摘要:

J. MATER. CHEM., 1994, 4( 5), 683-687 Studies of Vapour-phase Chemical Derivatisation for XPSAnalysis using Model Polymers Ian Sutherland, Enshan Sheng, Derek M. Brewis and Richard J. Heath Loughborough University of Technology, Loughborough, Leicesfershire LEI 7 3TU, UK Vapour-phase chemical derivatisation carried out on a vacuum adsorption rig has been investigated. Model polymers were used to evaluate the reactivity and selectivity of trifluoroacetic anhydride (TFAA) and hydrazine. Results have shown that TFAA is a good derivatising reagent for hydroxyl groups, while hydrazine is not suitable for carbonyl groups. Chemical derivatisation with TFAA of flame-treated polypropylene surfaces has shown that under various flame conditions an approximately constant proportion (ca.20%) of the oxygen introduced by flame treatment was present as hydroxyl groups. Selective removal of OH groups by derivatisation has a large effect on the adhesion of a flame-treated polypropylene surface to a reactive polyurethane paint, evidence of chemical reaction at the interface. X-Ray photoelectron spectroscopy (XPS or ESCA) is one of the most widely used surface analysis techniques available for the study of polymers. It has been shown to be a valuable surface analysis tool for elucidating molecular structure from chemical-shift data. However, even for very-high-energy reso- lution XPS, many functional groups cannot always be ident- ified owing to the overlap of chemically shifted peaks or to their low concentrations on the surface.To overcome this problem, chemical derivatisation can be used. It uses a chemi- cal reagent to react with a specific functional group, with the derivative having a unique element which is not previously present on the surface. Reagents containing fluorine are often used because of its large photoelectron cross-section. Various chemical derivatisation reactions have been reviewed by Briggsl and also Batich.2 Chemical derivatisation has been employed by many work- ers to analyse various functional groups after surface modifi- cation of a polymer. These include: trifluoroacetic anhydride (TFAA) for hydroxyl hydra~ine~'~and penta- fluorophenylhydrazine (PFPH)12 for carbonyl groups; sodium hydroxide (NaOH),11,12 trieth~lamine~.~ and trifluoroethanol (in the presence of a coupling agent and a catalyst) for carboxylic acid gro~ps.~~,~~,'~ Many of these reactions are solution-phase derivatisation with which some problems may be associated.Solvents may extract from a polymer lower molecular weight materials, which may be produced during a surface treatment, and at the same time may cause migration of functional groups away from the surface into the bulk of the polymer. The removal of the solvent after the derivatisation could be particularly problematic if ionic derivatives are employed as in the case of carboxylic acid derivatisation with NaOH where the amount of sodium incorporated has been found dependent on the washing conditions." Considering these possible problems associated with solu- tion-phase derivatisation, vapour-phase derivatisation can be used to eliminate these problems.Vapour-phase derivatisation for XPS analysis was first reported by Hammond et a1.: followed by several other worker^.^.^^ Successful chemical derivatisation requires that the derivatising reagent selectively and ideally completely reacts with the functional group. In this work, the reactivity and the selectivity of TFAA towards Table 1 Model polymers used for vapour-phase derivatisation polymer poly(viny1 alcohol) poly(acry1ic acid) poly(viny1 methyl ketone) poly(ethy1ene terephthalate) abbreviation supplier PVA Aldrich PAA Aldrich PVMK Aldrich PET ICI hydroxyl and hydrazine towards carbonyl groups were exam- ined using several model polymers.Effects of hydroxyl groups on the adhesion of flame-treated polypropylene (PP) with a polyurethane (PU) paint was investigated by vapour-phase chemical derivatisation. Experimental Poly(viny1 alcohol), poly(acry1ic acid), poly(viny1 methyl ketone) and poly(ethy1ene terephthalate) (PET) were used as model polymers for hydroxyl, carboxylic acid, carbonyl and ester groups. Details of these polymers are listed in Table 1. PET was supplied as 100pm film. It was cleaned in an ultrasonic bath with trichloroethylene for 30s and used directly as a model polymer for ester groups. Other model polymers were prepared by solution coating onto this cleaned PET film. The coating method involved dipping a clean piece of PET film into the solutions, i.e.10% PVA aqueous solution, 4% PAA aqueous solution or 4% PVMK solution in dimethylformamide (DMF). The film pieces were dried and kept in a desiccator. XPS and attenuated total reflection (ATR) infrared analysis showed that the PET film was com- pletely covered by the model polymers. Vapour-phase deriv- atisation reagents, trifluoroacetic anhydride (TFAA 99 +%) and hydrazine (anhydrous 98 YO)were purchased from Aldrich. Vapour-phase derivatisation was performed on a vacuum adsorption rig (Fig. 1). Vapour-phase derivatisation on a vacuum adsorption rig is considered superior to that under ambient conditions. Water vapour in the air and adsorbed molecules on sample surfaces may affect the derivatisation reaction. Adsorbed vapour on derivatised sample surfaces can be conveniently removed by pumping under the vacuum adsorption rig.Also, since it is an enclosed system it is safer to operate. There are three sub- rigs under the main adsorption rig. Each sub-rig was used exclusively for one derivatisation reaction. Two liquid-nitrogen traps were used to prevent hydrocarbon vapour from reaching the adsorption system. To avoid cross-contamination, different derivatisation reactions were carried out at a time interval of at least 1 day. Air in the flask (250 cm3) containing ca. 10 cm3 derivatising reagent was removed by pumping after the reagent had been frozen with liquid nitrogen. The sample (1.5 cm x 4.0 cm) was evacuated to a base vacuum of ca.10-5Torr and exposed to the derivatising reagent. During the reaction, the flask containing the reagent was kept at 20 "C (room temperature, ca. 25 "C) using a water bath during the reaction to avoid the conden- sation of the reagent on the sample surface. The water-bath temperature was found to have a significant effect on the rate J. MATER. CHEM., 1994, VOL. 4 vacuumTmain vacuum adsorption rig I I sub-rig 1 i P empty flask liquid nitrogen rotary traps gasoutlet Fig. 1 Schematic of vacuum adsorption rig used for vapour-phase derivatisation of the derivatisation rea~ti0n.I~ After the reaction, the flask was sealed and the specimen pumped for ca. 24 h to expel any physically adsorbed vapour.XPS analysis was normally performed on the same day. X-Ray photoelectron (XPS) spectra were recorded on a VG ESCALAB MKI spectrometer using an Al-Ka X-ray source at a pass energy of 85eV and a take-off angle of 90" with respect to the polymer surface. Elemental compositions were calculated by measuring the peak areas following the subtrac- tion of a Shirley-type background. Scofield photoelectron cross-sections15 were used. Correction was made for inelastic mean-free path,16 transmission of the energy ana1yse1-I~ and angular asymmetry in photoemission'* (where appropriate). Radiation damage was not found to be significant under the conditions used (X-ray source power, 200 W; acquisition time, <5 min). Flame treatment of a propylene homopolymer film (100 pm) manufactured by Neste was carried out as described in a previous paper.Ig A two-pack polyurethane paint (M615-122+M210-763, ICI) was air-sprayed onto the PP surface and cured at 90 "C for 30 min.Tensile adhesion was assessed using a composite butt adhesion test.19 In the test, the sample was bonded by an epoxy adhesive (AV100+HV100, Ciba-Geigy) between two cylindrical steel butts with a length of 50 mm and a diameter of 28 mm, and the tensile adhesion testing was carried out after the adhesive was cured. Results and Discussion Surface oxygen concentrations of model polymers, i.e. PVA, PAA, PVMK, and PET, were analysed with XPS. Results are listed together with their stoichiometric values in Table 2.Experimental values agreed broadly with stoichiometric ones. The slight difference (ca. 15% lower in all cases) might be attributed to the orientation of oxygen-containing groups away from the near surface or to the overestimation of the oxygen sensitivity factor. Derivatisationwith TrifluoroaceticAnhydride Trifluoroacetic anhydride (TFAA) was used to derivatise hydroxyl groups. Its reaction with PVA to form an ester can be written as +CH,--FH* OH PVA model polymer ~~~~~ ~ poly(viny1 alcohol) (PVA) poly(acry1ic acid) (PAA) poly(viny1 methyl ketone) (PVMK) poly(ethy1ene terephthalate) (PET) 00 ::II II + CF~-C-O-C-CF~ -+CH,--yHk + CF3-C-OH 0 ITFAA O=C-CF3 Table 2 XPS elemental compositions of model polymers oxygen concentration (atom%) functional group stoichiometric experimental -F-OH 33.3 28.7 0-&-OH 40.0 33.9 0 20.0 17.4-8-P 28.6 23.6-c-0 J.MATER. CHEM., 1994, VOL. 4 401 401 ’”i ./ 0 0.0 0.2 0.4 0.6 0.8 1.o conversion factor x Fig. 2 Theoretical oxygen (0)and fluorine (x) concentrations of PVA derivatised with TFAA according to eqn. (1) and (2) Assuming that all elements are homogeneously distributed within the XPS sampling depth both before and after the derivatisation, the relationship between the conversion factor and the surface atomic concentrations of oxygen and fluorine can be derived as follows. Neglecting hydrogen atoms, which are not detected, there is an original atomic percentage composition of PVA of [Cl0=71.3% and [010=28.7% as detected by XPS.As the sample is derivatised, each hydroxyl group that reacts introduces six extra atoms: 10, 2C and 3F. Taking the conversion factor as x (x =O no reaction, x =1 complete reaction), then the atomic percentage compo-sition of oxygen and fluorine measured by XPS can be written colo+xcolo = [C], +[010+6x [010 3x cola x 100cF1= [C]o+[O]o+6~[O]o [O] and [F] are plotted against conversion factor x in Fig. 2. This shows that the extent of reaction does not vary linearly with [F] detected, as is often mistakenly assumed. Fig. 2 also shows that [F] changes more rapidly than [O] as the reaction proceeds, indicating that [F] should be used to estimate the conversion factor.PVA was allowed to react with TFAA for various durations. Experimental [O] and [F] as detected by XPS are plotted against reaction time in Fig. 3. TFAA was found to react rapidly with PVA. Limiting fluorine incorporation on the surface was obtained at a reaction time of ca. 2 h. No significant increase in [F] was observed even after 16 h reaction. At 16 h reaction, the conversion factor was calculated to be ca. 80% according to eqn. (2) assuming that all the original oxygen existed as hydroxyl groups. This demonstrates that TFAA has a good reactivity towards hydroxyl groups. To evaluate the selectivity of TFAA, other model polymers were reacted with TFAA for 2 h under the same conditions. Results are shown in Table 3. Table 3 shows that only small amounts of fluorine were detected on PAA and PVMK surfaces after 2 h reaction with TFAA, indicating a good selectivity of TFAA towards hydroxyl groups in the presence of carboxylic acid and carbonyl groups.The reaction of TFAA with PAA is thought to be via the substitution of acid H atom and the reaction with PVMK could be through the reaction with enol groups. t O l d 0 4 8 12 16 reaction time/h Fig. 3 Experimental oxygen (0)and fluorine (x) concentr&ons of PVA as a function of reaction time with TFAA Table 3 XPS elemental compositions of model polymers reacted with TFAA for 2 h elemental compositions (atom%) polymer C 0 F PVA 51.3 23.3 25.4 PAA 65.2 31.2 3.6 PVMK 78.9 17.2 3.9 HPP untreated 97.8 1.1 1.1 Derivatisation with Hydrazine A hydrazone is formed as hydrazine reacts with PVMK: SCH2-FHk + NH2NH2 -SCHZ-THk + H20 c=o C=NNHa I I CH3 CH3 PVMK hydrazone The conversion factor (x) can be related in a similar way, as in the case of TFAA derivatisation, to [O] and [N] on derivatised surfaces by the following equations: (3) (4) where [010=17.4 is the original oxygen concentration before derivatisation.A similar plot as in Fig. 2 shows an almost linear relationship between x and [O] or [N]. Fig.4 shows the experimental [O] and [N] against derivatisation duration of PVMK with hydrazine. Again, there exists a limiting [N] value which corresponds to a conversion factor of .Y 0.5. Further prolonging of the reaction was found to have no significant effect on x.While an incomplete derivatisation of PVMK with hydra- zine was observed, the derivatisation with hydrazine towards carbonyl groups was found to be far less selective than that of TFAA towards hydroxyl groups. XPS analysis results of hydrazine-derivatised model polymers are shown in Table 4. Although the reaction of hydrazine with PVA is negligible, substantial reaction with PAA and particularly PET was found. White powder products were observed on the PET 2o T 0 2 4 6 8 10 12 reaction timeh Fig. 4 Experimental oxygen (0)and nitrogen (x) concentrations of PVMK as a function of reaction time with hydrazine Table 4 XPS elemental compositions of model polymers reacted with hydrazine for 2 h elemental composition (atom%) polymer C 0 N PVMK 75.0 8.9 16.1 PVA 70.6 28.5 0.9 PAA 62.7 27.0 10.3 PET 61.9 14.1 24.0 surface after the reaction. This is thought to be due to the chain-scission reaction of PET with hydrazine. Chemical Derivatisation of Flame-treated PP Polypropylene was treated at various air-to-gas (natural gas) ratios, flame intensities and distances from the polymer surface to the flame inner-cone tip.No oxygen was detected on untreated PP surface. After flame treatment, various oxygen concentrations up to 15 atom% were obtained. Except for those with very low oxygen concentrations (< ca. 2 atom%), the tensile adhesion level between flame-treated PP and polyurethane (PU) paint was so high (ca.26 MPa) that the failure of the test joint was cohesive within the PP film. Derivatisation of flame-treated PP surfaces with TFAA has shown that a substantial amount of oxygen was present as hydroxyl groups. Fig. 5 shows the concentration of oxygen present as -OH groups against the total oxygen concen- tration on the surface. It shows that ca. 20% of the total 4r oxygen concentration (atom %) Fig. 5 Correlation between -OH concentration and total oxygen concentration on flame-treated polymer surfaces J. MATER. CHEM., 1994, VOL. 4 oxygen on flame-treated PP surfaces was present as -OH groups. The role of hydroxyl groups on adhesion was investigated using chemical derivatisation with TFAA. Untreated PP did not show any fluorine on the surface upon TFAA derivatis- ation.PP was treated with a mild flame, and hydroxyl groups on the surface were derivatised with TFAA. Water contact angles (advancing and receding) on PP were observed to increase after the derivatisation. Significant decrease in adhesion level (from 26 to 13 MPa) was found with a two- pack polyurethane (PU) paint after the removal of hydroxyl groups on the PP surface by derivatisation with TFAA. This suggests that the existence of hydroxyl groups on PP is crucial in attaining very high adhesion when the PU paint is used. Hydroxyl groups may have reacted with the PU paint since the paint is a highly reactive system containing isocyanate. A fairly good adhesion level (13 MPa) with the PU paint was retained after the removal of the hydroxyl groups on the flame-treated PP surface, indicating that other functional groups may have some contribution to the level of adhesion achieved by flame treatment.Conclusions Vapour-phase chemical derivatisation with trifluoroacetic anhydride (TFAA) and hydrazine using a vacuum adsorption rig were evaluated using model polymers. The following conclusions can be drawn. (1) TFAA was found to derivatise hydroxyl groups to a high degree. The derivatisation was found to be selective in the presence of carboxylic acid and carbonyl groups. (2) Derivatisation with hydrazine for car- bony1 groups has been found problematic. Incomplete reaction and poor selectivity were found. This casts doubt on the validity of the use of hydrazine to derivatise carbonyl groups as previously rep~rted.~.~ Hydrazine reacts significantly with carboxylic acid and aromatic ester groups.It has been found by Chilkoti and Ratner6 that it does not react with aliphatic esters. (3) An approximately constant percentage (ca. 20%) of oxygen on flame-treated PP surfaces has been found to exist as hydroxyl groups. It has been further demonstrated that these hydroxyl groups are important in the adhesion with a two-pack polyurethane (PU) paint. Adhesion level was shar- ply reduced upon removal of these groups by TFAA derivatis- ation. While hydroxyl groups are important in the adhesion with the PU paint, they may not be so in other cases as of the adhesion between PP and an epoxy adhe~ive.'~ References 1 D.Briggs, in Encyclopaedia of Polymer Science and Engineering, ed. J. I. Kroschwitz, Wiley, Chichester 2nd edn., 1989,vol. 16. 2 C. D. Batich, Appl. Surf. Sci., 1988,32, 57. 3 J. S. Hammond, Polym. Prepr. Am. Chem. Soc. Div. Polym. Chem., 1980,21(1), 149. 4 R. A. Dickie, J. S. Hammond, J. E. de Vries and J. W. Holubka, Anal. Chem., 1982,542045. 5 E. Sheng, I. Sutherland, D. M. Brewis and R. J. Heath, Surf. Interface Anal., 1992, 19, 151. 6 A. Chilkoti and B. D. Ratner, Surf. Interface Anal., 1991,17, 567. 7 L. J. Gerenser, J. F. Elman, M. G. Mason and J. M. Pochan, Polymer, 1985,26, 1162. 8 J. M. Pochan, L. J. Gerenser and J. F. Elman, Polymer, 1986, 27, 1058. 9 W. R.Gombotz and A. S. Hoffman, J. Appl. Polym. Sci. Appl. Polym. Symp., 1988,42,285. 10 Y. Yakayama, T. Takahagi, F. Soeda, K. Hatada, S. Nagaoka, J. Suzuki and A. Ishitani, J. Polym. Sci. Part A: Polym. Chem., 1988,26, 559. 11 D. S. Everhart and C. N. Reilley, Anal. Chem., 1981,53,665. 12 D. Briggs and C. R. Kendall, Int. J.Adhesion Adhesives, 1982, 13. J. MATER. CHEM., 1994, VOL. 4 687 13 A. Chilkoti, B. D. Ratner and D. Bnggs Chem. Muter., 1991, 3, 18 R. F. Reilman, A. Msezane and S. T. Manson, J. Electron 14 51. E. Sheng, Ph.D. Thesis, Loughborough University of Technology, 1992. 19 Spectrosc. Relat. Phenom., 1976,8, 389. I. Sutherland, D. M. Brewis, R. J. Heath and E. Sheng, Surf. Interface Anal., 1990, 17, 507. 15 16 17 J. H. Scofield, J. Electron Spectrosc. Relat. Phenom., 1976,8, 129. M. P. Seah and W. A. Dench, Surf. Interface Anal., 1979,1,2. M. P. Seah, Surf. Interface Anal., 1980,2,222. Paper 3/07050F; Received 29th Novemher, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940400683

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 5), 689-694 Vapour-phase Synthesis of Titanium Nitride Powder Jan Pieter Dekker,*a9b Paul J. van der Put," Hubert J. Veringab and Joop Schoonmana a Laboratory for Applied Inorganic Chemistry, Delff University of Technology, Julianalaan 136, 2628 BL Dele, The Netherlands ECN, Energy Research Foundation, Westerduinweg 3, 1755 ZG Petten, The Netherlands Titanium nitride powder has been synthesized using titanium tetrachloride, ammonia and hydrogen. The influence of the reaction temperature on stoichiometry, particle size and production rate in the gas phase has been investigated. Crystalline titanium nitride powders were formed in all cases. The observed lattice parameter of the powders as a function of reaction temperature suggests that only at high reaction temperatures can a stoichiometric titanium nitride powder be formed.In the temperature range 900-1173 K the mean primary particle size varies between 100 and 200 nm, and the geometric standard deviation varies between 1.1 and 1.3 for the powders formed below a reaction temperature of 11 73 K. A significantly smaller primary particle size is observed at higher reaction temperatures. A fractal dimension analysis indicates that at low reaction temperatures the powders are not agglomerated. There is a demand for sintered ceramic products with increased hardness and strength. Submicrometre powders are desired to fabricate such products, because these yield a fine microstructure with uniform chemical composition. Non- oxide powders are attractive for the production of sintered parts because of their high melting points, high-temperature strengths, high hardness and corrosion resistance.Because of these requirements titanium nitride (TiN) is an attractive material. For example, it might be a substitute for tungsten carbide in cutting tools and wear-resistant parts.' In addition, the vapour-phase synthesis of TiN powder is an important prerequisite of the particle precipitation aided chemical vapour deposition process of TIN. Non-oxide powders can be pro- duced by gas-phase reactions,' solid-state reaction^,^,^ or thermal decomposition of ~olids.~ Gas-phase reactions at high temperatures, such as the oxidation of metal halides have been proven to be a cost-effective method to produce uniform submicrometre particles with high purity.6 The kinetics of formation of these oxide particles such as titania (Ti02) and silica (Si02) have been investigated e~tensively.~-' The forma- tion of several metal nitrides in the gas phase at high temperatures using their respective metal halides and ammonia (NH,) has been shown to be possible as Although the criteria for the synthesis of these solids in the gas phase are not well understood, the formation of a solid in the gas phase seems to be dependent on the thermodynamic equilibrium constant.In general, a homogeneous reaction will only occur if the equilibrium constant of the reaction is sufficiently high.2 TIN can be formed at high temperatures using titanium tetrachloride (TiCl,), hydrogen (H2) and NH,, according to the overall reaction, 2TiC14( g) +2NH3 (g) +H2( g) +2TiN(s)+8HC1( g) ( 1) We believe that the production of TIN powders can be a cost- effective method, because this chemical route involves rather cheap chemicals.However, reports on the formation of this metal nitride are scarce.2*11,12 have investi- Kato's group2*10*11 gated among other metal nitrides and metal carbides the TiN formation in the gas phase. The reduction of TiC1, with magnesium (Mg) in a nitrogen atmosphere has been reported to be effective for the production of TiN.l However, a disad- vantage is that Mg itself is an impurity source. We have investigated the synthesis of TIN powder using TiC14, NH,, and H, at a high temperature. The influence of the reaction temperature on stoichiometry, particle size, and production rate in the gas phase has been investigated.The stoichiometry is determined by XRD analysis, and particle sizes are obtained by quasi-elastic light scattering (QELS) analysis,13 sedimentation field flow fractionation (SdFFF) analy~is,'~.~~and SEM/TEM analysis. The diffusion coefficient of small particles in dilute liquid suspensions can be measured with QELS analysis or the photon correlation spectroscopy technique. Here, the autocorrelation function of the intensity of laser light, scattered by particles in a dilute liquid suspension is calculated. This power spectrum of scattered light allows determination of the particle diffusion coefficient, which can easily be correlated to a particle diameter if it is assumed that the particles are spherical and have a relatively monodisperse particle size distribution.A powder suspended in water can be fractionated by the SdFFF technique. In SdFFF a suspen- sion flows through a force field in a centrifuge. The mass distribution of the dispersed powder is exponentionally distrib- uted in the field as a result of the counteracting combination of Brownian diffusion and centrifugal force. Owing to this induced mass distribution within the laminar flow, the reten- tion time of particles in the apparatus is a function of their weight. The retention time of the particles can be correlated to their mass. This technique can be used to fractionate the powder to obtain dilute particle suspensions with narrow size distributions for QELS analysis.Often, gas-phase synthesis results in the formation of agglomerates. For many applications this is undesirable. The determination of the fractal dimension of the powder from TEM micrographs can provide information on the history of the agglomerate^.'^,^^ The fractal dimension of an agglomerate describes the relation between the mass (M) or the number of particles (N)and the radius (R)containing them: MCCRDf or NCCR~~ (2) The second relation is only valid for clusters consisting of monosized particles. The fractal dimension (D,) depends on certain features of the agglomerate-formation processes. For example, if the cluster is formed in two-dimensional space according to a particle-cluster diffusion-limited aggregation process, then the fractal dimension is expected to be cu.1.7, while a cluster-cluster diffusion-limited aggregation will yield a fractal dimension of <1.5.lS2O Ballistic aggregation pro- cesses will often yield higher fractal dimensions, e.g. a par- ticle-cluster ballistic aggregation will lead to a dimension of more than 1.9, and a cluster-cluster ballistic aggregation will give a dimension of more than 1.5.18-20 J. MATER. CHEM., 1994, VOL. 4 Experiment a1 Results and Discussion A cylindrical reactor (inner diameter =50 mm, length = 700mm) with, on the inside, a TiN coating in a three-zone furnace equipped with resistive heating was used for the synthesis experiments. In this set-up, the TiCl, is fed into the reactor by a hydrogen carrier gas.This hydrogen is bubbled through heated TiCl, in order to saturate the hydrogen stream with reactant vapour. The TiC1, concentration in the H2 stream is calculated assuming the TiC1, vapour pressure to be in equilibrium with the TiCl, liquid. A nozzle is used for the separated introduction of the reactants. H2 and NH3 flow through the inner side of this nozzle, whereas TiCl,, H, and N, are introduced at the entrance of the reactor outside the nozzle. Thus, the reactants for homogeneous reaction are mixed at a high temperature, i.e. at the beginning of the second heating zone of the furnace. An excess of H, is used for the reduction of the titanium-containing vapour species to minimize incorporation of chlorine in the powder.In the third heating zone of the reactor a cold finger is placed to collect a representative amount of the powder for further analysis. The cold finger is cooled from the inside by pressurized air. For the experiments as a function of reaction temperature an NH, excess of 10 with respect to TiCl, is used. The reaction temperature of the second heating zone of the reactor is varied from 900 to 1300 K, and the gas-phase temperature in the third heating zone of the furnace is kept constant at 865 K. For most experiments the temperature of the tip of the cold finger is kept constant at 800K. The experimental conditions are summarized in Table 1.The TIN powder collected on the cold finger in the third heating zone of the furnace is suspended in an aqueous ammonia solution (0.002mol drn-,) with a powder concen- tration of typically 1-2 wt.%. This suspension is fractionated by a Dupont SF3 analyser. The same instrument conditions were used as reported by Scarlett et u1.,l5 and the Dupont SF3 computer program was used for the particle-size distri- bution calculations. The fractionated suspensions are collected in optical cuvets, and these suspensions are analysed by a Coulter N4 light scattering analyser. TEM grids and polished silicon pieces are dipped in ethanol suspensions with a powder concentration of a few weight percent for TEM and SEM analysis, respectively. Screen printings of the TEM micro- graphs are used for the determination of the fractal dimension.The negative of the screen used yields white pixels on the TEM micrographs where particles are present. Each white pixel represents a certain amount of mass visualized by the screen on the TEM micrograph. The number of white pixels is counted as a function of the radius for each agglomerate. The fractal dimension can be obtained by fitting the data according to eqn. (2). Table 1 Process parameters for the synthesis of TIN powder inside nozzle NH3 flow 0-140 pmol s-' H, flow 0.39-0.20, 0.78 mmol s-' total flow 0.39, 0.78 mmol s-' outside nozzle: TIC& flow 7-24, 35 pmol s-' N, flow 0, 0.49 mmol s-l H, flow 0.38-0.49 mmol s-' total flow 0.54, 1.015 mmol s-' reaction time 1800, 3600 s reaction temperature 900-1300 K (second heating zone) gas phase temperature 865 K (third heating zone) tip of cold finger 800-865 K reactor pressure 1 x lo5 Pa The dark-brown titanium nitride powders formed were crys- talline in all cases. Three characteristic XRD diffractograms are presented in Fig.1. These results are in agreement with the reports of Hojo and Kato," who observed that TiC1, and NH, react in the gas phase above 800 K to form a crystalline TIN powder. The lattice parameter of the powders as a function of reaction temperature is presented in Fig. 2.21*22 There is an increase in lattice parameter with increasing reaction temperature. If it is assumed that the impurities in the powder are negligible, and that the composition of the powder varies only with respect to the stoichiometry then the lattice parameter can be correlated to the amount of nitrogen present in the TIN lattice.The lattice parameter of TIN, as a function of the stoichiometry number y is presented in Fig. 3. A comparison of Fig. 2 and 3 suggests that only at high -1111 200' 2201 311' ' 222 20 30 40 50 60 70 80 28/degrees Fig. 1 XRD patterns of TIN powder synthesized at three different temperatures: (a)923 K, (b)973 K and (c) 1023 K Oa4,,r 0.425-2 X a,.c. XE Xf 0.424-Q X xx X lx 0.4224 I 850 900 950 1000 1050 1100 1150 1200 VK Fig. 2 Lattice parameter of fcc TiN powder as a function of reaction temperature 0.4251-X 0.4201 I 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Y Fig.3 Lattice parameter of fcc TIN, as a function of nitrogen content y.Data from ref. 21 (x) and 22 (+) J. MATER. CHEM., 1994, VOL. 4 reaction temperatures can stoichiometric titanium nitride powder be formed. Particle sizes are obtained from SEM, TEM, QELS and SdFFF analyses. The particle size appears to be unaffected by the reactant concentrations. Only at lower TiCl, and NH, concentrations do the particle sizes tend to be smaller. In the temperature range 900-1173 K the mean primary particle size varies between 100 and 200 nm, as identified by TEM analysis. Particle-size distributions, typical for the experiments using a reaction temperature lower than 1173 K, are shown on the SEM and TEM micrographs in Fig.4 and 5, respectively. A significantly smaller primary particle size is observed at higher reaction temperatures as can be seen in Fig. 6. All these ~4 SEM micrograph of T~Npowder, typical for synthesis experiments within the temperature range 923-1 173 K H100 nm Fig. 5 TEM micrograph of TIN powder, typical for synthesis experiments within the temperature range 923-1 173 K 691 Fig. 6 TEM micrograph of TIN powder using a reaction temperature of 1223K particle sizes tend to be larger than the mean particle sizes reported by Kato et al.' as can be seen in Table 2. The mass mean diameters derived from SdFFF analysis of suspended powders are typically between 200 and 300nm, and the geometric standard deviation O(g4.13/50)varies between 1.1 and 1.3 for the powders formed below a reaction tempera- ture of 1173 K.These diameters are in reasonable agreement with the diameters obtained from the QELS analysis, where the fractionated suspension corresponds to the mean mass diameter of the SdFFF analysis, as can be Seen in Fig.7. There is good agreement between the QELS diameter:, using all the fractionated samples and their corresponding SdFFF Table 2 Specific surfaces and mean particle diameters derived from BET measurements and TEM micrographs Kato et al. this paper T/K d,,Jnm SBET/m2 8-l dT,Jnm SB,T/nl2 g-' 973 110 16 172 33 1073 133 7-32 1173 70 19 185 17 1273 16 1373 50 1000 M i ++ + + + + + + 10-1 I 850 950 1050 1150 1250 1350 VK Fig.7 Mean particle diameter as a function of reaction temperature using different particle size analysis techniques: x ,SdFFF; 0,QELS;+, TEM 692 diameters, as can be seen in Fig. 8. This is an indication that the powders are not agglomerated, because the QELS tech- nique determines a particle diameter independent of its mass, whereas the SdFFF technique determines the mass of the particles. Moreover, the TEM diameters of the primary par- ticles are of the same order of magnitude as the QELS/SdFFF diameters. Thus, from these results it can be concluded that the primary particle size is only a weak function of the reaction temperature, and that it is not likely that agglomer- ates in the gas phase are formed within the temperature region from 923 to 1173 K.At high reaction temperatures there is a considerable mismatch between the TEM particle diameters and the QELS/SdFFF diameters. This is an indication that the particles are agglomerated. Apparently, a high reaction temperature results in the formation of agglomerated particles. The conclusion that the particles become agglomerated during the synthesis process can be confirmed by a fractal-dimension analysis of the TEM micrographs. In all cases, agglomerates are found on the TEM micro- graphs. The degree of agglomeration is dependent on the powder concentration of the suspension. Thus, these agglom- erates have to be a result of the preparation method for TEM samples.Agglomeration in the liquid film on the TEM grid occurs during evaporation of the medium. This agglomeration process is a typical two-dimensional diffusion-limited process. In principle, two different agglomeration processes are likely to occur on the TEM grid, i.e. a particle-cluster aggregation or a cluster-cluster aggregation. If a fractal dimension is found which is characteristic for a particle-cluster aggregation then it is likely that all the agglomerates are formed on the TEM grid, and not in the gas phase during the synthesis process. It is very unlikely that particle-cluster agglomerates can be formed in the gas phase, because each cluster present in the gas phase will agglomerate with other clusters as well.The fractal dimension for different agglomerates formed at two reaction temperatures is determined according to the circle method on TEM micrographs.” TEM micrographs of agglomerates were taken which were thought to be rep- resentive for all the inspected agglomerates on the TEM grid. The experimental fractal dimensions along with the expected values based on computer simulations are given in Table 3. An example of a computer-simulated particle-cluster diffusion-limited aggregate (Of=1.7) is presented in Fig. 9. The choice of centre for the fractal dimension determination of a computer-simulated agglomerate by the circle method is straightforward. However, the determination of such a centre of an agglomerate on a TEM micrograph is far more difficult.We have chosen for the middle of the agglomerates %W,and %Las a centre for the circle method. In Table 3 the number of particles is equal to the total number of particles within the largest radius used for the fractal-dimension determination. 