Fretting Wear Behavior of Three Kinds of Rubbers under Sphere-On-Flat Contact.

Rubbers are widely used in various fields as the important sealing materials, such as window seal, door seal, valve, pump seal, etc. The fretting wear behavior of rubbers has an important effect on their sealing performance. This paper presents an experimental study on the fretting wear behavior of rubbers against the steel ball under air conditions (room temperature at 20 ± 2 °C and humidity at 40%). Three kinds of rubbers, including EPDM (ethylene propylene diene monomer), FPM (fluororubber), and NBR (nitrile-butadiene rubber), are considered in experiments. The sphere-on-flat contact pattern is used as the contact model. The influences of the displacement amplitude, normal force, frequency, and rubber hardness on the fretting wear behavior are discussed in detail. White light profiler and scanning electron microscope (SEM) are used to analyze the wear mechanism of the rubber surface. The fretting wear performances of three rubbers are compared by considering the effect of the displacement amplitude, normal force, frequency, and rubber hardness. The results show that NBR has the most stable friction coefficient and the best wear resistance among the three rubbers.

Experimental Studies on Fretting Wear Behavior of PVDF Piezoelectric Thin Films.

This paper discusses an in-depth experimental study on the fretting wear behavior of PVDF (polyvinylidene fluoride) piezoelectric thin film against a Si3N4 ceramic sphere under air conditions. A fretting wear device with a ball-on-plate contact configuration was applied. The changes of displacement amplitude, normal force, and applied voltage were taken into account. The friction logs were used to determine the contact state of the PVDF thin film during the fretting test. The 3D topography instrument and scanning electron microscope (SEM) were used to measure the details of the surface morphology and wear volume. The test results of PVDF thin films under different normal force, displacement amplitude, and applied voltage are summarized through the collection and analysis of experimental data. It is shown that the creep and plastic deformation lead to obvious winkles at the contact surface, which may decrease the specific wear rate of PVDF thin films.

Facile Preparation and Properties of Crosslinked Copolyether Elastomers with 1,2,3-Triazole and Urethane Subunit via Click Polymerization.

An azide terminated ethylene oxide-tetrahydrofuran copolymer with urethane segments (ATUPET) as a novel binder pre-polymer, has been prepared through ethylene oxide-tetrahydrofuran random copolymer (PET) end-capping modification via one-pot method. The structure characterization of the modifier has been analyzed by FTIR, 1H NMR, 13C NMR and GPC. In comparison with PET, ATUPET has a slightly higher viscosity because it has additional hydrogen bonding interaction generated by the urethane in ATUPET. Triazole cross-linked elastomers based on ATUPET with various functional molar ratios were prepared using tripropargylamine as a curing agent and cross-linker. Mechanical properties indicate that the modulus E and tensile strength σ b exhibit a parabolic dependence with the increase in R. At around the stoichiometric ratio, the modulus E and tensile strength σ b reach a maximum and the elongation at break exhibit an acceptable value at the same time. Swelling tests demonstrate that the apparent cross-linking densities (N0) have a maximum value at the stoichiometric ratio. Thermal analysis shows that the ATUPET prepolymer and its polytriazoles elastomers exhibit a satisfactory stability. The results demonstrated that ATUPET might be a promising polymeric binder for future propellant formulations especially in the field of isocyanate-free curing technology.

A theoretical insight into the reaction mechanisms of a 2,4,6-trinitrotoluene nitroso metabolite with thiols for toxic effects.

