Viscoelastic state rubbery flow

DMA methods are widely used by thermal analysts to determine the viscoelastic properties of pol5uners for a number of purposes (see Viscoelasticity). The primary application of these techniques to the study of polymeric solids and melts is well documented. Excellent general discussions covering the subject are provided in References 70-72. Linear Amorphous Polymers (qv) exist in a number of characteristic physical states depending on the time scale and temperature of measurement. These are illustrated in Figure 31 in terms of an arbitrary modulus fimction and are classified as glassy, leathery, rubbery, rubbery flow, and viscous (73). All linear amorphous polymers exhibit these five physical states when they... 

In the molten state polymers are viscoelastic that is they exhibit properties that are a combination of viscous and elastic components. The viscoelastic properties of molten polymers are non-Newtonian, i.e., their measured properties change as a function of the rate at which they are probed. (We discussed the non-Newtonian behavior of molten polymers in Chapter 6.) Thus, if we wait long enough, a lump of molten polyethylene will spread out under its own weight, i.e., it behaves as a viscous liquid under conditions of slow flow. However, if we take the same lump of molten polymer and throw it against a solid surface it will bounce, i.e., it behaves as an elastic solid under conditions of high speed deformation. As a molten polymer cools, the thermal agitation of its molecules decreases, which reduces its free volume. The net result is an increase in its viscosity, while the elastic component of its behavior becomes more prominent. At some temperature it ceases to behave primarily as a viscous liquid and takes on the properties of a rubbery amorphous solid. There is no well defined demarcation between a polymer in its molten and rubbery amorphous states. 

The equilibrium properties in the rubbery state are almost exclusively governed by the macromolecular scale structure crosslinking suppresses liquid flow, decreases the number of available network complexions, and the gap between equilibrium (unstretched) and fully stretched network states. With regard to time-dependent properties (viscoelasticity, time-... 

Gelation is defined as the point during polymerization when the polymer transforms from a hquid to a rubbery state [97]. At the molecular level, this correlates to the moment at which the molecular weight approaches infinity upon incipient formation of a cross-linked network. Macroscopically, gelation is defined as an abrupt increase in viscosity after which the polymer loses its ability to flow and develops viscoelastic properties. The macroscopic definition of gelation does not necessarily correlate to gelation at the molecular level, since hnear polymers... 

This is consistent with the previously described viscoelastic behaviour of gum rubber where there is a transition from the rubbery state to the flow state over a longer time scale. The longer time scale corresponds to the lower shear rate where the material is in the flow state. 

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