characterization of the effects of soft x-ray irradiation...

Journal of Electron Spectroscopy and Related Phenomena 122 (2002) 65–78www.elsevier.com/ locate /elspec

Characterization of the effects of soft X-ray irradiation on polymers1 *T. Coffey, S.G. Urquhart , H. Ade

Department of Physics, North Carolina State University, Raleigh, NC 27695, USA

Received 4 December 2000; accepted 23 July 2001

Abstract

The physical and chemical effects of the soft X-ray irradiation of polymers have been systematically evaluated for photonenergies just above the C 1s binding energy. This exposure causes radiation damage in the form of the loss of mass andchanges to the chemical structure of the polymers. These effects are evident in the Near Edge X-ray Absorption FineStructure (NEXAFS) spectra of the exposed polymers, posing a fundamental limit to the sensitivity of NEXAFSspectroscopy for chemical microanalysis. Quantitative understanding of the chemistry and kinetics of radiation damage inpolymers is necessary for the successful and validated application of NEXAFS microscopy. This paper outlines a method forquantifying this radiation damage as a function of X-ray dose, and applies these methods to characterize the loss of mass andloss of carbonyl group functionality from a diverse series of polymers. A series of simple correlations are proposed torationalize the observed radiation damage propensities on the basis of the polymer chemical structure. In addition, NEXAFSspectra of irradiated and virgin polymers are used to provide a first-order identification of the radiation chemistry. 2002Elsevier Science B.V. All rights reserved.

Keywords: NEXAFS spectroscopy; Polymers; Damage; Quantitative; Analysis; Radiation chemistry

1. Introduction from X-ray or electron spectroscopy necessarilycauses radiation damage to the exposed material.

Near Edge X-ray Absorption Fine Structure Polymers in particular are sensitive to radiation(NEXAFS) spectroscopy, performed with high spa- damage caused by X-ray and electron irradiationtial resolution in X-ray microscopy, is a powerful [7–11]. The particular risk for spectroscopic micro-method for the microchemical characterization of analysis is that the sample and its spectrum mightpolymer materials [1–4]. Like its cousin, Electron degrade faster than meaningful microanalysis can beEnergy Loss Spectroscopy in Transmission Electron performed. For chemically meaningful microanaly-Microscopy (e.g. TEM-EELS) [5,6], the combination sis, it is therefore critical to understand both the formof high spatial resolution with chemical sensitivity and the rate of the soft X-ray radiation damage. With

a quantitative understanding of the radiation damagekinetics, it can be possible to design experiments that

*Corresponding author. Tel.: 11-919-515-1331; fax: 11-919- work within a tolerable damage limit. Currently, the515-7331.

level of radiation damage for X-ray microscopy ofE-mail address: harald [email protected] (H. Ade).]1 polymers is not so severe as to prohibit the analysisPresent address: Department of Chemistry, University of Saskat-

chewan, Saskatoon, SK S7N 5C9 Canada. of most polymer materials [4,11]. However, the

0368-2048/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved.PI I : S0368-2048( 01 )00342-5

66 T. Coffey et al. / Journal of Electron Spectroscopy and Related Phenomena 122 (2002) 65 –78

inevitable push to higher spatial resolution and theelucidation of more subtle spectroscopic differenceswill necessarily make radiation damage a growingconcern.

There have been relatively few studies of the softX-ray radiation chemistry and radiation damage ofpolymers [11,12], particularly when compared to thenumerous studies of the chemical effects of highenergy electrons, hard X-rays, and gamma radiation[8–10,13,14]. In general, the radiation chemistry anddamage of polymers can take several forms, such asloss of crystallinity, loss of mass, or chemicalmodification [7]. We are primarily concerned herewith chemical modifications as manifested in NEX-AFS spectral changes, as NEXAFS spectroscopy isthe basis of compositional analysis in soft X-raymicroscopy.

The radiation chemistry of polymers and mole-cules can vary strongly between resonant versusnon-resonant core excitation [15–17]. As the chemi-cal sensitivity of NEXAFS spectroscopy is strongestat X-ray energies that correspond to specific resonant

Scheme 1. Chemical structures of polymers examined in this*excitations (e.g. C 1s→p transitions), the largestudy.

literature of non-resonant electron, g and hard X-rayirradiation may not be directly applicable to theradiation damage that occurs in resonant or near (PE), and poly(propylene oxide) (PPO). The chemi-resonant excitations. Experimental conditions and the cal structures of these polymers are presented inimpact of different characterization methods will Scheme 1.vary between different studies, and may therefore not Chemical changes in the radiation-damaged poly-be applicable to the specific environment in an X-ray mers were examined by comparing NEXAFS spectramicroscope. While our goal is to be as general as of the polymers acquired before and after soft X-raypossible, we have studied the radiation damage of irradiation. Several different effects are observed: thethese polymers in situ in the experimental conditions loss of mass from the polymer film; a decrease inin which NEXAFS microscopy is performed (e.g. intensity of specific spectral features, attributed tothin sections, He-rich environment, etc.). the loss of specific functional groups; and the

