摘要:

J. MATER. CHEM., 1994, 4( 5), 747-759 Effect of the Position of Lateral Fluoro Substituents on the Phase Behaviour and Ferroelectric Properties of Chiral I-Methylheptyl 4'-[(2-or 3-Fluoro-4-tetradecyloxyphenyl)propioloyloxy]biphenyl-4-ca rboxylates Christopher J. Booth," David A. Dunmur,b John W. Goodby,*" Jaskaran S. Kangb and Kenneth J. Toyne"" School of Chemistry, The University of Hull, Hull, UKHU6 7RX Centre for Molecular Materials, Department of Chemistry, University of Sheffield, Sheffield, UK S3 7HF The syntheses of four chiral laterally fluoro-substituted propiolate esters are described, along with transition tempera- tures, ferroelectric properties, phase diagrams and related data. The position of the fluoro-substituent was found to influence dramatically the formation of twist grain boundary (TGB A* and TGB C)phases as well as the magnitudes of spontaneous polarization and optical tilt angle in the ferroelectric smectic C*mesophases.Differential scanning calorimetric studies revealed the presence of a diffuse liquid-liquid transition above the clearing point in both of the 3-fluoro enantiomers, but not in the racemate. Circular dichroism and optical rotation measurements, carried out over a temperature range in which the diffuse peak occurs, appear to confirm the presence of a degree of chiral organization within the isotropic liquid. It is suggested that this phenomenon may be due to the presence of cybotactic groups or a network of entangled screw dislocations occurring close to the clearing point.The existence of materials that display the novel twist grain boundary phase (TGB A* phaset), as originally predicted by de Gennes' and developed later by Renn and Lubensky,' have now been well doc~mented.~~ The early structural studies performed by Goodby et al. on chiral 1-methylheptyl 4' -[( 4-alkoxyphenyl)propioloyloxy]biphenyl -4 -carboxylates (1) revealed that TGB A* phases were formed only when certain criteria were satisfied as follow^:^*^ 1 n=8to 16,t= (R)or(S) (i) the presence of a short-range smectic A* (Sz) phase and an Sz-smectic C* (Sz) transition close to the clearing point, thus allowing pretransitional fluctuations to stabilize the resulting twisted TGB A* phase; (ii) the transition from the cholesteric or liquid phase must approach being second order (this was often the case for the isotropic liquid to smectic A transitions for homologues of 1 where n =12-15); and (iii) the system must have strong chirality so as to be able to induce a twist through the smectic A phase.However, little work has been performed on the influence of molecular structure on the formation and phase stability of the TGB A* phase other than for increasing the terminal alkoxy substituent's chain length (although fluoro-substituents have been employed in a series of structurally isomeric tolanes'). Our planned work was to investigate the influence and effect of the positioning of lateral fluoro-substituents in the 4-alkoxyphenylpropiolate core on the formation of TGB A* phases and the ferroelectric properties of the smectic C* phases.The two materials selected for this study proved to have markedly contrasting properties, these were the (R)-, t The asterisk notation represents a phase with reduced symmetry caused by the inclusion of chiral molecules into the system, rather than using it to represent a helical macrostructure, hence smectic A compound of chiral compounds is denoted A*. and (S)-1-methylheptyl 4'-[(2-fluoro-4-tetradecyloxyphenyl) propioloyloxy] biphenyl-4-carboxylates (2) and (R)-and (S)-1-methylheptyl 4-[(3-fluoro-4-tetradecyloxyphenyl)propiol-oyloxy] biphenyl-4-carboxylates (3). The choice of the tetra- decyloxy substituent was deliberate in that it is a chain which is long enough to induce 'second-order' clearing behaviour in related systems, and it allows easy comparison with the parent non-fluoro-substituted series.334 F .O 3 t = (H)or (S) Experimental The phenylpropiolic acid core units were prepared as shown in Scheme 1.The first step in the synthesis is the al- kylation of the appropriately substituted 4-bromo-2/3-fluorophenol (compounds 4 and 5)with 1-bromotetradecane to give compounds 6 and 7, respectively. The individual cam- pounds were then cross-coupled with a protected terminal alkyne using palladium(0) tetrakis(triphenylphosphine), copper(1) iodide in anhydrous diisopropylamine to give the appropriately fluoro-substituted 1-(3-hydroxy-3-methyl-butynyl)-4-tetradecyloxybenzenes(compounds 8 and 9).'-'' Both compounds were subsequently deprotected using potass- ium hydroxide in refluxing toluene to furnish the ethynyl- benzenes (10 and 11) as low melting crystalline solids in J.MATER. CHEM., 1994,VOL. 4 HOsBr 4 X=F,Y=H 5 X=H,Y=F 1. C14H290 6 X=F,Y=H 7 X=H,Y=F 1. Ic t C14H290 10 X=F,Y=H 11 X=H,Y=F XY 12 X=F,Y=H 13 X=H,Y=F Scheme 1 Synthetic route to the (2-or 3-fluoro-4-tetradecyloxy-pheny1)propiolic acids. (a) CI4H2?Br, butanone, reflux; (b)3-methyl-but-1-yn-3-01, Pd( PPh,),, CuI, Pr',NH, N,, reflux; (c) KOH, toluene, N,, reflux; (d) (i) BuLi, THF, N,, -10"C; (ii) CO,(s), THF, -10"C to rt; (iii) HC1 (conc.). moderate yields.' Initial attempts to lithiate and carboxylate these terminal acetylenic compounds at -78 "C with butyl- lithium and then solid C02 failed, the starting terminal al- kyne was recovered. Carboxylation was eventually achieved, at the somewhat higher temperature of ca.-10 "C using the same reagents, to give the appropriate (2-or 3-fluoro-4-tetra- decyloxypheny1)propiolic acid (12 and 13). This approach to the propiolic acids differs slightly from the original article in which use was made of the Corey-Fuchs reaction that employs 4-alkoxy-j3,j3-dibromostyrenes prepared from the 4-alkoxy- benzaldehyde precursor^.^*'^ The (R)-or (S)-1-methylheptyl 4-hydroxybiphenyl-4-carboxylates (19 and 20) were synthesized as shown in Scheme 2.4'-Hydroxybiphenyl-4-carboxylicacid (15)was pre- pared by the acid hydrolysis of 4-cyano-4-hydroxybiphenyl (14, Merck). Compound 15 was then protected by treatment with methyl chloroformate in aqueous sodium hydroxide at ca.0 OC;I2 the 4-methoxycarbonyloxybiphenyl-4'-carboxylic acid (16) obtained was then esterified using (S)-or (R)-14 15 Ib 16 Id 12or 13 1 21 X= F, Y =H, (R) 22 X=F,Y=H,(S) 23 X=H,Y=F,(R) 24 X= H,Y = F, (S) Scheme2 Synthetic route to the (R)-and (S)-1-methylheptyl 4' [(2-or 3-fluoro-4-tetradecyloxyphenyl)propioloyloxy]biphenyl-4 carboxylates. (a) H,S04, AcOH, H,O, reflux; (b) (i) NaOH, CH30COC1,H,0,00C; (ii) 1 :1 HCl: H,O, pH= 5; (c) (R)-or (S)-octan-2-01, diethyl azodicarboxylate, PPh,, THF,N,, rt; (d) EtOH, NH3(aq), rt; (e)dicyclohexylcarbodiimide, 4-N.,Y-dimethylaminopyri-dine, CH2C1,, rt.octan-2-01, diethyl azodicarboxylate and triphenylphosphine (all obtained from Aldrich) to give the (R)-and (S)-protected esters (17 and 18)as colourless liquids after flash chromatogra- ph~;'~the reaction proceeds with inversion of the absolute configuration at the chiral centre, but with no detectable ra~emization.'~Compounds 17 and 18 were then deprotected using an ethanol-ammonia solution at room temperature to give the corresponding (R)-or (S)-1-methylheptyl 4'-hydroxy- biphenyl-4-carboxylate esters (19 and 20) in good yield.12 Each of these optically active biphenol esters were then esterified with the appropriate (2-or 3-fluoro-4-tetradecyloxy- J. MATER. CHEM., 1994, VOL. 4 pheny1)propiolic acid using dicyclohexylcarbodiimide and 4-N,N-dimethylaminopyridine at room temperature to give the four liquid-crystalline target compounds (21-24).15 Procedures The chemical structures of all intermediates and final products, were confirmed by a combination of the following techniques: 'H nuclear magnetic resonance (NMR) spectroscopy (JEOL GX NM270 FT-NMR spectrometer using tetramethylsilane as the internal standard), infrared spectroscopy (Perkin-Elmer 783 spectrometer) and mass spectrometry (MS; Finnigan 1020 GC-MS spectrometer).Specific optical rotations were per- formed using a Bendix-NPL Automatic Polarimeter Type 143 optical unit and controller; chloroform (SpectrosoL) was used as the solvent. At each stage of preparation the materials were purified by column chromatography (Fisons 60-120 mesh silica gel) or flash chromatography (May & Baker Sorbsil c60 40-60H pm silica gel) as described by Clark Still6 The purity of all intermediates and final compounds was checked by thin-layer chromatography (TLC; using Merck 60 F254 preformed aluminium- backed plates) and by normal- and reversed-phase high-performance liquid chromatography (HPLC) using Microsorb C18 or Si columns and acetonitrile (May & Baker, Chromanorm) as the mobile phase.Each of the final products was found to have a chemical purity in excess of 99.5%. All solvents used in reactions were dried, distilled and stored as described in Perrin and Armarego." The initial phase assignments and transition temperatures were determined by thermal polarized light microscopy using a polarizing microscope (Zeiss Universal) in conjunction with a hot-stage and controller (Mettler FP82 microfurnace and FP8O control unit).Differential scanning calorimetry (DSC; Perkin-Elmer DSC7 calorimeter, TAC7/PC controller and IBM system/2 Model 50Z computer) was used to determine both the transition temperatures and the heats of transitions. The instrument was calibrated against an indium standard (measured AH =28.35 J g-', literature value 28.45 J g-')'' and all enthalpies are quoted in kJ mol-l. Tilt angle and ferroelectric polarization measurements were performed in planar aligned 3.5 or 3.6 pm thick cells (Electronics Chemicals High Technology Group) with an active area of 0.25 cm2 indium-tin oxide electrodes, which had been previously treated with unidirectionally buffed poly- imide alignment layers.The electrical contacts were made directly to the internal surfaces. The cells were filled by capillary action while the materials were in their isotropic liquid states. Reasonably good alignment for optical and polarization studies was achieved by slowly cooling each material from the isotropic liquid into their respective smectic states. Cooling rates were of the order of ca. 0.2 "C min-l. Once filled, the cell was connected to an ac frequency generator (6 or 15 V peak to peak and 60 Hz), a dual trace oscilloscope and a Diamant bridge.lg Tilt angles (6) in the smectic C* phase were obtained by measuring the angle (26) between the optical extinction positions for the ferroelectrically switched states observed in the polarizing microscope.The ferroelectric polarization (P,) was measured using the Diamant bridge, while observing a characteristic hysteresis loop for switching at 60 Hz; polarization values are quoted in nC cm-2. A sensitive modulation technique using a photoelastic modulator was used to make precision optical rotation measurements in the presence of linear birefringence. Compound 22 was studied in a 100 pm thick glass cell with planar aligned walls over the temperature range from ca. 80 to ca. 110 "C.The full details of the technique will be described elsewhere. Before taking each measurement, the sample was 749 kept at constant temperature (i.e.kO.1 "C) for ca. 5 min. Circular dichroism (CD) measurements were carried out using the spectropolarimeter JASCO J-4OCS. Compound 22 was studied in a quartz cell of thickness 18 pm with homeotrop- ically aligned walls. The sample was maintained at constant temperature (ie. k0.2 "C) for ca. 5 min before running a spectrum. l-Bromo-3-fluoro-4-tetradecyloxybenzene,6 1-Bromotetradecane (9.15 g, 33.0 mmol) in butanone (20 ml) was added dropwise to a stirred, refluxing mixture of 4-bromo-2-fluorophenol (4) (6.02 g, 31.5 mmol), potassium carbonate (5.62 g; 40.7 mmol) and butanone (80ml). The resulting reaction mixture was heated under reflux for a further 20 h, TLC showed virtually complete reaction.The cooled reaction mixture was then filtered to remove the excess of potassium carbonate and precipitated potassium bromide. The filtrate was washed with 5% v/v sodium hydroxide (2 x 50 ml) then water (50 ml) and the organic layer was dried (MgSO,), filtered and the solvent evaporated off under reduced pressure to give a colourless oil, which was purified by column chromatography [silica gel; 1: 1 dichloro-methane-light petroleum (bp 40-60 "C)] to give a colourless liquid. This was then dried in U~CUO(P,O,, 0.20 mmHg, room temperature, 5 h). Yield, 11.82 g, 97%; mp 28-29 "C; 6, (270 MHz, solvent CDCl,, standard TMS); 0.87 (3 H, m), 1.35 (22 H, mI, 1.80 (2 H, quintet), 4.00 (2 H, t), 6.82 (1 H, t), 7.20 (2 13, m). v/cm-' (KBr disc): 2930, 2860, 1505, 1470, 1305, 1265. 1210, 1130, 875, 860, 800, 635 and 570.m/z 388 (M', 3%), 386 (M+, 5%), 191 (98%), 190 (loo%), 135 (lo%), 125 (5O/0), 111 (?&yo), 98 (21%). l-Bromo-2-~uoro-4-tetradecyloxybenzene,7 This was prepared using a similar method to that described for compound 6. Quantities used: compound 5 (4.03 g, 21.1 mmol), 1-bromotetradecane (6.41 g, 23.1 mmol), potass- ium carbonate (5.59 g, 40.5 mmol) and butanone (115 nil). Yield, 7.48 g, 92%; mp 30-31 "C; 6H (270 MHz, sdvent CDC13, standard TMS): 0.85 (3 H, t), 1.35 (22 H, m). 1.75 (2 H, quintet), 3.90 (2 H, t), 6.60 (1 H, d), 6.70 (1 H, d), 7.40 (1 H, t). v/cm-' (KBr disc): 2920, 2850, 1605, 1580, 1485, 1465,1320,1290,1170,1140, 1020,830 and 640. m/z 388 (M', 20%), 386 (M', 20?40), 191 (29%), 189 (28%), 134 (200/1,).3-Fluoro- 1-( 3-hydroxy-3-methylbut- l-ynyl)-4-tetradecyl oxybenzene, 8 A stream of dry nitrogen was bubbled through a stirred, dark- green mixture of compound 6 (5.40 g, 13.9 mmol), pal- ladium(0) tetrakis( triphenylphosphine) ( 1.05 g, 0.91 minol), copper(1) iodide (0.13 g, 0.68 mmol) and dry diisopropyl- amine (30ml) for a period of 10min. A solution of 3-methylbut-1-yn-3-01 (2.54 g, 30.2 mmol) in dry diisopro- pylamine ( 10 ml) was added dropwise at room temperature; the reaction mixture turned deep orange-brown. The reaction was then heated for 4 h under gentle reflux and under nitrogen. The cooled reaction was filtered through a pad of Hyflo- supercel II(BDH) and water (50 ml) added to the filtrate.The crude product was then extracted into diethyl ether (3 x 50 ml); the combined ether extracts were washed with brine (50 ml), dried (MgS04), filtered and the solvent was evaporated off to give a brown oil. The crude product was then purified by flash chromatography [fine mesh silica gel; 10% v/v ethyl acetate in light petroleum (bp 40-60 "C)] to give a white solid, which was recrystallized (cyclohexane) and dried in uucuo ( P205,0.40 mmHg, room temperature, 24 h). Yield, 4.16 g, 76%; mp 45-46 "C; 6, (270 MHz, CDCl,, standard TMS): 0.80 (3 H, t), 1.30 (22 H, m), 1.60 (6 H, s), 750 1.80(2H,quintet), 1.90(1 H, s, broad), 4.00 (2 H, t), 6.85 (1 H, t), 7.12 (2 H, d). v/cm-' (KBr disc): 3400,2990, 2960, 2920, 2850, 1615, 1570, 1520, 1470, 1310,1275, 1240, 1160, 1120, 950, 880, 815, 720 and 615.m/z 390 (M', lo%), 375 (5%), 194 (30%), 179 (100%).2-Fluoro-1-ynyl)-4-tetradecyl-1-(3-hydroxy-3-methylbut-oxybenzene,9 This compound was prepared using a similar method to that described for compound 8. Quantities used: compound 7 (5.40g, 13.95mmol), palladium (0) tetrakis(tripheny1phos-phine) (1.03g, 0.89mmol), copper(1) iodide (0.14g, 0.74mmol), 3-methylbut-1-yn-3-01 (2.55g, 30.4mmol) and diisopropylamine (50ml). The crude product was purified by flash chromatography [fine mesh silica gel; 10% v/v ethyl acetate in light petroleum (bp 40-60 "C)] to give a yellow solid, which was recrystallized from cyclohexane and dried in uucuo (P,O,, 0.20mmHg, room temperature, 18h).Yield, 2.34g, 43%, mp 44-45 "C; SH (270MHz, solvent CDCl,, standard TMS): 0.90(3 H, t), 1.40(22H, m), 1.65 (6H, s), 1.75 (2 H, quintet), 2.00(1 H, s), 3.95 (2 H,t), 6.60 (2H, m), 7.40(1 H, t). v/cm-' (KBr disc): 3460, 2920, 2850, 1620, 1500, 1465, 1285, 1230, 1160, 1115,960, 900 and 835. m/z 390(M+, 24%), 375 (loo%), 262(45%), 183 (23%), 179 (IOOYo),71 (45%). l-Ethynyl-3-Juoro-4-tetradecyloxybenzene,10 Compound 8 (4.06g, 10.4mmol), potassium hydroxide (0.62g, 11.1 mmol) and toluene (100ml) were stirred and heated