Anal. Chem. 1982, 5 4 , 2045-2049
2045
Surface Derivatization of Hydroxyl Functional Acrylic Copolymers for Characterization by X-ray Photoelectron Spect roscc)py R. A. Dlckie," J. S. Hammond,' J. E. deVries, and J. W. Holubka Engineering and Research Staff, Ford Motor Company, Box 2053, Dearborn, Michigan 48 12 1
The molecular speclation and structural analysis capability of X-ray photoelectron spectroscopy (XPS) can be substantlally extended by chemlcal derivatization techniques that result In the lncorporatlon of unlque elemental labels for reactlve functlonal groups of Interest. For a serles of methyl methacrylate-hydroxypropyl methacrylate copolymers, trlfluoroacetic anhydride has been used to derlvatlze surface hydroxyl groups. Angle-dependent photoemissionstudies, Interpreted uslng slngle and multlple overlayer models, lndlcate that the fluorine moletles are preferentlally orlented Into the bulk of the polymer. These results are conslstent wlth an lnltlal preferentlal orientation of hydroxy moletles away from the polymer surface and suggest thlat llttle reorlentatkon takes place under the derlvatlzation conditions employed.
Many different reactive functional groups are commonly incorporated in polymers used for coatings and adhesives. These include, for example, carboxyl, hydroxyl, and epoxy moieties. Although the presence of these functional groups at polymer surfaces can, in some instances, be confirmed by X-ray photoelectron spectroscopy (XPS), detailed studies of surface composition and structure rely on curve fitting techniques to resolve poorly separated components of the XPS carbon and oxygen spectra. We have previously shown that surface carboxylate residues can be identified in the presence of organic ester and hydroxyl moieties by treatment of the surface with dilute aqueous silver nitrate to form silver carboxylates (1). Qualitative chemical derivatization methods have been described that allow identification of surface functional groups on plasma-treated polyethylene surfaces (2). Solvent swelling and functional group mobility have been found to affect quantitative analyses based on liquid phase reagent derivatization techniques (3). In this paper, we report on the derivatization of surface hydroxyl functionality by treatment with gas-phme trifluoroacetic anhydride and discuss the implications of the results in terms of surface composition (concentration of functional groups near the surface) and surface structure (molecular orientation effects). A brief preliminary account of a portion of this -workhas been given elsewhere ( 4 ) .
EXPERIMENTAL SECTION Materials. Methyl methacrylate (MMA) and hydroxypropyl methacrylate (HPMA) were obtained from Rohm and Haas Co., Special Products Department, and were used without purification. 2-Heptanone and 2,2'-azobis(2-methylpropionitrile) were obtained from Aldrich Chemical Co. Trifluoroacetic anhydride was obtained from Matheson, Coleman and Bell. Preparation of Acrylic Copolymers. Acrylic copolymers with pendant hydroxyl functional groups were prepared from mixtures of methyl methacrylate and hydroxypropyl methacrylate Present address: Perk.in Elmer Corp., Physical Electronics Division, 5 Progress St., Edison, N J 08820.
by conventional solution polymerization techniques. In a typical synthesis, 28.8 g (0.2 mol) of hydroxypropyl methacrylate, 30.0 g (0.3 mol) of methyl methacrylate, and (as initiator for the polymerization) 2.0 g of 2,2'-azobis(2-methylpropionitrile)in 50 mL of 2-heptanone were added slowly to 25 mL of refluxing 2-heptanone under an argon atmosphere. Total addition time was 1 h. After addition of monomers, the reaction mixture was maintained at reflux for 1h, subsequently cooled to room temperature, and stored. All monomer mixtures were prepared by weighing to an accuracy of 3 ppt; all polymerizations were taken essentially to completion (at least 98%). Characterization of Bulk Polymers. Copolymer Composition. Bulk stoichiometry of the acrylic copolymers, calculated from the monomer mixture composition, was confirmed by transmission infrared spectroscopy. Copolymer compositions determined by least-squares fitting of the ratio of hydroxyl (3610 cm-l) to carbonyl (1730 cm-l) absorbance agreed with calculated values to within 3%. Precision of replicate analyses was 0.5%. All spectra were obtained with a Perkin-Elmer 283 infrared spectrometer. Copolymer Molecular Weights. Molecular weights were determined by gel permeation chromatography using a Waters Anaprep gel permeation chromatograph. The results indicate a number average molecular weight of about 3300 and a weight average molecular weight of about 7000 for all samples. No anomalies in molecular weight distribution were observed. Preparation of XPS Specimens. Test specimens were prepared by coating 7 mm X 17 mm steel tabs (vapor degreased in trichloroethylene)with a 25 wt % solution of acrylic copolymer in 2-heptanone. Solvent was allowed to evaporate at room temperature. Specimens were then subjected to surface derivatization and immediately inserted into the spectrometer for analysis. All specimens were lightly rinsed with Freon 113 to remove adventitious surface contamination before analysis. Derivatization was accomplished by exposing the specimen to trifluoroacetic anhydride (TFAA) vapor. About 2 WLof the reagent was injected onto the interior wall of a 3-mL sealed vial containing the specimen tab. After 3 min, the specimen was removed from the vial and inserted into the spectrometer. (In preliminary experiments, it was found that exposures as brief as 15 s sufficed to give nearly complete reaction. No additional incorporation of fluorine was observed for exposures longer than 60 8.)
