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Interchain Ion Formation in Secondary Ion Mass Spectrometry Resulting from the Double-Helical Structure of Isotactic Poly(methyl methacrylate) in Adsorbed Monolayers R. W. Nowak,† J. A. Gardella, Jr.,*,† T. D. Wood,† P. A. Zimmerman,‡ and D. M. Hercules§
Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14260-3000, Intel Corporation, 5000 West Chandler Boulevard, Chandler, Arizona 85226, and Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235
Results from time-of-flight secondary ion mass spectra (TOF-SIMS) of Langmuir-Blodgett monolayers of various isomers (isotactic and syndiotactic) of poly(methyl methacrylate) (PMMA) are reported. A detailed analysis of the repeating pattern of fragment ion clusters yields very different patterns for isotactic PMMA LB layers than for the syndiotactic and atactic forms. This is attributed to the resulting double-helical tertiary structure of isotactic PMMA, a structure that does not form for the syndiotactic and atactic PMMA polymer monolayers. The doublehelical structure of isotactic PMMA monolayers is verified using reflection absorption Fourier transform infrared spectroscopy. The repeating patterns of cluster ions in syndiotactic and atactic PMMA monolayers can be explained using statistical chain-breaking models for fragmentation of single isolated polymer chains. The repeating ion patterns from the TOF-SIMS of the isotactic PMMA monolayers are analyzed by considering bond breaking and ion formation between adjacent polymer chains, resulting in a newly proposed ion formation model due to the tertiary structure of the double-helical form. A rearrangement mechanism consistent with all ions that are formed is proposed. The study of secondary and tertiary structures of macromolecules at surfaces and interfaces is important in understanding how changes in structure due to adsorption, absorption, or denaturation translate into changes in function or properties.1-3 In particular, the stability and function (i.e., compatibility) of materials and the development of adhesion or adhesion in biological environments (so-called biocompatibility and bioadhesion) are effected through chemically and biologically specific interactions of substratum materials transmitted through structural changes in adsorbed biological macromolecules.4-8 Our research †
State University of New York at Buffalo. Intel Corp. § Vanderbilt University. (1) Vroman, L.; Adams, A. L. Surf. Sci. 1969, 16, 438. (2) Norde, W.; Lyklema, J. J. Biomater. Sci., Polym. Ed. 1991, 2, 183. (3) Andrade, J. D. In Surface and Interfacial Aspects of Biomedical Polymers. Vol. 2: Protein Adsorption; Plenum Press: New York, 1985. (4) Gristina, A. G. Science 1987, 237, 1588-95. ‡
10.1021/ac991447x CCC: $19.00 Published on Web 08/30/2000
© 2000 American Chemical Society
groups have been interested in the understanding of particular features of polymer surface chemistry that promote or control bioadhesion and protein adsorption.5,6 Both groups have been active in the development of methods to describe polymer surface structure.9-13 One approach is the development of methods to determine and quantify structure at surfaces via the use of monolayer and thin-film model systems.12,13 Secondary ion mass spectrometry (SIMS) has been utilized to study polymer surface structure and reactivity for many years. Early work documented sensitivity to monomer (including isomeric) structures, surface reactions and impurities.14-16 More recently, time-of-flight (TOF) mass analysis and high (time) mass resolution detection technology have extended the range of characterization to molecular weight distributions9-11,17 and imaging of lithographically modified polymers,18,19 among other applications. For some years, the Hercules group has made efforts to understand the fundamentals of high-mass fragmentation for ion formation from polymeric chains,9-11 At