Understanding the physical properties of liquid crystals (LCs) spatially confined to the mesoscopic scale is of fundamental and technological importance as a significant portion of LCs in device components is located near hard confining interfaces. Self-ordered nanoporous anodic alumina (AAO) membranes were used as an inorganic model matrix as it contains arrays of straight cylindrical nanopores, uniform in length and diameter, that can easily be infiltrated with LCs (5CB, E7 and HBC).  The nematic-to-isotropic, crystal-to-nematic, and supercooled liquid-to-glass temperatures were studied in the LCs confined in self-ordered nanoporous AAO. The nematic-to-isotropic and the crystal-to-nematic transition temperatures are reduced linearly with the inverse pore diameter. The finding that the crystalline phase is completely suppressed in pores having diameters of 35 nm and below yields an estimate of the critical nucleus size. The liquid-to-glass temperature is reduced in confinement as anticipated by the model of rotational diffusion within a cavity. These results provide the pertinent phase diagram for a confined LCs and are of technological relevance for the design of LC-based devices with tunable optical, thermal, and dielectric properties.  Similarly, another model LC system based on discotic mono-bromo hexa-peri-hexabenzocoronene (HBC-Br)  were studied. Structure formation, phase behavior, and dynamics of mono-bromo hexa-peri-hexabenzocoronene (HBC-Br) are also strongly affected by the confinement of cylindrical nanopores with rigid walls. Using self-ordered nanoporous AAO-containing arrays of aligned nanopores with narrow size distribution as a confining matrix, pronounced alignment of the HBC-Br columns along the nanopore axes was found to be independent of the pore diameter. Hence, arrays of one-dimensional supramolecular HBC-Br wires with the columns uniformly oriented along the wire axes on a macroscopic scale were obtained, unlike with discotics bearing smaller cores. The formation of the crystalline herringbone structure is shifted to lower temperatures in nanopores with diameters of a few hundred nanometers, whereas the formation of this low-temperature phase is completely suppressed when the pore diameter is below 20 lattice parameters. Moreover, the cylindrical confinement affects the disc axial dynamics as well as the distribution of relaxation times.<br/>

Structure formation, phase behavior, and dynamics of liquid crystals confined to nanopores

253rd National Meeting (2017)

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