under reflux and under nitrogen for 2h (TLC showed complete reaction). The acetone and toluene azeotrope was removed periodically using a Dean and Stark receiver and replaced with an equal volume of toluene.The cooled reaction mixture was poured into water (100ml) and the organic phase separated off. The aqueous phase was then washed with diethyl ether (3x 50ml) and recombined with the organic layer before being washed with brine (50ml), dried (MgSO,), filtered and the solvent evaporated off to give an orange oil. The crude product was purified by flash chromatography [fine mesh silica gel; 9: 1 dichloromethane-light petroleum (bp 40-60"C)]. The yellow solid obtained was dried in uacuo ( P205,0.35mmHg, room temperature, 6h). Yield, 3.08g, 89%; mp 34-35 "C; SH (270MHz, solvent CDCl,, standard TMS): 0.85(3 H, t), 1.35(22H, m), 1.83 (2H, quintet), 3.00(1 H, s), 4.03 (2 H,t), 6.85(1 H, t), 7.20 (2H, d). v/cm-' (KBr disc): 3300,2960, 2920, 2850, 1615, 1575, 1470, 1310, 1225, 1110,945, 880, 815, 720,625 and 600.WI/Z 332(M', 17%), 136 (loo%), 83(8%), 69 (19%), 55(45%). l-Ethynyl-2-$uoro-4-tetradecyloxybenzene,11 This compound was prepared using a similar method to that described for compound 10. Quantities used: compound 9 (2.35g, 6.02mmol), potassium hydroxide (0.39g, 6.95mmol) and toluene (70 ml). The crude product was purified by flash chromatography (fine mesh silica gel; dichloromethane). The yellow solid obtained was recrystallized (cyclohexane) and dried in uucuo (P,O,, 0.1mmHg, 40"C, 3h). Yield, 1.28g, 64%; mp 41-42"C; 6, (270MHz, solvent CDCl,, standard TMS): 0.85 (3 H, t), 1.30 (22H, m), 1.75 (2 H, quintet), 3.20(1 H, s), 3.95 (2 H, t), 6.65 (2 H, m), 7.35 (1 H, t).v/cm-l (KBr disc): 3300,2950, 2920, 2850, 1610, 1565,1500,1470, 1290, 1160, 1110,1010,850, 825, 720, 675 and 630.m/z 332(M+,40%), 165(15%), 136(100%). ( 3-Fluoro-4-tetradecyloxypheny1)propiolic Acid, 12 Butyllithium (3.8ml, 2.5mol I-' in hexanes) was added drop- wise to a stirred, cooled (ca. -10 to -7 "C) solution of compound 10 (3.03g, 9.1mmol) in dry tetrahydrofuran J. MATER. CHEM., 1994, VOL. 4 (35ml) under nitrogen. The resulting solution was kept at ca. -7"C for a further 10min before being poured onto a stirred slurry of crushed solid C02 and dry tetrahydrofuran (5ml) and allowed to warm to room temperature overnight. The solution was acidified with concentrated hydrochloric acid and water (100ml) was added. The product was extracted into diethyl ether (3x 50ml), the combined ether extracts were then washed with brine (50ml), dried (MgSO,), filtered and the solvent evaporated off to give a yellow solid.The product was purified by flash chromatography [fine mesh silica gel; dichloromethane (initially) and 9: 1 dichloro-methane-methanol (finally)], recrystallized (cyclohexane-ethyl acetate), washed [light petroleum (bp 40-60"C)] and dried in uucuo ( Pz05,0.30mmHg, room temperature, 10h). Yield, 2.47g, 72%; mp 91-93 "C; SH (270MHz, solvent CDCl,, standard TMS): 0.85(3 H, t), 1.25 (22 H,m), 1.85 (2H, quintet), 4.05(2H, t), 6.93(1 H, t), 7.35 (2 H, m), no carboxylic proton detected. v/cm-' (KBr disc): 2920, 2850, 2205, 1685, 1610, 1455, 1310,1275, 1250, 1230, 1125, 870, 810 and 610.m/z 376(M', trace), 136(100%).(2-Fluoro-4-tetradecyloxyphenyl)propiolicAcid,13 This compound was prepared using a similar method to that described for compound 12. Quantities used: compound 11 (4.50g, 13.6mmol), butyllithium (1.4ml, 10.0mol 1-' in hex- anes) and dry tetrahydrofuran (60ml). The crude product was purified by flash chromatography [fine mesh silica gel; dichloromethane (initially) and 9:1 dichloromethane-meth-anol (finally)] to give a yellow solid, which was recrystallized (cyclohexane) and dried in uucuo (P205,0.20mmHg, 40"C, 12h). Yield, 3.58g, 70%; mp 98-101 "C; 6, (270MHz, solvent CDCl,, standard TMS): 0.90(3 H, t), 1.30(22H, m), 1.65 (2H, m), 3.90 (2 H, t), 6.55 (2 H, t), 7.45(1 H, t), no carboxyl proton detected.v/cm-' (KBr disc): 3440, 2920, 2850, 2200, 1690, 1615, 1505,1465, 1235, 1215, 1170, 1115,850and 610. m/z 376 (M+, trace), 332 (M+-CO,, 6O/h), 253 (5%), 180 (16%), 136 (100%). 4'-Hydroxybiphenyl-4-carboxylicAcid,15 A mixture of concentrated sulfuric acid ( 115ml) and water (115ml) was added dropwise to a stirred suspension of 4-cyano-4'-hydroxybiphenyl,14 (25.62g, 131.4mmol), in gla- cial acetic acid (400ml). The mixture was heated under reflux for 48h; the cooled reaction mixture was then poured into water (600ml) with stirring and the white precipitate filtered off. The aqueous filtrate was washed with diethyl ether (4x 70ml); the combined extracts were then washed with water (50ml), dried (MgSO,).filtered and the solvent evapor- ated off to yield a white solid. Both crops of product wert combined, dried thoroughly and recrystallized (glacial acetic acid) and dried in uucuo (P,O,, 0.30mmHg, 50"C, 5h). Yield, 23.49g, 84%; mp 197-200"C; SH(270MHz, solvent CDCl,, standard TMS): 6.95 (2 H, d), 7.50(2 H, d), 7.60 (2H, d), 8.05 (2 H, d), 9.15(1 H, s, broad), no carboxylic proton was observed. v/cm-' (KBr disc): 3400, 1680, 1605, 1590, 1425, 1385, 1300,1195, 830, 770 and 720.m/z 214(Mi,loo%), 197 (48%), 168 (7%), 139 (15%), 115 (12%). 4'-Methoxycarbonyloxybiphenyl-4-carboxylicAcid,16 Compound 15 (15.02g, 70.2mmol) was added slowly to a vigorously stirred solution of sodium hydroxide (8.15g, 203.8mmol) in water (300ml) at -4 C. Methyl chlorc- formate (10.82g, 114.5mmol) was added dropwise and the temperature maintained at 0 "C.The resulting white slurry was then stirred under these conditions for a further 4h. The pH was then adjusted to 5 using concentrated hydrochloric acid solution (1 : 1,concentrated HC1-water) and the volumiii- J. MATER. CHEM., 1994, VOL. 4 ous white precipitate filtered off and washed with water. The white solid was dried and recrystallized (glacial acetic acid) and dried again in vucuo (P,O,, 0.20 mmHg, 40 "C, 4 h). Yield, 14.83 g, 78%; mp 256-260 "C, lit. transition temp. K 233-235 N > 300 ISO ("C)20;6, (270 MHz, solvent CDCl,, standard TMS): 3.90 (3 H, s), 7.37 (2 H, d), 7.60 (4 H, m), 8.10 (2 H, d), no carboxylic proton detected.v/cm-l (KBr disc): 2970, 2840, 2680, 2525, 1870, 1675, 1610, 1430, 1325, 1295, 1220, 1185, 1065, 1005,945, 930, 870, 825 and 775. m/z 272 (M', 60%), 228 (loo%), 213 (82%), 184 (43%), 138 (28%), 114 (13%). (R)-(-)-l-Methylheptyl-4'-Methoxycarbonyloxybiphenyl-4-carboxylate, 17 Triphenylphosphine (6.80 g, 25.9 mmol) and (S)-octan-2-01 (5.03 g, 38.6 mmol) in dry tetrahydrofuran (35 ml) were added dropwise to a stirred mixture of compound 16 (7.03 g, 25.8 mmol) and diethyl azodicarboxylate (4.50 g, 25.9 mmol) in dry tetrahydrofuran (40 ml) under nitrogen. The reaction was stirred for a further 24 h at room temperature; TLC indicated that the reaction had reached completion. The white precipitate was removed by filtration through a pad of Hyflo- supercel, the filtrate was then washed with brine (50 ml), dried (MgSO,), filtered and evaporated to give a white solid. This was purified by flash chromatography [fine mesh silica gel; 5% v/v ethyl acetate in light petroleum (bp 40-60 "C)] to give a colourless oil, which was dried in uucuo (P,OS, 0.20 mmHg, room temperature, 5 h).Yield, 8.06 g, 81%; 6, (270 MHz, solvent CDCl,, standard TMS): 0.85 (3 H, t), 1.25 (13 H, m), 3.90 (3 H, s), 5.16 (1 H, sextet), 7.25 (2 H, d), 7.60 (4 H, d), 8.10 (2 H, d). v/cm-' (KBr disc): 2960, 2930, 2860, 1760, 1720, 1610, 1500, 1440, 1380, 1265, 1185, 1110, 1010,945, 830 and 775. m/z 384 (M', 27%), 272 (lOOYo), 255 (32%), 228 (55%), 213 (~OYO), 139 (20%). = -12.5" (0.0409 g ml-', CHCl,). (S)-(+)-1-Methylheptyl4'-Methoxycarbonyloxybiphenyl-4-carboxylate, 18 This compound was prepared using a similar method to that described for compound 17.Quantities used: triphenylphos- phine (6.74 g, 25.7 mmol), (R)-octan-2-01 (5.03 g, 38.7 mmol), compound 16 (7.00 g, 25.7 mmol), diethyl azodicarboxylate (4.50 g, 25.9 mmol) and dry tetrahydrofuran (110 ml). The crude product was purified by flash chromatography [fine mesh silica gel; 5% v/v ethyl acetate in light petroleum (bp 40-60 "C)] to give a colourless liquid, which was dried in uucuo (P,O,, 0.20 mmHg, room temperature, 5 h). Yield, 7.98 g, 81%; SH (270 MHz, solvent CDCl,, standard TMS): 0.85 (3 H, t), 1.35 (13 H, m), 3.95 (3 H, s), 5.15 (1 H, sextet), 7.25 (2 H, d), 7.60 (4 H, d), 8.10 (2 H, d).v/cm-l (KBr disc): 2960, 2930, 2860, 1765, 1715, 1610, 1440, 1260, 11 10,1005,930,830 and 770. m/z 384 (M+,23YO),272 ( loo%), 255 (38%), 228 (64%), 213 (23%), 196 (lo%), 185 (14%), 168 (15%), 152 (12%), 139 (20%). +30.6" (0.0425 g ml -';CHC1,) (R)-(-)-l-Methylheptyl4-Hydroxybiphenyl-4-carboxylate,19 A solution of compound 17 (8.06 g, 20.9 mmol) in ethanol (30 ml) was added dropwise to a stirred mixture of ammonia (105 ml, 35% solution) and ethanol (180 ml) at room tempera- ture. TLC analysis showed complete reaction after a period of 30 min. The reaction was then poured into water (300 ml) and cooled in solid CO,; the precipitated product was then filtered off, dried and recrystallized (cyclohexane-ethyl acetate, 4: 1). The colourless crystals were then dried in vacuo (P,O,, 0.10 mmHg, 40 "C, 5 h).Yield, 5.52 g, 81%; mp 84-87 "C; 6, (270 MHz, solvent CDCl,, standard TMS): 0.85 (3 H, t), 1.35 (11 H, m), 1.65 751 (2 H, m), 5.00 (1 H, sextet), 5.20 (1 H, sextet), 6.95 (2 H, d), 7.55 (2 H, d), 7.60 (2 H, d), 8.10 (2 H, d). v/cm-' (KBr disc): 3350, 2960, 2910, 2840, 1680, 1600, 1585, 1560, 1530, 1495, 1350, 1290, 1270, 1195, 1110, 1060, 920, 830 and 770. VZ/Z 326 (M', 26%), 214 (loo%), 197 (21Yo), := -35.1" (0.0342 g ml-'; CHCl,). (S)-(+)-1-Methylheptyl4'-Hydroxybiphenyl-4-carbox).Iate,20 This compound was prepared using a similar method to that described for compound 19. Quantities used: compound 18 (7.51 g, 19.6 mmol), ethanol (210 ml) and ammonia (105 ml, 35% solution).Yield, 4.89g, 77%; mp 87-89 "C; 6, (270MHz. solvent CDCl,, standard TMS): 0.85 (3 H, t), 1.30 (11 H, m), 1.70 (2 H, m), 5.17 (1 H, sextet), 5.45 (1 H, s, broad), 6.95 (2 H, d), 7.55 (2 H, d), 7.60 (2 H, d), 8.20 (2 H, d). v/cm-' (KBr disc): 3360, 2960, 2920, 2850, 1685, 1605, 1590, 1535, 1500, 1407, 1300, 1280, 1200, 1115, 1060, 835, 770 and 730. m/z 326 M', 12%), 214 (100%), 197 (20%). [a]ET=7 +38.8" (0.0232 g m1-I; CHC1,) (R)-(-)-1-Methylheptyl4-[( 3-Fluoro-4-tetradecyloxyphenyl) propioloyloxy] biphenyl-4-carboxylate, 21 Dicyclohexylcarbodiimide (0.35 g, 1.7 mmol) was added to a stirred mixture of compound 12 (0.59 g, 1.6 mmol), compound 19 (0.50 g, 1.5 mmol), 4-N,N-dimethylaminopyridine(0.04 g, 0.3 mmol) and dry dichloromethane (10 ml) at room tempera- ture.The reaction mixture was stirred for a further 18 h before the precipitated 'urea' was removed by filtration through Hyflo-supercel and the filtrate washed successively with water (50 ml), 5% v/v acetic acid solution (2 x 50 ml) and water (50 ml). The organic phase was then dried (MgSO,), filtered and the solvent evaporated off to yield a brown solid. The crude product was purified twice by flash chromatography [fine mesh silica gel; 5% v/v ethyl acetate in light petroleum (bp 40-60 "C) and then fine mesh silica gel; 9: 1 dichloro-methane-light petroleum (bp 40-60 "C)].The colourless solid was recrystallized (cyclohexane x 3) and dried in uucuo(P,05, 0.10 mmHg, 40 "C, 6 h). Yield, 0.15 g, 14%; 6, (270 MHz, solvent CDCl,, standard TMS): 0.90 (6 H, t), 1.30 (33 H, m), 1.85 (4 H, quintet), 4.05 (2 H, t), 5.15 (1 H, sextet), 6.95 (1 H, t), 7.35 (4 H, m), 7.65 (4 H, m), 8.20 (2 H, d).v/cm-l (KBr disc): 2920, 2850, 2220, 1735, 1720, 1710, 1610, 1510, 1280, 1235, 1205, 1180. 1115, 880, 815 and 770. m/z 684 (M+, trace), 359 (55%), 345 I 15%), 214 (14%), 163 (loo%), 139 (11%). [a]:*=--21.1" (0.0095 g ml-'; CHC1,). Calc. for C4,H5,F0,: C, 77.16; H, 8.38. Found: C, 77.09; H, 8.27. (S)-(+)-l-Methylheptyl4-[( 3-Fluoro-4-tetradecyloxypheriy1)-propioloyloxy] biphenyl-4-carboxylate, 22 This compound was prepared using a similar method to that described for compound 21. Quantities used: dicyclohexylcar- bodiimide (0.37 g, 1.8 mmol), compound 12 (0.58 g, 1.5 mmol), compound 20 (0.50 g, 1.5 mmol), 4-N,N-dimethylaminopyrid-ine (0.03 g, 0.25 mmol) and dry dichloromethane (10 ml 1.The crude product was purified twice by flash chromatography [fine mesh silica gel; 5% v/v ethyl acetate in light petroleum (bp 40-60 "C) and then fine mesh silica gel; 9: 1 dichloro-methane-light petroleum (bp 40-60 "C)]. The colourless solid was recrystallized (cyclohexane) and dried in uucuo ( P,05, 0.30 mmHg, 40 "C, 17 h). Yield, 0.18 g, 17%; aH(270 MHz, solvent CDCl,, standard TMS): 0.90 (6 H, t), 1.30 (33 H, m), 1.75 (4 H, m), 4.05 (2 H, t), 5.20 (1 H, sextet), 6.95 (1 H, t), 7.30 (4H, m). 7.65 (4 H, m), 8.10 (2 H, d). v/cm-' (KBr disc): 2910, 2850, 2210, 1735, 1720, 1705, 1610, 1510, 1280, 1235, 1205, 1180, 1115, 880, 815 and 770.m/z 684 (M', trace), 359 (70%), 335 J. MATER. CHEM.. 1994,VOL.4 (9%), 214 (lo%), 163 (loo%), 139 (8%). [a]kT= +26.9" 1185, 1110, 1005, 920, 850, 815, 765, 730 and 605. rn/z 684 (0.0112 g ml-'; CHCl,). Calc. for C44H,7F05: C, 77.16; (M', lo%), 359 (lOOo/o), 335 (9%), 214 (lo%), 163 (70%). H, 8.38. Found: C, 77.31; H, 8.13. = +25.7" (0.0257 g ml-'; CHC1,). Calc. for C4,H,,F05: C, 77.16; H, 8.38. Found: C, 77.04; H, 8.32. (R)-(-)-l-Methylheptyl4'-[( 2-E;luoro-4-tetradecyloxyphenyl)-propioloyloxy] biphenyl-4-carboxylate, 23 This compound was prepared using a similar method to that Results described for compound 21. Quantities used: dicyclohexylcar- bodiimide (0.78 g, 3.8 mmol), compound 13 (1.09 g, 2.9 mmol), compound 19 (0.94 g, 2.8 mmol), 4-N,N-dimethylaminopyrid-ine (0.11 g, 0.9 mmol).The crude product was purified twice by flash chromatography [fine mesh silica gel; 4% v/v ethyl acetate in light petroleum (bp 40-60 "C) and then fine mesh silica gel; 5% v/v light petroleum (bp 40-60% "C) in dichloro- methane]. The off-white solid was recrystallized [ethanol (-72 "C) x 2, cyclohexane x 11 and dried in uacuo (P,05, 0.3 mmHg, 50 "C, 5 h). Yield, 0.80 g, 41YO;hH (270 MHz, solvent CDCl,, standard TMS): 0.88 (6 H, t), 1.27 (33 H, m), 1.80 (4 H, m), 3.98 (2 H, t), 5.18 (1 H, sextet), 6.70 (2 H, t), 7.29 (2 H, d), 7.55 (1 H, t), 7.64 (4 H, m), 8.12 (2 H, d). v/cm-' (KBr disc): 3420, 2960, 2920, 2850, 2200, 1710, 1615, 1510, 1465, 1300, 1280, 1230, 1190,1115,1000,860, 830 and 775.rn/z 684 (M+, trace), 359 (loo%), 214 (20?40), 163 (95%). [cx]:~= -13.8" (0.0217 g m1-I; CHC1,). Calc. for C4,H,,F05: C, 77.16; H, 8.38. Found: C, 77.09; H, 8.23. (S)-(+)-1-Methylheptyl4-[( 2-Fluoro-4-tetradecyloxyphenyl)-propioloyloxy] biphenyl-4-carboxyEate, 24 This compound was prepared using a similar method to that described for compound 21. Quantities used: dicyclohexyl- carbodiimide (0.60 g, 2.9 mmol), compound 13 (0.89 g, 2.36 mmol), compound 20 (0.73 g, 2.2 mmol), 4-N,N-dimethylaminopyridine (0.10 g, 0.8 mmol) and dry dichloro- methane (20ml). The crude product was purified twice by flash chromatography [fine mesh silica gel; 4% v/v ethyl acetate in light petroleum (bp 40-60 "C)and then fine mesh silica gel; 5% v/v light petroleum (bp 40-60 "C) in dichloro- methane].The colourless solid obtained was recrystallized [ethanol (-72 "C) x 2, cyclohexane x 11 and dried in ~UECMO (P,O,, 0.3 mmHg, 40 "C, 6 h). Yield, 0.46 g, 30%; hH (270 MHz, solvent CDCl,, standard TMS): 0.88 (6 H, t), 1.32 (33 H, m), 1.79 (4 H, quintet), 3.98 (2 H, t), 5.18 (1 H, sextet), 6.69 (2 H, m), 7.30 (2 H, d), 7.52 (1 H, t), 7.63 (2 H, d), 7.66 (2 H, d), 8.15 (2 H, d). v/cm-' (KBr disc): 2920, 2850, 2210, 1710, 1615, 1510, 1300, 1280, Optical Microscopy The phase sequences for the enantiomeric compounds 21-24 in addition to their racemates 25 and 26, as determined by optical microscopy, are listed in Table 1. On cooling from the isotropic liquid, the 3-fluoro enanti-omers (21 and 22) both form similar iridescent planar smectic C* textures at 75.3 and 75.5 "C, respectively, before crystalliz- ation occurs at much lower temperatures.This behaviour differs noticeably with the racemate, compound 25, which cools from the isotropic liquid to a focal-conic smectic. A texture (81.5 "C) before giving way directly to a Grandjean smectic C texture at 79.3 "C. No TGB A* phase behaviour was observed for the 3-fluoro enantiomers (21 and 22). It is interesting to note that the enantiomers do not display identical transition temperatures and they also have noticeably lower transition temperatures than the racemate. The 2-fluoro enantiomers (23 and 24), on cooling from the isotropic liquid, form the characteristic verrnis (filament) tex- ture of the TGB A* phase, before the filaments coalesce to give a platelet texture.34 Examples of the verrnis texture of the TGB A* phase is given in Fig.1. Further cooling resulted in the formation of homeotropic regions bordered by areas of focal-conic smectic A texture, which in turn gave a conventional schlieren smectic C* texture before finally recrystallizing. Can0 wedge cells were employed to investigate further the TGB A*-Sf transition in compounds 23 and 24, the surface orienting coating coupled with the separation of the glass plates giving improved textures of the mesophases. On heating, at the transition from S,* to TGB A*, the focal-conic fans of the smectic A texture develop transition lines and the texture gives way to the texture of a TGB A* phase, which is characterized by zones of filaments and Grandjean plane disclination lines.Rotation of the upper polarizer of the microscope clearly showed a colour dispersion for white light caused by the helicity and hence the optically active nature of the phase (unlike the lower smectic A* phase which showed no dispersion effect). The presence of Grandjean disclination Table 1 Transition temperatures for the (R)-, (S)-and (R,S)-l-methylheptyl 4'-[( 2-or 3-fluoro-4-tetradecyloxyphenyl)propioloyloxy] biphenyl-4-carboxylates compound number 21 22 23 24 25 26 X Y absolute transition temperatures/"C" X Y configuration Is0 TGB A* SAP** sc*/sc Kb F F H H F H H H F F H F (R)-(0 (R)-(SI-(R,s)-c (R,W .. . . . . 