2010) 1( 23مجلة ابن الھیثم للعلوم الصرفة والتطبیقیة المجلد الخواص الحراریة فيتاثیر اربع مجامیع مثیل لسلسلة ازو بلوریة سائلة ایمان صبیح حسن، عصام عبد الكریم عبد اللطیف جامعة بغداد ، ابن الهیثم –كلیة التربیة ، قسم الكیمیاء الخالصة ثنـــائي المكــــون لمركبـــات بلوریـــة ســـائلة تمــــت دراســـتها باســـتخدام مجهـــر الضــــوء اعشـــرون مخططـــ كـان اربعـة مخططـات منهـا الثبـات ان الطـور . (DSC)وجهاز المسـح التفاضـلي المسـعري ، المستقطب ،السمكتي هو نفسه في جمیع مركبات السلسلة (4-n-al koxy -2, 3, 5, 6-te tramethyl-4-n-al koxy az obenz ene ) (nPA4M). ( TBAA )والمركــب البلــوري الســائل المرجـع (6PA4M) والمخطـط الخــامس كــان بــین المركــب terephthl ylidene -bis(4-n-butyl anal ine الجـــل تشـــخیص الطـــور الســـمكتي ومعرفـــة نوعـــه للسلســــة ( مثیـل االربعـة الجانبیـة والموجـودة علـى امـا تـاثیر مجـامیع ال. وقورنت النتائج مع االدبیـات العلمیـة ، المذكورة فقــد تمــت مـــن خــالل خمســة عشـــر (nPA4M)احــدى حلقــات البنــزین علـــى الســلوك الحــراري للسلســـلة ةكـل مجموعـة مـن المخططــات تحـوي علـى خمســ، ى ثـالث مجــامیع علـثنـائي المكـون التــي قسـمت امخططـ (nPA4M)ت السلسـلة البلوریـة السـائلة االولـى بـین مركبــا ةاذ كانـت المجموعـ ،مخططـات ثنائیـة المكـون مع المركب المرجع 4-n-hexyloxyphe nyl -4-n`-hexyloxy azobenz ene (HPA) وكـان هــذا المركـب الیحتـوي علــى اي (nPA4M) نفسـها امـا المخططـات الخمسـة الثانیـة فكانـت بـین مركبـات السلسـلة . مجموعـة مثیـل جانبیـة (4B2M) والمركـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــب المرجعـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــي البلـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــوري الســـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــائل 4-butyl phe ny-4-{[(4-ethyl oxyphe nyl )carbonyl ]oxy}-3-me thyl benz oate ین امـا المخططـات ثنائیـة المكـون الخمسـة االخیـرة فكانـت بــ، الـذي یحتـوي علـى مجموعـة مثیـل جانبیـة واحـدة (nPA4M) والمركب المرجعي(4B2,3M) 4-butyl -2methyl phe ny4-{[(4-ethyl oxyphe nyl ) carbonyl ]oxy}-3-me thyl benz oate . الذي یحتوي على مجموعتي مثیل جانبیة IBN AL- HAITHAM J . FO R PURE & APPL. SC I VO L. 23 (1) 2010 The Effect of Four Lateral Methyl Groups on The Thermotropic Behavior of Azo Liquid Crystal Homologous Series I. K. Latif and E.S. Hassan Departme nt of Chemistry, College of Education, I bn Al-Haitham, Unive rsity of Baghdad Abstract Twenty binary liqu id cryst alline mixture d iagra ms were invest igated with p olarizi n g microscope and differential scannin g calorimeter (DSC). Four binary mixture diagrams were constructed to identify the smectic p hase which is found to be the same in all comp onents of the homologous series (4-n-alkoxy -2, 3, 5, 6-tetra methy l-4-n-alkoxy azo benzene) (nPA4M ). The fifth binary mixture diagram was between the (6PA4M ) and the reference liqu id cryst al comp ound, terep hthlylidene-bis (4-n-buty lanaline) (TBAA) to identify the type of smectic mesop hase of these comp ounds, and the results obtained were comp ared with the literature. To study the effect of the 4 methy l lateral group s on t he thermotrop ic behavior of the (nPA4M) homolo gous series, fifteen binary mixture diagr ams were divided in to three p arts, each p art conta ins f ive diagr ams . The f irst binary diagr ams w ere (nPA4M ) with reference liqu id cryst alline compound 4-n- hexy loxy p henyl-4-n`-h exy loxy azobenzene (HPA) ( compound without lateral group ). The second five d iagrams were between (nPA4M ) and reference liqu id cryst al compound 4-buty lpheny-4-{[(4- ethy loxy p henyl)carbony l]oxy }-3-m ethy l benzo ate (4B2M ) ( comp ound with one methy l lateral group ), and the last p art of binary diagr ams was between ( nPA4M ) and reference liquid cryst al compound 4-buty l-2methy lpheny4-{[(4- ethy loxy p henyl) carbony l]oxy }-3-m ethy lbenzo ate(4B 2,3M ) ( compound h as two methy l lat er al gr o u p s ) . . Introduction A wide range of lateral group s have been incorp orated into many liquid cry st al sy st ems. Initially, it may be thought that t hey disrup t molecular p acking and reduce liquid cry st al p hase st ability . Indeed such destabilization often occurs through lateral subst ituents [1, 2]. On the other hand, in many cases, this disrup tion may be also advantageous for mesomorp hism as well as p rop erties required for technical use. Such group s like –F,-CN, - NO2,-CH3, or -OCH3 alway s reduce the st ability of the smectic p hase more than of the nematic one. In general, the depression of the nematic-isot rop ic transition is p rop ortional to the size of the substituents irresp ective of its p olarity . For smectic p hases, the lamellar p acking of smectogens was destabilized by the increase in the size of lateral group s but enhanced by the increase in p olarity of side group s [3]. The lateral group s mentioned above introduce st rong side dipole moments altering intermolecular forces [4,5]. The methy l group often ap p ears as lateral subst ituents in st ructures of liquid cryst als and leads to a decrease of the clearing temp eratures and nematic ranges and increase the viscosity [6]. To identify any liquid cryst al p hase it must be shown that an invest igation of the behavior of binary sy st ems with comp onents forming liquid-cry st alline modifications leads to a classification of the liquid-cry st alline st ates [7]. The key to such a classification is the following emp irical rule of selective comp lete miscibility . All liquid-cry st alline modifications which exhibit comp lete miscilibility in binary sy st ems, without contradiction, can be marked with the same sy mbol. 'Without contradiction' means that modifications which have the same sy mbol in no case exhibit comp lete miscibility with modifications of another sy mbol [8]. In general, binary mesop hase sy st ems exhibit eutectic behavior in their solid-mesop hase IBN AL- HAITHAM J . FO R PURE & APPL. SC I VO L. 23 (1) 2010 transition temp eratures (Tm), whereas t heir mesop hase isotrop ic transition temp eratures (Tc) vary linearly with comp osition [9]. However, non-linear Tc-composition behaviors were reported for some mixtures when one component was chloro [10] or nitro [11, 12] substitut ed liquid cry st als. In most cases, a mixed mesomorp hic sy st em is emp loyed in order to depress the melting p oint (T m) of one, or both, of the pure comp onents so as to obtain its mesop hase over a more readily accessible temp erature [13]. The aim of the p resent work was to st udy the influence of the four methy l group s in the azo core (Figure 1) on the liquid cry st als thermotrop ic p rop erties, these methy l group s act as broadening group s on the geometrical shap e of the molecule i.e. they will change the ratio between the length and diameter (l/d) of the molecules, at the same time the molecules have four electron – donating CH3 group s on t he same azo benzene ring. Experimental 1. Preparation of material s The following homologous series compounds (4-n-alky loxy -2, 3, 5, 6–tetramethy l p henyl- 4-n-alkoxy azobenzene) (nAP4M ) were p repared according to the method p reviously described by Latif and Hassan [14]. The transition temp eratures and p hase behavior of this series are given in Table 1. R O N N R O R = CnH2n+1 n = 4-8 ( n, number of carbon atoms in the terminal alky l ) The following four reference liquid cryst alline comp ounds were used for miscibility exp eriments: i- Terep hthlylidene–bis-(4-n-buty l-anilin) (TBAA) N N M esop hase: Cr Sm5 SmH SmG SmC SmA N I Transition temp . °C: 113 74 89 144.5 172 199 233 ii- 4-n-hexy loxy p henyl -4-n-hexy loxy azobenzene. ( HPA ) O N N O M esop hase: Cr Sm N I Transition temp . °C : 102 114 iii- 