Direct Determination of Molecular Dimensions

Small-angle neutron scattering has also been applied to the analysis of networks that were relaxing after a suddenly applied constant uniaxial deformation (Boue et al., 1991). Results of dynamic neutron scattering measurements of Allen et al. (1972, 1973, 1971) indicate that segments of network chains diffuse around in a network, and the activation energies of these motions are smaller than those obtained for the center of mass motion of the whole chains. Measurements by Ewen and Richter (1987) and Oeser et al. (1988) on PDMS networks with labeled junctions show that the fluctuations of junctions are substantial and equate approximately to those of a phantom network model. Their results also indicated that the motions of the junctions are diffusive and 

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VII. Enthalpic and Entropic Contributions to Rubber Elasticity Force-Temperature Relations Vtn. Direct Determination of Molecular Dimensions IX. Single-Molecule Elasticity References... 

The advance of synthetic techniques in the last decade made possible the synthesis of model branched polymers of known and controlled structure and, equally important, relatively homogeneous in molecular weight and structure (204, 221, 228). Investigations (101, 136, 141, 196) (Fig. 9) of a variety of branched polymers of different branched chain configurations but of the same chemical stmctiure and total molecular weight permits direct determination of the effect of coil dimensions and configurations on rj. 

Determination of protein shape by photography with the electron microscope is of course, when it is feasible, the most direct method of determining the molecular dimensions. It has yielded vital information for some of the virus proteins and for certain other very large protein molecules. The evidence from electron microscope studies, however, cannot always be taken as final, for a preliminary drying of the protein preparation is essential before the photograph can be made, and the... 

The possibility of determining Avogadro s number from studies of the diffusion of colloidal particles was of great scientific importance, since it represented a breakthrough in Ostwald s restriction on the direct observation of molecular motion. In contrast, here NA is determined by the observation of the thermal motion of particles whose size significantly exceeds molecular dimensions... 

The factors that control separation and dispersion are quite different. The relative separation of two solutes is solely dependent on the nature and magnitude of the Interactions between each solute and the two phases. Thus, the relative movement of each solute band would appear to be Independent of column dimensions or particle geometry and be determined only by the choice of the stationary phase and the mobile phase. However, there is a caveat to this statement. It assumes that any exclusion properties of the stationary phase are not included in the term particle geometry. The pore size of the packing material can control retention directly and exclusively, as in exclusion chromatography or, indirectly, by controlling the access of the solute to the stationary phase in normal and reverse phase chromatography. As all stationary phases based on silica gel exhibit some exclusion properties, the ideal situation where the selective retention of two solutes is solely controlled by phase interactions is rarely met in practice. If the molecular size of the solutes differ, then the exclusion properties of the silica gel will always play some part in solute retention. 

Chain-type in crystals of linear polymers. In drawn fibres of these substances the molecules are approximately parallel to the fibre axis, and the unit cell dimension along the fibre axis is also the identity-period of the molecule itself. The fact that it is possible so simply to determine intra-molecular distances has far-reaching consequences. The magnitude of this identity-period may lead directly to a knowledge of... 

For a number of linear polymers, values of E for single crystals have been determined. This has been performed by direct measurements or by calculation from molecular dimensions and force constants for bond stretching and valence angle deformation (Treloar, 1960 Frensdorff, 1964 Sakurada and co-workers, 1962, 1970 Frank, 1970). 

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