Anal. Chem. 1995, 67, 3536-3540
Structure Determination of Polymeric Materials by Pyrolysis Gas Chromatography Frank Cheng-Yu Wang,* Bruce 6. Gerhart, and Patrick 6. Smith Analytical Sciences Laboratory, Michigan Division, The Dow Chemical Company, Midland, Michigan 48667
A pyrolysis gas chromatography method has been developed to investigate the microstructureof emulsion polymers. The number-average sequence length, which reflects the monomer arrangement in the polymer, was calculated using the proper formulas that incorporate the pure trimer peak intensities and hybrid trimer peak intensities. In this study, styrene and n-butyl acrylate copolymer systems were used to measure "the degree of structure" (Le., the number-average sequence length for styrene and n-butyl acrylate repeat units) and compared to a homogeneousnonstructured (or random) copolymer. The number-average sequence length information was further extended to calculate the composition. For the emulsion polymers examined in this study, the composition elucidatedfkom the number-averagesequence length matched the preparation recipe and/or what was measured by 13C NMR. The structure of a polymeric system usually can be determined by a spectroscopic method, such as NMR spectroscopy,' and by a structural degradation method, such as pyrolysis gas chromatography@-GC) .z Both methods have been developed for more than 20 years. There are many different polymeric systems that have been studied. Occasionally, the choice of which method to use mainly depends on which speci6c polymeric system is being studied and what kind of information needs to be generated. The number-average sequence length is most commonly used as an indicator of the degree of polymer structure. A structured copolymer, by our definition, contains regions within a polymer chain or polymer domain that are largely composed of one monomeric type. A nonstructured (or random) polymer would be one containing a completely homogeneous and random distribution of the monomeric types. The statistical method to calculate the number-average sequence length from the trimer distributions is well established.' The method is widely used in NMR spectroscopic analysis because NMR (especially 13CNMR) can supply triad molar fractions. In a copolymer system containing monomers A and B, the number-average sequence length of both monomers is expressed as
where AA and AB are the number-average sequence lengths of monomers A and B. NAAA,"+BAA, NBAB, NABA,NABB~BBA, and NBBBare the experimentally derived six distinguishable triad molar fractions or numbers of molecules. From the formulas above, if all six trimer molar fractions or numbers of molecules can be generated, the number-average sequence lengths of monomers A and B can be calculated. The higher value for the number-average sequence length implies a longer run of a particular monomer. Using a flame ionization detector in GC, the peak intensity is a direct reflection of concentration3 (molar fraction or number of molecules/unit volume). Thus, the peak intensity has been used as concentration or molar fraction. The number-average sequence length can be further used to calculate the mole percent of monomer in the copolymer. Utilization of the monomer-type molecular weights will allow for conversion to weight percent, which is the commonly used unit when emulsion polymers are designed and synthesized. Additionally, the weight percent of the monomers calculated from the number-average sequence length can serve as a check of the experimental procedure and the empirical determination. Similarities between these values assure the accuracy of the numberaverage sequence value for each monomer. Pyrolysis followed by gas chromatographic separation is a mechanism utilizing thermal energy to break down a polymeric structure to monomers and oligomers and separation of those units for quantitation. Because of the temperature limitations of the common silicone capillary column, only the dimer and trimers of the system studied here can be reliably separated and detected. The major mechanism of producing dimers and trimers with pyrolysis can be attributed to thermal degradation. A relatively small amount of dimers and trimers is formed as a result of a recombination of monomers. This mechanism is demonstrated as follows: Polymer
Pvolyair
-
Monomers
Dimers.Trlmers
The intensity of the various dimer and trimer peaks in a pyrolysis gas chromatogram will reflect the monomer sequence (1) Randall, J. C. Polymer Sequence Determination; Academic Press: New York, 1977; pp 41-69. (2) Jones, C. E. R; Reynold, G. E. J. BY. Polym. J. 1969,I , 197.
