Coil-Like Polymer Conformations

Polymers that have a relatively weak interaction between their segments assume a coiled, flexible, and more or less open structure in solution. The chains of such polymers are in continuous motion. At any instant, each individual molecule of a dissolved polymer population has a different shape and, likewise, the shape of each polymer molecule varies in time. It is therefore meaningless to consider the actual shape of the polymer molecule but the conformational state should rather be discussed in terms of the average shape. 

If the interactions between the polymer segments, between a polymer segment and a solvent molecule, and between solvent molecules are equally favorable, all rotational states around the covalent bonds in the polymer chain are equally probable. Assuming that the polymer has no volume, the conformation of the polymer molecule 

FIGURE 12.5 Random walk conformation of a polymer molecnle with chain elements of persistence length 

Equations 12.8 and 12.9 show that the size of the coil varies proportionally with Afp It follows that the molar volume Vp scales with Vp and, hence, the average segmental 

It is stressed again that since is an average quantity, the coil is spherical on average. However, each individnal polymer molecule has a nonspherical asymmetrical shape. Furthermore, the monomer distribntion in the statistical coil is Ganssian that is, the density of the polymer segments decreases outwards from the center. It can be proven that for a random walk conformation, the segmental density distribution Pp(r) is given by 

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The geometric properties of highly denatured states appear to be consistent with those expected for a random-coil polymer. For example, proteins unfolded at high temperatures or in high concentrations of denaturant invariably produce Kratky scattering profiles exhibiting the monotonic increase indicative of an expanded, coil-like conformation (Fig. 1) (Hagihara et al., 1998 see also Doniach et al., 1995). Consistent... 

Values of parameter a in the Mark-Kuhn-Hauvink equation, close to 0.5, which were obtained for silarylenecarboorganosiloxane fractions, allow correspondence of the polymer molecules to the coil- like type and toluene at 20 and 25°C to 0-solvents. As a consequence, extrapolation data of the Mark-Kuhn-Hauvink equation, silarylenecarboorganosiloxane molecules display the coil-like conformation, which is slightly disturbed in toluene at 20 and 25 °C by interaction with the solvent. 

L-Fonn hydriodide, average dp (or n) = 32. Transparent, solid, film-like polymer. Readily sol in water practically insol in the usual organic solvents. Transition of high mol -wt poly-L-lysine (dp 1500) in aq soln from a helical to a randomly coiled conformation under the influence of decreasing pH or increasing temp Applequist, Doty, C.A. 58, 6925b (1963). 

The differences between the spectra recorded in benzyl alcohol and m-cresol must be attributed to different polymer chain conformations in both solvents [66]. In the phenolic type of solvents, polyemeraldine protonated with CSA has an expanded coil-like conformation which facilitates interactions between adjacent polarons. As a result, the absorption observed in benzyl alcohol at around 800 nm, which is due to isolated polarons, is replaced by the intraband transitions within the half-filled polaron band (free carrier tail). 

When counterions are removed away from the doped polyaniline chain, the static repulsive interaction of the positive charges on the polymer backbone tends to extend this polymer chain from a more coil-like to a more expanded conformation. We think that solvents such as m-cresol probably work in this way. We note that these solvents are all phenols. The network of hydrogen bonds formed between phenol groups seems to provide an effective proton exchange medium through which the counterions can be removed from the polymer chain. 

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