J. MATER. CHEM., 1994, 4( ll), 1699-1703 1699 A New Type of Main-chain Liquid-crystal Polymer derived from 4'-Hydroxybiphenyl-4-carboxylic Acid and its Smectic Mesophase Behaviour Yasukazu Nakatat and Junji Watanabe" Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 7 52, Japan A homologous series of polymers (PHBC-n) has been prepared from 4'-hydroxybiphenyl-4-carboxylic acid and o-bromoalkan-1-01s containing five to nine methylene units which can serve as mesogen and flexible spacer groups, respectively. The PHBC-n polymers are characterized by having two different linkage groups, ether and ester groups, to connect the biphenyl mesogen to the methylene spacer, and are classified on a new type of main-chain liquid- crystal polymer.All polymers exhibit the enantiotropic smectic phase. The smectic behaviour and structure were examined by differential scanning calorimetry, optical microscopic and X-ray diffraction methods, and are discussed in comparison with the corresponding date of the BB-n polyesters, which include only the ester linkage group. In recent papers,lP3 Watanabe et a!. have reported the meso- can be observed only for the c-director but not for the n-phase properties and structure of a homologous series of director.2 Such an odd-even effect on the transition param- main-chain BB-n polyesters which were prepared from p,p'-eters and mesophase structure shows how the nature of the dibenzoic acid and alkanediols. flexible spacer is important for understanding the meso-morphic behaviour in this type of polyester.This effect is believed to result from a coupling of polymeric and mesogenic effects, in which the mesogenic groups are confined in .;pa~e.~*~ In this study, we prepared a new class of main-chain liquid- crystal PHBC-n polymers with the following formula These BB-n polyesters invariably form smectic mesophases and their isotropization temperature, ?I, and entropy, ASi, exhibit an odd-even oscillation in which the larger values are observed for polyesters with even n. The odd-even nature of the alkylene spacer also reflects the type of smectic liquid crystal.' In BB-n where 11 is even a normal SA phase is formed Here n is the number of methylene units in the flexible spacer.with both axes of the polymer chain and the biphenyl mesogen The PHBC-n polymers can be differentiated from the BB-n lying perpendicular to the layers [see Fig. l(a)]. In contrast, polyesters, since they possess two different linkage groups, an the smectic structure of BB-n with n odd was identified as a ester group on one side and an ether group on the other new type of smectic phase, S,,,$ in which the tilt direction of which connect the mesogen group to the alkylene spacer. The the mesogenic groups is the same in every second layer but mesophase properties and structures were examined by opposite between neighbouring layers [see Fig. 1(b)].The SCA differential scanning calorimetry (DSC), optical microscopy phase is quite novel and interesting since uniaxial ordering and X-ray measurements.t Present address: LINTEC Co., Nishikicho, Warabishi, Saitama Experimental335, Japan. In previous reports, this phase has been termed S,,, but Sc2 is a Syntheses confusing notation in the context of low molar mass systems. For this reason, we will hereafter adopt the notation SCA,which has been PHBC-n polymers were synthesized in two steps according to coined for the same type of smectic phase in low molar mass systems Scheme 1. by Fukuda and co-~orkers.