Krishnegowda, D

Kumaraswamy, S R

Revannasiddaiah, D

Somashekar, R

English

IACS Institutional Repository

Indian J. Phys. 66A (5), 683-6K6 (1992)Anisotropy of Lorentz field factors in two symmetric homologous seriesS R Kumara.swamy, D Krishncgowda, R Somashckar and D RcvannasiddaiahDepartm ent o f  Studies m Physics, IJnivcrsily o f  M ysore, M anasagangoin, M ysore 57 0  006 ,IndiaReceived 5 March 199J, accepted 14 Au^ ilsI 1991A b s tr a c t  : In this paper, we rcpori ihc results o f  cxir calculations on the oncntational order paraniciers and anisoiro^^ic l^)renl/ licld faclois (rom refractive index data in the nem atic phase lor  the fo llo w in g  tw o different h om ologou s sen es  *1. p, p’-< ii-n -a lk y l a /oxyb en zen e (sen es 1)2 p, p’-<Ji-n-alkoxy a /o x y b en /en c  (sen es II)It em erg es  from  these ca lcu lations lor both the series I and se n e s  II, that there is a Iincdi variation o l and (/-„ - L , ) \sith .S, the orientational order param eter, w hich is m goodagreem ent w ith the ex istin g  m odel o f the nem alic phase S econdly , it has bt*eri observed that the v a lu es  of and S evalu ated  at any icm pcralures b elow  nem atic-isotropic  Iransiiiontem perature for both the sen es  show aliem ation with the m olecular length, and m general, the valu e o f (Ly -  Lg) decrease'sK e y w o r d s  : Polari/ab ililics. orientational order parameter, Lorentz field factors P A C S  N o s . : 61 3 0 . G d, 78 20  hmAnisotropic properties of liquid crystalline materials in relation to the birefringence and' dielectric measurements have been studied by earlier investigators [1-41. A knowledge of local field experienced by a molecule in the medium is necessary to interpret the birefringence and dielectric measurements in terms of molecular properties. On the basis of a simple lattice ipodcl of oriented nematic liquid crystals, Dunmur [5] has shown the anisotropy in the local field is important in understanding the properties of liquid crystalline materials. Averyanov ei al [6] have shown for various uniaxial liquid crystals that there is a strong dependence of the local field tensor and Lorentz factor tensor in the specific features of the molecular structure. Subramhanyam and Krishnainurti [7] and Xue-Hua He and He-Yi Zhang f8J have pointed that Neugebauer relations lead to consistent results and at the same time provide indirect information regarding the anisotropic nature of the molecular distribution in the nematic pha^c. In this paper, we examine the following two symmetric homologous series of compounds which exhibit nematic phase on the basis of Lorentz field factors, calculated using Neugebauer relations.© 1992 lACS684 S R Kumaraswamy et alSeries I : C„H2m+i Q H , N:N (;0) Q H 4C„H2^ +i (m=3-7)Series II; C„.-,H2^ ,O C 6H4N:N(;OX)C6H4C„,,H2„_, (tn=4-9)Calculation of Lorentz field factors and orientational order param etersUsing the refractive index and density data reported by de Jeu and his coworkers [9,10] and employing Neugebaucr relations [11] the Lorentz field factors L , and and the orientational order parameter S al various relative temperatures (T  ^-  7) in the nematic phase were calculated. The Lorentz field factors and satisfy the condition + 2L„ -  1.The calculated values of S are in agreement with those estimated by others [9, 12] using different methods and hence in this note we have used our S values only for the interpretation of the temperature dependence of anisotropy of molecular distribution in different members of scries I and II.The anisotropic molecular distribution is generally expected in the case of molecules which arc long and lath-shaped and in all such ca.scs the effect of the molecular distribution is manifested as a difference between the Lorentz field factors (Lo -  L )^ or (Lj. -  Ln). As may be found from our calculations, the value of incrca.scs with increase of temperature and the difference between the Lorentz field factors (L„ -  L^) decreases with increase oftemperature. The anisotropy of molecular distribution is zero al the nematic-isotropic transition temperature. The decrease