Poly crystalline forms

The microtubule-bound complex of 74 revealed the F-F distance to be 6.5 0.5 A (Figure 6), which is larger than that observed in the poly crystalline form by ca. 1 A. It is very likely that the microtubule-bound conformation of 74 is achieved by a small distortion of the solution conformation (the recognition conformation), the latter being described by either conformer B or C (Figure 4). 

It is typical, for instance, of syndiotactic polystyrene (s-PS) [7-9] and syndiotactic poly- p-methylstyrene (s-PPMS) [10] to present crystalline forms with a transplant conformation of the chains (shown for s-PS in Fig. 1) as well as crystalline forms with sequences of dihedral angles of the kind TTG+G+ (or the equivalent G G TT), corresponding to a s(2/l)2 helical symmetry of the chains (shown for s-PS in Fig. 1). 

An analogous case, of identical chain conformations as well as of similar unit cell dimensions, have been described for the two crystalline forms of poly-p-phenylene terephtalamide [33-36] (better known with the trade name of Kevlar). The projections along the c axis of the packing of the chains proposed for the two forms [36] has been sketched in Fig. 8, corresponding to the localization of the chain axes in (0,0, z) and (1/2,1/2, z) for the more common polymorph, in (0, 0, z) and (1/2,0, z) for the other polymorph. 

Poly(/)-phenylenctcrcphthalamiclc) forms a liquid crystalline solution and can be spun into a fiber with a very high orientation these fibers have excellent tensile and thermal properties. These high-modulus fibers are suitable as reinforcing materials in technical applications. 

In this section, we will investigate the surface structure of the electrode in the potential range before a surface or bulk oxide starts forming, and will restrict ourselves to the adsorption of atomic oxygen only (not OH ) [Jacob and Scheffler, 2007]. Furthermore, in our simulations, we assume a single-crystal Pt(lll) electrode, which will be compared with the experimental CV curve (Fig. 5.9) for poly crystalline Pt. This simplification is motivated by the fact that our interest here is to describe the general behavior of the system only. 

The mechanism of anodic oxidation of CO at polycrystalline Au remains uncertain. Several groups have reported that the voltammetry of Au in acidic electrolytes is straightforward, with a well-formed oxidation wave/peak [Stonehart, 1966 Gibbs et al., 1977 Kita et al., 1985 Sun et al., 1999]. There is, however, no voltammetric evidence for the adsorption of CO on the Au surface, and spectroscopic studies indicate only a weak interaction of CO with poly crystalline Au surfaces in acidic solutions [Kunimatsu et al., 1986 Cuesta et al., 2003]. Moreover, there is little evidence for the formation of oxidizing species at the potential where the oxidation process is observed. Certainly, the oxidation of CO occurs at a potential over 500 mV less positive than that where bulk Au oxide is formed, and, indeed, the formation of this oxide strongly... 

In addition to the crystal forms, X-ray scattering studies indicate that when unoriented PEN fiber was drawn at 120 °C ( 7 g), a mesophase is generated. In this form, the molecular chains are in registry with each other in the meridional direction but not fully crystallized in the equatorial direction. This conclusion was based on the presence of additional meridional peaks not accounted for by the crystal structure obtained by X-ray scattering. The mesophase is a intermediate phase and its existence is strongly dependent upon the processing conditions consequently, it could have implications with respect to the properties of commercially produced fibers and films, since it appears to be stable and not easily converted to the crystalline form, even at elevated temperature [25, 26], The mesophase structures of PET, PEN and poly(ethylene naphthalate bibenzoate) were compared by Carr et al. [27], The phase behavior of PEN and PEN blends with other polymers has also been studied [28-32],... 

The word polyallomer is derived from the Greek words alios, meros. and poly. Alios means oilier and denotes a differentiation from the normal. The word meros means parts and the prefix poly is added to show that these materials are polymeric. Since allomerism is defined as a constancy of crystalline form with a variation in chemical composition, the polyallomers are examples of allomerism in polymer chemistry. 

Hasegawa, R., Kobayashi, M. and Tadokoro, H. Molecular conformation and packing of poly(vinylidene fluoride). Stability of three crystalline forms and the effect of high pressure, Polymer J. 3, 591 (1972) Hasegawa, R., Takahashi, Y., Chatani, Y. and Tadokoro, H. Crystal structures of three crystalline forms of poly(vinylidene fluoride). Polymer J. 3, 600 (1972)... 

