Abstract
High-resolution optical birefringence measurements are reported on the weakly first order nematic-isotropic (N–I) and the continuous nematic-smectic A (N–Sm A) phase transitions in smectic liquid crystal 8CB (octylcyanobiphenyl) doped with both pristine multi-walled carbon nanotubes (p-MWCNTs) and carboxyl group (-COOH) functionalized MWCNTs (f-MWCNTs). It has been observed that the inclusion of MWCNTs substantially decreases both the N–I and the N–Sm A transition temperatures compared to 8CB host while the order of the transitions remains the same being the I–N weakly first-order and the N-SmA continuous for all 8CB-MWCNT composites regardless of the surface functionalization. The optical birefringence data have been used to probe the temperature behavior of the nematic order parameter S(T) in the vicinity of both the N–I and the N–Sm A transitions. Sufficiently far away from the Sm A phase, the average value of the critical exponent β characterizing the limiting behavior of S(T) near the N–I transition was found to be 0.245±0.002 This value of β seems to be in accord with the tricritical nature (TCH) of the N–I transition. Thus, concluded is the fact that the nature of the N–I transition of nanomaterial-based composites dispersed in liquid crystals remains tricritical up to at least the reported dopant contents and within the experimental resolution. The pre-transitional smectic behavior has also been noted to continue up to 4.3K on average above TNA for all 8CB-MWCNT composites. From the detailed inspection of S(T) data in the immediate vicintiy of the N–Sm A transition TNA, we have then obtained the upper limits for a possible latent heat ΔHNA across TNA for all 8CB-MWCNT composites. A slight change in ΔHNA has been observed due to the inclusion of MWCNTs as compared to 8CB host. By exploiting the fact that the temperature derivative of the order parameter S(T) near TNA exhibits the same power law divergence as the specific heat capacity, we have also extracted the effective critical exponent α values characterizing the critical fluctuations near the N-SmA transition for all 8CB-MWCNT composites. From both α and ΔHNA values, we have concluded that the nematic and smectic fluctuations remain essentially bulk-like for both 8CB-p-MWCNT and 8CB-f-MWCNT composites and thus the fact that the coupling between the N and the Sm A order parameters for all composites are comparable to that of 8CB host has been documented.
Original language | English |
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Pages (from-to) | 47-58 |
Number of pages | 12 |
Journal | Fluid Phase Equilibria |
Volume | 495 |
DOIs | |
Publication status | Published - 1 Sept 2019 |
Bibliographical note
Publisher Copyright:© 2019 Elsevier B.V.
Funding
We thank BAYER (Germany) for providing us free samples of both pristine and –COOH functionalized MWNCTs. This work was supported by the Research Fund of Istanbul Technical University under Grants No: 40883 and 41101 . We appreciate to Dr. Selma Ergin, Faculty of Naval Architecture and Ocean Engineering-Ship Emissions Lab., for the usage of Ultra-Micro Balance during the preparation of both p-MWCNT and f-MWCNT doped samples. The authors also thank to the reviewers for their illuminating suggestions for improving the manuscript.
Funders | Funder number |
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Istanbul Teknik Üniversitesi | 40883, 41101 |
Keywords
- Critical exponents
- Latent heat
- Multi-walled carbon nanotubes
- Optical birefringence
- Polar smectic liquid crystal