molecules

Review CL-20-Based Cocrystal Energetic Materials: Simulation, Preparation and Performance

Wei-qiang Pang 1,2,* , Ke Wang 1, Wei Zhang 3 , Luigi T. De Luca 4 , Xue-zhong Fan 1 and Jun-qiang Li 1 1 Xi’an Modern Chemistry Research Institute, Xi’an 710065, China; [email protected] (K.W.); [email protected] (X.-z.F.); [email protected] (J.-q.L.) 2 Science and Technology on Combustion and Explosion Laboratory, Xi’an 710065, China 3 School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China; [email protected] 4 Department of Aerospace Science and Technology, Politecnico di Milano, 20156 Milan, Italy; [email protected] * Correspondence: [email protected]; Tel.: +86-029-8829-1765

Academic Editor: Svatopluk Zeman



 Received: 3 September 2020; Accepted: 17 September 2020; Published: 20 September 2020

Abstract: The cocrystallization of high-energy explosives has attracted great interests since it can alleviate to a certain extent the power-safety contradiction. 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12- hexaaza-isowurtzitane (CL-20), one of the most powerful explosives, has attracted much attention for researchers worldwide. However, the disadvantage of CL-20 has increased sensitivity to mechanical stimuli and cocrystallization of CL-20 with other compounds may provide a way to decrease its sensitivity. The intermolecular interaction of five types of CL-20-based cocrystal (CL-20/TNT, CL-20/HMX, CL-20/FOX-7, CL-20/TKX-50 and CL-20/DNB) by using molecular dynamic simulation was reviewed. The preparation methods and thermal decomposition properties of CL-20-based cocrystal are emphatically analyzed. Special emphasis is focused on the improved mechanical performances of CL-20-based cocrystal, which are compared with those of CL-20. The existing problems and challenges for the future work on CL-20-based cocrystal are discussed.

Keywords: molecular dynamic simulation; CL-20; cocrystal energetic materials; preparation; characterization

1. Introduction Energetic materials (EMs) are widely used in military and civilian applications, such as weaponry, aerospace explorations and fireworks. However, both the power and safety of energetic materials are the most concern in the field of energetic materials application, but there is an essential contradiction between them: the highly energetic materials are often not safe, and, at present, the rareness of pure low-sensitive and highly energetic explosive has been found [1–3]. For a long time, in order to obtain EMs with lower sensitivity, the modifications of existing explosives have often focused mainly on recrystallizing with solution and coating with polymer [4–6]. However, these traditional methods cannot markedly reduce the sensitivities of existing explosives by only modifying morphology or diluting power, due to the unchanging inherent structures of explosive molecules [7,8]. Due to the stringent requirements for both low-sensitivity and high-power simultaneously, the cocrystallization of explosive, a technique by which a multi-component crystal of several neutral explosive molecules forms in a defined ratio through non-covalent interactions (e.g., H-bond, electrostatic interaction, etc.) [9,10], has attracted great interest since it can alleviate, to a certain extent, the power-safety contradiction [11,12]. The new energetic cocrystals can potentially exhibit decreased sensitivity, higher performance through

Molecules 2020, 25, 4311; doi:10.3390/molecules25184311 www.mdpi.com/journal/molecules Molecules 2020, 25, 4311 2 of 19 increased packing efficiency or oxygen balance, or improve aging due to altered bonds to specific groups on constituent molecules [13–15]. Recently a lot of cocrystal explosives have been synthesized and characterized [16–20]. An evaluation of the power and safety of energetic cocrystals has been carried out, and cocrystallization is becoming increasingly hot in the field of energetic materials [21]. It was found that the stability (sensitivity) and detonation performance (energy, detonation velocity, and detonation pressure, etc.) of cocrystal explosive could be influenced by the molar ratio of molecular combination. Generally, when a cocrystal has too much content of high-energy explosives, the packing density and detonation performance will be increased, with possible increase in explosive sensitivity. On the contrary, the sensitivity will be decreased in a cocrystal explosive with a high content of low-energetic or non-energetic explosives. The searches for stable insensitive and highly-energetic explosive is the primary goal in the field of energetic material chemistry. Therefore, the molar ratios of two or more kinds of explosive components should be controlled in a reasonable scope, and it is very necessary to clarify the influence of the ratio of molecular combination on the stability and detonation performance of cocrystal explosives, such as packing density, oxygen balance, detonation velocity, and detonation pressure, etc. [22]. The compound 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) is currently known to be one of the most powerful explosives [23], and it is also a superior alternative to HMX for applications in low signature rocket propellants [24–26] and gun propellants [27]. However, the disadvantage of CL-20 is its increased sensitivity to mechanical stimuli and cocrystallization of CL-20 with other compounds, which may provide a way to decrease its sensitivity [28]. Moreover, the relationship of the reaction kinetic process with temperatures and densities for pyrolysis of CL-20/TNT cocrystal was investigated using a reactive force field (ReaxFF) molecular dynamics simulation. The evolution distribution of potential energy and total species, decay kinetics and kinetic parameters for thermal decomposition reaction of CL-20 and TNT were analyzed. It was shown that the breaking of NO bond from CL-20 molecules is the initial reaction pathway for the thermal decomposition − 2 of the cocrystal. With increasing the cocrystal density, the reaction energy barrier of CL-20 and TNT molecule decomposition increases correspondingly. The decomposition process of TNT has an inhibition action on the decomposition of CL-20 [29]. Meanwhile, the combustion behavior, flame structure, and thermal decomposition of bi-molecular crystals of CL-20 with glycerol triacetate (GTA), tris [1,2,5]oxadiazolo [3,4-b:30,40-d:30’,40’-f]azepine-7-amine (ATFAz), 4,40’-dinitro-ter-furazan (BNTF), oxepino [2,3-c:4,5-c’:6,7-c”] trisfurazan (OTF), oxepino [2,3-c:4,5-c’:6,7-c”] trisfurazan-1-oxide (OTFO) were studied [30]. It was found that the introduction of volatile and thermally stable compounds into the composition with CL-20 decreased the thermal stability of CL-20. The combustion mechanism of the CL-20 cocrystal depends both on the burning rate of the second component and its volatility. While there are few overview papers on CL-20-based cocrystal’s simulation, preparation and characterization. Thus, in this paper, the achievements in the intermolecular interaction of five types of CL-20-based cocrystals (CL-20/TNT, CL-20/HMX, CL-20/FOX-7, CL-20/TKX-50 and CL-20/DNB) by using molecular dynamic simulation are reviewed. The preparation and performance of CL-20-based cocrystals are emphatically analyzed, and the existing problems and challenges in the future work are reviewed. The aim of this work is mainly to explore the nature of the formation of the different CL-20-based cocrystal morphology and clarify the influence of the ratio of molecular combination on the stability and detonation performance of CL-20-based cocrystal explosives.

2. Modelling and Simulation

2.1. Molecular Structures of Cocrystal Monomer In order to simulate the properties of cocrystals, the molecular structures of each material can be constructed. Table1 lists the molecular structures of di fferent energetic materials formed cocrystals. Molecules 2020, 25, 4311 3 of 19

Molecules Table2020, 25Molecules, 1.x FORMolecular PEER2020, 25REVIEW, x FOR structures PEER REVIEW of di fferent energetic materials formed cocrystals.3 of 20 3 of 20 Molecules 2020, 25Molecules, x FOR PEER2020, 25REVIEW, x FOR PEER REVIEW 3 of 20 3 of 20 Molecules 2020, 25Molecules, x FOR PEER2020, 25REVIEW, x FOR PEER REVIEW 3 of 20 3 of 20 Samples Molecular Structures Samples Molecular Structures Samples Samples Molecular Structures Molecular Structures Samples Molecular Samples Structures Molecular Structures Samples Samples Molecular Structures Molecular Structures Samples Molecular Samples Structures Molecular Structures Samples Samples Molecular Structures Molecular Structures Samples Molecular Samples Structures Molecular Structures