~ EHW-n IAt Fig. 1 Schematic illustration of (a) the SA phase formed in even-membered BB-n and (b)the s,, phase in odd-membered BB-n. In the SCA phase, the mesogenic groups in each layer are tilted by about 25" to the layer normal but with opposite tilt directions in neighbouring layers.Scheme 1 1700 J. MATER. CHEILI., 1994, VOL. 4 Preparation of Ethyl 4'-(o-Hydroxyalkoxy)biphenyl-4-carboxylate (EHBC-n) Monomers 1 r In a dry, three-neck, 100ml round-bottom flask, equipped 0U with a magnetic spinner, condenser, additional funnel and Q) nitrogen inlet tube, was placed 3.0 g (1.24 mmol) of ethyl 4'-hydroxybiphenyl-4-carboxylate in 30.0 ml of N,N'-dimethyl- formamide and 1.3 g of anhydrous potassium carbonate. The additional funnel was charged with 15.0 mmol of w-bromo- Y alkan-1-01. After heating to 120 "C, 0-bromoalkan-1-01 was added dropwise. Stirring was continued for 4h after the 9050 130 170 210 addition was complete and the contents were then extracted TI'C with 500 ml of 1 mol 1-l HC1.Purification was achieved by column chromatography using silica gel and chloroform as Fig.2 DSC thermograms of (a) EHBC-8 monomer a nd (h) PHBC- eluent, followed by successive crystallizations from isopropyl 8 polymer alcohol-hexane. Preparation of PHBC-n Polymers The lowest transition at crystal transition, the hig hest one at T, correspon ?; to the isotropization ds to the crystal-liquid PHBC-n polymers were prepared by melt transesterification of the liquid crystal. The intermediat e transition at is due of the EHBC-n monomers, which was performed by placing to the transition between two liquid crystals. T he transition 1 g of EHBC-n with a small amount of tetraisopropyl orthotit- temperatures, as collected in Table 1 and plotted ag ainst n in anate as catalyst in a polymerization tube.The polymerization Fig. 3, decrease monotoni cally with in creasing YZ. Furthermore, tube was heated to 230 "C in a mantle heater, and a continuous it is found that the mono mers of EHB C-6 to EH BC-9 exhibit stream of nitrogen bubbles was passed through the melt by two mesophases while EH BC-5 show s only one me sophase. means of a capillary tube. The ethanol was distilled off for From optical microsco py observati ons, all of t he monomer 2 h, after which the temperature was raised to 250-300°C mesophases appear to be composed of thin rod s on cooling and the pressure was reduced to 0.2 mmHg; these conditions from the isotropic melt. After the t hin rods c oalesce upon were maintained for 1 h.The inherent viscosities of the further cooling, a mesop hase develops with ho meotropic polymers were measured at 30°C using a Ubbelohde vis- alignment which may resu lt from the t endency of t he molecules cometer for 0.5 g dl-' solutions in a 60:40 mixture of phenol to attach their polar hyd roxy end gro up to the glass surface. and tetrachloroethane. homeotropic mesophase, In this no birefringence can be Characterization of EHBC-n Monomers and PHBC-n observed. This feature rethe lower-temperature me sophase, indmains unalt icating thaered upon t bo tra nsition to th phases Polymers are optically uniaxial sme ctic phases. The calorimetric behaviour was studied using a Perkin-Elmer - DSC-I1 calorimeter.10 mg samples were heated and cooled 320 at a rate of 10°C min-' under a flow of dry nitrogen. In an (b) effort to provide a common thermal prehistory, the data were collected during the first cooling from an isotropic melt and - during the second heating from room temperature. Optical 240 microscopic observations were performed using an Olympus BH-2 polarizing microscope equipped with a Mettler FP-82 P hot stage and a Mettler FP-80 temperature controller. Wide- k angle X-ray diffractograms were recorded with a flat-plate 160 - camera using Ni-filtered Cu-Ka radiation. The distance from (a ) sample to film was calibrated using silicon powder. The sample temperature was regulated using the Mettler hot stage and controller.80 - Results 4 6 8 10 The EHBC-n monomers exhibit mesophases whose meso- n morphic behaviour and structure are briefly described here. Fig. 3 Variation of transitio n temperatur e with num ber of carbon A typical DSC thermogram is shown in Fig. 2(a) for EHBC- atoms in the methylene spa cer, n, for (a) EHBC-n m on omers and 8. All the EHBC-n monomers exhibit two or three transitions. (b)PHBC-n polymers; (0)T,, (a)?; and (0)?; Table 1 Characteristics of the EHBC-n monomers calorimetric data' monomer T,/"C T/"C 7,'/"C