Coil, random molecular

As an example chosen in the macromolecular field the C NMR spectrum of syndiotactic polypropylene might be mentioned In solution (averaged random coil conformation, molecular model corresponding to 7) it presents three signals in the crystal state, where a chiral rigid conformation exists [(2/1)2 helix], it shows four signals (Figure 17). 

Salamat-Miller, N., et al. 2005. A randomly-coiled, high-molecular-weight polypeptide exhibits increased paracellular diffusion in vitro and in situ relative to the highly-ordered alpha-helix conformer. Pharm Res 22 245. 

The first is the usual random coil/random coil mixing where enthalpic effects dominate the thermodynamics. The second case is "molecular composites", i.e., mixtures of rigid rods and random coils. In these systems, entropic effects which are based on conformational differences control the mixing characteristics. Between these two extremes lie the systems presently under investigation. 

Since most polymer molecules have relatively flexible backbone, they tend to be highly coiled and can be represented as random coils. A molecular coil, being very loose, inhabits a region significantly larger than the actual volume of the coil. In fact, the actual macromolecule accounts for only a few percent at most of the region of the coil. It may thus appear that two polymer coils can easily interpenetrate and the excluded volume and its effects are very small. However, even as approximate calculations for a fictitious case in Problem 3.11 show, this impression is totally untrue. 

The unperturbed dimensions of the high polymers depend very strongly on the relative populations of the two rotational isomers at the interflavan bond. If one rotational isomer in a homopolymer were populated to the exclusion of the other, the local conformation would describe a helix and the overall shape would be that of a rod. Significant population of the second rotational isomer converts the rod to a random coil. The molecular mechanics calculations do not provide the relative energies of the two rotational isomers with the accuracy required for description of the unperturbed dimensions. 

The interpretation of the experimental results can be accomplished similarly to Willhite and Dominguez (15,23) in terms of structure of the solutions and random molecular coils. Results from dynamic tests for a number of polymers a relation was found between the adsorbed amount and the viscosity, specifically, the... 

The viscosity of a polymer solution is one of its most distinctive properties. Only a minimum amount of research is needed to establish the fact that [77] increases with M for those polymers which interact with the solvent to form a random coil in solution. In the next section we shall consider the theoretical foundations for the molecular weight dependence of [77], but for now we approach this topic from a purely empirical point of view. 

In earlier chapters an unperturbed coil referred to molecular dimensions as predicted by random flight statistics. We saw in the last chapter that this thermodynamic criterion is met under 0 conditions. 

With appropriate caUbration the complex characteristic impedance at each resonance frequency can be calculated and related to the complex shear modulus, G, of the solution. Extrapolations to 2ero concentration yield the intrinsic storage and loss moduH [G ] and [G"], respectively, which are molecular properties. In the viscosity range of 0.5-50 mPa-s, the instmment provides valuable experimental data on dilute solutions of random coil (291), branched (292), and rod-like (293) polymers. The upper limit for shearing frequency for the MLR is 800 H2. High frequency (20 to 500 K H2) viscoelastic properties can be measured with another instmment, the high frequency torsional rod apparatus (HFTRA) (294). 

When polymer melts are deformed, polymer molecules not only slide past each other, but they also tend to uncoil—or at least they are deformed from their random coiled-up configuration. On release of the deforming stresses these molecules tend to revert to random coiled-up forms. Since molecular entanglements cause the molecules to act in a co-operative manner some recovery of shape corresponding to the re-coiling occurs. In phenomenological terms we say that the melt shows elasticity. 

A characteristic feature of thermoplastics shaped by melt processing operations is that on cooling after shaping many molecules become frozen in an oriented conformation. Such a conformation is unnatural to the polymer molecule, which continually strives to take up a randomly coiled state. If the molecules were unfrozen a stress would be required to maintain their oriented conformation. Another way of looking at this is to consider that there is a frozen-in stress corresponding to a frozen-in strain due to molecular orientation. 

Traditional rubbers are shaped in a manner akin to that of common thermoplastics. Subsequent to the shaping operations chemical reactions are brought about that lead to the formation of a polymeric network structure. Whilst the polymer molecular segments between the network junction points are mobile and can thus deform considerably, on application of a stress irreversible flow is prevented by the network structure and on release of the stress the molecules return to a random coiled configuration with no net change in the mean position of the Junction points. The polymer is thus rubbery. With all the major rubbers the... 

Step I. The time dependent structure of the interface is determined. Relevant properties may be characterized by a general function H(t), which for the ca.se of polymer melts can usually be described in terms of the static and dynamic properties of the polymer chains. For example, with symmetric (A = B) amorphous melt interfaces, H(t) describes the average molecular properties developed at the interface by the interdiffusion of random coil chains as [ 1,6J... 

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