1612
Anal. Chem. 1881, 63, 1612-1620
Static Secondary-Ion Mass Spectrometric Investigation of the Surface Structure of Organic Plasma-Deposited Films Prepared from Stable-Isotope-Labeled Precursors. 1. Carbonyl Precursors Ashutosh Chilkoti and Buddy D. Ratner* Department of Chemical Engineering and Center for Bioengineering, BF- 10, University of Washington, Seattle, Washington 98195 David Briggs ICI PLC,Wilton Materials Research Centre, P.O.Box 90,Wilton, Middlesbrough, Cleveland TS6 8JE, U.K.
Stabklrotopdabeied carbonyl precursors (acetaldehyde, ace to^, and l - b u t a m ~ . were ) wed to create p b m a d e podtod f l h a (PDF$), which were then exby podtlveand rwgcltltKknstalk SI-. Thb *wed h y d " (HC) fragments to be dhtlngulshod from oxygencontaining fragments in the statk SIMS spectra of these PDFs. Both the podtlve- and negativ+lom 8tatlc SIMS frcrqnentakn patterns of conventional HC and oxygen-contalning polymers were qualitatively examined In order to assign structural units on the PDF surface that could account for the salient features In the etatlc SIMS fragmentation patterns of these PDFs.
INTRODUCTION The plasma deposition of organics is an emerging technology, permitting the formation of ultrathin films (10-1OOO A) on a variety of substrates. Different applications will each require plasma-deposited f i b s (PDFs) with unique chemical and structural characteristics. However, the thin f i i nature of PDF8 and their complex chemistry, i.e., multifunctional, cross-linked, etc., has hindered their characterization. While infrared (IR) spectroscopy and nuclear magnetic resonance (NMR)spectroscopy have been used on occasion to elucidate PDF structure, the minute mass of material deposited precludes the use of these techniques for the analysis of ultrathin PDFs ( l a , 2-4). The partial characterization of these structurally complex, ultrathin films has been made possible by the use of X-ray photoelectron spedroscopy (XI'S) (5). The major advantages of XPS are ita surface sensitivity (sampling depth of -50 A) and ita ability to provide the atomic composition of the film (all elements except H). Molecularly specific information is provided by binding energy (BE)shifta of functionalized species. However, the identification and quantitation of functional groups by least-squares curve fitting of core-level photoemission lines is prone to ambiguity, particularly when these surfaces are functionalized by oxygen- or nitrogencontaining species. This is due to the multiplicity of such species present, many having virtually indistinguishable BE shifts (5). Furthermore, XPS is poor at discriminating hydrocarbon (HC) species because of extremely small BE shifts. This situation has been improved by the development of chemical derivatization methods to uniquely label functional groups of interest prior to XPS analysis (6). The advent of static secondary ion mass spectrometry (SIMS)as a surface characterization tool for polymers providea for the direct interrogation of molecular structure (7-9).Static SIMS is a surface analysis method (sampling depth of 10
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*Author to whom correspondence should be addressed. 0003-2700/91/03681612$02.50/0
A) (10) with a clear analogy to conventionalmass spectrometry (11). While isolated studies have appeared in the literature on the analysis of plasma-treated surfaces by static SIMS (12-15),to our knowledge no studies have been published on the characterization of PDF structure by static SIMS. The lack of such studies is related both to the recent introduction of static SIMS as a polymer surface analysis technique and the complexity of PDF structure. This investigation attempts to elucidate aapects of the surface structure of a set of oxygen-functionalized organic PDFs through the use of static SIMS. The aim of this prototype study is to determine the analytical utility of quadrupole-based static SIMS with regard to the characterization of structurally complex surfaces such as organic PDFs. The PDFs under investigation (Le., PDFs created from carbonyl precursors) were selected because of interest in their surface bioreactivity (16). The X P S and derivatization-XPS resulta for these PDFs have been published elsewhere and offer complementary information regarding their surface structure (17). The first step in the analysis of any unknown surface by static SIMS requires identification of the secondary ions. Since quadrupole-based static SIMS spectra of oxygen-containing organics (both low molecular weight compounds and polymers) are complicated by the inadequately resolved peaks displayed by both oxygen-containingand HC fragments, the unambiguous identification of secondary ions was done by analyzing PDFs prepared from stable-isotope-labeled precursors. This approach has been taken to distinguish between HC and oxygen-containing fragments in the positive static SIMS spectrum of poly(methy1 methacrylate) (PMMA) by using perdeuterated PMMA (18). In this regard, the advent of high mass resolution (4000-6000 m/Am in the range m / z 0-250) and high sensitivity time-of-flight mass analyzers should further simplify this task (15). The second step in the analysis requires translating the spectral information (i.e., the identity of the secondary ions) into structural features on the original surface. A preliminary attempt has been made in this paper to qualitatively relate the assigned secondary ions (both positive and negative) to possible structural featurea on the PDF surface by comparing the static SIMS fingerprint of the PDF with those of conventional HC and oxygen-containinghomopolymers. A more rigorous analysis using multivariate statistical analysis methods will be published elsewhere.
EXPERIMENTAL SECTION Materials. Unlabeled acetaldehyde, acetone, and 2-butanone (99% purity or greater) were acquired from Aldrich Chemical Co.
