Chain long range

Other Thermoplastics. Polyoxymethylene (POM) was imaged by afm, revealing oriented polymer chains parallel with the machine axis of sample extrusion (Fig. 15). Atomic scale resolution of the chains demonstrated the helical nature of the polymer chains. Long-range correlation between poljuner chains was observed as well (97). Imaging of extended chain crystals of POM closely matched molecular models for this material, allowing for the molecular packing and order in the extended chain crystal to be well understood. The authors were able to describe the polymer chain orientations with respect to the crystal (98). 

Cu(py)2Cl2 single crystal 80-0.1 monoclinic, Z=2 C.S. 3 linear, S=i, Heisenberg chain long range 74A19,... 

The ionic association induced by the addition of potassium iodide to poly(ethylene oxide) in methanol produces large changes in the nuclear spin-lattice relaxation (Tj) of the polymer protons. These changes can be described quantitatively in terms of free and charged units along the polymer chain. Long-range interactions, as evidenced in... 

CS. The true two-dimensional crystal with chains oriented vertically exists at low T and high ir in the CS phase. This structure exhibits long-range translational order. 

FIGURE 12.39 The proposed secondary structure for E. coli 16S rRNA, based on comparative sequence analysis in which the folding pattern is assumed to be conserved across different species. The molecule can be subdivided into four domains—I, II, III, and IV—on the basis of contiguous stretches of the chain that are closed by long-range base-pairing interactions. I, the 5 -domain, includes nucleotides 27 through 556. II, the central domain, runs from nucleotide 564 to 912. Two domains comprise the 3 -end of the molecule. Ill, the major one, comprises nucleotides 923 to 1391. IV, the 3 -terminal domain, covers residues 1392 to 1541. 

Monte Carlo simulations [17, 18], the valence bond approach [19, 20], and g-ology [21-24] indicate that the Peierls instability in half-filled chains survives the presence of electron-electron interactions (at least, for some range of interaction parameters). This holds for a variety of different models, such as the Peierls-Hubbard model with the onsite Coulomb repulsion, or the Pariser-Parr-Pople model, where also long-range Coulomb interactions are taken into account ]2]. As the dimerization persists in the presence of electron-electron interactions, also the soliton concept survives. An important difference with the SSH model is that neu-... 

Within the class of polymer crystals having, ideally, long-range positional order for all the atoms, different crystalline forms (polymorphs) may arise as a result of having different almost isoenergetic macromolecular conformations (of the main chain, in most known cases) or as a result of different, almost isoenergetic modes of packing of macromolecules with identical conformations [1-3]. 

Long-range positional order in three dimensional is maintained only for structural features which are not point-centered (e.g., for the chain axes, for which two periodicities only are sufficient to define a three-dimensional repetition) iii) Long-range positional order of some feature is maintained only in two or in one dimension (e.g., only along each chain axis). 

As an example of the second case, we may have conformationally disordered chains, but long-range order in the positions of the chain axes (condis crystals [5]). Fiber spectrum features are the occurrence of sharp reflections on the equator only and diffuse reflections on the other layer lines. 

A conformationally disordered mesomorphic form is present, for instance, in the high-temperature phase I of PTFE. In this form, a long-range 3-D order is present only in the periodic pseudohexagonal placement of the chain axes [49]. In fact intramolecular helix reversals would produce the conformational disorder [50-52] and a complete intermolecular rotational disorder would be also present [49, 52,53]. 

Differently from the cases of ETFE, PTFE, 1,4-trans-polybutadiene, and PE, in the mesomorphic form of i-PP it is believed that there is a nearly complete conformational order, in the chains packed with parallel axes, (intrachain long range 3-D order) and only short-range lateral order in the positioning of the chain axes [60, 61]. 

Theta temperature is one of the most important thermodynamic parameters of polymer solutions. At theta temperature, the long-range interactions vanish, segmental interactions become more effective and the polymer chains assume their unperturbed dimensions. It can be determined by light scattering and osmotic pressure measurements. These techniques are based on the fact that the second virial coefficient, A2, becomes zero at the theta conditions. 

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