Poly crystalline silicon (poly-Si) has been formed by the plasma-enhanced decomposition of dichlorosilane in argon at temperatures above 625 °C, a frequency of 450 kHz, and a total pressure of 27 Pa. Doped films have been deposited by the addition of phosphine to the deposition atmospheres (213). Approximately 1 atom % of chlorine was found in the as-deposited films. Annealing in nitrogen at temperatures above 750 °C caused chlorine to difluse from the film surface, grain growth to occur, and the film resistivity to drop. Such heat treatments were necessary to achieve integrated-circuit-quality films. 

The ester models, as stated above, were all LC, but two of the ether model compounds, the methoxy- and ethoxy-substituted derivatives, were not, despite the fact that each of these mesogens yielded some liquid-crystallinity in the poly(ether) form. Therefore it seems that the polymers in this system tend to be "more liquid crystalline" than the related small molecules. This hypothesis is supported by the fact that Memeger(H) found liquid crystallinity in allhydrocarbon polymers incorporating the distyrylbenzene mesogen, even in cases where the cis/trans ratio of the unsaturations was as large as 0.3, while Campbell and McDonald (10) noted that iodine isomerization to the all-trans form was essential for the observation of an LC phase in the small-molecule derivatives which they prepared. 

Hlavay and Poly k (2005) investigated the surface properties of an iron(III) hydroxide-coated alumina sorbent. Amorphous iron(III) hydroxide was coated onto a mixture of amorphous and various crystalline forms of AI2O3. The iron concentration of the sorbent was 56.1 mmol g-1. Depending on pH, batch experiments indicated that the sorbent was effective with As(V) and to some extent As(III). In a pH 5.6 solution with an ionic strength of 0.1 mmol L-1 and containing 1 mmol L-1 (75 mg As(V) L-1) of As(V), a dosage of 2.5 g sorbent 100 ml-1 of solution sorbed 0.3 mmol As(V) g-1 sorbent (22 mg As(V) g-1 sorbent). In comparison, the sorption capacity of As(III) was only about 0.06 mmol g-1 (4.5 mg As(III) g-1 sorbent) under the same conditions (Hlavay and Poly k, 2005, 76). 

The (1120) faces, dominant on poly crystalline samples, are characterized by highly shielded fivefold Fe3+ ions (5.43 A apart) and partial homopolar character. The situation is similar to that encountered on prismatic (1120) faces of (y-Cr20, after sintering under severe conditions. When probed with CO, Fe3+ ions exposed on the (1100) surfaces form weak Fe3+ CO adducts, characterized by a very modest shift of the C-0 stretching mode relative to the gas phase value (v = 2164-2165.5 cm A v = 20-22 cm-1). It is thus concluded that the local electric fields associated with Fe3+ ions at the surface of (1100) faces are very weak and similar to those reported for Mg2+ on MgO (001) and Cr3+ on (y-( r202 (1120) surfaces. 

In poly crystalline semiconductor samples, the excited-state lifetime of electron-hole pairs is so short that photocurrent collection is efficient only for carriers created within the space charge (depletion) region. Thin-film processes offer an inexpensive way to prepare large solar arrays, but the semiconductors formed by such processes are almost inevitably polycrystalline. It is not wise to use semiconductor films thicker than the depletion width in such devices because the additional thickness contributes only extra grain barrier boundaries for the majority of carriers to surmount on their way to the back contact. The additional thickness does not provide any additional photocurrent. 

In the above analysis, we used the concept of space charge layer, to be more precise, a depletion layer that would form in a doped, wide-gap semiconductor contacting another phase (a metal, an electrolyte solution, or vacuum). The poly crystalline diamond/metal junctions (where metal is Au, Pt, Pd, etc.) often show rectifying properties [67, 68] and their capacitance characteristics resemble those of a diamond/electrolyte solution junction. 

Anti will be used to describe only torsional angles within a few degrees of 180°. Deviations from exact A conformations are known for several polymers, for example poly(tetrafluoroethylene), which has two crystalline forms best described as all-T see J. J. Weeks, E. S. Clark and R. K. Eby, Polymer, 22, 1480 (1981). 

---