Kinetics for the Inhibited Polymorphic Transition of HMX Crystal after

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Kinetics for the Inhibited Polymorphic Transition of HMX Crystal after Strong Surface Confinement Feiyan Gong, Zhijian Yang, Wen Qian, Yu Liu, Jianhu Zhang, Ling Ding, Congmei Lin, Chengcheng Zeng, and Qi-Long Yan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01582 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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The Journal of Physical Chemistry

Kinetics for the Inhibited Polymorphic Transition of HMX Crystal after Strong Surface Confinement Feiyan Gonga*, Zhijian Yanga, Wen Qiana, Yu Liua, Jianhu Zhanga, Ling Dinga, Congmei Lina, Chengcheng Zenga, Qilong Yanb* aInstitute

of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang 621900, China

bScience

and Technology on Combustion, Thermo-structure and Internal Flow

Laboratory, Northwestern Polytechnical University, Xi’an 710072, China E-mail: [email protected]; [email protected] Abstract: The kinetics and the tuning mechanisms for the polymorphic transitions remain poorly understood. In this work, the mechanisms for the inhibited β→δ polymorphic transition of 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) crystal after strong surface confinement in poly(dopamine) (PDA) were studied thoroughly by non-isothermal and isothermal kinetics. The activation energy for the polymorphic transition of HMX was about 400 kJ mol-1, which was almost independent of conversion extent. The transition process followed three dimensional growth of nuclei (A3) model. As for HMX@PDA, the activation energies increased from 100 to 620 kJ mol-1 with increasing conversion from 0 to 1.0. The transition process could be divided in to two partially overlapped stages, i. e., the first transition step follows some mechanism between the 1st order reaction model and 2D diffusion (D2) model, the second step of HMX@PDA is approaching to the 2D nucleation and nucleus growth (A2) model. It

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has been demonstrated that the nucleation was the rate-limiting step during the polymorphic transition. The PDA coating significantly decreased the polymorphic transition rate, especially for the nucleation process. In particular, it was decreased by 108 times, comparing to the uncoated HMX. Based on the DFT calculation and systematical characterizations, the inhibition of this transition was attributed to the strong interactions between polymer chain of PDA and the function groups on the surface of HMX crystal, which blocked the formation of δ-nuclei at crystal surface. 1. Introduction Solid-solid polymorphic transitions ubiquitously exist in organic and inorganic crystals related with photoelectric materials,1,

2

pharmaceuticals,3 and energetic

materials.4 Since the properties of materials could be significantly changed accompanied with the polymorphic transition, understanding how the phase transforms and how to stabilize the phase form are fundamental questions in the research field correlated with polymorphic crystals.5,

6

Various reports have

demonstrated that the polymorphic transition can be tuned though substitution of some ions or doping with other particular ions.7-9 However, this strategy is not suitable for organic molecular crystals. Except for the ions doping strategy, researchers found that the polymorphic transition can be efficiently inhibited by surface functionalization.5,

10, 11

For example, Li successfully demonstrated that

surface coordination was an effective route to stabilize the monoclinic phase of VO2 from converting to rutile phase by coordinating L-ascorbic acid on VO2 nanobeam.11 The cubic to hexagonal polymorphic transition of FAPbI3 was effectively inhibited


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though the binding effect between long alkyl chain/aromatic ammonium cations and the surface of FAPbI3.10 These findings inspired new idea for modulating crystal phase by surface chemistry. It also implies that polymorphic transition of polymorphic crystals can be tuned by surface functionalization with strong interfacial interaction. The 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) crystal is a widely used high energetic material, which usually has α, β, δ and γ polymorphic forms.12, 13 It has been well documented that β-HMX can undergo thermal-induced solid-to-solid polymorphic transition to form δ phase at elevated temperatures (about 190 oC).4, 14-16 The polymorphic transition of energetic crystals would lead to defects formation due to large volume expansion17, generating hot spots that sensitizes material under external stimuli.4, 18 Various researches have demonstrated that great increase of HMX sensitivity at high temperature was correlated with the β→δ polymorphic transition.19, 20

Therefore, suppressing polymorphic transition and understanding the kinetics of

this process are essential for enhancing the performance and understanding the structure-property relationship in polymorphic energetic crystals. Firstly, in the case of the polymorphic transition inhibition, surface coating was demonstrated as an effective strategy to suppress undesired polymorphic transition. 21-24

Nevertheless, the highly efficient inhibition of polymorphic transition of

polymorphic energetic materials is still facing challenges. Typical difficulty is what kind of surface coating is the most efficient way to suppress the polymorphic transition by even using a minimum coating. The oxidation–polymerization of dopamine provides a facile and green way for coating materials.