400 300 2 c/)-I W !2 -0 200 100 100 200 300 400 d(SFFF)/nm Fig. 8 QELS diameter using the fractionated suspensions from the SdFFF analysis as a function of the corresponding SdFFF diameter J. MATER. CHEM., 1994, VOL. 4 Table 3 Mass fractal dimension for TIN agglomerates synthesized at two different temperatures number of calculated 2D expected 2D T/K particles L/Wa dimension dimension 1073 1591 1.3 1.69f0.09 1.71 (P-C)b 1073 77 1.4 1.80f 0.14 1.71 (P-C) 1073 193 1.75 1.68f0.08 1.71 (P-C) 1073 447 1.42 1.75& 0.06 1.71 (P-C) 1223 342 1.5 1.33 k0.04 1.44 (C-C)c 1223 312 1.67 1.18f0.07 1.44 (C-C) 1223 742 1.71 1.41 kO.02 1.44 (C-C) “Agglomerate length over width ratio; bParticle-cluster diffusion limited aggregation;18-” ‘Cluster-cluster diffusion limited aggregation.18-” W Fig.9 Computer-simulated diffusion-limited particle-cluster aggre-gation. The meaning of L, W,and R is illustrated. For agglomerates formed at a reaction temperature of 1073 K a fractal dimension is observed which is typical for a particle-cluster agglomeration. As mentioned before, it is very unlikely that such an process can occur during gas-phase synthesis. Hence, it can be assumed that this powder is not agglomerated. The observed fractal dimensions at a reaction temperature of 1223 K suggest that the particles collected in the reactor are already agglomerated. This is in agreement with the TEM and QELS/SdFFF analyses.It should be noted that a three-dimensional cluster-cluster diffusional aggre- gation will result in a fractal dimension of 1.78.18-20This value is close to the experimental fractal dimensions for powders formed at a reaction temperature of 1073 K. However, TEM images of the samples in a tilted position reveal that the agglomerates on the TEM grid are virtually two-dimensional. The driving force for powder collection on the cold finger in the reactor is thermophoresis. The thermophoretic velocity (v,) is proportional to the temperature gradient.23 v,= -K(;)B(T) (3) where K is a constant for a given particle size, v the kinematic viscosity of the gas, and T the temperature.In this case, where particles have a Knudsen number of order unity, K is only a weak function of the particle diameter.24 Indeed, an increase in the amount of deposited material is observed with increas- ing temperature gradient. However, a quantitative analysis of the collection efficiency is not possible, because the gas-phase temperature near the cold finger has a complex non-linear dependence with respect to the axial and radial position. However, if the temperature of the cold finger and the temperature of the gas phase are kept constant then the weight of the harvested powder, which is a measure of the total TiN mass concentration in the gas phase, can be used to study the kinetics of the gas-to-particle conversion in the J.MATER. CHEM., 1994, VOL. 4 tures the particle size characteristics are determined by the nucleus formation process itself. -*r Conclusions ‘E0-41 x G3 “ix I ‘\I -64 IX 0.7 0.8 0.9 1.0 1.1 1.2 1O~WT Fig. 10 Arrhenius plot of collected TIN powder mass rate on cold finger reactor. The powder weight collection rate as a function of the reaction temperature is presented in an Arrhenius plot in Fig. 10. The maximum experimental collection efficiency is 5 % given these experimental conditions. The gas-to-particle conversion shows an Arrhenius-like behaviour within the temperature range 923-1 173 K.The apparent activation energy of the powder-formation process in this reactor is 94 kJ mol-’. This apparent activation energy is determined by a combination of nuclei formation, particle growth and loss of reactant to the reactor walls. The contribution of the particle growth to the observed apparent activation energy can be eliminated by dividing the amount of harvested mate- rial by the particle mass with a mean primary particle diameter as derived from TEM analysis. This yields an apparent activation energy of 142 kJ mol-’ for the entire temperature region as can be seen in Fig. 11. This activation energy is related to the formation process of nuclei in the reactor. The observed decrease in particle growth can be ascribed to an increase in loss of reactant to the reactor wall or a change in growth mechanism. For example, a change in growth mechan- ism might be conceivable, if the reaction rate involving TiC1, as reactant is smaller than the reaction rate involving TiCl,.Because, thermodynamic equilibrium calculations reveal that in the temperature region where the decrease in powder formation is observed the homogeneous equilibrium concen- tration of TiC1, decreases, and TiCl, becomes the most abundant titanium gas-phase species.12 However, given the present experimental configuration, it can not be concluded what is the precise reason for the observed decrease in growth rate of the particles. Apparently, at low reaction temperatures the particle size characteristics are determined by a heterogen- ous growth on the nucleus, whereas at high reaction tempera- 35T -.-f 2gl 2510.7 0.8 0.9 1.0 1.1 1.2 ~O~WT Fig. 11 Arrhenius plot of collected mass on cold finger divided by the particle mass with the mean primary particle diameter, i.e.proportional the nucleation rate in the reactor Crystalline titanium nitride powders using TiCl,, NH,, and H2 were formed in all cases. The lattice parameter of the powders as a function of reaction temperature suggests that only at high reaction temperatures can a stoichiometric titanium nitride powder be formed. In the temperature range 900-1173 K the mean primary TEM particle size varies between 100 and 200nm. The mass mean diameters derived from SdFFF analysis of suspended powders are typically between 200 and 300nm, and the geometric standard devi- ation varies between 1.1 and 1.3 for the powders formed below a reaction temperature of 1173 K, and a comparison between the results of the QELS and the SdFFF techniques indicates that the powders are not agglomerated.At high reaction temperatures there is a considerable mismatch between the TEM particle diameters and the QELS/SdFFF diameters. This is an indication that these particles are agglom- erated. This phenomenon is confirmed by a fractal dimension analysis of the TEM micrographs. The gas-to-particle conver- sion shows an Arrhenius-like behaviour within the tempera- ture region 923-1173 K. The apparent activation energy of the powder formation process in this reactor is 94 kJ mol-’.From this result and the measured mass increase an activation energy of 142 kJ mol-’ for the homogeneous gas-phase reac- tion can be derived. Glossary mean primary particle diameter derived from dTEM TEM micrograph (nm) Df fractal dimension L length of an agglomerate on a TEM micrograph K thermophoretic velocity constant M mass of a part of an agglomerate within radius R N number of particles of an agglomerate within radius R R radius specific surface area derived from BET analy- SBET sis (m2g-’) T temperature (K) ut thermophoretic velocity (m s-’) W width of an agglomerate on a TEM micrograph Y stoichiometric number of TIN, c(84.13/50) geometric standard deviation V kinematic gas viscosity (m2 s-l) References 1 G.W. Elger, D. E. Traut, G. J. Slavens and S. J. Gerdemann, Metall. Trans. B, 1989,20,493. 2 A. Kato, J. Hojo and Y. Okabe, Mem. Fac. Eng. Kyushu Univ., 1981,41,319. 3 M. Yoshimura, M. Nishioka and S. Somiya, J. Muter. Sci. Lett., 1987,6, 1463. 4 H. Kudo and 0.Odawara, J. Muter. Sci.,1989,24,4030. 5 E. Rothman, J. Stitt and H. K. Bowen, Ceram. Industry, 19N5,24. 6 S. E. Pratsinis and S. V. R. Mastrangelo, Chem. Eng. Progr.. 1989, 85,62. 7 S. E. Pratsinis, H. Bai, P. Biswas, M. Frenklach and S. V. R. Mastrangelo, J. Am. Ceram. Soc., 1990,73,2158. 8 M. K. Akhtar, S. E. Pratsinis and S. V. R. Mastrangelo, J. Am. Ceram.SOC.,1992,75,3408. 9 W. G. French, L. J. Pace and V.A. Foertmeyer, J. Phys. Chem., 1978,82,2191. 10 Y. Okabe, J. Hojo and A. Kato, Yogyo Kyokai, 1977,85,173. 11 J. Hojo and A. Kato, Yogyo Kyokai, 1981,89,277. 694 J. MATER. CHEM., 1994, VOL. 4 12 J. P. Dekker, P. J. van der Put, H. J. Veringa and J. Schoonman, in Proc. Eighth Conf. Chemical Vapour Deposition, ed. M. L. Hitchman and N. J. Archer, J. Physique IV, 1991, 19 20 The Fractal Approach to Heterogeneous Chemistry, ed. D. Avnir, Wiley, Chichester, 1989. Trends in Aerosol Research, ed. A. Schmidt-Ott, Universitat 13 14 15 16 17 Measurement of Suspended Particles by Quasi Elastic Light Scattering, ed. B. E. Dahneke Wiley, 1983. J. C. Giddings, S. K. Ratamathanawongs and H. M. Moon, Kona, 1991,9,200. B. Scarlett, H. G. Merkus, Y. Mori and J. Schoonman, Particle Size Analysis, ed., P. J. Lloyd, Wiley, 1992, p. 107. J. L. LaRosa and J. D. Cawley, J. Am. Ceram. Soc., 1992,75,1981. R. J. Samson, G. W. Mulholland and J. W. Gentry, Langmuir, 1987,272. p. C2-592. 21 22 23 24 Duisburg, 1990. H. J. M. Rypkema, MSc Thesis, Delft University of Technology, 1986. W. Wakelkamp, PhD Thesis, Eindhoven University of Technology, 1991. P. G. Simpkins, S. Greenberg-Kosinski and J. B. MacChesney, J. Appl. Phys., 1979,50, 5676. L. Talbot, R. K. Cheng, R. W. Schefer and D. K.Willis, J. Fluid. Mech., 1980, 101, 737. 18 R. Julien and R. Botet, Aggregation and Fractal Aggregates, World Scientific Publishing, Singapore, 1986. Paper 3106945A;Received 22nd November, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940400689

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 5), 695-701 Co-pyrolysis of Hydrocarbons and SiEt, for the Synthesis of Graduated Si,C, -x Ceramic Thin Films by Chemical Vapour Deposition Jean M. Agullo, Florence Fau-Canillac and Francis Maury* Cristallochimie, Reactivite et Protection des Materiaux, CNRS-URA 445, Ecole Nationale Superieure de Chimie, 118 route de Narbonne, 31077 Toulouse cedex, France The thermal decomposition of hydrocarbons has been investigated under particular conditions in a low-pressure chemical vapour deposition reactor in order to select suitable carbon sources for the preparation of carbon-rich or graduated Si,C, --x layers. Growth of pyrolitic carbon thin films (pyro-C) starts at ca. 1050 K and only above 1273 K using C,H,Pr' and CH,, respectively. The microstructure of the pyro-C layers is more dependent on the deposition temperature than on the nature of hydrocarbons.Their co-pyrolysis with SiEt, used as Sic precursor has been achieved in the temperature range 1050-1250 K. As expected, the film composition does not change significantly at 1173 K using CH, as an additional C source. By contrast, the C content of the films deposited by co-pyrolysis of SiEt4 and C6H5Pri increases continuously from 0.48 to 1 by increasing the mole fraction ratio x(C6H5Pri)/[x(C,H,Pri) +x(SiEt,)] from 0 to 1. Multilayers and compositional gradient layers can be prepared by discrete or continuous changes of the gas- phase composition, respectively. These films were successfully used as interphase in ceramic-ceramic composite materials to weaken the fibre/matrix bond and to improve their ductility. Ceramic-ceramic composite materials such as Sic-Sic are highly resistant to oxidation at high temperature but their brittleness is a limiting feature for many applications.The deposition of a softer material on the Sic fibres, like pyrolitic- carbon thin films (pyro-C), before matrix formation improves significantly their fracture toughness.' This interphase weak- ens the fibre/matrix bond and protects the fibres against microcracks and chemical However, because pyro-C films have a low oxidation resist- ance, alternative solutions are required. From this point of view, since C and Si interphases improve the du~tility~,~ and the oxidation resistance7-' of ceramic composite materials, respectively, multilayers or graduated Si,C1 -x layers, in which the composition changes continuously from C (fibre interface) to Sic (matrix interface), would be suitable interphases.For this purpose, we undertook an investigation of the chemical vapour deposition (CVD) of non-stoichiometric Si,C1 -x films on flat substrates before extending the application of this process to the fibre treatment. Many silicon compounds have been used as molecular precursors for the growth of Sic thin films.g However, prefer- ring a process without chlorine because of the sensitivity of Sic Nicalon fibres to chlorine atmosphere," we have shown previously that SiEt, (Et =C2H5)was a suitable organometal- lic precursor for the deposition of SixC1-, at moderate temperature (700-900 OC)." Furthermore, Si enrichment of these films was achieved by SiH, addition in the gas phase." In the next step of this programme, the preparation of graduated SixC1 -layers requires an increase of the C content from ca.0.5 (Sic stoichiometry) to 1 (pyro-C). The first requirement for the selected hydrocarbons is to be readily decomposed in the conditions used for the pyrolysis of SiEt,, in order to control easily the carbon incorporation in the film by monitoring the gas-phase composition. Secondly, hydrocarbons have to be used also for the deposition of pyro-C films with a highly graphitic structure in order to facilitate the fibre slip in the ceramic matrix. It is assumed, for this objective, that the existence of aromatic rings in the hydrocarbons (sp2 C) will improve the degree of graphitization of the layer.This paper deals successively with the thermal decompo- sition of some hydrocarbons in a low-pressure CVD reactor and their co-pyrolysis with SiEt, in order to select suitable carbon sources for the preparation of either C-rich Si,C1 -x films or compositionally modulated SixC1 -x layers. Experimental Apparatus and Deposition procedure The thin films were deposited using a horizontal hot-wall low-pressure CVD apparatus. This set-up is used for the chemical vapour infiltration of ceramic films for the prep- aration of composite materials and details were rcported previ0us1y.l~ In summary, an electrical furnace with three independently regulated heating zones provides an isothermal length of ca.40 cm. Quartz reactors 90 cm long and 2.5 cm in diameter were used. The total pressure is measured indepen- dently of the gas composition and automatically monitored using a capacitance manometer and a throttle valve control system. The flow rate of gaseous precursors was directly monitored using mass Aowmeters and the partial pressure of the liquid precursors was adjusted both from the flow rate of the carrier gas (He) through their bubblers and their vapour pressure given by the Clausius-Clapeyron equations." Polycrystalline alumina plates and silicon wafers passivated by a thin film of Si3N, were used as substrates. Before being introduced in the reactor, they were degreased for 5 min in hot trichloroethylene and then for 5 min in hot acetone, and were dried under a nitrogen stream.In all experiments, the total pressure and the total flow rate were adjusted in order to keep constant the residence time of the species in the heating zone of the reactor. In co-pyrolysis experiments, the gas-phase composition is defined by the ratio of the mole fractions x(hc)/[x( hc) +x(SiEt,)], where x(hc) means the mole fraction of the hydrocarbon and x(SiEt,) that of the silicon precursor. The typical MOCVD conditions are reported in Table 1. General Instrumentation The degree of crystallinity of the pyro-C and C-rich Si,CI-, films was analysed by transmission electron microscopy (TEM) and selected area electron diffraction (SAED) using a Philips 100 kV microscope.For the TEM analyses, the tilt Table 1 Typical experimental MOCVD conditions used for the depos- ition of C-rich Si,C1-, thin films by co-pyrolysis of SiEt, and hydrocarbons (hc) deposition temperature/K 1050-1250 reactor pressure/Pa 400 total flow rate/sccm 175 carrier gas He reactor diameter/mm 25 residence time of species/s 0.2 mole fraction ratio: x(hc)/[x(hc)+x(SiEt,)] 0-1 angle between the primary electron beam and the surface of the film was 90" unless otherwise specified. The Si content of the films was determined by electron- probe microanalysis (EPMA) using a CAMECA microprobe (accelerating voltage: 5 kV) and an Si wafer as standard.The C content can be satisfactorily deduced by difference. The film composition of most samples was also determined by X-ray photoelectron spectroscopy (XPS) using a spectro-photometer (VG Escalab MK 11) operating with a non-monochromatized A1-Ka,,, X-ray source. After Ar + sputtering (3 kV, 150 PA, 5 min) to remove the surface contamination, the oxygen content was found to be only ca. 5atom% and the Si content was found to be in satisfactory agreement with the EPMA data. The layer thicknesses were measured on fractured cross- sections using a JEOL JSM-25 scanning electron microscope (SEM). Mass spectra of few precursors were recorded using a time-of-flight mass spectrometer (CVC Bendix) at a ioniz- ation potential of 70eV and using direct insertion into the ion source.Si,C, -Deposition from SiEt, Silicon carbide films were deposited by thermal decomposition of SiEt, under reduced pressure in the temperature range 1050-1250 K. The experimental conditions (Table 1) were optimized for the chemical vapour infiltration of Si,C1 -,of porous preform^.'^ In order to prepare C-rich Si,C1 -,films and according to thermodynamic calculations (vide infra), helium was preferred to hydrogen as carrier gas because the C incorporation in the film is facilitated to the detriment of CH, formation. Fig. 1 shows the variation of the growth rate uersus the deposition temperature. As usually observed in CVD pro- TI°C 950 900 850 800 0 0 lo-' 8.0 8.5 9.0 9.5 1O~WT Fig.1 Variation of the growth rate of Si,Cl-, as a function of the reciprocal temperature using SiEt, as a single source. The linear dependence at low temperature reveals an apparent activation energy of 192 kJ mol-' J. MATER. CHEM., 1994, VOL. 4 cesses, the linear dependence at low temperature (T<1123 K) indicates a thermally activated process with an apparent activation energy of 192 kJ mol-'. This is in satisfactory agreement with the value reported for the pyrolysis of this precursor in a cold wall CVD reactor under H, (197 kJ mol-') and He (155 kJ mol-l) atmosphere." This is consistent with a deposition process kinetically limited by the heterogeneous chemical reaction. The stabilization of the growth rate above 1173 K results mainly from the depletion of the reactives along the hot wall reactor but probably the diffusion of the species in the gas phase has a limiting contribution. The silicon content of the Si,C,-, films deposited between 1123 and 1233 K decreases slightly from x=0.5 to 0.4 (Fig.2). This result is different from that obtained for the pyrolysis of SiEt, in a cold-wall CVD reactor since, in the same tempera- ture range, the Si content was found to be 0.6-0.7." In the present process, the efficiency of the C enrichment originates probably from a more complete decomposition of the ethyl ligands. Although the atomic ratio is C :Si =8 : 1 in the starting molecule, the C incorporation in the film is not very high and it is not possible to increase drastically the C content of these Si,C1 -,films only by increasing the pyrolysis temperature.Selection of Hydrocarbons Thermodynamic Calculations in the Si, C, H system The thermodynamic approach frequently provides useful informations for CVD processes.15 Although calculations have been reported previously for the chemical system Si, C, H,16 we have made preliminary calculations using our particular experimental CVD conditions to obtain some clues about the selection of the precursors and the conditions leading to C enrichment of the films. The complex equilibrium calculations were performed using a version of the classical Solgasmix computer program based on the minimization of the Gibbs free energy of the total chemical system.17 The thermodynamic data of both the gaseous species (20 molecules and radicals) and solid phases were found in the usual tables and a data bank." SiEt, was assumed to be fully decomposed and, according to literature data, the C-rich Si,C1-, films were considered to be a mixture of P-Sic and graphitic C.16 Calculations for the system SiEt,-H, show that stoichio- metric P-Sic is obtained under atmospheric pressure at low temperature and high partial pressure of hydrogen. Increasing the pyrolysis temperature and/or decreasing the hydrogen partial pressure leads to C-rich films.This tendency is clearly enhanced under reduced pressure [Fig. 3(a)]. For example, under 0.1 kPa, the C content of the films is theoretically 0.6I I I t E 0.3 8 0.2 i7j 0.1 1 1050 1150 1250 TIK Fig.2 Influence of the substrate temperature on the silicon content x of Si,C1-, films deposited using SiEt, as a single source (0,EPMA data) and a mixture of SiEt, and C,H,Pr' with a mole fraction ratio X(C,H,Pr')/[x(C,H,Pr)+X(SiEt,)] =0.75 (A,EPMA data; .,XPS data) J. MATER. CHEM., 1994, VOL. 4 200ie He H2P gas composition gas composition v CI 1g) (c) 1 c. -500 ie CH4 gas composition gas composition Fig. 3 Thermodynamic calculations of the film composition in the systems SiEt,-H, (a), (b),SiEt4-CH,-H, (c) and SiEt,-CH,-He (d) versus the deposition temperature and the gas-phase composition. The mole fraction of SiEt, was fixed at 5x lop2 and the total pressure was (a) 10, (b)0.1 and (c), (d) 1 kPa higher than 80% in the temperature range 600-1200 K whatever be the hydrogen partial pressure [Fig.3(b)].The equilibrium C(S)+~H~*CH~ (1) where the solid C competes with the C in the gas phase in the form of CH, accounts for these results. The influence of an additional source of carbon has been investigated with the system SiEt,-CH4-H,. The total press- ure was fixed at 1kPa according to the experimental value of the deposition process using SiEt,. The typical results are shown by the Fig. 3(c) and 3(d). As expected, the C incorpor- ation in the films increases by increasing either the partial pressure of methane or the deposition temperature. This tendency is enhanced under an inert atmosphere since H2 facilitates the formation of CH, and is, of course, easily explained by eqn.(1). A carbon content as high as 95% can be expected at about 1273 K using the chemical system SiEt,-CH,-He. According to this result, it was not necessary to perform calculations using more complex hydrocarbons including heavier alkanes and aromatic compounds. Pyrolysis of CH4 Methane is the simplest hydrocarbon and it has frequently been used successfully in CVD processes as the C source; that is supported by the above thermodynamic analysis. Furthermore, its pyrolysis in a LPCVD reactor was reported to start at 973 K.19*20 However, no significant thermal decomposition of CH, was observed in our apparatus using the experimental conditions of Table 1. Even for temperatures higher than 1273 K, the residence time of the species in the reactor had to be increased from cu.0.2 to 200s in order to increase the yield of decomposition and to get acceptable growth rates of the pyro-C layers. Films deposited at 1323 K have a high degree of graphitization. TEM analvsis shows large hexagonal date-shaped crystal- -1 lites which have grown parallel to the surface of the substrate [Fig. 4(u)]. The corresponding SAED pattern contirms the graphitic structure [Fig. 4(b)].According to the literature,21’22 they exhibit two series of $iffraction spots with interplanar spacings of 2.1 1 and 1.21 A assigned to the (1Oz) and (llz) planes of a perfect crystal of graphite, respectively. The radial deformation of some diffraction spots reveals a trandational disorder of the graphitic planes.Furthermore, the multiplicity of the (lOz) spots originates probably from a rotational disorder of the basal planes along the stacking direction. The observation of Moire fringes, both on the bright-field [Fig. 4(u)] and on the dark-field images [Fig. 4(c)], is due to interferences between at least two diffracted beams and con- firms this structural analysis. Pyrolysis of C,H,Pr‘ Since the pyrolysis of CH4 occurs at temperatures higher than the temperature range of our process, less stable hydrocarbons should be found. From this point of view, when the pyrolysis process is initiated by homolytic bond breaking, which is frequently the case with this kind of compo~nd,~.’,~~two precursors are expected to be decomposed in a similar tem- Fig.4 TEM analysis of a pyro-C film obtained by pyrolysis of CH4 at 1323 K under a reduced pressure of 13kPa. Both the bright field micrograph, (a), and the corresponding SAED patterns, (1 I), were obtained with the surface of the sample perpendicular to the primary electron beam. The dark-field image of the same area (c), obtained using a ( 1lz) spot, exhibits the hexagonal plate responsible for the diffraction pattern (b).The arrows show Moire fringes J. MATER. CHEM., 1994, VOL. 4 perature range if they have comparable bond strengths. Table2 reports a comparison between the bond strength of SiEt, and a few hydrocarbons. Isopropylbenzene, C6H5Pr' (or cumene), seems to be a suitable candidate because the lability of the CH, groups is quite similar to that of Et groups of SiEt,.This is confirmed by the fragmentation of these mol- ecules under electronic impact in a mass spectrometer (Table 3). The fragmentation of the cumene is clearly initiated by the loss of a Me group generating the highest peak of the spectrum. The weakening of the C6H5cH(cH3)- (CH,) bond (275.9 kJ mol) results from the inductive effect of the aromatic cycle. Moreover, it was expected that the aromatic ring of this hydrocarbon could facilitate the graphitization of the pyro-C films. The mass spectrum of SiEt, recorded in the same conditions is typical of SiR, molecules;28 the loss of one or two Et groups occurs with a high probability and initiates the main decomposition pathway. Carbon layers were grown from C,H,Pr' between 1050-1273 K using experimental conditions close to those reported in Table 1 (residence time=0.9 s instead of 0.2 s).TEM analyses reveal that the films have a poor crystallinity with a mean size of microcrystallites of only a few nm [Fig. 5(a), 5(d)]. At 1173 K, the SAED pattern of a sample with the surface perpendicular to the electron beam exhibits two diffuse rings assigned to the (102) and (1lz) planes of a graphitic structure [Fig. 5(b)].After tilting the sample by 40" with respect to the electron beam, the SAED pattern does not change significantly indicating a very high disorder of the graphitic structure [Fig. 5(c)].This type of free C can be called amorphous because the basal planes are randomly rotated and they are not stacked in a single direction, for instance parallel to the surface of the film. The microstructure of the pyro-C films at 1273 K is different. SAED patterns give evidence for a turbostratic structure, i.e. the graphitic planes exhibit a rotational disorder but they have a preferential oriention parallel to the surface of the film. When the sample is perpendicular to the electron beam, the SAED pattern shows the two diffraction rings of the (102) and (1lz) planes (rotational disorder) and when it is tilted by 40" degrees diffuse diffraction spots appear with an interplanar spacing of Table 2 Principal bond dissociation energies (EBD) of molecular pre- cursors used in this work precursor bond EBD/kJmol-' ref.SiEt, Et,Si-Et 279.2 25 Et,SiCH, -CH3 372.0 26 CH4 H3C-H 439.7 27 C6H5Me C,jHsCH,-H 355.3 27 CsHs-MMe 418.0 27 C,H5Pr' C6H5CH(CH3)- CH3 275.9 27 C~HS-P? 405.5 26 H-c6H,Pr' 459.8 27 3.37 and 1.68 A corresponding to the (002) and (004) planes, respectively [Fig. 5(e), 501. In order to investigate the influence of the aromatic rings on the microstructure of the pyro-C films, CH,T and C,H,I were tried as C source in the same pyrolysis conditions as for the cumene. The low values of the bond strength between the organic group and the halogen indicates that they should be readily decomposed in these conditions (234 and 267 kJ mol-', respectively).At 1173 K, the films deposited using CH,I are amorphous whereas those grown from C6H,I are highly turbostratic (Table 4). This result argues for a better graphitization of the C layers when aromatic cycles exist in the precursor. Now, from a mechanistic point of view, the question is to know what is the origin of the carbon deposited using cumene? Analyses (gas chromatography, NMR) of the by-products trapped at the liquid-nitrogen temperature at the outlet of the reactor reveal that styrene is the principal product (including traces of C6H6, C6H5Me, C,H,Et). This confirms a preferential breaking of the bond C6H5CH(CH3)-cH3. The radical C6H,CH(CH,)' loses probably a hydrogen atom to form C,H,CH=CH,. The methyl radicals are more reactive and then could lead readily to C deposition and release of H,.This is supported by the fact that, under the same experimental conditions, the growth rate of a pyro-C film deposited using C6H4( Pri)2 is twice that using C,H,Pri indicating that the growth rate is strongly correlated with the number of 'CH, groups (Table 4). In summary, the degree of graphitization increases with the pyrolysis temperature of hydrocarbons (Table 4). For a fixed deposition temperature, aromatic cycles in the precursor induce a higher graphitization of the layers. Methane is probably too stable a hydrocarbon to be used as the carbon source in our process but cumene is an attractive candidate. Although the pyro-C deposited using C6H,Pr' probably orig- inates mainly from the methyl groups, it is not out of the question that the aromatic rings enhance the graphitization of the films.It is difficult to deposit graphite films below 1273 K even with aromatic corn pound^.