Shear-induced melting

However, for polymers with low degree of crystallization it was assessed that the melting point was depressed at high shear stresses (4). Shear Induced melting has been theoretically predicted from non-equilibrium molecular dynamic simulations (5,... 

Substantial interest has been raised in the problem of the structure and dynamics of suspensions in shear hydrodynamic fields. ° ° The experiments showed that both shear-induced melting and shear-induced ordering can be observed at different particle volume fractions and shear rates. The nonequilibrium microstructure of the suspension under shear can be investigated in these experiments and compared with the predictions from analytical theories and computer simulations. 

Earthquakes up to 700 hundred km deep occur in the mantle beneath subduction zmies. The physical causes of deep earthquakes are not weU understood. This is because minerals are expected to flow plastically at depths greater than about 30 km in normal areas. Processes that have been suggested to explain deep earthquakes include plastic instabilities, shear-induced melting, polymorphic phase transformations, and transformational faulting. Focal mechanisms can potentially shed light on this problem. 

Recent studies have contributed robustly to the realization that shear induced melt variations developed in runner sections will affect the manner in which a mold and part forming cavity will fill. These effects occur in, but are not limited to, multicavity molds. The filling patterns of single cavity molds are also affected by flow variation caused by shearing differences. These effects not only influence the way a single cavity mold will fill but how the polymers are oriented and consequently how the part will shrink and warp (1). The varying manner in which neat materials and filled materials shrink and how shear imbalances affect these two different types of materials is the focus of this study and subsequent write up. 

INVESTIGATION OF REVERSED SHEAR INDUCED MELT IMBALANCES IN... 

Figure 5 Probable mechanism of shear-induced exchange reactions during melt flow process [33]. (1) Interaction of zinc stearate, (RCOO)2 Zn, with ionic aggregates before melt flow. (II) Exchange reactions during melt flow.

For polymer melts or solutions, Graessley [40-42] has shown that for a random coil molecule with a Gaussian segment distribution and a uniform number of segments per unit volume, a shear rate dependent viscosity arises. This effect is attributed to shear-induced entanglement scission. 

As in PP-based nanocomposite systems, the extended Trouton rule, 3r 0 (y t) = r E (so t), also does not hold for PLANC melts, in contrast to the melt of pure polymers. These results indicate that in the case of P LANC, the flow induced internal structural changes also occur in elongation flow [48], but the changes are quite different in shear flow. The strong rheopexy observed in the shear measurements for the PLA-based nanocomposite at very slow shear rate reflects the fact that the shear-induced structural change involved a process with an extremely long relaxation time. 

Usually, synthetic polymers crystallize11 j15 from a melt or a solution in form of folded lamellae. Under specific circumstances it is sometimes also possible to obtain extended chain crystals which is the preferred arrangement in the crystallites of many natural polymers (cellulose, silk). Recently it has been found33 31 that in some cases another crystalline modification can be obtained, the so-called shish-kebabs, which are a sort of hybrid between folded lamellae and extended chain crystals. These shish-kebabs are obtained by shear-induced crystallization, a process in which the polymer crystallizes from solution under the influence of an elongated flow. 

Removal of the melt, also discussed in Section 5.1, is made possible, in principle, by two mechanisms drag-induced flow and pressure-induced flow (Fig. 5.4). In both cases, the molten layer must be sheared, leading to viscous dissipation. The latter provides an additional, important source of thermal energy for melting, the rate of which can be controlled externally either by the velocity of the moving boundary in drag-induced melt removal or the external force applied to squeeze the solid onto the hot surface, in pressure-induced melt removal. 

As viscosity increases with decreasing volatile content, the flash tank becomes inefficient as bubbles are entrapped and redissolved upon discharge. The falling-strand devolatilizer, shown schematically in Fig. 8.2, was developed to answer this problem, and represents an improvement over the ordinary flash tank. Here the polymer solution is pumped at high superheat into thin strands that fall gravitationally into the vacuum tank. Free of hydrostatic or shear-induced pressure fields, the bubbles nucleate, grow, coalesce, and rupture so that the volatiles are released before they get trapped in the melt of the cachepot. 

The decrease in expansion seems to occur at moisture levels above 30°/o moisture for both cereal (starch-based) polymer systems, and protein (soy grits). This corresponds to a point on their adsorption curves where water activity rises rapidly with added moisture that is, at a level where the water added to a mix has little effect on primary hydration of polymers, but behaves as a diluent. In mechanical terms, this may be explained by proposing that at above levels of 25°/o-30% water plasticisation of the polymers is complete, and further added water acts as a lubricant, reducing the shear-induced temperature rise and particle damage necessary for the formation of homogeneous melts. 

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