3536 Analytical Chemistry, Vol. 67, No. 19, October 1, 1995
(3) Irwin, W. J. Analytical Pyrolysis; Marcel Dekker, Inc.: New York, 1982; pp 135. 0003-2700/95/0367-3536$9.00/0 0 1995 American Chemical Society
and polymer structure when two conditions are met: (1) recombination contributes little to the pyrolysis products and (2) the formation of pyrolysis products is proportional to their existence in the copolymer. The literature4-14 discusses the determination of the structural sequence from dimer and trimer peak intensities. Since the probability of monomer recombination to trimers is considerably less than the same recombination to dimers, the determination of the number-averagesequence length will be little affected by peak intensities that result from inter- or intramolecular recombination. Thus, the use of all trimer peaks will provide more accurate and reliable results as compared to use of dimer peak intensities. Styrene and n-butyl acrylate emulsion polymers have been widely used in numerous coating and adhesive applications in the paper, paint, and construction industries. Mechanical and physical properties are an important aspect of the use of these polymers and can be governed by not only the composition but also by the structure of the polymeric molecules and the existence of homopolymer domains. Thus, the understanding of the relationship between mechanical, physical, and chemical properties becomes critically important. The polymer structure has a direct effect on such properties as modulus, glass transition temperature, film porosity, and minimum film formation temperature. By varying the monomer feed rates, initiator concentration, and chain transfer agent concentration, such properties as monomer sequence, molecular weight distribution, and particle size distribution can be affected. By knowing the monomer sequence in the final product, the emulsion polymer chemist will have a powerful tool for understanding to what degree an experimental parameter affects the polymer structure. There are several NMR spectroscopy studies dealing with determining the polymeric structure of n-butyl acrylate copolymer systems15J6 as well as n-butyl acrylate/styrene copolymer^^^ which are based on the same theory of determining the number-average sequence length from peak intensities corresponding to triads. However, the NMR structure determination becomes difftcult due to broadened peaks which result from high molecular weight. Pyrolysis gas chromatography of styrene (sTy)/alkyl acrylate systems have very clear and distinguishable trimer peaks that can be used in the determination of the number-average sequence length. The trimer peaks of STYIn-BA were identified by PyGC/MS in both electron ionization @I) and chemical ionization (CI) mode. The number-average sequence length was then calculated on the basis of eqs 1 and 2 and those trimer peak intensities. The weight percents of the monomers were eluci(4) Tsuge, S.; Ito, H.; Takeuchi, T. Bull. Chem. Sot. Jpn. 1970,17,3341. (5) Samamoto, Y.; Tsuge, S.; Takeuchi, T. Kobumhi Kagaku 1972,29(6), 407. (6) Alekseeva, IC V.; Khramova, L. P.; Solomatina, L. S. J. Chromatogr. 1973, 77 (l), 61. (7) Wallisch, IC L. J. Appl. Polym. Sci 1974,18,203. (8)Braun. D.; Canji. E. Angew. Makromol. Chem. 1974,36, 75. (9) Tsuge, S.; Hiramitsu, S.; Horibe, T.; Yamaoka, M.; Takeuchi, T. Macromolecules 1975,6,721. (10) Kalal, J.; Sevc, F.; Zachovel, J.; Kubat, J.J Polym. Sd., Polym. Lett Ed. 1979, 17,691. (11) Shimono, T.; Tanaka, M.; Shono, T. J. Anal. Appl. &olyis 1979,1, 77. (12) Blazso, M.; Varhegyi, G.; Jakab, E. J. Anal. Appl. pVrolysis 1980,2,177. (13) Tamaoka, A; Akihiro, I.; Ishida, Y.; Nishimurat, H. Asahi Garasu Kenkyu Hokaku 1987.37(2). 263. (14) Rao, M. R; Sebastian, T. V.; Radhakrishnan, T. S.; Ravindran, P. V.J. Appl. Polym. Sci. 1991,42,753. (15) Pichot, C.; Llauro, M. J. Polym. Sci., Polym. Lett Ed. 1981,42, 2619. (16) Brar, A S.; Sunita, S. C. V. V. Polymer 1993,34,3391. (17) Brar, A S.; Sunita, S. C. V. V. Polym. J. 1992,24,879.