high mass, common (5) Langer, R.; Peppas, N. Science 1994, 263, 1715-20. (6) Sevastianov, V. I. Crit. Rev. Biocompatibility 1988, 4, 109. (7) Ranieri, J. P.; Bellamkonda, R.; Jacob, J.; Vargo, T. G.; Gardella, J. A.; Jr.; Aebischer, P. J. Biomed. Mater. Res. 1993, 27, 917-25. (8) Bekos, E. J.; Ranieri, J. P.; Aebisher, P.; Gardella, J. A., Jr.; Bright, F. V. Langmuir 1995, 11, 984-9. (9) Bletsos, I. V.; Hercules, D. M.; Greifendorf, D.; Benninghoven, A. Anal. Chem. 1985, 57, 2384. (10) Bletsos, I. V.; Hercules, D. M.; van Leyen, D.; Benninghoven, A. Macromolecules 1987, 20, 407. (11) Bletsos, I. V.; Hercules, D. M.; Magill, J. H.; van Leyen, D.; Niehuis, E.; Benninghoven, A. Anal. Chem. 1988, 60, 938. (12) Gardella, J. A.; Jr.; Pireaux, J. J. Anal. Chem. 1990, 62, 419. (13) Li, J.-X.; Johnson, R. W., Jr.; Gardella, J. A., Jr. Secondary Ion Mass Spectrometry as Applied to Thin Organic and Polymeric Films Produced by Langmuir Blodgett and Self-Assembly. In Characterization of Organic Thin Films. Vol. 12. Materials Characterization Series: Surfaces, Interfaces, Thin Films of Materials; Ulman, A.; Ed.; Manning Publications: Greenwich, CT, 1995; pp 193-212. (14) Gardella, J. A., Jr.; Hercules, D. M. Anal. Chem. 1980, 52, 226-32. (15) Gardella, J. A., Jr.; Hercules, D. M. Anal. Chem. 1981, 53, 1879-84. (16) Gardella, J. A., Jr.; Novak, F. P.; Hercules, D. M. Anal. Chem. 1984, 56, 1371-5. (17) Hittle, L. R.; Altland, D. E.; Proctor, A.; Hercules, D. M. Anal. Chem. 1994, 66, 2302-12. (18) Vargo, T. G.; Thompson, P. M.; Gerenser, L. J.; Valentini, R. F.; Aebischer, P.; Hook, D. J.; Gardella, J. A., Jr. Langmuir 1992, 8, 130-4. (19) Vargo, T. G.; Gardella, J. A., Jr.; Calvert, J. M.; Chen, M.-S. Science 1993, 262, 1711-2.
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Figure 1. RA-FT-IR results from single monolayers of LB films of iso- and syn-PMMA and a solution-cast monolayer equivalent of iso-PMMA on polycrystalline silver substrata. Top shows region due to ester carbonyl stretching mode; bottom shows C-C and C-O stretching regions.
fragmentation pathways result in a series of repeating ion clusters, separated by the monomer mass. This is a result of chain breaking along the backbone with other possible bond-breaking, neutral molecule loss, radical cation stabilization, or rearrangement possibilities occurring. Consideration of all and any of these mechanisms can be summarized in statistical chain-breaking models which focus on the repeating ion cluster pattern. These have been useful in characterizing most ion formation pathways.20 A recent example of this is a systematic study summarizing all fragmentation pathways of a series of methacrylate polymers with different ester side-chain functionalities. A single scheme considered the polymers as independent extended chains on a surface and was successful in classifying all ion formation pathways that could be detected from methacrylate polymers prepared by submonolayer deposition, which produces dispersed polymer chains at a metal surface.20 One deviation from this successful model was subsequently reported,21 focusing on structurally pure isotactic poly(methyl methacrylate) (iso-PMMA) versus syndiotactic poly(methyl methacrylate) (syn-PMMA) and atactic poly(methyl methacrylate) (atactic-PMMA). While the statistical chain-breaking model easily accounted for all ion structures from syn-PMMA and atactic PMMA through established fragmentation pathways, none of the repeating cluster ions at high mass from iso-PMMA could be explained. The present investigation is one step in a study of high-mass ion formation mechanisms due to varied hydrogen bonding leading to different tertiary structures. We demonstrate the sensitivity of secondary ion formation mechanisms in SIMS to the (20) Zimmerman, P. A. Hercules, D. M.; Benninghoven, A. Anal. Chem. 1993, 65, 983-91. (21) Zimmerman, P. A.; Hercules, D. M. Appl. Spectrom. 1994, 48, 620-2.
