Time-of-Flight Secondary Ion Mass Spectrometric Analysis of Polymer

Apr 28, 2007 - Time-of-Flight Secondary Ion Mass Spectrometric Analysis of Polymer Tertiary Structure in Langmuir Monolayer Films of Poly(dimethylsilo...
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Anal. Chem. 2007, 79, 4126-4134

Time-of-Flight Secondary Ion Mass Spectrometric Analysis of Polymer Tertiary Structure in Langmuir Monolayer Films of Poly(dimethylsiloxane) Alan M. Piwowar and Joseph A. Gardella, Jr.*

Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14260

A series of model systems of poly(dimethylsiloxane) (PDMS) of molecular mass 2400 Da and low polydispersity (1.09) were prepared using the Langmuir-Blodgett technique to investigate the effects of tertiary structure on the ion formation probability in time-of-flight secondary ion mass spectrometry (TOF-SIMS). Using data from the measured surface pressure-area isotherm for PDMS ordered monolayer, films were transferred to silver mirror substrates from the various regions in the isotherm where structural changes are observed. Two particular conformations of the polymer are examined here: a linear caterpillar-like structure and a coiled helical structure. Reflection absorption Fourier transform infrared spectroscopic analysis indicates structural changes for the films related to adjustments to the orientation of the polymeric backbone consistent with the formation of two distinct structures. The polymeric changes translate into differences in ion formation probability of fragments in both the low (1000 Da) mass range. Data are also presented to analyze how tertiary structure may affect the apparent polydispersity index calculated from the TOF-SIMS spectra. The molecular characteristic central to an understanding of the physical properties of any type of polymer chain is its spatial configuration.1 Identifying surface tertiary structure can help to better understand molecular function and reactivity and can also help in designing surfaces to aid in adhesion, lubrication, corrosion protection, and bioorganic processes. In order to understand and identify surface structure, analytical techniques need to be extremely sensitive to small changes in orientation. As stated by Dong et al., “Mass spectrometry (MS) has been proven as a powerful method for the structural characterization of organic molecules due to its high sensitivity, high dynamic range, specificity, and selectivity and has been used to study polymers since the 1970s”.2 However, despite the obvious advantage of using mass spectrometry to investigate tertiary structure, only a few known articles have been reported in the literature dedicated to this topic.3-5 In 2000, Nowak et al. published a study documenting how differences in time-of-flight (TOF)-SIMS mass spectral ion formation mechanisms could distinguish between the tertiary structures * Corresponding author. E-mail: [email protected]. (1) Mark, J. E.Macromolecules 1978, 11 (4), 627-633. (2) Dong, X.; Proctor, A.; Hercules, D. M.Macromolecules 1997, 30, 63-70.

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of isotactic (double helix) and syndiotactic (single helix) poly(methyl methacrylate) (PMMA).3 Analysis of the repeating pattern of fragment ion clusters in the SIMS data yielded different patterns from the analysis of monolayer assemblies of double-helix isotactic PMMA as compared to monolayer assemblies of syndiotactic or atatic PMMA. This study was the first report of a verified tertiary structure leading to unique fragmentation mechanisms due to interchain rearrangements. In an effort to continue the development of this area of research for SIMS, we have begun a detailed investigation of another polymer, poly(dimethylsiloxane) (PDMS). The present study focuses on the differences in mass spectral ion formation probability between different tertiary structures of a macromolecule. PDMS is an excellent candidate for a tertiary structure investigation since it is known to adopt a series of structures at an air/water interface.6-9 At low surface pressures, the molecule assumes a flat orientation with its silicon and oxygen backbone in contact with the water surface and the methyl groups pointed toward the gas phase. Compression alters the orientation, and the polymer begins to adjust to the increased pressure by removing some of the silicon or oxygen atoms from the water subphase. Continued compression lifts the polymeric chains from the water, and eventually the molecules coil to form a helix. The polymer can be compressed and collected on a Langmuir trough, which provides an ideal environment for both the manipulation of the polymeric structure and Blodgett-style film transfer to a substrate suitable for SIMS analysis. The emphasis of this paper is to use differences in TOF-SIMS ion formation probabilities to distinguish tertiary conformations of model PDMS systems. We report results from the evaluation of fragmentation ions in the low- and high-mass range of TOFSIMS spectra of the model monolayer polymeric systems. The films were produced using the Langmuir-Blodgett technique of film preparation and structural differences were confirmed through (3) Nowak, R. W.; Gardella, J. A., Jr.; Wood, T. D.; Zimmerman, P. A.; Hercules, D. M. Anal. Chem. 2000, 72 (19), 4585-4590. (4) Rey-Santos, R.; Piwowar, A.; Alvarado, L. Z.; Gardella, J. A., Jr. Appl. Surf. Sci. 2006, 252 (19), 6605-6608. (5) Yan, W.-Y.; Gardella, J. A., Jr. Time of Flight Secondary Ion Mass Spectrometry Study of Ion Formation Mechanisms: Effects of End Group Chemistry; John Wiley and Sons: New York, 1998; Vol. XI, pp 451-454. (6) Fox, H. W.; Taylor, P. W.; Zisman, W. A. Ind. Eng. Chem. 1947, 39 (11), 1401-1409. (7) Noll, W.; Steinback, H.; Sucker, C. J. Polym. Sci., C 1971, 34, 123-139. (8) Lenk, T. J.; Lee, D. H. T.; Koberstein, J. T. Langmuir 1994, 10, 18571864. (9) Hahn, T. D.; Hsu, S. L. Macromolecules 1997, 30, 87-92. 10.1021/ac070056c CCC: $37.00

