‘Quasi-Tricritical’ and Glassy Dielectric Properties of a Nematic Liquid Crystalline Material

Round 1

Reviewer 1 Report

In the paper Quasi-tricritical and glassy dielectric properties of a nematic liquid crystalline material, the author discussed dielectric studies in the nematic and isotropic phase of 5CB. The author claims in the abstract that the changes in dielectric constant and NDE in the nematic phase are entirely dominated by the impact of pretransitional fluctuations. This claim seems strange to me. In my opinion, the changes in the dielectric tensor in the nematic phase are primarily a consequence of nematic ordering of molecules, and pretransitional fluctuations in the isotropic phase are also a consequence of that. Besides, it is not clear what is new in this work regarding the author previous work (in particular in the connection with the Ref. 36) or is this just a review of their work. This should be clarified. Some other major comments:

• There are statements in the introduction that are simply not true, e.g.,

There are five decades of studies indicating no pretransitional behavior for the IN transition of properties mentioned above.

Besides the papers published in co-authorship by the author of this paper, there are also others published more than a decade ago, just to mention one example: S. Sridevi, S. K. Prasad, D. S. S. Rao, and C. V. Yelamaggad, J. Phys.: Condens. Matter, 20, 465106 (2008).

• The alternative analysis of the decrease of dielectric constant (of an order of 1%) just before the phase transition to the nematic phase would be taking into account the pretransitional fluctuations of the nematic order parameter, which relaxation rate for 5CB in this temperature region strongly decreases from several 10 kHz down to a few kHz. It crosses the frequency at which dielectric constant presented in Fig. 1 is measured. This fluctuation mode should be in principle visible in a dielectric spectrum as a very weak mode. Comparison of dielectric spectra at different temperatures in this region may show this mode, if, of course, the signal to noise ratio is high enough. Did the author consider this and how would such analysis compare to the presented one?

In my opinion, the above comments should be addressed before the paper is taken into consideration for publication.

Author Response

REVIEWER I

• Reviewer 1: ‘The author claims in the abstract that the changes in dielectric constant and NDE in the nematic phase are entirely dominated by the impact of pretransitional fluctuations. This claim seems strange to me. In my opinion, the changes in the dielectric tensor in the nematic phase are primarily a consequence of nematic ordering of molecules, and pretransitional fluctuations in the isotropic phase are also a consequence of that.’

The response: I do not agree about this. The author's point of view is extensively explained in the CONCLUSION – see the new text between lines 329 and 371. Arguments ‘for the molecular pictures’ are first recalled, supported by new references. Subsequently, evidence related to the molecular’ picture is confronted with the ‘quasi-critical’ picture. As the check-point, only experimental facts are taking into account. No ‘subjective’ arguments from the Author. Experimental results are always the ultimate ‘check-point’.

• Reviewer 1: ‘There are five decades of studies indicating no pretransitional behavior for the IN transition of properties mentioned above.; was written in the paper. Besides the papers published in co-authorship by the author of this paper, there are also others published more than a decade ago, just to mention one example: S. Sridevi, S. K. Prasad, D. S. S. Rao, and C. V. Yelamaggad, J. Phys.: Condens. Matter, 20, 465106 (2008).

The response: This reference, and 2 more similar, is recalled in lines [87-90]:” In the last two decades, such in-deep studies taking into account Eq. (6) have been carried out also by ‘other’ authors extending the evidence confirming the appearance of the pretransitional effect in the isotropic liquid phase and the validity of Eq. (6), also for LC molecule beyond the basic rod-like geometry [28-30]

• Reviewer 1: ‘The alternative analysis of the decrease of dielectric constant (of an order of 1%) just before the phase transition to the nematic phase would be taking into account the pretransitional fluctuations of the nematic order parameter, which relaxation rate for 5CB in this temperature region strongly decreases from several 10 kHz down to a few kHz. It crosses the frequency at which dielectric constant presented in Fig. 1 is measured. This fluctuation mode should be in principle visible in a dielectric spectrum….

The response:  This issue is in-deep discussed between – lines 346-356, with recalling the paper, which may support frequency (relaxation rates) value recalled by the reviewer. The experimental evidence is here clear – 10 kHz or less cannot be linked to basic processes in the isotropic phase. The evolution of the primary absorption curve (high-resolution results), shown in the Appendix, clearly shows no additional relaxation process. Of course, it appears in the nematic phase and the two-phase region (the impact of impurities). All these are discussed in Conclusions.

• Reviewer 1: Besides, it is not clear what is new in this work regarding the previous author's work (in particular in connection with the Ref. 36), or is this just a review of their work. This should be clarified.

The response:  This issue is addressed at the end of the EXPERIMENTAL section.

Following the comments of the reviewer I decided to add the Appendix in which the spectrum of the imaginary part of dielectric permittivity is presented and discussed. The derivative analysis for TI-N + 0.1 K shows no evidence for a new relaxation process, which in my opinion, was suggested.

Reviewer 2 Report

The author deals with some unexplored details of the dielectric behaviour of a well known and thoroughly studied nematic, the 5CB. Analysis of the pretransitional effects and the nonlinear dielectric effect provides new insight and thus may warrant publication in Crystals.

Unfortunately the presentation of the interesting results is much weaker than the results themselves. The manuscript contains a lot of misprints, sentences with grammatical errors, which undermines the information transfer to readers.

