Research Journal of Recent Sciences ______ ______________________________ ______ ___ __ _ ISSN 2277 - 2502 Vol. 1( ISC - 2011 ), 122 - 127 (201 2 ) Res.J. Recent .Sci. International Science Congress Association 122 Synthesis and Study o f Main Chain Chalcone Polymers Exhibiting Nematic Phases Tandel R.C. , Gohil Jayvirsinh and Patel Nilesh K . Chemistry Department, Faculty of Science, The M.S. University of Baroda , Vadodara, Gujarat, INDIA Available online at: www.isca.in (Received 10 th Decembeer 2011, revised 7 th January 2012 , accepted 3 rd Feburay 2012 ) Abstract The mesogenic main chain polymers of general structure - I were synthesized and their different properties are studied. All the polymers exhibit nematic phases. Classical nematic textures are observed in these polymers. None of the dihydroxy, amino - hydroxy or dicarboxy compounds shows liquid crystalline properties. The role of flexible methylene spacers, degree of polymerization and central linkage on exhibition of all the polymers is discussed. All the polymers are characterized by standard methods . Keywords: : Liquid crystalline polymers , n ematic phase , chalcone linkage, c rystalline I – c rystalline II etc . Introduction The rarity of mesogenic compounds having chalcone linkage and potential of polymers with this linkage for application prompted 1 to study the polymers with chalcone linkage. The first mesogenic homologous series with chalcone linkage have been reported 2 and studied polyesters and polyester - amides by varying flexible methylene spacers from – (CH 2 ) 2 - to - (CH 2 ) 4 - in carboxy phenoxy dia cid moiety 3 , 4 . They also varied the flexibility by using rigid and flexible dicarboxylic acids as co - monomer. The chemistry and physics of high - performance fiber spinning based on main - chain liquid crystal polymer (MCLCP) solutions and melts was discussed, which is the largest industrial application of liquid crystal technology 5 . Recently Schiff - base chalcone linkage thermotropic mesogens have synthesized and study the effect of substituent on liquid crystalline properties 6,7 . The side chain liquid crystall ine photoactive polymers with chalcone pandent chalcone moiety were synthesized and characterized 8, 9 . However polymers exhibited relatively higher transition temperatures. The flexibility of the dicarboxylic acid moiety in the system increased by introducing ‘oxyetylene spacers’ in te place of methylene spacers 10, 11 . Solid to mesomorphic and mesomorphic to isotropic transition temperatures were drastically reduced but florescent behavior was not affected. One striking feature of "oxyethylene spac er" polymers was that smectic mesophase was eliminated. This prompted us to investigate polymeric ester and ester - amide having chalcone linkage with increased flexibility by increasing number of methylene spacers to 6 and 10 i.e. - (CH 2 ) 6 and - ( CH 2 ) 10 - . With this in view polyester - amides and polyesters containing chalcone linkage were synthesized by the route given in Figure 1 . It was proposed in the present study to concentrate on flexibility of carboxyl phenoxy diacid moiety by increasing flexibility with high number of methylene spacers to evaluate the effect of increased flexibility on mesogenic properties of polymers with this in view polymers were synthesized by route given in f igure 1. Figure - 1 Synthetic rout e to Polychalcones Measurements : The synthetic route adopted for the synthesis of polychalcones is shown in Figure:1. Elemental analysis were performed with a Perkin - Elmer 2400, C,H,N analyzer. The IR of polymers was recorded on Perkin - Elmer with KBr pellet. The optical textures of the polymers samples were studied with a Leitz Laborlux 12 Pol (Germany) polarizing microscope fitted with a Kofler heating stage. Ubbelohde Viscometer was used for the measurement of viscosity of polymers in dimethyl form amide as a solvent. Research Journal of Recent Sciences ______ _ _ _______________________________ ______________ _ ______ ISSN 2277 - 2502 Vol. 1( ISC - 2011 ), 1 22 - 127 (201 2 ) Res.J.Recent.Sci International Science Congress Association 123 Material and Methods Preparation of ifferent iacids ω - bis (4 - carboxy phenoxy) oligo ethers : Following different diacids were used for the synthesis of polymeric chalcones:1,6 - bis(4' - carboxy phenoxy) hexane (DA - I) , 1,10 - bis(4 ' - carboxy phenoxy) decane (DA - II) The diacids DA - I and DA - II) were synthesized by the same method reported in the literature 12, 13 for synthesis