2,4,6-Trinitrotoluene (TNT) is a class C carcinogen as rated by the Environmental Protection Agency. One of the toxicity mechanisms of TNT is the covalent binding of its metabolites to critical proteins. However, knowledge about their molecular reaction mechanisms is scarce. Herein, we have provided density functional theory (DFT) simulation evidences for the reaction mechanisms of the nitroso metabolite of TNT with the sulfhydryl group of model thiols for the first time. The results show that the solvent-mediated proton-transfer mechanism plays a significant role in the entire process. For the formation of semimercaptal, the mechanism is slightly different from the previous one where the thiolate anion attacks the nitroso group. The rearrangement of semimercaptal needs to be triggered by an acid or hydrated ion (H3O+), which is consistent with the previous assumption. The other pathway, the conversion of semimercaptal to hydroxylamine, has to overcome a higher barrier, although it does not need the participation of an acid or a hydrated ion. In addition, the details on transition states, intermediates and free energy surfaces for three reactions are given, which make up for the lack of experimental knowledge. These conclusions can help to deeply understand the toxic effects of TNT and other nitroaromatic explosives.

Theoretical insight into reaction mechanisms of 2,4-dinitroanisole with hydroxyl radicals for advanced oxidation processes.

The detailed degradation mechanism of an insensitive explosive, 2,4-dinitroanisole (DNAN), in advanced oxidation processes (AOPs) was investigated computationally at the M06-2X/6-311 + G(d,p)/SMD level of theory. Results obtained show that the addition-elimination reaction is the dominant mechanism. The phenol products formed can continue to be oxidized to benzoquinone radicals that are often detected by experiments and may be the initial reactants of ring-opening reactions. The H-abstraction reaction is an unavoidable competing mechanism; the intermediate generated can also undergo the process of addition-elimination reaction. The nitro departure reaction involves not only hydroxyl radical (•OH), but also other active substances (such as •H). More importantly, we found that AOP technology can easily degrade DNAN, similar to TNT and DNT. Thus, this method is worth trying in experiments. The conclusions of this work provide theoretical support for such experimental research. Graphical abstract Possible pathways of degradation by •OH radicals in advanced oxidation processes (AOPs) of the typical insensitive explosive 2,4-dinitroanisole (DNAN) were investigated by density functional theory (DFT) methods. Based on the Gibbs free energy barriers and intermediates, the dominant reaction mechanism was determined. The conclusions will be helpful in utilizing AOP technology to remove DNAN pollution.

Molecular dynamics simulations on ε-CL-20-based PBXs with added GAP and its derivative polymers.

Molecular dynamics simulations have been employed to study the ε-CL-20-based PBXs under COMPASS force field. ε-CL-20 was chosen as the base explosive due to its higher energy, density and detonation performance than conventional explosives. Four polymers, GAP, GAP-NH2, GAP-NO2 and GAP-NH2-NO2 were added into the ε-CL-20(001) crystalline surface to build the PBX models. The cohesive energy densities (CEDs), elastic coefficients, isotropic mechanical properties (Young's moduli, bulk moduli, shear moduli, Poisson's ratio, Cauchy pressure and K/G) and initiation bond length distribution were studied. It turned out that the CEDs order was A1 < A4 < A3 < A2 < A. The mechanical properties of pure ε-CL-20(001) were effectively improved by building PBX models. System A3 showed better comprehensive mechanical properties than the other three PBXs. A study on the initiation bond length distribution showed that the L max and L ave of N-NO2 increased with increasing temperature and they were related to the sensitivity of the explosives. The order of L max was A3 < A4 < A2 < A1 < A, which indicated that the PBXs owned lower sensitivity than system A. These studies are thought to provide guidance for further research on the application of GAP and its derivative polymers. Meanwhile, they are meaningful for the studies on ε-CL-20-based PBXs.

Comparative studies on structure, sensitivity and mechanical properties of CL-20/DNDAP cocrystal and composite by molecular dynamics simulation.