25, 26

The


poly(dopamine) can firmly attach to various solid materials with strong binding.

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27-29

We have previously demonstrated dopamine chemistry as a new route to coat energetic crystals and suppress the polymorphic transition.16 On the other hand, understanding the mechanisms of polymorphic transition can provide important guidance for manipulating the phase form of crystals and adjusting the properties of energetic materials.15,

30-33

To date, several transition mechanisms

have been proposed in literature. Henson presented a kinetic model based on SHG measurements of β→δ transition of HMX.15,

32, 33

The modeling of the kinetics

parameters involves complex optimizations and empirically determination. A thermodynamic and kinetic model was presented considering that the β→δ transition kinetics was governed by the thermodynamics of the melted δ-phase.31 In this model, the Ea for growth equals to the heat of fusion. As a development to the virtual melting mechanism, the phase-field approach discussed the combined effects of the external layer thickness and internal stresses on the transition.30 It implied that the polymorphic transition might be affected by the surrounding of surface layer.30 However, the kinetics and the tuning mechanisms of the polymorphic transitions remain poorly understood. In the present work, a strong interfacial interaction poly(dopamine) coating route was demonstrated to efficiently inhibit the β→δ transition of HMX. The mechanism was systematically investigated, including non-isothermal and isothermal kinetics, together with simulation of the interfacial interaction and systematic characterizations. We found that the key to inhibit the polymorphic transition is to control the surface


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nucleation rate, which is the rate-limiting step during the polymorphic transition. 2. Experimental and Theoretical Methods 2.1 Experimental The HMX crystals were surface confined by poly(dopamine) (PDA) via the in-situ polymerization of 3,4-dihydroxyphenethylamine (dopamine, molecular structures for HMX and dopamine shown in Fig. S1 in Supporting Information (SI)). For the sake of simplicity, HMX crystals coated with 0.5%, 0.8%, 1.2%, 1.5% and 1.8% PDA were called HP-1, HP-2, HP-3, HP-4 and HP-5, in which the content of PDA was adjusted by varying the coating time for 1h, 2h, 3h, 6h, and 24h, respectively. The detailed materials, preparation and characterization methods were given in SI. 2.2 Theoretical backgrounds for polymorphic transition kinetic models Firstly, Friedman’s isoconversional method

34, 35

and combined kinetic analysis

method 36 were used to study the nonisothermal kinetics of the polymorphic transition (see the detailed theoretical backgrounds in SI.). As for the isothermal kinetics model, the kinetics of β→δ polymorphic transition of HMX can be described as nucleation-growth phenomenon by the following schemes32.     HMX nucleus Nucleation:   HMX   kn

k

g     HMX Growth:   HMX nucleus    HMX  

At constant temperature, the reaction rate for the nucleation and growth process, which is presented by the change rate in the δ phase fraction, d[δ-HMX]n/dt and d[δ-HMX]g/dt, respectively, can be determined by the following equations:



d   HMX n dt d   HMX g dt

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 kn (1   )

(1)

 k g (1   )

(2)

where kn and kg is the rate constant (s-1) for the nucleation and growth process, respectively. α is the fraction of δ phase. The total reaction rate, d[δ-HMX]/dt, should be the sum of nucleation and growth: d   HMX  dt



d   HMX n dt



d   HMX g dt

d  kn (1   )  k g (1   ) dt

(3) (4)

By integrating Eq. 4, the transformed fraction can be described as: ( k  k )t kn e n g  1  ( k  k )t k g  kn e n g

(5).

3. Results and Discussion 3.1 Structure Characterizations and β→δ Polymorphic Transition Behaviors Firstly, SEM, Raman and XPS have been performed to demonstrate that HMX crystals had been fully coated with PDA (Fig. S2-S4 in SI). After careful deconvolution of the XPS spectra, the shift of the peaks to higher binding energies was noticed for the N1s and O1s of NO2 for HMX, as well as the O1s of C-OH for PDA. It might indicate the formation of hydrogen bonds between the NO2 groups in HMX crystal surface and the OH groups in PDA. The β→δ polymorphic transition of HMX involves a substantial entropy component. Typically, the appearance of endothermic profiles in DSC curves (see in Fig. 1(a)) during the heating process was due to the β→δ transition. Thermal analysis