75.3 75.5 85.8 87.1 81.5 87.7 . . 80.8 82.9 . . 72.6 73.5 79.3 76.8 49.6 62.7 33.9 39.2 44.8 33.6 . . . . . . "Recorded at a cooling rate of 2"C min-'. bRecrystallization temperatures determined by thermal optical microscopy. 'Racemic samples were prepared by weighing equal amounts of (R)-and (S)-enantiomers into clean glass vials and mixing whilst in the isotropic state. J. MATER. CHEM., 1994, VOL. 4 Fig. 1 TGB A* filaments in an isotropic background coalescing into a TGB A* platelet texture (compound 23) at 85.6 "C(100x) lines confirms the presence of a helical structure in the TGB A* phase; however, the absence of any effect that is related to helicity or optical activity in the smectic A* phase confirms the structure of this phase as being non-helical and conse- quently proves the presence of a TGB A* to Sz transition. Once again the racemate (26) displays different mesogenic behaviour to that shown by the enantiomers, and here the TGB A*-ST-S,X sequence is replaced by an SA-Sc sequence; the phase thermal stabilities of the racemate (26) are slightly higher than the phases shown by the enantiomers (23 and 24).Differential Thermal Analysis All of the enantiomers (21-24) and the racemates (25 and 26) were analysed by DSC at heating and cooling rates of 5 and 10 'C min-'; the transition temperatures and enthalpies are given in Table 2. The 3-flUOrO enantiomers, 21 and 22, clearly show sharp isotropic (Iso)liquid (Liq)-smectic C* transitions, as first observed by optical microscopy. However, both of the cooling thermograms of 21 and 22 show bizarre, diffuse transitions above the isotropic liquid-smectic C* peak, pos- sibly indicating some event taking place in the isotropic liquid.If this behaviour is compared with the racemate (25),in which no diffuse peak is observed (only an Iso-SA-S~ phase sequence), the 1s0-S~transition occurs at essentially the same temperature as the diffuse peak in thermograms for com-pounds 21 and 22. This is clearly demonstrated in Fig. 2, which depicts superimposed cooling thermograms of 21, 22 and 25 (all recorded at a cooling rate of 5°C min-l). WII I I I I I I1 I0 20 30 40 50 60 70 80 90 1 77°C Fig.2 DSC thermograms of the (R)-, (S)-and (R,S)-forms of 1-methylheptyl 4-[(3-Auoro-4-tetradecyloxyphenyl)propioioyloxyJ -4-carboxylates [compounds 21(b), 22(4 and 25(c) recorded at a cooling rate of 5 Oc min-'1 Summation of the enthalpies of the isotropic liquid-qmectic C* and liquid-liquid transition for 21 and 22 gives values that are roughly equivalent to the isotropic liquid-smectic A transition in the racemate (25).The 2-fluoro enantiomers (23 and 24) give normal thermo- grams in that no diffuse liquid-liquid transitions are observed; here only the Iso-TGB A* and S2-S; transitions arc seen, the S2-S: transition being second order in nature (AH=ca. 0.14 kJ mol-I). The TGB A*-S,* transition was not detected by calorimetry.However, as this transition simply corres'ponds to the expulsion of the twist from the phase, it would be expected to be accompanied by a relatively small cnergy change. The racemate (26) again shows similar phase events with enthalpies comparable to those of the enantiomers. Spontaneous Polarization and Tilt-angle Studies The spontaneous polarization (P,) of compounds 21-24 were measured as a function of temperature on cooling from the Curie point (Ts,* or TA*<*).The results for the (R)-enanti- omers 21 and 23 are shown in Fig. 3, and were recorded at 6 V (open symbols) and 15 V (filled symbols) at 60 €IT. The magnitude of the spontaneous polarization of the 3-flUOrO compound (21) was found to be slightly more than double that obtained for the 2-fluoro compound (23), i.e.for 21. P, = ca. 90 nC cmP2 and for 23, P,= ca. 40 nC cm-2 at 4 "C helow the Curie point. Similar behaviour was noted for the (S)-enantiomers, compounds 22 and 24. The factors determining the magnitude of P, are the tilt angle, transverse molecular dipole (pt) and the chirality-induced broken symmetry for rotation about the long molecular axis. In terms of a simple mean-field theory2' P, can be written as: P, = Po sin 0 =Npt(cos 4) where (cos 4) is a ferroelectric order parameter and N is the number density. Quadrupolar ordering of molecules about their long axes in tilted smectic phases is driven by the tilt angle, which generates macroscopic biaxiality, and in dis- cussing the effects of molecular structure it is better to consider the reduced polarization Po.Chirality in tilted smectic phases causes non-centrosymmetric phase symmetry perpendicular to the tilt plane, thus a molecule rotating about its tilted long 754 J. MATER. CHEM., 1994, VOL. 4 Table 2 DSC data for the (R)-,(S)-and (R,S)-1-methylheptyl 4'-[( 2-or 3-fluoro-4-tetradecyloxyphenyl)propioloyloxy]biphenyl-4-carboxylates transition temperatures/"C and enthalpies/kJ mol-la compoundnumber absolute configuration Liq-Liq' Is0 SAP2 s,*/sc K 21 22 (R)-(9- 79.7 79.5 [1.631 [1.403 74.5 74.3 [1.231 [1.11] 36.7 46.0 C22.181 [24.641 . . 23 (R)- C5.411 84.9 [0.17] 75.3 [34.521 33.9 . 24 25 26 (S)-(R,S)-(R,S)- [5.641 [3.521 [5.921 85.2 79.5 86.2 c0.121 [0.461 [0.061 75.7 77.6 75.4 [34.1 51 [22.461 [27.793 33.6 33.7 23.7 .. . "Data recorded at a cooling rate of 5 "C min-l. 'These values were taken from maximum peak temperatures not as onset temperatures. -10 -8 -6 -4 -2 0 reduced temperaturel'c Fig.3 Magnitude of the spontaneous polarization plotted as a function of reduced temperature of (R)-1-methylheptyl 4'-[( 3-fluoro and 2-fluoro-4-tetradecyloxyphenylj propioloyloxy] biphenyl-4-car-boxylates [compounds (a) 21 and (bj 23, respectively] axis experiences a potential of dipolar symmetry. This affects both the lateral steric dipole of the molecule and the transverse electric dipole responsible for P,. A maximal contribution to P, is achieved if the steric dipole is parallel to the electric dipole.Switching studies performed in polyimide aligned cells with a 9V dc source revealed that both the (R)-enantiomers, 21 and 23, have positive polarizations [Ps(+)] and conversely the (S)-enantiomers (22 and 24) have negative polarizations [P,(-)]. The electronically switched optical tilt angle charac-teristics for the (R)-enantiomers of both systems are given in Fig. 4. Here the 2-fluoro (R)-enantiomer (23), shows a much 030-0. 25 --* ._ 20 -c Q 15-0 oooooo 00 1 .10 . , . , . , 0, . , . , . , , 0,-16 -12 -8 -4 0 reduced temperature/"C larger optical tilt angle that appears to saturate in the region of ca. 30";in contrast, the 3-flUOrO (R)-enantiomer (21) satu-rates at a value of ca. 15" which, surprisingly, is a very low value for a smectic phase cooled directly from the isotropic liquid.This finding is the opposite to the trend encountered for the polarization properties. Standardizing the spontaneous polarization with respect to tilt angle (P,/Q)for the two systems shows that the 3-fluoro system's effective spontaneous polarization is approximately four times larger in comparison with the 2-fluoro system. This is shown in Fig. 5; the open and filled symbols denote data for the 3-fluoro and 2-fluoro compounds, respectively, at 15 V (squares) and 6 V (circles), respectively. Neglecting short-range interactions, we assume that the chiral centre in the 3-(21) and 2-(23) fluoro compounds causes a similar broken rotational symmetry potential for both materials.The average transverse electric dipole moment of 21 may be greater than that of 23 because of hindered rotation of the terminal C14H290-group caused by the 3-substituted fluorine atom; this might also lead to an increased steric dipole. Biaxial ordering in the tilted smectic C phase will reduce free rotation about the triple bond in the core of the molecule and 2-fluoro substitution increases the bulk of the molecule core. In order to accommodate this increased bulk at a similar packing density, the tilt angle for 23 increases in comparison with that for 21. The P, of 23 is less than that of 21, so that either there is a compensation of transverse dipole components in the core of 23, which is absent in the 3-flUOrO-materials, or the angle between the 9, Fig.4 Optical tilt angle measured as a function of reduced tempera-Fig. 5Standardized spontaneous polarization (Ps/O)as a function ture of (R)-l-methylheptyl4'-[(2-and 3-fluoro-4-tetradecyloxyphenyl) of reduced temperature of the (R)-1-methylheptyl 4-[(2-and 3-propioloyloxy] biphenyl-4-carboxylates [compounds 23 ( 0)and 21 fluoro-4-tetradecyloxyphenyl)propioloyloxy]biphenyl-4-carboxylates ( 0), respectively] [compounds (a) 23 and (b)21, respectively] J. MATER. CHEM., 1994, VOL. 4 electric dipole and the steric dipole is greater for 23 than for 21. Since biaxial ordering will tend to align the molecular core with the transverse component of the F-dipole perpen- dicular to the tilt plane, it is more likely that dipole compen- sation in 23 results in a reduced Ps.These speculations can only be substantiated by detailed molecular modelling. Optical Purity Studies The dependence of the diffuse peak, TGB A* and smectic C* phases on optical purity were studied by construction of full phase diagrams of both the 1-methylheptyl 4'-[( 3-fluoro-4-tetradecyloxypheny1)propioloyloxy] biphenyl -4 -carboxylates and 4'-[( 2-fluoro-4-tetradecyloxyphenyl)propioloyloxy]bi-phenyl-4-carboxylates. This was carried out by accurate weighing of the desired amounts of the (R)-and (S)-enanti-omers into a clean vial and then heating to the isotropic liquid and mixing thoroughly. The binary mixtures were then analysed by polarized-light microscopy of DSC (when required).The miscibility phase diagram for the 1-methylheptyl 4-[(3-fluoro-4-tetradecyloxyphenyl ) propioloyloxy] biphenyl- 4-carboxylates is shown in Fig. 6; all data points were obtained by optical microscopy studies, with the exception of the liquid-liquid transitions, which originate from DSC mixture studies. There are a number of striking features of the system. First, the dramatic and steep increase in the clearing point temperatures (ca. 7 "C)as the optical purity decreases. Second, although no TGB A* phases were noted for the pure enanti- omers, two distinctly different regions of induced TGB A* phase and what is believed to be TGB C* phase were observed by optical microscopy, occurring at compositions of between 10 and 25wt.% of either (R)-or (S)-enantiomer.The TGB A* phase was characterized by the usual filament texture, as exemplified earlier in Fig. 1. Furthermore, on cooling from liquid-liquid transitiont -isotropic liquid 741Ii 0 20 40 60 80 1 0 (R )-enantiomer (wt.%) Fig. 6 Miscibility phase diagram of binary mixtures of the (R)-and (S)-forms of 1-methylheptyl 4'-[( 3-fluoro-4-tetradecyloxyphenyl) propioloyloxy] biphenyl-4-carboxylates (compounds 21 and 22) (all data points obtained by optical microscopy unless otherwise indicated; +data points from DSC studies) the isotropic liquid the proposed TGB C* phase appears as a shimmering, grey schlieren texture; pitch bands being observed in certain domains.22 Other areas of the texture appeared as highly coloured (selectively reflecting) planar texture with clearly visible s= k1 brush defects and are believed to be ferroelectric S,* phase forming.This demon- strates that the two different helical axis orientations are associated with the TGB C* to smectic C* transition. Examples of the TGB C* phase texture are given in Fig. 7-9; Fig. 9 shows the disclination lines described earlier. Thermal studies on suitable mixtures failed to locate any phase transition between the TGB A* and TGB C* regions in the phase diagram, possibly for two reasons: first, because of the near-vertical nature of the transition line between the two phases (shown as a dotted line in Fig. 6):second, because of the low enthalpy expected to accompany such a transition in a mixed system.However, the vertical nature of a line separating the TGB A* and TGB C* phases is somewhat expected as the TGB A* phase appears above the smectic A* phase on heating and similarly the TGB C* phase is formed on heating the Sz phase. The relationship between the TGB-phase type and the structure of the ensuing smectic phase formed on cooling has been found to occur in other systems for individual, pure materials.*' Therefore the postulation that a vertical line separates the two TGB phases above their respective smectic phases is possible if not probable. As the system's optical purity decreases the TGB A* regions give way to a conventional smectic A* phase (ie. toward Fig. 7 Texture of the TGB C* phase observed for a binary mixture of compounds 21 and 22 (composition 14 wt.%, compound 21) at 78.6 "C(100 x) Fig.8 TGB C* phase just below the transition to the isotropic liquid for a binary mixture of compounds 21 and 22 (compsition 12 wt.%, compound 22) at 78.4 "C (100x) racemic composition). Furthermore, the transition tempera- tures of the diffuse liquid-liquid event (obtained from DSC studies on the mixtures) drop as the optical purity decreases, eventually disappearing at ca. 20% wt.wt. compositions. Underlying the whole combined TGB C*/TGB A*/smectic A* region was a continous smectic C* or smectic C phase region (depending on optical purity). From the above findings it is evident that the strong molecular chirality of this system not only stabilizes the TGB C* and TGB A* phases but appears to be capable of suppressing the clearing point and other phase transitions.The phase diagram for the (R)-and (S)-forms of 1-methylheptyl 4'-[(2-fluoro-4-tetradecyloxyphenyl)propiol-oyloxy] biphenyl-4-carboxylate is shown in Fig. 10. Again, the clearing point rises as the optical purity of the binary mixture decreases; however, it is not quite so marked as for the 3-flUO1-0 system (see Fig. 6). Two 'wing' like regions of the TGB A* phase appear between compositions of 0 and 5% wt.wt. of either (R)-or (S)-enantiomers, this contrasts to the pure 3-fluoro systems. It is interesting to note that the temperature ranges for these TGB A* 'wings' differ noticeably. As the enantiomers were determined to be in excess of 99.5% pure by HPLC, the differences in the phase range can only be attributed to very small differences in the optical purities of the two starting alcohols [(R)-and (S)-octan-2-01], indicat- ing the sensitivity of the system to optical purity, particularly at high enantiomeric excess.The presence of these TGB A* 'wings' also depresses the appearance of the SZ-Sz transition J. MATER. CHEM.. 1994, VOL. 4 Fig. 9 TGB C* schlieren texture for a binary mixture of compounds 21 and 22 (composition as for Fig. 8) at 77.4 "C ( LOO x) by a proportional amount; beyond compositions of 5% wt.wt. the Si-S,*/S, transition begins to rise to a higher and almost constant value. No induced TGB C* phases were observed for binary mixtures of the 2-fluoro compounds (23 and 24).Once again the high 'molecular' chirality associated with the 1-methylheptyl moiety appears to stabilize the formation of TGB A* phases and to some extent suppresses the clearing point. Optical Rotation Studies Compound 22 was found to exhibit strong optical activity just above the S$-Is0 phase transition. Fig. 11 shows that in the isotropic phase, the optical rotatory power of 22 increases from ca.