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 visible light of about 450 nm is not yet fully understood. To understand the nature of the induced material transport is of special interest to describe the dynamic processes occurring in thin films below the glass transition temperature $T_G.$ Thus, we investigated films made from the polar (poly {4^′-[2-(methacryloxy) ethyl]-ethyl}amino-4-nitroazobenzene, $T_G\text{=129\hspace{0.05em}°}\mathrm{C})$ and less polar {poly[4-(2-methacryloxy)-ethyl] azobenzene, $T_G\text{=80\hspace{0.05em}°}\mathrm{C}\}$ azobenzene side-chain homopolymers and performed temperature-resolved coherent x-ray and visible (VIS) light scattering measurements of the thermally induced erasure of the surface gratings. The simultaneous use of x-ray synchrotron light (λ=0.14 nm) and VIS laser light (λ=633 nm) allows the detection of the material flow on different lengths scales. We did not find remarkable differences in the thermal behavior of polar and nonpolar materials. Depending on the time used for inscribing the gratings the VIS signal starts vanishing at a critical temperature $T_K$ below the glass temperature $T_G.$ 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 components reach their intensity maxima at different temperatures and rise up in intensity with time constants characterized by an Arrhenius-like activation energy of about 2.6 eV. At ${\mathrm{T>T}}_G$ the grating peak intensities go to zero. Our measurements can be interpreted by a model of anisotropic viscosity. At ${\mathrm{T<T}}_G$ the erasing of the surface grating takes place by a material flow perpendicular to the initial surface. This is accompanied by the formation of an intrinsic density grating within the film against the shear tension of the polymer. At ${\mathrm{T>T}}_G$ the created lateral density modulation becomes equalized by a lateral material flow quantified by a diffusion coefficient of about ${\text{D=3×10}}^{\text{−13}}{\mathrm{\hspace{0.17em}cm}}^2 {\mathrm{s}}^{\text{−1}}.$