Thermal Properties and Non-Isothermal Crystallization Kinetics of Poly (δ-Valerolactone) and Poly (δ-Valerolactone)/Titanium Dioxide Nanocomposites

Round  1

Reviewer 1 Report

This paper presents the non-isothermal Crystallization Kinetics of the TiO2/PDVL nanocomposite, which is not studied elsewhere. The author gives a full consideration of the thermal and crystallization behaviors to understand the crystallization mechanism. Besides, this paper is well written. The reviewer recommend acceptance after following issues are well addressed.

a) The introduction section is not well organized with strong logic. Such as page 2, lines 49-55, the description of the TiO2 is too long and over complicated. These descriptions are just textbook like knowledge. Besides, line 60, composite material, not material composite

b) Page 2, lines 60-63, the author claims that there are several researchers, however, there is only one reference, [12] is cited.

c) Page 2, line 72, the author claims that "Combining PDVL as polymeric phase with TiO2 as inorganic nanofiller has at time not been studied." While, in the following, the author said "The crystalline microstructure and thermal and mechanical properties of PDVL/TiO2 hybrids depend in large part on the PDVL/TiO2 ratio, TiO2 phase particle size, distribution of the inorganic phase within the PDVL phase and on the interfacial forces between the two phases [29, 30]." If the second sentence is correct with the cited references, the reviewer is quite confused that why the author claims that there is no study of the PDVL/TiO2?

Most importantly, the main motivation of this paper seems that it is just there is no study of the PDVL/TiO2. However, the no reported work of the research object should not be the sole motivation.

d) How can conclude the potential engineering application of the PDVL/TiO2? Is there any supporting reports or published papers, or it is just the author's sole prediction?

e) Why the author studied the non-isothermal crystallization kinetics, not the isothermal crystallization kinetics? The author should carefully refer to following references and can find the advantages of the non-isothermal crystallization kinetics.

Non-Isothermal Crystallization Kinetics of Short Glass Fiber Reinforced Poly (Ether Ether Ketone) Composites, A viscoelastic model of compression and relaxation behaviors in preforming process for carbon fiber fabrics with binder

f) The figures in this paper can be improved with different colors. Current version of the figures are quite poor and old-fashioned.

Author Response

REVIEWER 1

We thank very much the reviewer for the attention given to this work and also appreciate their important comments. The manuscript is revised, all corrections and comments were satisfied without exception and our responses are listed below:

Comment 1: Page 2, line 49-55. The introduction section is not well organized with strong logic. Such as page 2, lines 49-55, the description of the TiO₂ is too long and over complicated. These descriptions are just textbook like knowledge. Besides, line 60, composite material, not material composite.

Answer 1: Ok, this comment is taken into consideration and corrected (see this new paragraph in the introduction section, in red )

“Titanium dioxide (TiO₂) is usually obtained from a variety of ores. The bulk metal oxide exists in three principal phases, rutile, anatase, and brookite [9]. The rutile phase is stable at high temperatures [10] and the titania phase is principally used in pigments, adsorbents, catalyst supports, filters, coatings, photoconductors, and dielectric materials”.

Comment 2: Page 2, Lines 60-63. The author claims that there are several researchers, however, there is only one reference, [12] is cited.

Answer 2: The reviewer is absolutely right, this comment is taken into account and corrected as presented in the paragraph below and reported in the manuscript (see the new manuscript in red)

The size of the TiO₂ particles is considered a key factor affecting its performance, notably when mixed with a polymer as a composite material. Indeed, several researchers have focused their investigations upon the reduction of the size of TiO₂ particles using different methods such as the sol-gel technique [12-16], homogenization followed by precipitation [17], hydrothermal [18], flame synthesis [19], relatively new molten salts [20] and mechanomechanical [21, 22].

Comment 3: Page 2, line 72. The author claims that “Combining PDVL as polymeric phase with TiO2 as inorganic nanofiler has at time not be studied”. While, in the following, the author said “The crystalline microstructure, thermal and mechanical properties of PDVL/TiO₂ hybrids depend in large part on the PDVL/TiO₂ ratio, TiO₂ phase size , distribution …  interfacial forces between the two [29, 30]. If the second sentence is correct with the cited references, the reviewer is quite confused that why the author claims that there is not study of the PDVL/TiO₂?

