Thomas Geue

Linear viscoelastic analysis of formation and relaxation of azobenzene polymer gratings.

Surface relief gratings on azobenzene containing polymer films were prepared under irradiation by actinic light. Finite element modeling of the inscription process was carried out using linear viscoelastic analysis. It was assumed that under illumination the polymer film undergoes considerable plastification, which reduces its original Youngs modulus by at least three orders of magnitude. Force densities of about 10(11) N/m3 were necessary to reproduce the growth of the surface relief grating. It was shown that at large deformations the force of surface tension becomes comparable to the inscription force and therefore plays an essential role in the retardation of the inscription process. In addition to surface profiling the gradual development of an accompanying density grating was predicted for the regime of continuous exposure. Surface grating development under pulselike exposure cannot be explained in the frame of an incompressible fluid model. However, it was easily reproduced using the viscoelastic model with finite compressibility.

Optical patterning in azobenzene polymer films

Thin azobenzene polymer films show a very unusual property, namely optically induced material transport. The underlying physics for this phenomenon has not yet been thoroughly explained. Nevertheless, this effect enables one to inscribe different patterns onto film surfaces, including one‐ and two‐dimensional periodic structures. Typical sizes of such structures are of the order of micrometers, i.e. related to the interference pattern made by the laser used for optical excitation. In this study we have measured the mechanical properties of one‐ and two‐dimensional gratings, with a high lateral resolution, using force–distance curves and pulse force mode of the atomic force microscope. We also report on the generation of considerably finer structures, with a typical size of 100 nm, which were inscribed onto the polymer surface by the tip of a scanning near‐field optical microscope used as an optical pen. Such inscription not only opens new application possibilities but also gives deeper insight into the fundamentals physics underlying optically induced material transport.

X-ray investigations of formation efficiency of buried azobenzene polymer density gratings

We present the results of time-dependent x-ray scattering measurements during the formation of a buried, modulated density structure below polymer film surface created at temperatures above the glass transition temperature TG during the thermal erasure of a holographically written surface relief grating. The surface relief gratings are initially formed on polymer films containing azobenzene side groups by exposure to a polarization holographic pattern made using light at 488 nm. The material responds to the actinic light by forming a sinusoidal surface relief pattern in a one-step process. Theoretical predictions show that the surface relief grating formation is accompanied by a density grating just below the film surface. Upon annealing above polymers glass transition temperature the surface relief grating melts while some of the density modulation in the bulk remains and a nonsinusoidal density grating is rebuilt after long term annealing at a temperature approximately 30 K above the glass transition te...

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.

Metal ion mediated mesomorphism and thin film behaviour of amphitropic tetraazaporphyrin complexes

Octa-substituted tetraazaporphyrins with amphiphilic 3,6-dioxaheptylthio and 3,6,9-trioxadecylthio chains and their metal complexes (Co(II), Ni(II), Cu(II), Zn(II)) were synthesised in order to compare their thermotropic and lyotropic mesomorphism as well as their thin film properties with related tetraazaporphyrins, phthalocyanines and triphenylenes, which have been reported previously. The Co, Ni, and Cu complexes melted into hexagonal columnar mesophases, whereas the Zn(II) complexes and the free-base porphyrins did not display mesomorphism. In fact, the zinc complexes did not crystallise on cooling, rather giving highly viscous isotropic oils at room temperature that solidified below −50 °C to glasses. This unusual behaviour may be caused by axial interactions between the zinc ion and the oxygen of the polyether groups. All 3,6,9-trioxadecylthio substituted derivatives solidified far below 0 °C giving rise to columnar hexagonal mesophases at room temperature for the Co(II), Ni(II), and Cu(II) complexes. The columnar hexagonal mesophases of the long chain derivatives spontaneously align homeotropically when sandwiched between two substrates, in contrast to the short chain analogues. Both the long chain and the short chain Co(II) complexes displayed a rather disordered hexagonal columnar packing as revealed by X-ray diffraction. Binary mixtures of the short chain derivatives with non-polar or polar organic solvents did not display additional lyotropic mesophases. However, the transition temperatures and enthalpies, as well as the texture of the hexagonal columnar mesophase were affected, assuming a solubility of the solvent in the thermotropic mesophase as shown for the Cu(II) derivative. All long chain derivatives were also soluble in water but, again, lyotropic mesomorphism was observed neither with water nor with organic solvents. The film forming properties of some derivatives were investigated by spin-coating and the Langmuir–Blodgett (LB) technique. Homogeneous films of the mesomorphic short chain copper complex were obtained by the spin-coating method. The films displayed a layer structure with edge-on orientation at ambient temperature, although they were of crystalline morphology, as shown by X-ray reflectivity measurements and polarised UV/VIS spectroscopy. Only monolayer films of short and long chain derivatives could be transferred by the LB method. Again, determination of the thickness of the monolayer by X-ray agreed with an edge-on orientation of the tetraazaporphyrins. In contrast, the extrapolated molecular areas on the water surface suggested a flat-on orientation of the macrocycle with the oligo(oxyethylene) chains being dissolved in the sub-phase.