4-buty lpheny 4-{[(4-ethy loxy p heny l)carbonyl]oxy }-3-methy l benzoate . (4B2M ). O O O O O M esop hase: Cr N I Transition temp °C: 91 179 IBN AL- HAITHAM J . FO R PURE & APPL. SC I VO L. 23 (1) 2010 iv- 4-buty l-2methy lp heny4-{[(4-ethy loxyp henyl)carbonyl]oxy }-3-methy lbenzoate. (4B2,3M ) O O O O O M esop hase: Cr N I Transition temp . °C: (143) 160 These compounds were kindly sup p lied by Prof Dr. Pelzl, Phy sical Chemist ry Dep artment, M artin Luther University , Halle, Germany . They were p repared according to the methods p reviously described [15-17]. 2. Di fferential S canni ng Calarime try (DS C) Calorimetric invest igations were carried out on 910 differential scanning calorimeter (DSC) 1090 thermal analy zer (Phillip s) by adopting a scanning rate of 10°C/min. 3. Preparation of binary mixtures The binary mixtures used in this st udy were p repared by weighing each of the p ure comp onents adequate p rop ortion to four p laces on an analytical balance into small glass vials, each mixture was heated until it became isotrop ic and mixed very well then allowed to cryst allize. This was repeated three times t o ensure a homogenous mixture. 4. Microscopy The transition temp eratures and the ty p e of mesop hase p resent in each mixture were determined by p olarized light microscopy using Oly mpus microscop e provided with a heating and cooling st age. Each mixture, mounted on a microscope slid and covered with a glass cover slip. Re sults and Discussion The the rmal behavior and the binary systems For t he invest igation of the monotrop ic smectic p hase in the homologous series nAP4M , (n = 4-8). It is necessary to p roof if all these smectic p hases (Table 1) [14]which ap p eard are the same smectic phase or not. Figure (1) represents the p hase diagrams of the four p ossible binary combinations made from the 6AP4M with the different alky loxy terminal group (n = 4, 5, 7, or 8). The binary diagrams show that all the homologies comp onents have the nematic and the same smectic p hase. On other hand our smectic p hase need to be identified with other known liquid cryst al comp onent which have a smectic p hase which can be miscible with it without contradiction. Figure (2) shows the binary mixture sy st em of the comp ound (6AP4M ) with the reference liquid cryst alline comp ound (TBAA).The smectic p hase of the comp ound (6AP4M ) is miscible with Sm5 p hase from comp ound (TBAA) without any contradictions and can take its sy mbol. The comp ressive of the thermody namic transition enthalpies ∆H values with top ology of the binary sy st ems diagrams Figure (2) and the transition enthalpies ∆H of the p ure liquid cryst als comp ounds (6AP4M and TBAA) give an identification image for the st ructural situation of the mesop hases. Phase transition temp eratures and transition enthalpy values of comp ounds (6AP4M and TBAA) are list ed in Table 2. By comp aring the transition between the various p hases ty p e in this table, we notify the following certain regularities: 1. The melting enthalpies p ossess the highest value comp ared with other data and it's normal[18] In certain substances with low melting enthalpy , transition could be detected due to several p hases in the solid st ate. In these cases t he transition enthalpies were added to t he melting enthalpies, t o y ield transition of ∆H. 2. The transition enthalpies connected with the liquid cry st als p hases do not exceed 7.080 kj/mol; in most cases t hey are considerably smaller. Certain regularities could be easily IBN AL- HAITHAM J . FO R PURE & APPL. SC I VO L. 23 (1) 2010 derived by taking into consideration the various group liquid cryst alline p hase ty p es. The transition enthalpies of the comp ound TBAA represented by small ∆H (Sm5 / SmH and SmH / SmG ),were