Anal. Chem. 1984, 56, 2762-2769
2762
tensity is recorded by using thermospray, along with monovalent ions formed by decomposition and by electron attachment.
CONCLUSIONS In the spectra compared in this study, the thermospray technique appears to produce more modes of fragmentation, more abundant fragment ions, and less abundant molecular ion species than does fast atom bombardment. When the same volatile buffer is used, the nature and ease of formation of positive ions varies with the basicity of the sample. These trends are analogous in both techniques and probably reflect the basicity of the sample relative to ammonia in the buffer. Although this behavior is analogous to that observed in chemical ionization, formation of ions in solution cannot be excluded by the present experiments. It should be pointed out that the presence of ammonium acetate produces a very different FAB spectrum, e.g., of p-nitrophenol glucuronide, than is recorded using plain thioglycerol (24). Finally, some of the thermospray ions are formed by reactions in the solution, e.g., thermally catalyzed acetylsis or ammonialysis. Organic anions can be readily analyzed by both techniques. Cluster ions are detected with both, having similar relative abundances, even while the factors which determine their relative abundances are a t present unknown. The facile production of divalent organic cations in thermospray but not in fast atom bombardment reveals a difference in the physical basis of the two processes, consistent with current understanding of the mechanisms operating for removal of ions from solution. LITERATURE CITED (1) Blakley, C. R.; Vestal, M. L. Anal. Chem. 1983, 55,750. (2) Barber, M.; Bordoli, R. S.; Elliott, G. J.; Sedgwlck, R. D.; Tyler, A. N. Anal. Chem. 1982, 54,645A.
Simons, D. S.; Colby, B. N.; Evans, C. H. Int. J. Mass Spectrom. I o n Phvs. 1974. 15. 291. Iribarne, J. V.; Thornson, B. A. J. Chem. Phys. 1976, 64, 2287. Giessman, U.; Rollgen, F. W. Int. J. rclass Spectrom. Ion Phys. 1981, 30, 267. Williams, D. H.; Bradley, C.; Bojeson, G.; Santikarn, S.; Taylor, L. C. E. J. Am. Chem. SOC. 1981, 103, 5700. Caprloli, R. M. Anal. Chem. 1983, 55, 2387. Fenselau, C.; Cotter, R. J.; Heller, D.; Yergey, J. J. ChromatoQr. 1983, 971. , ? -. -. I
Liberato, D. J.; Fenselau, C. C.; Vestal, M. L.; Yergey, A. L. Anal. Chem. 1983, 55, 1741. Yergey, A. L.; Liberato, D. J.; Millington, D. S. Anal. Biochem. 1984, 139 278
Cotter, R. J.; Fenselau, C. Biomed. Mass Spectrom. 1979, 6, 287. Rosenstock, H. M.; Draxl K.; Shiner, 6 . W.; Herron, J. T. "Energetics of Gaseous Ions;" J. Phys. Chem. Ref. Data, Suppl. 1977, 6, 1. Van Breemen, R. B.; Tabet. J.-C.; Cotter, R. J. Biomed. Mass Spectrom. 1984, 1 1 , 278. Harrison, A. G. "Chemical Ionlzation Mass Spectrometry"; CRC Press: Boca Raton, FL, 1982. Vestal, M. I n "Ion Formation from Organic Solids-IFOS 11"; Benninghoven, A. Ed.; Sprlnger-Velag: Berlin, 1983. Fenselau, C. I n "Ion Formatlon from Organic Solids-IFOS 11"; Benninghoven, A., Ed.; Springer-Verlag: Berlin, 1983. Fenselau, C. J. Net. Prod. 1984, 47, 215. Townsend, R. R.; Heller, D. N.; Fenselau, C. C.; Lee, Y. C. J. Biol. Chem., in press. Dell, A.; Ballou, C. E. Biomed. Mass Spectrom. 1983, IO, 50. Heller, D. N.; Fenselau, C.; Yergey, J.; Cotter, R. J. Anal. Chem., In press. Heller, D. N.; Yergey, J.; Cotter, R. J. Anal. Chem. 1983, 55, 1310. Chan, K. W. S.; Cook, K. D. Anal. Chem. 1983, 55, 1306. Ryan, T. M.; Day, R. J.; Cooks, R. G. Anal. Chem. 1980, 52, 2054. Fenselau, C.; Yelle, L.; Stognlew, M.; Liberato, D.; Lehman, J.; Feng, P.; Colvin, M., Jr. I n t . J. Mass Spectrom. Ion Phys. l98gV46, 411.
