CHARACTERIZATION OF THE EFFECTS OF SOFT X-RAY IRRADIATION ON POLYMERS.pdf

8/11/2019 CHARACTERIZATION OF THE EFFECTS OF SOFT X-RAY IRRADIATION ON POLYMERS.pdf

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Journal of Electron Spectroscopy and Related Phenomena 122 (2002) 6578

www.elsevier.com/locate/elspec

Characterization of the effects of soft X-ray irradiation on polymers

1 *T. Coffey, S.G. Urquhart , H. Ade

Department of Physics, North Carolina State University, Raleigh, NC 27695, U SA

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 photon

energies just above the C 1s binding energy. This exposure causes radiation damage in the form of the loss of mass and

changes to the chemical structure of the polymers. These effects are evident in the Near Edge X-ray Absorption Fine

Structure (NEXAFS) spectra of the exposed polymers, posing a fundamental limit to the sensitivity of NEXAFS

spectroscopy for chemical microanalysis. Quantitative understanding of the chemistry and kinetics of radiation damage in

polymers is necessary for the successful and validated application of NEXAFS microscopy. This paper outlines a method for

quantifying this radiation damage as a function of X-ray dose, and applies these methods to characterize the loss of mass and

loss of carbonyl group functionality from a diverse series of polymers. A series of simple correlations are proposed to

rationalize the observed radiation damage propensities on the basis of the polymer chemical structure. In addition, NEXAFS

spectra of irradiated and virgin polymers are used to provide a first-order identification of the radiation chemistry. 2002

Elsevier Science B.V. All rights reserved.

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

1. Introduction from X-ray or electron spectroscopy necessarily

causes 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 irradiation

tial resolution in X-ray microscopy, is a powerful [711]. The particular risk for spectroscopic micro-

method for the microchemical characterization of analysis is that the sample and its spectrum might

polymer materials [14]. 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 form

of high spatial resolution with chemical sensitivity and the rate of the soft X-ray radiation damage. With

a quantitative understanding of the radiation damage

kinetics, 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.

P I I : S 0 3 6 8 - 2 04 8 ( 0 1 ) 0 0 3 4 2 - 5


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66 T. Coffey et al. / Journal of Electron Spectroscopy and Related Phenomena 122 (2002) 6578

inevitable push to higher spatial resolution and the

elucidation of more subtle spectroscopic differences

will necessarily make radiation damage a growing

concern.

There have been relatively few studies of the softX-ray radiation chemistry and radiation damage of

polymers [11,12], particularly when compared to the

numerous studies of the chemical effects of high

energy electrons, hard X-rays, and gamma radiation

[810,13,14]. In general, the radiation chemistry and

damage of polymers can take several forms, such as

loss of crystallinity, loss of mass, or chemical

modification [7]. We are primarily concerned here

with chemical modifications as manifested in NEX-

AFS spectral changes, as NEXAFS spectroscopy is

the basis of compositional analysis in soft X-raymicroscopy.

The radiation chemistry of polymers and mole-

cules can vary strongly between resonant versus

non-resonant core excitation [1517]. As the chemi-

cal sensitivity of NEXAFS spectroscopy is strongest

at X-ray energies that correspond to specific resonantScheme 1. Chemical structures of polymers examined in this*excitations (e.g. C 1sp transitions), the largestudy.

literature ofnon-resonantelectron, g and hard X-ray

irradiation may not be directly applicable to the

radiation 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 in

impact 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 spectra

microscope. While our goal is to be as general as of the polymers acquired before and after soft X-ray

possible, we have studied the radiation damage of irradiation. Several different effects are observed: the

these polymers in situ in the experimental conditions loss of mass from the polymer film; a decrease in

in which NEXAFS microscopy is performed (e.g. intensity of specific spectral features, attributed to

thin 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 the

a wide range of common polymer structures, with a formation of new chemical structures. The kinetics

primary focus on polymers that contain the carbonyl of mass loss and the formation or loss of specific

functional 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 mass

terephthalate) (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 simple

sing 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


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T. Coffey et al. / Journal of Electron Spectroscopy and Related Phenomena 122 (2002) 6578 67

measure the difference in the radiation chemistry of cannot be easily measured, but the He flow rate was

polymers in the presence and absence of oxygen. kept constant for all quantitative damage experiments

