Anal. Chem. 1997, 69, 3791-3795
Composition and Microstructure of Acrylonitrile-Butadiene Copolymers by Pyrolysis-Photoionization Mass Spectrometry David. L. Zoller and Murray V. Johnston*
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716
The composition and microstructure of acrylonitrilebutadiene copolymers ranging from 19 to 51 wt % acrylonitrile were determined by pyrolysis-photoionization mass spectrometry. The high molecular weight samples chosen for this study (Mw > 100 000) are relatively insoluble and therefore difficult or impossible to quantitatively characterize by NMR. A simple model based upon the average number of acrylonitrile units in a group of oligomers having the same total number of monomers but variable amounts of acrylonitrile and butadiene was found to give an accurate and precise determination of the copolymer composition. In addition, Bernoullian and first-order Markovian models were used to interpret the mass spectral data. Although the copolymer compositions calculated from each model agreed quantitatively with the reported compositions, the Markovian model gave slightly more precise fits to the mass spectra. Number-average sequence lengths were also determined from the Markovian model. The acrylonitrile sequence lengths determined by pyrolysis-photoionization were larger than those reported previously for other copolymer samples. It is not known whether these differences reflect differences in the samples analyzed or artifacts of the analytical techniques. Many copolymers thermally degrade to produce, among other pyrolysates, oligomers representative of the copolymer chain. These oligomers can be analyzed by mass spectrometry to study the copolymer microstructure.1-3 However, interpretation of the mass spectra is often difficult owing to the complex distribution of ions formed. The numerous pyrolysates produced by thermal degradation are responsible for much of this complexity. Fragmentation of the pyrolysates during ionization in the mass spectrometer further complicates the mass spectra. “Soft” ionization techniques such as low-energy electron ionization,4 field ionization,5 and chemical ionization6 have been used to reduce fragmentation due to the ionization process. Although qualitative analysis of copolymer microstructure (e.g., distinguishing block (1) Schulten, H.-R.; Lattimer, R. Mass Spectrom. Rev. 1984, 3, 231-315. (2) Montaudo, G. Br. Polym. J. 1986, 18, 231-235. (3) Kyranos, J.; Vouros, P. J. Appl. Polym. Sci., Appl. Polym. Symp. 1989, 43, 211-240. (4) Hummel, D.; Dussel, H.; Czybulka, G.; Wenzel, N.; Holl, G. Spectrochim. Acta 1985, 41A, 279-290. (5) Schulten, H.-R.; Simmleit, N.; Muller, R. Anal. Chem. 1987, 59, 29032908. (6) Munson, B.; Shimuzu, Y. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 19912001. S0003-2700(97)00147-9 CCC: $14.00
© 1997 American Chemical Society
and random structures) is routinely performed using these techniques, quantitative analysis remains a formidable task. Recent studies in our laboratory indicate another soft ionization technique, vacuum ultraviolet (VUV) photoionization, is advantageous for microstructure characterization of copolymers. Pyrolysates are ionized by absorption of a single VUV photon with an energy just above the ionization threshold. Unlike field ionization, which may produce significant amounts of both protonated molecules (MH+) and radical cations (M•+) during ionization, photoionization results only in the formation of radical cations. Therefore, the ion distribution observed in the mass spectrum may match more closely the product distribution produced by thermal degradation. Qualitative sequence distributions of acrylonitrile-butadiene (AB) copolymers have been determined previously using NMR,7 pyrolysis-GC,8 and pyrolysis-mass