RECEIVED for review May 14,1984. Accepted July 26,1984. This research was supported in part by Grants PCM 82-09954 from the National Science Foundation and ST32CA09243 from the National Cancer Institute. FAB spectra were obtained at the Middle Atlantic Mass Spectrometry Facility, an NSF shared instrumentation facility.
Laser Mass Spectrometry of Poly(fluoroethy1enes) David E. Mattern, Fu-Tyan Lin, and David M. Hercules* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
Laser mass spectrometry (LMS) was applled to the analysis of poiy(fiuoroethy1enes) for the first tlme. The polymers investlgated ranged from poly(v1nyl fluoride) to poly(tetrafluoroethylene). A fragmentation mechanism common to each fluoropolymer yielded structurally relevant Ions Indicative of the orlentation of monomer unlts wlthln the polymer chaln. A unique set of structural fragments distingulshes the positive ion spectra of each homopolymer, ailowlng for Identtflcation. A quantitatlve study of the structural fragments formed from four poly(vlnyl1dene fluorlde) samples allowed determination of percent backward addition of monomer units within each sample. The results compared favorably wlth those obtalned from "F NMR spectroscopy. The appllcabillty of LMS to poly(chioroethyiene) analysis was also addressed.
The poly(fluoroethy1enes) are an important class of polymers due to the beneficial physical characteristics imparted by fluorine atoms. Chemical resistance and thermal stability are two characteristic properties which account for the extensive use of fluoropolymers in coatings. Analytical tech-
niques which can rapidly and accurately distinguish between the homopolymers are required since properties depend on the overall degree of fluorination. In addition to the degree of fluorination, the orientation of monomer units within a homopolymer chain also can have a significant effect on physical properties. A rapidly growing area of interest is plasma and ion beam induced fluqrination of polymer surfaces (1,2). Molecular information about the surface coating following controlled fluorination is essential to predict physical characteristics of the coating. Three analytical techniques which have been used to characterize poly(fluoroethy1enes) are ESCA, IR, and NMR spectroscopy. Each technique has an inherent advantage for solving one of the problems addressed above (i.e., distinguishing the homopolymerg, the orientation of monomer units, and surface analysis), but none is ideal for all applications. The well-defined chemical shifts of C 1s photoelectrons in a fluorinated environment are sufficiently large for fluoropolymer identification via ESCA (3). Unfortunately, charging effects common to organic polymers limit the ability of ESCA to probe molecular chain structure, because charging severely decreases spectral resolution. However, a growing application
0 1984 American Chemical Soclety 0003-2700/84/0356-2762$01.50/0
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
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Table I. Polymers Analyzed polymer poly(viny1 fluoride) poly(viny1idene fluoride) A B C D
poly(trifluoroethy1ene) poly(tetrafluoroethy1ene) poly(chlorotrifluoroethy1ene) A
acronym PVF PVFz
PVF:, PTFE PCTFE
B poly(viny1idene chloride/acrylonitrile) for ESCA is surface analysis of fluorine plasma treated materials (1,4). Careful interpretation of IR spectra can result in the identification of poly(fluoroethy1enes) and can provide fairly accurate determination of molecular orientation (5). The same type of structural information has been obtained by I9F NMR analysis (6). An inherent disadvantage of each of the above techniques is their rather stringent sample preparation requirements. Sample dimensions on the centimeter scale are required for most ESCA probes and contaminant-free surfaces are crucial. Transmission TR necessitates casting of thin films or preparation of pressed pellets; difficulties in obtaining optical contact between fluorinated surfaces and ATR cells have hindered internal reflectance IR (7). NMR spectroscopy requires reasonably soluble polymers, eliminating its application to highly fluorinated materials. Another technique which shows promise as an analytical tool for fluoropolymer analysis is mass spectrometry. Mass spectrometry has been used to characterize poly(tetrafluoroethylene) but has not been applied to the complete fluoroethylene series (8,9).Laser mass spectrometry using the laser microprobe mass analyzer (LAMMA-1000), which is manufactured by Leybold-Heraeus, offers several advantages: spatial resolution (1pm). However, Zn+ peaks were observed in the liquid-state PCTFE spectra. The relative ion intensities listed in the tables were obtained from the average of 10 or more individual spectra for each polymer. LAMMA peak intensities were reproducible to &5% RSD. Some mass spectra included in this paper are not representative of the average but were normalized to a particular peak intensity to highlight peaks of interest. The tabulated intensities are truly representative of each sample and these will be referenced throughout the paper.