We have examined a series of polymers that span observation of new spectral features, attributed to thea wide range of common polymer structures, with a formation of new chemical structures. The kineticsprimary focus on polymers that contain the carbonyl of mass loss and the formation or loss of specificfunctional group: poly(methyl methacrylate) functional groups was measured for a series of(PMMA), poly(bisphenol-A-carbonate) (PC), Nylon polymers using a ‘‘damage-monitor’’ image se-6, poly(vinyl methyl ketone) (PVMK), poly(ethylene quence technique. The radiation induced ‘‘massterephthalate) (PET), polyurethane (PU), poly- loss’’ for all polymers is determined by measuring(ethylene succinate) (PES). The easily damaged the rate of ablation as a function of X-ray exposure.carbonyl group [8] has a narrow and readily identifi- We develop a simple correlation that relates the

*able C 1s(C=O)→p transition that allows the rate of loss of the carbonyl functional group to theC=O

identification of numerous chemical moieties posses- carbonyl C 1s ionization potential, which is a simplesing carbonyl functionality [18]. For comparison, we metric for the local chemical and electronic environ-have also included polystyrene (PS), polyethylene ment of the carbonyl carbon atom. Finally, we

T. Coffey et al. / Journal of Electron Spectroscopy and Related Phenomena 122 (2002) 65 –78 67

measure the difference in the radiation chemistry of cannot be easily measured, but the He flow rate waspolymers in the presence and absence of oxygen. kept constant for all quantitative damage experimentsThe derived quantitative ‘‘critical doses’’ and quali- to best ensure a consistent atmosphere.tative insights should be useful to X-ray microscopy For energy scale calibration, CO gas was added2

practitioners in order to define boundary constraints to the He purge in the microscope and the transmis-for analytical experiments. sion spectrum of the mixture of the polymer and CO2

gas was recorded [22,23]. The energies of theCO →Rydberg transitions from the high-resolution2

NEXAFS spectra of Ma et al. [24] were used to2. Experimental calibrate these spectra.

2.1. Sample origin and preparation2.3. Detector and detector calibration

Thin films (|50 to 200 nm) of the polymers wereprepared for this study. Samples of PVMK and PES The X-ray transmission of the sample was mea-were obtained from Scientific Polymer Products, sured using a gas proportional counter mounted aPMMA from Aldrich Chemical, and PS from Poly- few millimeters behind the sample. Several copies ofmer Laboratories. The polyurethane (PU, see the same detector design were used in the course ofScheme 1) and poly(propylene oxide) (PPO) samples these experiments since the detector had a finitewere provided by Dow Chemical and have been operating lifetime. In order to properly account forpreviously described [19]. The Nylon-6 sample was the exchange of detectors, two variables were con-obtained from collaborators at AlliedSignal. The trolled: the detector position and the relative detectormolecular weight was not known or not defined for efficiency. The variable sample–detector distanceall polymers. Differences in molecular weight should was measured and corrected for by accounting forhave a minimal influence on the damage rate of the absorption of X-rays by the air /helium purge gasspecific functional groups but a larger effect on the mixture. It was not possible to measure the absolutemass loss damage rates. efficiency of the gas proportional counters against a

Thin polymer sections (|100 nm thick) of most known standard, although comparisons between thepolymers were prepared by ultramicrotomy, using a detected and anticipated photon count rates suggest aLKB Nova microtome or a Reichert-Jung Ultracut S detector efficiency between 10 and 40%. To accountcryo-ultramicrotome and were mounted on standard for the potential differences in efficiency of differentTEM grids. Thin films of PES and PS were prepared detectors, the rate of radiation damage for the C

*by solvent casting, from chloroform and toluene 1s(C=O)→p transition in polycarbonate (PC)C=O

respectively, and floated onto TEM grids. was used as an internal standard. The repeatability ofthis damage rate in identical conditions (same detec-tor, same atmosphere, and same sample thickness)

2.2. Microscope description was within 10%. This internal calibration methodprovides a relative comparison suitable for the

Data for this study were acquired using the Stony internally consistent comparison of the radiationBrook Scanning Transmission X-ray Microscope damage kinetics of different polymers. We have(STXM) at the National Synchrotron Light Source assumed a detector efficiency of 30% for our esti-(NSLS) in Brookhaven, NY [20,21] during several mate of the absolute critical dose and G-values,data acquisition runs. The STXM is operated at room cognizant that the systematic errors in these valuestemperature and inside a He purge enclosure at might be as large or even larger than 100%. Never-atmospheric pressure. The radiation damage studies theless, these results provide a meaningful com-were performed in this helium-rich environment parison of the effect of soft X-ray exposure betweenexcept for specific studies performed in air. The different polymers during NEXAFS microscopyprecise composition of the He purge atmosphere experiments.


