Mass Spectrometry of Synthetic Polymers - ACS Publications

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Anal. Chem. 2004, 76, 3417-3428

Mass Spectrometry of Synthetic Polymers Patricia M. Peacock and Charles N. McEwen*

DuPont Corporate Center of Analytical Sciences, Experimental Station, Wilmington, Delaware 19880-0228 Review Contents Scope Reviews Matrix-Assisted Laser Desorption/Ionization Fundamental Studies and Novel Sample Preparation Methods End Group Information Polymerization Mechanisms and Kinetics Copolymer Characterization Molecular Weight Distributions and Mass Moments Degradation and Fragmentation Information Quantitation Electrospray Ionization Kinetic and Mechanistic Studies Fragmentation Studies Characterization Studies Secondary Ion Mass Spectrometry Surface Characterization Plasma-Treated Surface Studies Fundamental Studies Gas Chromatography/Mass Spectrometry Pyrolysis Studies Pyrolysis and Thermogravimetric Analysis Identification of Additives Other Mass Spectrometric Methods for Studying Polymers, Copolymers, and Blends Thermogravimetric Analysis Pyrolysis and Thermal Degradation Other Methods Conclusion Literature Cited

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SCOPE The scope of this review is limited to articles published in 2002 or 2003 and includes only publications dealing with mass spectrometry of synthetic polymers. This literature review covers fundamental studies as well as applications of mass spectrometry (MS) to polymer characterization. The review does not cover mass spectrometry of natural polymers such as proteins and carbohydrates. Even with such a focused review, it is necessary to make subjective choices regarding which work to include and what portion of the work to mention in this review. This is especially true with regard to applications of matrix-assisted laser desorption/ionization (MALDI) MS to polymer characterization. It is clear, at least for polymer research, that MALDI MS has exceeded pyrolysis (Py) MS as a tool applied to polymer analysis. Secondary ion mass spectrometry (SIMS) continues to be an important tool for polymer surface characterization. Combining chromatographic techniques as well as thermogravimetric analysis (TGA) with mass spectrometry has been the subject of a number of publications. 10.1021/ac040064i CCC: $27.50 Published on Web 05/11/2004

© 2004 American Chemical Society

The intent of this review is to provide sufficient information to point the reader to recent literature on the use of mass spectrometry to characterize synthetic polymers. Sections are arranged according to mass spectrometric technique to facilitate finding the appropriate literature for the MS techniques available to the reader. REVIEWS The previous review in this series, published in 2002, covers 2000 and 2001 and includes reviews and books on MS of polymers published during this period as well as pertinent references to polymer characterization work involving MALDI, electrospray ionization (ESI), SIMS, and various gas-phase ionization methods used to characterize synthetic polymers (1). Other reviews include a review of the advantages of MALDI MS for polymer characterization by Macha and Limbach that concentrates on the enabling developments in MALDI methodology for analysis of polymers (2). The utility of polar and nonpolar matrixes is discussed as well as important solvent considerations and some limitations of MALDI in polymer analysis. Sato reviewed the advantages of combining size exclusion chromatography (SEC) and MALDI MS to characterize polymers (3). This combination can provide accurate molecular distribution analysis. Murgasova and Hercules reviewed the coupling of separation methods, especially liquid chromatography (LC), with MALDI and ESI mass spectrometry for the analysis of polymeric materials (4). These hyphenated techniques are very powerful for the molecular characterization of complex polymer systems. Wyttenbach and Bowers discussed the application of ion mobility spectrometry combined with mass spectrometry for studying fundamental processes such as the conformations of ions produced from low molecular weight polymeric materials, the kinetics of structural interconversion of these ions, the kinetics of dissociation processes, and the thermochemistry of ligand addition reactions (5). Dalluge reviewed MALDI MS primarily as a tool for characterizing biomaterials but covers aspects of synthetic polymer analysis (6). Several reviews appeared that were concerned with aspects of polymer surface characterization. Mathieu et al. briefly discusses the use of SIMS in the analysis of polymer surfaces (7). In a review of polymer surface analysis methods, St. John et al. discusses the role of SIMS (8). Weng and Chan reviewed the use of SIMS as a quantitative tool in the analysis of copolymers and polymer blends (9). Two books have been published dealing with polymer characterization using mass spectrometry in the period covered by this review. MALDI-TOF Mass Spectrometry of Synthetic Polymers by Pasch and Schrepp provides fundamental as well as practical information on experimental procedures and technical aspects in the analysis of polymers using MALDI MS (10). Included in this Analytical Chemistry, Vol. 76, No. 12, June 15, 2004 3417

book are descriptions of instrumentation, important experimental parameters, ionization mechanisms, matrix materials, sample preparation, and interpretation. Identification of polymers is discussed in several chapters. Also discussed are molar mass determination, analysis of complex polymers, and the case for using MALDI MS in conjunction with separation methods. An introductory chapter discusses the molecular heterogeneity of polymers and the fundamentals of light scattering, ultracentrifugation, viscometry, and size exclusion chromatography for the determination of molar mass distributions. Mass Spectrometry of Polymers, edited by Montaudo and Lattimer, also covers a number of aspects involved in polymer analysis using MALDI MS but, in addition, discusses laser desorption (LD) MS, field ionization (FI) MS, field desorption (FD) MS, SIMS, ESI MS, LC-ESI MS, Py gas chromatography (GC) MS, and Py-MS (11). The chapters in this book include an introduction to MS of polymers, polymer characterization methods, polymer characterization using Py-MS and Py-GC/MS, ESI and LC-ESI MS, FDMS and FIMS, FAB-MS, TOF-SIMS, LD-FTMS, MALDI MS, and LD MS. Wyttenbach and Bowers discussed the applications of ion mobility spectrometry coupled to MS for studying gas-phase ion conformations in a chapter in Topics in Current Chemistry (5). Numerous talks (with published abstracts) on polymer characterization using mass spectrometry were presented at national scientific meetings. These meetings include The American Society for Mass Spectrometry 50th (2002, Orlando) and 51st (2003, Montreal) conferences on mass spectrometry and allied topics, and the 223rd (2002, Orlando), 225th (2003, New Orleans), and 226th, (2003, New York) ACS National Meetings. The National Institute of Standards and Technology held workshops on synthetic polymer characterization using MALDI MS in Gaithersburg, MD, in November of 2002 and 2003. Information on MALDI MS analysis of polymers and presentations from the latest workshops can be found at http://polymers.msel.nist.gov/maldirecipes/ maldi.html. A summary of the 2002 workshop has been published (12). MATRIX-ASSISTED LASER DESORPTION/ IONIZATION With regard to synthetic polymers, the most prolific ionization method in the recent literature is matrix-assisted laser desorption/ ionization time-of-flight (MALDI-TOF) mass spectrometry. Publications during the past two years covered many topics in this arena, from practical applications to fundamental studies of the technique, and research to expand its capabilities for polymer characterization. MALDI-TOF MS is used to determine repeat units, ends, and molecular weight distributions of homopolymers, copolymers, and synthetic blends. It provided valuable insight into reaction mechanisms, kinetics, and degradation. Structural information from MALDI-TOF MS experiments complement traditional analytical methods such as nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC). Separation methods combined with MALDI-TOF extended the realm of its applications. Studies were conducted to evaluate the use of MALDI-TOF for quantitation of synthetic polymers. New preparation methods, automation, ionization of nonpolar species and insoluble polymers were explored as well. 3418