extremely small or even vanishing value (SmC/SmA), large transition enthalpy was (SmG/SmC) more than 1.500 kj/mol, and the comp ound 6AP4M showed a large ∆H (Sm5/N). The DSC thermograms Figure (3), show the transition enthalpies of the different mesop hase with different concentrations at (95, 75, 50, 20, and 5% of comp ound 6AP4M ). A large transition enthalpy (Sm5/SmH, SmH/SmG) at concentrations 5 and 20% of comp ound 6AP4M , this occurs also in the concentration 50% of comp ound 6AP4M between the p hases (Sm5/SmG). The concentrations 75% of comp ound 6AP4M , the transitions (Sm5/SmG, Sm5/N) large ∆H, and at 95% of comp ound 6AP4M have also a large transition ∆H between (Sm5/N). This large ∆H change indicates large st ructural change within the transitions from p hase to p hase. While the transition enthalpies within the same p hase group are combined normally with small structural change indicated by small ∆H value [19, 20]. Fifteen binary mixtures were rep resented in Figures (4, 5 , 6). Figure (4) shows t he binary mixtures between the (nPA4M ) and HPA (this reference compound has no lateral group in its st ructure), this Figure disp lay s linearly mesop hase isotrop ic transition temp eratures and app eared enantiotrop ic nematic p hase in wide concentration in the binary mixtures and the Sm5 as monotrop ic in a concentration region between (75-100% of nPA4M ). The Figure 5 shows the binary mixture sy st ems between nPA4M and 4B2M (this reference comp ound has one methy l lateral group in the middle core benzene ring), this also shows a linearly mesop hase isotrop ic transition temp erature, enantiotrop ic nematic p hase wide concentration area (0-96% of nPA4M ), and the monotrop ic Sm5 p hase in the concentration region of (77-100 % of nPA4M ). Figure 6 the binary mixtures were between nPA4M and 4B2,3M ( this reference compound contains two methy l terminal group s on t wo different core benzene ring ). The enantiotrop ic nematic p hase was on small concentration region of the eutectic transition temp erature (Tm) the maximum area was in diagram (a) in concentration between ( 56-94% of 4PA4M ) this area was small in comp arison with areas of nematic p hase in Figure 5, the monotrop ic Sm5 app eared in the diagrams (a-d), but in diagram (e) the nematic and the Sm5 p hases app eared enantiotrop ically and around the concentration where the eutectic transition temp erature of the Sm5 region was in the concentration ( 88-93 % of 8PA4M ). Conclusions Twenty p hase diagrams of t he all p ossible binary mixtures of the previously liquid cryst als comp ounds were constructed in order to invest igate the ty p e of the smectic p hase in the homologous series nPA4M . This smectic p hase was ( Sm5) which has a large transition enthalpy ∆H comp ared to the nematic p hase, which indicates that this p hase has large st ructural changes through the transition to the nematic p hase, which indicates that this smectic p hase has large order p arameter, this p hase till now is not identified by x-ray diffraction method to decide its ty p e. From the thermal p hase behavior and the lateral methy l group s effect on the thermotrop ic p rop erties of the homologues series one can be draw t he following conclusions:. 1. All the sy st ems containing HPA (a comp ound without a lateral methy l group ) tended to p ossess eutectic comp osition in resp ective of the difference between terminal chain lengths of t he two comp ounds in the binary sy st ems. 2. With the sy st ems containing 4B2M (a comp ound has one lateral methy l group ) and (4B2,3M ) a comp ound with two methy l lateral group s), the methy l lateral group s are a week electron-releasing therefore the melting p oints of their eutectic composition is much IBN AL- HAITHAM J . FO R PURE & APPL. SC I VO L. 23 (1) 2010 more depressed and accordingly led to the app earance of a wider comp osition range of nematic phase and allowed to app ear the enantiotrop ic Sm5 in the eutectic region. 