dated. These values were compared with recipe amounts and 13C NMR results, when available. EXPERIMENTAL SECTION Sample Preparation. (1) Emulsion Polymer. All homogeneous emulsion polymers were synthesized in our laboratories. The homogeneous polymer was synthesized on the basis of procedures given in an experimental polymer textbook.18 The structured polymer was synthesized in a similar way except monomers were fed in series. (2) Pyrolysis-GCand pyrOlysis-GC/MS. Samples of emulsion polymer (-1.5 pg) were deposited on the Pt ribbon and equilibrated for 10 min in a 250 "C interface connected to the injection port of a HP5890 gas chromatograph equipped with a flame ionization detector (FID). Samples were pyrolyzed (CDS 120 Pyroprobe Pt coil) at a set temperature of 500 "C. The coil was heat to 500 "C with 20 "C/ms and hold at 500 "C for a 2@s interval. The pyrolysis products were split in the 250 "C injection port, with 10 psi head pressure, 50:l split ratio, separated on a fused-silica capillary column U&W DB-5, 30 m x 0.25 mm id., l.@pm film thickness) using a linear temperature program (40 "C/4 min, 20 "C/min, to 120 OCA0 min and then 20 "C/min ramp to 300 "C/23 min), and detected by a FID. In the GUMS system, the FID was replaced with a VG Trio-1 mass spectrometer. The output from GC was transferred through a transfer line (280 OC) to the ion source of the mass spectrometer. The electron ionization mass spectrum was obtained every second over the mass range of 29-500 amu. Methane was used as reagent gas in the chemical ionization mode. RESULTS AND DISCUSSION
The reproducibility of pyrolysis data is always a concern when the technique is applied to any kind of quantitative study. Table 1shows the average and relative standard deviation of a numberaverage sequence length calculation based on six consecutiveruns of a 48%/52%STY/n-BA emulsion copolymer. Most of the terms show a relative standard deviation below 1%, which demonstrates the reliability of the pyrolysis method applied to the analysis of STY/n-BA copolymer. Pyrolysis of an emulsion polymer is performed on the dried film. The liquid emulsion is heated in the pyrolysis chamber at 250 "C for 10 min and allowed to coalesce to a solid. A volitility experiment showed there were no detectable materials released during this period. For the homogeneous emulsion polymers where every latex particle has a t "composition and structure, the film formation process will not alter the copolymer structure. For the blend emulsion polymer, the film formation process will have a significant affect on the interface volume of the polymers. The latex G with 50%styrene particles and 50%n-butyl acrylates particles will have no interface volume in the emulsion state, but after coalesence, an interface volume of styrene and butyl acrylate polymer will form. The response of the FID detector is assumed equal for all three styrenecentered trimers and for all three n-butyl acrylatecentered trimers in this study. Essentially, FID is a carbon atom counter; any components having the same number of carbon atoms should have the same response. The styrenecentered trimers have 2224 carbon atoms; the difference in carbon atoms is less than f 5 % (18) Collins, E. A; Bares, J.; Billmeyer, F. W., Jr. Erpen'ments in PolymerSciences; John Wiley & Sons: New Sork, 1973; pp 337-345.
Analytical Chemistry, Vol. 67, No. 79, October 1, 1995
3537
SMS
~~
Table 1. Multiple Runs for Number-Average Sequence Length Determination in One Emulsion Polymer from Pyrolysis Gas Chromatography Method
H1
H2
H3
H4
H5
H6
av
0.136 0.288 0.576 1.39 0.052 0.343 0.605 1.29
0.129 0.288 0.583 1.38 0.051 0.348 0.602 1.29
0.136 0.291 0.573 1.39 0.049 0.343 0.608 1.28
0.142 0.284 0.574 1.40 0.051 0.348 0.601 1.29
0.132 0.284 0.584 1.38 0.055 0.348 0.597 1.30
0.150 0.289 0.561 1.42 0.054 0.346 0.599 1.29
0.14 0.29 0.58 1.39 0.05 0.35 0.60 1.29
RSD (%)
normalized peak intens
sss
SSB+BSS BSB N(S) BBB BB+SBB SBS N@) mol % S B
0.73 0.27 0.83 1.50 0.23 0.25 0.41 0.51
380
(%I
385
ago
'
ads
1
'
'
I
'ao
'
T
+os
'
,Io
' ~ ' ' I ' ' ~ ~ I " ' ' 11 ' ~5 ' ' ~"' " ,(e 410 .is d o 44s a!o
' " X
w's
"-
Rmlenllon Time (minuln)
52 48
52 48
52 48
52 48
52 48
52 48
0.52 0.28 0.48 0.28
47 53
46 54
47 53
47 53
46 54
47 53
47 53
48 52
48 52
48 52
48 52
48 52
48 52
48 52
42 39
42 40
43 39
43 40
42 40
44 40
42 40
2.95 2.30
2.90 2.29
2.94 2.28
3.00 2.29
2.93 2.31
3.03 2.31
2.96 5.00 2.30 1.27
wt%
S B std wt % S B grouped S B group N(S) N(B)
I""I""
0
0.28 0.28
Figure 2. Trimer area of 50%/50% STYIn-BA homogeneous emulsion polymer with trimer assignments, where Ba = n-butyl acrylate and S = styrene.
