The influence of multi-walled carbon nanotube doping on liquid crystalline phase transitions of a smectogen octylcyanobiphenyl: A high-resolution birefringence study

Abstract

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 4.3 K on average above T N A for all 8CB-MWCNT composites. From the detailed inspection of S ( T ) data in the immediate vicintiy of the N–Sm A transition T N A , we have then obtained the upper limits for a possible latent heat Δ H N A across T N A for all 8CB-MWCNT composites. A slight change in Δ H N A 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 T N A 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 Δ H N A 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.