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Fundamental Studies and Novel Sample Preparation Methods. Among the more novel topics in the recent literature, Meier et al. applied ink-jet technology to the automated preparation of MALDI target plates (13, 14). They employed a multiplelayering approach where the matrix, cation, and analyte were deposited as separate layers. Poly(ethylene glycol) (PEG) and poly(methyl methacrylate) (PMMA) were used to evaluate the usefulness of the method. Resolved peaks were observed up to 35 000 amu. Trimpin et al. successfully applied solvent-free sample preparation methodology to the analysis of poly(9,9-diphenylfluorene) (15). The MALDI-TOF MS data they acquired provided mechanistic information about the polymerization process of the higher molecular weight insoluble fraction of the polymer. They also discovered that using a new matrix, 7,7,8,8-tetracyanoquinodimethane, avoided fragmentation of the analyte. Similarly, Gies et al. used the same preparation method to characterize an insoluble polyimide (16). Their research found that the quality of the spectra depended upon the choice of matrix and the particle sizes used after evaporative grinding. Other new matrix materials investigated were pentafluorobenzoic acid (PFBA) and pentafluorocinnamic acid (17). These matrix materials were used to ionize hydrophobic perfluoropolyethers by Marie et al. In these experiments, PFBA was the more effective matrix, yielding intact silver cationized ions. Progress was made toward the ionization of polyethylene during the past two years. Yalcin et al. published on substrateassisted laser desorption/ionization using cobalt, copper, nickel, or iron metal powders as a sample substrate, with silver nitrate as the cationization reagent (18). They were able to observe intact ions of polyethylene up to 5000 amu. Two papers described methods to derivatize hydrocarbon polymers prior to molecular weight experiments using MALDI-TOF MS. Lin-Gibson et al. brominated terminal vinyl groups of narrowly dispersed polyethylene and then reacted the analyte with triphenylphosphine (19). Subsequent MALDI-TOF MS analysis showed a decrease in fragmentation and an increase in the observed mass range compared to other reports of polyethylene data. However, the mass moments measured were lower than those from other analytical methods. The other derivatization procedure entailed sulfonating the olefin ends of polyisobutylene before MALDI-TOF MS analysis. The molecular weight information supplied by MS agreed with values obtained using laser light scattering- and vapor pressure osmometry, except for high molecular weight wide polydisperse samples (20). Researchers at the National Institute of Standards and Technology (NIST) made use of autocorrelation to recognize patterns in high-resolution spectra of synthetic polymers and copolymers (21). They found autocorrelation also was useful in determining where the ion signal met the baseline for molecular weight calculations. A comparison between traditional vacuum MALDI and atmospheric pressure (AP) MALDI has increased our fundamental understanding of the latter (22). Creaser et al. used PEG 1500 and various combinations of matrix materials and cations for the comparison. The expected cationized PEG molecular ions were observed using both methods. However, the AP-MALDI spectra also contained dimetalated matrix/analyte adducts for all matrix/

cation combinations used. Additionally, signal intensities in the AP-MALDI spectra fluctuated with changes in matrix, cationization reagent, and ion trap conditions. End Group Information. There were several papers involving the determination of end group masses of synthetic polymers using MALDI-TOF MS. In some cases, separation methods were utilized prior to the MALDI analyses, and in other cases, the polymer was analyzed directly. Much of the time, the information provided by MALDI-TOF MS complemented other more traditional analytical techniques, primarily NMR and GPC. In all cases, MALDI-TOF MS yielded valuable information about the analyte. Puglisi et al. noticed that nylon 6 and poly(butylene terephthalate) (PBT) generated drastically different signal intensities in MALDI MS experiments, even though the polymers used had similar molecular weight distributions and were present as an equimolar blend (23). The nylon-6 had a higher ionization efficiency than the PBT. The experimenters theorized that end groups are a primary parameter in determining the ionization efficiency of synthetic polymers. Two papers studied esterified ends of synthetic polymers. Luftmann et al. expected diester products of their modified telechelics, but found that if their reaction time was too short, a monoester byproduct was observed (24). These authors characterized several polyether-diols and polyester-diols having hydroxyl and ester ends using MALDI-TOF MS. Similarly, Kraft et al. used MALDI-TOF MS to study esterified low-mass poly(propylene glycol)s (25). The spectra showed the expected end group masses and also revealed a small abundance of monoesterified product that was the result of incomplete functionalization. Chevallier et al. utilized MALDI-TOF MS to analyze poly(amide ester)s that were synthetic products of two different routes: polycondensation and polytransesterification (26). Their data allowed the comparison of macrocyclics and linear oligomers in the products. Barner-Kowollik et al. used MALDI-TOF MS to study end groups formed by pulsed laser polymerization of methyl methacrylate at -34 °C (27). Interestingly, this experiment also allowed the calculation of the propagation rate coefficient, which agreed with that obtained from GPC data. On- and off-line coupling of MALDI-TOF MS with separation methods has proven to be quite useful for the analysis of synthetic polymers. MALDI-TOF MS was employed by Ji et al. to analyze TLC fractions of anionically polymerized polybutadiene and polystyrene in order to determine end groups (20). Another interesting coupling of a separation method with MALDI-TOF MS was work done by Meyer et al. (28). The capillary electrophoresis eluate of an emulsifier, Cremophor EL, was deposited directly onto a MALDI target plate, and then the components of the mixture were identified using accurate mass MALDI-TOF MS. Krueger et al. obtained molecular weight and functional distribution data, as well as structural information of benzylated polylactides by coupling LC at the critical condition with MALDI-TOF MS (29). Jiang et al. used LC at the critical condition in combination with MALDI-TOF MS and ESI in order to characterize reversible addition fragmentation chain transfer (RAFT) polymerization products of functionalized PMMA (30). In Jiang’s case, however, MALDI appeared to cleave the labile dithioester ends, while ESI did not. It was suggested that ESI is a softer ionization method than MALDI in this instance.

