Phase transitions in liquid crystals

Eleven series of dimeric liquid crystals, namely the α,ω-bis(4-n-alkylanilinebenzylidine-4′-oxy)alkanes, have been synthesized and their transitional properties characterized. The number of methylene groups in the flexible core was varied from one to twelve while the length of the terminal alkyl chains was increased from a methyl group to a decyl substituent. The rich smectic polymorphism of this family of compounds was studied using X-ray diffraction and polarizing microscopy; it includes several examples of rare phase transitions, for example, smectic F-smectic A and crystal G-isotropic, as well as novel modulated hexatic phases based on a two dimensional lattice. The transitional properties of a given series, where the length of the terminal chain is held constant while the length and parity of the flexible spacer is varied, exhibit a pronounced alternation. For example, the entropy change at the nematic-isotropic transition exhibits a strong alternation and this has been interpreted in terms ...

Smectogenic dimeric liquid crystals. The preparation and properties of the α,ω-bis(4-n-alkylanilinebenzylidine-4′-oxy)alkanes

Recent studies have established that many compounds composed of disc-shaped molecules exhibit stable thermotropic liquid crystalline phases. They are now referred to as discotic liquid crystals. Structurally, most of them fall into two distinct categories, the columnar and the nematic. The columnar phase, in its simplest form, has long-range translational periodicity in two dimensions and liquid-like disorder in the third, whereas the nematic phase is an orientationally ordered arrangement of discs without any long-range translational order. This review article describes the detailed structures of the various discotic phases and some of their important physical properties.

https://iopscience.iop.org/article/10.1088/0034-4885/53/1/002/pdf

Discotic liquid crystals

Thermotropic columnar mesophases from N–H⋯O, and N⋯H–O hydrogen bond supramolecular mesogenes

We present a joint molecular dynamics (MD)/kinetic Monte Carlo (KMC) study aimed at the atomistic description of charge transport in stacks of liquid-crystalline tetraalkoxy-substituted, metal-free phthalocyanines. The molecular dynamics simulations reproduce the major structural features of the mesophases, in particular, a phase transition around 340 K between the rectangular and hexagonal phases. Charge transport simulations based on a Monte Carlo algorithm show an increase by 2 orders of magnitude in the hole mobility when accounting for the rotational and translational dynamics. The results point to the formation of dynamical structural defects along the columns.

Theoretical Characterization of the Structural and Hole Transport Dynamics in Liquid-Crystalline Phthalocyanine Stacks

An extension of an earlier work by the present authors on the even-odd effect in smectic A-nematic (S-N) transition is presented. The earlier model showed too large an alternation in TSN. In the present work the mean field is modified by including the contribution from the orientation independent term of the Kobayashi pair potential. Numerical calculations on three homologous series with alkyl chains, namely 4-(4′-n-alkoxybenzylideneamino) biphenyls, (p'-n-alkoxybenzoyloxy) acetophenones and PAA show a marked diminution in alternation of TSN. The present mean field, though a definite improvement over the earlier one, is still found to be inadequate quantitatively, especially for samples with very small or virtually nonexistent alternation in TSN. It seems that a purely attractive pair potential may not be adequate in explaining this aspect of S-N transition. A possible role of repulsive interaction in suppressing this ‘even-odd’ amplitude is discussed.

On the Even Odd Effect in Smectic-Nematic Transitions

The Landau-de Gennes model of the nematic-isotropic phase transition with the inclusion of the density change and a higher order term (sixth-order term) in a simple way is examined. The temperature difference TNI−T* obtained is in good agreement with the experimental one. We also obtained the value of the ratio (Q*–QNI)/QNI. The value of (Q*–QNI)/QNI is not improved over the earlier attempt, but there is a definite improvement in the value of TNI–T* which is obtained as 1.045 K.

Landau-de Gennes model with the inclusion of density change for the nematic-isotropic phase transition

In an earlier work the present authors extended the variational method of Lee et al. for rod-like molecules to obtain the phase diagram of a discotic liquid crystal system. In the present work, the phase diagram of a discotic system is reproduced in a single particle potential model. It is seen that the single particle potential approach reproduces results very similar to those of the variational method but with considerable ease in computation. The equivalence of the two approaches is discussed.

A Mean Field Model for the Phase Diagram of a Discotic Liquid Crystalline System

The variational method of Lee et. al., for calculating the phase diagrams of systems consisting of rod-like molecules, has been extended to the discotic systems. Compared to the existing models of the discotic phase, the present results are in much better agreement with experimental observations. According to this theory the N-I transition temperature (T N-1) varies inversely as the square of the molecular diameter whereas in the work of Lee et. al., on rod-like molecules, T N-1 varies inversely as the molecular length.

Phase diagram for discotic liquid crystals

In the earlier intercalation model of the present authors, it was shown how gradual dimerization of polar molecules with lowering of temperature necessitates intercalation of neighboring layers for space filling and how the resulting decrease in layer thickness of SAd phase brings about a nematic reentrant phase. In the present work, an alternative mechanism for space filling is proposed which can account for SAd-Nre transition with an accompanying increase of layer thickness. It is also shown that the present mechanism of space filling coupled with intercalation can lead to the SAd-Nre transition at constant layer thickness. These are in conformity with experimental observations where the said transition often occurs with virtually no change in layer thickness and in some cases with an increase in layer thickness.

M. Saha

Papers

On the Even Odd Effect in Smectic-Nematic Transitions

Landau-de Gennes model with the inclusion of density change for the nematic-isotropic phase transition

A Mean Field Model for the Phase Diagram of a Discotic Liquid Crystalline System

Phase diagram for discotic liquid crystals

A Mean Field Model for Reentrant Polymorphism