2s
Anal. Chem. 1981, 53, 29-33 (8)
Schulten, H.R.; Beckey, H. D. Ofg, Mess Spectrom. 1973. 861-867. McEwen, C. N.; Bollnskl, H. G. Biomed. Mass Speckom. 1975, 2,
(10) (11)
Laughiin, R. G.; Mcaady. J., unpublished work. Schulten, H.R.; Beckey, H. D. Org. Mass Spectrom. 1972, 6, 885-895. High-temperature-actbated emitters were purchased from MMwest AnalyUcai Consukants, Inc., Champaign, IL. Linden, H. 6.; Winkler, H. U.; Beckey, H. D. J. phys. E . 1977, 70,
(9)
112-114.
(12) (13)
657-660.
D.; Schulten, H.R. Org. Mess Speckom. 1975, 70, 8 13-8 18. (15) Wood, G. W.; Oldenbug, E. J.; Lau, P. Y.; Wade, D. L. Can. J . Chem. 1978, 56, 1372-1377, (16) Roellgen, F. W.; Schulten, H A . Org. Mess Speckom. 1975, 70, (14)
Kuemmler,
660-668. (17) Rome, J. C.; Puo. G. Org. Mess Spectfom. 1977, 12, 26-32.
RECEIVEDfor review July
7,1980. Accepted October 2,1980.
Analysis of Styrene Polymers by ,Mass Spectrometry with Filament-Heated Evaporation Harold R. Udseth' and Lewis Friedman Department of Chemistry, Brookhaven National Laboratoty, Upton, New York 11973
A mass spectrometric study of moderate-sized polystyrene (nominal mol wt 2100) is reported. The polystyrene was evaporated from a probe filament heated at 1000 OC/s under both electron-impact (EI) and chemicai-lonlzatlon (CI)conditions. Under E1 conditions extenslve fragmentation and depoiymerizatlon were observed but nmers out to n = 11 were detected. Under CI conditions, nmers up to n = 27 were detected, and spectra that approximately reproduced the oligomer distribution were obtained. Temperature measurements and activation energies of evaporation rates for many of the ionic species were also measured.
Low molecular weight oligomers of polystyrene have been extensively studied by use of mass spectrometric techniques. Early attempts were made to identify complex involatile and thermally unstable materials by pyrolysis mass spectrometry (I) using simple heating techniques. This approach was continued by using laser heating and time-of-flight mass spectrometry (2), for rapid analysis of degradation products. More recently field desorption (FD) ionization techniques have been applied to the characterization of polystyrene oligomers separated by liquid chromatography (3). Matsuo (4) et al. have reported FD mass spectra of polystyrene samples with molecular weight distributions peaked at values of 600,2200,4000, and 8500. The FD ion source technique provides a valuable approach to the solution of the problem of obtaining m m spectra from thermally unstable involatile molecules. Recent work in chemical ionization ion sources with direct insertion sample probes and with FD emitters has shown that volatility enhancement of complex molecular systems can be achieved by interaction of the chemical ionization plasma and the sample on the probe or emitter surface (5-9). This volatility enhancement has been attributed to heterogeneous ion molecule reactions which give rise to species on the probe surface with lower activation energies of sublimation or desorption. Quantitative differences in activation energies of desorption of species responsible for the E1 and CI mass spectra of Pb(NOS)*have been observed (10). The purpose of this report is to present the result9 of a study of the mass spectra of samples of styrene polymer (mol w t 21001, obtained by rapid heating at rates of approximately loo0 O C / s . Smooth rhenium ribbon surface direct insertion 0003-2700/81/0353-0029$01.00/0
probes were used in both electron-impact and chemical-ionization sources. Rapid heating studies were made with measurement of both sample probe temperature and mass spectra. Polystyrene was selected as a relatively fragile involatile molecular system, capable of undergoing competitive thermal depolymerization during the desorption process. Quantitative determination of temperature dependences of rates of desorption were made to investigate the effect of rapid heating in the presence and absence of the CI plasma on competitive rates of polymer desorption.
