polymer (LCP) was introduced into a PEEK/CF system as a new strategy to address the aforementioned issues. Bearing aromatic rings on the main chains, LCP can strongly interact with PEEK by pi-pi interaction, which alters the crystallization behaviour and facilitates processing of PEEK/CF, eventually improving its mechanical performance. As a result, a high crystallinity (37.37%), a decreased equilibrium torque (8.902 Nm), and a high tensile strength (230.97 MPa) are realized with 5 wt% LCP. The current approach offers a new solution to simultaneously promote processing and mechanical performance of PEEK/CF and other polymer-based composites.