Crystalline superstructure

At first, however, this review will provide the reader with a critical overview over the most commonly used nanomaterials. The emphasis here will be particularly on those aspects of their synthesis, manipulation, and characterization that are of significant importance for their use as dopants in liquid crystalline phases or as precursors for the formation of liquid crystalline superstructures including size and size-distribution, shape, chemical purity, post-synthesis surface modifications, stability of capping monolayers, and overall thermal as well as chemical stability. 

Studies have been conducted on poly (tetramethylene oxide )-poly-(tetramethylene terephthalate) -segmented copolymers that are identical in all respects except for their crystalline superstructure (66,67,68). Four types of structures—type I, II, and III spherulites (with their major optical axis at an angle of 45°, 90°, and 0° to the radial direction, respectively), and no spherulitic structure—were produced in one segmented polymer by varying the sample-preparation method. Figures 10 and 11 show the stress-strain and IR dichroism results for these samples, respec-... 

In an investigation of partide-particle interaction in semiconductor nanociystal assemblies, Dollefeld et al. [17] examined (among other structures) the crystalline superstructure of [Cdi7S4(SCH2CH20H)26] (as described in Ref [7]). In the UV-visible absorption spectrum of this compound in solution, the first electronic transition was shifted to higher energies by about 150 meV as compared to the reflection spectrum of the crystalline material. In addition, the transition was broadened from a full-width at half maximum of approximately 390 meV to about 520 meV. Most likely, a complete description of the interaction of semiconductor nanocrystals in crystalline superstructures would include both electronic and dipole-dipole interactions. The electronic coupling may be introduced by covalent... 

The effects of the addition of two or more additives (i.e. talc, montmorillonite, Ceo, PDLA, and various polysaccharides) have been studied by Tsuji et al. [313], while those of talc and/or triphenyl phosphate as a plasticizer were investigated by Xiao et al. [330], Derivative N,N,N-tricyclohexyl-l,3,5-benzenetricarboxylamide was used as a model to tailor the crystalline superstructure of PLLA three crystal morphologies including conelike, shish-kebab, and needlelike structures were obtained by melt crystallization [331],... 

Liquid crystals exhibit a partially ordered state (anisotropic) which falls in-between the completely ordered solid state and completely disordered liquid state. It is sometimes referred to as the fourth state of matter . In recent years, interest in liquid crystalline thermosets (especially liquid crystalline epoxy) has increased tremendously [33-44]. If the liquid crystal epoxy is cured in the mesophase, the liquid crystalline superstructure is fixed permanently in the polymer network, even at higher temperature. Liquid crystal epoxies are prepared using a liquid crystal monomer [33-38] or by chemical modification of epoxy resin [43] which incorporates liquid crystal unit in the epoxy structure. Liquid crystalline epoxy resins with different types of mesogen such as benzaldehyde azine [33], binaphthyl ether [34, 35], phenyl ester [36, 37] and azomethine ethers [38, 39] have been reported. Depending on the chemical nature of the mesogen, the related epoxies display a wide range of thermomechanical properties. The resins can be cured chemically with an acid or amine [40, 41] or by photochemical curing in the presence of a photo-initiator [3]. Broer and co-workers [42] demonstrated the fabrication of uniaxially oriented nematic networks from a diepoxy monomer in the presence of a photo-initiator. 

HS and SS domains can organize to form crystalline superstructures, especially in the case of solution cast samples. Spherulites with diameters from several thousands of nm up to about 20 //m were observed by small-angle light scattering (SALS) [167,168], and by using optical and electron microscopic methods [169-171]. 

Under certain conditions, the lamellae of the TPEEs organize into a spherulitic structure, which is characteristic structure of the common semi-crystalline polymers. Based on the results of different methods, it was concluded that crystallization occurs by chain folding through which a spherulitic superstructure is formed. A well developed spherulitic crystalline superstructures, with diameters of about 5-20 pm, can be formed depending on the crystallization conditions [33]. Also, the soft, amorphous phase is embedded between radial crystalline fibrils of the hard segment spherulites. Under some conditions, other structures such as dendrites are developed. 

Recent examples in the spotlight are PtsCo and PtsNi alloys. Extended surfaces as well as nanoscale alloys of these materials showed promising activities and were reported to outperform Pt for the oxygen reduction reaction (ORR) which has been explained with a favorable shift in the d-band center [8,7]. In particular, PtsNi with its specific crystalline superstructure received attention in 2007, as Stamenkovic et al. shared their groundbreaking investigations with the scientific community [15]. 

To examine semicrystalline block copolymer morphology by electron microscopy, the specimens need to be sectioned and stained to provide sufficient imaging contrast. Though challenging, the development of new sample-preparation protocols has recently enabled the examination of individual crystals within block copolymer microdomains [Loo et al. 2000b]. Optical microscopy is a long-established technique for investigating crystalline superstructure, especially the presence and structure of spherulites. Recently, atomic force microscopy (AFM) has been applied to study the structures of thin supported films of crystallizable block copolymers. Optical and AFM measurements are nondestructive and can be conducted in or near real time so the process of crystallization can be tracked. 

Polymer dispersions normally consist of spherical particles. The dispersed particles scatter the light and are the cause of the milky appearance. This Mie scattering is utilized for particle size measurement Very small polymer particles hardly scatter visible light at all, those polymer dispersions have a translucent appearance. If all the particles are of the same size, the term monodisperse dispersions will be used. They are frequently recognized from a certain particle size merely from the iridescent appearance, which is caused by Bragg scattering at a crystalline superstructure of close packing of the particles. 

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