Elasticity molten polymer

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 final main category of non-Newtonian behaviour is viscoelasticity. As the name implies, viscoelastic fluids exhibit a combination of ordinary liquid-like (viscous) and solid-like (elastic) behaviour. The most important viscoelastic fluids are molten polymers but other materials containing macromolecules or long flexible particles, such as fibre suspensions, are viscoelastic. An everyday example of purely viscous and viscoelastic behaviour can be seen with different types of soup. When a thin , watery soup is stirred in a bowl and the stirring then stopped, the soup continues to flow round the bowl and gradually comes to rest. This is an example of purely viscous behaviour. In contrast, with certain thick soups, on cessation of stirring the soup rapidly slows down and then recoils slightly. 

This section will deal with suppression of flow in extension of molten polymers in the region of significant elastic strains21 24 . The study of polyisobutylene 11-20 23,35) failed to reveal such phenomena (the velocity of irreversible strain ep = d(ln[3)/dt increased strictly with time). Retardation of polymer fluid flow is considered on the example of homogeneous extension at constant strain velocity and force. Most experiments were carried out with commercial low-density polyethylene (LDPE) with molecular weight MW 105. Figure 7 gives experimental dependencies of tensile force F/S0 and irreversible strain In 3 ( 3 = e/ot) upon time t at different... 

Viscoelastic properties of molten polymers conditioning the major regularities of polymer extension are usually explained within the framework of the network concept according to which the interaction of polymer molecules is localized in individual, spaced rather far apart, engagement nodes. The early network theories were developed by Green and Tobolsky 49) and stemmed from successful network theories of rubber elasticity. These theories were elaborated more fully in works by Lodge50) and Yamamoto S1). The major elasticity. These theories is their simplicity. However, they have a serious drawback the absence of molecular weight in the theory. 

High-Elastic Properties of Molten Polymers and Filled... 

High-intensity vibration effect upon dissolved and molten polymers is relatively simple and can be obtained with the help of superimposition of high-frequency ultrasonic (US) vibrations. Periodic (cyclic) shear and volumetric strain of the medium is attained in this case without moving elements and due to elastic oscillations of US-vibrators of various design which function simultaneously as moulding elements (nozzles, cores) of extrusion heads. 

However, such viscous fluids as molten polymers can be very interesting objects for study from the point of view of ultrasonic (US) effects since they are characterized not only by viscosity but also by elasticity which alters radically their reaction to... 

Consequently, acoustic cavitation can also be expected in molten polymers under certain conditions at a relatively low intensity of acoustic treatment. High-viscous polymer systems characterized by elasticity or, in other words, demonstrating the properties both of liquids and elastic bodies simultaneously are a matter of special interest for a study of the behavior of materials in an acoustic field. 

C. McLuckie and M. Rogers, Influence of Elastic Effects of Capillary Flow of Molten Polymers, J. Appl. Polym. Sci., 13, 1049 (1969). 

As discussed before ( 5.2), a molten polymer shows also elastic behaviour, particularly on a short time-scale the fluid is visco-elastic. This can, in a simple experiment, be demonstrated in two ways. When we let a bar rotate around its axis in a viscoelastic fluid, then, after removal of the driving torque, it will rotate back over a certain angle. Moreover the fluid will, during rotation, creep upward along the bar, which indicates the existence of normal stresses next to shear stresses. 

Transportation of a molten polymer through a converging channel, e.g. with extrusion, goes accompanied with elongational flow when the rate of elongation is high, next to the viscous deformation also a considerable elastic component may be present. After the polymer leaves the channel, the elastic deformation will spontaneously recover, which is apparent as an increase in diameter of the extrudate, the so-called die-swell. 

At the upper limit of the ci range, v decreases to a minimum as the molecules are progressively immobilized, effectively making good and poor solvents functionally indistinguishable. In this regime, viscosity merges into elasticity, P becomes independent of c, and the dispersion simulates the behavior of a molten polymer. 

