c alibrated nexafs spectra of some common...

Journal of Electron Spectroscopy and Related Phenomena 128 (2003) 85–96www.elsevier.com/ locate/elspec

C alibrated NEXAFS spectra of some common polymersa a , b*O. Dhez , H. Ade , S.G. Urquhart

aDepartment of Physics, North Carolina State University, Raleigh, NC 27695,USAbDepartment of Chemistry, University of Saskatchewan, Saskatoon SK S7N 5C9, Canada

Accepted 27 September 2002

Abstract

Near edge X-ray absorption fine structure (NEXAFS) microscopy has evolved into a powerful characterization tool forpolymeric materials. The foundation of this utility depends crucially on the sensitivity of NEXAFS to the specific chemicalstructure of the polymer. Furthermore, for quantitative compositional analysis, reliable reference spectra with known energyresolution and calibrated energy scale are required. We report a set of NEXAFS spectra from 24 common polymers thatrepresent a range of chemical functionalities in order to create a database of calibrated polymer NEXAFS spectra to be usedfor compositional analysis. These spectra illustrate the sensitivity of NEXAFS spectroscopy to the polymer composition,illustrating the potential of NEXAFS for chemical analysis. 2002 Elsevier Science B.V. All rights reserved.

Keywords: NEXAFS; Polymers; Energy calibration; Reference spectra

1 . Introduction tronic delocalization for the functional groups [2–5].NEXAFS spectroscopy has been successfully applied

Near edge X-ray absorption fine structure (NEX- to a variety of systems and materials and continuesAFS) spectroscopy is a powerful means to identify to be enhanced through the utilization and increasingand distinguish small molecules and macromolecules availability of high intensity, high brightnesssuch as polymers [1]. This chemical sensitivity is synchrotron radiation facilities. New avenues oftypically manifest as sensitivity to functional groups, applications in the form of NEXAFS microscopywhere unique spectroscopic fingerprints can be as- have also evolved during the last decade [6–8].signed to specific chemical moieties and functional Complementary analytical tools, such as infrared andgroups. This approach has been very successful, and nuclear magnet resonance (NMR) spectroscopy havemolecular models have been frequently used to superior capabilities regarding composition, but theyinterpret the NEXAFS spectra of polymers (Refs. can not be used at very high spatial resolution or[2–5]). In general, a molecule must be large enough with very high surface sensitivity. Hence, characteri-to represent the spatial extent of the relevant elec- zation of surfaces and microscopy are the most

prominent and promising applications of NEXAFSspectroscopy for polymeric materials [8].*Corresponding author. Tel.:11-919-515-1331; fax:11-919-

In NEXAFS spectroscopy, the photoabsorption515-7331.E-mail address: harald [email protected](H. Ade). cross-section for the excitation of tightly bound core

]

0368-2048/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved.PI I : S0368-2048( 02 )00237-2

mailto:[email protected]



86 O. Dhez et al. / Journal of Electron Spectroscopy and Related Phenomena 128 (2003) 85–96

electrons into unoccupied molecular orbitals or the Tabular versions of the spectra are available atvacuum continuum is measured as a function of http: / /www.physics.ncsu.edu/stxm/polymer NEX-

]photon energy. Each element has a characteristicAFS.html.core binding energy (i.e., carbon 1s:|290 eV,nitrogen 1s:|400 eV, oxygen 1s:|530 eV, etc),making NEXAFS spectroscopy explicitly element 2 . Experimental methods and analysissensitive. The detailed spectral features observed inproceduresNEXAFS spectra correspond to transitions from theground state to a core excited state, where the initial 2 .1. Materialsand final states can be manifolds of states for a givenelement. The complexity of these manifolds depends We prepared thin films of 24 polymers, whoseon the variation in the chemical and hence electronic NEXAFS spectra were subsequently recorded inenvironments that a particular element can be found transmission. Most polymers that can be easilyin. The complex covalent bonding of carbon atoms dissolved by a common solvent were cast fromin organic materials makes carbon 1s NEXAFS a solution onto silicon substrates, glass sides, saltparticularly rich and detailed core absorption edge. crystals, or Si N membranes to appropriate thick-3 4

1C1s NEXAFS investigations are therefore well suited ness of about 100–150 nm . To evaporate all theto study synthetic polymers. solvent, these films were annealed at 808C–1208C

