Melting memory effect

It was found that melt memory effects are significant in polymers due to the topological nature [53]. It is considered that melt memory effects are mainly... 

Figure 32 shows a plot of ve against At as derived from Eq. 65. This figure also shows the experimental I(Af). We found that a decrease of I(At) with an increase of At correspond to an increase of ve(At) with an increase of At. Therefore, it is concluded that an increase of ve with an increase of At is clearly an important mechanism of the melt memory effect. 

Varga, J. (1986). Melting memory effect of the beta-modification of polypropylene. /. Therm. Anal Vol.31, Issue 1, pp.165-172. 

Alfonso GC, Scardigli P. Melt memory effects in polymer crystallization. Macromol Symp 1997 118 323-328. 

Just above the melting point the polymer is visually quite viscous and numerous observations have been made that the polymer exhibits a memory effect, that is to say, on recooling the melt crystallites will appear in the same sites where they had been before melting the polymer. Hartley, Lord and Morgan (1954) state It is reasonable to suppose that there will be a few localities in the crystalline polymer which have a very high degree of crystalline order, and therefore the melt can contain, even at considerable temperatures above the observed melting or collapse point, thermodynamically stable minute crystals of the polymer . Especially if the polymer has been irradiated so as to contain a few crosslinks as in irradiated polyethylene, then flow is inhibited and spherulites can be made to appear on recrystallization in the same sites that they had before the polymer was melted, Hammer, Brandt and Peticolas (1957). However, as mentioned above, the specific heat of irradiated polyethylene in the liquid state is identical with that of the unirradiated material, within the limits of experimental error. Dole and Howard (1957). 

There has been a general consensus among hydrate researchers that hydrates retain a memory of their structure when melted at moderate temperatures. Consequently, hydrate forms more easily from gas and water obtained by melting hydrate, than from fresh water with no previous hydrate history. Conversely, if the hydrate system is heated sufficiently above the hydrate formation temperature at a given pressure, the memory effect will be destroyed. Some experimental observations of the memory effect phenomenon are summarized in Table 3.3. 

The memory effect has important implications for the gas industry. For example, after hydrates initially form in a pipeline, hydrate dissociation should be accompanied by the removal of the water phase. If the water phase is not removed, the residual entity (i.e., residual structure, persistent crystallites, or dissolved gas) will enable rapid reformation of the hydrate plug. Conversely, if hydrate formation is desired, the memory effect suggests that hydrate formation can be promoted by multiple dissociation and reformation experiments (provided the melting temperature is not too high, or melting time is not too long). 

If the temperature for melting hydrate is close to the dissociation temperature, or insufficient time is given to melt hydrate, a memory effect is observed (attributed to residual structure, persistent hydrate crystallites remaining in solution, or dissolved gas) to promote future more rapid hydrate formation. This memory effect is destroyed at temperatures greater than 28°C, or after several hours of heating. 

A mathematical expression relating forces and deformation motions in a material is known as a constitutive equation. However, the establishment of constitutive equations can be a rather difficult task in most cases. For example, the dependence of both the viscosity and the memory effects of polymer melts and concentrated solutions on the shear rate renders it difficult to establish constitute equations, even in the cases of simple geometries. A rigorous treatment of the flow of these materials requires the use of fluid mechanics theories related to the nonlinear behavior of complex materials. However, in this chapter we aim only to emphasize important qualitative aspects of the flow of polymer melts and solutions that, conventionally interpreted, may explain the nonlinear behavior of polymers for some types of flows. Numerous books are available in which the reader will find rigorous approaches, and the corresponding references, to the subject matter discussed here (1-16). 

While we are concerned mainly with solutions, it is interesting that the parentage relationship appears also in the other possible passage of a solid polymer into a liquid phase, namely by melting. Already in 1945 Charlesby (17) pointed out the existence of what he called the memory effect in polyethylene films orientation was preserved even after prolonged heating... 

As well, such important concepts as hydrate nucleation and the often-observed memory effects are not well understood The memory effect, occurs when a hydrate is melted or decomposed and hydrate reformation upon cooling is much faster than the original hydrate... 

As can be imagined the radiation produced crosslinks and chain scissions act in opposite directions with respect to changing the physical properties of plastics. Crosslinks transform molten polyethylene from a liquid into an elastomer. The elastomeric properties thus produced make possible the so-called "memory effect" in which the irradiated PE, for example, when stretched at temperatures above its melting point and then cooled... 

It is only attacked by oxygen and oxidizing melts at more than 600 °C. Accordingly, it is employed in ultratrace analysis (no memory effects in crucibles of glassy carbon) and in the semiconductor industry. 

One of the widely used effects of irradiation of PE is the so-called memory effect [Charlesby, 1960, 1987 Silverman, 1992]. PE is irradiated at RT in an inert atmosphere, and heated above the melting point, then stressed and given a particular shape. During the subsequent cooling, the crystallization fixes the imposed shape. However, upon reheating, the crystals melt and the cross-linked PE returns to its original unstressed shape. This memory effect is the basis of the production of many heat-shrink products, e.g., food-packaging film and electrical connectors [Bradley, 1984 Silverman, 1992]. 

Melting endotherms Melting point depression Melting temperature, Memory effect Metal plating Metallocene catalyst... 

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