Molecular Dynamics Simulation of β-HMX Crystal Morphology Induced by Polymer Additives

Round 1

Reviewer 1 Report

It is an interesting study of simulation of the crystal morphology for important energetic material. I think that material is sound and should be accepted. I have only minor recommendation to add in the introduction the discussion of some other aspects of the polymer addition - beneficial changes of the mechanical properties and mechanical sensitivity (references: 10.1021/acsami.9b20323 and 10.1016/j.cej.2021.131363)

Author Response

Thanks for your valuable comments. Encapsulation coating of energetic crystals has been a hot topic in the industry. The mechanical sensitivity and friction sensitivity of explosive crystals after coating may become better. The discussion on the crystal sensitivity of explosives after polymer addition has been added to the introduction.

Reviewer 2 Report

The manuscript deals with the effect of different additives such as PVA, PAA and PEI on the growth morphology of beta-HMX crystal in mix water-DSMO solvents. It is well established that factors like impurities coverage, supersaturation, temperature, solubility etc. greatly affect the growth morphology ((J. Am. Chem. Soc., 2012, 134 (41), pp 17221–17233; Cryst. Growth Des., 2015, 15 (7), pp 3220–3234; Cryst. Growth Des. 2013, 13, 2413−2425). Authors work are unable to access the impact of these parameters on the growth morphology of beta-HMX crystal, the appropriate discussion about why they neglected the above mentioned factors in the benefit to the reader. Following are some specific comments which authors may find useful:

1. The present manuscript employed oversimplified growth model (ie. HP) to compute various energetics to predict crystal morphology. However, to account the effect of solvent, the authors have applied energy correction factor to the attachment energy. The morphology predicted solely on the basis of attachment energy best fit with the morphology obtained with vapour growth. Mere energy correction may not accurately predict the growth morphology from solvent and additive-mediated crystallization.
2. For accurate simulation of growth morphology of crystal from solution, one needs to compute step energies of the growing unit in different orientation and takes care of various controllable parameters such as driving forces for crystallization (supersaturation), temperature, solubility and additive coverage/concentration etc in the solution. The solute molecules generally directly adsorbs in the growing ledge of steps on the flat-faces rather than diffuse on the face as in the case of vapor growth.
3. How interaction of additive with crystal surface affect the morphology? An expression similar to Eq.(3-7) for additive may clarify the additive role on morphology.
4. Why simulated growth morphology of beta-HMX with 10% additive matches with experimental results?
5. In mix solvents (water-DSMO), the solubility of beta-HMX may varies that could also affect the morphology of beta-HMX. Please comments why the authors have neglected the variation of solubility in mix solvent for the benefit of readers.
6. In Table 5, the authors show the different computed energetics, which is, employed to determined Es and Eint as per Eq 3) and (7). However, it is generally thought that additive may greatly affect the step velocity of growth steps on the F-faces that ultimately determined the growth morphology of crystals. It is not clear how additives affect the morphology of beta-HMX crystal. The authors only calculated the modified Etot, Esur, Einit to substantiate the effect of additive on morphology. Do the chosen additive only sterically hinder the adsorption of solute and solvent only?

 

I do not evaluate the present form of the manuscript is publishable in Crystals as the manuscript does not add new insight into the effect of solvent and additives on the crystallization process. If the authors are willing to revise their manuscript, they are requested to incorporate some studies related to the factors mentioned above, so that readers find the manuscript interesting. In view of this, I recommend a major revision.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors present on the effects of polymer additives on the crystal morphology of beta-HMX through the use of molecular dynamics simulations (MDS). Ultimately the authors showed how the morphology of beta-HMX can be predicted when in the presence of solvent, water and polymer additives, which have a significant effect on the crystal habit. Being able to identify/control the crystal morphology of an energetic materials is critical to the safety and performance, and this computational study on the three selected polymer additives match well with previously published experimental HMX crystal data. This is a very nice paper that systematically addressed the issues seen in the published experimental results through the use of MDS and helps to add to the previous crystallization work on HMX. This paper should be published with minimal changes. The authors should address the minor changes on page 10 paragraph 2, 1) change wang to Wang and 2) fix the reference after “DMSO solution through a peristaltic pump. Error! Ref-erence source not found..”.

Author Response

Thanks for your affirmation and suggestions on this manuscript.

1) Due to my negligence, the case has been changed now.

2) ”Error! Ref-erence source not found” is “figure 8”, which has been changed now.

Reviewer 4 Report

This paper resorts to MD simulations to address an issue of great practical significance in the field of energetic materials, aimed at tuning the crystal morphology of an energetic material in view of reducing its sensitivity. A substantial amount of work has been done. However, they are serious issues with the manuscript that prevent its publication.