(Milwaukee, WI).CDSCDO,de-acetone,and de-2-butanone were acquired from Cambridge Isotope Laboratories (Woburn, MA) 0 1991 American chemicel sockty
ANALYTICAL CHEMISTRY, VOL. 63, NO. 15, AUGUST 1, l 9 Q l 1615
Table I. Summary of XPS Results for Carbonyl Precursor PDFs C1.__ spectra .
atomic precursor CHSCHO acetone [1,2,3-1sCs]acetone d,-acetone 2-butanone
%C
%O
94.1 91.6 88.7
5.9 8.4
91.6
93.0
11.3
8.4 7.0
% CH, (286 eV)
% COR (286.5 eV)
94.2 87.7 85.7 84.4 94.9
2.6 7.7
and had an isotopic labeling of at least 98 atom % as specified by the manufacturer. [ 1,2,3-lsCB]Acetonewas acquired from Isotec, Inc. (Miamisburg, OH), and had an isotopic labeling of 99 atom % '9c. All precursors were used as received. Untreated polystyrene (PS)substrates were supplied by Corning Glass Works (Horseheads,NY).They were cleaned,prior to plasma deposition, by ultrasonication in a 3% Ivory soap solution or reagent-grade 200 proof ethanol, followed by repeated rinses in deionized/reverse osmosis purified water. Samplea were dried in a laminar flow hood and stored in PS petri dishes prior to use. Poly(viny1 methyl ketone) (PVMK) was acquired from Scientific Polymer Products (Ontario, NY), poly(viny1ethyl ketone) (PVEK) was prepared by free-radical polymerization of the monomer (Aldrich), and poly(methy1isopropenyl ketone) (PMIsPrK) was acquired from Polysciences, Inc. (Warrington,PA). The conventional polymers were centrifugally cast on to 12-mm-diameter glass disks from 1-4% w/v solutions in spectrometric-grade solvents (PVMK, tetrahydrofuran; PVEK, toluene; PMIsPrK, chloroform)by using an EC-101 spincoater (Headway Research, Inc., Garland, TX). The glass disks on which the polymers were centrifugallycast were previously cleaned by ultrasonication in a 1.5% v/v Isopanasol (C.R. Callen Co., Seattle, WA) solution in deionized/reverse osmosis purified water, followed by repeated rinses in deionized/revewe osmosis purified water. All polymers were analyzed by XPS, prior to static SIMS analysis, to check for purity and stoichiometry. Plasma Deposition. A capacitively coupled, radio-frequency (rf) (13.56-MHz) plasma reactor was used to create the plasma (16). Backstreaming of oil from the mechanical pump was prevented by connecting a liquid nitrogen cold trap in line to the mechanical pump. In order to eliminate HC contamination of the PDF from the vapor inlet lines and the reactor walls,both the precursor inlet lines and the plasma reactor were baked out before every deposition. Concurrent to the bakeout, the reactor was cleaned by an Ar plasma for times ranging up to 2 h. The precursors, previously subjected to a freeze-thaw cycle under vacuum in order to remove diseolved air (to prevent contamination of the PDF with adventitious nitrogen and oxygen), were introduced into the reactor through a flow controller (UV2-21,Vacuum General, Inc., San Diego, CA). Prior to depositing the plasma films, the substrates were cleaned by an Ar etch at the following conditions: preasure (P)= 175mTorr, rf power (W) = 30 W, flow rate (8') = 4.0 cms min-l, time ( t ) = 5 min. In order to compare the plasma depositions from the three prBcurBors, the macroscopic reaction parameters were kept invariant and were P = 150mTorr, W = 10 W, F = 4.0 cm3min-l, and t = 10 min. Base pressures in the plasma reactor were 10 mTorr. XPS Analysis. The XPS experiments were done on a SSX100 spectrometer (Surface Science Instruments, Mountain View, CA). Details of the spectrometer and analysis conditions have appeared elsewhere (11, 17). Static SIMS Analysis. Static SIMS analysis of the PDFs was performed on an XPS/SIMS instrument based on the ESCALAB Mk 1 UHV system (VG Scientific), which has been described previously (19). A rastered 4keV Xe+ primary ion beam of 0.4 nA (current density 1nA cm-2) was used. The positiveand negativeion spectra were acquired on the same sample. The total ion doee corresponded to I2 X loa ions for positive-ion spectra (acquired first) and I5 x 10l2ions cm-2 for negative-ion spectra (acquired second). These conditions are well within the maximum primary ion dose of lOls ions cm-2 allowable for static SIMS (20). All positive-ion spectra were optimized on the m/z 43 peak, while negative-ion spectra were optimized on the m j z 59 peak. Charge neutralization was effected by flood electrons
-
-
6.6 10.5 4.0
% C=o
IC02R
3.2 4.5 6.3 4.3 3.4
0 0 1.4
(287.8 eV)
(289 eV)
0.7
0
with an accelerating potential of 700 V and a current density similar to that of the primary-ion beam. No electron-stimulated secondary-ion emission was observed when the ion beam was switched off while irradiating the sample with the electron flood.
RESULTS AND DISCUSSION The averaged XPS results for the unlabeled PDFs have been reported previously (17). For the sake of clarity, however, the X P S results (both atomic percentages calculated from 0 to lo00 eV binding energy (BE) scans and the percentages of various carbon-oxygen species calculated from a Gaussian least-squares fit of the XPS C1, spectral envelope) for both the unlabeled and stable-isotope-labeled PDFs that were subjected to static SIMS analysis are summarized in Table I. The XPS results reported are consistent with the results of a previous study, wherein the surface chemistry of PDFs created from acetaldehyde, acetone, and 2-butanone was observed to be similar (17). Specifically, a large fraction of the oxygen atoms in the precursor are lost during the deposition process, leading to the formation of PDF8 with low levels of oxygen incorporation (6-10 atom %). COR (R = H or C,,H,,,) and C=O type species predominate, while COzR (R = H or C,H,,,) type species, if present, are in low concentration. The full width at half-maximum (FWHM) of the peaks corresponding to these carbon species in the least-squares curve fit of the high-resolution C1, spectral envelope were 1.3-1.7 eV. These FWHM are significantly higher than those obtained for conventional oxygen-containing model polymers. Since the effect of sample charging on these PDFs was minthe larger FWHM (as compared to oxygen-conimal tainiig model polymers) of the deconvoluted peaks in the C1, spectra of the PDFs is probably due to subtle differences in the bonding environment of each functional species. Changes in the nearest-neighbor and next-nearest-neighbor environment would be expected to lead to shifts in the BE of the peak corresponding to each functional species (e.g., COR, RC-O, etc). Since these differences in peak BE for a particular carbon species would have an extremely small dynamic range (a few tenths of an electronvolt), they would be incorporated in a composite peak, which leads to the peak broadening observed in the C1, spectra of these PDFs. In the present study, the similarities between the XPS results of the PDFs prepared from the unlabeled and labeled precursors, notably, acetone, [1,2,3-13C3]acetone, and d6acetone) provide confidence that the information accruing from the static SIMS analysis of the stable-isotope-labeled PDFs can be related to the unlabeled PDFs. Since these PDFs were prepared from completely labeled precursors ('9c or D), isotope effects arising from substitution of C or H by their stable isotope analogues (W or D) are not a problem as long a~ comparison of results between PDFs mated from unlabeled and stable-isotope-labeled precursors is qualitative. The positive static SIMS spectra of the PDFs created from the three precursors are shown in Figures 1 and 2. Visual inspection of the three spectra reveals that the fingerprints of the PDFs are similar,which corroborates previous fidings, both from Fourier transform infrared (FTIR) and XPS analyses, that the surface chemistry of these PDFs is quali-
(In,
1014 ANALYTICAL CHEMISTRY, VOL. 63, NO. 15, AUGUST 1, 1991
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50 60 70 80 90 100 m/z Flguro 1. Posttive-lon static SIMS spectra (mlz 0-100) of (a)acet-
Flgur. 2. Posithreion static SIMS spectra (mlz 100-200) of (a) acetaldehyde PDF, (b) acetone PDF, and (c) 2-butanone PDF.
