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

The observed phase transition sequence N-SA-Nre Sre with a lowering in temperature in pure compounds of molecules with a strongly polar end group is reproduced in a mean field model which is taken as an extension of the McMillan potential. This model shows how the dimers in presence of space filling monomers can account for the high temperature SA phase. As the temperature is lowered, more and more monomers form dimers leaving empty spaces and this initiates an intercalation of dimers belonging to the neighbouring layers. This intercalation is responsible for the lowering of the strength of layering interaction in a McMillan type potential and thus a reentrant nematic phase occurs. This model for the first time successfully accounts for the thickness of the low temperature smectic phase.

A Molecular Mean Field Theory of Reentrant Phases

In this paper we propose to estimate the role of chain conformations in bringing about reentrant polymorphism in a nonpolar sample of mesogens. To this end we also show that the reentrant phenomenon is built in the McMillan model, if one explicitly incorporates the e8'ect of tail-chain conformations in the molecular potential instead of treating the chains as an extension of the rigid part. The model is next utilized to predict the single reentrance I-N-Sm-N„with lowering of ternperature in a nonpolar system. I. INTRQDUCTIOlV Since the discovery of reentrant polymorphism in a binary mixture of two polar compounds, ' a large number of investigations have been carried out in this field. Liquid-crystalline systems exhibiting such reentrant polymorphism I X Sm X„-, a-nd -the double (or triple) reentrant phase sequence consist of organic molecules usually with three or four aromatic rings with ester linkages and having polar cyano or nitro-end groups. Apart from pure compounds, ' such reentrant polymorphism with a lowering of temperature is also exhibited by binary mixtures of polar-polar, ' ' polar-nonpolar, ' or even by nonpolar-nonpolar compounds. "' The last one however only shows the single reentrant phase sequence. Further, as a given homologous series is ascended the reentrant phase sequence is shown by the higher homologs (e.g. , usually by the octyloxy, nonyloxy, etc. members) which are neither very short nor very long. These findings indicate that the dipolar force plays a crucial role in the reentrant polymorphism. There are theoretical models' ' that emphasize this role of dipolar force to bring about the phase sequence. A number of theories assume some sort of bimolecular organization (dimers)' ' or even trimers or n-mers with antiparallel association that compensates (not always fully) the dipole moments. This system of dimers or n-mers together with existing monomers can bring about the desired phase sequence. Such theories can also explain the variation of layer thickness' ' as an interdigitation or reorientation' (in the case of constant layer thickness) of the component systems in a mean-field approach. These theories essentially show that the high-temperature smectic phase is an induced phase and the reentrant nematic phase is brought about by a competition between two incommensurate lengths. This two-length theory is also one of the main ingredients of the Landau theory of the phase transition developed by Prost and Barois. ' A further review of theories and experiments for reentrant nematic phases can be found in Refs. 24 and 25. However, the occurrence of single reentrance I-X-SmX„(Refs. 11 and 12) in a binary mixture of nonpolar compounds cannot possibly be due to such dipolar forces. In fact, for such systems, observation' on layering thickness shows no hint for the kind of dimerization as is found in reentrant systems with polar compounds. The above result, together with the observation that in all the systems discussed the reentrant phase usually occurs for certain members (usually eighth or ninth) of the homologous series, indicate that the tail chains should have some active role in bringing about the reentrance. Dowell proposed a lattice model for condensed phases that predicted reentrance in a single-component nonpolar system. In that model, it is seen that a segregated packing of cores beside cores and chains beside chains occurs with a lowering of temperature leading to a usual smectic phase. With further lowering of temperature, the chains become less flexible, and packing differences between the rigid cores and tail chains decrease. Thus, the need for segregated packing of rigid cores with cores (and tail chains with tail chains) is overcome by entropy of unsegregated packing, leading to the disappearance of the smectic-A phase and the appearance of the reentrant nematic phase. In other words, the Dowell model holds the change in chain configuration responsible for reentrance. We present in this section some results we have obtained on the role of chains in bringing about reentrance in a single-component nonpolar sample. As in the case of polar systems, ' ' our present study is also based on a molecular mean-field approach and the starting point, so to say, is again the McMillan model. Here, however, we show that the reentrant phenomenon is built in the McMillan model, if one explicitly incorporates the efFeet of tail-chain conformations in the molecular potential instead of treating the chains as an extension of the rigid part. The model is used next to predict the single reentrant phase sequence I-iV-Sm-X„with lowering of temperature in an idealized nonpolar system. Our results also indicate that chain conformations alone can give rise to reentrance for certain intermediate members of a homologous series only under some very restrictive conditions.

Role of end chains in the reentrant behavior of a nonpolar system.

A mean field model for a rectangular columnar to hexagonal columnar phase transition, assuming thatthe molecules are biaxial in both the phases, is presented. The theory is developed for a face centeredrectangular lattice taking the molecular cores to be tilted in a particular direction with respect to thecolumn axis by the same angle at each lattice site. Phase diagrams for a homologous series for twovalues of biaxiality parameter are presented. With increasing temperature a phase sequence rectangular-hexagonal-nematic-isotropic is found for shorter members while for higher members of a homologousseries the nematic phase is absent in the calculated phase sequence.

A Molecular Mean Field Model for a Rectangular to Hexagonal Phase Transitionin Discotic Liquid Crystals

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.

M. K. Roy

Papers

A Molecular Mean Field Theory of Reentrant Phases

Role of end chains in the reentrant behavior of a nonpolar system.

A Molecular Mean Field Model for a Rectangular to Hexagonal Phase Transitionin Discotic Liquid Crystals

Chain ordering and smectic a phase

On the Even Odd Effect in Smectic-Nematic Transitions