CL-20CL-20 CL-20 TNT TNT TNT CL-20 CL-20 TNT TNT CL-20 CL-20 TNT TNT

HMX HMX FOX-7 FOX-7 HMXHMX HMX FOX-7 FOX-7 FOX-7 HMX HMX FOX-7 FOX-7

TKX-50 TKX-50 DNB DNB TKX-50 TKX-50 DNB DNB TKX-50TKX-50 TKX-50 DNB DNB DNB

Simulation details:Simulation Andersen details: was Andersen set as the was temperature set as the controltemperature method control (298.15 method K). COMPASS (298.15 K). COMPASS Simulation details:Simulation Andersen details: was Andersen set as the was temperature set as the temperaturecontrol method control (298.15 method K). COMPASS (298.15 K). COMPASS force-fieldSimulation wasforce-field assigned. details:Simulation wasAndersen Summation assigned. details: was Andersen methods Summationset as the forwas temperature methodselectrostatic set as the for controltemperature andelectrostatic van method der control andWaals (298.15 van method were K).der COMPASSEwaldWaals (298.15 andwere K). COMPASSEwald and force-field wasforce-field assigned. was Summation assigned. methodsSummation for methodselectrostatic for andelectrostatic van der andWaals van were der EwaldWaals andwere Ewald and Simulationforce-fieldAtom-based, details:was force-fieldAtom-based,respectively. assigned. Andersen was Summation respectively.The assigned. accuracy was methodsSummation set The for as accuracythe for Ewaldelectrostaticmethods temperature for method the for Ewaldandelectrostatic was controlvan method1.0 der × andWaalsmethod10 was− 4van kcal·mol were 1.0 der (298.15× Ewald Waals10−1. −4Cutoff kcal·mol andwere K). COMPASS Ewald−1. Cutoff and Atom-based, Atom-based,respectively. respectively.The accuracy The for accuracythe Ewald for method the Ewald was method1.0 × 10 was−4 kcal·mol 1.0 × 10−1. −4Cutoff kcal·mol −1. Cutoff Atom-based,distance and Atom-based,distancebufferrespectively. width and bufferrespectively.forThe the accuracy width Atom-based forThe for the accuracythe methodAtom-based Ewald forwere method the method 15.5 Ewald Åwas andwere method1.0 2.0 15.5 × Å, 10 Årespectively;was−4 andkcal·mol 1.0 2.0 × Å, 10−1 .respectively; 1.0−4Cutoff kcal·mol f. of −1 . 1.0Cutoff f. of force-fielddistance was and assigned. distancebuffer width and Summation bufferfor the width Atom-based methods for the Atom-basedmethod for electrostatic were method 15.5 Å andwere and 2.0 15.5 van Å, Årespectively; derand 2.0 Waals Å, respectively;1.0 were f. of Ewald 1.0 f. and of distancetime step and was distancetimebuffer set forstep width MDand was processes, bufferfor set thefor width Atom-basedMD and processes, for the the tota methodAtom-basedl anddynamic the were tota timemethod 15.5l dynamic was Å andwere perfor time2.0 15.5 medÅ, was Årespectively; withand perfor 2.0100,000 med4Å, respectively;1.0 withfs. f. All of 100,000 1 1.0 fs. f. All of Atom-based,time step respectively. wastime set forstep MD was The processes, set accuracy for MD and processes, the for tota thel anddynamic Ewald the tota time methodl dynamic was perfor was timemed 1.0was with perfor10 100,000−medkcal withfs. molAll 100,000 − . Cutofs. All ff timethe MD step calculations wastimethe set MD forstep were calculationsMD was processes,carried set for wereout MD and with processes,carried the MS tota 7.0outl and [31].dynamicwith the MS tota time7.0l [31].dynamic was perfor timemed was with perfor 100,000med withfs. All 100,000 fs. All the MD calculationsthe MD werecalculations carried wereout with carried MS 7.0out [31].with MS 7.0 [31]. × · distancethe and MDBecause bu calculationsffer thethe width MD bindingBecause were calculations for carriedenergiesthe the binding Atom-based wereout of withcocrystals carriedenergies MS 7.0out ofobtained method [31].withcocrystals MS from were7.0 obtained [31].the 15.5 MD from Åcalculations and the 2.0MD Å,arecalculations respectively;of different are of different 1.0 f. of Because the bindingBecause energiesthe binding of cocrystals energies ofobtained cocrystals from obtained the MD from calculations the MD arecalculations of different are of different time stepnumbers wasBecause set of formolecules, numbersthe MD bindingBecause processes, of i.e., molecules, energies thedifferent binding and of i.e.,supercells cocrystals energies thedifferent total and of obtainedsupercells dynamicdifferentcocrystals from andmolecular timeobtained differentthe wasMD molar from calculations molecular performed rathetios, MD molarthose arecalculations with ofof ra differentthetios, 100,000 same those are ofof fs. thedifferent All same the numbers of molecules,numbers of i.e., molecules, different i.e.,supercells different and supercells different andmolecular different molar molecular ratios, molarthose of ra thetios, same those of the same numbersnumber ofof molecules,moleculesnumbersnumber ofof i.e.,should molecules,molecules different be adopted i.e.,supercellsshould different asbe and aadopted supercellsstandard different as for andmoleculara standardassessing different molar for molecularthe assessing racomponenttios, thosemolar the of interactionracomponent thetios, same those ofinteraction the same MD calculationsnumber of weremoleculesnumber carried of should molecules out be with adopted should MS 7.0asbe aadopted[ 31standard]. as fora standardassessing for the assessing component the interactioncomponent interaction numberstrengths of and moleculesnumberstrengths stabilities of andshould ofmolecules cocrystals.stabilities be adopted should ofTherefore, cocrystals. asbe aadopted accordingstandard Therefore, as fortoa ouraccording standardassessing recent forto investigationthe our assessing component recent investigation[32],the interaction componentan energy [32], interaction an energy Becausestrengths the and bindingstrengths stabilities energiesand of cocrystals.stabilities of ofcocrystalsTherefore, cocrystals. according Therefore, obtained to ouraccording from recent the toinvestigation our MD recent calculations investigation [32], an energy are [32], of an di energyfferent strengthscorrection and formulastrengthscorrection stabilities for andbinding formulaof cocrystals.stabilities energy for binding ofTherefore, was cocrystals. used energy according to Therefore, standardizewas used to ouraccording to (uniform) standardizerecent toinvestigation ourthe (uniform) recentdifferences investigation [32], the causedan differences energy by [32], causedan energy by correction formulacorrection for binding formula energy for binding was used energy to standardizewas used to (uniform)standardize the (uniform) differences the caused differences by caused by numberscorrectiondiverse of molecules, supercells formulacorrectiondiverse and for i.e., supercells bindingdifferent formula different energyand mofor lecularbindingdifferent supercells was molar used energy mo lecular toratiosand standardizewas asmolar used di follows:fferent toratios (uniform)standardize molecularas follows: the (uniform) differences molar the caused ratios,differences by those caused of by the diverse supercellsdiverse and supercells different and mo leculardifferent molar molecular ratios asmolar follows: ratios as follows: same numberdiverse ofsupercells moleculesdiverse and supercells different should and mo be leculardifferent adopted molar mo aslecular ratios a standard asmolar follows: ratios for as assessing follows: the component interaction Eb* = Eb·N0/Ni,E b* = Eb·N0/Ni, (1) (1) b* b 0 i b* b 0 i E = E ·N /N ,E * = E ·N /N , (1) (1) strengths and stabilities of cocrystals. Therefore,Eb* = Eb according·N0/Ni,E b = Eb to·N0 our/Ni, recent investigation(1) [32 ], an energy(1) where Eb* denoteswhere the Eb *binding denotes energy the binding after corrected,energy after and corrected, Ni and N and0 are N thei and number N0 are of the molecules number offor molecules for b* b* i 0 i 0 correctionwhere formula E * denoteswhere for the binding E *binding denotes energy energy the binding after was corrected,energy used after to and standardize corrected, N and N and are (uniform) N the and number N are the of the molecules dinumberfferences offor molecules caused for by wheredifferent Eb supercells denoteswheredifferent the andEb bindingsupercells denotes a standard energy the and bindingpattern a after standard (molarcorrected,energy pattern ratio after and in (molarcorrected, 1:1),Ni and respectively. ratio N and0 arein 1:1),N thei and numberrespectively.The N 0binding are of the molecules energynumberThe binding E offorb is molecules energy E forb is different supercellsdifferent and supercells a standard and pattern a standard (molar pattern ratio in (molar 1:1), respectively. ratio in 1:1), respectively.The binding energyThe binding Eb is energy Eb is diverse supercellsdifferentcalculated supercells by anddifferentcalculated the following di andff supercellserent bya standard the formula: molecular following and pattern a standard formula: molar(molar pattern ratio ratios in (molar 1:1), as follows: respectively. ratio in 1:1), respectively.The binding energyThe binding Eb is energy Eb is calculated bycalculated the following by the formula: following formula: calculated bycalculated the following by the formula: following formula: Eb = Etot − (nECL-20Eb = +E mtot E− CM(n),E CL-20 + mECM), (2) (2) Eb = EEtot − (nEECL-20Eb =N +E mtot EN− CM(n),E CL-20 + mECM), (2) (2) Eb = Etotb −* (n=ECL-20Ebb = +E m0tot/ E− CM(ni,),E CL-20 + mECM), (2) (2) (1) where Etot, ECL-20where or ECMtot, isECL-20 single or EpointCM is energysingle pointof cocrystal energy· or of monomer;cocrystal or n monomer;and m are then and number m are ofthe number of where Etot, ECL-20where or ECMtot, isECL-20 single or EpointCM is energysingle pointof cocrystal energy or of monomer;cocrystal or n monomer;and m are then and number m are ofthe number of wheremonomers Etot, EinCL-20wheremonomers cocrystal. or ECMtot, isEinCL-20 singlecocrystal. or EpointCM is energysingle pointof cocrystal energy or of monomer;cocrystal or n andmonomer; m are then and number m are ofthe number of where Emonomers* denotes inmonomers thecocrystal. binding in cocrystal. energy after corrected, and N and N are the number of molecules for bmonomers inmonomers cocrystal. in cocrystal. i 0 different2.2. supercells CL-20/TNT2.2. and System CL-20/TNT a standard System pattern (molar ratio in 1:1), respectively. The binding energy Eb is 2.2. CL-20/TNT2.2. System CL-20/TNT System 2.2. CL-20/TNT2.2. System CL-20/TNT System calculated byCL-20 the followingis a complexCL-20 formula:iscage a complex compound, cage whichcompound, can be which regarded can beas aregarded six-membered as a six-membered ring and two ring and two CL-20 is a complexCL-20 iscage a complex compound, cage whichcompound, can be which regarded can beas aregarded six-membered as a six-membered ring and two ring and two five-memberedCL-20 isfive-membered a complexringsCL-20 arranged iscage a complexrings compound, together, arranged cage and whichcompound, together, it has can six beand nitrowhich regarded it hasgroups. can six asbe nitro Thearegarded six-membered preparationgroups. as Thea six-membered ofpreparationring CL-20/TNT and two ofring CL-20/TNT and two five-memberedfive-membered rings arranged rings together, arranged and together, it has six and nitro it hasgroups. six nitro The preparationgroups. The ofpreparation CL-20/TNT of CL-20/TNT five-memberedcocrystal by five-memberedcocrystalthe rings solvent arranged by methodthe rings solvent together, E hasarranged= attractedmethodE andtot together, it has has(nmuchE attractedsix and attentionnitro it+ hasgroups.muchmE worldwidesix attentionnitro The), preparationgroups. due worldwide to The its of preparationhigh-energy, CL-20/TNTdue to its of high-energy, CL-20/TNT (2) cocrystal by cocrystalthe solvent by methodthe solvent hasb attractedmethod− has much attractedCL-20 attention much worldwideCM attention due worldwide to its high-energy, due to its high-energy, cocrystalinsensitive by and cocrystalinsensitivethe cheap solvent explosives. by and methodthe cheap solvent Thehas explosives. densityattractedmethod and Thehas much melting densityattracted attention point and much melting of worldwide the attention CL-20/TNT point dueof worldwide the tococrystal CL-20/TNT its high-energy, due explosive tococrystal its high-energy, explosive insensitive andinsensitive cheap explosives. and cheap The explosives. density and The melting density point and melting of the CL-20/TNT point of the cocrystal CL-20/TNT explosive cocrystal explosive insensitive(1.91 cm, 136 andinsensitive(1.91 °C) cheap cm,are explosives. between136 and °C) cheap areCL-20 The explosives. between density (2.04 g/cmandCL-20 The melting 3density, 210(2.04 °C) point g/cmand and melting 3of, 210 TNTthe CL-20/TNT°C) point(1.70 and ofg/cm TNTthe cocrystal3 CL-20/TNT, 81(1.70 °C), g/cm explosive and cocrystal3, the81 °C), explosive and the where Etot(1.91, E cm,CL-20 136or(1.91 °C)ECM cm,areis between136 single °C) areCL-20 point between (2.04 energy g/cmCL-20 of3, 210(2.04 cocrystal °C) g/cm and3, or 210TNT monomer; °C) (1.70 and g/cm TNTn3, and81(1.70 °C), mg/cm and are3, the81 °C), number and the of (1.91sensitivity cm, 136 is (1.91sensitivitynearly °C) cm,are double between136 is nearly °C)reduced areCL-20 double betweenthat (2.04 reducedof CL g/cmCL-20-20 that 3[11]., 210(2.04 of Figure CL°C) g/cm-20 and 13[11]., shows 210TNT Figure°C) (1.70the and intermolecular 1 g/cm shows TNT3, 81(1.70the °C), intermolecular g/cmhydrogen and3, the81 °C), hydrogen and the monomerssensitivity in cocrystal. is sensitivitynearly double is nearly reduced double that reducedof CL-20 that [11]. of Figure CL-20 1[11]. shows Figure the intermolecular 1 shows the intermolecular hydrogen hydrogen sensitivitybond of CL-20/TNT is sensitivitynearlybond of doublecocrystal. CL-20/TNT is nearly reduced doublecocrystal. that reducedof CL-20 that [11]. of Figure CL-20 1[11]. shows Figure the intermolecular 1 shows the intermolecular hydrogen hydrogen bond of CL-20/TNTbond of cocrystal. CL-20/TNT cocrystal. bond of CL-20/TNTbond of cocrystal. CL-20/TNT cocrystal. 2.2. CL-20/TNT System CL-20 is a complex cage compound, which can be regarded as a six-membered ring and two

five-membered rings arranged together, and it has six nitro groups. The preparation of CL-20/TNT cocrystal by the solvent method has attracted much attention worldwide due to its high-energy, insensitive and cheap explosives. The density and melting point of the CL-20/TNT cocrystal explosive 3 3 (1.91 cm, 136 ◦C) are between CL-20 (2.04 g/cm , 210 ◦C) and TNT (1.70 g/cm , 81 ◦C), and the sensitivity is nearly double reduced that of CL-20 [11]. Figure1 shows the intermolecular hydrogen bond of CL-20/TNT cocrystal. Molecules 2020, 25, 4311 4 of 19

Molecules 2020, 25, x FOR PEER REVIEW 4 of 20

FigureMolecules 1. IntermolecularFigure 2020 ,1. 25 Intermolecular, x FOR PEER hydrogen REVIEW hydrogen bond bond of of CL-20 CL-20/TNT/TNT co cocrystalcrystal [8], [with8], with permission permission from Han from Neng4 Han of 20 Neng Cai Liao, 2012.Cai Liao, 2012.