The 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 the

CO Rydberg transitions from the high-resolution2NEXAFS spectra of Ma et al. [24] were used to

2. Experimental calibrate these spectra.

2.1. Sample origin and preparation

2.3. Detector and detector calibration

Thin films (|50 to 200 nm) of the polymers were

prepared 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 of

mer Laboratories. The polyurethane (PU, see the same detector design were used in the course of

Scheme 1) and poly(propylene oxide) (PPO) samples these experiments since the detector had a finite

were provided by Dow Chemical and have been operating lifetime. In order to properly account for

previously 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 detector

molecular weight was not known or not defined for efficiency. The variable sampledetector distance

all polymers. Differences in molecular weight should was measured and corrected for by accounting for

have a minimal influence on the damage rate of the absorption of X-rays by the air/ helium purge gas

specific functional groups but a larger effect on the mixture. It was not possible to measure the absolute

mass 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 a

LKB Nova microtome or a Reichert-Jung Ultracut S detector efficiency between 10 and 40%. To account

cryo-ultramicrotome and were mounted on standard for the potential differences in efficiency of different

TEM 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 of

this damage rate in identical conditions (same detec-

tor, same atmosphere, and same sample thickness)2.2. Microscope description was within 10%. This internal calibration method

provides 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 values

temperature 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 between

except for specific studies performed in air. The different polymers during NEXAFS microscopy

precise composition of the He purge atmosphere experiments.


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2.4. Measurement of radiation damage where IandI are the transmitted and incident X-ray0flux, respectively, m is the energy dependent mass

A 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. The

irradiated 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 the

dose from the acquisition of these spectra would be hole) provided an internal and nearly instantaneous

below the threshold for observing radiation damage, measurement of the incident beam intensity (I ) at0based on the measured critical doses described the time the image was recorded. This internalI was0below. used to obtain the optical density of the material at

In 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 to

specific 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 1sp transition. Since the featureless ioniza-C=Ocarbon atoms in the polymer. Quantitative determi- tion cross-section at 315 eV is proportional to both

nation of the polymer radiation damage kinetics was the sample thickness and density, the images used to

obtained through a series of X-ray imaging measure- damage the polymer at this energy also contain

ments performed in the X-ray microscope. A series the mass-thickness and can be used to measure

of 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 optical

imaging 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 and

The effect of the radiation damage was measured by typically exponential way. In order to quantify the

imaging 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=Oexcitation at 290.5 eV in polycarbonate (PC). The

*OD5OD 1C 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 criticalcradiation dose, 1/e or 63% of the total attenuation of

a specific spectroscopic feature (or mass-thickness)

has occurred. This metric can be used to compare the3. Calculations

relative 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 Beers 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

`

OD5 2 ln(I/I )5mrt (1) mass loss. Crosslinking and chain scissioning are0


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hypothesized as likely radiation damage outcomes geometries were determined using the program

based 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 hole

Virtual 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 of

polymers. The target polymers and the molecular 4. Results and discussion

models used for these calculations are presented in

Scheme 2. 4.1. Qualitative chemical and spectroscopic

Calculations on these species were carried out observations

using Kosugis GSCF3 package [25,26]. These

calculations 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 mass

into account the core hole in the HartreeFock loss for many of the polymers investigated in this

approximation 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 that

the 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 the

potential (IP) with a typical accuracy of 1 eV, NEXAFS spectra of some polymers that are totally

reflecting 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 a

He-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=Otransition is clearly observed. In the NEXAFS spec-

trum of irradiated PET (Fig. 1), a decay in the C*1s(CH)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, PVMK

and 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=Oused below to track radiation damage to the carbonyl

functional group as a function of radiation dose.