tatively similar (17). The positive-ion spectra of these PDFs are dominated by HC fragments, namely C,H,+ excursions. This is to be expected since the XPS C/O ratio of these PDFs is -10. The static SIMS spectra (in particular the pitive-ion spectra) of the PDFs do not provide much more information about their structure. In order to distinguish between oxygen-containing fragments and HC fragments, and between aliphatic and aromatic HC fragments, static SIMS of stable-isotope-labeledPDFs was performed. Pertinent results are presented below. A. Identification of Secondary Ions. Positive-Ion Static SIMS Analysis. The Cs cluster is the base cluster in the positive-ion static SIMS spectra of all the carbonyl PDFs investigated herein. The primary peak in this cluster is at m/z 43 for the unlabeled PDFs, with secondary peaks at m / z 41 and 39 and a minor peak at m/z 45 (Figure 1). Since these organic PDFs are oxygen containing, possible assignments for these peaks are CsH3+(mlz 39), C3H6+and CHCO+ (mlz 411, C3H,+ and CH3CO+(mlz 43), and C2H50+( m / z 45). Examination of the positive-ion spectra of the PDFs created from the completely labeled precursors (D or 13C) allows for a distinction to be made between the overlapped HC and ox-
ygen-containing secondary ions. In this regard, analysis of the positive-ion spectrum of the [1,2,3-'W3]acetone PDF is the most informative. lSC analogues of the C3H3+,C3Ha+,and C3H7+ions are observed at m/z 42 (l8CSHs+),m / z 44 (18C3H6+),and m/z 46 (l3CsH,+)(Figure 3b) for the [1,2,3-1SC]acetonePDF. Additionally, peaks indicative of oxygen-containingcations are observed at m / z 43 (13CH13CO+),m / z 45 (13CHS13CO+), and m / z 47 (l3C2HSO+).The dominant peak in this cluster is at m/z 45, which suggests that a significant fraction of the m/z 43 intensity in the unlabeled acetone PDF positive-ion spectrum is due to the CHsCO+ ion. Similarly, the low intensity observed for the m/z 43 ion ('WH'WO+) in the positive-ion spectrum for the [1,2,3-13C3]acetonePDF suggests that the contribution of the CHCO+ ion to the observed intensity of the m / z 41 peak in the unlabeled acetone PDF paitive-ion spectrum is minimal. Finally, the extremely low intensity peak at m/z 45 (C2H6O+)in the acetone PDF p a itive-ion spectrum is paralleled by a m / z 47 ion of similar intensity in the pitive-ion spectrum of the [1,2,3-'Bcs]acetone PDF, which may be assigned as a *3C2H60+ion. Corroboration of these assignments can be made by examining the positiveion spectrum of the &acetone or CDSCDO
0
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ANALYTICAL CHEMISTRY, VOL. 63,NO. 15. AUGUST 1, 1991
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Figure 3. Positive-ion static SIMS spectra (mlr 0-100) of (a) acetone PDF, (b) [ 1,2,3-'%C3]acetonePDF, and (c) d,-acetone PDF.