Molecular dynamics simulation was performed on 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), 2,4-dinitro-2,4-diazapentane (DNDAP), and CL-20/DNDAP cocrystal and composite under COMPASS force field at different temperatures. The binding energy (E bind), radial distribution function (RDF), trigger bond length, cohesive energy density (CED) and mechanical properties were studied and compared. The results show that the binding energy of the cocrystal is evidently higher than that of the composite at the same temperature. RDF analysis reveals that hydrogen bonds and vdW forces between CL-20 and DNDAP exist in both CL-20/DNDAP cocrystal and composite, and the interactions in the cocrystal are stronger than those in the composite. The maximum trigger bond length decreases in the order ε-CL-20 > CL-20/DNDAP composite > CL-20/DNDAP cocrystal. Moreover, the rigidity and stiffness of the cocrystal and composite decrease compared to that of CL-20, while the ductility and elasticity are better than that of the two pure components. These results demonstrate that CL-20/DNDAP cocrystal might be very promising in explosive applications.

Interactions between poly-(phthalazinone ether sulfone ketone) (PPESK) and TNT or TATB in polymer bonded explosives: a molecular dynamic simulation study.

The glass transition temperature (T g) and density of poly-(phthalazinone ether sulfone ketone) (PPESK A) were estimated by molecular dynamic (MD) simulation. A novel poly-(phthalazinone ether sulfone ketone) (PPESK B) was constructed by introducing nitrol and amini energetic groups into PPESK A, and T g and density were also simulated for PPESK B. The estimated T g values of PPESK A were very close to experimental results, while for PPESK B three estimated values differed by < 5 K. The interactions between explosives and polymer binders of polymer bonded explosives (PBXs) were simulated by MD. Comparison of the cohesive energy densities (CED) and solubility parameter (δ) values of PBXs, polymer binders, and mono-explosives indicate that, upon introducing polymer binders, the CED and δ values of PBXs decreased compared with those of corresponding mono-explosives. The binding energies (E bind) imply that 2,4,6-trinitrotoluene-based PBXs are more stable than 1,3,5-triamino-2,4,6-trinitrobenzene (TATB)-based PBXs. The mechanical properties, Young's modulus E, shear modulus G, bulk modulus K, Poisson's ratio γ and Cauchy pressure (C 12 -C 44) of the PBXs were assessed. The rigidity of the PBXs was found to be lower than that of mono-explosives. All K/G values were positive, indicating that PBXs are flexible. Based on these mechanical properties results, PBXs using PPESK B as a binder are superior to those using PPESK A as a binder. Due to the low C 12 -C 44 values of the PBXs, the ductility of the materials of the fracture surface is poorer, especially for TATB-based PBXs.

Porous Triphenylbenzene-Based Bicyclooxacalixarene Cage for Selective Adsorption of CO2/N2.

A bicyclooxacalixarene cage with triangular prism structure was synthesized by a one-pot SNAr reaction. The structure of the oxacalixarene cage 1 was characterized by NMR, MS spectra, and X-ray crystal structure analyses. In the solid state, the molecular cage was assembled into an interlaced porous network structure. Gas adsorption studies indicated that cage 1 exhibited high CO2/N2 selectivity of 106.

The effect of HNS on the reinforcement of TNT crystal: a molecular simulation study.

The effect of crystal modifier 2,2',4,4',6,6'-hexanitrostillbene(HNS) on the reinforcement of crystalline 1,3,5-trinitrotoluene (TNT) was investigated by molecular simulation. The intermolecular interactions between HNS and TNT were revealed by quantum chemistry calculations in detail, strong attractive forces were found between HNS and TNT. The solid interface models of TNT/HNS along three crystalline directions were studied, the distance between HNS molecule and TNT system was narrowed after optimization; the mechanical properties were calculated, showing the mechanism of the reinforcement.

Theoretical investigation on the structure and performance of N, N'-azobis-polynitrodiazoles.

