Molecular Dynamics Modeling for the Determination of Elastic Moduli of Polymer–Single-Walled Carbon Nanotube Composites
Round 1
Reviewer 1 Report
In the publication, the authors conducted an intriguing investigation on the impact of nanotubes on the elastic moduli of polymer composite materials using atomistic simulation. The study delved into the mechanical properties of composites, with a particular focus on the concentration of Single-Walled Carbon Nanotubes (SWCNTs) and their effect on the strength characteristics and glass transition temperature of the composite. The paper can be accepted for publication after major revision.
Please add an explanation of why polypropylene, poly(ethyl methacrylate), and polystyrene were chosen for theoretical predictions.
It is completely not clear, except for highly specialized experts, why the molecular weight of the polymers varied in the models for different concentrations of the nanotubes. This should be explained in detail.
In conclusion, the authors noted, ".....at low concentrations of SWCNTs, the value of the strength characteristics increases. However, after reaching the critical mass, their values decrease sharply". However, any discussion or explanation for this phenomenon was not provided in the text. In general, it is a well-known fact that is related to the fabrication of nanotubes agglomerates.
The paper did not consider information on the degree of nanotube dispersion and nanoarchitectonics at the surface of the nanotubes, which strongly influence the mechanical properties of composites. Including a discussion on the role of nanotube dispersion and the effect of nanoarchitectonics on the mechanical properties of the composites will enrich the study and contribute to a more comprehensive understanding of the results.
I suggest the citation of the paper where a polymer composite with carbon nanotubes was developed: https://doi.org/10.1016/j.apsadv.2021.100104
Minor editing of English language required.
Author Response
Dear referee,
Thanks for your comments that helped improve the article.
Below we answer questions raised by the reviewer and point out some corrections to our manuscript that we hope will improve clarity.
- We have added an explanation why polypropylene, poly(ethyl methacrylate) and polystyrene were chosen for the theoretical prediction.
«These materials are thermoplastic polymers, which are widely used in the manufacture of products for the aviation, medical industry, construction, instrumentation and household products. The addition of CNTs would help to increase the strength of these materials and extend their service life.»
- The molecular weight of the polymers varied in the model for different concentrations of nanotubes varied in order to achieve the desired ratio of CNTs to polymer. At high degrees of polymerization (more than 400), the effect of the length of the polymer chain on the strength and glass transition temperature is insignificant. However, this is precisely what makes it possible to smoothly reach "small" concentrations of CNTs (2%, 3%, 4%...%).
- We have added in the conclusion your remark about the discussion of adding CNTs after reaching the critical value.
«It is a well-known fact that the agglomeration of the filler leads to a decrease in the strength of the composite, which was shown in this study.»
- Thanks for the link to an interesting article. We have carefully studied it. Our article did not touch on the role of nanotube dispersion and the effect of nanoarchitectonics on the mechanical properties of composites, since consideration of these issues is a topic for another large work. In our work, the constructed nanotubes were not modified or improved in any way. We have added a link to the paper you suggested, as its results may improve understanding of the impact of nanoarchitectonics for the creation of polymer composites.
«Modification of CNTs can help improve the interaction between CNTs and the polymer matrix. In the article [17] it was noted that The NH2-MWCNTs modified with MPIs are easily dispersed in a thermoplastic elastomers matrix and improved the mechanical properties of nanocomposites in comparison with non-modified fillers. »
Reviewer 2 Report
This paper presents results on the mechanical properties of several CNT/polymer composites. The authors used MD simulations to study the influence of CNTs on the elastic moduli and Tg of different polymers. They analyzed the effect of size, concentration and structure of CNTs.
I think the authors should rewrite the sections related to the calculation methodology and model construction, since they are difficult to understand. Besides, they tried to study the influence of CNT concentration, size and structure on polymer properties comparing polymers with different degree of polymerization, which could also influence these properties. It is difficult to separate the effect of CNT characteristics from that of polymer molecular weight. One of them must be kept constant.
The paper present some interesting results but several important issues should be clarified and improved before publication. I cannot recommend publication in present form.
1. Models:
The authors must give a more detailed description of the models they built.
-How many polymeric chains did they use in each cell? Only one with different degree of polymerization?
-How many CNTs did they use in each model for low concentrations? (Table 1 does not show the CNT number)
-What are the densities of the equilibrated cells?
-Did they create a trajectory for each system, or build 30 different random models for each concentration and polymer and analyzed only the final resulting structure after molecular dynamics?
2. The stages of the calculations are difficult to understand. The authors must rewrite the methodology they used to relax and analyze their models. For instance, the authors state that, “at the third stage, the resulting cell density was set”. What do they mean? Please, clarify the methodology. It is hard to follow your procedure.
3. The authors state that “the arrangement of nanotubes also plays an important role: the more ordered it is, the more rigid the composite in the direction of most nanotubes arrangement” Can they prove this statement from their calculations? This can be related to the number of CNTs used in their models. It is important to clarify this point. Do they have some images showing the arrangement of several CNTs contained in the same cell? They only include the image of a cell containing one CNT.
4. The authors state that for high CNT concentration, the mechanical properties degrade and state that “This may be due to the fact that an insufficient amount of polymer cannot hold the SWCNT together by van der Waals forces, and the composite is destroyed at the same values of the mechanical stress”. As in point 3, could the authors show an image of CNT aggregation? (Table 2 shows that there are systems containing several CNTs). The same holds for Tg values.
5. Table 3. Diameter, length and molecular weight are varying at the same time. How much do each one contribute to the value of mechanical properties?
6. How was Tg calculated? Please, describe.
7. The authors state that, “It can be seen from the obtained values that an increase in the SWCNT diameter in polypropylene leads to an increase in Young's modulus, bulk modulus, and shear modulus. That agrees with conclusion made by Tran Huu Nam et al. for a CNT/epoxy composite: it is better to use smaller diameter aligned CNTs to achieve high mechanical properties.” The authors are using one single CNT in their models. What does it align with? I guess experimental results are based on systems containing large amounts of CNTs. The authors have only one.
8. Effect of chirality: please, describe CNT length and characteristics of composites used for this study.
To make this manuscript publishable, the authors must separate the effect of polymer from that of CNT. They cannot compare polymers of different molecular weight, as it is well known that the degree of polymerization affect both mechanical properties and Tg values. They can do so by fixing the degree of polymerization and changing the number of chains in the cell. Results in table 1 are no reliable. The same holds for Table 2, second line (19%) or Table 3.
Author Response
Please see the attachment
Author Response File: 
Author Response.pdf
Round 2
Reviewer 1 Report
The authors have answered all my comments since I recommend accepting the paper in its present form.
Minor editing of English language required.
Author Response
We have checked the English carefully and have made the corrections to the final text
Reviewer 2 Report
Although the authors have tried to address some of my concerns, the models and methodology they used are not suitable to get accurate results and draw reliable conclusions.
Author Response
The second Reviewer suggested:
“Although the authors have tried to address some of my concerns, the models and methodology they used are not suitable to get accurate results and draw reliable conclusions.”
Indeed, molecular dynamics forcefield method do not provide the exact solutions, however, the methodology of simulated dilatometry was widely used previously for the polymers various properties prediction. Even if it does not provide the accurate quantitative results, the qualitative predictions are trusty. The most conclusions wade in the paper agree with available published previously results obtained by other methods.