A relatively detailed primer about the NEXAFS in vacuum for up to a day. Polymers with aT aboveg

spectroscopy of polymers has been previously pro- room temperature were transferred to transmissionvided by Ade and Urquhart [8], and was recently electron microscopy (TEM) grids by floating thecomplemented by a series of NEXAFS spectra polymer from the substrate onto water or anotherilluminating the evolutionary trend of the spectral suitable liquid and by subsequent pickup with a TEMsignature of carbonyl functional groups as the nearest grid. Polymers with aT below room temperatureg

neighbor environment is changed progressively and were investigated on 100 nm thick Si N mem-3 4

systematically from a ketone to a carbonate group branes. Some polymer thin films, as noted below,[9]. Complementary, but somewhat limited, over- were prepared by ultramicrotomy. Generally, molec-views of polymer NEXAFS spectra have been ular weight, polydispersity index, and minutepreviously presented by Kikuma and Tonner [10], amounts of stabilizers and additives do not materiallyUnger et al. [11] and Ade [12,13]. Here, we present affect the NEXAFS spectrum and are thus not notedthe most comprehensive compilation of polymer or discussed.carbon 1s NEXAFS spectra to date, including spectra For the purpose of discussion and relative com-of previously not investigated polymers. Further- parison, the polymers examined are grouped alongmore, we enhance the utility of several NEXAFS common functional group themes as much as pos-spectra of some previously examined polymers by sible and are identified in the five different schemesacquisition of NEXAFS spectra of these polymers on detailed below. Scheme 1 groups together unsatu-a common energy scale and energy resolution. This rated polymers that have carbon–carbon doublewas accomplished by utilizing the same instrument bonds in the main-chain, so-called diene polymers:for data acquisition and by recording the NEXAFS trans-1,4-polyisoprene (t-PI; Scheme 1.A),cis-1,4-spectrum with a small amount of CO for energy polyisoprene (c-PI; Scheme 1.B), polybutadiene2

calibration [14].Our primary goal with this manuscript is the

1presentation ofaccurately calibrated NEXAFS spec- For NEXAFS experiments at the carbon K-edge in transmission,tra of polymers, rather then discussing specific the optimum thickness for polymer sections is|100 to 150 nm.

Samples thinner than|80 nm often suffer from rapid beamspectroscopic assignments. For details on the generaldamage and the spectra have a poor signal to noise ratio. Spectraprinciples of NEXAFS spectroscopy and micro-from samples thicker than 200 nm are more readily distorted by

scopy, the reader is referred to the literature detector darkcounts, higher order spectral contamination, or[1,8,12,15]. The energy calibration procedure is sample inhomogeneities on the size scale of the X-ray beam,discussed in detail. including a diffuse, large halo.

http://www.physics.ncsu.edu/stxm/polymer_NEXAFS.html.











O. Dhez et al. / Journal of Electron Spectroscopy and Related Phenomena 128 (2003) 85–96 87

Scheme 2.

Scheme 1.

(PBD; Scheme 1.C), and polychloroprene (neoprene-NP; Scheme 1.D).t-PI andc-PI were obtained fromAldrich and thin films were prepared by solventcasting onto Si N windows. PBD were obtained3 4

from J. Dias (Exxon). The PBD was spun cast by M.Rafailovich’s group from toluene and thin sectionswere floated onto a TEM grid. NP was obtained fromJ. Dias (Exxon) in the form of cryomicrotomedsections mounted on coated EM grids.

Scheme 2 groups together polymers with main-and side-chain carbonyl groups: poly(vinyl methylketone) (PVMK; Scheme 2.A), poly(vinyl pyrrol-idone) (PVP; Scheme 2.B), poly(methyl methacryl-ate) (PMMA; Scheme 2.C); poly(ethylene succinate)(PES; Scheme 2.D); and poly-e-caprolactam(Nylon6; Scheme 2.E). PVMK, PVP, and PES wereobtained from Scientific Polymer Products, PMMAfrom Aldrich Chemicals, and Nylon6 from Allied-Signal. The PVMK, PMMA, and Nylon6 weremicrotomed and mounted on TEM grids. Thin filmsof PVP were spun-cast on salt from a solution of 30mg/ml in methanol and floated onto a TEM grid.The low molecular weight PES (M |10 000) was Scheme 3.W


solvent cast from chloroform and floated onto acarbon coated TEM grid.

