Effect of Ion Concentration on the Electro-Optic Response in Polymer-Stabilized Cholesteric Liquid Crystals

Round 1

Reviewer 1 Report

The manuscript is another move in the ongoing research at the Air Force Research Laboratory on electro-optical response in polymer stabilized cholesteric liquid crystals. In this paper the effect of ionic concentration on the selective transmission of light with stationary applied electric field is investigated. The are many sources of ions in the electrolyte such as cholesteric liquid crystal mixture that may come from synthetic impurities, ion injection from the bounding polyimide coated plates, photopolymerization initiation dopant or UV light ionization. In this work authors utilized, previously reported by this group, initiatorless polymerization of the diacrylate liquid crystal monomers mixed in a cholesteric matrix which eliminates the need for photoinitiator in the mixture. Thus, the ion concentration can be more carefully monitored and the effect of DC electric field polarization of the polymer network on the selective transmission through the planar cholesteric structure as the function of ion concentration can be studied. The authors report transmission bandwidth broadening in cholesteric with low ion concentration and red shift of the band in high ion concentration case with the applied DC electric field and propose the mechanism for this electro-optic response such that it is ion-facilitated electromechanical deformation of the polymer stabilizing network. The study adds the important aspect of ions concentration to the consideration for the future optical application of electrically tunable polymer stabilized cholesteric liquid crystal devices. Overall presented experimental results, methods, and materials used in this work are well described with sufficient details. The study is well designed, technically sound and the analysis performed with the highest technical standards. The results interpreted appropriately and justified with the defined goal for the investigation. The conclusions could be interesting for the readership of the Journal, especially in the field of tunable photonic materials and liquid crystal devices. I would recommend this work to be published in Crystal after minor revision and after authors clarified the following inquiries. Broad comments: 1) Add to the discussion on the difference between broadening and red shifting of the transmission band with different ion concentrations described in Figures 5-6? How is cholesteric pitch modulated through the cell gap for different ion concentrations during DC field application? Is it a smooth pitch modulation or rather sharp? Both Figure 6 a and c look similar to me with two primary periodicities corresponding to peak that move away from each other with electric field. 2) What is the reason for the imbalance of ions trapped in the polymer network? Would you expect the electro-mechanical deformation of the polymer network to be the same near both negative and positive ITO plates as the results of ion screening at the boundaries? 3) It is mentioned in the paper that the sample was rotated during the UV exposure to avoid heat build- up in experiments to Figure 5. I can imagine UV light to provide the ion imbalance by ionizing one side of the cell more and thus creating the excess of charges on that side. Can you comment on this specific experimental preparation? Specific comments: 1) No mentioning of Figure 1(a-c) panels in the text. What is the initiator type effect onto the bandwidth and peak position and how it is related to the ion concentration? Either expand a bit in the text on a-c panels, or remove completely and leave panel d only. If you do decide to keep it, do all transmission curves at different voltages between 0 and 100V overlap behind the blue curve on panel a, 2) Figure 7 is not mentioned in the text either, although last paragraph before the conclusion clearly describes this figure. Add reference to Figure 7 to text. 3) Figure 4. Inset information is missing in the caption, complete the sentence with the information on the inset. 4) Large bandwidth increase to is an somewhat exaggerated in page 6 when describing Figure 5a, at most I see doubling of the bandwidth. 5) Include tetrahydrofuran when first mentioning THF for clarity.

Author Response

Thanks for the comments and corrections are in the red color in the text.

Broad comments: 

1) Add to the discussion on the difference between broadening and red shifting of the transmission band with different ion concentrations described in Figures 5-6? How is cholesteric pitch modulated through the cell gap for different ion concentrations during DC field application? Is it a smooth pitch modulation or rather sharp? Both Figure 6 a and c look similar to me with two primary periodicities corresponding to peak that move away from each other with electric field. 

