Formation of a buried density grating on thermal erasure of azobenzene polymer surface gratings

The transformation of a lateral surface relief grating inscribed on a polymer film containing azobenzene moieties into a density grating of equal spacing buried under a smooth surface was found for the polymer poly{(4-nitrophenyl)[4-[[2-(methacryloyloxy)-ethyl]ethylamino]phenyl]diazene} (pDR1M) using an X-ray scattering experiment. Annealing a polymer sample pre-inscribed with a surface grating for several hours above the glass transition temperature creates a nearly sinusoidal lateral density difference up to about 10%. The new state is stable between room temperature and the decomposition temperature of the original polymer when the formation was performed under the influence of VIS light. Growth of liquid-crystalline aggregates is proposed as a most probable explanation for the process.