Rich polymorphism of a rod-like liquid crystal (8CB) confined in two types of unidirectional nanopores

Abstract

We present a neutron and X-rays scattering study of the phase transitions of 4-n-octyl-4'-cyanobiphenyl (8CB) confined in unidirectional nanopores of porous alumina and porous silicon (PSi) membranes with an average diameter of 30 nm. Spatial confinement reveals a rich polymorphism, with at least four different low temperature phases in addition to the smectic A phase. The structural study as a function of thermal treatments and conditions of spatial confinement allows us to get insights into the formation of these phases and their relative stability. It gives the first description of the complete phase behavior of 8CB confined in PSi and provides a direct comparison with results obtained in bulk conditions and in similar geometric conditions of confinement but with reduced quenched disorder effects using alumina anopore membranes.

Physics of fluids confined in nanochannels: A shared space for neutrons and simulation

“…In some cases, these crystalline forms correspond to well-known metastable states of the bulk system, the nucleation of which is favored by confinement. In other cases, these confined crystalline structures have never been observed in the bulk [9]. Moreover, it has been shown for benzene that crystallization can be definitively avoided by confinement.…”

“…Moreover, diffraction experiments reveal possible changes in the sequence of phases of the confined system with respect to its bulk counterpart. For various systems spanning from water to liquid-crystals, crystalline structures that differ from the normal bulk one can be formed in confinement [8,9]. In some cases, these crystalline forms correspond to well-known metastable states of the bulk system, the nucleation of which is favored by confinement.…”

“…Smectic order of nanoconfined 8CB has been analyzed in detail by neutron diffraction and small angle neutron scattering [9,29]. Rocking curve measurements have shown that the smectic layers are aligned perpendicular to the pore axis.…”

Individual Liquids and Liquid Solutions Under Confinement

“…New properties of the confined LCs have been documented, including previously unknown phase transitions, extreme optical turbidity, and sluggish, glassy-like dynamic behavior. Two major areas of the SAS applications in the area of confined LCs may be outlined as investigation of the structure of LC-polymer composites [64,65], and modification of the LC structure and phase behavior in porous media [66][67][68][69][70].…”

Theoretical Study of Nematic to Isotropic Transition in Porous Media

“…Hence, the equilibrium director configuration depends on the elastic constants, surface coupling, the size of the system and its density, as well as on applied external fields. There are several studies mainly with dielectric spectroscopy [5][6][7], differential scanning calorimetry [8], NMR [4,9] and X-ray scattering [10], on the molecular dynamics of different alkylcyanobiphenyls in porous media. Also, self-ordered nanoporous aluminum oxide (AAO) [11,12] has been widely used.…”