in the anisotropy of Lorentz Held factors is consistent with the expectation that the molecular distribution should be less ani.sotropic with increase of temperature. It may also be seen from our calculations that in both the scries, there is a linear variation of the Lorentz field factors and with the orientational order parameterS. The values Lu and L^  ^ correspond to the case when 5 = 1 are given in Table 1. It isTabic 1. Ijorcni/ field factors of senes I and IT at S  = /Series Im 3 4 5 6 71^1 0.268 0.270 0.273 0.273 0.268L , 0.366 0.365 0.364 0.364 0.3660.098 0.095 0.091Scrievs II0.091 0.098m 4 5 6 7 8 91^1 0.258 0.258 0.255 0.255 0.260 0.2600.371 0.371 0.373 0.373 0.370 0.3700.113 0.113 0.118 0.118 0.110 0.110evident from our calculations that the anisotropy of the Lorentz field factors (L„ -increases with increase in S, which is in confirmity of the nematic model suggested by Saupe and Maier theory [13]. It should be mentioned here that (L ^ -L ^  at any particular (Tg-T) when plotted against the number of carbon atoms (m) for both the series, shows odd-Anisotropy of Lorentz field factors etc. 685even effect (see Figure 1) as observed by Marcclja [14]. It is interesting to note that in general (L  ^-  in the nematic phase decreases with increase in the chain length and this isdue to the fact that the pseudo>pair potential seen by the molecule increases with the increase in chain length [15]. From Table M t is clearly seen that the variation of L„, Lj.and ~ f-ii ) with the chain length is almost constant showing that when methylene groups arc added symmetrically on either side of the central rigid portion, thechange in Lorentz field factors is almost negligible. TTiis is not surprising since numerous configurations are possible for the end chains.Figure 1. Variation of anisotropy of Ijorenlz field factors f-.) with m d l ( J ^ - T )  =  10°C.Following are some of ihc conclusions which emerge from the above calculations.1. The orientational order parameter 5 at a given (7^-V) was found to be higher in those substances which show smectic and nematic phases, when compared with substances exhibiting only nematic phase.2. The trend in the variation of the anisotropy of Lorentz field factors (L„ -  LJi with (Tg -  T) for all members of the scries I and II is similar.3. The value of anisotropy of Lorentz field factors {Lo~^^ increases linearly with increase in the value of the orientational order parameter S.4. The value of (L„ -  L^) at any given relative temperature alternate as the number of methylene groups in the end chain is increased.5. The presence of C -0  group on either side of the central rigid portion in series II docs not affect much the values of L\\ and Li  observed in scries 1 (sec Table 1).A cknow ledgm entOne of us (SRK) is thankful to the University of Mysore, Mysore, for the award of University Post-graduate Research Fellowship.6 8 6  S R Kumaraswamy ct alReferences1112 |3 |41516 |7 |8 19110111|12113114115W Maier and G Meier 1961 Z Naiurforsch 16a 262G W Gray 1962 Molecular Structure and the properties of Liquid Crystals (New York :Academic Press) K F Carr 19637 Chem Phys 38 1536,39 1979B K Vainshiein, 1 G Chisiyakov, li A Kosicrin and H M Chailkovskii 1967 Sov Phys Dokiady 12 405 DA Dunmur 1971 Chem Phys Lett 1049E M Averyanov, V A Zhuikov, V Ya Zyry'anov and V F Shabanov 1986 Mol Crysl Liq Cryst 133 13511S Subramhanyam and D Knshnamurti V)l'iMol Cryst Lq Cryst 22 239Xuc-IIua lie and Hc-Yi Zhang 1981 Mol Cryst Liq Crust. 74 189W H de Jcu and W AP Claassen 1977./ Chem Phys 67 3705W 11 dc Jeu and P Bordcwijk 1978 J Chem. Phys 68 10911 li J Ncugebauer 1954 Can J Phys 32 1EG Hanson and Y K Shen WlftMol Cryst Liq Cryst 36 193A Saupc and W Maier 1959 Z Naiurjorsi.h 15a 882, 196i') ibtd 15a 287S Marcelja 19747 Chem Phys 69 359')A J Leadbeller and M A Ma/.id 1979 Mot Cry.st Liq Crysl 51 85

Anisotropy of Lorentz Field Factors in Two Symmetric Homologous Series

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Anisotropy of Lorentz Field Factors in Two Symmetric Homologous Series

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