2.4. Measurement of radiation damage where I and I are the transmitted and incident X-ray0

flux, respectively, m is the energy dependent massA simple determination of the character of the soft absorption coefficient, r is the polymer density, and

X-ray radiation chemistry of the polymers examined t is the sample thickness.in this study was obtained by comparing the NEX- The polymer film region chosen for the damage-AFS spectra of the virgin (undamaged) and the X-ray imaging series included an open area or a hole. Theirradiated polymer films. These spectra were ac- X-ray transmission measured in this open area (i.e.quired with a defocused beam so that the radiation the intensity in the image pixels corresponding to thedose from the acquisition of these spectra would be hole) provided an internal and nearly instantaneousbelow the threshold for observing radiation damage, measurement of the incident beam intensity (I ) at0

based on the measured critical doses described the time the image was recorded. This internal I was0

below. used to obtain the optical density of the material atIn this work, radiation damage was induced using the imaging energy (Eq. (1)). In the images used to

an X-ray energy in the C 1s ionization continuum monitor the radiation damage, these images provide(315 eV). This energy is above the chemically the optical density of the particular feature used tospecific NEXAFS absorption features and where the monitor chemical change in the polymer, such as the

*X-rays are absorbed non-preferentially by all of the C 1s→p transition. Since the featureless ioniza-C=O

carbon atoms in the polymer. Quantitative determi- tion cross-section at 315 eV is proportional to bothnation of the polymer radiation damage kinetics was the sample thickness and density, the images used toobtained through a series of X-ray imaging measure- ‘‘damage’’ the polymer at this energy also containments performed in the X-ray microscope. A series the ‘‘mass-thickness’’ and can be used to measureof images was acquired in which the material was the degree of mass loss from the polymer.alternatively exposed to ‘‘damaging’’ photons by With increasing radiation dose (d), the opticalimaging at 315 eV, and then monitored at an energy density of a monitored spectral feature or the mass-corresponding to a specific spectroscopic transition. thickness at 315 eV will change in a monotonic andThe effect of the radiation damage was measured by typically exponential way. In order to quantify theimaging at the energy of a specific spectroscopic damage rate, the optical density has been fitted to the

*feature, such as, for example, the C 1s(C=O)→p following exponential expression:C=O

excitation at 290.5 eV in polycarbonate (PC). The*OD 5 OD 1 C exp(2d /d ) (2)dwell time used in recording these images was ` c

adjusted so that the radiation exposure in the‘‘damaging’’ images at 315 eV was significantly where OD is the remaining optical density after`

greater than the radiation exposure from the moni- infinite radiation dose, (C1OD ) is the optical`

toring images. This ‘‘exposure-monitor’’ sequence density prior to irradiation, d is the radiation dose,was repeated for about 50 image pairs. and d is the critical radiation dose. At the criticalc

radiation dose, 1 /e or 63% of the total attenuation ofa specific spectroscopic feature (or mass-thickness)has occurred. This metric can be used to compare the3. Calculationsrelative radiation sensitivity of different polymersand different functional groups, as a smaller critical3.1. Quantitative determination of the radiationdose implies a faster damage rate. For mass loss, thedamage kineticsvalue of OD from the fit of Eq. (2) can also be used`

to characterize the nature of the radiation chemistry.Since NEXAFS spectroscopy measured in trans-If the extrapolated optical density after infinite dosemission obeys Beer’s law, the optical density (OD)tends to zero (i.e. OD →0), then the polymer loses`at any energy can be obtained from transmissionmass during radiation damage, while if the extrapo-X-ray images aslated OD is close to 1, the polymer is resistant to`

OD 5 2 ln(I /I ) 5 mrt (1) mass loss. Crosslinking and chain scissioning are0


hypothesized as likely radiation damage outcomes geometries were determined using the programbased on these observations. GAMESS [28] with a 6-21G* level basis set. For the

GSCF3 calculations, a Huzinaga [29] basis set is3.2. Ionization potential calculations employed: (621/41) contracted Gaussian type func-

tions were used on the heavy atoms (C, N and O);To aid the discussion of the radiation damage rates (41) on H; and a higher quality basis set (411121/

and chemistry for these polymers, ab initio Improved 3111/*) on the heavy atom onto which the core holeVirtual Orbital (IVO) calculations have been per- is placed.formed to determine the carbon 1s ionization po-tential (IP) of the carbonyl carbon atom in a series ofpolymers. The target polymers and the molecular 4. Results and discussionmodels used for these calculations are presented inScheme 2. 4.1. Qualitative chemical and spectroscopic

Calculations on these species were carried out observationsusing Kosugi’s GSCF3 package [25,26]. Thesecalculations are based on the Improved Virtual The nature of the chemical changes induced byOrbital approximation (IVO) which explicitly takes radiation damage and the susceptibility towards massinto account the core hole in the Hartree–Fock loss for many of the polymers investigated in thisapproximation and are highly optimized for calcula- paper can be observed qualitatively in Figs. 1 and 2.tions of core excited states [27]. The difference in Fig. 1 presents the NEXAFS spectra of polymers thatthe total energy between the core ionized and ground lose mass upon X-ray irradiation (PET, Nylon-6,states energies (DSCF) gives the core ionization PVMK, PMMA and PE) and Fig. 2 presents thepotential (IP) with a typical accuracy of ¯1 eV, NEXAFS spectra of some polymers that are totallyreflecting the limitations of the IVO approximation. resistant to mass loss (PS, PC and PU) and by

Optimized (minimum total energy) molecular implication should crosslink under X-ray irradiation.All spectra except that of PE have been acquired in aHe-rich environment.