Lines 45-47: the sentence is unclear; ’… properties of such properties as…” did you mean ’… properties as …’ ?

Line 58: Formula (5) has an error. It must not contain the same multiplier (n₀) as Eq. (4).

Lines 71-72: part of the sentence is repeated.

Lines 76-77: should not be new paragraphs, but continuation of a sentence.

Lines 79-80: The sentence is not clear. Maybe ’but’ should be deleted?

Line 115: The abbreviation ’BDS’ is not defined.

Line 152: A comma is missing after ‘ε*=11.197’, ‘K’ at the end of the line should be deleted

Line 155: Grammatical error. Did you mean „Experimental data presented in Fig. 1 make it possible …”?

Line 160: Misprint. Did you mean ’α=0.49±0.02’ ?

Line 161: Delete ’the’ after ’pretransitional’.

Line 165: No subject. Did you mean ’ … transition, Δε(T) should follow …’

Fig. 2: What do you mean by ’β=0.25+0.04’ ? Shouldn’t it be ’β=0.25±0.04’ ?

Lines 184-185: Did you mean „The nematic phase was reached on cooling, and the additional DC electric field (U=40V) was applied just at the phase transition at T_I-N.” ?

Line 186: ’… changes of ε(T) …’ ?

Line 192: Did you mean „… possibility may provoke experimental …”

Fig. 3: The symbols for 20Hz are missing from the figure.

Lines 198-199: The order of words is unconventional. Did you mean „From the application point of view, the impact of ionic impurities on dielectric properties is meaningful, becoming significant for the low frequencies domain.” ?

Line 212: ’preparatory’ is an adjective, not a noun. What is the spatial praparation technique and the special design of the capacitor mentioned?

Fig. 4: The vertical axis (T²/H_a) should have a dimension, isn’t it? The inscriptions ’Supercooled Nematic’ and ’DSM’ are hardly visible.

Line 219: „… capacitor and fats cooling …” should be „… capacitor and fast cooling …”

Line 224: „foe” should be „for”

Line 234: Did you mean „The amplitude is D=D_TT₀=const” ?

Line 235: Did you mean „Comparing Eqs. (10) and (11), one obtains” ?

Line 237: The abbreviation ’VFT’ is not introduced.

Line 250: The abbreviation SA has been previously introduced related to Eq. (11), not for Eq. (10) ! Either ’one can show’ or ’we can show’, select one.

Line 253: Either ’for the linear’ or ’in the linear’, select one.

Line 260: „Notable there are qualitatively…”

Line 269: Did you mean „In the case of the isotropic phase of the nematic liquid crystal, the rod-like symmetry causes the increase of the number of neighbors …” ?

Line 274: Did you mean „and Δε^f at the applied measuring radio-frequency. ” ?

Line 289: Pentylcyanobiphenyl !!

Line 296: „studies in the”

Line 297: Delete „the order parameter”, as β is an exponent only.

Lines 297-298: ’indicated the tricritical’

Lines 298-300: The last sentence has no meaning in its present, mistyped form.

Line 304: „The results of this report show that the static …”

Line 305: „... LC material, are entirely …”

Line 307: „… transition is a challenging …”

Publication may become possible after the necessary corrections.

Author Response

REVIEWER II

The reviewer stated: The author deals with some unexplored details of the dielectric behaviour of a well known and thoroughly studied nematic, the 5CB. Analysis of the pretransitional effects and the nonlinear dielectric effect provides new insight and thus may warrant publication in Crystals.

Subsequently, the in-deep list of corrections (mainly related to language) was given. I aver grateful for this impressive work.

The response: Exactly, all suggestions of the Reviewer were taking into account in the text. This also includes the correction of Fig. 4.

Concluding all comments and suggestions of Reviewers have been taken into account.

I am very grateful to Reviewers for such in-deep work and comments. All these make this report better for readers who look for broad insight into the given problem. Additionally, I made an extra correction – supported by the native speaker. Note that -the Appendix is added to meet all expectations of the Reviewer I. The Appendix address all issue realted to determing the primary relaxation time from BDS spectrum.

Round 2

Reviewer 1 Report

In the revised paper Quasi-tricritical and glassy dielectric properties of a nematic liquid crystalline material the author addressed my comments, although not entirely satisfactory. In particular, they added data in the appendix claiming there is no process in the 10 kHz range, but not showing the temeprature dependence of the dielectric spectrum in that frequency range.

Author Response

Dear Referee, 

In Appendix  Figure 3 A  which shows spectra of the real and imaginary part of dielectric permittivity from f = 1 Hz to f = 1 GHz  - in the immediate vicinity of the I-N temperature (T_IN+0.1 K ) and well beyond T_I-N +100K) is presented. 

This is associated with the derivative of experimental data. to reveal even subtle 'hidden' processes. There is no doubt that there are no 'special events' in the spectrum within  1 kHz - 10 kHz frequency range.and also well below.  

The spectrum shows also the very high quality of the tested sample, regarding the 'ionic' or 'heterogeneous' contaminations. 

I hope that this very clear evidence answer your last comment. 

Thank You once very much for Your efforts which really improved the paper and - in fact - notably changed it. 

Yours Sincerely 

Aleksandra Drozd-Rzoska