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studies revealed the significant retardation of polymorphic transition by PDA coating. Specifically, for the 1.2% and 1.5% PDA coated HMX (HP-3 and HP-4), the peak increased to about 247.6 oC, with a broad shoulder appeared at 220 oC and 225 oC, respectively; when the PDA amount was increased to 1.8%, the shoulder around 220 oC

disappeared, leaving only a profile with peak at 247.7 oC. Although various other

polymers would increase the polymorphic transition temperature of HMX as well, the most important advantage of using PDA coating is the highly efficient inhibition by a minimum amount. (Fig. S5 in SI). The morphology of naked HMX crystals, HP-4 and HP-5, after heating to various temperatures (at 10 oC min-1) and cooling down to room temperature, are subject to detailed observation by SEM, as showed in Fig. S6. The pristine HMX crystals maintain compact at 180 oC, but cracked at 190 oC. Almost all the naked HMX crystals cracked at 200 oC. However, both HP-4 and HP-5 kept intact at 200 oC and started cracking at 220 oC. The cracking of HMX crystals was attributed to the rapid release of the accumulated stress in the crystals. The temperatures for the cracking were consistent with the polymorphic transition of the HMX, indicating the efficient suppressing of HMX polymorphic transition by PDA coating.



Fig. 1. DSC curves and XRD patterns for HMX and HMX@PDA. (a) DSC curves for HMX with various coating amount of PDA. Content of PDA is 0%, 0.5%, 0.8%, 1.2%, 1.5% and 1.8%. (b-d) Temperature-dependent XRD patterns for (b) HMX, (c) HP-4 and (d) HP-5 from 180 oC to 200 oC, with the XRD patterns for pure β and δ phase HMX for reference. The indexes in red color correspond to those of δ phase from the ICDD database. The phase compositions of HMX and HMX/PDA have been determined by temperature-dependent XRD spectra, which were obtained under various temperatures for pristine HMX, HP-4 and HP-5 are shown in Fig.1(b-d). As presented in Fig. S7, the phase compositions for β-HMX and δ-HMX were determined by Rietveld refinement.18 It can be seen that the δ phase began to appear at 185 oC, as characterized by the emergence of the dominant (113) and (104) face of δ-HMX. After coating with PDA, the δ-HMX diffraction peaks show up at higher temperature and the content of δ-HM form was obviously lower than that of the naked sample. As shown in Fig. S7, 37.3% β phase was changed to δ phase for the naked sample at 200


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oC,

whereas only 6.9% and 3.9% transition in the cases of HP-4 and HP-5,

respectively. Detailed insight into the transition behavior of HMX and HMX/PDA will be performed by studying the non-isothermal and isothermal kinetics, taking HMX and HP-4 as typical comparison. 3.2 Non-isothermal kinetics for the polymorphic transition To understand the delaying mechanism for the transition of HMX by PDA, firstly, the non-isothermal transition kinetics has been investigated. The α-T curves for crystal transitions of both pure HMX and HP-4 (HMX@PDA) after deconvolution are shown in Fig. S8. The corresponding linear regression plots by a combined kinetic method using Eq. S3 (SI) for fitting was also included. The best linear fitting parameters of the plots were listed in Table S1. In order to comprehensively compare the Ea of naked HMX and HP-4, the dependence of Ea on α was plotted in Fig. 2(a) and summarized in Table S2. The activation energy for the β→δ polymorphic transition of HMX was about 400 kJ mol-1, which was almost independent on the extent of conversion. As for HP-4, the activation energies for the first and second step started at about 100 kJ mol-1 and 350 kJ mol-1 and showed apparent increase to about 350 kJ mol-1 and 620 kJ mol-1 with the increasing extent of conversion from 0 to 1.0, respectively. To better compare with the results, the functions are all normalized, see in Fig. 2(b). It can be seen that the transition process of pure HMX follow three dimensional growth of nuclei (A3) model, which is logical that the crystallization need nucleation and growth, and also approved by the results of our isothermal kinetic evaluation. As



the crystals of HMX were coated with PDA, the crystal transition process has been greatly changed. First of all, as shown above, the process could be divided in to two partially overlapped stages. Secondly, the physical model for the first transition step of HP-4 follows some mechanism between the first order reaction model (F1) and the 2D diffusion model (D2). It is probably due to strong interfacial interaction between HMX crystals and PDA molecular network, where the latter blocked the configuration transition of HMX. It is also very interesting that the second step of HP-4 is approaching to the 2D nucleation and nucleus growth model (A2), indicating the active sites for nuclear growth was decreased for HMX under PDA coating.