Several general trends can be observed. In poly-mers that contain carbonyl functional groups, a

*decrease in the intensity of the C 1s(C=O)→pC=O

transition is clearly observed. In the NEXAFS spec-trum of irradiated PET (Fig. 1), a decay in the C

*1s(C–H)→p transitions (284.8 and 285.4 eV) isC=C

also observed in addition to the attenuation of the C*1s(C=O)→p transition (289 eV) (previouslyC=O

observed by Rightor et al. [11]). Similarly, PVMKand Nylon-6 have a pronounced decrease in the C

*1s(C=O)→p transition (286.8 and 287.3 eV,C=O

respectively), as well as the growth of a new featureat |285 eV. The change in the intensity of the

*‘‘carbonyl’’ C 1s(C=O)→p absorption will beC=O

used below to track radiation damage to the carbonylfunctional group as a function of radiation dose.

The observation of new spectroscopic transitionsat |285 eV in the NEXAFS spectra of manyirradiated polymers, including PE, PVMK, andScheme 2. Chemical structures of the molecular models used forNylon 6, are characteristic of unsaturated C–Cab initio Improved Virtual Orbital (IVO) calculations of the C 1s

ionization potential of the carbonyl group in a series of polymers. bonds (i.e. phenyl, ethylene and ethyne functional


Fig. 1. Comparison of the C 1s NEXAFS spectra of polymers thatpredominantly lose mass upon irradiation: poly(ethylene tere-phthalate) — PET, Nylon-6 — N6, poly(vinyl methyl ether) —PVMK, poly(methyl methacrylate) — PMMA and polyethylene

Fig. 2. Comparison of the C 1s NEXAFS spectra of polymers that— PE, before (———) and after (– ? –) X-ray exposure. Theare resistant to mass loss: polystyrene — PS, polycarbonate — PCtotal dose administered is indicated. PE was exposed in a He/airand polyurethane — PU, before (———) and after (– ? –)mixture. The total dose has not been determined. The spectra areX-ray exposure. The total dose administered is indicated.included as an illustration of spectral changes due to damage. For

details about the dependence of PE damage on the presence ofoxygen, see text.

in the 286–287 eV energy range in the spectra of the*irradiated polymers. Typically, a C 1s(C–R)→pC=C

groups). Additional new features can be observed in transition originating from the substituted phenylthe NEXAFS spectrum of irradiated Nylon-6 (Fig. ring carbon atoms is present in this energy range1), particularly a relatively well-resolved spectro- (e.g. C–R atoms, where R are substituents withscopic feature is introduced at 286.75 eV. The energy electron inductive properties). For example, in theof this new feature closely corresponds to the C spectrum of virgin polyurethane (Fig. 2), the C

* *1s→p transition in a polyacrylonitrile [1,30] and 1s(C–R)→p transition at 286.5 eV is attributedC≡N C=C

*acetonitrile [31], and the C 1s→p transition in to phenyl ring carbon atom sites that are substitutedC=N

imidazole [32], which suggests that this new feature by the carbamate group [19], above the C 1s(C–*could be from the formation of C=N unsaturation in H)→p transition from the ‘‘C–H’’ phenyl carbonC=C

Nylon-6. atoms. In all spectra of the radiation damagedIn polymers containing phenyl functional groups aromatic polymers, the energy range 286–287 eV is

(PET, PS, PC, PU), new features are also observed ‘‘filled in’’ by broad or discrete transitions, sug-


gesting a modification of the phenyl rings is apotential outcome of the radiation damage of thesepolymers.

The chemical origin of these features and thechemical pathways leading to them has not beenunambiguously identified, although NEXAFS spectraprovide valuable indications of the likely chemistry.