In spite of these limitations, MALDI-TOF MS is quite useful in complementing other analytical characterization methods. Schilli et al. utilized several analytical techniques to characterize products of the RAFT polymerization of n-isopropylacrylamide (31). GPC was used to provide molecular weight distribution information, while MALDI-TOF MS spectra provided the end group information. In a study of dibutyl-substituted poly(3,4propylenedioxythiophene), GPC was used for molecular weight information and MALDI-TOF MS to determine end groups (32). The complementary nature of NMR, GPC, and MALDI-TOF MS was exploited by Farcet et al., as well (33). Living poly(butyl acrylate) was prepared in bulk and in a miniemulsion to study the effect of chain transfer to polymer. NMR was used to look at branching and unsaturation, and MALDI showed that the primary constituents of the products had the desired ends. MALDI-TOF MS, NMR, and SEC were employed by Gillies and Frechet to monitor the synthesis of polyester dendrimer/poly(ethylene oxide) hybrids used for drug delivery (34). By changing the dendrimer or PEO chains, they could tailor the molecular weight, architecture, and drug-loading capability of the hybrid molecules. D’Agosto et al. combined the power of NMR and MALDI-TOF MS to characterize the product of RAFT polymerization of N-acryloylmorpholine (NAM) and amphiphilic poly(NAM)block-polystyrene (35). Similarly, MALDI-TOF MS and NMR were employed by Coppo et al. to determine the structure of poly(cyclopentadithiophene)s prepared with various catalysts and synthetic protocols (36). The MALDI data in this case revealed the occurrence of a side reaction that produced chains capped with methyl ends. The MALDI-TOF MS information gathered by Quirk et al. supplemented their NMR data by providing chain end information on functionalized polymeric organolithium compounds (37). End groups and cyclic species of hyperbranched aliphatic polyesters and polyamides were studied by Chikh et al., using MALDI-TOF MS and NMR (38). Valuable information on ring equilibrium and side reactions was obtained. Polymerization Mechanisms and Kinetics. Determining the ends of a polymer chain aids prediction of the polymer’s physical and behavioral properties, and monitoring end group formation provides insight into polymerization mechanisms and reaction kinetics. Such was the case with Liu et al. when they used MALDITOF MS to characterize poly(n-vinylpyrrolidinone) (39). Chen et al. also obtained useful kinetics information when they monitored the hydrolysis reaction of ester ends of poly(butyl methacrylate) using MALDI-TOF MS (40). Similarly, Meier et al. used their automated multilayer approach to monitor the living cationic ringopening polymerization of 2-ethyl-2-oxazoline with MALDI-TOF MS (41). Kricheldorf and Schwarz applied MALDI-TOF MS methods to the analysis of step-growth polymerization products (42). The study included polyesters, polycarbonates, polyamides, poly(ether sulfone)s, poly(ether ketone)s, and polyurethanes. They found that when reaction conditions favored high molecular weights, the amount of cyclics produced increased, as did their size. Included in the paper was an interesting comparison of cyclization of synthetic polymers versus biopolymers. Another application in the literature used MALDI-TOF MS to study enzyme-based polymerization reactions. Xu et al. were able to follow the effect of varying reaction conditions on each mer in Analytical Chemistry, Vol. 76, No. 12, June 15, 2004

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the system. This information could not have been acquired by other, more classical analytical techniques (43, 44). Copolymer Characterization. While homopolymers are now analyzed routinely using MALDI-TOF MS, the technique is seeing an increase in copolymer characterization applications as well. Since MALDI-TOF MS is capable of providing the mass of each copolymer chain, the composition of individual mers can be determined. The technique may also generate information on the block length of a copolymer. One recent study used MALDI-TOF MS to examine the melt mixing of nylon 6,6 having acid ends (nyl6,6-COOH) with nylon6,10 at 290 °C (45). While the experimenters did not find NMR helpful, MALDI-TOF MS allowed them to monitor the copolymerization at various times during the heating process. After 10 min of heating, there was more nyl6,6-COOH in the mix than expected, and the copolymer appeared to be blocky. After 30 min of heating, however, the amounts of both nylon monomers observed corresponded to the feed composition, and the copolymer appeared to be random. Longer heating times caused a small amount of degradation but otherwise did not significantly affect the copolymer composition. Pretreatment of some copolymers was necessary for successful characterization. For example, when Murgasova et al. used SEC and MALDI-TOF MS to characterize polyester/polyurethane copolymers, partial acid hydrolysis was used to degrade the hard segment before it could be analyzed by MALDI (46). Analysis of the soft blocks was more direct, and molecular weights of these blocks obtained by MALDI-TOF MS were comparable to those derived from MALS/SEC data. Cox et al. cleverly found a way to distinguish between the isobaric monomers in ethylene/CO copolymers (47). They chemically reduced the CO units using hydrogenation, thereby adding two mass units per CO. The modified copolymer was subsequently analyzed by MALDI FTICR, which had sufficient resolution to resolve individual mers and allow composition information to be obtained. Other studies were more straightforward. For example, Meier et al. used MALDI-TOF MS to help characterize supramolecular block copolymers based on terpyridine metal complexes (48). Schrod et al. used MALDI-TOF MS data to determine structures of phenol-urea-formaldehyde cocondensates (49). Chen et al. studied seven active components in blue light-emitting diodes using MALDI-TOF MS as well (50). The composition, ends, and presence of macrocyclics for these conjugated copolymers were all derived using MALDI MS data. Interestingly, Hong et al. characterized low molecular weight polystyrene/poly(ethylene oxide) copolymers, and used the MALDI-TOF MS data to construct a block-length distribution map (51). Diblock copolymers of polystyrene/poly(ethylene oxide) were examined by MALDI-TOF MS to help explain unexpected FT-NIR data in work performed by Schmalz et al. (52). Ultimately, the MS data also furthered their understanding of the mechanism of copolymerization. An unusual result was observed during the investigation of five poly(ethylene oxide)/PPP rod-coil diblock copolymers (53). Several different cation salts were employed in an effort to study cation affinity. It was noted that varying the type and size of the cation affected the cation affinity and the observed molecular weight distribution. One paper involved the analysis of a terpoly3420