Scheme 3 groups together polymers that havemain-chain aromatic and main-chain carbonylgroups: poly(ether ether ketone) (PEEK; Scheme3.A), poly(oxybenzoate-r-2,6-oxynaphthoate)(Vectra� 950; Scheme 3.B), poly(butylene tere-phthalate) (PBT; Scheme 3.C); poly(ethylenenaphthalate) (PEN; Scheme 3.E), and polycarbonate(PC; Scheme 3.F). The PEEK was obtained fromAldrich, Vectra� 950 from Hoechst Celanese, PBTfrom General Electric Plastics, PET from ScientificPolymer Products, PEN from Dupont, and PC from3M. Samples of PEEK, Vectra�, PBT, PET, andPEN were microtomed and mounted on standardTEM grids.

Scheme 4 groups together poly(acrylonitrile)(PAN; Scheme 4.A), poly(a-methyl styrene) (PaMS;Scheme 4.B), and poly(2 vinyl pyridine) (P2VP;Scheme 4.C) without a strong overarching functionalgroup theme. PAN and P2VP was obtained fromScientific Polymer Products, and PaMS was ob-tained from Aldrich. Samples of PaMS were mi-crotomed and mounted on TEM grid. Samples ofPAN and P2VP were spun-cast on salt from asolution of 30 mg/ml in DMF and floated onto TEM Scheme 5.grid.

Scheme 5 groups together the following saturatedpolymers: poly(methylene oxide) (PMO; Scheme poly(vinyl alcohol) (PVA; Scheme 5.C), poly-5.A), ethylene propylene rubber (EPR; Scheme 5.B), propylene (PP; Scheme 5.D), poly(isobutylene)

(PIB; Scheme 5.E), and polyethylene (PE; Scheme5.F). PMO, EPR, and PVA were obtained fromAldrich (purity.99%). PP and PE was provided byDow Chemical. PIB (vistanex LM 120) was pro-vided by Exxon. Samples of PMO, EPR, PE and PPwere microtomed or cryo-microtomed and mountedon TEM grid. Thin films of PVA were depositedfrom H O/MeOH solution.2

2 .2. Experimental details

All NEXAFS experiments were performed usingthe Stony Brook scanning transmission X-ray micro-scope (STXM) located at X1A beamline of theNational Synchrotron Light Source (NSLS) atBrookhaven National Laboratory (BNL) [16]. Thespectra were acquired with an atmospheric pressure

Scheme 4. He/air mixture (primarily He) inside a microscope


enclosure to increase X-ray transmission and to very rarely had to reject an energy scan. Had therereduce radiation damage facilitated by oxygen [17]. been any radiation damage to the samples, thisThe energy resolution utilized was about 120 meV. procedure would have also revealed a systematicI-zero normalization scans were repeatedly inter- spectral change in sequential energy scans. As aspersed with the sample measurements. All measure- defocused beam was used, no signs of radiationments were performed with a slightly defocused damage were observed in the spectra reported here,X-ray beam. The microscope permits the identifica- except as indicated.tion of suitable, homogenous sections with an eventhickness.

2 .2.2. Energy calibration2 .2.1. Reproducibility The dispersion and linearity of the monochromator

The reproducibility of the spectra was carefully was determined and maintained by X1A personnelchecked and verified by acquisitions of typically four by using the zero order beam, as well as oxygen,to five energy scans per sample that were compared nitrogen and CO absorption features. The energy2

to each other. Absorption spectra were derived from calibration procedure assumes that the mono-the transmitted X-ray intensity by a sequence of chromator and grating geometry, i.e. the includedconversions and quality checks. Each data set con- angle and groove density and hence the dispersion, issisted of four or five energy scans. These spectra determined initially, is fixed and stable thereafter,were pair-wise compared to each other to detect any and that any calibration error is a wavelength offsetpotential beam instability, sudden energy scale shifts due to lost stepping motor steps or drifts in theor any other artifact. Only if at least four spectra monochromator actuating mechanism. The spectraoverlapped within the signal to noise of the data was were acquired during a number of different ex-an average calculated from the common spectra. We perimental runs with the Stony Brook microscope,