4.2. Mass loss — observations

Fig. 3 presents the mass loss for a series ofpolymers as a function of radiation dose. A summaryof the critical dose values and fractional massremaining at infinite dose obtained from the fit of thedata to Eq. (2) is presented in Table 1. Somepolymers undergo damage until no polymer wouldremain for infinite dose (OD 50), while others`

would reach a finite, constant mass thickness(OD .0). The polymers PU, PC, PS, and PE`

exhibited negligible mass loss and the respective dataFig. 3. Summary of polymer mass loss results. The y-axisrepresents the fraction of the polymer thickness remaining after are not displayed in Fig. 3.the indicated dose. Polymers can lose mass due to bond breaking

Table 1Summary of the rate and extent of chemical damage by mass loss for a series of polymers exposed in a He-rich environment. The ‘‘criticaldose for mass loss’’ represents the X-ray dose needed to reduce the projected sample thickness by 1/e, and the ‘‘fractional mass’’ is the massremaining after an extrapolation to infinite exposure

aPolymer Density Mass loss Fractional Propensity for crosslinking3 b,c,d(g /cm ) critical dose mass,

Previous This3(eV/nm ) OD /(OD 1C)` ` work work

PC 1.2 None 1.0 Unknown Yes (strongly)PU 1.24 None 1.0 Unknown Yes (strongly)PES 1.175 830 0.32 Unknown SomePMMA 1.2 350 0.72 Some [12] YesNylon-6 1.12 800 0.85 Yes [8] YesPVMK 1.12 1400 0.74 Unknown YesPET 1.385 58,700 0 Some [33] NoPS 1.05 None 1.0 Yes [8] Yes (strongly)PE 0.92 None 1.0 Yes [34] Yes (strongly)PPO 1.0 430 0 No [8] No

a Densities from Scientific Polymer Products compilation, http: / /www.scientificpolymer.com/resources /poly dens alpha.htm.] ]b All experimental (relative) errors are estimated to be 10%. Systematic errors are dominated by uncertainty about the absolute detector

3 3efficiency and might be as large as 100%. The measured radiation dose is presented in eV/nm rather than SI units of Grays, as eV/nm canbe directly related to the measurement units of the microscope (spatial scale, sample thickness and photon energy).

c 3 5Conversion: eV/nm 5(1.602310 Grays) /r (where r is the polymer density).d Assumed 30% detector efficiency.


(scissioning) in the main chain or in side groups. polymers tested, those with aromatic groups (PET,While some polymers might undergo both scission- PS, PC, PU) lost less than 10% of their original mass

3ing and crosslinking upon irradiation, one mecha- at a dose of 1500 eV/nm . The five polymers (PPO,nism usually dominates. Scissioning is the dominant Nylon-6, PVMK, PMMA, and PES) that lost aprocess in electron damage studies of PET [33] and significant fraction (.10%) of their mass for thisPMMA [8,12,13]; crosslinking dominates in PE [34], dose do not contain aromatic groups. This resultwhile crosslinking and scissioning are considered to agrees with previous electron irradiation studies [8,9]be comparable in Nylon-6 [8]. Based on the OD which indicate that the presence of aromatic groups`

values shown in Table 1, our data is consistent with protect polymers from radiation damage. It is inter-these previous results: PE and PS do not lose mass esting to note that aromatic groups seem to onlywhich can be attributed to high crosslinking, PET stabilize the polymers, but not to control the prefer-loses mass, while Nylon has OD 50.85 and PMMA ence for scission or crosslinking (i.e. they change the`

has OD 50.72, which would indicate that some rate but not the outcome). For example, our data`

scissioning occurs, but that crosslinking is somewhat indicate that PET eventually completely scissions,more dominant. Among the other polymers investi- but does so only very slowly, while PC, PS and PUgated, PVMK both scissions and crosslinks, with a lost virtually no mass. An unexplored aspect is the

*slight dominance of crosslinking, PES predominantly effect that resonant C 1s→p excitation and theC=C

scissions, while PPO scissions completely. PU ex- potential for substantially different de-excitationhibits negligible mass loss and therefore should be pathways will have on the radiation chemistry ofhighly crosslinking. aromatic polymers.

Previous studies [8] suggest that a reasonable In addition to kinetic stabilization due to theprediction of the propensity for crosslinking in presence of aromatic groups, crosslinking uponpolymers can be based on the structure of the irradiation can directly protect a polymer from masspolymer backbone. Scheme 3 presents two general- loss. According to previous experiments [8], Nylon-ized polymer structures: Structure A with the tertiary 6, PMMA, polyethylene, and polystyrene crosslinkbackbone carbon atom highlighted, and Structure B, upon irradiation with electrons. Of these four poly-with the quaternary backbone carbon atom high- mers, Nylon-6 retained 85% of its total mass andlighted. Polymer chains with tertiary carbon atoms PMMA retained 72% of its total mass upon irradia-are likely to crosslink, while polymer chains with tion with 315 eV X-rays, while polyethylene andquaternary backbone carbons are likely to scission. polystyrene lost a negligible fraction of their mass.We note that our data for the radiation damage of PS The spectra of Nylon-6 and polyethylene which wereand PVMK supports this prediction. However, most acquired after irradiation with soft X-rays show theof the other polymers investigated cannot be fitted growth of a new, broad peak at |285 eV (see Fig. 1),

*within this scheme on account of their more compli- where C 1s→p transitions associated with C=CC=C

cated chemical structure. unsaturation have been observed in many otherThe presence of an aromatic group has a signifi- species [35]. An increase in C=C bonds has been

cant impact on the damage rate of polymers. Of the associated with crosslinking in polymers such as PE[8], but might arise in general from mechanisms thatare not directly related to crosslinking. In soft X-rayirradiated PMMA, main chain and side chain scis-sioning is the dominant damage mechanism. How-ever, after sufficiently large doses, PMMA mightcrosslink [36]. This is due to the loss of the methylester side chain, which has a 1:1 correspondencewith the formation of C=C bonds [13]. In damagedPMMA, Zhang et al. attributed a feature at 286.6 eVto C=C bonds and interpreted it as a sign of

Scheme 3. crosslinking [12,13], even though C=C bonds typi-


cally have a resonance near 285 eV. Our results showslightly different spectral features for damagedPMMA, but agree with prior results in as far asPMMA retained a large percentage of its mass and isthus crosslinking.