A,S/J mol-' K-' AJ/J K-l AiS/J mol-'mol-' Kpl d(&Jb;A d(S,)b/A EHBC-5 124 146 14.2 25.9 20.7 EHBC-6 107 111 140 7.1 4.6 22.2 22.7 22.0 EHBC-7 96 109 134 7.0 4.2 27.2 23.7 23.0 EHBC-8 93 105 129 16.7 5.0 31.8 25.0 24.2 EHBC-9 91 102 126 61.3 4.6 30.5 26.3 25.5 'Calorimetric data are collected on heating.'d is the layer spacing. J. MATER. CHEM., 1994, VOL. 4 4 6 8 10 n Fig. 4 Variation of d-spacings with n for (a)EHBC-n monomers and (b)PHBC-n polymers: (@) S,; (0)SA The higher-temperature mesophase can be assigned to the smectic A (S,) phase from its characteristic X-ray pattern which includes a sharp inner reflection (the layer reflection), and a broad outer reflect+. The spacing of the layer reflection varies from 20.7 to 25.5 A with a variation of n from 5 to 9, while the sp!cing of the outer broad reflection is constant at around 4.5A. On cooling to the lower-temperature meso- pha!e, the outer reflection becomes sharp with a spacing of 4.4 A.This, together with the optical microscopic observations, suggests that the lower-temperature mesophase is smectic B (SB).The layer-reflection spacings are plotted against n in Fig.4. Although !he spacing in the SB phase is somewhat larger by 0.7-0.8 A than that in S,, the spacing in each phase increases linearly with incre?sing n. The average increment of spacing per unit of n is 1.2 A for both phases. This increment, as well as the value of the layer spacing, is reasonable for the uniaxial smectic phases in which the molecules, in nearly extended form, lie perpendicular to the layers. Fig. 2(b) shows the DSC thermograms of the PHBC-8 polymer. Two peaks can be observed upon heating and cooling, All other polymers also exhibit two enantiotropic peaks.The peak at the lower temperature (T,) corresponds to the crystal-liquid crystal transition and the higher-tempera- ture peak at to the isotropization of the liquid crystal. The thermodynamic data obtained from the heating curves are listed in Table 2. In Fig. 3, the transition temperatures are plotted against the number of methylene units, n, in the flexible spacer. As n increases, the vertical spacing of the two transition tempera- tures, which represents the temperature span of the mesophase, becomes narrow, showing a marked odd-even oscillation. The isotropization entropy, AJ, also exhibits odd-even oscil-lation with the larger value found for PHBC-n with n odd as illustrated in Fig.5. A fan-like texture was observed for all of the polymer mesophases by optical microscopy showing the smectic nature of the mesophase. X-Ray diffraction also verifies the smectic layered mesophase; the diffraction pattern consists of sharp inner and diffuse outer reflections. The spacings for the inner- layer reflections are listed in Table 2 and plotted against n in n Fig. 5 Variation of smectic-phase isotropization entropy with n in the polymeric PHBC-n system Fig. 4. One can also find here a marked odd-even oscillation, with the larger value occurring for polymers with odd n. To determine the structure of the smectic phase in detail, X-ray diffraction of the oriented fibre specimens, which can be prepared by drawing the isotropic melt, was undertaken.Typical oriented X-ray patterns are shown for PHBC-7 and PHBC-8 in Fig. 6(u)and (b),respectively, where the jibre axis corresponding to the chain axis is placed in the vertical direction. From Fig. 6(u), it can be seen that in PHWC-7 the layer reflection is placed at the meridian and the outer broad reflections appear on the equator. A similar pattern is observed in two other odd-n homologues, PHBC-5 and PHBC-9. Therefore, we conclude that the polymers with odd n form an SA phase, such as that depicted in Fig. l(u). In the polymers with even n (PHBC-6 and PHBC-8), in contrast, the outer broad reflections are split into two portions, lying above and below the equator, the layer reflection remaining on the meridian [Fig.6(b)].This suggests the formation of SCA,as illustrated in Fig. l(b), the structural details of which have been described in previous paper^.