RESULTS AND DISCUSSION Fluoropolymers: Positive Ion Spectra. Common Fragment Ions. This section will deal with those ions which are common to two or more polymers and thus are not useful for distinguishing between them. All common ions and their respective relative intensities are listed for each polymer in Table 11. The base peak for all fluoropolymers is CF+ at m/z 31 and this ion will be used to reference the relative intensities of other fragments. Similarly, chloropolymers yield CCl+ a t mlz 47 and 49; their detection in the PVF, sample indicates the presence of a chlorine-containing contaminant. Another common ion is CF3+at m / z 69 which is present in all spectra except for PVF. The absence in PVF suggests that the ion forms from a perfluoromethylene precursor. Additional common fragments include contaminants (Le., Na+, K+), carbon-fluorine and carbon-hydrogen cluster ions. The carbon-hydrogen ions are of the general form CnHm+,where n = 1-18 and m = 0-3. These C-H cluster ions are deleted from Table I1 for brevity, as are the structurally relevant ions which will be discussed below. Structural Fragment Ions. In addition to the common ions listed in Table 11, LAMMA positive ion spectra yielded structurally relevant fragments that are characteristic of each poly(fluoroethy1ene). These fragments are indicative of the way in which monomer units combined during the polymerization process. They could ideally be used for quantifying the various possible types of linkages between monomer units. Uniformity of monomer linkages along the molecular chain can affect a polymer's physical properties, i.e., crystallinity; thus their detection is crucial. The following section will discuss the various linkage types and their associated terminology, the origin of structural fragments, characteristic structural ions for each sample, and percent backward addition calculations for PVF2. Most polymers formed from vinyl precursors polymerize by the free radical route and tend to orient monomer units
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
Table 11. Common Fragments in the Positive-Ion Laser Mass Spectra of Fluoropolymers fragment
mlz
PVF
PVF2
PVF,
PTFE
PCTFE A
PCTFE B
CF' CC1' CF3+ CF< CFC12'
31 47,49 69 85, a7 101, 103, 105
100
100
100
100
21
5 17
57
100 11 1 63 31
100 12 41 73 19
C3F' CdF+ CEF' CGF' CTF' CBF' CgF'
55 67 79 91 103 115 127
18 1 19 1 10 1 5
7
10
12 1 9 1 5
9
C8,'
93
5
a
19
370 3 10 8 16 3 3
2 1
-A
-B
-H -CH2CF2CH=CF2
+ -CH2*
41
15
+
-F CHyCFCH2CP2-
+b cleavage
CF~SCH-CF~(+)
(+)CH2CF=CH2
CF2-
-CH2
(+)CF2CH=CF2
CH2=CFCH2(+)
CF2-
Figure 1. Structural ion fragmentation mechanisms for poly(fluoroethy1enes).
in a normal or head-to-tail (H-T) fashion. An example of a PVF2 molecular chain is
P-CHzCFz*
t CHz=CFz
-
Fl
P-CH~CF~CHZCF~*
However, due to the reaction temperature used and/or normal statistical probability, some backward additions occur producing head-to-head (H-H) linkages, i.e., -CH2CF2CF2CH2-. Monomer addition subsequent to a H-H type linkage has been proven to occur via a tail-to-tail (T-T) linkage by lgFNMR (11). A PVFz chain incorporating a backward addition would be as follows: H-T
H-H
T-T
H-T
nnnn
-CH~CF~CH~CF~CF~CH~CHZCF~CH~CF~-
Normal polymerization conditions produce molecular chains containing less than 20% backward additions. Laser irradiation of poly(fluoroethy1ene) films was found to form fragments which correspond to the linkage type of the particular polymer. These ions are of the general form C3H,,F,+ where n + m = 5; they are probably allyl cations due to resonance stabilization; [CH2-CF-CH2]+. Two possible mechanisms for formation of these cations are shown in Figure 1. Mechanism A involves chain scission followed by the loss of hydrogen and/or fluorine anions resulting in odd-electron chain ends. These ionized chain ends are known to rapidly undergo cleavage to produce the desired three-chon ions (12). Mechanism B, which would be energetically preferred under LMS conditions, differs from A in that hydrogen and/or fluorine atoms are proposed as leaving groups resulting in
allyl-type chain ends. Cleavage of the carbon-carbon bond fl to the double bond is known to be favorable and has been confirmed by thermal degradation studies (13). Two processes competing with formation of three-carbon ions are dehydrofluorination and depolymerization. The former is most pronounced for PVF because of its lower thermal stability and is of decreasing importance as the degree of fluorination increases. Dehydrofluorination is analogous to dehydrochlorination (to be discussed later) and involves molecular elimination of HF resulting in double-bond formation along the polymer chain. Ions are not directly produced by dehydrofluorination, but the process hinders structural ion formation since mechanisms A and B require linear and saturated polymer segments for structural ion formation, Figure 1. The unsaturated segments resulting from dehydrofluorination cannot participate in fragmentation mechaniims A and B and thus restrict structural ion formation and detection. Depolymerization of poly(fluoroethy1enes) involves unzipping of polymers into monomer units via a free radical mechanism. Mass spectrometry can monitor depolymerization through detection of ionized monomer units. The depolymerization mechanism was most prevalent in PCTFE and PTFE, a result confirmed by thermal degradation studies (13).