Fig. 1. High energy resolution absorption spectrum of CO overplotted with a spectrum of P2VP acquired in the presence of CO after2 2

proper alignment, and thus energy calibration, of the CO /P2VP spectrum with the CO spectrum.2 2


but any individual spectrum was calibrated and ALS is plotted with a calibrated CO /P2VP spec-2

normalized within the same synchrotron fill. trum from X1A. Due to the experimental step sizeTwo sets of spectra were acquired for each and resolution, the C 1s→3s (v50) and the C

polymer, one without a calibration gas and one in the 1s→3s (v51) Rydberg vibrational features cannotpresence of CO . By using the sharp, well-defined be resolved in the X1A data. The alignment of the2

spectral features of CO [18], the absolute energy Rydberg features and the effectiveness of this meth-2

scale can be calibrated simultaneously with each od is demonstrated by this figure. The residualpolymer spectrum. CO gas was added to the He systematic calibration and instrument errors are2

environment of the microscope enclosure, and the estimated to be about 20–30 meV.transmission spectrum of the polymer and CO gas2

mixture was recorded [14]. Any offset of the sinebar2 .2.3. Data processing

of a grating is essentially linear in wavelength ratherAfter checking the reliability of the energy scans,

than energy, so the energy scale of the spectra werethe optical densityOD is calculated by inversion of

calibrated by shifting thewavelength of the polymer /Beer’s law as OD 5 2 ln(I /I ), where I is the0CO spectrum to that of the CO reference spectrum.2 2 transmitted intensity andI the incident intensity.0After proper alignment, the spectra are converted toThe optical density is related in a straightforward

an energy scale. The location and shape of the entireway to the mass absorption coefficient, sample

Rydberg series was used for the energy calibration,density and thickness:

with the greatest emphasis in the alignment pro-cedure given to the well resolved 3p Rydberg peak at OD(E)5 2 ln(I /I )5m(E)*r* t0294.96 eV. In Fig. 1, a high energy-resolution CO2

gas phase spectrum recorded at beamline 7.0 at thewhere m(E) is the mass absorption coefficient at

Fig. 2. NEXAFS spectrum of PaMS fitted with 6 Gaussian peaks and one arctan function at 290.2 eV. Inset shows expanded low energyp*peak and a single gaussian peak, delineating the limitations of our fitting approach and clarifying the meaning of the tabulated energies. (Seetext for details.).


energy E, r is the density, andt is the sample since the low energy peak at 285.3 eV has chemicalthickness. In analytical NEXAFS microscopy, this and vibrational components [20] and should there-allows one to determine the results of a detailed fore be asymmetric (see the inset to Fig. 2). Thecompositional analysis in multicomponent systems in actual peak energy will typically be lower than theterms of mass thickness (r* t) or, if the density is energy quoted by up to 80–100 meV. Nonetheless,known, as thicknesst [8]. we tabulate the gaussian peak location as an indica-

For the purposes in this manuscript and the goal to tion of where the main NEXAFS peaks should be increate a database, theOD spectrum was subsequently carefully calibrated spectra (see below), along withnormalized and converted to absolute oscillator peak assignments (when possible) based on priorstrength. The conversion to absolute oscillator literature or related polymer NEXAFS spectra.strength is obtained by scaling the pre-edge (,284eV) and post-edge (.310 eV) of theOD spectra tothe oscillator strength of C1s ionization (taken from 3 . Results and discussion[19]).

A series of Gaussian peaks were used to fit these Spectroscopic assignment is a delicate, and atspectra. A fitted NEXAFS spectrum of PaMS is times, controversial task. In this paper, we focus onshow in Fig. 2 as an example. The spectrum is fitted the most prominent peaks and do not try to fit orwith 6 gaussian peaks and an arctangent stepfunction consider every detail. We are motivated by providingat 290.4 eV (for the ionization continuum onset) to reliable reference spectra and not by providingarchive the best fit. This is inadequate at some level, detailed assignments and interpretation.

Fig. 3. (a) C 1s NEXAFS spectra oftrans-1,4-polyisoprene (1.A),cis-1,4-polyisoprene (1.B), mixed isomer polybutadiene (1.C), and mixedisomers polychloroprene (Neoprene) (1.D). (b) Expanded low energy region of the same spectra. (Labels for polymers refer to Scheme 1.).