-0.5" mm-' at ca. 108 "C to a maximum of ca.-5.2" mm-' at ca. 87 "C. Below ca. 80 'C the optical rotatory power changes sharply indicating the entrance to another phase, i.e. the S$ phase. This suggests the existence of strong chiral interactions in the isotropic phase, but this behaviour is unlike normal pretransitional behaviour.For example, the optical rotatory power of 22 in the pretransition region is found to be ca. 5 times larger than that of the pretransitional behaviour of CE6 (a common cholesteric liquid-crystal material). Furthermore, the reciprocal optical rotation plots of 22 and CE6 are very different near the liquid crystal to isotropic phase transition. The onset of strong pretransitional chiral interactions in the isotropic phase coincides in temperature with the broad peak detected bj DSC. Similar behaviour has been reported by Frame et al." J. MATER. CHEM., 1994, VOL. 4 TGB A' isotropic liquid woQ88i lo---.. ,TGB A* I 78 08ol 0 20 40 60 80 100 (R)-enantiomer (wt.%) Fig.10 Miscibility phase diagram of binary mixtures of the (R)-and (S)-forms of l-methylheptyl4-[( 2-fluoro-4-tetradecyloxyphenyl) propioloyloxy] biphenyl-4-carboxylates (compounds 23 and 24) (all data points obtained by optical microscopy) t.1 .... I.,..l....l....I....1....1 80 85 90 95 100 105 T/"C Fig. 11 Optical rotatory power as a function of temperature in the isotropic phase of compound 22 However, in their case, enhanced optical activity was observed just above the Sz-Is0 phase transition. Circular Dichroism Studies Optical rotation provides a useful monitor of the bulk chiral interactions whereas CD probes the local molecular chirality. The CD absorption observed in the region 300-400 nm in 22 can be attributed to the group X-CO-OR*, where X is the biphenyl ring (see Fig.12). A strong enhancement of the CD signal is observed just above the S,*-Iso phase transition. This is illustrated in Fig. 12, which shows that a peak that is related 77.8 3 x -3 x lo4 500 . 13 h/nm Fig. 12 Circular dichroism as a function of temperature in the isotropic phase of compound 22 r """""""""""''1 10000-8000-vd 1 6000-4000 -1 . 1 .... I....,....I....,..I 78 79 80 81 82 77°C Fig. 13 Reciprocal circular dichroism in the wavelength region 355-370 nm as a function of temperature of compound 22 to the intrinsic CD absorption develops at Lax=355-1370 nm on cooling from the 'true' isotropic to just above the S,*phase. The CD diverges as the S$-Iso transition is approached from above according to T-Tc*-Iso,as illustrated in Fig.13. On entering the S,* phase the CD spectrum changes completely (as shown in Fig. 14) to a broad band associated with selective reflection. The negative intrinsic CD measurements correlate well with the negative optical rotation results in the Optical Rotation Studies section and occur in the same region as the DSC anomaly. Discussion The diffuse transition observed in cooling DSC thermograms of 21 and 22, as well as being reproducible, have been noted in other homologues of this series,24 in the original parent compounds (1, n= 14)3and in related highly chiral systems.25 The authenticity of this event in related systems has been reinforced by X-ray diffraction st~dies.~ However, the diffuse transition is usually associated with the appearance of frus-trated phases, such as the TGB A* phase.For 21 and 22 (the outer fluoro systems) no frustrated phases apparently occur, and so this is the first time that this type of transition is seen with what hitherto appears to be normal phase behaviour. Optical rotation and circular dichroism spectral studies have 5 x 0 <4 I -5 x lo4 6007L 3 Wnm Fig. 14 Circular dichroism at 76.8 “C in the smectic C* phase of compound 22 also confirmed the presence of a change in the liquid phase, and the results clearly show the presence of strong local chiral interactions. A tentative explanation for the liquid-liquid transition in other systems is related to the ensuing disordering of the TGB A* phase str~cture.~ It is believed that the TGB A* phase is stabilized by the presence of screw dislocations (hence, it is frequently termed ‘a lattice of screw dislocations’).A situation might then arise in which the screw dislocations ‘melt’ at the clearing point leaving cybotactic clusters of smectic A regions in an isotropic background, the smectic A regions may then melt completely to give an amorphous isotropic liquid. Conversely, the smectic A clusters may first melt leaving a network or entanglement of material that originally formed the dislocation ‘cores’ in an isotropic background, this network then eventually melts to give the amorphous isotropic liquid. The results of optical rotation and CD measurements suggest that the latter is a more likely explanation.It should be noted that other workers have observed the persistence of long- range order above the clearing points of a number of unrelated ionic amphiphilic materials;26 however, whether this is a related phenomenon is not at all clear. Similarly, in ceramic systems both entangled and disentangled flux phases have been found to occur.27 However, this model cannot account for the appearance of the isotropic liquid-isotropic liquid-smectic C* sequence observed in the thermal studies of the enantiomers 21 and 22 because of the lack of a TGB A* phase. Consequently, we may have to question the actual nature of the so-called S,* phase. The optical purity and miscibility studies show that the clearing points and other transitions become depressed as the optical purity of the system increases, this is not only in agreement with earlier unrelated TGB A* system studie~,~ but also with de Gennes’ original hypothesis.’ Thus, the physical studies and physical nature of the material suggest that a frustrated phase must be present in 21 and 22.However, as no smectic A* phase was detected, this indicates that the smectic C* phase cannot be ‘normal’ and may possibly have a frustrated structure. Consequently, the ferroelectric C* phase may actually be a TGB C* phase, for which two possible structures are suggested: one in which there is a twist in the tilt direction within the blocks of smectic layers making up J.MATER. CHEM., 1994, VOL. 4 the macroscopic helical structure, and one in which this twist is absent.28 Detailed structural investigations are currently underway in order to unravel this mystery, the results of which will be reported later. We now turn to the transition occurring in the liquid state. As the thermogram shows a very broad diffuse peak, this event may not constitute a real phase transition, but may in fact be related to a structural change occurring in the liquid. This could be a conformational or a packing change rather than the presence of an entangled or disentangled fog phase, as found at TGB A* to isotropic transitions. Thus, this gives us two possibilities, either the C* phase is normal and the liquid-liquid transition is related to a structural change, or else the entangled/disentangled fog phase is present and the C* phase is in fact a frustrated phase (ie.a TGB C* phase).28 Turning now to the peculiarities found in the ferroelectric properties of the two related systems. It is clear that the 3-flUOrO (outer) compounds have about twice the value of the spontaneous polarization in comparison with the 2-fluoro (inner) systems, whereas they have half the value of the tilt angle. Normalizing with respect to tilt angle (PJO) shows that the effective spontaneous polarization is four times larger for the outer fluoro systems in relation to the inner fluoro compounds. This is remarkable as the change in the position of the fluoro-substituent is taking place at the opposite end of the molecular to the ‘chiral moiety’, which strongly influ- ences the polarization. Conclusions The position of fluoro-substituents in the 1-methylheptyl 4-[(4-tetradecyloxyphenyl)propioloyloxy]biphenyl-4-carb-oxylate system has clearly been shown to influence the forma- tion of TGB A*, TGB C* and S,* phases in the enantiomeric materials and in addition was found to affect markedly the ferroelectric and optical tilt angle behaviour in the S,* phases. The calorimetric and circular dichroism data have further confirmed the authenticity and reproducibility of the diffuse liquid-liquid event in the isotropic liquid.Finally, the two types of material synthesized provide further examples of the influence of optical purity (enantiomeric excess) on the trans- ition temperatures of phase transformations.The authors thank the SERC for their financial support (to C.J.B. and J.S.K.) and the MOD (DRA Malvern). They also thank Mrs. B. Worthington, Mr. R. Knight and Mr. A. D. Roberts for their assistance in the various spectroscopic measurements and Dr. A. J. Slaney for advice on polarization and tilt angle measurements. Circular dich roism measure-ments were made at the SERC National CD Service, University College London, and the assistance of Dr. Alex Drake in making these measurements is gratefully acknowl- edged. Finally, our thanks are expressed to Dr. R. Pindak (AT&T Bell Laboratories, USA) for his insights into the nature of the TGB C* phase.References 1 P. G. de Gennes, Solid State Commun., 1972, 10,753. 2 S. R. Renn and T. C. Lubensky, Phys. Rev.A., 1988,38,2132. 3 J. W. Goodby, M. A. Waugh, S. M. Stein, E. Chin, R. Pindak and J. S. Patel, J. Am. Chem. SOC.,1989, 111, 8119. 4 J. W. Goodby, M. A. Waugh, S. M. Stein, E. Chin, R. Pindak and J. S. Patel, Nature (London), 1987,337,449. 5 A. J. Slaney and J. W. Goodby, Liq. Cryst., 1991,9, 849. 6 A. J. Slaney and J. W. Goodby, J.Muter. Chem., 1991, 1, 5. J. MATER. CHEM., 1994, VOL. 4 7 A. Bouchta, H. T. Nguyen, M. F. Achard, F. Hardouin, C. Destrade, R. J. Twieg, A. Maaroufi and N. Isaert, Liq. Cryst., 1992,12, 575. 8 T. X. Neenan and G. M. Whitesides,J. Org. Chem., 1988,53,2489. 9 G. Just and R. Singh, Tetrahedron Lett., 1987,48,5981. 10 A. Carpita, A. Lessi and R. Rossi, Synth. Commun., 1983,571. 11 E. J. Corey and P. L. Fuchs, Tetrahedron Lett., 1972,36,3769. 12 E. Chin and J. W. Goodby, Mol. Cryst. Liq. Cryst., 1986,141,3 11. 13 0.Mitsunobu, Synthesis, 1981, 1. 14 I. Nishiyama and J. W. Goodby, J. Muter. Chem., 1993,3, 149. 15 B. Nieses and W. Steglich, Angew. Chem., Int. Ed. Engl., 1978, 17, 522. 16 W. Clark Still, M. Kahn and A. Mitra, J. Org. Chem., 1978, 43, 2923. 17 D. D. Perrin and W. L. F. Arrnarego, Purification of LaboratoryChemicals, Pergamon Press, Oxford, 3rd edn., 1988. 18 CRC Handbook of Physics and Chemistry, ed. R. C. Weast, CRC Press, Boca Raton, 68th edn., 1988. 19 H. Diarnant, Rev. Sci. Instrum., 1957,28,30. 20 A. J. Slaney, Ph.D. Thesis, University of Hull, 1992. 21 B. Zeks, Ferroelectrics, 1988,84,3. 22 L. Navailles, H. T. Nguyen, P. Barois, C. Destrade and N. Isaert, Liq. Cryst., 1993,15,479; J. Phys. II (France), 1992,2, t889; Phys. Rev. Lett., 1993,71, 545. 23 K. C. Frame, J. L. Walker and P. J. Collings, Mol. Cryst. Liq. Cryst., 1991, 198,91. 24 C.J. Booth, J. W. Goodby and K. J. Toyne, unpublished results. 25 I. Nishiyama and J. W. Goodby, unpublished results. 26 V. Busico, A. Ferraro and M. Vacatello, Mol. Cryst. Liq. Cryst., 1985,128,243. 27 P. L. Gammel, D. J. Bishop, G. J. Dolan, J. R. Kwo, C. A. Murray, L. F. Schneemeyer and J. Waszaczak, Phys. Rev. Lett.. 1987, 59, 2592. 28 S. R. Renn and T. C. Lubensky, Mol. Cryst. Liq. Crvst., 1991, 209, 349. Paper 3/04521H; Received 28th July, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940400747

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 5), 761-763 761 Free Radical Generation during Thermal Decomposition of Azoisobutyronitrile in Nematic Liquid Crystal Mixturest Tatyana 1. Shabatina: Yurii K. Yarovoy,b Vladimir A. Batyuk" and Gleb B. Sergeefl a Moscow State University, 119899 Moscow, Russia Ukraine Institute of Physical Chemistry, Odessa, Ukraine Moscow Bauman State Technical University, 107005 Moscow, Russia Free radical generation during the thermal decomposition of azoisobutyronitrile (AIBN) in nematic 5CB :50CB mixtures has been studied by electron paramagnetic resonance (EPR) at 308-343 K. The kinetic model of primary radical cage escape in nematic mesophases is considered from the point of view of rotational and translational evolution of primary radical pairs ordered by anisotropic intermolecular energy potentials.An expression has been obtained to describe the cage effect of free radicals escaped into the bulk as a function of the order parameter of a nematic matrix. The investigation of free radical reactions in liquid crystals is of great interest, particularly in respect of means of effective chemical stabilization of liquid-crystalline materials. This problem is especially acute for a number of cholesteric and nematic liquid crystals.lv2 The effects of molecular orientational ordering on the kinetics of some photo-induced free radical reactions in nematic and smectic liquid crystals has been discussed In the present work we analyse the specific features of active radicals generated thermally in a nematic matrix during thermal decomposition of a model azocom- pound, i.e.AIBN which is widely used as a free-radical initiator.6 The factors which determine the radical reaction kinetics in a molecular organized mesophase are considered. Experimental Materials AIBN was purified twice by recrystallization from ethanol. The liquid crystals, 4-pentyl-4'-cyanobiphenyl (5CB) and 4-pentyloxy-4-cyanobiphenyl(50CB)of highest purity grade were employed without further purification. The stable radical 2,2,6,6-tetramethyl-4-piperidine-N-oxide(TEMPO) was puri- fied by sublimation under vacuum conditions. Kinetic Investigation The kinetics of free radical formation during the thermal decomposition of the azocompound dissolved in a liquid- crystalline 5CB :50CB mixture of varying component ratios has been studied by EPR using an EPR Rubin spectrometer (made in Russia) of 3 cm range over a temperature range of 308-343 K.The free radicals in the sample volume were determined by measuring the expenditure of stable TEMPO which was used as a counter of active cyanoisopropyl radicals. The rate constant of free radical generation (ki) during thermal decomposition of AIBN was measured using the kinetics of the expenditure of the TEMPO via its interaction with primary radicals escaped in the sample's bulk. The value of the cage effect (e) was determined from the relationship e = W/2kD[In], (1) where is the free radical generation rate measured from the initial decrease of the TEMPO EPR signal during thermal TThis paper was submitted in association with the 1st International Conference on Materials Chemistry, Aberdeen, July 1993. decomposition of the azocompound; [In] is the initial AIBN concentration and k,, is the rate constant of AIBN decompo-sition.The last value was measured in different matrices, and it was found that kD is independent of the nature of the matrix at a given temperature and can be described as k, = 1.58 x 1015exp(-129 kJ/RT).7 Dynamic Investigation The stable nitroxide radical TEMPO was used in our investigation as a free radical acceptor ('counter' of active radicals) and as a spin probe, which characterized, by means of EPR, the molecular dynamics of the system. The evaluation of micro-viscosity values (Q) of the reactive system of different compositions and at different temperatures has been described previously.' The stable radical has almost globular shape* and was not oriented by the liquid-crystalline matrix.Hence its EPR spectra were not influenced by matrix anisotropy and reflect the effective value of Q at given temperature. Results and Discussion Thermal decomposition of AIBN takes place as a simul-taneous break of two C-N bonds and corresponding formation of a geminal radical pair: where R is (CH3)2-C'-CN. The translational and rotational evolution of these radical pairs in an isotropic or anisotropic solvent cage determines the relationship between the prob- abilities of primary cyanoisopropyl radicals to recombine or to escape into the bulk.We have determined the rate constant of cyanoisopropyl radical generation (ki) and the value of the cage effect in a liquid-crystalline 5CB :50CB system of different component ratios and at different temperature conditions, including isotropic and nematic phase states. Temperature dependences of the initial free radical generation rate constant in the nematic system 5CB :50CB (70 :30 wt./wt.) matrix are pre- sented in Fig. 1. It was shown that a decrease in temperature is accompanied by the transition of the system from an isotropic to a nematic state and leads to a sharp decrease in the value of ki. This phase transition can also be caused by changing the composition of the liquid-crystalline system.In varying 762 -5.51 'r\ I -7.0 t "3:050 3:lOO 31.150 3:200 31250 1O~WT Fig. 1 Initial rate constant Versus temperature for free cyanoisopropyl radical formation during thermal decomposition in a 5CB :50CB (70: 30) nematic mixture, [AIBN] = 2.8 x mol dmT3 the content of alkoxycyanobiphenyl from 10-20 wt.