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the use of reflection-absorption Fourier transform infrared spectroscopy (RA-FT-IR). An ideal outcome of the interpretation of the mass spectrometry ion formation mechanisms would be their use to distinguish tertiary structures at the surface of thick polymer films using SIMS. EXPERIMENTAL SECTION Materials. Poly(dimethylsiloxane) end capped with trimethylsiloxy and isobutyl groups was purchased from Polymer Source Inc., Dorval, QC, and was used as received or diluted in reagent grade pentane (98% min.) purchased from Fischer Scientific. The PDMS used in this study had a weight average molecular weight (Mw) of ∼2400 (PDMS-2400) and a polydispersity index of 1.09 (as determined by GPC by the manufacturer). PDMS-2400 was also investigated by further by mass spectrometry.10,11 PDMS was transferred as monolayers or submonlayers onto silver substrates. The silver substrates were silicon wafers coated first with chromium and then with silver by vacuum evaporation. The silicon wafer was purchased from Atomergic Chemetals Corp. The wafer was boron-doped silicon (111) with a 60-mm diameter with a thickness of 350 µm. The wafer was first cleaned with acetone and then methanol and air-dried. Chromium was vapor deposited (51 Å) followed by the silver (∼300 Å). The vapor deposition was performed by Dr. Wayne Anderson and Joondong Kim in the UB Materials Research Instrument Facility Clean Room, located in the Department of Electrical Engineering at the University of Buffalo. The chromium layer was necessary to increase the adhesion of the overlayer material. Each mirror was broken into two halves for the RA-FT-IR experiment or broken into approximately 1 cm × 1 cm pieces for the TOF-SIMS experiment. The mirrors were cleaned by dipping them in a saturated KOH/ 2-propanol solution for ∼1 min followed by scrubbing and separate rinses with 2-propanol and triply distilled water. The mirrors were stored in triply distilled water until their use. RA-FT-IR. The data for the RA-FT-IR was acquired on a Nicolet Magna FT-IR spectrometer 550 (Nicolet Instruments Corp., Madison WI). A Harrick versatile reflection attachment with retromirror assembly and Harrick double-diamond polarizer, (Harrick Scientific Corp., Ossining, N.Y.) was used to collect the reflection absorption spectra. Each sample was aligned at 78° to the incoming beam for grazing angle reflectance. The sample spectra were an average of 1000 scans. The resolution of the instrument was 2 cm-1 with Happ-Genzel apodization. A DTGS detector was used. All of the data were analyzed using the spectral program Grams 386 (Galactic Software) and by Microsoft Excel. Langmuir-Blodgett Trough. All films were prepared using a KSV model 2200 thermostated Langmuir trough, with Blodgettstyle film-transfer device (KSV Instrument Ltd., Helsinki, Finland). The solutions of polymer were deposited via a 10-µL Hamilton microsyringe (Reno, NV) onto triply distilled water in the trough. The solution concentrations were ∼1.0 mg/mL. The dilute solution was chosen to ensure polymeric chain extension. The initial surface pressure at compression varied from 0.1 mN/m for caterpillar films up to 1.5 mN/m for helical coil films. Isotherms (10) Yan, W.-Y.; Gardella, J. A., Jr.; Wood, T. D. Am. Soc. Mass Spectrom. 2002, 13, 914-920. (11) Yan, W.-Y.; Ammon, D. M., Jr.; Gardella, J. A., Jr.; Maziarz, E. P.; Hawkridge, A. M.; Grobe, G. L.; Wood, T. D.Eur. J. Mass Spectrom. 1998, 4, 467-474.

