Raman investigation of stable and metastable states of4−octyl−4′-cy

Abstract

We study the polymorphism of solid phases of 4-octyl-4(')-cyanobiphenyl (8CB) by Raman spectroscopy. For bulk 8CB, the Raman spectrum of the CN stretch is featured by a single peak, which shifts abruptly at the smectic-A-crystal transition. In confinement, the CN peak splits both at high and low temperatures. In the isotropic and liquid crystal phases, the signature of the liquid crystal bulk (LC) coexists with another peak that is assigned to LC molecules interacting with the matrice interface. We find correlations between the volumic fractions of interfacial liquid and the texture of the matrices. At low temperatures, we assign the splitting of the CN peak to the coexistence of different metastable solid phases. For strong confinements, the temperature dependence of the CN stretching frequency extends to that of the liquid, which suggests the existence of frozen-in smecticlike solid phases. We discuss the structure of these metastable solid phases in the light of neutron diffraction measurements. We also report on the peculiar analogy between the effect of quenched disorder due to the porous matrices and the effect of thermal quenching.

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

“…4. Fehr et al have mentioned that 8CB could crystallize in a variant form of the K phase, denoted K', which presents such a Bragg peak at 0.65Å -1 [17,18].This is also consistent with the splitting of the endothermic peak (B)…”

“…The occurrence of a single peak at 0.25Å -1 has already been observed in confinement in aerogels [17]. It has been assigned to a metastable phase of 8CB, which is promoted by confinement and denoted Ks.…”

“…At temperatures lower than the smectic transition, the phase behavior of 8CB confined in disordered porous silica glasses shows a rich polymorphism [17,18]. For an average pore size of 20 nm, Fehr et al have shown that 8CB forms a phase (denoted K s ), which is only observable in the bulk state after a strong thermal quench.…”

Molecular dynamics of a short-range ordered smectic phase nanoconfined in porous silicon

“…40 The spatial resolution (FWHM) of the confocal micro Raman spectroscopy is about 1.5 ìm (calibrated on a silicon plate substrate). A direct scan and check of the 8CB loaded in PSi across the porous layer is obtained by changing the location of the volume analyzed by the confocal microscope device.…”

“…These phases, which do not correspond to stable crystalline phases of bulk 8CB have been reported in confined geometry. 40,44 In the temperature region covered on cooling from 250 K to 220 K, three phases of confined 8CB coexist: two crystalline polymorphs and the short-range smectic phase.…”