Langmuir 1989,5, 140-144
140
the bulk equilibrium value cannot be explained by the heat effects. (c) The surface charge may alter the interfacial equilibrium. In the present study the charge on the droplets ranged from lo4to lOe elementary charges. For the droplet size range (2 pm 5 a 5 20 pm) encountered in this study the effect of surface charge density on the equilibrium composition is negligible. Moreover, the observed moisture content shows no correlation with the surface charge density.
Conclusions We have measured the evaporation and growth of single glycerol droplets by suspending them individually inside an electrodynamic balance in a stream air of precisely controlled humidity and temperature. By changing the relative humidity of the air stream, we have obtained the growth and evaporation rates of the same droplets over a wide range of relative humidities. The growth and evaporation rates of the droplets have been obtained from two independent methods: light scattering and balancing voltage data. The data obtained by these two methods are in excellent agreement. We have also demonstrated that the resonant peaks found in an intensity spectrum can be used to obtain droplet size and refractive index simultaneously with high precision. From the experimental data, we draw the following conclusions. (i) The time required by a droplet to reach the maximum size is considerably higher than the time predicted by the diffusion theory for the reported value of the liquid-phase diffusion coefficient of a glycerol-water system. The un-
usually slow growth rates indicate either the existence of interfacial resistance or the presence of other rate-controlling mechanisms. (ii) During the evaporation period, the square of the droplet radius changes linearly with time. The slope of the a2 versus t plot decreases as the relative humidity increases. At a given relative humidity, the observed slopes are highly reproducible. (iii) From the ratio of the slopes in humid air to that in dry air we have obtained the thermodynamic parameter Y A X A P A I W U as a function of relative humidity. The experimentally measured slope ratios are significantly higher than the theoretically predicted values based on the equilibrium composition with respect to water vapor in the bulk gas phase, indicating that the droplet interface contains glycerol in excess of the equilibrium value. (iv) For a given relative humidity, the maximum (Le., steady state) water content found in a droplet is always lower than the value predicted by the glycerol-water bulk equilibrium data. The steady-state water content increases with increasing droplet size. The steady-state water content data also raise the possibility of rate-controlling steps other than diffusion being involved in the growth and evaporation processes.
Acknowledgment. We are grateful to the National Science Foundation, Brown and Williamson Tobacco Corp., Tennessee Eastman Co., and Dow Corning Corp. for their generous support under the Presidential Young Investigator Award (Grant No. CPE-831190). Registry No. Glycerol, 56-81-5.
Surface Characterization of Calcium Hydroxylapatite by Fourier Transform Infrared Spectroscopy Tatsuo Ishikawa,* Masato Wakamura, and Seiichi Kondo School of Chemistry, Osaka University of Education, 4-88 Minamikawahori-cho, Tennoji-ku, Osaka 543, J a p a n Received March 16, 1988. I n Final Form: August 29, 1988 Surface characterization by FTIR spectroscopy was carried out on heat-treated and water-exposed colloidal nonstoichiometriccalcium hydroxylapatite (HAP), synthesized from orthophosphoricacid and calcium hydroxide in an aqueous phase. Two IR bands were newly found at 3682 and 3673 cm-' besides an already-reported 3659-cm-l band for HAP treated at 573 K and are assigned to surface P-OH groups. The nature of adsorbed water is also discussed.
Introduction Calcium hydroxylapatite, Calo(P04)6(OH)2(HAP), is biologically important and is used as a bioceramic and adsorbent for chromatography. There have been many studies on the nature of the surface of HAP by adsorption of various molecules, ions, and biopolymers in the liquid phases.'-7 Among a few IR spectroscopic studies of this (1) Misra, D. N. Adsorption on and Surface Chemistry of Hydroxyapatite; Plenum: New York, 1984; pp 1-176. (2) Bell, L. C.; Posner, A. M.; Quirk, J. P. J. Colloid Interface Sci. 1973, 42, 250. (3) Barton, S. S.; Harrison, B. 409.
H. J. Colloid Interface Sci. 1976, 55,
material in the gaseous phase, Cant et al.8 reported a weak absorption band at 3660 cm-' for well-crystallized HAP, which was assigned to either surface Ca(OH),, surface OH ions, or 03POH. Joris and Amber2 assigned this band to ~~
~
(4) Hlady, V.; Furedi-Milhofer, H. J. Colloid Interface Sci. 1979,69, 460. (5) Rawls, H. R.; Bartels, T.; Arends, J. J. Colloid Interface Sci. 1982, 87, 339.
