Characterization of the Effects of Soft X-Ray Irradiation on Polymers

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

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

T. Coffey, S.G. Urquhart1 , 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 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 ®rst-order identi®cation 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 [7±11]. The particular risk for spectroscopic micro- method for the microchemical characterization of analysis is that the sample and its spectrum might polymer materials [1±4]. Like its cousin, Electron degrade faster than meaningful microanalysis can be Energy 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, the 515-7331. E-mail address: harald [email protected] (H. Ade). level of radiation damage for X-ray microscopy of ] 1Present address: Department of Chemistry, University of Saskat- polymers is not so severe as to prohibit the analysis 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. PII: 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 the elucidation of more subtle spectroscopic differences will necessarily make radiation damage a growing concern. There have been relatively few studies of the soft X-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 [8±10,13,14]. In general, the radiation chemistry and damage of polymers can take several forms, such as loss of crystallinity, loss of mass, or chemical modi®cation [7]. We are primarily concerned here with chemical modi®cations as manifested in NEX- AFS spectral changes, as NEXAFS spectroscopy is the basis of compositional analysis in soft X-ray microscopy. The radiation chemistry of polymers and mole- cules can vary strongly between resonant versus non-resonant core excitation [15±17]. As the chemical sensitivity of NEXAFS spectroscopy is strongest at X-ray energies that correspond to speci®c resonant excitations (e.g. C 1s→p* transitions), the large Scheme 1. Chemical structures of polymers examined in this study. literature of non-resonant electron, 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 speci®c 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 ®lm; a decrease in in which NEXAFS microscopy is performed (e.g. intensity of speci®c spectral features, attributed to thin sections, He-rich environment, etc.). the loss of speci®c 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 speci®c 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 identi®- We develop a simple correlation that relates the = → able C 1s(C O) pC*=O transition that allows the rate of loss of the carbonyl functional group to the identi®cation 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 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 ¯ow 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, CO2 gas was added practitioners in order to de®ne 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 → CO2 Rydberg transitions from the high-resolution NEXAFS 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 ®lms (|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 Scienti®c Polymer Products, sured using a gas proportional counter mounted a PMMA 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 ®nite 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 de®ned for ef®ciency. The variable sample±detector distance all polymers. Differences in molecular weight should was measured and corrected for by accounting for have a minimal in¯uence on the damage rate of the absorption of X-rays by the air/helium purge gas speci®c functional groups but a larger effect on the mixture. It was not possible to measure the absolute mass loss damage rates. ef®ciency of the gas proportional counters against a Thin polymer sections (|100 nm thick) of most known standard, although comparisons between the polymers were prepared by ultramicrotomy, using a detected and anticipated photon count rates suggest a LKB Nova microtome or a Reichert-Jung Ultracut S detector ef®ciency between 10 and 40%. To account cryo-ultramicrotome and were mounted on standard for the potential differences in ef®ciency of different TEM grids. Thin ®lms 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) pC*=O transition in polycarbonate (PC) respectively, and ¯oated onto TEM grids. was used as an internal standard. The repeatability of this damage rate in identical conditions (same detector, 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 radiation Brook Scanning Transmission X-ray Microscope damage kinetics of different polymers. We have (STXM) at the National Synchrotron Light Source assumed a detector ef®ciency 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 speci®c studies performed in air. The different polymers during NEXAFS microscopy precise composition of the He purge atmosphere experiments. 68 T. Coffey et al. / Journal of Electron Spectroscopy and Related Phenomena 122 (2002) 65 ±78

