Review Reports - A Study on the Effects of Vacuum, Nitrogen, and Air Heat Treatments on Single-Chain Cellulose Based on a Molecular Dynamics Simulation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Introduction,

It is necessary to indicate why bamboo was selected for the study. Why a wood species was not chosen for the study?

The section on molecular dynamics is confusing, explain its relevance more clearly.

The text is included with unnecessary citations, Only include the most essential references.

Material and Methods

Please indicate the bamboo species, and provide some physical properties such as density,….

The terms like "degree of polymerization" and "molecular chains" does not consistently define them. This may confuse readers unfamiliar with polymer chemistry. A clear definition of these terms should be provided early in the text.

While the text provides details about simulation parameters, it lacks justification for the chosen values. For example, why was a degree of polymerization of 20 selected? Providing references or reasoning for these choices.

Why specific methods like NVT and NPT were chosen for the simulations?

Are the temperature ranges for kinetic simulation based on previous studies, or do they represent a novel exploration?

Results and discussion

The comparison between the vacuum, nitrogen and air models is interesting, Please Consider discussing the implication of these findings for practical application, such as how different environments might be selected based on the desired properties in cellulose-based materials.

Conclusion

I would recommend shortening this section and focusing on the obtained results

Comments on the Quality of English Language

Minor editing of English language required

Author Response

Dear reviewer,

We feel great thanks for your professional review work on our article. As you are concerned, there are several problems that need to be addressed. According to your nice suggestions, we have made extensive corrections to our previous draft, the detailed corrections are listed below.

Introduction

Comment 1: It is necessary to indicate why bamboo was selected for the study. Why a wood species was not chosen for the study?

Response 1: We sincerely thank the reviewer for careful reading. The research background of this paper is mainly referenced to the changes in the properties of wood such as beech as well as pine under heat treatment in different media, the chosen research object is wood, which is reflected in reference [7] [8]. The paper also refers to the change process of heat treatment of many bamboo materials, which is reflected in reference [6] [33]. In addition, the focus of this paper is mainly on the effects of heat treatment on the cellulose-related properties of wood from a microscopic point of view. Although there are many species of wood and bamboo, their main component is cellulose, so the specific species may not have an impact on the study. Due to the limited ability to write a thesis book, we are very sorry that we were not able to make it clear to you the first time, and we would like to explain it to you in detail.

Comment 2: The section on molecular dynamics is confusing, explain its relevance more clearly.

Response 2: Thank you very much for your suggestion. In order to more clearly explain the correlation between molecular dynamics and this study. In lines 85-87, this paper adds that molecular simulation technology provides a new idea for the study of wood heat treatment, and this method provides a good theoretical basis for the existing macroscopic experiments. In addition, in lines 111-114, it is mentioned that the current research focus in the field of wood heat treatment is mostly focused on the experimental stage, but it is still insufficient to reveal its internal mechanism. In this paper, molecular dynamics was used to explain the effect of temperature change on the related properties of wood during heat treatment.

Comment 3: The text is included with unnecessary citations, Only include the most essential references.

Response 3: We're really sorry for our careless mistake. We have checked and modified the references in this part.

Material and Methods

Comment 4: Please indicate the bamboo species, and provide some physical properties such as density,….

Response 4: Thank you very much for your careful reading. This paper focuses on the effects of heat treatment on the cellulose-related properties of wood from a microscopic point of view, and therefore the species of wood is not covered in detail in this paper. Although there are many species of wood as well as bamboo, their main component is cellulose, so the specific species may not have an impact on the study.

Comment 5: The terms like "degree of polymerization" and "molecular chains" does not consistently define them. This may confuse readers unfamiliar with polymer chemistry. A clear definition of these terms should be provided early in the text.

Response 5: We were really sorry for our careless mistakes. In line 136 and line 150, the definition of molecular chain and degree of polymerization are added to help readers better understand the article.

Comment 6: While the text provides details about simulation parameters, it lacks justification for the chosen values. For example, why was a degree of polymerization of 20 selected? Providing references or reasoning for these choices.

Response 6: Thank you very much for your question. First of all, the setting of relevant simulation parameters in this study was determined after consulting relevant literature, and the specific literature has been marked in the paper. In line 160 of the paper, it is mentioned that the scale setting of the simulation model constructed by Wang Yunting et al. [20] was referenced in this study. Secondly, the reason why a cellulose chain with a polymerization degree of 20 was selected in this study is explained in the supplementary explanation in lines 152-158. In the molecular dynamics simulation, the simulation results are consistent with the actual situation when the polymerization degree of cellulose is greater than 10, and the different polymerization degrees of cellulose have little influence on the simulation results of material properties. Although the actual chain length of cellulose is quite large, the increase in chain length during the simulation will increase the simulation time and complexity. Therefore, cellulose chains with a polymerization degree of 20 were selected in this study.

