EELS analysis of PMMA at high spatial resolution

We performed the electron energy-loss spectroscopic (EELS) analysis of electron-sensitive polymers in the analytical transmission electron microscope in order to evaluate the possibility to obtain chemical information on polymers at a nanometre scale (i.e, at 2.4 nm diameter probe). In the acquired spectra, we propose an identification of the ELNES fine structure to the different chemical bonding in agreement with molecular orbital calculations (EHT) and with previous XANES experiments. The main results confirm that poly(methyl mettacrylate) (PMMA) is very sensitive to electrons when a large probe size is used, with a critical dose of about 10(2) C m(-2). However a high dose rate in a nanometre diameter electron beam is less destructive and the EELS spectra of far less degraded PMMA could be obtained even at 10(7) C m(-2). Irradiation damage was however thought to be the main limitation of the field-emission gun microscope, since high electron doses are required to acquire an EELS spectrum. This surprising behaviour was already observed in the case of poly(ethylene terephthalate), which is however more resistant to the electron beam (Varlot et al., 1997. Ultramicroscopy 68 (2, 123-133). Several possible explanations were studied, such as the influence of the accelerating voltage, a wrong calculation of the electron dose, the excitation delocalisation and the electron dose rate. (C) 2001 Elsevier Science Ltd. All rights reserved.