Polymer melt, solid-liquid behavior

Transition from liquid behavior to solid behavior has been reported with fine particle suspensions with increased filler content in both Newtonian and non-Newtonian liquids. Industrially important classes are rubber-modified polymer melts (small rubber particles embedded in a polymer melt), e.g. ABS (acrylo-nitrile-butadiene-styrene) or HIPS (high-impact polystyrene) and fiber-reinforced polymers. Another interesting suspension is present in plasticized polyvinylchloride (PVC) at low temperatures, when suspended PVC particles are formed in the melt [96], The transition becomes evident in the following... 

The amorphous state is the characteristic of all polymers at temperatures above their melting points (except under special circumstances where liquid crystals may form). If a molten polymer retains its amorphous nature on cooling to the solid state, the process is called vitrification. In the vitrified amorphous state, the polymer resembles a glass. It is characteristic of those polymers in the solid state that, for reasons of structure, exhibit no tendency toward crystallization. The amorphous solid state is characterized by glass transition (Tg), which is described in a later section. We consider below only the behavior of polymer melt. 

Just as a crystalline solid melts into an isotropic liquid at a particular temperature, if a polymer were to be perfectly crystalline it also would similarly melt. This temperature is the melting temperature or T - On the other hand, the thermal behavior of an amorphous polymer is slightly different. As a polymer melt is cooled down continuously, at a critical temperature, the polymer motion ceases completely. This temperature is the glass transition temperature (Tg) of a polymer. Below the Tg the polymers behave like inorganic glasses and become hard and brittle. What is the difference between the melting temperature and glass transition tempera-... 

In contrast to simple elastic solids and viscous liquids, the situation with polymeric fluids is somewhat more complicated. Polymer melts (and most adhesives are composed of polymers) display elements of both Newtonian fluid behavior and elastic solid behavior, depending on the temperature and the rate at which deformation takes place. One therefore characterizes polymers as viscoelastic materials. Furthermore, if either the total strain or the rate of strain is low, the behavior may be described as one of linear or infinitesimal viscoelasticity. In such a case, the stress-deformation relationship (the constitutive equation) involves not just a single time-independent constant but a set of constants called the relaxation spectrum,(2) and this, too, may be determined from a single stress relaxation experiment, or an experiment involving small-amplitude oscillatory motion. 

All of these melt studies were based on consideration of the melt as a liquid, but the early work on application properties where from the solid mechanics point of view. Elastic effects were also very important in the melt state. Consideration of the viscoelastic characteristics of polymers in both the liquid and solid states led to the investigation of the elastic, or solid like, behavior of polymer melts. Studies of the recoverable strain in polymer melts indicated that if the molecular weight was high enough the melt exhibited almost a one-hundred percent recovery. Obviously true understanding of melt processing required consideration of both the viscous and elastic behavior. 

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