Answer 3: Excuse me, the reviewer is perfectly right, this is a mistake of inattention. We confirm the originality of this work and the two references report in general other works showing the influence of incorporation of inorganic materials on in polymer matrices

(this expression is now correctly written as in the new version in the manuscript, see the new version and copy below)

“Combining PDVL as polymeric phase with TiO₂ as inorganic nanofiller has at time not been studied. The crystalline microstructure, thermal and mechanical properties of polymer/TiO₂ hybrids depend in large part on the polymer/TiO₂ ratio, TiO₂ phase particle size, distribution of the inorganic phase within the polymeric phase and on the interfacial forces between the two phases. Indeed, the investigation carried out by Zhang et al. [29] on the poly(phenylenevinylene)/Titanium oxide (PPV/TiO₂) nanocomposite revealed that the optical properties of this nanomaterial depended on the interfacial structure between PPV acid TiO₂ nanoparticles.  In this same sense, Kamal et al. [30] investigate the same metal oxide combined with PCL in a hybrid material fiber for use in biomedical domain. The results obtained were the smaller particles of the anatase phase exhibited significant enhancement of important number of properties compared with that prepared by the rutile phase. It was also revealed from the data obtained that the better interactions between polymer chain and anatase particle leads to better mechanical and biocompatibility properties.  

Comment 4: How can conclude the potential engineering application of the PDVL/TiO2? Is there any supporting reports or published papers, or it is just the author's sole prediction?

Answer 4: This investigation is just my prediction basing on works reported on the properties of PDVL and those of TiO₂ separately.

This work is only a part of a work on the preparation of materials with desirable properties for their application in the biomedical field notably as tissue engineering, in which the incorporation of TiO2 to PDVL (polymer biocompatible and quickly biodegradable) gives this polymer complementary properties such as the transparency, antibacterial effect etc.

Comment 5:  This investigation is the study the thermal and crystallographic properties of PDVL/TiO2 nanocomposite.

Answer 5: Yes, it is. The thermal study and the crystallographic properties of this new material give an excellent idea about homogeneity, nanofiller dispersion in the matrix polymer and in general thermodynamic stability and thus to have a desirable and applicable material.

Comment 6: Why the author studied the non-isothermal crystallization kinetics, not the isothermal crystallization kinetics? The author should carefully refer to following references and can find the advantages of the non-isothermal crystallization kinetics.

Non-Isothermal Crystallization Kinetics of Short Glass Fiber Reinforced Poly (Ether Ether Ketone) Composites, A viscoelastic model of compression and relaxation behaviors in preforming process for carbon fiber fabrics with binder

Answer 6: Ok, this suggestion is taken into account and added to the introduction (see the new version in red and copy below)

 “This method, although it is known more complex, was chosen in this work to study the kinetics of crystallization of the PDVL and the prepared PDVL/TiO2 composite, because this route is very compatible with the different industrial treatment practices compared to that of isothermal crystallization kinetics. This technique can also offer great potential in the rapid processing of industrial production”.

Comment 7: The figures in this paper can be improved with different colors. Current version of the figures are quite poor and old-fashioned.

Answer 7: Good, this suggestion is taking into account (see the new version of the Figures)

Author Response File: Author Response.docx

Reviewer 2 Report

Please see attached file.

Comments for author File: Comments.pdf

Author Response

REVIEWER 2

We thank very much the reviewer for the attention given to this work and also appreciate their important comments. The manuscript is revised, all corrections and comments were satisfied without exception and our responses are listed below:

Comment 1: How was TiO₂ used in this work prepared? How much size is it? Could you indicate SEM image if possible.

Answer 1: As described in the experimental section, the TiO₂ powder used in this work has a particle size of 21 nm and directly provided from Sigma Aldrich company

Comment 2: The reviewer thinks Ti mapping image of the composite display in detail. Could you indicate Ti mapping image of the composite if possible?

Answer 2: This comment is vague and other reviewers request the reduction of the size of the text, on the other hand, we will try to go a little deeper in our future investigations on this subject. 

Comment 3: Why were the intensities of TiO₂-2/PDVL and TiO₂-5/PDVL around 37° weak in Figure 1? 

