Poly crystalline structure

We note that such a transition from a circular to a quadratic envelope of the crystals has also been reproduced by computer simulations [13,14]. There, this transition is due to the reduction of the growth front nucleation probability. Wliile the disk-like pattern consists of multiple crystals, the square-shaped pattern represents a single crystal. We thus assume that, for the given film thickness, we observed a transition from a poly crystalline structure to a single crystal within the temperature interval from 45 to 50°C. 

Just as an example, the X-ray diffraction patterns of compression moulded samples of PVDF, poly(vinylfluoride), and of some VDF-VF copolymers of different compositions are shown in Fig. 17 [90]. The degrees of crystallinity of the copolymer samples (40-50%) are high and analogous to those of the homopolymer samples. This indicates a nearly perfect isomorphism between the VF and VDF monomeric units [90, 96], The diffraction patterns and the crystal structures of the copolymers are similar to those of PVF, which are in turn similar to the X-ray pattern and crystalline structure of the P form of PVDF. On the contrary, the X-ray pattern of a PVDF sample crystallized under the same conditions (Fig. 17 a) is completely different, that is typical of the non-piezoelectric a form [90]. 

PAs have also been copolymerized with other polymer systems and, in particular", with polyesters and poly ethers. In the copoly esteramides the crystallinity is decreased by copolymerization, as the crystalline structure of the amide unit is very different from the ester unit. However, alternating polyesteramides behave as homopolymers with a glass ttansition temperature and a melting temperature intermediate to the polyester and the PA polymer (Figs. 3.10 and 3.11).23,24 Polyesters, such as PBT and PET, modified with a small amount of diamide are also copolymers that have a high order.24,73... 

Poly crystalline boron nitride films, with a structure similar to rhombohedral boron carbide and a ratio of boron to nitrogen of 3 1, were produced by hot-filament CVD. This work indicates the possible existence of other boron-nitride structures. 

A square and triangular Pt nanoparticle obtained by using poly-NIPA and poly-NEA, respectively, was observed by high resolution TEM (HRTEM) (JEM-2010F). The images (Figure 4) show a crystalline structure with clearly resolved lattice fringes. The square Pt nanoparticle... 

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. 

CXRS entered the commercial market in the mid-to-late 1990s after a long development period. The basic principles have been known for many years, but practical development faced many hurdles. Most explosives have a crystalline structure. Because the crystals are small and randomly orientated, the structure is sometimes referred to as poly crystalline. These crystals exhibit a strong coherent scatter at certain angles that depend on the X-ray energy and the crystal lattice spacing. This coherent scatter (also called diffraction) is a property of the crystal lattice and is unrelated to... 

In the case of crystals, both intramolecular (conformational) and packing energies should be taken into account simultaneously. Such a total energy minimization method, with suitable crystallographic constraints, has been applied in different steps of the analysis of crystalline structures of three different synthetic polymers. Structures of these molecules, namely, isotactic trans-1,4-poly-penta-1,3-diene (ITPP), poly-pivalolactone (PPVL), and isotactic cis-1,4-poly(2-methyl-penta-1,3-diene)(PMPD), do not have troublesome features such as charged groups, counterions, and solvent molecules. 

Joshi S, Grigorian S, Pietsch U, Pingel P, Zen A, Neher D, Scherf U (2008) Thickness dependence of the crystalline structure and hole mobility in thin films of low molecular weight poly(3-hexylthiophene). Macromolecules 41 6800... 

Figure 1 A generalized diagram of the structure of the cetostearyl alcohol gel found in topical and vaginal creams. The bilayers are formed principally of cetostearyl alcohol. The hydrophilic poly(oxyethylene) chains attached to the 5-carbon sorbitan rings in Polysorbate 60 retard water drainage from the interlamellar space and keep the lamellae from collapsing into a dense crystalline structure.

Preliminary data on the crystalline structure of some poly-a-olefins obtained from optically active and racemic monomers were obtained by G. Natta, P. Corradini, I. W. Bassi (80) and are reported in Table 6. 