Comment 7: Why specific methods like NVT and NPT were chosen for the simulations?

Response 7: Thank you for your question. In molecular dynamics simulations, the selection of specific ensemps (such as NVT and NPT) is based on an understanding of the simulation objectives, etc., aimed at efficiently simulating and understanding the behavior of materials under different conditions. First, the reasons for choosing NVT ensemble when adding kinetic relaxation in lines 205-209. The NVT ensemble can keep the number of particles (N), volume (V), and temperature (T) of the system constant. This is because after geometric optimization, we are more concerned about the vibration behavior of the system at a specific temperature, and the change of volume is not the main concern at this stage. Through the NVT ensemble, the thermal motion of the system at normal temperature can be simulated, and the system can reach the thermal equilibrium state. Secondly, the reasons for choosing NPT ensemble when adding molecular dynamics simulation in lines 217-221. The NPT ensemble not only keeps the particle population and temperature of the system constant, but also allows the volume of the system to change. This is essential for studying properties such as phase transition, thermal expansion and compressibility of materials at different temperatures and pressures.

Comment 8: Are the temperature ranges for kinetic simulation based on previous studies, or do they represent a novel exploration?

Response 8: Thank you very much for your question. The temperature range of the kinetic simulation is based on the results of the combination of previous research and new exploration, and the temperature range of 423-503K is determined, and the simulation experiment is conducted on this basis. On the one hand, the upper limit for this study, based on Bal and Candelier et al., is 503K. On the other hand, with the continuous development of computational techniques and simulation methods, this study attempts to simulate the behavior of materials at more temperatures. These new simulations make it possible to explore a wider range of temperatures.

Results and discussion

Comment 9: The comparison between the vacuum, nitrogen and air models is interesting, Please Consider discussing the implication of these findings for practical application, such as how different environments might be selected based on the desired properties in cellulose-based materials.

Response 9: I'm sorry we didn't discuss practical applications enough in this section. In lines 464-477, we add a discussion of this section. The discussion in this paragraph is mainly divided into two practical applications of decoration architecture and art design. According to the specific application scenarios and required characteristics of wood, the comprehensive consideration is made to maximize the potential of wood and meet the diversified needs of cellulose based materials in different fields.

Conclusion

Comment 10: I would recommend shortening this section and focusing on the obtained results.

Response 10: Thank you very much for your advice. In response to your suggestions, we have shortened this section to focus on the results obtained.

Thank you very much for your attention and time. We look forward to hearing from you soon.

Yours sincerely,

Youna Hua

Reviewer 2 Report

Comments and Suggestions for Authors

1. Include some numerical values in the conclusion.

2. Correct the mistake in this "TranslatFFigure 2"

3. Give technical inference for figure 2, why that specific wave pattern occurred.

4. According to table 1, the range variation in the density is very less, give the technical reason to it.

5. Give the technical data for Mean square displacement graph according to figure 3, in detail.

6. Explain figure 4, why there is hydrogen bond variation occurring due to temperature

7. According to table 2, for different temperature air provide the better results, state why?

8. Explain the figure 5, why vacuum gives higher young's and shear moduli.

9. Overall, inference part can be improved in the manuscript.

Comments on the Quality of English Language

Moderate language correction is required.

Author Response

Dear reviewer,

Comment 1: Include some numerical values in the conclusion.

Response 1: Thank you very much for your careful examination. We added some numerical values to the conclusion to better support the conclusion.

Comment 2: Correct the mistake in this "TranslatFFigure 2"

Response 2: We were really sorry for our careless mistakes. In lines 264-265, we have corrected the error.

Comment 3: Give technical inference for figure 2, why that specific wave pattern occurred.

Response 3: Thank you very much for your question. In lines 248-254, we give a detailed explanation of the wave in Figure 2. At the beginning of the simulation, the system usually starts from a specific initial state, which can result in uneven energy and temperature distribution. Therefore, in the early stages of the simulation, significant fluctuations in energy and temperature can be observed, which are manifestations of the transition of the system from non-equilibrium state to equilibrium state. Over the simulation time, if the system gradually reaches an equilibrium state, then the fluctuations in energy and temperature will stabilize and fluctuate around some average value.

Comment 4: According to table 1, the range variation in the density is very less, give the technical reason to it.

Response 4: According to your suggestion, we have added the reasons for the small density change in lines 295-297. As a kind of polymer material, cellulose has a lot of hydrogen bonds and other interaction forces between its molecular chains to maintain a relatively stable structure. Although vacuum, nitrogen and air environments have different effects on the interaction of cellulose chains, they do not lead to fundamental changes in the overall structure of the cellulose chains. Therefore, the range of variation in density may be relatively small.

Comment 5: Give the technical data for Mean square displacement graph according to figure 3, in detail.