of 1,2 - bis (4' - carboxy phenoxy)ethane. p - Hydroxy benzoic acid (0.2 mole) was dissolved in 100 ml 22.4% potassium hydroxide solution (0.4 mole) and 30 ml of alcohol in round bottom flask. Approximate dibromide (0.11 mole) was added to the flask and whole mass was refluxed for 8 - 10 hours. Reaction mixture was allowed to cool, then acidified with cold 1: 1 hydrochloric acid. Solid mass obtained was filtered and washed with water and dilute alcohol. Diacids obtained were recrystallized several times from DMF solvent till constant melting points were obtained. DA - I : 290 o C, Reported 290 - 292 o C 12,13 , DA - II : 273 o C, Reported 273 - 274 o C 12,13 Synthesis of Diacid Chlorides of Diacids DA - I and DA - II 14 : Diacid chlorides were prepared by reacting the corresponding diacids with excess of thionyl chloride and heating on a water - bath till the evolution of hydrogen chloride gas ceased. Excess of thionyl chloride was distilled off under reduced pressure using vacuum pump and the diacid chlorides left behind as a residue was used in next rea ction without further purification. Synthesis of different Chalcone : Synthesis of 4,4' - dihydroxy chalcone: 4,4' - dihydroxy benzylidene acetophenone (HC): 4,4' - dihydroxy chalcone was synthesized according to the hot condensation process 17 13.6 gms (0.1 mole) 4 - hydroxy acetophenone and 13.2 gms (0.1 mole) 4 - hydroxy benzaldehyde were dissolved in minimum amount of ethanol, 55 ml of 50% potassium hydroxide was added to the above solution. The flask was heated at 50 o C for twenty hours. The solution was ac idified by cold 6 N HCl solution (congored), Yellow crystalline solid separated, which was filtered and washed with water. It was recrystallized from ethanol melting point 204 o C . Figure 2 Syntesis of 4,4’ - dihydroxy chalcone Synt hesis of 4 - amino - 4’ - hydroxy chalcone: 4 - amino - 4’ - hydroxy benzylidene acetophenone (AHC): 4 - Amino - 4’ - hydroxy chalcone was prepared by extending the procedure 18 for the para substituted derivative. 13.5 gms (0.1 mole) 4 - Amino acetophenone, 12.2 gms (0.1 mole) 4 - hydroxy benzaldehyde and a few drops of piperidine in absolute ethanol (40 ml) were taken in round bottom flask and were refluxed for twelve hours. The reaction mass was concentrated up to its half the volume and then reaction mass was po ured to ice - water mixture with stirring, solid separated, which was filtered, dried and crystallized from ethanol, M.P. 217 o C. Figure - 3 Synthesis of 4 - amino - 4’ - hydroxy chalcone Polycondensation of different diacid chloride with different chalcones : The diacid chloride of DA - I and DA - II were condensed with respective chalcones by solution polycondensation reaction, pyridine was used as a solvent. The appropriate diacid chloride (0.005 mole) was dissolved in 10 ml dry pyrid ine and cooled to 0 o C in an ice - bath. The solution of respective chalcone (0.005 mole) in 5 ml dry pyridine was added to the diacid chloride with constant stirring. The reaction mixture was guarded against moisture absorption. The temperature of mixture wa s allowed to rise to room temperature (28 - 30 o C) and stirred for two more hours. It was finally poured into ice - hydrochloric acid mixture and solid separated was filtered, washed with water followed by alcohol to remove unreacted starting materials. Polym ers were purified by solvent - non solvent method. DMF was used as a solvent and methanol was used as a non - solvent. The transition temperatures and viscosity data are recorded in t able 1 and 2, respectively. In the IR spectrum of EP - 1 and EP - 2, the characteristic keto - ester linkage was observed at 1720 and 1740 cm - 1 , keto of - CH = CHCO – at 1660 and 1655 cm - 1 , - CH=CH - Ar at 1560 and 1555 cm - 1 and ν C - H bending of alkane at 1460 and 1455 cm - 1 , respectively and IR spectrum of EP - 3 and EP - 4 the characteristic keto - ester linkage was observed at 1740 and 1720 cm - 1 , - NH - bending at 1690 and 1690 cm - 1 , - CONH - R at 1510 and 1500 cm - 1 , keto of amide at 1600 and 1590 cm - 1 , - CH=CH - Ar at 1570 and 1575 cm - 1 , respectiv ely. ν C - H bending of aromatic ring obtained at 760, 750 cm - 1 etc. Research Journal of Recent Sciences ______ _ _ _______________________________ ______________ _ ______ ISSN 2277 - 2502 Vol. 1( ISC - 2011 ), 1 22 - 127 (201 2 ) Res.J.Recent.Sci International Science Congress Association 124 Table - 1 Transition temperatures of polychalcones Results and Discussion Reference to t able 1 indicates that