Photoisomerization-controlled phase segregation in a submicron confined azonematic liquid crystal.

Deuteron nuclear magnetic resonance is used to study the phase segregation behavior of photoisomerizable liquid crystal diheptylazobenzene (7AB) confined into cylindrical pores of Anopore membranes. It is demonstrated that the concentration of both components in a binary trans-7AB and cis-7AB mixture can be controlled dynamically using UV-illumination stimulated trans-to-cis photoisomerization and thermally induced cis-to-trans backisomerization. The so far elusive temperature-concentration phase diagram of such system is determined by comparative analysis of the behavior in bulk, thin-planar, and Anopore-confining geometry.

Deuteron NMR study of monolayer thick films of nematogenic molecules

We present a deuteron NMR study of molecular thickness films of a nematogenic material in cylindrical pores. Compared to liquid crystal in bulk or completely filling the pores, a reduced quadrupole splitting is found. It strongly depends on thickness, but only weakly on temperature, even crossing the bulk nematic-isotropic transition. This demonstrates the presence of a 2D-like film that eventually coexists with a bulklike phase suggesting a dewetting behavior. Motionally averaged angular patterns with biaxiallike features reveal unexpectedly fast surface molecular diffusion.

Critical behavior of thermal parameters at the smectic-A-hexatic-B and smectic-A-smectic-C phase transitions in liquid crystals.

High temperature resolution measurements of thermal conductivity, thermal diffusivity, and specific heat, with simultaneous polarized light visual inspection of the sample, have been performed at two different liquid crystal phase transitions: the SmA-SmC (Smectic-A-Smectic-C) and the SmA-HexB (Smectic-A-hexatic-B) in racemic A7 [4-(3-methyl-2-chlorobutanoyloxy)-4(')-heptyloxybiphenyl] and 65OBC (n-hexyl-4(')-n-pentyloxybiphenyl-4-carboxylate) compounds, respectively. In the past, anomalies in the thermal conductivity at the transitions have been reported. Our results indicate a nonsingular behavior of the thermal conductivity at both transitions, similarly to what has been previously reported for the smectic-A-nematic phase transition. It is also shown how, in several cases, the nature of the transition can be affected by the sample thermal history due to the presence of strain annealing phenomena.

Chain segment order in ultrathin polymer films: a deuterium NMR study.

The orientational order of monomerically thin films of polydimethylsiloxane melt deposited on the surfaces of cylindrical pores is investigated in this paper by solid-state deuterium nuclear magnetic resonance. Spectra demonstrate substantial orientation of the methyl groups by the surface. The quadrupolar splitting of the oriented film component saturates below one monomer layer, but decreases stepwise for thicker films. Comparisons of experimental and simulated spectra suggest a flat chain conformation in the first polymer monolayer.

Anchoring and structural transitions as a function of molecular length in confined liquid crystals.

Using deuteron nuclear magnetic resonance to study liquid crystals confined to cylindrical pores, an anchoring transition has been found. The transition exhibits an unexpected sharp dependence of the anchoring strength on cyanobiphenyl liquid crystal molecular length. A structural transition from a parallel axial to a planar radial configuration occurs due to an anchoring transition from planar to weakly homeotropic orientation at the walls. The anchoring strength is at a minimum near the decylcyanobiphenyl (10CB) liquid crystal length. Long chain liquid crystal configurations depend on thermal cycling and on the equilibrium atmosphere leading to a bistable SmA structure. Orientational order wetting in the isotropic phase also depends on molecular length.

Aerosil dispersed in a liquid crystal: magnetic order and random silica disorder.

Dispersions of silica spheres (aerosil) in octylcyanobiphenyl (8CB) liquid crystal were investigated with deuteron NMR. For the lowest silica density dispersions, there is a field-induced liquid crystal alignment as the field anneals most elastic strains. Increasing the silica density, the molecular alignment is stabilized by the spheres and is retained after sample rotation in the field; in the process, a few silica strand links are broken by the field, which realigns some molecules. At higher densities, the aerosil dispersions are aerogel like and disordering effects are apparent.