spectrometry.9 Within these copolymer chains, the acrylonitrile units have been found to exist primarily in BAB triads when the acrylonitrile content is below 50 wt %. Pyrolysis-field ionization mass spectrometry10 has also been used to investigate the sequence of AB copolymers. We report here composition and microstructure characterization of AB copolymers by pyrolysis-photoionization mass spectrometry. The high molecular weight samples chosen for this study are insoluble and therefore could not be quantitatively characterized by NMR. Both Bernoullian and first-order Markovian models are used to interpret the mass spectral data, and the results are compared to reported values. EXPERIMENTAL SECTION Pyrolysis was performed directly in the source region of a reflectron time-of-flight (RETOF) mass spectrometer (R. M. Jordan, Co.) using an insertion probe (Vacumetrics, Ventura, CA) and temperature-programming system (Omega, Stamford, CT). In the experiments reported here, ∼1 mg of polymer sample was loaded into a pyrex vial and placed in the end of the insertion probe. Initially, the sample was heated just below the temperature of polymer degradation for 15 min in order to bake off additives within the polymer. Polymer thermal degradation was then performed by heating at a rate of 20 °C/min to a final temperature of 500 °C. The RETOF mass spectrometer had an open source design and was held at room temperature. As the samples were heated, gaseous products effused into the center of the source (7) Harwood, J. Rubber Chem. Technol. 1982, 55, 769-809. (8) Shimono, T.; Tanaka, M.; Shono, T. Anal. Chim. Acta 1978, 96, 359-365. (9) Pausch, J.; Lattimer, R.; Meuzelaar, H. Rubber Chem. Technol. 1984, 56, 1031-1044. (10) Schulten, H.-R.; Plage, B. Angew. Makromol. Chem. 1991, 184, 133-146.
Analytical Chemistry, Vol. 69, No. 18, September 15, 1997 3791
Scheme 1
region where they were photoionized. The distance from the end of the probe to the photoionization laser beam was 1.25 cm. Mass spectra (64k record length) were continuously recorded and averaged over 32 laser shots with a 500 MHz transient digitizer (Model 9846, Precision Instruments, Knoxville, TN) mounted in a personal computer. Individual spectra were typically recorded at 8 Hz although recent hardware upgrades have improved this to 100 Hz. Data analysis was performed using an averaged mass spectrum (640 laser shots) over the first 30 °C of polymer backbone degradation. Photoionization was performed with radiation derived from a Scanmate dye laser/frequency-doubler system (Lambda Physik, Inc., Acton, MA) pumped by the second harmonic of an Infinity 40-100 Nd:YAG laser (Coherent, Inc., Santa Clara, CA). Radiation at 354.6 nm was produced by frequency doubling the dye laser output at 709.2 nm. The near-ultraviolet radiation was frequency tripled in a phase-matched mixture of xenon and argon to yield VUV radiation at 118.2 nm (10.5 eV). Physical separation of the ultraviolet and VUV radiation was accomplished by off-axis focusing of the ultraviolet radiation into the frequency-tripling cell. Additional experimental details concerning VUV generation are discussed elsewhere.11 Polybutadiene (Mw ) 420 000), polyacrylonitrile (Mw ) 86 200), and acrylonitrile-butadiene copolymers with reported acrylonitrile contents of 19-22, 30-32, and 37-39 wt %, respectively, were obtained from Aldrich (Milwaukee, WI). An acrylonitrilebutadiene copolymer with a reported acrylonitrile content of 51 wt % was obtained from Scientific Polymer Products, Inc. (Ontario, NY). All of the acrylonitrile-butadiene copolymers had Mw > 100 000 and were synthesized using an emulsion polymerization.
Figure 1. Pyrolysis-photoionization mass spectra of the tetramer region for (a) polybutadiene and (b) polyacrylonitrile.