Six novel high energy density compounds of N, N'-azobis-polynitrodiazoles were designed. Their optimized geometric and electronic structures, band gaps, and heats of formation were explored at B3LYP/aug-cc-pVDZ level of density functional theory (DFT). Detonation properties were predicted by Kamlet-Jacobs equations. Results show that the designed compounds have high densities (1.80 to 1.84 g · cm⁻³) and excellent detonation performance (D 8.51 to 9.02 km · s⁻¹, P 32.16 to 36.58 GPa). In addition, the bond dissociation energies of C-NO2 bonds were found to range from 223.59 to 240.46 kJ · mol⁻¹. All of them appear to be potential explosives compared with the well known ones, 1,3,5-trinitro-1,3,5-triazine (RDX, 8.75 km · s⁻¹, 34.70 GPa) and octahydro- 1,3,5,7-tetranitro-1,3,5,7-tetraazocane (HMX, 8.96 km · s⁻¹, 35.96 GPa), especially R3 (8.98 km · s⁻¹, 36.19 GPa) and R6 (9.02 km · s⁻¹, 36.58 GPa). Finally, the position and number of nitro groups in the N, N'-azobis-polynitrodiazoles determine the heat of formation, stability, sensitivity, density, and detonation performance of these compounds.

Easy methods to study the smart energetic TNT/CL-20 co-crystal.

2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) is a high-energy nitramine explosive with high mechanical sensitivity. 2,4,6-trinitrotoluene (TNT) is insensitive but by no means a high performance explosive. To reveal the significant importance and smart-material functionality of the energetic-energetic co-crystals, the stability, mechanical and explosive properties TNT/CL-20 co-crystal, TNT crystal and CL-20 crystal were studied. Non-hydrogen bonded non-covalent interactions govern the structures of energetic-energetic co-crystals. However, it is very difficult to accurately calculate the non-covalent intermolecular interaction energies. In this paper, the local conformation and the intricate non-covalent interactions were effectively mapped and analyzed from the electron density (ρ) and its derivatives. The results show that the two components TNT and CL-20 are connected mainly by nitro-aromatic interactions, and nitro-nitro interactions. The steric interactions in TNT/CL-20 could not be confronted with the attractive interactions. Moreover, the scatter graph of TNT crystal reveals the reason why TNT is brittle. The detailed electrostatic potential analysis predicted that the detonation velocities (D) and impact sensitivity for the compounds both increase in the sequence of CL-20 > TNT/CL-20 co-crystal > TNT. Additionally, TNT/CL-20 co-crystal has better malleability than its pure components. This demonstrates the capacity and the feasibility of realizing explosive smart materials by co-crystallization, even if strong hydrogen bonding schemes are generally lacking in energetic materials.

Theoretical studies on densities, stability and detonation properties of 2D polymeric complexes Cu(DAT)2Cl2 and its new analogues Zn(DAT)2Cl2.

A novel environmentally friendly octahedrally coordinated 2D polymeric complexes bis(1,5-diaminotetrazole) -dichlorozinc(II) (Zn(DAT)2Cl2) was first designed based on the the crystal data of bis(1,5-diaminotetrazole)- dichlorocopper(II) (Cu(DAT)2Cl2). Density functional theory (DFT) was used to predict the optimized geometries at TPSSTPSS/6-311G(d, p) level. Densities and detonation properties were evaluated using the electron cloud enclosed volume and VLW equation of state (VLW EOS), respectively. Calculation results show that the density of Zn(DAT)2Cl2 (2.117 g · cm(-1)) is a bit more than that of Cu(DAT)2Cl2 (2.106 g · cm(-1)). The calculated high positive heat of formation (HOF) predicts that the stabilities of the title compounds decrease in the order Zn(DAT)2Cl2 > Cu(DAT)2Cl2, which agrees with the result of bond dissociation energies (BDE). Even though they have the same molecule structures, their first scission steps are different. Furthermore, the title two compounds show good detonation velocities and pressures compared with that of bis-(5-nitro-2H-tetrazolato-N (2)) tetraamminecobalt(III) perchlorates (BNCP), and they are potential candidates for high-energy-density materials (HEDM).

Computational investigation on the new high energy density material of aluminum enriched 1, 1-diamino-2, 2-dinitroethylene.