Fig. 2. (a) The plots of Ea with α for crystal transition HMX under non-isothermal conditions. (b) The normalized kinetic models with some ideal models for HMX and HMX@PDA (HP-4) by both master plots and combined kinetic analysis methods. As shown in Fig. S9, the isothermal curves for crystal transition of HMX at 179 °C and 183 °C have been predicted based on the parameters by the combined kinetic methods. One has to note that the theoretical curves follows well with the experimental ones only after correction of the cA value, since this value has a certain uncertainty according to the theory of the combined kinetic method. The cA for combined kinetic method could be finally determined by using this strategy, which


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should be 3.7×1040 for crystal transition of HMX. The isothermal kinetics for HMX-PDA can not be predicted by the above method, since it is very difficult to predict the isotherms for multi-step kinetic processes. 3.3 Isothermal kinetics for the polymorphic transition 3.3.1 Isothermal kinetics by Eq. 5 To quantitatively determine the rate of the polymorphic transition reaction and further understand the inhibited transition mechanism of HMX by PDA, isothermal heating DSC experiments were carried out at various temperatures and normalized for the degree of conversion, α(t) (Fig. 3). Considering that the calorimeter only recorded the thermal behavior of the polymorphic transition, but not directly measure the crystal phase, XRD measurements were carried out in order to verify the transition fractions derived from DSC. The procedure was: HMX sample was firstly isothermally heated by DSC, quickly cooled to room temperature and measured by XRD immediately (within 10 minutes) to avoid the inverse polymorphic transition from δ to β. As summarized in Fig. S10-S12, it shows that the transformed fraction derived from DSC curves were in good agreement with the XRD data. This correlation allowed us to use DSC curves to measure the isothermal phase-transition kinetics accurately. Firstly, in order to obtain the isothermal polymorphic transition kinetics parameters, the data for the polymorphic transition of HMX and HP-4 were modeled by using Eq. 5. As summarized in Table 1, the correlation factor R was above 0.998, indicating these fitted curves were consistent with experimental data for both HMX



and HP-4. For HMX, in the investigated temperature range (179 oC-183 oC), the rate constant obtained from the fit of Eq. 5 was 10-5 s-1 and 10-3 s-1 for nucleation and growth, respectively. As for HP-4, the temperature should be raised to 212oC, in order to get the thermal signals for the endothermic polymorphic transition reaction. In the investigated range (212oC-224oC), the nucleation and growth rate constant for HP-4 fell to 10-9 s-1 and 10-3 s-1, respectively, corresponding to an inhibition of the nucleation reaction by a factor of 104 although at much higher temperatures. Therefore, it clearly demonstrated PDA coating significantly decelerate the polymorphic transition rate, particularly, the nucleation process.

Fig. 3. The isothermal kinetic curves for HMX and HP-4 at various temperatures. DSC curves for (a) HMX and (c) HP-4; plots of extent of conversion against time (t) for (b) HMX and (d) HP-4. The solid lines are the fitting plots according to Eq. 5.


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Table 1. Fitting parameters for kinetics models of polymorphic transition of HMX and HP-4 by Eq. 5. Sample

T (oC)

kn (s-1)

kg (s-1)

R

179

3.5×10-5

6.1×10-3

0.9996

181

7.0×10-5

8.4×10-3

0.9996

182

9.3×10-5

11.6×10-3

0.9996

183

12.0×10-5

14.3×10-3

0.9988

212

6.7×10-9

5.7×10-3

0.9982

216

4.1×10-9

7.6×10-3

0.9990

218

4.6×10-9

8.8×10-3

0.9993

224

5.4×10-9

13.0×10-3

0.9998

HMX

HP-4

3.3.2 Isothermal kinetics parameters obtained by Logistic function Comparing the rate constant values of nucleation and growth, as shown in Table 1, clearly, kg >> kn, i.e., the nucleation is the rate determining process during the polymorphic transition reaction. Then, the Eq. 5 can be written as 1

 1

kg kn

e

1



 ( kn  k g ) t

e

( kn  k g ) t



kg

(6).

kn

Considering kg >> kn, therefore, 1 e

( kn  k g ) t



kg

≈0

(7).

kn

Accordingly, Eq. 6 can be rewritten as



1

a 1

kg kn

e

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(8).