First, the authors provide virtually no data to validate the the force field with respect to the problem addressed. Before undertaking large-scale MD simulations like reported here, it must be checked that the model is consistent with available data, either experimental or theoretical. In the present work, this implies that the potential must provides reasonable estimates of available densities, sublimation enthalpies, perhaps solvation free energies…

Moreover, beyond validating the model against experiment, the authors must compare it to earlier similar models. This is all the more important as according to my own experience with Materials Studio, this tool is rather designed for “quick and dirty” simulations in support of experiment like those reported for HMX morphology in a recent study https://link.springer.com/article/10.1007/s12039-017-1250-1. In general, results obtained using this software are not reproductible as the authors fail to report all simulation details. For instance, the very large error of less than 10% reported here regarding lattice parameters appears inconsistent with the lattice parameters (also very far from experiment) reported in another similar study [CrystEngComm, 2019, 21, 4910], perhaps due to a different version of COMPASS. This shows that all simulations details must be reported, especially with using this software in which default options and force fields may change from one version to another.

Anyway, I disagree with the authors statement that such a huge error is “acceptable”. This reflects a poor description of intermolecular forces, which may hamper the value of the calculated surface energies, interactions with solvents and polymers… It is a pity that the authors did not shift to a more accurate force field before embarking on the whole study, especially when HMX is a very well studied explosive for which there are simple and proven force fields, such as Smith's [J. Phys. Chem. B, 1999, 103(18), 3570–3575].

Surprisingly, the authors mention previous works (“A large number of previous studies have shown that…” on p.3, “researchers find that… on p.4,…) without providing any references. This is not acceptable in a research paper.

Generally speaking, simulation details are either lacking or unclear, for instance: “Atom-based method used to calculate van der Waals force” => of course it is atom-based, but providing the cut-off radius would be more informative.

The simulations appear to rely heavily on MD. Unfortunately, the details of how the energies needed in eqs. 3-8 are not given. There is a procedure which is especially popular in the community of MS users to predict morphologies as a step-by-step procedure was published by an early user (a German PhD as far as I remember). However, this procedure does not basically relies on MD, which is used only to obtain an amorphous layer of solvent molecules to mimic the fluid phase, although it is simply relaxed to a local equilibrium from which the relevant energies are computed. Therefore, it is very important that the authors make it clear whether they use this procedure, or alternatively, true MD simulations where the energies must be averaged in some way.

On p.8 it is stated that “the crystal volume decreases with the increase of water mass fraction”. The significance of this volume is specially unclear. For me, it is an input of the model and has no special significance, except for the fact that it must be adjust to avoid artefacts, taking also into also on the solvent composition which could require a larger simulation box.

The conformations shown on Fig.6 are also dubious as minima obtained from simulated annealing. In gas-phase, such flexible chains adopt much more coiled conformations due to van der Waals interactions. Were they optimized in an implicit or explicit solvent? Do they result from an incomplete optimization? This sounds really puzzling…

I cannot see that the morphology similated in DMSO with >=5% PVA [(d) and (e) on Figurer 7] is similar to experiment [(a) on Figure 8]. One should find a better way to compare experiment and simulated morphologies. In my eyes they differ significantly, but the comparison is made difficult due to the distribution of shapes observed on some pictures.

Notwithstanding these major issues, the qualitative results reported and the discussion are rather interesting, especially regarding the role of polymer chains on the diffusion of small molecules and on crystal morphologies. Therefore, the authors should strive to improve the manuscript so that it may be eventually published. Ideally, the about 10% error on lattice parameters should be fixed, but I am afraid this would be too much work.

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 5 Report

The authors present an interesting computational study on crystal morphology of β-HMX crystallized in different polymer additives in the solution, and the simulation results were in good agreement with the experiment. Effect of water on the crystal morphology of β-HMX and the effect of additive PVA on the solute and solvent diffusion ability during crystal crystallization were studied. I think the revised manuscript could be of general interest of the journal audience and I recommend its publication.

Author Response

Thank you very much for your affirmation.

Round 2

Reviewer 2 Report

I do not feel that the authors have adequately addressed the reviewer's comments that could further improve the manuscript. For "Accurate morphology prediction" one should also consider the kinetic factors. The morphology predicted using energetics do not represent the growth morphology morphology.

Author Response

Thank you for your comments, all the words “accurate morphology prediction” have been changed to “morphology prediction”. In this paper, only the crystal morphology is predicted by energetics. Due to my limited knowledge, I do not know how to use kinetic methods to carry out experiments under molecular dynamics simulation. And for graduation, I have no time to do more. In fact, the relevant articles in this paper are also designed by the energy method, and the kinetic scheme is rare.