of PDMS are not dependent on the initial surface pressure.12 The compression for each film was 4.0 mm/min and was held at constant surface pressure when a film was being transferred. Caterpillar-type films were transferred at a surface pressure of 0.9 mN/m while the helical coil films were transferred at a surface pressure near 8.5 mN/m. The film was withdrawn from the water/ polymer interface at a speed of 2.0 mm/min. Surface pressure measurements were made using a platinum Wilhelmy plate, which was flame cleaned prior to each experiment. The temperature of the subphase was maintained by circulating temperature-controlled water using a model CFT-25 refrigerated recirculator (NESLAB, ThermoScientific, Waltham, MA). To minimize vibrations from the recirculator to the Langmuir-Blodgett trough, the water flow from the cooling system was connected to a mounted adapter by neoprene tubing. TOF-SIMS. The TOF-SIMS used in this work was a Physical Electronics model 7200 equipped with an 8-kV Cs+ primary ion source used at that voltage for all experiments. The ion gun was operated in the pulse mode with the slit aperture set at 10 mil and the blanking aperture set to 10 mil. Positive ion spectra were recorded for all samples. A typical run for PDMS 2400 was a survey set for 0-3500 Da, at a resolution of 1.1 ns/step. A total of 500 frames were collected for each sample. The primary ion flux dosage was calculated to be 4.0 × 1012 ions/cm2, which is within the static limit. Beam current was measured at 0.6 pA or ∼400 ions/pulse with a spot diameter of 50 µm. The primary ion incident angle was 60°. The extractor was operated in the positiveion mode at 3 kV, and the reflectron voltage was maintained at 78 V. The operating pressure of the instrument was 4.5 × 10-9. The samples were conductive so no charge neutralization was necessary, and only positive secondary ions were analyzed. The data was analyzed using the ToFPak software program from Physical Electronics (Eden Prarie, MN). RESULTS AND DISCUSSION Polymeric monolayer systems of PDMS-2400 were prepared using the Langmuir-Blodgett method of film collection and were differentiated by RA-FT-IR analysis, described in detail elsewhere.13 Briefly, PDMS-2400 is dissolved in pentane as a dilute solution (1.0 mg/mL) to ensure chain extension and is pipetted onto a water subphase held constant at 20 °C. After the solvent evaporates (10 min after pipetting), the polymer is compressed until the desired structure (caterpillar or helical coil) has developed at the characteristic surface pressure.13 A silver mirror substrate is then pulled through the air/water interface transferring a monolayer or submonolayer polymer film. The structure of the polymer monolayer films was analyzed using RA-FT-IR to investigate the differences between the transferred films. Differences in tertiary structure were examined using two methods: comparison of the ratios of the 1050-cm-1 peak with the 1110-cm-1 peak and through a comparison of relative film thickness using the peak intensity at 1265 cm-1. These methods are described in detail elsewhere.13 Figure 1 shows the results of typical infrared data for the caterpillar and helical coil structures. These data are representative (12) Granick, S. Macromolecules 1985, 18, 1597-1602. (13) Piwowar, A.; Gardella, J. A. J. Reflection-Absorption Fourier Transfrom Infrared Spectroscopic Study of Poly(dimethylsilxoane) Films Transferred using the Langmuir-Blodgett Technique. In preparation.

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Figure 1. RA-FT-IR data comparing monolayer films of the caterpillar-like and helical coil structures of PDMS-2400 transferred on silver substrates.

of the results of RA-FT-IR measurements on three separate films, each transferred from a separate Langmuir film preparation. Intensities were measured for peaks corresponding to the SiO-Si asymmetric stretch and the Si-O-Si symmetric stretch located at 1050 and 1110 cm-1, respectively, for each spectrum from each film. The asymmetric stretch has a net dipole moment change parallel to the backbone, and the symmetric stretch has a net dipole change perpendicular to the backbone.14 Ratios of those intensity values were calculated and compared between the two sets of films. The ratio of I1050/I1110 for the helical coil and caterpillar films is 0.418 ( 0.008 (n ) 3) and 0.33 ( 0.06 (n ) 3), respectively. The difference in ratio values is consistent with changes in polymeric backbone orientation. The higher value for the helical coil films suggests that the polymer backbone has a higher degree of perpendicularity to the substrate surface, which is consistent with the formation of a flat helical coil. The relative thickness of the monomolecular films was also used to examine structural differences. The calculated molecular diameters of the caterpillar and helical coil structures are 5.6 and 12.5 Å, respectively.6 Since the Langmuir-Blodgett method of film casting intrinsically forms monolayer systems, it is assumed that a change in thickness for the films is due to the tertiary structure and density of the polymeric backbone and is not due to the formation of a multilayer system. The thicknesses of the films should therefore be proportional to the thicknesses of the theoretical structures. Further, differences in absorbances from the infrared data should correlate to the changes film thickness. A comparison was therefore made between the theoretical thickness values and the relative absorbance ratio measured from the infrared data. For that assessment, ratios of the theoretical thicknesses were compared to ratios of the 1265-cm-1 peak intensity. The peak located at 1265 cm-1 (Si-CH3 symmetric bend) was utilized because of its sharp peak shape and narrow width. The ratio of the caterpillar films to the helical coil films for the peak at 1265 cm-1 (Cat I1265/Hel I1265) was 0.45 ( 0.07 (n ) 3 for each film type). The high degree of error (further explained in the discussion section) is associated with the formation of the caterpillar films, which are transferred near the beginning of another structural transition.13 The ratio of the calculated thicknesses determined previously by Fox et al. for the caterpillar and 4128