Structure of thermally treated oxadiazoleamide Langmuir-Blodgett films

Abstract The thermal treatment of Y-type Langmuir-Blodgett (LB) films formed from the amphiphilic derivative of 2,5-diphenyl-1,3,4-oxadiazole 1 results in changes of the molecular packing. These changes have been analysed by a combination of X-ray specular reflectivity data, X-ray grazing incidence diffraction data and scanning force microscopy images. On the basis of these experimental data we have simulated possible supramolecular structures. These simulations provide insight into the intermolecular interactions giving rise to the observed structural transitions. The crystalline structure induced by thermal treatment of the LB films is chracterized by a uniaxial texture, which is correlated with the dipping direction during deposition of the LB film.

X-ray reflectivity from sinusoidal surface relief gratings

The sinusoidal shape of light-induced surface relief gratings of polymers can be probed by x-ray scattering techniques. A particular approach of kinematic x-ray scattering theory is developed to interpret experimental scattering curves. The simulations demonstrate the particular sensitivity of x-ray reflectivity for very small grating amplitudes. At angles of incidence close to the critical angle of total external reflection a grating amplitude h < 2 nm already provides measurable grating peaks. In general the grating amplitude h can be measured from the envelope function over the grating peak maxima. The capability of the approach is demonstrated by simulation of the reflection curve recorded from a polymer sample with non uniform grating height.

Vacuum deposition films of oxadiazole compounds: formation and structure investigation

Abstract The search for alternative routes of organic thin film formation is stimulated by the outstanding properties of these films in such fields as nonlinear optics, photonic data processing and molecular electronics. The formation of highly ordered multilayer structures by thermal vacuum deposition (VD) of organic compounds is an essential step toward the application of supramolecular organic architectures in technical systems. The VD of an amphiphilic substituted 2,5-diphenylene-1,3,4-oxadiazole 1 onto silicon substrates at defined temperature was used for the formation of ultrathin films. The structural data obtained for the VD-films of oxadiazole 1 by means of X-ray reflectivity, X-ray grazing incidence diffraction and atomic force microscopy (AFM) investigations indicate the formation of well ordered oxadiazole multilayers. The structure of the VD-multilayers is compared with that of Langmuir–Blodgett (LB) films and thermally treated LB-multilayers prepared from the same compound.

Competition of Alignment and Aggregation? Phenomena in Constrained Films of LC Poly (Olefin Ulfone)s and Maleic Anhydride Co- and Terpolymers

Abstract Anisotropic films of functionalised polymers such as the liquid crystalline poly (olefin sulfone)s (POS) and maleic anhydride co- and terpolymers (MAC, containing azobenzene side-groups too) are studied. Starting from isotropic spin-coated films annealing results in homeotropically aligned films by spontaneous self-assembling. X-ray specular scattering (XRS) and X-ray grazing incidence diffraction (GIXD) indicate a layered structure across the film parallel to the interface and significant in-plane order in thin films. Polarised UV-VIS spectroscopy investigations show that the homeotropic orientation of the side groups causes the formation of “card packed” or H-aggregates. For azobenzene containing MAC terpolymer an initial UV-irradiation followed by a polarised VIS-irradiation causes a light-induced co-operative reorientation of both types of side-groups and results in a lamellar stacking which is tilted. Orientation and aggregation are therefore driven by the liquid crystallinity and the segreg...

Monolayers and multilayers of uranyl arachidate: in-plane structure of uranyl arachidate multilayers

Abstract The molecular in-plane structure of uranyl arachidate Langmuir-Blodgett (LB) films formed at different subphase pH values was analysed by means of X-ray grazing-incidence diffraction. For multilayers formed at low subphase pH a reorganisation of the arachidic acid film structure is confirmed. At appropriate subphase pH values, reorganisation of the film structure e.g. via the formation of three-dimensional crystallites, is prevented by the presence of the uranyl ions and by the subsequent introduction of conformational disorder (gauche defects) in the alkyl chains. The observation of a macroscopic flow-induced in-plane texture in these uranyl arachidate LB films has profound implications for the design of ordered, supramolecular structures by the Langmuir-Blodgett technique.