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mer of CO/styrene/methylstyrene (54). MALDI-TOF MS was useful in determining the structure of the terpolymer; however, the molecular weight distribution data were lower than that measured by SEC. Presumably, this was due to the high polydispersity of the copolymer. Molecular Weight Distributions and Mass Moments. While it is generally recognized that MALDI-TOF MS is limited to measuring molecular weight distributions of polymers with low polydispersity, its usefulness in providing molecular weight information is evidenced by the number of papers published on this topic. Perhaps the most notable of these is the work by Malvagna et al. (55). Their study of high molecular weight (40 000 and 160 000) poly(vinylpyrrolidone) (PVP) yielded correct molecular weight distributions by MALDI-TOF MS. This is especially surprising, considering the polydispersities of the polymers were 1.8 and 2.2. The researchers theorized that the ability of PVP to complex the MALDI matrix overcame the shortcomings of MALDI. This result led these authors to experiment with a new sample preparation technique, which involved flash freezing and then freeze drying the analyte with the matrix. This protocol was used successfully to acquire valid molecular weight distributions for several additional samples having moderately high polydispersities. Another remarkable contribution came from Kassalainen and Williams, who combined the strengths of thermal field-flow fractionation and MALDI-TOF MS for synthetic polymer analysis (56). One example of their accomplishments was successfully analyzing polystyrene up to 575 000 amu. Their efforts enabled MALDI-TOF MS to accurately measure molecular weight distributions of wide polydisperse polymers and mixtures, from low-mass oligomers up to several hundred thousand amu. Williams et al. had promising results, as well (57). They studied individual oligomers of poly(butylene glutarate) and observed uniform response by MALDI-TOF MS over the entire mass range, from a degree of polymerization (dp) of 8 up to a dp of 64.This data was then used to calibrate the GPC. Petkovska et al. generated molecular weight distribution information for acyclic diene polymers with polydispersities less than 1.5 (58). The MALDI-TOF MS values agreed with those from GPC studies. Another study by Xie et al. was aimed at investigating the accuracy of molecular weight measurements obtained by SECmultiangle laser light scattering (MALLS) (59). The analyte was low-mass, low-polydispersity poly(diisopropyl trimethylene-1,1dicarboxylate) that was synthesized using anionic ring-opening polymerization. Molecular weights obtained by MALDI-TOF MS, vapor pressure osmometry and SEC-MALLS all agreed. MALDI-TOF MS data of polystyrene, PEG, and PMMA analyses was used to determine Mark-Houwink-Sakurada parameters by Tatro et al. (60). The authors recommended depicting the molecular weight distribution as the ratio of distribution width at half-height to the mass of the monomer unit, instead of the traditional Mw/Mn ratio. A comparison of ESI-MS and MALDI-TOF MS analyses of polyethyleneimine noted significant differences between the data provided by the two techniques (61). The study involved four polymers with varying degrees of branching and low polydispersity. Three matrixes were evaluated for the MALDI experiments. The molecular weight distribution measured by MALDI-TOF MS

was dramatically affected by the choice of matrix and also affected somewhat by laser energy. Alternatively, preferential ion attachment was observed in the ESI data. The authors speculated that the differences in the results arose from preferential desorption/ ionization, degradation, and fragmentation from the MALDI desorption process or the aforementioned preferential ion attachment in ESI. Differences in ESI and MALDI were also noted by Liu et al. (62) In the comparison of automated GPC-MALDI-TOF MS and on-line ESI-TOF MS, the molecular weight distributions of poly(dimethyl siloxane) for both techniques showed evidence of mass discrimination. While the GPC-MALDI-TOF MS data underrepresented the low-mass oligomers, the GPC-ESI-TOF MS data underrepresented high mass oligomers. Degradation and Fragmentation Information. MALDI-TOF MS has also demonstrated its usefulness when investigating the degradation and fragmentation of synthetic polymers. Carroccio et al. acquired MALDI-TOF mass spectra of photooxidized nylon6, and identified over 40 compounds (63). The structural data they gathered provided new, detailed information on the mechanism of degradation. The same group also found that MALDI-TOF MS could provide insight into the photooxidative degradation mechanism of a polyetherimide (64). In addition, these researchers combined MALDI, SEC, and NMR to monitor the thermal degradation of poly(bisphenol A carbonate) (65). Structural information on the degradation products was supplied by MALDI and confirmed by NMR. SEC curves followed the degradation and formation of low-mass oligomers. Lattimer employed both direct probe pyrolysis with chemical ionization (Py-CI-MS) and MALDITOF MS in the study of pyrolyzed poly(acrylic acid) (66). The MALDI data suggested dehydration and decarboxylation processes occurred. Biodegradable polymers were the focus of Sato’s work (67). Molecular weight changes as well as end group changes from enzymatic degradation were identified using MALDITOF MS. To supplement the data, Py/GC provided information on the degree of biodegradation and compositional changes during degradation. Polyamide degradation received quite a bit of attention over the period of this review. Montaudo et al. monitored thermal and oxidative degradation of nylon 6 using MALDI-TOF MS (68), and Nyden et al. performed a similar study utilizing both FT-IR and MALDI-TOF MS (69). Kalugina et al. went one step further and used IR, NMR, GPC, and MS to identify thermal and thermooxidative degradation products of polyamides (70). Their study was aimed at understanding the mechanism of degradation to devise a way to stabilize these materials. Postsource decay (PSD) was exploited to study the fragmentation behavior of three discrete oligomers of PMMA (71). The 20-, 60-, and 100-mer were analyzed using MALDI PSD with lithium, cesium, and potassium cations. In this experiment, Laine et al. found that the best PSD results could be obtained when the size of the cation increased with increasing chain length. Quantitation. Promising results utilizing MALDI-TOF MS for the quantitation of synthetic polymers have been reported. Yan et al. compared signal intensities of two poly(dimethyl siloxane)s (PDMS) with different molecular weights (72). PMMA was then added to the mix, and the signal intensities of the PDMSs were compared again. Signal intensity plotted against analyte concentration revealed a linear relationship from 1000 to 10 000 amu with