RESULTS AND DISCUSSION Polybutadiene and Polyacrylonitrile. Thermal degradation of polybutadiene12-14 is known to produce butadiene, vinylcyclohexene, and butadiene oligomers. Preferential cleavage of the polybutadiene chain occurs at β(C-C) bonds with respect to C-C double bonds along the polymer backbone. The radicals formed by bond breaking undergo secondary reaction by either disproportionation or intramolecular radical transfer mechanisms. Scheme
1 illustrates the generation of a trimer from the pyrolysis of transpolybutadiene. The processes denoted by -H• and +H• do not necessarily involve a hydrogen atom but rather indicate a secondary reaction which produces either a saturated or unsaturated end group. Three products are possible: an oligomer with two unsaturated ends (monoisotopic mass, 54n - 2 where n is the number of butadiene monomers), an oligomer with two saturated ends (54n + 2), and an oligomer containing both an unsaturated and a saturated end (54n). A detailed description of possible reaction pathways for these products is given elsewhere.15 Figure 1a shows the pyrolysis-photoionization mass spectrum of polybutadiene (Mw ) 420 000) in the vicinity of the tetramer. Analogous to Scheme 1, products at m/z 54n - 2, 54n, and 54n + 2 are observed. Three additional products at m/z 54n - 1, 54n + 1, and 54n + 3 correspond to molecules having one 13C substitution. Other ions in the mass spectrum correspond to more complex processes and exhibit very low peak areas. Distributions similar to Figure 1a are observed for oligomers up to 16 monomer units in length. The photoionization spectrum in Figure 1a is qualititatively similar to those obtained by field ionization.15,16 Photoionization mass spectra from five separate runs were obtained to quantitatively determine the relative amounts of the 54n - 2, 54n, and 54n + 2 products. The actual product ratio is dependent on the heating ramp used and oligomer length. In this and subsequent experiments, peak area ratios were averaged over the first 30 °C of polymer backbone degradation. After a slight correction for the isotopic contribution of the 54n product at the 54n + 2 monoisotopic mass, the relative ion signals for the 54n - 2 and 54n + 2 products were determined to be equal, within experimental error (SD ( 3%), at each oligomer length from 3 to 10 monomer units. The product ratio of 54n - 2 to 54n to 54n + 2 was approximately 1:3:1 for the trimer and tetramer series and approximately 1:2:1 for the pentamer through decamer series. Thermal degradation of polyacrylonitrile17-19 is known to produce ammonia, HCN, acrylonitrile, and acrylonitrile oligomers.
(11) Van Bramer, S.; Johnston, M. Appl. Spectrosc. 1992, 46, 255-261. (12) Golub, M.; Gargiulo, R. J. Polym. Sci., Polym. Lett. 1972, 10, 41-52. (13) Brazier, D.; Schwartz, N. J. Appl. Polym. Sci. 1978, 22, 113-124. (14) Radhakrishnan, S.; Rama Rao, M. J. Polym. Sci., Polym. Chem. Ed. 1981, 19, 3197-3208.
(15) Lattimer, R. J. Anal. Appl. Pyrolysis 1997, 39, 115-127. (16) Schulten, H.-R.; Plage, B.; Lattimer, R. Rubber Chem. Technol. 1989, 62, 698-708. (17) Ballisteri, A.; Foti, S.; Montaudo, G.; Scamporrino, E. Makromol. Chem. 1979, 180, 2935-2942.
3792 Analytical Chemistry, Vol. 69, No. 18, September 15, 1997
Figure 2. Pyrolysis-photoionization mass spectra of acrylonitrilebutadiene copolymer (a) additives at 200 °C and (b) pyrolysis products at 275 °C.
