Polycrystalline materials, crystal polymers

Carrier injection and transport in OLEDs has been treated in detail by, among others, Kalinowski.113 Most of the organic electroluminescent materials, small molecules and conjugated polymers are low-conductance materials. The h+ mobility in these materials is typically 10 7—10 3 cm2/(Vs), and the e mobility is typically lower by a factor of 10-100. However, it is now clear that the low mobility is due to the disorder in the amorphous or polycrystalline materials. Indeed, in high-quality single crystals of pentacene, the h+ and e mobility are 2.7 and... 

The Laue function in Eq. 2 for small size crystals, as in crystalline polymers, is characterized by spurious, undesirable maxima which, in a polycrystalline material are quite absent, owing to the large distribution of crystal dimensions. A Bernoulli-type distribution of the size of the crystallites may be in these cases assumed substituting the product of Laue functions in Eq. 2 with a product of fimctions of kind [150,151] ... 

The first group includes ISEs with solid membranes, consisting of glass, single crystals, polycrystalline materials, or a matrix of polymer containing particles of a polycrystalline active materials... 

When LCEs are synthesized in the absence of external fields, so-called polydomain LC elastomers are obtained, which show macroscopically isotropic properties similar to polycrystalline materials. This resembles, e.g., bulk material of a low molar mass liquid crystal, where thermal fluctuations prevent a uniform director orientation over the whole sample. For nematic elastomers the overall isotropic behavior also indicates an overall isotropic conformation of the polymer chains, which is the consequence of the maximization of the chain entropy. 

Birefringent samples have a refractive index that depends on the direction of the electric field in the light. They may be single crystals, or polycrystalline or amorphous polymers with oriented regions [23, 69]. Materials containing... 

In die late nineteenth century, scientists quickly adopted flie seminal publications of the Curie brothers. Consequently, piezoelectricity and electrostriction were first discovered and investigated on inorganic, mono- or polycrystalline materials (Katzir 2006). Therefore, the theoretical treatment of tire relevant electromechanical properties has been based on the physics and in particular on the structure and the anisotropy of crystals (Newnham 2005 Tichy et al. 2010). Semicrystalline or amorphous polymers are usually less anisotropic flian crystals, and the symmetry... 

Polymers in the solid state can be completely amorphous, partially crystalline, or almost completely crystaUine. Polymer crystals have the requiranent that they must accommodate the covalent axis within an ordered structure. For some time the interpretation of X-ray diffraction patterns was that individual polymer molecules were partly crystalline and partly amorphous. The longest dimension of the crystallites in polycrystalline materials is usually about 5-50 nm, which is a small fraction of the length of a fully extended polymer molecule. A graphical representation of this once popular model is shown in Figure 3.12. Here a long polymer chain wanders... 

It is difficult to prepare stretched oriented fibres and such fibres may differ in their polymer chain conformation compared with the unstretched materials. Furthermore the quality of the single crystal X-ray data is poor and difficult to interpret. In contrast power X-ray data of relatively high quality may be obtained from polycrystalline polymer films. Lightfoot, Mehta and Bruce (1992) have obtained the first crystal structure of a polymer-salt complex, PEOjiNaClQ from powder X-ray data. Fig. 5.11(h). The structure is similar to the corresponding PEOjiNal structure, the PEO chains are wrapped around the Na ions with each Na ... 

The major feature of polymers that have been bulk crystallized under quiescent conditions are polycrystalline structures called sphemlites. These are roughly spherical supercrystalline structures which exhibit Maltese cross-extinction patterns when examined under polarized light in an optical microscope. Spheruliies are characteristic of semicrystalline polymers and are also observed in low-molecular-weight materials that have been crystallized from viscous media. Sphemlites are aggregates of lamellar crystallites. They are not single crystals and include some... 

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