Poly(viny1 fluoride) (PVF). The positive ion spectrum of PVF is shown in Figure 2 and the structurally significant fragments are listed in Figure 3. The relative intensities of the structural fragments (CF+ = 100) are listed for each polymer in Table 111. As mentioned previously, dehydrofluorination seems to be the dominant fragmentation process for PVF. Evidence for this mechanism is the low relative intensities of structural ions and the high intensities of carbon
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
I
5
10
59
2765
PVF2
PVF
(CH2 c F2 1"
20
0
C F+
0
4
.
_
1
M00 /Z
60
1 40
Figure 2. Positiie ion LMS of PVF, PVF2, PVF,, and PTFE. (NOTE: The PVF spectrum is not representative of the average but Is shown because the peaks of interest are highlighted.) Table 111. Positive Ion Relative Intensities (CF+= 100)
ion CF+ C3HS+ C3H4F+ C3H3F2f C&Fa CaHF4" C85+ C3Ft6Cl+ C3F431C1+ C3F$%12+ C3FaasC13'C1+ C3F331C12+
mass (rn/z) PVF 31 41 59 77 95 113 131 147 149 163 165 167
100 1 1 1
PVFz (A)
PVFz (B)
PVFz (C)
PVFz (D)
PVF3
PTFE
100
100
100
100
100
100
100
100
14 6
3 11 4 29
2 9 4 34
5 9 2 8 5
21
115 100 21 40 24
34 31 11 20 13 2
2
21
clusters. The observed relative intensities can be explained if the rates for structural ion formation by the mechanisms described above are equal to or less than the rate of H F elimination. The rapid loss of fluorine atoms from the polymer backbone accounts for the carbon-hydrogen cluster ion dominance in the PVF spectrum. Quantitative determination of PVF linkage types is restricted for two reasons other than the low ion yields of structural fragments. As shown in Figure 3, a mlz 59 fragment occurs for each of the possible linkage types but is the only fragment characteristic of a T-T linkage. This would necessitate determination of the percent contribution from each linkage type to the overall mlz 59 peak intensity. Such a determination requires specific knowledge of the ionization efficiences for each fragment. The calculations would be nontrivial and are beyond the scope of the present study. The
4
25
PCTFE (A) PCTFE (B)
isobaric interference of the mlz 41 and 59 fragments by 41K+ and CaF', respectively, is another limitation. Contributions to the peak intensity could be estimated by measurement of the 99K+and @Ca+peak intensities. However, this would limit accuracy due to the low intensities for the molecular fragments. Poly(viny1idene fluoride) (PVF,). An increase in the relative intensities of the structural ions with a corresponding decrease in carbon cluster intensities is readily discernible when comparing the positive ion spectrum of PVFz with that of PVF. Figure 3 shows the structural fragments which can be produced from three linkage types for PVF,. In contrast to the other fluoroethylenes, each structural ion has a unique empirical formula and therefore different m / z values. The m / z 77 and 95 ions are characteristic of T-T and H-H linkages, respectively, while the mlz 59 and 113 ions corresponding
