TOF CID Study of Polysulfone

Mar 25, 2009 - Sparkle T. Ellison, Anthony P. Gies*, David M. Hercules and Stephen L. Morgan. Department of Chemistry and Biochemistry, University of ...
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Macromolecules 2009, 42, 3005-3013

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Py-GC/MS and MALDI-TOF/TOF CID Study of Polysulfone Fragmentation Reactions Sparkle T. Ellison,† Anthony P. Gies,*,‡ David M. Hercules,‡ and Stephen L. Morgan† Department of Chemistry and Biochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208, and Department of Chemistry, Vanderbilt UniVersity, 7330 SteVenson Center, NashVille, Tennessee 37235 ReceiVed January 23, 2009; ReVised Manuscript ReceiVed March 2, 2009

ABSTRACT: We report a combination of the evaporation-grinding MALDI sample preparation method, MALDI-TOF/TOF CID, and Py-GC/MS to examine the fragmentation mechanisms of polysulfone (PSF). MALDI-TOF/TOF CID studies yield a wealth of information about the mass, structure (linear or cyclic), and end-groups of species generated in the synthesis and fragmentation of these polymers. Additionally, Py-GC/MS experimental data are presented, for comparison of the multimolecular free radical reactions in pyrolysis with the unimolecular fragmentation reactions of MS/MS. TOF/TOF CID results indicate that linear PSF undergoes main chain fragmentation at characteristic locations. However, cyclic species produce “unexpected” fragment ions, presumably due to their different gas-phase configuration and the multiple main-chain breaks required to generate CID spectra. These results are supported by Py-GC/MS data and are consistent with our proposed degradation mechanisms.

Introduction Poly(aryl ether sulfone)s are transparent, amorphous thermoplastics that exhibit mechanically tough and rigid properties as well as temperature and thermal oxidative resistance due to the diarylsulfone group in the main chain backbone.2,3 The most commercially utilized poly(aryl ether sulfone)s is polysulfone (PSF).4 The sulfone group in PSF draws electrons from the benzene rings, thus making the entire group resistant to oxidation.4 The presence of the isopropylidene linkage in the repeat unit also lowers PSF’s use-temperature and resistance to organic solvents, when compared with other poly(aryl ether sulfone)s.3 The ether and isopropylidene linkage impart flexibility to PSF, which increases the polymer’s toughness, and improves the melt properties which facilitates processing at lower temperature.4 The degradation pattern of a polymer is an important step in choosing a material for product manufacture. For instance, PSF finds many uses in cookware, fluid-handling pipes and medical devices; however, its outdoor use is limited due to poor UV stability.3,4 PSF’s extensive applications (and likelihood of human exposure) render its degradation behavior worthy of examinations in particular with the recent health concerns regarding polymers synthesized with bisphenol A. Traditionally, analytical pyrolysis has been employed to study the structure of high-temperature polymeric materials which are difficult to fragment and study by any other means.5 Almen and Ericsson6 used Py-GC/MS and Py-GC-FID/FPD to determine the formation rates of SO2 during the pyrolysis of polysulfone resin and poly(1,4-phenylene ether-sulfone). Ohtani et al.7 used Py-GC/MS to characterize PSF when pyrolyzed in the presence of tetramethylammonium hydroxide. Montaudo et al.8 used direct pyrolysis-mass spectrometry to study the thermal degradation of poly(ether-sulfone) and poly(phenylene oxide). Perng9 applied stepwise Py-GC/MS and TGA to poly(ether-sulfone) and PSF to determine pyrolysis products, thermal stabilities and develop degradation mechanisms. However, until the present * Corresponding author. Telephone: (615) 343-5980. E-mail: A.Gies@ Vanderbilt.edu. † Department of Chemistry and Biochemistry, University of South Carolina. ‡ Department of Chemistry, Vanderbilt University.