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helical coil is 0.448. The agreement between the values supports the formation of the caterpillar and helical coil structures. Therefore, the infrared data support the formation of two different polymeric tertiary structures, and films transferred under these conditions were used for the TOF-SIMS study. The most interesting SIMS information in this study is found in the high-mass range (HMR) of the spectra from about 1000 to 3000 Da. However, the mass range below 250 Da was also investigated for structurally related information. The dominant peaks found below 250 Da belong to characteristic peaks for PDMS and were used for identification and calibrations. These peaks were as follows: 15 (CH3+), 28 (Si+), 43 (CH3Si+), 73 (CH3)3Si+, 107/109 (1Ag+), 147 (C5H15OSi2+), 207 (C5H15O3Si3+), and 221 Da (C7H21O2Si3+).2,15 For the investigation of the low-mass range (LMR), peak ratios were calculated and examined. In static SIMS, absolute peak intensities are an unreliable means of comparison because they can be affected by matrix effects, electronic effects, fluctuations in primary ion dose, and drift in primary beam current, all of which influence the intensity of ions.16,17 The use of ion intensity ratios is the simplest way to correct for these errors and it provides a reliable means of comparison. Figure 2 displays the ratio values for the LMR data collected for the caterpillar (n ) 3) and helical coil films (n ) 3) on the TOF-SIMS. These ratios were calculated by dividing the intensity of the major low-mass peaks (identified above) by the intensity for the peak located at 15 Da (CH3+). Potential contributions of contamination will be covered in the discussion section. Ratios for other peaks in the low-mass range were also calculated and show relative intensity differences as well (see Supporting Information). For Figure 2, a difference is observed between the ratio values for the two films. The data clearly show statistically significant differences in fragmentation (14) Tsao, M.-W.; Pfeifer, D.-H.; Rabolt, J. F.; Castner, D. G.; Haussling, L.; Ringsdorf, H. Macromolecules 1997, 30, 5913-5919. (15) Newman, J. G.; Carlson, B. A.; Michael, R. S.; Moulder, J. F.; Hohlt, T. A. Static SIMS Handbook of Polymer Analysis; Perkin-Elmer Corp.: Eden Prairie, MN, 1991. (16) Leeson, A. M.; Alexander, M. R.; Short, R. D.; Briggs, D.; Hearn, M. J. Surf. Interface Anal. 1997, 25, 261-274. (17) Vickerman, J. C. Surface Analysis: The Principal Techniques; John Wiley & Sons: Chichester 1997; pp 135-213.

Figure 2. Ratio values for TOF-SIMS low-mass-range data calculated for the caterpillar and helical coil films of PDMS-2400. The intensity of the identified peaks was divided by the intensity of the peak at 15 Da (CH3+). The diamonds correspond to data from caterpillar films, and the squares correspond to the data from the helical coil films.

Figure 3. High-mass range (1000-3000 Da) TOF-SIMS data for Langmuir-Blodgett films of PDMS-2400. Top: representative data for a caterpillar film. Bottom: representative data for a helical coil film.

for the two sets of films. The ratio data suggest that the difference in tertiary structure in films is affecting the fragment intensities in the LMR. Figure 3 displays the HMR (1000-3000 Da) TOF-SIMS spectra for the two types of Langmuir-Blodgett films collected. The HMR was the area primarily investigated because it provides the most data about the tertiary structure through information about the neutral long-chain oligomers and fragmentations along the chain. In the high-mass range, a series of five repeating peaks is observed for all films examined. Elman et al. described a typical PDMS spectra consisting of an intense series of peaks that arise from cationization by silver (e.g., M + Ag+) of intact oligomers.18 They also described the formation of weaker ensembles of peaks that arise from cationized oligomers with further fragmentation processes, involving the loss of a species containing the end group. It is widely noted that for PDMS a series of peaks form that also correspond to an integral number of repeat units cationized by silver.2,18-21

The peak series of highest intensity corresponds to the intact linear oligomer cationized with one Ag+ atom [M + Ag+]. Peak assignments for the oligomers were verified using the isotopic distribution simulation program Googly.22 The numbered peaks observed in the HMR in Figure 3 are therefore identified by the number of monomers present. For example, a peak labeled n ) 12 represents an oligomer with 12 monomeric units, end group 1 (isobutyl), end group 2 (trimethylsiloxy), and cationized with one silver atom. The molecular formula for n ) 12 would be C21H90O12Si13Ag+ with a molecular mass of 1128 Da. Cationization with silver is commonly observed in SIMS experiments when Ag substrates are used.19 The spacing between each oligomer peak is equal to the mass of 1 repeat unit (74 Da). The repeat pattern of three cationized oligomer peaks is observed in all films. The pattern results from ionization of oligomers formed from the three different initiatiors in the anionic polymerization conditions, with lithium silanates of one, two, and three monomer units formed from the ring opening.10,23 In addition to the oligomer peaks, four clusters of peaks are also observed in the HMR (see Figure 4). These peaks repeat every 74 Da and are located between the cationized neutral oligomer peaks. They are believed to be due to fragmentation from the neutral oligomer as described above. The masses of the four peaks correspond to average losses of 17, 32, 47, and 59 Da from an oligomer peak. The relationship between fragment losses of 17 Da (fragment I) and 59 Da (fragment IV) was investigated for this study to quantify intensity formation differences for the systems. Losses of 32 (fragment II) and 47 Da (fragment III) were not investigated due to low peak intensity. Panels a and b in Figure 5 show a comparison of fragments I and IV divided by the intensity of the oligomer at the immediate higher mass (In-17/In for fragment I and In-59/In for fragment IV) (18) Elman, J. F.; Lee, D. H. T.; Koberstein, J. T. Langmuir 1995, 11, 27612767. (19) Bletsos, I. V.; Hercules, D. M. Macromolecules 1987, 20, 407-413. (20) Dong, X.; Hercules, D. M. J. Phys. Chem. B 2001, 105, 3942-3949. (21) Bletsos, I. V.; Hercules, D. M.; Magill, J. H.; vanLeyen, D.; Niehuis, E.; Benninghoven, A. Anal. Chem. 1988, 60, 938-944. (22) Proctor, A. Googly; 1994. (23) Hawkridge, A. M.; Gardella, J. A., Jr. Am. Soc. Mass Spectrom. 2003, 14, 95-101.

