X-ray investigations of the molecular mobility within polymer surface gratings

The physical origin of surface relief patterning on amorphous polymer films containing azobenzene-side chains induced by holographic exposure with vis ible light of about 450 nm is not yet fully understood. To understand the n ature of the induced material transport is of special interest to describe the dynamic processes occurring in thin films below the glass transition te mperature T-G. Thus, we investigated films made from the polar (poly {4'-[2 -(methacryloxy) ethyl]-ethyl}amino-4-nitroazobenzene, T-G=129 degrees C) an d less polar {poly[4-(2-methacryloxy)-ethyl] azobenzene, T-G=80 degrees C} azobenzene side-chain homopolymers and performed temperature-resolved coher ent x-ray and visible (VIS) light scattering measurements of the thermally induced erasure of the surface gratings. The simultaneous use of x-ray sync hrotron light (lambda=0.14 nm) and VIS laser light (lambda=633 nm) allows t he detection of the material flow on different lengths scales. We did not f ind remarkable differences in the thermal behavior of polar and nonpolar ma terials. Depending on the time used for inscribing the gratings the VIS sig nal starts vanishing at a critical temperature T-K below the glass temperat ure T-G. Up to T-G the x-ray grating peak intensities increase to a maximum even if the VIS signal is almost zero. Probing the grating in a different depth below the surface, the first and second order x-ray Fourier component s reach their intensity maxima at different temperatures and rise up in int ensity with time constants characterized by an Arrhenius-like activation en ergy of about 2.6 eV. At T > T-G the grating peak intensities go to zero. O ur measurements can be interpreted by a model of anisotropic viscosity. At T < T-G the erasing of the surface grating takes place by a material flow p erpendicular to the initial surface. This is accompanied by the formation o f an intrinsic density grating within the film against the shear tension of the polymer. At T > T-G the created lateral density modulation becomes equ alized by a lateral material flow quantified by a diffusion coefficient of about D=3x10(-13) cm(2) s(-1). (C) 2000 American Institute of Physics. [S00 21-8979(00)08011-7].