study, there has been no MS/MS study to examine the fragmentation and degradation of isolated PSF chains. Previous work has shown that a combination of both Py-GC/MS and TOF/TOF CID is needed to fully explain degradation of polymers.10-12 Here we report a unified explanation for PSF degradation observed both for the multimolecular free radical fragmentation of Py-GC/MS and the unimolecular fragmentation of TOF/ TOF CID. Comparisons will focus on relating the continuous input of kinetic energy during Py-GC/MS with the “pulsed” input of kinetic energy during TOF/TOF CID to support our unified degradation mechanisms for PSF. These studies examined a series of model 25-, 50-, and 100-mer PSF polymers which allowed us to custom tailor the content of dichloro-capped linears (Figure 1A), cyclic (Figure 1B), and chloro-hydroxycapped linears (Figure 1C) within each polymeric mixture. Experimental Section Polysulfone Synthesis. Three model Bisphenol A polysulfone (PSF) samples (n ) 25, 50, and 100 - Mn ≈ 11 000, 22 000, and

Figure 1. Structure of PSF oligomers: (A) dichloro-capped linear; (B) cyclic; (C) chlorohydroxy-capped linear.

10.1021/ma900161y CCC: $40.75  2009 American Chemical Society Published on Web 03/25/2009

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Table 1. Structural Assignments for Pyrolysis Products Observed in the Py-GC/MS Pyrograms Reported in Figure 2

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Figure 2. Py-GC/MS pyrograms of PSF Mn ) 22 000 Da (50-mer) taken at: (A) 700 °C; (B) 850 °C. Sample size: 0.5 mg.

44 000), were synthesized via step-growth polymerization.13 All reagents were obtained from commercial sources and used as received after analysis by Py-GC/MS to verify their purity. The experimental conditions (i.e., monomer reaction ratios) were chosen, based upon Carother’s equation, to produce the desired degree of polymerization. For example, to produce a polymer with 25 repeat units (n ) 25): 50 moles of 4,4′-dichlorodiphenylsulfone were reacted with 48 moles of Bisphenol A. Likewise, n ) 50 and n ) 100 polymers require dichlorosulfone:diol mole ratios of 50:49 and 50:49.5, respectively.13 Number-averaged molecular weights (Mn) were determined by size exclusion chromatography (SEC) and MALDI-TOF MS. The structures of the oligomers are shown in Figure 1. MALDI-TOF/TOF CID Measurements. All samples were analyzed using an Applied Biosystems 4700 Proteomics Analyzer MALDI-TOF/TOF MS (Applied Biosystems, Framingham, MA) equipped with 355-nm Nd:YAG lasers. All spectra were obtained in the positive ion mode using an accelerating voltage of 8 kV for the first source and 15 kV for the second source and a laser intensity of ∼ 10% greater than threshold. The grid voltage, guide wire voltage, and delay time were optimized for each spectrum to achieve the best signal-to-noise ratio. The collision energy in both TOF instruments is defined by the potential difference between the source acceleration voltage and the floating collision cell; in our experiments this voltage difference was set to 1 kV. Air was used as a collision gas at pressures of 1.5 × 10-6 and 5 × 10-6 Torr (which will later be referred to as “low” and “high” pressure, respectively). All spectra were acquired in the reflectron mode with a mass resolution greater than 3000 fwhm; isotopic resolution was observed throughout the entire mass range detected. External mass calibration was performed using protein standards from a Sequazyme Peptide Mass Standard Kit (Applied Biosystems) and a three-point calibration method using Angiotensin I (m ) 1296.69 Da), ACTH (clip 1-17) (m ) 2093.09 Da), and ACTH (clip 18-39) (m ) 2465.20 Da). Internal mass calibration was subsequently performed using a PEG standard (Mn ) 2000; Polymer Source, Inc.) to yield monoisotopic masses exhibiting a mass accuracy better than ∆m ) ( 0.1 Da. The instrument was calibrated before every measurement to ensure constant experimental conditions. All samples were run in Dithranol (Aldrich) doped with sodium trifluoroacetate (NaTFA, Aldrich). The samples were prepared using the evaporation-grinding method (E-G method)1,14,15 in which a 2 mg sample