PDF. Since the spectra of both PDFs are qualitativelysimilar, our discussion will be confined to the de-acetonePDF spectrum. For the d6-acetonePDF, the m / z 42 peak is assigned solely to a C&+ ion (Figure 3c). The inversion in the intensity of the m / z 46 and 50 peaks as compared to the m / z 41 and 43 peaks in the positive-ion spectrum of the acetone PDF suggests that the m / z 46 peak in the de-acetone PDF positive-ion spectrum is composed of CD3CO+ and C3D6+ions, while the m/z 50 peak can be attributed mainly to C3D7+ions (with a small contribution from C2D60+). Moreover, if the C3H7+ion were the dominant contributor to the intensity of the m / z 43 peak in the positive-ion spectrum of the acetone PDF, the intensity of the m / z 50 peak (C3D7+)in the positive-ion spectrum of the de-acetone PDF would have been greater. This confirms that a major fraction of the intensity of the m/z 43 peak in the positiveion spectrum of the acetone PDF is due to the CH3CO+ion. Peaks in the C4 cluster of the acetone PDF positive-ion spectrum are observed at m/z 51,53,55, and 57 (Figure 3a). Equivalent ions in the [1,2,3-13C3]acetonePDF positive-ion spectrum are observed at m / z 55 and 57-62 (Figure 3b). The presence of C4H3+,C4H6+,C4H7+,and C4H9+ions in the acetone PDF is confirmed through the observation of '9C4H3+
1615
( m / z 551, l3C4H6+(m/z 571, "C4H7+ ( m / z 59), and 13C4H9+ ( m / z 62) ions in the [l,2,3-'SC3]acetone PDF positive-ion spectrum. Additionally, the presence of even mass ions at m/z 58 (l3C3H3O+), m/z 60 ('V3H6O+),and m/z 62 ('V3H70+)in the positive-ion spectrum of the [1,2,3-13C3]acetonePDF indicates the presence of their unlabeled analogues in the acetone PDF positive-ion spectrum. Corroboration of these asaignments from the equivalent cluster in the d6-acetonePDF positive-ion spectrum is complicated by the isotopic shifts displayed by the HC and oxygen-containing fragments. Reference to Figure 3c shows that the C4D6+,C4D7+,and C4D9+ ions are overlapped with the C3D30+,C3D50+,and C3D7+ions, respectively. Furthermore, since the relative intensities of all three oxygen-containing fragments are not large (as ascertained from the [1,2,3-W3]acetonePDF spectrum), shifts in the intensities of the deuterated analogues are not dramatic (as was the case for the C3 cluster). Hence, changes in the relative intensities of the C4 cluster in the positive-ion spectrum of the d6-acetonePDF cannot be used to corroborate these assignments, though the peak positions are consistent with them. The c6 cluster in the positive-ion spectrum of the acetone PDF has peaks at m / z 63 (C6H3+),m / z 65 (CsH6+),m / z 67 (C5H7+),m / z 69 (C6H9+),and m / z 71 (C5H11'). These peak assignments are corroborated in the positive-ion spectrum of the [1,2,3-13C3]acetonePDF by the presence of peaks at m / z 68 (13CsH3+),m/z 70 ("C,&+), m / z 72 (l3C5H7+),m/z 74 ('%A+), and m/z76 (1%J311+). Additionally, low intensity peaks at m/z 71,73,75, and 77 can be attributed to ions of the type W4H,0+ (m = 3,5,7,9). Similarly, in the c6 cluster of the acetone PDF positive-ion spectrum, the C6H6+( m / z 77), C6H6' (m/Z 781, C&+ (m/Z 79), C6H9+ (m/Z 811, and C&11+ (m/z 83) can be identified by their 13C analogues. Low-intensity even mass peaks at m / z 86,88, and 90 in the [1,2,3-13C3]acetonePDF postive-ion spectrum indicate the presence of 13C6H,0+ ions (m = 5,7,9). In clusters c5 and greater, both aliphatic and aromatic secondary ions are present. Aromatic ions such as Cd-13+(m/z 63), C,5H6+ (m/Z 65), Ca.5' (m/Z 77), c&+ (m/Z 79), C7H7+ (m/z 911, C d 7 +(m/z 1031, C&+ (m/z 105), C&+ (m/z 1151, ClJ-18+ (m/z 1281, and ClJ-19+ (m/z 129) in the positive-ion spectrum of the acetone PDF (Figure 3a,b) are identified by the presence of their %labeled analogues, notably 13C6H3 (m/Z 68), 13C5H5+(m/Z 70), '%&+ (m/Z 83), "C&+ (m/Z 85), 13C7H7+( m / z 981, 13C8H7(m/z lll),13C8H9+(m/z 113), '%!&I7+ (m/z 1241, '9ClJ-I8+ (m/z 1381,and 1%!&9+ (m/z 139) (Figures 3b and 4b). Similarly, aliphatic fragments (i.e., C6H9+,CSH11+, C&,+, and C&ll+, C7H11+, csH13+) can be identified by their corresponding 13Clabeled analogues in the [1,2,3-13C3]acetonePDF spectrum (Figures 3b and 4b) and perdeuterated analogues in the &acetone PDF positive-ion spectrum (Figures 3c and 4c). It must also be noted that oxygen-containing fragments could not be unequivocally discerned in clusters beyond Cs. This is because of the presence of even mass HC ions in the positive-ion spectra of these PDFs, particularly above m/z 100, which would overlap with oxygen-containing fragments. However, this does not preclude the possibility that such ions may be present in low concentration. Negative-Ion Static SIMS Analysis. The negative-ion static SIMS spectra of the CH3CH0,acetone, and 2-butanone PDFs are qualitatively similar, which further confirms the similarities in their structure. The principal HC fragments in the negative-ion spectrum of the acetone PDF (shown in Figure 5a) are of the type C,- and C,H- ( x = 1-4), which are characteristicof both aliphatic and aromatic HC polymers (11, 21, 22). Corroboration for these assignments is found by comparing the acetone PDF negative-ion spectrum with that
1818
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Flgure 4. Positlvslon static SIMS spectra (mlz 100-200) of (a) acetone PDF, (b) [ 1,2,3-'%&]acetone PDF, and (c) d,-acetone PDF. observed for the [ 1,2,3-l3C3]acetoneand d6-acetone PDFs (Figure 5b,c). Specifically, 13CI-, 13C,H-, and '9C,H2- ions are observed in the negative-ion spectrum of the [ 1,2,3-13C3]acetone PDF (Figure 5b), while C,; C,D-, and C,D2- ions are observed for the d6-acetonePDF (Figure 5c). The presence of mainly W,D- fragments ([1,2,3-1sC3]acetonePDF) and C,Dfragments (&acetone PDF) is significant for two reasons. First, it confirms that contamination of the PDFs by adventitious HC (from the inlet lines and reactor walls) was minimal. Second, it strongly suggests that the PDFs are uniformly deposited on the underlying substrate. Note that the 'epeak intensity in Figure 5b is barely above background. In the event that these PDFs were patchy, the relative intensity of l2C) the negative-ion containing atomic anions (e.g., W-, l 2 c ~in spectrum of the [1,2,3-'9C3]acetone PDF would be higher since the carbon isotope in the substrate (PS) is primarily 12C. Similarly, the extremely low peak intensity ratio of H- to Din the negative-ion spectrum of the d,-acetone PDF (Figure 5c) confirms that no effect of the substrate can be discerned in the static SIMS spectra of these PDFs. The 8-12-A sampling depth suggested for static SIMS provides an extremely conservative lower bound for the film thicknesses of these PDFs (10). Realistically, the thicknesses of these PDFs exceed 100 A, as suggested by angular-dependent XPS (17).