Figure 1b shows the pyrolysis-photoionization mass spectrum of a polyacrylonitrile sample (Mw ) 86 200) in the vicinity of the tetramer. Products corresponding to acrylonitrile oligomers with one saturated end plus one unsaturated end (monoisotopic mass, 53n, where n is the number of acrylonitrile monomers) and with two saturated ends (53n + 2) are observed in the mass spectrum with approximately equal peak areas. Products at 53n + 1 and 53n + 3 correspond to molecules containing a 13C or 15N substitution. Unlike polybutadiene, the product corresponding to an acrylonitrile unit with two unsaturated ends (53n - 2) has a very low peak area. Figure 1b is analogous to that obtained by field ionization,10 but the field ionization mass spectrum is complicated by the production of protonated oligomers as well. Acrylonitrile-Butadiene Copolymer. Four AB copolymers with different compositions and high molecular weights (Mw > 100 000) were analyzed using pyrolysis-photoionization mass spectrometry. At low temperatures, additives within the copolymer are evolved from the sample. Figure 2a shows a typical mass spectrum dominated by additives, in this case a 30-32% acrylonitrile copolymer at 200 °C. At higher temperatures, AB copolymers thermally degrade to produce, among other pyrolysates, oligomers of acrylonitrile and butadiene subunits. Figure 2b shows the pyrolysis-photoionization mass spectrum for the 30-32% acrylonitrile copolymer at 275 °C. As with polybutadiene or polyacrylonitrile, preferential cleavage occurs at the β(C-C) bond to either the nitrile bond in an acrylonitrile unit or the C-C double bond in a butadiene unit. This cleavage produces oligomers that are grouped according to the total number of monomers along the backbone. For example, the ions in group 5 of Figure 2b correspond to oligomers containing a total of five monomers. The mass of each oligomer in the group depends upon the relative number of acrylonitrile and butadiene units (53x + 54y, where x and y correspond to the number of (18) Mailhot, B.; Gardette, J. Polym. Deg. Stab. 1994, 44, 223-235. (19) Suzuki, M.; Wilkie, C. Polym. Deg. Stab. 1995, 47, 217-221.
Figure 3. Pyrolysis-photoionization mass spectra expanded in the octamer region for acrylonitrile-butadiene copolymers with reported compositions of (a) 19-22, (b) 30-32, (c) 37-39, and (d) 51 wt % acrylonitrile.
acrylonitrile and butadiene monomers, respectively, in the oligomer). Oligomers containing a greater number of acrylonitrile units appear on the low-mass side of the distribution while oligomers containing a greater number of butadiene units appear on the high-mass side of the distibution. Superimposed upon the distribution of oligomers within a group are products containing zero, one, or two unsaturated ends as indicated in Scheme 1 and Figure 1. Oligomer distributions up to the 11-mer are produced with high enough signal intensity to permit microstructure calculations. Figure 3 shows the octamer series for four different composition AB copolymers in the mass range from a pure acrylonitrile octamer with two unsaturated ends (m/z 422) to a pure butadiene unit with two saturated ends (m/z 434). As expected, the centroid of the distribution shifts to a lower m/z as the acrylonitrile content increases. Composition. The centroids of the oligomer distributions in Figures 2 and 3 may directly give quantitative compositional information if several conditions are met. First, consistent with the polybutadiene spectrum in Figure 1a, the AB copolymer is assumed to thermally degrade with equal probabilities of forming oligomers with two unsaturated ends (M - 2) and two saturated ends (M + 2). Although this assumption is inconsistent with the polyacrylonitrile spectrum in Figure 1b, the results presented below suggest that it is valid for AB copolymers, presumably because the butadiene content of the copolymers is high. Second, the probabilities for forming each oligomer in a series must only depend on the relative amount of each sequence within the copolymer chain. For example, the relative amounts of AA, AB, BA, and BB formed by pyrolysis must depend only upon the relative occurrence of these sequences in a copolymer chain. Third, the mass spectrum in the range for each oligomer series Analytical Chemistry, Vol. 69, No. 18, September 15, 1997
3793
Table 1. Copolymer Composition from Oligomer Centroid acrylonitrile, % reported
calculateda
19-22 30-32 37-39 51
21 ( 1 32 ( 1 38 ( 1 50 ( 1
a Calculated from the slopes of the lines in Figure 4. Uncertainties represent 95% confidence intervals from 10 runs.
