Random coil-like configuration

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. 

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. 

Based on properties in solution such as intrinsic viscosity and sedimentation and diffusion rates, conclusions can be drawn concerning the polymer configuration. Like most of the synthetic polymers, such as polystyrene, cellulose in solution belongs to a group of linear, randomly coiling polymers. This means that the molecules have no preferred structure in solution in contrast to amylose and some protein molecules which can adopt helical conformations. Cellulose differs distinctly from synthetic polymers and from lignin in some of its polymer properties. Typical of its solutions are the comparatively high viscosities and low sedimentation and diffusion coefficients (Tables 3-2 and 3-3). 

The complexity and diversity of structures in the native proteins eluded any attempt to produce some simple conformation that accounted for their interfacial properties. The study of synthetic polypeptides with non-polar side chains has provided good evidence to support the view that the a-helix can be stable at the air-water interface (5), and it is therefore possible that the interfacial denaturation of proteins is mainly a loss of the tertiary structure (6, 7, 8). Since for a typical protein an a-helix takes up about the same area per residue as the p conformation, it can be accommodated as easily. Moreover, like the p conformation but unlike a more randomly coiled structure, it is linear and therefore compatible with a plane surface without loss of configurational entropy (5). In this respect a plane surface may favor an ordered over a more random structure. The loss of solubility of the spread protein can then be attributed to intermolecular association between hydrophobic side chains exposed as a result of the action of the interface on the polar exterior of the molecules. 

The assumption that the contraction process is ideally adiabatic, while perhaps not entirely permissible practically, seems indicated by modern theory of the behavior of molecular chains, which pictures these as undergoing, when freed of restraints, a sort of segmental diffusion, much like the adiabatic expansion of an ideal gas into a vacuum (155). In the case of the molecular chain, it diffuses to the most probable, randomly coiled configuration, which is much less asymmetric, hence shorter, than an initially extended chain. Because rubber most nearly presents this ideal behavior, those fibers which develop increased tension (a measure of the tendency toward assumption of the contracted form) when held isometrically under conditions of increasing temperature (favoring the diffusion ) are said to be rubber-like. Most normal elastic solids upon stress are strained from some stable structure and relax as the temperature is raised. 

The simplest model of rubber-like behaviour is the phantom network model. The term phantom is used to emphasize that the configurations available to each strand are assumed to depend on the positions of the junctions only. Consequently, the configurations of one chain are independent of the configurations of neighbouring strands. For many purposes, the strands can be treated as Gaussian random coils. Even in this simplest case, an exact solution is not a trivial task as will be outlined in Sect. 3. 

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