Patterning nano-objects is an exciting interdisciplinary research area in current materials science, arising from new optical and optoelectronic properties and the need to miniaturize electronic components. Many techniques have been developed for assembling nanoparticles into two- and three-dimensional arrays. Most studies involving liquid crystals as templates have dealt with colloidal particles and nematic and smectic phases. Here, we demonstrate the long-range ordering of nanoparticle assemblies that adopt the helical configuration of the cholesteric liquid crystalline phase. Because we used glass-forming cholesterics, the nanostructures could be examined by transmission electron microscopy. The platinum nanoparticles form periodic ribbons that mimic the well-known 'fingerprint' cholesteric texture. Surprisingly, the nanoparticles do not decorate the original cholesteric texture but create a novel helical structure with a larger helical pitch. By varying the molar fraction of cholesterol-containing mesogen in the liquid crystal host, we show that the distance between the ribbons is directly correlated to the pitch. Therefore this inherent lengthscale becomes a simple control parameter to tune the structuring of nanoparticles. These results demonstrate how such an assembly process could be modulated, providing a versatile route to new materials systems.

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It is shown that the natural ultraviolet light absorbing properties of the liquid crystal constituent during the photoinduced elaboration of a liquid crystalline gel induce the broadening of the reflection bandwidth. The polymer component is then included in a resin by preserving its spatial distribution, and transmission electron microscopy investigations of cross sections show the existence of a structure gradient, which is at the origin of the broadening phenomenon. Such reflectors may be of interest for reflective polarizer-free displays or smart windows for the control of solar light for which a broadband reflection is required.

A way to make cholesteric films reflecting in a broad wavelength band consists in associating different cholesteric pitches in the same film. In two previous papers, we proposed an efficient method to produce variable pitch films, based on a thermal processing, and we studied the optical properties of these films with respect to the time of processing. In the present paper, we study the microstructure of such films with respect to the processing time by means of transmission electron microscopy. The cholesteric phase is shown to be very well ordered. Within a wide range of annealing times, its periodicity is progressive from one face to the other of the film. A description of the evolution of the structure with respect to the processing time is given, and classified in three stages, corresponding to the three stages already stated from the optical properties. The relationship between the evolution of the structure and the optical properties is discussed. PACS. 61.30.Eb Experimental determinations of smectic, nematic, cholesteric, and other structures -61.30.-v Liquid crystals -61.16.Bg Transmission, reflection and scanning electron microscopy (including EBIC)

Metallic nanoparticles dispersed in a cholesteric liquid crystal can order in accordance with the helical structure of the chiral phase. Since the liquid crystals we used have a glassy state, the nanostructures may be examined by transmission electron microscopy.The platinum nanoparticles form periodic ribbons which mimic the well-known fingerprint cholesteric texture. The particles do not decorate the pristine texture but create a novel structure with a larger periodicity. The distance between the ribbons is directly correlated to the helical pitch which therefore becomes a simple control parameter to tune the structuring of nanoparticles. Investigations of cross-sections show how the particles are arranged in the volume; a selective segregation proceeds at the periphery of the film and the particle ordering is localized close to the film-air interface. On the fingerprint patterning of nanoparticles, we do an analogy with the positive staining of polymer films with heavy-metal-containing compounds for transmission electron microscopy investigations and we discuss the accumulation of particles in the sites with the highest energy of director distortions.

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Aqueous suspensions of aggregated silica particles have been dewatered to the point where the colloidal aggregates connect to each other and build a macroscopic network. These wet cakes have been compressed through the application of osmotic pressure. Some cakes offer a strong resistance to osmotic pressure and remain at a low volume fraction of solids; other cakes yield at low applied pressures, achieving nearly complete solid/liquid separation. We used small angle neutron scattering and transmission electron microscopy to determine the processes by which the particles move and reorganize during cake collapse. We found that these restructuring processes follow a general course composed of three stages: (1) at all scales, voids are compressed, with large voids compressed more extensively than smaller ones; the local order remains unchanged; (2) all voids with diameters in the range of 2-20 particle diameters collapse, and a few dense regions (lumps) are formed; and (3) the dense lumps build a rigid skeleton that resists further compression. Depending on the nature of interparticle bonds, some cakes jump spontaneously into stage 3 while others remain stuck in stage 1. To elucidate the relation between bond strength and compression resistance, we have constructed a numerical model of the colloidal network. In this model, particles interact through noncentral forces that are produced by springs attached to their surfaces. Networks made of bonds that break upon stretching evolve through a plastic deformation that reproduces the three stages of restructuring evidenced by the experiments. Networks made of bonds that are fragile jump into stage 3. Networks made of bonds that can be stretched without breaking evolve through elastic compression and restructure only according to stage 1.

A series of cholesteric side chain elastomers with odd and even numbers of atoms along the spacer chain were prepared. Varying the content of odd and even spacer groups resulted in anisotropic main chain conformations changing from prolate to oblate. The conformation anisotropy was characterized by temperature-dependent measurements of the sample dimensions. The transmission spectra for polarized light of the elastomer films were recorded under uniaxial mechanical stress, applied perpendicular to the helical axis of the cholesteric liquid crystal. With increasing strain a blue shift of the selective reflection due to a contraction of the helix along its axis was observed. In the case of a nonvanishing conformation anisotropy in addition a distortion of the regular helical structure was indicated by changes in the transmission spectra for linearly polarized light. This distortion was found to be much weaker than predicted by theory.