Il Nuovo Cimento D

, Volume 4, Issue 3, pp 229–244

A model for the orientational order in liquid crystals

  • S. K. Ghosh
Article

Summary

A model for the orientational-order parameter in liquid crystals from the microscopic viewpoint and in conformity with its current understanding well established in the field of phase transitions is proposed here. This model has a close resemblance with the original ideas of Maier and Saupe. However, it is independent of the nature of molecules, rigid or flexible, and enables us to describe a phase with a single-order parameter tensor\(\mathop S\limits^ \leftrightarrow\) as expected. Moreover, an important advantage of this model in contrast to the prevailing ones is its, ability to separate the contributions of long-range order from local information inherent in any microscopic measurement such as by NMR and EPR techniques with some reasonable assumptions. All these are demonstrated here with the observed quadrupole splittings for different deuterium sites of long-chain molecules forming the uniaxial nematic phase. The role of molecular conformations on the observed splittings is also explicitly demonstrated by recovering quadrupole coupling constants and bond orientations at different molecular sites. These results are in reasonable agreement with available data.

PACS. 61.30

Liquid crystals 

Riassunto

Si propone qui un modello per il parametro d’ordine orientazionale nei cristalli liquidi dal punto di vista microscopico e in conformità con la sua comprensione corrente ben stabilita nel campo delle transizioni di fase. Questo modello ha una stretta somiglianza con le idee originali di Maier e Saupe. Comunque, ciò è indipendente dalla natura delle molecole, rigida o flessibile, e non ci permette di descrivere una fase con un tensore di parametro d’ordine singolo\(\mathop S\limits^ \leftrightarrow\) come ci si attendeva. In piú, un importante vantaggio di questo modello in contrasto con quelli equivalenti è la sua capacità di separare i contributi d’ordine a largo raggio da informazioni locali inerenti a misurazioni microscopiche come per mezzo di tecniche NMR e EPR con alcuni presupposti ragionevoli. Tutti questi sono dimostrati qui con i tagli quadrupolari osservati per diverse posizioni del deuterio in molecole a lunga catena che formano la fase nematica uniassiale. II ruolo delle conformazioni molecolari nei tagli osservati è pure dimostrato esplicitamente reinstaurando costanti di accoppiamento quadrupolare e orientamenti di legame in diverse posizioni molecolari. Questi risultati sono in ragionevole accordo con dati disponibili.

Резюме

Предлагается моделъ ориентационного порядка в жидких кристаллах с микроскопиъки зрения и в соответствии с имеющимся пониманием фазовых переходов. Эта моделъ имеет тесхую связъ с первонаъалыми идеями Майера и Соупа. Моделъ не зависит от природы молекул (жестких или гибких) и позволяет описатъ фазу с помощъю тензора\(\mathop S\limits^ \leftrightarrow\). Кроме того, важное преимущество этой модели по сравнению с предыдущими моделями заключается в возможности разделения вкладов далънодействующего порядка от локалщной информации, свойственной микроскопиъеским измерениям типа ЯРМ и ЭПР. Возможности этой модели иллюстрируются на примере наблюдаемых расщеплений для разлиъных местоположений дейтерия для длинных цепных молекул, образующих одноосную нематиъескую фазу. В явном виде покаывается влияние молекулярных конформаций на наблюдаемые расщепления, восстанавливая постоянные квадру-польиой связи и ориентации связей при различных положениях молекул. Полекул. Полученные результаты хорощо согласуются с имеющимися данными.

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Copyright information

© Società Italiana di Fisica 1984

Authors and Affiliations

  • S. K. Ghosh
    • 1
    • 2
  1. 1.Istituto di Fisica dell’UniversitàL’AquilaItalia
  2. 2.Unità del Gruppo Nazionale di Struttura della Materia del C.N.R.L’AquilaItalia

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