Preparation of Fluorine-Free Superhydrophobic Paper with Dual-Response of Temperature and pH

Round 1

Reviewer 1 Report

Dear Authors,

I think that your work is up to date and interesting.

The manuscript (MS) can be published after some minor corrections.

In the following, some points to improve the MS are given. Please use these suggestions.

• Why you use the term preparation in the title? You also characterize …
• Line 5 of abstract: please substitute “are” with “is” before called.
• Please separate the number and the measure unit in the temperatures, volumes, etc..
• Figure 1 must be improved in quality and the name of molecules should be more clear.
• Section 2.2.2: please add the temperature of the ultrasonic treatments also if they are performed at room temperature.
• Figure 2: pay attention because LCST is explained after the figure 2 and you use acid and basic, sample treated... You must better describe all before showing the figure 2. Please also enlarge the words inside this figure.
• Section 2.2.3: Please put figure 3 near the appropriate text.
• Please better describe the acid, neutral and basic solutions.
• Figure 4 please enlarge the marker in the micrographs.
• Section 3.3: substitute prove that with verify… and correct ev with eV.
• Section 3.4: please better justify the presence of this characterization.
• Conclusions: line 3, the characterizations not found! Please change the verb. Line 7 please change state with conditions. Please better argument the last sentence of the conclusions because the present one is too poor.

Best regards

Author Response

please see Attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Bin Du et al., report on the study of the wettability and permeability of paper on which a copolymer has been deposited. The copolymer is produced by radical polymerization and combines thermo-sensitive (N-isopropylacrylamide (NIPAm)), pH-sensitive (2- (dimethylamino) ethyl methacrylate (DMAEMA)) and hydrophobic (butyl methacrylate (BMA) and hydroxyethyl methacrylate (HEMA)) monomers. Paper is coated by this copolymer from toluene solutions. The main result of the study is quite interesting: the paper can be turned superhydrophobic and impermeable to water at temperatures above 25 degC, while it is wettable and permeable with water at temperatures below 25 degC. Other results include the fact that when pre-treated with acidic (resp. basic) water, the paper is hydrophilic (resp. superhydrophobic). However, the manuscript presents major shortcomings, which need to be corrected before the paper can be accepted in Coatings. In short, the manuscript is too shallow, does not analyze data completely, has a confuse expression and does not support all the claims in an appropriate manner.

- The word “superhydrophobic” is introduced page 1 for description of a surface with certain wetting properties. No indication is given for what this word means when applied to a polymer (page 3, line21).

- The synthesis of the polymer is not fully characterized. The FTIR spectra is not completely convincing. Peak attribution should be illustrated on the graph to be more convincing. NMR or GPC data could be useful.

- There is no description of the paper used for the experiment: what paper is it? which thickness? which surface roughness? etc.

- The SEM images tell very little, and I disagree with the claim that they allow to “judged that the layer of material was coated copolymer with dual-response of temperature and pH in the experiment” (page 6, line 3). They are also insufficient to claim that “the copolymer was … uniformly coated on the paper” (page 12, line 13).

- The TGA data is also inconclusive. A 3% difference in the residue might be within experimental error. If not, it would mean that the polymer has a slightly relative smaller weight loss than the neat paper. Since we do not know what the paper is, it is not helpful.

- The study shows that the paper permeability switches from water to oil at 25°C and the authors claim this corresponds to the LCST of the copolymer. However, the volume transition temperature of the copolymer was not determined separately, and the permeability transition temperature of the paper could be given with more precision. Also the authors could tell what is happening in the liquid collection experiment very close to the transition.

- In the same way, the pH variation of the paper wettability would need to be studied more precisely and related to the pKa of the copolymer.