The non-bondThe non-bond distances distances of O of (3),O (3), H H (5) in in CL-20 CL-20 and andH (8), H O (13) (8), in O TNT (13) are in 0.231 TNT nm areand 0.2310.244 nm and nm, respectively, which are smaller than the sum of their van der Waals radii (0.272 nm [33]), 0.244 nm,indicating respectively, the existence which areof intermolecular smaller than hydr theogen sum bonds. of their Through van the der interaction Waals radii of C-H…O (0.272 nm [33]), indicatinghydrogen the existence bond between of intermolecular the intermolecular, hydrogen CL-20 and bonds. TNT molecules Through are connected the interaction as the form of of C-H ... O hydrogen“zigzag bond between chain” structure, the intermolecular, and combined CL-20to form and the TNTstable moleculesstructure of areCL-20/TNT connected cocrystal, as the form of indicating that the intermolecular hydrogen bond makes the arrangement of CL-20 and TNT “zigzag chain” structure, and combined to form the stable structure of CL-20 /TNT cocrystal, indicating molecules in the crystal more regular, and the molecular packing is close with higher crystal density that the intermolecular(1.84Figure g·cm− 31.). Intermolecular hydrogen hydrogen bond bond makes of CL-20/TNT the arrangement cocrystal [8], with of permission CL-20 and from TNTHan Neng molecules in the 3 crystal more regular,CaiMeanwhile, Liao, 2012. and the the cohesive molecular energy packing density is(CED) close of withCL-20/TNT higher cocrystal crystal and density its CL-20/TNT (1.84 g cm ). · − composite mixture at different temperatures were investigated [11], results show in Table 2 and Meanwhile,The non-bond the cohesive distances energy of O (3), density H (5) in CL-20 (CED) and ofH (8), CL-20 O (13)/TNT in TNT cocrystal are 0.231 nm and and its0.244 CL-20 /TNT Figure 2. With the increase of temperature, CED and its component VDW force and electrostatic force compositenm, mixture respectively, at diff erentwhich temperaturesare smaller than were the sum investigated of their van [11 der], results Waals radii show (0.272 in Table nm 2[33]), and Figure2. decrease gradually, which is negatively related to the law that the sensitivity increases gradually with indicating the existence of intermolecular hydrogen bonds. Through the interaction of C-H…O With the increasethe increase of temperature,of temperature. CEDUnder andcertain its conditio componentns, CED, VDW which force reflects and the electrostaticenergy required force for decrease hydrogen bond between the intermolecular, CL-20 and TNT molecules are connected as the form of gradually,phase which transition, is negatively can be used related to determine to the the law relative that thermal the sensitivity sensitivity. increases gradually with the “zigzag chain” structure, and combined to form the stable structure of CL-20/TNT cocrystal, increase of temperature. Under certain conditions, CED, which reflects the energy required for phase indicatingTable 2.that The the cohesive intermolecular energy density hydrogen (CED) of bondCL-20/TNT makes cocrystal the arrangement and its CL-20/TNT of CL-20 composite and at TNT transition, can be used to determine the relative thermal sensitivity. moleculesdifferent in thetemperatures crystal more [11], regular,with permission and the from molecular Han Neng packing Cai Liao, is close 2016. with higher crystal density (1.84 g·cm−3). −3 −3 −3 Meanwhile,Samples the cohesiveT /Kenergy VdWdensity Energy/kJ·cm (CED) of CL-20/TNTElectrostatic Energy/kJ·cmcocrystal and itsCED/kJ·cm CL-20/TNT Table 2. The cohesive energy density195 (CED) 0.37 of CL-20(0.01) /TNT cocrystal 0.54 and(0.01) its CL-20/TNT 0.91 (0.01) composite at differentcomposite temperatures mixture [at11 ],different with245 permission te mperatures 0.36 from were(0.00) Han investigated Neng Cai [1 Liao, 0.521], (0.01)results 2016. show in Table 0.88 (0.01) 2 and FigureCL-20/TNT 2. With thecocrystal increase of temperature,295 CED 0.36 (0.00) and its component VDW 0.50 (0.01) force and electrostatic 0.86 (0.01) force decrease gradually, which is negatively345 related 0.35 (0.00)to the3 law that the sensitivity 0.48 (0.01) increases gradually3 0.83 (0.01) with 3 Samples T/K VdW Energy/kJ cm− Electrostatic Energy/kJ cm− CED/kJ cm− the increase of temperature. Under395 certain 0.34conditio· (0.00) ns, CED, which 0.47 reflects (0.01) the energy· required 0.81 (0.01) for · phaseCL-20/TNT transition, composite195 can be used to 295 determine 0.37 (0.01) the 0.31 relative(0.01) thermal sensitivity. 0.45 0.54 (0.01) (0.01) 0.76 (0.01) 0.91 (0.01) Deviations are listed in the parentheses. 245 0.36 (0.00) 0.52 (0.01) 0.88 (0.01) CL-20/TNTTable 2. The cohesive energy density (CED) of CL-20/TNT cocrystal and its CL-20/TNT composite at cocrystal different temperatures295 [11], with 0.36permission (0.00) from Han Neng Cai Liao, 0.50 2016. (0.01) 0.86 (0.01)

Samples345 T/K 0.35VdW (0.00) Energy/kJ·cm−3 Electrostatic 0.48 Energy/kJ·cm (0.01)−3 CED/kJ·cm 0.83−3 (0.01) 395195 0.34 (0.00) 0.37 (0.01) 0.54 0.47 (0.01) (0.01) 0.91 (0.01) 0.81 (0.01) 245 0.36 (0.00) 0.52 (0.01) 0.88 (0.01) CL-20/TNTCL-20/TNT cocrystal 295 0.36 (0.00) 0.50 (0.01) 0.86 (0.01) 295 0.31 (0.01) 0.45 (0.01) 0.76 (0.01) composite 345 0.35 (0.00) 0.48 (0.01) 0.83 (0.01) 395 0.34 (0.00) 0.47 (0.01) 0.81 (0.01) CL-20/TNT composite 295Deviations are 0.31 listed (0.01) in the parentheses. 0.45 (0.01) 0.76 (0.01) Deviations are listed in the parentheses.

Figure 2. CED (a), vdw (b) andelectrostatic (c) energies of CL-20/TNT cocrystal vs. temperatures [11], with permission from Chemical Journal of Chinese Universities, 2016.

2.3. CL-20/HMX System

Figure 2. CEDFigure (a), 2. CED vdw (a), (b) vdw andelectrostatic (b) andelectrostatic (c) (c) energiesenergies of of CL-20/TNT CL-20/TNT cocrystal cocrystal vs. temperatures vs. temperatures [11], [11], with permissionwith permission from Chemical from Chemical Journal Journal of of Chinese Chinese Universities, 2016. 2016.

2.3. CL-20/HMX System

Molecules 2020, 25, 4311 5 of 19

2.3. CL-20/HMX System For HMX, if CL-20 and HMX can form the cocrystal in a certain proportion, the sensitivity of CL-20 can be significantly reduced on the basis of its energy reduce little; on the other hand, the cost of explosive can be significantly reduced and its application range can be expanded [32]. The properties of hexanitrohexaazaisowurtzitane (CL-20)/cyclotetramethylenete-tranitramine (HMX) cocrystal were simulated and compared with those of CL-20/HMX mixture with the molar ratio of CL-20 and HMX as 2:1 [2]. Simulation and calculation results show that the cocrystal process of CL-20/HMX can significantly improve the anti-deformation ability and ductility of the system, and the tensile modulus of the cocrystal structure is greater than that of the mixture. The maximum bond length (Lmax) decreases in the order of: [CL-20/HMX mixture] > [ε-CL-20] > [β-HMX] > [CL-20/HMX cocrystal]. The structure of CL-20/HMX mixture is sensitized by the predominant interaction of Van der Waals force (Table3), showing that the two components in the mixture system can be stably adsorbed and have good physical compatibility. The cohesive energy density (CED) value of CL-20/HMX cocrystal structure is far greater than that of CL-20/HMX mixture, and with the increase of temperature, the cohesive energy density of the two components decreases and the structural stability becomes worse (Table4). The low sensitivity of CL-20 /HMX cocrystal system is caused by the existence of hydrogen bond CH–O with relatively short length.

Table 3. The binding energy between each component of CL-20/HMX mixture, kJ mol 1 [2], · − with permission from Han Neng Cai Liao, 2016.

Interaction Etotal ECL-20 EHMX Einter Ebind E 25,926.37 6256.29 395.88 5718.20 5718.20 − − − − vdW 1656.07 632.68 1187.37 3363.27 3363.27 − − Electrostatic 23,541.63 7939.87 15,181.47 420.30 420.30 − − − − Etotal is the single point energy of the equilibrium structure, ECL-20 is the single point energy of ε-CL-20, EHMX is the single point energy of HMX, E is the total energy of each structure, vdW is the energy of each structure obtained by vdW interaction, electrostatic is the energy of each structure obtained by electrostatic interaction, Ebind is binding energy of CL-20 with HMX, Einter is interaction energy of CL-20 with HMX.

Table4. CED of CL-20/HMX cocrystal and mixture systems at different temperatures [2], with permission from Han Neng Cai Liao, 2016.

T/K Samples Parameters 200 250 298 350 400 CED 1.167 1.162 1.166 1.151 1.145 CL-20/HMX vdW 0.080 0.072 0.074 0.054 0.054 cocrystal electrostatic 1.087 1.090 1.092 1.097 1.091 CED 0.069 0.036 0.032 - 0.021 CL-20/HMX vdW 0.034 mixture electrostatic 0.035 0.038 0.036 0.034 0.036

2.4. CL-20/FOX-7 System For CL-20/FOX-7 cocrystal, the unit cell models of CL-20 and FOX-7 were constructed according to their experimental cell parameters, respectively. Initial models were obtained by discovering module in the COMPASS force field; 1.0 10 5 kcal mol 1 of accuracy was required. The CL-20 and FOX-7 × − − crystal morphologies in vacuum were predicted by the Growth morphology model. Different cocrystal molar ratios can be treated by the substituted method: molecules of CL-20 supercells were substituted by an equal number of FOX-7 at molar ratios of 8:1, 5:1, 3:1, 2:1, and 1:1 (CL-20/FOX-7). Molecules of FOX-7 supercells were substituted by an equal number of CL-20 (ε-, γ- and β-forms) at molar ratios Molecules 2020, 25, 4311 6 of 19 of 1:2, 1:3, 1:5, and 1:8 (CL-20/FOX-7). Substituted molecules in this method were determined by the Miller indices hkl. For the substituted models, NVT ensembles were selected. Five growth faces (0 1 1), (1 01), (1 0 -1), (0 0 2), and (1 1 -1) and random face of ε-, γ- and β-CL-20 were selected to study the binding energies of the cocrystals with FOX-7 in different molar ratios. These growth faces and a random face of FOX-7were selected to study the binding energies of * cocrystals. Based on the MD simulation of substituted models, Eb of the ε-, γ-, and β-CL-20 cocrystal explosives with FOX-7 on the different cocrystal faces in the different molar ratios are calculated (Table5). As can be seen, except for the random trend, there is a trend that the strongest binding energies * Eb in the γ-CL-20/FOX-7 are larger than those in ε-CL-20/FOX-7 and β-CL-20/FOX-7. Furthermore, * for γ-CL-20/FOX-7, the binding energies Eb on the (1 1 0) and (1 0 -1) cocrystal faces of FOX-7 in 1:2 and those on the (1 0 -1) and (1 1 0) faces of γ-CL-20 in 1:1 are larger than the other cases. These results indicate that FOX-7 may prefer cocrystalizing with γ-CL-20 on the (1 1 0) and (1 0 -1) faces of FOX-7 in 1:2, or on the (1 0 -1) and (1 1 0) faces of γ-CL-20 in 1:1. It is noted that, as mentioned above, for the cocrystal with the excess ratio of FOX-7, the FOX-7 supercells were substituted by an equal number of CL-20, while in the cocrystal with the excess ratio of CL-20, the CL-20 supercells were substituted by FOX-7.

