Solid state polymorphism of liquid crystals in confined geometries

Abstract

Solid polymorphism of 4-alkyl-4'-cyanobiphenyl (nCB) was studied so far as a function of thermal history. In this paper we show that metastable solid phases of 4-octyl-4'-cyanobiphenyl (8CB) are also formed when the mesogens are confined in porous silica matrices and we study their structure by neutron diffraction and by Raman spectroscopy. Three metastable solid states are identified: one crystalline phase K', two frozen-in smectic-like phases K(s) and K'(s). We discuss the relation between the structure of the metastable solid phases and that of the mesomorph phases.

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

“…A second interpretation implies the K' phase. The high-q signatures of the K' phase have always be observed in co-existence with the lamellar Ks Bragg peak [18]. It suggests that the K' phase most likely nucleates from the Ks one.…”

“…At 300 K, a clear diffraction ring appears for an angle of diffraction corresponding to a value of the momentum transfer q equals to 0. 1.69 Å -1 and can be assigned to the stable crystalline phase of 8CB, denoted phase K [18].…”

“…This usually gives a central role to the thermal rate on the nature of the obtained phase or the proportion of coexisting phases. Indeed, the Ks phase has been obtained in bulk conditions only after a thermal quench, whereas slower cooling rates promotes the more stable K phase [18]. For 8CB in PSi, temperature scan rates ) obviously promotes the Ks phase, which is more stable than the K's phase.…”

“…The use of porous materials permeated by macroscopically aligned linear channels has provided invaluable information about the structure of these unusual lamellar phases, which goes beyond the present understanding of the rich polymorphism of 8CB in confinement, as essentially described in the case of random porous materials [17,18]. Ks and K's phases present a preferential alignment of the nematic director along the pore axis.…”

“…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.…”