In PE, the formation of C=C bonds and theappearance of a 285 eV feature is attributed tohydrogen abstraction, which also leads to cross-linking [8]. The 285 eV peak associated with cross-linking is also present in the radiation-damagedspectrum of poly(vinyl methyl ketone) (see Fig. 1).Since PVMK retains a high percentage (74%) of itsmass after irradiation, it is likely that PVMK pre-dominantly crosslinks upon irradiation.

4.3. Loss of carbonyl functional group

Polymers containing carbonyl groups have aFig. 4. Summary of carbonyl group loss results. The y-axis*characteristic C 1s(C=O)→p transition that isC=Orepresents the normalized change in optical density at the carbonyluseful for chemical analysis. This functional group istransition.known to damage easily, which will have a large

effect on the sensitivity of spectral analysis of thesematerials. We have therefore determined a ‘‘critical and PVMK), the critical dose calculated should bedose’’ for the loss of the carbonyl functional group close to the true critical dose. The uncertainty isfor the following polymers: PMMA, PC, Nylon-6, largest for PES, for which mass loss is a substantialPVMK, PET, PU, and PES. The experimental data process and therefore the calculated critical dosesand a fit of the dose dependence of the C should be used with some caution.

*1s(C=O)→p transition to Eq. (2) are displayed Table 2 summarizes the critical dose for theC=O

in Fig. 4 for all polymers except for PET. carbonyl group, the atomic fraction of carbonylSince mass loss and chemical changes to a specific carbon atoms in the polymer, the carbonyl-normal-

functional group can occur simultaneously, the criti- ized damage rate, the G-value for this damage, andcal dose obtained by fitting the raw OD of the C the calculated C 1s ionization potential of the

*1s(C=O)→p transition to Eq. (2) will involve carbonyl carbon atom. The carbonyl-normalizedC=O

some uncertainty, particularly when the mass loss is radiation damage rate accounts for differences in theunrelated to the damage of the carbonyl functional polymer stoichiometry, permitting a direct compari-group. Since we do not have a priori knowledge of son of the ‘‘carbonyl’’ critical doses. The G-values inthe detailed mechanisms for mass loss, we can only Table 2 are inversely related to the un-normalizedevaluate the critical dose of the optical density for critical doses in that they quantify the extent of

*the C 1s(C=O)→p transition intensity. This radiation damage for a given dose [8]. The carbonylC=O

parameter substitution can either under- or overesti- G-values calculated provide a measure for ‘‘carbonylmate the actual critical dose for the carbonyl group events’’ per 100 eV dose to the whole sample. Foritself, since this intensity is convoluted with the loss polymers that exhibit significant mass loss, the G-of the polymer itself. For polymers that lose no mass, values calculated have relatively large uncertainty,such as PU and PC, there will be no additional but are included in order to facilitate comparisonuncertainty. For polymers where the mass loss is with previous electron irradiation studies. A directsmall relative to the attenuation of the C comparison might be limited by the experimental

*1s(C=O)→p transition (i.e. Nylon-6, PMMA differences in sample environment and the particularC=O


Table 2Summary of the chemical damage rates of the loss of the carbonyl functional group for a series of polymers exposed in a He-richenvironment, in comparison to the calculated C 1s ionization potential for the carbonyl carbon atom in these polymers

Polymer C=O [ C=O carbon Normalized G-value Calculateddamage atoms to total [ C=O C 1s C=O

a,b arate carbon atoms damage rate ionization3 3(eV/nm ) per monomer (eV/nm ) potential (eV)

PMMA 520 1/5 104 0.86 295.43PES 530 2/6 177 1.10 295.98PC 580 1/16 36 0.31 297.79PU 740 2/13 114 0.43 296.63PVMK 1600 1/4 400 0.38 294.00Nylon6 2300 1/6 383 0.17 294.58PET 22,000 2/10 4400 0.03 295.87

a All experimental (relative) errors estimated 65%. Systematic errors are dominated by uncertainty about the absolute detector efficiency3 3and might be as large as 100%. The measured radiation dose is presented in eV/nm rather than SI units of Grays, as eV/nm can be directly

related to the measurement units of the microscope (spatial scale, sample thickness and photon energy).b Assumed 30% detector efficiency.