- The English expression often requires editing.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript given by Du and co-workers reports on the production of a superhydrophobic coating for papers with dual response induced by temperature and pH variation. The coating is synthesized by means of free radical polymerization and applied to the substrate by a dip coating process. Using SEM, IR, XPS and TGA the polymer coated papers were examined. In addition, the LCST of the copolymer is determined by a separation experiment of oil and water. The superhydrophobicity, as well as the temperature and pH value dependency of the coating is determined by means of static contact angle measurement. While the story of the manuscript at hand is clear and straight-forward, the reviewer has some major concerns regarding the interpretation of the authors' findings. From the reviewer’s point of view this manuscript could be taken into consideration for publication in MDPI Coatings, if the authors take the following major inconsistencies and some additional investigations into account:

1. First of all, the manuscript need English polishing by a native speaker.
2. Introduction, page 2: Environmentally friendly materials are mentioned at the beginning of the manuscript, but it is not clear why NIPAM or DMAEMA should be environmentally friendly or why these materials are more suitable compared to other classical acrylates and amides. Moreover, the syntheses protocols comprising the application of organic solvents seems not to be very environmentally friendly, so the term eco-friendly should be used with caution and deleted in this case.
3. Introduction, page 2:
4. A superhydrophobic block copolymer is mentioned. How can a block-copolymer be produced by this free radical polymerization strategy? A block copolymer would behaves totally different for the application studies. From all analytical data, a statistical copolymer was obtained. Please clarify and change accordingly.
5. Within the same context: what are the copolymerization parameters of the used monomers?
6. From the reviewers point of view, it remains unclear which component turns this polymer into a superhydrophobic coating? However, PNIPAM coatings are rather polar, PDMAEMA coatings are rather polar. What is the reason – on a more molecular level – for these high contact angles against a sessile water drop?
7. Introduction: while references 26-33 give several external triggers for switching the surface polarity, redox as potential trigger as been only mentioned for PANI, which is not highly efficient. The authors could also add the following references for polarity switching on coating by stimuli-responsive moieties: J. Mater. Chem B 2013, 1, 828-834; Langmuir 1994, 10, 1493−1497; Organometallics 2013, 32, 5873-5878; Polymer 2016, 98, 429-436
8. Materials, page 3: some solvents are missing here.
9. Methods, page 3: Again, the block copolymer is described, but there is no proof that it is a block copolymer, a random polymer or only a blend of 4 homopolymers is present (which has to be proven, too). DSC analysis could give information for clarification along with SECX measurements.
10. Minor: ‘N’ in nitrogen-containing compounds have to be written italic.
11. Section 2.2.1: The authors wrote ‘When the reaction was completed…’. How was this determined? Was it a full consumption of all used monomers by means of free radical polymerization? Again, in this section the authors mention ‘a superhydrophobic block copolymer’, which is not correct.
12. Figure 1: This figure is confusing as it shows the same monomers and polymers and reaction conditions in the upper and lower figure. Shouldn’t there be a difference? Please clarify/change. Moreover, the ‘co’ in copolymers should be written italic (same counts for the legend)
13. Figure 2: from my point of view, this figure should be part of the respective section in the R&D section.
14. Section 2.2.3 and else: the term ‘critical’ response is scientifically incorrect and imprecise. Do the authors mean the ‘lower critical solution temperature’ here?
15. Section 2.2.3: LCST and UCST is typically determined by turbidimetry measurements, which should also provided here.
16. How where the XPS spectra calibrated?
17. Section 3.1: the abbreviations for the respective copolymers are not consistent and change over the whole manuscript. Please unify.
18. Section 3.1, SEM discussion: The sentence ‘Compared with the single fiber of the original paper, it could be judged that the layer of material was coated copolymer with dual-response of temperature and pH in the experiment.‘ sounds strange to the reviewer and seems to be scientifically incorrect. How thick was the layer? From the corresponding images it is only obvious that in increase in roughness could be determined.