( 6 ) Simabayashi, S.; Sumiya, S.; Nakagaki, M. Chem. Pharm. Bull. 1984,32,3824.
(7) Christoffersen, M. R.; Christoffersen, J.; Ibsen, P.; Ipsen, H. Colloids Surf. 1986, 18, 1. (E) Cant, N. W.; Bett, J. A. S.; Wilson, G. R.; Hall, W. K. Spectrochim. Acta 1971,27A, 425.
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Surface Characterization of Calcium Hydroxylapatite
Langmuir, Vol. 5, No. 1, 1989 141
U al
c n L
Ll 0
n m
O b
0'2
0'6
0'4
0'8
1'0
PI P o
Figure 1. Adsorption isotherms of nitrogen (solid lime) and water (broken line) on HAP treated in vacuo at 573 K.
the stretching vibration of free OH groups of water molecules trapped in lattice vacancies of OH- ions produced by a deficiency of calcium ions in nonstoichiometric HAP. However, there are few experimental evidences supporting this interpretation, such as the physical and chemical behavior of this band. The purpose of this study is to characterize the surface of HAP through the study of the change of IR spectra by heat treatment and water and ion adsorption.
4 000
3500 3000 wave number [ c m - I )
2
0
Figure 2. IR spectra of HAP treated in vacuo at 423, (l),573 (2), and 773 K (3). The weight of sample was 15 mg. 8.0r
1
Experimental Section Materials. HAP was synthesized by a method mentioned elsewhere.'O Orthophosphoricacid solution (7.5 cm3of 20 wt %) was added instantly into 2 dm3 of solution containing 3 g of calcium hydroxide under a nitrogen atmosphereat 298 K and was aged for 4 h by boiling this suspensionin a Pyrex or polypropylene container at pH 10 measured at room temperature. The white
precipitates were washed thoroughly with distilled water under ultrasonic agitation until the pH of washing water was lowered to below 9, and the sedimented mass was dried at 343 K for 10 h. The molar ratio of Ca/P04,1.59, of this material obtained by the EDTA titration and molybdenum-blue colorimetry, respectively, was less than the stoichiometric ratio of HAP, 1.67. This may indicate the existence of lattice vacancies of calcium ions in this material. The X-ray diffraction (XRD) pattern of this material was of well-crystallized HAP. The IR spectrum had no absorption bands of CO?-. By prolonged aging of this material in a Pyrex glass container under the same conditions as above, an absorption band appeared at 3740 cm-' which correspondsto that of free silanol groupsof silica This result shows that the surface of HAP was partly covered with silica by the dissolution of Pyrex glass of the container. The surface of the material used in this investigationis easentially free of silica, since no 3740-cm-' band was found for this material. The crystalline particles of this material have rod shapes, the approximate size of which was about 72 X 24 X 24 nm by TEM. The nitrogen BET specific surface area computedfrom the adsorption isotherm of Figure 1, was 91 m2g-', which is close to 72 m2 g-' estimated from the particle sizes mentioned above. Methods. TransmissionIR spectra were taken with a Digilab FTS-15E FTNIR spectrophotometer in the wavenumber region 2000-7700 cm-' with 2-cm-' resolution. The vacuum cell for HAP samples is capable of heat treatment and gas adsorption in situ, so that the absorbances can be quantitatively estimated. HAP disks of 10-rnm diameter for IR measurement were made by pressing powder of 15-100 mg at 50 kg cm-2. The transmittance of these samples was quite high, and the signal to noise ratio of the 4675-cm-' band in Figure 5, for instance, was about 15 after 200 scans. The sample was pretreated at the temperatures from (9) Joris, S. J.; Amberg, C. H. J. Phys. Chem. 1971, 75, 3172. (10) Bett, J. A. S. Christner, L. G.; Hall,W.K.J. Am. Chem. SOC. 1967,89, 5535.
0' 200
'
'
400
I
600
I
600
1000
temperature ( K )
Figure 3. Thermogravimetricand differential thermal analysis
curves of HAP.
