High-resolution x-ray study of the nematic–smectic-Aand smectic-A–smectic-Ctransitions in liquid-crystal–aerosil gels

Abstract

The effects of dispersed aerosil nanoparticles on two of the phase transitions of the thermotropic liquid crystal material 4-n-pentylphenylthiol-4'-n-octyloxybenzoate (8S5) have been studied using high-resolution x-ray diffraction techniques. The aerosils hydrogen bond together to form a gel which imposes a weak quenched disorder on the liquid crystal. The smectic-A fluctuations are well characterized by a two-component line shape representing thermal and random-field contributions. An elaboration on this line shape is required to describe the fluctuations in the smectic-C phase; specifically the effect of the tilt on the wave-vector dependence of the thermal fluctuations must be explicitly taken into account. Both the magnitude and the temperature dependence of the smectic-C tilt order parameter are observed to be unaffected by the disorder. This may be a consequence of the large bare smectic correlation length in the direction of modulation for this transition. These results show that the understanding developed for the nematic to smectic-A transition for octylcyanobiphenyl (8CB) and octyloxycyanobiphenyl (8OCB) liquid crystals with quenched disorder can be extended to quite different materials and transitions.

Confinement-Induced Orientational Order in a Ferroelectric Liquid Crystal Containing Dispersed Aerosils

“…The instrumental resolution does not allow for the detailed line shape analysis carried out by Clegg et al for the SmA-SmC transition of 8S5 [9]. Figure 1 illustrates the line shapes in the SmA [1:5 K above T C , Fig.…”

“…While theoretical predictions have not yet been formulated, the experimental evidence points towards a minimization of the random disorder effects, possibly due to the long-range interactions driving the transition. The recently published x-ray diffraction experiments [9] on mixtures of 8S5 with silica nanoparticles revealed that the weak coupling of the liquid crystal with the hydrophilic aerosil surface does not influence the overall molecular orientation and the character of the transition [9]. Contrary to these results, dielectric spectroscopy measurements on the SmA-SmC phase transition of the liquid crystal S--[4-(2 0 -methyl butyl) phenyl 4 0 -n-octylbiphenyl-4-carboxylate] (CE8) containing dispersed silica aerosils demonstrated that aerosils strongly affect the transitional behavior [10].…”

Smectic-Aand smectic-Cphases and phase transitions in8¯SFreelon1, Ramazanoglu2, Chung3 et al. 2011Phys. Rev. ESelf Cite15170View full textAdd to dashboardCite

“…[7] In certain liquid crystal systems, as the temperature is lowered below the SmA phase a transition to a tilted or smectic-C (SmC) molecular phases occurs; in a previous study, the smectic correlation lengths have been found to either saturate or diminish on cooling into the SmC phase. [8] Recently, the role of QRD on tilted smectics (SmC) has been further investigated.…”

“…al. [8] presented high-resolution X-ray diffraction data that suggested that both the SmA and SmC phases and phase transitions were largely unaffected by the random fields originating from the aerosil gel network. This finding is also in disagreement with the results of subsequent lower resolution X-ray diffraction studies of doping-induced trends observed for 8 5 S [10] and with the phenomenology deduced from studies of a number of other LC-aerosil gel systems.…”

Surface and disorder effects in aerosil dispersed liquid crystals

“…The properties of the phase transitions and the dynamics and structure of these dispersions have been studied by e.g. theoreticians [4][5][6], calorimetry [7,8], NMR [9], X-ray scattering [10][11][12], light scattering [13,14], computer simulations [15], and ESR [16]. Given the large number of papers in this field, the selection in this introduction should be considered indicative.…”