ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
initial characterization of new chemical systems and for fundamental mechanistic studies. Its use for quantitative chemiluminescence measurements is more limited because of the very low levels usually measured. This is not totally due to the limitations of the IDA but also to the fact that analytical chemiluminescence measurements are normally made without any wavelength discrimination in order to maximize the amount of luminescence detected.
ACKNOWLEDGMENT We thank Tracor Northern for loaning us the TN-1710 signal analyzer.
LITERATURE CITED (1) R. E. Santini, M. H. Milano, and H. L. Pardue, Anal. Chem., 45, 915A (1973). (2) G. Horlick and E. G. Codding, Anal. Chem.. 45, 1490 (1973). (3) Y. Talmi, Am. Lab., March, 79 (1978). (4) G. Horlick, Appl. Spectrosc., 30, 113 (1976). (5) D. E. Osten, Ind. Res., 82, October (1975). (6) J. A. Hass, L. J. Perko, and D. E. Osten, Ind. Res., May (1977). (7) Y. Talmi, Anal. Chem., 47, 658A (1975). (8) Y. Talmi, Anal. Chem., 47, 699A (1975). (9) K. W. Busch and G. H. Morrison, Anal. Chem., 45, 712A (1973). (IO) J. D. Winefordner, J. J. Fitzgerald, and N. Omenetto, Appl. Spectrosc., 29, 369 (1975). (1 1) R. P. Cooney, G. D. Boutilier, and J. D. Winefordner, Anal. Chem., 49, 1048 (1977). (12) D. F. Brost, B. Malloy. and K. W. Bush, Anal. Chem., 49, 2280 (1977). (13) H. L. Felkel, Jr., and H. L. Pardue, Anal. Chem., 50, 602 (1978). (14) N. G. Howell and G. H. Morrison, Anal. Chem., 49, 106 (1977). (15) H. L. Felkel, Jr., and H. L. Pardue, Anal. Chem., 49, 1112 (1977). (16) F. S. Chaung, D. F. S. Natusch, and K . R. O'Keefe, Anal. Chem., 50, 525 (1978). (17) G. Horlick and E. G.Codding, Appl. Spectrosc., 29, 167 (1975). (18) K. M. Aldous, D. G. Mitchell, and K. W. Jackson, Anal. Chem., 47, 1034 (1975). (19) T. L. Chester, H. Haraguchi, D. 0. Knapp, J. D. Messman. and J. D. Winefordner, Appl. Spectrosc., 30, 410 (1976). (20) T. E. Cook, M. J. Mihno, and H. L. Pardue, Clin. Chem., ( Winston-Salem, N.C.), 20, 1422 (1974). (21) E. G. Codding and G. Horlick, Spectrosc. Lett., 7, 33 (1974). (22) T. E. Edmonds and G. Horlick, Appl. Spectrosc., 31, 536 (1977). (23) M. Franklin, C. Baber, and S. R. Koirtyohann, Spectrochim. Acta, Parl B , 31, 589 (1976). (24) K. R. Betty and G. Horlick, Appl. Spectrosc., 32, 31 (1978). (25) G. Horlick, E. G. W i n g , and S. T. Leung, Appl. Spectrosc.,29, 48 (1975). (26) K . W. Busch, N. G. Howell, and G. H. Morrison, Anal. Chem., 46, 2074 (1974). (27) N. G. Howell, J. D. Ganjei, and G. H. Morrison, Anal. Chem., 48, 319 (1976).