The observation of new spectroscopic transitions

at |285 eV in the NEXAFS spectra of many

irradiated polymers, including PE, PVMK, andScheme 2. Chemical structures of the molecular models used for

Nylon 6, are characteristic of unsaturated CCab initio Improved Virtual Orbital (IVO) calculations of the C 1sionization potential of the carbonyl group in a series of polymers. bonds (i.e. phenyl, ethylene and ethyne functional


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Fig. 1. Comparison of the C 1s NEXAFS spectra of polymers that

predominantly 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 of

oxygen, see text.

in the 286287 eV energy range in the spectra of the*irradiated polymers. Typically, a C 1s(CR)pC=C

groups). Additional new features can be observed in transition originating from the substituted phenyl

the NEXAFS spectrum of irradiated Nylon-6 (Fig. ring carbon atoms is present in this energy range

1), 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 the

of this new feature closely corresponds to the C spectrum of virgin polyurethane (Fig. 2), the C* *1sp transition in a polyacrylonitrile [1,30] and 1s(CR)p transition at 286.5 eV is attributedCN C=C

*acetonitrile [31], and the C 1sp transition in to phenyl ring carbon atom sites that are substitutedC=Nimidazole [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 CH phenyl carbonC=CNylon-6. atoms. In all spectra of the radiation damaged

In polymers containing phenyl functional groups aromatic polymers, the energy range 286287 eV is

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


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gesting a modification of the phenyl rings is a

potential outcome of the radiation damage of these

polymers.

The chemical origin of these features and the

chemical pathways leading to them has not beenunambiguously identified, although NEXAFS spectra

provide valuable indications of the likely chemistry.

4.2. Mass loss observations

Fig. 3 presents the mass loss for a series of

polymers as a function of radiation dose. A summary

of the critical dose values and fractional mass

remaining at infinite dose obtained from the fit of the

data to Eq. (2) is presented in Table 1. Some

polymers 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 1

Summary of the rate and extent of chemical damage by mass loss for a series of polymers exposed in a He-rich environment. The critical

dose for mass loss represents the X-ray dose needed to reduce the projected sample thickness by 1/ e, and the fractional mass is the mass

remaining after an extrapolation to infinite exposure

aPolymer Density Mass loss Fractional Propensity for crosslinking

3 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 Some

PMMA 1.2 350 0.72 Some [12] Yes

Nylon-6 1.12 800 0.85 Yes [8] Yes

PVMK 1.12 1400 0.74 Unknown YesPET 1.385 58,700 0 Some [33] No

PS 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

aDensities from Scientific Polymer Products compilation, http:/ / www.scientificpolymer.com/ resources/ poly dens alpha.htm.

] ]bAll 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 can

be directly related to the measurement units of the microscope (spatial scale, sample thickness and photon energy).c 3 5

Conversion: eV/nm 5(1.602310 Grays)/r (where r is the polymer density).d

Assumed 30% detector efficiency.


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(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 mass3

ing 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 a

process 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 result

while 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 only

which 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 PU

gated, 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 1sp excitation and theC=Cscissions, while PPO scissions completely. PU ex- potential for substantially different de-excitation

hibits negligible mass loss and therefore should be pathways will have on the radiation chemistry of

highly crosslinking. aromatic polymers.

Previous studies [8] suggest that a reasonable In addition to kinetic stabilization due to the

prediction of the propensity for crosslinking in presence of aromatic groups, crosslinking upon

polymers can be based on the structure of the irradiation can directly protect a polymer from mass

polymer 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 crosslink

backbone 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 and

lighted. 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 and

quaternary 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 were

and PVMK supports this prediction. However, most acquired after irradiation with soft X-rays show the

of 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 1sp transitions associated with C=CC=C

cated chemical structure. unsaturation have been observed in many other

The 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 that

are 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 might

crosslink [36]. This is due to the loss of the methyl

ester side chain, which has a 1:1 correspondence

with the formation of C=C bonds [13]. In damaged

PMMA, Zhang et al. attributed a feature at 286.6 eV

to C=C bonds and interpreted it as a sign ofScheme 3. crosslinking [12,13], even though C=C bonds typi-