comparable desorption efficiencies for the two PDMS samples, even when PMMA was present. A blend of polystyrene and poly(R-methylstyrene) with similar molecular weights and polydispersities was also measured quantitatively (73). Murgasova et al. coupled SEC and MALDI for this analysis. MALDI-TOF MS was used to characterize narrow molecular weight fractions, because SEC could not resolve the two polymers. SEC yielded the quantitation information. A plot of concentration ratio versus peak area ratio produced a good fit using linear regression analysis. Another interesting quantitative endeavor consisted of two parallel studies: one based on quantifying a mixture of two poly(ethylene glycol)s with significantly different end groups and the other focused on the quantitation of two completely different polymers (one PEG, one conjugated) (74). Plots of molar ratio versus relative intensity ratio for the first study showed good linearity with a slope of nearly 1 and an intercept at 0. The data were reproducible and agreed with the proportions present in the blend. In the study of two different polymers, the plot gave linear response with a good fit and intercept also at the origin. However, the slope was 0.083 and the calculated quantities did not agree with the stoichiometry. The authors suggested this was due to differences in ionization efficiency of the two polymers. The promising aspect of this result is the possibility of using the data to fit a linear calibration curve for quantitation. ELECTROSPRAY IONIZATION Electrospary ionization is a far more important ionization method for characterization of biological than for synthetic polymers. The two most important reasons for this are the added complexity of multiple charging and the more stringent solvent requirements of ESI compared with other mass spectrometric methods. Nevertheless, ESI has some advantages such as more readily available instrumentation for collision-induced dissociation (CID) studies and the ease of interfacing with chromatographic methods. ESI is also well suited for kinetic and mechanistic studies. Kinetic and Mechanistic Studies. Nierengarten et al. found ESI-MS to be the only analytical tool capable of unambiguous characterization of copper(II) coordination polymers (75). They used kinetic and thermodynamic ESI-MS studies to exclude the possibility that the complexes formed in the electrospray process. A nitroxide trapping technique was combined with LC (UV detection) and ESI-MS to identify and quantify 15 nitroxide-trapped oligomers in the free radical polymerization of styrene and to estimate 14 of the propagation rate constants (76, 77). The single chromatographic peak observed for each trapped oligomer indicates negligible rate constants for formation of branched products. Negative ion ESI-MS has been used to study the polymerization of poly(isobutylsuccinic acid) and commercially available poly(isobutylene) (78). ESI-MS was suggested as a way to determine the succinic ratio in poly(isobutylsuccinic acid). Differences in the mass spectra of BF3- and AlCl3-catalyzed poly(isobutylene) implicated different polymerization mechanisms for the two catalysts. Fragmentation Studies. MS/MS was combined with ESI and end group oxidation to unambiguously assign structures for polymeric methyl acrylates generated by cumyl dithiobenzoate, cumyl p-fluorodithiobenzoate, and 1-phenylethyl dithiobenzoateAnalytical Chemistry, Vol. 76, No. 12, June 15, 2004

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mediated RAFT polymerization (79). Chen et al. found ESI MS/ MS fragmentation of polyglycol esters to be different from polyglycol ethers whose functional end groups are linked to the polymer chain through oxygen ether bonds (80). The authors demonstrated the utility of the MS/MS method by examining two surfactants based on fatty acid methyl ester ethoxylates. In another study, MALDI and ESI MS/MS were used to characterize in detail end groups formed in living anionic polymerization (81). The ability to detect both minor and major products of the polymerization provides insight into the reaction mechanisms. Among other things, these authors addressed the degree of functionalization and oligomerization achieved upon reaction of the living anion with electrophiles. Adhiya and Wesdemiotis studied the effects of polar versus nonpolar solvents in the MALDI MS postsource decay and in the ESI MS CAD of poly(propyleneimine) dendrimers (82). These authors found that even in the gas phase the M + H+ ions had memory of the solvent, but M + 2H+ ions formed in ESI showed no such memory effects, apparently because charge repulsion dominated in the gas phase. Simonsick et al. used SEC interfaced with ESI-Fourier transform (FT) MS with sustained off-resonance irradiation CID to characterize 1,4-cyclohexenedicarboxylic acid/ neopentyl glycol polyesters (83). In addition to the structural information provided by CID, accurate mass measurement of individual oligomers could be obtained using SEC-ESI-FTMS. MSn, with n up to 5, was applied to the characterization of soluble oligomers produced by electrochemical oxidation of o-phenylenediamine using ESI on an ion trap MS (84). Prebyl demonstrated the use of ESI MS and CID to distinguish copolymer samples of styrenesulfonic acid-co-maleic acid having different monomer ratios (85). In a fundamental study applying ESI and MS/MS, Bogan and Agnes determined for poly(ethylene glycols) that the sites for alkali metal ion attachment are nonselective with respect to the repeating unit of the polymer, but there is selectivity with respect to metal ion interaction with the ether oxygen atoms in an inner coordination sphere (86). Interestingly, the highest mass resolution for PEG (MW average 1000) was obtained with Cs+ ion coordination and the lowest with Na+ (lithium was not studied). These results were explained in terms of the alkali ion radius and the number of unique coordination structures affecting the mobility of the complex. Characterization Studies. A few publications discussed polymer characterization and ESI MS. In combination with 1H and 13C NMR, ESI MS was used to study the details of the polymeric structure generated by the acid-catalyzed polycondensation reaction of terephthalaldehyde with 2-amino-2-hydroxymethyl-1,3propanediol or with 2,2′-(1,4-phenylene)bis-1,3-(4,4-dihydroxymethyl)oxazolidine (87). These same techniques were used to chemically characterize the biocompatible polymer poly(styreneco-maleic acid), which had been derivatized with linear B disaccharide (88). Both MALDI and ESI were used to determine the purity of the functionalization of polypropylene glycols esterified with p-O2NC6H4COCl followed by reduction of the nitro group (25). Ouyang et al. used ESI MS and X-ray crystallography to aid in the characterization of novel two-dimensional coordination polymers (89). Functional PMMA polymers synthesized by RAFT polymerization were characterized by combining critical liquid chroma3422

