Viscoelastic behavior segmental relaxation

The experimental observation that viscoelastic behavior can be shifted and superimposed, at least to a first approximation, means that all the relaxation processes involved have the same (or nearly the same) temperature dependence. Because the relaxation processes that occur in polymers involve a whole range of length scales, by which we mean they involve coupled motions that range from local rearrangements of just a few chain segments to motions of the chain as a whole, this implies that frictional forces encountered by a chain act in the same or a very similar manner on all the segments. This is important if you intend to be in the business of developing theories of polymer dynamics, but concerns us no further here. 

As an example of assignments of mechanisms for a specific polymer, the temperature-frequency loci shown in Fig. 15-11 for polystyrene have been attributed to the following motions " a, segmental motions of the main chain governed by the monomeric friction coefficient (i.e., the transition zone of viscoelastic behavior) j(3, local mode torsional oscillations of the main chain y, rotation of the phenyl group around the bond joining it to the main chain 5 (observed in dielectric measurements only, and nearly absent in isotactic polymer), relaxations of resultant... 

Polycarbonate s high toughness is based on its relaxation potential at low temperatures. Any accumulation of foreign molecules in individual chain segments influences relaxation capability and thus viscoelastic behavior. For example, methylene chloride or chloroform accumulates on the carbonate group, which is the reason for polycarbonate s good solubility in these media. 

The relaxation processes described above apply to linear viscoelastic behavior. If the deformation is not small or slow, the orientation of the chain segments may be sufficiently large to cause a nonlinear response. We will see that this effect alone can be accounted for in rheological models by simply replacing the infinitesimal strain tensor by one able to describe large deformations no new relaxation mechanism needs to be invoked. Nonlinear effects related to orientation, such as normal stress differences, can be described in this manner. 

Some information concerning the intramolecular relaxation of the hyperbranched polymers can be obtained from an analysis of the viscoelastic characteristics within the range between the segmental and the terminal relaxation times. In contrast to the behavior of melts with linear chains, in the case of hyperbranched polymers, the range between the distinguished local and terminal relaxations can be characterized by the values of G and G" changing nearly in parallel and by the viscosity variation having a frequency with a considerably different exponent 0. This can be considered as an indication of the extremely broad spectrum of internal relaxations in these macromolecules. To illustrate this effect, the frequency dependences of the complex viscosities for both linear... 

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