UV-Triggered Coumarin-PDMS Dimerization for Robust and Easy-Cleaning Polyurethane Coatings

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript presents a polyurethane coating system incorporating coumarin-functionalized PDMS, where UV-triggered cycloaddition is used to induce dimerization, enhancing coating robustness and imparting self-cleaning properties. The topic is relevant and aligns well with current interests in smart coatings, UV-responsive materials, and surface functionalization. Furthermore, Integration of coumarin photochemistry with PDMS is conceptually attractive. I believe this work is very interesting and suitable for publication in Coatings, MDPI after “Major Revisions” and after carefully addressing the following comments:

Comments:

1. The paper in its current form lacks sufficient mechanistic evidence for coumarin dimerization. The central claim of UV-triggered coumarin dimerization is not convincingly demonstrated (i.e., no clear spectroscopic confirmation (e.g., UV-Vis decrease at ~320 nm or FTIR changes, lack of reversible behavior demonstration (cyclodimerization vs cleavage) and no quantitative degree of dimerization).
2. The authors are required to provide direct evidence of [2+2] cycloaddition via UV-Vis spectral evolution, FTIR peak changes and possibly NMR (if available).
3. The authors should demonstrate reversibility using different wavelengths.
4. Although the manuscript reports improvements in hydrophobicity, mechanical properties and self-cleaning, however, the link between structure and performance is not rigorously established.
5. The authors need to show more clearly the correlation between crosslink density and mechanical strength and surface chemistry and contact angle.
6. Introduction: (Lines 39-41): The authors need to cite the recent work in the open literature: https://doi.org/10.3390/polym14193993

(Lines: 51-52): https://doi.org/10.1021/acsapm.3c01740

7. Role of PDMS vs coumarin not clearly separated. Please elaborate more on this.
8. The authors should provide systematic comparison: PU only vs PU–PDMS vs PU–coumarin–PDMS and discuss how UV affects network structure.
9. Self-cleaning property is a key claim but appears to be not strongly demonstrated. The authors should provide clear quantitative analysis (e.g., dirt removal efficiency).
10. A contact angle <150° is not sufficient for self-cleaning. The authors need sliding angle measurements and contaminant removal tests.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

I have read the manuscript provided by the authors and I find appealing their idea. Their systems have been characterized extensively and showed potential applications. However, the authors should clarfify some points, add some references and better explain some results. Please see the pdf attached with all the comments.

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Authors described a photochemical strategy that replaces uncontrollable or irreversible crosslinking methods with a controllable UV triggered approach. This topic is interesting, and the article is written well. However, before publication, several weak points of the work need to be addressed.

[1] The research method should be better defined. It should be more precisely indicated what problem the authors chose to solve. It should also be justified why these particular research methods, and not others, will be optimal for solving the title problem.

[2] Why was 365 nm UV irradiation chosen? This should be explained.

[3] The quality of Fig. 4 is poor. The multiplet lines are not visible. Perhaps key regions of the spectrum should be enlarged. Furthermore, the spectrum description does not take into account spin-spin couplings.

[4] The readability of the analyses in Fig.7 should be improved.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Publish as it is

Author Response

We thank Reviewer 1 for the positive recommendation and agree that the manuscript can be published in its current form. We appreciate the reviewer’s time and efforts.

Reviewer 2 Report

Comments and Suggestions for Authors

I have read the revised manuscript from the authors, and I have to say they have done a great job in the review. As far as I can see, they have addressed all comments and responded clearly. The new revised manuscript has greatly improved. My only concern is related to comment 6. According to the reviewers, the final material contains various PDMS species. I think this needs clarification in the manuscript. Do these species affect the final properties of the systems? Are they misleading the results, for example, in H-NMR? I think this needs clarification before accepting the manuscript.

Author Response

Comment: I have read the revised manuscript from the authors, and I have to say they have done a great job in the review. As far as I can see, they have addressed all comments and responded clearly. The new revised manuscript has greatly improved. My only concern is related to comment 6. According to the reviewers, the final material contains various PDMS species. I think this needs clarification in the manuscript. Do these species affect the final properties of the systems? Are they misleading the results, for example, in H-NMR? I think this needs clarification before accepting the manuscript.

Response: We sincerely thank the reviewer for his positive comments and thorough evaluation of our work. We agree that the potential impact of the statistical PDMS species in the intermediate product on the final results needs to be clarified. Based on our experimental design and analysis, we can confirm that these species do not mislead our conclusions for the following reasons.

The reviewer is concerned that the presence of multiple species (mono-NH₂, di-NH₂, and non-functional PDMS) might complicate or bias the ¹H NMR assignment. Indeed, the NMR spectrum represents an average signal of all PDMS species. As shown in Figure 4, the key signals we use to confirm successful synthesis are:

1. The characteristic Si-CH₃ signal at 0.1-0.2 ppm (a-c), which is common to all PDMS species.
2. The urea NH signal at 8.0 ppm (t), which confirms that the amino groups have successfully reacted.
3. The characteristic coumarin signals at 6.06 ppm, 6.71-7.98 ppm, and 2.34 ppm, which confirm the successful grafting of coumarin.
4. we quantitatively analyzed the ¹H NMR spectrum by integrating the characteristic peaks. The integrated area of m and n (-CH₂-O-, adjacent to the coumarin ring) is 1, and the integrated area of f-l (-CH₂- of the HDI unit) is 3.2. For the desired mono-amino-terminated product, the theoretical ratio of F-L to m-n is 3:2. The measured ratio (3.2:2) is very close to the theoretical value (3:2), confirming that the majority of the product is the desired mono-amino-terminated PDMS. The slight excess of 0.2 is likely due to the presence of a small amount of di-amino-terminated or non-functional PDMS.

Even with minor amounts of di-amino-terminated or non-functional PDMS, the final coating performance is not negatively affected. In fact, di-amino-terminated PDMS can act as an additional crosslinker, and non-functional PDMS may facilitate chain migration to the surface, potentially contributing synergistically to the overall properties. We have added a sentence in the NMR analysis section (lines 185-192) clarifying that the statistical mixture does not interfere with the spectral assignments. And the figure 4 has been updated, with the specific integral areas marked out.

Figure 4. ¹H NMR spectrum of HNP.

The ¹H NMR spectrum of HNP was further analyzed by integrating characteristic peaks. The integrated area of m and n (-CH₂-O-, adjacent to the coumarin ring) is 1, and the integrated area of f-l (-CH₂- of the HDI unit) is 3.2. For the desired mono-amino-terminated product, the theoretical ratio of f-l to m and n is 3:1:1. The measured ratio (3.2:1:1) is very close to the theoretical value, confirming that the majority of the product is the desired mono-amino-terminated species. The slight excess of 0.2 is attributed to the presence of minor amounts of di-amino-terminated or non-functional PDMS species, which do not interfere with the structural assignments.

Author Response File: Author Response.pdf