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tography (LC at the critical conditions) and both MALDI MS and on-line and off-line ESI MS (30). The power of this combination resides in the complementary nature of the chromatography in which molecular weight has almost no influence on the separation, and mass spectrometry that measures oligomer mass. Interestingly, the authors found that labile ends such as dithioester groups on the PMMA were not observed in the MALDI experiments but were observed intact by ESI. Only the ESI experiments provided evidence that the RAFT polymers still exhibited living characteristics in the form of the dithio moiety. Liu et al. conducted a comparative study of automated GPC-MALDI-TOF-MS and online GPC-ESI-TOF-MS (62). These authors concluded that ESI MS is especially useful in calibrating the low-mass range where MALDI MS is more problematic at low mass. SECONDARY ION MASS SPECTROMETRY Surface Characterization. Surface characterization of polymeric materials using SIMS received considerable attention during this period. Pisciotti et al. studied the surface and bulk, through surface analysis of cross sections, of injection-molded high-flow and low-flow grade commercial polystyrene using TOF-SIMS (90). These authors were able to detect differences in the surface spectra for the two polystyrene grades. Spin-coated thin films of dendritic polyester macromolecules prepared form bis(hydroxymethyl)propionic acid as well as polyether macromolecules prepared form 3-ethyl-3-(hydroxymethyl)oxetane were studied by TOF-SIMS (91). Cationized (Na+, Ag+, Cu+) molecular ions were observed up to 3000 amu for some species, allowing identification of the polymers, and providing information about end groups and incomplete reactions. Typically, TOF-SIMS is one of several surface analytical methods used for characterization. Lei et al. used TOF-SIMS along with atomic force microscopy (AFM), X-ray photoelectrom spectroscopy (XPS), and contact angle measurements to characterize the surface of blends of the condensation polymers made from 1,8-dibromooctane reacted with bisphenol A and 4,4’-(hexafluoroisopropylidine)diphenol (92). Roux et al. used the same surface analysis techniques to characterize the surface of a gold electrode coated with a polypyrole derivative containing carboxylic acid pendant groups that were subsequently reacted with styrene by surface-initiated radical polymerization (93). Huang et al. used TOF-SIMS along with FT-IR, SEM, and contact angle measurements to characterize the surface properties of C60-containing poly(ethyl methacrylate) polymers and blends of this polymer with poly(ethyl methacrylate) (94). Novel perfluoropolyether-urethane ionomers from aqueous dispersions were characterized by Canteri et al. using XPS and TOF-SIMS combined with principal component analysis (PCA) (95). These authors found surface enrichment in fluorine. PCA was also used by Medard et al. to quantify low levels of the antioxidant Irgafos 168 at the surface of thin amorphous layers of poly(ethylene terephthalate-ethylene isophthalate) (96). These authors found that ∼1% of the additive in the bulk resulted in almost uniform coverage of the polymer surface. An interesting application of the PCA multivariate statistical method with TOFSIMS data to obtain quantitative information was reported by Coullerez et al. (97). These authors were able to determine that the fragmentation pattern observed by SIMS for hyperbranched

2,2-bis(hydroxymethyl)propionic acid polymers was highly affected by the functional end groups and that fragments that were assigned to the ethoxylated pentaerythritol core show decreasing abundance with increasing molecular weight of the polymer. These authors were able to calibrate the molecular weight of the hyperbranched polyesters on the surface using this ion abundance dependence. Plasma-Treated Surface Studies. Several publications dealt with characterizing polymer surfaces after electric discharge plasma treatment. Oiseth et al. looked at thin spin-coated highdensity polyethylene films that were modified using argon as well as oxygen radio frequency glow discharge plasma. As with most of the studies of this kind, TOF-SIMS was used in combination with other analytical methods: in this case, XPS and contact angle measurements (98). O’Hare et al. used the same analytical techniques and in addition atomic force microscopy to study the surface of poly(ethylene terephthalate) films that were treated with corona discharge plasma of varying energy (99). These authors identified phenolic-OH, carbonyl, and carbolylic acid functionalities as having been added to the film by the plasma treatment. Plasma polymerization of acrylic polymers fabricated using a millisecondpulsed plasma was studied using XPS and TOF-SIMS (100). In this study, electron impact ionization was also used to study neutral and charged species in the pulsed plasma as a function of the plasma off time. The authors found an increased importance of radical chemistry in polymer growth as a function of the length of time the plasma was off. Several analytical surface techniques including TOF-SIMS were used to characterize fluoropolymer films deposited on plasma-treated copper foils (101) and on pristine oxide-covered silicon and hydrogen-terminated silicon surfaces (102). Both groups synthesized the fluoropolymer films by plasma polymerization of allylpentafluorobenzene using argon glow discharge conditions. The results, in the study by Yang et al. indicate that the fluorinated aromatic structure was preserved in the deposited film. Fundamental Studies. A number of SIMS studies were carried out with the objective of providing fundamental knowledge. A series of fluorinated polyether-polysulfones with defined fluorocarbon segmental lengths were studied by positive and negative ion TOF-SIMS to elucidate the relation between secondary ions observed and polymer structure (103). Lee and Gardella used quantitative TOF-SIMS to analyze the oligomeric biodegradation products at the surface of biodegradable poly(glycolic acid), poly(L-lactic acid), random poly(D,L-lactic-co-glycolic acid), and other poly(hydroxy acids) (104). These authors also reported on the simultaneous TOF-SIMS quantitative measurement of triphenylamine (Ph3N) in a 20:80 wt % Ph3N/poly(L-lactic acid) blend and the degradation kinetics of the polymer (105). The amount of Ph3N detected at the surface was related to the rate of poly(lactic acid) degradation. Gilmore et al. studied the surfaces of a blend of poly(vinyl chloride) and polycarbonate with static SIMS and AFM (106). They found that static SIMS did little damage to the surface, but AFM showed significant subsurface damage had occurred. It was recommended that, in using the combination of AFM and static SIMS, AFM investigations be done prior to static SIMS. Hu et al. used dynamic SIMS to measure the effects of organosilicate clay on the tracer diffusion coefficient in polystyrene

