Lakshmi, Vijaya K

Ramakrishnancharya, M

Sastry, Sreehari

IACS Institutional Repository

IndianJ. Phys.m{2), 173-176(2006)S y n th e s i s ,  c h a r a c t e r i z a t i o n  a n d  d i l a to m e t r i c  s tu d i e s  o n  N - ( p - n - t r i  a n d  t e t r a  d e c y I o x y b e n z y l id e n e ) - p - to lu id in eS Sreehari Sastry*, K Vijaya Lakshmi, M Rsiinakrishnancharya, Nanchara Rao, O Sahaya Baskaranand C Ra^achandra Prabhu'Department of Physics. Acharya Nagarjuna Uiaiversity. Nagarjunanagar 522 510, Andhra Pradesh. India ' Faculty of Physics, Department of Applied Sciences, Higher College of Technology,P.O Box -74, Al^Khuwair-133, Muscat, OmanE-mail srccflari. ssastry @yahoo comReceived 17May 2005, accepted II November 2006abstract Synthesis, characterization and phase transition studies are earned out on two Schiff base liquid crystal materials vie.. N-(p-/i-ridct.vloxybenzyhdenc)-/7-toluidine, 130 1 and N-(/7-/i-tetradecyloxybenzylidene)-p-ioluidine, 140.1. The characterization studies (pha.scs and ihasc transition temperatures) are carried out using polarising microscope equipped with hot stage and differential scanning calorimeter. The phase lansition studie.s are carried out using dilatometric technique The results infer that these two compounds possess only one monotropic liquid rystalline phase viz., smectic-A. The order of phase transition across isotropic to smectic-A is found to be of first order l^ie results are discussed in iit light o f  the data available on other nO.m liquid crystal compoundskeywords Liquid crystal, synthesis, phase transition, dilatometry. density measurements •ACS Nos. 42.70 Df. 6 l30E b  64.70.MdIntroduction"he homologous series, viz., N-(p-n-alkoxybenzylidene)-/?-/t- IkyUinilines, popularly known as nO.ms, exhibit rich but subtle l^ymorphism [1-31. The compounds are model systems to study he different types o f melting and attracted the interest of both hcoreticians and experimentalists. Dilatometric technique is one t the powerful tools to study the order o f phase transitions [4j.a part o f systematic studies on nO.m compounds, this ommunication presents the synthesis, characterization and ilatometric studies on two Schiff base compounds, ndecyloxybenzylidene)-/7-toluidine (130.1) and N-(p-/i- ;tradecyloxybcn2yIidene)-p-toIuidine, (140.1), The density udies are carried out to infer the nature o f isotropic-smectic- phase transition. The pressure dependence o f transition emperatures across the transition, is estimated from the leasured volume jumps and enthalpy data. The results arc iscussed in the light o f the reported data on the nO.ms’ liquid •■ystalhne compounds, which exhibit isotropic to smectic-A "ansition.j Corresponding Author2. ExperimentalAn Olympus polarising microscope (BX 50) along with Instec temperature controller with ±0.1 temperature accuracy was used for the textural identification o f liquid crystal phases and the determinations o f phase transition temperatures. Perkin- Elmer DSC-7 was used to determine the transition temperatures and relevant enthalpy o f transitions. A U-shaped bi-capillary pyknometer in conjunction with a cathetometer, was used to study the temperature variation o f density. The absolute error in the determination o f density is ±0.1 kg.m'^. The cooling rate followed during the density measurements was l®C/h in general, 0.5^/h at the vicinity of isotropic to smectic—A transition.The compounds in the present study are synthesized following the standard procedure [5]. The molecular structure for the compounds is given below...Ons 13 and 14©2006IACS3. Results and discussion174 S S reekari Sastry et a IFigure 1. Miscibility diagram of 130.1 in 20.16.This phase o f 130.1 is co-miscible with the phase o f 20.16. This confirms that the identified liquid crystalline phase o f 130.1 