Cholesteric Liquid Crystal Polymer Network Patterns with a Golden Structural Color

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

I have reviewed the manuscript entitled "Cholesteric Liquid Crystal Polymer Network Patterns with a Golden Structural Color" which reports the development of cholesteric liquid crystal polymer network (CLCN) films exhibiting thermochromic structural colors via a two-step photopolymerization process. The approach and results are interesting for several applications such as decoration and anti-counterfeiting. However, I have identified several issues that should improve to enhance the clarity, completeness, and scientific rigor of the manuscript. I believe the manuscript has potential but requires significant revision to meet the standards of publication. I encourage the authors to address the below concerns in detail.

Comments

1. The authors should clarify the novelty of their study in relation to prior works on color tuning in cross-linked cholesteric liquid crystal elastomers, such as Broer et al. (Nature, 1995). Proper citation and comparison with relevant literature are necessary.
2. The procedure for preparing rubbing-oriented PET films is not described. This information is essential for reproducibility.
3. The conditions under which the CD spectra (Figure S2) were measured should be stated explicitly. The author should describe the measurement detail (i.e. Solution or film?).
4. For Figure S1 (DSC), the authors should report phase transition enthalpy to support the identification of the phase transition from liquid crystalline to isotropic phase.
5. In Figure 3, the statement regarding the origin of the 332 nm CD signal may be misleading. The authors had better to measure CD spectra of (R)-C6P in solution as a control to support their interpretation. (I think the same CD spectra also be obserbed in solution state.)
6. It is unclear which data correspond to pre- and post-polymerization states in Figures S1–S4 and Figure 1. Labels should be added accordingly. In Figure S4, the presence of Tg suggests polymerization, but this should be explicitly stated.
7. The manuscript discusses differences between Hg and LED lamps. However, if the only difference lies in intensity, the discussion should focus on intensity rather than lamp type.
8. In Figure 1d–g, the layout could be improved. At minimum, Figures 1e and 1f should not be presented at an angle.
9. The sentence "This redshift should be driven by the thermochroism of (R)-C6P" lacks supporting discussion. The authors should describe the mechanism with references or experimental evidence.
10. The blue shift discussed in relation to polymerization shrinkage should be better illustrated. For easily understanding, the authors had better to plot peak reflection wavelengths at various temperatures and include them in the Supporting Information. In addition, they should use consistent color coding to aid comparison.
11. The figure captions lack sufficient detail. Information such as polymerization conditions should be included in the captions, not just in the main text.
12. The reflection spectra in Figure 2d show a single peak, while Figure 2c shows multiple peaks. The authors should explain this difference.
13. The FT-IR discussion is unclear. If the authors aim to address oxygen inhibition, they should explain correlation between the inhibition thickness and the polymerization conversion using SEM-measured pitch and surface area in IR, quantitatively.

I believe that addressing these points will significantly improve the manuscript’s quality and impact. I encourage the authors to revise the paper accordingly.

Author Response

Comments: I have reviewed the manuscript entitled "Cholesteric Liquid Crystal Polymer Network Patterns with a Golden Structural Color" which reports the development of cholesteric liquid crystal polymer network (CLCN) films exhibiting thermochromic structural colors via a two-step photopolymerization process. The approach and results are interesting for several applications such as decoration and anti-counterfeiting. However, I have identified several issues that should improve to enhance the clarity, completeness, and scientific rigor of the manuscript. I believe the manuscript has potential but requires significant revision to meet the standards of publication. I encourage the authors to address the below concerns in detail.

1. The authors should clarify the novelty of their study in relation to prior works on color tuning in cross-linked cholesteric liquid crystal elastomers, such as Broer et al. (Nature, 1995, 378, 467–469). Proper citation and comparison with relevant literature are necessary.

Our response: We have significantly expanded the discussion of our work's novelty compared to Broer et al. (Nature, 1995, 378, 467–469) in the Introduction (Page 2).This study demonstrates a novel approach for independently controlling double reflection bands in cholesteric liquid crystal polymer network (CLCN) films through oxygen-inhibited, temperature-modulated two-step photopolymerization. This method fundamentally differs from the continuous pitch gradient strategy reported by Broer et al., which achieved broadband reflection through molecular diffusion during the photopolymerization process.