2.4. Measurement of radiation damage where I and I0 are the transmitted and incident X-ray ¯ux, respectively, m is the energy dependent mass A simple determination of the character of the soft absorption coef®cient, 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 ®lm 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 ®lms. 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 (I0 )at based on the measured critical doses described the time the image was recorded. This internal I0 was below. 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 speci®c 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 pC*=O transition. Since the featureless ioniza- carbon 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 speci®c 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 speci®c spectroscopic damage rate, the optical density has been ®tted to the = → feature, such as, for example, the C 1s(C O) pC*=O following exponential expression: excitation at 290.5 eV in polycarbonate (PC). The dwell time used in recording these images was OD 5 OD` 1 C* exp(2d/dc ) (2) adjusted so that the radiation exposure in the

``damaging'' images at 315 eV was signi®cantly where OD` is the remaining optical density after greater than the radiation exposure from the moni- in®nite 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 dc is the critical radiation dose. At the critical radiation dose, 1/e or 63% of the total attenuation of a speci®c spectroscopic feature (or mass-thickness) 3. Calculations has occurred. This metric can be used to compare the relative radiation sensitivity of different polymers 3.1. Quantitative determination of the radiation and different functional groups, as a smaller critical damage kinetics dose implies a faster damage rate. For mass loss, the value of OD` from the ®t of Eq. (2) can also be used Since NEXAFS spectroscopy measured in trans- to characterize the nature of the radiation chemistry. mission obeys Beer's law, the optical density (OD) If the extrapolated optical density after in®nite dose → at any energy can be obtained from transmission tends to zero (i.e. OD` 0), then the polymer loses X-ray images as mass during radiation damage, while if the extrapolated OD` is close to 1, the polymer is resistant to

OD 52ln(I/I0 ) 5 mrt (1) mass loss. Crosslinking and chain scissioning are T. Coffey et al. / Journal of Electron Spectroscopy and Related Phenomena 122 (2002) 65 ±78 69 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 is 3.2. 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 potential (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 Kosugi's GSCF3 package [25,26]. These calculations are based on the Improved Virtual The nature of the chemical changes induced by Orbital approximation (IVO) which explicitly takes radiation damage and the susceptibility towards mass into account the core hole in the Hartree±Fock 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 re¯ecting 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 polymers 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 spectrum of irradiated PET (Fig. 1), a decay in the C → 1s(C±H) pC*=C transitions (284.8 and 285.4 eV) is also observed in addition to the attenuation of the C = → 1s(C O) pC*=O transition (289 eV) (previously observed by Rightor et al. [11]). Similarly, PVMK and Nylon-6 have a pronounced decrease in the C = → 1s(C O) pC*=O transition (286.8 and 287.3 eV, respectively), as well as the growth of a new feature at |285 eV. The change in the intensity of the = → ``carbonyl'' C 1s(C O) pC*=O absorption will be used 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 Scheme 2. Chemical structures of the molecular models used for irradiated polymers, including PE, PVMK, and ab initio Improved Virtual Orbital (IVO) calculations of the C 1s Nylon 6, are characteristic of unsaturated C±C ionization potential of the carbonyl group in a series of polymers. bonds (i.e. phenyl, ethylene and ethyne functional 70 T. Coffey et al. / Journal of Electron Spectroscopy and Related Phenomena 122 (2002) 65 ±78

Fig. 1. Comparison of the C 1s NEXAFS spectra of polymers that predominantly lose mass upon irradiation: poly(ethylene terephthalate) Ð PET, Nylon-6 Ð N6, poly(vinyl methyl ether) Ð PVMK, poly(methyl methacrylate) Ð PMMA and polyethylene Ð PE, before (ÐÐÐ) and after (± ? ±) X-ray exposure. The Fig. 2. Comparison of the C 1s NEXAFS spectra of polymers that total dose administered is indicated. PE was exposed in a He/air are resistant to mass loss: polystyrene Ð PS, polycarbonate Ð PC mixture. The total dose has not been determined. The spectra are and polyurethane Ð PU, before (ÐÐÐ) and after (± ? ±) included as an illustration of spectral changes due to damage. For X-ray exposure. The total dose administered is indicated. details about the dependence of PE damage on the presence of oxygen, 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 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 with scopic 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 → → 1s pC**≡N transition in a polyacrylonitrile [1,30] and 1s(C±R) pC=C transition at 286.5 eV is attributed → acetonitrile [31], and the C 1s pC*=N transition in to phenyl ring carbon atom sites that are substituted 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) pC*=C transition from the ``C±H'' phenyl carbon Nylon-6. atoms. In all spectra of the radiation damaged In polymers containing phenyl functional groups aromatic polymers, the energy range 286±287 eV is (PET, PS, PC, PU), new features are also observed ``®lled in'' by broad or discrete transitions, sug- T. Coffey et al. / Journal of Electron Spectroscopy and Related Phenomena 122 (2002) 65 ±78 71