1
n
LateXH
0.83 0.41
n Latex c
BaBaS+SBaLia
I 3
l ' " ' 1 ~ " ' 1 " " l " ~ ' l ' " ' I ' ' ~ ' l " ' ' / ' ' ~ ' -0 $5 10 2e so 31 40
s
R*lmlion nm. (minuter)
"
9
L.bxB
1
4L
Figure 1. Typical pyrogram of a 50%/50% by weight STYIn-BA homogeneous emulsion polymer.
around those trimers. This fact makes the equal response assumption valid. The same argument also applied to It-butyl acrylatecentered trimers. The 500 OC pyrolysis temperature was chosen to obtain a higher yield of trimer for both styrene and n-butyl acrylate. Figure 1 shows the typical pyrogram of a 50%/50%by weight STY/n-BA homogeneous emulsion polymer. The identification of pure trimers was accomplished by comparing retention times with both 100%styrene polymer and 100%It-butyl acrylate polymer as well as Py-GC/MS in the electron ionization mode and chemical ionization mode. The identification and assignment of hybrid trimers (isomers) was accomplished by Py-GC/MS(ED, Py-GC/ MS(CI), interpretation of E1 spectra, and comparison made to similar structural compound spectra in the NIST library. Figure 2 shows an expansion of the trimer area of Figure 1 with trimer assignments. 3538 Analytical Chemistry, Vol. 67, No. 19, October 1, 1995
I
h Ratentionlime (minuter)
Figure 3. Pyrogram of the trimer area for five different compositions of STYhBA. The pure trimer peaks directly reflect the relative abundance of the monomers, where Ba = n-butyl acrylate and S = styrene.
Figure 3 shows the pyrogram of the trimer area for five different compositions of STY/n-BA The number-average sequence lengths were calculated for all five polymers. The peak areas were normalized on the basis of the summation of NS, NsBa+Bas, and NB&Ba equaling 1and the summation of N B ~ B ~ ~ ,
Table 2. Number-AverageSequence Length for Different Compositions of Homogeneous Emulsion Polymers from Pyrolysis Gas Chromatography Method
A
B
C
D
E
SSS SSB+BSS BSB
0.069 0.058 0.872
0.116 0.221 0.663
0.168 0.322 0.511
0.610 0.305 0.085
0.733 0.210 0.057
N(S) BBB BBS+SBB SBS
1.11
1.29
1.49
4.21
6.17
0.374 0.401 0.224
0.076 0.379 0.545
0.058 0.323 0.620
0.005 0.123 0.872
0.031 0.106 0.863
2.35
1.36
1.28
1.07
1.09
32 68
49 51
54 46
80 20
85 15
28 72
44 56
49 51
76 24
82 18
25 75
43 57
50 50
74a 26a
82 18
13 78
34 46
49 38
91 13
94 14
4.39 3.86
3.05 2.40
3.04 2.36
6.00 2.08
8.98 2.59
normalized peak intens
N(B) mol %
S B
expt wt % S B
std wt % S B grouped (%) S B
a
BaBaSISBaBa
Weight percentage determined by I3C NMR analysis.