^^^^^ This was success- fully realized from aromatic precursors by dehydrogenation in a halogen atmosphere31 but for our application, a halogen- free process is required to avoid corrosion of the ceramic fibres. Co-pyrolysis Results and Discussion Co-pyrolysis of SiEt4and CH, Although CH, does not look very promising as an additional carbon source in our Si,Cl-, deposition process since it is quite undecomposed, co-pyrolysis experiments with SiEt, were undertaken because chemical reactions between both precur- sors are possible at these temperatures.Fig. 6 shows that, at 1173K and using the CVD conditions reported in Table 1, the composition of the films does not change significantly when the mole fraction ratio x(CH,)/[x(CH,) +x( SiEt,)] Table3 Principal ions obtained by fragmentation of SiEt, and C6H5Pri under electronic impact in a mass spectrometer using an ionizing potential of 70 eV SiEt, C&,Pr' assignment mlz rel. intensity assignment mlz rel. intensity SiEt,' SiEt,+ HSiEt, + H,SiEt + SiEt+ SiH3+ 144 115 87 59 57 31 25 98 100 43 13 10 25 100 6 12 12 6 J. MATER. CHEM., 1994, VOL. 4 Fig. 5 TEM micrographs and corresponding SAED patterns of pyro-C films obtained by pyrolysis of C6H5Pr at 1173 K, (u)-(c), and 1273 K, (d)-cf), under a reduced pressure of 200Pa.The tilt angle between the electron beam and the surface of the sample was 0" for the SAED patterns (b)and (e) and 40" for (c) and (f). The magnification is the same than that of Fig. 4 Table 4 Some features of pyro-C thin films deposited by thermal decomposition of various hydrocarbons in the same MOCVD reactor using the experimental conditions: total pressure =200 Pa, mol fraction =0.1 1 and He as carrier gas deposition growth rate/ precursor temperature/K nm h-l structure' -CH4 1273 no deposition 1323 - graphitic C,H.+k! 1273 42 amorphous 1343 1866 turbostratic C6H5Pri 1173 132 amorphous 1223 162 turbostratic 1273 222 turbostratic C,H,( Pr' h 1223 312 - CH,I 1223 - amorphous C6H5T 1223 turbostratic 'Determined by TEM analyses.increases from 0 to 0.93. This is of course in agreement with the results on the pyrolysis of CH, and it can be considered that this hydrocarbon is non-decomposed in such pyrolysis conditions. Moreover, addition of methane in the gas phase has a Fig. 6 Variation of the silicon content x of the Si,C,-, films deposited by co-pyrolysis of SiEt, and hydrocarbons as a function of the gas- phase composition: SiEt,-CH, (0,EPMA data), SiEt4-C6H5Pr' (H, EPMA data; A,XPS data). The deposition temperature was 1173 and 11 18 K using CH4 and C,H,Pr', respectively negative effect on the growth rate of the Si,C, -,films. [ndeed, the deposition rate decreases continuousIy when the mole fraction of CH, increases (Fig. 7).This can be partly explained by a decrease of the diffusion coefficient of SiEt, in a He-CH, mixture when the mole fraction of CH, increases. The variation of the diffusion coefficient of SiEt,, calculated using the formula of Fuller et a1.,32as a function of the gas-phase J. MATER. CHEM., 1994, VOL. 4 I I 1 I I1 T -500 I E -400 $ 0).--300 8 -200 .-g u) -100 5 Fig. 7 Variation of the growth rate of SiXC1-, thin films (a)deposited using a mixture SiEt,-CH, as a function of the gas-phase composition. The binary diffusion coefficient of SiEt, is also reported (dotted curve). The mole fraction of SiEt, was fixed at 7 x and the deposition temperature was 1173 K composition supports this assumption (Fig.7). This indicates that the decomposition process of SiEt, is not purely kin- etically controlled above 1173 K but that diffusion phenomena in the gas phase also play a role. However, the variation of the two curves of Fig. 7 is not similar indicating more complex phenomena such as inhibition effects due to radical trapping or other mechanisms. Co-pyrolysisof SiEt, and C,H',Pr Co-pyrolysis experiments between SiEt, and C6H,Pr' were carried out at 1118 K. The gas-phase composition was changed by increasing the mole fraction of cumene and keeping constant that of SiEt,. The Si content of the films decreases continuously from 0.5 to 0, this means the carbon content increases from the Sic stoichiometry to pyro-C by increasing the ratio of the mole fractions x(C,H,Pr')/[x( C,&Pr')+X(SiEt,)] from 0 to 1 (Fig.6). The films have a uniform morphology and thickness. Furthermore, a satisfac- tory agreement is found between XPS (after Ar' sputtering to clean the surface) and EPMA analyses, revealing a constant distribution of the elements through the thickness of the films. Keeping constant the gas-phase composition [x(c,H,Pr')/[x(C,H,Pr') +x(SiEt,)] =0.75, the deposition temperature was increased from 1050 to 1200 K. Fig. 2 shows that the carbon content of the films is higher than in those deposited using SiEt, as a single source whatever the tempera- ture. Furthermore, the Si content decreases by increasing the pyrolysis temperature, that is probably the result of a more efficient decomposition of cumene at high temperatures.By contrast with the chemical system SiEt,-CH,, the growth rate is thermally activated between 1050 and 1200K using the mixture SiEt4-C&Pr' with an apparent activation energy of ca. 109 kJ mol-'. Analysis of the microstructure of these carbon-rich Si,C1-, layers has been recently reported in an abbreviated form.33 Concluding Remarks Cumene is a suitable additional carbon source for the prep- aration of carbon-rich SixC1 --x films by thermal decomposition of SiEt, under reduced pressure. The graphitization of pyro-C layers deposited using various hydrocarbons increases with the temperature but they are generally quite amorphous below ca.1273 K if the decomposition occurs without a halogenated atmosphere, even if the precursor contains aromatic rings. rl600 800 sputtering time/s Fig. 8 Typical SIMS depth profiles of SixC1 -x multilayers deposited at 1118K and under a reduced pressure of 400 Pa using a +mixture SiEt,-C,H,Pr' (Ar sputtering). The C content of the four successive layers was adjusted from the gas-phase ratio x(C,H,Pf)/[x( C,H,Pf)+x(SiEt,)] fixed at (a) 1, (b) 0.85, (c) 0.5 and (d) 0. For clarity, each composition profile has a different intensity scale. H,C'; a, Si+; jl, Sil; +, O+ The pyro-C layers grown using C6H,pr' are effectively amorphous at 1173 K but start to be turbostratic at 1223 K. Using a mixture SiEt,/C&Pr', the carbon incorporation in the films can be increased either by increasing the deposition temperature or by increasing the mole fraction of cumene in the gas phase.This last procedure is more versatile for the CVD process because of the high inertia of the big furnaces used in industrial set-ups. Using computer-monitored mass flowmeters, multilayers and compositional gradient layers can be easily prepared by discrete or continuous changes of the gas-phase composition, respectively. Fig. 8 illustrates Si,C, -, multilayers (four layers) prepared by this process at 1118 K. A pyro-C film is grown first on a silicon substrate and the carbon content of the next layers is adjusted from the gas- phase composition. A slight oxygen contamination is observed in the Si-rich layers as this frequently occurs with this highly reactive element.Details both on the preparation of C-rich Si,C1-, films by this process and on their structural charac- terization is reported in a further paper.34 Furthermore, this deposition process was recently applied to prepare specific interphases in ceramic-ceramic composite materials. It was found that pyro-C and graduated SixC1 -,interphases, grown using cumene and SiEt4--C&Pr', respectively, weaken the fibre-matrix bond and improve the ductility of the cer-amic-ceramic composite materiaL6 The authors would like to thank Professor R. Morancho for the fruitful discussions during this work, Dr. F. Teyssandier for his help concerning the thermodynamic calculations and J.Poujardieu for TEM experiments. This work was partially supported by the SociCtk Europeenne de Propulsion, France, under contract No. 449334. References 1 J. Homeny, W. L. Vaughn and M. K. Ferber, J. Am. Ceram. SOC., 1990,73, 394. 2 T. M. Besmann, R. A. Lowden, D. P. Stinton and T. L. Starr, J. Phys. (Paris), Colloq. CS,1989,50,229. 3 R. Naslain, Proceedings of the 1st International Conference on Functionally Gradient Materials, ed. M. Yamanouchi, M. Koizumi, T. Hirai and M. Shiota, Society of Non-Traditional Technology (FMG Forum), Sendai, 1990, p. 71. 4 J. J. Brennan, Mater. Sci. Eng. A, 1990,126,203. 5 A. J. Caputo, D. P. Stinton, R. A. Lowden and T. M. Besmann, Am. Ceram. SOC.Bull., 1987,66,368. 6 J. M. Agullo, F. Maury and J.M. Jouin, J. Phys. IV (Paris), Colloq. C3, 1993,3, 549. J. MATER. CHEM., 1994, VOL. 4 701 7 R. A. Lowden and D. P. Stinton, Ceram. Eng. Sci. Proc., 1988, 9, 705. 22 A. Oberlin, J. Goma and J. N. Rouzaud, J. Chim. Phys., 1984, 81, 701. 8 9 10 11 12 13 14 15 16 17 18 19 20 21 K. L. Luthra, J. Am. Ceram. SOC., 1986,69, C-231. J. Schlichting, Powder Metal. Int., 1980,12, 141. D. P. Stinton, A. J. Caputo and R. A. Lowden, Am. Ceram. Soc. Bull., 1986,6, 347. F. Maury, A. Mestari and R. Morancho, Muter. Sci. Eng. A, 1989, 106,69. A. Mestari, F. Maury and R. Morancho, J. Phys. (Paris), Colloq. C5, 1989,50, 769. J. M. Agullo, F. Maury and R. Morancho, Thin Solid Films, 1992, 209, 52. Handbook of Chemistry and Physics, ed. R. C. Weast, C.R.C. Press, Boca Raton, FL, 66th edn.1985, D-196. C. Bernard, Y. Deniel, A. Jacquet, P. Vay and M. Ducarroir, J. Less-Common Met., 1975,40, 165. A. 1. Kingon, L. J. Lutz, P. Liaw and R. F. Davis, J. Am. Ceram. SOC.,1983,66, 558. G.Eriksson, Acta Chem. Scund., 1971,25,2651. (a) M. W. Chase Jr., C. A. Davies, J. R. Downey Jr., D. J. Frurip, R. A. McDonald and A. N. Syverud, JANAF Thermochemical Tables, 3rd. Edn., J. Phys. Chem. Ref Data, Suppl. 1, 14, 1985; (b) SGTE databank, Scientific Group Thermodata Europe, BP 66,38402 Saint Martin &Heres, France. P. A. Tesner and 0.G. Shein, Kinet. Katal., 1986,26,1259. E. V. Denisevic and P. A. Tesner, Neftekhimija, 1974,20, 390. G. Schiffmatcher,H. Dexpert and P. Caro, J. Microsc. Spectrosc. Electron., 1980, 5,729. 23 24 25 26 27 28 29 30 31 32 33 34 R. J. Bogaert, T. W. F. Russel, M. T. Klein, R. E. Rochelean and B. N. Baron, J. Electrochem. SOC.,1977,124,790. T. L. Chu, S. S. Chu, S. T. Ang, A. Duong, Y. X. Han and Y. H. Liu, J. Appl. Phys., 1986,60,4268. A. E. Pope and H. A. Skinner, Trans. Faraday SOC.,1964,60,1404. D. Griller, J. M. Kanabus-Kaminska and A. Maccoll, J. Mol. Struct., 1988, 163, 125. Handbook of Chemistry and Physics, ed. R. C.Weast, C.R.C. Press, Boca Raton, FL, 66th edn., 1985, F-185. M. R. Litzow and T. R. Spalding, in Mass Spectiometry of Inorganic and Organometallic Compounds, ed. M. F Lappert, Elsevier, Amsterdam, 1973, p. 233. R. J. Bard, H. R. Baxman, J. P. Bertino and J. A. O’Rourke, Carbon, 1968,6,603. M. L. Lieberman, Proceedings of the 3rd international Conference on Chemical Vapor Deposition, ed. F. A. Glaski, American Nuclear Society, 1972, p. 95. P. H. Chang and M. M. Labes, Chem. Muter., 1989,1,523. E. N. Fuller, K. Ensley and J. C. Giddings, J. Phys. Chem., 1969, 73, 3679. J. M. Agullo, F. Maury, R. Morancho and R. Carles, Muter. Lett., 1991, 11,257. F. Maury and J. M. Agullo, submitted for publication. Paper 3/06761K; Received 1lth Novemher, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940400695

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 5), 703-705 Bi,W, -xCuxOG-x(0.7<x 60.8): A New Oxide-ion Conductort Vandana Sharma, Ashok K. Shukla and Jagannatha Gopalakrishnan Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-5600 72, lndia Anion-deficient Aurivillius phases of the general formula, Bi,W, -,CU,O~-~, possessing orthorhombic/tetragonal Bi,WO,-like structures, have been synthesized by quenching the oxide melts. The tetragonal phase stabilized for the compositions 0.7 <x <0.8 is a good oxide-ion conductor in the temperature range 500-900 K, the x =0.7 composition exhibiting the highest conductivity in the series. Solid oxide-ion electrolytes have applications in sensors, oxygen pumps and in high-temperature electrolyser-fuel-cell hybrid systems.All the solid electrolytes known, however, exhibit low oxide-ion conductivities below 1100 K, limiting their application. Therefore, there is a need for fast oxide-ion conductors that are operative at temperatures around 700 K.',2 Recently, efforts have been directed towards developing such materials. Abraham et aL3 discovered that Bi4V20 which belongs to the Aurivillius layered perovskite family, exhibits high oxide-ion conductivity in its high-temperature tetragonal phase (Tr=850 K). Subsequently,e it has been possible to stabilize the tetragonal phase at room temperature by substituting appropriate amounts of copper, titanium or niobium for vanadium in Bi4V2011; all the tetra- gonal phases stabilized by substitution exhibit high oxide-ion conductivities in the temperature range 500-900 K, the highest oxide-ion conductivity observed to date in this family of oxides being ca.1x lop4Q-' cm-' at 700 K for the composi- tion Bi,V .8Ti0.2010.9.6 Here, we report the synthesis of a new series of oxide-ion conductors, Bi,W, -,Cu,06 -,,, derived from the Aurivillius phase Bi2W06 by substitution of Cu" for Wvl. The highest oxide-ion conductivity of ca. 1 x R-' cm-' at 700 K is exhibited by the composition Bi,Wo,3Cuo,,04,6; this value is about an order of magnitude higher than the conductivities of copper/titanium-substituted Bi4V2OI1 reported previously.6 The significance of this work lies in that, unlike Bi4V2011= Bi,VO,,, no.,,the parent Aurivillius phase, Bi2W06, of the present series is a stoichiometric compound without oxygen vacancies. Accordingly, the present study opens up the pos- sibility of 'engineering' new oxide-ion electrolytes starting from other Aurivillius phases7 of the general formula (Bi202)(An- 'Bn03"+ ') by appropriate chemical substitution. Experimental Various members of the Bi2Wl -,CU,O~-~, family were syn- thesized by melting stoichiometric mixtures of appropriate amounts of the oxides followed by quenching the melt.The melting temperature varied between 1320 and 1110 K. The compositions with higher copper content melted at lower temperatures. The products were characterized by X-ray powder diffraction (JEOL JDX-8P diffractometer) using Cu-Ka radiation.Infrared (IR) spectra of the oxides were recorded on a Briiker IFS 113 V FT-IR spectrometer at room temperature in a KBr matrix. Samples were compressed into pellets of diameter 9 mm and thickness 1.5 mm. The pellets were sintered at 873 K in air for 24 h. Impedance data on the sintered pellets of ca. 95% theoretical density coated with platinum paint were obtained at various partial pressures of TContribution no. 1007 from the Solid State and Structural Chemistry Unit. oxygen in the temperature range 500-900K, employing a 4194-A Hewlett-Packard impedance-gain phase analyser in the frequency range 100 Hz-15 MHz interfaced to an IBM-PC. Samples were equilibrated at constant temperature for ca. 45 min prior to each set of impedance measurements.Results and Discussion Typical X-ray diffraction patterns of copper-substituted bis- muth tungstates, Bi,W1-,Cu,O,-,, (O<x<0.8), are shown in Fig. 1 and the lattice parameters derived from the XRD patterns are listed in Table 1. Solid solutions of Bi,W,-,Cu,O,-,, exist for the range O<x<O.8. Phases up to x=0.65 adopt the orthorhombic Bi2WO6 structure, while those in the narrow range 0.7dxGO.8 stabilize with the tetragonal structure. We see that the c parameter and the unit-cell volume change abruptly at x =0.7 when the structure changes from orthorhombic to tetragonal (Table 1). A similar volume change also occurs across the orthorhombic-tetragonal transition in Bi4V2011.3 Since X-ray diffraction studies reflect only gross structural features, we recorded the IR spectra in the region 1000-500 cm to examine the local structural changes (d1 / A A* J I I 1 I IJ 0 20 30 40 50 60 2Bldeg rees Fig. 1 XRD patterns of (a) Bi2W0.3C~0.704.6r(b) Bi2W0.7C~0,305.4, Bi2W06 and (dl Bi2W0.2Cu0.804.4 J.MATER. CHEM., 1994, VOL. 4 Table 1 Synthesis conditions and cell parameters of copper-substituted bismuth tungstates synthesis crystal sample temperature/K system 1073 orthorhombic 1073 orthorhombic 1323 orthorhombic 1223 orthorhombic 1173 orthorhombic 1153 orthorhombic 1140 orthorhombic 1123 tetragonal 1110 tetragonal 1110 tetragonal accompanying the phase transformation. The IR spectra for typical copper-substituted bismuth tungstates along with the spectrum of the parent Bi,WO, are shown in Fig.2.The spectrum of Bi,WO, is similar to that reported by Bode et a[.,*showing absorption bands at 822, 743, 696, 596 and 548 cm-'. The spectra of Bi2W1-,Cu,O,~,, (O<xG0.65) orthorhombic phases are similar to the Bi,WO, spectrum. However, the spectrum of the tetragonal Bi2W0.3C~0,704.6 is completely different, revealing the absence of the features found for Bi,W06, and Bi2W0.7C~0.305.4. This indicates that the local structure around the W/Cu atoms in the tetragonal Bi2W0,3C~0.704,6is significantly different from that of the orthorhombic Bi,WO, and Bi,Wo.7Cuo.305~4. Indeed, the spectral features of Bi2W0.3C~0.704.6 are typical of fast-ion conducting materials such as CSHSO,.~~'~ Electrical conductivity values for various specimens of copper-substituted bismuth tungstates were obtained from their complex impedance plots.Typical impedance data for Bi,Wo,3Cuo.704,, are shown in Fig. 3. The bulk resistance of the sample is taken as the intercept of the high-frequency arc on the real Z axis, while the low-frequency intercept of the combined arcs amounts to the total pellet resistance compris- ing the bulk resistance and the resistance due to the grain boundaries. For example, the bulk and grain boundary resist- ances for a sample of Bi2Wo~3Cuo,704.6 at 650 K are approxi- mately equal (1.4 ka), amounting to a total pellet resistance ~~ 1000 900 800 700 600 500 wavenum berkm-' Fig.2 IR spectra of (a) Bi2W0,,,Cuo.,04.6, (b) Bi2W0.7C~0.305.4 and (C) Bi2WO6 a/A b/A C/A volume/A3 cell 5.47(5) 5.48 5.43(6) 5.47 16.42(7) 16.38 -492 5.49 5.47 16.33 490 5.48 5.46 16.35 489 5.49 5.47 16.34 49 1 5.48 5.45 16.39 490 5.48 5.45 16.37 489 3.95b - 16.66 518 3.95b - 16.72 522 3.95b - 16.75 522 r I IVI I I 4, 0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 Z'/kQ Fig. 3 Impedance data for B~2Wo,3Cuo.,04,6 at 650 K in the frequency range 100 Hz-15 MHz. (a) lo6Hz, (b) lo4Hz. of 2.8 kQ, as shown in Fig. 3. The conductivity data for Bi2Wl -,CU,O,-~, as calculated from the bulk resistances of the respective samples is shown in Fig. 4. The temperature dependence of the conductivity of the x =0.7 member at 1atm oxygen partial pressure, together with the variation in conduc- tivity of the series with x at 850 K and po, = 1 atm are given in this figure.We see that there is an abrupt (about an order of magnitude) increase in the conductivity across the orthorhombic-, tetragonal phase transformation at x=0.7. r I 1 I I I I) 1.2 1.4 1.6 1.8 2.0 2.2 1O~WT Fig. 4 Temperature dependence of oxide-ion conductivity for Bi2w0.3cu0.704.6 "t PO2 atm (-A-)and Bi4V1.8Ti0.2010.9 at PO2 atm (-.-), (B1203)0.8(Er203)0.2 (-*-), (zr02)0.9 (y203)0.1(-).The inset shows variation of the conductivity of Bi2W, -xCu,06-,, with x (O<x 60.80) at 850 K at po2= 1 atm. J. MATER. CHEM., 1994, VOL. 4 t A A 1o-~t-tti 1o-~ 0 10 20 -In Po2 Fig.5 Variation in conductivities of BiZW0,3Cu0,704,6(A) and Bi2Wo,,Cu,,,0,., (0)with oxygen partial pressure (Brouwer diagram) at (a) (c) 693 K and (b)(d)793 K. The conductivities for Bi2W0.3C~0,704.6 are between and 10-3 a-1 cm-' and the activation energy value is 0.35 eV in the temperature range 500-900 K. In Fig. 4 are also included the conductivity data for Zr02-Y203,11 Bi203-Er20312 and Bi4Vl.8Tio,2010.96for comparison. We see that the conductivity of Bi2Wo~3Cuo,704~6 is higher than those of Bi4V1.8Ti0.2010.9 and ZrO, -Y203 over the entire temperature range, while it is lower than the conductivity of Bi,03-Er203. The conduc- tivity of Bi2Wo~,Cuo~704,6 shows little variation with oxygen partial pressure (Fig.5), suggesting that the electrical conduc- tion is mainly ionic. Transport-number measurements are, however, required to establish the exact ionic and electronic components to the electrical conductivity. In Fig. 5 are also included the data on the dependence of conductivity on oxygen partial pressure for the orthorhombic Bi2W0.7C~0.305.4.The data do show the presence of an electronic component of the conductivity in this material at higher oxygen pressures. Our results in this respect are similar to those reported by Goodenough et all3 for Ba21n,0,. Goodenough et al. suggested that do cations, such as TiIV and NbV, which exhibit 'ferroelectric' displacement in octa- hedral oxygen coordination, and Cu" which occurs in four, five or six coordination readily in oxides, provide a low-energy barrier for oxide-ion migration, giving rise to the high oxide-ion conductivity of Bi4V201 substituted with Ti", NbV or Cu".An abrupt increase in the c parameter of the tetragonal phase of Bi2Wl -xCux06-2x (Table l), suggesting a cooperat- ive displacement of Wvl towards the apical oxygen. reveals that the Goodenough model is likely to be applicablc for the series reported in this paper. Determination of the crystal structures of the orthorhombic and tetragonal phases is required to establish details of the structure and mechanism of oxide-ion conduction of the Bi2Wl -xCux06-2x series. We thank Professor C. N. R. Rao, FRS for his continuing encouragement. We also thank Dr H. N. Vasan for his assistance in recording IR spectra.References 1 J. B. Goodenough and A. K. Shukla, in Solid State Ionic Devices, ed. B. V. R. Chowdhari and S. Radhakrishna, World Scientific, Singapore, 1988, pp. 573-604. 2 J. B. Goodenough, A. Manthiram, M. Paranthaman and Y. S. Zhen, Mater. Sci. Eng. B, 1992, 12, 357. 3 F. Abraham, M. F. Debreuille-Gresse, G. Mairesse and G. Nowogrocki, Solid State Ionics, 1988,28-30, 529. 4 F. Abraham, J. C. Boivin, G. Mairesse and G. Nowogrocki, Solid State Ionics, 1990,40-41,934. 5 T. Iharada, A. Hammouche, J. Fouletier, M. Kleitz, J. <'.Boivin and G. Mairesse, Solid State Ionics, 1991,48,257. 6 Vandana Sharma, A. K. Shukla and J. Gopalakrishnm, Solid State Ionics, 1992,58, 359. 7 B. Frit and J. P. Mercurio, J. Alloys Compounds, 1992,188,27. 8 J. H. G. Bode, H. R. Kuijt, M. A. J. Th. Lahey and G. Blase, J. Solid State Chem., 1973,8, 114. 9 Ph. Colamban, M. Pham-Thi and A. Novac, Solid Stare lonics, 1986,20, 125. 10 V. Varma, N. Rangavittal and C. N. R. Rao, J.Solid State Chem., 1993,106,164. 11 T. H. Etsell and S. N. Flengas, Chem. Rev., 1970,70,339. 12 M. J. Verkerk, K. Keizer and A. J. Burggraaf, .[. Appl. Electrochem., 1980, 10, 81. 13 J. B. Goodenough, J. E. Ruiz-Diaz and Y. S. Zhen, Sorid State lonics, 1990,44,21. 14 R. W. Wolfe, R. E. Newnham and M. I. Kay, Solid State Commun., 1969,7, 1797. Paper 3/06603G; Received 4th November, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940400703

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(5), 707-711 Crystal Structures of Two Sodium Yttrium Molybdates: NaY(MoO,), and Na,Y(MoO,), Nicola J. Stedman; Anthony K. Cheethamb and Peter D. Battle"* a lnorganic Chemistry Laboratory, South Parks Road, Oxford, UK OX7 3QR Materials Department, University of California, Santa Barbara, CA 93106, USA The crystal structure of NaY(MoO& has been determined from powder X-ray diffraction data [space group /4,/a, Z= 2, a =5.1 9890(5) A, c =11.3299(1) A]. The Na and Y atoms are disordered over the eight-coordinate sites of a scheelite structure. The crystal structure of Na,Y(MoO,), has been determined from single-crystal X-ray diffraction data [space group 14,/a, Z=4, a =11.374(3) A, c =11.440(5) A] and found to be scheelite related, with the tetrahedral sites occupied by an ordered arrangement of Mo and Na in a 4:1 ratio.Comparisons are drawn between the structures of these two closely related phosphor materials. Alkali-me tal rare-earth-me ta1 molybdates and tungs ta tes, which have been known since the 1880s,' were ascribed a wide range of formulae by early workers. The situation was clarified in 1943 by the elegant work of Sillen and Sundvall,, which showed that many of the preparations reported were, in fact, orientated overgrowths of two phases, AM(XO,), and A,M(XO,),, where A= Li, Na or K, M =La or Bi and X = Mo or W. A study of crystals of the sodium lanthanum molybdate NaLa(MoO,), and the sodium lanthanum tung- state Na,La( W04), suggested that the former adopts the scheelite structure with Na and La disordered over the eight- coordinate site (the calcium site in scheelite, CaW0,3) and that the latter adopts a related but more complex structure.However, although this work identified the structure type, it did not provide a detailed description of the crystal structure, including the accurate location of the anions. NaLa( has subsequently proved an ideal starting point for studies of the effect of small variations in host structure on luminescent properties4'' because each of the three types of cation can be replaced in turn (sodium by lithium, lanthanum by another lanthanide, yttrium or a combination of these, and molybdenum by tungsten), thus giving rise to a wide range of compositions whilst always retaining the same basic Relatively broad peaks were observed in the luminescence spectra of phosphors based on this type of structure, probably due to the inherent disorder of the lanthanide and alkali-metal cations, but little or no concentration quenching was ~een.~~,~' In contrast, phosphors based on AM(XO,), (A=K, Rb, Cs), which adopt structures in which the alkali-metal and rare-earth-metal ions are ordered, gave sharper spectra but were more prone to concen- tration quenching.,' Neodymium-or erbium-doped NaLa(MOO,), have lately proved promising as laser materials.21 The phases A,M(XO,), (A =Li, Na; M =La to Lu, Y; X = Mo or W) have also proved to be good phosphors, showing very little concentration quen~hing.'~,~~-~~ They are, again, isomorphous across the lanthanide series22v26 and since the early work by Sillen and Sundvall the crystal structures of the tungstates and molybdates Na,La(MO,),, Na,Tb( MO,), and Na,Lu(MO,), (M=Mo or W) have been fully deter- mined, confirming that the structure is scheelite related.27-29 Despite the high level of interest in the optical properties of these materials, no close structural comparison of the two series of compounds has been carried out.Curiously enough, this is not due to a lack of structural information regarding the more complex A,M(XO,), phases, but due to a lack of data regarding the simple scheelites AM(X04)2. We therefore undertook a study of the two sodium yttrium molybdates NaY(MoO,), and Na,Y(MoO,),, beginning our work at a time when no AM (XO,), compound had been fully character- ised. During the course of our work an account of the crystal structure of NaLa(MoO,), was published,21 but the quality of the refinement was not good enough to be considered definitive for the whole structure type.The results of our own work are described below. Experimental The initial synthesis of the two phases was entirely serendipit- ous; an attempt to crystallise a reduced yttrium molybdenum oxide by dissolution in a sodium molybdate flux at 1100 "C under an inert gas, followed by a slow cooling, led instead to crystals of molybdenum dioxide and Na,Y(MoO,), in a melt containing NaY(MoO,),. It was shown that the product contained two distinct Na-Mo-Y-0 phases by the analysis of individual crystallites using a JEOL 2000FX analytical electron microscope in transmission mode together with a Tracor Northern TN5500 energy-dispersive X-ray detector (Table 1).Thin crystallites (partly transparent to the electron beam) were analysed as the effects of fluorescence and absorp- tion can be considered negligible in the thin-crystal limit,30 thus enabling the composition of each crystallite to be inferred from the relative peak intensities in the X-ray spectra.Comparison of the results with those obtained on analysis of an yttrium molybdate of known composition showed that particles of type A contained yttrium and molybdenum in the ratio 1:4,and that particles of type B contained yttrium and molybdenum in the ratio 1:2. It was also apparent that particles of type A contained over twice as much sodium Table 1 Analytical electron microscopy results for individual crystal- lites of the two Na-Y-Mo oxides particles of Type A particles of Type B "a5Y(MoO4 )4 1 "aY (MOO, 121 Y-Ka/Mo-Ka Na-KalMo-Kr Y-KalMo-Ka Na-Ka/M o-Ka 0.33 0.50 0.63 0.21 0.36 0.47 0.60 0.23 0.33 0.52 0.67 0.24 0.31 0.47 0.64 0.25 0.35 0.53 0.65 0.23 0.33 0.57 0.30 0.46 Particles containing molybdenum only were also observed, corre- sponding to molybdenum dioxide.(relative to molybdenum) as particles of type B, suggesting the formulae given above. In order to confirm these formulae, and because good single crystals of NaY(MoO,), had not been obtained in the first experiment, a pure polycrystalline powder of each phase was prepared by standard solid-state techniques.In each case, the appropriate stoichiometric mix- ture of MOO, (Aldrich, 99.5%), Y203 (Aldrich, 99.99%, dried before use) and Na2C03 (BDH AnalaR, 99.9%, dried before use) was ground well in an agate mortar, pressed into a pellet and heated for 2 days at 500 "C in an alumina crucible, reground and repelleted, then heated for 5 days at 800 "C. The ratio of yttrium to molybdenum in each of these powders was checked by analytical electron microscopy, giving results which correlated well with those given in Table 1, and the sodium content of each was confirmed by atomic absorption spectroscopy (NaYMo,O,: calc.5.32%, expt. 5.87%. Na5Y(Mo04),: calc. 13.63%, expt. 13.97%). Na5Y(Moo4), was studied by single-crystal X-ray diffrac- tion. A small, approximately spherical (diameter z0.2 mm), colourless, transparent crystal suitable for data collection was selected and mounted on a four-circle kappa-geometry Enraf- Nonius CAD-4 automatic diffractometer controlled by .a PDPll/23 microprocessor. Mo-Ka radiation (2=0.710 69 A) was used. 25 reflections were located from a preliminary Polaroid photograph, centred and then indexed on a triclinic unit cell. However, examination of the Niggli matrix of this first unit cell suggested that the true symmetry was tetragonal. The unit cell was transformed accordingly and a number of additional reflections were then measured, centred and used in a least-squares refinement to obtain accurate unit-cell parameters.Shells of intensity data were collected in the angular range 0"<6< 37.5". A horizontal slit width of 4 mm was used with a &dependent o scan width of (1.10+0.35 tan 6)'. Three strong reflections were chosen as intensity standards and were monitored every hour, and three chosen as orientation con- trols were checked after the measurement of every 250 reflec- tions. At the end of data collection the intensities of two reflections with x close to 90" were measured as a function of @ (from 0 to 360'), and these data were used to correct for absorption (p= 73.51 cm-l; min/max =2.59/2.73). Powder diffraction data were collected on NaY(MoO,), by J.MATER. CHEM., 1994, VOL. 4 step scanning over the angular range 5"<28<100' using a Philips PW1710-based diffractometer operating in Bragg-Brentano geometry with a step size of 0.02 O 28 andoa count time of, 10 s per step. Cu-Ka radiation (& =1.540 51 A, A2 = 1.544 33 A) was used. Results Study of the systematic absences in the data collected on Na5Y(Mo04), led to the assignment of the space group as I4,/a (International Tables for Crystallography Volume A, no. 88, origin !t -131) with 274 and the unit-cell parameters a =11.374( 3) A, c = 11.440(5) A. The data were merged (R= 4.43%) and corrected for polarisation, the Lorentz factor and absorption. Friedel pairs were not averaged as a correction for anomalous scattering was made.Of the 2074 reflections remaining at this stage, 1403 with I >3a(I) were used for structure solution and refinement. The structure was solved using the automatic Patterson interpretation routine in the program SHELXS3,All the atoms were located. The model from SHELXS was then refined using the least-squares routines in the program CRYSTALS3, In the final stages of refinement, all atomic coordinates and anisotropic temperature factors were allowed to vary (a total of 59 parameters) and a three-term Chebyshev polynomial (coefficients 6.41, -3.13, 3.91) was introduced as a weighting scheme. The refined atomic positions and tempera- ture factors are listed in Table 2 and selected bond lengths and angles are given in Table 3.The final agreement factors were R=3.14% and R,=3.38%; the strongest pe$k in the last difference Fourier map had a height of 0.18 e A-,. The structure is represented in Fig. 1. The powder diffraction data collected on NaY (MOO,), were smoothed using the Philips Automated Powder Diffraction Software34 and sutsequently indexe$ on a tetra- gonal unit cell [a=5.19890(5) A, c= 11.3299( 1) A], consistent with space group I4Ja (International Tables Volume A, no. 88,l with 2 =2. The data were analysed using the program GSAS.35 In the starting model, yttrium and sodium were placed on the 4(b) site and constrained to half occupancy, and molybdenum was placed on the 4(a) site, as proposed originally by SillCn and Sundvall., The oxygen site was then Table 2 Atomic coordinates and temperature factors for Na,Y (Mo0J4 atom site X Y Z Ueauiv/A2 ~ ~ ~~ 0.0 0.25 0.625 0.023 1 0.7044(2) 0.5 0.1296(2) 0.25 0.5940( 2) 0.375 0.0138 0.006 1 0.8189( 3) 0.8512( 3) 0.61 1 l(2) 0.6798(2) 0.8240( 3) 0.34359( 2) 0.2 194 (2) 0.31 19( 2) 0.3343( 2) 0.4652( 3) 0.38685(2) 0.4727( 3) 0.5382( 2) 0.3 133 (2) 0.4791 (3) 0.0072 0.01 39 0.0105 0.01 10 0.0142 u22 u33 u23 u13 Na(1) 0.031 (1) 0.03 1 (1) 0.013(2) 0.0 0.0 0.0 W2) 0.0145( 7) 0.01 66( 7) 0.0116(6) O.O026( 5) -O.OOlO( 5) 0.001 1 (5) Y(1) O.O067( 1) 0.0067( 1) O.O052( 1) 0.0 0.0 0.0 Mo(1) 0.0075( 1) 0.0076( 1) 0.0067( 1) -O.O003( 1 ) 0.0000( 1) 0.0oO0( 1) O(1) 0.016( 1) 0.012( 1) 0.016(1) O.O04( 1) O.O03( 1) 0.002(1) O(2) 0.012( 1) 0.001( 1) 0.01 (2) -O.O006(8) -O.O033( 8) 0.0014(8) O(3) 0.0090( 9) 0.016( 1) 0.010(1) O.O024(8) -0.0007(8) -0.0019(8) (34) 0.023( 1) 0.013(1) 0.012( 1) -0.0059(9) O.Ool(1) -O.OOO( 1) T =[-27~~(h~a*~u,, +12c*2u33+k2b*2~2z +2hka*b*u12+2hla*c*uI3+2klb*c*u2,)].Uequiv=(U1U2U3)"3,where Ui are the principal components of the thermal displacement tensor. Structure factors for this compound have been deposited on magnetic tape at the Chemical Crystallography Laboratory, 9 South Parks Road, Oxford. J. MATER. CHEM., 1994, VOL. 4 Table3 Bond distances (‘/A)and bond angles (‘/degrees) in Na5Y(MOO,), Na( 1)-O( 1) 4 x 2.454( 3)O(1)-Na( 1)-O( 1) 4 x 120.3(l),2 x 89.5( 1) Na(2)-O( 1) 2.400(3)Na(2)-0( 1)’ 2.486( 3) Na(2)-O(2) 2.415(3)Na( 2)-0(3) 2.358( 3) Na( 2)-O(4) 2.459( 3) Na(2)-O(4)’ 2.310( 3) 4 x 2.362(2) 4 x 2.366( 2) Mo( 1)-O( 1) 1.759( 3) Mo( 1)-0(2) 1.775( 3) Mo( 1)-0( 3) 1.796 (2) Mo( 1)-O(4) 1.741(3) O(1)-Mo( 1)-0(2) 106.5(1) O(1)-Mo( 1)-0(3) 1133 1) O(1)-Mo( 1)-0(4) 107.0( 1) O(~)-Mo( 1)-0( 3) 112.5( 1) O(2)-Mo( 1)-0(4) 105.7( 1) 0(3)-Mo( 1)-0(4) 111.1( 1) C 0Nal”j F oy n Wh Fig.1 Crystal structure of Na5Y(MOO,),. MOO, tetrahedra are shaded. located using a difference Fourier synthesis. In the final cycles of refinement, a zero-point error, the unit-cell parameters a and c, the coordinates of the oxygen atom, an isotropic temperature factor for each atom (constrained to be equal for yttrium and sodium) and four terms for a pseudo-Voigt peak shape were refined simultaneously against the intensities of 82 Bragg reflections.The background was defined by linear interpolation between fixed points selected by eye. The final observed, calculated and difference diffraction profiles are shown in Fig. 2. The final atomic positions are given in 709 Table 4 Atomic parameters for NaY(MoO,j, atom site X Y z T ‘is,/A2 Nap 4(b) 0.5 0.75 0.125 0.0076(6)Mo 4(a) 0.0 0.25 0.125 0.0055(4)0 16(f) 0.1464(7) 0.4813(7) 0.2122(3) 0.012(2) Table 5 Bond distances (/A) and bond angles (/degrees) in NaY (MOO,), Na/Y-0 4 x 2.435(4), 4 x 2.511(4j Mo-0 4 x 1.733(3)0-Mo-0 4 x 108.9(l), 2 x I10.5(2) 0 Fig. 3 Crystal structure of NaY(MoO,),. MOO, tetrahedra are shown.Shaded circles represent a disordered distribution of Na and Y. Table 4. Selected bond angles and distances are given in Table 5 and the structure is drawn in Fig. 3. The final agree- ment factors were R,=5.49%, R, =9.37% and R, =12.52%, with x2=7.22. Discussion Na5Y(Mo04)4 was found to be isostructural with the other phases NaSLn(XO,), (Ln =La, Tb or Lu and X =Mo or W) for which the crystal structures have been deter~nined.