EXPERIMENTAL SECTION Evaporation studies were carried out in both a chemical ionization and an electron impact ion source and in each case the products were extracted and analyzed by a computer-controlled quadrupole mass spectrometer. A DEC PDP 8/E computer was used to control the Extranuclear Laboratories Inc. quadrupole power supplies which operated an Extranuclear Laboratories Inc. 3/8 in. diameter quadrupale rod mass analyzer assembly. Studies below 1200 amu were made with an Extranuclear Laboratories Inc. power supply operating at a frequency of approximately 1.7 MHz. A special low-frequency (292 IrHz)supply capable of mass analysis in our experiments up to 70 OOO m u was used for the mass range of lOO0-4Wl amu. Details of the computer-controlled mass analysis and data acquisition system have been previously presented (10). Samples were evaporated from a rhenium ribbon filament that was heated at a rate of approximately loo0 O C / s . The spectra were taken at scanning rates of from 2 to 11amu/ms and scanned in steps ranging in width from 0.28 to 10 amu. The CI spectra were taken with argon and methane as reagent gases et a pressure of 0.1 torr. Temperature measurements of rates of sample evaporations were made by using the resistance of the rhenium as a thermometer. The probe filament was heated with a constant current rapid heating pulser. The voltage drop across the filament was measured and recorded in the computer during the course voltage to frequency converter. of the evaporation with a 5-m Accurate values for the change in resistance of the filament and correspondingly the change in temperature could thus be recorded for each 1.2-ms time period of the sample evaporation. The precision of sample probe temperature measurement is estimated at f3 K. The sample probe surface is not uniformly heated, with a considerable temperature gradient from the center to ends supported on heavy wire leads. Consequently, the absolute temperature of sample desorption is uncertain by a value larger than the precision of measurement. This uncertainty is estimated to be less than 25 K. Measurements of slopes of plota of log relative ion intensity vs. reciprocal absolute temperature depend on differences in temperature rather than absolute measurements 0 1980 American Chemlcal Society
30
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
and are not sensitive to the larger estimated systematic error in temperature that may be encountered in our experiments. Measurement of the temperature of desorption of tryptophane samples from a probe equipped with a thermocouple using single heating techniques and from a filament probe rapidly heated indicated an internal consistency of temperature measurement to better than h5 K. Corrections were made for the transmission of the quadrupole mass analyzer as a function of resolution by use of the equation (11,221
In (Trz/Trl) = a(R1 - Rz)
where R1 and Rz are values of the resolution and Trl and Tnare the respective values of ion transmission at these resolutions. The constant a was previously measured for this quadrupole (13)and a value of a = -0.0174 was obtained. The resolution at each peak in the mass spectrum was measured, and the resolution as a function of mass was used to obtain an approximate correction of relative ion intensities. The mass scale in the lower region of the spectra was calibrated by use of perfluorotributylamine as an internal standard. Higher masses were identified by extrapolation of the low mas calibration and by the assumption in the higher mass region, that in the very clean oligomer spectrum produced by argon CI, that the maximum and minimum masses were separated by a mass difference of 104n where n - 1was the number of equally spaced peaks between the peaks identified as min or m a . Absolute masses were derived by assigning the mass 104n + 58 to a peak where n could be unambiguously determined by independent calibration. The mass sequence 104n + 58 is expected and confirmed in independent studies ( 3 , 4 ) . This calibration of the mass scale facilitated interpretation of the methane CI spectra with attendant solvated ion peaks. Limitations in the resolution of the mass analysis system used indicate a potential error in mass assignment of approximately 4 amu at the higher masses observed in this work. The polystyrene sample used in this study was obtained from the Pressure Chemical Co. of Pittsburgh, PA. The polymer was produced by anionic butyllithium catalysis and a methanol termination step (14-26). Thus the oligomers have an m/e = 57 butyl group initiator, a proton terminator, and a narrow molecular weight distribution. The sample used was from lot no. 12b with a nominal molecular weight of 2100 and a weight distribution M , / M , of less than 1.10. The polystyrene was deposited on the sample probe from benzene solutions (1.0 nmol/wL) with approximately 0.7 nmol of polystyrene used per run. The probe surface was located approximately 0.5 cm from the center of the exit hole in the ion source in CI experiments. This position was not varied by more than a factor of 1 mm in the experiment covered. RESULTS AND DISCUSSION Mass spectra were taken under conditions of electron impact and methane and argon chemical ionization. The data generated in these rapid heating experiments include a set of mass spectra for each thermal desorption and independently determined temperature dependences of specific selected relative ion intensities. The spectra, taken as a function of time during sample heating, produced information on the composition of species desorbed during each stage of the sample probe heating process. Wide variations in relative ion intensities were observed. The spectra presented are composite, obtained by summing relative intensities of the respective ions. Such composite spectra would have been obtained if the rapid heating experiments had been carried out in a spectrograph using photoplate detection. The correlation between the temperature dependence of relative ion intensity and the time sequence of observation of respective ions is sufficiently good to permit omission of time resolved spectra with no significant loss of information. Thus in Figure 1 electron impact mass spectra are presented graphically. Numbers above selected ions indicate the temperature a t which counting rates of 500 counts/ms were observed. The numbers inside the parentheses give the number of styrene monomer units and the respective masses of the ion. Tem-
M4SS lomu1
Flgwe 1. Partial E1 spectrum of polystyrene. The sdid llnes are the nmer spectrum, and the numbers h parentheses give Rrst the nunber of monomer units present and second the assigned monoisotoplc mass. The numbers not in parentheses glve the evaporation temperature in kelvin. The dashed lines are fragment sequence peaks.