Table 5. The corrected binding energy (in kJ mol 1) of the substituted models of CL-20/FOX-7 [10], · − with permission from the Journal of Mol Model, 2016.

8:1 5:1 3:1 2:1 1:1 1:2 1:3 1:5 1:8 (0 1 1) 337.2 625.4 982.3 1204.2 1165.2 1262.8 1224.7 844.1 607.2 − − − − − − − − − (1 1 0) 303.0 578.6 748.5 1125.2 1383.6 1417.7 1232.2 1115.3 642.2 − − − − − − − − − (1 0 -1) 348.2 619.5 807.7 1125.9 - 1183.1 1189.3 806.4 677.0 ε − − − − − − − − (0 0 2) 323.2 523.8 839.4 946.4 1136.5 1206.7 787.9 728.4 761.1 − − − − − − − − − (1 1 -1) 365.2 490.0 967.1 1079.2 1290.4 1074.0 1267.2 911.7 624.6 − − − − − − − − − (0 2 1) 372.5 572.9 964.2 1031.1 1368.5 − − − − − (1 0 1) 416.6 523.0 938.2 1184.7 1422.7 1122.6 927.9 871.6 681.2 − − − − − − − − − Random 381.5 581.4 946.3 1097.3 1589.0 1361.4 1446.0 1078.0 822.1 − − − − − − − − − (0 1 1) 393.6 503.1 918.0 1023.5 1231.6 1288.0 1140.1 1028.2 518.3 − − − − − − − − − (1 1 0) 363.5 511.2 940.2 1101.6 1456.7 1605.1 1387.0 1179.4 735.1 − − − − − − − − − (1 0 -1) 452.6 543.1 815.2 1128.4 1489.2 1588.2 1311.8 1056.6 726.0 γ − − − − − − − − − (0 0 2) 428.3 511.6 762.3 878.5 1003.9 1257.3 1157.3 967.8 732.3 − − − − − − − − − (1 1 -1) 420.3 493.1 659.3 815.2 979.2 1137.0 1024.5 922.3 657.1 − − − − − − − − − (0 2 1) 466.0 526.3 762.1 912.1 1218.1 − − − − − (1 0 1) 431.8 527.6 988.6 1215.1 1345.5 1322.1 1252.3 977.8 511.6 − − − − − − − − − Random 378.9 515.8 927.5 1288.4 1587.2 1369.7 1201.5 835.6 417.5 − − − − − − − − − (0 0 1) 367.0 578.9 913.4 1007.5 1137.6 β − − − − − (0 1 0) 389.1 618.0 985.6 1123.7 1165.2 − − − − − Random 408.5 579.3 866.2 1252.8 1443.6 1532.7 1276.8 828.1 497.6 − − − − − − − − −

2.5. CL-20/TKX-50 System TKX-50 is an ionic salt structure with two hydroxylamine cations, and it can easily form hydrogen + bonds between H of -NH3 in TKX-50 and O of –NO2 in CL-20, which is in good agreement with the results of surface electrostatic potential energy analysis. The surface electrostatic potential energy analysis of CL-20 and TKX-50 is useful for exploring the formation mechanism of CL-20/TKX-50 cocrystal. CL-20/TKX-50 cocrystals with the mole ratio of 1:1, 1:2, 1:3 and 2:3 were prepared by means of the solvent–nonsolvent method [34]. Compared with the raw materials and the mixture, the XRD spectra of CL-20/TKX-50 cocrystals have no obvious change when the feed ratio is 1:1, 1:3 and Molecules 2020, 25, 4311 7 of 19

2:3, and it can be considered that the eutectic of CL-20/TKX-50 cocrystals has not been successfully prepared. Finally, the optimal mole ratio is 1:2. Figure3 shows the surface electrostatic potential energy distribution map of CL-20 (a) and TKX-50 (b), and the equilibrium structure of CL-20/TKX-50 cocrystal Moleculesmodel (c),2020 a, 25 schematic, x FOR PEER diagram REVIEW of intermolecular interaction of CL-20 and TKX-50 model (d). 7 of 20

Molecules 2020, 25, x FOR PEER REVIEW 7 of 20

FigureFigureFigure 3. 3. SurfaceSurface 3. Surface electrostatic electrostatic electrostatic potential potentialpotential en en energyergyergy distributiondistribution distribution map map map of of CL-20 CL-20 of CL-20 (a) ( anda) (anda TKX-50) and TKX-50 TKX-50 (b), and(b), the (andb), andthe equilibriumthe equilibriumequilibrium structure structure structure of CL-20/TKX-50of ofCL-20/TKX-50 CL-20/TKX-50 cocrystal cocrystal cocrystal model model ( (cc),), CL-20 CL-20 (c), and CL-20 and TKX-50 TKX-50 and models TKX-50 models (d) models [34],(d) [34], with (d )[with 34], permissionwithpermission permission from from from Huo Huo Huo Zha Zha ZhaYao Yao YaoXue XueXue Bao, Bao, Bao, 2020. 2020. 2020. 2.6. CL-20/DNB System 2.6. CL-20/DNB2.6. CL-20/DNB System System As aAs cheap a cheap and and insensitive insensitive explosive, explosive, DNB DNB is is often often usedused as an alternative explosive explosive for for TNT, TNT, but but its As a cheap and insensitive explosive, DNB is often used as an alternative explosive for TNT, but energyits isenergy not ideal. is not Ifideal. CL-20 If CL-20 and DNB and DNB can becan combined be combined in the in the same same lattice lattice through through noncovalent noncovalent bond its energy is not ideal. If CL-20 and DNB can be combined in the same lattice through noncovalent at thebond molecular at the molecular level through level cocrystalthrough cocrystal technology technology to form to an form explosive an explosive crystal crystal with uniquewith unique structure, bond at the molecular level through cocrystal technology to form an explosive crystal with unique it is expectedstructure, thatit is theexpected sensitivity that th ande sensitivity cost of CL-20 and willcost beof greatlyCL-20 will reduced be greatly without reduced significant without energy structure, it is expected that the sensitivity and cost of CL-20 will be greatly reduced without reduction,significant so as energy to expand reduction, the so application as to expand range the application of CL-20 range [12]. of In CL-20 order [12]. to improve In order to the improve hazardous significantthe hazardous energy reduction,performance so of as CL-20, to expand 1,3-dinitrobenze the application (DNB) range was introduced of CL-20 [12].to CL-20 In order to form to improveCL- performance of CL-20, 1,3-dinitrobenze (DNB) was introduced to CL-20 to form CL-20/DNB cocrystal the hazardous20/DNB cocrystal performance with 1:1 ofmole CL-20, ratio. 1,3-dinitrobenzeThe primitive cell of(DNB) CL-20/DNB was introduced cocrastal was to shown CL-20 in to Figure form CL- with 1:1 mole ratio. The primitive cell of CL-20/DNB cocrastal was shown in Figure4. 20/DNB4. cocrystal with 1:1 mole ratio. The primitive cell of CL-20/DNB cocrastal was shown in Figure 4.

FigureFigure 4. The 4. The primitive primitive cell cell of CL-20of CL-20/DNB/DNB cocrystal cocrystal [3 ],[3], with with permission permission from from Han Han Neng Neng CaiCai Liao,Liao, 2015. 2015. The molecular dynamics simulation of 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (CL-20)/1,3- FigureThe 4. molecularThe primitive dynamics cell of simulation CL-20/DNB of cocrystal2,4,6,8,10,12-hexanitrohexaazaisowurtzitane [3], with permission from Han Neng (CL-20)/1,3- Cai Liao, dinitrobenzene (DNB) cocrystal and CL-20/DNB cocrystal with two polymer binders, hydroxyl- 2015.dinitrobenzene (DNB) cocrystal and CL-20/DNB cocrystal with two polymer binders, hydroxyl- terminated polybutatiene (HTPB) and polyethylene glycol (PEG) respectively were conducted [3]. terminated polybutatiene (HTPB) and polyethylene glycol (PEG) respectively were conducted [3]. ResultsTheResults indicate molecular indicate that thatdynamics the the binding binding simulation energy energy ofof CL-20CL-20/2,4,6,8,10,12-hexanitrohexaazaisowurtzitane/DNB/PEGDNB/PEG is islarger larger than than that that of CL-20/DNB/HTPB, of CL-20/ (CL-20)/1,3-DNB/HTPB, dinitrobenzenepredictingpredicting that that the(DNB) the stability stability cocrystal and and compatibility andcompatibility CL-20/DNB of the the co former formercrystal is isbetterwith better thantwo than thosepolymer those of the of binders,latter the latter (Table hydroxyl- (Table 6). 6). terminatedIn comparison polybutatiene with CL-20/DNB (HTPB) cocrystal,and polyethylene an addition glycol of a small (PEG) amount respectively of binder were (HTPB conducted or PEG) [3]. Resultsdecreases indicate the that elastic the bindingconstants energy (Cij), tensile of CL-20/ modulusDNB/PEG (E), bulk is larger(K) and than shear that modulus of CL-20/DNB/HTPB, (G), while predictingCauchy that pressure the stability (C12–C44 and) and compatibility K/G value increase, of the showing former thatis better the stiffness than those of the of polymerbonded the latter (Table 6). explosives (PBXs) system is weaker, and its ductibility is better. In comparison with CL-20/DNB cocrystal, an addition of a small amount of binder (HTPB or PEG) decreases the elastic constants (Cij), tensile modulus (E), bulk (K) and shear modulus (G), while Cauchy pressure (C12–C44) and K/G value increase, showing that the stiffness of the polymerbonded explosives (PBXs) system is weaker, and its ductibility is better.

Molecules 2020, 25, 4311 8 of 19

In comparison with CL-20/DNB cocrystal, an addition of a small amount of binder (HTPB or PEG) decreases the elastic constants (Cij), tensile modulus (E), bulk (K) and shear modulus (G), while Cauchy pressure (C12–C44) and K/G value increase, showing that the stiffness of the polymerbonded explosives (PBXs) system is weaker, and its ductibility is better.

Table 6. Total energies, component energies, binding energy and its normalized values of two different polymerbonded explosives (PBXs) [3], with permission from Han Neng Cai Liao, 2015. Molecules 2020, 25, x FOR PEER REVIEW 8 of 20 Samples Etotal Ebase Epolymer Ebind E’bind Table 6. Total energies, component energies, binding energy and its normalized values of two 51,789.1 51,322.6 343.0 809.4 185.2 differentCL-20/DNB polymerbonded/HTPB explosives− (PBXs)− [3], with permission from Han Neng Cai Liao, 2015. (114.1) (108.0) (29.5) (31.3) (7.2) Samples Etotal Ebase Epolymer Ebind E’bind 51,451.5 −51789.151,367.3 −51322.6 343.0 858.4 809.4 185.2 942.6 214.2 CL-20/DNB/PEGCL-20/DNB/HTPB− − (134.5)(114.1) (124.9) (108.0) (29.5) (42.1) (31.3) (7.2) (42.6) (9.7) −51451.5 −51367.3 858.4 942.6 214.2 CL-20/DNB/PEG (134.5) (124.9) (42.1) (42.6) (9.7) In addition, the initial thermal decomposition pathways, as well as some important products In addition, the initial thermal decomposition pathways, as well as some important products generating mechanism of CL-20/DNB cocrystal at high temperatures (2000, 2500 K and 3000 K), generating mechanism of CL-20/DNB cocrystal at high temperatures (2000, 2500 K and 3000 K), were werestudied studied by reactive by reactive molecular molecular dynamics dynamics simulations simulations using ReaxFF using force ReaxFF field [4]. force Results field show [4]. that Results showwith that the with increasing the increasing of temperature of temperature during the th duringermal decomposition the thermal decomposition process, the time process, to balance the time to balanceand potential and potentialenergy decrease, energy while decrease, the quantity while theof products quantity increases. of products All the increases. CL-20 molecules All the CL-20 moleculesdecompose decompose faster than faster that thanof DNB, that and of DNB,as the andtemperature as the temperature rises, the decomposition rises, the decomposition rate of DNB rate of DNBincreases increases significantly. significantly. According According to the toprod theuct product identification identification analysis, analysis, the main the mainthermal thermal decompositiondecomposition products products areare NO22, ,NO, NO, N2 N, H2,H2O, 2HNOO, HNO3, HON,3, HON, HONO HONO and CO and2 for the CO cocrystal.2 for the The cocrystal. major initial decomposition mechanism is the breaking of N−NO2 in the CL-20 and C−NO2 in the The major initial decomposition mechanism is the breaking of N NO2 in the CL-20 and C NO2 in DNB, which contributes to the formation of NO2. Then, the number− of NO2 increases to the peak− the DNB, which contributes to the formation of NO2. Then, the number of NO2 increases to the peak rapidly and decreases subsequently (Figure 5). After the NO2→ONO rearrangement, it participates rapidly and decreases subsequently (Figure5). After the NO 2 ONO rearrangement, it participates in in other reactions and eventually N2, NO, HONO, HON, H2O→ occur, and so on. In addition, the othersimulation reactions results and eventually indicate that N 2carbon-containing, NO, HONO, HON, clusters H2O formed occur, in and the solater on. stage In addition, of decomposition the simulation resultsat 2500 indicate K and that 3000 carbon-containing K, which is a common clusters phenom formedenon in the during later stage the detonation of decomposition of rich carbon- at 2500 K and 3000containing K, which explosives. is a common phenomenon during the detonation of rich carbon-containing explosives.