: A sentence “As shown in Figure 1(d), the deformed polymer network induced by DC field application modulates the pitch across the cell gap. For the bandwidth broadening samples, the pitch deforms linearly,[11] while non-linear pitch deformation is reported for the red tuning samples across the cell gap.[13]” has been added to page 6. The concentration of the ions trapped in the polymer network influences magnitude of polymer deformation, resulting in bandwidth broadening for the sample with smaller ion concentration and red shifting tuning for the samples with higher ion concentrations.

 

2) What is the reason for the imbalance of ions trapped in the polymer network? Would you expect the electro-mechanical deformation of the polymer network to be the same near both negative and positive ITO plates as the results of ion screening at the boundaries? 

: The potential mechanism for the EO response of PSCLC is: Ions in the LC mixture, especially positive ions, are trapped in the polymer network during the photopolymerization process. When a DC voltage is applied, the free ions in the PSCLC move to the counter electrodes (charge screening), and then the positive ions trapped in the polymer network move to the negative electrode. The movement of the trapped positive ions distorts the polymer network, resulting in pitch variation – pitch contraction and expansion near the negative and positive electrodes, respectively (Figure 1(d)). The magnitude of the polymer deformation is affected by ion concentration and viscoelastic properties of the polymer network.

 

3) It is mentioned in the paper that the sample was rotated during the UV exposure to avoid heat build- up in experiments to Figure 5. I can imagine UV light to provide the ion imbalance by ionizing one side of the cell more and thus creating the excess of charges on that side. Can you comment on this specific experimental preparation? 

: Sample rotation is primarily intended to avoid heat build-up in the sample. Heat accumulation creates a non-uniform polymer network due to thermal expansion of the polymer network and destroys the helical structure of the CLC, yielding in a focal-conic structure. We also studied the effect of UV exposure on one side and both sides of the sample, and no difference was observed for the EO response of these samples, indicating that no charge imbalance occurred through the thickness of the cell.

 

Specific comments: 

1) No mentioning of Figure 1(a-c) panels in the text. What is the initiator type effect onto the bandwidth and peak position and how it is related to the ion concentration? Either expand a bit in the text on a-c panels, or remove completely and leave panel d only. If you do decide to keep it, do all transmission curves at different voltages between 0 and 100V overlap behind the blue curve on panel a, 

: Figure 1(a-c) is mentioned on page 2.  We have corrected Figure 1(a) from “0V to 100V” to “0V and 100V”. These two curves are overlapped (Black curve at 0V and blue curve at 100V).

 

2) Figure 7 is not mentioned in the text either, although last paragraph before the conclusion clearly describes this figure. Add reference to Figure 7 to text. 

: We have added “in Figure 7” to the text on page 7.

 

3) Figure 4. Inset information is missing in the caption, complete the sentence with the information on the inset. 

: We have added information about the inset, such as “Inset is an image of the recrystallized solution of C3M monomer.”

 

4) Large bandwidth increase to is an somewhat exaggerated in page 6 when describing Figure 5a, at most I see doubling of the bandwidth. 

: We have deleted “large” in the paragraph.

 

5) Include tetrahydrofuran when first mentioning THF for clarity.

: We have added “tetrahydrofuran (THF)” on page 4.

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript entitled „Effect of Ion Concentration on the Electro-optic Response in Polymer Stabilized Cholesteric Liquid Crystals” is devoted to investigate the effect of ions trapped in the polymer network on the position and/or bandwidth of the selective reflection notch in polymer stabilized cholesteric liquid crystals. The topic of the manuscript fits well the scope of Crystals journal in the field of thermotropic liquid crystals. The manuscript contains novel results which should have impact on the specific field of research but also on the broader soft matter and liquid matter communities. I expect the manuscript will be appropriately cited if published. Despite of this I have several remarks which described in Comments to the authors. So, I recommend this paper for publication but after minor revision.

I expect to take into account the following remarks and questions:

(A) In Figure 1, there is a mistake in the description: "...Schematic of the polymer network in a deformable PSCLC with Δe< 0: (a) with no applied field, (b) with a DC field applied between the top and bottom substrates...". I think it should be "...(d) Schematic of the polymer network in a deformable PSCLC with Δe< 0: (on the left) with no applied field, (on the right) with a DC field applied between the top and bottom substrates...".