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Figure 4. TOF-SIMS data of PDMS-2400 in the high-mass range showing the four clusters of fragments identified as I, II, III, and IV located between cationized neutral oligomers.

for the oligomers corresponding to 3n + 2 repeat units. The graphs show an increase in fragmentation ratio for caterpillar films up to nearly 2100 Da when compared to the helical coil films. Past 2400 Da, the fragment peak intensities are too low to retrieve usable information. The intensity ratio differences indicate that the tertiary structure in the caterpillar films either increases the fragmentation probability for the fragment clusters or has lower oligomer intensity values in this region. These data are typical for all ratio comparisons of this type (fragment/oligomer at higher mass (i.e., to the right)). While it is likely that the cluster peaks are related to the loss of mass from neutral oligomers, ratios were also calculated for clusters to other oligomers to investigate other possible relationships. Panels a and b in Figure 6 show the plots of fragments I and IV as a ratio to the lower mass oligomer peak located to the left (In+59/In for fragment I and In+17/In for fragment IV) for oligomers corresponding to 3n + 2. The plots are similar to the Figure 5a and b in that the caterpillar film ratio is higher than the helical coil film ratio. Values for the other oligomers (3n and 3n + 1) showed similar ratio separation. Ratios of the fragment peaks to the oligomer peaks either at higher or lower mass have shown statistical differences between the films. The difference supports the hypothesis that the tertiary structure affects fragmentations in the HMR. Another investigation was conducted in an attempt to normalize the peak intensities by plotting the ratio of the fragment peak by the sum total of that fragment (i.e., intensity of fragment I/Σ intensity of fragment I) as shown in Figure 7. Also, the fragment peaks were divided by the sum total for the oligomer intensity (i.e., intensity of fragment I or IV/Σ intensity of nx) and plotted in Figure 8. For Figure 7, both the fragment I ratio and the fragment IV ratio follow a similar pattern. Caterpillar films have a higher ratio value up until ∼1800 Da, where the values briefly become statistically equal after which helical coil films begin to have a higher ratio value. In Figure 8, a similar pattern forms, only with more error due to the integration areas for the neutral oligomer 4130

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peaks. The data show that, regardless of the normalizing function, the films follow a consistent pattern of caterpillar films having a higher ratio value in the early part of the HMR followed by helical coil having a higher ratio value past 2000 Da. This is in agreement with the ratio values discussed from Figures 5 and 6 and will be explored further in the discussion section. Values for the number-average molecular weight (Mn), weightaverage molecular weight (Mw), and polydispersity (PD) were calculated using the oligomer distributions from the TOF-SIMS data following procedures described by Elman et al.18 For this study, the area of each oligomer ion was recorded by integrating the area under each peak in the isotopic cluster individually and summing the total area as the intensity. The masses of the oligomer peaks were adjusted for Ag+ by subtracting the molecular weight of Ag from each oligomer peak. Oligomer peaks from n ) 10 to n ) 40 were included in the calculation. Oligomer peaks past n ) 40 were not used because they did not have an intensity of at least three times the background signal.16 In all, three films were used for the caterpillar calculation and three films for the helical coil calculations. The results are summarized in Table 1. The polymer sample information reported by the manufacturer was determined using size exclusion chromatography in THF at 35 °C. As with the intensity ratios in the HMR and the peak normalization calculations, the polydispersity data show differences between the two types of films. Specifically, the polydispersity data indicate that caterpillar films are composed of oligomers with lower average Mw and Mn than the helical coil films. DISCUSSION Infrared Data and Langmuir-Blodgett Film Preparation. The reflection absorption FT-IR data provide enough evidence to support the formation of two sets of films with different tertiary structures. One set of films consists of a caterpillar-like structure whose structure may be interpreted as having the polymer backbone lying flat on the substrate surface in a linear chain extension. For the second set of films, it is believed that the polymer chain is coiled into a flat helix with between six to eight monomers per turn.6,13,24 It should be noted that the region where the caterpillar structure is believed to exist is prior to a steep increase observed in a surface pressure-area isotherm, which indicates the transition to another structural state. The ideal location to transfer the caterpillar film would be at 0.1 mN/m. Films transferred at 0.1 mN/m do not absorb sufficiently in the infrared spectra due to poor substrate coverage; therefore, films were instead withdrawn at 0.9 mN/m, where useful infrared data could be collected. This led to an increase in error associated with the caterpillar-like films. However, the caterpillar films and the helical coil films have significantly different infrared spectral intensities and peak intensity ratios supporting the assignment of two distinct tertiary structures. An assumption throughout the presentation of the data is that the only difference between the two sets of films is the tertiary structure. Another discrepancy between the films that will be discussed briefly is the amount of time spent on the LangmuirBlodgett trough for collection. Helical coil films require ∼45 min of extra collection time compared to the caterpillar films. The increased time may affect the film surface by allowing for (24) Albouy, P.-A. Polym. Commun. 2000, 41, 3083-3086.