FigureFigure 5. Time5. Time evolution evolution of consumption of consumption of CL-20 of CL-20 and DNBand andDNB main and productsmain products at various at various temperatures. (a) CL-20temperatures. and DNB; (a) (CL-20b) 2000 and K; DNB; (c) 2500 (b) K;2000 (d) K; 3000 (c) K;2500 [4 ],K; with (d) 3000 permission K; [4], with from permission Han Neng from Cai Han Liao, 2016. Neng Cai Liao, 2016.

Meanwhile, the effect of CL-20/DNB cocrystallizing and mixing on the sensitivity, binding energy, mechanical properties and thermal decomposition on the molecular level were investigated under the condition of the COMPASS force field. Results indicate that the cocrystallizing and mixing

Molecules 2020, 25, 4311 9 of 19

Meanwhile, the effect of CL-20/DNB cocrystallizing and mixing on the sensitivity, binding energy, mechanical properties and thermal decomposition on the molecular level were investigated under the condition of the COMPASS force field. Results indicate that the cocrystallizing and mixing can reduce the sensitivity of CL-20, increase that of DNB, and the cocrystallizing effect is more obvious and stable [35].Molecules 2020, 25, x FOR PEER REVIEW 9 of 20

can reduce the sensitivity of CL-20, increase that of DNB, and the cocrystallizing effect is more 3. Preparation obviousMolecules 2020 and, 25stable, x FOR [35]. PEER REVIEW 9 of 20 For CL-20/TNT cocrystal, the CL-20/TNT cocrystal with 1:1 mole ratio was prepared by means of 3.can Preparation reduce the sensitivity of CL-20, increase that of DNB, and the cocrystallizing effect is more the recrystallization method at room temperature, and with ethyl acetate as solvent [8]. It was found obviousFor andCL-20/TNT stable [35]. cocrystal, the CL-20/TNT cocrystal with 1:1 mole ratio was prepared by means that the morphology of CL-20/TNT cocrystal is different from that of CL-20 and TNT. The crystal of of the recrystallization method at room temperature, and with ethyl acetate as solvent [8]. It was CL-20 is “spindle”found3. Preparation that shapethe morphology and TNT of CL-20/TNT is an irregular cocrystal block is different crystal. from However, that of CL-20 CL-20 and/ TNTTNT. The cocrystal is a prismatic crystalcrystalFor of withCL-20/TNT CL-20 a smoothis “spindle” cocrystal, and shapethe complete CL-20/TNT and TNT surface cocrystais an andirregularl with uniform 1:1 block mole sizecrystal. ratio (Figure was However, prepared6). The CL-20/TNT by average means particle size is 270cocrystalofµ m,the indicatingrecrystallization is a prismatic that crystalmethod the designingwith at room a smooth temperatur and andpreparation completee, and withsurface ofethyl and cocrystal acetate uniform ascan size solvent e(Figureffectively [8]. 6).It wasThe change the average particle size is 270 μm, indicating that the designing and preparation of cocrystal can shape andfound size ofthat explosive the morphology cocrystals. of CL-20/TNT cocrystal is different from that of CL-20 and TNT. The effectivelycrystal of CL-20change is the “spindle” shape and shape size andof explosive TNT is cocrystals.an irregular block crystal. However, CL-20/TNT cocrystal is a prismatic crystal with a smooth and complete surface and uniform size (Figure 6). The average particle size is 270 μm, indicating that the designing and preparation of cocrystal can effectively change the shape and size of explosive cocrystals.

Figure 6. ScanningFigure 6. Scanning electron electron microscope microscope (SEM) (SEM) photographsphotographs of CL-20, of CL-20, TNT and TNT CL-20/TNT and CL-20 cocrystal./TNT cocrystal. (a) CL-20; (b) TNT; (c) CL-20/TNT cocrystal [8], with permission from Han Neng Cai Liao, 2012. (a) CL-20; (b) TNT; (c) CL-20/TNT cocrystal [8], with permission from Han Neng Cai Liao, 2012. AtFigure the 6.same Scanning time, electron ultrafine microscope CL-20/TNT (SEM) cocrysta photogralphs explosive of CL-20, was TNT prepared and CL-20/TNT by a spray-dryingcocrystal. At themethod same(a) CL-20;[20]. time, Results (b) TNT; ultrafine show (c) CL-20/TNT that CL-20 the preparedcocrystal/TNT [8], cocrystalsamples with perm are explosiveission not the from mix Han wasof Neng CL-20 prepared Cai and Liao, TNT 2012. bybut arather spray-drying method [20ultrafine]. Results CL-20/TNT show cocrystal that the explosives. prepared The samples particle sizes are notof the the cocrystal mix of explosives CL-20 andwere TNT lower but rather ultrafine CL-20than At1 /μ TNTthem and same cocrystal they time, can ultrafineaggregate explosives. CL-20/TNT into many The microparticles,cocrysta particlel explosive sizes which of was the are prepared spherical cocrystal byin explosivesshapea spray-drying and 1–10 were lower μmethodm in size [20]. (Figure Results 7). showThe thermal that the decomposition prepared samples process are notcan thebe dividedmix of CL-20 into two and stages. TNT but The rather peak than 1 µm and they can aggregate into many microparticles, which are spherical in shape and 1–10 µm temperaturesultrafine CL-20/TNT of exothermic cocrystal decomposition explosives. The for particlethe first sizesand secondof the cocrystalstages are explosives 218.98 °C wereand 253.15lower in size (Figure°C,than respectively 17 μ).m Theand they (Figure thermal can 8).aggregate decompositionThe characteristic into many dropmicroparticles, process height canof CL-20/TNT which be divided are spherical cocrystal into inexplosives two shape stages. and is 1–1049.3 The peak temperaturescm,μm whichin of size exothermic increases(Figure 7). by decompositionThe 36.2 thermal cm compared decomposition for with the that firstpr ofocess raw and canCL-20 second be (Tabledivided stages 7). into aretwo 218.98stages. The◦C andpeak 253.15 ◦C, respectivelytemperatures (Figure8 ).of Theexothermic characteristic decomposition drop for height the first of and CL-20 second/TNT stages cocrystal are 218.98 explosives°C and 253.15 is 49.3 cm, °C, respectively (Figure 8). The characteristic drop height of CL-20/TNT cocrystal explosives is 49.3 which increases by 36.2 cm compared with that of raw CL-20 (Table7). cm, which increases by 36.2 cm compared with that of raw CL-20 (Table 7).

Figure 7. SEM photographs of explosive samples. (a) Raw CL-20 (×100); (b) Raw TNT (×100); (c) CL- 20/TNT cocrystal (×900); (d) CL-20/TNT cocrystal (×4200); (e) Spry drying CL-20 [20], with permission from Han Neng Cai Liao, 2015. Figure 7. FigureSEM 7. photographs SEM photographs of of explosive explosive samples. samples. (a) Raw (a) CL-20 Raw (×100); CL-20 (b) (Raw100); TNT ((×100);b) Raw (c) CL- TNT ( 100); 20/TNT cocrystal (×900); (d) CL-20/TNT cocrystal (×4200); (e) Spry drying CL-20× [20], with permission × (c) CL-20/TNT cocrystal ( 900); (d) CL-20/TNT cocrystal ( 4200); (e) Spry drying CL-20 [20], from Han Neng Cai Liao,× 2015. × with permission from Han Neng Cai Liao, 2015.

Molecules 2020, 25, 4311 10 of 19 Molecules 2020, 25, x FOR PEER REVIEW 10 of 20

Figure 8. Differential scanning calorimetry (DSC) curves of explosive samples [20], with permission Figure 8. Differential scanning calorimetry (DSC) curves of explosive samples [20], with permission from Han Neng Cai Liao, 2015. from Han Neng Cai Liao, 2015. Table 7. The lattice parameters of CL-20/DNB cocrystal were calculated by the ReaxFF force field Tablecompared 7. The with lattice the experimentalparameters of value. CL-20/DNB cocrystal were calculated by the ReaxFF force field compared with the experimental value. Lattice Parameter Experimental Value [5] ReaxFF Error/% Lattice Parameter Experimental Value [5] ReaxFF Error/% a/Å a/Å 9.47039.4703 9.5606 9.5606 0.95 0.95 b/Å b/Å 13.458913.4589 13.5872 13.5872 0.95 0.95 c/Å 33.620 33.9406 0.95 c/Å Density/g·cm−3 33.6201.880 33.94061.8268 −2.8 0.95 Density/g cm 3 1.880 1.8268 2.8 · − − The ultrafine CL-20/HMX cocrystal explosive was prepared by an ultra-highly efficient mixing methodThe [6]. ultrafine The prepared CL-20/HMX samples cocrystal were regular explosive block-like was prepared ultrafine by CL-20/HMX an ultra-highly cocrystal efficient explosives mixing withmethod a uniform [6]. The particle prepared size samples of less werethan regular1 μm (Figure block-like 9), which ultrafine appeared CL-20/HMX new stronger cocrystal diffraction explosives peakswith a at uniform 11.558°, particle 13.264°, size 18.601°, of less 24.474°, than 1 33.785°,µm (Figure 36.269°.9), which The purity appeared of the new CL-20/HMX stronger di cocrystalffraction explosive was 92.6%. peaks at 11.558◦, 13.264◦, 18.601◦, 24.474◦, 33.785◦, 36.269◦. The purity of the CL-20/HMX cocrystal explosive was 92.6%. Moreover, the micro/nano CL-20/HMX energetic cocrystal materials were prepared by mechanical ball milling, and characterized using SEM [7]. Results show that after a milling time of 120 min, CL-20/HMX cocrystals prepared under optimal conditions are spherical in shape and the particle size is 80–250 nm. Compared with respective energetic monomers, the prepared micro/nano CL-20/HMX energetic cocrystal materials exhibit unique crystal structure and thermal decomposition properties (Figure 10). In addition, nano-sized CL-20/HMX cocrystals with a mean particle size of 81.6 nm were prepared under the conditions of the 0.3 mm diameter of milling balls [9]. The impact sensitivity was conducted to evaluate the impact safety performance of the CL-20/HMX cocrystal (Table7). The micro-morphology of the explosives is near-spherical and the particle size reveals a normal distribution (Figure 11). Before and after milling, the element composition and molecular structure of CL-20/HMX did not change in comparison to the raw CL-20 and HMX, while there are new crystal phases in the formed cocrystal. The characteristic drop height (H50) of CL-20/HMX was 32.62 cm with 5 kg hammer, which is lower than that of raw materials and mixture, indicating that the mechanical ball milling method is not simply to mix the two explosives physically, but to form the cocrystal between the explosive molecules through the hydrogen bond. Meanwhile, the crystal technology can improve the impact safety of explosives.