and 140.1 is smectic-A.The DSC thermo grams o f  130.1 and 140.1 are shown in Figures 2 and 3. The transition temperatures along with enthalpy values from thermal microscopy and DSC, are presented in Table 1.3.2 Dilatometric studies on 130.1 and 140.1 :The density measurements are carried out on both these compounds using dilatometer technique, which is equipped with temperature variation accessory. The variations o f density andTable 1. Phase tran.sition temperatures (in®C) in heating aiul cooi„, cycles due to thermal microscopy. DSC and density measureineuts3.1 Characterization and phase identification in 130.1 and 140.1 :The compounds 130.1 and 140.1, exhibit a single-phase variant in between solid crystal and isotropic liquid. The observed phase is monotropic.On c(K)ling from isotropic liquid, a liquid crystalline phaseseparates out in the form o f batonnets at 76.3‘XI? and 76.5 ”C in 130.1 and 140.1, re.spectively. These batonnets float in the isotropic liquid and finally, coalesce U> form focal conic fan structure, a characteristic texture o f smectic-A pha.se. This phase also exhibits homeotropic texture indicating it as an orthogonal phase. These observations indicate that the phase is smectic- A.For further confirmation o f this phase, miscibility studies are carried out. The compounds, 130.1 and 140.1 are mixed with 20.16 (independently), which also exhibit a single-phase smectic-A variant. As a representative case, the miscibility diagram o f 130.1 in 20.16 is shown in Figure 1.Compound Crysial- Smeclic-A-Hcati ng/< 'ooli ng Smectic-A isotropic Meliii130.1C TM 65.0 76.3H DSC 84C DSC* 64 2 76.5C Density 76.3140.1f: TM 68.6 76 5H DSC 8? <>(.C DSC 67 2 76 6c Density 76 520  -  1 5130 1Heating^  Cooling1 J' 5®C/Minute U  10"C/Minute60 65 70 75 80 85Temperature (®C)90Figure 2, DSC thermogram of 130 I heating and cooling foi ditfti scan ratesFigure 3. DSC thermogram of 140.1 heating and cooling for scan ratesSynthesis, characte risa tion  and diiatometric studies etc 175ermal expansion coefficient (a )  with temperature o f these ,mpKiunds are shown in Figures 4 and 5.gure 4. Variation of density and thermal expansion coefficient M^ i^th nporaturc in 130 1jure 5. Variation of density and thermal expansion coefficient with jperature in 140.1The estimated contribution o f molar volume per methylene )up increment o f nO,l series in the isotropic liquid at Iniind to be 16.4 x 10'  ^m"' mol ‘ in 130.1 and 15.5 x 10^ m  ^’ in 140.1. These are found to be in good agreement with : reported values o f other nO.m compounds 40 .m [6], 50.m 1, 60.m [8,9], 70.m [10,11], 80.m [9,12, 13,14,11], 90.m 5,16,17,11], lOO.m 118] and 120.m [19] series o f nO.m mpounds. It is further noticed that these results are in cment with the reported values o f 16.42 x 10'  ^to 17.2 x 10'  ^’ mol ‘ for methylene group contribution for isotropic liquids.The density jumps { A p f 100) and the computed ermal expansion coefficient maxima at isotropic-smectic>A Insition = 1 / M vid M vjd T ) for these compounds along 1th other compounds [20] which exhibit isotropic-smectic-A ase transition, are given in Table 2.2. Smectic-A thermal range, density jump, thermal expansion P maxima and pressure dependence of transition temperatures !~A transition in nO.m compounds.P'PoundI I^^'-Thcrnml A pfp  X 100 dT .