19. Figure 4/discussion: How stable are those layers upon cycles of washing?
20. The first sentence in Section 3.2 should be corrected with respect to English.
21. Figure 5: Why is the signal intensity of the – most important – copolymer measurement so low?
22. General, major: there is only insuffienct copolymer analysis (which should comprise NMR, DSC and SEC).
23. Figure 7: for determining the exact amount of coated copolymer, a statistic should be given. Was this only one measurement?
24. Section 3.5.1: Discussion and determination of the LCST should be accompanied with turbidimetry measurements to be more precise.
25. Figure 9: for determining the CA for sessile water drops, the surface must be smooth. Otherwise the error for the measurement is too large. Please give full statistics for these measurements along with an error (how many places were measured and so on).
26. Figure 9: Values for the pristine substrate, PNIPAM and so on have to be given for comparison.
27. Section 3.6.: Treatment of the substrates AND the polymers with an alkaline solution of pH 14 will cause several side-reactions. Please provide blank sample measurements of the cellulose substrate without polymer. A polymer characterization after treatment with an alkaline solution should be given.
28. Figure 12: Resolution of Fig 12 is poor, while a covalent linkage of the NH group should be also given.
29. Chapter 2.2.2, page 4:
30. What kind of paper substrates are involved? Why do these papers need to be washed
31. Figure 2, page 4:The right part of the figure comes without explanation or sufficient caption, in my opinion this part of the figure would be better in Figure 3.
32. There is a typo in the figure caption.
33. Chapter 2.3, page 5:What is the pH value of these?
34. Acidic and basic solutions with pH values lower and higher than 7 are described: Which acids and bases are involved here?
35. Figure 3, page 5:
36. The figure is not explained very much, so that it is not clear that the polymer-coated papers seal the sample tubes. Perhaps a clearer caption would be a good solution.
37. Chapter 3.1, page 6:
38. Why should the paper become rougher when a thin polymer coating is applied? How much polymer is on the paper?
39. Figure 4, page 6:
40. The dimension bars are chosen very small. Can they be extended (e.g. Picture a: instead of 100 µm better apply 500 µm)
41. Figure 5 page 7:
42. Can you compare IR spectra of single monomers with a copolymer? It might be better to compare the homopolymers with the copolymer.
43. Figure 6, page 7: Does “original” mean the pure paper substrate?
44. The labeling of the spectra is insufficient. Is it the pure copolymer or the copolymer on the paper substrate?
45. Figure 7, page 8:The graph implies that the untreated paper burns at a higher temperature than the treated one. Why is this so?
46. Also, here the labeling of the thermograms is not sufficient. At which temperatures do the samples burn exactly?
47. Chapter 3.5.1, page 8: Is the paper hydrophilic or hydrophobic?
48. One sentence is written here: “It indicates that the paper is hydrophilic and hydrophobic after the two temperature treatment.”
49. Figure 8, page 9: The photos of these experiments are certainly more interesting, because they show the temperature range in which the LCST is located?
50. It is described that oil and water separation experiments were also performed in the temperature range between 20 and 35°C.
51. Chapter 3.5.2, page 9:Which component makes the coating superhydrophobic?It is described that the CA is still very high after 3 h at 55°C: How can a temperature of 55°C be maintained for 3 hours during CA measurement?
52. What contact angle does the untreated paper have?
53. Why is the CA of paper with polymer so high?
54. Chapter 3.5.2, page 9:What makes it more hydrophilic than just hydrophilic?
55. "superhydrophilic paper" sounds strange to me.
56. Figure 10, page 10:
57. Why is the left fiber hydrophobic when water interacts with the polymer here?
58. Chapter 3.6, page 11:How do you infiltrate a super-hydrophobic paper with an aqueous solution?
59. Which acids and bases are involved?
60. Figure 12, page 12:
61. Quality of the picture is very low.
62. in the entire manuscript:The cross-linking of the polymer on the fiber is mentioned once, can this cross-linking be proven?
63. Why is this polymer ecofriendly?
64. In general, I am missing the explanations for what reason the components BMA and HEMA are used?

Author Response

please see Attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

The authors addressed all aspect raised by the reviewer. From my point of view the manuscript can be accepted in ist present form.

Author Response

Please see the attachment.

Author Response File: Author Response.docx