ReteMlon Time (mlnutro)
Figure 4. Pyrogram of the trimer area for three different structures of STY/n-BA emulsion polymers all having the same 50/50 wt % composition, where Ba = n-butyl acrylate and S = styrene. ~~~~
NSB*+W~S,and NSSGequaling 1. All normalized peak areas were then used in eqs 1and 2 to calculate the number-average sequence length for both styrene and n-butyl acrylate. The mole percentage and weight percentage were also calculated to compare with the weight percentage obtained by preparation recipe or by the NMR technique. Table 2 lists results of the normalized peak intensities, the calculated number-average sequence length, and the calculated weight percent composition. All of the emulsion polymers (AE) were homogeneously polymerized and considered nonstructured (random). Thus, the number-average sequence length is strictly a reflection of the composition of the polymer. When the calculation is extended to obtain the monomer weight percent of the polymer, a match is made to that expected from the recipe or an NMR determination to within f3%.This assures the accuracy and precision of the number-average sequence length value. Figure 4 shows the pyrogram of the trimer area for three different structures of STYIn-BA emulsion polymers all having the same 50/50 wt % composition. The first polymer (G) consists of a physical blend of an all-styrene and an all-n-butyl acrylate emulsion. The second 0 consists of core/shell emulsion particles made with two distinct domains, a n-butyl acrylate core and a styrene shell. The third (C) is a homogeneous STYIn-BA polymer made by the simultaneous feed of the individual monomers. The significant differences in the pyrograms can be seen in the hybrid trimer peak intensities. These peak intensities decrease from the completely homogeneous polymer system (C) , to the core/shell particle structure with a finite interface (Fj, to the physical blend of particles with no distinct polymer interface (G). Table 3 shows the data for the three structured polymers (C, F, G) . The number-average sequence length strongly reflects the “structural differences” of the polymers, and concurrently, the
l
~
Table 3. Number-AverageSequence Length for the Same Composition but Different Structural Emulsion Polymers from Pyrolysis Gas Chromatography Method C
F
G
0.168 0.322 0.511 1.49
0.909 0.030 0.061 13.20
0.973 0.014 0.013 49.77
0.058 0.323 0.620
0.847 0.106 0.047
0.956 0.037 0.006
1.28
9.977
39.775
54 46 49 51
57 43 52 48
56 44 50 50
50 50
50 50
50 50
49 38
94 95
3.04 2.36
62.22 17.93
normalized peak intens
sss
SSB+BSS BSB N(S) BBB BBS+SBB SBS N(B) mol % S B S B std wt % S B grouped (%) S B
‘
99 99
grouped N(S) N(B)
143.47 53.08
weight percent monomer composition matches the composition based on monomer feed. This is a strong demonstration that the number-average sequence length can be used as a measure of the degree of structure and composition. Advances in emulsion polymerization technology have allowed the polymer chemist to synthesize the emulsion polymer with a degree of structure. Variation in the monomer feed sequence can produce emulsion polymers with the same composition but vastly different physical and mechanical properties as a result of Analytical Chemistty, Vol. 67, No. 19, October 1, 1995
3539
structural differences. A prime example is the comparison of the homogenous polymer and the monomer domain separated (core/ shell) polymer. The number-average sequence length can be an important measurement of the degree of structure. Additionally, it may be important to determine the percent of two or more of the same type of monomers bonded together. Such parameters are termed the percent of grouped monomers and the numberaverage sequence length of grouped monomers. For monomer A in the AB copolymer,these two terms are expressed as follows: % group of monomer A = (NM
+ ”+BAA)
x
100% or (1- NBAB) x 100% number-average sequence length of grouped monomers =
NAAA
+ ”+BAA
(1/2)”+BAA
For the emulsion polymers studied, the information pertaining to the percent of grouped monomers is shown in Tables 1and 2. Examination of the percent number of grouped monomers and the number-average sequence length of grouped monomers as calculated using the previous formulas reveals the relationship between the relative monomer concentration and the structure of the polymer chains. This is shown in Table 2, where the percent of grouped monomers varies with composition as one might expect. This mainly results from the relative concentration of the monomers under the homogeneous polymerization conditions. Table 3 values of percent of grouped monomers is even more dramatic. Here the variation is strictly related to the degree
3540 Analytical Chemistry, Vol. 67, No. 79, October 1, 7995
of structure in the emulsion polymer, not to composition. The differences are extreme as the homogeneous emulsion polymer (C) reveals 49% of the styrene as grouped, the core/shell 0 reveals 94%of the styrene as grouped, and the blend is what one would expect with 99%of the styrenes grouped. CONCLUSIONS
By applying the proper statistical formula and the data obtainable from pyrolysis gas chromatography, the numberaverage sequence length as well as the monomer composition of an emulsion copolymer can be explored. The structure of a copolymer of two monomeric types can be quantified by deriving the percent of grouped monomers and the number-averagelength of grouped monomers. This method could be extended to any copolymer system as long as all six trimer peaks can be identified and the peak intensities obtained by assuming that these intensities represent the polymer compositions. This method extends the capabilities of pyrolysis not only in the quantitative study of monomer composition but also in the realm of polymer structure investigation.
Received for review May 30, 1995. Accepted July 18,
1995.a AC950515M
@
Abstract published in Adounce ACS Absfracts, September 1, 1995.
