ORGANIC POLYMER SURFACE-ANALYSIS BY HIGH-RESOLUTION ELECTRON-ENERGY LOSS SPECTROSCOPY - DIPOLE VERSUS NONDIPOLE CHARACTER AND RESOLUTION ENHANCEMENT

High-resolution electron energy loss spectroscopy (HREELS) has been used to study the surface vibrational characteristics of approximately 100-angstrom-thick films of bisphenol-A-polycarbonate, polyethylene, and poly(2-vinylpyridine). Several vibrational modes of each polymer show significant intensity variations with primary beam energy, indicating electron interaction with the polymer surface via temporary negative-ion resonances. In high-density polyethylene, a simple dipole-allowed infrared-like spectrum exists for beam energies < 3 eV, whereas resonant enhancement of nondipole modes characterize the spectrum at higher beam energies. In the polycarbonate there is an enhancement of the carbonate modes at approximately 3.5 eV and the CH (aliphatic and aromatic) stretching modes at approximately 4.5 and approximately 5.0 eV, respectively. A comparison of the loss intensities of the polymer films is made with modes exhibited in infrared and Raman data which leads to insight into the relative contributions of dipole and other scattering mechanisms. These observations indicate that temporary negative-ion resonance scattering plays a major role in electron scattering from polymer films. Resolution enhancement methods have also been applied to the loss spectra in order to further improve the interpretability of the data. Maximum likelihood restoration has been found to produce very high quality data which allows for both the unique characterization of polymers and for the investigation of more subtle variations in the polymer surface. This is seen in the identification of carboxylic acid functionalities on the surface of bisphenol-A-polycarbonate.