~~-~~ The molybdenum site has an approximately regular tetra- hedral coordination geometry and the eight-coordinate yttrium site is approximately square prismatic. In contrast to the structure of N~Y(MOO,)~,two distinct sodium sites are found. Most of the sodium in the structure “a(?)] is accommodated in a highly irregular six-coordinate site, in which Na-0 bond lengths range from 2.310(3) to 2.486(3) A.The remainder [Na(l)] is found in a site of hiGher (S,) symmetry, coordinated by four oxygens at 2.454(3) A. Our refinements confirm that NaY(MoO,), adopts the scheelite structure with sodium and yttrium disordered over the 4(b) site. Molybtenum is coordinated by four oxygen atoms at 1.733(3) A in a tetrahedral site, and the yttrium/sgdium site is eight-coor$inate, with four oxygens at I 1 I I1-20 30 40 50 60 70 80 90 100 2.435(4)A and four at 2.511(4)A. The disorder of sodium and yttrium in this structure is also likely to affect the o~ygen site. Each oxygen atom forms part of the coordination sphere of three cation sites, one which is always occupied by molyb- denum and two which can be occupied by either sodium or 2Hdegrees Fig.2Observed (...), calculated (-) and difference X-ray diffraction profiles of NaY(Moo,),. Reflection positions are marked. J. MATER. CHEM., 1994, VOL. 4 yttrium; four different local environments are thus possible for each oxygen atom in the structure (the oxygen site is not equidistant from the two nearest sodium/yttrium sites, so there are two possible ways in which each oxygen may be coordinated by one molybdenum, one yttrium and one sodium). The site located in our refinements will thus be the centre of an averaged electron density distribution, the average being taken over the actual oxygen sites in the structure. Residual electron density near O(1) but displaced slightly towards each of the two nearest yttrium/sodium sites was observed in the final difference Fourier maps, but the disorder could not be modelled successfully. Despite this, the model obtained for NaY ( MOO,), appears to be significantly better than any published model for the series AM(X04)2.The most recent of these, that for NaLa(MoO,),, is clearly unsatisfac- tory; our calculations for the bond lengths in this phase using the parameters given in ref. 2b resulted in a remarkably short Mo-0 bond length (1.69 A) and a short 0-0 contact (1.91 A), which suggests that the oxygen position quoted is in error. In order to compare the structures of these two compounds it is useful to envisage the cation sublattice of Na5Y(Mo04), as being composed of layers perpendicular to the c axis, with four such layers, together with the anion sublattice, being contained in one unit cell.One layer is shown in Fig.4 together with the corresponding layer of the simple scheelite structure; the scheelite unit cell is also shown, lying with the axis a' at an angle of ca. 60 O to the a axis of Na,Y( MOO,),. In general, the structure of the scheelite-related phases A,M(XO,), can be derived from the structure of AM(XO,), by replacing one-fifth of each X with A, and altering the random 1: 1 distribution of A and M on the eight-coordinate sites such that every fifth site is now occupied by M, the remainder being occupied by A. This is then followed by small shifts in the positions of some of the cations, and larger changes in the anion sublattice, some oxygens being lost altogether.The cations X (Mo or W) remain tetrahedrally coordinated, but are displaced slightly out of the planes containing the cations M. Of the two new A sites, the six- (a1 0Na V V0 0 0 0 0 0 Fig.4 (a) Cation layer perpendicular to [OOl] in Na,Y(MoO,),. MOO, tetrahedra are shaded, the larger unfilled circles represent Na, the smaller circles represent Y. The unit cell is shown. (b)The same layer in NaY (MOO,),. coordinate sites, derived from an eight-coordinate M site in the scheelite structure, are also displaced slightly out of the planes containing the remaining M cations, while the A cations in the four-coordinate sites, derived from a tetrahedral X site in the scheelite structure, remain coplanar with the M cations.The disorder in the scheelite phase leads to ambiguity in the interpretation of the bond distances in Nay(M00,)~ as yttrium-oxygen and sodium-oxygen distances cannot be dis- tinguished. In the light of typical Na-0 and Y-0 bond lengths,36 use of the average oxygen position given in Table 3 is likely to lead to an Na/Y-0 bond length that on the one hand underestimates the true Na-0 distances and on the other overestimates the true Y -0 distances present. This would explain the apparent difference in Y-0 bond lengths in the two phases; the yttrium site appears smaller in Na,Y(MoO,), than in NaY(MoO,),. The two sodium sites in Na5Y(Mo0,), have lower coordination than the sodium/yttrium site in NaY(MoO,), and the Na-0 bond lengths are therefore shorter in the former.It is also possible that the Mo-0 bond lengths present in NaY(MoO,), are slightly underestimated as a result of the disorder as they are certainly smaller than those found in Na,Y (MOO,),. This may, however, be simply a result of the difference in the two structures; the sodium-rich phase adopts a much more open structure than that of the simple scheelite. Conclusions The structures of NaY MOO^)^ and Na,Y (MOO,), have been determined. Rietveld analysis of powder X-ray diffraction data showed that NaY(MoO,), adopts the scheelite structure with sodium and yttrium disordered over the eight-coordinate site.Residual electron density near the oxygen site suggested that this is also disordered, but it did not prove possible to model this. Na,Y( MOO,), was studied by single-crystal X-ray diffraction and was shown to adopt a scheelite-related structure. References 1 A. G. Hogbom, Ofvers. Akad. Stockholm, 1885,42, 1. 2 L. G. Sillen and H. Sundvall, Ark. Kemi, Mineral. Geol., 1943, 17A, 10. A. Zalkin and D. H. Templeton, J. Chem. Phys., 1964,40, 501. L. G. Van Uitert, J. Appl. Phys., 1960,31, 328. L. G. Van Uitert and R. R. Soden, J. Chem. Phys., 1962,36,517. L. G.Van Uitert and R. R. Soden, J. Chem. Phys., 1962,36,1289. L. G. Van Uitert, R. R. Soden and R. C. Linares, J. Chem. Phys., 1962,36,1793. 8 L. G.Van Uitert and R.R. Soden, J. Chem. Phys., 1962,36,1797. 9 L. G. Van Uitert, J. Chem. Phys., 1962,37,981. 10 L. G. Van Uitert and S. Iida, J. Chem. Phys., 1962,37,986. 11 L. G. Van Uitert, J. Electrochem. Soc., 1963,110,47. 12 M. M. Schieber and L. Holmes, J. Appl. Phys.. 1964,35,1004. 13 M. M. Schieber, Znorg. Chem., 1965,4, 762. 14 M. V. Mokhosoev, V. I. Krivobok, S. M. Aleikina, N. S. Zhingula and N. G. Kisel, Znorg. Muter., 1967,3, 1444. 15 M. V. Mokhosoev and E. I. Get'man, Russ. J. Inorg. Chem., 1969, 14, 1236. 16 R. Heindl, F. Damay, R. Der Agobian and J. Loriers, C. R. Acad. Sci. Paris, 1965,261, 3335. 17 S. Preziosi, R. R. Soden and L. G. Van Uitert, J.Appl. Phys., 1962, 33, 1893. 18 M. V. Savel'eva, I. V. Shakhno and V. E. Plyuschev, Znorg.Muter., 1970, 6, 1466. 19 B. F. Dzhurinskii, L. N. Zorina and G. V. Lysanova, Inorg. Muter., 1980, 16,86. 20 J. P. M. Van Vliet, G. Blasse and L. H. Brixner, J. Solid State Chem., 1988,76,160. 21 S. B. Stevens, C. A. Morrison, T. H. Allik, A. L. Rheingold and B. S. Haggerty, Phys. Rev. B Conden. Muter., 1991,43, 7386. J. MATER. CHEM., 1994, VOL. 4 71 1 22 23 24 25 26 27 28 29 30 V. K. Trunov, T. A. Berezina, A. A. Evdokimov, V. K. Ishunin and V. G. Krongauz, Russ. J. Inorg. Chem., 1978,23,1465. H. Y-P. Hong and K. Dwight, Muter. Res. Bull., 1974,9,775. J. Huang, J. Loners, P. Porcher, G. Teste de Sagey, P. Car0 and C. Levy-Clement,J. Less Common. Metals, 1983,94,251. J. Huang, J. Loners, P. Porcher, G. Teste de Sagey, P. Car0 and C. Levy-Clement, J. Chem. Phys., 1984,80,6204. M. V. Mokhosoev, F. P. Alekseev and E. I. Get’man, Russ J. Inorg. Chem., 1969,14,310. V. A. Efremov, T. A. Berezina, I. M. Averina and V. K. Trunov, Sou. Phys. Crystallogr., 1980,25, 146. V. A. Efremov, V. K. Trunov and T. A. Berezina, Sou. Phys. Crystallogr., 1982,27,77. R. F. Klevtsova, L. A. Glinskaya, L. P. Kozeeva and P. V. Klevtsov, Sou. Phys. Crystallogr., 1973, 17, 672. A. K. Cheetham and A. J. Skarnulis, Anal. Chem., 1981,53,1060. 31 32 33 34 35 36 International Tables for Crystallography, ed. T. Hahn. Kluwer, Dordrecht, 1989,vol. A. G. M. Sheldrick, Program for the Solution of Crystal Structures, University of Gottingen, Germany, 1986. D. J. Watkin, J. R. Carruthers and P. W. Betteridge, CRYSTALS User Guide, Chemical Crystallography Laboratory, University of Oxford, Oxford, 1985. Philips PW 1877 Automated Powder Diffraction Software, Version 3.50. A. C. Larson and R. B. von Dreele, Los Alamos Report LAUR R. D. Shannon and C. T. Prewitt, Acta Crystallogr., Sect B, 1969, 25,925. 86-748,1987. Paper 3/06485I; Received 29th October, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940400707

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(5), 713-717 Electret Behaviour of Di= and Tri-nuclear Iron Hydrazone- Hexacyanoferrate Compounds studied by the Thermally Stimulated Depolarization Current Technique Alex Bonardi: Rosanna Capelletti,*' Corrado Pelizzi*" and Pieralberto Tarasconia a lstituto di Chimica Generale ed Inorganica, Universita di Parma, Wale delle Scienze, 43100 Parma, Italy 'Dipartimento di Fisica, Universita di Parma, Wale delle Scienze, 43700 Parma, Italy A series of di- and tri-nuclear iron hydrazone-hexacyanoferrate complexes have been prepared and characterized by vibrational and electronic spectroscopies, thermal analysis and by the thermally stimulated depolarisation current (TSDC) technique. The TSDC spectra, measured in the range 110-300 K, have confirmed the reorientation of (1) dipoles in the terminal aromatic radicals, belonging to the molecule and (2) water molecules, whenever they are present in the compound or are adsorbed on it.A fraction of water molecules is loosely bound to the compound and can be removed by pumping on the sample; the related dehydration and hydration kinetics have been analysed. Binuclear and polynuclear metal complexes containing similar or different metal ions are of interest for practical purposes in different fields, i.e. in homogeneous catalysis' and biochem- istry,,,, and for ele~trical~,~ and magnetochemica16 studies. In particular, the use of polydentate Schiff as ligands to form binuclear metal complexes has made it possible to bring pairs of metal atoms into close proximity, thus favouring the production of peculiar magnetic, optical or electric properties.Our research work on the synthesis and characterization of homo- and hetero-nuclear metal complexes with polyfunc- tional nitrogen ligands,lO,ll with the aim of isolating new electronic and magnetic materials, has been recently devoted to the investigation of a series of di- and tri-nuclear iron hydrazone-hexacyanoferrate complexes.12 As an extension of this research program we now report the characterization of this series of complexes by means of infrared and ultraviolet absorption spectroscopy, thermal analysis and a dielectric technique (TSDC).13-15 This technique has been employed previously to study a variety of electric polarization phen- omena in insulating materials such as ionic crystals, polymers and biopolymers, where the electronic conductivity is negleg- ible.The dielectric processes which have been analysed pre- viously are reorientation of ionic dipoles, the space charge and the interfacial polarization phenomena originating in inhomogeneous dielectrics. In this case the technique is used to study polarization phenomena induced by reorientation of groups in the presence of an electric dipole moment belonging to the molecule itself (e.g. see Fig. 3, later) or present in the sample as a consequence of the synthesis process (from water or methanol molecules). In this way it is possible to monitor the changes in the dipole concentration and/or environment as a consequence of specific treatments.In fact, the reorien- tation of water molecules, as detected by TSDC peaks, has already been studied in a variety of materials such as ice,16 inorganic hydrated compounds such as La( S04),17 (where TSDC peaks shift as a consequence of deuteriation) and biopolymers (melanin,ls lysozyme,19 casein,,' homopeptides2' and keratin2,). Application of the TSDC technique to biopoly- mers enabled the identification of different water sites and the analysis of the dehydration kinetic^.^^^^^ The reorientation of water molecules adsorbed on solid surfaces, for instance Si0223 or the hygroscopic NaI surface,15 has also been detected by this technique. Experimentalt [Fe( H,daps)Cl,].H,O (l),[Fe( H2dappc)C1,]-2H20 I 2), and [Fe( H2dapt)C12]-2H20 (3) were synthesized and characterized as previously de~cribed.2~ The complexes K, [Fe(daps)] [Fe( CN),12.2MeOH (4), [Fe( H,dappc)], [Fe( CN),] C1-4H20 (5)and K[Fe(H,dapt)] [Fe(CN),].4MeOH (6)were prepared by combining separately methanolic solutions of the above reported compounds (1.0 mmol in 80 ml of MeOH 99.9%) with an aqueous solution of K,[Fe(cN),] (2.0 mmol in 80 ml of water).$ The solutions were stirred at room temperature for ca.30min. After the solutions had been filtered, the resultant green microcrystalline products were washed with water and methanol and dried in vacuum. In a similar way, when 1 was treated with K,[Fe(CN),] in a 2: 1 molar ratio, a fine green precipitate of K,[Fe(dap~)]~[Fe(CN),].2W,0 (7) was collected by filtration.The yields of the purified compounds were >80%. The IR absorption measurements were performed at room temperature using a Perkin-Elmer 283B and a Jasco 702 G recording spectrophotometer, both operating in the 4000-200 cm-' range. In order to monitor the absorption changes induced by the sample dehydration process, the pellets were assembled in a cell (with KRS5 windows) which could be evacuated. The UV absorption spectra were meas- ured at room temperature using a Jasco 505 recording spectro- photometer. The thermal analysis was performed using a Perkin-Elmer Delta series TGA7 thermobalance operating in the range 30-400 "C, usually at a heatinglcooling rate of 20 "C min-l. Elemental analyses were carried out using a Perkin-Elmer 240 and a Philips PU7450 ICP atomic emission spectrometer. The TSDC measurements were performed using a home- made apparatus, typically as follows.The sample was assembled in a gas-exchange cryostat, placed between two 7 H,daps =2,6-diacetylpyridine bis(salicyloy1h ydrazone), H2dappc = 2,6-diacetylpyridine bis(2-pyridinecarbonylhydrazone), H,dapt = 2,6-diacetylpyridine bis( 2-thenoylhydrazone). $Elemental analyses (C, H, N, S, and Fe) are in agreement with the proposed formulations. Anal. Calc. (found 1 for K6Fe3C37H27N1,06(4): C, 36.79 (36.50%); H, 2.25 (2.36%); N, 19.72 (20.17%); Fe, 13.87 (14.55%). Anal. Calc. (found) for Fe,C48H,6N20C10, (5):C, 46.72 (47.15%); H, 3.76 (4.15%); x, 22.70 (22.68%); Fe, 13.58 (13.65%).Anal. Calc. (found) for KFeZC29H33N1106S2(6):C, 41.15 (40.92%); H, 3.93 (4.12%); N, 18.20 (18.35%); S, 7.56 (8.20%) Fe, 13.19 (13.50%). Anal. Calc. (found) for K3Fe3C52H42N16010(7): C, 46.75 (47.05%); H, 3.17 (3.33%); N, 16.78 (16.42%); Fe, 12.54 (13.01%). J. MATER. CHEM., 1994, VOL. 4 metal electrodes and polarized by an electric field E, (ca. lo3V cm-l), at a temperature Tp (usually ca. 300 K) at which the electric dipoles are mobile. The sample was then cooled to a temperature (usually 110 K) at which the dipoles are no longer mobile; the electric field was then turned off. The polarization induced by the field is frozen in because the dipoles remain aligned along their preferred orientations.The sample was then connected to an electrometer, then warmed at a constant rate (ca. 0.1 K s-'). During this step the dipoles gain mobility and lose their preferred orientations, giving rise to a displacement depolarization current which is detected by the electrometer (Cary 401), that is able to monitor currents as low as 10-15A. In this way a plot is obtained of the thermally stimulated depolarization current (TSDC) us. temperature, which is displayed by a recorder (Leeds & Northrup Speedomax XL682 recorder). The elec- trometer signal and the voltage supplied by the chromel-con- stantan thermocouple (which monitors the sample temperature) are acquired by means of a Labmaster interface and transferred to an IBM personal computer for data processing.In the present case the TSDC spectra were moni- tored in the temperature range 110-300 K. For further details of the apparatus used for TSDC measurements see ref. 15. Blocking electrodes were obtained by inserting A-type samples (see below) in a PTFE box which was then placed between the electrodes. In order to avoid changes in the hydration level, the TSDC measurements were never performed in dynamic vacuum, but under a dry nitrogen atmosphere at a pressure of 100Torr. Between subsequent TSDC runs the nitrogen pressure was increased to 600 Torr. A clean dynamic vacuum (2 x Torr) for dehydrating the samples was obtained by using a diffusion pump followed by a trap cooled to liquid-nitrogen temperature. Different kinds of samples were used A-type samples, i.e.pure-compound pellets (4= 13mm, x~0.3mm); B-type samples, i.e. compound (ca. 1mg) dispersed in KBr (100 mg) pellets and C-type samples, i.e. solution of the synthesized compound in DMSO (lo-' mol 1-l). A-type samples were used for TSDC measurements; B-type samples were used for IR measurements, since they were transparent enough to allow detection of the absorption spectrum in the IR range; C-type samples were used for UV spectroscopy. Results and Discussion The IR spectra of 1, 2 and 3 and of the parent cyano- derivatives are very similar, suggesting that there is no substan- tial change in the coordination of the hydrazone ligand towards the iron@) atoms caused by the substitution reaction.The v(CN) bands in 4-7 are in the range 2060-2040 cm-l, in agreement with bands reported for terminal and non-linearly bridging CN gro~ps.'~-~~ The IR spectra of 5 and 6 exhibit v(NH) bands comparable with those of 2 and 3, respectively, while in the spectra of 4 and 7 these bands are not present, in agreement with the doubly deprotonated nature of the hydrazone ligands. The complexes, dissolved in DMSO (lo+ mol l-'), show UV-VIS peaks at 315 <A/nm <330 with shoulders in the range 350<A/nm<460. It is difficult to assign these bands unequivocally because transitions of different origin, such as iron d-d transitions, charge transfer (ct) from the ligand (CN-, H,daps, H'dappc or H,dapt) to the metal, and ct between the metals, are expected to occur in this range.Free K3[Fe(CN)J in DMSO has bands at 424 and 320 nm due to M+L(CN-) ct and d-d transitions of the iron(m) crystal- field, respectively. By analogy with these findings, the peak and the first shoulder present in the spectra of the cyano- complexes are tentatively assigned to cyanoferrate absorption. The thermogravimetric analysis (TG) of the cyano-complexes 4-7 shows a first weight loss in the range 50-170 "C which corresponds to two MeOH molecules for 4, four water and one HC1 molecule for 5 (see Fig. 1) one MeOH molecule for 6, and two water molecules for 7; a second weight loss, in the range 180-330 "C, corresponds to three, four and three HCN molecules for 4, 7 and 5, respectively, three MeOH and one HCN molecule for 6.Typical TSDC spectra of the compounds investigated in the present paper are shown in Fig. 2. They are related to pellets (A-type samples) polarized in the range Tp=290 K and T,= 110K (T, =polarization temperature, at which dipoles can be easily oriented by the field; T,=temperature at which the field is switched off, since the polarization is frozen in). Fig. 2(a) concerns 5, while Fig. 2(b) concerns 6:12 the former shows a broad band peaking at ca. 192 K and a rise towards the high-temperature side (T>250 K), while the latter shows a huge band at 210 K and a shoulder at ca. 150 K. Note that the Fig. 2(b) does not exhibit any rise on the high-temperature side. These results support those reported in ref.12 for a different sample of the same compound [described by Fig. 2(a) of the present work] and for 4. The TSDC plot of the hydrated compound 5 shows a peak at ca. 192 K and a current rise in 'O0I90 8ol 111, , , , , , , , , , Illlj-//lllltt50 0 100 200 300 400 T/"C Fig. 1 TG of 5: (a) weight loss 0s. temperature; (b)first derivative of weight loss us. temperature NI5 0.15 a I 0.10 : 125 2250.00La-150 175 200 LL-LU-d 250 TIK Fig. 2 TSDC plot for a pellet of (a) 5 and (b) 6 with Tp=290 K, T,= 110 K and V,= 100V J. MATER. CHEM., 1994, VOL. 4 the high-temperature region (above 230 K); such a rise is the beginning of the band peaking at ca. 290 K, shown in ref. 3 and not shown in the present figure.The compound, including MeOH molecules, does not show such a rise, but only a broad band at 210K (peaking at ca.220K) in agreement with that of 4 reported in ref. 12. When blocking electrodes were used, the spectra were qualitatively the same, even if their amplitude is reduced. This result rules out charge-carrier injection being responsible for the observed spectra. Previously12the 290 K band was attributed to the reorien-tation of water molecules embedded in the crystal structure (see formula of compound 5): in Fig. 2 the presence of the displacement current rise on the high-temperature side of Fig. 2(a) (compound 5) and its absence in the spectrum in Fig. 2(b) (compound 6)support this interpretation. The peaks at lower temperatures exhibited by both curves are attributed to the reorientation of dipoles associated to the terminal aromatic radicals2*(see Fig.3): for the compounds described in Fig. 2 they are picolinic and salicylic groups. Such low-temperature relaxations are absent in compounds in which the terminal aromatic groups do not possess a dipolar moment.12 Moreover, the intensity of the low-temperature peak has a linear dependence on the strength of 043 OH H2hPPC H2daps H2dapt P = 2.22D 1.60 D 0.54D Fig. 3 Bis(acy1hydrazone)s of 2,6-diacetylpyridine ligands and the related dipole moments' (expressed in D; 1D x3.335 64 x lop3'C m) 715 1.5 cu5 1.0 a 70 :F 0.5 0.0 ' ' ' ' 1 " 1 ' I '1 ' 1 I LI 100 150 200 250 300 77K Fig.5 Effect of the dehydration process induced on the TSDC plots by the exposure of a pellet of 5 to a dynamic vacuum at room temperature for increasing times t,: (a)0 min, (b) 15 min, (c) 30 min, (d)60 min, (e) 120min, (f) 225 min, fg)1240min. For all curves T,, Tf, and V, are the same as in Fig. 1. the applied electric field, as expected if the process responsible for the TSDC peak is the reorientation of non-interacting dip01es.l~The results of this analysis are shown in Fig. 4 for 5: the 180 K peak? increases with increasing applied voltage and the peak amplitude scales linearly with the strength of the applied electric field, at least up to ca. 2 x lo4V cm-' (insert to Fig. 4). Note that the TSDC peaks in Fig. 4 and Fig. 2 (a)are broad (a shoulder is also present at cu.215 K). This means that the peak is caused by dipoles that have a distribution of relaxation times'* rather than a unique relaxation time z. To obtain further support for the hypothesis that reorien-tation of water molecules is responsible for the rise on the high-temperature side of the spectrum, dehydration and rehy-dration experiments were performed at room temperature and the effects on the TSDC plots were monitored. Fig. 5 shows the results of the dehydration process, obtained by exposing a pellet of 5 at room temperature to dynamic vacuum for increasing times. The rise on the high-temperature side of the plot is progressively reduced until it is completely suppressed for long times of sample exposure, supporting its attribution to the reorientation of water molecules.The release of water molecules is confirmed also by the simultaneous decrease of the typical IR absorption bands, associated with the stretching modes of the water moleculesi2and by TG results (see above).v).= 1 In the present case water molecule desorption takes place h l::y,,j C3 even at room temperature (ca. 300 K), i.e. at temperatures 0 lower than that at which the first minimum in the TG 0 50 100 150 200 500 c. c 0" 250 g I,,, I, 0 50 100 150 (a) 200 250 77K Fig. 4 Dependence of the low-temperature TSDC peak on the applied electric field for a pellet of 5 ca. 0.1 mm thick kept for ca. 2 h in a dynamic vacuum at room temperature.The TSDC plots were all obtained by polarizing in the range 290-110 K (as for Fig. 1) at different voltages V,: (a) 5 V, (b) 10 V, (c) 25 V, (d) 50 V, (e) 100 V, 200V. The insert shows the amplitude of the peak 11s. the applied voltage. derivative plot occurs [Fig. l(b)], and the desorption is induced by the dynamic pumping on the sample. Note, moreover, that this release of water molecules occurs in a time that is much longer than that required for the TG scan. By increasing the time of exposure of the sample to dynamic vacuum, the low-temperature band, peaking at 192 K in the freshly prepared sample [Fig. 2(u)], shifts gradually towards low temperatures down to a saturation value [Fig. 6(u)J. The changes induced by the dehydration process at room tempera-ture on the TSDC spectra displayed in Fig. 5, are summarized in Fig.6(b)and (c), where the current density, taken at three characteristic temperatures Ti, T2 and & in the TSDC plot of Fig. 5, is plotted us. the time of exposure of the sample to the t The shift in the peak from 192 K (for the freshly prepared sample, see curve 1 of Fig. 2) to 180 K (see Fig. 4) is accounted for by having stored the sample for ca. 2 h in a dynamic vacuum (see below). J. MATER. CHEM., 1994, VOL. 4 o-201saturation value 0.15 160 0 50 100 150 200 250 0 50 100 150 200 250 annealing time in vacuum/min Fig.6 Time evolution of the TSDC plots along the dehydration process displayed by Fig. 4 for a pellet of 5 exposed to a dynamic vacuum at room temperature.