Aluminum enriched 1, 1-diamino-2, 2-dinitroethylene (Al-FOX-7) crystal, as a new high energy density material (HEDM), was designed and investigated using grand canonical monte carlo (GCMC), NVT+NPT-molecular dynamics (MD) and GGA-PBE-density functional theory (DFT) methods. The results show that, Al atoms break out H-bond of functional group of FOX-7 crystal, and form new Al-H and Al-O bonds. Their atomic content (x) influences the surface electronic states, friction sensitivities and cj detonation properties of Al-FOX-7 crystals. While x is 4 atoms, the crystal has the highest friction sensitivities and cj detonation temperatures, which are about 1.5 times to that of FOX-7 crystal.

Synthesis, structural investigation and thermal properties of a novel manganese complex Mn2(DAT)2Cl4(H2O)4 (DAT=1,5-diaminotetrazole).

A novel manganese complex Mn(2)(DAT)(2)Cl(4)(H(2)O)(4), where DAT is 1,5-diaminotetrazole, was synthesized by the reaction of manganese(II) chloride tetrahydrate and 1,5-diaminotetrazole (DAT) in ethanol, and characterized by elemental analysis, FT-IR spectroscopy. The crystal structure was determined through X-ray single crystal diffraction. The molecular unit of Mn(2)(DAT)(2)Cl(4)(H(2)O)(4) has a distorted octahedral structure, containing two central manganese cations, which coordinated by a mono-dentate terminal chloride, a bulky DAT ligand and two water molecules, and linked by two bidentate bridging chloride ligands. There are two main exothermic peaks and one endothermic peak in the thermal decomposition process, investigated by differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA), the final residue of the title compound at 600 degrees C is MnO. The kinetic parameters of the endothermic process and two main exothermic processes were studied by applying the Kissinger's and Ozawa-Doyle's methods.

Theoretical studies on the heats of formation, densities, and detonation properties of substituted s-tetrazine compounds.

Substituted s-tetrazine compounds were designed and investigated in order to find comprehensive relationships between the structures and performances of high-nitrogen energetic compounds. Density functional theory (DFT) was used to predict the optimized geometries, electronic structures, heats of formation and densities, and the detonation properties were evaluated by using the VLW equation of state (EOS). Calculation results show that there are good linear relationships between heats of formation, densities, detonation properties and the number of N atom in all designed high-nitrogen compounds. Furthermore, several designed high-nitrogen compounds show good detonation velocities and pressures compared with octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX), making them potential candidates for high-energy-density materials (HEDM).

Theoretical studies on the tautomerism and intramolecular hydrogen shifts of 5-amino-tetrazole in the gas phase.

The tautomerism and intramolecular hydrogen shifts of 5-amino-tetrazole in the gas phase were studied in the present work. The minimum energy path (MEP) information of 5-amino-tetrazole was obtained at the CCSD(T)/6-311G**//MP2/6-311G** level of theory. The six possible tautomers of 1H, 4H-5-imino-tetrazole (a), 1H-5-amino-tetrazole (b), 2H-5-amino-tetrazole (c), 1H, 2H-5-imino-tetrazole (d), the mesoionic form (e) and 2H, 4H-5-imino-tetrazole (f) were investigated. Among these tautomers, there are 2 amino- forms, 3 imino- forms, and 1 mesoionic structure form. In all the tautomers, 2-H form (c) is the energetically preferred one in the gas phase. In the imino- tautomers, the energy value of the compound d is similar as that of the compound f but it is higher than the energy value of the compound a. The potential energetic surface (PES) and kinetics for five reactions have been investigated. Reaction 2 (b-->c) was hydrogen shifts only in which the 1-H and 2-H rearrangement. This means that the reaction 2 (b-->c) is energetically favorable having an activation barrier of 45.66 kcal.mol(-1) and the reaction energies (DeltaE) is only 2.67 kcal.mol(-1). However, the reaction energy barrier for tautomerism of reaction 1 (b-->e) is 54.90 kcal.mol(-1). Reaction 1 (b-->a), reaction 3 (c-->d), and reaction 5 (c-->f) were amino- -->imino- tautomerism reactions. The energy barriers of amino- -->imino- tautomerism reactions required are 59.39, 65.57, 73.61 kcal.mol(-1) respectively in the gas phase. The calculated values of rate constants using TST, TST/Eckart, CVT, CVT/SCT and CVT/ZCT methods using the optimized geometries obtained at the MP2/6-311G** level of theory show the variational effects are small over the whole temperature range, while tunneling effects are big in the lower temperature range for all tautomerism reactions.

