Calorimetric and small angle x-ray scattering study of phase transitions in octylcyanobiphenyl-aerosil dispersions

Abstract

High-resolution calorimetric studies have been made of the liquid crystal phase transitions for several dispersions of 70-Å-diam silica spheres ͑aerosil͒ in octylcyanobiphenyl ͑8CB͒ as a function of silica density S. The excess specific heat peaks associated with the nematic-isotropic (N-I) and the nematic-smectic-A (N-SmA) transitions both exhibit shifts to lower temperatures, decreases in the specific heat maximum values, and decreases in the transition enthalpies as S is increased. Two distinct regimes of S-dependent behaviors are observed with a crossover between them at S Х0.1 g cm Ϫ3. For lower silica densities, sharp second-order C p peaks are observed at the N-SmA transitions, characterized by effective critical exponents that decrease monotonically with S from the pure 8CB value toward the three-dimensional XY value, and two closely spaced but distinct first-order C p features are observed at the N-I transition. For higher silica densities, both the N-SmA and the N-I transitions exhibit a single rounded C p peak, shifting in temperature and decreasing in total enthalpy in a manner similar to that observed in 8CBϩaerogel systems. Small angle x-ray scattering data are qualitatively aerogel-like and yield temperature-independent mass-fractal dimensionalities for aerosil aggregates that differ for samples with silica densities above and below the crossover density.

Smectic ordering in liquid-crystal–aerosil dispersions. II. Scaling analysis

“…Additionally, the ease at achieving nearly arbitrary silica densities for the aerosil system allows for greater control of the disorder. For the 8CB+aerosil system, thermal evidence for two regimes of behavior has been found [4]: low-density gels where pseudo-critical behavior is closely related to that for the pure LC and higher density gels where all transition features appear to be smeared. More rigid aerogels in LC+aerogel systems are crudely like the LC+aerosil gels in the high density regime but differ in some important ways since the elastic strain imposed by the random anchoring surfaces of aerogels is fully quenched.…”

“…When dispersed in an organic liquid medium, aerosil particles comprising 3 to 4 lightly fused spheres and having a mean radius of gyration of ≈ 240Å [4] will attach to each other and form a gel by a diffusion-limited aggregation process. This gel can be thought of as a randomly crossing "pearl necklace" of silica and a cartoon depiction is given in Fig.…”

“…[4] and [19] for a more detailed derivation. Strictly speaking, this definition of l o is valid only in the dilute regime where the addition of more solid does not significantly change the specific surface area.…”

“…In spite of the loss of smectic LRO, Paper I [12] and an earlier calorimetric study of 8CB+aerosils [4] show that smectic thermal fluctuations still play an important role in LC+aerosil systems. Many concepts from pure materials such as finite-size scaling, two-scale universality, and tricritical-to-XY crossover due to variable de Gennes coupling need to be considered in addition to intrinsic new quenched random effects that dominate at temperatures below an effective transition temperature.…”

“…They include the still enigmatic superfluid transition of 4 He in aerogels and porous glasses, the superfluid transition and phase separation of 4 He- 3 He mixtures in silica aerogels [1] and doped magnet systems [2]. Relatively recent efforts with liquid crystal (LC) -silica composites [3,4,5,6,7,8,9] have demonstrated that these are especially interesting model systems. They are of particular importance as a way to access "soft" (elastically weak) phases of continuous symmetry, which are directly coupled to surfaces and external fields.…”