(a) Peak position TM0s. t,; (b)current density j(&) measured at & (indicated by an arrow in Fig.4); (c) current densities j(T,) and j(T,) measured at TIand & (indicated by arrows in Fig. 4). dynamic vacuum. Fig. 6(b) shows the fast decay of j(q), related to the high-temperature side of the TSDC plot; note that the ordinate at the origin is as high as ca. 5000 and is not displayed in Fig. 6(b)to avoid compression of the curve. Fig. 6(c) shows how the decrease in the current at is accompanied by an increase in the current at q.This can again be ascribed to the role played by the water molecules: their release during the sample exposure to dynamic vacuum causes a partial reorganization of the crystal structure around the dipoles associated with the terminal aromatic radicals (see Fig.3), the reorientation of which is responsible for the low- temperature peak in Fig. 1 and 3. This rearrangement might induce a change of the dipole reorientation parameters (acti- vation energy E, and pre-exponential factor zo) in the dipole relaxation time z, given by z=zo exp(EJJk,T) (1) and therefore a change of the temperature TM at which the peak occurs. For non-interacting dipoles TMis given by where /3 is the heating rate used during the TSDC recording process.15 In the present case the dipole moments related to the terminal aromatic radicals are reoriented more easily when water is removed, since the low-temperature TSDC peak shifts to still lower temperatures.Similar behaviour was observed in polymers of biological interest, such as rnelanin,l8 lysozyme," casein2' and homopeptides,21 and in dinuclear copper complexes.29 The shift of the 192 K peak observed during the dehydration process at room temperature is reversed if the sample is rehydrated at room temperature, i.e. it is exposed to the ambient moist atmosphere. Fig. 7 shows the changes induced in the low-temperature peak shape with increasing time of exposure of the sample to the atmosphere and Fig. 8 shows the time evolution of the peak at position TM.This process is accompanied again by the fast growth on the high-temperature 125 150 175 200 225 250 TIK Fig. 7 Effect of the rehydration process induced on the TSDC plots by the exposure to moist atmosphere at room temperature of a pellet of 5, previously dehydrated (see Fig.4) for different exposure times ta: (a) 22 h 50 min, (b) 45 h 50 min, (c)265 h 50 min, (d) 331 h 50 min, (e) 718 h 50 min, (f) 2275 h 50 min. For all curves Tp, Tf and V, are the same as in Fig. 1. t 0 2000 4000 6000 tlh Fig.8 Time evolution of the TSDC peak position TM along the rehydration process at room temperature displayed by Fig. 7 for a pellet of 5 exposed to a moist atmosphere side of the TSDC spectrum and the recovery of the IR absorption spectra related to the H20 stretching modes,12 which provides further support for the observed changes being attributed to the rehydration process. However, by comparing Fig.5(a)(for a freshly prepared sample) with Fig. 70 (for the same sample submitted to an initial dehydration process for a long time and then exposed to the moist atmosphere for a very long time), it turns out that the recovery is not complete. Moreover, Fig. 8 shows that the peak does not shift back to the original TM=192 K, notwithstanding that the sample has been exposed to the moist ambient atmosphere for an extremely long time (ca. 6000 h). This suggests that the removal of molecules induced by exposure of the sample to a dynamic vacuum has caused, even to a limited extent, some irreversible change in the crystal structure. Irreversible changes are indeed expected due to a partial release of HCl molecules (suggested by the TG results; see above and Fig.1)induced by pumping. This behaviour is similar to that reported for Ni( H,dapa)13C1.H20,12 where iodine is released as a conse- quence of pumping on the sample. However, when HC1 and iodine are removed from the crystalline structure they cannot J. MATER. CHEM., 1994, VOL. 4 be replaced by exposing the sample to the ambient moist atmosphere, as the water molecules can. Note that TSDC measurements performed on the second hydrated complex (7),investigated in the present work, give quite similar results, confirming the above interpretation on the role played by the water molecules. In this case, however, the removal of water molecules induced by dynamic pumping of the sample at room temperature causes a shift in the low- temperature TSDC peak towards higher temperature.This means that the reorientation of the dipoles associated with the terminal aromatic radical becomes less easy when reor- ganization of the crystalline structure around the dipole occurs as a consequence of the dehydration process. The exposure of the compounds containing methanol mol- ecules, such as 4 and 6 to dynamic vacuum does not suppress any of the TSDC peaks (at variance with the hydrated compounds), at least in the temperature range investigated in the present work, but causes again a shift in the low-temperature TSDC peak, attributed to the reorientation of the dipoles associated with the terminal aromatic radicals. In fact the dynamic pumping removes a fraction of methanol molecules, as supported by TG and by IR spectra, again causing a rearrangement of the environment around the reorienting dipole and, as a consequence, a change in the reorient at ion parameters.Generally the TSDC peaks of all the compounds investi- gated in the present work are broad and can be described by a distribution of relaxation times rather than by a unique z. This is also true of the TSDC peaks of the samples exposed to dynamic vacuum, but the relaxation time distribution is obviously different from that for the freshly prepared com-pound. The existence of a relaxation time distribution suggests that there are slightly different environments around the reorienting dipole which affect the reorientation parameters and, as a consequence, the relaxation time given by eqn. (1).Conclusions The di- and tri-nuclear iron hydrazone-hexacyanoferrate com-pounds behave as electrets, i.e. they are able to store electric polarization. The TSDC technique, used to investigate this feature, is able to confirm the reorientation of dipolar species, even in complex compounds, and the changes induced on their reorientation parameters by changing the hydration level. The TSDC technique is a sensitive tool for monitoring even weak loss of water molecules which escape detection by the more frequently used TG technique. The authors are indebted to Mr. Carlo Mora for technical help in performing the TSDC measurements. This work was supported by the Minister0 dell'Universit8 e della Ricerca Scientifica e Tecnologica (MURST, Roma) and by the Consorzio Interuniversitario di Struttura della Materia.References 1 J. C. Bailar Jr., Coord. Chem. Rev., 1980,31, 53. 2 F. L. Urbah, in Metal Ions in Biological Systems, ed. H Siegel, Marcel Dekker, New York, 1984, vol. 13, p. 73. 3 R. C. Long and D. N. Hendrickson, J. Am. Chem. Sot., 1983, 105,1513. 4 B. M. Hoffman and J. A. Ibers, Ace. Chem. Res., 1989,16, 15. 5 R. Gross-Lannert, W. Kaim and B. Olbrich-Deussner Inorg.Chem., 1990,19,5046. 6 0.Kahn, Angew. Chem., Int. Ed. Engl., 1985,24, 834. 7 U. Casellato, P. A. Vigato and M. Vidali, Coord. Chem. Ref?., 1977, 23, 31. 8 H. Adams, N. A. Bailey, W. Daniel Carlisle, D. E. Fenton and G. Rossi, J. Chem. SOC.,Dalton Trans., 1990, 1271.9 P. A. Vigato, S. Tamburini and D. E. Fenton, Coord. Chem. Rev., 1990, 106, 25. 10 S. Ianelli, G. Minardi, C. Pelizzi, G. Pelizzi, L. Reverberi, C. Solinas and P. Tarasconi, J. Chem. SOC., Dalton Trans., 1991, 2113. 11 M. Carcelli, C. Ferrari, C. Pelizzi, G. Pelizzi, G. Predieri and C. Solinas, J. Chem. Soc., Dalton Trans., 1992,2127. 12 A. Bonardi, R. Capelletti, C. Pelizzi and P. Tarasconi, Proc. Int. Symp. on Electrets ed. R. Gerhard-Multhaupt, W. Kunstler, L. Brehmer and R. Danz, IEEE Press, Piscataway, NJ 1991, pp. 123-128. 13 C. Bucci, R. Fieschi and G. Guidi, Phys. Rev., 1966,148,816. 14 J. van Turnhout, in Electrets, ed. G. M. Sessler, Springer-Verlag, Berlin, 1989, p. 81. 15 R. Capelletti, in Defects in Solids, ed.A. V. Chadwick and M. Terenzi, Plenum Press, New York, 1986, p. 407. 16 P. Dansas, S. Mounier and P. Sixou, C. R. Acad. Sci. Paris, Ser. B, 1968,267,1223. 17 F. Rull, L. F. Sanz and J. A. de Saja, J. Electrostatics, 1980,8,221. 18 M. Bridelli, R. Capelletti and P. R. Crippa, Bioelectrochem. Bioenerg., 1981,8, 555. 19 M. Bridelli, R. Capelletti, G. Ruani and A. Vecli, Proc. 5th Int. Symp. Electrets, IEEE Press, New York, 1985. 20 A. Anagnostopoulou-Konsta and P. Pissis, J. Phys. D, 19x7, 20, 1168. 21 M. G. Bridelli, R. Capelletti, A. Vecli and M. Zaniboni, Prclc. Int. Symp. on Electrets ed. R. Gerhard-Multhaupt, W. Kiinstler, L. Brehmer and R. Danz, IEEE Press, Piscataway, NJ, 1991, pp. 720-725. 22 J. L. Leveque, J. C. Garson, and G. Bouduris, Biopolymers. 1977, 16, 1725. 23 F. Ehrburger and J. B. Donnet, J. Appl. Phys., 1979,50,1478. 24 A. Bonardi, C. Carini, C. Merlo, C. Pelizzi, G. Pelizzi, P. Tarasconi, F. Vitali and F. Cavatorta, J. Chem. Soc., Dalton Trans., 1990,2771. 25 M. Wicholas and T. Wolford, Inorg. Chem., 1974, 13,316. 26 0.P. Anderson, Inorg. Chem., 1975,14,730. 27 J. Metz and M. Hanack, J. Am. Chem. Soc., 1983,105,828. 28 C. P. Smyth, Dielectric Behavior and Structure, McGraw -Hill, New York, 1955. 29 A. M. Manotti Lanfredi, F. Ugozzoli, A. M. Camus, N. Mmich and R. Capelletti, Inorg. Chim. Acta, 1993,206, 173. Paper 3/06403D; Received 26th October, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940400713

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994,4(5), 719-727 Liquid-crystalline Polyethers based on Conformational Isomerism Part 33.+-Therrnotropic Polyethers based on a Mesogenic Group containing Rigid and Flexible Units: 1-(4'-Hydroxybiphenyl=4~yl)-2~(4=hydroxyphenyl)propane Virgil Percect Peihwei Chu," Goran Ungar,b Stephen 2. D. Cheng" and Yeocheol Yoon" a Department of Macromolecular Science, Case Western Reserve University, Cleveland, OH 44 106, USA School of Materials and Centre for Molecular Materials, The University of Sheffield, Sheffield, UK SI 3DU " Institute of Polymer Science, The University of Akron, Akron, OH 44325, USA The synthesis of a mesogenic group containing rigid and flexible units: 1-(4'-hydroxybiphenyl-4-yl)-2-(4-hydroxy-pheny1)propane (TPP) is presented. TPP was polyetherified under phase-transfer catalysed conditions with a,o-dibromoalkanes containing 4-20 methylenic units.The resulting polyethers, TPP-x (where x =number of methylenic groups in the spacer), were characterized by a combination of techniques consisting of differential scanning calorimetry, thermal optical polarized microscopy and small- and wide-angle X-ray scattering experiments. All TPP-x polyethers displayed multiple crystalline phases whose nature was determined by the spacer length. TPP-x with x of less than 9 exhibited crystalline phases in which the mesogenic and spacer were intermeshed. Polymers with longer spacers displayed crystalline phases in which the mesogen and spacer were separated in separate layers of different electron densities.TPP-x with x =5, 7, 9 and 11 also exhibited a nematic mesophase. The traditional pathway used in the synthesis of molecular and macromolecular liquid crystals is based on the concept of a rigid-rod-like mesogenic unit. In 1988 we advanced the concept of a flexible rod-like mesogenic unit or rod-like mesogenic unit based on conformational isomerism. This concept was used to synthesize main-chain liquid-crystalline polyethers based on conformational isomerism without' and with2 flexible spacers. It has been demonstrated that this synthetic strategy can be employed to tailor-make linear polymers exhibiting ~ne'-~ or two4 uniaxial nematic, smectic5 and columnar hexagonal mesophases.6 Polyethers based on very flexible mesogenic groups such as 1,2-bis(4-hydroxyphenyl)ethane (BPE)5,6 and 1-(4-hydroxy- pheny1)-2-(2-methyl-4-hydroxypheny1)ethane(MBPE)2.3 and qwdibromoalkanes display mostly virtual, but also mono- tropic and even enantiotropic mesophases.2 Transformation of virtual or monotropic mesophases into enantiotropic mesophases can be accomplished by copolymerization.' Copolymerization experiments can also be used to determine thermal transitions and thermodynamic parameters of the virtual mesophases of the homopolymers.Thermodynamic schemes that explain the interconversion between virtual, monotropic and enantiotropic mesophases have been elabor- ated.7 These schemes suggest that the transformation of a virtual mesophase into an enantiotropic mesophase can also be accomplished by increasing the rigidity and decreasing the degree of order of the macromolecule or of its structural units or by a combination of both. Increased rigidity and decreased degree of order (i.e.increased conformational flexibility) have been used to design the mesogenic unit 1-( 4-hydroxybiphenyl- 4-y1)-2-( 4-hydroxyphenyl) butane (TPB), which, by polyetheri- fication with a,o-dibromoalkanes, leads to soluble polyethers displaying only nematic mesophases.' Polyethers based on TPB and flexible spacers are presently employed as models for main-chain liquid-crystalline polymers in a variety of physical investigations.' At the same time, TPB and its architectural variants are used in molecular engineering of molecular and macromolecular liquid crystals with complex architecture such as macrocyclics'(d~~'Oand dendrimers." Some of these developments have recently been reviewed.12 The polyethers based on TPB and flexible spacers are soluble t Part 32: ref 9(4.in conventional solvents and exhibit only nematic meso-phases.' We are also interested in the design of soluble, main- chain polyethers with more ordered mesophases than the nematic one. In principle, this can be accomplished by increas- ing the cylindrical shape of the TPB-like mesogen. The first aim of this paper is to report the synthesis of such a novel mesogenic unit, this is achieved by replacing the 1,2-substituted butane group of TPB by 1,2-substituteti pro- pane, i.e. l-(4-hydroxybipheny1-4-yl)-2-(4-hydroxyphenyl)-propane (TPP).The second aim of this paper is to describe the synthesis and characterization of polyethers based on TPP and a,w-dibromoalkanes containing 4-20 methylenic groups. Experimental Materials 1,4-Dibromobutane (99"/0), 1,5-dibromopentane (97%), 1,6-dibromohexane (97%), 1,7-dibromoheptane (97%), 1,8-dibromooctane (98YO), 1,9-dibromononane (W%), 1,lO-dibromodecane (97%) and 1,ll-dibromoundecane ( 98%) (all from Aldrich) were used after vacuum distillation. 1,12-Dibromododecane (technical, Aldrich) was purified by recrystallization from methanol. 1,16-Dibromohexadecane (mp 56-57 "C; Pfaltz and Bauer) and 1,18-dibromooctadecane (K and K Laboratories) were used as received. 1,13-Dibromotride~ane'~(bp 106-162 "C under 0.5 mmHg), 1,14-dibrornotetrade~ane'~(mp 50 "C),1,15-dibromopenta-decane15 (mp 28 "C), 1,17-dibrom~heptadecane~(mp 37-38 "C), 1,19-dibromononade~ane~(~)(mp 49.5 "C) and 1,20-dibromoeicosane'~d~(mp 69 "C) were synthesized as described previously. All other chemicals were commercially available and were used as received.Techniques Proton nuclear magnetic resonance ('H NMR) spectroscopy (200MHz) was carried out by use of a Varian XL-200 spectrometer with TMS as internal standard and CDCI, or [2H,]acetone as solvent. The purity of the products was determined by a combination of thin-layer chromatography (TLC) on silica gel plates (Kodak) with fluorescent indicator and high-performance liquid chromatography (HPLC).Relative molecular weights of the polyethers were determined 720 by gel permeation chromatography (GPC). HPLC and GPC analyses were carried out with a Perkin-Elmer series 10 LC equipped with an LC-100 column oven and a Nelson Analytical 900 series data station. The measurements were made by using a UV detector, CHC1, or THF as solvent (1ml min-', 40 "C), two PL gel columns of 5 x lo2 and 1 x lo4 A and a calibration plot constructed with polystyrene standards. A Perkin-Elmer DSC-7 differential scanning calorimeter was used to measure the thermal transitions. Heating and cooling rates were always 20 "C min-'. Indium was used as a calibration standard. First-order transitions (crystal-crystal, crystal-liquid crystal, liquid crystal-isotropic, etc.) were read at the maxima or minima of the endothermic or exothermic peaks.Glass transition temperatures (T,) were read at the middle of the change in the heat capacity. All heating and cooling scans after the first heating scan were identical. A Carl Zeiss optical polarizing microscope (magnification 100 x) equipped with a Mettler FP 82 hot-stage and a Mettler FP 800 central processor was used to observe the thermal transitions and to characterize the anisotropic textures. X-Ray scattering patterns were recorded using a flat-plate wide-angle (WAXS) vacuum camera (at room temperature and elevated temperatures) or a helium-filled flat-film pinhole- collimated small-angle (SAXS) camera (at room temperature). Ni-filtered Cu-Ka radiation was used.The samples were either in the form of prepared polymers, free-standing powders, fibres or bulk samples in Lindemann thin-walled 1 mm capil- laries cooled from the melt. The temperature stability of the X-ray heating cell was kO.1 "C. WAXS experiments were also carried out with a Rigaku X-ray generator. The point-focused beam was monochromatized with a graphite crystal (Cu-Ka radiation). Diffractograms were recorded as a function of temperature (k0.5 "C cell stability) from polymer films obtained upon melting. Synthesis of 4-Acetoxybiphenyl,2 Compound 2 was prepared by the esterification of 4-phenyl phenol 1 (80 g, 0.47 mol) with acetic anhydride (67 ml, 0.70 mo1).8 The product was recrystallized from 95% ethanol to yield 90 g (89?40) of white crystals.Purity (HPLC) 99.5%; mp 86-88 "C (1it.l6, mp 86-87 "C); 'H NMR (CDCl,, TMS)6,: 2.32 (3 H, s, CH,-), 7.16 (2 H, d, ortho to acetoxy of the substituted phenyl ring, J 8.0 Hz), 7.44 (3 H, m, meta and para of the unsubstituted phenyl ring), 7.56 (2 H, d, meta to acetoxy of the substituted phenyl ring, J 8.0 Hz), 7.59 (2 H, d, ortho of the unsubstituted phenyl ring, J 8.0 Hz). Synthesis of 4-Hydroxyphenylacetic Acid, 4 Compound 4 was prepared by the demethylation of 4-methoxyphenylacetic acid (3) (80 g, 0.30 mol) with hydro- bromic acid (181 ml, 0.99 mol) in 400 ml acetic acid.8 The product was recrystallized from 100ml water to yield 68g (93.5%) of white, needle-like crystals. Mp 146-152 "C (liti7, mp 148 "C); 'H NMR (C2H,]acetone, TMS)GH: 3.50 (2 H, s, -CH,-), 6.79 (2 H, d, ortho to hydroxy of the phenyl ring, J 8.0 Hz), 7.13 (2 H, d, meta to hydroxy of the phenyl ring, J 8.0Hz).The 'H NMR spectrum showed that 4 is free of unreacted methoxy groups. Synthesis of 4Acetoxyphenylacetic Acid, 5 Compound 5 was prepared by the esterification of 4 (75 g, 0.49 mol) with acetic anhydride (93 ml, 0.74 mol) according to the procedure described for the synthesis of 2.8 After washing with water several times and filtration, 76 g (80%) of J. MATER. CHEM., 1994, VOL. 4 a fine white powder was obtained. This was used in the next reaction step without further purification. Mp 105-108 "C (lit.", mp 108-110 "C); 'H NMR (CDCl,, TMS): 6 2.29 (3 H, s, CH,-), 3.64 (2 H, s, -CH2-), 7.06 (2 H, d, ortho to acetoxy of the phenyl ring, J 10.0 Hz), 7.31 (2 H, d, meta to acetoxy of the phenyl ring, J 10.0 Hz).Synthesis of 1-(4-Acetoxybiphenyl-4-y1)-2-(4acetoxyphenyl) ethanone, 7 Compound 7 was prepared by the Friedel-Crafts acylation of 2 with 6. Compound 5 (45g, 0.23mol) and S0C12 (25m1, 0.35 mol) were placed in a 250 ml three-necked flask equipped with a nitrogen inlet-outlet. After adding a few drops of N,N-dimethylformamide (DMF), the reaction mixture was stirred at room temperature for 2 h, and excess SOCl, was removed under reduced pressure to produce a yellow solid, which was used directly in the acylation reaction. The conversion was checked by NMR (-CH2- shift from 3.59 to 4.10).Compound 2 (59 g, 0.28 mol) was dissolved in 250 ml CH,Cl, in a 1000 ml three-necked flask equipped with a nitrogen inlet-outlet, a thermometer and a dropping funnel. The solution was cooled to below 10 "C using an ice-water bath after which time anhydrous AlCl, (110 g, 0.83 mol) was added. 4-Acetoxyphenylacetyl chloride (6') dissolved in 200 ml anhy- drous CH,Cl, was added dropwise maintaining the reaction temperature below 10 "C. After completing the addition, the deep-red solution was stirred at the same temperature for 3 h. The reaction mixture was poured into a mixture containing 120ml concentrated HCl, 1OOOml iced water and 600ml CHCl,. The organic layer was separated and washed twice with 500 ml water, dried over MgSO,, filtered and the solvent was removed on a rotary evaporator.The product was recrystallized from 1000 ml toluene to yield 63 g (70%) white crystals. Purity (HPLC) 99.5%; mp 196-198 "C; 'H NMR (CDCl,, TMS)GH: 2.29 (3 H, S, CH3COO--Ph-CH,-), 2.33 (3 H, s, CH,C00-biphenyl), 4.30 (2 H, s, -CH,-), 7.07 (2 H, d, ortho to acetoxy of the monophenyl ring, J 8.0 Hz), 7.20 (2 H, d, ortho to acetoxy of the biphenyl ring, J 8.8 Hz), 7.31 (2 H, d, meta to acetoxy of the monophenyl ring, J 8.0 Hz), 7.65 (2 H, d, meta to acetoxy of the biphenyl ring, J 8.8 Hz), 7.65 (2 H, d, meta to carbonyl of the biphenyl ring, J 8.0 Hz), 8.08 (2 H, d, ortho to carbonyl of the biphenyl ring, J 8.0 Hz). Synthesis of 1-(4-Methoxybiphenyl-4yl)-2-( Cmethoxyphenyl) propanone, 8 Compound 7 (20 g, 0.052 mol), iodomethane (10 ml, 0.16 mol) and tetrabutylammonium hydrogen sulfate (TBAH; 5.4 g, 0.016 mol) were dissolved in 300 ml THF in a 1000 ml three- necked flask equipped with reflux condenser.NaOH (300 ml, 50%) was quickly added at 40 "C and the reaction was stirred for 50min. During the reaction the colour of the solution changed from orange to light yellow. The reaction mixture was poured into 800 ml water and 600 ml CHCl,, after which time the organic layer was separated, stirred for 30 min with 300 ml dilute HCl, washed with 300 ml water and dried over MgSO,. The solvent was removed on a rotary evaporator to give a yellow solid, which was recrystallized from 95% ethanol to yield 7.8 g (44%) light-yellow crystals.The dimethylated compound was eliminated by recrystallization. Purity (HPLC) 99.5%; mp 128-132 "C. 'H NMR (CDCl,, TMS)G,: 1.52 (3 H, d, CH,-CH-, J 7.4 Hz), 3.76 (3 H, S, CH3O-Ph), 3.85 (3 H, s, CH,O-biphenyl), 4.66 (1 H, q, CH,-CH-), 6.84 (2 H, d, ortho to methoxy of the monophenyl ring, J 8.4 Hz), 6.97 (2 H, d, ortho to methoxy of the biphenyl ring, J 8.8 Hz), 7.23 (2 H, d, meta to methoxy of the monophenyl ring, J 8.4 Hz), J. MATER. CHEM., 1994, VOL. 4 7.55 (2 H, d, meta to methoxy of the biphenyl ring, J 8.8 Hz), 7.55 (2 H, d, meta to carbonyl of the biphenyl ring, J 8.4 Hz), 8.00 (2 H, d, ortho to carbonyl of the biphenyl ring, J 8.4 Hz). Synthesis of 1-( 4-Methoxybiphenyl-4yl)-24Qmethoxyphenyl)propane, 9 Compound 9 was prepared by the reduction of 8 with LiAlH,-AlCl,.AlCl, (35 g, 0.26 mol) was placed in a 200 ml three-necked flask equipped with a dropping funnel and a nitrogen inlet-outlet, and cooled in an ice-water bath, after which time 95 ml anhydrous diethyl ether was added dropwise. LiAlH, (4.0 g, 0.12 mol) was placed in a 250 ml three-necked flask equipped with a dropping funnel and a nitrogen inlet-outlet, and cooled in an ice-water bath. To the flask containing LiAlH, were added successively 95 ml anhydrous diethyl ether, AlC1,-diethyl ether complex solution, 95 ml anhydrous CHCl, and then the solution of 8 in 95 ml anhy- drous CHCl, was added dropwise to the reducing agent solution at 0 "C. The reaction mixture was stirred at room temperature for 5 h, after which time 30% HC1 (290 ml) was added dropwise with stirring to decompose the LiAlH,-AlCl, complex. The product was extracted with 400ml CHCl,, washed twice with 100 ml water and dried over MgS0,.After filtration, the solvent was evaporated off to yield a light- yellow solid, which was recrystallized from 1100 ml ethanol to produce 13 g (88%) of white, needle-like crystals. Purity (HPLC) 99.5%; mp 128-129 "C;'H NMR (CDCl,, TMS)G,: 1.24 (3 H, d, CH,-CH-, J 7.2Hz), 2.67-2.94 (3 H, m, -CH,-CH-), 3.79 (3 H, S, CH3O-Ph), 3.85 (3 H, S, CH30-biphenyl), 6.83 (2 H, d, ortho to methoxy of the monophenyl ring, J 9.1 Hz), 6.96 (2 H, d, ortho to methoxy of the biphenyl ring, J 9.2 Hz), 7.12 (2 H, d, ortho to methylene of the biphenyl ring, J 8.7 Hz), 7.12 (2 H, d, meta to methoxy of the monophenyl ring, J 9.1 Hz), 7.42 (2 H, d, meta to methoxy of the biphenyl ring, J 9.2Hz), 7.51 (2 H, d, meta to methylene of the biphenyl ring, J 8.7 Hz).Synthesisof 1-(4-Hydroxybiphenyl-4yl)-2-( Qhydroxyphenyl) propane, 10 Compound 10 was prepared by the demethylation of 9. To a 500 ml three-necked flask equipped with a dropping funnel and a nitrogen inlet-outlet and placed in a dry ice-acetone bath, were added 100 ml anhydrous CH,Cl,, BBr, (1.0 mol 1-l in CH2C12, 97 ml, 0.096 mol), followed by 9 (13.34 g, 0.04 mol) dissolved in 177 ml anhydrous CH2C1, at -78 "C. After addition, the reaction mixture was stirred at room temperature for 16 h, after which time 100ml water and 400ml diethyl ether were added.The organic layer was separated, washed with 100 ml water twice, dried over MgSO,, filtered and the solvent evaporated off. The product was purified by column chromatography (silica gel, diethyl ether) and the white product recrystallized twice from toluene to yield 9.1 g (74.5%) of white, needle-like crystals. Purity (HPLC) ~99.5%; mp 189-189.5 "C; 'H NMR (CDCl,, TMS)S,: 1.24 (3 H, d, CH,-CH-, J 8.0 Hz), 2.71-3.04 (3 H, m, -CH,-CH-), 4.57 (1 H, s, hydroxy of the monophenyl ring), 4.73 (1 H, s, hydroxy of the biphenyl ring), 6.76 (2 H, d, ortho to hydroxy of the monophenyl ring, J 8.0Hz), 6.89 (2 H, d, ortho to hydroxy of the biphenyl ring, J 8.0 Hz), 7.09 (2 H, d, ortho to methylene of the biphenyl ring, J 8.0 Hz), 7.09 (2 H, d, meta to hydroxy of the monophenyl ring, 2d, J 208.0 Hz), 7.44 (2 H, d, meta to hydroxy of the biphenyl ring, J 8.0 Hz), 7.44 (2 H, d, meta to methylene of the biphenyl ring, J 8.0 Hz). 721 Synthesis of Polyethers Conventional liquid-liquid two-phase (organic solvent-aqueous NaOH solution) phase-transfer-catalysed polyether-ification conditions were used for the preparation of polyethers.2(") The polyetherifications were accomplished under a nitrogen atmosphere at 80 "C in an o-dichloro-benzene-10 mol 1-' NaOH two-phase system (10-fold molar excess of NaOH versus phenol groups) in the presence of TBAH as phase-transfer catalyst. The molar ratio of nucleophilic to electrophilic monomers was always 1.0 : 1.0.An example of the polyetherification is as follows.To a 25 ml single-necked flask equipped with a condenser and a nitrogen inlet-outlet were successively added 0.60 mmol (0.1826 g) of monomer TPP (lo),1.2 ml of o-dichlorobenzene, 0.60 mmol (0.1632 g) of 1,8-dibromooctane, 1.2 ml of 10 mol I-' NaOH and 1.24 mmol (0.0815 g) of TBAH. The reaction mixture was stirred at 1100 rpm with a magnetic stirrer at 80 "C. After 3 h of reaction the organic and aqueous layers were diluted with CHC1, and water and the aqueous layer separated off. The organic layer was washed once with water, once with dilute HCl and three times with water. The polymer solution was precipitated into methanol to yield 0.2412 g (97%) of white fibrous material. The polymer was further purified by four successive precipitations from CHC1, solution into acetone, then once from CHCl, solution into methanol and finally twice from THF solution into water.Results and Discussion Synthesis and Thermal Characterization Scheme 1 presents the synthesis of racemic TPP (10). The synthesis of TPP was accomplished via a synthetic route similar to that employed in the preparation of TPB.' As illustrated in Scheme 2, TPP has a stereocentre. However, the sequence of reactions outlined in Scheme 1 leads to the racemic mixture of the two enantiomers of TPP. The individ- ual enantiomers of TPP can be prepared via the same procedure as that used in the synthesis of the two enantiomers of TPB.~o(~) There are a few details for the preparation of TPP that we should mention here.Although 4 is commercially available, we prefer to prepare it from 3 since this route is less expensive, The alkylation of 7with ethyl iodide leads to a mixture of C- and 0-alkylated products. Their separation was difficult. The 0-alkylated product was cleaved in situ and then was realkyl- ated with ethyl iodide to increase the overall yield of the C-alkylated product.' In the alkylation of 7 with ethyl iodide only low amounts of 0-alkylated and C-dialkylated products are obtained. The pure compound 8 can be separated from the C-dimethylated and 0-methylated products by recrys- tallization from 95% ethanol. The reduction of 8 to 9 was performed with LiAlH,-AlCl,~Et,O in CHCl,." For reasons discussed previously,' it is essential that complete reduction is accomplished at this step.The demethylation of 9 to 10 was most conveniently performed with BBr, in CHC1, 2o The synthesis of the polyethers of TPP with a,u-dibromoalkanes (TPP-x) was performed under phase-transfer catalysed polyetherification conditions developed in our labor- atory' (Scheme 2). As illustrated in Scheme 2 the resulting TPP-x polyethers are in fact copolyethers derived from the four constitutional and stereoisomers of TPP. TPP-x contain- ing even number x is soluble only at high temperature in chlorobenzene, THF or chloroform. TPP-x with odd number x is soluble at room temperature. TPP-x with x=4, 6 and 8 is so sparingly soluble that the molecular weight could not be determined by GPC.TPB-x with both odd and even x is very soluble even at room temperature.8 J. MATER. CHEM., 1994, VOL. 4 DSC traces of the first, second and subsequent heating scans are shown in Fig. l(u), (b) and (c).The phase-transition temperatures reported in Table 1were collected from the DSC traces of Fig. 1 and were assigned by a combination of thermal optical polarized microscopy and X-ray scattering experi- ments. Both these experiments will be discussed in a later section of this paper. The DSC traces from Fig. 1 demonstrate that TPP-x presents multiple phase transitions during both heating and cooling. All TPP-x are crystalline polymers with polymorphic crystalline phases. Only TPP-x with x=5, 7, 9 and 11 exhibit above their highest crystalline melting trans- ition also an enantiotropic nematic mesophase. The highest phase-transition temperatures of TPP-x present an odd-even dependence on spacer length regardless of whether this trans- ition refers to the melting of a crystal or of a liquid-crystalline phase into an isotropic liquid [Fig.2(b)]. With increasing 1 5 spacer length this dependence disappears. The second highest melting temperature is almost independent of spacer length [Fig. 2(u)]. Fig. 2(b) shows a plot of the highest transition PMF(i.e., isotropization) and the nematic-crystalline transition 2 6 7 CH31, TBAH, NaOH(aq)-THF, 40OC1 8 9 BBr&H2CI2 or HBr-CH,CO~BAH1 Scheme 1 Synthesis of TPP Table 1summarizes the yields, molecular weights and phase- transition temperatures together with the corresponding thermodynamic parameters of all TPP-x.Although the mol- ecular weights reported are only relative to polystyrene stan- dards, they are all larger than the values below which they become molecular-weight dependent.s temperatures for TPP-x with odd number x. There is a continuous increase of the nematic-crystalline and a continu- ous decrease of the isotropization transition temperatures. At x= 13 these two dependences intercept each other and there- fore, for x> 13 the nematic mesophase of TPP-x disappears. X-Ray Characterizationof TPP-XPolymers In agreement with the DSC data, the X-ray diffraction experi- ments showed a clear difference in the behaviour of TPP-x polymers with x=even and x=odd.As was expected, this difference was most pronounced in polymers with short spacers. With increasing spacer length the phase behaviours of even and odd polymers gradually converged. This trend was in agreement with that observed by using DSC [Fig. 2(u)]. Polymers With Odd-numbered Spacers x <9 X-ray scattering experiments confirmed that TPP-x (x=odd) polymers with short spacers, i.e. TPP-5, TPP-7 and TPP-9, are crystalline with a nematic phase at higher temperatures. The two main DSC peaks (Fig. 1) were thus identified as crystal-nematic(N) and nematic-isotropic(1) transitions, respectively. The powder pattern of the crystalline state is dominated by two intense, closely spaced reflections in the wide-angle range and no clear diffraction features at lower angles.For TPP-5 the Bragg spacings corresponded to the two wide-angle reflections 4.77 and 4.54 A, at room tempera- ture. On slow cooling from the nematic melt and after further annealing, additional weak diffraction features appeared. At 123 "C,i.e. between the temperatures of melting from the first DSC scan [i.e. 53 "C;see Table 1 and Fig. l(u)] and the small DSC endotherm at 148 "C, the pattern thus recorded had the same global appearance as that at room temperature prior to annealing, with th,e two strong reflections corresponding to 4.80 and 4.635 A. However, ic addition there were two weak reflections at 4.98 and 5.11 A. Upon cooling below the weak solid-solid transition the distribution of the satellite reflections on the low-angle side of the main wide-angle doublet changed into a short series of equidistant lines.In addition a very weak diffractjon feature developed at lower angles, corresponding to 20.5 A. The position of the dominant wide-angle doublet did not seem to change discontinuously at the solid-solid transition. The main features of the fibre pattern of TPP-5 are shown schematically in Fig. 3. The two dominant reflections are seen to be on the same row line, one of the reflections being equatorial. The maximum annealing temperature was only 80 "C, which did not produce resolvable satellite reflections J. MATER. CHEM., 1994, VOL. 4 Br -(CH&-Br C d Scheme 2 Synthesis of polyethers based on 1-(4-hydroxybiphenyl-4-y1)-2-(4-hydroxyphenyl) propane and a,w-dibromoalkanes containing x methylenic units (TPP-x) Table 1 Characterization of polyethers based on TPP and a,w-dibromoalkanes (TPP-x) with different numbers of methylenic units Ix). Data collected from first heating, cooling (both on first line) and second heating DSC (on the second line) scans thermal transitions/"C and corresponding enthalpy changes/kJ mru- ' in parentheses" x yield(Yo) 04")GPC (wv/M")GPC heating coo1in g 4 5 6 97.7 94.8 93.3 -11,200 - -2.53 - K 65(0.5) K 144(2.68) K 280(21.77) I G 46 K 150(2.95) K 282(20.55) I K 53(0.35) K 148(3.57) N 183(7.52) I G 48 K 149( 3.08) N 183(4.56) I K 59(1.25) K 126(2.22) K 249(20.35) I I272(-18.7) K 134(-3.22) K 34 (; I 173(-4.76) N lox(-3.05) K 39 G I 234(- 19.36) K 121(-0.44) K 7 8 9 10 11 12 13 97.5 97.5 99.0 91.2 91.3 90.2 97.5 25,000 -29,500 19,700 3 1,800 18,300 29,200 2.60 -2.30 3.32 1.68 2.90 2.00 G 38 K 135(0.56) K 249( 19.97) I K 49(0.48) K 150(5.0) N 178(5.88) I G 37 K 150(5.11) N 178(5.31) I K 55(0.33) K 141(2.16) K 221(19.51) I K 146(2.52) K 213 K 221(19.15) I K 48(0.56) K 154(7.43) N 173(6.69) I G 37 K 154(7.35) N 173(6.57) I K 52(0.5) K 145(12.19) K 204(20.51) I K 144( 12.08) K204(20.73) I K 49(0.5) K lll(9.9) K 155(8.7) N 165(6.2) I K lll(9.75) K 155(8.6) N 165(6.24)I K 51(0.6) K 144( 12.78) K 187( 19.54) I G 31 K 144( 13.48) K 186( 19.8) I K 50( 1.1) K 108( 15.86) K 162( 18.84) I I 167(-5.81) N 126(-4.08) K 33 <i I 207(- 18.63) K 135(- 1.79 ) K I 161(-7.32) N 133(-5.36) K 27 <; I 186(-21.94) K 120(-12.67) K I 156(-8.0) N 136(-6.7) K 95(-9.27) K I 166(-19.48) K 131(-12.77) K 19 G I 147(-7.0) K 140(-10.3) K 97(-14.14) K 28 G 14 15 16 17 18 92.3 93.3 93.1 95.2 96.7 16,500 20,900 28,700 34,500 23,500 2.03 2.02 3.42 1.89 2.92 G 26 K 108( 15.48) K 162( 18.25) I K 50(0.68) K 143( 17.12) K 176(21.56) I G 30 K 143(22.2) K 176(22.24) I K 49(0.66) K 118( 19.93) K 158(20.41) I G 27 K llg(21.7) K 158(19.48) I K 49(0.82) K 140(19.96) K 170(21.8) I G 27 K 142(22.99) K 170(21.9) I K 47(0.65) K 121(22.81) K 154(23.84) I G 23 K 122(26.2) K 154(23.51) I K 50(0.94) K 137(23.47) K 159(19.65) I I 161(-21.92) K 132(-18.49) K 32 G I 137(- 18.77) K 105(-21.03) K 23 G I 151(-21.67) K 129(-22.41) K 32 G I 132(-23.31) K 108(-24.91) K 21 G I 135(- 19.57) K 124(-23.9) K 30 G 19 89.4 23,100 2.01 G 27 K 137(24.98) K 158( 19.54) I K 47(0.73) K 119(23.98) K 156(23.86) I I 137(-23.0) K 108(-25.05) K 23 G 20 92.0 37,300 2.28 G 26 K 120(26.54) K 157(23.35) I K 50( 1.14) K 135(23.54) K 156(23.76) I G 29 K 137(27.63) K 157(23.41) I I 139(-22.66) K 126(-28.06) K 31 G ~~ " mru = molecular repeat unit.J. MATER. CHEM., 1994, VOL. 4 I I I I I -20 30 80 130 180 230 280 3 3 -20 30 80 130 180 230 280 3 TIT 77°C I I I I I -20 30 80 130 180 230 280 3 0 T/"C Fig. 1 First heating (a), first cooling (b), and second heating (c) DSC thermograms (20 "C min-l) of polyethers based on TPP and a,w-dibromoalkanes containing x methylenic units (TPP-x) or the 20.5 A feature.A sharp laxer-line streak was present sitional correlation. This correlatiyn greatly improves on slow with the meridional spacing of 6.0 A (indicated in Fig. 3). This melt crystallization. The 20.5 A periodicity which thus was attributed to a maximum in the molecular structure factor. develops corresponds reasonably well with the calculated A summary interpretation of the diffraction data on TPP-5 extended monomer length of 23.5 A. The solid-solid transition is that the crystal structure is based on a distorted hexagonal at 143 "C appears to be associated with axial shift between packing of tilted chains, with initially poor interchain tran- neighbouring chains. J. MATER. CHEM., 1994, VOL. 4 P 0 4 8 12 16 20 24 it-200 100 K K G 0 8,111 temperatures-of TPP-x as a function of x.Data collected from second heating scans. U, T,; 0,T,; 0,TK-N; A, TI;and (b)dependence of the transition temperatures of TPP-x with x containing an odd number of methylenic groups as a function of x. Data collected from cooling scans. A, TPNor TbK; 0, TN-K; .,TG;and a, TK-K I I I I I I r Fig.3 Main features of the X-ray diffractogram pattern of TPP-5 GI..., fGh,-qv;c T,a,-t;p,,ll In TPP-5 the departure from the hexagonal chain cross- section remained approximately constant with temperature, as judged by the constant splitting of the dominant, wide- angle reflection. In some TPP-x (x =odd) homopolymers with larger x, as well as in some copolymers of TPP with two different spacers (which will be described in a subsequent paper), the splitting decreased with temperature.This is illustrated in Fig. 4for TPP-9. Here, with increasing tempera- ture the cross-section of the unit cell perpendicular to the chain direction became progressively closer to the hexagonal. This may be associated with a reduction in the chain tilt. Polymers With Odd-numbered Spacers x 3 11 TPP-x homopolymers with x 3 13 behaved quite differently from those described above, with TPP-11 being an intermedi- ,,I ,,,,l,llll,,*ll,,,,l1,,,l1111 i,,, 5 10 15 20 25 30 35 2B/degrees Fig. 4 X-Ray diffractograms of TPP-9 (powder) as a funcr.ion of temperature. Temperature of each thermogram/"C: (a)rt; (b)40.