Fig. 3 displays the spectra of the unsaturated diene observed in the C 1s spectra of fluorinated ethanespolymers shown in Scheme 1. The main C1s(C– [21]. Energies and assignments of the most signifi-H)→p* peak transitions near|285 eV can be cant features are summarized in the table as part ofC=C

readily correlated with the presence ofp bonds in Fig. 3.these polymers. The carbon atoms forming the Fig. 4 shows the spectra of polymers with main-double bond in the isoprene are inequivalent due to or side-chain single carbonyl groups (see Scheme 2).the presence of the methyl group, leading to the All spectra show a single, strong C1s(C=O)→p* C=O

separation into two components. Subtle differences transition, the energy of which depends on theare observed in the peak shape, location, and intensi- ‘chemical’ environment, i.e. near neighbor composi-ty of NEXAFS spectra oftrans- and cis-1,4,-poly- tion, of the carbonyl group. The systematic trends ofisoprene. The carbon atoms in the double bond of this dependence has been recently explored andcis-1,4-polybutadiene result in a single, vibrationally discussed by Urquhart and Ade [9]. Various weakerbroadened, and asymmetric peak at 285.14 eV. The features are due to C1s(C–H)→s* and C1s(C–C–H

carbon atoms in neoprene are very inequivalent due C)→s* transitions. The C1s(C=O)→p* tran-C–C C=O

to the strong inductive effect of the chlorine atom. sition is most intense in PVMK due to the relativeAs a result, the two carbon atoms participating in the abundance of carbonyl groups in this polymer (onedouble bond in the polymer backbone give rise to the in four carbon atoms) and least intense in Nylon6distinct C1s(C–H)→p* and C1s(C–Cl)→p* and PVP (one in six carbon atoms). Energies andC=C C=C

features at 285.11 eV and 286.37 eV, respectively. assignments of the most significant features areThe large intensity of the neoprene 287.91 eV peak summarized in the table as part of Fig. 4.appears to be similar to the potential barrier effects Fig. 5 displays the spectra of main chain aromatic

Fig. 4. (a) C1s NEXAFS spectra of poly(ether ether ketone) (PEEK) (2.A), poly(oxybenzoate-r-2,6-oxynaphthoate) (Vectra�) (2.B),poly(butylene terephthalate) (PBT) (2.C), poly(ethylene terephthalate) (PET) (2.D), poly(ethylene naphthalate) (PEN) (2.E), andpolycarbonate (PC) (2.F). (b) Expanded low energy region of the same spectra. (Labels for polymers refer to Scheme 2.).


Fig. 5. (a) C1s NEXAFS spectra of poly(vinyl methyl ketone) (PVMK) (3.A), poly(vinyl pyrrolidone) (PVP) (3.B), poly(methylmethacrylate) (PMMA) (3.C), poly(ethylene succinate) (PES) (3.D), and poly-e-caprolactam (Nylon6) (3.E). (b) Expanded low energyregion of the same spectra. (Labels for polymers refer to Scheme 3.).

and carbonyl containing polymers detailed in C1s(C=O)→p* transition of PEEK at 286.7 eV.C=O

Scheme 3. The spectra are now significantly more The most extensive conjugation, and hence delocal-complex due to the presence of two different func- ized orbitals, and thus energy splitting occurs intional groups withp bonds as well as inductive PEN. As a result, PEN has by far the lowest energyeffects of various side-groups. In addition, in PEN feature of this set of polymers at 284.4 eV. Energiesand Vectra�, the naphthalene group has a complex and assignments of the most significant features arespectrum of its own, and in PET [22], PBT, and summarized in the table as part of Fig. 5.PEEK, the aromatic group interacts with the car- Fig. 6 shows the spectra of PAN, PaMS, andbonyl groups. The numerous spectral features thus P2VP. PAN has a single, very intense peak at 286.58reflect the more complex electronic structure of the eV, due to thep* orbitals of the triple bonds of thepolymers. Generally, features below 287 eV corre- nitrile group present in the polymer at high con-spond to various C1s(C–H)→p* and C1s(C– centration. PaMP has a spectrum very similar toC=C

R)→p* transitions as indicated in the table polystyrene, in which the lowest energy peak atC=C

below Fig. 5. An exception is the low energy of the 285.18 eV consists of slightly shifted peaks due to


Fig. 6. (a) C1s NEXAFS spectra of poly(acrylonitrile) (4.A), poly(a-methyl styrene) (PaMS) (4.B), poly(2-vinyl pyridine) (P2VP) (4.C),and polyethylene (4.D). (b) Expanded low energy region of the same spectra. (Labels for polymers refer to scheme 4.).