 

2. The procedure for preparing rubbing-oriented PET films is not described. This information is essential for reproducibility.

Our response: The rubbing-oriented PET films were not prepared by us. They were given by Wuxi Wanli Adhesive Material Co., Ltd. Typically, they were prepared by rubbing with cotton cloth. The source have been described in Section 2.1 (Page 2).  

3. The conditions under which the CD spectra (Figure S2) were measured should be stated explicitly. The author should describe the measurement detail (i.e. Solution or film?).

Our response: As the suggestion, we have added the measurement condition in “Section 2.7. CD spectra of (R)-C6P and CLC mixture”.

4. For Figure S1 (DSC), the authors should report phase transition enthalpy to support the identification of the phase transition from liquid crystalline to isotropic phase.

Our response: Thanks for the suggestion, we have added the phase transition enthalpy of (R)-C6P to support the identification of the phase transition from liquid crystalline to isotropic phase in the SI file (Figure S2).

 

5. In Figure 3, the statement regarding the origin of the 332 nm CD signal may be misleading. The authors had better to measure CD spectra of (R)-C6P in solution as a control to support their interpretation. (I think the same CD spectra also be observed in solution state.)

Our response: For the CD spectrum of the (R)-C6P solution in THF (1.0 ´ 10⁻² M), one positive CD signal was identified at 302 nm, and two negative CD signals were identified at 276 and 231 nm (Figure S4). No CD signals originated from the stacking of neighboring molecules were identified (Page 5, 1^st paragraph).

6. It is unclear which data correspond to pre- and post-polymerization states in Figures S1–S4 and Figure 1. Labels should be added accordingly. In Figure S4, the presence of Tg suggests polymerization, but this should be explicitly stated.

Our response: Thanks for the suggestions, we have defined them in Figure 1. Acrylate-type CLC mixture polymerize under UV irradiation to form the cholesteric liquid crystal polymer network (CLCN). The existence of the glass transition temperature should be driven by the high viscosity of the CLC mixture (Page 5 and Figure S6).

7. The manuscript discusses differences between Hg and LED lamps. However, if the only difference lies in intensity, the discussion should focus on intensity rather than lamp type.

Our response: As the suggestion, we have supplemented the details in “Section 2.1. Chemical Reagents and Instruments” (Page 3) and “Section 3.1. CLCN Films with a Broad Reflection Band or Double Reflection Bands” (Page 6).

The photopolymerization was carried out using a high-pressure Hg lamp (MINHIO 4012−20, 1.0 kW) produced by MINHIO Intelligent Equipment Co., Ltd (Shenzhen, China). The light intensities of UVV (> 390 nm), UVA (320-390 nm), UVB (280-320 nm) and UVC (< 280 nm) were measured to be 75.4, 96.0, 87.2, and 0 mW cm^-2, respectively. The UV LED series equipment (UVSF81T, 365 nm, 260 mW cm⁻², output power) was produced by FUTANSI Electronic Technology Co., Ltd (Shanghai, China).

Comparison of the LED and high-pressure Hg lamps, the high-pressure Hg lamp has a strong UVB (87.2 mW cm⁻²) light which is proposed to drive a high photopolymerization rate (Page 6).

8. In Figure 1d–g, the layout could be improved. At minimum, Figures 1e and 1f should not be presented at an angle.

Our response: As the suggestions, Figure 1 was improved.

9. The sentence "This redshift should be driven by the thermochroism of (R)-C6P" lacks supporting discussion. The authors should describe the mechanism with references or experimental evidence.

Our response: This redshift should be driven by the thermochroism of (R)-C6P which was shown in the CD spectra taken at different temperatures (Page 6 and Figure S3).

10. The blue shift discussed in relation to polymerization shrinkage should be better illustrated. For easily understanding, the authors had better to plot peak reflection wavelengths at various temperatures and include them in the Supporting Information. In addition, they should use consistent color coding to aid comparison.