1777
(28) D. W. Johnson, J. B. Callis, and G. D. Christian, Anal. Chem., 49, 747A (1977). (29) A. E. McDowell, R. S. Harner, and H. L. Pardue, Clin. Chem. Winston-Salem, N . C . , 22, 1862 (1976). (30) M. J. Milano and K. Y. Kim, Anal. Chem., 49, 555 (1977). (31) G. M. Ridder and D. W. Margerum, Anal. Chem., 49, 2090, 2098 (1977). (32) A. E. McDowell and H. L. Pardue, And. Chem., 49, 1171 (1977). (33) R. E. Dessy, W. D. Reynolds, W. G. Nunn, C. A. Titus, and G. F. Moler. Clin. Chem. ( Winston-Salem, N.C.), 22, 1472 (1976). (34) D. A. Yates and T. Kuwana, Anal. Chem., 48, 510 (1976). (35) T. A. Nieman, F. J. Holler, and C. G. Enkc, Anal. Chem., 48, 899 (1976). (36) E. Kohen, C. Kohen, J. M. Salmon, Mikrmhim. Acta, 1976 11, 195 (1976). (37) J. P. Fillard, M. deMurcia, J. Gasiot, and S. Chor, J . Phys. E., 8, 993 ( 1975). (38) J. R. Jadarnec, W. A. Saner, and Y. Talmi, Anal. Chem., 49, 1316 (1977). (39) R. P. Cooney, T. Vo-Dinh, and J. D. Winefordner, Anal. Chim. Acta, 89, 9 (1977). (40) R. P. Cooney, T. Vc-Dinh, G. Walden, and J. D. Winefordner, Anal. C b m . , 49, 939 (1977). (41) H. Steinhart and J. Sandmann, Anal. Chem., 49, 950 (1977). (42) A. L. Broadfoot and B. R. Sandel, Appl. Opt., 16, 1533 (1977). (43) S. S. Vogt, R. G. Tull, and P. Kelton, Appl. Opt.. 17, 574 (1978). (44) B. R. Sandell and A. L. Broadfoot, Appl. Opt., 15, 3111 (1976). (45) J. F. McNall, Imaging in Astronomy, p FB8-1, June 18-21, 1975. (46) J. F. McNall and K. H. Nordsieck, "An Intensified Self-scanned Array Detector System that is Photon Noise Limited", preprint, University of Wisconsin, Madison, Wis. (47) S.B. Mende and F. H. Chaffee, Appl. Opt., 16, 2698 (1977). (48) R. G. Tull, J. P. Choisser, and E. H. Snow, Appl. Opt., 14, 1182 (1975). (49) R. L. Wilson and J. D. Ingle, Jr., Anal. Chem., 49, 1060 (1977). (50) S. D. Hoyt and J. D. Ingle, Jr., Anal. Chim. Acta, 87, 163 (1976). (51) R. L. Wilson and J. D. Ingle, Jr., Anal. Chem., 48, 1641 (1976). (52) E. E. Edwin, R. Jackman, and N. Hebert. A~lyst(London),100, 689 (1975). (53) W. I.M. Holman, J . Biochem., 38, 388 (1944). (54) R. Strohecker and H. M. Henning, "Vitamin Assay-Tested Methods", Verlag Chemie, Germany, 1966. (55) "Methods of Vitamin Assay", Assoc. oi Vitamin Chemists, Inc., Interscience, New York, 1966. (56) G. Rindi and V. Perri, Int. Z. Vitaminforsch, 32, 398 (1962). (57) "Official Methods of Analysis of the Association of Official Analytical Chemists", William Horwitz. Ed., AOAC, Washington, D.C., 1970. (58) R. L. Wilson and J. D. Ingle, Jr., Anal. Chem., 49, 1066 (1977). (59) R. L. Wilson and J. D. Ingle, Jr., Anal. Chim. Acta, 92, 417 (1977). (60) J. R. Totter, Photochem. Photobiol., 3, 231 (1964). (6 1) Larry A. Montano, Master's Thesis, Oregon State University, Corvallis, Ore., 1978. (62) J. Slawinsk, Photochem. Photobiol., 13, 489 (1971). (63) E. J. Bowen and R. A. Lloyd, Proc. Chem. Soc., October, 305 (1963). (64) G. 8. Melvzova and R. F. Vassil'ev, Mol. Photochem., 2, 251 (1970).
RECEIVED for review July 12,1978. Accepted August 18, 1978. Acknowledgement is made to the NSF (grant CHE-76-16711) for partial support of this research and one of us (M.A.R.) gratefully acknowledges an NSF graduate fellowship.
Quantitative Measurement of Ethylene Incorporation into Propylene Copolymers by Carbon- 13 Nuclear Magnetic Resonance and Infrared Spectrometry John R. Paxson" and James C. Randall Phillips Petroleum Company, Research and Development, Bartlesville, Oklahoma 74004
Carbon-13 NMR has been used successfully to characterize a series of ethylene-propylenecopolymers for weight per cent ethylene. Once characterized, this copolymer series served as callbration standards for development of a faster infrared method. An excellent correlation was obtained between the techniques. These copolymers contained >95 % propylene, with the ethylene units present as isolated entities between two head-to-tail propylene units.