gesting a modi®cation 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 been unambiguously identi®ed, although NEXAFS spectra provide valuable indications of the likely chemistry.

4.2. Mass loss Ð 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 in®nite dose obtained from the ®t of the data to Eq. (2) is presented in Table 1. Some polymers undergo damage until no polymer would

remain for in®nite dose (OD` 50), while others would reach a ®nite, constant mass thickness

(OD` .0). The polymers PU, PC, PS, and PE Fig. 3. Summary of polymer mass loss results. The y-axis exhibited negligible mass loss and the respective data represents 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 in®nite exposure Polymer Densitya Mass loss Fractional Propensity for crosslinking (g/cm3 ) critical dose b,c,d mass, Previous This (eV/nm3 ) 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 Yes PET 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 a Densities from Scienti®c Polymer Products compilation, http://www.scienti®cpolymer.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 ef®ciency and might be as large as 100%. The measured radiation dose is presented in eV/nm33 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). c35Conversion: eV/nm 5(1.602310 Grays)/r (where r is the polymer density). d Assumed 30% detector ef®ciency. 72 T. Coffey et al. / Journal of Electron Spectroscopy and Related Phenomena 122 (2002) 65 ±78

(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 ing and crosslinking upon irradiation, one mecha- at a dose of 1500 eV/nm3 . The ®ve 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 signi®cant fraction (.10%) of their mass for this PMMA [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 1s pC*=C excitation and the scissions, 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 ®tted growth of a new, broad peak at |285 eV (see Fig. 1), → = within this scheme on account of their more compli- where C 1s pC*=C transitions associated with C C cated chemical structure. unsaturation have been observed in many other The presence of an aromatic group has a signi®- 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-ray irradiated PMMA, main chain and side chain scissioning is the dominant damage mechanism. How- ever, after suf®ciently 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 of Scheme 3. crosslinking [12,13], even though C=C bonds typi- T. Coffey et al. / Journal of Electron Spectroscopy and Related Phenomena 122 (2002) 65 ±78 73 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 crosslinking [8]. The 285 eV peak associated with crosslinking 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 predominantly crosslinks upon irradiation.

4.3. Loss of carbonyl functional group

Polymers containing carbonyl groups have a = → characteristic C 1s(C O) pC*=O transition that is Fig. 4. Summary of carbonyl group loss results. The y-axis useful for chemical analysis. This functional group is represents the normalized change in optical density at the carbonyl known to damage easily, which will have a large transition. 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 ®t of the dose dependence of the C should be used with some caution. = → 1s(C O) pC*=O transition to Eq. (2) are displayed Table 2 summarizes the critical dose for the in Fig. 4 for all polymers except for PET. carbonyl group, the atomic fraction of carbonyl Since mass loss and chemical changes to a speci®c 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 ®tting the raw OD of the C the calculated C 1s ionization potential of the = → 1s(C O) pC*=O transition to Eq. (2) will involve carbonyl carbon atom. The carbonyl-normalized 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. The G-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) pC*=O transition intensity. This radiation damage for a given dose [8]. The carbonyl 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 signi®cant 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) pC*=O transition (i.e. Nylon-6, PMMA differences in sample environment and the particular 74 T. Coffey et al. / Journal of Electron Spectroscopy and Related Phenomena 122 (2002) 65 ±78