peratures of desorption were arbitrarily defined in terms of 500 counts/ms which corresponded to approximately twothirds of the maximum intensity of the relatively low abundance ions in Figure lb. Electron-Impact Mass Spectra. The electron-impact data in Figure 1 are divided into two molecular weight regions which overlap at the trimer ion at m l e = 370. The solid linea in the figure are used to indicate ions which consist of styrene monomer units and the butyl initiator, with masses given by the relation m = 104n 58. These ions are designated as the A oligomer sequence. Ions with values of n ranging from 1 to 11are found in the electron-impact mass spectrum. The A sequence constitutes the only ions observed above m l e = 780. The dashed lines represent ions, with massea that do not fit the A sequence, produced by thermal surface reactions and/or electron-impact decompositions. Sequential mass distributions can also be found in the dashed line spectra. For example there is a set of ions with masses, 291,395,499,603, and 707, which differ by the mass of a styrene monomer unit. The major focus of attention in this work is on the A sequence of ions and the information they provide on thermal desorp tion of polystyrene. Temperatures of desorption, shown in Table I show a gradual increase for A oligomer ions with increasing molecular weight. The observation of lower molecular weight ions at lower temperatures and during earlier stagea in the desorption is inconsistent with they hypothesis that these ions are electron-impact fragmentation products. Temperatures of desorption establish the lower molecular weight ions as primarily products of thermal depolymerization reactions. Low molecular weight fragment ions not part of the A sequence with massa at 325 and 351, respectively, are included in Table
+
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
Table I. Evaporation Temperatures and Activation Energies for Some of the Peaks in the E1 Spectrum mass, amu
kcal/mola
E* 9
temp,
266 325 351 370 474 578 682 786 890 994 1098 1202
51 17 17 41 26 27 26 27 29 21 18 18
353 650 633 352 36 2 388 410 425 44 8 465 478 505
5000
31
1
K
Estimated uncertainty 23 kcal/mol.
I. These ions, indicated by dashed lines in Figure la, appear late in the desorption process and at high enough temperatures to be either products of pyrolysis or electron-impact decompositions of the highest molecular weight A sequence oligomers detected. With the exception of these fragment ions, the correlation of increasing mass and temperature of desorption of the A sequence is clearly shown in Table I. The second column in Table I gives activation energies for the rate limiting processes responsible for the generation of the respective ions in the electron-impact spectrum. These activation energies are calculated from slopes of linear plots of log relative ion intensity w.reciprocal absolute temperature. Data used in these plots were limited to ion intensities measured prior to sample exhaustion as indicated by a decrease of relative ion intensity with increasing sample probe temperature. Samples were sufficient to cover the probe with roughly 100 molecular layers. Under these circumstances desorption does not deplete the concentration of molecules on the probe surface until the last layer is exposed. Consequently, the kinetics of desorption prior to sample exhaustion is that of a zero-order process in surface molecule concentration. No normalization is required for calculation of activation energies from the slope of a plot of log relative intensity vs. reciprocal absolute temperature. An additional assumption in the activation energy calculation is that relative ion intensities are directly proportional to concentration of neutral gaseous desorption products. Activation energies in Table I for A sequence oligomers are higher for the n = 2 and n = 3 oligomers, relatively constant for n = 4 through 8, and lowest for n = 9-11. If the respective rate limiting processes were desorption in this homologous series of oligomers, a gradual increase in activation energy with molecular weight would be expected. If oligomer ions were formed exclusively by electron-impact decomposition of the higher molecular weight polymers, then both activation energies and temperatures of desorption of the respective ions would be expected to be almost identical. The dimer and trimer ions have virtually the same temperatures of desorption, but the 10 kcal/mol difference in activation energy supporta the conclusion that these ions are formed in surface thermal depolymerization reactions at different rates. Differences in temperatures of desorption for oligomer ions with n = 3-8 support the argument that, in spite of similar activation energies, these oligomers are not for the most part produced from the same parent species by electron-impact dissociation. Sample plots of log relative intensity vs. 1 / T are given in Figure 2. A complex mechanism for the production of neutral species which are parents of the trimer ions is shown by the segmented plot of the temperature dependence of the relative intensity of the trimer ion. There is, a t lower temperatures, the slope that gives a 41 kcal/mol activation energy and a
c
\
501
\
20
\ I
lo 2 '6
2'7
2 8
I
29 IOOO/T
t I
x
L x 3 0
I
31
3 2
O K - [
Flgure 2. Evaporation plots from rapMly heated pdystyene obtained under E1 condltkns. The Xs are a plot of an evaporation of the 5mer and the Os are a plot of an evaporatlon of the 3mer.
