Self-supporting thin-film

Polyurethanes based on the HDI cyclic trimer show a rubberlike mechanical behavior. In fact, the Z1030/1072 and Z1031-H films show a low elastic modulus E, no yielding, and an ultimate, widely reversible, deformation beyond 100-150%, likely underestimated owing to the difficulty in assessing the ultimate properties of self-supported thin films. 

The analysis of thin films is often performed by infrared spectroscopy. Compared to reflection absorption measurements on metal surfaces using p-polarized fight at grazing incidence, investigation of self-supported thin films or of thin films on transparent substrates shows weaker infrared absorption bands. Weaker absorption bands are caused by the absence of the surface enhancement mechanism and the poorer reflectivity. On the other hand, due to the absence of the metal selection rule, spectra of p- and s-polarization can be recorded in the case of freestanding films or of transparent substrates. Complex spectral features may arise... 

The set-ups developed for studying membranes cannot be directly applied to thermotropic liquid crystals, as most of them - especially the nematics - are usually not able to form self-supporting thin films they require solid containers. The technique developed by the authors copes with this... 

Let us first consider self-supporting thin-film (pellicle) beamsplitters of thickness d. Chamberlain et al. [45] have shown that the beamsplitter efficiencies for parallel and perpendicular polarized radiation are given by... 

Thin self-supporting clay films (appropriate for IR measurement) readily take up organic amines such as cyclohexylamine with displacement of the major fraction of the intercalated water. For the Ua -exchanged sample the majority of the amine is present in the unprotonated form - there being insufficient Bronsted acidity generated by the interlayer cation. When Al + is the exchangeable cation, however, a major fraction of the intercalated amine becomes protonated (see Figure 2). 

Reformate obtained from low to moderate sulfur liquid fuels (< 15-30 ppm S by weight) would contain about 1-2 ppm sulfin species by volume. This sulfur concentration is low enough to be tolerated by the 60wt.% Pd-40wt.% Cu system according to the literature. Kulprathipanja et al. give an excellent discussion of the impact of sulfur [23] and other components of reformate [24], CO, CO2 and H2O on self supporting Pd-Cu foil and supported thin film Pd-Cu membranes. 

CdS films and thin disks as a base material for the CdS-Cu S heterojunctions have also been prepared by sintering techniques. The preparation method for sintered films consisted in silk-screen printing on a ceramic substrate followed by a drying and sintering process at temperatures of about 600 C /15,16/. A different approach is based on compressing CdS powder in self supporting thin disks which are sintered at temperatures between 700 and 85o C. Grain sizes have been reported in this case of 5 to lO ym /16/. 

Reflectometry is a useful probe with which to investigate the structure of multilayers both in self-supporting films and adsorbed on surfaces [51]. Specular X-ray reflectivity probes the electron density contrast perpendicular to the film. The X-rays irradiate the substrate at a small angle (<5 °) to the plane of the sample, are reflected, and are detected at an equal angle. If a thin film is present on the surface... 

Specimens on grids are an alternative to self-supporting disks. Specimens in the form of small particles may be supported on a thin film (e.g. an amorphous carbon film) before being placed on the grid. Thin slices of the material may be produced by an instrument known as an ultramicrotome. The principal advantage of the technique is that it leaves the chemistry unchanged, and it may be employed to create uniform thin films of multiphase material. The instrument operates by moving... 

The various morphological variants available in bead form can be repHcated in thin films ( 2 cmx8cmx50-100 pm) produced simply by photo-initiated free radical polymerization of comonomer mixtures introduced by capillary action into an appropriate mold formed with microscope sHdes [48]. With appropriate choice of comonomers, and porogen in the case of macroporous films, reasonably mechanically robust self-supporting films can be removed from the mold for further exploitation (Fig. 1.9). 

Acidity of the PILC. Acidic properties were studied by infrared spectrometry adsorbing pyridine as probe molecule on self supported wafers prepared by pressing the PILC into thin films (15... 

For some applications it is desirable to prepare aerogels as thin hints that are either self-supporting or supported on another substrate. All common coating methods such as dip coating, spin coating, and spray coating can be used lo prepare gel films. 

Extremely thin, self-supporting film electrodes have been successfully produced. For example, polypyrrole films of 120 nm thickness have been used successfully to grow microcrystallites, followed by removal of the film plus crystallites from the substrate for characterization of the crystallographic structure and orientation of the microcrystallites by transmission electron microscopy with selected area electron diffraction [23]. Thin-film electrodes have been applied to allow transmission of high-energy beams, including x-rays, for in situ... 

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

Preparation of self-supporting ang thin films, in which the clay particles are oriented normal to the c axis, is one of the more useful drying methods. A simple apparatus in which such films can be prepared is illustrated in Figure 4. The sample suspension is transferred through the ball valve and placed onto the drying surface, which may be either a porous plate, porous filter overlying the porous plate, or a non-porous window resting on the porous plate. 

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