Flexible chain molecules polymer-solvent interaction

This kind of perfect flexibility means that C3 may lie anywhere on the surface of the sphere. According to the model, it is not even excluded from Cj. This model of a perfectly flexible chain is not a realistic representation of an actual polymer molecule. The latter is subject to fixed bond angles and experiences some degree of hindrance to rotation around bonds. We shall consider the effect of these constraints, as well as the effect of solvent-polymer interactions, after we explore the properties of the perfectly flexible chain. Even in this revised model, we shall not correct for the volume excluded by the polymer chain itself. 

At the beginning of this section we enumerated four ways in which actual polymer molecules deviate from the model for perfectly flexible chains. The three sources of deviation which we have discussed so far all lead to the prediction of larger coil dimensions than would be the case for perfect flexibility. The fourth source of discrepancy, solvent interaction, can have either an expansion or a contraction effect on the coil dimensions. To see how this comes about, we consider enclosing the spherical domain occupied by the polymer molecule by a hypothetical boundary as indicated by the broken line in Fig. 1.9. Only a portion of this domain is actually occupied by chain segments, and the remaining sites are occupied by solvent molecules which we have assumed to be totally indifferent as far as coil dimensions are concerned. The region enclosed by this hypothetical boundary may be viewed as a solution, an we next consider the tendency of solvent molecules to cross in or out of the domain of the polymer molecule. 

Real polymer processes involved in polymer crystallization are those at the crystal-melt or crystal-solution interfaces and inevitably 3D in nature. Before attacking our final target, the simulation of polymer crystallization from the melt, we studied crystallization of a single chain in a vacuum adsorption and folding at the growth front. The polymer molecule we considered was the same as described above a completely flexible chain composed of 500 or 1000 CH2 beads. We consider crystallization in a vacuum or in an extremely poor solvent condition. Here we took the detailed interaction between the chain molecule and the substrate atoms through Eqs. 8-10. 

The HPAM molecule is a flexible chain structure sometimes known as a random coil in polymer chemistry. There is essentially no permanent secondary structure in polyacrylamide which affords it some degree of rigidity in the way that the helical structure acts in xanthan. Like xanthan, HPAM is a polyelectrolyte, and as such it will interact quite strongly with ions in solution. However, since the polyacrylamide chain is flexible, it may respond much more to the ionic strength of the aqueous solvent, and its solution properties are much more sensitive to salt/hardness than are those of xanthan. This is illustrated schematically in Figure 2.11, in which the effect of ionic strength on the hydrodynamic size of the molecule is shown. The effects of ions on the solution properties of polyacrylamide are discussed in more detail in Chapter 3. 

When both chains are flexible, they thus interact as hard spheres. This is not, however, always the case for polymers with a more general conformation. Two rods, or a rod and a flexible chain in a good solvent interpenetrate almost freely and are diaphanous. We discuss now an example of a marginal situation, a solution of rodlike molecules and flexible chains in a 0-solvent. 

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