Carbon-13 nuclear magnetic resonance has proved to be an excellent technique for analyses of sequence distributions and 0003-2700/78/0350-1777$01,00/0
comonomer contents in ethylene-propylene, E-P, copolymers (1-5). These analyses are particularly straightforward if one of the monomer units is present a t a level of 95% or greater because the other monomer will occur primarily as an isolated unit. The branching content has been examined previously for ethylene-propylene copolymers containing predominantly ethylene (6);however, corresponding quantitative measurements have not been reported for the ethylene content in copolymers containing predominantly propylene. Accessible, accurate, and reasonably fast routine analytical methods are needed for process control and development. Infrared (IR) spectrometry is probably the most extensively applied spectral technique for such applications ( 7 , 8 ) . It is conceivable that C 1978 American Chemical Society
1778
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
0 -
___
4 @0 ~
1 r
3L
PPI4
11s
Figure 1. A 13C NMR spectrum at 25 2 MHz and 125 OC of a 97/3 propylene-ethylene copolymer in 1,2,4-trichlorobenzeneand perdeuterobenzene The internal standard is hexamethyldisiloxane, HMDS
most IR bands used for determining copolymer compositions are sensitive to sequences of both monomers. Thus, IR methods for compositional analysis can be calibrated if (a) known standards of similar constitution to the copolymers being analyzed are available and (b) assignments and behavior of the calibration bands are well established; preferably the absorptivities of these bands should be relatively independent of the position of monomer units in t h e chain. Thus, quantitative infrared analysis of copolymers depends primarily on the standards employed whose composition can be determined directly and reliably. In this study, 13CNMR is used t o provide such reference standards for the less time-consuming infrared measurements. Because it is relatively inexpensive and easy to operate, IR equipment is more feasible than 13C NMR for quality or process control in commercial applications. This study shows that an excellent correlation, which can be used for quantitative analysis, is obtained between I3C NMR and IR results on a series of E-P copolymers containing greater than 95 wt% propylene.
EXPERIMENTAL The copolymers examined in this study were prepared by a method described previously (9). The ethylene contents varied from 1 to 3 wt%. Nuclear Magnetic Resonance Measurements. NMR spectra were measured from polymer samples dissolved in a mixture of 1,2,4-trichlorobenzene and perdeuterobenzene at concentrations between l@l5 wt% . Sufficient perdeuterobenzene was added to maintain a lock signal on a Varian XL-100-15 NMR spectrometer at a temperature of 125 "C. The spectrometer system was equipped with Varian's FT-100 pulsed NMR-Fourier transform system and a disk accessory. The 13C NMR spectra were obtained utilizing the following conditions for the pulse sequences: pulse angle 90" transients 2000-5000 accumulated pulse delay 8.5 s spectral width 5000 Hz acquisition 1.5 s no signal time enhancement used A pulse interval of 10 s is sufficient to satisfy the spin-lattice relaxation time, T1, of the methyl group, which is the slowest relaxing nucleus with a T, of 2 s at 125 "C ( I O , 11). The number of transients accumulated depended upon the ethylene content and, in each case, accumulations were allowed to continue until a satisfactory signal-to-noise ratio was achieved. As shown by a typical example in Figure 1, each 13C NMR spectrum was recorded with proton noise-decoupling to remove unwanted 13C-lH scalar couplings. No corrections were made for differential nuclear Overhauser effects (NOE) since constant NOEs were assumed (12,131in agreement with previous workers (1-4). Constant NOEs for all major resonances in low ethylene content ethylene-pro-
pylene copolymers have been reported recently ( 1 4 ) . Infrared Measurements. Infrared spectra were obtained on molded films of a nominal 1-mm thickness. Approximately 0.52 g of sample was placed between 31.8-mm diameter aluminum foil disks on a 1-mm brass spacer of 19.0-mm i.d. and 31.8-mm 0.d. resting in a Buehler 20-2112 specimen mold. The temperature of the assembled mold was maintained to 175 "C by a standard thermocouple temperature controller. After the sample was pressed to 6000 psig (40.34 megapascals) in a Buehler 1315 AB specimen mold and cooled to 35 "C under pressure, the film was removed and mounted in a holder. Five measurements with a Starrett Model 230-M micrometer were averaged to provide a film thickness for the ethylene calculations. Since small variations in thickness introduce substantial errors in final calculations, the film was discarded if the readings varied by more than 0.05 mm and another sample was molded. Infrared spectra were obtained on a Digilab Model 15 Fourier Transform system at four wavenumber resolution in double beam operation. An adequate signal-to-noise ratio resulted from 1000 scans. Standard double precision DigilabIData General computer software was used to present data properly scale expanded in absorbance form. The absorbance of the 732 cm-' infrared band, attributed to y r (CH2)3(7)was recorded. Comparable transmission spectra were obtained on a typical plant instrument, the Perkin-Elmer 257.