polymers EP - 1 to EP - 4 exhibit only nematic mesophases. The comparison of mesogenic properties of different polymers is a difficult task as the mesogenic properties of polymers depend not only on the chemical constitution but also on the molecular weight and polydispersity of the system. In the present study the structure of all the polymers vary uniformly hence i ntrinsic viscosity values [η int ] are taken to compare the properties of polymers. Due to the solubility parameters of the polymers molecular weight determination could not be done by other method. Number of researchers has studied the effect of increased flexibility of methylene spacers on differe nt polymeric systems 19 - 22 . A common observation is that the increase in flexibility lowers transition temperatures and induces smectic phases in certain cases 23 - 25 . Reference to t able 1 indicates that increase in flexibility in each system affects solid mesomorphic as well as mesomorphic - isotropic transition temperatures. Compared to EP - 1, EP - 2 polymers, EP - 3 and EP - 4 polymers exhibit lower solid - nematic and nematic - isotropic transitions. Normally an amide linkage enhances mesogenic thermal stability. It seems the unsymmetrical linkages (ester and amide) in the repeat unit may be responsible in lowering of solid - nematic and nematic - isotropic transitions of EP - 3 and EP - 4 polymers. Reference to table:1 further shows that when amide linkage (EP - 3 and EP - 4), t he transition temperatures are severally affected. These results indicate that amide linkage bring down solid - nematic as well as nematic - isotropic transition temperatures. The polymers exhibit very fine texture similar to nematic phase in small molecules. This is an interesting aspect, normally a good texture is observed after keeping a polymer sample for sometime on the hot stage of the microscope ( figure - a and b ). The absence of smectic phase in all the polymers is quite surprising. Even with ten spacer s and one of the amide linkage EP - 3 and EP - 4 do not exhibit smectic mesophase. This may be due to the increased flexibility of the systems. Polymers EP - 1, EP - 2 and EP - 3 were studied by using Mettler DSC - 4000. Polymer EP - 1 does not exhibit endothermic peak for nematic - isotropic transition temperature. Only one endothermic peak is obtained for crystalline - nematic transition temperatures. However on cooling the melt an exothermic peak is observed for isotropic - nematic transition temperatures. Polymer EP - 2 exh ibits one additional endothermic peak between the two recorded transitions Solid - Nematic and Nematic - Isotropic. It is difficult to account this endothermic peak. However on cooling the melt exothermic peak are not observed for any of the transitions. In the case of polymer EP - 3 an endothermic peak for Solid - Nematic transition temperatures is observed. An additional peak between the two transition temperatures is obtained in this polymer also. The DSC results are in conformity with certain unusual mesogeni c series 26, 27 . Reference to table - 1 show s that the polymers EP - 1 to EP - 4 exhibit nematic mesophase only. The comparison of mesogenic properties of different polymers is a difficult task as the mesogenic properties of polymers depend not only on th e chemical constitution but also on the molecular weight and polydispersity of the system. In the present study the structures of all the four polymers vary uniformly hence intrinsic viscosity values [  ] are taken to compare the properties of polym ers. The nematic. - i sotropic. Transition temperatures do not differ much in the case of EP - 1 and EP - 3 even though - 0 - is replaced by - NH - in EP - 3. However, the transition temperature of EP - 2 is much higher compared to all the other three polymers which is difficult to explain. A little difference can be explained on viscosity results, but here difference is large enough. Molecular weight and polydispersity data can through some light which could be obtained for EP - 1 (molecular weight 2028) and EP - 4 (molec ular weight 1960) only. An interesting aspect worth noting is that polymer EP - 3, even after cooling exhibits nematic texture. This trend is not observed in other similar polymers. The intrinsic viscosity obtained by using one point method 21 , indicates t hat this procedure can be used. The procedure followed by condensation polymerisation gives consistent data. Intrinsic viscosity does not defer markedly from EP - 1 to EP - 4. DSC Results ( t able 4 ) indicate typical behavior. Except polymer EP - 1 none of th e polymers exhib it endothermic for nematic - i sotropic transition temperatures. In the case of polymer EP - 3 an extra peak not matching with microscopic results is observed at 67 o C. Reexamination of the slide of polymer EP - 3 indicated that there is no phase change at this Code no. X = m = Transition Temperatures o C Nematic Isotropic EP - 1 - O - 6 140.0 190.0 EP - 2 - O - 10 150.0 280.0 EP - 3 - NH - 6 124.0 190.0 EP - 4 - HN - 10 141.0 169.0 Research Journal of Recent Sciences ______ _ _ _______________________________ ______________ _ ______ ISSN 2277 - 2502 Vol. 1( ISC - 2011 ), 1 22 - 127 (201 2 ) Res.J.Recent.Sci International Science Congress Association 125 temperature. This indicates that the endothermic peak at 67 o C in the case of EP - 3 may be due to Crystalline I - c rystalline II transition. Enthalpy change in pol yesters EP - 2 to EP - 4 from s olid - n ematic differs and is higher for EP - 4. However in the case of EP - 3 te ΔH value (J/g) is muc less due to second crystalline modification. The major enthalpy change in c rystalline - I to c rystalline - II phase change in EP - 3 su ggests that it might be a highly ordered phase. Only X - ray study can through some light on this aspect. Figure - 4 Intrinsic viscosity against different concentration Figure - 4 Optical polarized microphotograph of Nematic liquid crystalline phase (Thread like texture) of EP - 2 on heating at 245 0 C Figure: 5 Optical polarized microphotograph of Nematic liquid crystalline phase (Thread like texture) of EP - 4 on cooling at 154 0 C Table - 2 Viscosity data of the Polychalcones Solvent: Dimethyl Formamide Concentration: 0.5% Temperature: 34.4 o C Code  rel  sp  red  inh  int EP - 1 1.0665 0.0665 0.1330 0.1288 0.1298 EP - 2 1.0705 0.0705 0.1410 0.1363 0.1374 EP - 3 1.0679 0.0679 0.1358 0.1314 0.1325 EP - 4 1.0724 0.0724 0.1449 0.1397 0.1411 Calculated by One Point method (15) Table - 3 Viscosity data of Polymers Solvent: N - Methyl - 2 - Pyrrolidone, Temperature: 34.4 o C Polymer Concentration (%)  rel  sp  intrinsic (dl/gm) One pt. method Graphical EP - 2 1.0 1.1500 0.1500 0.1423 0.75 1.1107 0.1107 0.1419 0.1340 0.50 1.0705 0.0705 0.1374 0.25 1.0345 0.0345 0.1363 Research Journal of Recent Sciences ______ _ _ _______________________________ ______________ _ ______ ISSN 2277 - 2502 Vol. 1( ISC - 2011 ), 1 22 - 127 (201 2 ) Res.J.Recent.Sci International Science Congress Association 126 Table - 4 Calorimetric data of Polychalcones and Polycarbonates Heating rate = 10 o C Sr.No. Polymer Wt. mg. Phases Peak Temp. o C (Microscopic Reading)  H J/g  S J/g. o K Total  S J/g. o K 1 EP - 1 17 K - Nematic 144.8 (140.0) 2.44 0.00584 Nematic - Iso. 190.3 (190.0) 0.39 0.00084 0.0067 2 EP - 2 17 K - Nematic 141.1(150.0) 4.51 0.0109 Nematic - Iso. - (256.0) - - 0.0109 3 EP - 3 17 Extrapeak 67.0 5.38 0.0158 K - Nematic 121.1(124.0) 0.79 0.0020 0.0178 Nematic - Iso. - (190.0) - - 4 EP - 4 10 K - Nematic 143.1 (141.0) 9.09 0.0219 Nematic - Iso. - (169.0) - - 0.0219 ( ) Values indicate microscopic data, K indicates crystalline, Iso. = isotropic Table - 5 GPC Data of the Polymers Polymer Mn Mw Mw/Mn Molecular weight EP - 1 633 2100 3.318 2028 EP - 4 955 2453 2.569 1960 Conclusion Polyesters and polyesteramides having chalcone linkage with increased flexibility by increasing number of methylene spacers show higher thermal stability than monomers. All the polymers exhibit nematic meso phases. Except polymer EP - 1 none of the polymers exhibit endothermic for Nematic - Isotropic transition temperatures. Acknowledgement We are indebted to Professor B.V. Kamath, Head, Chemistry Department, Faculty of Science for taking interest in the work and providing necessary facilities. W e extend our sincere thanks to the Head, Applied Chemistry Department, Faculty of Technology and Engineering, M.S. University of Baroda, Vadodara for providing Polarizing Microscope facility. We are also thankful to research scholars for elemental and spec tral analysis. References 1. Vora R.A. and Sheth K. A., Presented at the International Conference of Liquid Crystal Polymers , Bordeaux, France, July 20, (1987) 2. Chudgar N. K . and Shah S. N., Liq. Cryst ., 4 (6) , 661 (1998) 3. Gaissman T.A. and Clinton R.O., J. Am. Chem. Soc ., 68 , 697 (1984) 4. Kumar A., Sinha J.,Bhargava K.P., and Shanker K., Ind. J. Chem., 23 B, 589 (1999) 5. Pickena S.J., Sikkemab D. J ., Boerstoelc H., Dingemansd T. 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