Abstract
Experimental results referring to the transformation of smectic phases, mainly smectic A, into nematic and reentrant nematic phases are reviewed. A new explanation of some experimental results is proposed. Factors which are responsible for the depression of smectic phases in mixtures of polar mesogens are discussed and the possibility of forming mixtures with a broad temperature range of nematic phase from smectic compounds, which can be useful for liquid crystal displays (LCDs), is shown. A nematic gap observed in some cases between monolayer (SA1) or monolayer and partially bilayer (SAd) smectics results from the differences in the organization of the molecules in the smectic layers. It is concluded that polar phase from smectic A1 phases can be divided into two groups: (a) the first one is characteristic for compounds with the -NCS, -F, -CI, -I or ‒COC m H2m + 1 terminal group. The spacing of the smectic layer slowly expands with the increase in alkyl chain length and the structure of the smectic A1 phase slowly changes to be more like the smectic Ad phase (d/1 > 1). It is proposed that such a smectic is called an enhanced monolayer smectic (SA1e (b) the second one is typical for compounds with the -CN terminal group. This kind of smectic A1 phase is rapidly transformed into the smectic Ad phase with increasing alkyl chain length. These latter monolayer mesogens easily form the reentrant nematic phase when they are mixed with other polar smectic mesogens.