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=O ratea,b carbon atoms damage ratea ionization (eV/nm3 ) per monomer (eV/nm3 ) 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 a All experimental (relative) errors estimated 65%. Systematic errors are dominated by uncertainty about the absolute detector ef®ciency and might be as large as 100%. The measured radiation dose is presented in eV/nm33 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 ef®ciency. 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- ¯uorocarbons Ð which have a higher relative ioniza- ues, we see that the carbonyl group in polycarbonate tion potential [37] Ð 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 damage observed 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 is more 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/ nm3 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 Fig. 5. Normalized carbonyl critical dose vs. ionization potential spot, while we raster-scanned the beam to expose a of the carbonyl carbon. larger area. This would suggest a potential dose rate T. Coffey et al. / Journal of Electron Spectroscopy and Related Phenomena 122 (2002) 65 ±78 75 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. Speci®cally, the spectral feature of the carbonyl group at 288.2 eV is 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) pC*=O transition intensity (288.2 eV 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 identical helium ¯ow 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 in damage chemistry a helium-rich environment. The PE irradiated in air only received a 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 an atmosphere 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 radiation 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 note immediately 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 environment (600 eV/nm3 ) than in a helium-¯ushed environment (58,700 eV/nm3 ). In both cases, the Fig. 7. Comparison of fractional mass loss upon irradiation for polyethylene terephthalate damage in air (an oxygen-rich environ- extrapolated thickness at in®nite dose is zero, that is, ment) and in a helium-rich environment (the data for the PET PET completely scissions. For PE, the results are damaged in a helium-rich environment have a large scatter due to even more remarkable. For PE damaged in air, the noise in the incident X-ray beam). 76 T. Coffey et al. / Journal of Electron Spectroscopy and Related Phenomena 122 (2002) 65 ±78 critical dose for mass loss was 400 eV/nm3 , and the linking for irradiation in a vacuum. Therefore, the extrapolated thickness at in®nite 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 is on 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 pC*=C peak at 288.3 eV in the C 1s NEXAFS spectrum of transition as a function of dose, for PE irradiated in PET (Fig. 1) has been assigned as the C = → air and in a He-¯ushed environment. This C 1s(C O) pC*=O transition of the carbonyl group → 1s pC*=C transition is a classic sign of radiation [11,39]. In a helium-¯ushed environment, this car- damage in saturated polymers, and has been associ- bonyl transition decays with a critical dose of 22,000 ated with polymer crosslinking through carbon±car- eV/nm3 (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 pC*=C transition is not ob- eV/nm . Much of this decrease in critical dose can served. In a helium-rich environment, an increase of be attributed to increased mass loss in the presence the X-ray absorption of this transition is observed, of oxygen. However, when normalizing for the mass with a critical dose of 260 eV/nm3 (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 1s pC*=O transition) upon irradiation for 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. T. Coffey et al. / Journal of Electron Spectroscopy and Related Phenomena 122 (2002) 65 ±78 77 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 polymers 5. Conclusions damaged in an oxygen-rich vs. an inert environment is also drastically different. This paper presents a detailed description of soft Future, re®ned 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 ci®c 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 of chemical change by tracking the loss of a speci®c Acknowledgements chemical group. (iv) Examining the effect of atmosphere on the We would like to thank R. Spontak, V. Knowlton, radiation damage by soft X-rays. and A.P. Smith for their assistance in polymer ultramicrotomy. These data were acquired using the By documenting NEXAFS spectral changes, we Stony Brook STXM developed by the groups of C. hope to provide an initial ``catalog'' of damaged Jacobsen and J. Kirz from SUNY at Stony Brook spectra for users of NEXAFS and EELS spectros- with support from the Of®ce of Biological and copy tools. By examining the rate and extent of mass Environmental Research, U.S. DOE under contract loss, we explored some general rules for polymer DE-FG02-89ER60858, and the NSF under grant mass loss: polymers that contain aromatic groups DBI-9605045. We thank C. Jacobsen for making his and/or crosslink upon irradiation are more resistant ``stack'' code [40] available to us for adaptation for to mass loss than polymers which do not contain this experiment, and Sue Wirick for microscope aromatic groups or scission upon irradiation. By maintenance and support. Zone plates utilized were examining the rates of chemical changes, we de- developed by S. Spector and C. Jacobsen of Stony veloped a rule of thumb for chemical group loss: the Brook and Don Tennant of Lucent Technologies Bell susceptibility of a chemical group to radiation dam- Labs, with support from the NSF under grant ECS- age depends on the local electronic structure Ð 9510499. We most gratefully acknowledge technical quanti®ed here by the ionization potential Ð of the assistance by A. Scholl and D.A. Winesett in the chemical group. We noted that as the ionization acquisition of the damage spectrum of PE. A potential of the chemical group investigated here GAANN fellowship and an NSF Young Investigator increases, the susceptibility of that group to radiation Award (DMR-9458060) supported this work. 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 References method for choosing the spectral and image acquisition times which will not cause extensive damage to [1] H. Ade, X. Zhang, S. Cameron, C. Costello, J. Kirz, S. their polymer samples. Finally, we analyzed the Williams, Science 258 (1992) 972. 78 T. Coffey et al. / Journal of Electron Spectroscopy and Related Phenomena 122 (2002) 65 ±78

