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D. Ghose

Bio: D. Ghose is an academic researcher from University of Calcutta. The author has contributed to research in topics: Discotic liquid crystal & Liquid crystal. The author has an hindex of 5, co-authored 8 publications receiving 55 citations.

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TL;DR: 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.
Abstract: 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.

9 citations

Journal ArticleDOI
TL;DR: In this article, a mean field model for a rectangular columnar to hexagonal columnar phase transition, assuming that the molecules are biaxial in both the phases, is presented, taking the molecular cores to be tilted in a particular direction with respect to the column axis by the same angle at each lattice site.
Abstract: 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.

7 citations

Journal ArticleDOI
TL;DR: In this article, the phase diagram of a discotic system is reproduced in a single particle potential model and the equivalence of the variational method and the particle potential approach is discussed.
Abstract: 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.

5 citations

Journal ArticleDOI
TL;DR: In this paper, the variational method of Lee et al. was extended to the discotic phase diagram of rod-like molecules, and the N-I transition temperature (T N-1) varies inversely as the square of the molecular diameter.
Abstract: 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.

5 citations


Cited by
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Journal ArticleDOI
TL;DR: A comprehensive overview of phase transition studies can be found in this article, where the authors identify the essential key concepts and points of difficulty associated with the study of phase transitions and discuss the most widely used experimental techniques for measuring these transition properties.

308 citations

Journal ArticleDOI
TL;DR: In this article, the detailed structures of the various discotic phases and some of their important physical properties are described. But they are classified into two distinct categories, the columnar and the nematic.
Abstract: 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.

221 citations

Journal ArticleDOI
TL;DR: In this paper, the structures of columnar mesophases that depend on the presence of either N-H⋯O, or H-O-Bonds (subsumed under the term ‘N|H|O-bonds') are reviewed.

117 citations

Journal ArticleDOI
TL;DR: The results point to the formation of dynamical structural defects along the columns in stacks of liquid-crystalline tetraalkoxy-substituted, metal-free phthalocyanines.
Abstract: 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.

82 citations

Journal ArticleDOI
TL;DR: In this article, the authors present experimental and theoretical results for the critical behavior in the isotropic phase of nematogens and discuss the possibility of tricritical behavior at the nematic-isotropic transition.
Abstract: The low value of , where is the nematic-isotropic phase transition temperature and denotes the virtual transition temperature, is a long-standing puzzle in the physics of liquid crystals. The present review presents experimental and theoretical results on this long-standing problem. New experimental and theoretical results for the critical behaviour in the isotropic phase of nematogens are reviewed. We calculate in a unified approach the low value of , at both critical and tricritical points. The possibility of tricritical behaviour at the nematic-isotropic transition is also discussed by means of Landau theory. The various predictions are compared with the available experimental results.

62 citations