% to 30-50 wt.%, we also observed a decrease of ki. The dynamic properties of the system do not change under these conditions to any practical extent. The results obtained are presented in Fig. 2. The reaction rate of the thermal decomposition of AIBN does not depend on the chemical nature of the solvent' and it is almost constant for different temperatures across the range under investigation.Thus, the lowering of the yield of cyanoisopropyl radicals escaped into the bulk in the case of the nematic mesophase compared with the isotropic system probably occurs as a result of an increase in the probability of primary radicals recombining in an ordered solvent cage of the nematic matrix. Recently we have shown" that the phase transitions of cholesteric liquid-crystalline systems (with local molecular ordering the same as in nematic mesophases) influence free radical recombination kinetics. The transition of the system from an isotropic state to a mesophase was accompanied by a jump increase of the peroxide radical recombination rate. A study of the kinetics of free radical escape (e) determined by molecular ordering (S) and micro-viscosity (Q) has been made for the liquid-crystalline system 5CB :50CB (70:30).Alteration of the molecular order of this nematic system was achieved by adding increasing amounts of the solvent chlorobenzene. The results obtained are presented in Table 1. The data obtained show that the introduction of small amounts of chlorobenzene leads to a lowering of the temperature of the nematic-isotropic phase transition of the system under investigation. This is followed by a definite decrease in the order parameter of the nematic system. Adding chlorobenzene in amounts greater than 8 wt.% led to the destruction of the nematic order of the system. It can be seen in Table 1 that Q decreases slowly with the increase of chlorobenzene. This causes a corresponding increase in e.The * 1 0 10 20 30 40 50 50CB (wt.%) Fig. 2 The initial rate constant of free cyanoisopropyl radicals 5CB :50CBa J. MATER. CHEM., 1994, VOL. 4 Table 1 Cage escape (e) for free radical generation during thermal decomposition by chlorbenzene of AIBN in a nematic mixture 5CB :50CB chlorbenzene added (wt.%) T,"/"C Q,/cP S 1Oe 0 61.0 28.1 0.67 1.98 1.63 56.2 27.2 0.65 2.02 3.92 48.9 26.0 0.59 2.13 5.42 45.1 25.6 0.52 2.35 6.80 41.5 25.4 0.46 2.55 7.30 40.5 22.4 0.35 2.99 8.20 37.5 22.0 0 3.00 12.0 -21.2 0 3.21 15.1 18.8 0 3.31 20.2 16.4 0 3.42 24.0 14.4 0 3.63 36.2 10.2 0 4.18 42.0 7.8 0 4.59 a = the temperature of the nematic-isotropic phase transition of the reactive system, determined by DTA. dependence of e on media micro-viscosity is presented in Fig.3. The data obtained show that in the isotropic phase a linear correlation exists between (1-e)-l and T,I'Q according to Noyes' equation." In the nematic phase there are strong deviations from linear dependence. It has been shown by the spin-probe method', that molecular-dynamic and bulk-viscosity properties of liquid-crystalline systems do not change significantly during the isotropic-nematic transition. Hence the deviations observed in the nematic mesophase can be caused only by a change in the molecular ordering of the system. The process of free radical formation during thermal decomposition of AIBN can be examined using a model of rotational and translational evolution of primary geminal radical pairsI3 where R' is a primary radical escaped into bulk; [R', Re]*is a primary radical pair with a mutual orientation of radicals favouring their recombination; [R', R'] is a primary radical pair with mutual orientation which does not favour their recombination; a is the probability of obtaining a primary radical pair with mutual orientation of radicals that favours their recombination; a1 is the probability of obtaining nitrogen and primary radicals escaped into bulk; v1 and v2 are the frequencies of transitions between primary radical- pair states of favoured and unfavoured radical recombination and vD diffusion frequency of primary radicals.The value of e in this case can be obtained from (p+ 1)e-1 -N +-1-CC (2)(p+ l)(e+ 1) -k,*(& + a) + c( where, N = vD + v,; g =v,/v,; k,* is the recombination rate constant for a primary radical pair with mutual orientations of radical particles favouring radical recombination; f is that part of primary radical pairs with mutual orientations favouring radical,!3 recombination; is the ratio of theformation during AIBN thermal decomposition in nematic mixture versus its composition T = 313 K, [AIBN] = translational diffusion constant of N, and primary radicals.2.8 x mol dmP3 Eqn. (2) can be reduced to the following equation which is J. MATER. CHEM., 1994, VOL. 4 1.8-1.6-.--I -? 7v 1.4-i 1.2-+ -r Fig.3 The dependence of free radicals cage escape on media bulk viscosity ,, 0.3 0.4 0.5 0.6 0.7 S Fig.4 The dependence of free radical cage escape on the order parameter of nematic systems linear relative to f l-e (3)e-(1 +B)-' =af+b where a =g/[(N/K:) +(1 -a)]; b =aa/g.In the nematic mesophase, radical diffusion contacts of primary radicals leading to their recombination are deter- mined by nematic ordering of the system and f can be represented by f =A +a,S (4) where, is that part of a primary radical pair with mutual orientations of radical particles favouring their recombination in isotropic phase; a, is an empirical parameter, that reflects the guest-host interactions; S is an order parameter of nematic systems. For a general case, a, can be positive if reagent ordering in the liquid-crystalline matrix favours the chemical interaction and negative if reaction molecule ordering hinders the reaction.Using eqns. (3) and (4)we obtained eqn. (5) connecting the function of primary radical escape [#(e)]with a nematic order parameter (S): (5) where A and B are correlation coefficients. The dependence of the experimental function of free radical cage escape during thermal decomposition of AIBY in a nematic 5CB :50CB system, $(el, on the order parameter of a nematic matrix is presented in Fig. 4. It can be seen that kinetic data obtained for free radical formation are well described by eqn. (5) for S from 0.35 to 0.67. Conclusions The formation of cyanoisopropyl radicals during the thermal decomposition of AIBN has been studied in nematic liquid- crystalline systems.It has been shown that the phase transition from isotropic to the nematic state leads to a lowering of primary radical escape in the bulk. This is probably due to an increased probability of primary radical recombination in ordered solvent cages of nematic matrices. The kinetic model of primary radical cage escape in a nematic mesophase is considered on the basis of rotational and transliitional evolution of primary radical pairs ordered by an anisotropic intermolecular energy potential. An expression has been obtained to describe the yield of free radicals escaped in volume as a function of the order parameter of a ncmatic matrix.It describes the experimental kinetic data for ncmatic order parameter values from 0.35 to 0.67. References 1 I. I. Gorina, Zh. Vses. Khim. Obshch., (in Russian), 1983, 28, 223. 2 R. L. Vardanyan, G. E. Dingchan and B. Khanukaev, Kinet. Katal., 1978, 19, 72. 3 D. A. Hrovat, J. H. Liu, N. J. Turro and R. G. Weiss, J. Am. Chem. Soc., 1984, 106, 5291. 4 V. A. Batyuk, T. I. Shabatina, Yu. N. Morosov and G. B. Sergeev, Mol. Cryst. Liq. Cryst., 1988, 161, 109. 5 V. A. Batyuk, T. I. Shabatina, Yu. N. Morosov and G. B. Stbrgeev, Mol. Cryst. Liq. Cryst., 1990, 186, 87. 6 N. M. Emanuel, E. T. Denisov and Z. K. Maizus, Hydro( arbon Oxidation Chain Reactions in Liquid Phase, (in Russian), Nauka Press, Moscow, 1985, 270. 7 B. E. Krysuk, E. N. Ushakova, A. P. Griva and S. T. Denisov, Dokl. Acad. Sci. USSR, 1984, 277,N3, 630. 8 Spin Labeling. Theory and Applications, (in Russian I, ed. L. J. Berliner, Mir, Moscow, 1979, p. 640. 9 N. M. Emanuel, D. Gal, Ethylbenzene Oxidation, (in Russian), Nauka Press, Moscow, 1984, 376. 10 Yu. K. Yarovoi, T. I. Shabatina, V. A. Batyuk and G. B. Sergeev, Mol. Cryst. Liq. Cryst., 1990, 191, 283. 11 R. M. Noyes, J. Am. Chem. SOC., 1954, 77, 2042. 12 Yu. N. Morosov, A. V. Reiter, T. I. Shabatina and V. A. Batyuk, Molecular Materials, 1992, 2, 35. 13 A. P. Griva, L. N. Denisova and E. T. Denisov, Zh. Phys. Chem., (in Russian), 1985, 59, 2944. 14 M. F. Grebenkin and A. V. Ivashenko, in Liquid CrystaIline Materials, (in Russian), Khimiya, Moscow, 1989, p. 288. Paper 3/04341J; Received 21st July, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940400761

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(5), 765-770 Statistical Analysis of Apatitic Tricalcium Phosphate Preparation Hassan Chaair, Jean-Claude Heughebaert, Monique Heughebaert and Michel Vaillant INPT, ENSCT, CNRS URA 445, Laboratoire des Materiaux, Equipe de Physico-chimie des Solides, 38 rue des 36 Ponts, 31400 Toulouse, France Apatitic tricalcium phosphate was obtained by a continuous (3-5 kg per 24 h) process using the conventional double decomposition method between an aqueous calcium nitrate solution, Ca(NO,),, and an aqueous ammonium phosphate solution, (NH,),HPO,. To check the effect of certain variables on the reaction, a fractional central composite design was set up taking six variables into account: pH, (Ca/P),,e,,,, concentration of the calcium nitrate solution ([Ca2+ I), temperature (T),duration of precipitation (R) and speed of stirring (S).The limiting factors of precipitation for apatitic tricalcium phosphate are discussed.Calcium phosphates are used as bioceramics for prosthetic application^.'-^ They are based mainly on hydroxyapatite [HAP; Cal0( PO,),(OH),] and P-tricalcium phosphate [P-TCP; Ca,(PO,),]. The difficulty with most of the conventional precipitation methods used is in obtaining well defined and reproducible solids, i.e. a solid with a given Ca/P rati~.~.~Factors governing the precipitation, pH, temperature, (Ca/P),,,,,,,, , speed of stirring, etc., are not usually precisely controlled; consequently, the solid precipitated is, in fact, a mixture of various calcium phosphates that, after heating to 900°C in air, gives a solid with approximately the desired Ca/P ratio.The purpose of this paper is to optimize the continuous synthesis of tricalcium phosphate (3-5 kg per 24 h) by looking for a possible optimum in the response surface representing the relationship between the atomic Ca/P ratio of the precipitate (ca. 1.50), and investigating the variables governing precipitation. In this work the relationship between the atomic ratio of the precipitate and six quantitative variables, i.e. pH, atomic ratio of the reagents [(Ca/P),,,,,,,], concentration of the calcium solution ([Ca" I), temperature (T), duration of precipitation (R)and speed of stirring (S),was determined by a polynomial of the second degree in a set of experiments according to a fractional central composite Experimental The precipitates were obtained by a wet process using a conventional double decomposition method between a cal- cium solution, Ca( NO,),, and an ammonium phosphate solution, (NH4)2HP04.9 A schematic diagram of the appar- atus is shown in Fig.1. A 3 dm3 reactor was maintained at constant temperature, both the reagents were introduced at the same constant flow rate and the pH was adjusted to a constant value with a pH-stat, which controlled the addition of base (or acid) as necessary. The precipitate ran out of the overflow of the reactor and was filtered, washed with de-ionized water, air-dried at 80 "C and heated to 900 "C in air. The resulting material was studied by X-ray diffraction, infrared ( IR) spectroscopy and chemical analysis.X-ray diffraction patterns were recordet at room temperature with Co-Ka, radiation (A= 1.78892 A) and a Seeman-Bohlin camera, the presence of impurities was checked by comparison with the American Society for Testing and Materials (ASTM) file." Precise Bragg angles for the samples were measured with respect to lines for NaC1, which was used as internal standard." IR spectra were recorded with a Perkin-Elmer FTIR 1600 spectrophotometer using pellets consisting of 1mg powder in 300mg KBr. The phosphorus content was t (HAP or TCP) Fig. 1 Block diagram for the continuous synthesis of calcium phosphate. R,=reactor; M =stirrer; R, = pH-stat; F = filtration system; Se =dryer; Br =grinder; and T = sieve analyzed by the colorimetric method described by Gee and Deitz.12 The relative error for phosphorus determination was ca.0.1%. Calcium was determined by atomic absorption spe~trometry,'~with a relative error of 0.3%. Statistical Analysis Experimental analysis is used in agriculture, biology and ~hemistry'~to study the empirical relationship between one or more measured responses and a number of variables. This part of the paper discusses the principles governing the construction and analysis of a fractional central composite design. Calculation: two-variable design The matrix of the complete central composite design can be written: Xl x2 -+ -+ 22 factorial design (NF)+ + -a 0 7 +a 0 I axial points 2 x 2 (N,) 0 -a 0 +a 0 0 }centre points (N,)0 0 The central composite design 22 has a total number of experiments of ~I=N,+N,+N~=~~+(~x2)+No=8+No. The theoretical model equation is therefore: YI =Po +PJl+ P2X2 +PlJ: +P22Z +P12XlX2 Note, this composition plan is not centred.It is sufficient to calculate the interactions and the quadratic variables, as with the matrix of the theoretical model: xo x1 x2 x: x; XlX2 + --+ + + + +-+ + -1+-++ + -NF+ ++ + + + + -a 0 a2 0 0 + +a 0 a2 0 0X= I+ 0-a 0 a2 0 Na+ O+a 0 a2 0 + 00 0 0 0 + 00 0 0 0 1No sum 8+No 0 0 4+2a2 4+2a2 0 The sum of X; (or of X:) equals 4+ 2a2. The coefficient parameters may be written: b =(X'X)-'X'y, and the matrix XX can be calculated: n O 0 2a2+4 2a2+4 0 2a2+4 0 0 0 0 XX= 2a2+4 0 0 0 2a2+4 4 0 SYMMETRY 2a2+4 0 4 Since X: and X2 are not centred, non-zero terms can result in X'X:2a2+4 on the first row and, for reasons of symmetry, in the first column.X: is thus not orthogonal to Xi, hence the 4 appears in the triangle at X:X; (or X;X: by symmetry). X'X is not a