Thermal reactivity of some nitro- and nitroso-compounds derived from 1,3,5,7-tetraazabicyclo[3.3.1]nonane at contamination by ammonium nitrate.

Thermal reactivity of 3,7-dinitro-1,3,5,7-tetraazabicyclo[3.3.1]nonane (DPT), 3,7-dinitroso-1,3,5,7-tetraazabicyclo[3.3.1]nonane (DNPT), 1,3,5-trinitroso-1,3,5-triazinane (TMTA or R-salt), 1,3,5-trinitro-1,3,5-triazinane (hexogen or RDX), 1,5-diacetyl-3,7-dinitro-1,3,5,7-tetrazocane (DADN), alpha-modification of the 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (octogen or HMX) and of their mixtures with 2wt.% of ammonium nitrate (AN) has been examined by means of non-isothermal differential thermal analysis. The resulting data were analyzed according to the Kissinger method. The reactivity was expressed as the E(a)R(-1) slopes of the Kissinger relationship. A relatively high reactivity has been found with mixtures of DPT and DNPT with AN. Electronic charges q(N) at nitrogen atoms in molecules of the compounds studied were calculated by means of ab initio DFT B3LYP/6-31G** method. The relationships were confirmed between the slopes E(a)R(-1) and the q(N) values for the nitrogen atoms primarily undergoing reaction. On the basis of these relationships it is stated that the destabilizing effect of AN is due to acidolytic attack of nitric acid (resulting from dissociation of ammonium nitrate) at the nitrogen atoms with the most negative q(N) values in the molecules of the compounds studied.

A new method to evaluate the stability of the covalent compound: by the charges on the common atom or group.

A new method of comparing and analyzing the electrostatic potential (ESP) charges of the common atom or group to evaluate and compare the stabilities of covalent compounds was introduced. That is, covalent compounds will become more stable when the electron acceptors accept adequate electrons and possess adequate negative charges, and the electron donors donate adequate electrons and possess adequate positive charges accordingly. All calculations were performed by density functional theory (DFT) and the general gradient approximation (GGA) method with the Beck-LYP hybrid functional and the DNP basis set in Acceryls' code Dmol3. Calculation results verified the method considering the molecular structure is well applied in the covalent molecule systems of hydrides, oxides, alkyl radicals, and nitro compounds. Furthermore, the method has good operability, for the charges can be easily obtained by simple calculation.

Investigation of correlation between impact sensitivities and nitro group charges in nitro compounds.

A new method of calculating the Mulliken net charges of the nitro group, Q(NO)()2, to assess impact sensitivities for nitro compounds is established. All calculations including optimizations and Mulliken population and frequency analyses are performed by density functional theory (DFT) and the general gradient approximation (GGA) method in Acceryls' code Dmol(3) with the Beck-LYP hybrid functional and the DNP basis set. As a result, the charges on nitro group can be regarded as a structural parameter to estimate the impact sensitivity on the bond strength, oxygen balance, and molecular electrostatic potential. The compound with more -Q(NO)()2 will be insensitive and gives a large value of impact sensitivity H(50)(). This method considering the molecular structure is applicable for almost all nitro compounds when the C-NO(2), N-NO(2), or O-NO(2) bond is the weakest in the molecule. According to the results in this paper, the compounds with -Q(NO)()2 >0.23e show H(50)()