and poly(methyl methacrylate) (107). Anderson et al. used TOFSIMS and XPS to study the interaction of oxygen at the poly(pphenylenevinylene)/calcium interface (108). The interface was produced by thermal evaporation of calcium on a spin-coated 2,5bis(decyloxy)poly(p-phenylenevinylene) layer. The SIMS results suggested that the mechanism of oxidation begins with carbide bond formation between the calcium and polymer during evaporation. The carbide bond oxidizes on exposure to oxygen. An example of the use of imaging TOF-SIMS in high-throughput analyses was given by Xu et al. for the analysis of combinatorially synthesized polymer resins (109). They demonstrated that the TOF-SIMS method, while being an order of magnitude faster than ESI-MS or LC/MS, gave equivalent chemical specificity and sensitivity. GAS CHROMATOGRAPHY/MASS SPECTROMETRY Pyrolysis Studies. GC/MS was frequently used to study polymer pyrolysis and thermal degradation (110). A universitylevel laboratory experiment utilizing Py-GC/MS of polystyrene and ABS polymer was reported as a teaching tool (111). Jin et al. and Huang et al. investigated the thermal degradation mechanism of high-performance polyimide with norbornene ends using PyGC/MS (112, 113). The composition and distribution of the pyrolysates incorporating norbornene end groups were related to cross-linking reactions that occur during pyrolysis. In a study of organic material in fine atmospheric particulates using Py-GC/ MS, Fabbri et al. pyrolyzed (poly(styrene-co-isoprene), polylimonene, and polypinene to identify markers for these organic materials (114). Bertini et al. studied the thermal degradation of alkyne-containing polystyrenes using Py-GC/MS (115). In the thermal analysis of acrylonitrile polymerization and cyclization in the presence of N,N-dimethylformamide, Aviles et al. identified occluded molecules of DMF and several compounds containing nitrogen that were produced during the thermal degradation (116). Krauze et al. identified 64 products from the thermal degradation of polystyrene in nitrogen and in air and at several different temperatures using Py-GC/MS and Fourier transform infrared spectroscopy (117). Py-GC and MALDI MS were used to evaluate the biodegradability of several polymers (67). GC/MS was also used to analyze the decomposition products of the UV decomposition of styrene polymerized with 1-phenylethyl phenyldithioacetate (118). This study identified both 1-phenylethyl and benzyl radicals produced during the decomposition. The pyrolysis mechanisms for polyacrylonitrile and its copolymers, acrylonitrile-styrene, acrylonitrile-butadiene, and acrylonitrile-butadiene-styrene, were studied using Py-GC/MS by Wang and Zhao (119). In a study of the degradation kinetics of poly(ethylene terephthalate) in supercritical methanol for the purpose of developing a chemical recycling process for waste plastics, Goto et al. used GC/MS as one means of analyzing the products of degradation (120). Pyrolysis and Thermogravimetric Analysis. The combination of Py-GC/MS and TGA was employed by Nishida, et al. to study the initial thermal degradation process for poly(p-dioxanone) (121). Arii used TGA/MS and TGA-GC/MS to study the thermal decomposition of polystyrene (122). Monomer, dimer, and trimer were identified as the major degradation products. The thermogravimetric kinetics of the degradation of high-density polyethAnalytical Chemistry, Vol. 76, No. 12, June 15, 2004

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ylene alone and with silicoaluminophosphate catalyst was studied by Araujo et. al (123). GC/MS was used in this study to identify the products of thermal and catalytic cracking of the polyethylene, and TGA was used to obtain information on the kinetics of degradation. TGA/MS and Py-GC/MS were used by Jakab and Blazo to study the effect of carbon black on the thermal decomposition of vinyl polymers in an inert atmosphere (124). The thermal degradation products observed were influenced by the nature of the substituents on the hydrocarbon chain. Carbon black had no effect on the decomposition of PMMA with quaternary carbon atoms on the polymer chain but hindered the decomposition of polyethylene, polystyrene, and polyacrylonitrile while promoting the decomposition of polypropylene. The thermal decomposition mechanism of nine cyanate ester resins was studied using TGA, IR, and Py-GC/MS (125). The thermal decomposition of polycyanurates was shown to begin with hydrocarbon chain scission and cross-linking reactions that occurred with negligible weight loss followed, at higher temperature, by decyclization of the triazine rings and rapid weight loss. Identification of Additives. Py-GC/MS was used as a rapid method for identifying polymer additives (126). In this work by Herrera et al., samples of recycled ABS were compared to virgin ABS. Watanabe et al. combined evolved gas analysis, purge-andtrap sampling, and GC/MS to characterize polymeric materials with regard to substrate polymers and additives (127, 128). Nerin et al. employed headspace, liquid extraction, and supercritical fluid extraction with GC/MS and inductively coupled plasma MS to analyze recycled polyethylene terephthalate flakes for contaminants (129). Polycarbonate baby bottles were analyzed for bisphenol A monomer using GC/MS before and after use by dishwashing, boiling, and brushing (130). The authors reported a significant increase in migration of bisphenol A due to use as compared to new bottles. GC/MS was used by Ezerskis and Jusys to study the electropolymerization of chlorinated phenols on a platinum electrode (131). GC/MS revealed dimers, trimers, and tetramers were present in the polymer mixtures and showed the ether-linked nature of these oligomers. Information was obtained on the mechanism of polymerization. Polyisobutylenes partially functionalized with isothiocyanate groups were characterized using SEC, LC, and GC/MS (132). OTHER MASS SPECTROMETRIC METHODS FOR STUDYING POLYMERS, COPOLYMERS, AND BLENDS Thermogravimetric Analysis. Combining thermogravimetric analysis (TGA) and mass spectrometry provides a powerful analytical tool for studying polymer degradation. The combination of TGA with GC/MS was discussed in the section on gas chromatography/mass spectrometry. TGA can be combined directly with mass spectrometry as a means of identifying evolved gases as a function of temperature. Arii, as discussed in the section on GC/MS, used TGA-GC/MS as well as TGA/MS in a study of the degradation of polystyrene (122). Jakab and Blazso also used both TGA-GC/MS and TGA/MS to study the effect of carbon black on the thermal decomposition of vinyl polymers (124). TGA/ MS was used by Hurduc et al. to study the degradation of some aromatic polyethers containing flexible spacers (133, 134). In this study, either bis(2-chloroethyl) ether or 1,6-dichlorohexane was 3424

