Viscoelastic phenomena

FIGURE 7.4 Schematic of a typical extrusion die showing the various parts of the die (side view). The manifold distributes the melt from the extruder uniformly over the width of the die while the restrictor can be used to compensate for variations in flow rate across the width of the die. The lips give the melt stream the desired dimensions and shape. (Note Because of die swell the dimensions of the extrudate may be considerably different from those of the die lips.) 

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The various elastic and viscoelastic phenomena we discuss in this chapter will be developed in stages. We begin with the simplest the case of a sample that displays a purely elastic response when deformed by simple elongation. On the basis of Hooke s law, we expect that the force of deformation—the stress—and the distortion that results-the strain-will be directly proportional, at least for small deformations. In addition, the energy spent to produce the deformation is recoverable The material snaps back when the force is released. We are interested in the molecular origin of this property for polymeric materials but, before we can get to that, we need to define the variables more quantitatively. 

The Maxwell and Voigt models of the last two sections have been investigated in all sorts of combinations. For our purposes, it is sufficient that they provide us with a way of thinking about relaxation and creep experiments. Probably one of the reasons that the various combinations of springs and dash-pots have been so popular as a way of representing viscoelastic phenomena is the fact that simple and direct comparison is possible between mechanical and electrical networks, as shown in Table 3.3. In this parallel, the compliance of a spring is equivalent to the capacitance of a condenser and the viscosity of a dashpot is equivalent to the resistance of a resistor. The analogy is complete... 

Viscoelastic phenomena always involve the change of properties with time and, therefore, the measurements of viscoelastic properties of solid polymers may be called dynamic mechanical. Dynamic mechanical thermal analysis (DMTA) is a very useful tool for studying... 

The mechanical behavior of plastics is dominated by such viscoelastic phenomena as tensile strength, elongation at breaks, stiffness, and rupture energy, which are often the controlling factors in a design. The viscous attributes of plastic melt flow are also important considerations in the fabrication of plastic products. (Chapter 8, INFLUENCE ON PERFORMANCE, Viscoelasticity). 

The time/temperature-dependent change in mechanical properties results from stress relaxation and other viscoelastic phenomena that are typical of these plastics. When the change is an unwanted limitation it is called creep. When the change is skillfully adapted to use in the overall design, it is referred to as plastic memory. 

They produced high performance electrets from thin polymer films metallized so as to yield high capacitance. Both electrical and mechanical properties of these transducers have been remarkable examples of how applications of science of solids, including knowledge of electron traps, conduction processes in insulators and the viscoelastic phenomena of semicrystalline polymers, can be combined.(6) Incidentally, similar ideas have been applied to optimization of the properties of particle microphones, through assemblies of perfectly microspherical polymer carbon systems. These have shown what limits of performance... 

Fredrickson AG. Viscoelastic phenomena in thick liquids Phenomenological analysis. In Acrivos A, ed. Modern Chemical Engineering, vol. I Physical Operations. New York Reinhold Publ., 1963 197-265. 

One important aspect of polymer structure should be mentioned here, however, viz. the very important role of very long polymer molecules in the viscoelastic behaviour. Accordingly, the viscoelastic phenomena are also highly dependent on the molecular mass distribution. 

Functional structural groups, 129 Fundamental quantities of mass and volume, 72 Fundamental viscoelastic phenomena, 406... 

This behavior results from stress relaxation and other viscoelastic phenomena that are typical of TPs. In addition to using heat TPs such as polyolefins, neoprenes, silicones, and other cross-linkable TPs are example of plastics that can be given memory either by radiation or by chemically curing. Fluoroplastics need no such curing. When this phenomenon of memory is applied to fluoroplastics such as TFE, FEP, ETFE, ECTFE, CTFE, and PVF, interesting and useful high-temperature or wear-resistant applications become possible. 

Polymer solutions may have the memory effects observed in viscoelastic phenomena. This requires additional relaxation terms in the constitutive equations for the viscous pressure tensor, which may be affected by the changes in the velocity gradient. Besides this, the orientation and stretching of the macromolecules may have an influence on the flow. 