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cally have a resonance near 285 eV. Our results show

slightly different spectral features for damaged

PMMA, but agree with prior results in as far as

PMMA retained a large percentage of its mass and is

thus crosslinking.In PE, the formation of C=C bonds and the

appearance of a 285 eV feature is attributed to

hydrogen abstraction, which also leads to cross-

linking [8]. The 285 eV peak associated with cross-

linking is also present in the radiation-damaged

spectrum of poly(vinyl methyl ketone) (see Fig. 1).

Since PVMK retains a high percentage (74%) of its

mass 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 these

materials. We have therefore determined a critical and PVMK), the critical dose calculated should be

dose for the loss of the carbonyl functional group close to the true critical dose. The uncertainty is

for the following polymers: PMMA, PC, Nylon-6, largest for PES, for which mass loss is a substantial

PVMK, PET, PU, and PES. The experimental data process and therefore the calculated critical doses

and 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 carbonyl

Since 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, and

cal 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 the

unrelated 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. TheG-values in

the detailed mechanisms for mass loss, we can only Table 2 are inversely related to the un-normalized

evaluate 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 carbonyl

mate the actual critical dose for the carbonyl group events per 100 eV dose to the whole sample. For

itself, 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 comparison

uncertainty. For polymers where the mass loss is with previous electron irradiation studies. A direct

small 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


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Table 2

Summary of the chemical damage rates of the loss of the carbonyl functional group for a series of polymers exposed in a He-rich

environment, 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 Calculated

damage atoms to total [ C=O C 1s C=Oa,b a

rate carbon atoms damage rate ionization3 3

(eV/ nm ) per monomer (eV/ nm ) potential (eV)

PMMA 520 1 / 5 104 0.86 295.43

PES 530 2 / 6 177 1.10 295.98

PC 580 1 / 16 36 0.31 297.79

PU 740 2 / 13 114 0.43 296.63

PVMK 1600 1 / 4 400 0.38 294.00

Nylon6 2300 1 / 6 383 0.17 294.58

PET 22,000 2 / 10 4400 0.03 295.87

aAll experimental (relative) errors estimated 65%. Systematic errors are dominated by uncertainty about the absolute detector efficiency

3 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-excited

maximum 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 sensitive

has 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 polyurethane

potentially useful correlation between the carbonyl- foams [3]. Further, the substantially more rapid mass

normalized critical dose and the calculated ionization loss observed for poly(propylene oxide) (PPO) than

potential 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 or

atoms with a higher C 1s ionization potential have a more oxygen atoms. The overall correlation in the

lower critical dose. The ionization potential is depen- present study and these other supporting observations

dent on the local electron density of the carbon atom suggest a relationship between the sensitivity of

site, which varies with the electronegativity and carbonyl functional groups to radiation damage and

their local chemical environment. We observe about

an order of magnitude more damage to a carbonate

functionality than to a ketone. The one exception to

this 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 may

add aromatic stabilization to the carbonyl functional

groups.

Also of note is the substantially lower 7700 eV/3

nm 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 single

spot, while we raster-scanned the beam to expose aFig. 5. Normalized carbonyl critical dose vs. ionization potentialof the carbonyl carbon. larger area. This would suggest a potential dose rate


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dependence in the radiation damage kinetics, a theme

not explored in this paper. In addition, the spectral

shapes observed by Rightor after damage are slightly

different 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, while

the feature at 289.1 eV appears equally damaged in

both data sets. We will show explicitly (Section 4.4)

that the relative oxygen content in the sample

environment 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 environment

between the Rightor et al. and our data might

account for the observed differences. Since all

samples for the data presented here have been

exposed in the same manner and with identicalhelium flow rates during two data runs, a comparison

between different polymers within our data sequence

is self-consistent and meaningful.