Figure 5. (a) Ratio values for the intensity of fragment IV (corresponding to a loss of 59 Da) divided by the oligomer intensity of immediate higher mass. Data corresponding to the caterpillar films are identified as diamonds while the data corresponding to the helical coil films are identified as squares. (b) Ratio values for the intensity of fragment I (corresponding to a loss of 17 Da) divided by the oligomer intensity of immediate higher mass. Data corresponding to the caterpillar films are identified as diamonds while the data corresponding to the helical coil films are identified as squares.

Figure 6. (a) Ratio values for the intensity of fragment I (corresponding to a gain of 59 Da) divided by the oligomer intensity of immediate lower mass. Data corresponding to the caterpillar films are identified as diamonds while the data corresponding to the helical coil films are identified as squares. (b) Ratio values for the intensity of fragment IV (corresponding to a gain of 59 Da) divided by the oligomer intensity of immediate lower mass. Data corresponding to the caterpillar films are identified as diamonds while the data corresponding to the helical coil films are identified as squares.

increased contamination. Nevertheless, PDMS is known to be “chemically inert” and has low surface energy, making the formation of a contamination layer rather difficult. We believe that the time difference spent during film preparation should therefore

have negligible effects on contamination levels. Also, PDMS is believed to have a higher volatility of low-mass oligomers than high-mass oligomers.10 The increased amount of time that the polymer spends on the water surface may allow for desorption of Analytical Chemistry, Vol. 79, No. 11, June 1, 2007

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Figure 7. (a) Ratio values of the intensity of fragment I divided by sum total of intensities for fragment I. Data corresponding to the caterpillar films are identified as diamonds while the data corresponding to the helical coil films are identified as squares. (b) Ratio values of the intensity of fragment IV divided by the sum total fragment IV intensities. Data corresponding to the caterpillar films are identified as diamonds while the data corresponding to the helical coil films are identified as squares.

Figure 8. (a) Ratio values of the intensity of fragment I divided by sum total of all detectable oligomer intensities. Data corresponding to the caterpillar films are identified as diamonds while the data corresponding to the helical coil films are identified as squares. (b) Ratio values of the fragment IV intensity divided by the sum total of all detectable oligomer intensities. Data corresponding to the caterpillar films are identified as diamonds while the data corresponding to the helical coil films are identified as squares.

lower molecular weight oligomers, thereby altering the polydispersity (discussed below). Low-Mass Range Ratios. The TOF-SIMS data show differences in either fragmentation probability or fragmentation mechanism for a polymer with different tertiary structures. The LMR ratio comparison shows a difference of values for caterpillar and helical coil films. The ratio values in Figure 2 show a distinct difference in the formation ability of either the 15-Da peak (CH3+) or the main peaks used for the ratio. The CH3+ peak is acknowledged by the authors to be possibly due to other species; 4132

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specifically hydrocarbon contamination. However, the reproducibility of the values as evidenced by the low degree of error supported the use for this study. Further, other ratios showed similar separation of values between the systems and substantiates the fragmentation differences between the films for the majority of the LMR peaks examined. Although the LMR peaks cannot be directly linked to the HMR oligomers, the presence of a difference between the ratios is supportive evidence of fragmentation differences based on tertiary structure.

Table 1. Number-Average Molecular Weight (Mn), Weight-Average Molecular Weight (Mw), and Polydispersity As Determined by the Identified Structure or Techniquea film type

Mn

Mw

polydisperisty

manufacturer caterpillar helical coil

2200 1810 ( 30 1990 ( 20

2400 1980 ( 40 2130 ( 20

1.09 1.098 ( 0.003 1.072 ( 0.002

a The manufacturer’s data were determined using GPC by Polymer Source Inc. (Dorval, QC).