(c) 60 (d) 80; (e) 100; cf) 110 6)120; (h) 130; (i) 140; (j)150; (k) 160; r,l) 170 (m) 180; (n) 190; (0)200; (p) 210; (4)220. ate case. The x3 13 polymers did not exhibit the nematic phase although, as the thermograms showed, they underwent a strong transition below the final isotropization temperature. This, however, was a crystal-crystal transition and the poly- mers did not show a liquid-crystal phase at all (Figs. 1 and 2). Neither of the two crystal forms was isomorphous with those described above. For illustration, Fig. 5 shows the temperature evolution of the diffractogram of TPP- 13. The low-temperature form was characterized by a number of intense Bragg peaks in the wide-angle region and by three orders of layer reflections at lower angles.The structure was clearly of lower symmetry than those described so far. On the other hand, the scattering pattern of the high- temperature form was rather simple and was dominated at wide angles by a strong maximum with a medium-intensity reflection on either side of it. The strong low-angle funda- mental was retained, but the higher orders were weakened, compared with the low-temperature form. The lowest angle reflection in both forms closely corresponded to the monomer spacings calculated for the extended monomer units. The considerable intensity was in contrast with the extremely weak or non-existent low-angle diffraction intensity of the TPP-x polymers with x=5, 7 and 9. A strong layer reflection in the long spacer polymers suggested that aromatic mesogens and aliphatic spacers were segregated into separate layers of different electron densities.On the other hand, the absence of strong reflections in the low-angle region, as in polymers with 5 <x <9, signified full or partial intermeshing.21 Polymers TPP-15, TPP-17, TPP-19 and TPP-20 displayed 5 10 15 20 25 30 35 2Wdegrees Fig.5 X-Ray diffractograms of TPP-13 (powder) as a function of temperature. Temperature of each thermogram/"C: (a) 30; (b) 40; (c) 60; (d) 80 (e) 100; cf) 110; (g) 120; (h) 130; (i) 140; (j)150; (k) 160; (I) 170; (m)180; (n)190; (0)200; (p) 210; (4) 220. the same two crystal forms as TPP-13, and melted without undergoing a liquid-crystal phase transformation.As will be shown further below, the present high-temperature crystal phase was also found in TPP polymers with long even spacers (x 10). TPP-11 showed three strong first-order transitions. X-Ray scattering from the highest temperature phase (156-166 "C) consisted only of diffuse scattering and confirmed the nematic assignment made by optical polarized microscopy. The lowest temperature phase was isomorphous in the low-temperature crystal form of higher odd-numbered polymers ( 13<x <19). The phase in the intermediate temperature region, between 113 and 156 "C, was a new high symmetry crystal form. The powder diffraction pattern ?as dominated by one intense wide-angle reflection at !.70 A and one medium-intensity low- angle reflection at 22.1 A.In addition, there were several weab bands on both the wide- and low-angle side of the 4.70A reflection. The appearance of these side bands, particularly the low-angle ones, suggested that the structure was a tilted hexagonal, i.e. either Crystal-G or Crystal-H.22 This intermediate phase could be rapidly quenched, partially or completely. Even at room temperature, the formation of the low-symmetry low-temperature phase was rather slow. Similarly, precipitation from solution resulted in a mixture of Crystal-G and Crystal-H and the poorly developed low- temperature crystal form, which had disappeared at 48 "C during the first DSC heating scan. Polymers with Even-numbered Spacers x d 8 The even-numbered spacer polymers also showed a break in their phase behaviour around the same spacer length as the odd-numbered spacer polymers.This was already suggested by the appearance of DSC thermograms (Fig. 1). For TPP-x J. MATER. CHEM., 1994, VOL. 4 polymers with x <8 the low-temperature transition endotherm was small whereas for polymers with x 2 10 it was considerably larger (Fig. 1, Table 1). X-Ray diffraction patterns of TPP-6 and TPP-8 were recorded as a function of temperature. Both the low- and the high-temperature phases ?ere characterized by a strong sharp diffraction peak at 4.6A. The low-temperature phase had additional weak reflections on the high-angle side and one reflection at lower angles, corrFsponding >o approximately half the monomer repeat: 12 A (=24/2 A) for TPP-8, as compared with the extended monomer length of 27A.The observed reflections for TKP-8 could b? indexed on a hexag- onal unit cell with a= 5.31 A and c =21 A, as shown in Table 2. The discrepancy between the measured (12.0) and calculated ( 10.5) dooz spacing is not understood. Attempts at producing well oriented fibres were unsuccessful. A significant feature of the low-temperature phase is the absence of the 001 reflection, indicating the existence of a glide plane parallel to the chain axis. Thus neighbouring chains are translated by half the monomer length with respect to each other. This seems to be a rather general feature of the main-chain polymers with shorter spacers and also parallels the behaviour of the TPP-x (x=odd) polymers.In the high-temperature phase only the strong sharp 100 hexagonal reflection remains. At lower angles two very weak diffractioq features were observed, which correspond to 11.5 and 14.8 A. As in the loy-temperature form, no reflection was observed in the 20-30A region, which would correspond to monomer periodicity. Had it not been for the two weak reflections, the high-temperature phase would have been described as hexagonal columnar. It may be that these were not true Bragg reflections but rather sharp molecular trans- formation maxima, in which case the description of the phase as columnar would have been valid. Further attempts to prepare well oriented fibres of TPP-8 and TPP-6 are planned, with a view to resolving this question.Polymers with Even-numbered Spacers x 3 10 The main feature of the powder diffractograms of the low- temperature phase of the polymers with longer even-numbered spacers was common to the whole series from TPP-10 to TPP-18. There were a number of strong reflections at wide angles, indicating low crystal symmetry (see Fig. 5). In addition, there was always a reasonably strong low-angle reflection corresponding closely to monomer periodicity, as well as its second- and sometimes third-order. For example, for TPY-12 the observed reflections corresponded to 26.3 and 26.5/2 A, respectively, compared with the calculated extended monomer length of 32.2A. The appearance of the strong reflection in the region of the full monomer repeat suggests that mesogens and spacers form separate layers, in contrast to the observation with shorter-length spacer TPP polymers.The overall situation was similar to the odd-numbered spacer TPP series, except that the actual crystal structure was different in the two series. With increasing temperature some diffraction peaks shifted Table 2 Observed reflections of TPP-8 spacinglii reflection measured calculated 100 4.60 4.60 101 4.48 4.49 103 3.86 3.84 104 3.43 3.46 002 12.0 10.5 J. MATER. CHEM., 1994, VOL. 4 considerably, leading to merging of certain reflections [Fig. 6(d)and (e)].This was associated with a heat capacity anomaly, which became apparent as a broad shoulder in the DSC traces of some of the polymers in the present group, e.g.in TPP-16 at ca. 100 “C. Above the main transition, associated by the lower of the two large DSC endotherms (Fig. l), the powder pattern was greatly $mplified, leaving one dominant wide-angle reflection at 4.70A and a weaker reqection on either side; these corre- sponded to 5.16 and 4.23 A (data for TPP-12). In addition, two orders of low-angle reflections remained, although they were shifted to somewhat smaller fundamFnta1 spacing. For TPP-12 the shift was from 26.4 to 24.2 A. Thus, the high- temperature phase in TPP-N polymers with long, even-numbered spacers x 210 was isomorphous with the high- temperature form of the polymers having long odd-numbered spacers with x 3 13.It was not surprising that the convergence of the structures of x =even- and x =odd-numbered polymers should occur for long spacers and at high temperatures. Increasing both spacer length and temperature reduced the probability of finding the spacer in its minimum-energy conformation. The fact that the spacers were conformationally disordered in the high-temperature phase was suggested by the abov5-mentioned average monomer shrinkage of more than 2A upon the transition from the low-temperature form to the high-temperature form. For a long disordered spacer, orientational correlation between its terminal bonds became too low for the even-odd variation to determine mutual orientation of successive mesogens and hence influence crystal structure.The nematic mesophase of TPP-x polymers with x =5, 7, 9 and 11 exhibited characteristic schlieren textures. I\ -5 10 15 20 25 30 35 2Bldegrees Fig.6 X-Ray diffractograms of TPP-18 (powder) as a function of temperature. Temperature of each thermogram/”C: (a) 30; (b) 40; (c) 60; (d) 80; (e) 100; (f) 110; (g) 120; (h) 130; (i)140; (j)150; (k) 160; (I) 170; (m) 180; (n) 190; (0)200; (p) 210; (4)220. Conclusions The TPP-x polymers described in this paper provided a very complex and comprehensive system in which the structure of the polymers was strongly influenced by their spacer length. This system contrasted with the TPB-x polymers, which showed only a nematic, and/or a nematic and a crystalline phase.Since some of the phases of TPP-x polymers exhibited various hexagonal or distorted hexagonal crystalline phases, they could be of great potential for the molecular design of polymers displaying a columnar hexagonal liquid-crystalline phase by suitable copolymerization experiments. Financial support by the National Science Foundation, Materials Research Group DMR-9122227 and NATO is gratefully acknowledged. References 1 V. Percec and R. Yourd, Macromolecules, 1988,21,3379. 2 (a) V. Percec and R. Yourd, Macromolecules, 1989, 22, 524; (b) V. Percec and Y. Tsuda, Macromolecules, 1990,23,5; (c)V Percec and Y. Tsuda, Macromolecules, 1990, 23, 3229; (d)V. Percec and Y. Tsuda, Macromolecules, 1990, 23, 3509; (e) V. Percec and Y. Tsuda, Polymer, 1991,32,661 and 673.3 G. Ungar, J-L. Feijoo, A. Keller, R. Yourd and V. Percec, Macromolecules, 1990,23, 341 1. 4 G. Ungar, V. Percec and M. Zuber, Macromolecules, 1992,25,75. 5 G. Ungar, J-L. Feijoo, V. Percec and R. Yourd, Macromolecules, 1991,24,1168. 6 G. Ungar, J-L. Feijoo, V. Percec and R. Yourd, Macromolecules, 1991, 24, 953; V. Percec, M. Zuber, S. Z. D. Cheng and A. Q. Zhang, J. Muter. Chem., 1992, 2,407; V. Percec, M. Zuber, G. Ungar and A. Alvarez-Castillo, Macromolecules, 1992,25,439. 7 V. Percec and A. Keller, Macromolecules, 1990,23,4347;A. Keller, G. Ungar and V. Percec, Advances in Liquid Crystalline Po/ymers, ed. R. A. Weiss and C. K. Ober, ACS Symposium Series 435, American Chemical Society, Washington, DC, 1990, p.308 8 V. Percec and M. Kawasumi, Macromolecules, 1991,24,6318. 9 (a) D. F. Gu, A. M. Jamieson, M. Kawasumi, M. Lee and V. Percec, Macromolecules, 1992, 25, 2151; (b) D-F. Gu, A. M. Jamison, M. Lee, M. Kawasumi and V. Percec, Liq. Cryst., 1992, 12, 961; (c)D. Y. Yoon, G. Sigaud, M. Sherwood, C. Wade and V. Percec, Bull. Am. Phys. SOC., 1992,37( l),369; (d)V. Percec and M. Kawasumi, Macromolecules, 1993,26, 3663. 10 (a) V. Percec, M. Kawasumi, P. L. Rinaldi and V. E. Litman, Macromolecules, 1992, 25, 3851; (b)V. Percec and M. Kawasumi, Adv. Muter., 1992, 4, 572; (c) V. Percec and M. Kawasumi, Liq. Cryst., 1993, 13, 83; (d) V. Percec and M. Kaw,.isumi, Macromolecules, 1993, 26, 3917; (e)V. Percec and M. Kaw.isumi, Chem. Muter., 1993, 5, 826; (f)V. Percec and M. Kaw,tsumi, J. Muter. Chem., 1993, 3, 725; (g)V. Percec and M. Kawasumi, J. Chem. SOC., Perkin Trans. 1, 1993, 1319; (h) J-F. Li, V. Percec and C. Rosenblatt, Phys. Reu. E, 1993, 48, R1; (i) J-F. Li, V. Percec, C. Rosenblatt and 0.D. Lavrentovich, Europhys Lett., in the press. 11 V. Percec and M. Kawasumi, Macromolecules, 1992,25,3843. 12 V. Percec and D. Tomazos, in Comprehensive Polymer Science First Suppl., ed. G. Allen, Pergamon Press, Oxford, 1992, p.299-383. V. Percec and D. Tomazos, Ind. J. Technol., 1993, 31, 339. 13 V. Percec and Y. Tsuda, Polym. Bull., 1989,22,489. 14 V. Percec and Y. Tsuda, Polym. Bull., 1989,22,497. 15 V. Percec and Y. Tsuda, Polym. Bull., 1990,23,225. 16 V. Percec, M. Lee and H. Jonsson, J. Polym. Sci. Part A, I’olym. Chem., 1991,29,327. 17 E. Salkowski and H. Salkowski, Chem. Ber., 1879,12,650. 18 V. Percec and D. Tomazos, J. Polym. Sci. Part A, Polym. Chem., 1989,21,999. 19 R. F. Nystron and C. R. Berger, J. Am. Chem. SOC.,1958,80,2896; W. L. Albrecht, D. H. Gustafson and S. W. Horgan, J. Org. rhem., 1992,31,3355. 20 J. F. W. McOmie, M. L. Watts and D. E. West, Tetrahedron, 1968, 24, 2289. 21 G. Ungar and A. Keller, Mol. Cryst. Liq. Cryst., 1988, 155, 3 13. 22 Smectic Liquid Crystals: Textures and Structures, ed., G. W. Gray and J. W. Goodby, Leonard Hill, Glasgow, 1984,p. 134. Paper 3/06318F; Received 20th October, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940400719

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 5), 729-734 Kinetic and Mechanistic Aspects of Iron(ii) Coordination to Bipyridyl- based Hydrogel Polymer Membranes Andrew L. Lewis* and J. David Miller The Speciality Materials Research Group, The Department of Chemical Engineering and Applied Chemistry, Aston University, Aston Triangle, Birmingham, UK B4 7ET A range of hydrophilic membranes composed of copolymers of 4-methyl-4’-vinyl-2,2’-bipyridyl with 2-hydroxyethyl methacrylate have been synthesized. These membranes readily coordinate iron(ii) from aqueous solution to form the tris(2,2’-bipyridyI)iron(ii)species, but at a rate very much slower than that of the free ligand in solution. Kinetic studies on the rate of development of these colour centres have shown the process to be anion-dominated and pseudo-first order for iron(ii) sulfate and second order for the chloride and perchlorate.The extent of coordination within the membrane is also dependent on the salt used, as the anion influences the concentration of Fe” partitioned within the gel matrix, and ultimately the position of the equilibrium established between the mono and tris complexes. Mechanisms are proposed incorporating the known water-structuring effects that anions impose on this type of hydrogel environment and accounting for the presence of ion pairs and their effect on the molecular reorganisations that are necessary in order for tris complexation to occur. 2,2’-Bipyridyl (bipy), its derivatives and structural relatives, are known to form complexes with a large number of metal ions.’-4 In 1888, Blau first observed and isolated the well known iron(1x) bipyridyl system.’ In aqueous solutions above pH 5, formation of the tris(2,2’-bipyridyl)iron(11)species is rapid, driven by a spin change accompanying the addition of the third ligand.This complex is intensely red in colour, due to a metal-to-ligand charge-transfer transition, which can easily be monitored spectrophotometrically to provide quanti- tative data. In solutions of pHG3, the rate is retarded by protonation of the bipyridyl nitrogens, and data for the rate of complex formation have been shown to fit fourth-order kinetics:2 This suggests a stepwise formation: Fe2++bipy K1 Fe( bipy)2 + (i) K2 Fe( bipy)2’ +bipy-Fe( bipy)?’ (ii) k3 Fe( bipy)? + +bipy CFe(bipy); + (iii) k-3 Steps (i) and (ii) are rapidly established equilibria and step (iii) is the rate-determining step in solutions of pH>,5.The values of the stability constants for the complex formation of the mono, bis and tris species [K1, K2 and K3 (k3)]are 104.3, 103.7and 109.5,re~pectively.~The consequence of these values is that, in solution, the concentration of Fe(bipy);’ is almost always very small compared to those of the mono and tris forms. The excellent ion-binding capability, the ability to stabilise unusual oxidation states and the potential to form complexes with catalytic activity or photoactivity make this ligand a desirable choice for use after its immobilisation into polymers.The preparation of polymers containing bipyridyl groups, and their application in metal-ion-specific absorption studies and as supports for catalytic complexes, are well documented in the literature.“12 However, the majority of hetero-and co-polymers described are hydrophobic and not ideally suited for use in aqueous solution. In earlier paper^'^'' we have described the preparation and some uses of hydrogel copolymer membranes composed of 2-hydroxyethyl methacrylate (HEMA) containing small percentages by weight of 4-methyl-4’-vinyl-2,2‘-bipyridyl (vbipy) and of a cross-linking agent. These materials swell when exposed to water, attain large equilibrium water contents and behave in ways comparable to those of aqueous solutions of the related monomeric organic molecules.We have already reported on studies of the complexes formed when these materials, and related membranes containing vinylpyridine, interact with solutions containing some transition-metal ~a1ts.I~The complexes are very similar to those found in true solution, but the rate at which equilibrium is attained is much slower. Here we report studies of the rate of formation of the [Fe( bi~y)~]” complex centre within membranes immersed in solutions of iron@) salts. The rates are unusually slow, but the main kinetic and mechanistic features of the process have been elucidated. These provide both an appreciation of how the polymer network influences the complexation process and of the nature of the interaction between the permeating ions and the hydrogel environment.Experimental Preparation of the Monomers, Complexes and Copolymers The 4-methyl-4’-vinyl-2,2’-bipyridyl(vbipy) used in this work was synthesized from 4,4’-dimethyl-2,2’-bipyridyl (Aldrich) uia the method of Abruna et and the monomeric Fe(vbipy)g+ complex was prepared by a method analogous to that reported for the tris(bipy) comp1ex.l’ The chelating membranes were fabricated using the glass plate membrane mould described previously.” The appropriate weight percent of vbipy was dissolved in the HEMA monomer, along with 0.5 wt.% azoisobutyronitrile (AIBN) initiator and an amount of ethylene glycol dimethacrylate (EGDM) cross-linker to ensure mechanical integrity. The solution was degassed with dry nitrogen for lOmin, after which it was injected into the membrane mould.The mould was placed horizontally in an oven at 60°C for 3 days, followed by 2 h at 90°C postcure. The resulting membrane was allowed to soak in distilled water for at least 7 days, with frequent changes of water. EGDM concentrations in the range 1-20 wt.% were investi- gated, the result being a reduction in the rate of complexation and the final absorbance value of the membrane as the cross- linked density was increased. We therefore standardised on 1 wt.% EGDM for all further studies. Coordination Studies using Visible Spectroscopy The preparation of copolymer samples as membranes is ideal for spectrophotometric investigation. While there are vari- ations in composition and thickness from one prepared batch to another, they are slight and data obtained from different specimens are reasonably reproducible.The random errors found in the measurement of rate constants are greater than those obtained for solution studies, as can be seen in the tabulated data, but their size is insufficient to mask the underlying trends that arise as experimental parameters are changed. In our estimation, rate data are accurate to f15%. The main experimental problems posed by the use of mem- branes reside in their lack of rigidity. We tried various ways of keeping a piece of membrane upright in a spectrophoto- meter lightpath, but none was completely satisfactory. For the present studies, in which observations need only be taken every few minutes, we found that the most convenient way of taking readings was to immerse the membrane sample in a capped thermostatted vessel containing reagent solution, quickly to remove the membrane when a reading was to be taken and to let it adhere by surface tension to the outer face of a clean spectrophotometric cuvette.Measurements were taken using an SP8-100Pye-Unicam spectrophotometer. This procedure has obvious drawbacks. Studies can only be carried out close to room temperature, with reagents that are not unduly air-sensitive. The advantages are that the technique is simple and that the reagent solution can permeate the mem- brane from both sides, thus reducing the period over which complex formation is controlled by the rate of diffusion of reactant solution into the membrane.In order to keep the visible absorbance within the spectro- photometric range it was necessary to restrict our work to thin membranes made up from a monomer mixture containing 0.5% by weight of the substituted bipyridyl monomer (see Fig. 1). In the swollen membrane this gives a concentration of 0.033 mol of 2,2’-bipyridyl per dm3 of membrane or 0.096 mol dm-3 in the imbibed water, if all the ligand groups are available in that phase. When a 4 cm2 piece of membrane was immersed in 20cm3 of an Fe2+ salt solution, even at Fig. 1 Structure of the poly( HEMA-vbipy) copolymer (m:n=300 :1 in 0.5 wt.% copolymer) J. MATER. CHEM., 1994, VOL. 4 0.01 mol dm-3 concentration there was a large excess of the salt. The total meta1:ligand ratio was ca.26: 1 in that case. Therefore, once the salt concentration in the imbibed water is at equilibrium with that in the external solution, the concentration of Fe2+(aq) in the membrane will be constant. Initially we decided to investigate the rate of development of the colour of the Fe(bipy)g+ centre using a concentration range of 0.01 d[Fe2+]/mol dm-3 <0.48. Since there is ample evidence that the anion is often dominant in determining the properties of the salts in hydrogels, e.g. their permeability coefficients18 we extended our study to three different iron@) salts: the chloride (BDH), the perchlorate (Johnson Matthey) and the sulfate (BDH).Results and Discussion Equilibrium Studies The first parameter we consider here is the ‘infinity reading’, the absorbance value measured when the reaction appears to be essentially complete. In these studies the absorbance, and therefore the fraction of the 2,2’-bipyridyl groups converted into Fe(bipy)g+ centres, was found to vary with the concen- tration of Fe2+ used. It also varies with the anion used. The data are collected in Table 1, together with calculated values derived after making the following assumptions: (a) that the fraction of the 2,2’-bipyridyl groups in the copolymer available for coordination is independent of the Fe2+ concentration, but may vary from one anion to another; (b)that the absorp- tion coefficient at 534 nm for the Fe(bipy)i+ centres will be the same as that found for the copolymer we produced separately from HEMA and [Fe(vbipy),] SO4 monomers1’ (see Table2) and (c) that the only equilibrium affecting the Table 1 Observed and calculated ‘infinity readings’ for the develop- ment of colour at 534nm (the parameters used in the calculations are given at the foot of the columns) ~~~ ~ ~ we2+I/rnol dm-’ FeC12 absorbance obs.calc. Fe(c104)2absorbance obs. calc. FeSO, absorbance obs. 0.01 1.63 2.27 - 1.50 1.76 0.02 - 2.18 1.38 1.40 - 0.04 1.75 2.04 1.23 1.25 2.00 0.08 1.91 1.83 1.06 1.04 1.87 0.12 1.67 1.66 0.95 0.88 1.86 0.16 1.37 1.52 0.68 0.76 2.10 0.20 1.39 1.41 0.63 0.66 1.72 0.24 1.17 1.30 0.53 0.58 2.15 0.32 1.09 1.12 0.53 0.45 2.05 0.40 0.98 0.98 0.3 0.36 1.89 0.48 0.92 0.86 0.33 0.29 1.90 fraction converted: 0.83 0.57 0.66 & 0.05 Kiv 16 8.2 2-200 Table 2 Visible absorption spectral details for vbipy complex formation with Fe” ~~~~ precomplexed vbipy(wt.%) ~ absorbance (at 534 nm) El dm3 mol-’ cm-’ 0.05 0.282 6503 0.1 0.565 6514 0.25 1.420 6548 0.375 2.110 6487 0.5 2.800 6457 Membranes soaked in 0.25 rnol dmP3 FeS04 solution.coordinated from solution absorbance (at 534 nm) 0.206 0.431 1.110 1.700 2.260 €1 %vbipy involved dm3 rno1-I cm-’ in tris formation 6507 73 6539 76 6563 78 6453 81 6434 81 J. MATER. CHEM., 1994, VOL.4 observed absorbance value is 3[Febipy12+F==[Fe( bip~)~]~+ +2Fe2+ (iv) The stability constants for the equilibria involving Fe2+ and free bipyridyl in water are well known.4 They can be used to show that, at concentrations similar to those prevailing within the imbibed water of our swollen hydrogels, the concentrations of the free ligand and of the bis complex are negligible compared to those of the mono and tris complexes, while for the free ligand the equilibrium constant for equilib- rium (iv) is 4 x lo4. In the mechanism proposed below this constant is designated Kiv. When the copolymer is allowed to equilibrate in iron@) sulfate solution, the final recorded absorbance readings show some scatter, but no significant variation with the salt concen- tration is apparent.Therefore, 0.66 & 0.05 of the 2,2'-bipyridyl groups appear to be available for tris complex formation and a lower limit of Kiv>200 can be deduced. When the counter- ion is either chloride or perchlorate a definite variation of absorbance with [Fe"] is seen, which means that Ki, must be considerably smaller in these cases and least-squares fitting gives values of 8.2 and 16 for the equilibrium constant and 0.83 and 0.57 for the available ligand fractions. Given the scatter of the experimental results, the agreement between observed and calculated absorbance values is very good (Fig. 2). The only doubt concerns the values at very low concentrations of FeCl,, where the fit is poor. We believe that the rate of diffusion of salts into the membranes becomes dominant at low concentrations, and unexpectedly low values are then observed because the final equilibrium would only be attained after a longer period has elapsed than we were able to allow in our studies.That is, the final absorbance values had not been reached when we eventually took our readings for experiments at low iron concentrations. This point is amplified below when the kinetic observations are discussed. The nature of the anions involved is known to have a marked impact on the properties of hydrogels exposed to salt solutions and this is attributed to their interaction with the imbibed water, i.e. their structure-making or structure-breaking properties." We note that iron@) salts of the two structure-breaking, singly charged anions exhibit much lower values of Kiv than does the structure-making sulfate.The 3.0 T 2.5 -Fig. 2 Observed [x, Fe(ClO&; 0, FeCl,] and calculated (-) variations of the infinity absorbance for membranes soaked in iron(n) salt solutions 731 variations in the fractions of ligand groups utilised by the salts may correlate with the positions of the anions in the Hofmeister series and may also vary inversely with the changes in the equilibrium water contents of poly-HEMA samples when immersed in salts of these anions." Howerfer, more than three points are needed to prove such relationships, and so the need for further studies with other anions is indicated. If the variation in the final absorbance readings can be attributed correctly to an [Fe2+]-dependent equilibri um, then the attribution can be verified in extra experiments.A change in the Fe2+ concentration made after a reaction system has been left to stand should cause a change in the system's visible absorbance. We carried out experiments in which such changes were used to demonstrate qualitatively the involvement of equilibrium (iv). We did not use them to obtain quantitative support, as the achievement of re-equilibration requires much longer elapsed times than we felt able to use (weeks rather than single days). However, we observed the changes summar- ised in Table 3, in which the absorbance readings obtained for experimental sequences in which membrane samples were immersed in 20cm3 aliquots of Fe(C104)2 solutions for 24 h before their visible absorbances were measured.The mem- branes were then washed and immersed in solutions of different concentrations until the following day. The whole operation was repeated five times. As the concentration of aquated Fe2+ ions is altered, so the concentration of tris- bipyridyl complexes changes in the sense that equilibrium (iv) requires. The absorbances of the specimens on day 5 were observed to change further when the systems were left unchanged in the same salt solution for a further period, showing that the move towards re-equlibrium had not been completed in the 1 day allowed per measurement. The time scales involved prevented us from making further attempts to continue this aspect of the investigations.Kinetic Studies The kinetic behaviour of our copolymer in its reactions with solutions of iron@) salts varies with the anion used, as also does its equilibrium behaviour. Therefore we discuss our results with different salts separately. First we consider the development of the red colour of the tris centre observed when membranes are immersed in solutions of iron@) sulfate. Kinetic observations were made by taking a membrane specimen, immersing it in a thermostatted solution of the chosen iron(1r) salt from which air was excluded to minimise unwanted oxidation. At known time intervals the membrane was removed from the solution and its visible absorbance at 534nm was quickly measured.The specimen wau then returned to the salt solution. In our earliest studies we began by using low-concentration iron(i1) solutions, but with vol- umes sufficient to ensure a large excess of Fe2+ over ligand, but we were unable to find a simple rate equation for the plot of absorbance against time. Up to concentrations of ca. Table 3 Changes in the absorbance measured at 534 nm of membranes after immersion in solutions of Fe( C10,)2 of various concentrations concentrations concentrations decreasing daily increasing dailj day CFe''l/mol dmP3 absorbance w+1/mol dmP3 absorbance ~~~ 0.40 0.31 0.01 1.59 0.20 0.38 0.05 1.50 0.10 0.45 0.10 1.37 0.05 0.54 0.20 1.26 0.01 0.61 0.40 1.