373 to 773 K under Pa for 2 h, after which there was no change in weight and IR spectrum of this material. The protons of surface hydroxyl groups of these samples were deuteriated by repeated adsorption-desorption cycles of heavy water under proper conditions until equilibrium was reached. The adeorption isotherm of water in the gas phase was measured after pretreatment at 573 K under Pa for 2 h by means of an automatic gravimetric adsorption apparatus at 298 K, as is shown in Figure 1. The ion exchange of cadmium and copper(I1) ions was carried out by immersing 100 mg of HAP in 100 cm3of 0.1 mol dm-3cadmium and copper(I1) nitrate solutions at pH 5.5 and 303 K for 15 h. In case of fluoride ions, 100 mg of HAP was immersed in 100 cm3 of 0.1 mol dm-3 sodium fluoride solution at 303 K for 10 h at pH 5.0. HAP samples were removed afterward from these solutions by centrifugation and were used for IR measurement. Simultaneous TG and DTA were carried out in air at a heating rate of 5 K min-'.
Results and Discussion Spectrum 1 of Figure 2 of HAP pretreated at 423 K in vacuo has a strong and sharp band at 3570 cm-' which was assigned to OH- ions on lattice sites of the HAP ~rystals.~ This spectrum has also a very broad absorption band at about 3300 cm-'. When the sample is heated up to 773 K, this band almost disappeared, as is seen in spectrum
142 Langmuir, Vol. 5, No. 1, 1989
Ishikawa et al.
a 0
C
m
.c L
0 bl
o m
t-"-'
- --.-.L---L 3700
3750
3650
3600
w a v e number (cm-I)
Figure 4. IR spectra of HAP treated in vacuo at 373 (l),473 (2), 573 (3), 673 (4), and 773 K (5). The sample weight was 100 mg*
3 of Figure 2. Accompanied by this change, the thermogravimetric (TG) curve of this material of Figure 3 exhibits a steep weight decrease from 300 K to about 700 K together with an endothermic heat of desorption in the DTA curve. Furedi-Milhofer and co-workers" attributed this weight loss to the desorption of adsorbed water inside micropores with a high energy of adsorption. However, the t-curve12obtained from the nitrogen adsorption isotherm in Figure 1 showed that this material does not have microporosity. Also, there seem to be no ultramicropores, the diameter of which is smaller than that of nitrogen molecule, since the adsorbed volume of water in liquid state at low relative pressure in Figure 1 is almost equal to that of nitrogen. Therefore, the weight loss by heating mentioned above might be caused by the dehydration and/or thermal dehydroxylation of chemically adsorbed water. P-OH Sites. Weak absorption bands appeared at about 3700 cm-' simultaneously with a decrease of the 3300-cm-' band in Figure 2. These weak bands are shown in detail in the IR spectra 1 , 2 , 3 , 4 , and 5 of HAP heat treated at 373, 473, 573, 673, and 773 K, respectively, of Figure 4. Spectrum 1 pretreated at 373 K has a broad band ranging from 3650 to 3700 cm-', but at least three bands at 3682, 3673, and 3659 cm-' became obvious by heat treatment at 573 K. Cant et ale8and later Joris and Ambergg found an absorption band at about 3660 cm-' which may be the same one as the 3659-cm-l band in this study. The former two bands are newly found in this work. The weight decrease started from room temperature in Figure 3. This decrease slowed down and reached equilibrium at about 700 K, but again an endothermic weight decrease started from about 700 K. Following the weight loss above 700 K, the 3682-, 3673-, and 3659-cm-' bands became gradually weaker. H-D isotope exchange of this material resulted in the disappearance of 3682-, 3673-, and 3659-cm-' bands, and the respective OD bands appeared at 2721,2712, and 2705 cm-'. Only a very small portion of the 3570-cm-' band showed an isotope shift with the isotope wavenumber ratios of the original bands and OD bands ( Y ~ ~ / Y ?of~ ) 1.357. The isotope ratios of the three bands mentioned (11) Furedi-Milhofer, H.; Hlady, V.; Baker, F. S.;Beebe, R. A.; Wikholm, N. W.; Kittelberger, J. S.J. Colloid Interface Sci. 1979, 70, 1. (12)Lippens, B. C.; de Boer, J. H. J . Catal. 1965, 4, 319.