Figure 4. Acrylonitrile content determined from the centroid of each oligomer series. Error bars reflect the 95% confidence intervals for 10 runs of each copolymer. Several data points are missing due to interference from additive peaks.
must be free from interference due to other pyrolysates and additives. Finally, the relative ionization and detection efficiencies of each oligomer within a series must be similar. In order to determine whether the oligomer distributions do indeed give accurate copolymer compositional information, a weighted mean mass peak (MPn) for each n-oligomer series was calculated using eq 1, where m is the m/z of a peak in the series 54n+2
∑
MPn )
(m)(aream)
m)53n-2
(1)
54n+2
∑
m
m)53n-2
and aream is the peak area at that m/z. The average number of acrylonitrile units for the n-oligomer series was then calculated using eq 2 where Ai is the monoisotopic mass of an acrylonitrile
av no. of A units )
(Bi)(n) - MPn Bi - Ai
(2)
unit and Bi is the monoisotopic mass of a butadiene unit. Figure 4 shows a plot of the average number of acrylonitrile units for each oligomer series from the 5-mer to the 11-mer for each of the copolymer samples. Error bars reflect the 95% confidence intervals. Several oligomer series were not included in Figure 4 owing to significant interference from additives. The linearity of these plots indicates that the four conditions in the preceding paragraph are met, and the slopes of the lines give the acrylonitrile mole fractions. The results, tabulated as weight percents in Table 1, show excellent agreement between the measured and reported compositions. It should be noted that the agreement between the measured and reported compositions is achieved without the use of any adjustable parameters in the data analysis procedure. 3794
Analytical Chemistry, Vol. 69, No. 18, September 15, 1997
Microstructure. Since oligomers having the same composition but different sequences of monomers are detected at the same m/z, statistical modeling20,21 must be used to determine the relative amounts of these sequences. The Bernoullian model assumes a completely random distribution throughout the copolymer chain dependent only on the relative amounts of each monomer. Application of the Bernoullian model to pyrolysis-photoionization requires the following steps. First, an initial value for the probability of finding an acrylonitrile unit in the copolymer chain, F(A), is guessed. The probability of finding a butadiene unit, F(B), is then given by 1 - F(A). For each oligomer series, the probabilities for producing each possible sequence of monomers is calculated from F(A) and F(B). The probabilities are then summed for all sequences containing the same number of acrylonitrile and butadiene units to find the relative product yields at each monoisotopic mass. (The monoisotopic mass is based upon 12C, 1H, and 14N and corresponds to an oligomer containing one saturated and one unsaturated end.) Next, the product yields at each monoisotopic mass are adjusted for the probability of 13C and/or 15N isotopic substitution. Finally, the isotopically corrected product yields are adjusted for the production of oligomers with two unsaturated ends or two saturated ends in addition to oligomers having one unsaturated and one saturated end. In this last step, the probabilities of forming oligomers containing zero (M - 2) and two (M + 2) saturated units are assumed to be equal as indicated in Figure 1a for polybutadiene. The average ratio of the M - 2, M, and M + 2 products for all oligomer series is given a single variable, X. (Recall that for polybutadiene, X varies between 2 and 3 depending upon oligomer length. The value of X determined by the procedure described below is typically between 2.1 and 2.3 for the copolymer samples.) After these adjustments are made, the calculated product distribution is compared to the measured ion distribution in the mass spectrum. Agreement factors between the two, given by AFn-series in eq 3, are calculated for each oligomer series and then the mean agreement factor is determined (where %mexpt is the normalized experimental peak area and %mmodel is the peak probability determined from the model). 54n+2
AFn-series )
∑
|%mexpt - %mmodel|
(3)
m)53n-2
The values of F(A) and X are then incremented until the mean agreement factor is minimized and the calculated product distribu(20) Montaudo, M.; Ballistreri, A.; Montaudo, G. Macromolecules 1991, 24, 5051-5057. (21) Montaudo, M.; Montaudo, G. Macromolecules 1992, 25, 4264-4280.
Table 2. Copolymer Composition from Bernoullian Model acrylonitrile, % reported calculateda 19-22 30-32 37-39 51 a
23 ( 1% 34 ( 1 38 ( 1 49 ( 1
agreement factor
ion current described by model, %
0.29 0.24 0.26 0.35
84.5 88.0 87.0 82.5
Uncertainties represent 95% confidence intervals from 10 runs.