diagonal matrix and neither is (X'X)-'; it appears that (X'X)-' is almost diagonal since it presents numerous zero values, i.e. it is almost orthogonal. Full orthogonality is obtained leading to a diagonal (XX)-',if the variables X: and X; are replaced by centred variables. If the sum of X: is equal to 2a2+4,in n experiments, J. MATER. CHEM., 1994, VOL. 4 the new variable is named U:.The same is true for Xi: 2a2+4u;ox; -~ n Finally, a is calculated by expressing Ul orthogonally with respect to U;. The theoretical model is then written: r =Pb +Pix1+Pix2 +PilUI+Pi2G +&2XlX2 Example Let us consider a complete central composite design 22,with one experiment in the centre. In other words n=8+ No= 8 + 1=9. Therefore: ui", -2a2+4 ~ 9 u?2ux?22a2+4 -___9 hence the new variables: UfI =9Xt' -(2a2+4)=9x;' -2a2-4 The model matrix can thus be written: + --5-2a2 5-2a2 + -+ + -5-2a2 5-2a2 NF=22 -+-+ 5-2a2 5-2~~ + + + 5-2a2 5-2a2 + X= + -a 0 7a2-4 -4-2a2 0 + +a 0 7a2-4 -4-2a2 0 Na=2x2+ 0 -a -4-2a2 7a2-4 0 + 0 +a -4-2a2 7a2-4 0 + 0 0 -4-2a2 -4-2a2 0 No= 1 sum 9 0 0 0 0 0 The experiments are centred but Ut is not orthogonal to U;.By making U: orthogonal to U;: 9c (u;1)(u;2)=4(5-2a2)+4(7a2-4)(-4 -2a2) i=l +(-4 -2a2)2 = 180 -144~~-36a4 3 0 *5 -4a2-a4=0 which can be written: (a-l)(a+ 1)(a2+5)=0 where the real solutions are: a= f1, the two others being imaginary. J. MATER. CHEM., 1994, VOL. 4 Calculation with More than Two Variables On calculation, with more than two variables, orthogonality can also be written: i=l Application For k =2 (k= number of explanatory variables), and No = 1 (the same as before): (4 + 2a2)222-V2+2a2I2 4--022 + (2 x 2) + 1 =O; 9 so: (a-l)(u + l)(a + 5)2=0; real solutions: a = f1 For k = 6, and No =5 (complete central composite design): (26 + 2a2)2 (64 + 2~~)~ 26-~64-=O*a= f224+(2 x 6)+5 81 For k-p=6-1, NF=2k-p=26-1=2s=32 and N0=6 (as for this case, fractional central composite design): (26-+ 2a2)2 (32 +2~~)~ =32--O*a= +226-1 -26-' +(2 x 6) + 6 50 In the present case, a fractional central composite design is analysed in which the response (y) is the atomic Ca/P ratio of the washed solid obtained, heated to 900°C in the air [(Ca/P),,,], and the variables xj are: pH, (Ca/P),,,,,,,,, [Ca2+], T, R and S, hereafter respectively called x1, x2, x3, xq, x5, and x6.Table 1 shows the fractional central composite design presented according to the standard order; the values of the coded variable Xj are dimensionless. The values of the natural variables are summarized in Table2.The 50 experiments to be run are of orthogonal design (which means that the coefficients do not change when any model parameter changes). They are the following (Table 1). (i) The first 32 experiments belong to a 26-1 factorial fractional design; the & 1 coded values Xj were obtained by calculating: Xj =(xj-~j)/Ax The additional variable, x6, is confounded (i.e. confused) with the product XlX2X3X4Xs. (ii) The next 12 experiments were the points on the six axes, at a distance +a from the centre. (iii) In theory, we should have performed six experiments in the centre, but it is usually accepted that only three or four central experiments are sufficient to account for the coefficients of the estimated model with good accuracy (see Appendix).The distance c1 was calculated so as to obtain square values of the variables X: that are mutually orthogonal; in the present design, which consisted of six variables belonging to 26-1 and six experiments in the centre, we have a=2. The estimated model was: 6 66 6 jj =bo + C bjXj i-C 1 bjj, XjXj, + 1bjjU5 j= 1 j=1 j' = 1,j#j' j=1 Let b,X, be the general term of 9, the 28 terms (1constant + 6 variables j + 15 interactions jj' + 6 squared variables jj = 28) generally used for the construction of the model must be mutually orthogonal, and the normal equation gives the b, coefficients with the least-squares method: n b, = 7,r, where r, = c Xiuyi, i= 1c x u i= 1 Xi,and yi being the Xu and y values for the ith experiment; Y, is named contrast.Results Table 1 shows the experimental data for each atomic ratio of precipitate washed and heated at 900°C (yi). The run was performed in a randomized order (Table 1). The 28 terms were easily calculated by substituting data values in the expressions for the least-squares estimates of the coefficients (Table 3). The fitted response surface is, if expressed in real variables:-(Ca/p),,, x lo3= 1487.1+24.4Xi + *.* -7.2x6 + 2.8X1X, f ..' -3.OX5X6 + 2.9XIl-k '** + 3.7x66 (1) From this equation, it is possible to compute estimated values jji and the corresponding residuals ei = yi-9, (Table 1). An estimate of the variance of the experimental error (sf) was obtained by dividing the residual sum of squares, C,eZ (Table l), by v (number of degrees of freedom =number of experiments minus number in the model, i.e.50-28=22), see Table 4: s; =(12.44 x 10-3)/22 = 5.65 x 10-4 The statistical errors for P and Ca of 0.1 and 0.3%, respectively, agreel23l3 with a well defined product, after many measurements. But an error of s, =0.024 and relative error of 0.024/1.500 = 1.6% after only one measurement is expected (colorimetric and atomic absorption). The estimated variances of the coefficients sb2, given in Table 3 were therefore calculated by the following formulae: Ii The significance of the effects can be estimated by comparing the values of the ratio bi/si, to a critical value for the F distrib~tion,'~as indicated in Table 3.It appears that only the main effects of pH, (Ca/P),,,g,nts, [Ca"], T and S, the interactions pH-[Ca2+], T-R, (Ca/P),,,,,n,,-S, [CLi2+1-S and T-S, and the quadratic term R2 are significant. The best fitting response function is then conveniently written as follows: (cTP)whs x lo3= 1487 +24x1 + 11x2 f 7x3 + 11x4 --7x6 (f4) (f4) (+4) (f4) (f4) + 8x1 X3 +9xZx6 -SX3X6 + 7X4X6 (f4) (f4) (i4) (f4) + loX4x5 -10x55 (2)(+4) (*3) The numbers in parentheses below the coefficients represent the standard deviations (Table 3), for example: sbjj= 48.84 x = 0.0030 = 3 x lop3 Discussion Investigation of eqn. (2) showedAat if XI =0, X2=0, X3= 0, X4= 1, X, = 1 and & =0, (Ca/p),,, x 1.50 (Fig. 2). This 768 J. MATER. CHEM., 1994, VOL.4 Table 1 The fractional central composite design presented according to the standard order. (Measurements not actually carried out are in parentheses, see Appendix) order coded units of variables (Ca/p)whs exp (Ca/p)whs cal residues x3 x4 X, x6 Yi 9i lo3eiactual logical Xl x, 1 1 ------1.484 1.466 + 18 14 2 + ----+ 1.47, 1.453 + 22 37 3 -+ ---+ 1.44, 1.456 -10 21 4 + + ----1.46, 1.469 + 00 42 5 --+ --+ 1.43, 1.427 + 08 8 6 + -+ ---1.54, 1.528 + 21 36 7 -+ + ---1.46, 1.488 -20 20 8 + + + --+ 1.516 1.522 -06 29 9 ---+ -+ 1.459 1.462 -03 47 10 + --+ --1.456 1.479 -23 32 11 -+ -+ --1.45, 1.444 + 08 23 12 + + -+ -+ 1.536 1.517 + 19 13 13 --+ + --1.47, 1.457 + 18 41 14 + -+ + -+ 1.484 1.486 -02 15 15 -+ + + -+ 1.470 1.458 + 12 28 16 + + + + --1.501 1.517 -16 11 17 ----+ + 1.41, 1.403 + 16 27 18 + ---+ -1.45, 1.470 -12 35 19 -+ --+ -1.44, 1.447 + 01 18 20 + + --+ + 1.46, 1.487 -18 33 21 --+ -+ -1.440 1.459 -19 40 22 + -+ -+ + 1.44, 1.455 -09 49 23 -+ + -+ + 1.461 1.439 + 22 2 24 + + + -+ -1.550 1.548 + 02 3 25 ---+ + -1.48, 1.479 + 05 4 26 + --+ + + 1.533 1.514 + 19 16 27 -+ -+ + + 1.481 1.503 -22 48 28 + + -+ + -1.51, 1.528 -09 43 29 --+ + + + 1.42, 1.425 -01 45 30 + -+ + + -1.54, 1.539 + 09 12 31 -+ + + + -1.461 1.484 -23 25 32 + + + + + + 1.546 1.565 -19 17 33 -2 0 0 0 0 0 1.44, 1.450 -06 30 34 +2 0 0 0 0 0 1.55, 1.548 + 10 50 35 0 -2 0 0 0 0 1.43, 1.464 -34 26 36 0 +2 0 0 0 0 1.54, 1.510 + 39 38 37 0 0 -2 0 0 0 1.45, 1.463 -05 7 38 0 0 +2 0 0 0 1.500 1.490 + 10 39 39 0 0 0 -2 0 0 1.45, 1.464 -09 31 40 0 0 0 +2 0 0 1.52, 1.506 + 14 9 41 0 0 0 0 -2 0 1.41, 1.439 -24 46 42 0 0 0 0 +2 0 1.482 1.454 + 28 10 43 0 0 0 0 0 -2 1-53, 1.516 + 20 5 44 0 0 0 0 0 +2 1.47, 1.487 -15 6 45 0 0 0 0 0 0 1.49, 1.487 + 03 (44) 46 0 0 0 0 0 0 ( 1.490) 1.487 + 03 (24) 48 0 0 0 0 0 0 (1.48,) 1.487 -02 (34) 50 0 0 0 0 0 0 (1.48,) 1.487 -05 22 47 0 0 0 0 0 0 1.48, 1.487 -02 19 49 0 0 0 0 0 0 1.48, 1.487 -05 Table 2 Factorial levels used to optimize apatitic-TCP natural variables (xj) coded variables X,, X,, X,, X,, X,, Xga x1 =pH 5.25 5.75 6.25 6.75 7.25 x2 = (Ca/P)reagents 1.44 1.47 1.50 1.53 1.56 x, = [Ca2+ ]/moll-0.75 1.oo 1.25 1.50 1.75 x4 = T/"C 57.5 60.0 62.5 65.0 67.5 x5 = R/h 2.00 3.oo 4.00 5.00 6.00 x6 = S/rpm 400 500 600 700 800 OX, = (XI -6.25)/0.50; x,= (X2 -1.50)/0.03; xs = (Xj -1.25)/0.25; x4 = (X4 -62.5)/2.50 x5 = (Xg -4.00)/1.00; and x6 = (xg -600)/100.3. MATER. CHEM., 1994, VOL. 4 769 Table 3 Analysis of variable effect coefficient variance F-value (bU) (&> (b,2/&) significance 6 1.4871 ---0.0244 1.41 x 42.11 **** 0.0115 1.41 x 10-5 9.35 *** 0.0067 1.41 x 10-5 3.17 * 0.0106 1.41 x 10-5 7.95 *** 0.0036 1.41 x 10-5 0.92 NS -0.0072 1.41 x 10-5 3.66 * 0.0028 1.77 x 10-5 0.44 NS*0.0082 1.77 x 10-5 4.15 0.0037 1.77 x lo-’ 0.77 NS 0.0027 1.77 x 10-5 0.4 1 NS -O.OOO6 1.77 x 10-5 0.02 NS -0.0065 1.77 x 10-5 2.39 NS “ 3 0.0048 1.77 x 10-5 1.31 NS $ 0.0044 1.77 x 10-5 1.09 NS -0.0017 1.77 x 10-5 0.16 NS**0.0098 1.77 x 10-5 5.43 0.0022 1.77 x 10-5 0.27 NS ‘t.*0.0086 1.77 x 10-5 4.18 * 0.75 1.00 1.25 1.50 1.75-0.008 1 1.77 x 10-5 3.71 0.0074 1.77 x 10-5 3.09 * x, = [ca2+]/rnolI-’ -0.0030 1.77 x 10-5 0.51 NS Fig.2 Response function contour lines [eqn. (l)], Xl =pH =0; X2= 0.0029 8.84 x 0.95 NS (Ca/P),,,,,,, =0; X4 = T = 1; and X,= S =0. Broken line shows the o.OOO1 8.84 x 0.04 NS experimental range. -0.0025 8.84 x 0.7 1 NS -0.0004 8.84 x lop6 0.02 NS***-0.0102 8.84 x 11.77 0.0037 8.84 x lop6 1.55 NS (Fig. 2). When the calcium concentration of the C’a(N0,)2 ****: significant at a level of 0.1% (Fo.ml(1,22) = 14.38); ***: signifi-solution and the duration of the precipitation increasecant at a level of 1 % (Fo,ol(1,22) = 7.95); **: significant at a level of together, or when the duration of the precipitation increases, (1,22) =4.30); *: significant at a level of 10% (Fo.lo5% (F0.05 (1,22)= the calcium concentration of the Ca( N03)2 solution remaining 2.95); and NS: non-significant.unchanged, then the Ca/P ratio of the precipitate increases up to 1.50. Table 4 Regression variance analysis for model ratio Ca/P of the solid washed, air dried and heated to 900°C -Conclusionssource of sum of degrees of mean variation squares freedom square Fexpu Sb The precipitation of apatitic tricalcium orthophosphate, has been studied using a factorial central composite design. The regression 0.05807 27 0.002 15 3.80 -‘ response equation for the atomic ratio Ca/P of the precipitate residue 0.01244 22 0.000565 --sum 0.07051 49 ---obtained was established. From this equation, it was possible to forecast the optimal conditions to obtain 8-TCP from ‘Fexp: Snedecor factor.bS: significance test; and ‘: significant apatitic tricalcium orthophosphate, after heating to 900 “C, approximatively at a level of 0.1% (Fo,wl(27,22) = 3.85)14. and also to determine the Ca/P ratio of the precipitates. Appendix Calculations are elementary for Table 1, which show:, the six results required for the centre, this means 50 measurements. However, it is preferable to recalculate the results from the 47 measurements actually carried out. Some confounding appears only with the six square values, Xjj.So, the six normal equations have to be solved simultaneously. Because the central composite design is well balanced, the six normal diagonal coefficients of the unknown bjj are all equal to 1408/47: (-1): 1487+ ... -10(-1)’=1477 (0): g=1487 40’ (+1): Q = 1487 + ... -lo(+ 1)’ = 1477 = [( +(1)’ + ... + ( 1)2 +(4)’+ (q2]-47 1600 1408=64--=-47 47 The same holds for the 32 non-diagonal coefficients of the 7 70 unknowns bjfj,(j’# j)= -96/47: c (Xjj-XJJ)(xjrj. 47 -x,.,.)= c(XjjXjjj,)-c xjjcxjrj. j#j’ 40 x 40 1600 ~= (32 + 0) -47 = 32 --47 96-_--47 So, by addition of the six equations, we obtain a 7th equation in which all the coefficients of the left-hand member are equal to [1408 -(6 -1) x 96]/47 = 928/47.Therefore, by adding the jth normal equation to the 7th equation, with appropriate multipliers 29 and 3, the parameter bjj is isolated: bjjC(29 x 1408 + 3 x 928)]/47 =(3 + 29)xj x 3 1 Tcj, j’fj i.e. 928bjj= 32Xj + 3 1YJtj, j’#j and the corresponding contribution is: Finally, the six contributions bjj are in the j order from 1 to 6, i.e. 0.0027, -0.0001, -0.0028, -0.0006, -0.0104 and 0.0035. As in Table 3, the X,, contribution is much greater than the others. The new value of b,, is -0.0104, which is close to the -0.0102 in Table 3. Glossary b = coefficient matrix. b, = coefficient of the polynomial model. (Ca/P),,, = Ca/P ratio of washed heated solid. ei = residual of the ith experiment, ei = yi-ji.k = number of explanatory variables. p = number of complementary variables. s& =estimated variance of coefficient b,. s;? =residual variance, s;?= xie;/v. X= matrix of the model. J. MATER. CHEM., 1994, VOL. 4 xj = natural variable x for element j, and .‘cj its mean, i.e. either pH, (Ca//P)reagents, [Ca2+], temperature (T),duration of precipitation (R) or speed of stirring (S). Xj = coded variable X for element j. Xjj= centred squared variable Xj (Xjj= Xj -Xg). yi = measured response for the ith experiment. ji= calculated response for the ith experiment. Y,= contrast of a linear combination of values of y,, where the sum of the coefficient is zero. IX = distance from the centre of the design. Ax = difference between x and X.v = number of degrees of freedom = number of experiments minus number of coefficients in the model. References 1 J. G. J. Peelen, B. V. Rejda and K. de Groot, Ceramurgiu lnt., 1978, 4, 71. 2 M. Heughebaert, R. Z. Legeros, M. Gineste and G. Bonel, J. Biomed. Muter. Rex, 1988, 32(A3), 257. 3 J. C. Chae, J. P. Collier, M. B. Mayor and V. A. Surprenant, in Bioceramics: Material Characteristics versus in viro behavior, ed. P. Ducheyne and J. Lemons, Ann. New York Acad. Sci., 1988, 523, 81-90. 4 M. Jarcho, R. L. Salsbury, M. B. Thomas and R. H. Doremus, J. Muter. Sci., 1979, 14, 142. 5 J. C. Heughebaert and G. Montel, Calc. Tiss. Inr., 1982, 34, 103. 6 J. C. Heughebaert and G. Montel, Bull. Soc. C‘him. Fr., 1970, 8-9,2923. 7 G. E. P. Box and K. B. Wilson, J. R. Stat. SOC., 1951, B13, 1. 8 G. Sado and M. C. Sado, Les plans d’experiences. De l’expkrimenr- ation a l’assurance qualite, AFNOR technique, Paris, 1991. 9 J. C. Heughebaert, These de Doctorat d’Etat, INP, Toulouse, 1977. 10 ASTM, Powder diflraction jile, no. 9-1 69, fl-calcium orthophos- phate Ca, (PO4)*,ASTM, Philadelphia, PA. 11 H. E. Swanson and R. K. Fuyat, NBS, Circular 539, vol. 11, 1953. 12 A. Gee and V. R. Deitz, Anal. Chem., 1954, 25, 1320. 13 M. Pinta, Sgectrometrie d’absorption utomique, Tome 11, ed. S. A. Masson, Paris, 1972. 14 G. E. P. Box, W. G. Hunter and J. S. Hunter, Statistics for Experimenters, An introduction to Design, Data Analysis and Model Building, Wiley, New York. 1978. 15 G. E. P. Box and N. R. Draper, Empirical Model-Building and Response Surfaces, Wiley, New York, 1987. Paper 31035765; Revised 22nd June, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940400765