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tertiary structure of well-ordered monolayers of PMMA. Secondary structures of PMMA, in particular stereoregular PMMAs, are well known as model systems for investigations into the fundamentals of solid-state and surface structure. PMMA has a simple carbon backbone with a methyl group side chain and a methyl carboxylate side chain attached to the R-carbon of the backbone. The two different stereoisomers of PMMA investigated in the present study are distinguished by stereoisomerization; in particular the stereorelationship of the methyl carboxylate side groups for each: isotactic (same side) and syndiotactic (alternating side). The isomeric purity is measured by the triad tacticity percentage and is over 97% for this polymer.22 The isomers differ only in the longrange arrangement of the side groups in relationship to one another. These different secondary structures give rise to different tertiary configurations due to steric hindrance and interchain molecular forces. The secondary structure of iso-PMMA has been shown to produce a double helix in the solid-state crystal23,24 and, subsequently, upon production of a well-ordered LangmuirBlodgett (LB) monolayer.25,26 Syndiotactic and atactic PMMA both form single helices when prepared as a LB monolayer.27,28 By using this ordered monolayer model system, we hope to observe the influences of the different helical tertiary structures on secondary ion formation at the monolayer/surface. The differences observed in the high-mass range of the TOF-SIMS results are a (22) Brinkhuis, R. H. G.; Schouten, A. J. Macromolecules 1992, 25, 6173-8. (23) Kusanagi, H.; Tadokoro, H.; Chatani, Y. Macromolecules 1976, 9, 531-2. (24) Bosscher, F.; ten Brinke, G.; Eshuis, A.; Challa, G. Macromolecules 1982, 15, 1364-8. (25) Brinkhuis, R. G. H.; Schouten, A. J. Makromol. Chem., Macromol. Symp. 1991, 46, 335-9. (26) Brinkhuis, R. G. H.; Schouten, A. J. Macromolecules 1992, 25, 6174-8. (27) Apel, U. M.; Hentschke, R.; Helfrich, J. Macromolecules 1995, 28, 1779. (28) Belopol’skaya, T. V. Vysokomol. Soedin, Ser. A 1972, 14, 640-5.
Figure 3. Schematics of possible intrachain rearrangement reactions for iso-PMMA: (1) All possible combinations of how the methyoxy group on side chain reacts with each possible atom in adjacent side chain, e.g., methoxy-methoxy, methoxy-ester single bond oxygen, methoxy-ester carbon, methoxy-carbonyl oxygen, and methoxy backbone carbon. (2) All possible combinations of how the oxygen group on side chain reacts with each possible atom in adjacent side chain. (3) Same as previous except reaction involves C atom of the arbonyl site on side chain. (4) Same as previous except with O atom of the carbonyl site on side chain. (5) Same as previous except with backbone C with side group attachment.
Figure 2. High-mass fragmentation pattern from monolayer LB firms of syn- and iso-PMMA. Top compares region m/z ) 1000-3000. Bottom shows expansion of region m/z 1025-1175 and shows repeating cluster patterns.
consequence of the unique chain coiling of the iso-PMMA in comparison to syn-PMMA27,28 giving rise to our assertion that the different tertiary structures cause different ion formation mechanisms. EXPERIMENTAL SECTION In the present work, LB methods are used to produce isoPMMA and syn-PMMA and transfer these to cleaned29,30 polycrystalline silver (Aldrich, 99.9%) at 12 mN/m, which produces crystalline, close-packed monolayer films.17 The polymers were obtained from Scientific Polymer Products Inc. (Ontario, NY). The silver was washed three times with detergent (Sparkleen, Fisher) and rinsed with triply distilled water. The substrate was then sonicated in methanol, chloroform, and hexane, each for 10 min. The substrate was then placed in a radio frequency glow discharge cleaning chamber (Harrick Plasma Cleaner PDC-23G) and cleaned in an argon plasma on a low setting for 10 min. The substrates were stored under triply distilled water until use. The polymers were dissolved in chloroform (Fisher HPLC grade) to a concentration of ∼1% weight/volume. A KSV2200 Langmuir-Blodgett trough was filled with triply distilled water, and the substrate was submersed below the air-water interface. The solutions were spread (∼50 µL) on trough water at the (29) Dybal, J.; Krimm, S. Macromolecules 1990, 23, 1301-8. (30) Cohen, L. R. H.; Hercules, D. M.; Karakatsanis, C. G.; Rieck, J. N. Macromolecules 1995, 28, 5601-8.
