REFRACTIVE INDICES, ORDER PARAMETER AND DENSITY STUDY OF BKS/B07 NEMATIC LIQUID CRYSTAL

Dpartmnt f Physical Chmistry 4-1 Rgina Elisabta Blvd, District, Bucharst phn: +40-1-14508; fax: +40-1-15949 pissn: 10-871X ISSN: 1844-0401 ARS DOCENDI PUBLISHING HOUSE Ss. Panduri 90, District 5 Bucharst phn/fax: +40-1-410575 -mail: arsdcndi@yah.cm REFRACTIVE INDICES, ORDER PARAMETER AND DENSITY STUDY OF BKS/B07 NEMATIC LIQUID CRYSTAL V.S. Chandl, R. Manhar and J.P. Shukla abstract: Th papr discusss th rsults f rfractiv indics, dnsitis, rdr paramtr and mlcular plarizabilitis f BKS/B07 nmatic liquid crystal at diffrnt tmpraturs. Th rintatinal rdr paramtr has bn dtrmind frm th calculatd plarizability valus btaind frm th masurd rfractiv indics. Dnsity masurmnt indicats that th nmaticistrpic transitin is f th first rdr. Th rdr paramtr as statd abv was dtrmind using dirct xtraplatin mthd, Nugbaur s mthd, Saup and Mair s mthd, Vuks and mdifid Vuks mthd. Th bsrvd valus diffr slightly in crystal stat but ar in gd agrmnt in th nmatic phas. ky wrds: rdr paramtr; rfractiv indics; plarizability; nmatic liquid crystal. rcivd: July 11, 011 accptd: Sptmbr 15, 011 1. Intrductin Liquid crystals hav grat imprtanc in display dvics and in many thr scintific uss nwadays. Th mst imprtant prprty f a liquid crystal which gvrns almst all its physical prprtis is its rintatinal rdr [1-4]. Th rintatinal rdr S is xprssd in trms f micrscpic rdr paramtr. Th valu f S lis btwn 1 (fr cmplt rdr) and 0 (fr cmplt disrdr in istrpic liquid). Th micrscpic rdr paramtr S diffrs frm th macrscpic rdr paramtr Q, which can b dtrmind by nuclar magntic rsnanc, dilctric prmittivity, diamagntic suscptibility, rfractiv indics, X-ray scattring, UV and IR dichrism, lastic cnstants, tc. In this papr rdr paramtr f th liquid crystal has bn rprtd frm th masurmnt f rfractiv indics and dnsitis at diffrnt tmpraturs in nmatic and istrpic phas. Th ptical anistrpy is an imprtant prprty and is vry usful in display dvics, which als gvrns th thicknss f th liquid crystal layr t b usd in such dvics. Study f ptical bhavir f liquid crystals is thus imprtant. Fr practical applicatins th liquid crystal shuld hav a lw birfringnc and high rdr paramtr. Th rdr paramtr has bn calculatd using dirct xtraplatin mthd, Nugbaur s Mthd, Saup and Mair s Mthd, Vuks and mdifid Vuks mthd. Ordr paramtr f nmatic liquid crystals has bn rprtd using ptical anistrpy data by many wrkrs [5-9]. Dpartmnt f Physics, Intgral Univrsity, Lucknw-606, India. crrspnding authr -mail: chandl.intgral@gmail.cm Dpartmnt f Physics, Lucknw Univrsity, Lucknw-6007, India. Anall UnivrsităŃii din Bucurşti Chimi (sri nuă), vl 0 n. 0, pag. 155 16 011 Anall UnivrsităŃii din Bucurşti

156 V.S. CHANDEL R. MANOHAR J.P. SHUKLA. Matrial and Mthds Th liquid crystal usd fr th prsnt invstigatin, namly BKS/B07 was prcurd frm Prf. B. K. Sadashiva f Raman Rsarch Institut, Bangalr and was usd dirctly withut furthr purificatin. Th phas transitins ar as fllws: Crystallin 16 C Nmatic Th structur f nmatic liquid crystal is as givn blw. 10 C Istrpic. Exprimntal.1. Rfractiv indx masurmnt Th rfractiv indics n and n fr th xtrardinary ray and rdinary ray in th nmatic phas and rfractiv indx n in th istrpic phas at diffrnt tmpraturs wr masurd by mans f an Abb s rfractmtr [10]. Th tmpratur f th rfractmtr was maintaind cnstant within ±0.1 0 C by mans f a micr prcssr basd tmpratur cntrllr (Julab, mdl n. F-5 HD).. Dnsity masurmnt Th dnsitis f th liquid crystal in th nmatic and istrpic phass wr dtrmind by intrducing th wightd sampl f liquid crystal insid a glass capillary tub in a thrmstat. Th lngth f th clumn in th capillary was masurd at diffrnt tmpraturs with a travling micrscp. Th dnsity was calculatd aftr crrcting fr th xpansin f th glass tub [10-1].. Thry (rdr Paramtr) Dirct xtraplatin mthd In hmgnus alignmnt a nmatic liquid crystal bhavs lik a uniaxial crystal. Th rlatin btwn macrscpic rdr paramtr and rfractiv indics in paralll and prpndicular dirctin f th lng mlcular axis wr btaind by mdifying th quatin fr uniaxial crystal as [1]: n = n+ Q. n (1) 1 n = n Q. n ()

