EELS analysis of PMMA at high spatial resolution

We performed the electron energy-loss spectroscopic (EELS) analysis of elec tron-sensitive polymers in the analytical transmission electron microscope in order to evaluate the possibility to obtain chemical information on poly mers 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 di fferent 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 sens itive 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 elec tron beam is less destructive and the EELS spectra of far less degraded PMM A could be obtained even at 10(7) C m(-2). Irradiation damage was however t hought to be the main limitation of the field-emission gun microscope, sinc e high electron doses are required to acquire an EELS spectrum. This surpri sing behaviour was already observed in the case of poly(ethylene terephthal ate), which is however more resistant to the electron beam (Varlot et al., 1997. Ultramicroscopy 68 (2, 123-133). Several possible explanations were s tudied, such as the influence of the accelerating voltage, a wrong calculat ion of the electron dose, the excitation delocalisation and the electron do se rate. (C) 2001 Elsevier Science Ltd. All rights reserved.