^.^ Discussion Compared with the monomeric EHBC-n, PHBC-n polymers show two distinct features of mesophase behaviour arid struc- ture: (i) the PHBC-n polymer smectic mesophases appear in a higher-temperature region than those of EHBC-n monomers, as shown in Fig. 3; (ii) the marked odd-even alternation appears in the thermodynamic parameters of the transition as well as in the smectic structure (see Fig. 3-5). These can be explained by the polymeric effect, which results from the linkage of mesogenic groups into the polymer backbone through the flexible spacer.6 The odd-even alternation of the smectic structure is interes-ting.As stated in Introduction, this was first observed in the BB-n polyesters and was believed to result from the different conformational constraints on the angular displacement of the mesogenic groups for the odd- and even-n Supplementary evidence for this conformational constraint has been derived from the computational analyses.' Abes Table 2 Characteristics of the PHBC-n polymers calorimetric data' polymer qinhb/dlg ~ T,/"C TJT A,S/J mol-' K-' AiS/J mol - K - &JA PHBC-5 0.43 270 307 8.8 19.2 16.2 PHBC-6 0.37 178 215 3.8 14.2 16.1 PHBC-7 0.33 238 261 10.9 23.0 18.5 PHBC-8 0.41 178 204 5.9 19.6 18.3 PHBC-9 0.35 210 22 1 10.9 25.1 20.7 'Calorimetric data are collected on heating.vinh is the inherent viscosity. 'Layer spacing. J. MATER. CHEM., 1994. VOL. 4 (a) Fig. 6 X-Ray diffraction patterns for the oriented smectic phases of (a) PHBC-7 and (b)PHBC-8 fibres. The fibre was prepared by drawing the isotropic melt. The axis is placed in a vertical direction. performed a conformational analysis, within the framework of the rotational isomeric state model, and evaluated the angle, 0, defined by unit vectors attached to two successive mesogenic groups, for all possible conformations of the alky- lene spacer. The results indicate that the angular distribution of main-chain polyesters such as BB-n is different, depending on the odd-even parity of n. When n is even, 0 values are distributed in the two regions 0-30" and 85-130".For odd n most of the angles are located in the region 50-90", and to some degree orientations are also permitted in the region above 150". In each system, the conformers with smaller angular displacements of the successive mesogens are in the more extended form, whereas those with larger angular dis- placements are in the folded form. Comparing the calculations with the observations for the BB-n polyesters,132 we have arrived at the following conclusions as to the conformational constraint imposed by the smectic field. (1) Where n is even, parallel orientation of the successive mesogenic groups is allowed, conforming more or less to the concept of an ordinary uniaxial ordering of the n-director, but where n is odd, uniaxial orientation of successive mesogenic groups is not expected.(2) The conformers with smaller angular displacements in both the odd and even systems are those which participate in the experimentally observed smectic layer structures. These points help to explain how the different types of smectic phases appear to be dependent on the odd-even parity of n and, at the same time, why the new type of smectic phase, SCA,is formed from the BB-n polyesters with odd n. The computational analyses by Abe have also shown that similar angular distributions can be attained in main-chain polyethers in which the mesogen and alkylene spacer are connected by an ether linkage group.' This suggests that the polyethers may form two types of smectic structures, in the same way as the polyesters.So far, the main-chain polyethers have been synthesized with a variety of mesogenic groups. However, these tend to form nematic liquid cry~tals.