STUDY OF BKS/B07 NEMATIC LIQUID CRYSTAL 157 whr n is th avrag rfractiv indx and n is th birfringnc crrspnding t cmplt alignmnt and fr uniaxial alignmnt and fr uniaxial crystal n = n, n = n and δ n= n n [14,15]. Frm bth th quatins 1 and, w gt n n n n δn Q= = = n n n Th valu f macrscpic rdr paramtr Q bcms 1 at abslut tmpratur that is at 0K δn = n. This can b dtrmind by xtraplating δn fr T = 0K. This xtraplatin is dn n th linar prtin f th graph drawn btwn birfringnc (δn) vrsus ln T Tc 1 as dn by thr wrkrs [1, 16]. Mdifid Vuks mthd In rdr t find ut th micrscpic rdr paramtr (S), th rfractiv indics n and n hav bn analyzd fllwing th mthd f Hallr [17] and Hrn [18]. This mthd uss Vuks rlatin [19] δα (n n ) S = α (n + n ) whr δα is th plarizability anistrpy and α is th mlcular plarizability. Th principal plarizabilitis ar dtrmind as dscribd by Sarna t al. [0]. Fr th dtrminatin f ffctiv plarizabilitis α and α fllwing thr mthds ar usd. Nugbaur s mthd In this mdl th anistrpic intrnal fild is givn as n -1 =4πNα (1-Nα γ ) -1 (5) () (4) n -1 =4πNα (1-Nα γ ) -1 (6) Whr N is th numbr f mlculs pr c.c., n and n ar rfractiv indics fr rdinary and xtra-rdinary rays, and γ and γ ar th intrnal fild cnstants. Th xprssin fr calculating th ffctiv plarizabilitis α and α btaind frm quatin 5 and 6 and using th cnditin γ +γ =4π, prducs 1 4πN n + (n+ ) + = + α α n 1 n 1 (7) 4π In istrpic phas γ = γ 0 = α =α 0 =α

158 V.S. CHANDEL R. MANOHAR J.P. SHUKLA 9 n 1 α + α = α= 4 N π n + (8) Vuk s mthd This mdl f istrpic fild was mdifid frm Lrntz-Lrnz quatin as fllws whr Saup and Mair s mthd n 1 4πNα = n + n 1 4πNα = n + (9) (10) 1 n = (n + n ) (11) This mdl f anistrpic intrnal fild whr intrnal fild is assumd as having singl intrnal fild cnstant can b givn as: n 1 4πNα = n + + a n 1 ( ) n 1 4πNα = n + + a n 1 ( ) Hr th intrnal fild cnstant a can b calculatd using th rlatin: (1) (1) n 1 1 n 1 (n 1) V is = + V n + n + a(n 1) n + + a(n 1) Nm (14) Th ffctiv plarizabilitis α and α hav bn btaind by slving th quatins (7), (8) and (9), (10) and (1), (1) shwn abv fr thr diffrnt mdls. Th rdr paramtr is givn by S = ( α α ) ( α α ). Sinc w hav nly α and α valus and nt th principal plarizabilitis α and α, th paralll and prpndicular cmpnnts rspctivly t th lng axis f th mlcul, w us th xtraplatin mthd fr calculating th valus f α and α. Th xtraplatin is dn fr th valus f (α -α ) n th plt f (α -α ) against ln T 1 fr T = 0 K. Tc