Molecules 2020, 25, 4311 11 of 19

Molecules 2020, 25, x FOR PEER REVIEW 11 of 20

Figure 9. SEM photographs of CL-20, HMX and CL-20/HMX cocrystals at different mixing time. (a) Figure 9. SEM photographs of CL-20, HMX and CL-20/HMX cocrystals at different mixing time. CL-20; (b) HMX; (c) CL-20/HMX cocrystals for 5 min; (d) CL-20/HMX cocrystals for 15 min; (e) CL- (a) CL-20; (b) HMX; (c) CL-20/HMX cocrystals for 5 min; (d) CL-20/HMX cocrystals for 15 min; 20/HMX cocrystals for 30 min; (f) CL-20/HMX cocrystals for 45 min; (g) CL-20/HMX cocrystals for 60 (e) CL-20/HMX cocrystals for 30 min; (f) CL-20/HMX cocrystals for 45 min; (g) CL-20/HMX cocrystals min [6], with permission from Han Neng Cai Liao, 2020. for 60 min [6], with permission from Han Neng Cai Liao, 2020. Molecules 2020, 25, x FOR PEER REVIEW 12 of 20

Moreover, the micro/nano CL-20/HMX energetic cocrystal materials were prepared by mechanical ball milling, and characterized using SEM [7]. Results show that after a milling time of 120 min, CL-20/HMX cocrystals prepared under optimal conditions are spherical in shape and the particle size is 80–250 nm. Compared with respective energetic monomers, the prepared micro/nano CL-20/HMX energetic cocrystal materials exhibit unique crystal structure and thermal decomposition properties (Figure 10).

Molecules 2020, 25, x FOR PEER REVIEW 12 of 20

Moreover, the micro/nano CL-20/HMX energetic cocrystal materials were prepared by mechanical ball milling, and characterized using SEM [7]. Results show that after a milling time of 120 min, CL-20/HMX cocrystals prepared under optimal conditions are spherical in shape and the particle size is 80–250 nm. Compared with respective energetic monomers, the prepared micro/nano MoleculesCL-20/HMX2020, 25 energetic, 4311 cocrystal materials exhibit unique crystal structure and thermal decomposition12 of 19 properties (Figure 10).

Figure 10. SEM photographs of CL-20/HMX mixture at different milling time. (a) 10 min; (b) 20 min; (c) 30 min; (d) 40 min; (e) 50 min; (f) 60 min; (g) 90 min; (h) 120 min; (i) 180 min; (j) 300 min; (k) 480 min; (l) CL-20; (m) HMX [7], with permission from Initiators & Pyrotechnics, 2018.

In addition, nano-sized CL-20/HMX cocrystals with a mean particle size of 81.6 nm were prepared under the conditions of the 0.3 mm diameter of milling balls [9]. The impact sensitivity was conducted to evaluate the impact safety performance of the CL-20/HMX cocrystal (Table 7). The micro-morphology of the explosives is near-spherical and the particle size reveals a normal distribution (Figure 11). Before and after milling, the element composition and molecular structure of CL-20/HMX did not change in comparison to the raw CL-20 and HMX, while there are new crystal phases in the formed cocrystal. The characteristic drop height (H50) of CL-20/HMX was 32.62 cm with 5 kg hammer, which is lower than that of raw materials and mixture, indicating that the mechanical Figure 10. SEM photographs of CL-20/HMX mixture at different milling time. (a) 10 min; (b) 20 min; ball millingFigure 10. methodSEM photographs is not simply of CL-20 to mix/HMX the mixture two explosives at different physically, milling time. but (a) 10to min;form ( bthe) 20 cocrystal min; ((cc)) 3030 minmin;;( d(d)) 40 40 min; min; (e ()e 50) 50 min; min; (f )( 60f) 60 min; min; (g) ( 90g) min;90 min; (h) ( 120h) 120 min; min; (i) 180 (i) min;180 min; (j) 300 (j) min; 300 (min;k) 480 (k) min 480; between the explosive molecules through the hydrogen bond. Meanwhile, the crystal technology can (min;l) CL-20; (l) CL-20; (m) HMX (m) HMX [7], with [7], permissionwith permission from Initiatorsfrom Initiators & Pyrotechnics, & Pyrotechnics, 2018. 2018. improve the impact safety of explosives. In addition, nano-sized CL-20/HMX cocrystals with a mean particle size of 81.6 nm were prepared under the conditions of the 0.3 mm diameter of milling balls [9]. The impact sensitivity was conducted to evaluate the impact safety performance of the CL-20/HMX cocrystal (Table 7). The micro-morphology of the explosives is near-spherical and the particle size reveals a normal distribution (Figure 11). Before and after milling, the element composition and molecular structure of CL-20/HMX did not change in comparison to the raw CL-20 and HMX, while there are new crystal phases in the formed cocrystal. The characteristic drop height (H50) of CL-20/HMX was 32.62 cm with 5 kg hammer, which is lower than that of raw materials and mixture, indicating that the mechanical ball milling method is not simply to mix the two explosives physically, but to form the cocrystal between the explosive molecules through the hydrogen bond. Meanwhile, the crystal technology can Figure 11. SEMSEM photographs photographs and and particle particle size size distribution distribution of nano-sized of nano-sized CL-20/HMX CL-20/HMX cocrystal. cocrystal. (a) improve the impact safety of explosives. (50,000;a) 50,000; (b) 20,000; (b) 20,000; (c) Frequency (c) Frequency distribution; distribution; (d) Cumulative (d) Cumulative frequency frequency distribution distribution [9], with [9], withpermission permission from fromGu Ti Gu Huo Ti HuoJian Ji Jian Shu, Ji Shu,2018. 2018.

Nano-scale CL-20/HMX cocrystal of CL-20 and HMX in 2:1 molar ratio with a mean size below 200 nm was prepared by bead milling [36]. Figure 12 shows the morphological evolution of the crystal particles. It was found that most of the coformer crystal particles are ~1 µm (Figure 12a). The mean particle size of the discrete coformers substantially decreased after 10 min of milling (Figure 12b), with no conversion to the cocrystalline material. Plate-like crystal particles with dimensions less than 500 nm started to appear in the specimen being milled for 20 min, as indicated by the arrow in Figure 12c. The plate-like particles were assigned to the 2CL-20 HMX cocrystal as (1) the 2CL-20 HMX · · cocrystal is known to have a plate-like morphology; (2) the appearance of these particles and the diffractionFigure peaks11. SEM of photographs the 2CL-20 HMXand particle cocrystal size distribution in the XRD of pattern nano-sized occurred CL-20/HMX at the cocrystal. same time; (a) and · (3) more50,000; plate-like (b) 20,000; particles (c) Frequency were observed distribution; in the specimens(d) Cumulative upon frequency further milling distribution (Figure [9], 12 withd and e, respectively).permission The from observation Gu Ti Huo ofJian these Ji Shu, relatively 2018. large cocrystal particles seems to be contradicting to the intensive collisions between the grinding media and particles and between the particles themselves occurring during the milling process. It is possible that some growth occurs during the drying of the sampled specimens. Molecules 2020, 25, x FOR PEER REVIEW 13 of 20

Nano-scale CL-20/HMX cocrystal of CL-20 and HMX in 2:1 molar ratio with a mean size below 200 nm was prepared by bead milling [36]. Figure 12 shows the morphological evolution of the crystal particles. It was found that most of the coformer crystal particles are ~1 μm (Figure 12a). The mean particle size of the discrete coformers substantially decreased after 10 min of milling (Figure 12b), with no conversion to the cocrystalline material. Plate-like crystal particles with dimensions less than 500 nm started to appear in the specimen being milled for 20 min, as indicated by the arrow in Figure 12c. The plate-like particles were assigned to the 2CL-20·HMX cocrystal as (1) the 2CL-20·HMX cocrystal is known to have a plate-like morphology; (2) the appearance of these particles and the diffraction peaks of the 2CL-20·HMX cocrystal in the XRD pattern occurred at the same time; and (3) more plate-like particles were observed in the specimens upon further milling (Figure 12d and e, respectively). The observation of these relatively large cocrystal particles seems to be contradicting to the intensive collisions between the grinding media and particles and between the particles themselvesMolecules 2020 ,occurring25, 4311 during the milling process. It is possible that some growth occurs during13 ofthe 19 drying of the sampled specimens.

Figure 12. 12. SEMSEM images images (a– (af)– fof) ofspecimens specimens sampled sampled at milling at milling times times between between 0 and 0 60 and min. 60 [36], min with [36], withpermission permission from fromCryst. Cryst. Eng. Comm, Eng. Comm, 2015. 2015.

Preparation procedure: procedure: 438.0 438.0 mg mg (1 (1 mmol) mmol) CL-20 CL-20 and and 472.3 472.3 mg mg (2 (2mmol) mmol) TKX-50 TKX-50 were were dried dried in ain beaker a beaker at 40 at °C 40 ◦forC for3 h, 3 30 h, mL 30 mLDMF DMF was was added added to the to beaker, the beaker, heat heatto 80 to °C 80 in◦ Cthe in water the water bath, bath, and ultrasonicand ultrasonic for 30 for min 30 minuntil until the drug the drug is completely is completely dissolved. dissolved. Then, Then, 100 mL 100 chloroform mL chloroform was wasput into put theinto flask. the flask. The completely The completely dissolved dissolved solution solution was dro waspped dropped into the into flask the flaskat the at rate the of rate 0.8 of mL/min, 0.8 mL/ min,and theand magnetic the magnetic stirring stirring speed speed was was kept kept at at1000 1000 r/min. r/min. After After droppi dropping,ng, keep keep the the original original speed speed and continue stirring for 1 h. After After standing standing for for 3 3 h h and and filtering, filtering, the the filtered filtered samples were dried in a vacuum for 3 h to obtain CL-20/TKX-50 ultrafine cocrystal samples. The performance was compared with that of CL-20/TKX-50 mechanical mixture in the same mole ratio. To decrease the sensitivity of CL-20, the CL-20/TKX-50 cocrystal explosive was prepared by solvent-nonsolvent method. The surface electrostatic potentials of CL-20 and TKX-50 were analyzed and the possible non-covalent bonding between cocrystal molecules was predicted [34]. Results shown that the prepared CL-20/TKX-50 cocrystal has a flat sheet shape, the formation, disappearance, shift and change of intensity of peaks been proved the formation of a new lattice structure. The crystal morphology of CL-20 and TKX-50 is nearly spherical shape, and the particle size distribution is relatively uniform, and the particle size is 1 µm. However, the morphology of CL-20/TKX-50 cocrystal shows a long and thin lamellar structure, which is quite different from that of CL-20 and TKX-50, and the particle size of CL-20/TKX-50 cocrystal is 10 µm, indicating that the cocrystal process can not only change the morphology of the original crystal, but also prove the formation of a new crystal (Figure 13). Molecules 2020, 25, x FOR PEER REVIEW 14 of 20 Molecules 2020, 25, x FOR PEER REVIEW 14 of 20 vacuumvacuum forfor 33 hh toto obtainobtain CL-20/TKX-50CL-20/TKX-50 ultrafineultrafine cocrystalcocrystal samples.samples. TheThe performanceperformance waswas comparedcompared withwith thatthat ofof CL-20/TKX-50CL-20/TKX-50 mechanicalmechanical mixturemixture inin thethe samesame molemole ratio.ratio. ToTo decreasedecrease thethe sensitivitysensitivity ofof CL-20,CL-20, thethe CL-CL-20/TKX-5020/TKX-50 cocrystalcocrystal explosiveexplosive waswas preparedprepared byby solvent-nonsolventsolvent-nonsolvent method.method. TheThe surfacesurface electrostatielectrostaticc potentialspotentials ofof CL-20CL-20 andand TKX-50TKX-50 werewere analyzedanalyzed andand thethe possiblepossible non-covalentnon-covalent bondingbonding betweenbetween cocrystalcocrystal moleculesmolecules waswas predictedpredicted [34].[34]. ResultsResults shownshown thatthat thethe preparedprepared CL-20/TKX-50CL-20/TKX-50 cocrystalcocrystal hashas aa flatflat sheetsheet shape,shape, thethe formation,formation, disappearance,disappearance, shiftshift andand changechange ofof intensityintensity ofof pepeaksaks beenbeen provedproved thethe formationformation ofof aa newnew latticelattice structure.structure. TheThe crystalcrystal morphologymorphology ofof CL-20CL-20 andand TKX-50TKX-50 isis nearlynearly sphericalspherical shape,shape, andand thethe particleparticle sizesize distributiondistribution isis relativelyrelatively uniform,uniform, andand thethe particleparticle sizesize isis 11 μμm.m. However,However, thethe morphologymorphology ofof CL-20/TKX-50CL-20/TKX-50 cocrystalcocrystal showsshows aa longlong andand thinthin lamellarlamellar structure,structure, whichwhich isis quitequite differentdifferent fromfrom thatthat ofof CL-20CL-20 andand TKX-50,TKX-50, andand thethe particleparticle sizesize ofof CL-20/TKX-50CL-20/TKX-50 cocrystalcocrystal isis 1010 μμm,m, indicatingindicating thatthat thethe cocrcocrystalystal processprocess cancan notnot onlyonly changeMoleculeschange thethe2020 morphology,morphology25, 4311 ofof thethe originaloriginal crystal,crystal, bubutt alsoalso proveprove thethe formationformation ofof aa newnew crystalcrystal (Figure(Figure14 of 19 13).13).

