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

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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 chemi- cal 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 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 radiation Brook Scanning Transmission X-ray Microscope damage kinetics of different polymers.
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