Random coil configurations

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... 

The crossover 2d 2d behavior can be described in a similar manner to the case of a tube confinement. For the chain, trapped between two parallel plates a distance D apart, one again has N/g blobs but they arrange to a two-dimensional random coil configuration ... 

In the unstressed state the molecules of an elastomer adopt a more-or-less randomly coiled configuration. When the elastomer is subjected to stress the bulk material experiences a significant deformation, as the macromolecules adopt an extended configuration. When the stress is removed, the molecules revert to their equilibrium configurations, as before, and the material returns to its undeformed dimensions. 

Typical materials that exhibit liquid crystalline behaviour are made up of long, thin molecules. Hence in principle polymers ought to show the basic requirement for liquid crystal behaviour. Conventional polymers, however, are too flexible and tend to adopt random coil configurations in the melt. They are thus not sufficiently anisotropic to exhibit a mesophase. 

Nevertheless, the values for different systems derived from measurements carried out in several laboratories are strikingly consistent, and they lend support to the view that the value of is the same in different systems, provided of course that the molecules conform to a random coil configuration. The best value for appears at present to be 2.1 ( 0.2) X10, r being expressed in cm., M in units of molecular weight, and [rj] in deciliters per gram. 

Size exclusion chromatography (which is also known as gel permeation chromatography) is based on the premise that a polymer molecule in solution adopts a random coil configuration, which encompasses a volume (known as its hydrodynamic volume) that is proportional to its molecular weight. We fractionate polymers according to their hydrodynamic volumes to generate a molecular weight distribution plot. 

V, is the molar volume of polymer or solvent, as appropriate, and the concentration is in mass per unit volume. It can be seen from Equation (2.42) that the interaction term changes with the square of the polymer concentration but more importantly for our discussion is the implications of the value of x- When x = 0.5 we are left with the van t Hoff expression which describes the osmotic pressure of an ideal polymer solution. A sol vent/temperature condition that yields this result is known as the 0-condition. For example, the 0-temperature for poly(styrene) in cyclohexane is 311.5 K. At this temperature, the poly(styrene) molecule is at its closest to a random coil configuration because its conformation is unperturbed by specific solvent effects. If x is greater than 0.5 we have a poor solvent for our polymer and the coil will collapse. At x values less than 0.5 we have the polymer in a good solvent and the conformation will be expanded in order to pack as many solvent molecules around each chain segment as possible. A 0-condition is often used when determining the molecular weight of a polymer by measurement of the concentration dependence of viscosity, for example, but solution polymers are invariably used in better than 0-conditions. 

Despite its plausibility and the apparent absence of driving forces to produce other large scale arrangements, the random coil configuration has been questioned on various grounds. It seems worthwhile to review the evidence at this point, since most molecular theories for amorphous polymers are based on the random coil picture. 

For polystyrene fractions in diethyl phthalate solution (30000average value of 1.6 x 10 18 ( 50%). In dilute solution e/36M is 1.27 x 10 18 for polystyrene (21). No systematic variations with concentration, molecular weight or temperature were apparent, the scatter of the data being mainly attributable to the experimental difficulties of the diffusion measurements. The value of Drj/cRT for an undiluted tagged fraction of polyfn-butyl acrylate) m pure polymer was found to be 2.8 x 10 18. The value of dilute solution data for other acrylate polymers (34). Thus, transport behavior, like the scattering experiments, supports random coil configuration in concentrated systems, with perhaps some small expansion beyond 6-dimensions. 

More quantitative chemical evidence for random coil configuration comes from cyclization equilibria in chain molecules (49). According to the random coil model there must be a very definite relationship among the concentrations of x-mer rings in an equilibrated system, since the cyclization equilibrium constant Kx should depend on configurational entropy and therefore on equilibrium chain and ring dimensions. Values of /Af deduced from experimental values on Kx for polydimethylsiloxane, both in bulk and in concentrated solution, agree very well with unperturbed dimensions deduced from dilute solution measurements(49). 

An isolated linear macromolecule generally tends to assume a random coil configuration. Only for very stiff polymers a rod like configuration is assumed. Several types of measurements can be used to determine the dimensions of the random coil configuration. Conversely, if the appropriate relationships have been established, the same measurements can be used to determine the average molar mass of a given polymer. 

Molecular weights are weight averages obtained from intrinsic viscosities of solutions in solvents in which PBLG adopts random coiled configurations ... 

In the polymers of Roviello and Sirigu discussed above, the chain direction was perpendicular to the layers, yet the fully extended repeat unit of the polymer with mesogenic structure 2 had a length of approximately 29 A while a spacing of only 20.6 A was observed These examples indicate that the spacer may not always be completely extended in the liquid crystalline state, but the spacer is certainly much more extended than if it were in a random coil configuration. 

The large size of the hyaluronic acid molecules and their random-coil configuration lead to molecular interactions, even in dilute solution. As a result of these interactions, solutions of the polymer exhibit non-Newtonian and elastoviscosity (Bll). Synovial fiuid shows an increase in viscosity with reduced shearing force and possesses structural rigidity which is reversibly broken down by shearing. Such viscosity behavior makes synovial fluid an ideal lubricant between joint surfaces, which move slowly under considerable pressure for most of the time but which may be required to accelerate violently (03). 

Examination of the random coil configuration of polypeptide copolymers has also led to very interesting results. The study of copoly-... 

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