Table 3. Copolymer Composition and Microstructure from First-Order Markovian Model ion current acrylonitrile, % agreement described a reported calculated factor by model, % 19-22 30-32 37-39 51 a
21 ( 1 32 ( 1 38 ( 1 50 ( 1
0.14 0.12 0.13 0.15
93.0 94.0 93.5 92.5
a
For each oligomer series, the probabilities for producing each possible sequence of monomers are calculated from PAA, PBB, PAB, and PBA. The probabilities are then summed for all sequences containing the same number of acrylonitrile and butadiene units to find the relative product yields at each monoisotopic mass. The product yields are then adjusted for isotopic substitution and the formation of products containing zero, one, or two unsaturated ends as described for the Benoullian model. The calculated product distribution is then compared to the measured ion distribution and agreement factors are determined. Once values of PAA and PBB are determined, the percent composition and number-average sequence lengths can be tabulated. Number-average sequence lengths are given by
NA ) (1 - PAA)-1
(4a)
NB ) (1 - PBB)-1
(4b)
a
NA
NB
1.3 ( 0.1 1.3 ( 0.2 1.7 ( 0.1 3.3 ( 0.2
4.8 ( 0.1 2.8 ( 0.3 2.8 ( 0.2 3.3 ( 0.2
Uncertainties represent 95% confidence intervals from 10 runs.
tion closely resembles the measured ion distribution. Table 2 shows the reported composition, calculated composition, mean agreement factor, and percentage of the ion current described by the best-fit model (calculated from the agreement factor) for 10 runs of each AB copolymer. Good agreement between the reported and calculated compositions is observed. The first-order Markovian model describes distributions where the sequence of monomers depends on both the relative amounts of each monomer and the reactivity of the monomers toward each other. Application of the first-order Markovian model to pyrolysisphotoionization requires the following steps. First, initial values for the probability of an acrylonitrile unit adding to an active copolymer chain ending with an acrylonitrile unit (PAA) and the probability of a butadiene unit adding to an active copolymer chain ending with a butadiene unit (PBB) are guessed. The probability of a butadiene unit adding to an active copolymer chain ending with an acrylonitrile unit is given by PAB ) 1 - PAA. The probability of an acrylonitrile unit adding to an active copolymer chain ending with a butadiene unit is given by PBA ) 1 - PBB. (22) Katritsky, A.; Weiss, D. J. Chem. Soc., Perkin Trans. 2 1974, 13, 15421547. (23) Anachkov, M.; Stefanova, R.; Rakovsky, S. Br. Polym. J. 1989, 21, 429432. (24) Fang, T. Macromolecules 1990, 23, 2145-2152. (25) Hill, D.; O’Donnell, J.; Perera, M. J. Polym. Sci., Polym. Chem. 1996, 34, 2439-2454.
Table 3 shows the reported composition, calculated composition, number-average sequence lengths, mean agreement factor, and percentage of the ion current described by the best-fit model (calculated from the agreement factor) for 10 runs of each copolymer sample. The percentage of the ion current described by the Markovian model represents a slight improvement over that described by the Bernoullian model. Significant differences in the number-average sequence lengths are observed for copolymers having low (19-22%), medium (30-32%, 37-39%), and high (51%) acrylonitrile contents. Due to the low solubility of these high molecular weight samples, quantitative NMR data could not be obtained for comparison. Previous NMR studies22-25 of other AB copolymer samples has shown acrylonitrile diads are virtually absent from copolymer samples with acrylonitrile contents below 35%. The number-average sequence lengths obtained using pyrolysis-photoionization indicate higher acrylonitrile diad levels than are typically obtained by NMR. It is not clear whether this difference is due to differences in the specific samples analyzed or to artifacts in the analytical procedures. Future work will explore this issue as well as applications to other copolymer systems. ACKNOWLEDGMENT This research was supported in part by the National Science Foundation, Grants CHE-9300644 and CHE-9629672. Received for review February 5, 1997. Accepted June 18, 1997.X AC970147H X
Abstract published in Advance ACS Abstracts, August 1, 1997.
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