High-Mass Range Ratios. For the ratio comparison of the fragments with the oligomer peaks, the difference in values is clearly noticeable. For the peaks where intensity values were able to be distinguished from the background, the caterpillar films fragment with higher intensity than helical coil films. The ratio difference diminishes as the values approach higher masses. The normalization of fragment I or IV using the respective sum total of the fragments shows a similar trend. The data show that caterpillar films have a higher fragmentation ratio than the helical coil for fragment I up to 1852 Da and for fragment IV until 1810 Da. If we follow the trend that the fragment peaks are a loss from a neutral oligomer, an approximation to the values at which fragments I and IV begin to favor the helical coil ratios corresponds to n ) 22. It can be safely stated then that the fragment ratios favor the caterpillar films from ∼10 monomer units up to∼22 monomer units, after which helical coil films begin to have a higher ratio value. The change in ratio may be due to the presence of polymers with longer molecular chains for the helical coil films, which is supported by the polydispersity data. Polydispersity. The polydispersity values calculated may also be influenced by tertiary structure. The data show that a difference of Mw, Mn, and PD is observed between the two systems. This leads to two possible conclusions: either the polydispersity index calculated from the TOF-SIMS data is affected by the tertiary structure or the transferred films contain different distributions of polymers with varying molecular weight due to compression. It appears from the data that the caterpillar films form predominately from the lower molecular weight polymer chains while helical coil films consist of slightly higher molecular weight polymer chains. Again, both sets of films are transferred from the same solution consisting of the same polymer oligomer distribution. But as pointed out in the discussion of the compression time, it is possible that the lower molecular weight fragments are evaporating from the film surface in the helical coil collection. This may affect the polydispersity calculation. Fragmentation Mechanism. The peaks of most importance to this study are found in the HMR. For the present study, we compared results from the HMR with the results of other studies of high-mass TOF-SIMS analysis of PDMS to determine the similarities and differences in assignments for common ion structures. Dong et al. described the formation of two intense clusters of ion signals from monolayer/submonolayer films of trimethylsilyl (also referred to as trimethylsiloxy)-terminated PDMS in the high-mass range; (i) a silver-cationized linear fragment having a formula identical with that of the intact oligomer and (ii) a series corresponding to an integral number of repeat

units cationized by silver that likely have a cyclic structure.2 They discussed the use of chain scission as the simplest fragmentation mechanism for which initial bond-breaking events occur at weak points in the polymer chain, but denounced its effectiveness based on unfavorable bond dissociation energies. Dong et al. assumed that the siloxane chain forms a random coiled structure rather than a linear orientation and stated that, “during the fragmentation process two siloxane bonds are broken while two new siloxane bonds form simultaneously, requiring no net energy change”, forming cyclic fragments of the polymer. In the high-mass range, the cyclic species increase in intensity when moving to higher molecular weight samples with respect to the peaks corresponding to the linear species. Additionally, they studied the effects of end groups on the fragmentation mechanism from which they determined that, when a Si-O bond or a Si-C bond is cleaved, the charge resides on silicon instead of carbon or oxygen. For the high-mass range comparison of the siloxane polymer terminated with various end groups, the previously mentioned two intense clusters (linear and cyclic) form. Additionally, two other series of clusters were detected in the HMR that may be related to the formation of ions with intrinsic charge (no Ag+ cationization) through the loss of end group fragments. Elman et al. examined monolayer and multilayer films of R,ωfunctional PDMS oligomers with pentylamine and propylcarboxy end groups, which were manufactured using both the Blodgett method of film transfer and by spin coating.18 They also calculated the molecular weight distribution of the films using end group titration, size exclusion chromatography, and the SIMS data. For the pentylamine spin-coated samples, two homologous families of peaks occurs with one series corresponding to an oligomer species, where a single pentylamine chain end has been removed in the fragmentation process and where the second peak series is assigned to the intact oligomer chain cationized by one silver ion. For the Langmuir-Blodgett-Kuhn (LBK) film of the same polymer type, an additional series of peaks forms, which the authors proposed to be due to the formation of an [M + H]+ species, which corresponds to a protonated oligomer. The LBK film of the propylcarboxy-terminated PDMS contained only one series of peaks corresponding to an [M + H]+ species. Bletsos et al. examined TOF-SIMS spectra in the HMR for 1-mm-thick films of PDMS cross-linked with methylvinylsiloxane using a Ni conducting grid to avoid charging.21 For the thick sample, there are two intense series of peaks corresponding to PDMS fragments or cyclic species cationized with Ni ([nR + Ni]+) and fragments with lower intensity corresponding to a [nR - CH3 + Ni]+ species. Elman pointed out that a later interpretation of this less intense family was designated as [M - CH3]+.18 Bletsos et al. also examined cast films of PDMS deposited on silver substrates.19 The fragmentation pattern again is explained to consist of intact polymer chains cationized with Ag+ and peaks corresponding to [nR + Ag]+. Comparison of these four studies2,18,19,21 results in a welldocumented cationization mechanism of siloxane oligomers, leading to peaks corresponding to an oligomer with a defined number of monomers and end groups cationized with a metal (either Ag+ or Ni+). For our study, this corresponds to the series of peaks of highest intensity in our HMR. The present work shows a comparison of peak location and relative intensity of the Analytical Chemistry, Vol. 79, No. 11, June 1, 2007

