Chain configurations, localized

Structures [VIII] and [IX] are not equivalent they would not superimpose if the extended chains were overlaid. The difference has to do with the stereochemical configuration at the asymmetric carbon atom. Note that the asymmetry is more accurately described as pseudoasymmetry, since two sections of chain are bonded to these centers. Except near chain ends, which we ignore for high polymers, these chains provide local symmetry in the neighborhood of the carbon under consideration. The designations D and L or R and S are used to distinguish these structures, even though true asymmetry is absent. 

Fig. 9.6. In plastics of the polyamide type, the skeletal chain acquires local dipole moments at the points where the (NH)-(CO) configuration occurs.

Fig. 48. EPR spectra in Yj j Qdo jBajCujOs j,. (a) Evolution of the 1 ) <-> 1 f) transition as a function of the oxygen content x. Spectra were taken at 9.1 GHz in the temperature range 77-92 K. Lines A, B, D, F and G correspond to well-detemiined CuO chain configurations - i> - 2. nd K, respectively, (b) Concentration dependence of the relative amounts of the local CuO chsun configurations. Open square, open triangle, asterisk, solid triangle, and solid square, represent the 0, 1, 2, 3 and 4 fold occupied configurations, respectively. The calculated intensities are represented by the solid curves (from Pekker et al. 1991, Rockenbauer et al. 1992).

Statistical treatments with such a local set of conframational probabilities then enables the calculation of a chain configurational partition function, or an average vector property for a chain of any length. This is most conveniently done by matrix... 

The measurement of the dipole moments of copolymers and its analysis in terms of both sequence distribution and local chain configurations has received attention Modern computer aided analytical procedures provide in ght into the dependence of mean square dipole moment per residue on reactivity ratios, polymer composition and rotamer probabilities. One such calculation for atactic cc ly-(p-chlorostyrene-p-methylstyrene) has shovm that at constant composition, the dipole moment is quite sensitive to the sequence distribution and thus to the reactivity ratios. This dependence would be even more marked for syndiotactic chains. For cop61y(propylene-vinyl chloride) and copoly(ethylene-vinyl chloride) d le moments are again very sequence dependent, much more so than the diaracteristk ratio. It would appear that in copolymer systems dielectric measurements can be a powerful method of investigating sequence distributions. Two copolymers, p-dilcxo-styrene with styrene and with p-methylstyrene have been examined experimentally The meamrements were made on solid amorphous samples above the ass-rubber transition temperature (Tg) and they are consistent with the predictions of the rotational isomeric state model udi known reactivity ratios and rea nable replication probabilities . However, it is the view of this author that deduc-... 

A somewhat delicate question is how often one performs the identity switch (Fig. 7.5(b)) relative to the local moves (Fig. 7.5(a)) needed to equilibrate the chain configurations. For asymmetric compositions (0 < ), which occur in the one phase region in thermal equilibrium choosing A/i > 0, the mean square gyration radii gs) the two types of... 

Simulating mesophase formation in block copolymers, one encounters similar problems as for the simulation of macroscopic phase separation in polymer blends. Again the need arises to equilibrate both the local structure of the chain configurations and the long wavelength collective properties. 

Experimental study of the mechanism of folding depends on development of methods which can detect local, partially ordered structures This, in turn, leads to the regime of statistical approaches, searching for distributions of chain configurations and dynamics of segmental motions At this point, concepts and methodologies used by polymer chemists are useful for the protein chemists and vice versa ... 

Fig. 4 The upper panel describes schematically the chain configuration of an AB-type diblock copolymer, in which Da and Db denote the domain widths of block A and block B, respectively, with D being the periodicity distance, and the lower panel describes schematictilly the concentration profiles, in which (pA and (ps denote the local concentration profiles for block A and block B,...

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