Our response: As the suggestion, we have supplemented the peak reflection wavelengths at various temperatures in the SI file (Figure S8).

11. The figure captions lack sufficient detail. Information such as polymerization conditions should be included in the captions, not just in the main text.

Our response: As the suggestions, figure captions with the key polymerization conditions were added (See figure captions).

12. The reflection spectra in Figure 2d show a single peak, while Figure 2c shows multiple peaks. The authors should explain this difference.

Our response: The multi-reflection bands should be formed by the partial overlap of two reflection bands (Page 6).

13. The FT-IR discussion is unclear. If the authors aim to address oxygen inhibition, they should explain correlation between the inhibition thickness and the polymerization conversion using SEM-measured pitch and surface area in IR, quantitatively.

Our response: As the suggestion, FE-SEM analysis was employed to quantitatively evaluate the oxygen inhibition effect on the photopolymerization of CLC mixture (Figure S10). After the first polymerization step, unreacted compounds were removed by washing with acetone. A CLCN film with a thickness of 1.5 μm was identified on the PET substrate (Figure S10a). For the CLCN film prepared through the two-step polymerization approach, the thickness was about 3.5 μm, revealing that the thickness of oxygen inhibition layer was approximately 2.0 μm (Figure S10b). These experimental observations provide direct evidence for the spatial penetration depth of molecular oxygen (Page 8).

I believe that addressing these points will significantly improve the manuscript’s quality and impact. I encourage the authors to revise the paper accordingly.

Reviewer 2 Report

Comments and Suggestions for Authors

The work presented for evaluation is of an applied nature and mainly focuses on cholesteric liquid crystal network (CLCN) films with composite structural colors, which may have potential applications in decoration and anti-counterfeiting. A cholesteric liquid crystal mixture based on thermochromic acrylate was used, the structural color of which of the CLCN film can be controlled by the photopolymerization temperature. CLCN films with double reflection bands were prepared using a two-step photopolymerization method. The distance between the two reflection bands was controlled by the polymerization temperatures of these two steps.

In the preparation of the films discussed here, well-known properties of cholesteric liquid crystals were used, namely the dependence of the helical pitch on temperature and the phenomenon of selective light reflection. Moreover, the wavelengths of the reflection bands were controlled by the two polymerization temperatures.

The optical properties of the produced CLCN foils are documented in a number of diagrams and photographs. The quality of the graphs and photographs is sufficient to understand the observed phenomena and foil parameters.

The conclusions summarize and discuss the results obtained in the work.

The literature references cited in the work are sufficient to justify the results obtained in the work.

It would be recommended to give temperatures in Kelvins as the official units throughout the work.

Author Response

Comments: The work presented for evaluation is of an applied nature and mainly focuses on cholesteric liquid crystal network (CLCN) films with composite structural colors, which may have potential applications in decoration and anti-counterfeiting. A cholesteric liquid crystal mixture based on thermochromic acrylate was used, the structural color of which of the CLCN film can be controlled by the photopolymerization temperature. CLCN films with double reflection bands were prepared using a two-step photopolymerization method. The distance between the two reflection bands was controlled by the polymerization temperatures of these two steps.

In the preparation of the films discussed here, well-known properties of cholesteric liquid crystals were used, namely the dependence of the helical pitch on temperature and the phenomenon of selective light reflection. Moreover, the wavelengths of the reflection bands were controlled by the two polymerization temperatures.

The optical properties of the produced CLCN foils are documented in a number of diagrams and photographs. The quality of the graphs and photographs is sufficient to understand the observed phenomena and foil parameters.

The conclusions summarize and discuss the results obtained in the work.

The literature references cited in the work are sufficient to justify the results obtained in the work.

It would be recommended to give temperatures in Kelvins as the official units throughout the work.

Our response: We appreciate the reviewer’s attention to unit standardization. The phase temperature ranges are generally using Kelvins. Herein, Celsius is also OK. When we try to change it to Kelvins, the temperatures became complicated. We will take account of using Kelvins in the future publications.