E
Figure 5. IR spectra of HAP adsorbing various amounts of water: (1) 0, (2) 0.34, (3) 0.81, (4) 1.1, and (5) 2.3 mmol g-l. The temperature of pretreatment was 573 K. The sample weight waa 100 mg.
above were 1.353,1.354, and 1.352, respectively, which are almost equal to each other. This probably means that the bond nature of these OH groups is almost identical with each other. These bands can be assigned to P-OH groups, since the wavenumbers of these bands are close to the 3666-cm-' band assigned to surface P-OH groups of phosphoric acid supported on silica gels.13 These P-OH groups may constitute surface acidic phosphate ions, HPOt-. These ions would have been converted from PO4* ions in order to maintain the overall charge balance of calcium-deficient HAP.14J6 The symmetry of distorted tetrahedral Pod3-ions or their sp3orbitals might have been changed in this surface structure. It is difficult to assign these three bands to only one kind or a number of different kinds of P-OH groups. However, it should be mentioned that these groups may be free or isolated OH groups of similar bond nature, looking at the almost identical H-D isotope ratios, quite high wavenumbers, and ion-exchange properties of these bands mentioned below. All of these bands disappeared on immersing this material in cadmium and copper(I1) nitrate solutions, but there was no observable decrease of the 3570-cm-' band by this procedure. This suggests not only the ion exchange of lattice calcium ions of HAP, as is well-known,16but also that of protons of the P-OH groups for these ions under discussion. The immersion of this material in a sodium fluoride solution of pH 5.0 did not influence these three bands, although the absorbance of the 3570-cm-' band decreased to a small extent by the exchange of OH- ions to fluoride ion, as was reported elsewhere." Water Adsorption. At the beginning of water adsorption on HAP pretreated at 573 K, the 3300-cm-' band assigned to chemically adsorbed water mentioned before reappeared and grew much larger than that of spectrum 1of Figure 2. Simultaneously, a broad and very weak band was detected at about 5250 cm-l which may be assigned to a combination band of water.le This behavior can be (13) Low,M.J. D.; Ramamurthy, P. J.Phys. Chem. 1968, 72, 3161. (14) Berry, E.E.J. Inorg. Nucl. Chem. 1967, 29, 317. (15) Joris, S.J.; Amberg, C. H. J. Phys. Chem. 1971,20, 3167. (16) Suzuki, T.; Hatsushika,T.; Hayakawa, Y. J. Chem. Soc., Faraday Trans. 1 1981, 77, 1059. (17) Menzel, B.; Amberg, C.H. J.Colloid Interface Sci. 1972,38, 256. (18) Yamatera, H.; Fitpatrick, B.; Gordon, G. J. Mol. Spectrosc. 1964, 14, 268.
Langmuir, Vol. 5, No. 1, 1989 143
Surface Characterization of Calcium Hydroxylapatite
a,
u c
m
n
L
0 Lo
n
m
0'5 1'0 1'5 210 2'5 amount o f adsorbed w a t e r ( m m o l / g ) 3750
3700 3650 w a v e number (cm-1)
3600
Figure 6. IR spectra of HAP adsorbing various amounts of water: (1)0, (2) 0.34, (3) 0.48, and (4) 1.1mmol g-l. The temperature of pretreatment was 573 K. The sample weight was 100 mg.