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984 Iiead-to-Tail
-c H2CHFCH2CHFCH2C HFPVF
(+)CH CF=CH2
( 5 3 m/z)
Head-to-Head -CH2CFHCFHCH2CH2CHF-
(+) CHFCH=CHF ( 7 7m/z 1
or
T a i l - t o-Tail FHC FHCH2CH2C HF-
-CH$
(+) CH2CFCFH ( 7 7 m/z)
or CH2CH=C HF (59n/z)
PVF3
-CFHCF2CFrlCFlCFHCF2(+) CEH-CF=CFH ( ? 5 1n/z)
-CFHCF2CF2CFIiCr€I-CF2-
(+) CFzCF=CF2
-CFHCFZCF~CFHCFPCF~-
CF2=CF-CFH(+) ( 1 1 3 m/z)
( 1 3 1 m/z)
(+) CFHCF=CF2
( 1 1 3 m/z)
or
or
(+)CFH-CH=CF2 ( 9 5 m/z) He ad-t o-Tai 1 -CFClCF2CFClCF2CFClCF2-
He ad- t o-Head
T a i l - t o-Ta i1
-CFClCF2CF2CFClCFClCF2-
-CFClCF2CF2CFClCFCLCF2-
FC TFE (+)CFClCF=CFC1 (+)CFzCFCF2 ( 1 6 3 m/z) ( 1 3 1 m/z)
CF2=CF-CFCl(+) (147 m/z)
(+)CFClCF=CF2 ( 1 4 7 m/z)
or
or (+) CFClCCl-CF2 ( 1 6 3 m/z)
De polyme riza t i o n
-CF2CF2CF2CF2PTPE
shv (+) CY2CF=CF2 ( 1 3 1 m/z)
-CF2CF2CFy
CF2-
s -CF2*
+
CF2 = CF2 ( 1 0 0 m/z)
Flgure 3. Listing of all possible three-carbon structural ions formed from poiy(fluoroethy1enes).
to normal, H-T linkages. It is important to determine if these fragment ion intensities can be quantified and used for determining the percentage of backward additions (%BW) along a polymer chain. Prior to using laser mass spectrometry for such determinations, it is important to review the same type of measurement performed by 19FNMR, which will be used as a reference method. The calculation of backward additions by 1 9 NMR ~ is a standad method in polymer science. Interpretation of NMR spectra for the complete poly(fluoroethy1ene) series is well documented with special emphasis placed on PVF2 ( 6 , I I ) . An example of an NMR spectrum for PVF2 (sample D) is shown in Figure 4. The fluorine atoms of the H-H linkage experience a chemical shift of -25 ppm downfield from the resonance energies of fluorine in a H-T environment. These
two peaks labeled as C and D are indicative of backward addition of monomer units along the PVF2 molecular chain. The equation used for quantifying backward addition is
%BW =
(
(Ac + A , ) P AT
)
X
100
(1)
where Ac and AD are the areas under peaks C and D and AT is the total area under all peaks. The %BW values for PVFz by eq are listed in IV. Calculation of %BW from mass spectra would be expected to a format
%BW =
xHH,TT(Ii) CHH,TT(Ii)
+ CHT(Ii)
x 100
(2)
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984 19
F NMR
A
A
C D
B
A
A
H H T T
C
D
Flgure 4. "F NMR of PVF, (sample D) in DMF.
Table IV. %BW Values Obtained from I9F NMR and
LAMMA A
PVF, sample B C
D
'F NMR 2.5 f 0.8%= 4.8 f 0.5% 4.0 f 0.5% 4.7 f 0.5% 4.8 f 4% 3.9 f 5% 4.3 f 20% LAMMA 2.5 f 4%
Error listed as relative standard deviation in percent. Scaling factor, a! = 9.6.