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cationized neutral oligomer peaks with a Googly isotopic distribution simulator, which are nearly identical, with a peak intensity and peak location linear regression of 0.99 for n ) 16 (an oligomer with 16 repeat units). A second series of peaks observed in this study corresponds to the formation of cyclic groups of PDMS monomers cationized with Ag+, which has been proposed elsewhere.2,19,21 For this model, it appears likely that the polymer backbone fragments twice during the bombardment process excising both end groups. The formation of this species corresponds to fragment IV from our data. A comparison of the experimental data with the isotopic spectral program yielded a linear regression of peak location of 0.99, with a linear regression comparing intensities of the peaks of 0.78 for n ) 16. There is no detectable series of peaks in the HMR in this study corresponding to protonated oligomer species [M + H+] described by Elman.18 Bletsos’ proposed fragmentation21 of [M CH3]+ is also not detected in our spectra. Another possible fragmentation pathway discussed by some of the authors is the loss of a chain fragment containing the end groups, giving rise to an oligomer species with intrinsic charge.2,18 Information provided by the manufacturer describes our polymer chain as possessing both a trimethylsiloxy and sec-butyl end group. For the following discussion of end group fragmentation we will refer only to the loss of species from an intact oligomer chain and not discuss additions to the polymer backbone since this would be redundant. Fragment losses of 57 (sec-butyl or SB) and 73 Da (trimethylsiloxy or TMS) were therefore considered, without silver cationization. The first consideration was for a neutral oligomer with the loss of 57 Da, corresponding to the fragmentation loss of SB. A series of peaks corresponding to fragment I is located near a loss corresponding to [M - 57]+, but the isotopic cluster of the spectral simulation differs by 2 Da. The loss of a TMS end group is complicated due to the presence of an oxygen. If just the end group were to fragment, it would leave an unstable oxygen, which is unlikely to carry the positive charge required for separation and detection. Instead, we investigated a loss of the TMS end group and an oxygen atom from the backbone (loss of 89 Da) and determined that there were no detectable series of peaks corresponding to this species. An examination of chain fragmentation of the end groups and cationization by silver was also explored. One potential fragment would be the loss of the sec-butyl end group followed by cationization. This would leave the chain with an intrinsic positive charge, and the presence of an Ag+ atom would make the fragment a double-charged species. It is possible, however, that during the loss of the sec-butyl end group a hydrogen transfer takes place from the carbon atom adjacent to the silicon during the fragmentation process. This would give a species that is singly charged (due to the Ag+) that has a mass which corresponds to fragment IV observed in our spectra. A linear regression of peak location for this species is 0.99 while the linear regression of peak intensities is 0.79, only slightly higher than the linear regression for the proposed cyclic structure. In considering the loss of the TMS end group, a possible hydrogen transfer to an oxygen is possible, but the location of this fragment would overlap with the [M + Ag+] series of peaks and cannot be deconvoluted for investigation. A slightly modified version of this fragmentation 4134

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would include a loss of the SB group with a hydrogen transfer to an adjacent oxygen of the chain, along with Ag+ cationization. This species would correspond to fragment I from our data. The peak position linear regression and the linear intensity regression for this species is 0.99 and 0.60, respectively. Another possibility for the fragment I series corresponds to the cyclic structure, which instead of being cationized with Ag+ is protonated [nR + H]+. This species has a linear regression of intensity of 0.86 and a linear regression of peak location of 0.99. In total, it is not possible to associate a series of peaks with a particular fragment without an extensive MS/MS study of those fragments. We can, however, rule out any detectable presence of peaks corresponding to [M + H]+, [M - CH3], or intrinsically charged fragments that form from the loss of a single end group. The comparison with isotopic simulation software is an extremely useful tool, but can only be used properly if adequate data sufficiently devoid of noise can be collected. From the presentation of the data, it is clear that there are a number of theoretical fragments that can correspond to the observed location of peaks in experimental spectra. Further investigation will have to be performed to elucidate the structures of the fragments found in the high-mass range of the TOF-SIMS. With this in mind, for the data collected for this study, we conclude that the highest intensity of peaks in the HMR belongs to neutral oligomers cationized by silver [M + Ag+], Fragment IV is due to the detection of cyclic groups of the polymer cationized by silver [nR + Ag+], and fragment I is due to the detection of protonated cyclic groups of the polymer [nR + H+]. Fragment II is of low intensity, and assignments cannot easily be made. Fragment III has no precedence in assignments from other studies and has not been assigned a structure in the present study. CONCLUSION We reported differences in the intensities of low-mass ions and high-mass repeating fragment ions for different tertiary structures of PDMS-2400 in TOF-SIMS. The data reported support the conclusion that the linear caterpillar films fragment with higher intensity in the early parts of the high-mass range (n ) 10 to n ) 22) after which the helical coil films fragment with a higher intensity ratio. A difference in polydispersity is observed between the two sets of films, but may be due to compression time on the Langmuir-Blodgett trough and will be explored further. The fragmentation mechanism was also explored for this study. ACKNOWLEDGMENT The authors thank Dr. Richard Nowak for discussions. This work was supported by NSF grants CHE-0316735 and CHE0616916. NOTE ADDED AFTER ASAP PUBLICATION This article was released ASAP on April 28, 2007, the text associated with ref 2 was not designated as a direct quote. The current version was posted on May 1, 2007. SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review January 10, 2007. Accepted March 23, 2007. AC070056C