Answer 3:  Sorry, the reviewer is absolutely right, in principle there is no reason why these two spectra do not follow the others. This anomaly has been corrected and the analysis has been redone (see the new spectra in Figure 1)

Comment 4: How were Tc estimated in Figure 8?

Answer 4:  As described in the text in the experimental part (section 2.4, in the end of the paragraph) . The crystallization temperature was collected from the top of the corresponding DSC peak

Author Response File: Author Response.docx

Reviewer 3 Report

There is a significant problem for publication. In paper, "the minimum crystallization temperature is observed at 1.5 and 1.0 wt%, respectively, whereas the maximum is reached at 4.0 and 5.0 wt%,respectively. This finding indicates the incorporation of TiO2 in the PDVL matrix at amounts ranging from 1.0 to 1.5 wt% accelerates the crystallization process, and an excess amount of TiO2 in the composite has the opposite effect, i.e., slowing crystalline growth."

However, this interpretation is wrong. The decreased crystallization temperature indicates the slowing crystalline growth. The increased crystallization temperature indicates the accelerated the crystallization process. This paper should be rejected. 

Author Response

REVIEWER 3

We thank very much the reviewer for the attention given to this work and also appreciate their important comments. The manuscript is revised, all corrections and comments were satisfied without exception and our responses are listed below:

Comment 1: There is a significant problem for publication. In paper, "the minimum crystallization temperature is observed at 1.5 and 1.0 wt%, respectively, whereas the maximum is reached at 4.0 and 5.0 wt%, respectively. This finding indicates the incorporation of TiO₂ in the PDVL matrix at amounts ranging from 1.0 to 1.5 wt% accelerates the crystallization process, and an excess amount of TiO2 in the composite has the opposite effect, i.e., slowing crystalline growth."

However, this interpretation is wrong. The decreased crystallization temperature indicates the slowing crystalline growth. The increased crystallization temperature indicates the accelerated the crystallization process. This paper should be rejected.

Answer 1: Sorry, the reviewer has perfectly reason. A significant error was occurred during the data acquisition.  These results are revised, corrected, the discussion are modified and rewritten as presented in last paragraph of the section 3.4 . Noting that all the results related to this part were also revised.

 (see the new version and figure 8).

Author Response File: Author Response.docx

Reviewer 4 Report

This work reports the crystallization of a novel nanocomposite system based on PDVL and TiO2 nanoparticles. After some structural characterization, a careful study on the crystallization kinetics is carried out. There are many interesting insights here reported and the topic matches well with the Journal. Accordingly, I suggest accepting this manuscript for publication after following corrections:

·      Please, correct grammar mistakes: For instance, in line 167: remove capital letter from “Polymer”.

·      I think PDVL/TiO2 is the correct form for defining polymer nanocomposites.

·      Please, correct following sentence: “This polymer is characterized by a semicrystalline aliphatic structure, low melting point (~-63 °C), and glass transition temperature (~ 58 °C), and exhibits less elastomeric behaviour compared to PCL”.

·      I suggest removing Table 1 since it is not necessary.

·      In Figure 1, a new peak emerges at 2θ of about 36 ° but only for a given samples. Could authors comment on that?

·      Regarding Figure 2, authors should provide a more clear scale bar. I would also expect higher-magnification images given the small diameter (21 nm as stated by authors) of the nanoparticles. This would allow a better observation of the morphology.

·      I suggest removing Figures of XRD and FTIR results into the Supporting Information as no noticeable changes are observed upon nanoparticle introduction. This will condense the paper solely to into the most relevant aspects.

·      I think DSC results are rather interesting. I found Tg behaviour especially interesting as an increase of 15 °C is achieved upon TiO2 incorporation. Previous works suggested that the interaction of the hydroxyl groups on PDVL may interact with the surface of the nanoparticles to enhance Tg (refer to: J. Appl. Polym. Sci. 135 (2018) 46035).

·      I think this paper is too large for its content. A shorted manuscript would be easier for reading. Therefore, suggest shortening some “theory” sections devoted to the explanation of diverse models and also including some Figures such as 10, 12 and 16 into Supporting Information.