RESULTS AND DISCUSSION Since the infrared method depends upon the availability of reference copolymer and 13C NMR is used for the purpose of providing well-characterized copolymers, t h e 13C NMR method will be discussed first. Ethylene-propylene copolymers have been the subject of extensive I3C NMR investigations (for example, see references 3, 4, and 5) and complete spectral assignments are available. Thus, these copolymers containing predominantly propylene should be readily characterized by the 13C NMR technique. T h e I3C NMR spectrum of an ethylene-propylene copolymer, containing approximately 97 70 propylene in primarily isotactic sequences, is shown in Figure 1. The major resonances are numbered consecutively from low to high field. Chemical shift data and assignments, according to Ray, Johnson, and Knox ( 4 ) in a study of related E-Pcopolymers, are listed in Table I. Greek letters are used to distinguish the various methylene carbons and designate t h e location of the nearest methine carbons as suggested by Carman ( I ) . Reference chemical shift data from t h e study of Ray et al., a predominantly isotactic polypropylene and an ethylenepropylene copolymer containing 97% ethylene, are also listed in Table I. The latter two polymers establish a base for comparing the various chemical shift data and lend support to the assignments of Ray, Johnson, and Knox. (Chemical
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
1779
Table I. Observed and Reference I3CNMR Chemical Shifts in ppm for Ethylene-Propylene Copolymers and Reference Polypropylenes as Measured with Respect to an Internal TMS Standard reference PP 97/3a sequence crystalline amorphous 3/97 E/P line carbon E/Pa [Ray et al. ( 4 ) ] E/P assignment (12) (12) 1
aa-CH,
46.4
46.3
PPPP
46.5
47.0-47.5 r 46.5 m
2 3 4 5
aa-CH, ay-CH, CH CH
46.0 37.8 30.9 28.8
45.8 37.8 30.7 28.7
PPPE PPEP PPE PPP
28.5
28.8 mmmm 28.6 mmmr 28.5 rmnir 28.4 m r t rr
6 7
PP-CH, CH 3
24.5 21.8
24.4 21.6
PPEPP PPPPP
21.8
21.3-21.8 mm 20.6-21.0 mr 19.9-20.3 rr
8 9
CH 3 CH, CH, CH a-CH, P-CH,
21.6 20.9
21.4 20.7 19.8
PPPPE PPPEP EPE EPE EPE EPE EEE
-(CH2)na
19.8 33.1 37.4 27.3 29.8
33.1
29.8
As measured in this laboratory.