Response 5: Thank you very much for your suggestion. In lines 337-342 and 349-351, we add the detailed data of the mean azimuth shift chart. Therefore, it is more intuitive to find that the average orientation displacement value of the cellulose chain fluctuates gently in the vacuum model, while the average orientation displacement value of the cellulose chain fluctuates sharply in the air model.

Comment 6: Explain figure 4, why there is hydrogen bond variation occurring due to temperature.

Response 6: Thank you very much for your question. In lines 390-396, we add the reason why the temperature increase causes the hydrogen bond change. With the increase of temperature, the speed and frequency of molecular movement will increase, resulting in more frequent intermolecular collisions, making the distance and relative position between molecules change, and the hydrogen bond within the chain is destroyed or even broken, resulting in the overall number of hydrogen bonds reduced.

Comment 7: According to table 2, for different temperature air provide the better results, state why?

Response 7: Thank you very much for your careful reading. It can be seen from the data in Table 2 that the γ of the air model is higher than that of the vacuum and nitrogen models at any temperature. In lines 458-461, we add an explanation of this result. The oxygen in the air can promote the oxidation reaction on the surface of the wood and form an oxide layer, which can maintain the moisture and humidity inside the wood to a certain extent, and is conducive to maintaining the plasticity of the wood.

Comment 8: Explain the figure 5, why vacuum gives higher young's and shear moduli.

Response 8: We are sorry that we did not explain in the paper why the vacuum environment provides better Young's modulus (E) and shear modulus (G). In lines 446-450, we add an explanation of this reason. The vacuum environment contributes to the rapid discharge of water inside the wood during the heat treatment process, thus promoting the densification of the wood. With less moisture, the wood becomes more rigid to resist deformation. Therefore, the vacuum environment has a higher Young's modulus (E) and shear modulus (G).

Comment 9: Overall, inference part can be improved in the manuscript.

Response 9: Thank you very much for your advice. In this section, we have made modifications and adjustments. Add more dimensions to the analysis, delve deeper into the reasons behind the data, and ensure logical coherence between the various parts of the reasoning process.

Thank you very much for your attention and time. We look forward to hearing from you soon.

Yours sincerely,

Youna Hua

Reviewer 3 Report

Comments and Suggestions for Authors

In this manuscript, the authors employ molecular dynamics (MD) to study the effects of different environments (vacuum, air, and nitrogen) on the properties of amorphous cellulose at elevated temperatures. Properties assessed include cell parameters, densities, mean square displacement, hydrogen bonding, and mechanical properties. The motivation for the study is to better understand how thermal modifications treatments of wood under the same environments may be affecting the amorphous cellulose in the wood during treatment. However, the structures studied by MD only contained a single cellulose chain with a degree of polymerization of 20. It is not possible to make meaningful property measurements from a single polymer chain in MD. Furthermore, in wood the amorphous cellulose is surrounded by other amorphous cellulose chains, as well as other wood polymers that are embedded inside of cell walls, which is much different than the single cellulose chain surrounded by vacuum or gases in MD simulations of this paper. So even if meaningful property measurements could be made, it is unclear how the results could relate back to wood undergoing thermal treatment in any meaningful way.

Comments on the Quality of English Language

Paper was well written.

Author Response

Dear reviewer,

Comment 1: The structures studied by MD only contained a single cellulose chain with a degree of polymerization of 20.

Response 1: Thank you very much for your careful reading. In this paper, a single cellulose chain is used, which is determined on the basis of consulting a lot of relevant data. Both Ouyang et al. and Jiang et al. used cellulose chains in their studies and were recognized(DOI:10.3390/f13060903;10.3390/f14061208.). Single-strand simulations may not fully reflect the true behavior of amorphous cellulose in wood. However, due to the limitations of laboratory funds and time, it is not possible to conduct multi-chain simulation. We plan to increase the complexity of the simulation system in future studies, simulating systems containing multiple cellulose chains to simulate the interactions between cellulose chains in wood while considering the effects of other polymers such as hemicelluloses. Thanks again for your question.

Comment 2: In wood the amorphous cellulose is surrounded by other amorphous cellulose chains, as well as other wood polymers that are embedded inside of cell walls, which is much different than the single cellulose chain surrounded by vacuum or gases in MD simulations of this paper.

Response 2: There is a difference between the simplified model constructed by the molecular dynamics simulation, in which a single cellulose chain is surrounded by a vacuum or gas, and the complex environment of the cellulose chain in real wood. However, this paper mainly studies the microscopic mechanism of the effect of different temperatures and media on the properties of cellulose amorphous region, and more intuitively explains how it acts on cellulose, the main component of wood, which is crucial to improve the utilization rate of wood. Therefore, in this paper, individual cellulose is used as a representative study, and our simulation aims to provide a preliminary, microscopic perspective to explore the effects of different environments on the cellulose chain.