where C",TT(li) is the summation of ion intensities for the m / z 77 and 95 fragments and C H T ( l i ) is the summation of ion intensities for H-T fragments at m/z 59 and 113. The one-half factor in the numerator is not carried over from eq 1because each backward linkage can fragment into either a H-H or T-T ion but not both. The %BW values obtained by using eq 2 were in the 20-25% range, a substantial difference from the NMR results. Factors leading to this discrepancy could be differing efficiencies of formation and ionization of the individual ions. It was observed that the %BW values from 2 followed the same numerical order as from the NMR data, which led to the assumption that these factors are consistent between samples. A scaling factor (cy) was introduced in eq 2 to normalize the NMR and LAMMA values. The operational equation then becomes
where 01 was calculated by using the NMR determined %BW value of a standard sample and solving eq 3 for cy. The value of 01 was calculated to be 9.6 by using sample B as a reference standard. The results listed in Table IV show the agreement in %BW values between NMR and LMS. The NMR and LAMMA values agreed within the experimental error for all samples, but the values for sample D differed by 0.4%. One explanation for this deviation would be branching and/or crosslinking sites within the polymer chain, which would change the fragmentation characteristics of the polymer. However, direct comparison of sample D spectra with the other PVF, spectra gave no indication of an altered fragmentation pattern. 19FNMR spectroscopy also gave no indication of numerous branch sites within sample D. The factor of 5 difference in the relative standard deviation between sample D and the other polymers may indicate poor sample uniformity. This commercial PVF, could be a mixture of samples obtained under different polymerization conditions, causing a distribution of backward additions throughout the polymer. 19F NMR would not detect this effect since its sampling volume
2767
is orders of magnitude larger than LAMMA, and any nonuniformity would cancel out. A peak of varying intensity at m / z 40 was detected in every PVF, sample and was considered to be from calcium contamination. Its presence is significant since CaF+ would be a probable fragment formed by laser ionization of calcium on a fluoropolymer. The m/z value of 59 for CaF+ is equivalent to that of the C3H4F+ion and, therefore, the CaF+ ion intensity would contribute to the CHT(lJ term in eq 3. Attempts to alleviate this problem, sample cleaning and using the ion intensity of Ca+ to estimate the CaF' contribution to the m / z 59 intensity, did not result in a signficant change in the %BW values measured. Therefore, the CaF+ contribution is either negligible or its effect is consistent between samples. Within the established error limits, laser mass spectrometry proved that it can be quick and effective technique for determining %BW values in PVFz, if a standard sample is available. Poly(trifluoroethy1ene) (PVF3). A typical spectrum for PVF, is shown in Figure 2 and its structural fragments are given in Figure 3. Unlike PVF2, the three linkage types do not fragment into structurally relevant ions of unique mass. The m / z 113 ion can be produced by fragmentation and ionization of a chain segment corresponding to either a H-T, H-H, or T-T linkage. Knowledge of the efficiencies for m/z 113 ion formation from each of the three linkages is essential for determining %BW values from LAMMA spectra. Although specific structural information cannot be abstracted from the LAMMA spectra, PVF3 can be distinguished from the other fluoropolymers by the three structurally significant peaks at m / z 95, 113, and 131. Poly(tetrafluoroethy1ene) (PTFE). The positive ion spectrum of PTFE, Figure 2, is similar to PTFE spectra reported for various mass spectrometric techniques (8,9). The fragments previously used to "fingerprint" PTFE are all present: CF+, CF,+, CF3+,C3F3+,C2F4+, and C3F5+a t m / z 31, 50,69,93,100, and 131, respectively. The structural peaks of interest, m / z 100 and 131, correspond to the PTFE monomer unit and a three carbon, allyl fragment, as shown in Figure 3. Detection of the monomer unit correlates well with the depolymerization mechanism proposed from thermal degradation studies (13). Depolymerization is the dominant degradative process in thermal analysis but is less significant in LAMMA analysis as determined by the low relative intensity of the C2F,+ ion. Structural ion formation via mechanisms A and B competes with depolymerization under LMS conditions and results in a peak at m / z 131, C3F5+. As expected, C3F5+is the only structural fragment present in PTFE spectra. The appearance of the CZF4+ and C3F5+fragments along with the lack of carbon-hydrogen cluster ions readily distinguishes PTFE from the other fluoroethylenes. Poly(chlorotrifluoroethy1ene)(PCTFE). The structural fragents formed by irradiation of PCTFE with the laser are shown according to linkage type in Figure 3. The isobaric nature of the possible fragments inhibits quantification of percent backward additions. However, the intensity ratios between the three peaks suggest mostly head-to-tail linkages along the PCTFE molecular chain. The most intense peak a t mlz 131 corresponds to a H-T linkage only, whereas the mlz 147 and 163 peaks correspond to H-T linkages in addition to H-H and T-T. A direct comparison between the spectra for the two PCTFE samples indicates dependence of polymer fragmentation patterns on physical state. The spectra and relative intensities of samples A and B are shown in Figure 5 and Table 111. The high relative intensities of the structural fragments for sample A (low molecular weight liquid) along with the absence of carbon cluster ions, excluding halogenated sin-
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ANALYTICAL CHEMISTRY, VQL. 56, NO. 14, DECEMBER 1984
Table V. Negative Ion Relative Intensities
fragment
mlz
a 37Cl-
5
19 35 370 24 36 48 60 72 84 96 108 120
F-
43 55 79 103 127
C2FC3FCSFC7FCBF-
c137c1czc3c4C5c.5C7cc c9-
PVF
PVFp
PVF3
PTFE
PCTFE A
PCTFE B
100
100
100
104 58 98 72 51 19 16 4
62 38 64 36 12 13 3 2
100 51 56 57 30 37 15 18 7 8 3 2
100 122 76 29 31 48 10 9 6 5 2
100 223 153 162 151 211 96 53 45 45 37 3
2
3
7
41
GO-
70 44 66 26 119 8 9 5 2 9 1 21 8
14 4 14 4
intensity is inflated by the C3H- contribution. 0
7
.