higher temperature process that gives an activation energy lower than 27 kcal/mol or lower than that of the pentamer ion. Comparison of the spectra shown in Figure 1 with results of the electron-impact study of pyrolysis of polystyrene by Bradt, Mohler, and Dibeler (I), shows the effect of rapid heating on the pyrolysis4esorption process. Monomer ions were observed as the most abundant ions in the earlier work. Small yields of dimers and trimers and a trace of tetramers were reported. Figure l b shows that with more rapid sample heating much higher molecular weight species can be detected. With slower heating, depolymerization exhausts the sample before temperatures can be reached for desorption of higher molecular weight oligomers. The observation of the higher molecular weight oligomers under conditions of rapid heating with electron-impact ionization suggests that most of the decomposition observed by Bradt, Mohler, and Dibeler occurred prior to electron-impact ionization. Thermal degradation studies on polystyrene have yielded activation energies as low as 22 kcal/mol(I7). This value is close to activation energies observed for formation of oligomer ions n = 4 through n = 11. Thermogravimetric analysis (18) of styrene degradation has given activation energies foi depolymerization up to 60-77 kcal/mol. These relatively high activation energies are more consistent with our results on the more abundant dimer oligomers. The lower values of activation energies for styrene depolymerization were attributed to random rupture of weak links in initial rate studies while the higher activation energies in the thermogravimetric analysis were taken as more representative values for the bulk degradation of the polymer. Chemical-IonizationM a s s Spectra. Chemical-ionization mass spectra taken with methane and argon reagent gases are presented in Figures 3 and 4, respectively. These spectra were taken to determine the effect of contact of a weakly ionized plasma with the heated sample surface on the competitive depolymerization and desorption processes. The possibility of chemical interactions of neutral reagent gases with the sample is, in the case of methane, remote. With argon, neutral moleculesurface chemical interactions can be eliminated from
32
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
Table 11. Evaporation Temperatures and Activation Energies for Some of the Peaks in the Argon C I Spectrum E,, kcal/mola mass, temp, amu initial final K
100 >.
k W
z W
-
5
50
500
1000
1500
2000
2500
3000
M A S S lamu)
Flgure 3. Partial argon C I spectrum of polystyrene. The solid lines are the nmer spectrum and the dashed lines are intermedhte fragment peaks. The f m t for the numbers Is the same as that used In Figure
1.