Thank you very much for your attention and time. We look forward to hearing from you soon.

Yours sincerely,

Youna Hua

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

There are no further comments

Author Response

Dear reviewer:

Thank you very much for your professional review which has greatly improved our manuscript. Please accept my sincere thanks and best wishes.

Yours sincerely,

Youna Hua

Reviewer 3 Report

Comments and Suggestions for Authors

Thank you for your responses to my comments. However, I disagree that meaningful physical properties like density and mechanical properties can be obtained from a simulation with a single molecular chain. Furthermore, it remains unclear how the simulations relate to a cellulose chain inside of a wood cell wall.

Because it is not possible for the authors to do a more realistic amorphous cellulose material volume with multiple cellulose chains, it is recommended that the authors rewrite the manuscript to focus only on effects of the treatments on the cellulose chain chemistry. The link between these simulation results on a cellulose chain and the amorphous cellulose in wood is very weak at best.

Comments on the Quality of English Language

Paper is easy to read and understand.

Author Response

Dear reviewer:

Thank you for your decision and constructive comments on my manuscript. We have carefully considered the Reviewer's suggestion and made some changes. We have tried our best to improve and made some changes to the manuscript. It has been modified according to your comments, and the specific modification is described as follows:

After a group discussion, all of our co-authors unanimously acknowledge and appreciate your suggestion. These suggestions prompted us to significantly optimize and adjust the research framework and presentation details of the paper. More importantly, we have precisely shifted our focus to the field of molecular dynamics simulation of cellulose single chains. In the 2-4 lines of the paper, we change the title to the molecular dynamics simulation of cellulose single chains and shift the focus of the paper to the study of cellulose chains. In particular, the introduction of lines 32-55 of the article was rewritten, and the research was carried out with cellulose as the research entry point. Since this cellulose chain is used in many heat treatment experiments on wood, it can be preliminarily concluded that our experimental results can also be helpful to related heat treatment experiments.
Given the complexity of cellulose chains in wood and the challenges of comprehensive simulation, we chose a widely validated single chain structure of cellulose as the basis for our research, and data on key physical properties such as density provide solid and effective support for our research. In addition, the single-chain structure of cellulose selected in this paper has been confirmed by many studies, and we list the following references as a solid foundation for our research work. At the same time, we honestly recognize that despite our efforts to improve, there are still some limitations in this paper, mainly due to the initial exploration of cellulose multi-chain systems, different kinds of cellulose chains, and even the complexity of lignin chains. Looking to the future, we are committed to expanding our research horizons and exploring more diversified analytical paths, including but not limited to the dynamic behavior of cellulose multi-chain systems and the interaction mechanisms between different cellulose chains, to contribute more comprehensive and in-depth insights to the field.

[1]Wang, W.; Ma, W.; Wu, M.; Sun, L. Effect of Water Molecules at Different Temperatures on Properties of Cellulose Based on Molecular Dynamics Simulation. BioResources 2022, 17, 269–280. https://doi.org/10.15376/biores.17.1.269-280.

[2]Wang, W.; Li, J. Effect of Vacuum and Heat Treatment on a Single Chain of Cellulose: Molecular Dynamics Simulation. BioResources 2023, 18, 187-197. https://doi.org/10.15376/biores.18.1.187-197.

[3]Ouyang, F.; Wang, W. Effect of Thermo-Hydro-Mechanical Treatment on Mechanical Properties of Wood Cellulose: A Molecular Dynamics Simulation. Forests 2022, 13, 903. https://doi.org/10.3390/f13060903.

[4]Guo, Y.; Wang, W.; Jiang, X. Molecular Dynamics Study on Mechanical Properties of Cellulose with Water Molecules Diffusion Behavior at Different Oxygen Concentrations. Forests 2023, 14, 371. https://doi.org/10.3390/f14020371.

[5]Tao, J.; Zhan, H.; Luo, C.; Hu, S.; Duan, X.; Liao, M. Diffusion Properties of Gas Molecules in Oil–Paper Insulation System Based on Molecular Dynamics Simulation. Energies 2024, 17, 3811.https://doi.org/10.3390/en17153811.

Although a single cellulose chain cannot fully replicate the complexity of the entire cell wall, it can represent the basic building blocks of cellulose microfilaments in the cell wall. On the one hand, cellulose acts as a skeleton in the cell wall and has an important influence on the strength of the material. On the other hand, the hydrogen bond between cellulose chains is the key to maintaining the stability and mechanical strength of wood cell walls. Molecular dynamics simulation of the formation and breaking of hydrogen bond networks of individual cellulose chains can help to understand the stability changes of cellulose under different conditions. Therefore, the simulation can be considered valid.

Thank you very much for your attention and time. We look forward to hearing from you soon.

Yours sincerely,

Youna Hua