the inequivalent carbon atoms of the aromatic group mers grouped together in Scheme 5. The largestthat are convoluted with vibrational contributions spectral differences between different polymers are[20]. The shift of the carbon atoms in the conjugated associated with the presence of oxygen heteroatoms,side-chain ring is further increased in P2VP due to shifting the ionization potential of the respectivethe presence of the nitrogen. This leads to a pro- carbon atom. In PMO, in which each carbon atomnounced split of the lowest energy peak, with has two oxygen neighbors but is otherwise structural-features at 284.87 eV, 285.39 eV, and 285.68 eV, ly similar to PE, the spectrum is shifted by about 3respectively. The feature near 287.3 eV in PaMS is eV relative to that of PE. Some radiation damage wastypically assigned to a C1s(C–H)→s* transition observed in PMO, which changed the relative in-C–H

and the 288.67 eV feature to a C1s(C–H)→2p* tensity of the two peaks in the first band. In PVA,C=C

transition [1]. The inductive effect of the nitrogen with only one carbon atom out of two backbone.

atom in the ring in P2VP, presumably due to core carbon atoms with an oxygen neighbor, thelevel shifts, seems to also effect the C1s(C– heteroatom shift results in C1s(C–H)→s* andC–H

H)→s* and C1s(C–H)→2p* transitions in C1s(C–OH)→s* peaks that are 1.6 eV apart.C–H C=C C–H

P2VP by splitting them up and thus lowering their The differences in C–C and C–H bonding arrange-peak intensity. Energies and assignments of the most ments in the polymers without oxygen, i.e., EPR, PE,significant features are summarized in the table as PIB and PP, yield only very small spectral changes.part of Fig. 6. An understanding of the subtleties of the spectra of

Fig. 7 displays the spectra of the saturated poly- this family of polymers is still evolving. Recent


Fig. 7. (a) C1s NEXAFS spectra of poly(methylene oxide) (PMO) (5.A), ethylene propylene rubber (EPR) (5.B), poly(vinyl alcohol) (PVA)(5.C), polypropylene (PP) (5.D), poly(isobutylene) (PIB) (5.E), and polyethylene (PE) (5.F). (b) Expanded low energy region of the samespectra. (Labels for polymers refer to Scheme 5).

experiments on a sequence of various ethylene-1- 4 . Conclusionsalkane copolymers, i.e. well-controlled polyethylenepolymers of various densities, have shown a clear We have recorded the NEXAFS spectra of 24matrix effect that primarily affects the spectral polymers and demonstrated the uniqueness of eachintensities rather than energies of the prominent spectrum. The spectra have been carefully calibratedspectral contributions [23]. The relative strength of against CO spectral features and are made available2

the C1s(C–O)→s* features in PMO is unex- in a database as reference spectra. It is alreadyC–H

plored, but could be related to matrix effects similar known that in some cases higher energy resolutionto those observed in the polyethylene materials. results in resolving some finer details [20]. ThisEnergies and assignments of the most significant might apply for many of the polymers included infeatures are summarized in the table as part of Fig. 7. this study. Higher energy resolution revealing in-

In many instances the width of the spectral peaks creased spectral details should, thus, further improveobserved is significantly larger than the core-hole the analytical capabilities of NEXAFS spectroscopylifetime contribution of about 100 meV. Several by increasing our ability to discriminate betweenpeaks relating to inequivalent carbon atoms as well increasingly subtle chemical structure changes. Weas vibrational overtones must be underlying these are looking forward to ever increasing instrumentbroad, often asymmetric peaks similar to the situa- capabilities and the exploitation, particularly intion in polystyrene [20,24]. We expect this to be the NEXAFS microscopy, of polymer NEXAFS spec-case for all aromatic polymers. troscopy.


[6] H. Ade, X. Zhang, S. Cameron, C. Costello, J. Kirz, S.A cknowledgementsWilliams, Science 258 (1992) 972.