interpreted as follows. Most of water initially added to the material, from 0 to 0.34 mmol g-', might have been used for rehydroxylation and/or hydration, or strongly adsorbed on a Lewis acid site suggested by Joris and Amberg.l6 Also, a part of the water was physisorbed in addition to rehydroxylation and/or hydration. This interpretation agrees with the result of the calorimetric study by Dry and Beebe,lgwho observed a large exotherm in the initial stage of adsorption of water. This band grew stronger proportionally to the amount of adsorbed water from 0.34 m o l g-' in Figure 5. This band splits into 5205and 5308-cm-l bands. The reason for this splitting will be discussed in the last section. Parallel to this increase of the water combination band, the absorbances of the P-OH fundamental stretching bands mentioned above decreased, as is seen in spectra 1, 2,3, and 4 as a function of the adsorbed amount of water of 0, 0.34, 0.48, and 1.1mmol g-l, respectively, of Figure 6. The same behavior of intensity change was observed above for the 4675-cm-' band, as is shown in Figure 5. Since this 4675-cm-I band disappeared completely after H-D exchange by adsorption of D20, the functional group having this absorption band is located on the surface. Therefore, this band might be assigned to a combination of a vibration band of the fundamental stretching vibration of surface P-OH groups at about 3670 cm-', and its deformation vibration possibly at around 1000 cm-l is expected to be observed in the near future. The absorbance decrease of the possible P-OH band (4675 cm-') following water adsorption was negligibly small from 0 to 0.34 mmol g-l, as is illustrated in Figure 7. This behavior is consistent with that of the combination band of water in this adsorption range. Interaction of Water with the Surface P-OH Groups. Figure 7 shows that the absorbance of the 4675-cm-I P-OH band decreased to about one-third of the total absorbance from 0.34 to 1.1mmolg-1 of adsorbed water, being equal to about 5 water molecules per nm-2. Therefore, two-thirds of the total P-OH groups interact with these water molecules. The population of P-OH groups on the HAP surface was estimated as mentioned below. The most developed crystalline surface of HAP is the ac or (1010) plane, and there seems to be about two phosphorous atoms in this plane of a unit cell of area a
Figure 7. Relation between the absorbance of the 4765-cm-' band and the amount of adsorbed water.
= 0.943 X c = 0.688 nrnsZo The crystal structure of HAP indicates that one of these phosphorous atoms may have one and the other may have two OH groups in this plane.21 Therefore, the resulting surface population of phosphorus atoms per nm2 is 3 or 4.5 P-OH groups on the average. Therefore, two-thirds of the total of 4.5 P-OH groups nm-2 or 3 P-OH groups nm-2 would be available for the adsorption of five water molecules. However, it is difficult to determine which of these three bands observed here can be assigned these two kinds of P-OH groups. Above 1.1 mmol g-l of BET monolayer capacity of water adsorption, the absorbance of the P-OH band remained constant, as is seen in Figure 7. This suggests that there would still be P 4 H groups not interacting with water molecules even after multilayer adsorption of water; the reason for this is not known. In conclusion, the absorbance change of P-OH bands on water adsorption suggests that there seem to be at least three energetic steps of water adsorption: (1) decomposed, hydrated, or strongly adsorbed water; (2) water adsorbed by hydrogen bonding with P-OH groups; and (3) water adsorbed by hydrogen bonding and an adsorption interaction such as that due to dipolar and dispersive forces with the HAP surface and/or the adsorbed water in the first step. The configuration of adsorbed water in the second and third steps may be inferred from the analysis of two separated combination bands of adsorbed water at 5308 and 5205 cm-l (named as a and b bands, respectively) in spectra 4 and 5 of Figure 5 mentioned previously. The wavenumber of the a band is close to that of water vapor (5332 cm-l).18 This suggests that the perturbation of the OH bond of OH groups of adsorbed water assigned to the a band is much weaker than that of the OH bond showing the b band. The adsorbed water having the b band may have a configuration such as that of protons of adsorbed water molecules forming a first layer by hydrogen bonding with the oxygen lone pairs of surface P-OH groups and/or of water molecules already adsorbed. Above the BET monolayer adsorption capacity (1.1mmol g-l), the absorbance of the b band increased with the increase in the adsorbed amount of water. Band a can be assigned to one or two free OH groups of an adsorbed water molecule, with one free and the other hydrogen bonded to functional groups, in the former case, or less probably with both free (20) Kukura, M.; Bell,L. C.; Posner, A. M.; Quirk,J. P. J. Phys. Chem. 1972, 76, 900.
(19)Dry, M. E.; Beebe, R. A. J. Phys. Chem. 1960,64,1300.
(21) Kay, M. I.; Young, R. A.; Posner, A. S. Nature (London) 1964, 204, 1050.