^ tdPrange I0-4OC-I K.k.bar 'I 1.3 0.57 98.7 14.864.5 0.93 133.7 15.85The isotropic-smectic-A transition is accompanied by a change from a disordered molecular arrangement o f isotropic liquid to a long-range orientation o f the molecules with simultaneous growth o f periodic layer arrangement in smectic- A  phase. Generally, this transition is regarded as t)f first order. The density jumps {Ap/p>iUyO) estimated from the vertical distance between density values tif extrapolated curve from isotropic and smectic-A regions at the observed transition temperatures across the transitions are 0,51^ /v for 130.1 and 0.93% for 14.01 (Table 1). The larger density jump observed across / ~ transition than at /-nematic or /-cholesteric transition is due to the transformation o f random orientation o f molecules in isotropic phase into ordered molecular phase (which possess both long range orientational order with one dimensional periodic t>rdering) compared to only long range orientational order in nematic phase. .The reported [6-19] minimum and maximum jumps across I-smectic-A transition are 0.35% and 2%. This shows that the values o f density jumps observed for these two compounds fall in the middle range. The large density jump and the calculatedthermal expansion coefficient maximum ( ) 98.7 x 10 ** ’and 133.7 x 10"^%? * in 130.1 and 140.1, respectively, confirm the order o f the transition as first order.The higher slope value o f density for the smectic-A phase (33.4 X I f o r  both the compounds) suggests the denser packing o f molecules and higher structural ordering in smectic-A phase compared to isotropic phase. It is interesting to note that the density slope in the isotropic phase o f 9.34 x 10'** fur 130.1 is found to be in agreement with value reported for other compounds o f nO.m series while the value o f 18.9 x 10'** for 140.1 IS found to be higher.The /- 5  ^ transition can also be visually observed in the pyknomeler bulb. 'The translucent smectic-A phase grew uniformly in the lower part o f the bulb and the less dense isotropic liquid appeared to float above smectic-A phase with a clear boundary between them. This type o f co-existence and the embryos appearing simultaneously at the bottom o f the bulb suggest nucleation growth analogy o f the isotropic to smectic-A transition. Further, the embryos, which are devek>ped at the bottom o f the bulb appeared with a characteristic translucent appearance rather than with a uniform, opaque, milky nematic appearance throughout the bulb. This suggests the simultaneous development o f orientational and translational order.The pressure dependence o f  transition temperatures estimated from the Clausius-CIapeyron equation l(d l\./dP) = 7'^ , (d M /A H )] (where 7^, dM ,^, AH are transition temperature, change in volume and enthalpy, respectively) is 14.86 K.k b a r ’ and 5.85 K.k bar ’ in the compounds 130.1 and 140.1,176 S Sreehari Sastry et alrespectively. The observed poor agreement in the pressure dependence o f transition temperature (Table 1) among different compounds, which accounts the mesophase thermal stability in the domain o f chemical environment, may be due to the parameters being obtained in different experimental conditions.3.3 DSC characterization o f  the order o f  pfiase transition:The differential scanning calorimetry is an extensive technique not only for the detection o f phase transitions, quantitative determination o f the associated enthalpy changes at the transition and for the qualitative analysis o f materials (purity, isothermal crystallization efc.), but also for the determination o f the order o f transition whether first or second order. Navard and Haudin \21 ] proposed an elegant and simple methtxi to determine the order o f phase transition from the DSC peak heights or material quantities. The order o f phase transition can be determined from the magnitude o f the ratio (AO o f different enthalpy peak heights performed with two different scanrates (/'  ^and Tj), which is a function o f the ratio o f the scanning rates,  ^(=  /?