Poly , crystallographic data

Wegner, however, established that radiation-induced solid-state polymerization of BCMO leads to a polymer morphology, which is incompatible with the so-called topochemical polymerization, i.e., a process in which monomer molecules are transformed into polymer without destruction of the crystal lattice 36). Electron microscopy, X-ray analysis and electron diffraction studies, have shown that polymerization starts at the edges and imperfections of the monomer crystals and that amorphous polymer is formed initially. Further transition from the amorphous state leads to the thermodynamically unstable monoclinic p-form. Density measurements indicate that the polymer is only 45-50% crystalline. The density of the amorphous poly-BCMO is 1.368 g/cm3 the density calculated for the crystalline polymer from crystallographic data of the p-form is 1.456 g/cm3. The density of the product of the radiation-induced solid-state polymerization is 1.41 g/cm3 36). 

Table 6.2 (27) summarizes the crystallographic data of some important polymers. Several polymers have more than one crystallographic form. Poly-tetralluoroethylene (Teflon ), for example, undergoes a first-order crystal-crystal transition at 19°C. 

Isotactic poly(butene-l), PBl, may exist in at least three distinct crystal modifications. The crystallographic data of PBl are reported in Table 4. The different modifications are commonly referred to as I, II, and III. 

B. Crystallographic Data and Modifications I. Mechanical Properties of Poly(propylene) ... 

VI/22 CRYSTALLOGRAPHIC DATA AND MELTING POINTS FOR VARIOUS POLYMERS TABLE 4. POLY(DIENES) AND POLY(DIYNES)... 

The drop of the voltammetric crurent is associated with Pt surface oxidation, and the drop on the negative-going mn is due to Reaction (12.9) (surface poisoning by CO) and the Tafehan kinetics of Reaction (12.8). Further, the shift between curves in Fig. 12.13a and b indicates that in the potential range between 0.5 and 0.6 V, methanol oxidation occms with zero or low level atop CO smface intermediate. The amplitudes on Fig. 12.13 on both scans nearly equal to each other indicate a high level of preferential (111) crystallographic orientation of the poly crystalline Pt surface used for this work, as inferred from data in [Adzic et al., 1982]. 

Up to March 2006 the molecular structures of about 40 alkylidyne complexes ligated by poly(pyrazolyl)borates had been reported, and structural data for the crystallographically characterised alkylidyne-metal complexes are compiled in Table II. Of these, Tp- and Tp -ligated complexes represent more than 80%, with one third of the total known structures being bi- and polymetallic complexes. Structural features of alkylidyne complexes in general have been discussed elsewhere and, accordingly, an attempt to restrict this discussion to features which are unique or characteristic of the scorpionate alkylidyne complexes has been made. 

Figure 25.14 Adapted from crystallographic coordinates deposited with the Proten Data Bank PDB ID 2SK, Fossey, S.A., Nemethy, G. Gibson, K.D., Scheraga, H.A. Conformational Energy Studies of Beta-Sheets of Model Silk Fibroin Peptides. I. Sheets of Poly (Ala-Gly) Chains. Biopolymers 31, 1529 (1991).

The periodic arrangement of polymer chains in crystallites of semicrystalline polymers has been in the focus of early AFM investigations, in which known, but also unknown, crystal stmctures have been reported. Due to the local character of the experiment, ensemble averaging is avoided, unlike in conventional X-ray crystallographic methods. It is, however, important to note that most data published in the literature is contact mode AFM data, which represent, in most cases, lattice resolution only. One prominent example is the structure of poly(oxy methylene) unveiled by Snetivy and Vancso (Fig. 6.13) [24]. The information on the periodic arrangement of molecules is hence averaged over the length scale of the tip-sample contact. Consequently, point defects and other deviations in the periodic structure cannot be resolved. 

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