Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Polymer 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... [Pg.176]

Alfonso GC, Scardigli P. Melt memory effects in polymer crystallization. Macromol Symp 1997 118 323-328. [Pg.240]

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). [Pg.261]

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). [Pg.510]

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... [Pg.387]

Shrinkablefilm and tubing Cross linked semicrystalline thermoplastics display rubberlike properties at temperatures above their melting points. On deformation followed by fast cooling the polymer maintains its deformed shape. The polymer returns to its original shape when reheated. This memory effect is applied in the production of heat shrinkable films and tubing. Radiation doses of the order of 40 — 1(X) kGy are used in the production of heat shrinkable products. [Pg.189]

On the other hand, in a non-Newtonian fluid, the viscosity depends on the shear rate. Besides showing very high non-Newtonian viscosities, polymers exhibit a complex viscoelastic flow behavior, that is, their flow exhibits memory , as it includes an elastic component in addition to the purely viscous flow. Rheological properties are those that define the flow behavior, such as the viscosity and the melt elasticity, and they determine how easy or difficult is to process these materials, as well as the performance of the polymer in some applications. The rheology of the polymers and its effect on the processing of these materials are studied in Chapters 22 and 23. [Pg.4]

Figure 3.1 Various molecular structures of shape memory polymers (SMPs). A stable network and a reversible switching transition are the prerequisites for the polymers to show the shape memory effect (SME). The stable network can be molecule entanglement, chemical cross-hnking, crystaUization, and IPN the reversible switching transition can be the crystalUzation/melting transition, vitrification/glass transition, anisotropic/isotropic transition, reversible chemical cross-linking, and association/disassociation of supramolecular structures. Source [22] Reproduced with permission from Elsevier... Figure 3.1 Various molecular structures of shape memory polymers (SMPs). A stable network and a reversible switching transition are the prerequisites for the polymers to show the shape memory effect (SME). The stable network can be molecule entanglement, chemical cross-hnking, crystaUization, and IPN the reversible switching transition can be the crystalUzation/melting transition, vitrification/glass transition, anisotropic/isotropic transition, reversible chemical cross-linking, and association/disassociation of supramolecular structures. Source [22] Reproduced with permission from Elsevier...
These polymers also display the memory effect, such that when heated to slightly above the melting point, the material remembers its premelt chain conformation. This could have implications concerning delamination resistance because, if laminates are processed in the temperature range within which the memory effect is operative, inadequate resin interpenetration through adjacent plies may result. [Pg.537]

Orthodontics is an area in which polymers are desirable for both their esthetic appeal and shape-memory effect. In 2007, Eliades published an opinion paper on projected future materials for orthodontics and discussed research into polymer-based archwires [67]. In the following year, Jung and Cho demonstrated the use of shape-memory polyurethanes for arch wires [68]. An in vitro dental model was used to test the correction of misaligned teeth and can be seen in Fig. 10. The melt spun polymer, synthesized from 4,4 -methylene bis(phenylisocyanate) and PCL-diol, was stretched to the length required to realign the teeth and attached to stainless... [Pg.158]

Die swell is a complex rheological phenomenon [1], It can be observed as an extrudate with a cross-section (D which is greater than the die cross-section DJ. This effect, also known as extrudate swell, Barus effect, or % memory, is defined as the ratio D /Dq = B and is a feature of polymer melt flow. Die swell is associated with the viscoelastic nature of polymer melts as it exceeds the swelling of constant viscosity (Newtonian) fluids. Accordingly, for laminar flow situations, the swelling due to velocity profile rearrangements or mass balance considerations accounts for only 10-20% and cannot explain the 50-300% increase in extrudate cross-section of the polymer emerging out of a die. [Pg.158]

One way to explain die swell is to consider the ability of the polymer melt to remember its flow history. The idea is to imagine a fluid element moving from the reservoir into a capillary die as a short, fat cylinder getting squeezed into a long, slender cylinder. If the residence time of the fluid element in the die is shorter than the time of its fading memory, it will try to return to its original shape and produce the die swell effect. [Pg.158]

Viscoelastic phenomena may be described through three aspects, namely stress relaxation, creep and recovery. Stress relaxation is the decline in stress with time in response to a constant applied strain, at a constant temperature. Creep is the increase in strain with time in response to a constant applied stress, at a constant temperature. Recovery is the tendency of the material to return partially to its previous state upon removal of an applied load. The material is said to have memory as if it remembers where it came from. Because of the memory effect, in transient flows the behavior of viscoelastic fluids wUl be dramatically different from that of Newtonian fluids. Viseoelastie fluids are fiiU of instabilities. Some examples inelude instabilities in Taylor-Couette flow, in eone-and-plate and plate-and-plate flows (Larson 1992). The extrudate distortion, commonly called melt fraeture, is a notorious example of viscoelastic instability in polymer processing. The viseoelastie instability in injection molding can result in specific surface defects such as tiger stripes (Bogaerds et al. 2004). [Pg.8]

Thermal history, also known as heat history, describes the memory effect in polymers. In simple terms, polymers are affected by hot or cold temperatures and remember the last effect. This is considered a variable that should be controlled in the laboratory. Thermal history has a strong influence on the material s glass transition, melting, and crystallization temperatures. Temperature changes, both hot and cold temperatures, induce thermal stresses in the material. The extent and implication of thermal stresses to a polymer are usually unknown and uncontrolled. This variable may be of interest for those who want to know more about the sample as it was received in the laboratory or how it reacts to an uncontrolled end-use environment. Usually this is of interest, but because it is uncontrolled it is very difficult to rely on for material comparisons. [Pg.113]

Molecular dynamics of a macromolecular chain involves both cOTiformational and rotational motions. Along these lines, the backbone dynamics of poly(n-alkyl methacrylates) has been elucidated by advanced solid state NMR, which enables conformational and rotational dynamics to be probed separately [41], The former is encoded in the isotropic chemical shift. The latter is probed via the anisotropic chemical shift [14] of the carboxyl group with unique axis along the local chain direction. Randomization of conformations and isotropization of backbone orientation occur on the same time scale, yet they are both much slower than the slowest relaxation process of the polymer identified previously by other methods [40]. This effect is attributed to extended backbone conformations, which retain conformational memory over many steps of restricted locally axial chain motion (Fig. lb, c). These findings were rationalized in terms of a locally structured polymer melt, in... [Pg.299]


See other pages where Polymer melting memory effect is mentioned: [Pg.390]    [Pg.281]    [Pg.183]    [Pg.197]    [Pg.166]    [Pg.218]    [Pg.66]    [Pg.163]    [Pg.510]    [Pg.166]    [Pg.229]    [Pg.229]    [Pg.668]    [Pg.388]    [Pg.111]    [Pg.112]    [Pg.132]    [Pg.137]    [Pg.60]    [Pg.776]    [Pg.36]    [Pg.163]    [Pg.209]    [Pg.715]    [Pg.141]    [Pg.387]    [Pg.316]    [Pg.399]    [Pg.407]    [Pg.46]    [Pg.149]    [Pg.219]    [Pg.7554]    [Pg.7559]    [Pg.165]   
See also in sourсe #XX -- [ Pg.390 ]




SEARCH



Melted polymer

Melting memory effect

Memory effects

Polymer melts

© 2024 chempedia.info