interface. The solvent was allowed to evaporate for 30 min, and the film was compressed to a surface pressure of 12 dyn/cm at a velocity of compression (Vc) of 1.0 mm/min. The substrate was pulled through the interface at a velocity of deposition (Vd) of 1.0 mm/min. A monolayer of each polymer was deposited on the Ag substrate. Infrared spectra were acquired with a Nicolet Magna FT-IR spectrometer 550, Nicolet Instruments Corp., Madison, WI. A Harrick versatile reflection attachment with retro-mirror assembly (no. VRA-XXX-1) with a Harrick double diamond polarizer (no. PDD-J2R), Harrick Scientific Corp. Ossining, NY, was used to collect the reflection absorption spectra. All spectra were recorded with an 85° angle to the surface normal. The sample and reference spectra were an average of 5000 scans. The resolution of the instrument was 4 cm-1 with Happ-Genzel apodization. A DTGS detector was used. Ag foil was used as a substrate. The FT-IR was purged with dry N2 for 3 h at the start of the day’s experiments and 1 h after each sample introduction. We recorded time-of-flight SIMS spectra from 0 to 3500 Da of all LB monolayer samples under conditions similar to those for analysis of submonolayer films reported previously.24-26 Details of the Ion-ToF III TOF-SIMS31 have been described elsewhere.21 The instrument has a 10-keV argon source which was operated with a current of ∼0.5 pA and a pulse width of ∼1.0 ns. The spectra were taken in the ion counting mode. The data acquisition time was 300 s, with a cycle time of 200 µs and a time resolution of 1.25 ns/channel. The ion gun was operated in the pulsed mode at 5000 Hz. The primary ion dosage was less than 1013 ions/cm2, which is considered the static mode. Three LB firms of each (31) Nowak, R. W. Ph.D. Dissertation, Department of Chemistry, SUNY Buffalo, August 1996. 1995, 28, 5601-8.
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Figure 4. Typical spreadsheet illustrating combinatorial consideration of reactions between chain.
Figure 5. Proposed structure of resulting fragment which is cationized by silver to form fragment ion. Formula for reaction product after rearrangement is illustrated at left.
stereopolymer were prepared and analyzed in three locations per sample. RESULTS AND DISCUSSION We verified the structure of the LB monolayer film of iso-PMMA as a double helix using reflection absorption Fourier transform infrared spectroscopy (RA-FT-IR) measurements in a manner similar to that reported previously for multilayers of iso and syn-PMMA.25 The salient results from analysis of the monolayers are presented in Figure 1 and can be interpreted in comparison to Brinkhuis and Shouten’s infrared analysis.25 4588
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First, we compare the spectra recorded from monolayers of syn-PMMA and iso-PMMA in the region assigned to vibrations due to the carbonyl functional group near 1740 cm-1 (Figure 1A). There is a difference in band position and shape when the results are compared from the different preparations of iso-PMMA and syn-PMMA. This must be due to the differences in environment near the carbonyl between the two ordered films, and the present result is exactly consistent with data recorded by Brinkhuis. Figure 1A shows the carbonyl peak for iso-PMMA LB films is symmetrical and clearly centered at 1735 cm-1, while the same region for synPMMA shows a main peak at 1740 cm-1, with a low-frequency shoulder near or at the same frequency as the iso-PMMA LB film. A second comparison can be made between results from the analysis of iso-PMMA LB film and a solvent-cast monolayer equivalent of iso-PMMA. The latter preparation was accomplished to test the thesis that such a preparation would likely produce a film of lower crystallinity, and this should show a different band shape and position. Note that the carbonyl region of the iso-PMMA solvent-cast film shows a strong intensity at 1745 cm-1, with a low-frequency shoulder, and is clearly different from the (evidently) more crystalline iso-PMMA LB film. Similar shifts are also exhibited in the region between 1400 and 1100 cm-1 (Figure 1B), which represents vibrations due to C-C bonding and the C-O portion of the ester side chain. As Brinkhuis assigned, clear differences in the C-C bands near 1300 are noted between isoand syn-PMMA. For example, the shoulder at 1296 cm-1 is assigned to vibrations of the delocalized backbone and methylene carbon-carbon bonds characteristic for crystalline iso-PMMA.