criterion used to judge an event (here, 63% of number of heteroatoms around the core-excitedmaximum NEXAFS spectral change). atom. We note that previous studies have shown that

Using the carbonyl-normalized critical dose val- fluorocarbons — which have a higher relative ioniza-ues, we see that the carbonyl group in polycarbonate tion potential [37] — are very radiation sensitivehas a higher propensity towards damage than all [38], and that carbamate functional groups (NH–other polymers, including the aliphatic poly(ethylene C(O)–O–R) damage more readily than urea (NH–succinate). We note a somewhat surprising but C(O)–NH) functional groups present in polyurethanepotentially useful correlation between the carbonyl- foams [3]. Further, the substantially more rapid massnormalized critical dose and the calculated ionization loss observed for poly(propylene oxide) (PPO) thanpotential of the carbonyl C 1s atom, which can be in PE supports a model in which more damageobserved from Fig. 5. In general, carbonyl carbon occurs faster for polymers with polarized bonds oratoms with a higher C 1s ionization potential have a more oxygen atoms. The overall correlation in thelower critical dose. The ionization potential is depen- present study and these other supporting observationsdent on the local electron density of the carbon atom suggest a relationship between the sensitivity ofsite, which varies with the electronegativity and carbonyl functional groups to radiation damage and

their local chemical environment. We observe aboutan order of magnitude more damage to a carbonatefunctionality than to a ketone. The one exception tothis correlation to note is PET. In PET, the carbonyl

*groups are stabilized by p delocalization [11], i.e.mixing of carbonyl and aromatic p orbitals, which ismore extensive than in the other polymers. This mayadd aromatic stabilization to the carbonyl functionalgroups.

Also of note is the substantially lower 7700 eV/3nm critical dose reported by Rightor et al. [11]. A

possible explanation might be that Rightor et al.exposed the polymer with a focused beam in a singlespot, while we raster-scanned the beam to expose aFig. 5. Normalized carbonyl critical dose vs. ionization potential

of the carbonyl carbon. larger area. This would suggest a potential dose rate


dependence in the radiation damage kinetics, a themenot explored in this paper. In addition, the spectralshapes observed by Rightor after damage are slightlydifferent than in the data of this study. Specifically,the spectral feature of the carbonyl group at 288.2 eVis higher relative to other features in our data, whilethe feature at 289.1 eV appears equally damaged inboth data sets. We will show explicitly (Section 4.4)that the relative oxygen content in the sampleenvironment plays a crucial role in the evolution of

*the C 1s(C=O)→p transition intensity (288.2 eVC=O

in PET), and differences in sample environmentbetween the Rightor et al. and our data mightaccount for the observed differences. Since allsamples for the data presented here have beenexposed in the same manner and with identicalhelium flow rates during two data runs, a comparisonbetween different polymers within our data sequenceis self-consistent and meaningful.

Fig. 6. A comparison of fractional mass loss upon irradiation for4.4. Effect of atmospheric oxygen on the radiation polyethylene damaged in air (an oxygen-rich environment) and in

a helium-rich environment. The PE irradiated in air only receiveddamage chemistrya low total dose due to the rapidity of its mass loss.

The atmosphere in which the polymer is exposedto radiation can drastically affect the nature and rateof the radiation damage. In an inert atmosphere, suchas vacuum or an unreactive gas, X-ray generatedradicals are understood to be more stable [8]. In anatmosphere containing oxygen, the radicals can reactto form peroxides or hydroperoxides [8,14], oroxygen itself can be photoexcited or photoionizedand become reactive, accelerating the rate of radia-tion degradation or the extent of the radiationdamage [8,14].

To explore the effect of the atmosphere on theradiation chemistry of polymers, PE and PET wereirradiated in air in addition to the helium-richenvironment presented above. These results arepresented in Fig. 6 and 7, respectively. We noteimmediately that both PE and PET lose mass at amuch faster rate when oxygen is present. The criticaldose for mass loss from PET is nearly two orders ofmagnitude smaller in an oxygen-containing environ-

3ment (600 eV/nm ) than in a helium-flushed en-3 Fig. 7. Comparison of fractional mass loss upon irradiation forvironment (58,700 eV/nm ). In both cases, the

polyethylene terephthalate damage in air (an oxygen-rich environ-extrapolated thickness at infinite dose is zero, that is, ment) and in a helium-rich environment (the data for the PETPET completely scissions. For PE, the results are damaged in a helium-rich environment have a large scatter due toeven more remarkable. For PE damaged in air, the noise in the incident X-ray beam).


3critical dose for mass loss was 400 eV/nm , and the linking for irradiation in a vacuum. Therefore, theextrapolated thickness at infinite X-ray dose is zero. presence of oxygen dramatically changes the chemi-In contrast, PE loses a negligible fraction of its mass cal pathways under which radiation damage to thein a helium-rich environment. polymer occurs.