Polysulfone Fragmentation Reactions 3007 of polysulfone was ground to a fine powder using an agate mortar and pestle. Then molar ratios (with respect to the moles of polymer) of twenty-five parts matrix and one part cationizing agent (NaTFA) were added to the finely ground polymer along with 60 µL of distilled tetrahydrofuran (THF, Fisher). The mixture was ground until the THF evaporated after which the residue that accumulated on the sides of the mortar was pushed down to the bottom of the vessel. The mixture was then ground again to ensure homogeneity. A sample of the mixture was then pressed into a sample well by spatula on the MALDI sample plate. MS and MS/MS data were processed using the Data Explorer 4.9 software (Applied Biosystems). Py-GC/MS Measurements. Polysulfone samples (Mn ≈ 11 000, 22 000, and 44 000) were analyzed using a Pyroprobe 2000 heated filament pyrolyzer (CDS Analytics, Oxford, PA) interfaced to an HP G1800C GC/MSD instrument (Palo Alto, Ca) (electron ionization - 70 eV) via a CDS 1500 valve interface (CDS Analytics, Oxford, PA) kept at a constant temperature of 270 °C. The injection port and transfer line temperatures were both set at 270 °C. A 0.5 mg polymer sample and quartz wool (which prevents any nonvolatile components from escaping the sample tube) were placed into a quartz sample tube; the tube was positioned inside of the coil filament and allowed to purge under helium for 3 min in the pyrolysis interface. The pyrolyzer coil was then heated to 400 °C at a rate of 20 °C/ms and held for 10 s. The mass spectrometer was scanned from 40 to 500 Da. After obtaining the GC/MS, the quartz insert was removed from the pyrolyzer coil and flame cleaned. Additional samples were analyzed with the pyrolyzer coil temperature raised in 100 °C increments, until a temperature of 1200 °C was reached. For the 50-mer PSF sample, additional measurements were made at final pyrolysis temperatures of 750 and 850 °C. The separation was performed using a 30 m (0.25 i.d. and 0.25 µm film thickness) HP-5 MS capillary column (Agilent, Palo Atlo, CA). The column oven temperature was programmed to start at 50 °C for 3 min, ramp at 7.0 °C/min for 31.50 min, and held at 270 °C for 4.50 min, for a total heating cycle of 39 min.

Results and Discussion While previous Py-GC/MS studies have reported mechanistic information for the thermal degradation of PSF,6,9,16 to date none has compared data from the multimolecular free radical reactions of Py-GC/MS with data from the unimolecular fragmentation of MS/MS. We will first present results from Py-GC/MS that include fragmentation mechanisms and order of preferential bond cleavage. MS/MS data will then be presented, followed by a comparison of these mechanisms with those from Py-GC/MS. Pyrolysis-GC/MS. The pyrolysis products were identified using library matching software, where a match of 85% or greater was accepted. The following terminology will be used when relating the pyrolysis products shown in Table 1 with their peak intensities observed for the 50-mer PSF (Mn ) 22 000 Da) pyrograms shown in Figure 2: major (>50% base peak, BP); medium (30-50% BP); small (7-30% BP); minor (3-7% BP); trace (700 °C). This compound is observed in Py-GC/MS from 850 to 1200 °C, however, it is most abundant at 900 °C, where it is approximately 5% of the base peak. At temperatures higher than 900 °C, diphenylsulfone most likely exudes SO2 to form biphenyl (species 1-15), which

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Table 3. Structural Assignments for Peaks in the MALDI-TOF/TOF CID Mass Spectra Reported in Figure 4

Scheme 4. TOF/TOF CID Fragmentation for Linear PSF Oligomer

is consistent with the pyrolysis data showing a decrease in species 1-31 and increase in species 1-15 at these temperatures.8 Previous work by Perng,16 involving similar instrumentation and analysis conditions, could have detected these sulfonated species. However, no assignments were reported for a single sulfonated PSF fragment. The second “new” species is 4-chlorodiphenylsulfone (species 1-32). The presence of chlorine on this compound suggests that this species is a fragment from a linear PSF chain with the end group (Cl) still intact. Since the sample is a combination of both linear and cyclic chains, the low intensity of this peak is attributed to its small percentage in the cyclic rich 50-mer PSF sample. MALDI-TOF/TOF CID. By correlating the results of MALDI-TOF MS and collision induced dissociation (CID), additional structural information about polymers can be determined from fragment peaks of precursor ions, along with identification of end-groups. MALDI-TOF MS was initially performed on each sample to determine the “ideal” peak series for CID fragmentation. CID fragmentation was conducted on a variety of polysulfone precursor ions; however, it was discovered that the low effective kinetic energy of our TOF/TOF CID instrument (with respect to PSF fragmentation) was limited to the lower molecular weight portion of the PSF molecular mass distributionsmolecular masses above 2000 Da did not generate usable CID spectra for PSF. Furthermore, cyclic PSF species presented an additional challenge for CID fragmentation, Vida infra. For the sake of simplicity, we will focus our discussion on the fragmentation of the dichloro-capped linear species at m/z ) 1635.2 (n ) 3) (Figure 3) and the cyclic species at m/z