X-ray Photoelectron Spectroscopy. The XPS spectra were obtained with a Vacuum Generators ESCA I11 X-ray photoelectron spectrometer interfaced to a Vacuum Generators 3040 data system. An aluminum anode X-ray source producing A1 Kcu X-rays at 1486.6 eV was used. The hemispherical analyzer was operated in a constant electron pass energy mode to achieve a resolution of 1.25 eV fwhm (full width at half maximum peak height) on the Au(4f7,,) line for gold foil. The extent of sample charging varied somewhat from specimen to specimen for the polymer films, resulting in shifta in observed binding energies and some broadening of the spectra. The binding energies for the X P S spectra were referenced to the methyl-methylene component of the C Is spectrum at 285.0 eV. Observed half-widths on the C 1s line for the polymer samples ranged from 1.5 to 2.2 eV. An instrumental vacuum of at least 1 X lo4 torr was maintained for all analyses. Data were acquired at 15, 25, 35, and 4 5 O takeoff angles relative to the plane of the sample surface. All photoelectron peak areas were measured by using computer numerical integration. A linear interpolation algorithm was used
0003-2700/82/0354-2045$01.25/00 1982 Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
20
0 L 0
I - #
~
200
1
4
IO3 [ - O H ] , moles/cm3
Y 0
40
mole % H P M A I 2
I
I
400
,
I
,
600
I
800
,
1
1000
BINDING ENERGY,eV
survey spectra (at 45' takeoff angle) of a hydroxyfunctional acrylic copolymer (20 mol % HPMA, 80 mol % MMA) before (A) and after (B) derivatization with trifluoroacetic anhydride. The unlabeled peak at high bindlng energy in A is the 0 KLL Auger line; in addltion, the spectrum in B shows a F KLL Auger component at ca.
Observed atomic % fluorine as a function of copolymer composition. The dashed line is calculated based on specimen homogeneity and complete reaction of available hydroxyl groups with trifluoroacetic anhydride. Flgure 2.
Figure 1. XPS
830 eV.
i
I
1
20
40
I
0.4
.-c0 0
L
C
for background correction. Elemental compositions were calculated by using the photoelectron escape depth values of Clark and Thomas (5), the photoelectron cross sections of Scofield (6),and the formalism of Wagner (7). The least-squares curve-fitting computer routine employed (8) assumed that all peaks were Gaussian in shape without an inelastic tail. The routine computed the peak positions, peak intensities (integrated areas), and fwhm common to all components within a spectrum.