consideration. The methane CI spectrum was scanned from the mass of the monomer in the A sequence of ions up to mass 3000. With argon the lower molecular weight region was not investigated. The lowest oligomer shown in the argon CI spectrum is the tetramer, m / e = 473. The CI spectra were taken with relative low resolution, f 4 mass unit, in the mass range between 500 and 3000 amu. Resolution limitations are reflected by presentation of data as monoisotopic mass spectra. For ions containing on the order of 100 or more carbon atoms with approximately 1.1% 13C, ions having one or more 13C atoms would be the most abundant species in a polyisotopic spectrum. Higher molecular weight ions were not sufficiently well characterized, with respect to mass, to establish the A sequence ions in the methane CI spectrum as protonated species. Nor were the solvated A sequence ions, indicated by the dashed lines in the higher molecular weight region of the methane CI spectrum, identified precisely. The addition of a two or three carbon, hydrocarbon, fragment was established. The graphical data in Figure 3 and 4 accurately reflect relative intensities of oligomer and fragment ion peaks but are schematic in that they are presented as monoisotopic masses. The methane and argon CI spectra both show higher molecular weight species not seen under similar conditions of sample heating with electron-impact ionization. Temperatures of desorption presented for all of the A sequence ions in the argon spectrum are quite similar to temperatures observed in the electron-impact source. Some small differences may be noted but there is no conclusive evidence for volatility enhancement in which the argon CI plasma in contact with the sample gives rise to lower temperatures of desorption. Temperatures of desorption for selected ions in the methane CI spectrum are slightly higher than those found for electron impact or argon chemical ionization. In both CI spectra the correlation of increasing temperature of desorption with increasing ion molecular weight for the A sequence oligomer ions is maintained. Here again the lower molecular weight ions appear earlier in the sample heating process and cannot be taken as products of exothermic gas-phase ionic decomposition reactions. Such reactions might be expected with argon because of the relatively large difference between the recombination energy of argon ions and estimated ionization potentials of styrene polymers (approximately 4 or 5 eV). In view of this potential exothermicity, the argon CI spectrum is unusually clean and free of fragmentation prod-
474 57 8 682 786 890 994 1098 1202 1306 1410 1514 1618 1722 1826 1930 2034 2138 2242 2346 2450 2554 2658 2762 2866
32 32 34 36 35 37 45 50 49 49 62 62 49 47 44 63 52 36 38 43 38 28 35 37
48 55 41 42 40 42 56 45 52 65 64 64
37 3 39 5 40 7 420 436 450 472 483 483 488 493 49 8 51 2 521 526 532 535 540 546 552 565 568 575 577
a Estimated uncertainty * 3 kcal/mol through mass 1618 and * 6 kcal/mol for higher masses.
Table 111. Evaporation Temperatures and Activation Energies for Some of the Peaks in the Methane C I Spectrum activation mass, energy evaporation E,, kcala temp, K amu 579 607 633 1203 1224 1253 1619 1665 2139 2659
22 24 20 31 32 33 44 42 51 51
44 6 47 6 476 532 521 532 526 535 562 573
Estimated uncertainty 3 kcal/mol. ucts. These results may be explained by a very efficient collisional deexcitation of oligomer ions by neutral argon in the CI source or by deexcitation of products of heterogeneous ion-molecule reactions on the surface of the sample probe. The only evidence in the maas spectrum for a somewhat more gentle ionization with methane than argon is the shift to lower molecular weight in the maximum of the argon spectrum. With argon CI, the n = 19 oligomer is the most abundant high molecular weight ion. In the methane CI spectrum the most abundant ion in this mass region has 22 styrene monomer units. In contrast to the values found in the electron-impact mass spectrum, activation energies for the CI spectra tend to increase with increasing molecular weights. Unfortunately comparisons are limited because many of the ions detected in the CI spectra are not seen under electron-impact conditions. With argon CI one can compare the nmers with n = 4 through 8 and see a systematic 6-9 kcal increase in activation energy with argon chemical ionization. However, with higher molecular weight oligomers n = 9-11 the argon activation energies are 16-32 kcal/mol larger than those observed with
33
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
M A S S lornu)
Flgure 4. Partial methane C I spectrum of polystyrene. The sdid llnes are the nmer sequence and the dashed lines are sdvatbn OT fragment peaks. The format for the numbers is the same as that used in Figure 1.
