Dendritic multilayers

Watanabe and Regen 81 reported the construction of ordered, dendritic multilayers (10) via a bridged, outer sphere —outer sphere mode of assembly (Figure 9.5) whereby the transition metal Pt was used as a connector moiety. Although amine-terminated PAMAM-type dendrimers 76 were employed for this particular example, this process could easily be extended to other types of macromolecules. 

Many polymers form more complex single crystals when crystalhzed from dilute solution including hollow pyramids that often collapse on drying. As the polymer concentration increases, other structures occur, including twins, spirals, and multilayer dendritic structures with the main structure being spherulites. 

A two- or multilayer separator is generally used in which a strong fibrous element is incorporated to prevent internal short circuits by zinc dendrites... 

The cell body of a mammalian motor neuron has an extended process termed an axon that branches at its tip to make multiple contacts (synapses) with a muscle cell (fig. S1.1). Some axons actually reach lengths of several meters. Much of the axon may be encased in myelin sheaths, which are multilayered membranes formed by other cells that wrap themselves around the neuron. Shorter processes (dendrites) extend from the cell body to make contacts with other neurons. If a motor neuron is stimulated electrically or is triggered by its connecting neurons, an electric signal called an action potential sweeps down the axon and is transmitted to the muscle cell, which then proceeds to contract. Other... 

A typical method for fabricating multiple complex layers is illustrated in Figure 2.11,12 First, an Au/mica or Au/ITO plate is immersed in a chloroform solution of tpy-AB-SS-AB-tpy (tpy=2,2 6, 2" -terpyridyl), providing Au-S-AB-tpy SAM on the plate. In the case of connecting the Fe(II) ion, the tpy-terminated plate is immersed in 0.1 M Fe(BF4)2 aq or (NH4)2Fe(S04)2 aq to form a metal complex. Subsequently, the metal-terminated surface is immersed in a chloroform solution of the ligand Lj or L2 to form a bis(tpy)iron structure (Fig. 2b). The latter two processes are repeated for the preparation of multilayered bis(tpy)iron (II) complex films with linear structures. When L3 is used instead of Lj or L2, the resulting molecular wires have a dendritic structure (Fig. 2c). 

They have observed linear growth of the film thickness, which is consistent with multilayer ordering. The thickness of an individual molecular layer for the generation G4 is about 5 nm, which indicates preservation of the globular shape of the dendritic macromolecules within the multilayer films (Fig. 4.13c). 

Fig. 4.13 General scheme of dendritic macromolecules within molecular layers at interfaces (a) compressed Langmuir bilayer at air-water interface (b) adsorbed and self-assembled monolayers of neutral dendrimers (left) and dendrimers with sticky surface groups (right) (c) multilayer self-assembled films obtained by layer-by-layer deposition of dendrimers low molar mass ions (left) and two adjacent dendrimer generations (right). (From ref. [106])...

Huskens et al. exploited host-guest interactions between dendritic guest molecules and CD-modified nanoparticles for the formation of organic/metal nanoparticle multilayers on a PDMS stamp (Fig. 13.15).88 The multilayer stacks were transferred to a complementary host surface, while no materials remained on the protruding areas of the PDMS stamp. These multilayers showed a well-defined thickness control of 2 nm per bilayer. 

In respect to thin films of dendritic polymers, many investigators have fabricated different types of films such as multilayers of amine-terminated PAMAM dendrimers on a silicon wafer [34], covalent attachment of amine-terminated PAMAM dendrimers [35] and poly(iminopropane-l,3-diyl) dendrimers [36] on thiol SAMs, self-assembled composite multilayer films from amine- and... 

Metallic copper nanoparticles within covalently bonded multilayered dendritic ultrathin films made of pamam, using supercritical CO2 as a processing medium, were described by Puniredd and Srinivasan . The nanoparticles were obtained in higher yield, in a denser and more stable distribution, and showed greater stability towards polar solvent attack than the analogous products of liquid solvent processes, for example, in tetrahydrofuran, which was explained by the facile solvent separation and transport. 

Several articles have addressed the possibility of using dendritic molecules as electroluminescent materials in organic light-emitting diodes (LEDs) [140-148]. For example, compound 70 works as an excellent hole transport material, and has been incorporated into multilayer devices that exhibit high luminance efficiency and significant thermal stability [140, 141, 144],... 