[7] H. Ade, A.P. Smith, S. Cameron et al., Polymer 36 (1995)We are grateful to A.P. Smith and V. Knowles for1843.

microtoming some of our samples. G. Mitchell, J. [8] H. Ade, S. Urquhart, in: T.K. Sham (Ed.), Chemical Applica-Dias, C. Sloop, D. Liu, M. Rafailovich and M. tions of Synchrotron Radiation, World Scientific Publishing,

Singapore, 2002.Zharnikov were kind enough to provide some of the[9] S.G. Urquhart, H. Ade, J. Phys. Chem. B 106 (2002)samples. Data was recorded using the Stony Brook

8531–8538.STXM at the NSLS, developed by the groups of J.[10] J. Kikuma, B.P. Tonner, J. Electron Spectros. Relat. Phenom.

Kirz and C. Jacobsen, with support from the U.S. 82 (1996) 53.Department of Energy, Office of Biological and ¨[11] W.E.S. Unger, A. Lippitz, C. Woll, W. Heckmann, Fresenius

J. Anal. Chem. 358 (1997) 89.Environmental Research (contract DE-FG02-[12] H. Ade, in: J.A.R. Samson, D.L. Ederer (Eds.), Experimental89ER60858) and the National Science Foundation

Methods in the Physical Science, Vol. 32, Academic Press,(grant DBI-9605045). The zone plates at X1A were1998, p. 225.

developed by S. Spector and C. Jacobsen of Stony[13] H. Ade, Trends Polym. Sci. 5 (1997) 58.Brook and Don Tennant of Lucent Technologies Bell [14] H. Ade, A.P. Smith, H. Zhang et al., J. Electron Spectrosc.Labs, with support from the NSF under grant ECS- Relat. Phenom. 84 (1997) 53.

[15] H. Ade, in: J. Kroschwitz (Ed.), Encyclopedia of Polymer9510499. We particularly thank Sue Wirick for herScience and Technology, 3rd Edition, Wiley, New York, (inhelp in maintaining the X1A microscope and beam-press). Web version available at http: / /www.mrw.inter-

line. Work at NCSU supported by NSF grants DMR- science.wiley.com/epst / index.Ltml9458060 and DMR-0071743, and work at the Uni- [16] M. Feser, M. Carlucci-Dayton, C. Jacobsen, J. Kirz, U.versity of Saskatchewan is supported by NSERC and ¨Neuhausler, G. Smith, B. Yu, in: I. McNulty (Ed.), X-ray

Microfocusing: Applications and Techniques, Vol. 3449,the University of Saskatchewan.SPIE Proc, 1998, p. 19.

[17] T. Coffey, S.G. Urquhart, H. Ade, J. Electron Spectrosc.Relat. Phenom 122 (2002) 65.

R eferences [18] Y. Ma, C.T. Chen, G. Meigs, K. Randall, F. Sette, Phys. Rev.A 44 (1991) 1848.

[19] B.L. Henke, P. Lee, T.J. Tanaka, R.L. Shimabukuro, B.K.¨[1] J. Stohr, NEXAFS Spectroscopy, Springer-Verlag, Berlin,Fujikawa, At. Data Nucl. Data Tables 27 (1982).1992.

[20] S.G. Urquhart, H. Ade, M. Rafailovich, J.S. Sokolov, Y.[2] A.P. Hitchcock, S.G. Urquhart, E.G. Rightor, J. Phys. Chem.Zhang, Chem. Phys. Lett. 322 (2000) 412.96 (1992) 8736.

˚ [21] A.P. Hitchcock, Physica Scripta T31 (1990) 159.¨[3] L.G.M. Pettersson, H. Agren, B.L. Schurmann, A. Lippitz,[22] E.G. Rightor, A.P. Hitchcock, H. Ade et al., J. Phys. Chem.W.E.S. Unger, Intl. J. Quantum Chem. 63 (1997) 749.

B 101 (1997) 1950.[4] S.G. Urquhart, A.P. Hitchcock, A.P. Smith, H. Ade, E.G.¨[23] A. Scholl, R. Fink, E. Umbach, G. Mitchell, S. G. Urquhart,Rightor, J. Phys. Chem. B 101 (1997) 2267.

H. Ade, Chem. Phys. Lett. (submitted)[5] S.G. Urquhart, A.P. Hitchcock, A.P. Smith, H.W. Ade, W.Lidy, E.G. Rightor, G.E. Mitchell, J. Electron Spectrosc. [24] Y. Liu, T.P. Russell, M.G. Samant et al., Macromol. 30Relat. Phenom. 100 (1999) 119. (1997) 7768.

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