Figure 6. Proposed interchain ion formation mechanism. For details, see text.
Specific shifts of the C-O band from 1148 (iso-PMMA LB film) to 1162 cm-1 (iso-PMMA solvent cast) are also exhibited. The similarity of the infrared results to those published previously25,27,28 indicates that the iso-PMMA is transferred as a monolayer in a crystalline state in which the polymer chains are organized in double-helical structures. The differences in environment of the iso-PMMA LB film in a crystalline structure are evidence of changes in the strength of interchain (hydrogen-like) interaction at the ester linkage between chains.27 This would be consistent with the crystalline structure of the double helix determined by X-ray diffraction and molecular dynamics calculations.5,6,26 The shift of the carbonyl band from 1745 cm-1 in the amorphous iso-PMMA to a value of 1735 cm-1 in crystalline LB films of iso-PMMA, a shift of 10 cm-1, is similar in magnitude to the shifts on ester carbonyl vibrations induced by solvent association of esters with acidic solvents such as phenol.32 This suggests a complex chemical interaction which weakens the carbonyl bonds due to the interaction of the methyl carboxylate side chain from the adjacent polymer chain in the double helix. For the TOF-SIMS results, a distribution of peaks characteristic of the repeating pattern of clusters of ions from fragmentation of the chains and cationization by silver was detected in the range from a few hundred to several thousand daltons. A portion (10003000 Da) is shown in Figure 2A, with an expansion of the range in Figure 2B to note the 100.12-Da spacing between repeating clusters. This difference corresponds to the monomer mass and represents chain breaking at subsequent points along the chain, yielding different chain lengths. Each different cluster in the data
from syn- (and atactic, vide infra) PMMA could be reconciled with the statistical chain-breaking scheme reported previously for the solvent-cast films.20,21 Again, that scheme considers all possible rearrangements for a single, isolated polymer chain. The results for iso-PMMA shown in Figure 2B (labeled fragments A-C at 55, 85, and 41 Da, respectively), demonstrate that the three repeating ion clusters are unique. They are not observed in the spectra from syn-PMMA. These fragments could not be described by the statistical ion formation scheme reported for the methacrylate series.20 They must obviously be due to some other ion formation mechanism.21 In the present results, the infrared characterization of the LB films allows a guarantee of the structure of the films, where previous preparations using solution submonolayer casting followed by annealing20,21 did not allow for independent structural determination. The close proximity of the chains in the double helix of LB preparations of iso-PMMA would suggested to us that an ion formation mechanism that incorporates fragmentation and rearrangement between adjacent polymer chains in the helix might be considered. As noted above, the proximity of the side chains in the double helix leads to a significant shift in the carbonyl vibrational frequency, similar to that of an acidic solvent association. Before considering interchain-initiated fragmentations and rearrangements, we first reviewed all commonly known gas-phase rearrangement reactions to find possible mechanisms for ion formation.33 None were consistent with the fragments identified in the isotactic PMMA. To consider interchain-initiated fragmentation and rearrangements, we noted that crystal structures of crystalline iso-PMMA15 show the acrylate groups rotated slightly
(32) Bellamy, L. J. The Infrared Spectra of Complex Molecules, Chapman and Hall: New York, 1980; Vol. 2, p 157.
(33) McLafferty, F. W., Ed. Mass spectrometry of organic ions; Academic Press: New York, 1963.