Reviewer 3 Report

Comments and Suggestions for Authors

I have the following comments:

1. CLCN must be defined in the main text, not just in the Abstract.

2. Delta n and PSLC should be defined where they are introduced in the text.

3. The Introduction should discuss and focus on the originality of this study in relation to other polymeric CLCs based on acrylates.

4. How was the helical pitch extracted from the results obtained by SEM?

5. Compound (R)-C6P was analysed by DSC and POM, and two phases were assigned N* and N. While a typical texture for N* is clearly visible in Figure 1c, the POM picture presented in Figure 1b is not convincing for the assignment of the N phase.

6. The NMR spectra of (R)-C6P should be included in the SI file.

Author Response

Comments: I have the following comments:

1. CLCN must be defined in the main text, not just in the Abstract.

Our response: Sorry for making this mistake. The CLCN have been defined in the main text (Page 2).

2. Delta n and PSLC should be defined where they are introduced in the text.

Our response: As the suggestion, delta n and PSLC have been defined in the main text (Page 1 and 2). Δn = n_e − n_o, where n_e and n_o are the extraordinary and ordinary indices, respectively. The Polymer-stabilized cholesteric liquid crystal (PSCLC) and CLCN films have been prepared by the diffusion of less or non-reactive compounds during the preparation process [22,31].

3. The Introduction should discuss and focus on the originality of this study in relation to other polymeric CLCs based on acrylates.

Our response: Thanks for the suggestion, we have compared with relevant literatures in page 2, 2^nd paragraph.

4. How was the helical pitch extracted from the results obtained by SEM?

Our response: The cholesteric helical structure exhibits a periodic layered morphology, where one full pitch (P) corresponds to a 360° molecular rotation. The observed layer spacing in SEM images represents half-pitch (P/2) due to the alternating alignment of molecular directors. Multiple periodicities were measured from SEM images using software with calibrated scale bars.

5. Compound (R)-C6P was analysed by DSC and POM, and two phases were assigned N* and N. While a typical texture for N* is clearly visible in Figure 1c, the POM picture presented in Figure 1b is not convincing for the assignment of the N phase.

Our response: Thanks a lot for the suggestion. We have taken again for the POM image. During the cooling process, a marbled texture and a Grandjean one were observed in the polarized optical microscopy (POM) images at 60.0 and 45.0 °C, respectively (Figure 1b and c).

6. The NMR spectra of (R)-C6P should be included in the SI file.

Our response: As the suggestion, the NMR spectra of (R)-C6P added to in the SI file (Figure S1).

Reviewer 4 Report

Comments and Suggestions for Authors

In this paper, a thermochromic acrylate-based cholesteric liquid crystal mixture was prepared. The structural colour of the CLCN film can be controlled by the photopolymerisation temperature. Based on oxygen inhibition of the acrylate group, CLCN films with double reflection bands were prepared using a two-step photopolymerisation method. The distance between these two reflection bands was controlled by the polymerisation temperatures of these two steps.
Remarks.

1- The literature review is very poorly described. A more detailed analysis should be presented and the role of nanomaterials for the polymer network of cholesteric liquid crystals should be pointed out.
2. SEM and DSC analysis should be described in more detail. Especially the issues of sample preparation.
3. There is no analysis of the error of the studies, both in the methodology and in the discussion of the results.
4. No comparison with the results of other authors. Especially numerical data that would allow comparison of similar studies with the results shown in the paper.
5. The conclusions should be improved. The conclusions should be made more specific.

 

Author Response

Comments: In this paper, a thermochromic acrylate-based cholesteric liquid crystal mixture was prepared. The structural colour of the CLCN film can be controlled by the photopolymerisation temperature. Based on oxygen inhibition of the acrylate group, CLCN films with double reflection bands were prepared using a two-step photopolymerisation method. The distance between these two reflection bands was controlled by the polymerisation temperatures of these two steps.
Remarks.

1. The literature review is very poorly described. A more detailed analysis should be presented and the role of nanomaterials for the polymer network of cholesteric liquid crystals should be pointed out.

