Quantitative Evaluation of Radiation Damage to Polyethylene Terephthalate by Soft X-rays and High-energy Electrons Jian Wang, Gianluigi A. Botton, Marcia M. West, and Adam P. Hitchcock* Brockhouse Institute for Materials Research, McMaster UniVersity, Hamilton, ON, Canada L8S 4M1 ReceiVed: September 17, 2008; ReVised Manuscript ReceiVed: December 12, 2008 The chemical changes and absolute rates in radiation damage to polyethylene terephthalate (PET) caused by soft X-rays and energetic electrons have been measured using a scanning transmission X-ray microscope (STXM). Electron beam damage at two different dose rates and a range of doses was performed in an 80 keV transmission electron microscope (TEM). The STXM beam was used to create damage patterns with systematically varied doses of monochromatic soft X-rays on an adjacent piece of the same PET sample. NEXAFS spectroscopy at the C 1s and O 1s edges was used to study the chemistry of the radiation damage and to determine quantitative critical doses for PET damage by both types of radiation. The spectral changes were similar for damage by electrons and X-rays, indicating the radiation chemistry is dominated by secondary processes, not the primary event. The critical dose for chemical changes determined from C 1s spectral features is 4.2(6) × 10 8 Gy and was the same for soft X-rays and electrons within measurement uncertainties. The critical dose for specific damage processes (as defined by changes in several different, bond-specific spectral features) was found to be similar in the C 1s region and was comparable between C 1s and O 1s edges for electron beam damage. There were statistically different critical doses for soft X-ray damage as probed by changes in O 1s spectral features related to carbonyl and ester bonds. 1. Introduction Radiation chemistry of polymers is important in a number of areas, including lithography technologies, 1 polymer degrada- tion, 2 and processing of polymer materials to improve physical or chemical properties. 3 Two major radiation sources used in these applications are X-rays and electron beams. Radiation damage of polymers induced by X-rays, particularly soft X-rays, is attracting growing attention. 4-11 Better understanding of damage mechanisms and more accurate determinations of damage rates for polymers are not only important to the areas mentioned above, but also to the field of soft X-ray spectro- microscopy, which is increasingly being applied to organic materials 12-15 and biological samples. 14,16,17 A complete bibli- ography of soft X-ray spectromicroscopy has recently been published in association with a recent review of polymer applications 15 and updates can be obtained from http://unicorn. mcmaster.ca/xrm-biblio/xrm_bib.html. Relative to electron energy loss spectroscopy in transmission electron microscopes (TEM- EELS), there is an advantage to using near edge X-ray absorption spectroscopy (NEXAFS) in soft X-ray microscopes for inner shell excitation based analysis of radiation sensitive materials since the electron beam excites orders of magnitude more valence than inner shell ionizations, whereas monochro- matic X-rays excite inner shell transitions essentially exclusively. That advantage was quantified in an early comparative study of damage to polyethylene terephalate (PET) by TEM-EELS versus nonspatially resolved NEXAFS. 5 That study also reported a difference in critical doses for photon versus electron damage, which was noted at the time as unexpected but possibly associated with the very large difference in the dose density in the electron microscope versus the comparison between radiation damage of the same material in electron and X-ray microscope. As the technology of both microscopies has improved im- mensely in the past decade, it is important to revisit the issue of relative damage rates, critical doses, and mechanisms for radiation damage to polymers. Polyethylene terephthalate (PET, structure in Figure 1) is one of the most important commercial polymers with many excellent features, such as good radiation resistance with blockage of ultraviolet rays, high chemical resistance, low gas permeability, high heat resistance, and good transparency. 18 Rates and critical doses for radiation damage of PET by different radiation sources and techniques have been reported. 5,6,10 Rightor et al. 5 compared the rate of damage to PET by soft X-rays, as measured by X-ray absorption spectroscopy (XAS), and by 100 keV electron beams, as measured by TEM-based electron energy loss spectroscopy (TEM-EELS). In their study, the critical dose of PET was found to be 5∼13-fold larger for damage by soft X-rays than by 100 keV electrons. In addition, a ∼500-fold advantage in terms of analytical information per unit damage was determined for XAS over TEM-EELS. However, within resolution limitations of the EELS system used in the experiments, the change in the near edge spectra was very similar in the two cases, indicating the damage products and mechanisms were most likely the same. This paper reports an experimental study of the radiation damage of PET by X-ray and electron microscopes. Relative to the earlier study, 5 the new aspects of these measurements include: (1) a scanning transmission X-ray microscope (STXM) with a focused spot size of ∼30 nm was used for the X-ray damage, resulting in a much higher dose density than the previous work using the 1996 version of the NSLS X-1A STXM in which a 500 nm beam size was used; (2) the analysis of the damage chemistry and quantitative changes was carried out by STXM for both X-ray and electron beam damage, which reduces possible inconsistencies associated with use of different analyti- cal techniques and instruments for quantitative analysis. A brief preliminary report of this work has been published. 10 It indicated * Corresponding author: aph@mcmaster.ca; phone:(+1) 905 525-9140 x24749; fax: ( +1) 905 521-2773. J. Phys. Chem. B 2009, 113, 1869–1876 1869 10.1021/jp808289e CCC: $40.75  2009 American Chemical Society Published on Web 01/28/2009