0
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60
70
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Figure 8. OK/CW peak intensity ratio versus XPS O/C
created from carbonyl precursors.
ratio for PDF
The presence of oxygen in these PDFs is indicated by peaks at m / z 16 and 17 (0-and OH-) (Figure 5). While the spectra are qualitatively similar, differences in the oxygen concentration, as quantified by the OH-/CH- (OD- for the perdeuterated PDFs and 13CH-for the [1,2,3-'9C3]acetone PDF) peak intensity ratio, are apparent in Figure 6. The intensity of atomic oxygen peaks relative to atomic carbon peaks in
PrNALYTICAL CHEMISTRY, VOL. 63, NO. 15, AUGUST 1, 1881 1617
static SIMS has been observed to correlate well with the XPSmeasured O/C ratio for conventional oxygen-containing polymers also (23-25). The presence of a peak at m/z 43 (13CH13CO-) in the negative-ion spectrum of the [1,2,3-'3C3]acetonePDF and a peak at m/z 42 (CDCO-) in the negative-ion spectrum of the d6-acetonePDF allows the peak at m/z 41 in the negative-ion spectrum of the acetone PDF (and the acetaldehyde and 2-butanone PDFs) to be assigned as a CHCO- ion. Low-intensity peaks at m/z 45 ('%H3'3CO-) and m/z 46 (CD,CO-) in the negative-ion spectra of the [1,2,3-'9C3]acetone PDF and d6-acetone, respectively, allow the m/z 43 ion in the negative-ion spectra of the carbonyl precursor PDFs to be identified as a CH,CO- ion. Similarly, the presence of peaks at m / z 60 ('3C3Ha0-) and m/z 61 (l%H,l%OO-) for the [1,2,3'%]acetone PDF and m / z 62 (C3D60-and CD3COO-) for the d6-acetonePDF permits assignment of the peaks at m/z 57 and 59 present in the negative-ion spectra of the unlabeled PDFs as C3H60- ( m / z 57) and CH3COO- ( m / z 59), respectively. It must be noted that the intensity of the m/z 57 and 59 anions is extremely weak in the acetone PDF negative-ion spectrum, while the intensity of these peaks is much higher in the CHBCHOPDF negative-ion spectrum. The negative-ion static SIMS data for these PDFs provide validity to our assumption that C,H,02+ fragments are not present in the positive-ion spectra of these PDFs. The existence of significant concentrations of these ions would complicate peak assignments in the positive-ion spectra of the carbonyl precursor PDFs, since these ions would overlap in some cases with C,H,+ ions and would not be discriminated in the positive-ion spectrum of the '%-labeled PDF. CnHmOz+ ions in the positive-ion spectrum of conventional oxygencontaining polymers are accompanied by prominent C,H,O; ions in the negative-ion spectrum (7,24-28). Since the only C,H,O; type ion in the negative-ion spectra of the PDFs is CH3COO- (present as a low intensity peak), this strongly suggests that C,H,02+ type ions are largely absent in the positive-ion spectra of the PDFs under investigation. Deriuatization SZMS. The unlabeled acetone PDF, the &acetone PDF, and the [1,2,3-W3]acetone PDF were derivatized with trifluoroacetic anhydride (TFAA) and hydrazine by using the methodology described previously (17). Subsequent static SIMS analysis of the derivatized PDFs revealed the incorporation of the TFAA and hydrazine label via fluorine- and nitrogen-containing secondary ions, which corroborated the derivatization XPS results. However, these peaks were not characteristic of the PDFs but were fragments of the (reacted) derivatizing reagents. In a previous study of a poly(styrene/4-hydroxystyrene)copolymer series (231,it was shown that derivatization SIMS is analytically useful when the secondary ions observed in the SIMS spectrum are from derivatized fragments of the original surface. This is contingent upon selection of appropriate derivatizing reagents, which are capable of forming stable ions that reflect PDF surface structure. TFAA and hydrazine are useful for derivatization XPS because they can be used for vapor-phase derivatization, which has been shown to be preferable to liquid-phasereactions (7).An additional requirement imposed upon the selection of derivatization reagents for static SIMS is that they modify the fragmentation pattern of the surface such that the yield of structurally significant secondary ions is enhanced. In this respect, "FAA and hydrazine proved to be poor choices as derivatizingreagents. In view of the above, detailed results are not presented. B. Assignment of Structural Units. Since the aim of this investigation is to elucidate the surface chemistry of these PDFs, the kinds of structural units from which both the HC and oxygen-containing secondary ions can be created must
Table 11. Relative Intensities of Oxygen-Containing Secondary Cations for the [1,2,3-*F8]AcetonePDF
mlz
assignment
re1 intensity
43 45 47 58
1sCH18CO+ WHS1FO+ '3czH6O+
60
'aCSH60+
62
13cS~,0+
86
'8CfiO+ '*CIH70+ '9C;HBO+
4.9 100.00 2.8 2.8 4.2 1.4 2.1 8.5 9.2 1.4 2.1 4.9 2.1
88 90
~~C~H~O+
aBase peak in the positive-ion spectrum of the [1,2,3-1SCS]acetone PDF.
