Melting behavior memory effects

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). 

Li et al. reported that immiscible high-density polyethylene (HDPE)/ poly(ethylene terephthalate) (PET) blends, prepared by means of melt extrusion with ethylene-butyl acrylate-glycidyl methacrylate (EBAGMA) terpoly-mer as a reactive compatibilizer, can exhibit shape memory effects [32]. They observed that the compatibilized blends showed improved shape memory effects along with better mechanical properties as compared to the simple binary blends. In the blend, HDPE acts as a reversible phase, and the response temperature in the shape recovery process is determined by of HDPE. The shape-recovery ratio of the 90/10/5 HDPE/PET/EBAGMA blend reached nearly 100%. Similar behavior was observed for immiscible HDPE/ nylon 6 blends [33]. The addition of maleated polyethylene-octene copolymer (POE-g-MAH) increases compatibility and phase-interfacial adhesion between HDPE and nylon 6, and shape memory property was improved. The shape recovery rate of HDPE/nylon 6/POE-g-MAH (80/20/10) blend is 96.5% when the stretch ratio is 75%. 

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). 

Due to the crystal memory effect, the starting structure, temperature (T and time of fusion prior to crystallization might influence the polymorphic behavior of polymers in the following cold or melt crystallization. An interesting example is s-PS. The a-form aystals of s-PS are exclusively obtained upon melt crystallization when traces of the a-form nuclei are present and not erased completely at low Tj (e.g., 280 °C). In contrast, once the crystal nuclei are completely removed at high (e.g., 380°C), only the thermodynamically stable p-form crystals are obtained under the same crystallization conditions [125]. 

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. 

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