The presence of oxygen also has a dramatic effect The radiation damage and chemistry of PET ison the radiation chemistry of PE. Fig. 8 presents the also quite different in the presence of oxygen. The

*change in the intensity of the 285 eV C 1s→p peak at 288.3 eV in the C 1s NEXAFS spectrum ofC=C

transition as a function of dose, for PE irradiated in PET (Fig. 1) has been assigned as the C*air and in a He-flushed environment. This C 1s(C=O)→p transition of the carbonyl groupC=O

*1s→p transition is a classic sign of radiation [11,39]. In a helium-flushed environment, this car-C=C

damage in saturated polymers, and has been associ- bonyl transition decays with a critical dose of 22,0003ated with polymer crosslinking through carbon–car- eV/nm (see Fig. 9). In air, the optical density at

bon double bond formation from photo-radicals on 288.3 eV decreases much more rapidly upon irradia-adjacent polymer chains. For irradiation in air, an tion, with a critical dose for this decrease of 1900

3*increase in the C 1s→p transition is not ob- eV/nm . Much of this decrease in critical dose canC=C

served. In a helium-rich environment, an increase of be attributed to increased mass loss in the presencethe X-ray absorption of this transition is observed, of oxygen. However, when normalizing for the mass

3with a critical dose of 260 eV/nm (1/e dose for the loss, the OD at 288.3 eV is actually increasingincrease in the intensity of this feature, see Ref. slightly. NEXAFS spectra of PET irradiated in air[11]). Similar behavior has been observed for elec- were not obtained.tron and g-irradiation damage in polypropylene: These observations, and the discrepancy to Right-degradation in the presence of oxygen, but cross- or et al. [11] for the critical dose for PET, indicate

Fig. 8. Changes in the optical density at 285 eV upon irradiation Fig. 9. Fractional change in optical density at 288.3 eV (energy*for polyethylene damaged in air (an oxygen-rich environment) and corresponding to the C 1s→p transition) upon irradiation forC=O

in a helium-rich environment. The optical density at 285 eV is polyethylene terephthalate damaged in air (an oxygen-rich en-scaled by normalizing the optical density at 315 eV to 1. vironment) and in a helium-rich environment.


that the polymer damage and radiation chemistry are effects of different environments upon radiationsensitive to the acquisition conditions in the X-ray chemistry: we determined that irradiating polymermicroscope. samples in an oxygen-rich environment causes more

extensive and faster mass loss than in an inertenvironment. The radiation chemistry of polymers

5. Conclusions damaged in an oxygen-rich vs. an inert environmentis also drastically different.

This paper presents a detailed description of soft Future, refined experiments regarding radiationX-ray radiation damage to several polymers under damage of polymers in a STXM could include thetypical operating conditions in a STXM. We accom- control over some or all of the following variables:plished this by: dose rate, sample thickness, molecular weight, use of

radical scavengers and antioxidants, and the oxygen(i) Acquiring NEXAFS spectra of virgin and level in the He environment. Improvements in instru-radiation damaged polymers to look at the spe- mentation presently under way to better control thecific chemical changes caused by soft X-ray He environment should prove to be helpful inirradiation. minimizing radiation damage.(ii) Characterizing the rate and the extent of massloss in polymers irradiated by soft X-rays.(iii) Characterizing the rate and the extent of

Acknowledgementschemical change by tracking the loss of a specificchemical group.

We would like to thank R. Spontak, V. Knowlton,(iv) Examining the effect of atmosphere on theand A.P. Smith for their assistance in polymerradiation damage by soft X-rays.ultramicrotomy. These data were acquired using theStony Brook STXM developed by the groups of C.By documenting NEXAFS spectral changes, weJacobsen and J. Kirz from SUNY at Stony Brookhope to provide an initial ‘‘catalog’’ of damagedwith support from the Office of Biological andspectra for users of NEXAFS and EELS spectros-Environmental Research, U.S. DOE under contractcopy tools. By examining the rate and extent of massDE-FG02-89ER60858, and the NSF under grantloss, we explored some general rules for polymerDBI-9605045. We thank C. Jacobsen for making hismass loss: polymers that contain aromatic groups‘‘stack’’ code [40] available to us for adaptation forand/or crosslink upon irradiation are more resistantthis experiment, and Sue Wirick for microscopeto mass loss than polymers which do not containmaintenance and support. Zone plates utilized werearomatic groups or scission upon irradiation. Bydeveloped by S. Spector and C. Jacobsen of Stonyexamining the rates of chemical changes, we de-Brook and Don Tennant of Lucent Technologies Bellveloped a rule of thumb for chemical group loss: theLabs, with support from the NSF under grant ECS-susceptibility of a chemical group to radiation dam-9510499. We most gratefully acknowledge technicalage depends on the local electronic structure —assistance by A. Scholl and D.A. Winesett in thequantified here by the ionization potential — of theacquisition of the damage spectrum of PE. Achemical group. We noted that as the ionizationGAANN fellowship and an NSF Young Investigatorpotential of the chemical group investigated hereAward (DMR-9458060) supported this work.increases, the susceptibility of that group to radiation

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