·      Cyrstallization half time provides interesting insights about the nucleation effect of nanoparticles, although no references to previously reported works could be found. Authors are encouraged to compare their results to similar recent works where the effect of TiO2 on polymer crystallization has been studied (European Polymer Journal 91 (2017) 10–20; Polymer Bulletin 74 (2017) 445–464).

·      I encourage authors to compare the data shown in Table 3 with similar systems reported so far since this table reports the central work of this paper.

·      Many grammar mistakes are found: In line 326: “As can be seen, the t1/2 of the sample containing 1 wt% TiO2 dramatically decreased from 14.28 to 65.40 s when the cooling rate varied from 5 to 20 °C min”; Line 418: “thereby indicating a significant improves in its thermal stability.”

·      The dual effect of inorganic nanoparticles into the crystallization of polymers is a widely reported effect.

·      A notable enhancement on the thermal stability is obtained. This is very useful from the application point of view. I think authors should underline that part.

·      Conclusion section should be shortened to display only the most interesting/relevant findings.

Author Response

REVIEWER 4

We thank very much the reviewer for the attention given to this work and also appreciate their important comments. The manuscript is revised, all corrections and comments were satisfied without exception and our responses are listed below:

This work reports the crystallization of a novel nanocomposite system based on PDVL and TiO₂ nanoparticles. After some structural characterization, a careful study on the crystallization kinetics is carried out. There are many interesting insights here reported and the topic matches well with the Journal. Accordingly, I suggest accepting this manuscript for publication after following corrections:

Comment 1: Please, correct grammar mistakes: For instance, in line 167: remove capital letter from “Polymer”

Answer 1: This comment is taken into consideration and corrected (see the new version, correction in red)

Comment 2: I think PDVL/TiO₂ is the correct form for defining polymer nanocomposites.

Answer 2: Ok, this is exact, this comment is taken into consideration and corrected (see the new version, in red)

Comment 3: Please, correct following sentence: “This polymer is characterized by a semicrystalline aliphatic structure, low melting point (~-63 °C), and glass transition temperature (~ 58 °C), and exhibits less elastomeric behaviour compared to PCL”.

Answer 3: Ok, the reviewer is right, this sentence is not correctly written. The correct expression is now corrected (see the new version in the text in red)

Comment 4: I suggest removing Table 1 since it is not necessary.

Answer 4: Sorry, this suggestion is not retained in this case, because the experimental conditions are necessary and highly recommended by other reviewers, it is for this reason that it must be presented.

Comment 5: In Figure 1, a new peak emerges at 2θ of about 36 ° but only for a given samples. Could authors comment on that?

 Answer 5: Absolutely, the reviewer means the peak centered at 2θ = 37 ^o with those at 25.3 and 48.1 ^o which increase with the concentration of TiO₂ in the nanocomposite characterize the anatase phase of the TiO₂ nanoparticles (004), (101) and (200), respectively, as mentioned in the XRD section.

Comment 6: Regarding Figure 2, authors should provide a clearer scale bar. I would also expect higher-magnification images given the small diameter (21 nm as stated by authors) of the nanoparticles. This would allow a better observation of the morphology.

Answer 6: Ok, the reviewer is absolutely right. This comment is taken into consideration and the quality of these images are more improved (see the new version of the SEM images.

Comment 7: I suggest removing Figures of XRD and FTIR results into the supporting information as no noticeable changes are observed upon nanoparticle introduction. This will condense the paper solely to into the most relevant aspects.

Answer 7: Sorry, ok for Figure 5 (FTIR), but for General FTIR spectrum and XRD other reviewers need more details on these figures.

Comment 8: I think DSC results are rather interesting. I found Tg behaviour especially interesting as an increase of 15 °C is achieved upon TiO₂ incorporation. Previous works suggested that the interaction of the hydroxyl groups on PDVL may interact with the surface of the nanoparticles to enhance Tg (refer to: J. Appl. Polym. Sci. 135 (2018) 46035).

