Calorimetric study of the nematic to smectic-Aand smectic-Ato smectic-Cphase transitions in liquid-crystal–aerosil dispersions

Abstract

A high-resolution calorimetric study has been carried out on nanocolloidal dispersions of aerosils in the liquid crystal 4-n-pentylphenylthiol-4'-n-octyloxybenzoate (8S5) as a function of aerosil concentration and temperature spanning the smectic-C to nematic phases. Over this temperature range, this liquid crystal possesses two continuous XY phase transitions: a fluctuation-dominated nematic to smectic-A transition with alpha approximately alphaXY=-0.013 and a mean-field smectic-A to smectic-C transition. The effective critical character of the N-SmA transition remains unchanged over the entire range of the introduced quenched random disorder while the peak height and enthalpy can be well described by considering a cutoff length scale to the quasicritical fluctuations. The robust nature of the N-SmA transition in this system contrasts with cyanobiphenyl-aerosil systems and may be due to the mesogens being nonpolar and having a long nematic range. The character of the SmA-SmC transition changes gradually with increasing disorder but remains mean field like. The heat capacity maximum at the SmA-SmC transition scales as rho with an apparent evolution from tricritical to a simple mean-field step behavior. These results may be generally understood as a stiffening of the liquid crystal (both the nematic elasticity as well as the smectic layer compression modulus B) with silica density.

CdSe nanoparticles dispersed in ferroelectric smectic liquid crystals: Effects upon the smectic order and the smectic-Ato chiral smectic-Cphase transition

“…While the T c drifts have not been observed in the case of 8S5 + aerosil mixtures [11], they have been recorded in the case of calorimetric measurements of 7O.4 + aerosils [43]. The evolution of the C p shape to step-like for the χ = 0.20 sample, however, is in line with what has been reported for the 8S5 + aerosil mixtures [22]. On the contrary, the CE8 + aerosils mixtures exhibited a substantially different behavior where, analogously to the tilt angle, the heat-capacity profiles evolved into broadened and smeared supercritical anomalies as a function of increasing χ , with a rapidly decreasing enthalpic content.…”

“…Extensive studies of the N -SmA transition of the thermotropic liquid crystal 4-n-pentylphenylthiol-4-noctyloxybenzoate (8S5) reveal that the critical character remains unaffected by the aerosil-introduced quenched-random disorder. Even though the N -SmA is a pseudotransition, due to the destruction of the quasilong-range smectic order, the critical exponents derived from high-resolution heat-capacity data remain three-dimensional XY -like, for a large range of nanoparticle concentrations included in the study [22]. In addition, investigating the effects of aerosils upon the smectic order of aligned and nonaligned gels of octylcyanobiphenyl (8CB), it was demonstrated that quenched randomness in aligned gels leads to constant three-dimensional XY critical exponents while otherwise a random field with disorder strength linearly dependent upon the nanoparticle concentration must be considered [9,[23][24][25].…”

“…For the SmA-SmC transition of 8S5, the systematic analysis of high-quality calorimetric data has provided evidence for a tricritical to classical mean-field behavior of the heat-capacity peaks as well as for a substantial decrease of the heatcapacity jump as a function of aerosil concentration [22]. On the other hand, high-resolution x-ray diffraction experiments measuring the temperature dependence of the order parameter revealed nanoparticle-induced extensive coexistence regions and smearing of the data near the transition temperature [26].…”

Smectic-Aand smectic-Cphases and phase transitions in8¯SFreelon1, Ramazanoglu2, Chung3 et al. 2011Phys. Rev. ESelf Cite18170View full textAdd to dashboardCite

“…It should be noted that high-resolution calorimetric data on 8 5 S +sil samples [16] show that the N-SmA transitions exhibit a critical variation in the excess heat capacity C p that, except for a narrow region of finite-size rounding, is as well-defined as that for the pure LC. Furthermore, the excess heat capacity associated with the SmA-SmC transition for LC+sil samples is smeared and rounded relative to the Landau mean-field behavior seen in pure 8 5 S , in qualitative agreement with the X-ray behavior observed here.…”

“…1(a), is the primary order parameter of the SmC phase. Detailed high-resolution calorimetry work [16] on 4-n-pentylphenylthiol-4'-n-octyloxybenzoate ( 8 5 S ) in aerosil gels shows that both the N-SmA and SmA-SmC phase transitions are dramatically affected by the presence of the aerosil gel. In contrast to the calorimetry work, Clegg et.…”