RESULTS AND DISCUSSION The Derivatization Reaction. The objective of chemical derivatization in XPS analysis is the conversion of specific reactive organic moieties to more readily identifiable and quantifiable forms. In the present work, gaseous trifluoroacetic anhydride has been employed to label surface hydroxyl groups by esterification to form trifluoroacetate moieties
The reaction proceeds at room temperature to apparent completion in less than 1min, and, for the acrylic copolymers of this study, provides a unique elemental label. There appear to be no significant side reactions. In control experiments, poly(methy1 methacrylate) films were treated either with TFAA or with a 1:l mixture of TFAA and trifluoroacetic acid; no fluorine could be detected on these surfaces, nor were there any other significant changes in elemental composition. X-ray beam degradation effects have also been assessed. No significant change in composition of derivatized specimens was observed during the X-ray beam exposure times (ca. 15 min) normally employed for the analyses. Qualitatively, the effect of the derivatization is to introduce a fluorine 1s peak in the X P S spectrum, as illustrated in Figure 1. The principal components of the low-resolution survey spectrum before derivatization are carbon 19, oxygen Is, and oxygen KLL Auger peaks. After derivatization, a well-separated, clearly defined fluorine 1s peak is observed at 689 eV; fluorine KLL Auger peak is observed at 830 eV. Elemental Composition of Derivatized Surfaces. Elemental compositions calculated from high-resolution XPS spectra of the acrylic copolymers before and after derivati-
.-
0 c
:0.2
c
a2 C C
s 0
mole % HPMA I
1
I
0
2
4
, moles/crn3 Flgure 3. Observed concentration ratio, F/O, as a function of copolymer composition. The dashed line is calculated as for Figure 2. 1O3[-OH]
zation are given in Table I. Also included in Table I are theoretical values calculated assuming specimen homogeneity and complete reaction of hydroxyl groups with trifluoroacetic anhydride. The experimentally observed fluorine concentrations are somewhat lower than predicted. The reasons for the discrepancy are discussed in detail subsequently, but first it should be noted that a reasonable analytical working curve can be constructed based on the elemental analyses, as illustrated in Figure 2. The observed fluorine content (calculated as an atomic percent of total observed fluorine, carbon, and oxygen) is plotted as a function of copolymer composition; the XPS detection limit for surface hydroxyls (expressed in terms of HPMA-MMA copolymer composition) is about 2 mol % . The composition of the polymer samples can be expressed more generally (Le., independent of polymer type or comonomer selection) in terms of the concentration of functional groups per unit volume. Included in Figure 2 is a calculated concentration scale, in moles of hydroxyl groups per cubic centimeter of polymer. In the calculation of this concentration scale it is assumed that the density of the surface layers is the same as that of bulk polymer (about 1.19 g/cm3) and thus the surface concentration is probably overestimated somewhat. The elemental analyses of Table I indicate that all samples bear a higher level of carbon than expected. The effect of excess surface carbon on the analysis of fluorine incorporation can be partly compensated for by basing the analytical curve on the observed ratio of fluorine to oxygen concentration, as
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
2047
-
Table I. Summary 07 XPS Results: Trifluoroacetic Anhydride Derivatization of Hydroxy-Functional Acrylic Copolymers compositio n, HPMA/ MMA takeoff mol % angle, deg ratio
element analyses, atomic % after derivatization before derivatization calcd obsd calcd obsd C
0
C
0
C
0
F
high-resolution C I s spectra ester/rnethyl ratio
C
0
F
calcd
obsd
10/90
45 35 25 15
71.3
28.7
75.4
24.6
68.4
27.8
3.3
73.5 74.9 74.1 72.8
24.1 23.0 24.5 25.9
2.4 2.1 1.4 1.3
0.72
0.60 0.53 0.30 0.24
20180
45 35 25 15 45 35 25 15
71.0
29.0
76.1
23.9
65.9
27.3
6.5
29.1
77.7
22.3
63.9
26.8
9.3
23.7 21.6 21.1 20.6 23.9 22.2 21.8 20.0
4.7 3.3 3.3 3.6 6.8 6.1 5.2 4.0
0.76
70.9
71.6 75.1 75.5 75.8 69.3 71.1 75.0 76.0
0.61 0.51 0.47 0.40 0.61 0.55 0.44 0.44
45 35 25 15
‘70.7
29.3
75.7
24.3
62.3
26.4
11.3
66.5 68.5 69.4 70.6
23.6 22.6 22.3 21.4