In 1968, Paul Langerhans discovered the presence of a particular type of epidermal cell in humans, which is now named after him. The Langerhans cell (LC) is a mobile dendritic cell that has no desmosomes and is found in most multilayered malpighian epithelia. LCs make up 2-5% of epidermal cells. They come from the medulla and migrate towards and within the skin and stay within the epidermis for 2 weeks at the most. 

Figure 3. SEM images of dendritic gold clusters formed on an ITO electrode modified with a polyelectrolyte multilayer by electrochemical deposition. Reproduced with permission from [30]. Copyright 2004 American Chemical Society.

Cell construction for the Ni-Zn cell is similar to that for the sealed spiral wrap cylindrical and prismatic Ni-Cd and Ni-MH cells for portable electronic applications but with some differences. The Ni-Zn cell incorporates a multilayer micro-porous separator with wicking components used to maintain uniform electrolyte distribution, as well as acting as a buffer/barrier to prevent shorting from zinc dendrite formation. The barrier also slows oxygen gas diffusion and lowers the recombination rate that can lead to pressure buildup inside the cell. The KOH electrolyte contains CaO additions to calcium zincate that can control the free zinc ions in solution. In the cell electrolyte, the CaO reacts with zinc ions to form calcium zincate. Other improvements center on the use of fiber (steel wool) current collectors for the zinc electrode. The Ni-Zn cells have excellent high rate and low temperature performance. 

For many polymers, the single crystals are not simple flat structures. The crystals often occur in the form of hollow pyramids, which collapse on drying. If the polymer solution is slightly more concentrated, or if the crystallization rate is increased, the polymers will crystallize in the form of various twins, spirals, and dendritic structures, which are multilayered (see Figure 6.11) (34). These latter form a preliminary basis for understanding polymer crystallization from bulk systems. 

In the previous sections it was observed that when polymers are crystallized from dilute solutions, they form lamellar-shaped single crystals. These crystals exhibit a folded-chain habit and are of the order of 100 to 200 A thick. From somewhat more concentrated solutions, various multilayered dendritic structures are observed. 

Thermal Shock. Multilayer ceramic capacitors may crack if exposed to thermal transients exceeding 4°C/s. These cracks are usually invisible, but may be the sites for dendritic growth in service, when the assembly is exposed to moisture under applied bias. Capacitors with high values and larger thicknesses are most susceptible. These failures can be avoided by following the manufacturer s requirements for maximum temperature excursion and rate of temperature change. 

Zhang etalP showed that the surface covered with dendritic gold clusters, which was formed by electrochemical deposition onto indium tin oxide (ITO) electrode modified with a polyelectrolyte multilayer, showed superhydrophobic properties after further deposition of an -dodecanethiol monolayer. The chemical bath deposition (CBD) has also been used to make nanostrucmred surfaces, thus, Hosono etaL fabricated a nanopin film of brucite-type cobalt hydroxide (BCH) and achieved a contact angle of 178° after further modification of lauric acid (LA). Shi et al7 described the use of galvanic cell reaction as a facile method to chemically deposit Ag nanostructures... 

To these sets of primary and secondary reactions related to solvents, one has to add the eontributions of salt anion reduction, which usually forms metal halides and M AXy species (A is the main high oxidation-state element in the salt anion and X is a halide, such as chloride or fluoride). Most of the produets of aetive metal surface reactions are ionic compounds that are insoluble in the mother solution, and therefore, precipitate as surface films. It should be added to this picture that possible polymeric species can be formed, espeeially in alkyl carbonate solvents, whose reduction forms polymerizable species sueh as ethylene or propylene. Hence, the surface films formed on active metal electrodes are very complicated. They have a multilayer structure perpendicular to the metal surface, and a lateral, mosaic-type composition and morphology (i.e. containing mixtures and islands of different compounds and grains). Such a structure may induce very non-uniform current distribution upon metal deposition or dissolution processes, which leads to dendrite formation, a breakdown of the surface films, etc. These situations are demonstrated in Fig. 13.6 active metal dissolution leads to the break-and-repair of the surface films, thus forming mosaic-type structures. 

Vestbeig, R., Malkoch, M., Kade, M. et al. (2007) Role of architecture and molecular weight in the formation of tailor-made ultrathin multilayers using dendritic macromolecules and click chemistry. Journal of Polymer Science Part A-Polymer Chemistry, 45,2835. 

Fig. 12 Alternative models to explain the non-linear growth mechanism of polyelectrolyte multilayer. The substrate is the horizontal slab. The film resulting from successive polymer adsorption steps is shown in different level of gray, a Island model, b Dendritic model. Reprinted with permission from Ref. [162]. Copyright (2011) American Chemical Society...

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