3. Figs. (4a, 5a and 6a) show the eutectic movement toward a high concentration of (4PA4M 0). This p henomenon app eared in all sy st ems by the same manner. Therefore Sm5 ap p eared in fig. (6e) enantiotrop ic with nematic phase. Re ferences 1.Weissflo g, W. and De mus, D.(1983) Cry st. Res. Technol. 18: K21. 2.Weissflo g, W. and De mus, D.(1984) Cry st. Res. Technol. 1955. 3. Collings,P.J. ; Harid,M . and Gray, G.W.; Goodby J.W. (1997) "Introduction to liquid crystals, pp . 70-77, Tylor&Francis , London. 4. Orz eszko,J.K. ; M aurin,D. and M elon-Ksyta, Z. (2001) Naturforsch. 55b, 1035 5. Orz eszko,J.K.; M elon-Ksyta, K. and Czup ry nski, Z. (2003)Naturforsch. 58b:1015 6. Demus,D. (1985).Cryst. Res. Technol. 10:1413-1420 7. Sack mann,H. and Demus,D. (1973). M ol. Cryst. Liq. Cry st. 21:239 8. Sack mann, H. (1979).Pure App li. Chem. 38: 503 9. Gray ,W. (1962)."M olecular Structure and Prop erties of Liquid Crystals", New York, Academic Press 10. Sack mann, H. and Demus,D. Z . (1963).Phys. Chem., 224:177 11. Sack mann, H. and Demus, D. Z . (1965). Phys. Chem., 230:285 12. Dove ,S. ; Patel,P. R. and Vasnath, K. L. (1966). Indian J. Chem.,4 :505 13. Rushdy , M. (2006).Egy pt. J. Solids, 29(2):227-240 14. Latif,I. ; Hassan,E. S. (2009) Jorna l of Diyala, 36 : 368. 15. Goodby ,J. W. ; Gray G. W. and M osley ,A. (1978).M ol. Cry st. Liq. Cryst . 41(lett.):183 16. Wegand,C. ; Gab ler,R. (1940).J. Prakt. Chem. 155,332 17. Youn g,W.R. ; Ha ller, I. and Green, D. C. (1972) J. Org. Chem. 37:3707 18. Wieggeleben,A. ; Richter,L. ; Deresch ,J. and Demus , D. (1980).M ol.Cry st. Liq. Cry st. 59:329 19. Arnold,H. ; Demus,D. ; Koch,H. J. ; Nelles ,A. and Sackmann, H. (1969). Z. Phy s.Chem. 240:185 20. M arzotko ,A. and Demus, D. (1975). Liq. Cryst. Proc. Int. Conf., 1973: 189 Table (1): The transition te mperatures (°C) for series nAP4M Transition temp eratures (C) --------------------------------------- Compound R= CnN2n+1 No. n Cr Sm N I 1AP4M 1 • 129 - - 2AP4M 2 • 126 - - 3AP4M 3 • 103 - - 4AP4M 4 • 89 • (83) • (85) 5AP4M 5 • 87 • (59) • (62) 6AP4M 6 • 80 • (48) • (63) 7AP4M 7 • 69 • (47) • (54) 8AP4M 8 • 65 • (45) • (61) IBN AL- HAITHAM J . FO R PURE & APPL. SC I VO L. 23 (1) 2010 Table (2) Transition enthalpies of the pure liquid crystal s components (A&B) 6PA4M M esop hase: Cr Sm5 N I Transition enthalpy ∆H (J/mole): 21950 (1980) (4080) TBAA M esop hase: Cr Sm5 SmH SmG SmC SmA N I Transition enthalpy: 19800 410 1300 4370 b 590 1100 ∆H (J/mole) ( ) mean a monotrop ic transition. b: mean just a jump in the Cp . Fig.(1): The four binary combinati ons of compound 6PA4 M with the different alkyloxy terminal group a) 4PA4M, b) 5PA4M c) 7 PA4 M and d) 7PA4M. IBN AL- HAITHAM J . FO R PURE & APPL. SC I VO L. 23 (1) 2010 Fig.(2): The binary mixture of compound 6PA4 M with the reference liquid crystalline compound TB AA. IBN AL- HAITHAM J . FO R PURE & APPL. SC I VO L. 23 (1) 2010 Fig.(3):DS C the rmograms of the transition enthalpies of the different mesophase with different concentration 5-95% of compound 6 AP4M. IBN AL- HAITHAM J . FO R PURE & APPL. SC I VO L. 23 (1) 2010 Fig.(4). The binary mixture diagrams of nPA4M and refe rence compound HPA, a) 4PA4M, b) 5PA4M, c) 6PA4M, d) 7PA4M and e) 8PA4M. IBN AL- HAITHAM J . FO R PURE & APPL. SC I VO L. 23 (1) 2010 Fig.(5):The binary mixture diagrams of nPA4M and refe rence compound 4B2M, a) 4PA4M, b) 5PA4M, c) 6PA4M, d) 7PA4M and e) 8PA4M. IBN AL- HAITHAM J . FO R PURE & APPL. SC I VO L. 23 (1) 2010 Fig.(6): The binary mixture diagrams of nPA4M and refe rence compound (4B3,2M) a) 4PA4M, b) 5PA4M, c) 6PA4M, d) 7PA4M and e) 8PA4M.