4. Rsults and discussin STUDY OF BKS/B07 NEMATIC LIQUID CRYSTAL 159 Frm Fig. 1 it is bsrvd that th slp f th xtrardinary rfractiv indx n changs at th tmpratur 16 C indicating th phas transitin. Similar typ f th bhavir can b sn frm th data f rdinary rfractiv indx n which changs slightly at 16 C, indicating th crystal nmatic phas transitin. Th rdinary rfractiv indx n incrass slwly, n th thr hand xtrardinary rfractiv indx n dcrass fastr with incras in tmpratur. At 10 C th rfractiv indx changs sharply, which xhibits th nmatic istrpic phas transitin. Aftr th transitin frm nmatic t istrpic phas, th valu f th rfractiv indx rmains almst cnstant, shwing th istrpic phas f th matrial. This typ f bhavir has als bn rprtd fr diffrnt nmatic mixturs by svral thr wrkrs [1-9]. Fig. 1 Variatin f n and n with tmpratur fr th nmatic sampl BKS/B07. It is bsrvd frm Fig. that birfringnc f th sampl BKS/B07 dcrass slwly with incrass in tmpratur and th curv changs its slp at 16 0 C indicating crystallin t nmatic transitin. Als it can b bsrvd that in istrpic phas th liquid crystal bhavs lik a nrmal rganic liquid, [9,0] giving nly n rfractiv indx rsulting almst int zr birfringnc in th valu δn. Th zr birfringnc valu at 10 0 C indicats th istrpic phas f th matrial. Fig. shws th variatin f dnsity f nmatic sampl BKS/B07 with tmpratur, which rmains almst cnstant with incras in tmpratur. Aftr th tmpratur f th sampl incrass furthr, a suddn discntinuity in dnsity curv f th sampl at 16 C appars. This indicats th crystallin t nmatic phas transitin f th first rdr [1-5]. In th nmatic phas th dnsity f th sampl dcrass with incrass in tmpratur and again th curv changs its natur at 10 C indicating th nmatic t istrpic transitin. Th slight bradning f th transitin tmpratur is prbably du t th prsnc f sm small impurity cmpnnts in th sampl. In istrpic phas th dnsity dcrass with

160 V.S. CHANDEL R. MANOHAR J.P. SHUKLA incras in tmpratur [0, 5]. Dmus t al. [1] and Singh t al. [1] hav als bsrvd similar typ f bhavir. Als Chang and Mayr [6] hav rprtd pr-transitin tmpratur f 11 K in a msgns. Furthr it may b ntd that th phas transitins f th nmatic matrial btaind by diffrnt mthds that is rfractiv indics masurmnt tchniqu and dnsity masurmnt tchniqu ar almst sam. Fig. Variatin f birfringnc with tmpratur fr th nmatic sampl BKS/B07. Fig. Variatin f dnsity with tmpratur fr th nmatic sampl BKS/B07.

STUDY OF BKS/B07 NEMATIC LIQUID CRYSTAL 161 In Fig. 4, th micrscpic rdr paramtr (S) as calculatd using dilctric data matchd vry wll with macrscpic rdr paramtr (Q) dtrmind using rfractiv indics data fr th nmatic sampl BKS/B07 in all phass. Th micrscpic rdr paramtr (S) and macrscpic rdr paramtr (Q) bth dcras as th tmpratur f th sampl incrass and shws a slight chang in its valu at tmpratur 16 C shwing crystal t nmatic phas transitin and at 10 C rdr paramtr shws an abrupt chang and bcm zr indicating th istrpic phas. Th rdr paramtrs (S and Q) dcras frm 0.4048 at 16 C t 0.081 at 19 C and thn bcm zr in th istrpic phas. Th natur f th rdr paramtr curv is almst similar t that bsrvd fr thr nmatic sampls [11,16,7-9]. It can b sn frm figur 4 that th micrscpic rdr paramtr (S) calculatd frm Vuks, Nugbaur s and Saup and Mair s mthds ar in gd agrmnt with th macrscpic rdr paramtr (Q). Th macrscpic rdr paramtr (Q) may nt ssntially b sam t that f micrscpic rdr paramtr (S). Hwvr, W. Kuczynski t al. [9] has suggstd that n can xpct similar valus within a givn lcal fild as has bn fund in th prsnt study. Fig. 4 Variatin f rdr paramtr with tmpratur fr th nmatic sampl BKS/B07. Th variatin f rati f principal plarizability with tmpratur dcrass with incras in tmpratur as shwn in Fig. 5 and at th crystal t nmatic transitin that is at 16 C its valu dcrass mr rapidly than that in crystallin phas. At nmatic t istrpic transitin its rati bcms 1 shwing th istrpic phas. Th chang in th valu f α is mainly du t th chang in th rdr paramtr. It shws that rintatin f lng α 0 mlcular axis with dirctr, changs and th valu f θ incrass with incras in tmpratur which in turn implis that th sampl is mr disrdrd.

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