 ( kn  k g ) t

Interestingly, the Eq. 12 is similar with the Logistic function as shown below y

a 1  be  kt

(9),

Here, by combing the Eq. 8 with Eq. 9, we can obtain k=kn+kg, a=1 and b= kg/kn, indicating k and b in logistic function can represent the total rate of conversions and the ratio between growth and nucleation rate, respectively. As can be seen from Fig. 4 and Table 2, the polymorphic transition kinetics for both HMX and HP-4 followed the Logistic equation with good correlation. The value of “a” was near to 1. The reliability of Eq. 9 was further verified by comparison of kn and kg. For HMX, the rate constant for nucleation and growth was 10-5 s-1 and 10-3 s-1, respectively. For HP-4, over a temperature range from 212 oC to 224 oC, the rate constant for nucleation and growth was 10-9 s-1 and 10-3 s-1, respectively. The rate constant values generated from Eq. 5 were consistent with those from Eq. 9, which further demonstrated that the nucleation was the rate determining process during the nucleation-growth polymorphic transition mechanisms. Furthermore, the Arrhenius temperature dependence of the HMX polymorphic transition data yielded a linear plot (Fig. 4(b)), the slope of which gave En = 333 kJ mol-1 and Eg =488 kJ mol-1, representing the activation energy for nucleation and growth, respectively. In the case of HP-4 (Fig. 4(d)), linear regression of these data (solid line) gave the activation energy of 100 kJ mol-1 and 131 kJ mol-1, with lnA of 5.1 and 27.4, for the nucleation and growth process, respectively. It was noted that the


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activation energy for HP-4 were lower than those of HMX, due to the higher temperatures for HP-4. Comparing with the activation energy calculated by non-isothermal kinetics, the values for both HMX and HP-4 were consistent with the initial values in the non-isothermal analysis (Fig. 2(a)).

Fig. 4. Isothermal kinetics plots of fraction reacted against time (t) and (b, d) natural logarithm of the nucleation (kn) and growth (kg) rate constant versus 1000/T for (a-b) HMX and (c-d) HP-4, respectively. The solid lines are fitting plots according to the Logistic function. Although the isothermal kinetics for HMX and HP-4 were measured at different temperature region, the Arrhenius plots enable us to predict the kinetic parameters and compare them at same temperature. According to Arrhenius equation, the predicted nucleation and growth rate constant for HMX at 212 oC would be 0.24 s-1 and 3.9 s-1, respectively. It clearly demonstrated that the PDA coating in HP-4 significantly decreased the polymorphic transition rate, especially for the nucleation process, which was decreased by eight orders of magnitude, comparing with the naked HMX.



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Table 2. Fitting parameters for kinetics of polymorphic transition of HMX and HP-4 by Logistic function. Sample T (oC)

a

b

k (s-1)

kn (s-1)

kg (s-1)

R

179

1.0037

187.6

6.2×10-3

3.3×10-5

6.2×10-3

0.9995

181

0.9998

119.6

8.7×10-3

7.2×10-5

8.7×10-3

0.9995

182

1.0021

138.0 11.9×10-3

8.6×10-5

11.8×10-3 0.9994

183

0.9998

136.9 14.8×10-3 10.7×10-5 14.7×10-3 0.9993

212

0.97418

2.0

6.1×10-3

3.0×10-9

6.1×10-3

0.9990

216

0.99496

2.1

7.7×10-3

3.6×10-9

7.7×10-3

0.9991

218

0.99502

2.3

9.0×10-3

3.9×10-9

9.0×10-3

0.9994

224

0.99943

2.4

13.2×10-3

5.5×10-9

13.2×10-3 0.9998

HMX

HP-4

3.4 Proposed inhibition mechanisms of PDA on HMX polymorphic transition PDA was coated onto the HMX crystals via the in-situ polymerization of dopamine monomers. Only physical interaction acts between HMX and PDA, because there are no reactive groups for chemical bonding reaction on the HMX crystals. The interfacial interactions between HMX crystal surfaces and PDA segments were calculated by means of molecular dynamics simulations (see SI for details calculations). When the optimized PDA segments model was employed to the HMX surfaces with a 5 nm vacuum layer between them, as shown in Fig. 5 and Fig. S13 (SI), both the cross-linked and linear PDA segments tightly bonded onto the HMX surfaces after the simulations. There exist considerably large interaction energy between the HMX crystal and PDA molecules, in which Van der Waals’ force contributes largest (Table


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S3 and Table S4). Combined with the nonisothermal and isothermal kinetics analysis, it is probably that, the strong interfacial interaction limited the freedom of the surface HMX molecular. It explains why the activation energy was increased, the reaction model was changed from three dimensional to two dimensional (Fig. 2), and the nucleation rate was significantly decreased (Table 1 and Table 2).

