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

Abstract

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 TG. Thus, we investigated films made from the polar (poly {4′-[2-(methacryloxy) ethyl]-ethyl}amino-4-nitroazobenzene, TG=129 °C) and less polar {poly[4-(2-methacryloxy)-ethyl] azobenzene, TG=80 °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 TK below the glass temperature TG. Up to TG 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 T>TG the grating peak intensities go to zero. Our measurements can be interpreted by a model of anisotropic viscosity. At T<TG 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 T>TG the created lateral density modulation becomes equalized by a lateral material flow quantified by a diffusion coefficient of about D=3×10−13 cm2 s−1.

Photoinduced Motions in Azo-Containing Polymers

“…401 X-ray investigations are also very powerful in analyzing the surface morphology and the molecular mobility. 402,403 Some very interesting developments have appeared in the recent literature. In a series of papers, Japanese researchers from Toyota have demonstrated that the optical near filed induced by depositing a series of polystyrene spheres on an azo polymer film allows the creation of very regular patterns on the film surface.…”

“…Time dependent analysis has proven useful to investigate the formation of half-period SRG as well . X-ray investigations are also very powerful in analyzing the surface morphology and the molecular mobility. , …”

Inscription of local surface relief gratings with a Scanning Near-field Optical Microscope on an azo-polymer film

“…, which has a glass transition temperature of T g = 129 °C and a broad maximum of absorption at approximately λ ≈ 500 nm [18] .…”

Design of Collective Motions from Synthetic Molecular Switches, Rotors, and Motors

“…Typically, SRG have a period in the range of micrometers and a modulation depth of a few hundreds of nanometers, meaning that the motion takes place at a scale several orders of magnitude higher than that of the photochromic molecule alone (Figure c). The grating spacing Λ g can be modulated by tuning the incident angle θ of the interfering beams at a wavelength λ, following the equation: Even if SRG are widely explored in the literature, the mechanism of their formation is still under debate. ,− Briefly, two main ideas are usually opposed: (i) the photofluidization concept and (ii) the alignment effect. For the former, it is suggested that the continuous trans → cis and cis → trans isomerization processes soften the polymer matrix around the photochromic molecules like a thermal fluidization.…”