[2] S. Zhu, Y. Liu, M.H. Rafailovich, J. Sokolov, D. Gersappe, [21] M. Feser, M. Carlucci-Dayton, C. Jacobsen, J. Kirz, U. D.A. Winesett, H. Ade, Nature 400 (1999) 49. Neuhausler,È G. Smith, B. Yu, in: I. McNulty (Ed.), X-ray [3] S.G. Urquhart, A.P. Hitchcock, A.P. Smith, H.W. Ade, W. Microfocusing: Applications and Techniques, SPIE. Proc, Lidy, E.G. Rightor, G.E. Mitchell, J. Electron Spectrosc. Vol. 3449, 1998, pp. 19. Relat. Phenom. 100 (1999) 119. [22] A.P. Smith, T. Coffey, H. Ade, in: J. Thieme, G. Schmahl, E. [4] H. Ade, S. Urquhart, in: T.K. Sham (Ed.), Chemical Applica- Umbach, D. Rudolph (Eds.), X-ray Microscopy and Spec- tions of Synchrotron Radiation, World Scienti®c, Singapore, tromicroscopy, Springer, Berlin, 1997. 2001. [23] H. Ade, A.P. Smith, H. Zhang, G.R. Zhuang, J. Kirz, E. [5] R.F. Egerton, Electron Energy Loss Spectroscopy in the Rightor, A. Hitchcock, J. Electron Spectrosc. Relat. Phenom. Electron Microscope, Plenum Press, New York, 1986. 84 (1997) 53. [6] M.M. Disko, C.C. Ahn, B. Fultz (Eds.), Transmission [24] Y. Ma, C.T. Chen, G. Meigs, K. Randall, F. Sette, Phys. Rev. Electron Energy Loss Spectrometry in Materials Science, A 44 (1991) 1848. Minerals, Metals and Materials Society, Warrendale, 1992. [25] N. Kosugi, Theor. Chim. Acta 72 (1987) 149. [7] D. Vesely, D. Finch, Electron Microsc. Anal. 7 (1985). [26] N. Kosugi, H. Kuroda, Chem. Phys. Lett. 74 (1980) 490. [8] K. Dawes, L.C. Glover, Effects of Electron Beam and [27] W.J. Hunt, W.A.I. Goddard, Chem. Phys. Lett. 3 (1969) 414. g-Irradiation on Polymeric Materials, AIP Press, Woodbury, [28] M.W. Schmidt, K.K. Baldridge, J.A. Boatz et al., J. Comput. NY, 1996. Chem. 14 (1993) 1347. [9] L.C. Sawyer, D.T. Grubb, Polymer Microscopy, Chapman [29] S. Huzinaga, J. Andzelm, M. Klobukowski, E. Radzio- and Hill, New York, 1987. Andzelm, Y. Sasaki, H. Tatewaki, Gaussian Basis Sets for [10] G. Beamson, D. Briggs, High Resolution XPS of Organic Molecular Orbital Calculations, Elsevier, Amsterdam, 1984. Polymers: The Scienta ESCA300 