9.8 8.9 8.3 8.0
30170
40160
illustrated in Figure 3 The curves of Figures 2 and 3 are essentially equivalent except that the difference between calculated and experimentally observed quantities is substantially smaller in the latter case. Qualitatively, the elemental analysis results are consistent with the presence of a carbon-rich surface layer. This has been confirmed in studies of the dependence of observed composition on detector takeoff angle. Significant dependence of observed composition on detector takeoff angle is presumptive evidence of compositional inhomogeneity in the sample thickness direction. Surface layers are accentuated relative to the bulk as the takeoff angle is reduced (9-11). The takeoff angle dependence of olbserved composition is summarized in Table I. There is a significant decrease in oxygen and fluorine concentrations, and a corresponding increase in carbon concentration, with decreasing takeoff angle for most of the samples. These resulb indicate a surface enrichment in carbon and suggest that either surface orientation or surface contamination effects may substantially influence functional group quantification for these samples. Models of Surface Structure. Quantitative interpretation of the angular dependence of observed composition is facilitated by compari,son of results with models of surface structure (9-13). A straightforward analysis of angle-dependent data in terms of overlayer models is possible for systems such as metals bearing a metal oxide film for which it is possible to determine the photoelectron intensity from the underlying substrate in an independent experiment (e.g., after sputtering away the oxide) (14). For the polymer analysis attempted here, there lis no such reference state or material available for surface analysis, and a somewhat less direct method of analysis hari been used based on the simple flat surface models of Figure 4, the known bulk composition of the samples, and the obuervation that the molar concentration of carbon in the samples is essentially constant. Photoelectron intensity equations for tlhe models used have been summarized by Fadley et al. (12). Far the homogeneous material of Figure 4A, for each spectral component i, the observed intensity is given by
I’ = SX”x”
(1)
where A’ is the electron inelastic mean free path and xiis the atomic concentration; S is a sensitivity factor (6. Wagner (15)). The sensitivity factor S is the product of the photoionization cross section 0, the instrumental detection efficiency T,the
A
0.80
0.82
0
0.63 0.59 0.54 0.47
C
Flgure 4. Models of surface structure.
efficiency of production from the photoelectron process y, the angular efficiency factor 4, and the X-ray flux F. Scofield’s values (6) for photoionization cross sections were used, T i n the constant pass energy mode was assumed to be inversely proportional to kinetic energy, Y and 4 were assumed constant since only 1s spectra were included, and F was maintained constant throughout the analysis. The atomic compositions of Table I have been calculated by using equations of this form and correspond to fractional relative compositions f
f =X’/CXJ j=1
(2)
where the superscript indexes identify the elements of interest. The summation has been carried out over all elements present excluding only hydrogen. For a specimen comprising an overlayer of material A of thickness d on a substrate of bulk material B, observed a t takeoff angle 0
IAz = SxA’XL[l- exp(-d/X’ sin e)] IB’
= SXB’X’ exp(-d/X’ sin 8)
(3)
(4)
where the subscripts A and B denote the layer; in Figure 4B, A consists of carbon and B of carbon, fluorine, and oxygen. For a double surface layer configuration with a topmost layer A, an intermediate layer B, and bulk C
IAk = SxA’X’[l - exp(-dl/X’ sin e)]
IB) = SxBIXL[exp(-dl/X’sin 8) - exp(-d2/X’ sin e)] IC’ = Xxc’X1 exp(-d2/Xk sin 0)
(5) (6)
(7) In Figure 4C, A consists of carbon, B of carbon and oxygen, and C of carbon, oxygen, and fluorine. The total observed intensity for an element present in more than one layer is the s u m of the contributions from each layer. (It is assumed that,
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 12,OCTOBER 1982 14
I
'
I
'
I
I
r
1 4 , ,
I
UFV
12
1
,
l
HPMAIMMA 10190
20/80 30/70
t // l
0
1
,
1
1
I
2
3
4
I/sin
Flgure 5.
40/60
0 3 0 0
I
lo:
0
i
slope :0 0 4
1
0 0
I
e Flgure 8.