>, keeping the weight o f compound constant. It follows that for a first order transition, N  varies as the square root o f y?, while for second order transition, N  should be directly proportional to R. Here, wc have used R—2 {i.e., keeping the mass o f the sample constant, the DSC thermograms are obtained for scan rates 10 ‘X" /min and 5 °C/min).The DSC thermograms scanned at 10 °C/min and 5 °C/min are shown in Figures 2 and 3 for the two compounds, respectively. The heights calculated for 10 ®C/min {hf^ and 5 ®C/ min (h^) are 4.5 and 3.1 in 130.1 and 4.1 and 3.1 in 140.1, respectively. The Navard and Haudin parameters, (h 2 /hj = R) are 1.45 and 1.37 for both the compounds, respectively. The values suggest that the order o f  transition in these compounds is o f the first order.4. Conclusions(i) The thermal microscopic textural observations show that the compound exhibits a monotropic smectic-A phase.(ii) The data available on a number of nO.m compounds which exhibit isotropic to smectic-A transition, reveal that the density jumps across this transition varies between 0.36% to 1.99%. The density jump in the present compound (0.93 %) falls in the middle range of the observed values (0.36% to 1.99%).(iii) It is interesting to note that the Isotropic-smectic-A phase transition can be visually observed in the pyknometer with the smcctic-A phase settling to the, bottom and the isotropic phase floating over it.(iv) The observed density jump and the calculated them expansion coefficient ( a  ) confirm the order cw i transition as first order, which is further contiim from the value o f Navard and Haudin paranKti {h  ^/hj = 1.37) obtained form DSC.AcknowledgmentsWe are thankful to Prof.V.G.K.M. Pisipati for providi, experimental facilities at the Centre for Liquid Crystal Resear, and Education, Acharya Nagarjuna University. Finatiu support rendered by University Grants Commission (No.I i( 18/2003(SR)J, New Delhi is greatly acknowledged.References[11 K Kelker, B J Schcurle, R Hartz and W Bartsch u (//(/,,.962 (1970)[2 ] G W  Smith and Z  G Garlund 7 Cht'nt Ptiys 59 3214[3 ] A Wiegeleben, L Richter, J Deresch and D Demus Mol ( CrvM. 59 329(1980)[4 ] D Demus, G Konig, D Marzotko, R Ruramki, A Hauler I Lm C Salhmann and W Wcissflog Cryst K(\s Terhnol 2V 44 (1994)[5 ] V G K M Pistpati, A K George, C'h Srinivasii and N MiiriN , Naturforsrh. 58a 103 (2003)[6 ] J V Rao, N V S Rao, V G K M Pisipati and C R K Muity /<< / h- Phys Chem. 84 1137 (1980)[71 V G K M Pisipati, N V S Rao, P V Datta Prasad and P R AIjiuh Z-NaturforscJi. 40a 472 (1985)[8 ] V G K M Pisipati, N V S Rao. M K Rao, D M Potukuclii .uullH, Aiapati Moi. Crvst. Ltq Cryst. 146 89 (1987)[91 D Saran, B Bahadur and V G Bhide Mol Cryst IJq. C rvsi 51 i! | (1979)[10] N V S Rao. V G K M Pisipati. Y Gouri Sankar and J S R Mun Acustica 60 163 (1986)[11] Jitendra Nadh. C G Rama Rao, M Srinivasulu, D M Potukii and V G K M Pisipati Mol. Cryst. l.iq. Cryst 366 457 (2I)()[121 V G K M Pisipati and N V S Rao Phase Trans 3[13] C Rama Chandra Prabhu and V G K M Pisipati Cry^ st /?< s 7c(/w 37 269 (2002)[14] P Bhaskara Rao, N V S Rao, V G K M Pisipati and D .Saran fri Res. Teehnol. 24 723 (1989)[15] P R  Aiapati, D M Potukuchi, P Bhaskara Rao. N V S Hao. \  ^M Pisipati and A S Paranjpe Uq.Cry.st. 5 545 (1989)[16] V G K M Pisipati, N V S Rao, M W  Nagi Reddy. C G Kama K and G Padmavathi Cryst. Res. Teehnol. 26 709 (1991)[17] N V S  Rao. D M  Potukuchi, P V Sankara Rao and V  Ci Pisipati Liq. Cryst. 12 127 (1992)[18] C Rama Chandra Prabhu Ph.iX Thesis. (Nagarjuna linnets' India) (1998)[19] M Srinivasulu, D M Potukuchi and V G K M Naturforsch. 52 7 13 ( 1997)[20] Ch Srinivasu Ph.D Thesis. (Nagarjuna University, India)[21] P Navard and J M Haudin J. Thermal Analy^^  ^(1984)

Synthesis, Characterization and Dilatometric Studies on N-(p-n-Tri and Tetra Decyloxybenzylidene)-p-Toluidine

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Synthesis, Characterization and Dilatometric Studies on N-(p-n-Tri and Tetra Decyloxybenzylidene)-p-Toluidine

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