Interaction forces, polymer chain segments

Forces of interaction between polymer chain segments... 

Because of the van der Waals interactions, the plates attract a larger number of polymer segments into the gap between the plates than when there are only electrostatic interactions with the chain segments. As a result, the interaction between plates, with the latter at constant electrical potential, becomes more attractive at distances larger than about 12 A than when the van der Waals interaction is neglected (Fig. 2). This behavior occurs because (p2 increases and hence the number of bridges increases. The force between plates at distances smaller than 12 A becomes, however, more repulsive when the van der Waals interaction is taken into account than when it is neglected. This increased repulsion can be attributed to the third term in Eq. (17), which provides a positive contribution to the force and becomes dominant at short distances. 

The interactions between solvent and polymer depend not only on the nature of the polymer and type of solvent but also on the temperature. Increasing temperature usually favors solvation of the macromolecule by the solvent (the coil expands further and a becomes larger), while with decreasing temperature the association of like species, i.e., between segments of the polymer chains and between solvent molecules, is preferred. In principle, for a given polymer there is a temperature for every solvent at which the two sets of forces (solvation and association) are equally strong this is designated the theta temperature. At this temperature the dissolved polymer exists in solution in the form of a nonexpanded coil, i.e., the exponent a has the value 0.5. This situation is found for numerous polymers e.g., the theta temperature is 34 °C for polystyrene in cyclohexane, and 14 °C for polyisobutylene in benzene. 

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