Determination of overlayer thicknesses using eq 9.
since the samples are all organic polymers of approximately constant density, X' is a function only of electron kinetic energy. The values used-calculated from those given in ref 5-are XF = 1.2 nm, ho = 1.4 nm, and Xc = 2.3 nm.) For the single overlayer model of Figure 4B, provided that xBc = xAC, it can be shown that [(XBo/%BC)/@/F)]
P
(8)
where the ratio x B 0 / x B c is estimated from the known composition of the bulk specimen. Similar expressions can be written for fF/lfc. Central to the applicability of eq 8 is the assumption that x B C = xAC. From known molecular structure and measured values of density, it is calculated that xc ranges mol/cm3 for the polymers used in this from 5.7 to 5.9 X study. It is estimated that derivatization reduces these values mol/cm3. For typical alkane slightly, to about 5.4 to 5.8 X hydrocarbons, xc is about 5.7 x mol/cm3. Thus, even for an alkane hydrocarbon overlying the polymers of the present study, the ratio xBC/xAc ranges only from about 0.96 to 1.02, allowing the use of eq 8. The overlayer thickness can be estimated as the slope of a plot of d/sin 0 (as calculated from eq 8) vs. l/sin 0. Data for oxygen and fluorine are plotted in this way in Figure 5. There is considerable scatter in the data, especially at the 15' takeoff angle (l/sin 0 = 3.9). Neglecting the 15' data, the remaining oxygen data are consistent with a carbon overlayer thickness of about 0.2 nm. The fluorine data for the 40% PIPMA sample are also consistent with an overlayer thickness of about 0.2 nm, but the fluorine data on the remaining three samples are more nearly consistent with an overlayer thickness of about 0.4 nm. This suggests that for these samples fluorine is, on average, further from the surface than is oxygen. It also suggests that the double overlayer model of Figure 4C might be more appropriate for these samples. For this model, if it is assumed that xAC= xBc = xcC, and that xc0 = xB0, then In [ ( x ~ " / x ~ ~ ) / ( f ~ / f=o )(&/AF l
4
3
2 I/sin 8
Determinatlon of overlayer thicknesses using eq 8.
d/sin 0 r ho In
1
SYMBOL
x
ov
I
KEY -
- dl/Ao)/sin 0 (9)
A plot of the left-hand side of eq 9 vs. l/sin 0, as in Figure 6, should yield a line of slope (d2/XF- dl/Xo). For the three samples that showed a significant separation of fluorine and oxygen in Figure 5, the slope is found to be 0.16; the predicted value based on values of dl and d2from Figure 5 is 0.14, well within experimental uncertainty. Further confirmation of the overlayer model can be obtained by analysis of high-resolution XPS carbon and oxygen spectra (a typical spectrum is shown in Figure 7 ) . The oxygen spectra comprise a broad single peak that can be resolved into
284
288
292
532
536
BINDING ENERGY, eV
Hlgh-resolution carbon Is and oxygen 1s spectra (at 45' takeoff angle) for a hydroxy functional acrylic copolymer (30 mol % HPMA, 70 mol % MMA) derivatized wlth trifluoroacetic anhydride. Experimental spectra are shown as squares; the dashed lines show the computer resolution into components as discussed in the text. The circles represent the final computer fit to the experimental spectra. Flgure 7.
a doublet corresponding to the two ester oxygen species present. The carbon spectra show four components: methyl-methylene defined for reference at 285.0 eV, ester carbon singly bonded to oxygen at ca. 286.7 eV, and ester carbonyl carbon at ca. 289.0 eV as well as a fluorinated carbon (CF,) species at ca. 293.1 eV. Except for the 40 mol % HPMA copolymer sample, the fluorinated carbon component was found to be too low in intensity to be accurately quantified, and the remainder of the analysis was therefore based on the oxygenated carbon species. In the analysis of the high-resolution carbon spectra, the ratio of integrated peak area attributable to ester carbons to the area attributable to methyl-methylene carbons was determined; these values are given in Table I. Denoting this ratio of areas as fE8/fMe, it can be shown that, for a single overlayer model (Figure 4B) d/sin 0 = Xc In
[(xBEs/xBMe)(xBMe/xAMe)
(fMe/fEB)
X
- (xBMe/XAMe)
+ 11 (10)
where superscripts Es and Me denote ester and methylmethylene components, respectively. The ratio X B ~ / can X ~ be calculated from the known bulk composition of the specimen; the ratio xBMe/xAMe is less certain but can be estimated from the known bulk composition of B and the assumed elemental carbon concentrations of A and B, as discussed
~
~
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
"Ti
I2l 10
I
0
HPMA/MMA
SYMBOL
IO/EICI ZOlElO 3O/i'O 4O/EiC)
0 a 0
-1
1
i
J
0
3 J
O 2 I 0
/
A
1
L L L L L A 0
I
2
3
4
e Flgure 8. Determination (of overlayer thicknesses using eq 10. I/sm
2049
is the sample containing the highest level of hydroxyl groups. The layer thicknesses are on the order of transverse molecular dimensions, as illustrated in Figure 9. This suggests that, at least for lower levels of hydroxyl group content, there is an initial preferential orientation of hydroxyl moieties away from the surface and that this initial orientation is not disturbed by the derivatization procedure so that the trifluoromethyl groups introduced lie below the original ester moieties. These results are consistent with the surface orientation expected from surface energy arguments for hydroxyl groups in a relatively nonpolar copolymer; it is surprising, however, that the introduction of the trifluoroacetate group does not result in sufficient molecular mobility near the surface to allow the fluoromethyl moieties to become concentrated at the film surface. Additional experiments, based on lower glass transition temperature polymers and on heating of the derivatized samples used in this work to temperatures near the glass transition temperature prior to analysis, may help to clarify the questions of molecular mobility at the film surfaces. In any case the present results do serve to underscore both the sensitivity of XPS to surface molecular configuration and the potential utility of chemical derivatization techniques for surface analysis of polymers.