) 907.2 (n ) 2) (Figure 4), where the number of repeat units “n” corresponds to the mass numbers found. Table 2 summarizes the masses of the ion peak series observed in MALDI-TOF/ TOF CID mass spectra from a linear polysulfone oligomer ion: PSF, 1635.2 Da (n ) 3). Table 3 summarizes the masses of the ion peak series observed in MALDI-TOF/TOF CID mass spectra from a cyclic polysulfone oligomer ion: PSF, 907.2 Da (n ) 2). All values listed are for sodium-cationized polysulfone ions, unless noted otherwise. MS/MS allows the study of unimolecular polymer degradation (due to gas phase/low pressure dilution) in which a complete array of fragment ion masses is detected in one single, highly resolved spectrum.12 Based on the Py-GC/MS results (Scheme 1), one would expect primary CID fragmentation of PSF to occur at the Ph-SO2 bond. The CID “pulse” input of kinetic energy is much shorter in duration than the continuous kinetic energy input of Py-GC/MS, therefore, fragmentation at the Ph-O and Ph-C(CH3)2 linkages will occur at a much lower rate in CID. Most of the secondary reactions should involve radicals formed by the cleavage of the Ph-SO2 bond. The MS/MS spectrum of linear PSF (Figure 3) shows fragment peaks of greatest intensity at 1017.2 and 1459.4 Da corresponding to fragment 2-1 (n ) 2 and 3, respectively), which are formed by a single main-chain fragment at the Ph-SO2 bond. The intensity of the 2-1 fragment peaks confirms that this was the major fragmentation pathway. Of the seven fragment series noted in Table 2, three involve fragmentation of the Ph-SO2 bond (2-1, 2-5, and 2-6). Series 2-5 and 2-6 require the fragmentation of two Ph-SO2 bonds. Only

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Macromolecules, Vol. 42, No. 8, 2009 Scheme 5. TOF/TOF CID Fragmentation for Cyclic PSF Oligomer

two series, 2-2 and 2-4, are formed by a single main chain fragmentation at the Ph-O cleavage, whereas series 2-7 is formed by two main chain cleavages at the Ph-O bond and a methyl shift. Series 2-3 involves a single main chain fragmentation of the Ph-C(CH3)2 bond. The fragmentation of Ph-C(CH3)2 and Ph-O bonds show that MS/MS has enough energy to induce cleavage at these higher-strength bonds, however, the predominance of Ph-SO2 bond fragmentation shows that this bond prefers to break. The fragmentation pattern for linear PSF is shown in Scheme 4. The MS/MS fragmentation of cyclic PSF requires more kinetic energy than linear PSF fragmentation. This increase in kinetic energy is a direct result of cyclic structures needing to break in two places along the main chain, instead of one, to produce fragment ions. As a result, fewer fragments are formed, even at such high kinetic energy, as shown in Figure 4. Also, cyclic PSF is a bit of an “oddball” in that its predominant fragmentation pathway (Scheme 5) is not consistent with the major degradation pathways observed by Py-GC/MS or CID fragmentation of linear PSF, Schemes 1 and 4, respectively. The observed bond breaking for cyclic PSF is not the result of bond cleavage at the Ph-SO2 but rather at the Ph-O bond. The MS/MS spectrum for cyclic PSF shows three intense peaks at 225.1, 449.1 and 673.2 Da that correspond to fragments 3-1, 3-2, and 3-3, respectively. Each of these fragments (Table 3) is a result of two Ph-O bond breaks. Table 3 also shows other fragments (formed via cleavage of Ph-O bonds) whose peak intensities are much smaller but observable, nonetheless. A reasonable explanation for this prevalent cleavage is that the Ph-O bond is stretched due to a strained cyclic conformation, thus weakening this bond. The atoms of the diaryl sulfone group are fixed in a rigid spatial configuration, while the ether linkage is more flexible.4 By stretching the Ph-O bond, the amount of energy required to fragment this bond is much lower. This will result in preferential cleavage of the Ph-O bond, even though the Ph-SO2 bond may be the weakest link, based upon BDEs for linear PSF. This explanation is supported by our attempts to fragment dimeric, trimeric, and tetrameric cyclic PSF. We were only able to fragment the cyclic dimer, but not