~-'~ Only polyethers based on the biphenyl mesogen have been reported to form smectic me so phase^,'^,^^ but structural analyses have not been made in relation to the odd-even character of the flexible spacer. Therefore, the question of how the odd-even character of the flexible spacer in the polyethers affects the smectic structure remains unsolved. On this point, it is interes- ting that the present PHBC-n polymers showed a similar odd-even alternation to the BB-n polyesters. The PHBC-n polymers have an ester linkage on one side of the biphenyl mesogen, but an ether linkage on the other, and hence it is suggested that the ether linkage may serve to produce similar odd-even effects to the ester linkage.Note that the odd-even alternation of the smectic structures appears with ah opposite trend for the PHBC-n and BB-n polymers. The SA phase is observed here for PHBC-n with odd n, while it is formed from even-n polymers in the BB-n series. Also, the SCAphase is observed in even-u PHBC-n and odd-n BB-n. The layer thickness, as well as the thermodynamic parameters of the transition, also shows opposite oscillations with n. This may be due to the fact that the number of atoms in the spacer directly connecting two successive biphenyl mesogens is fewer by a unit in PHBC-n than in BB-n, because of the lack of a carbonyl group in one of the two linkage groups.Therefore, for the angular displacements of the biphe- nyl groups in PHBC-n and BB-(n-1) are similar. This can be understood from Fig. 7, where the molecules in a fully extended form are illustrated for PHBC-7 and BB-6. This simple argument is also supported by the observation that the absolute value of the layer thickness, as well its odd-even oscillation, is comparable for the PHBC-n and BB-(n-1) systems (see Fig. 8). Fig. 7 Comparison of the extended-chain structures of (a) PHBC-7 and (b)BB-6 J. MATER. CHEM., 1994, VOL. 4 1703 22 r 3 J. Watanabe, M. Hayashi, A. Morita and T. Niori, hlol. Cryst. Liq. Cryst., in the press.4 A. D. L. Chandani, E. Gorecka, Y. Ouchi, H. Takezoe and 20-'5 18--0 5 6 A. Fukuda, Jpn. J. Appl. Phys., 1989,28, L1265. Y. Takanishi, H. Takezoe, A. Fukuda and J. Watanabe, Phys. Ret.. B, 1992,45,7684. A. Ciferri, in Polymer Liquid Crystals, ed. A. Ciferri, 16 /+---d- W. R. Krigbaum and R. B. Meyer, Academic Press. London, 1982, pp. 63-83. 7 J. Watanabe, H. Komura and T. Niori, Liq. Cryst., 199?, 13,455. 8 A. Abe, Macromolecules, 1984,17,2280. 4 6 8 10 9 S. Antoun, R. W. Lenz and J-I. Jin, J. Polym. Sci., Polvm. Chem. n Ed., 1981, 19, 1901. Fig. 8 Comparison of the smectic layer spacings of the PHBC-n (0) and BB-(n -1) (0)polymers 10 11 A. C. Griffin and S. J. Havens, J. Polym. Sci., Polym. Phys. Ed., 1981, 19, 951. J-I.Jin, E-J. Choi, S-C. Ryu and R. W. Lenz, Polym. J., 1988, 18,63. Finally, note that the main-chain PHBC-n polymers have non-centrosymmetric arrangements of the repeating unit within a polymer chain. In other words, the polymer has a head-to-tail character and a non-centrosymmetric alignment of the dipole moments. In addition to having two different 12 13 14 15 A. Abe and H. Furuya, Macromolecules, 1989,22,2982. J-I. Jin and J-H. Park, Eur. Polym. J., 1987,23,973. T. Shaffer and V. Percec, J. Polym. Sci., Polym. Lett. Ed., 1985, 23, 185. T. Shaffer, M. Jamaludin and V. Percec, J. Polym. Sli, Polym. Chem. Ed., 1986, 24, 15. linkage groups, this structural characteristic places the PHBC-n polymers as a new class of main-chain liquid-crystal poly- mer.l6 In this new type of polymeric system, ferroelectricity (one of the most predominant properties in a liquid-crystalline field) can be achieved if the polymers are packed into smectic 16 17 18 A. M. Ahmed, W. J. Feast and J. Tsibouklis, Polymer. 1993, 34, 1297. R. G. Petscheck and K. M. Wiefling, Phys. Ret.. Lett. 1987, 59, 343. J. Watanabe, Y. Nakata and K. Shimizu, J. Phys. I1 Fr., 1994, 4, 581. layers with a non-centro-symmetric alignment.17*'s This matter is currently under investigation. Paper 4/02999B; Received 20th hfuy, 1994 References 1 J. Watanabe and M. Hayashi, Macromolecules, 1989,22,4083. 2 J. Watanabe and S. Kinoshita, J. Phys. I1 Fr., 1992,2, 1237.