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inward to the center of the double helix in close proximity to each other. The entanglement of the double helix provides a close locked-in conformation which may increase the likelihood of mechanisms otherwise energetically improbable. There are several possible interchain bond-breaking and reformation reactions that could be considered between the two chains. To examine all potential ion structures that could result from such a reaction/rearrangement, a combinatorial analysis using spreadsheets was developed. Thirty different spreadsheets were constructed, each calculating values within the cells due to different masses resulting from unique reaction products (i.e., the creation of a new bond from bond breaking). The possible reaction combinations are detailed in Figure 3 and a spreadsheet in Figure 4. All possible combinations of bond breaking and subsequent bond re-formation between two polymer side groups in close proximity to yield this reaction product were considered. Each spreadsheet result considers one reaction product and all the possible end groups reported in the previous study.20 The fragment resulting from the interchain reaction is detailed in four parts in Figure 5: reaction product, monomers, and two end groups. The reaction product mass in the upper left cell of the spreadsheet in Figure 4 includes the sum of the mass of the reaction product from Figure 5, the mass of a multiple number of monomers, and the mass of a cationized Ag+ ion. The two end groups are represented by the average (i.e., weighted average of the isotopes) masses in the heads of the columns and rows (in bold). The reaction of the two chains has created a fragment with two end groups. The other cells of the spreadsheet represent the masses of all possible fragments created during the intrachain reaction. The spreadsheet illustrated in Figure 5 is one of the 30 possible different reactions explored and detailed in Figure 3. It is the particular reaction described from site 4 (large arrow in Figure 3). This sheet was selected for display because it contains all of the fragments observed in the isotactic PMMA SIMS spectra in Figure 2B. These fragment masses were not observed in any of the other 29 spreadsheets. The results in Figure 4 reveals three repeating clusters for the iso-PMMA result from a single reaction product. These are fragments A (m/z ) 1055 (C48O18H82Ag)), B (m/z ) 1085 (C48O20H80Ag), and C (m/z ) 1141 (C52O20H88Ag)). That reaction product was shown to result from the loss of C4H4O2 from the two monomer units. A proposed structure for this formula is shown in Figure 5, which provides a typical structure for the (34) Wolynes, P. G. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 2426-7. (35) Wood, T. D.; Chorush, R. A.; Wampler, F. M., III.; Little, D. P.; O’Conner, P. B.; McLafferty, F. W. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 2451-4.
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cluster labeled fragment B, at 1085.02 Da. The loss of C4H4O2 indicates that the new fragment contains two fewer double bonds and /or rings. Figure 6 presents a proposed rearrngement mechanism consistent with the strength of the interactions between iso-PMMA strands in the double helix and the close proximity of the two side chains. This mechanism is only a “possible “ mechanism and cannot be proven by the methods described in this paper. Part 1 illustrates the transfer of a hydrogen from an adjacent ester methyl group to the carbonyl oxygen from the side groups, followed by the ester oxygen attachment to the methylene on the main chain to create a new chain from the two chains. We hypothesize that the acidic-like interactions between ester groups lead to the 10cm-1 shift in the carbonyl vibration, as evidence of the possibility of this first step. Part 2 illustrates a methyl shift from the ester oxygen to the carbon on the ester carbonyl. Part 3 depicts the elimination of C4H4O2 to yield the final reaction product, a new polymer chain formed from two adjacent chains. There is some precedence for invoking intrachain rearrangements to explain the TOF-SIMS fragmentation ion results from functional polymers. Cohen et al.30 used such a mechanism to explain a series of fragment ions with cyclic structures from the TOF-SIMS analysis of polyester polyurethanes. However, this is the first known report of a verified tertiary structure leading to unique fragmentation mechanisms which could only be due to interchain rearrangements. Further, the fact that the structure which leads to this ion formation mechanism is a double-stranded helix suggests that significant efforts in mass spectrometry to characterize the tertiary structure of macromolecules34,35 would hold promise for understanding their fundamental structural and reactivity properties. In our case, determination of the existence of such a structure at a material surface, and its quantification, may lead to a better understanding of how surface chemistry elicits structural changes in biological macromolecules to signal specific cellular processes such as bioadhesion and growth.7,8 Secondary ion mass spectrometry among other mass spectrometry methods would seem to provide valuable information for this determination. We are continuing to pursue this approach.31
Received for review December 16, 1999. Accepted June 27, 2000. AC991447X