be considered. Since the negative-ion static SIMS spectra of these PDFs are sparse in molecular oxygen-containii anions, a variety of oxygen-containing structural units that exhibit characteristicmolecular anions can be ruled out based on the absence of such peaks in their negative-ion spectra. Such information is often not unambiguously available by comparison of the positive-ion spectra of conventional polymers and PDFs, in view of the large number of peaks in the PDF spectra, many of which are due to inadequately resolved oxygen-containing and HC cations. Thus, comparing both the positive- and negative-ion spectra of conventional HC and oxygen-containing polymers with those of the PDFs provides complementary information regarding the structural units that could account for the oxygen-containing secondary ions observed in the SIMS spectra of these PDFs. The prominent oxygen-containing molecular secondary ions in the negative-ion spectra of the carbonyl precursor PDFs are CHCO- ( m / z 41) and CH3COO- (m/z 59), with minor contributions from C2H30-( m / z 43) and C3H60- (m/z 57). The intensitiesof the oxygen-containing positive ions that were unambiguously identified are shown in Table 11. The three major secondary cations for the unlabeled PDFs are CH3CO+ ( m / z 431, C4H60+( m / z 69), and C4H70+( m / z 71). These oxygen-containing ions were considered, in addition to the C,H,+ ions, in evaluating the structural units that account for their presence in the static SIMS spectra of PDFs. The following rationale was adopted in comparing the static SIMS spectra of the PDFs with those of conventional polymers: (1) The presence of one or more prominent oxygen-containing secondary anions in the negative-secondary-ion spectrum of a conventional polymer that is absent in the negative-ion spectra of these PDFs ruled out the structural unit represented by the polymer from consideration. However, the absence of one or more peaks in the negative-ion spectrum of the conventional polymer that are present in the negative-ion spectra of the PDFs does not preclude the possibility that the structural unit of the polymer may be present in the PDFs, since other kinds of structural units present in the PDFs, formed by plasma rearrangement of the precursor during deposition, may lead to fragments that would only be found in the PDFs. Since PDFs are known to be multifunctional and probably possess a variety of structural units, this is probable. (2) The positive-ion spectrum of the conventional polymer under consideration was then examined in order to verify if peaks correspondingto the major oxygen-containing secondary cations in the PDF positive-ion spectra were present in the spednun of the conventionalpolymer. If both (1)and (2) were satisfied, then the structural unit of the conventional oxy-
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 15, AUGUST 1, 1991
gen-containing polymer or a fragment thereof was considered to be present. (3) Only prominent ions in the spectra of conventional polymers were used for this kind of qualitative analysis (i.e., m / z 43,69,and 71 in the positive-ion spectra and m / z 41 and 59 in the negative-ion spectra). This is because low-intensity fragments in the spectrum of the conventional polymer may not necessarily appear in the spectra of these PDFs, since their intensity may be below the SIN of the spectrometer. (4)Finally, in examining the positive-ion spectra of conventional oxygen-containing polymers, only secondary cations that would be created from one repeat unit or less were considered. This is because, even if the structural unit represented by the conventional polymer were present in the PDFs, given the marked cross-linked and HC nature of these PDFs, it is statistically unlikely that adjacent units of the conventional polymer (required to form ions with mlz > M, where M is the repeat unit of the polymer) would be found in the PDFs. Least-squares curve fitting of the high-resolution XPS Ct spectra of these PDFs suggested the presence of a low concentration of acidlester groups in their surface region (27). It has been observed that poly(alky1acrylates) and poly(alky1 methacrylates) display negative-ion static SIMS spectra that are rich in structural information (27,28). Poly(alky1 methacrylate) homopolymers display low mass peaks at m / z 31 (CH,O-), mlz 41 (C,HO-), and m/z 55 (CH,CHCO-) and a characteristic peak at m / z 85 (CH,C(CH,)COO)- (27,28). Since the mlz 85 anion is usually present as a prominent peak in the negative-ion spectra of poly(alky1 methacrylate) homopolymers (since it is formed by R-group cleavage), its conspicuous absence in the negative-ion spectra of the PDFs (along with the absence of mlz 31 and 55 in the negative-ion spectra) strongly suggests that alkyl methacrylate type units are not present in the carbonyl percursor PDFs. A similar rationale can be used to exclude the possibility of poly(alky1 acrylate) units being present in these PDFs, based on the absence of peaks at m/z 71 and 81 (characteristic of this class of homopolymers) in the negative-ion spectra of the PDFs under investigation. Additionally, structural units where the ester is present in the backbone such as in poly(1actic acid) (PLA) or poly(g1ymlic acid) (PGA) are unlikely, based on wide disparities between their fingerprint and those of the PDFs (25). On the other hand, comparison of the negative-ion spectrum of poly(hydr0xybutyrate) (PHB) with those of the PDFs, notably, the relative intensities of the m/z 41,57,and 59 anions, suggests that fragmenta of PHB type units may be present. The absence of peaks at m/z 85 and 87 ([M - HIand [M + H]-) in the negative-ion spectra of the PDFs confirms that the complete repeat unit of PHB is absent (25). Homopolymers containing acid groups, such as poly(acrylic acid) (PAA) and poly(methacry1ic acid) (PMAA), are characterized by a set of peaks in their negative-ion spectra indicative of the polymer repeat unit (28,29). The presence of an isolated unit of PAA would result in the formation of molecular anions at m / z 41,59,and 71 in static SIMS (29), while the corresponding anions for PMAA would be observed at m / z 55,73,and 85 (28).The absence of peaks at mlz 71 and 85 indicates the absence of complete units of PAA or PMAA, while the low intensity of peaks at m/z 55 and 71 suggests that even parts of the polymer unit are unlikely to be present in the PDFs under investigation. Comparison of the negative-ion spectrum of poly(viny1 acetate) (PVAc) with those of the PDFs suggests that acetate groups could easily account for the mlz 59 anion observed in the negative-ion spectra of these PDFs, since the m/z 59 anion is the dominant molecular fragment in the PVAc negative-ion spectrum (7, 29). This is corroborated by the lack of other molecular