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(5),771-772 771 MATERIALS CHEMISTRY COMMUNICATIONS Conducting Polymer-Clay Composites for Electrochemical Ap pl ica tions Peter W. Faguy," Wanli Ma," J. Alan Lowe," Wei-Ping Panb and Terri Brownb a Department of Chemistry, University of Louisville, Louisville, KY 40297, USA Department of Chemistry, Western Kentucky University, Bowling Green, KY 42707, USA Pyrrole can be polymerized within montmorillonite clays via chemical means utilizing Fe3+ and Cu2+ as the oxidizing species. The resultant composite has properties of both the conducting polymer and the host material. Vibrational spectroscopy, thermal analysis and conductivity data all indicate that polypyrrole is present in the interlayer region of the clays utilized. Electrochemically, the conducting polymer-clay composite shows promise for both sensor and electrolysis applications.A widely studied and potentially useful facet of heteronuclear aromatic ring systems is their ability to polymerize oxidatively, often forming conjugated electronically conducting poly- mers.1-2 Both electrochemical and chemical polymerizations can be carried out within porous or layered materials to realize a composite material with attributes of both the conducting polymer and the host material. Conducting poly- mers spontaneously polymerized inside aluminosilicates are a class of these composite materials which show much promise in electrochemical application^,^,^ but have yet to be investi- gated for electrocatalytic properties. A large body of work exists which details in situ polymerizations of pyrrole, thio- phene and aniline intercalated into clays,5 zeolites: layered metal oxide/halides: and various silicates.' These studies have focused on conductivity and structural information, very little electrochemistry has been performed. Our laboratory has recently begun to prepare and to characterize polypyrrole- montmorillonite clay composites.The research stems from an expectation that some of the electrochemical and catalytic properties of exchangeable clays can be coupled with the electrochemical and electronic properties of conducting polymers. The nature and extent of polymerization of pyrrole in transition-metal-exchanged montmorillonite clays are depen- dent on the particular transition metal and solvent used.For the present study, reactions were carried out in distilled water and with distilled pyrrole. The clay used was a commercially available montmorillonite catalyst support KSF (Aldrich). From preliminary X-ray powder patterns and TG results it appears that the clay is disordered, containing some non- montmorillonite clay-mineral phases. When there is Fe3+ or Cu2 present the initial reaction slurry changes from colour- + less to green, through blue and finally to a shiny black. Very similar observations were made for other intercalative poly- merization~.~-~Two-point conductivity measurements show that with intercalation of the conducting polymer, conductivit- ies increase by three or more orders of magnitude if no copper (Fe3+ only) is present in the clay and by a factor of ca.10 if more than 0.3% (m/m) copper is present. A possible expla- nation of this phenomenon is the strong interaction of Cu2+ with the polymer which localizes electronic charge and reduces conductivity. Thermal analysis and scanning electron microscopy provide indirect evidence that the conducting polymer is intercalated. The SEM shows, even for pyrrole: clay loadings of 3: 1, no detectable morphological changes in the clay particles. If the pyrrole was oxidizing on the particle surface it would be expected that polymeric dendrites would be evident. Two major observations can be made from the TG md DTA results: the initial dehydration of the disordered mont morillon- ite occurs at 105°C and is lowered by ca.14" with pyrrole treatment; and an endothermic transition in the CPCC samples at 580 "C is associated with the intercalated polypyr- role. The increased volatility of interlayer water may be evidence that the polypyrrole exists at the ion-e vchanged oxide surface in the interlayer region, displacing Rater. The fact that no pyrrole or low-molecular-weight oligimers are detected points to a thermally stable polymer. Similar thermal studies of pyrrole intercalated in FeOCl layered solids showed the same polymer ~tability.~ Infrared and Raman measure- ments of CPCC powder indicate that the polymeric material -1 00 200 500 V/mV vs. SCE Fig. 1 Cyclic voltammetry of carbon paste electrodes in 0.1 mol 1-' HC104 at 10 mV s-' with (-) and without (---) 5 mmol 1-' ascorbic acid: (a) carbon only, (b) carbon; CPCC (4:l) and (c) as in (b) but held at -0.1 V for 10 min prior to anodic sweep in the clay is structurally very similar to electrochemically synthesized polypyrrole.’ The potential for CPCC electrodes in sensor applications is shown in Fig.1. The onset of ascorbic acid oxidation on CPCClcarbon paste electrodes [Fig. 1 (b)] occurs ca. 50 mV before it does on carbon paste electrodes alone [Fig. l(a)]. This favourable shift in potential occurs despite diffusion overvoltages associated with adsorption processes. Such a polarization shifts the uncatalysed oxidation of dopamine’ more than 50mV positive, CPCC/carbon paste relative to carbon paste alone.From the Cu2+/Cuo couple in the clay [Fig. l(a) dotted line; ca. 220 mV] and the onset of ascorbic acid oxidation it would seem that Cu2+ sites in the CPCC act electrocatalytically. Pre-concentration via adsorption can lead to increased oxidation currents as is evident by comparing Fig. l(b) and l(c). This preliminary report demonstrates that conducting poly- mer-clay composites do possess many characteristics of both polypyrrole and the clay host. Their initial applicability to electrocatalytic oxidations has been indicated, with catalysis and adsorption both playing roles. The unique electronic, electroactive and adsorptive characteristics of these com-posites produce an electrode material which could be utilized J.MATER. CHEM., 1994, VOL. 4 in such electrochemical applications as energy storage and waste water remediation. The authors wish to acknowledge the National Science Foundation and the Commonwealth of Kentucky for financial support through the NSF-Kentucky EPSCoR Advanced Development Program (grant EHR-9108764). References 1 A. F. Diaz and J. Bargon, in Handbook of Conducting Polymers, ed. T. A. Skotheim, Marcel Dekker, New York, 1986,vol. 1, p. 81. 2 R. J. Waltman and J. Bargon, Can. J. Chem., 1986,64,76. 3 D. R. Rolison, R. J. Nowak, T. A. Welsh and C. G. Marsh, Talanta, 1991,38,27. 4 C. M. Castro-Acuna, F. F. Fan and A. J. Bard, J. Electroanal. Chem., 1987,234,347. 5 T. H. Chao and H. A. Erf, J. Catal., 1986,100,492. 6 P. Enzel and T. Bein, Synth. Met., 1993,55-57 1238. 7 M. G. Kanatzidis, L. M. Tonge, T. J. Marks, H. 0. Marcy and C. R. Kannewurf, J. Am. Chem. Soc., 1989,111,4138. 8 V. Mehrotra and E. P. Giannelis, Sol. State Comrnun., 1991,77, 155. 9 P. W. Faguy, W. Ma, J. A. Lowe and W. E. Brewer, Electrochim. Acta, submitted. Communication 4/014491; Received 1st March, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940400771

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994,4(5), 773-774 (BaTiO,),(Gd,Ce),Cu20,: A New Homologous Series of Layered Cuprates containing Various Layers of Perovskite Units Rukang Li Department of Applied Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China Two compounds in a new homologous series (BaTiO,),(Gd, Ce),Cy207 with m =2 and 3 have been synthesized and characterized. The tetragonal unit-cell parameters, a =3.874 04(4) A, c =36.888(1) A (for m =2) and a =3.881 45(5) A, c =44.775(1) A (for m =3) for the two compounds were determined by X-ray and electron diffraction. From the diffraction patterns, we deduce that the structures of the two compounds are built up by alternative stacking of multiple perovskite layers, copper-oxygen planes and double fluorite layers.The unusual structural characteristics in the series are the wide separations between the CuO, planes. Although both compounds contain CuO, planes, as is common in high-T, cuprates, we have not yet succeeded in making the samples superconducting. As it is now widely accepted that copper-oxygen planes in cuprates are crucial for the appearance of superconductivity, we have aimed to explore cuprates containing new interleaving layers which connect or separate the CuO planes as potential high-T, superconductors.' A single perovskite layer, MTiO, has long been incorporated into the high-T, cuprates for connecting the Cu-0 planes.2 Recently, Gormezano and Weller3 found that a double perovskite layer can also act to joing the CuO, pyramidal layers.Based on these studies, we have succeeded in preparing a new cuprate: Ti,(Ba,Gd) (Gd,Ce)2Cu20y, which is built up by alternative stacking of (Gd,Ce),02 fluorite layers and a double perovskite unit joining CuO, pyramidal layers4 in the sequence (Gd,Ce),O2-CuO5-MTiO3-MTiO3-Cu0,-( Gd,Ce),O,. The structure of the compound has been assigned as Ti-2322, in accordance with the structures of other layered cuprates. However, if one rewrites the chemical formula as: (BaTiO3),(Cd,Ce),Cu2O7, the Ti-2322 compound corre-sponds to the m=2 member in this family. Thus it is plausible to expect other members in the family with different m could also exist. Here, I report the preparation and the characteriz- ation of new cuprate compounds with m=2, 3 in the (BaTi03),(Gd,Ce)3Cu207 family.The (BaTiO,),Gd, -,Ce,Cu,O, samples were prepared by solid-state reaction from appropriate ratios of the starting compounds Ti0,(99.99%), BaC0,(99%), Gd203( 99.95%), Ce0,(99.9%) and CuO(99%). The thoroughly ground mix- tures of the reagents were first heated at 1050 "C for 48 h in covered corundum crucibles with an intermittant grinding. After cooling to room temperature, the samples were reground, pelleted and finally calcined at 1100 "C for another 48 h, followed by furnace cooling. The as-prepared samples were then subjected to X-ray powder diffraction, electron diffrac- tion, resistance measurements and further heat treatment. The X-ray diffraction (XRD) patterns were recorded with a Rigaku D,,, -rA diffractometer equipped with a Cu-Ka rotating anode source.A step width of 0.02" (28) and a counting time of 1 s were applied. A curved graphite mono- chromator was set in the diffraction path and silicon powder added as the internal standard. Selected-area electron diffrac- tion (SAED) was performed on a Hitachi H-800 transmission electron microscope. The sample resistances were measured by the standard four-probe method down to 15 K in a commercial He-circulating refrigerator. Taking account of the proposed structural relationship among members in the (BaTi03),(Gd,Ce)3Cu207 family, we derive the structural models shown in Fig. 1 (up to m= 3). All Fig. 1 Ideal structural models of the members in the (BaTi0,),(Gd,Ce),Cu20, family (FL=fluorite layers, PU .=perov-skite units).The upper parts of the models are shifted by (3, +:I relative to the lower parts the models can be viewed as combinations of two types of structural unit. One unit is the multiple perovskite layer connecting copper-oxygen layers, i.e. Cu05-( Ba?'iO,),-CuO, (m=O, 1, 2,3). In combination with the other type of unit, the double fluorite layers Ln202, the units of the per- ovskite containing CuO, bilayers are shifted alternatively by a translation 1/2, 1/2 in the basal plane. This translation causes the primitive lattice to become body centred. The members of the family differ from each other by the number of perovskite layers between pairs of CuO, planes.Thus the lattice constant c correspondingo to each structure should follow the relation: c, + =c, +8 A. Since (Y,Ce)2SrCuFeOy(c,=20.5 A), and NbSr,(Nd,Ce),Cu,O, (cl =28.8 A), may be taken as model examples for the structures with m=O and 1, respectively, the c paramtters of the mepbers with m=2 and 3 would be c2=36.8 A and c3=44.8 A from the above relation. Fig. 2 shows the XRD patterns of the as-prepared samples with the nominal compositions of Ti2Ba2Gd2.33Ce0.67(:u20, (rn=2) and Ti3Ba3Gd2.4 Ceo&u20y (rn=3). The m =2 samples are nearly single phase with a minor phase of BaTiO,. For the samples with m =3, a large amount of BaTiO, was found to coexist with the main phase. Although the patterns for the samples m=2 and 3 seemed to resemble each other, we have J.MATER. CHEM.. 1994, VOL. 4 Fig. 2 X-Ray and electron diffraction patterns (insets) of the Ti,Ba,Gd2,33Ceo,67C~,0y and Ti,Ba,Gd,,,Ce,,,Cu,O, samples. The asterisked peaks in the patterns are due to BaTiO, indexed the two patterns successfully by least-squares fitting the XRD peaks in the range 20-65" and 9btained the cell parameters: a 3.87404(4) A, c,= 36.888(1) A for m =2, and a =3.88145(5) A, c =44.755(1)A for m =3, respectively. The systematic absences of diffraction indices, h +k +1 =odd, as well as the magnitudes of the c parameters all agree with the ideal structure models [Fig. l(c), (d)] and space group The SAED patterns of the two samples (Fig. 2, inset) generally agree with our indexing of the XRD patterns.For m=2 samples, the strongest 001 spot is 0018 (the 9th spot) whereas that for m=3 is 0022 (the 11th spot). This indicates that the m=2 sample has a nine-layer structure and the rn= 3 has an eleven-layer structure. Such evidence further supports the proposed structural models for the two compounds. However, broad, weak streaks along the c* direction at (3,2, 1) were observed in a pattern of sample with m=3. Those streaks reveal the existence of distortion from the ideal 14,mmm structure. The distortion may originate from rotation of the TiO, octahedra in the perovskite units similar to the SnO, rotations found in L~,B~,S~,CU,O,,.~ It is worth noting that an important structure feature of the compounds in this family is that the CuO planes are well separated in the unit cel!.Normally, the thickness of the double fluorite layer is 6 A, whereas those of !he perovskite layers in the m=2, 3 compounds are 8 and 12 A, respectively. If one assumed that the Cu0,-Ln,O,-CuO, units were the source for superconductivity, the present compounds would provide model examples to study whether well isolated CuO planes are able to support high-T superconductivity. In fact, both samples showed low room-temperature resistances and semiconductive behaviour down to 15 K. Although the resis- tivities and their temperature dependences became smaller and weaker, all the samples remained semiconductive after treatment in flowing oxygen at 1100 "C. A preliminary high oxygen pressure treatment (20 atm, 800 "C) was also unsuc- cessful to make the samples superconducting or metallic.In conclusion, we have succeeded in preparing two new layered cuprates with the general composition (BaTiO,),(Gd,Ce),Cu,O,: rn =2 and 3. The compounds con- tain units with multiple layers of BaTiO, sandwiched between CuO, planes. The structures of the compounds are alternative stacking of this unit and double fluorite layers. Although the structures possess complete CuO, planes, we have not yet succeeded in inducing superconductivity in them. Financial support from the Fok Ying Tung education foun- dation is gratefully acknowledged. References 1 Li Rukang, Appl. Phys. Commun., 1992,11,295. 2 Li Rukang, Zhu Yiangjie, Xu Cheng, Chen Zuyao, Qian Yitai and Fan Chengao, J.Solid State Chem., 1991,94,206. 3 A. Gormezano and M. T. Weller, J. Muter. Chem., 1993,3,771. 4 Li Rukang, J. Solid State Chem., in the press. 5 Li Rukang, Tang Kaibin, Qian Yitai and Chen Zuyao, Muter. Res. Bull., 1992,27, 349. 6 M. T. Anderson K. Poeppelmeier, J. P. Zhang, H. J. Fan and L. D. Marks, Chem. Muter., 1992,4, 1305. Communication 4/01335B; Received 8th February, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940400773

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(5), 775 CORRIGENDA Corrigendum to Effects of Sintering Conditions on Hydroxyapatite for Use in Medical Applications: A Powder Diffraction Study Isaac Abrahams, Department of Chemistry, Queen Mary and Westfield College, University of London, Mile End Road, London, UK El4NS, Jonathan C. Knowles, IRC in Biomedical Materials, Queen Mary and Westfield College, University of London, Mile End Road, London, UK El 4NS J. Mater. Chem, 1994,4,185. Please note that the footnote on p. 186 should read: t Supplementary data available (SUP 57007,40 pages); details from Editorial Office. Corrigendum to Preparation and Characterization of a Sodium Insertion Compound of Hydrogen Molybdenum Bronze, Na0.*5(H20)y[H0.21 MOO31 Kazuo Eda, Noriyuki Sotani, Department of Chemistry, faculty of Science, Kobe University, Nadu-ku, Kobe 657, Japan, Flumikazu Hatayama, School of Medical Sciences, Kobe University, Tomogaoka, Suma, Kobe 654-07, Japan, Masakazu Kunitomo, Toshiro Kohmoto, Department of Physics, Faculty of Science, Kobe University, Nadu-ku, Kobe 657, Japan J. Mater. Chem, 1994, 4, 205. Please note that the footnote on p. 205 should read: 1'Supplementary data available (SUP 57006, 2 pages); details from Editorial Office.