100 "+
NEGATIVE I O N
PC T F E :A PVFZ
5
I - 50 J-J
i ,
J 5
1. J
u
.~
Figure 6. Negative ion IMS of PVF,.
Flgure 5. Positive ion LMS of PCTFE: sample A (viscous liquid) and sample B (powder B (powder).
gle-carbon ions, suggests three-carbon, structural ion formation as the dominant fragmentation/ionization mechanism for the polymer. In contrast, these structural ion peaks for sample B (high molecular weight powder) are dwarfed by intense carbon-fluorine and/or -chlorine cluster peaks. As with the other fluoropolymers, the relative intensities were not found to be a function of laser energy or sample support. The role of depolymerization in the fragmentation patterns of both samples is evident by spectral comparison. The PCTFE monomer unit at m / z 116 is detected with a relative intensity of 15 in sample B but is absent from the spectrum of sample A. Depolymerization occurs at the expense of structural ion formation (mechanisms A and B), so the overall relative intensity of structural ions should be inversely proportional to the degree of depolymerization. Although depolymerization would contribute to the intensity differences, it is not expected to be the sole cause.
A significant distinction between samples A and B is their physical state: viscous liquid vs. powder. The effect of physical state on fragmentation mechanisms may be a function of the degree of crystallinity. Crystallinity is characterized by closely packed polymer segments experiencing strong intermolecular forces. The opposite is true of amorphous segments. PCTFE is known to possess a high degree of crystalline character and should therefore consist of many tightly packed segments. The liquid sample would possess far fewer crystalline segments so intermolecular forces should not have a significant effect on its fragmentation pattern. Whether crystalline character plays a significant role in fragmentation mechanisms will require further study. Fluoropolymers: Negative Ion Spectra. The negative ion spectra of the poly(fluoroethy1enes) are all similar; the spectra are dominated by carbon cluster ions, F and the C,F series. The relative intensities (F- = 100) of selected ions are tabulated for each sample in Table V. Figure 6 shows the negative ion spectrum of poly(viny1idene fluoride) as a typical example. The only significant trend from the tabulated results is the increase in C,F- relative intensities with increasing fluorination. The chlorine-containing contaminant in the PVF3 sample is readily evidenced by the C1- peaks at mlz 35 and 37. The C1- ion is detected as the base peak in the negative ion spectra of PCTFE, as well as in most chloropolymers. The labile character of chlorine under laser irradiation is discussed further in the chloropolymer section. There is a striking
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
I i'
NEGATIVE ION
100
1
,
50(
iI
l
20
POLY VINYLIDENECHLORIDE- ACRYLONITRILE a0:20
40
60
80
100 M/Z
120
140
160
180
Flgure 7. Positive a n d n e g a t i v e ion LMS of poly(viny1idene chloride/ acrylonitrile), 80:20 m o n o m e r ratio.