oxygen-containing peaks in the PVAc negative-ion spectrum and by the fact that the primary oxygen-containing fragment in the PVAc positive-ion spectrum is at m / z 43 (7, 29). Furthermore, all the other peaks in both the positive- and negative-ion spectra of PVAc are present in the PDF spectra. Based on the above analysis, it appears that the esterlacid type speciea present in thew PDFs are more likely to be PVAc type structural units rather than acrylate, methacrylate, acrylic, methacrylic acid, glycolic acid, or lactic acid ester type structural units. The derivatization XPS results for these PDFs suggested that hydroxyl groups were present in these PDFs (17). Both the positive- and negative-ion spectra of PVA have been published (30).The major peaks in the negativeion spectrum of PVA are at m/z 16,17,41,43,57, and 59 (with the exception of Cx-, C,H-, and C,H,- negative ions), all of which are consistent with the oxygen-containing secondary ions present in the negative-ion spectra of these PDFs. The absence of peaks at m/z 79 and 81 (present in low intensities for PVA) may be due to the relatively low concentration of PVA type units in the PDFs or alternately due to the presence of contaminants in the PVA spectrum (e.g., Br). Similarly, the major positive ions observed for PVA, namely, at m/z 41,43,55,57,67,69, etc., are observed for the PDFs also. Thus, examination of both the positive- and negative-ion spectra of PVA suggests that PVA type structural units could account for the presence of hydroxyl groups in these PDFs. Similarly, XPS results suggested that ether groups were present in these PDFs. The negative-ion spectrum of poly(ethylene oxide) (PEO) displays OH(CH2CH20).CH2CHz0and H(CH2CH20),CH2CH20-ions (7). The fact that such clusters are not observed in the negative-ion spectra of these PDFs indicates that adjacent PEO type units are not present on the PDF surface. However, such units may be present but not in molecular proximity, which would prevent the formation of OH(RO),- and H(R0); cluster ions. If isolated PEO type units are present, then the only prominent oxygen-containing anion in the PEO negative-ion spectrum would be the m / z 41 anion. The positive-ion spectrum of PEO contains clusters due to ions derived from the repeat unit with the general formula [(CH2CH20),H]+.Peaks at m/z 45,89,133,and 177 correspond to n = 1-4. While the absence of ions corresponding to more than one repeat unit may be attributed to the absence of adjacent PEO type units, the absence of a significant m/z 45 peak indicates that even isolated PEO type units are probably not present (7). The base peak in the positive-ion spectrum of poly(propy1eneglycol) (PPG), namely m / z 59,is present as a low intensity peak in the positive-ion spectrum of the acetone PDFs (31). Thus, a low concentration of PPG type units may be present in these PDFs. On the other hand, all the major fragments associated with the positive fingerprint of poly(tetramethy1eneglycol) (PTMG) below m / z 100, i.e., m / z 55, 71,and 85,are also present in the positive-ion spectra of these PDFs (31). However, a prominent fragment in the PTMG positive-ion spectrum at m/z 73 (which is the base peak for PTMG (MW = 1000),is absent in the acetone PDF positive-ion spectrum. The above analysis suggests that backbone type ether units, if present, are low concentration PPG type units, rather than PEO or PTMG type units. Since derivatization XPS indicated that free carbonyl groups were present in these PDFs, we examined the static SIMS spectra of conventional carbonyl-functionalized homopolymers, poly(viny1methyl ketone) (PVMK), poly(viny1 ethyl ketone) (PVEK), and poly(methy1isopropenyl ketone) (PMIsPrK). While the details of their fragmentation pattern will be published elsewhere (29), comparison of the static SIMS spectra of PVMK and PMIsPrK with the PDF spectra
ANALYTICAL CHEMISTRY. VOL. 83, NO. 15, AUGUST 1, 1991
r'
1818
(a) Poly(viny1 methyl ketone)
U
41 mlz CHCO'
( b ) Poly(methyl lsopropnyl
1 0
E i
91
1
L 0
10
20
1
1 ,
1
,L ; (b) Poly(methy1 isopropenyl ketone)
ketone)j
;;
'.
41 mlz CHCO'
30
50
60
70
80
90 100
0
10
mlz
Flgwe 7. Posltlveion static SIMS spectra (mlz 0-100) of (a) PVMK and (b) PMIsPrK.
reveals the subtle structural features static SIMS is capable of elucidating for polymeric systems. The positive-ion spectra of PVMK and PMIsPrK, shown in Figure 7, indicate that, based on the fingerprint of these polymers, the ketone group present in the PDFs under study are likely to be either PVMK or PMIsPrK type units. Specifically, the presence of a base peak at m/z 43 (CH3CO+)for both polymers, and the absence of peaks (up to one repeat unit), not present in the PDFs, suggests that these units are likely to be present. Similarly, PVEK type units are unlikely to be present based on the fact that the dominant peak in the positiveion spectrum of PVEK (spectrum not shown) is at m / z 57, which is a minor peak in the positive-ion spectra of these PDFs. However, comparing the negative-ion spectra of PVMK and PMISPrK (Figure 8) with the those of the carbonyl precursor PDFs (e.g., Figure 5a) reveals that the relative intensity of the m/z 57 [M - CHIpeak in the negative-ion spectnun of PVMK is much greater than in the PDFs under investigation. On the other hand, all the molecular anions present in the negative-ion spectrum of PMIsPrK are present in the PDFs. Also, the relative intensities of these molecular anions are similar to those in the negative-ion spectra of these PDFs, suggesting that isolated PMIsPrK typeunits are more likely to be present in the PDFs than PVMK type units. Transmission FTIR of acetone plasma deposited on KBr powder (subsequently pressed into a pellet and analyzed) indicated the presence of carbonyl groups by the presence of a band at 1710 cm-I (17). Finally, the nature of the HC species present on the PDF surface must be considered. Clusters C1-C5display prominent HC ions of the type CnH2n+l,CnHsn4, and CnH2n-3.If the contribution of the CH,CO+ ion to the m/z 43 peak is removed, the C1-C5 fragmentation patterns for the PDFs are remarkably similar to the positive-ion spectra of saturated HC polymers. Specifically, it appears that, in these clusters, the positive-ion spedra of acetone and 2-butanone PDFs resemble that of polyethylene (PE) closely (7,21,22). However, the CH3CH0 PDF positive-ion spectnun displays relatively higher intensity peaks at m/z 55 and 69, which is characteristic of polypropylene (PP). It thus appears that the CH3CH0 PDFs may have more PP type units. It is interesting that these