Answer 8: Ok, good.  (this comment is revised and rewritten with new revealing, see the new version in the text or the comment in below)

 Figure 3 presents a comparison between the FTIR spectra of PDVL/TiO₂ nanocomposites and those of their components and reveals no particularly shift on the absorption bands of PDVL. However, a slight widening of the carbonyl band at 1730 cm^-1 at the half of its height is observed, indicating the presence of interaction between the polymer and the nanofiler. This observation is also confirmed by the decrease of the broad band localized between 550 and 650 cm^-1 attributed to the Ti-O-Ti of TiO₂ in the hybrid materials. According to the literature [46, 47], a depression in the carbonyl peaks of PCL in PCL/TiO₂ nanocomposites, in which PDVL is one of its family, is caused by the presence of physical interaction between PCL and TiO₂ nanoparticles. Basing on this principle, we can confirm that TiO₂ is effectively incorporated into the PDVL to form nanocomposites.

Comment 9: I think this paper is too large for its content. A shorted manuscript would be easier for reading. Therefore, suggest shortening some “theory” sections devoted to the explanation of diverse models and also including some Figures such as 10, 12 and 16 into Supporting Information.

 Answer 9: Sorry, the reviewer is right, however, given the large amount of results and feedback from the reviwers, it is very difficult to reduce the manuscript in such a short time.

Comment 10: Cyrstallization half time provides interesting insights about the nucleation effect of nanoparticles, although no references to previously reported works could be found. Authors are encouraged to compare their results to similar recent works where the effect of TiO₂ on polymer crystallization has been studied (European Polymer Journal 91 (2017) 10–20; Polymer Bulletin 74 (2017) 445–464).

Answer 10: Ok, this comment is taken into account and the suggestions of reviewer are added (see the new version in red), however other article was found in the literature on the PCL/TiO₂ hybrid material which is comparable to our investigation was taken for comparison ( see aricle. Z. Wei, G. Wang, P. Wang. L. Liu, M. Qi, DOI 10.1002/pen.22165.

Comment 11: I encourage authors to compare the data shown in Table 3 with similar systems reported so far since this table reports the central work of this paper.

Answer 11: Ok, your suggestion is very valuable, however other reviewers suggested the reduction of huge data in the manuscript.

Comment 12: Many grammar mistakes are found: In line 326: “As can be seen, the t1/2 of the sample containing 1 wt% TiO₂ dramatically decreased from 14.28 to 65.40 s when the cooling rate varied from 5 to 20 °C min”; Line 418: “thereby indicating a significant improves in its thermal stability.”

Answer 12: These sentences are corrected as: Line 326: As can be seen, the t_1/2 of the sample containing 1% by weight of TiO₂ decreased considerably from 14.28 to 65.40 s when the cooling rate varied from 5 to 20 ° C min.

Comment 13:  Line 418: thereby indicating a significant improvement in its thermal stability

Answer 13:  This sentence is taken into account and corrected

Comment 14: The dual effect of inorganic nanoparticles into the crystallization of polymers is a widely reported effect.

Answer 14: Yes, however, I don’t know what do you mean by this sentence

Comment 15: A notable enhancement on the thermal stability is obtained. This is very useful from the application point of view. I think authors should underline that part.

Answer 15:  Yes, absolutely. However this article is only a part of our project and this property will be supported in coming reports.

Comment 16: Conclusion section should be shortened to display only the most interesting/relevant findings.

Answer 16:  Yes, the conclusion is shortened (see the new version)

Author Response File: Author Response.docx

Round  2

Reviewer 3 Report

In couclusion part, "The DSC analysis used at different cooling rates, indicated in general that the
incorporation of 1.0 to 1.5 wt% of TiO2 in the PDVL matrix accelerates the crystallization process, and the use of excess TiO2 in the composite had the opposite effect of slowing the growth of the
crystallites"  This part is wrong. This part should be corrected.

Author Response

 I thank you very much for the interest that gives to this work and  for the benefic remarks which go into the interest of improving the quality of this work to publish it in this “Crystal” journal. 

Reviewer 3

Comment: In conclusion part, "The DSC analysis used at different cooling rates, indicated in general that the incorporation of 1.0 to 1.5 wt% of TiO₂ in the PDVL matrix accelerates the crystallization process, and the use of excess TiO₂ in the composite had the opposite effect of slowing the growth of thecrystallites” This part is wrong. This part should be corrected.

Answer: Sorry, absolutely this expression is wrong in the conclusion. This expression is now considered and corrected (see the new version in red)

Author Response File: Author Response.docx