Database, Wiley, New [30] A.P. Hitchcock, I. Koprinarov, T. Tyliszczak et al., Ultramic- York, 1992. roscopy 88 (2001) 33. [11] E.G. Rightor, A.P. Hitchcock, H. Ade, R.D. Leapman, S.G. [31] A.P. Hitchcock, M. Tronc, A. Modelli, J. Phys. Chem. 93 Urquhart, A.P. Smith, G. Mitchell, D. Fisher, H.J. Shin, T. (1989) 3068. Warwick, J. Phys. Chem. B 101 (1997) 1950. [32] E. Apen, A.P. Hitchcock, J.L. Gland, J. Phys. Chem. 97 [12] X. Zhang, C. Jacobsen, S. Lindaas, S. Williams, J. Vac. Sci. (1993) 6859. Technol. B 13 (1995) 1477. [33] D.T. Turner, in: M. Dole (Ed.), The Radiation Chemistry of [13] J.A. Moore, J.O. Choi, Degradation of Poly(Methyl Meth- Macromolecules, Vol. II, Academic Press, New York, 1973, acrylate): Deep UV, X-ray, Electron-beam, and Proton-beam pp. 137. Irradiation, American Chemical Society, 1991. [34] B.J. Lyons, Radiat. Phys. Chem. 22 (1983) 135. [14] J.H. O'Donnell, Chemistry of Radiation Degradation of [35] A.P. Hitchcock, D.C. Mancini, J. Electron Spectrosc. Relat. Polymers, American Chemical Society, 1991. Phenom. 67 (1994) 1. [15] M.C.K. Tinone, K. Tanaka, J. Maruyama, N. Ueno, J. Chem. [36] E.M. Lehockey, I. Reid, I. Hill, J. Vac. Sci. Technol. 6 Phys. 100 (1994) 5988. (1988) 2221. [16] D.M. Hanson, Adv. Chem. Phys. 77 (1990) 1. [37] W.L. Jolly, K.D. Bomben, C.J. Eyermann, At. Data Nucl. [17] E. Ikenaga, K. Isari, K. Kudara, Y. Yasui, S.A. Sardar, S. Data Tables 31 (1984) 433. Wada, T. Sekitani, K. Tanaka, K. Mase, S. Tanaka, J. Chem. [38] B.J. Lyons, Radiat. Phys. Chem. 45 (1995) 159. Phys. 114 (2001) 2751. [39] S.G. Urquhart, A.P. Hitchcock, A.P. Smith, H. Ade, E.G. [18] S.G. Urquhart, H. Ade, J. StohrÈ (submitted for publication). Rightor, J. Phys. Chem. B 101 (1997) 2267. [19] S.G. Urquhart, H.W. Ade, A.P. Smith, A.P. Hitchcock, E.G. [40] C. Jacobsen, S. Wirick, G. Flynn, C. Zimba, J. Microsc. 197 Rightor, W. Lidy, J. Phys. Chem. B 103 (1999) 4603. (2000) 173. [20] X. Zhang, H. Ade, C. Jacobsen, J. Kirz, S. Lindaas, S. Williams, S. Wirick, Nucl. Instrum. Meth. A A347 (1994) 431.