shift measurements are known to vary a few tenths of a ppm between laboratories.) From the chemical shift assignments, it can be established t h a t the E - P copolymers examined in this study contain principally isolated ethylene linkages. The nine resonances shown in Figure 1can be easily attributed to isolated ethylene linkages ( 4 ) as shown below:
Table 11. Carbon-13 NMR Intensity Data for 3/97 and 1/99 Ethylene-Propylene Copolymers relative areas peak heights _
line
3/97
1
155.0 18.0 14.0 17.0 169.0 6.0 c176.0
6.5 195.0 2.0 c176.0
21.0
16.5
2 3 4
5 6
7 8 9 With the structure of these E-P copolymers firmly established, the 13C NMR relative intensities can be used to determine the ethylene-propylene content and thereby establish reference copolymers for the faster infrared method. One advantage of the 13C NMR approach is that the ethylene content can be measured independently from more than one region of the I3C NMR spectrum. Conceivably, six different resonances could be used t o establish the ethylene content while the weight per cent propylene can be measured in three ways. These relationships are given below:
(P)= k(17 + 1 8 + 1 9 ) (P)= k(14 + 1 5 ) (PI = k(1, + 12 + (1/2)13)
(1) (2)
(3)
(E) = k1,
(4)
(E) = (1/2)k13
(5)
(E) = (1/2)k14
(6)
(E) = (1/2)k12
(7)
(E) = ( l / Z ) k I s = (l/Z)k19
(8)
The quantitative relationships given by Equations 1 and 8 are from methyl resonances only, Equations 3 , 4 , 5, and 7 utilize methylene resonances only, and Equations 2 and 6 are developed from methine resonances only. (Note that k is the
1/99 186.5 14.0 7.0
_
1/99 1217 40 40 52 1447
_
~
3/97 202 17 17 23 245
18
8
1477 74 57
220 25 22
NMR proportionality constant that, includes all of the variables in an NMR analysis that become fixed when a specific set of experimental conditions is selected. Such factors include the number of molecules in the vicinity of the receiver coil, instrumental response, and recorded attenuation. It is unnecessary to evaluate k because every resonance has the same value for k . I , is the intensity of the peak x.) Although the ethylene and propylene measurements are certainly overdetermined through the use of Equations 1 through 8, the accuracies of the various ways that (E) and (P) can be measured are not the same. For example, there is considerable overlap between peaks 1 and 2 and also among peaks 7,8, and 9. The second point of concern is tacticity. The methyl, methine, and methylene carbon resonances of polypropylene display different degrees of sensitivity toward configurational differences (15). As shown in Table I, resonances from propylene units in configurational sequences other than isotactic may overlap with corresponding resonances from isotactic propylene units associated with the isolated ethylene linkage. This overlap could be a factor in an analysis of both the methylene resonances, 1 and 2, and the methyl resonances 7,8, and 9. As a consequence, the methine resonances, 4 and 5, should be the best suited for quantitative measurements because the methine carbon resonance is the least sensitive toward configurational differences (15) and also the least affected by overlap from neighboring resonances. Otherwise, the accuracy of the 13C NMR method for (E), (P) may be related to the ethylene content. As the ethylene content drops to a level comparable to the various resonances from atactic
1780
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
Table 111. Precision of Determination of Ethylene in Ethylene-Propylene Copolymers from Infrared Analysis ethylene levela
95% confidence level
0.90-1.29 1.21-1.58 1.4 2- 1.7 8 1.83-2.17 1.93-2.27 2.13-2.4 8 2.33-2.68 2.52-2.89 2.62-2.99 From I3C N M R , weight per cent. 1.1
1.4 1.6 2.0 2.1 2.3 2.5 2.7 2.8
CI
The final equation relating the infrared absorbance to the ethylene content is: Y = 2.465X + 0.451 (9) Figure 2. An infrared spectrum of a low ethylene content ethylenepropylene copolymer. T h e 732 wavenumber band indicates random ethylene monomer units. A typical base line is illustrated
polypropylene sequences, t h e calculated ethylene content could be considerably different from the true value. This point is illustrated in Table I1 where intensity data are listed for resonances 1 through 9 for two ethylene-propylene copolymers with 3/97 and 1,199 comonomer distribution, respectively. The peak intensities for resonances 2, 8, and 9, which should be identical, are enhanced to different extents by configurational overlap in the 1/99 ethylene/propylene copolymer sample. The best quantitative results, therefore, are obtained by using methine resonances 4 and 5 to determine t h e comonomer composition. Similar results are obtained whether one uses peak heights or peak areas as shown below: weight per cent ethylene by peak
by peak
sample
heights
areas
3/97 ( E ) / ( P ) 1/97 (E)/(P)
2.8
3.0
1.1
1.1
T h e composition of the remaining ethylene-propylene copolymers were determined by peak heights using the methine resonances only. I n no instance was there any evidence for an inclusion of consecutive ethylene units. Thus, composition data from 13C NMR could now be used to establish an infrared method based on a correlation with the 732 wavenumber band which is attributed to the rocking mode, -yr, of the methylene trimer, -(CH2)3-, between tertiary carbon atoms (16). Figure 2 shows the infrared spectral region and baseline used for the measurement of the random ethylene monomer unit content in these polymers. Typical calibration curves were based on data obtained in single application of the infrared technique. Peak height measurements were made on the 732 cm-' band, which, once again, is characteristic of an ethylene unit isolated between two head-to-tail propylene units. To set 95% confidence limits on t h e ethylene concentration, we used a procedure (17) to calculate upper and lower calibration curves that can be set on a daily constructed calibration line in much the same way as confidence bands are set on regression lines. However, the upper and lower calibration curves differ from confidence bands in that they apply back to the independent variable (the ethylene concentration) used in determining the calibration curve. Using this procedure, an ethylene concentration can be predicted, given a n observed peak height, and a confidence interval can be set on this predicted value for which a probability statement applies. Table I11 shows t h e results of such a process on calibration data obtained in a single application of the infrared method. Identical precision was obtained for data sets from both the FTS-15 and PE-257.