the same mass ions under electron-impact ionization conditions. Temperatures of desorption and activation energies are presented for the CI experiments in Tables I1 and 111. Two sets of activation energies are given for the argon CI studies, because with higher molecular weight ions, plots of log intensity vs. 1/T gave segmented lines which were resolvable into low- and high-temperature processes. This phenomenon was not observed in the methane CI studies but was reproducible in the argon experiments. Comparison of the data on selected A sequence oligomers shows differences in activation energy for rate-limiting processes leading to desorption in the argon and methane CI spectra. For example, for the n = 15 ion, the respective activation energies are 62 kcal/mol with argon CI and 44 kcal/mol with methane CI. No difference was found for the activation energy of the n = 20 oligomers in the methane spectra and during the hightemperature part of the argon spectra. An 18 kcal/mol difference is noted for the n = 11oligomer ion at mass 1202 in the argon spectrum and the assigned mass of 1203 in the methane CI spectrum. These data all indicate a significant interaction of reagent ions with molecules on the surface of the sample probe during the sample heating process. There is evidence in the lower molecular weight portion of the methane CI spectrum of production of relatively large yields of lower molecular weight depolymerization products. The major differences in spectra, temperatures of desorption, and activation energies found in the E1 and CI studies suggest that sample exhaustion via low-temperature depolymerization reactions was in part inhibited by interaction of gaseous ions with species on the surface of the sample probe. This inhibition could have been an ionic catalysis of recombination or polymerization reactions that reduce the concentration of lower molecular weight oligomers which desorb at lower temperatures. The lower activation energies in the E1 spectra, particularly for nmers 9-11, may be the result of the failure of the basic assumption of zero-order kinetics for their desorption. These oligomers are detected at temperatures well above those re-
quired for desorption of 99% of the sample and are produced in the desorption of the last few layers of sample. Under these circumstances sample depletion can play an important role in giving activation energies that are too low. Correction for this type of error is difficult. Suffice it to say that activation energies for ions produced after most of the sample has been desorbed are probably lower limits of values of activation energies of desorption and are useful only to establish the presence or absence of genetic relationships between these ions and lower molecular weight species. The result of the interaction of the gaseous ions in the CI source with the solid sample surface is to facilitate observation of spectra which can be well correlated with the structure of the solid polymer. If one assumes that the lower molecular weight oligomers of the A sequence, with values of n = 1-3, are primarily thermal depolymerization products, then the remaining mass spectrum can be used to calculate average molecular weights of the polymer on the probe surface. Number and weight average molecular weights are calculated from the methane CI spectrum to be 1987 and 2143, respectively, on the basis of the latter assumption. Values of M,, and M , calculated from the argon CI spectrum are 1800 and 1970, respectively. The ratios of M,/M,, in the argon and methane CI spectra are 1.10 and 1.08, respectively, in good agreement with the value obtained from the Pressure Chemical Co. for this sample of polystyrene. The nominal molecular weight of the polystyrene used in this work, given by this supplier, was 2100.
LITERATURE CITED (1) . . Bra&. P.: Dibek.. V. H.:. Mohler. F. L. J. Res.
MU.Bu. Sfand. ( U S . ) .
I
1953;50, 201.
(2) Cobff, S. 0.;Vanderborgh, N. E. AM/. Chem. 1973. 45,1507. (3) Lattimer, R. P.; Harmon, D. J.; Welch, K. R. AM/. Cham. 1970, 57. 1293. (4) Matsw. T.; Metsuda, H.; Katakuse, I. AM/. Chem. 1970,51, 1329. (5) Holland, J. F.; Soltmann, B.; Sweeley, C. C. 8iomed. Mass Spectrom. 1976,3. 340. (8) Hansen, G.; Munson, B. Anal. Chem. 1078,50, 1130. (7) Hunt. D. F.; Shaban~witr.J.; Bok, F. K.; Brent, D. AM/. Chem. 1977, 49, 1160. (8) Candl, D. I.; DzMk, I.; Homing, M. G.; Montgomery, F. E.; Nowlln, J. G.; SBken, R. N.; Thenot, J. P.; Homing, E. C. A M I . Chem. 1079,51, 1858. (9) Nowlin, J. G.; Candl, D. 1.; Dridic, I.; Horning, M. 0.; StlllweH, R. N.; h n i n g , E. C. AM/. Left. 1979, 72, 573. (10) Radus, T. P.: Udseth, H. R.; Friedman, L. J. f h y s . Chem. 1970, 83, 2969. (11) Vasile, M. J.; Jones, G. R.; Falconer, W. E. Int. J . Mass Spectrom. Ion phys. 1972, 70,457. (12) BNbaker, W. M. A&. Mass Specfrom. 1968, 4,293. (13) Gaffney, J. S.; Pierce, R. C.; FrWman, L. Int. J. Mass Smctrm. Ion phys. 1977, 25,439. (14) Abres, T., Jr.; Wymon, D. P.; Allen, V. R. J . folym. Sci., Part A iB64. 2. . ... , 4533. . .. . (15) Jentoft, R. E.; Quw, T. H. J . folym. Sci., Part 8 1960, 7,811. (18) Kato, Y.; Kklo, S.; Watanabe, H.; Yamamoto, M.; Hashlmoto, T. J. Appl. Polym. Scl. 1975, 19, 829. (17) J e W , H. H. G. J. folym. Scl. 1048,3, 850. (18) Fwss, R. M.; Salyer, I. 0.; Wllson, H. S. J . folym. Scl., f e r t A 1964, 2, 3147.
Received for review March 31, 1980. Accepted October 9, 1980. Research carried out under contract with the U.S. Department of Energy and supported by its Office of Basic Energy Sciences.