difference in the negative ion intensities between the two PCTFE samples. The low molecular weight sample fragmented into fewer carbon cluster ions than the powder, as evidenced by the lower relative intensities and fewer total fragments. These results are similar to those of the positive mode in that the liquid sample forms fewer cluster ions than the solid sample. Chloropolymers. The success of laser mass spectrometry (LMS) for distinguishing between poly(fluoroethy1enes) does not extend to the poly(chloroethy1enes). The lack of structurally significant ions in the LMS spectra of poly(viny1 chloride) (PVC) has been reported previously (9). The only fragments characteristic of PVC were CCl+ and C3H2Cl+in the positive mode and Cl- in the negative mode. The spectra in both modes were dominated by carbon-hydrogen cluster ions. We have studied various linear chloropolymers to test the feasibility of LMS for chloropolymer analysis. The positive and negative ion spectra of poly(viny1idene chloride/acrylonitrile) are shown in Figure 7 and are typical of the spectra obtained from linear chloropolymers. After removing peaks attributable to carbon-hydrogen cluster ions, only one chlorine-containing ion could be confirmed in the positive ion spectrum, CC1+ at m / z 47 and 49. The distinguishing characteristics in the negative ion spectra of linear chloropolymers are the high CF intensity at m/z 35 and 37 and the groups of peaks in the mlz 120-200 region. The high mass peaks probably correspond to cyclic ions and some proposed structures are shown in Figure 7. The molecular formulas for the ions are C5H3C12+at m / z 133, 135, 137; C6H3C12+at m / z 145,147,149, and C5H3C13+at m / z 168,170, 172, 174. The relative intensities of the peaks within each cluster do not exactly correspond to those expected from 37Cl isotope ratios for reasons well-explained elsewhere (14). The same cyclic fragments were detected in the negative ion spectra of poly(viny1idene chloride/vinyl chloride) samples having varying monomer ratios. The fragmentation patterns of all samples were to similar to allow accurate identification from the mass spectra.
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The fragmentation pattern observed for poly(viny1idene chloride/acrylonitrile) is not surprising in view of the results obtained from thermal degradation studies on chloropolymers. The thermal characteristics of PVC have been studied extensively (13, 15) and poly(viny1idene chloride) (PVC2) has received much attentian as well (15, 16). A dehydrochlorination mechanism is responsible for the degradation of PVC and PVC2 at elevated temperatures. Dehydrochlorination involves either the molecular elimination of HC1 or free radical loss of hydrogen and chlorine from the molecular backbone, resulting in conjugated segments along the polymer chain. Subsequent intramolecular rearrangement of these conjugated segments yielding benzene molecules from PVC and trichlorobenzene and tetrachloronaphthalene from PVC2 has been verified by both pyrolysis/mass spectrometry (16) and thermal degradation studies (15). Although the fragmentation patterns produced by LMS and pyrolysis/MS do not completely match for PVC2, the dehydrochlorination mechanism is significant in both techniques as evidenced by the cyclic ions and lack of saturated fragments. Applicability of LMS to Polymer Analysis. We have shown LMS to be a valuable technique for poly(fluor0ethylene) analysis. In addition to the capability of distinguishing between homopolymers, LMS was used to determine the molecular structure of selected polymers. The structural ions appearing in the positive-ion spectra were used for identifying each homopolymer in the series. Spectral reproducibility, which was approximately *5 % between successive laser shots, was sufficient for obtaining reliable data. Wnfortunately, LMS was not able to distinguish poly(ch1oroethylenes) because of the labile character of chlorine atoms. The percent backward addition values calculated from LAMMA spectra of poly(viny1idene fluoride) samples were consistent with those obtained from I9F NMR. The acquisition of structural information from the LAMMA spectra of other polymers should be viable if standard samples are available. The limiting factor in abstracting structural information from the other poly(fluoroethy1enes) proved to be the isobaric interference of the structurally relevant ions.
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86,3364-3367. (3) Clark, D. T.; Feast, W. J.; Kilcast, D.; Musgrave, W. K. R. J. Polym. Sci., Polym. Chem. Ed. 1973, 7 1 , 389-41 1. (4) Clark, D. T. Pure Appl. Chem. 1982, 5 4 , 429-438. (5) Painter, Paul C. "The Theory of Vibrational Spectroscopy and Its Application to Polymeric Materials"; Wiley: New York, 1982. (6) Tonelli, A. E.; Schilling, F. C.; Chais, R. E. Macromolecules 1982, 75, 849-853. (7) Blackwell, C. S.;Degen, P. J.; Osterholtz, F. D. Appl Spectrosc. 1978, 32, 480-484. (8) Briggs, D.; Wooton, A. B. S I A , Surf. Interface Anal. 1982, 4 , 109-115
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RECEIVED forreview May 23,1984. Accepted August 27, 1984. This work was supported by the National Science Foundation under Grant CHE-8108495.
