Trans-crystalline structure

If growth occurs in contact with a nucleating surface then tran -crystalline structure is observed. The nucleating object will normally have a flat surface or a fibril. These surfaces become covered with a high density of nucleations resulting in a one-dimensional (columnar) growth in a direction parallel to the normal of the surface. The thickness of the /raAw-crystalline layers depends on the balance between surface nucleation and bulk nucleation. 

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. 

On the other hand, a remarkable difference between catalysis by Y and 13 zeolites has been found for the Claisen-Sohmidt condensation of acetophenone and benzaldehyde (Table 5). When the cross aldolic reaction is carried out in the presence of HY, together with the expected trans and ois chalcones 5, the 3,3-diphenylpropiophenone 6 is also formed, this product being not detected on 13 zeolites. A likely explanation for the absence of 6 using zeolite beta is that the crystalline structure of this zeolite exerte a spatial constraint making difficult the formation of a big size molecule like 6, especially in the smaller channel. Similar effects due steno limitations on 6 catalysis have been found for the formation of multi-branched products during the cracking of alkanes (ref 8). 

Fig. 11.7 Comparison of the photocatalytic activities of different Ti02 powders for producing trans-2,3-epoxyhexane and cis-2,3 -epoxyhexane from 2-hexene (trans/cis = 1.7). The reactions were performed by photoirradiation for 19 h in pure 2-hexene suspended with TiQ2 powder under a stream of oxygen. The crystal structures and surface areas are listed underneath the graph. The main crystalline structure of each TiD2 powder is denoted by A (anatase) and F (rutile). The ratio trans/cis epoxyhexane produced photocatalytically...

The crystalline structure of the stereocomplex has not yet been elucidated. Liquali et al.301) proposed a model for the stereocomplex (see Eq. (13)) involving two flat syndiotactic chains with the same number of trans- and gauche-conformations which may lie in the grooves of one isotactic 5rhelix. More recently, Tadokoro et al.302) found that iso-PMMA forms a lOj-helix. Further-... 

Later, the structure of the trimethylarsine-palladium bromide compound, [ (CH3)3As 2(PdBr2)2], was determined by x-ray crystal analysis (Mann and Wells, 1938) and found to possess the trans symmetric structure, XLIIIB, with a planar molecule 40), It should be added that each of the many bridged compounds prepared in these investigations appeared to be entirely homogeneous in the crystalline state. 

The dependence of the polymer microstructure on the ratio of catalyst components is related to the nature of these components. The structure of polybuta-diene obtained with an aluminum triisobutyl (AIBU3)-titanium tetrachloride catalyst system is a function of the Al/Ti molar ratio (Table II). Polybutadiene prepared at Al/Ti ratios of 0.5 to 8 in benzene or heptane and at 3° or 25° C. contain at least 90% 1,4- units. Polymerizations carried out at ratios of 1.0 and less at 25° C. in heptane and at ratios of 1.25 or less at 3° C. in heptane or benzene give crystalline polymers containing more than 96% trans-, A- structure (6). A similar temperature dependence of polymer structure has been reported in the polymerization of butadiene with a diethylcadmium-titanium tetrachloride catalyst system (3). Polybutadiene obtained with a triethylaluminum-titanium tetrachloride catalyst system at a 0.9 Al/Ti ratio at 30° C. in benzene is reported to contain 67% cis-1,4- units (19). 

The true conformation in the crystalline structure of a polymer is more complicated, the backbone chain often taking a helical shape in which alternate chain bonds take trans and gauche positions, such that the carbon chain is not in a plane. For example. 

Monodisperse diblock oligo (ethylene oxide) mono-n-alkyl ethers have also been prepared and studied.143 Oligomers with hydroxy-ended E-blocks formed bilayer crystals (cf. Section II.G and Figure 22), and the methoxy-ended oligomers formed monolayer crystals. The helical oxyethylene blocks were perpendicular to the layer plane, while the alkyl blocks were generally tilted at 30° to the layer normal. Monodisperse triblock oligomers with a central methylene block and outer oxyethylene blocks were also studied subsequently144 and both fully crystalline and partly crystalline structures were found, with chains in wholly trans-planar and mixed trans-planar/helical conformations. 

The crystalline structure of solvated MCI provides a basis for the assumption that replacement of a methyl group by a more bulky group at the N(T) atom in the indoline ring can hinder the solvation of the phenolate oxygen atom. To confirm this assumption, an X-ray diffraction analysis of the pentyl-substituted MC2 (Figure 6) was carried out.37,38 The pentyl substituent at the N(l ) atom is similar to the methyl group in terms of its electronic properties. The MC2 molecules in the crystals are not solvated. MC2 molecules (Figure 6) have an almost planar trans-cis structure, similar to that of MCI. 

BM) and are believed to possess a trans octahedral structure (49). The corresponding reactions of ReCl4(NCMe)2 with methanol and ethanol give the green crystalline complexes ReCl4[NH=C(Me)OR]2 (M = Me or Et) (50) in the case of the methanol reaction, Cs2[Re(0-Me)Cl5] (Section 43.6.1.9) can be isolated from the reaction filtrate upon addition of CsCl. 

Robinson C (1956) Liquid-crystalline structures in solutions of a polypeptide. Trans Faraday... 

ATR-IR spectroscopy at various angles of incidence was used to study ultradrawn films of PE. A gradient of molecular and crystalline structures was found from the surface into the core. The skin was highly orthorhombic. Moving inside the film, both monoclinic and orthorhombic lattices coexisted. Anew phase with isolated trans-planar sequences was observed in the core. Various kinds of collective and oriented or isolated conformational disorder were identified. A molecular model was proposed for the process of orientation during drawing (346). 

PVDF is well known for its polymorphism, with at least four phases found experimentally. The different phases of PVDF can be characterized by the conformations of the -CH2- and -CF2- in tiie polymer diain. The main phases are the all-trans p-phase, TGTG a-phase and T3GT3G y-phase [Du-07]. These phases exhibit different properties which are also reflected in foeir surface properties. It was shown that the phases have an influence on surface roughness and also surfoce energy [JEE-07]. While there has been a lot of research in the crystalline structures of PVDF in understanding the origination of its ferroelectric b vior, most of this research has been limited to the polymer as a film. 

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