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used as the flexible spacers in various bisphenol polymers. These authors proposed a degradation mechanism based on chain transfer reactions. Focarete et al. used TGA/MS along with several other analytical techniques to study the solid-state properties of polylactide, poly(lactide-co-glycolide), and blends of these polymers with atactic poly(3-hydroxybutyrate) (135). Shibaev et al. used TGA with MS and DSC to study the thermal and thermooxidative degradation of PMMA blends with C60 fullerene (136). These authors found that C60 inhibits the thermal and thermooxidative degradation of PMMA. They proposed that the enhanced stability was the result of the thermal production of C60-containing polymer. Kilian et al. prepared high molecular weight core cleavable star polymers by the coupling of living anionic poly(alkyl methacrylate) arms with either dicumyl alcohol dimethacrylate or 2,5dimethyl-2,5-hexanediol dimethacrylate (137). TGA/MS detected evolution of the diene byproduct of the core degradation, confirming the mechanism of degradation proposed from TGA and NMR studies. Boeker et al. used TGA and thermal desorption mass spectrometry to study the thermal degradation of block copolymers to which perfluoroester, fluorinated urethane, or fluorinated carbonate side chains had been attached to the polymer pendant hydroxy groups (138). Pyrolysis and Thermal Degradation. Other mass spectrometric methods have been combined with pyrolysis or thermal degradation methods to analyze polymeric materials. A simple, but often effective approach is to use the solids probe inlet of the mass spectrometer. Fazlioglu and Hacaloglu studied the thermal decomposition of glycidyl azide polymer using the direct insertion probe inlet (139). They found that the initial thermal degradation involved the side groups of the polymer structure. Badawy and Dessouki were able to determine that the pyrolysis products from polymer produced by radiation polymerization of acrylonitrile in a viscous system with styrene at ambient temperature corresponded to oligonitriles with styrene end groups (140). Hong et al. placed polymers on an electrode at the sampling aperture of a quadrupole mass spectrometer for the purpose of investigating plasma-polymer surface interactions (141). These authors identified molecular fragments from polypropylene, polyethylene, polyethylene terephthalate, and polyimide that were liberated by chain scissions caused by bombarding ions and vacuum UV photons in the plasma. Other Methods. Both electron impact (EI) ionization and metastable atom bombardment (MAB) MS were used to identify thermal decomposition products from a polyurethane (142). MAB MS uses discrete energy stored in metastable atoms for ionization. Modulation of the ionization energy is achieved by changing the ionizing gas. Thus, with N2 gas, only molecular ions of the pyrolyzates were observed. The authors of this study detected isocyanic acid, methylene isocyanate, ethylene isocyanate, propyl isocyanate, and butyl isocyanate from pyrolysis of diphenylmethane diisocyanate polyurethane using EI and MAB ionization. EIMS was also used by Ramjit to study the mechanism and kinetics of disulfide-disulfide interchange in polysulfide polymer melt blends (143, 144). The mass spectrometry experiments were designed to monitor concentrations of the monomer reactants as well as the randomized copolymer dimer units. Mass spectrometry was also used in other studies involving thermal degradation of

polymers (70, 145). Laser ablation TOF MS was used to study polystyrene and acrylonitrile-styrene copolymer-based silver nanocomposites (146). Cox et al. studied the thermal degradation of ethylene-methyl acrylate copolymers and blends of this copolymer with polyethylene or poly(methyl acrylate) homopolymers to determine the feasibility of quantifying the monomer composition of the copolymer (147). These authors were able to create a model using chemometric analysis of the thermal decomposition products ionized by the soft photoionization method that was predictive of the copolymer composition between 2.8 and 20.5% methyl acrylate. Determining the copolymer composition in blends of the copolymer with the respective homopolymers proved to be more challenging. In other work, vacuum photolysis of poly(benzyl methacrylate) using 147- and 123.6-nm light was studied by Dorofeev and Vainer using mass spectrometry (148). Koster et al. compared electron capture dissociation (ECD) and low-energy collisionally activated dissociation (CAD) of doubly protonated hyperbranched polyesteramide oligomers for obtaining structural information (149). ECD was shown to be useful for structural characterization of large oligomers where CAD may fail to produce fragmentation, but for lower mass oligomers, ECD did not provide complementary structural information as compared to CAD. CONCLUSION Mass spectrometry continues to grow as a tool for the characterization of synthetic polymers. Part of the appeal of this technology is the complementary nature of the information obtained compared with other methods applied to polymer characterization. Interfaced with pyrolysis, TGA, or chromatography, mass spectrometry becomes an especially powerful method for determining structural details of many polymeric systems. Patricia M. Peacock is a Staff Technologist in the Dupont Corporate Center for Analytical Sciences in Wilmington, DE. She studied materials engineering at Virginia Polytechnic Institute and State University, and chemistry at the University of Delaware. She has several publications and 19 years experience in polymer characterization. Her current research interests include practical applications of mass spectrometry for the characterization of a wide variety of industrial and research polymers. She is Chair-Elect of the Delaware Valley Mass Spectrometry Discussion Group.

(9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37)

Charles N. McEwen is a Research Fellow in the Dupont Corporate Center for Analytical Sciences in Wilmington, DE. In addition to numerous publications, he has taught short courses and coauthored or coedited books and book chapters dealing with diverse areas of mass spectrometry. His most recent interest has been in applying mass spectrometry to polymer characterization. He received a B.S. in chemistry from the College of William and Mary, M.S. in chemistry from Atlanta University, and Ph.D. in chemistry from the University of Virginia. He is a member of the American Chemical Society and the American Society for Mass Spectrometry.

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