In summary, the differences observed between DSA and TBA can be accounted for to some extent by careful consideration of the meaning of the observed viscoelastic phenomena. An explanation of the frequency dependence of the magnitude of the first peak and strong frequency dependence of the time to the second peak by DSA for Resin 5208 is beyond the scope of this work. 

The transient net work model is an adaptation of the network theory of rubber elasticity. In concentrated polymer solutions and polymer melts, the network junctions are temporary and not permanent as in chemically crosslinked rubber, so that existing junctions can be destroyed to form new junctions. It can predict many of the linear viscoelastic phenomena and to predict shear-thinning behavior, the rates of creation and loss of segments can be considered to be functions of shear rate. 

Doi molecular theory adds a probability density function of molecular orientation to model rigid rodlike polymer molecules. This model is capable of describing the local molecular orientation distribution and nonlinear viscoelastic phenomena. Doi theory successfully predicts director tumbling in the linear regime and two sign changes in the first normal stress difference,as will be discussed later. However, because this theory assumes a uniform spatial structure, it is unable to describe textured LCPs. 

Rheology is the science that deals with the deformation and flow of matter under various conditions an example is plastic melt flow. The rheology of plastics, particularly TPs, is complex but manageable. These materials combine the properties of an ideal viscous liquid (pure shear deformations) with those of an ideal elastic solid (pure elastic deformation). Plastics are therefore said to be viscoelastic (Figure 17). The mechanical behavior of plastics is dominated by the viscoelastic phenomena such as tensile... 

Boltzmann superposition principle A basis for the description of all linear viscoelastic phenomena. No such theor) is available to serve as a basis for the interpretation of nonlinear phenomena—to describe flows in which neither the strain nor the strain rate is small. As a result, no general valid formula exists for calculating values for one material function on the basis of experimental data from another. However, limited theories have been developed. See kinetic theory viscoelasticity, nonlinear, bomb See plasticator safety. 

An interesting example of the use of the EQCM to monitor both interfacial electrochemical and subsequent chemical processes is the deposition of Hg on thin TSM resonator-immobilized graphite electrodes [115]. As the authors conceded, there are a variety of complicating factors - notably spatially inhomogeneous deposition of Hg droplets and viscoelastic phenomena associated with these fluid deposits. Nevertheless, in chloride media, when electrodeposition of elemental Hg was followed by setting the electrode to open circuit, the electrode unequivocally continued to gain mass this was interpreted in terms of the comproportionation of deposited Hg(0) and dissolved Hg(II), to produce surface-immobilized Hg2Cl2. 

Theoretical treatments for the acoustically thick film case have only recently been developed (see Sect. 2.7.2.1.2) thus this topic has not been reviewed previously. The field is at an interesting stage of development the existence of viscoelastic phenomena in this context are generally accepted, the basic theoretical concepts have been delineated [18-20, 22, 23, 25],... 

PVF provides a good example of how the EQCM can be used to explore solvation processes via their influence over film viscoelastic properties. It also provides a representative example of progress in this area an early study [133] of PVF electrodeposition clearly identified the presence of film viscoelastic phenomena, which were subsequently parameterized in terms of... 

In Sect. 2.7.3.7.1, appropriate control of polymer composition of a redox polymer (PVF) was shown to lead to the introduction of viscoelastic phenomena and to thermal sensitivity. For polypyrrole, deposition from micellar surfactant media (dodecylsulfate and dodecylbenzenesul-fonate) also leads to changes in film morphology and viscoelastic behavior [139]. 

The mechanical spectra and temperature dependencies derived from DMA provide, as such, no immediate insight to their molecular origin. Qualitatively the various viscoelastic phenomena are linked to the energy-elastic deformation of bonds and the viscous effects due to large-amplitude movement of the molecular segments. The latter are based on internal rotation causing conformational motion to achieve the equilibrium entropy-elastic response. 

Owning to large normal stress differences, the following-listed viscoelastic phenomena are unique in polymer flows. 

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