Figure 13. SEM photographs of CL-20, TKX-50 and CL-20/TKX-50 cocrystal. (a) raw CL-20; (b) Raw Figure 13. SEM photographs of CL-20, TKX-50 and CL-20CL-20/TKX-50/TKX-50 cocrystal. ( a)) raw CL-20; ( b) Raw TKX-50; (c) CL-20/TKX-50 cocrystal [34], with permission from Huo Zha Yao Xue Bao, 2020. TKX-50; (c) CL-20CL-20/TKX-50/TKX-50 cocrystalcocrystal [[34],34], withwith permissionpermission fromfrom HuoHuo ZhaZha YaoYao XueXue Bao,Bao, 2020.2020.

4.4. EnergeticEnergetic PerformancePerformance InIn orderorder order toto to studystudy study thethe the thermalthermal thermal decompositiondecomposition decomposition propprop propertiesertieserties ofof CL-20/TNTCL-20/TNT of CL-20/TNT cocryatalcocryatal cocryatal explosives,explosives, explosives, thethe samplesthesamples samples ofof CL-20/TNTCL-20/TNT of CL-20/TNT cocryatalscocryatals cocryatals werewere were testedtested tested byby DSCDSC by DSC (TA(TA (TA Instruments,Instruments, Instruments, NewNew New Castle,Castle, Castle, DE,DE, DE, USA)atUSA)at USA) thethe at −1 1 heatingthe heating rate rate of 10 of °C·min 10 C min−1, the, thecurve curve shown shown in Figure in Figure 14. 14 As. As is isshown shown there there are are three three stages stages of of the the heating rate of 10 °C·min◦ · , the− curve shown in Figure 14. As is shown there are three stages of the thermalthermal decompositiondecompositiondecomposition process processprocess of CL-20ofof CL-20/TNTCL-20/TNT/TNT cocrystal, cocrystal,cocrystal, the maximum thethe maximummaximum endothermic endothermicendothermic peak temperature peakpeak temperatureistemperature 143.5 ◦C, which isis 143.5143.5 is °C,°C, higher whichwhich than isis higher thathigher of than thethan melting thatthat ofof the pointthe meltingmelting of TNT pointpoint (81 ◦ofofC[ TNTTNT37]). (81(81 With °C°C [37]). the[37]). increase WithWith thethe of increasetemperature,increase ofof temperature,temperature, the hydrogen thethe bond hydrogenhydrogen between bond thebond cocrystal betweenbetween molecules thethe cocrystalcocrystal breaks molecules andmolecules the molecular breaksbreaks structureandand thethe molecularismolecular destroyed structurestructure [38]. There isis destroyeddestroyed are two [38].[38]. processes ThereThere are ofare exothermic twotwo processesprocesses decomposition ofof exothermicexothermic at decompositiondecomposition 222.6 ◦C and 250.1 atat 222.6222.6◦C, °Crespectively.°C andand 250.1250.1 °C, Compared°C, respectively.respectively. with the ComparedCompared maximum withwith exothermic thethe maximummaximum peak valueexothermicexothermic of CL-20 peakpeak (321.5 valuevalue◦C[ ofof 33 CL-20CL-20]) and (321.5(321.5 TNT °C(245°C [33])[33])◦C[ andand39]), TNTTNT the exothermic(245(245 °C°C [39]),[39]), peak thethe ofexothermicexothermic CL-20/TNT peakpeak cocrystal ofof CL-20/TNTCL-20/TNT shifts, indicating cocrystalcocrystal that shifts,shifts, the indicatingindicating cocrystal changesthatthat thethe cocrystalthecocrystal thermal changeschanges decomposition thethe thermalthermal characteristics decompositiondecomposition of the characcharac raw materialsteristicsteristics ofof and thethe endows rawraw materialsmaterials the cocrystal andand withendowsendows unique thethe cocrystalthermalcocrystal decomposition withwith uniqueunique thermalthermal behavior. decompositiondecomposition behavior.behavior.

Figure 14. Figure 14. DSCDSC curve curve of of the the CL-20/TNT CL-20/TNT cocrystal cocrystal explosive explosive [8], [8], with with permission permission from from Han Han Neng Neng Cai Cai Liao,Figure 2012. 14. DSC curve of the CL-20/TNT cocrystal explosive [8], with permission from Han Neng Cai Liao,Liao, 2012.2012.

The impact sensitivity value of CL-20/TNT cocrystal is H50 = 28 cm. Compared with CL-20

( H50 = 15 cm), the impact sensitivity of CL-20 is significantly reduced by 87%. Low-sensitivity TNT and high-sensitivity CL-20 combine to form a cocrystal at the molecular scale through eutectic technology, which changes the internal composition and crystal structure of explosives compared with traditional methods. In addition, due to the intermolecular hydrogen bond, on the one hand, it increases the stability of the molecular system of the cocrystal, on the other hand, it improves the anti-vibration of the cocrystal molecule to the mechanical external force, so the desensitization effect is obvious. Through the eutectic technology, it can effectively realize the desensitization of high sensitive explosive and improve its safety performance. The crystal morphology, particle size and sensitivity of prepared CL-20/HMX cocrystal were characterized [6]. The thermal decomposition process of cocrystal explosives had only one exothermic decomposition stage with peak temperatures of 248.3 ◦C. The enthalpy for the exothermic decomposition Molecules 2020, 25, x FOR PEER REVIEW 15 of 20

The impact sensitivity value of CL-20/TNT cocrystal is H50 = 28 cm. Compared with CL-20 (H50 = 15 cm), the impact sensitivity of CL-20 is significantly reduced by 87%. Low-sensitivity TNT and high-sensitivity CL-20 combine to form a cocrystal at the molecular scale through eutectic technology, which changes the internal composition and crystal structure of explosives compared with traditional methods. In addition, due to the intermolecular hydrogen bond, on the one hand, it increases the stability of the molecular system of the cocrystal, on the other hand, it improves the anti-vibration of the cocrystal molecule to the mechanical external force, so the desensitization effect is obvious. Through the eutectic technology, it can effectively realize the desensitization of high sensitive explosive and improve its safety performance. The crystal morphology, particle size and sensitivity of prepared CL-20/HMX cocrystal were Molecules 2020, 25, 4311 15 of 19 characterized [6]. The thermal decomposition process of cocrystal explosives had only one exothermic decomposition stage with peak temperatures of 248.3 °C. The enthalpy for the exothermic ofdecomposition the cocrystal (2912.1of the cocrystal J g 1) was (2912.1 remarkably J·g−1) higherwas remarkably than that of higher the physical than that mixture of the of physical CL-20 and mixture HMX · − (1327.3of CL-20 J gand1) HMX (Figure (1327.3 15). The J·g−1 friction) (Figure sensitivity 15). The friction of CL-20 sensitivity/HMX cocrystal of CL-20/HMX explosive cocrystal was 84%, explosive which · − waswas decreased84%, which by was 16% decreased compared by with 16% original compared CL-20, with and original the characteristic CL-20, and height the characteristic of the cocrystal height was increasedof the cocrystal by 28.6 was cm andincreased 11.5 cm by compared 28.6 cm withand original11.5 cm CL-20compared and HMX,with original respectively CL-20 (Table and8 ).HMX, The compatibilityrespectively (Table of CL-20 8)./HMX The cocrystalcompatibility with componentsof CL-20/HMX of solid cocrystal propellant with shown components that the preparedof solid CL-20propellant/HMX shown cocrystal that wasthe compatibleprepared CL-20/HMX with NG/BTTN, cocrystal AP andwas Alcompatible powder, whilewith NG/BTTN, incompatible APwith and HGAP,Al powder, N-100. while incompatible with HGAP, N-100.

Figure 15. Figure 15. DSC curves of different different samples [[6],6], with permissionpermission fromfrom HanHan NengNeng CaiCai Liao,Liao, 2020.2020. Table 8. The mechanical sensitivities of different samples.

1 Samples F/% H50/cm P/MPa D/m s · − 19.3, 13.1 [20]; 13.1 CL-20 100 [6,7] 44.9 [34]; 43 [8] 9385 [34]; 9500 [8] [7]; 15 [8,9,34] 36.4 [6]; 19.6 [7]; HMX 28 [6]; 84 [7] 39.6 [34] 9048 [34] 27.2 [9]; 13.9 [34] Ball milling of CL-20 72 [7] 42.3 [7] Ball milling of HMX 60 [7] 47.8 [7] 96 (mole ratio is 15.4 (mole ratio is CL-20/HMX mixture 2:1) [7] 2:1) [7], 20.1 [9] Ball milling of CL-20/HMX 68 [7] 43.5 [7] mixture (mole ratio is 2:1) Micro/nano CL-20/HMX 60 [7] 47.6 [7], 32.6 [9] cocrystal Ultrafine CL-20/HMX 84 [6] 47.9 [6] cocrystal TNT 102 [8], 157.2 [20] 21 [8] 6900 [8] CL-20/TNT cocrystal 28 [8], 49.3 [20] 35 [8] 8600 [8]

CL-20/TNT mixture 18.8 [20] TKX-50 55.4 [34] 41.0 [34] 8524 [34] CL-20/TKX-50 mixture 26.0 [34] CL-20/TKX-50 cocrystal 34.0 [34] 43.8 [34] 9264 [34] 1 H50 is characteristic drop height, cm; F is friction sensitivity, %; D is detonation velocity, m s− ; P is detonation pressure, MPa. The hammer weight: (2.500 0.002) kg, sample weight: (35 1) mg for impact· sensitivity in [7,20], the hammer weight: (1.5 0.01) kg, sample weight:± (20 1) mg, pressure: (2.45± 0.07) MPa, swing angle: (80 1)0 for friction sensitivity in± [7]; the hammer weight: 2 kg,± sample weight: 30 mg± for impact sensitivity, for friction± sensitivity in [6]; the hammer weight: 2 kg, sample weight: (30 1) mg in [8]; the hammer weight: 5 kg, sample weight: (35 1) mg in [9,34]. ± ± Molecules 2020, 25, 4311 16 of 19 Molecules 2020, 25, x FOR PEER REVIEW 17 of 20