( j7 0.01 mol dm-3, the profile indicated the occurrence of two distinct processes.Only when we used an excess of Fe2+ at a concentration 3 0.04 mol dm-3 could we successfully describe the profile from 5 min onwards by a single-termed rate equation. At these concentrations a first-order rate equation provides a good description to at least 80% achievement of the final absorbance, i.e. more than two half-lives. No other rate equation that we investigated offered a comparable standard of curve fitting; a point of note, see below. The results are typically reproducible to f 15% for membrane specimens coming from the same prepared sheet, but less so for specimens from different batches. This level of reproduc-ibility is less than that typifying kinetic studies in homo- geneous solutions, but is sufficient to warrant the deductions made below.In Table4 we quote the rate constants deter- mined for membrane samples immersed in FeSO, solutions over a 12-fold range of concentrations, from 0.48 down to 0.04 mol dm-3. We also quote the datum for 0.01 mol dmP3 Fez+, up to 50% completion. Even with the large potential errors imposed by the limitations of our technique, it is clear that a pseudo-first-order rate equation can be used to describe the data, and that the rate constant shows no clear dependence on the Fez+ concentration in the salt solution. This fact helps us to explain the change in the reaction profile at low concentrations. When a piece of an iron-free membrane is immersed in a solution of an iron@) salt, the salt must first diffuse into the imbibed water of the membrane before it can react with the ligand groups of the copolymer.The rate of the diffusion J. MATER. CHEM., 1994, VOL. 4 process is concentration dependent, unlike the dependence found for complex formation. Therefore at sufficiently low FeS04 concentrations the diffusion of ions into the membrane will become the only rate-determining process. At intermediate concentrations (0.01 mol dm-3) both processes will be import- ant, but at higher concentrations (20.04 mol dm-3) complex formation will be the slowest step. Elsewhere we have already reported detailed studies of the permeation of divalent trans- ition-metal salts through copolymer membranes which exhibit rates of permeation consistent with this explanation.” In Table 4 we also quote the rate constants determined for the formation of [Fe( bipy),12+ centres when iron-free copoly- mer membrane specimens are immersed in solutions of FeC1, or Fe(ClO,),.Most of the points made above still apply, with two notable exceptions. The value of the visible absorbance finally achieved by the membrane is [Fe2+]-dependent, see above, while the rate equation which describes the colour development up to at least 80% completion is now a second- order equation. A first-order equation cannot be used to describe these experimental data. Again at low concentrations diffusion becomes important, and again the computed rate constants appear to be independent of the concentrations of Fe2+ used.When once an iron@) salt, in the concentration range we have used, has diffused into one of our hydrogel membranes essentially all the 2,2’-bipyridyl groups will be present initially as mono complexes. Fig. 3 shows the visible absorption spectra of a membrane exposed to iron(I1) per- chlorate, taken at various intervals of elapsed time. Early on, when the absorbance due to the tris complex is not too Table 4 Rate constants at 25 “C for the formation of the [Fe( bi~y)~]’+ colour centre within the swollen copolymer membranes [Fe2+]/mol cm-3 FeSO,k/w4 min-l FeCl,k/dm3 mol-I min-l Fe(C10412k/dm3 mol -min- 0.01 63 - - 0.04 64 29.6 16.8 0.08 93 28.9 14.4 0.12 101 29.6 6.5 0.16 118 22.9 10.4 0.20 106 23.6 7.4 0.24 66 25.8 11.2 0.32 76 24.9 9.8 0.40 86 23.0 32.2 0.48 69 20.9 19.3 0.3 0.2 8 a % v)13 (d 3.1 500 400 300 600 500 460 wavelengthhm 3.0300 Fig.3 (a)Visible absorption spectra for mono- and tris-(2,2’-bipyridyl) iron@) (after Krumholz2’): (-) Fe(bipy),+; (---) Fe( bipy)<+ (value of E to be multiplied by 20). (b) Visible spectra of early kinetic stages for the complexation of 0.48 mol dmP3 Fe(C104), to 0.5 wt.% vbipy-HEMA copolymer membrane: (i) 10; (ii)15; (iii) 40; (iv) 120; and (v) 210 min. J. MATER. CHEM., 1994,VOL. 4 P-bipy +Fe2+-SO,'-+P-bipy .Fe2+-SO,'-P-bipy -Fez+ * SO,'-+( SO,'-)P-bipy -Fe2+ 2(SOi-)P-bipy Fe2+(S0,'-)( P- bipyj2*Fe2+ +Fez+.SO',-(SO:-)( P-bipy), -Fe2++P-bipy .Fez+* SO,'-+(SO:-)(P-bipyj, Fe2+ +Fez+* SO:-Scheme 1 P-bipy +Fe2+.(X 3, P-bipy Fe2+ (X-j, P- bipy * Fez+ * (X-),+( X -)P-bipy * Fe2+ X -+2(X -)P-bipy Fe2 X -+((X -)P -bipy} * Fez+ +Fe2+ -(X 3, {(X -)P- bipy) 2* Fe2++(X-)P- bipy -Fe2+.X2+ {(X-)P- bipy}, -Fe2++ Fe2+* (X 7, Sche'me2 733 fast (v) rate-determining step (vi) fast (vii) fast (viii) fast (v') fast (ix) rate-determing step (x) fast (xi) intense, that there is an additional absorbance at ca.400 nm which can be attributed to the combination of the spectra arising from the [Fe( bi~y)~]~' and [Fe( bipy)12+ species (literature2' La,z 410 nm, 1=320 dm3 mol-I cm- '). There-fore we have independent evidence supporting the argument for the predominant initial mono complex formation.The argument that the concentration of the bis complex will be negligible should still hold. Therefore the order of the rate constant for the formation of the [Fe( bipy),12+ centre will correspond to the number of moles of the mono complex taking part in the rate-determining step. The involvement of the bis complex in a rate equation would require an inverse dependence upon [Fe2+] to occur, which is not observed. Therefore we can write the rate expression for the sulfate as: and that for X=Cl or ClO, as dCFe(bipy)3X21=kx[Fe(bipy)2+]2 (3)dt The rate constants quoted in Table4 are many orders of magnitude smaller than those describing complex formation between Fe2+ and 2,2'-bipyridine in aqueous s~lution.~ We believe that difference to be an intrinsic property of ligand- containing copolymers.In order to form the tris complex it is necessary to bring different segments of polymer chain into close proximity, a process which will demand that some rearrangement occurs to present them in suitable relative configurations. If we are correct in attributing the very slow rates of reaction to the need for some polymer-chain reorganis- ation to occur, then this reorganisation need occur for only 1 mol of the mono complex when the anion is sulfate, but for 2 mol when the anion is chloride or perchlorate. We attribute this difference to the differing charges on the anions. When a salt diffuses into the swollen hydrogel it must do so as ion pairs for electrostatic reasons, and ion-pairing will continue to exist after complexation has occurred or a build-up of positive charge would occur. Thus the mono complex initially formed is probably best written as P-bipy.Fe2+.SOz-. We deliberately show the polymer chain and the anion at opposite sides of the cation to indicate that, in this configuration, the anion will block the access of other ligands to the cation.Before a bis complex can be formed the cation must be made more accessible by rearranging the structure. We choose to write such a rearranged structure as (SOi-)P-bipy.Fe2+. A plausible mechanism for the first-order formation of the sulfate product can now be written as in Scheme I.In this scheme the back reactions are assumed to be slow and unimportant, and are not shown.The formation of products with singly charged anions in a second-order process can be explained by the slightly modified Scheme 2. The occurrence of this mechanism, rather than a first-order alternative, requires an encounter between two (X-)P-bipy.Fe2'.X-species to be more probable than the formation and sub- sequent reaction of the ion-paired configuration (X-)2P-bipy.Fe2+, i.e.there must be a structural factor which militates against the latter alternative. This does not seem too unlikely. Conclusions We have demonstrated that hydrogel membranes made by the copolymerisation of 2-hydroxyethyl methacrylate with 4-methyl-4'-vinyl-2,2'-bipyridyl are capable of coordinating iron (11) in aqueous solution.When hydrated, typically 40% of the membrane's weight is due to imbibed water, which provides a medium through which metal salts can difl'use and coordinate with the appended 2,2'-bipyridyl ligands. Both the permeating anion type and the three-dimensional polymer network contribute to produce rates of complexation that are very much slower than the analogous reactions of the free species in solution. At low iron concentrations this can be attributed in part to a diffusion-controlled process, but at higher iron concentrations there is no doubt that the nature of the anion becomes the dominant factor. Within the polymer, in excess of 80% utilisation of irnmobi- lised ligands in tris complexes is possible, a phenomenon that reflects the high degree of polymer chain flexibility, despite the fact that every complex centre is essentially a three-centred cross-link.14 However, the extent of coordination is once again shown to be influenced by the type of salt used, which also determines the position of the equilibrium between the mono and tris complexes.We wish to thank SERC for their financial support of this work through the provision of a research studentship to A.L.L. References 1 W. W. Brandt, F. P. Dwyer and E. C. Gyarfas, Chem. Re?>., 1954, 54,959. 2 H.Irving and D. H. Mellor, J. Chem. Soc., 1962, 5222. 3 L. F. Lindoy and S. E. Livingstone, Coord. Chern. Rev., 1967, 2, 173. 4 J.D.Miller and W.R. McWhinnie, Adv. Inorg. Chem. Radiat. Chem., 1969,12,135. 734 J. MATER. CHEM., 1994, VOL. 4 F. Blau, Berichte., 1888,21, 1077. 13 A. L. Lewis and J. D. Miller, J. Chem. Soc., Chem. Commun., R. J. Card and D. C. Neckers, J. Am. Chem. Soc., 1977,99,7733. 1992,1029. R. J. Card and D. C. Neckers, Inorg. Chem., 1978,17,2345. 14 A. L. Lewis and J. D. Miller, Polymer, 1993,34,2453. C. E. Carraher, J. E. Sheats and C. U. Pittman Jr., Metal-15 A. L. Lewis and J. D. Miller, J. Mater. Chem., 1993,3, 897. containing Polymeric Systems, Plenum Press, New York, 1985, 16 H. D. Abruna, A. 1. Breikss and D. B. Collum, Inorg. Chem., 1985, p.385. 24, 987. 9 D. C. Neckers, J. Macromol. Sci., Polym. Chem. Ed., 1983,21,3115. 17 F. W. Cagle Jr. and G. F. Smith, Anal. Chem., 1947,19,384. 10 D. C. Neckers and K. Zhang, J. Polym. Sci., Polym. Chem. Ed., 18 C. J. Hamilton, S. M. Murphy, N. D. Atherton and B. J. Tighe, 1983,21,3115. Polymer, 1988,29, 1879. 11 J. D. Miller and D. S. Morton, J. Chem. Soc., Dalton Trans., 19 A. L. Lewis and J. D. Miller, Polymer, in the press. 1983,1511. 20 P. Krumholz, J. Am. Chem. Soc., 1949,71,3654. 12 C. M. EIIiott, C. J. Baldy, L. M. Nuwaysir and C. L. Wilkins, Inorg. Chem., 1990,29, 389. Paper 3/05994D; Received 7th October, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940400729

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 5), 735-739 New Route for Dispersion of Inorganic Salts onto the Channel Surfaces of Microporous Crystals: High Dispersion of CuCI, in Zeolites using a Microwave Technique Feng-Shou Xiao, Wenguo Xu, Shilun Qiu and Ruren Xu* Key Laboratory of Inorganic Hydrothermal Synthesis and Department of Chemistry, Jilin University, Changchun 730023, People's Republic of China A series of CuCI,-zeolites with a weight ratio of CuCI,.2H,O:zeolites of 0.1 0-0.80 have been prepared. After the reaction of CuCI,.2H20 with zeolite in a microwave oven for 10-20 min at ambient temperature, the CuCI,-zeolites (CuCI,.2H20 :NaZSM-5 =0.10-0.50; CuCI,.2H,O :NaY =0.1-0.60) showed only those X-ray peaks assigned to zeolites, the characteristic peaks of the CuCI,.2H20 having disappeared completely, which suggests that the CuCI, is highly dispersed in the channels of NaZSM-5 and NaY zeolites.Similar phenomena were observed for the samples of CuCI,-silicalite-I, AuCI,-Nay, AuCI,-NaZSM-5, NiCI,-NaZSM-5, RuCI,-Nay, Li2S04-AIP04-1 1. The n-hexane isotherms of CuC12-NaZSM-5 treated in a microwave oven exhibited a higher adsorption pressure to reach saturated adsorption of n-hexane, compared with those of NaZSM-5 and a mechanical mixture of NaZSM-5 and CuCI,.2H20 without treatment in a microwave oven. IR spectra of CO adsorbed on CuCI,-NaZSM-5 treated in a microwave oven showed much stronger adsorption bands than those of CuZSM-5 ion-exchanged by CuCI, solution, which indicates that the concen- tration of Cu2+ (CO) and Cuf (CO) on the CuCI,-NaZSM-5 is larger than those on the CuZSM-5.Furthermore, CuC12.2H20-NaZSM-5 (weight ratio 0-0.50) exhibited no peak at the melting point of CuCI, in DTA curves, because the CuCI, was highly dispersed in the channels of NaZSM-5. However, increasing the CuCl2.2H,O loading in NaZSM-5 up to 0.60, beyond the high-dispersion capacity, the DTA curve exhibited peaks at 770 K assigned to the melting point of CuCI, due to residual crystalline CuCI,. The forms of the active components present in heterogeneous In the present paper, we try to develop a new route for the catalysts are of importance to catalysis. A supported catalyst dispersion of some inorganic salts onto channel surfaces of usually consists of active components dispersed on a support microporous crystals.As an example, we study the dispersion with a highly specific surface. The degree of dispersion of the of CuCl, in NaZSM-5 and NaY zeolites using the microwave active component on the support is of economic consequence method, and it is found that the CuC1, highly disperses in the and influences the activity and selectivity of the catalysts.'-5 pores of NaZSM-5 and NaY zeolites. One method for obtaining high dispersity of the active compo- nent onto the support is the so-called 'monolayer disper- sion',&-'' which is thermodynamically stable. Recently, Iwamoto et ~l."-'~ reported that copper ion- Experimental exchanged zeolites, in particular copper ion-exchanged Preparation of SamplesNaZSM-5 zeolites, are very active for catalytic decomposition of nitrogen monoxide (NO).This and the stoichiometry of CuCl2-NaZSM-5 and CuC1,-NaY were prepared from the reaction were further confirmed by the other research- CuC1,.2H20 (purity >99.99%) with NaZSM-5 (Si :A1 =40, ers,'+-'' and increasing interest in this field has been fuelled surface area 500 m2 g-') and NaY (Si:Al=2.75, surface area by various environmental concerns.18-20 The activity and 750 m2 g-I), respectively. The NaZSM-5 (Nay) powder selectivity in catalytic decomposition of NO are strongly (2.00 g) was mixed mechanically with CuC1,.2H20 crystals influenced by the copper loading in zeolites and the properties (0-1.60 g), and the sample was placed in the microwave oven of zeolite^.'^^'^^'^ The preparation of Cu-zeolites is usually (Microwave Products, Shunde, China; model, E-100EA; fre- performed by the ion-exchange method.Owing to the limi- quency, 2450 MHz; power, 800 W). After reaction for tation of the ion-exchanged amount in zeolites,2'*22 it is 10-60 min, the sample was characterized by X-ray diffraction difficult to prepare Cu-zeolites with a high Cu content (e.g. (D/max-IIIA, Rigaku). The CuZSM-5 sample (Cu :Al =0.40)samples with Cu: Al> 1.0).Additionally, it is well known that was prepared from Cu2+-exchange in 0.1 mol 1-' CuC1,zeolites containing various cations exhibit high catalytic aqueous solution with NaZSM-5 at room temperature for activity and selectivity in many catalytic reaction^:^-^^ and several hours. the preparation of zeolites exchanged by cations with higher content such as ReY (Re =rare-earth cation) and NH4ZSM-5 is not simple.Generally, these samples were prepared by repeated cycles of cation-exchange of zeolites, drying and Hexane Isotherm Measurements calcination.26 In this decade commercially available microwave ovens The hexane isotherms on various samples were carried out have been used in many laboratory including with a Cahn-2000 electron recording balance, equipped with the preparation of short-lived radio- temperature control and weight monitor units, connected to organic synthe~is,~~,~' pharmaceutical^,^' the dissolution of geological samples in a vacuum system consisting of an oil diffusion pump backed mineral acid,33 the reaction of solids in the synthesis of up by a mechanical pump.The sample was first placed into inorganic and the crystallization of NaA zeo- a sample cell, and evacuated at 473 K for 3 h. After the sample lite.35 More recently, we have successfully used the microwave had been cooled to room temperature, it was exposed to technique to synthesize some molecular ~ieves~~?~~ hexane and its weight change was recorded. The sensitivity ofat ambient temperature with short reaction time (5-30 min). the electron recording balance was 0.1 pg. 736 Infrared Spectroscopy The sample was pressed into a self-supporting disc (10 mm diameter, 5-5.5 mgcm-, mass) and placed in an infrared quartz cell with CaF, windows. After evacuation from room temperature to 573 K for 5 h and at 573 K for 2 h to remove adsorbed water and other impurities, the sample disc was cooled to room temperature.Then CO 100Torr was intro- duced into the cell, and infrared spectra were measured using a Fourier transform spectrometer (Nicolet 5DX) with a reso- lution of 2 cm-' in the region 4000-1200 cm-'. Thermal Analysis Differential thermal analysis (DTA) was recorded in a flow of pure nitrogen on a PE-1700 by programmed heating at 10 K min-'. Results and Discussion X-Ray Diffraction Fig. 1 shows the XRD patterns of NaZSM-5 zeolite with CuC12.2H20 under microwave conditions. As observed in Fig. l(u) and l(b), the two XRD patterns give the same peaks at 7.9", 8.9" and 23.1", characteristic of NaZSM-5, which indicates that the framework structures of NaZSM-5 are stable under microwave conditions.The mechanical mixture u.-v)c u .-c J. MATER. CHEM., 1994, VOL. 4 of CuC12.2H,0 and NaZSM-5 (CuC12.2H,0 : NaZSM-5 = 0.10; Cu: Al= 1.5) shows peaks at 16.29", 21.94- and 33.96", assigned to crystalline CuC12.2H20, in addition to those of the NaZSM-5 zeolite [Fig. l(c)]. It is of interest to note that the characteristic peaks assigned to crystalline CuCI2.2H2O disappear completely when the sample is treated in a micro- wave oven for 10 min, as given in Fig. 1(d).The disappearance of the XRD peaks of crystalline CuClz.2H,0 in CuCl,. 2H20-NaZSM-5 can be explained by the high dispersion of CuC1,.2H20 in the NaZSM-5 zeolite channels, where the CuCl2.2H20 no longer exists in the crystalline ~tate.~,~',~~ Increasing the CuC12-2H,0 mass loading in NaZSM-5 zeolite to 0.50 (Cu:Al=7.6), we observed that the XRD peaks of NaZSM-5 maintained their positions, and we could not observe the XRD peaks of crystalline CuCl,.2H20, as shown in Fig.l(e)-(h). With a further increase in CuC12.2H20 mass loading in NaZSM-5 zeolite to 0.60-0.70 (Cu: A1 =9.1-10.6), the characteristic peaks assigned to crystalline CuC12-2H20 appear [Fig. l(i)-(l)]. Fig. 2 shows the XRD patterns of NaY zeolite mixed with CuC1,-2H20 under microwave conditions. The results demon- strate that CuC12.2H20 is highly dispersed in the pores of NaY zeolite with a CuCl2.2HZO mass loading of 0-0.60 (CU:A1=1.0). In contrast to those samples prepared from the copper ion-exchanged method, Cu/NaZSM-5 prepared from the microwave technique exhibits the following features: (i) the -10 20 30 10 20 30 10 20 30 20ldegrees Fig.1 XRD patterns of NaZSM-5 and CuC12.2H20 under various conditions: (a)NaZSM-5; (b) NaZSM-5 in microwave oven for 60 min; (c)mechanical mixture of CuC1,.2H20 and NaZSM-5 with CuC12.2H20 loading of 0.10 g g-' (Cu:Al= 1.5); (d)sample (c) in microwave oven for 10 min; (e)mechanical mixture of CuC12.2H20 and NaZSM-5 with CuC12.2H20 loading of 0.20 g g-' (Cu :A1 =3.0) in microwave oven for 10 min; (f)mechanical mixture of CuC12.2H20 and NaZSM-5 with CuCl,.2H20 loading of 0.30 g g-' (Cu: A1 =4.5) in microwave oven for 10 min;(g) mechanical mixture of CuC12.2H20 and NaZSM-5 with CuC1,*2H20 loading of 0.40 g g-' (Cu :A1=6.0) in microwave oven for 10 min; (h) mechanical mixture of CuC12-2H20 and NaZSM-5 with CuC12.2H20 loading of 0.50 g g-' (Cu :A1 =7.5) in microwave oven for 10 min; (i) mechanical mixture of CuCI2.2H2O and NaZSM-5 with CuCl,.2H20 loading of 0.60 g g-' (Cu: Al=9.0); (j)after i, the sample in microwave oven for 20 min; (k)mechanical mixture of CuCl,.2H20 and NaZSM-5 with CuC1,.2H20 loading of 0.70 g g -' (Cu :A1 = 10.5); (1) sample (k)in microwave oven for 20 min (V,characteristic peaks of CuC12-2H,0) J.MATER. CHEM., 1994, VOL. 4 -10 20 30 10 20 3 2eldegrees Fig.2 XRD patterns of NaY and CuC12.2H20 under various con- ditions: (a)Nay; (b)NaY in microwave oven for 60 min; (c) mechanical mixture of CuC12-2H20 and NaY with CuC12-2H20 loading of 0.20 g g-' (Cu: Al= 0.34); (d) sample (c) in microwave oven for 10min; (e) mechanical mixture of CuCl,.2H20 and NaY with CuC1,.2H20 loading of 0.40 g g-' (Cu: A1 =0.67) in microwave oven for 10min; (f)mechanical mixture of CuC12.2H20 and NaY with CuCl2.2H20 loading of 0.60 g g-' (Cu :Al= 1.0) in microwave oven for 10min; (g) mechanical mixture of CuC12.2H20 and NaY with CuC12.2H20 loading of 0.80 g g-' (Cu: Al= 1.34); (h) sample (9) in microwave oven for 20 min (V,characteristic peaks of CuC12*2H20) dispersion loading of CuC1, in NaZSM-5 is very large; we can prepare the CuC12.2H,0-NaZSM-5 with CuC1,-2H2O loading up to 0.50-0.6Og g-' (Cu:Al=7.6-9.1 g g-'), the higher Cu loading in zeolites may produce a better catalyst for NO, decomposition; (ii) it takes a very short time; generally, the dispersion of CuC12-2H20 in zeolites takes only 10 min; (iii) the preparation of samples is very simple without stirring in solution, drying and calcination steps.Using the microwave technique, we have recently prepared the series of samples: CuC1,-silicalite-I, AuC1,-NaZSM-5, AuC1,-Nay, RuC1,-Nay, NiC1,-NaZSM-5 and Li,S04-AlPO,-11, and it is found that metal salts such as CuCl,, AuCl,, NiCl,, RuC1, and Li,S04 can be highly dispersed in the channels of various zeolites such as silicate-I, NaZSM-5, Beta, and AlPO,-11, which are well characterized by X-ray diffraction.,O In some conditions, the above-mentioned mate- rials exhibit some specific chemical properties.For example, the sample of AuCl,-NaY prepared from highly dispersed AuC1, in NaY zeolite under microwave conditions is very active for the decomposition of NO,, and even at room temperature partial CO and NO can be converted into CO, and N2 over an AuC1,-NaY The Li2S04-AlP04- 1 1 sample prepared from highly dispersed Li2S0, in AlPO, zeolite under microwave conditions has a much higher electronic conductivity than does Li2S0,.42 More recently, we have studied the rare-earth ion (Ce3+, Eu3+ and Sm3+) exchanged zeolites (Beta, NaY and NaX) in aqueous solution under microwave condition^,^^ and it is found that the rate of ion-exchange of zeolites under micro- wave conditions is ca. 60 times that of the sample without using the microwave.Generally, a complete exchange of Ce3+ with Na-Beta zeolite takes only 8min under microwave conditions. Comparatively, the same Ce3 +-exchanged content in Na-Beta zeolite takes at least 8 h. Hexane Isotherms Fig. 3 shows the hexane isotherms on various samples. It is clear that all samples exhibit Langmuir-type isotherms.26 For NaZSM-5 and CuZSM-5, the hexane isotherms are the same, indicating that NaZSM-5 and CuZSM-5 have the same channels and surface areas. For a mechanical mixture of CuC1,.2H2O with NaZSM-5, the shape of the hexane iso- therm is the same as those of NaZSM-5 and CuZSM-5, but the adsorption amount of hexane is reduced, which is in good agreement with the theoretical value for the CuC12.2H20-NaZSM-5 sample estimated by the isotherm of NaZSM-5 [Fig.3(a)], where NaZSM-5:(CuC1,.2H20 + NaZSM-5)=0.91 and only NaZSM-5 can adsorb hexane. It is interesting to note that hexane adsorption on the sample (CuC1,.2H20 :NaZSM-5 =0.10 g g-I), treated in a microwave oven for lOmin, requires a higher hexane pressure to reach saturated adsorption [Fig. 3(c)], compared with those on NaZSM-5, CuZSM-5 and the mechanical mixture of CuC12.2H20 with NaZSM-5 without the treatment of micro-wave oven. This phenomenon may be interpreted by the different arrangement of channels in various samples. The microwave effect for the CuC12.2H,0-NaZSM-5 sample may result in the high dispersion of CuC1, into channels of NaZSM-5, leading to a change in channel shape in the zeolite, '1o! 0 0.1 0.2 0.3 0.4 0.5 PIP0 Fig.3 Hexane isotherms on various samples: (u)NaZSM-5 treated in microwave oven for 60min; (b) CuZSM-5 exchanged by CuC1, aqueous solution with NaZSM-5; (c) mechanical mixture of CuC12.2H20 with NaZSM-5 (weight ratio of 0.10 g g-', Cu :A1 = 1.5); (d) mechanical mixture of CuC1,-2H20 with NaZSM-5 (weight ratio of 0.10 g g-', Cu: Al= 1.5) in microwave oven for 10 min; (e)mcchan-ical mixture of CuC1,-2H20 with NaZSM-5 (weight ratio of 0.30 g g-',Cu :A1=4.5) in microwave oven for 10 min which strongly influences the adsorption of hexane on the samples. Accordingly, the higher the copper loading on NaZSM-5, the larger the change in hexane isotherm. In fact, the sample with mass ratio CuC12.2H,0 :NaZSM-5 =0.30 exhibits a lower adsorption amount and a higher adsorption pressure for saturated hexane adsorption, in contrast to those of the other samples, as shown in Fig.3(e). The CuCl,-ZSM-5 sample with CuC1,.2H20 :NaZSM-5 =0.70 g g- (Cu :A1 = 10.5) shows a small adsorption capacity, suggesting that at the higher Cu loading, the pores are either completely filled, or the pore windows are obstructed with residual material. IR Spectra of CO Adsorbed on Samples Fig.4 shows the IR spectra of CO adsorbed on CuZSM-5 and CuC1,-NaZSM-5 treated in the microwave oven. CuZSM-5 [Fig. 4(a)] exhibits bands at 2170 and 2156 cm-', which could possibly be assigned to Cu2+(CO) and Cu'(C0) species, respecti~ely.~~~ The CuC1,-NaZSM-5 sample also gives rise to bands at 2168 and 2156cm-', with intense CO adsorbed species, as shown in Fig. 4(b).This result indicates that the concentrations of Cu2+(CO) and Cu'(C0) complexes on CuC1,-NaZSM-5 is very high, compared with that on CuZSM-5. Comparatively, we can not observe the bands at 2180-2150 cm-' assigned to Cu"+(CO) (n= 1,2) for CO adsorption on the CuCl,.2H20-NaZSM-5 sample not treated in the microwave oven but pretreated by evacuation for 2 h at 473 K. The above results also indicate that CuCl, is highly dispersed in the channels of NaZSM-5 zeolite. Thermal Analysis Fig. 5 shows the curves of DTA for various samples. The sample of NaZSM-5 shows one peak at 361 K in the DTA curve, which is assigned to the desorption of water adsorbed on NaZSM-5.The DTA curve of CuC12.2H,0 shows two peaks, at 400 and 773 K [Fig. 5(b)],which are attributed to the dehydration of CuC1,-2H20 and the melting point of CuCI,, respectively. The mechanical mixture of CuCI2.2H,O with NaZSM-5 (weight ratio, 0.10 g g-', Cu: Al= 1.5) gives 0.25AI (b ) -2200 2025 1850 1E 5 wavenumbedcm-' Fig.4 Differential IR spectra between CO adsorbed on the sample and the background of the sample: (a)CuZSM-5 exchanged by CuCl, aqueous solution with NaZSM-5; (b) mechanical mixture of CuCI,-2H20 with NaZSM-5 (weight ratio of 0.30 g g-', Cu :A1 =4.5) in microwave oven for 10 min J. MATER. CHEM., 1994, VOL. 4 400 I I I I 383 523 663 803 TIK Fig.5 DTA curves of (a) NaZSM-5 in microwave oven for 10min, (b)CuC1,.2H20, (c)mechanical mixture of CuCl2.2Hz0 and NaZSM-5 with weight ratio of CuC1,.2H20 :NaZSM-5 of 0.10 g g-' (Cu :A1= lS), (d) mechanical mixture of CuC12.2H,0 and NaZSM-5 with weight ratio of CuC12.2H20:NaZSM-5 of 0.10 g g-' (Cu:Al= lS), followed by treatment in microwave oven for 10 min, (e) mechanical mixture of CuC12.2H20 and NaZSM-5 with weight ratio of CuC1,.2H20 :NaZSM-5 at 0.30 g g-' (Cu :A1 =4.5), followed by treatment in microwave oven for lOmin, (f)mechanical mixture of CuCl,*2H20 and NaZSM-5 with weight ratio of CuC1,.2H2O :NaZSM-5 at 0.60 g g-' (Cu :A1=9.0), followed by treatment in microwave oven for 20 min two strong peaks at 390 and 603 K, as shown in Fig.5(c). The peak at 390K is very similar to the peak at 400K assigned to the dehydration of CuC1,.2H20 in Fig. 5(b),and thus we assigned this peak to the dehydration of the CuC1,.2H2O-NaZSM-5. The peak at 603 K may be due to the 'monolayer' dispersion of CuC1, in the NaZSM-5, in the temperature range 573-723 K. Similar phenomena have been studied extensively by Xie et aL6 It is very interesting to find that, after the reaction of CuC1,.2H2O-NaZSM-5 in a micro- wave oven for 10 min, the sample profile exhibits only a peak at 380 K assigned to the dehydration of the sample, the peak at 603 K in Fig. 5(c) and the peak at 773 K in Fig. 5(b) completely disappeared. These results may be interpreted by the high dispersion of CuC1,-2H2O in NaZSM-5 zeolite under microwave conditions.6 When the copper loading in NaZSM-5 was increased to give a weight ratio of CuC1,.2H2O :NaZSM-5 =0.30 g g -'(Cu :A1 =4.5) treated in the microwave oven, the sample DTA curve still exhibited one main peak at 400K.With further increase in CuC12.2H,0 loading in NaZSM-5 up to 0.60 g g-' (Cu :A1 =9.0), followed by treatment in the microwave oven for 20min, the sample profile gave the peaks at 400 and 770 K. The peak at 400 K is assigned to the dehydration of the sample, and the other peak, at 770K, is assigned to the melting point of CuC1, because of the residual crystalline CuCl, in CuC1,.2H20-NaZSM-5. The above phenomena are the same as those for CuC1,-zeolites prepared via 'monolayer disper- J.MATER. CHEM., 1994, VOL. 4 739 sion' of CuCl,-NaZSM-5 by heating a mixture of CuC1, and NaZSM-5 for 12 h at 623 K and demonstrate that the CuCl, is highly dispersed in the channels of NaZSM-5, and that the saturated amount for CuC1,.2H20 dispersion in NaZSM-5 is a weight ratio of CuC1,.2H20-NaZSM-5 of ca. 0.50 g g-'. 2 3 4 5 G. Poncelet, P. Grange, and P. A. Jacobs, in Preparation of Catalysts 111, Elsevier, Amsterdam, 1983. H. C. Yao, Y. F. Yu Yao and K. Otto, J. Catal., 1979,%, 21. G. Y. Lee and J. C. Zhao, Petrochem. Technol. (China), 1987, 16, 266. T. Mikae, K. Sekizawa, T. Hironaka, M. Nakano, S. Fujii and Y. Tsutsumt, New Develop. Zeolite Sci. Technol., Proc. 7th Znt. Conclusions Zeolite Con$, Kodansha, Tokyo, 1986, p. 747. Y.-C. Xie and Y.