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

http://mather.syr.edu/Mather/2013/BurkeMather_Microstructure_POLYMER(2013).pdf

Evolution of microstructure during shape memory cycling of a main-chain liquid crystalline elastomer

A comprehensive review of the recent developments regarding the phenomenon of reentrant phase transitions (RPT) in liquid crystals is presented. In addition to liquid crystals this phenomenon has been observed in amazingly diverse systems. A critical assessment of the experimental investigations concerning single and multiple reentrances is given. A brief account of the theoretical efforts is also given. The article ends with the identification of the factors which impede the proper understanding of the phenomenon.

Reentrant phase transitions in liquid crystals

The double critical points of binary mixtures, liquid crystals and ferroelectrics are investigated. The classification of double critical points is done and thermodynamics in their vicinities is considered. It is shown that the isomorphism hypothesis could be proved for a wide class of intermolecular potentials. A possible symmetry of the systems having double critical points and its consequences are discussed.

The classification of double critical points and thermodynamical properties in their vicinities

Chain ordering and smectic a phase

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

In an earlier paper, the authors reported a mean field model for reproducing the double reentrant phase sequence of a pure compound with strong polar end groups. The present work is an extension of the earlier model to the case of a binary mixture consisting of a polar and a nonpolar component. The calculated phase diagram shows that the double reentrant polymorphism ceases to exist beyond a certain fraction of the nonpolar component. This aspect and other features of the phase diagram are in good qualitative agreement with experimental observations in some cases.

A Mean Field Theory of Reentrant Phases in Some Binary Mixtures of Polar and Nonpolar Compounds

Formation of tilted smectic-C liquid crystal phase in polar Gay–Berne molecules

Our extension of Marcelja's theory of nematic-isotropic transition to the smectic A phase is used to calculate the smectic A-nematic (Sm-N) transition temperatures for the homologous series 4–(4′- n-alkoxy benzylideneamino) biphenyls. In a mean field calculation, explicit inclusion of the conformational energies of the alkyl chain of a molecule in the partition function shows an even-odd effect generally observed in experiments on Sm-N transitions. It is found that the members of this series showing the smectic phase will have first order Sm-N transitions and the transition entropies for individual members are calculated.

D. Ghosh

Papers

Chain ordering and smectic a phase

On the Even Odd Effect in Smectic-Nematic Transitions

A Mean Field Theory of Reentrant Phases in Some Binary Mixtures of Polar and Nonpolar Compounds

Formation of tilted smectic-C liquid crystal phase in polar Gay–Berne molecules

Chain Ordering and Sm-N Transition in Different Members of the Homologous Series 4-(4′- n-AIkoxy Benzylideneamino) Biphenyls