At thethe same same time, time, the the DSC DSC curves curves of CL-20 of /CL-20/HMX cocrystalHMX cocrystal were prepared were prepared by means ofby mechanicalmeans of ballmechanical milling, ball and milling, compared and compared with the CL-20 with the/HMX CL-20/HMX mixture [mixture7]. The [7]. mechanical The mechanical sensitivity sensitivity of the microof the /micro/nanonano CL-20 /CL-20/HMXHMX energetic ener cocrystalgetic cocrystal materials materials was reduced was reduced obviously obviously compared compared to that to of that raw HMX,of raw whileHMX, the while energy the energy output output property property was equivalent was equivalent to that ofto rawthat CL-20.of raw CL-20. In orderorder to to compare compare the etheffect effect of the preparedof the prepared CL-20/TKX-50 CL-20/TKX-50 cocrystal on cocrystal the thermal on decomposition the thermal ofdecomposition each component, of each the component, DSC curves the of DSC CL-20, curves TKX-50, of CL-20, CL-20 TKX-50,/TKX-50 CL-20/TKX-50 mixture and mixture CL-20 /andTKX-50 CL- cocrystal20/TKX-50 were cocrystal investigated were investigated [34] (Figure [34] 16 (Figure). The 16). exothermic The exothermic decomposition decomposition peaks of peaks CL-20 of andCL- TKX-5020 and TKX-50 were 240.2 were and 240.2 234.8 and◦C, 234.8 respectively, °C, respective and therely, and were there two were exothermic two exothermic decomposition decomposition peaks in thepeaks decomposition in the decomposition process ofprocess the CL-20 of the/TKX-50 CL-20/TK mixture,X-50 mixture, indicating indicating that the decompositionthat the decomposition process ofprocess the CL-20 of the/TKX-50 CL-20/TKX-50 mixture mixture is obviously is obviously a simple a simple superposition superposition of CL-20 of CL-20 and TKX-50. and TKX-50. The firstThe decompositionfirst decomposition peak peak appears appears at 229.8 at 229.8◦C, °C, corresponding corresponding to the to the exothermic exothermic decomposition decomposition of partof part of TKX-50;of TKX-50; the the second second peak peak appears appears at 242.5 at 242.5◦C, corresponding °C, corresponding to the to exothermic the exothermic decomposition decomposition of CL-20. of ForCL-20. the For CL-20 the/TKX-50 CL-20/TKX-50 cocrystal cocrystal decomposition, decomposition, with the increasewith the of increase temperature, of temperature, the first exothermic the first decompositionexothermic decomposition peak of CL-20 peak/TKX-50 of CL-20/TKX-50 cocrystal appears cocrystal at 171.6 appears◦C, which at 171.6 can be°C, attributed which can to thebe destructionattributed to of the hydrogen destruction in the of cocrystal hydrogen structure in the co andcrystal the decompositionstructure and the of adecomposition small amount ofof TKX-50.a small Subsequently,amount of TKX-50. a large Subsequently, number of cocrystal a large number products of begincocrystal to decompose, products begin and theto decompose, second exothermic and the decompositionsecond exothermic peak decomposition appeared at 222.8 peak◦C, appeared indicating at the 222.8 formation °C, indicating of hydrogen the formation bond and theof hydrogen existence ofbond a new and structure the existence between of a CL-20new structure/TKX-50 between cocrystal CL-20/TKX-50 molecules. cocrystal molecules.

Figure 16. DSC curves of CL-20, TKX-50, CL-20/TKX- CL-20/TKX-5050 mixture and CL-20/TKX-50 CL-20/TKX-50 cocrystal [34], [34], with permission from HuoHuo Zha YaoYao XueXue Bao,Bao, 2020.2020. In addition, the DSC curves of CL-20, DNB, CL-20/DNB cocrystal were shown in Figure 17. In addition, the DSC curves of CL-20, DNB, CL-20/DNB cocrystal were shown in Figure 17. There are three (one is endothermic melting and two exothermic decompositions) thermal There are three (one is endothermic melting and two exothermic decompositions) thermal decomposition stages. The melting temperature of cocrystal is 136.4 C, 44.7 C higher than that of raw decomposition stages. The melting temperature of cocrystal is 136.4◦ °C, 44.7◦ °C higher than that of material DNB (91.7 C) in the melting stage, indicating that the cocrystal decomposes into liquid DNB raw material DNB (91.7◦ °C) in the melting stage, indicating that the cocrystal decomposes into liquid at this point. There are two exothermic peaks at 216.9 C and 242.9 C in the exothermic decomposition DNB at this point. There are two exothermic peak◦s at 216.9 °C◦ and 242.9 °C in the exothermic stage, which was attributed to the exothermic decomposition of two single components after cocrystal decomposition stage, which was attributed to the exothermic decomposition of two single decomposition. Moreover, the crystal density of CL-20/DNB is higher than that of CL-20/TNT, and the components after cocrystal decomposition. Moreover, the crystal density of CL-20/DNB is higher sensitivity of DNB is lower than that of TNT. Thus, the sensitivity of CL-20/DNB is lower than that of than that of CL-20/TNT, and the sensitivity of DNB is lower than that of TNT. Thus, the sensitivity of CL-20/TNT. In addition, the cost of the DNB is significantly lower than that of TNT. Therefore, the CL-20/DNB is lower than that of CL-20/TNT. In addition, the cost of the DNB is significantly lower cocrystal may become an excellent explosive with high-energy, insensitive and low-cost prospective than that of TNT. Therefore, the cocrystal may become an excellent explosive with high-energy, ingredients in explosives and propellants. insensitive and low-cost prospective ingredients in explosives and propellants.

Molecules 2020, 25, 4311 17 of 19 Molecules 2020, 25, x FOR PEER REVIEW 18 of 20

Figure 17. DSC curves of CL-20, DNB and CL-20/DNB cocryatal [5], with permission from Chinese Figure 17. DSC curves of CL-20, DNB and CL-20/DNB cocryatal [5], with permission from Chinese Journal of Energetic Materials, 2013. Journal of Energetic Materials, 2013. The impact sensitivity, the calculated detonation of the CL-20, TKX-50, CL-20/TKX-50 mixture and The impact sensitivity, the calculated detonation of the CL-20, TKX-50, CL-20/TKX-50 mixture CL-20/TKX-50 cocrystal are shown in Table8. The characteristic drop height of CL-20 /TKX-50 cocrystal and CL-20/TKX-50 cocrystal are shown in Table 8. The characteristic drop height of CL-20/TKX-50 is lower than that of TKX-50, but significantly higher than that of CL-20, β-HMX and CL-20/TKX-50 cocrystal is lower than that of TKX-50, but significantly higher than that of CL-20, β-HMX and CL- mixture, indicating that the impact sensitivity of CL-20/TKX-50 cocrystal is much lower. The detonation 20/TKX-50 mixture, indicating that the impact sensitivity of CL-20/TKX-50 cocrystal is much lower. velocity and detonation pressure of CL-20/TKX-50 cocrystal are slightly lower than that of CL-20, but the The detonation velocity and detonation pressure of CL-20/TKX-50 cocrystal are slightly lower than detonation performance is obviously improved compared with β-HMX, indicating CL-20/TKX-50 that of CL-20, but the detonation performance is obviously improved compared with β-HMX, cocrystal has good detonation performance. indicating CL-20/TKX-50 cocrystal has good detonation performance. 5. Existing Problems and Challenges 5. Existing Problems and Challenges As we all know, it is of great significance for the preparation of energetic materials with high-energy and low-sensitivityAs we all know, to useit is eutecticof great technologysignificance to for modify the preparation the single high-energy of energetic explosives. materials with While high- the developmentenergy and low-sensitivity of an energetic to cocrystal use eutectic is still technology in the exploratory to modify stage, the the single main high-energy challenges for explosives. cocrystal atWhile present the aredevelopment as follows: of (1) an the energetic optimal cocrystal preparation is still method in the exploratory of energetic stage, cocrystal the main still needs challenges to be simplifiedfor cocrystal and at simulated.present are The as follows: solvent evaporation(1) the optimal method preparation is mainly method used toof exploreenergetic the cocrystal preparation still conditionsneeds to be of simplified cocrystal and by experience simulated. at The present solvent and evaporation many experiments method is with mainly different used stoichiometricto explore the ratiospreparation cannot conditions meet the engineering of cocrystal requirements. by experience (2) at Thepresent effective and cocrystalmany experiments formation with principle different and formationstoichiometric mechanism ratios cannot of energetic meet the compounds engineering under requirements. the guidance (2) ofThe thermodynamics effective cocrystal are formation not fully revealed,principle whichand formation greatly hindersmechanism the controllableof energetic constructioncompounds under of energetic the guidance cocrystal of thermodynamics compounds [40]. Theare not main fully challenges revealed, for which us in thegreatly future hinders are possibly: the controllable (1) the design construction and formation of energetic mechanism cocrystal of energeticcompounds cocrystal [40]. The can main be drawchallenges lessons for from us in the the otherfuture research are possibly: fields, (1) the the cocrystal design and is formed formation by self-assemblymechanism of of energetic hydrogen cocrystal bonding can and beπ– drawπ stacking lessons interactions; from the other (2) on research the basis fields, of the existingthe cocrystal mature is eutecticformed by technology, self-assembly a safe, of ehydrogenfficient and bonding practical and synthesis π–π stacking method interactions; suitable for (2) energetic on the basis cocrystal of the shouldexisting be mature improved, eutectic promoting technology, the theoreticala safe, efficient research and of practical cocrystal synthesis formation, method and simulate suitable and for designenergetic the cocrystal formation should of energetic be improved, cocrystal promoting from the theoretical the theoretical level research [41]. of cocrystal formation, and simulate and design the formation of energetic cocrystal from the theoretical level [41]. Author Contributions: Conceptualization, W.-q.P. and X.-z.F.; resources, W.-q.P. and K.W.; data curation, W.-q.P.; writing—originalAuthor Contributions: draft preparation, conceptualization, W.-q.P.; writing—reviewW.-q.P. and X.-z.F.; and resour editing,ces, W.Z. W.-q.P. and L.T.d.L.; and K.W. project; data administration, curation, W.- X.-z.F.q.P.; writing—original and W.-q.P.; funding draft acquisition, preparation, J.-q.L. W.-q.P.; All authorswriting—review have read and and editing, agreed toW.Z. the publishedand L.T.d.L.; version project of the manuscript. administration, X.-z.F. and W.-q.P.; funding acquisition, J.-q.L. All authors have read and agreed to the published Funding:version of Thisthe manuscript. research was funded by the National Natural Science Foundation of China, grant number 21703167 and the Science and Technology on Combustion and Explosion Laboratory Foundation of China, grantFunding: number This 2019SYSZCJJ. research was funded by the National Natural Science Foundation of China, grant number 21703167 and the Science and Technology on Combustion and Explosion Laboratory Foundation of China, grant Acknowledgments: The authors are thankful to Juan Zhao from Analysis and Measurement Center on Energetic Materials,number 2019SYSZCJJ. Xi’an Modern Chemistry Research Institute for the particle micrograph experiments and Xiao-ning Ren from Science and Technology on Combustion and Explosion Laboratory, Xi’an Modern Chemistry Research Acknowledgments: The authors are thankful to Juan Zhao from Analysis and Measurement Center on Energetic Materials, Xi’an Modern Chemistry Research Institute for the particle micrograph experiments and Xiao-ning Ren from Science and Technology on Combustion and Explosion Laboratory, Xi’an Modern Chemistry Research

Molecules 2020, 25, 4311 18 of 19

Institute for the thermal decomposition tests. We are especially thankful to Fengqi Zhao from Science and Technology on Combustion and Explosion Laboratory, Xi’an 710065, China and RuiQi Shen from Nanjing University of Science and Technology for providing many helpful suggestions and English statements. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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