the cyclic trimer or tetramer. The trimer and tetramer are larger and not as strained as the cyclic dimer, thus the Ph-O bond is not as stretched in larger cyclic structures. Conclusions The multimolecular free radical reactions in pyrolysis were compared with the unimolecular fragmentation reactions of MS/ MS to establish fragmentation patterns for PSF. Using Py-GC/ MS, we were able to not only predict preferred fragmentation located based on BDE, but we also identified two previously unreported sulfone-containing fragments. We found that CID fragmentation of higher molecular weight PSF is not easy due the high energy needed to fragment the bonds of this high strength polymer. We were also able to make predictions CID fragmentation of low molecular weight linear PSF using fragmentation patterns that were establish by Py-GC/MS. The CID fragmentation of cyclic PSF did not follow the same patterns, possibly as a result of its conformation. Acknowledgment. The authors would like to thank Jon F. Geibel, formerly of Chevron Phillips Chemical Company, LP, for his helpful synthesis advice. Supporting Information Available: Text giving further details about the mass spectra and figures showing the polysulfone overall MALDI-TOF mass spectra, uncropped TOF/TOF CID spectra, and Py-GC/MS electron ionization fragmentation spectra, and a table showing the structures of species SI-1-SI-4. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Gies, A. P.; Nonidez, W. K.; Anthamatten, M.; Cook, R. C.; Mays, J. W. Rapid Commun. Mass Spectrom. 2002, 16, 1903–1910. (2) El-hibri, M. J.; Nazabal, J.; Equiazabal, J. I.; Arzak, A. Poly (aryl ether sulfone)s. In Handbook of Thermoplastics; Olabisi, O., Ed.; Marcel Dekker: New York, 1997; pp 893-950. (3) Odian, G., Principals of Polymerization, 3rd ed.; John Wiley & Sons, Inc: New York, 1991; pp 155-157.

Macromolecules, Vol. 42, No. 8, 2009 (4) Harris, J. E., Polysulfone. In Engineering Thermoplastics: properties and applications, Margolis, J. M., Ed.; Marcel Dekker: New York, 1985; pp 177-200. (5) Wampler, T. P. Applied Pyrolysis Handbook, 2nd ed.; Wampler, T. P., Ed.; CRC Press: New York, 2007; pp 1-46. (6) Alme´n, P.; Ericsson, I. Polym. Degrad. Stab. 1995, 50, 223–228. (7) Ohtani, H.; Ishida, Y.; Ushiba, M.; Tsuge, S. J. Anal. Appl. Pyrolysis 2001, 61, 35–44. (8) Montaudo, G.; Puglisi, C.; Rapisardi, R.; Samperi, F. Macromol. Chem. Phys. 1994, 195, 1225–1239. (9) Perng, L. H. Poly. Sci. Part A: Poly. Chem. 2001, 81, 2387–2398. (10) Gies, A. P.; Ellison, S. T.; Vergne, M. J.; Orndorff, R. L.; Hercules, D. M. Anal. Bioanal. Chem. 2008, 392 (4), 627–642. (11) Gies, A. P.; Vergne, M. J.; Orndorff, R. L.; M., H. D. Anal. Bioanal. Chem. 2008, 392, 609–626.

Polysulfone Fragmentation Reactions 3013 (12) Gies, A. P.; Vergne, M. J.; Orndorff, R. L.; Hercules, D. M. Macromolecules 2007, 40, 7493–7504. (13) Yang, H. H., Aromatic High-Strength Fibers; John Wiley & Sons, Inc.: New York, 1989; p 873. (14) Gies, A. P.; Nonidez, W. K. Anal. Chem. 2004, 76, 1991–1997. (15) Gies, A. P.; Nonidez, W. K.; Anthamatten, M.; Cook, R. C. Macromolecules 2004, 37, 5923–5929. (16) Perng, L. H. J. Polymer Sci. Polymer Chem. 2000, 38, 583–593. (17) Cottrell, T. L. The Strengths of Chemical Bonds, 2nd ed.; Academic Press, Inc: London, 1958; pp 242-243. (18) Kiran, E.; Gillham, J. K.; Gipstein, E. J. Appl. Polym. Sci. 1977, 21, 1159–1176.

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