where Y is the infrared absorbance a t 7 3 2 uavenumhers divided by the film thickness in centimeters and X is the wt% ethylene. The standard errors are 0.110 for the intercept and 0.051 for the slope. In conclusion, it is apparent from the data in Table 111 that the infrared absorbance at 732 cm is sufficiently sensitive to the ethylene incorporation to determine the wt7r ethylene to within 0.1-0.27~a t the 95% confidence level. Of course, this uncertainty in part reflects the uncertainty in the '.'T NMR measurements; however, it is gratifying that, collectively, t h e measurements can be made within the indicated limits a t the 95% confidence level. From the 13CNMR data, it can be concluded that the propylene units occur in predominantly isotactic, head-to-tail sequences and that the ethylene units are incorporated as isolated units only. Thus, this structural prerequisite is a requirement for application of this method because it has not been tested on copolymers containing propylene configurational irregularities or ethylene sequences two units and longer. This method does illustrate the feasibility, however, in combining infrared and '"C KMMK data to develop quantitative, analytical methods for polymer structural analysis. ACKNOWLEDGMENT The authors express their appreciation to J. P. Butler and F. T. White for the infrared measurements and to F. L. Tilley for the NMR measurements. Appreciation is also expressed to the Phillips Petroleum Company for permission to publish this work. LITERATURE CITED (1) C. J. Carman and C. E. Wilkes, Rubber Chem. Techno/., 44, 781 (1971) (2) C. E. Wilkes, C. J. Carman, and R . A. Harrington, J . Polym. Sci., 43, 237 (1973). (3) C. J. Carman, R. A. Harrington, and C. E. Wilkes, Macromoiecules. 10. 536 (1977). (4) G. J. Ray, P. E. Johnson, and J. R. Knox, Macromolecules, I O . 773 (1977). (5) J. C. Randall, Macromolecules, 11, 33 (1978). (6) J. C. Randall, J . folym. Sci., Polym. Phys. E d . , 11, 275 (1973). (7) H. V. Drushel, Grit. Rev. Anal. Chem.. 1. 161-192 (1970). (8) C. Tosi and F. Ciampelli, Adv. Polym. Scl., 12, 88-130 (1973) and references therein. (9) R. J. Perry, U.S. Patent 3 736 307, issued May 29, 1973. (10) D. E. Axelson, L. Mandelkern, and G. C. Levy, Macromolecules, I O . 557 (1977). (11) J. C. Randall, J . folym. Sci., Polym. Phys. Ed.. 14, 1693 (1976) (12) J. Schaefer and D. F. S. Natusch, Macromolecules, 5 , 416 (1972). (13) Y. Inoue. A. Nlshioka, and R. ChGjB, J . Polym. Sci.. Polym. Phys. Ed.. 11, 2234 (1973). (14) J. M. Sanders and R. A. Komoroski. Macromolecules, I O . 1214 (1977) (15) J. C. Randall, "Polymer Sequence Determination", Academic Press, New York. 1977. Chao. 1. (16) C. Tosi, Spectrochim. Acta, Part A, 24, 2157 (1968). (17) H. Scheffe, "A Statistical Method Calibration", University of California, Berkeley, paper presented at Gordon Research Conference on Statistics, July 10, 1972.
RECEIVED for review March 30, 1978. Accepted August 16, 1978.