(B) In Materials and Method section, in sentence "...and Δε < 0 nematic LC MLC-2079 (Merck) with TNI = 102 °C, Δε = -6.1, and Δn = 0.15 at λ = 589 nm" it is not necessary to mention that this is negative dielectric anisotropy mixture as values and sign of Δε is presented.

(C) What was method used in real-time FTIR measurements? In solution or in cell?. As I understand in range of wave numbers from 985 cm^-1 to 980 cm^-1 and from 1683 cm^-1 to1633 cm^-1 there are no bands from other groups existed in chiral dopants or components of LC mixtures?

(D) I am not sure that description of method for measurement of ion density on page 5 is necessary as similar information were presented in Materials and Method section.

(E) In description of Figure 4 there is perhaps something missing: "Inset is...."?

(F) It would be valuable to describe result of bandwidth broadening of the reflection band after using monomer C3M and C6M (in the first case bandwidth increase is described but for the second one not); this would make it easier for the reader to find the effect of terminal monomer chain length on these effects. Authors made such comparison for non-chiral and chiral monomers.

(G) Where is "Video 1" file used in sentence "Video 1 shows the large and reversible bandwidth broadening response of a PSCLC sample prepared using 6 wt% chiral LCM"?

Author Response

Thanks for the comments and corrections are in the red color in the text.

(A) In Figure 1, there is a mistake in the description: "...Schematic of the polymer network in a deformable PSCLC with Δe< 0: (a) with no applied field, (b) with a DC field applied between the top and bottom substrates...". I think it should be "...(d) Schematic of the polymer network in a deformable PSCLC with Δe< 0: (on the left) with no applied field, (on the right) with a DC field applied between the top and bottom substrates...".

: As the reviewer mentioned, we have corrected “(a) to (left image) and (b) to (right image)”.

(B) In Materials and Method section, in sentence "...and wΔε < 0 nematic LC MLC-2079 (Merck) with TNI = 102 °C, Δε = -6.1, and Δn = 0.15 at λ = 589 nm" it is not necessary to mention that this is negative dielectric anisotropy mixture as values and sign of Δε is presented.

: The “-“ sign has been deleted.

(C) What was method used in real-time FTIR measurements? In solution or in cell?. As I understand in range of wave numbers from 985 cm-1 to 980 cm-1 and from 1683 cm-1 to1633 cm-1 there are no bands from other groups existed in chiral dopants or components of LC mixtures?

: A sentence “The LC mixture was placed between KBr plates.” has been added in the Experimental section (page 3).

(D) I am not sure that description of method for measurement of ion density on page 5 is necessary as similar information were presented in it is necessary to Materials and Method section.

: In the Materials and Method section, the method of ion density measurement is described, while the ion density value of the samples prepared using as-received, purified, and residual monomers is reported for the study of the dynamic response of the samples (page 5).

(E) In description of Figure 4 there is perhaps something missing: "Inset is...."?

: We have added information about the inset, such as “Inset is an image of the recrystallized solution of C3M monomer.”

(F) It would be valuable to describe result of bandwidth broadening of the reflection band after using monomer C3M and C6M (in the first case bandwidth increase is described but for the second one not); this would make it easier for the reader to find the effect of terminal monomer chain length on these effects. Authors made such comparison for non-chiral and chiral monomers.

: We have added a paragraph on page 7, such as “The EO response of PSCLC is related to the ion density and viscoelastic properties of the polymer network. For PSCLCs with similar ion densities, the viscoelastic properties of the polymer network influence the magnitude of the dynamic response of the PSCLC. PSCLCs prepared using C6M (Figure 6) show larger bandwidth broadening or red shifting tuning response than PSCLCs prepared using short alkyl chain length C3M monomer (Figure 5) due to the large molecular weight between crosslinks.”

(G) Where is "Video 1" file used in sentence "Video 1 shows the large and reversible bandwidth broadening response of a PSCLC sample prepared using 6 wt% chiral LCM"?

: We have sent the Video 1 to the editor.

Author Response File: Author Response.docx