Our response: Thanks for the suggestion, while our current study focuses on pure liquid crystal systems without the addition of nanomaterials.

2. SEM and DSC analysis should be described in more detail. Especially the issues of sample preparation.

Our response: We have increased the DSC process of sample preparation in “Section 2.1. CD spectra of (R)-C6P and CLC mixture” (Page 2, 3 and 4). Before taking the FE-SEM images, platinum was spattered on the surfaces of the samples. The DSC measurements were conducted on the solid samples of (R)-C6P and a LC242/(R)-C6P/CA-iso/Irgacure 907 (w/w/w/w, 55.6/37.0/4.6/2.8) mixture, which were performed using about 5 mg of (R)-C6P or the CLC mixture with the cooling and heating rates of 10 °C min⁻¹. The measurements were cycled between −50 and 100 °C for three times.

3. There is no analysis of the error of the studies, both in the methodology and in the discussion of the results.

Our response: We have increased the error limits in “Section 2.1. Chemical Reagents and Instruments” (Page 3). Error limits were estimated as follows: wavelength, + 1 nm; transmittance, + 0.1%.

4. No comparison with the results of other authors. Especially numerical data that would allow comparison of similar studies with the results shown in the paper.

Our response: More discussions were added in the Results and Discussion section.

5. The conclusions should be improved. The conclusions should be made more specific.

Our response: Revised as the suggestion. This work paves the way for the next-generation smart optical systems combining broadband reflection with dynamic color modulation, and promising applications in decorative coatings, security labeling and other photonic devices (Page 9).

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thank you for the careful revisions and your detailed response to my previous comments. The manuscript has been significantly improved, and I commend the authors for their thorough work. Overall, I believe the study can contribute valuable insights to the field. I have only a few minor points that I suggest to further enhance the clarity and impact of the work.

1. The observation of thermochromism for (R)-C6P in Figure S3 is intriguing. While the authors mention this phenomenon, I encourage them to briefly discuss their hypothesis on the underlying mechanism. Even a speculative explanation, such as the role of conformational changes, molecular aggregation, or interaction with the surrounding matrix, would strengthen the readers' understanding
2. To highlight the novelty of this study more clearly, I recommend citing your recent publication following below. Including this reference will emphasize the originality of the approach.

"Better Understanding of the Composite Colored Cholesteric Liquid Crystal Polymer Network Film Prepared through Polymerization-Induced Chiral Dopant Diffusion."

These are relatively minor suggestions, and I believe the manuscript is very close to being ready for publication. I look forward to seeing the final version.

Author Response

Thank you for the careful revisions and your detailed response to my previous comments. The manuscript has been significantly improved, and I commend the authors for their thorough work. Overall, I believe the study can contribute valuable insights to the field. I have only a few minor points that I suggest to further enhance the clarity and impact of the work.

1. The observation of thermochromism for (R)-C6P in Figure S3 is intriguing. While the authors mention this phenomenon, I encourage them to briefly discuss their hypothesis on the underlying mechanism. Even a speculative explanation, such as the role of conformational changes, molecular aggregation, or interaction with the surrounding matrix, would strengthen the readers' understanding

Our response: Thanks for the suggestion. The thermochromism of (R)-C6P may be driven by the temperature dependency of molecular conformation [39,40] (Page 6).

2. To highlight the novelty of this study more clearly, I recommend citing your recent publication following below. Including this reference will emphasize the originality of the approach. "Better Understanding of the Composite Colored Cholesteric Liquid Crystal Polymer Network Film Prepared through Polymerization-Induced Chiral Dopant Diffusion."

Our response: Thanks for the suggestion. It was cited and briefly discussed (Page 2, Ref. 33). Similarly, based on oxygen inhibition and molecular diffusion, a two-step photopolymerization method for the preparation of the CLCN film with double reflection bands was developed [32,33].

These are relatively minor suggestions, and I believe the manuscript is very close to being ready for publication. I look forward to seeing the final version.

Reviewer 4 Report

Comments and Suggestions for Authors

Accept in present form

Author Response

Accept in present form.

Thanks for the suggestion.