  Although polymers are widely used in laser-irradiation research, their
microscopic response to high-intensity ultrafast XUV or X-ray irradiation is
still largely unknown. Here we comparatively study homologous series of
alkenes. XTANT-3 hybrid simulation toolkit is used to determine their damage
kinetics and irradiation threshold doses. The code simultaneously models the
nonequilibrium electron kinetics, the energy transfer between electrons and
atoms via nonadiabatic electron-ion (electron-phonon) coupling, nonthermal
modification of the interatomic potential due to electronic excitation, and the
ensuing atomic response and damage formation. It is shown that the lowest
damage threshold is associated with local defect creation such as
dehydrogenation, various group detachment from the backbone, or polymer strand
cross-linking. At higher doses, the disintegration of the molecules leads to a
transient metallic liquid state. We identify nonthermal effects as the leading
mechanism of damage, whereas the thermal (electron-ion coupling) channel
influences the kinetics only slightly in the case of femtosecond-pulse
irradiation. Despite notably different properties of the studied alkene
polymers, the ultrafast-X-ray damage threshold doses are found to be very close
to ~0.05 eV/atom in all three materials: polyethylene, polypropylene, and
polybutylene.