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Langmuir 1989, 5, 144-147
Figure 8. Model of the adsorption of water on HAP. and lone-pair electrons of its oxygen atom attached to the various sites in the latter case. The absorbance of the a band increased until the BET monolayer capacity of 1.1 mmol g-' was reached but stayed constant above this water content. The reason for this result could be that these free OH groups exist only on the surface of adsorbed layers or
may remain inside the layer without hydrogen bonding. A band at 3691 cm-l in spectrum 4 of Figure 6 appeared simultaneously with band a. Therefore, this 3691-cm-' band may be the fundamental stretching vibration band of adsorbed water, since the wavenumber of this band is close to those of the fundamental stretching vibration of water vapor at 3756 and 3652 cm-1.22 A model of the configuration of the surface P-OH groups and adsorbed water is postulated in Figure 8, as a summary of the discussion described above. Registry No. Calo(P04)6(OH)2, 12167-74-7. (22)Yamauchi, H.;Kondo, S. J. Colloid Polymer Sci., in press.
Spreading Transition of a Liquid Film Steve Granick* and Daniel J. Kuzmenka Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801
Stephen J. Clarsont and J. A. Semlyen Department of Chemistry, University of York, York YO1 5 0 0 , U.K. Received June 13, 1988. I n Final Form: August 30, 1988 Liquids of poly(phenylmethylsi1oxane) spread at the air-water interface undergo, with increasing temperature, a transition from the spread to the nonspread state. This was manifested as a vanishing of the surface pressure, T . It was also visualized. In spreading from a droplet of macroscopic size, i.e., from an essentially infinite reservoir, x vanished at 55 f 2 "C for a liquid with number-average molecular weight M, = 2240 pmol-'. However, when the amount spread was limited to 2 and 1mgm-2,the temperatures at which the surface pressure vanished were 42 and 37 f 2 "C, respectively. The same pattern held, but at higher transition temperatures, for a liquid species with M, = 1470 gmol-l. The dependence on the amount spread might be owing to molecular weight distribution in the polymer fractions but might also reflect increased equilibrium film thickness when the spreading coefficient is small (a "pancake"), as predicted by Joanny and de Gennes but apparently not previously observed. In this system, it is possible by varying the temperature to tune the spreading coefficient so that it passes continuously from positive to negative.
Introduction If a drop of nonvolatile liquid is placed onto a liquid or solid surface, there are three possibilities: it may spread completely to wet the substrate (form a film of zero contact angle), it may partially wet the substrate (form a film of finite contact angle), or it may dissolve. If the liquid is immiscible then only the first two possibilities remain. Which possibility occurs can be expressed as the outcome of competition between macroscopic interfacial energies described by the spreading coefficient.' Qualitatively, wetting reflects preferential adsorption of the spreading liquid to the interface, while partial wetting reflects preferential contact between the two other phases. Recent interest in these ancient questions has focused on the dynamics of spreading and on the transition between complete and partial wetting as the chemical potential is varied. The state of the art is discussed in recent revie~s.2~ On the experimental side, the wetting transition has been studied in detail for a handful of experimental syst e m ~ But . ~ ~from ~ experiment, little is known about the familiar physical situation, the spreading of a liquid droplet. The problem is to find a suitable experimental system. 'Present address: Dept. of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221-0012.
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Earlier4 we reported measurements of surface pressure as a function of surface coverage for poly(phenylmethy1siloxane) liquids spread at the water-air interface at 22.8 "C. The behavior was unlike that characteristic of conventional amphiphiles such as pentadecanoic acid, for which the amphiphilic groups of the molecule are separated from the other end of the molecule, and layers beyond the monolayer do not form.' For liquids of number-average molecular weight M, = 1470 and 2240 gmol-l, smooth transitions to multilayers and eventually films of macroscopic thickness was observed. It was concluded that these films wet the water-air interface. However, fractions of this polymer of higher molar mass failed to spread. This "drying" transition5 as a function of molar mass, i.e., a transition from preferential wetting of water by liquid to its preferential wetting by air, is reminiscent of the observation, for linear hydrocarbons, that pentane, hexane, and heptane spread at the water-air interface but octane (1) Adamson, A. W. Physical Chemistry of Surfaces, 4th Ed.; wiley: New York, 1982. (2) de Gennes, P.-G. Rev. Mod. Phys. 1985,57,827. (3) Dietrich, S. In Phase Transitions and Critical Phenomena; Domb, C., Lebowitz, J., Academic: London, 1987. (4) Granick, S.; Kuzmenka, D. J.; Clarson, S. J.; Semlyen, J. A. Macromolecules, in press. (5)Pandit, R.;Schick, M.; Wortis, M. Phys. Reu. B: Condens. Matter 1982, 26, 5112.
0 1989 American Chemical Society