-Q.Tang, Adv. Catal., 1990,37, 1. The important conclusions of this study may be summarized as follows. (a) After reaction of CuCI2.2H,O with zeolites in a microwave oven for 10-20 min at ambient temperature, the samples of CuCl,-zeolites (CuC12.2H,0 :NaZSM-5 = 0.1-0.50 g g-'; CuCl,-2H20:NaY =0.1-0.60 g g-') showed only those peaks assigned to the zeolite, and the characteristic peaks of CuCl2-2H,O disappeared completely, suggesting that the CuC1, is highly dispersed on the channel surfaces of the zeolites under microwave conditions. Using the microwave technique, some samples having catalytic activities could be prepared, such as CuC1,-silicalite-I, AuC1,-Nay, 10 11 12 13 14 15 J. M. J. G. Lipsch and G. C. A. Schuit, J. Catal., 1969,15174. F. E. Mossoth, J.Catal., 1973,30,204. N. Giordano, J. C. J. Bart, A. Vaghi, A. Castellan and G. Martinofti, J. Catal., 1975,36,81. J. Sonnemans and P. Mars, J. Catal., 1973,31,203. M. Iwamoto, S. Yokoo, K. Sakai and S. Kagawa, J. C hem. SOC., Faraday Trans. I, 1981,77,1629. M. Iwamoto, H. Yahiro, Y. Mine and S. Kagawa, Cirem. Lett., 1989,213. M. Iwamoto, H. Yahiro, K. Tanada, N. Mizuno, Y. Mine and S. Kagawa, J. Phys. Chem., 1991,95,3727. Y. Li and W. K. Hall, J. Phys. Chem., 1990,94,6415. Y. Li and W. K. Hall, J. Catal., 1991,129,202. AuC1,-ZSM-5, RuC1,-Nay, NiC12-ZSM-5 and Li2S04-AlP04-11. In these cases, CuCI,, AuCI,, NiCl,, RuCl, and Li,SO, could be highly dispersed in the pores/channels of the molecular sieves silicalite-I, ZSM-5, Beta and AIP04-11, respectively. Therefore, it is suggested that the reaction of some inorganic salts mixed with zeolites under microwave conditions may be a new route for the dispersion of some inorganic salts onto the channel surfaces of zeolite crystals.(b) Compared with the ion-exchanged methods, the reaction of CuCI2.2H,O mechanically mixed with zeolites such as NaZSM-5 under microwave conditions exhibits the following features: (i) the dispersion loading of CuCl, in NaZSM-5 is very large; highly dispersed CuC1,-2H20-NaZSM-5 can be prepared with Cu :A1=0-9.0; comparatively, the maximum Cu:A1 of CuZSM-5 prepared from copper ion-exchange is ca. 0.5-0.75; (ii) it takes a very short time; generally, the high dispersion of CuC12.2H20 in NaZSM-5 zeolite takes only 10 min; (iii) the preparation of samples is very simple, without stirring in solution, drying and calcination; (iv) in a microwave oven, CuC1,.2H20 can disperse easily in silicalite-I zeolite which does not have ion-exchange capability. The Cu-silicalite-I sample cannot be prepared by the ion-exchange method. (c) Hexane isotherms of CuCl,-NaZSM-5 treated in a microwave oven exhibit a higher adsorption pressure to reach saturated adsorption of hexane, compared with those of NaZSM-5 and a mechanical mixture of NaZSM-5 and CuC1,-2H2O without treatment in the microwave oven, which 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 Y.Li and J. N. Armor, Appl. Catal., 1991,76,21. M. Shelef, Catal. Lett., 1992, 15,305. W. K. Hall and J. Valyon, Catal. Lett., 1992,15, 311. Y. Li and J. N. Armor, US Pat., 1992,5149512.C. H. Bartholomew, R. Gopalakrishna, J. Davidson, P Stafford and W. C. Hechker, 13th North American Meeting of the Catalysis Society, PA-47, Pittsburgh, May 2-6, 1993. D. W. Breck, in Zeolite Molecular Sieves, Wiley, New York, 1974. G. D. Stuckey and F. G. Dwyer, in lntrazeolite Chemistry, ACS Symposium Series 218, American Chemical Society, Washington D.C., 1983. J. A. Rabo, in Zeolite Chemistry and Catalysis, ACS Monograph 171, American Chemical Society, Washington D.C., 1976 B. Imelik, C. Naccache, Y. Ben Taarit, J. C. Vedrine, G. Coudourier and H. Praliaud, in Catalysis by Zeolites, Elsevier, Amsterdam, 1980. Y. Murakami, A. Iijima and J. W. Ward, in New Developments in Zeolite Science and Technology, Elsevier, Amsterdam, 19x6. D. W. Breck, Zeolite Molecular Sieves, Wiley-Interscience, New York, 1974.D. M. P. Mingos and D. R. Baghurst, Chem. SOC. Rev., 1901,20,1. R. A. Abramovitch, Organic Preparation and Procedures bit., 1991, 23(6), 683. D. R. Baghurst and D. M. P. Mingos, J. Organomet. Chein., 1990, 384, C57. R. Gedye, F. Smith, K. Westaway, H. Ali, L. Baldisera and J. Rousell, Tetrahedron Lett., 1986,27,279. R. Gedye, F. Smith and K. Westaway, Can. J. Chem., 1986,66,17. D. R. Hwang, S. M. Moeriein, L. Long and M. J. Welsch, J. Chem. SOC., Chem. Commun., 1987,1799. L. B. Fischer, Anal. Chem., 1986,58,261. D. R. Gaghurst and D. M. P. Mingos, J. Chem. SOC., Chem. Commun., 1988,829. may be interpreted as the partial change in channel shape of 35 P. Chu, F. G. Dwyer and J. C. Vartull, Eur. Pat.App, 1990, NaZSM-5 zeolite due to the high dispersion of CuC1, on the channel surfaces. (d) In the DTA curves, the peaks assigned to the melting point of CuCl, in sample CuC1,.2H20-NaZSM-5 (CuC1,.2H20 :NaZSM-5 =0-0.50, Cu :A1=0-7.5) treated in a microwave oven were not observed, in good agreement with the fact that there is no crystalline CuCl, in CuC1,-NaZSM-5 because of the high dispersion of CuC1, on the channel surfaces. 36 37 38 39 358, 827. T. Y. Song, G. N. Xu, W. G. Xu, X. P. Meng, X. S. Liu, F. C).Zhou and R. R. Xu, Chem. J. Chin. Univ. (Chinese Ed.), 1992,13, 1209. X. P. Meng, W. G. Xu, S. Q. Tang and W. Q. Pang, Chin Chem. Lett., 1992, 69. Y. C. Xie, H. X. Zhang and R. H. Wang, Sci. Sin. (Chit). Ed.), 1980, 337; Sci. Sin. (Engl. Ed.), 1980,23, 980. Y. C. Xie, L. L. Gui, Y. J. Liu, B. Y. Zhao, N. F. Yang, Y. F. Zhang, Q. L. Guo, L. Y. Duan, H. Z. Huang, X. H. Cai and Y. Q Tang, Proc. 8th Znt. Congr. Catal., Dechema, Frankfurt-am-Main, 1984, vol. 5, p. 1470. This work has been supported by the National Natural 40 W-G. Xu, F-S. Xiao, S-H. Feng, S-L. Qiu and R-R. Xu, unpub-lished results. Foundation of China, the Foundation of Pang-Deng Plan in 41 S-L. Qiu, R. Ohnishi and M. Ichikawa, J. Chem. Soc., Chem. China, the Foundation of Key Laboratory for Inorganic Hydrothermal Chemistry in China, and the Foundation of the Committee of Science and Technology in Jilin Province of China. 42 43 44 Commun., 1992,1425. S-H. Feng, W-G. Xu and R-R. Xu, unpublished results. Y-G. Tian, PhD Thesis, Department of Chemistry, Jilin University, Changchun, China, 1992. G. L. Millar, C. H. Rochester and K. C. Waugh, J. Chem SOC., Faraday Trans., 1991,87,1467. References 45 46 Y-Y. Huang, J. Am. Chem. Soc., 1973,95,6636. C. L. Angel1 and P. C. SchaRer, J. Phys. Chem., 1966,70,14J 3. 1 B. Delmon, P. A. Jacobs and G. Poncelet, in Preparation of Catalysts, Elsevier, Amsterdam, 1976. Paper 3/05934K; Received 4th October, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940400735

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994,4(5), 741-745 741 Determination of Acid-Base Properties of Solid Materials by Inverse Gas Chromatography at Infinite Dilution A Novel Empirical Method based on the Dispersive Contribution to the Heat of Vaporization of Probes Mohamed M. Chehimi* and Emmanuelle Pigois-Landureau lnstitut de Topologie et de Dynamique des Systemes, Associe au CNRS (URA 34), Universite Paris 7 Denis Diderot, 7 rue Guy de la Brosse, F-75005 Paris, France We introduce a novel empirical method based on AH&,, the dispersive component of the heat of vaporization of probes, to assess the acid-base properties of solid surfaces quantitatively by inverse gas chromatography (IGC) at infinite dilution. In this method, AGa, the free energy of adsorption of probes, is related to AH&.As in the methods of Sawyer, Papirer, Schultz and Donnet we obtain a straight line for alkanes when AGa is plotted vs. For polar probes interacting by both dispersive and specific forces, the experimental data lie above the alkane linear plot. AGP, the specific acid-base contribution to AGa calculated by our new method, matches that determined by the four methods mentioned above. In particular, in the case of self-associated probes (e.g. tetrahydrofuran and ethyl acetate) it yields AG;' values similar to those determined by the method of Donnet et a/., whereas for non-self-associated probes (e.g, chloroform and diethyt ether) AG:' values match those obtained by the methods of Papirer and of Sawyer. A/-&p for polar probes (for alkanes AH&,=AHvap) can easily be determined from the heat of mixing at infinite dilution in apolar solvents, as recommended by Fowkes.The pioneering work of Fowke~l-~ on the role of acid-base interactions in the science and technology of adhesion, mixing and solubility of polymers and other materials have stimulated extensive research work on those topics by several labora- torie~.~As stressed by Fowkes,' the need for methods to assess acid-base properties of materials is urgent. Acid-base inter-actions of materials in the condensed phase are studied by several methods4 including inverse gas chromatography (IGC),6 where 'inverse' means that the stationary phase rather than the mobile phase is the main object of investigation.IGC is very suitable for the characterization of finely divided materials such as polymers, fibres and fillers. In the case of characterization of polymeric materials by means of IGC, the column can either be packed directly with the polymer of interest (powder, beads, fibres, et~.),~or be coated on the inside with the polymer to create a capillary column8 or be packed with small inert spheres coated with a thin polymeric film.g Volatile probe molecules are injected at infinite dilution (zero surface coverage) in order to allow stationary phase-probe molecule interactions only and rule out probe-probe interactions, When semi-crystalline or amorphous polymers are characterized at temperatures above the glass transition temperature, Tg,both surface adsorption and bulk sorption of the probes contribute to the retention times of the probes."*" On the contrary, if IGC experiments are performed at a temperature below Tg,adsorption of the probes on the polymer surface govern their retention times," and sorption by the bulk, which is progressive with time, may be slow enough to be neglected within the timescale of the chromatographic process.12 This constitutes one of the advan- tages of IGC over static methods to study adsorption by polymers.The fundamental retention parameter in GC is the net retention volume of the volatile probe, VN, which corresponds to the volume of the carrier gas required to elute a zone of solute vapour. Of greater importance is the specific retention volume, Vg, which can be defined as V, per unit mass of stationary phase.Depending on the physical state of the packing material at the working temperature, Vg can be related to AGmix or AGa, the free energy of mixing and the free energy of adsorption of the injected probe, respectively. In the case of amorphous or semi-crystalline polymers at a temperature beyond Tg: AG-=RT In [n, In$, +n2 1n$2+~,~n,$,] where subscripts 1 and 2 denote the probe and the polymer, respectively; n,and q5i are the number of moles and the volume fraction of component i, x12is the Flory-Huggins interaction parameter, R the gas constant and T the working temperature. In this expression of AG-, x12can be related to Vgby: x12=ln(273.15 Rv2/PyVgV,)-1+ V1/M2v2 +11-Vl)P?/RT where v2 is the specific volume of the liquid phase (the polymer) and M2 is its molecular weight; V,, Py and B1, are the molar volume, the vapour pressure and the second virial coefficient of the probe in the gaseous state, respectively. For high-molecular-weight polymers, 1/,/M2v2can be neglected in the above expression for x12.When one deals with crystalline solids (inorganic fillers, fibres) or amorphous polymers below their Tg, adsorption phenomena govern the retention of the probes on the material surfaces. Therefore, Vg is related to the surface partition coefficient,K,, defined as the ratio between the concentration of probe in the stationary phase and in the mobile phase, respectively, by: Vg = VN/m =KsAs where m and As are the weight and the specific surface area of the solid of interest, respectively.From Ks, AG, can be derived and expressed as a function of VN by: -AGa=R T In (VN) +C where R is the gas constant, T the working temperature and C a constant which takes into account the weight and the specific surface area of the material, and the standard states of the probes in the mobile and the adsorbed phases. Adsorption of apolar probes (e.g. alkanes) results from London dispersive interactions only, whereas for that of polar probes capable of specific acid-base interactions with the packing material, both London and acid-base interactions contribute to AGa:576 AG, =AG: +AG:~ where AG; and AGtB are the dispersive and acid-base contri- butions to AG,, respectively. However, since only one chroma- tographic signal is recorded for a given polar probe, one has to find a method to distinguish between both contributions to AG,.In the several approaches which were proposed in the literature13-16 to split AGa into its two contributions, AG, [or RTIn(V,)] is plotted uersus a given physicochemical property of the probes. With alkanes a straight line is obtained as shown schematically in Fig. 1. In the case of ‘polar’ probes such as the Lewis acid, chloroform (CHCl,) and the Lewis base tetrahydrofuran (THF), interacting specifically with the solid material, the values of RTln(VN) will lie above the straight line. In Fig. 1,the vertical distance between the alkane reference line and the molecular probe of interest is referred to as: -A G,AB = -(AGa -AG:) =RTIn(VN/VN,ref) where I/N,ref is the net retention volume of a hypothetical reference alkane having the same value of the physicochemical property at the working temperature.The abscissa coordinate on Fig. 1 can be: Tb,the boiling point;13 log(P,), the logarithm of the vapour pressure;14 a(yf)ll2, the product of the cross- sectional area (a)and the square-root of the dispersive contri- bution to the surface energy of the probes [(yt)”2];15 the deformation polarizability.16 In the last approach, Donnet et~2.’~related AG, to London’s equation by: J. MATER. CHEM., 1994, VOL. 4 calculated on the basis of van der Waals models) might not reflect the real situation, especially when specific interactions are concerned.Depending on the nature of the substrate, the probe molecule may have a different geometry” which yields a different contact surface area. In their IGC study of untreated and heat-treated natural Madagascar graphite (high-surface-energy materials), using the approaches of both Papirer14 and Schultz,15 Donnet et all6 found that RTln(VN) values for several probes lay below the dispersive interaction line defined by the alkanes, yielding positive values of ActB. They suggested another AG, data treatment, based on deformation polarizability, for the determination of AGtB. This approach led to negative AGtB values. Donnet et criticized the methods of Schultz” and Papirer14 because in these methods the interaction of an isolated probe with a surface is compared to the probe-probe interaction in the liquid state. However, Donnet et a1.I6 describe the molecular probe-solid interaction by a mol-ecule-molecule interaction.For this reason, their approach may probably not tell the whole story. In this paper we suggest another empirical method for assessing AGtB, where AGa [or RTln(V,)] values are related to AH:,,? the dispersive contribution to the heat of vaporiz- ation of the probes. This approach can be understood as follows. First, according to Trouton’s rule: where LVa?is the latent heat of vaporization and Tb the RTIn(V,)+C =K(h~~)~~~~l~,~(hv~)~~~~l~,~boiling point. where C is a constant, hvi the ionization potential of the ith interacting material, cto the deformation polarizability and K a constant which takes into account the permittivity in vacuum, the distance of adsorbate-adsorbent interaction and Avogadro’s number. S and L refer to solid and liquid.The main interest of this method lies in the fact that the probes are characterized by an intrinsic property derived from London forces. We have used the approach of Saint Flour and Papirer14 to study the dispersive and acid-base properties of conducting polypyrrole (PPy) Panzer and Schrei ber17 have recently compared the methods of in their study of polycarbonate surfaces and found similar AGtB results for each polar probe. They concluded that the approach of Sawyer and Brookman13 was the most convenient since Tb of most widely used probes are readily available in the literature.Nevertheless, Schultz et a2.’s15method is attractive since the parameter characterizing the probe contains yf. Such an approach was actually sought by Gray1* a decade ago. However, the major difficulty of this method lies in the determination of a. Conceptually, the value of a (usually physicochemical property Fig. 1 Method for the evaluation of AG,AB between polar probes and the solid stationary phase: 0,alkanes; H,polar probe Secondly, the Clausius-Clapeyron equation states that AH,,, and log(P,) are related by: log(Po)= -AH,,,/RT+constant At this stage, one is tempted to correlate AGa [or RTln(V,)] with AH,,, since both equations involve AH,,,.However, in the case of self-associated liquids, Trouton’s rule does not hold since the ratio is significantly higher than 80 J K-’ mol-1.20 Moreover, Fowkes’ has shown that, for self-associated probes, AH,,, includes a significant acid- base contribution. It obviously follows that these interactions contribute also to log(P,) and Tb. We thus suggest that RTln(VN) be related to AHtaP rather than AHvap, since the former reflects a dispersive property of the injected probe. After a brief account of the method for the evaluation of AH:ap, we shall apply our approach to the retention data of apolar and polar probes injected in chromatographic columns packed with chloride-doped polypyrrole (PPyCl). The ‘AHtaP’approach is applicable to PPyCl since this polymer is not soluble in any common solvent and not fusible.Added to this, PPyCl is a very rigid material and does not soften at all at the column temperature we have used. Thus the retention data are due solely to the adsorption phenomena of the molecular probes. Experimental Chemical Synthesis PPyCl powders were chemically synthesized by oxidative polymerization by the method of Rapi et at 0 “C using urea as a buffer. The concentrations of pyrrole (Aldrich) and oxidizing agent, FeCl, (Prolabo) were 0.1 mol 1-1 and 0.3 moll-’, respectively. The black PPyCl powder was washed with doubly distilled water, dried in a vacuum desiccator and then sieved to less than 100 pm. J. MATER. CHEM., 1994, VOL. 4 IGC Stainless-steel columns of 1/8 in ( 1 in =2.54 cm) outer diam- eter (od) and 30cm length were packed with ca.160mg of PPyCl powder. A gas chromatograph (Girdel 330) fitted with a flame ionization detector was used. CH, was the non-interacting marker and helium was the carrier gas. The flow rate was 16 ml min-l. The oven temperature was 53 "C as measured by a digital thermometer. The injector and detector tempera- tures exceeded that of the column by ca. 20 "C only to avoid temperature gradients at the inlet and outlet of the column. The columns were conditioned at 110 "C for 15 h. Probe molecules were injected manually by a Hamilton gas-tight syringe at extreme dilution. The signals were recorded with a Delsi 21 digital recorder and the retention times determined graphically according to the method of Conder and Young.12 Results and Discussion In this section, we first recall how we evaluated AH:,, values of the injected probes and then we compare the various methods which can be used to calculate AG2B values.Evaluation of AHLpValues Fowkes has suggested an easy method to split AHvapinto its dispersive and acid-base contributions for self-associated liquids using AHZix, the limiting heat of mixing at infinite dilution in hexane or cyclohexane (apolar solvent^):^ A U:,, =(VP-AHZix+A U,,,)'/4 Vd2 (1) where V is the molar volume of the probe, 6 the solubility parameter of the apolar solvent, AU,,, is the total energy of vaporization of the probe and AU:,, its dispersive component.Once AUd is determined by eqn. (l), one can calculate AH:ap, A Ut$:acid-base contribution to AU,,,) and the degree of self-association (%SA): (i) AH:,^ =AU:,, +RT (ii) AU:?, =AU,,, -AU:,, and (iii) %SA=(AU~&/AUV,,)lOO% We have used the values of AH:ap published by Fowkes5 for the polar probes. Those for 1,4-dioxane (DXN) and CH2C12 were lacking. They were determined as follows. Fig. 2 shows the heat of mixing of DXN and CH2C12 in hexane us. the molar fraction x at 25 "C. AHZix is the limiting slope at 18001 1 1200' c 1 .I -5 9001El -I2 600 aJ= 3001a, I 0.0 0.1 0.2 0.3 0.4 0.5 0.6 mole fraction of probes in hexane Fig. 2 Heats of mixing of 1,4-dioxane (0)and of CH2C12 (A)into hexane uemm the mole fraction, at 25 "C. The variations of the heats of mixing with x were fitted by: H(CH,C12)= -5741.2x2+ 5845.1~ 9164.9~~-6070.1~~ -0.256 and H(dioxane)= 3893.1x4+ 136.1~~ -9121.3x2+7197.8x-O.438 x=O.The heats of mixing were reported by Christensen .~et ~1 Using~ AHZx in eqn. (l),the determination of AH:,, for DXN and CH2C1, is straightforward. The values of AH:!, and AH,,, for alkanes and polar probes are reported on Table 1. For DXN, AH:,, differs significantly from AH,,, and yields a percentage of self-association of 19%. This is quite surprising for a 'pure' Lewis base for which Gutmann's acceptor number is negligibly CH,CI, is self-associated to the extent of 13% although it was expected to have a low %SA comparable to those of CHCI, arid CCl,.5 However, such a %SA is in line with a recent IR study which demonstrated that CH,Cl, dimerizes by double hydrogen bonding in inert solvents.26 Evaluation of AGF We compare six methods of evaluating AGtB, where R T ln(V,) values are related to the abscissa coordinates which are labelled as follows: I, AH:,,; 11, AH,,,; 111, T,,; IV, log(Po); V, a(~d,)l'~;VI, (~V)~/~CX,~O~~.Table 2 reports RT ln(VN) values for alkanes and polar probes adsorbed on PPyCl powder at 53 "C. The probes are indeed adsorbed and not absorbed in the PPyCl bulk, as evidenced by the heats of adsorption which vary in the range of 25-43 kJ mol-l on going from hexane to nonane7" and are thus comparable to the AH,,, range for these alkanes.Following the work of Dorris and Gray27 on poly(ethy1ene terephtalate), we found for AG2H2,the free energy of adsorption of a methylene group in the alkane series, a value of 2.7 kJmol-l. This value is almost equal to 2.76 kJmol-l which is that of AGZH2, the free energy of liquefaction of a methylene group in the same series. Fig. 3 shows a plot of RT ln(VN) us. AH:,, for probes Table 1 Values of AH:ap and AH,,, for alkanes and polar probes probe AH:,,)CJ mol-' AH,,,/W mo1-l hexane 31.5 31.5 heptane 36.5 36.5 octane 41.5 41.5 nonane 46.4 46.4 decane 51.4 51.4 CCl, 31.9 32.4 CHC1, 30.4 30.8 CH2C1, 27.8" 3 1.7' ether 25.8 27.4 DXN 29.4" 35.8b THF 23.3 30.9 EtAc 29.3 34.9 AH,,, for n-alkanes were taken from ref.23; AH,,, and AH$,, for other probes from ref. 5. 'AH:,, calculated according to ref. 5 and using the heat of mixing of DXN and CH2C12 in hexane from ref. 22. bFrom ref. 24. Table 2 Values of RT In(&) for alkanes and polar probes adsorbed on PPyCl at 53 "C probe RT In(V,/ml min-l)/kJ mol-' hexane -1.67 heptane 0.97 octane 3.96 nonane 6.35 CCI, -0.11 CHCI, 3.24 CH2Cl, 4.52 ether -0.86 DXN 7.07 EtAc 6.25 THF 5.00 J. MATER. CHEM., 1994, VOL. 4 81 -l1 6t 0 Fig. 3 Plot of RTln(V,) us AH:a, for alkanes and polar probes adsorbed on PPyCl at 53 "C: 0, C,-C,; a, DCM; 0, CHCI,; t,CCl,; A,ether; 0,THF; V,EtAc; +, DXN adsorbed on PPyC1. It is very interesting to note that for alkanes a linear correlation is obtained, as with the previously published methods (111-VI). The alkane line has the same slope as that determined using AHvap,since AH:ap =AHvap for the alkane series.For polar probes interacting specifically with the substrate, their corresponding markers are above the 'dispersive' line defined by the alkanes. Table3 lists %SA and AGtB values for the polar probes determined by methods I-VI on the basis of RTln(VN) values in Table 2. Table 3 shows that: (i) AGtB values calculated by our new method (I) are included in the range of those determined by all other methods (11-VI); (ii) for any polar probe, the AGkB value obtained by method VI is invariably greater than those of methods I-V; (iii) with the exception of EtAc, methods I, I11 and IV lead to AGF values invariably greater than those of method V (this is in line with a recent IGC study of birchwood meal by methods IV and V7d); (iv) for the poorly self-associated liquids (CHC13, CC14 and ether) we obtain AGtB values similar to those with methods 11-V.This holds also for CH2C12, although it has a higher %SA. (v) As far as probes of higher %SA are concerned (DXN and THF), AGtB values are comparable to those obtained by method VI but significantly greater than those determined by methods 11-V, since in these methods the self- association aspect of the probes is ignored. Although in method V the self-association character is considered, one has to keep in mind that for the polar probes used in this work Jyt NN JyL. Moreover, the experimental evaluation of a using apolar reference solids31 may lead to values which differ from those obtained in the real situation involving a polar adsorb- ent.(vi) When we applied the approach of Donnet et a1.16 to CCl,, we surprisingly determined a very high value of -Table 3 Values of AGf' for polar probes adsorbed on PPyCl at 53 "C and determined by several methods -AG,AB/kJ mol-' probes %SA I I1 I11 IV v VI CCl, 1.6 1.4 1.1 1.1 0.8 0.0 4.0 CHCI, 1.6 5.5 5.3 5.8 5.7 4.1 8.3 ether 6 3.9 3.0 4.4 4.2 3.3 5.4 CH,Cl, 13 8.2 6.1 9.0 8.9 7.7 13.7 Et Ac 18 9.1 6.1 7.2 7.0 8.2 11.1 DXN 19 9.9 6.4 5.7 5.5 4.0 10.5 THF 27 11.1 7.0 7.0 6.9 6.8 12.5 MeJhods I11 and IV: Tb and log(Po) were from ref.24. Method V: a in A' from ref. 28; yt from ref. 29 for alkanes, from ref. 28 for THF, ether and EtAc, from ref. 30 for halogenomethanes and DXN. Method VI: (h~)'/~a,lO~~values in C3/21/2rn-l/' from ref. 16 except for DXN, CHzClz and CCl, which were calculated by us according to ref. 16. ~ 14 Ii 7 E. 0 $61 8 0 *+ iz 0.8 ~ D I I -2L CCI4 CHCl3 ether EtAc THF-self-associated probes Fig. 4 Comparative evaluation of AG?, by six different methods, for polar probes adsorbed on PPyCl: 0,AH:,,; U, AH,,,; A, log Po; + a(y$)'; Tb; 0 (hv)'ao x AGtB (4.0kJmol-'), whereas it has a value in the range 0-1 kJ mol-I by methods 111-V. Obviously, this is a great discrepancy, especially in the case of the quasi-neutral CCl, for which we obtain a 'reasonable' AGtB value (-1.4 kJ mol-l).Although this probe was mentioned in their paper,16 Donnet et al. did not report any AGtB for CC14 adsorbed on untreated and heat-treated Madagascar graphite. Generally, Table 3 clearly shows that for chloromethanes, AGtB values by method VI deviate significantly from those determined by all other methods. Fig. 4 summarizes this comparative study by a plot of AGtB determined with the six methods for some polar probes characterized by %SA and adsorbed on PPyCl at 53 "C. The results obtained with PPyCl and discussed above fall in the most frequent situation of polar probes data lying above the dispersive alkane line. However, some system^'^.^^ yield positive AGtB values.In other words, the RTln(V,) values for polar probes lie below the reference line. Although Donnet et suggested the 'deformation polarizability' method to overcome this shortcoming, one could question these observations when the classical approaches are applied as follow^.'^-^^ Is this due to the method applied to the raw retention data or is there a real thermodynamic effect, like perhaps repulsion forces?33 Is the use of alkanes, for sub- tracting the London contribution to AGa, a universal method or perhaps should IGC users choose perfluoroalkanes to account for dispersive interaction^?^^^^' It has been shown that n-alkanes may diffuse in porous and lamellar materials but not bulky This phenomenon may induce an additional mechanical retention which 'raises' the alkane reference line, thus leading to positive AGtB for the polar probes.A key to this diffusion may be found in the determi- nation of indexes of morphology as suggested by Papirer et ~ 1 . ~ ~ Conclusions We have introduced a novel empirical method for the quanti- tative assessment of acid-base gas-solid interactions by means of IGC and based on AHtap, the dispersive contribution to the enthalpy of vaporization of probes. It is shown not only that the AH:ap approach is linked to those of Sawyer and of Papirer but, more importantly, takes into account the self- association character of the polar probes, as recommended by Fowkes.' For polar probes of low percentage of self- association (%SA) our approach yields AGtB values compar- able to those obtained by the classical methods of Sawyer, Papirer and Schultz.On the other hand, it yields AGtB values matching those obtained by the method of Donnet et al., when applied to probes of high %SA. In the special case of J. MATER. CHEM., 1994, VOL. 4 745 chloromethanes, our method leads to AG," values which 10 A. Exteberria, J. Alfageme, C. Uriarte and J. J. Truin, disagree with those obtained by Donnet's method. At this stage it is reasonable to restrict our method to temperatures close to ambient since AHtaPis calculated using calorimetric measurements at 25 "C.In the same manner as the IGC approach of Gray18 for determining the dispersive 11 12 13 J.Chromatogr., 1992,607,227. J. E. Guillet, M. Romansky, G. J. Price and R. van der Mark, ref. 6, ch. 3. J. R. Conder and C. L. Young, in Physicochemical Memurements by Gas Chromatography, Wiley-Interscience, New York, 1979. D. T. Sawyer and D. J. Brookman, Anal. Chem., 1968,40,1847. contribution to the surface energy of solids, our new method of estimating AGtB is only applicable to adsorption phen- omena and it should not be used when strong polymer bulk diffusion of the probes is suspected. Given that, the 'AH:ap' method of determining the acid-base properties of polymers 14 15 16 17 C. Saint Flour and E. Papirer, J. Colloid Interface Sci., 1983, 91,69. J. Schultz, L. Lavielle and C. Martin, J.Adhesion, 1987, 23,45. J. B. Donnet, S. J. Park and H. Balard, Chromatographia, 1991, 31,434.U. Panzer and H. P. Schreiber, Macromolecules, 1992,25,3633. and other solid materials is interesting and promising, as it considers the self-association character of the probes. 18 J. Anhang and D. G. Gray, in Physicochemical Aspects of Polymer Surfaces, ed. K. L. Mittal, Plenum Press, New York, 1483, vol. 2, p. 659. The authors would like to thank Dr. John S. Lomas (ITODYS, Universitt Paris 7 Denis Diderot) for his editorial criticism. 19 20 S. R. Cain, J. Adhesion Sci. Technol., 1990,4, 333. J. Israelachvili, Intermolecular and Surface Forces, 4cademic Press, London, 2nd edn., 1992. 21 S. Rapi, V. Bocchi and G. P. Gardini, Synth. Met., 1988, 24,217. 22 J. J. Christensen, R. W. Hanks and R. M. Izatt, Handbooh of Heats References of Mixing, Wiley, New York, 1982.F. M. Fowkes, J. Adhesion, 1972,4, 155. F. M. Fowkes and M. A. Mostafa, Ind. Eng. Chem. Res. Dev., 1978, 17, 3. F. M. Fowkes, J.Adhesion Sci. Technol., 1987,1,7. 23 24 25 26 R. R. Dreisbach, Adv. Chem. Ser. 1955,15; 1959,22 and 1961,29. CRC Handbook of Chemistry and Physics, ed. R. C. Weast, CRC Press, Boca Raton, Florida, 67th edn. 1986. F. L. Riddle and F. M. Fowkes, J.Am. Chem. SOC., 1990,112,3259. J. C. Dobrowolski, M. H. Jamroz and A. P. Mazurek, J. Mol. Acid-base Interactions: Relevance to Adhesion Science and Struct., 1992,275,203. Technology, ed. K. L. Mittal and H. R. Anderson Jr. VSP, Utrecht, 1991. 27 G. M. Dorris and D. G. Gray, J. Colloid Interface Sti, 1980, 77, 353. F. M. Fowkes, J.Adhesion Sci. Technol., 1990,4,669. inverse Gas Chromatography. Characterization of Polymers and 28 M. Nardin, E. M. Asloun and J. Schultz, Polym. Adv. rechnol., 1991,2, 109. Other Materials, ed. D. R. Lloyd, T. C. Ward and H. P. Schreiber 29 R. J. Good and E. Elbing, in Chemistry and Physics of Znterfaces- ACS Symposium Series 391, American Chemical Society, Washington DC, 1989. (a)M. M. Chehimi, E. Pigois-Landureau and M. Delamar, J. Chim. Phys., 1992, 89, 1173; (b) M. M. Chehimi, M.-L. Abel, E. Pigois-Landureau and M. Delamar, Synth. Met., 1993,60, 183; (c) M.-L. Abel and M. M. Chehimi, Polymer, in the press; (d) D. P. Kamdem, S. K. Bose and P. Luner, Langmuir, 1993,9,3039. D. Arnould and R. L. Laurence, in inverse Gas Chromatography. Characterization of Polymers and Other Materials, ed. D. R. Lloyd, T. C. Ward and H. P. Schreiber, ACS Symposium Series 391, 30 31 32 33 34 35 36 II, ACS Publications, Washington D.C., 1971. F. M. Fowkes, F. L. Riddle, W. E. Pastore and A. A. Weber, Colloids Surf., 1990,43, 367. J. Schultz,L. Lavielle and C. Martin, J. Chim. Phys., 1987,84,231. E. Morales, M. V. Dabrio, C. R. Herrero and J. L. Acosta, Chromatographia, 1991,31, 357. C.J. van Oss,J. Dispersion Sci. Technol., 1991,12,201. M. Nardin and E. Papirer, J. Colloid inte$ace Sci., 1990, 137, 534. J. H. Burness and J. G. Dillard, Langmuir, 1991,7, 1713. E. Papirer and H. Balard, Prog. Org. Coatings, 1993,22, 1 American Chemical Society, Washington DC, 1989, ch. 8. Z. A. Al-Saigh and P. Munk, Macromolecules, 1984,17,803. Paper 3/05453E; Received 13th September, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940400741

出版商:RSC    

年代:1994

数据来源: RSC