In the film blowing process where a continuous stable parison is blown from an annular die, it is crucial that the molten polymer exhibits certain elastic extensional properties and it is here that the viscoelastic nature of the polymer is beneficial.If, however, the manufacturer is concerned with profile and surface finish of an extrudate, viscoelastic effects of the polymer may well present difficulties. Both die swell and most polymer extrusion instabilities are linked to viscoelastic effects and as such different levels of viscoelasticity give rise to different extrusion characteristics. 

M 24. Mtosa, C.. V. Tabling, and A. Nasini Elasticity of molten polymers from stress relaxation data. J. Appl. Polymer ScL 5, 574—579 (1961). 

In summarizing, it can be concluded that the microhardness of elongational flow injection moulded PE is influenced by a local double mechanical contribution (a) a plastic deformation of crystal lamellae under the indenter, and (b) an elastic recovery of shish-fibrils parallel to the injection direction after load removal. Further, the Shish-crystals are preferentially formed when high orientation occurs, i.e. at zones near the centre of the mould and at an optimum processing temperature Tp around 145-150 °C. Below this temperature overall orientation decreases due to a wall-sliding mechanism of the mbber-like molten polymer. 

The study of polymer melts and especially their elasticity was one of the areas that drove the development of commercial DMAs. Although a decrease in the melt viscosity is seen with temperature increases, the DMA is most commonly used to measure the frequency dependence of the molten polymer as well as its elasticity. The latter property, especially when expressed as the normal forces, is very important in polymer processing. 

Many commercial experiments have been carried out over the years with many different types catalyst supports, calcined at various temperatures [407]. The concentration of chromium has been varied from as little as 0.01 wt% to as much as 6.0 wt% Cr. Higher chromium loading tends to increase the elasticity of the molten polymer. Many of the seemingly contradictory responses observed in Tables 23 and 24 as a result of changing the activation temperature were also exhibited when the chromium coverage was raised The breadth of the MW distribution remained constant whereas the rheological breadth increased. The low shear melt viscosity went up without an accompanying increase in MW. An Arnett... 

The CY-a values characterizing these polymers are also listed in Table 29 (as explained in Section 9). The CY-a value is considered as a measure of the relaxation time distribution of a molten polymer [526,527], When the MW distributions are similar, as in these samples, the CY-a value is a sensitive indicator of LCB. High LCB levels in the polymer tend to produce a low CY-a values. The CY-a values shown in the table reinforces the conclusions drawn from the JC-ot values, namely that both alkaline aging of the catalyst and its pore volume can influence melt elasticity. 

Time dependency also enters into the consideration of the rheological response of any viscoelastic system. In the steady-state testing of such materials as molten polymers, the selected time scale should be sufficiently long for the system to reach equilibrium. Frequently, the required period, t > 10" sec, is comparable to that in thixotropic experiments. More direct distinctions between these two types of flow are the usual lack of elastic effects and larger strain values at equilibrium observed for the thixotropic materials (see Table 7.4). There is a correlation between these two phenomena, and theories of viscoelasticity based on thixotropic models have been formulated by Leonov [1972, 1994]. Inherent to the concept of thixotropy is the yield stress. 

The mechanisms governing deformation and breakup of drops in Newtonian liquid systems are relatively well understood. However, within the range of compounding and processing conditions the molten polymers are viscoelastic liquids. In these systems the shape of a droplet is determined not only by the dissipative (viscous) forces, but also by the pressure distribution around the droplet that originates from the elastic part of the stress tensor. Therefore, the characteristics of drop deformation and breakup in viscoelastic systems may be quite different from those in Newtonian ones. Some of the pertinent papers on the topic are listed in Table 9.3. 

There are two main corrections that have to be applied to the information obtained from the capillary rheometer. First, there is an entrance pressure drop when the molten polymer enters the capillary, which is taken into account through the entrance or Bagley correction. This pressure drop is related to elastic deformations of the melt at the entry of the capillary [15]. Secondly, the non-Newtonian shear rate is expressed in terms of an apparent viscosity (defined in terms of a Newtonian flow). The relationship between the non-Newtonian and Newtonian shear rates, expressed as in the following equation, is known as the Rabinowitch correction [13, 16] ... 

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