Pochan, J. M., Hinman, D. F. and Froix, M. F. Morphological studies on the viscous crystalline phase of poly(diethylsiloxane) including the dynamics of phase formation and the relationship of viscous crystalline structure and crystalline structure. Macromolecules 9, 611 (1976)... 

Crystallization from the melt often leads to a distinct (usually lamellar) structure, with a different periodicity from the melt. Crystallization from solution can lead to non-lamellar crystalline structures, although these may often be trapped non-equilibrium morphologies. In addition to the formation of extended or folded chains, crystallization may also lead to gross orientational changes of chains. For example, chain folding with stems parallel to the lamellar interface has been observed for block copolymers containing poly(ethylene), whilst tilted structures may be formed by other crystalline block copolymers. The kinetics of crystallization have been studied in some detail, and appear to be largely similar to the crystallization dynamics of homopolymers. 

Corundum is aluminum oxide, q -A1203, which has a hexagonal crystalline structure that is analogous to hematite. However, water treatment systems most often use activated alumina, which is typically produced by thermally dehydrating aluminum (oxy)(hydr)oxides to form amorphous, cubic (y), and/or other polymorphs of corundum (Clifford and Ghurye, 2002, 220 Hlavay and Poly k, 2005 Mohan and Pittman, 2007). When compared with corundum, amorphous alumina tends to have higher surface areas, greater numbers of sorption sites, and better sorption properties. 

As pointed out above with relation to the data at 87 °C, the Tic of the crystalline-amorphous interphase is appreciably longer than that of the amorphous phase, suggesting the retention of the helical molecular chain conformation in the interphase. We also note that a Tic of 65-70 s for the crystalline phase is significantly shorter than that for other crystalline polymers such as polyethylene and poly-(tetramethylene oxide), whose crystalline structure is comprised of planar zig-zag molecular-chain sequences. In the crystalline region composed of helical molecular chains, there may be a minor molecular motion in the TiC frame, with no influence on the crystalline molecular alignment that is detected by X-ray diffraction analyses. Such a relatively short TiC of the crystalline phase may be a character of the crystalline structure that is formed by helical molecular chain sequences. 

This is illustrated by the case of poly (tetramethylene terephthalate) (4GT). Three independent determinations have been made of the crystalline structure of the cx-phase of this material. (1,2,3). The conformation angles are given in Table I (see Figure 1 for key) from which it will be seen that they all... 

Abstract. Three independent determinations have been made of the crystalline structure of the a-phase of poly (tetra-methylene terephthalate). The data on which these determinations have been based are used to asses the contributions to the uncertainties of the structural parameters caused by errors in the unit cell parameters, structure factors, and bond parameters. The effects of differences in the model from which refinement is started are also assessed. The major contribution to uncertainty arises from errors in the structure factors (the "R-factor" between structure factor sets from two different laboratories can be greater than 20%) but errors in bond parameters also make a sizeable contribution. Hamilton s test indicates that one of the structure factor sets used in this study is less inaccurate than the other two and using this the best model satisfying all the other data is estimated together with the uncertainties in its parameters. 

A number of effects lead to loss of profile depth resolution. The effect of crystallite orientation on sputtering rate is shown in Fig. 4.20, in which 10 keV Kr+ ions at 50° incidence were used to sputter poly crystalline iron [76]. This ion etching may be useful to bring out grain structure but leads to loss of depth resolution... 

As in any electrode process, the potential applied to the electrode determines the reaction rate. In electrodeposition, we expect that it affects the rate of deposition and thence the structure of the deposit a low overpotential signifies more time available to form an electrodeposit of perfectly crystalline structure. This can be observed experimentally (Fig. 15.7). Another factor arises from differences in current density between different parts of the electrode owing to electrode shape, which affects mass transport and thus accessibility to the cations to be deposited. Generally, it is best to apply a potential corresponding to the formation of poly crystalline deposits. A more perfect crystalline structure would be desirable, but the low rate of electrodeposition does not compensate for using such low overpotentials. 

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