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The pendant drop technique was used to determine p/A isotherms of docosanic acid spread on the drop surface of an aqueous polymer solution. Two water soluble polymers were used, poly(dimethyl-diallyl-ammoniumchloride) and sodium poly(styrene sulfonate-b-ethylethylene). By fast changes of the drop volume, the monolayers were compressed and dilated. The stress relaxation was monitored and surface rheological dilation parameters were obtained. It is shown that the fatty acid monolayer can be mechanically stabilized by both interacting anionic and cationic polymers. In the case of the anionic polymer, the interaction becomes more pronounced in the presence of salts in the subphase (counterions). Brewster angle microscopy shows that the typical tilt-orientation of crystalline domains of the fatty acid monolayers transforms into a more uniform and fluid-like structure caused by the polymer/monolayer interaction. The surface rheological behavior is dramatically influenced by the polymer binding. The interaction results in surface dilational viscoelastic properties and show that there is a strong resistance against expansion of the complex fatty acid/polymer layer.
Langmuir-Blodgett films of zinc 11,18,25-tri(tert-butyl)-4-sulfo-phthalocyanine (ZNPctSO3Na) have been deposited onto hydrophilic and hydrophobic silicon wafers. Y-type films were formed on both types of substrate, and the transfer ratio was very close to unity. The organization of the films on the molecular level was probed by X-ray specular reflectivity.
This paper describes the formation and structure investigation of Langmuir monolayers and Langmuir-Blodgett multilayers formed from amphiphilic derivatives of 2,5-diphenyl-1,3,4-oxadiazole. The 2,5-diphenyl-1,3,4-oxadiazole group as a functional unit with interesting physical and chemical properties is maintained, while the head group, the length of the alkyl chain and the structure of the coupling unit between aromatic and aliphatic part of these linear short-chain amphiphiles is systematically varied in order to explore the influence of this change on the film forming properties and the stability of Langmuir and Langmuir-Blodgett films. Molecular mechanics simulations are shown by these systematic variations to be suitable for the prediction of optimal chemical structures allowing for a stable stratified molecular packing. The combination of a detailed structure investigation of the multilayers based on scanning force microscopy and X-ray data with molecular mechanics simulations yields an insight into the packing of the molecules and the intermolecular interactions.
The possibilities and limits of structure refinement of Langmuir-Blodgett films by means of symmetrical reflection of X- rays are described using the example of a stearic acid multilayer. Three different techniques for the determiantion of the electron density profile from reflectivity data are compared; a Fourier method, a Patterson method, and model calculations. The important role of the a priori information for finding the besft structure model is outlined.
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.
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 characterized by a uniaxial texture, which is correlated with the dipping direction during deposition of the LB film.
Study of gas transport through composite membranes with a stabilised Langmuir-Blodgett skin layer
(1995)
The Langmuir-Blodgett (LB-) technique is used to deposit molecular reinforced separation layers on porous polymer substrates resulting in composite membranes for gas separation. The adsorption of a polycation to the arachidic acid Langmuir layer and the subsequent transfer of the highly ordered and stabilised monolayer onto a polypropylene membrane (Cellgard 2400) yields a laminated separation layer combining the advantageous high degree of order of fatty acid films and the stability of thin polymeric films. X-ray reflectivity data of these films confirm the transfer of the assembled polymer layer together with the fatty acid monolayer and the formation of ordered Y-type LB-films. SFM pictures show a dense film without pinholes completely covering the porous support. Gas permeation measurements are used to study the transport process of different gases through the composite membrane.
The structure of mono- and multilayers of amphiphilic disc-shaped pentaynes wa inbestigated by Brewsterangle microscopy, X-ray specular reflection and grazing incidence diffraction (GID). X-ray specular reflection experiments confirm the "edge on" arrangement of the molecular discs. The molecular modelling of the Langmuir-Blodgett (LB)- multilayers predicts a columnar in-plane packing of the molecules. A GID experiment with monochromatic synchrotron radiation was used to verify the predicted multilayer structure on molecular level, while the Brewsterangle microscopy gave a deeper insight in the monolayer in-plane structure on micron scale.
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.
Langmuir-Blodgett(LB) multilayers were prepared from disc-shaped multiyne mesogens based on amphiphilic alkyl pentakis(aryl-ethynyl)benzene ethers. The two compounds used are characterized by five hydrophobic flexible chains and one hydrophilic substituent at the terminal position of the alkoxy chain. The LB films were analysed by X-ray scattering and spectroscopic measurements. An edge-on arrangement of the two discotic pentaalkynes within Y-type bilayers with a different packing density proved to be possible for the LB films of both compounds.
For bilayer stacks of non-centrosymmetrical molecules, formed by the Langmuir-Blodgett (LB) technique (Y deposition), 00l peaks for the bilayer period are usually observed in the X-ray specular reflectivity curve, in this study, we show that the introduction of stacking disorder at lower transfer ratios and the subsequent partial repair of defects by a parallel shift of oppositely oriented molecules into the vacancies may result in the formation of antiphase domains and in an increase in the average monolayer symmetry followed by quenching of the bilayer peaks. The X-ray reflectivity data, obtained for multilayers of penta-alkynes, are used to demonstrate this real structural effect, which is expected to be of general importance for X-ray and neutron reflectivity data reduction and for the structural modelling of multilayers.