ACKNOWLEDGMENT The contributions of Anne Durisin, who synthesized the polymers used in this study, are gratefully acknowledged. LITERATURE CITED
Flgure 9, Schematic illustration of molecular dimensions in methyl methacrylate-hydroxypropyl methacrylate copolymers. Only the carbon and oxygen atoms are shown for clarity. The asterisks identify hydroxyl groups available for reaction with triffluoroacetic anhydride. earlier. Plotting d/sin 0 as calculated from eq 10 vs. l/sin 0 yields (Figure 8) a straight line of slope d = 0.24 nm, in good agreement with the value 0.22 nm obtained by analysis of the elemental composition data. Overall, the best description of the sample surfaces is given by the double overlayer model of Figure 4C. It is not possible to distinguish between carbon contamination and orientation effects in the case of the carbon-rich surface layer. There is, however, a separation beetween fluorine and oxygen that does indicate a preferential orientation of the fluorine-containing moieties away from the specimen surface and into the bulk of the polymer for all but one of the samples. The exception
(1) Hammond, J. S.;Holubka, J. W.; Dickie, R. A. J. Coat. Techno/. 1979. No. 655. 45. (2) Everhart, D. S.;'Reilly, C. N. Anal. Chem. 1981, 52, 655. (3) Everhart, D. S.;Reilly, C. N. Surf. Interfaces Anal. 1981, 3(3), 126. (4) Hammond, .I. S. Polvm. Preor., Am. Chem. SOC.. Div. Polvm. Chem. 1980, 27(1), i49. . (5) Clark, D. T.; Thomas, H. R. J . Polym. Sci., Polym. Chem. Ed. 1977, 15 2843 . ., - - .-. (6) Scofleld, J. ti. J. Electron Spectrosc. 1976, 8, 129. (7) Wagner, C. D. Anal. Chem. 1977, 49, 1282. (8) Fadley, C. S. Ph.D. Thesis, University of California, Berkeley, CA, 1970 (LBL Report UCRL-19535 1970). (9) Fadley, C. S.J . Necfron Spectrosc. 1974, 5, 725. (10) Drelllna M. J. Surf. Sci. 1978. 71. 231. (1 1) Carlsoh, T. A. "Photoelectron and Auger Spectroscopy"; Plenum: New York, 1975. (12) Fadley, C. S.;Balrd, R. J.; Siekhaus, W.: Novakov, T.; Bergstrom, S. A. L. J. EleCtfOn Spectrosc. 1974, 4 , 93. (13) Castle, J. E. Surf. Sci. 1977, 68, 583. (14) Ansel, R. 0.;Dlckenson, T.; Fovey, A. F.; Sherwocd, P. M. A. J . Electron Soectroec. 1977. 7 7 301. (15) Wagner, C. D. I n "Quantitative Surface Analysis of Materials", ASTMSTP 643; McIntyre, N. S.,Ed.; American Society for Testing and Materials: Phlladelphla, PA, 1978; pp 31-46.
RECEIVED for review October 28, 1981. Accepted June 15, 1982.