20
30
40
50 mlz
60
70
80
90
100
Flgufs 8. Negatlvekn static SIMS spectra (mlz 0-100) of (a) PVMK
and (b) PMIsPrK. PDFs show less similarity in the C1-C6 clusters with those observed for branched HC polymers such as polyisobutylene (PIB) and poly(1-butene) (PlB). The above observations suggest that these PDFs display mainly PE type units, that the CH3CH0 PDF displays a higher relative concentration of PP type units, and that alkyl branches longer than a methyl substituent (as in PP) are not present in these PDFs. Also, methyl substituents at the same carbon atom (asin PiB) are unlikely. The onset of aromaticity was observed in the clusters C5 and higher (in addition to the aliphatic fragments that are present throughout) and confirmed by 13C and D labeling. Examination of the positive-ion spectra of the unlabeled carbonyl precursor PDFs reveals that the relative intensity of aromatic fragments (e.g., m/z 77,91,105,115,128, and 165) is higher than the more hydrogenated fragments present in the same cluster. The presence of aromatic ions may not necessarily be indicative of aromatic structures on the PDF surface. Rather, these fragments may indicate unsaturation in these PDFs, as evidenced by the presence of such ions in the positive-ion spectra of poly(cis-butadiene) and polyisoprene (21,22). The cross-linked nature of these PDFs must also be taken into consideration, which has been suggested by a number of studies on organic PDFs (2,3,32-34). Since cross-linking would effectively dehydrogenate the PDF surface, it is possible that the presence of aromatic fragments in the positiveion spectra of these PDFs is indicative not only of unsaturation but also of cross-linking. In general, the negative-ion spectra of HC polymers are structurally uninformative. However, the C-/CH2- ratio in the negative-ion spectra has recently been suggested to be a measure of unsaturation in these polymers (22). If this trend can be validated for oxygen-containing polymers, it may be possible to ascertain the degree of hydrogenation of these oxygen-containing organic PDFs. Efforts to systematically investigate the effects of cross-linking on the static SIMS spectra of HC and oxygen-functionalized polymers are currently under progress. CONCLUSIONS The static SIMS analysis of PDFs created from stable-
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 15, AUGUST 1, 1991
Table 111. Possible Oxygen-ContainingStructural Units Present in Carbonyl Precursor PDFs
polymer
PVA
X
H
Y
2
OH
dehydrogenation caused by HC cross-linking. However, at the present time, delineation between these two effeds cannot be achieved due to the lack of studies on model HC and oxygen-containing polymeric systems that incorporate welldefied levels of cross-linking and/or unsaturation. This will be a focus of future research In addition, we are investigating the use of multivariate statistical analysis techniques in order to systematize and quantify similarities of SIMS spectral features of organic PDFs to conventional homopolymers. Registry No. PS, 9003-53-6; CH3CH0,75-07-0; acetone, 6764-1; 2-butanone, 78-93-3.
LITERATURE CITED Table IV. Probable HC Structural Units Present in Carbonyl Precursor PDFs polymer
polyethylene
polypropylene poly(cia-butadiene) polyisoprene
structure -[CH,CH,-l, -[CH,CH(CHs)-l, -[CH,CH--CHCH,-], -[CH,CH=C(CHs)CHt-],
isotope-labeled carbonyl precursors allowed the HC secondary ions to be distinguished from oxygen-containingfragments. Specifically, the static SIMS analysis of the [1,2,3-1SC3]acetone PDF revealed that CH3CO+ was the primary oxygen-containing secondary cation in the positive-ion static SIMS spectra of these PDFs. Secondary cations of the general formula C,H,O+ (with lower intensities) could also be identified up to mlz 100. In this regard, it was observed that positive-ion static SIMS analysis of D-labeled PDFs was ambiguous. The negative-ion static SIMS spectra were dominated by C; and C,H- ( x = 1-4) clusters, which are characteristic of HC polymers. This corroborated the XPS and positive-ion SIMS results, both of which revealed these PDFs to be largely HC. Low-intensity, simple oxygen-containing molecular secondary anions (CHCO-, C3H60-,and CH3COO-) were also observed. A qualitative comparison of the oxygen-containing secondary ions (both positive and negative) with the spectra of simple homopolymers published in the literature provided some insights into the probable structural units that these secondary ions could arise from. The probable oxygen-containing species that are likely to be present in these PDFs are summarized in Table 111. Based on this analysis, it was suggested that polymeric vinyl oxygen-functionalized structural units, such as represented by PVA (hydroxyl), PMIsPrK (ketone), and PVAc (acetate),could account for the prominent oxygen-containing secondary ions present in the positive- and negative-ion static SIMS spectra of these PDFs. Additionally, acrylate, methacrylate, and acid groups were largely absent, due to marked dissimilarities between the static SIMS spectra of these PDFs and those of poly(alkyi acrylate) homopolymers, poly(alky1 methacrylate) homopolymers, PAA, and PMAA. The HC units probably present on the surface of these PDFs are summarized in Table IV. While the HC characteristics of these PDFs could be largely accounted for by PE type units, some branching in these PDFs is likely, based on the similarity with the static SIMS spectra of PP. The effect of unsaturation in these PDFs was suggested by the presence of aromatic fragments, which are observed predominantly above CBin the positive-ion spectra of these PDFs. The presence of such aromatic fragments may also be related to
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s..
m.
RECEIVED for review October 29,1990. Accepted March 28, 1991. ”his research was supported by the National Institutes of Health under Grant RR01296.
