Entropy restrictions, conformational

The principle of depletion is illustrated in Figure 1. If a surface is in contact with a polymer solution of volume fraction , there is a depletion zone near the surface where the segment concentration is lower than in the bulk of the solution due to conformational entropy restrictions that are, for nonadsorbing polymers, not compensated by an adsorption energy. The effective thickness of the depletion layer is A. Below we will give a more precise definition for A. 

At first thought, it might be considered that the steric restrictions posed by the large fluorine atoms would cause conformational rigidity due to restricted bond rotations. Hsu, however, showed that the conformations of the TFE chains in the crystalline regions in Nafion are in fact dynamic in that they can undergo helix reversals that is, the handedness of the helix is easily reversed.These helix reversals are also seen in PTFE.2 o This disorder phenomenon causes considerable conformational entropy and is... 

The critical block sizes needed for domain formation are greater than those needed for phase separation in physical mixtures of the corresponding homopolymers. This is because the conformational entropy of parts of molecules in the block domains is not as high as in mixtures, since placement of segments is restricted by the unlike components to which they are linked. Thus the minimum molecular weights of polystyrene and c/i-polybutadiene for domain formation in AB block copolymers of these species are about 5,000 and 40,000, respectively [29]. 

Numerical results for a 10% solution of polymer [N - 1000) in Its own monomer are given in fig. 5.15 (solid curves). It is clear that long chains try to avoid the surface region because of the incurred loss of conformational entropy. The available space Is occupied by the monomer which does not suffer from these entroplcal restrictions. Hence, the polymer is depleted. It can be shown that at low (p (below coil overlap) the thickness of the depletion zone is proportional to Vw, see sec. 5.3e. At higher (as in fig. 5.15), the osmotic pressure pushes the chains closer to the surface, making the depletion layer thinner and its thickness more weakly dependent on chain length. 

The apparent contributions of different functional groups and/or bond types to overall binding energies derived from the various studies reviewed above, are summarized in Table 20.1. Also included are corresponding values used or suggested for the overall loss of rotational and translational entropy, TAS, and the loss of conformational entropy resulting from restriction of free rotation, E. ... 

Conformation of macromolecules in solid state and in solution exhibits both many contingencies and restrictions. Coiled conformation of macromolecules is most appropriate for majority of methods employed for molecular characterization of polymers. Statistical coils of macromolecules in equilibrium exhibit large conformational entropy. Any external intervention that leads to a change in the conformation of macromolecules has to surmoimt considerable resistance, which is cormected with the loss of overall conformational entropy. 

The surfactant properties of macromolecules (and hence their ability to self-assemble into functionally interesting motifs) can be controlled with high sensitivity by manipulating molecular architecture without changing the chemical identity of the amphiphilic moieties. In addition to differences in surfactant properties between macromolecules in different architectural classes, subtle variations in an architectural class also lead to significant effects. This is due to the importance of conformational entropy for self-assembly processes of polymers. This notion of choosing the nature of the connections between the amphiphilic moieties to control surfactant properties may prove useful in applications where the choice of chemical structure is restricted (e.g., for concerns related to biocompatibility or toxicity). 

Linear chains experience a decrease in conformational entropy when they are restricted to those conformations that bring the two ends into contact [30]. If Aa = 1 and Ab is sufficiently large so that Gaussian statistics apply, and excluded volume is ignored, the loop entropy is given by... 

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