Fig. 6. A comparison of fractional mass loss upon irradiation for

4.4. Effect of atmospheric oxygen on the radiation polyethylene damaged in air (an oxygen-rich environment) and ina 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 exposed

to radiation can drastically affect the nature and rate

of the radiation damage. In an inert atmosphere, such

as vacuum or an unreactive gas, X-ray generated

radicals are understood to be more stable [8]. In anatmosphere containing oxygen, the radicals can react

to form peroxides or hydroperoxides [8,14], or

oxygen itself can be photoexcited or photoionized

and become reactive, accelerating the rate of radia-

tion degradation or the extent of the radiation

damage [8,14].

To explore the effect of the atmosphere on the

radiation chemistry of polymers, PE and PET were

irradiated in air in addition to the helium-rich

environment presented above. These results are

presented in Fig. 6 and 7, respectively. We noteimmediately that both PE and PET lose mass at a

much faster rate when oxygen is present. The critical

dose for mass loss from PET is nearly two orders of

magnitude smaller in an oxygen-containing environ-3

ment (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).


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3critical dose for mass loss was 400 eV/nm , and the linking for irradiation in a vacuum. Therefore, the

extrapolated 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 the

in 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 1sp peak at 288.3 eV in the C 1s NEXAFS spectrum ofC=Ctransition 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*1sp 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 22,0003

ated with polymer crosslinking through carboncar- 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 19003

*increase in the C 1sp 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 mass3

with a critical dose of 260 eV/nm (1 /e dose for the loss, the OD at 288.3 eV is actually increasing

increase 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 1sp 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.


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that the polymer damage and radiation chemistry are effects of different environments upon radiation

sensitive to the acquisition conditions in the X-ray chemistry: we determined that irradiating polymer

microscope. samples in an oxygen-rich environment causes more

extensive and faster mass loss than in an inert

environment. The radiation chemistry of polymers5. Conclusions damaged in an oxygen-rich vs. an inert environment

is also drastically different.

This paper presents a detailed description of soft Future, refined experiments regarding radiation

X-ray radiation damage to several polymers under damage of polymers in a STXM could include the

typical 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 the

cific chemical changes caused by soft X-ray He environment should prove to be helpful in

irradiation. minimizing radiation damage.(ii) Characterizing the rate and the extent of mass

loss in polymers irradiated by soft X-rays.

(iii) Characterizing the rate and the extent ofAcknowledgements

chemical change by tracking the loss of a specific

chemical 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 polymer

radiation damage by soft X-rays.ultramicrotomy. These data were acquired using the

Stony Brook STXM developed by the groups of C.By documenting NEXAFS spectral changes, we

Jacobsen and J. Kirz from SUNY at Stony Brookhope to provide an initial catalog of damaged

with 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 grant

loss, we explored some general rules for polymerDBI-9605045. We thank C. Jacobsen for making his

mass loss: polymers that contain aromatic groupsstack code [40] available to us for adaptation for

and/or crosslink upon irradiation are more resistantthis experiment, and Sue Wirick for microscope

to mass loss than polymers which do not containmaintenance and support. Zone plates utilized were

aromatic groups or scission upon irradiation. Bydeveloped by S. Spector and C. Jacobsen of Stony

examining the rates of chemical changes, we de-Brook and Don Tennant of Lucent Technologies Bell

veloped 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 technical

age depends on the local electronic structure assistance by A. Scholl and D.A. Winesett in the

quantified here by the ionization potential of theacquisition of the damage spectrum of PE. A

chemical group. We noted that as the ionization GAANN fellowship and an NSF Young Investigatorpotential of the chemical group investigated here

Award (DMR-9458060) supported this work.increases, the susceptibility of that group to radiation

damage also increases. We believe that these rules of

thumb will aid users of soft X-ray and even electron

analysis techniques by providing them a comparative Referencesmethod for choosing the spectral and image acquisi-

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