Thickness and interfacial roughness changes in polymer thin films during X-irradiation

Despite the obvious occurrence of synchrotron X-ray damage to organic thin films, few attempts have been made to qualitatively determine changes to their structural parameters during X-ray exposure. We report here the use of X-ray reflectivity to study X-radiation damage to thin films of poly(tert-butyl acrylate) and polystyrene at various incident flux densities and sample temperatures. At the flux densities studied, (0.4-6.7) × 10 ⁹ photons/(s mm ²), the polyacrylate film thickness decreased during irradiation at rates ranging from -0.1 to -4 Å/min, while the surface roughness increased. The volume of polymer removed per incident photon ranged from 150 to 1400 Å ³/photon. The rate of the thickness decay of the films was found to be linear with flux density over the range studied. The damage rate also appears to be directly related to the amount of X-ray energy deposited in the film, not the amount of energy available for creation of secondary electrons at the substrate. At comparable flux densities, polystyrene films heated above the glass transition temperature were found to behave similarly to the polyacrylate films, losing between 40 and 250 Å ³/photon; however, at room temperature polystyrene films instead slightly increased in thickness.