Fig. 5. Simulation of the interfacial interactions between cross-linked PDA and HMX crystal surfaces. (a) Typical morphology of β-HMX particle. (b) Model for the cross-linked PDA polymer segments. (c-h) The interfacial model between HMX crystal surfaces and cross-linked PDA segments before (c-e) and after (f-h) the simulation for (1 0 0) (c, f), (0 1 1) (d, g) and (1 1 0) (e, h). The inhibition of the polymorphic polymorphic transition might originate from



the interaction between NO2 in HMX and OH groups in PDA. β-HMX has chair-like configuration with the NO2 groups two above the ring plane and the other two below. As for the δ-HMX, all the -NO2 in the molecular configuration located on the same side of the ring plane (boat-like configuration). The HMX molecular involves turning the -NO2 group over to achieve the polymorphic transition. Consequently, the nucleation of δ phase nuclei prefers to be initiated from the free surface of crystals, where the hindrance is lowest, especially for the high-quality crystals where internal defects can be avoided. As shown in Fig. 6, in the case of naked HMX, the nucleation starts from the surface, which is barrier free. As soon as the surface nucleation starts, the polymorphic transition quickly spreads to the entire crystal. In the case of HMX@PDA, combining the simulations with kinetics studies, it implies that the interfacial interaction played a key role in slowing down the nucleation rate. XPS spectra noticed the shift to higher binding energies for the N1s and O1s of NO2 for HMX, as well as the O1s of C-OH for PDA, suggesting the existing of strong interaction between the NO2 groups in HMX crystal surface and the OH groups in PDA (Fig. S4). Due to strong interaction, the configuration transition was restricted, which in turn decreased the nucleation rate.


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Fig. 6. Illustrated mechanism for the inhibited transition of HMX after surface confinement by PDA. Interestingly, the polymorphic transition of PDA coated HMX was correlated well with the pressure effect on the polymorphic transition of HMX. Combined the nonisothermal curves with the pressure-temperature relationship reported by Brill and Karpowiz,14,

33

it was noted that the temperature at which the pressure dependence

becomes asymptotic is about 247 °C. This temperature was coincident with the polymorphic transition peak for HP-4 and HP-5 (see the DSC curves in Fig. 1(a)) reported in this work. It implies that the inhibition of PDA on the polymorphic transition for HP-4 and HP-5 corresponds to at least 200 MPa pressure effect on the naked HMX. Coincidently, the inhibition of polymorphic transition of CH3NH3PbI3 induced by PMMA coating was also attributed to the increase of the stress in CH3NH3PbI3 layer (about 200~300 MPa with respect to its phase diagram).37 The correlation of these phenomena indicates that the inhibition effect of PDA coating on the polymorphic transition of HMX might be partly attributed to the stress due to



strong interfacial interaction between PDA and HMX. The stress limited the nucleation of the δ phase nuclei from the surface. Furthermore, the strong interaction between PDA and HMX brought constraint effect to the HMX crystals. This effect postponed the stress release, protected the crystals from cracking, prevented the exposure of new free surfaces and suppressed the polymorphic transition. In a word, the strong interfacial interaction played a key role in inhibiting the polymorphic transition. This inspires us that surface functionalization with strong interfacial interaction might provide a promising approach to suppress the polymorphic transition of the polymorphic crystals. Conclusions In summary, we understood the mechanisms for strong surface confinement by PDA to inhibit the polymorphic transition of HMX, which was supported by systematical polymorphic transition behavior, molecular dynamics simulations, non-isothermal and isothermal kinetics modeling. The strong interfacial interaction between the NO2 in HMX crystal surface and the OH groups in PDA restricted the chair-like to boat-like configuration transition, decreasing the nucleation rate of new phase. Suppressing the polymorphic transition can improve the thermal stability, which is practically important for the safety of the energetic materials. The strong interfacial interaction coating approach and kinetics models in this work can be not only extended to the phase stabilization of other polymorphic energetic materials (etc., CL-20 and FOX-7), but also provide insights for other organic functional materials. Supporting Information Detailed experimental, theory background and parameters


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Kinetics for the Inhibited Polymorphic Transition of HMX Crystal after

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