Critical Behavior and Scaling Relationships at the SmA_d−N and N−I Transitions in Nonyloxycyanobiphenyl (9OCB)

Abstract

Different kind of measurements were performed on the liquid crystal nonyloxycyanobiphenyl (9OCB) to carry out a study of the molecular dynamics in the smectic A (SmA), nematic (N), and isotropic (I) phases as well as an exhaustive analysis of both the SmA-to-N and N-to-I phase transitions. For the dynamic study, broadband dielectric spectroscopy (102 to 1.8 x 109 Hz) was used. Two orientations (parallel and perpendicular) of the molecular director with regard to the probing electric field were investigated. From this study, the static dielectric permittivity was obtained in both alignments and, in addition, the molecular motions that contribute to each one were discussed. The static dielectric data together with specific heat and volumetric determinations were analyzed, proving that both phase transitions are weakly first order, displaying a nearly tricritical behavior. However, the width of metastable regions seems to be dependent on the physical magnitude, although specific heat and volumetric determinations allow for comparable results. It should be noticed that the temperature derivative of the static dielectric permittivity, specific heat, and isobaric thermal expansion coefficient data derived from volumetric determinations are related to each other by scaling relationships.

Tricritical behavior of the nematic to smectic-A phase transition in the binary mixture of liquid crystal

“…On the contrary, as far as the stability spinodal temperatures they were determined to be strongly dependent on the experiment. Thus, for the I-N phase transition, similar T * * I N values were reported for 9OCB from dielectric spectroscopy, specific heat, and density measurements 24,26,29,[31][32][33][34] whereas values of T * I N strongly differ by a factor two (when comparing specific heat and density) or three (when the aforementioned techniques are compared to dielectric measurements). As for the N-SmA phase transition, results are scarce and when existing, disparate when the spinodal temperatures are compared.…”

“…16,[28][29][30] Due to the dielectric anisotropy, proportional to the order parameter, of the liquid crystal phases the metastable limits as well as the critical exponents of the order parameter (β) can be determined from dielectric spectroscopy. Results have been found in reasonably agreement with the tricritical hypothesis (β = 0.25).…”

Twist, tilt, and orientational order at the nematic to twist-bend nematic phase transition of 1″,9″-bis(4-cyanobiphenyl-4′-yl) nonane: A dielectric,H2NMR, and calorimetric study

“…The critical behavior of the heat capacity around the 350 N -I phase transition follows the standard expressions 351 [15,30,32,33] in a region of no more than ±3 K around 352 T NI :…”

Influence of internal flexibility on the double glass transition in a series of odd non-symmetric liquid crystal dimers characterised by dielectric measurements

“…Temperature was controlled by means of a System Quatro from Novocontro with stabilization at different temperatures with a precision of the order of 20 mK. Additional details of the experimental technique can be found elsewhere [39,40].…”

“…The MDSC technique, in addition to specific heat data, simultaneously provides phase shift data (Φ) that allow determining the coexistence region in weakly first-order phase transitions. Experiments were performed on 3 − 7 mg samples sealed in aluminium pans on cooling from the I phase down to the mesophase and next on heating; the temperature rate on cooling was 2 K/min and on heating were of 1, 2.5 and 5 K/min, with a modulation temperature amplitude of ±0.5 K and a period of 60 s. Extensive details about this technique can be found elsewhere [39,40].…”