Temperature- and time-resolved X-ray scattering at thin organic films

Multilayers of an Fe(II)-polyelectrolyte-amphiphile complex (Fe-PAC) were investigated simultaneously by energy-dispersive X-ray reflectivity and in-plane diffraction at the bending-magnet beamline at BESSY II. By recording spectra between room temperature and about 348 K with a time resolution of about 60 s, two phase transitions were identified and the respective activation energies were determined. Owing to a fixed geometrical set-up, an energy-dispersive experiment is suitable for measuring X-ray reflectivity and in-plane X-ray diffraction of thin organic films simultaneously. Installed at a bending magnet at BESSY II and using an energy range of 4 < E < 25 keV, both reflectivity and diffraction spectra can be collected within about 60 s. The performance is demonstrated by temperature- and time-resolved measurements of the phase transition behaviour of multilayers of an Fe-PAC deposited on a silicon support by means of the Langmuir-Blodgett (LB) technique. By measuring the X-ray reflectivity while increasing the temperature, it is shown that the original LB phase is not stable and transforms irreversibly into a liquid-crystalline (LC)-like phase at about 318 K. At the same temperature the in-plane diffraction signal vanishes reversibly, reflecting rotational disorder of the hexagonal arrangement of amphiphilic chains. Its activation energy is determined to be about 1.3 eV. At about 338 K a second irreversible phase transition occurs to another LC phase with smaller vertical layer spacing. This transition is reversible between 329 K and 338 K.