Reassessment of structure of smectic phases: Nano-segregation in smectic E phase in 4-n-alkyl-4′-isothiocyanato-1,1′-biphenyls

Abstract

Based on new diffraction data from aligned samples of smectic E (SmE) phase of 4-n-alkyl-4'-isothiocyanato-1,1'-biphenyls, systematics against the alkyl chain length n is analyzed. In order to perform the analysis, the molecular form factor approximated by a box-shaped distribution is calculated while taking the rounding of the distribution at corners into account. The analysis clearly shows the nano-segregated layered structure, which does not fit to the traditional structural view of SmE phase but does fit to the model the present authors proposed recently. Some implications of this conclusion are discussed in relation to the importance of the molten state of alkyl chains in most of real mesogens revealed previously through thermodynamic analyses.

Chemical Programming of the Domain of Existence of Liquid Crystals

“…For the lipophilic cyanobiphenyls n ‐LC i ′, j , a chain length of n ≥7 is sufficient for the demixing of the apolar parts, leading to nanoscale segregation and the formation of thermotropic liquid‐crystalline phases for each member of the series (except for 12 ‐ LC 2′,2 ; Figure ). As expected, substitution (i.e., methylation) of the rigid aromatic core has only a minor influence on the melting temperatures controlled by the entropic contribution of the molten flexible alkyl chains . With the help of the CFED concept, the latter effect can be associated with a cohesive entropy density of Δ S m / V mol =0.253(8) J K −1 cm −3 .…”

“…As expected, substitution (i.e.,m ethylation) of the rigid aromatic core has only am inor influence on the meltingt emperatures controlled by the entropic contribution of the molten flexible alkyl chains. [13,16] With the help of the CFED concept, the latter effect can be associated with ac ohesive entropyd ensity of DS m /V mol = 0.253(8) JK À1 cm À3 .O nt he other hand, the clearing temperatures correspond to decorrelations of the rigid aromatic cores, and thus, strongly depend on the methylation of the aromatic partofthe cyanobiphenyls in n-LC i',j .Again, the application of the CFED concept provides aq uantitative assessment of the effect of peripheral methylation onto the clearing temperaturew ith ac ohesive entropyd ensity of DS m /V mol = 0.0054(9)JK À1 cm À3 .I ns impler words, methyl substitution of the cyanobiphenyl core can be exploited to tune the clearing temperatures (T clearing ), while the meltingt emperatures (T m )a re essentially invariant (the relative effect can be estimated as 0.253/0.0054 % 47). Finally,t he CFED versus T tr correlation highlightedi nF igure 8c an be combined with the associated local linear H/S compensations (Figure 7) to give aset of two empirical equations [Eqs.…”

“…However, contrary to Trouton's rule for the vaporization of liquids, the entropy changes, Δ S tr , accompanying the melting process that transforms a solid into a liquid or a liquid crystal, or the isotropization process that transforms a liquid crystal into an isotropic liquid, are not constant. They depend on the nature of the material and contribute to the stabilization of the different states of matter . For instance, the large and variable entropic contributions of the molten flexible chains to the Gibbs free energy have been exploited for the stabilization of the nanosegregated liquid‐crystalline phases found in alkylcyanobiphenyls (smectic A mesophase), bis‐alkyloxybenzoylhydrazines (cubic mesophase), and alkylisothiocyanatibiphenyls (smectic E mesophase) …”

“…However,c on-trary to Trouton's rule for the vaporization of liquids, the entropy changes, DS tr ,a ccompanying the melting process that transforms as olid into al iquid or al iquid crystal,o rt he isotropization process that transforms al iquid crystal into an isotropic liquid, are not constant.T hey depend on the nature of the materiala nd contribute to the stabilization of the different states of matter. [13] For instance, the large and variable entropic contributions of the moltenf lexible chains to the Gibbs free energy have been exploited for the stabilization of the nanosegregatedl iquid-crystalline phases found in alkylcyanobiphenyls (smectic Am esophase), [14] bis-alkyloxybenzoylhydrazines (cubic mesophase), [15] and alkylisothiocyanatibiphenyls (smectic E mesophase). [16] Consequently,the square of the Hildebrands olubility parameter, d 2 ,p ertinent to the vaporizationo fl iquids, has been replaced by the concept of cohesive free energy density,C FED = ÀDG ref tr /V mol in liquid crystals, in which DG ref tr = DH tr ÀT ref DS tr measures the intermolecular cohesion free energy in the solid or in the liquid-crystalline phase estimated at af ixed reference temperature, T ref (DH tr and DS tr are the enthalpy ande ntropy changes of the incriminated phase transition, respectively).…”

Positron annihilation in liquid crystals

“…They form herringbone array in layers with highly disordered vertical orientation of molecules (head-to-tail disorder). The SmE phase of nTBC have been extensively investigated and new models of the SmE lamellar structure with two types of sublayers consisting of aromatic cores and alkyl chains of molecules has been proposed [30,31]. This is connected to the previous reports concerning molten state of the alkyl chains in the SmE phase which has been confi rmed by thermodynamic, structural, and spectroscopic studies including PALS results [28,32,33].…”