ISSN:0959-9428    

DOI:10.1039/JM9940400775

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( S), 777-779 BOOK REVIEWS Liquid Crystals. By S. Chandrasekhar. Cambridge University Press. 1992. Second Edition. Pp. xvi +460. Price f22.95. ISBN 0-521 -41 747 3 (Hardback); 0-521 -42741 -X (Softback). Those familiar with the first edition of Professor Chandrasekhar’s ‘Liquid Crystals’ published in 1977 will welcome the appearance of this second edition, which has been considerably extended by the addition of around a further 100 pages of text, and doubling the references to more than 900. For newcomers to the fascinating field of liquid crystals, the book provides a reasonably complete overview of the physics of thermotropic low-molecular-weight liquid crystals. When the first edition appeared in 1977, the subject matter was largely of academic interest, and applications were restricted to simple calculators and watches.Indeed at that time there was considerable discussion amongst experts about which type of display technology (liquid crystal or non-liquid crystal) would be successful in challenging the existing display technologies. Nowadays the widespread availability of liquid crystal watches, calculators, digital indicators, computer screens and increasingly TVs, means that all scientists and most of the general public, will at least have heard of liquid crystals. The success of liquid-crystal science in the area of displays can be attributed in large measure to the development of suitable materials, so it is entirely appropriate to review this new edition in a journal devoted to materials chemistry.To set the subject matter of the book in context, it is important to note that liquid-crystal science is concerned with the study of a wide range of materials, including lyotropic liquid crystals formed by solution of amphiphilic molecules in suitable solvents (usually water), biological membranes, main- and side-chain liquid crystal polymers and thermotropic low-molecular weight liquid crystals which form when certain solid crystals melt. This book is largely concerned with the physics of thermotropic liquid crystals, although lyotropics and polymer liquid crystals do get a very brief mention. It has to be admitted that chemists and materials scientists lacking a good physics base will have a hard time assimilating the contents but the effort will be rewarded with a thorough understanding of the principles of liquid-crystal science.The study of liquid crystals is accurately described as interdisciplin-ary, involving scientists from many backgrounds, and the preparation of a monograph addressed to all should address this diversity in readership. Chandrasekhar attempted to do this in the first edition by ‘presentCing] as far as possible a self-contained treatment of ... different aspects of the subject’, and this approach is perpetuated in the second edition. Thus this is less of a textbook but more of a source book on the physical behaviour of liquid crystals, a point reinforced by the extensive referencing of each of the six chapters. Although wide-ranging in the topics covered, there has, of necessity, been some selection, which reflects to an extent the particular interests of Professor Chandrasekhar and the research group at Bangalore over the years.What is remarkable is the number of areas of liquid crystal studies that the scientists from Bangalore have contributed to, and there are very few topics that do not get a mention. This 400+ page book begins with a brief introductory chapter describing the different structural types of liquid crystals; many of the more recently discovered structural developments, including the polymorphism of smectic A and C liquid crystals, and columnar and discotic liquid crystals are covered in later chapters. Chapter 2 gives a clear account of the statistical theories of nematics, while the third chapter contains an extensive discussion of both static and dynamical aspects of continuum theory, with descriptions of a variety of electric-field effects, including the more esoteric topics of electro-hydrodynamics, flexoelectricity and order electricity.Chapter 4 entitled ‘Cholesteric liquid crystals’, contains a useful introduction to the optics of twisted structures using Jones matrices, and a brief mention of the doubly twisted cubic phases known generically as blue phases. The essential feature of these, and indeed other twisted smectic and colum- nar phases is their intrinsic chirality-the relationship to derivatives of cholesterol is more and more tenuous, and hopefully the old classification of ‘cholesteric liquid crystals’ will soon vanish.The chapter on smectic liquid crystals has been considerably extended from the first edition and now contains details of modulated smectic A and smectic C phases as well as the twisted chiral smectic A (twist grain boundary) phase and the chiral ferroelectric smectic C phase. which is the basis of a new generation of fast-switched ferroelectric liquid crystal displays. Since Professor Chandrasekhar and his group art: credited with the discovery in 1977 of a new category of thermotropic liquid crystals based on disc-shaped molecules, it is fitting that the new and final chapter of ‘Liquid Crystals’ should be devoted to discotic liquid crystals.This includes details of structural types of discotic liquid crystals as well as their theory and a description of the defects observed. Perhaps it is worth noting that throughout this book the fluid-state defects and disclinations characteristic of all liquid crystalline phases are discussed in some detail, and this provides a recurring theme through the text. Intermediate in structure between the nematic phases of rod-like (long-axes ordered) and disc-like molecules (short-axes ordered) is the biaxial nematic in which both long and short molecular axes are ordered, and the final section of the book is devoted to a description of such a phase. It must be said that the existence of a low-molecular-weight thermotropic biaxial nematic is still the subject of controversy, but the structural basis for the phase is clear enough.The selection of topics means that there are a few subjects that get little or no attention. For example hexatic phases (smectics B, F and I) have a very brief mention, while there is little on device physics, liquid crystalline materials for applications and the relationships between molecular proper- ties and liquid crystalline behaviour. This is in no sense a criticism, and it is remarkable how many aspects of liquid- crystal science are dealt with in this book. The second edition of Chandrasekhar’s ‘Liquid Crystals’ can be recommtmded to any liquid crystal scientist wishing to have a comprehensive account of the physics of liquid crystals. D.Dunmur Receioed 13th January, 1994 Photochemical Vapour Deposition. By J. G. Eden. Volume 122 in Chemical Analysis: A Series of Monographs on Analytical Chemistry and Its Applications. Wiley-lnterscience, 1992. Pp. xi +194.Price f43.95.ISBN 0-471 -55083-3. Chemical vapour deposition (CVD) represents a well-established technology for the successful production of thin films for a variety of advanced materials applications, all of which rely on the ability of the technique to deposit films of controllable composition and uniform thickness. Photochemical vapour deposition is a more recent develop- ment of CVD in which the rate and products of deposition are influenced by the presence of light. The introduction of visible, ultraviolet or vacuum ultraviolet photons to a CVD reactor offers new possibilities in respect of (i) the production of novel materials and (ii) overcoming some of the short- comings associated with conventional CVD, in particular the requirement for high growth temperatures and consequent less than ideal selectivity of deposition.As stated in the preface the purpose of this book is to provide an overview of photochemical vapour deposition with the following audiences in mind: the student approaching photodeposition for the first time, the thin-film engineer wishing to evaluate photochemical vapour deposition for a particular application and possibly to adapt it to an existing process, and the experienced researcher desiring a review of the work completed to date.Meeting these diverse require- ments within a single volume would appear to be a rather daunting task, but since the field of photochemical vapour deposition is still young the author has been able to achieve this satisfactorily with a timely book which is organized in the following manner. The first three chapters comprise intro- ductory material describing the basic principles underlying photochemical vapour deposition and practical aspects of designing appropriate reactor systems, with particular em-phasis on the choice (and limitations) of optical sources. Subsequent chapters include detailed discussions of the current status of the technique with respect to the growth of metal (Chapter 4, 26 pp.), semiconductor (Chapter 5, 45 pp.) and dielectric (Chapter 6, 19 pp.) films.Other applications such as the growth of metal oxide films constituting the high T, superconductor compositions and light-enhanced surface pro- cessing such as cleaning and nitridation are briefly covered. The concluding chapter provides an assessment of the future prospects for photochemical vapour deposition. As indicated above, the emphasis of the contents of the book concerns the versatility of the technique for growing metal, semiconductor and dielectric films. Applications receiv- ing special attention include the photodeposition of dielectric films for the fabrication of VLSI devices and the growth of amorphous hydrogenated silicon for photovoltaic appli-cations. These aspects are comprehensively covered and, in many cases, comparisons are provided with the results from other more well-established deposition routes, e.g.plasma CVD, MOCVD, MBE etc. The coverage includes much tabulated information that will serve as an excellent source of reference for both academic and industrial scientists from a range of disciplines including chemistry, physics, electronics, electrical engineering and materials science. The advantages offered by photochemical vapour deposition clearly lie in the lower processing temperature requirement, which frequently results in enhanced selectivity of deposition and lower levels of contaminants in the resultant films; particularly, the bane of conventional CVD, adventitious carbon. In view of the recent MOCVD studies in which preformed adducts of parent precursors have been successfully used to provide some ‘tailor- ing’ of the behaviour of the parent during the decomposition process it is perhaps surprising that this does not yet appear to have been used in tandem with photochemical vapour deposition.The book has been written by an active and expert prac- titioner of the subject in a clear and easy to read style. It contains few typographical errors and is well referenced. However, the index is barely adequate. As with any rapidly advancing field the obvious danger lies in the fact that the book will rapidly become out of date. Nevertheless, it provides both an excellent ‘state of the art’ view of the field in the early 1990s and a very good reference source of the subject area.Although a variety of films have been grown successfully by conventional CVD processes, the author acknowledges that the development of photochemical vapour deposition as a J. MATER. CHEM., 1994, VOL. 4 reliable tool for the device engineer or the materials scientist is still in its infancy. Indeed, in terms of applications and understanding, photochemical vapour deposition probably occupies a similar position to CVD 10-15 years ago. Much remains to be discovered and understood. Hopefully, the specific advantages that are highlighted in this book will focus the attention of device engineers and materials scientists and enhance the general awareness of the technique. R.Whyman Receiued 7th January, 1994 Superconductivity Today (An Elementary Introduction).By T. V. Ramakrishnan and C. N. R. Rao. Wiley Eastern. 1992. Pp. x +116. Price Rs 55.00. ISBN 81-224-0391-3. ~~~~~ This slim volume is intended as an elementary introduction to the field of superconductivity, mainly for graduate and postgraduate students. Written in a readable way, it encompasses historical perspectives and a phenomenological description of superconductivity, together with an account of superconducting materials. The book also includes a chapter containing convenient ‘potted’ accounts of the development of theories of superconductivity up to 1956, including the London equation, Ginsburg-Landau and BCS theories. A separate chapter is devoted to the anomalous properties of cuprates, together with descriptions of recent theories put forward to explain their normal state and superconducting properties.As a chemist, I found the two chapters describing theories to be particularly useful and well written; both are presented at a very accessible level for graduate or postgradu- ate students, and while not especially rigorous, they give a good flavour of the basic ideas behind the different theoretical developments. The book concludes with short sections on applications of superconductivity, and the challenges still before us. In this rapidly developing area, any text-book runs the risk of becoming out-of-date before publication; casualties here are the doped C60 superconductors (mentioned only in a footnote in the materials chapter, and briefly in the final section), and the Hg-containing cuprates.The description of superconducting materials as a whole is probably one of the less impressive parts of the book; it contains adequate descrip- tions of a range of superconducting materials from conven- tional alloys to cuprates (but very little on organic CT compounds), but the discussion of the features of a structure which are important for superconducting behaviour is not well developed. I suspect these parts of the book would leave a rather confused impression in the mind of a student not already familiar with the materials discussed. Since 1987, I have been looking for a short introductory text that I can recommend to 3rd year undergraduates and 1st year postgraduates in chemistry, in order to give them a concise overview of superconductivity without swamping them in formulae.This book is by no means perfect in this respect since, in order to be concise, much is glossed over, and there are a few points where the text does not flow well as a result. There are a number of typographical errors in formulae and some errors in the text (for example the description of the Jahn-Teller distortion of the CuO, octahedra in the cuprates). Nevertheless, the book is very accessible and covers a wide range of topics in an interesting and stimulating way. It will certainly be useful to any student, and given the nature of the subject matter, perhaps no authors could hope for more.The book appears to be written to some extent for the Tndian market (for example, the final section comments specifically J. MATER. CHEM., 1994, VOL. 4 on the challenges facing the Indian scientific community) and is priced very attractively at Rs 55.00. Whatever the price on the wider market, the book is probably worth the cover price for the concise, palatable theory chapters alone. W. Flavell Received 7th January, 1994 Solid State Chemistry: Compounds. Ed. A. K. Cheetham and P. Day. Clarendon Press, Oxford, 1992. Price €40.00. Pp. xii +304. ISBN 0-19-855166-5. This is a multi-author companion to Solid State Chemistry: Techniques published in 1988 and the new volume is a valuable addition to the literature devoted to solid state chemistry.The one disappointment attaching to it is that, while the title implies a general coverage of the subject, the reader discovers that the book deals with selective topics only. However, the topics included have been well chosen, covering electronic structure as an essential starting point followed by treatments of several classes of materials that have been prominent in solid state chemical chemistry research in recent years. In the preface the editors express the hope that they will be able to deal with optoelectronic materials, magnetic materials and solid electrolytes in a further volume. The material is presented lucidly by a distinguished team of contributors and the editors have achieved an effective structure that results in a ‘good read’.The classes of materials included are: chain compounds, superconducting materials (inorganic only), metal-rich compounds, heterogeneous cata- lysts, intercalated layered compounds, zeolites and ferroics (a generic grouping including ferro-magnetic, ferro-electric and ferro-elastic materials). There are no weak chapters and the surveys of superconducting materials, metal-rich compounds and intercalation in layered compounds are particularly useful. The volume is well referenced and well indexed. It should prove useful for final year undergraduates studying special topics and for first year graduate students embarking on research in the field. P. T. Moseley Received 23rd December, 1993 Fatigue of Materials.By S. Suresh. Cambridge Solid State Science Series. Cambridge University Press, 1992. Pp. xviii +618. Price €24.95,ISBN 0-521-43763-6. As the author clearly states, the fatigue behaviour of load- bearing materials is of utmost concern in a wide range of engineering applications. In spite of this, very few complete and well-balanced text-books are presently available on the market. ‘Fatigue of Materials’ certainly goes a long way to resolving this problem. The book presents a comprehensive treatment covering both the physics as well as the mechanics of crack initiation and propagation in materials as diverse as ductile metals, brittle ceramics and high performance com-posite materials. The author refers to almost 1000 scientific articles covering virtually all aspects of the fatigue behaviour of modern materials.The book begins with an interesting review of’ the early pioneering work undertaken in the late nineteenth and early twentieth century. The introduction continues by presenting the different approaches that are currently adopted in acade- mia and industry for characterizing the long-term behaviour of structures and components. The early chapters consider the problem of deformation and fatigue crack initiation in a range of ductile materials. Here, the influence of grain boundaries in metals, cyclic hardening in polycrystals and other relevant effecr s are dis- cussed in terms of microstructural and environmental aspects. Following this, a short chapter examining phenomenological continuum approaches for characterizing the total fatigue life of a material is presented.Chapter 5 outlines the principles of fracture mechanics and their application to fatigue loading. Here, one feels that the chapter is perhaps misplaced since it somewhat breaks the rhythm between the chapters on fatigue crack initiation in ductile solids and on fatigue crack growth inductile solids. Perhaps this section would have been better placed in an earlier chapter. The subsequent chapters consider fundamental subjects, such as stress concentrations, small fatigue cracks and variable amplitude fatigue loading. Chapter 12 contains a well written introduction to environ- mental effects covering corrosion cracking and high tempera- ture fatigue response in an interesting and detailed manner. The later chapters present the problem of fatigue of brittle, semicrystalline and non-crystalline solids. In these sections considerable attention is given to failure in ceramics as well as polymeric materials. The final chapter outlines ~t number of fascinating (although sometimes fatal) case studies. Here, many of the concepts presented in earlier chapters are re-discussed putting them very much in an application-minded context. The book also contains a significant number of problems suitable for undergraduates as well as postgraduates. Without doubt, ‘Fatigue of Materials’ represents an invalu-able addition to an engineers or researchers library. Certain chapters would be well suited to postgraduate courses in either materials science or mechanical engineering. It IS written in a style that is both clear and easy to read. The figures are well presented and pertinent. The relatively low price of E25 makes it excellent value for money! W. J. Cantwell Received 17th Febru,zry, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940400777

出版商:RSC    

年代:1994

数据来源: RSC