Electron energy loss spectroscopy of polytetrafluoroethylene: experiment and first principles calculations

We have performed electron energy-loss spectroscopy (EELS) on a 200 kV transmission electron microscope (TEM) equipped with a monochromator to investigate molecular conformation of polytetrafluoroethylene (PTFE). The experimental spectra show several unique features in the low-loss region and the onset of carbon K-edge for PTFE. Density function theory (DFT) methods are employed to calculate the low-loss and core-loss spectra of PTFE with consideration of the effects of phase transitions, chain orientation and polarization. The shape and width of the characteristic peaks of the experimental spectra are well reproduced in DFT calculations. By comparing the spectra from experiments and theory, the detailed information about the conformational dependence of EEL spectra for PTFE can be obtained. In the present work, we have demonstrated an application of combining high-resolution EELS and DFT calculations in both low-loss and core-loss regions to discriminate changes of chain conformation and orientation for the polymer with complex phase transition behavior.