Rheological melt elasticity

Polyolefin melts have a high degree of viscoelastic memory or elasticity. First normal stress differences of polyolefins, a rheological measure of melt elasticity, are shown in Figure 9 (30). At a fixed molecular weight and shear rate, the first normal stress difference increases as MJM increases. The high shear rate obtained in fine capillaries, typically on the order of 10 , coupled with the viscoelastic memory, causes the filament to swell (die swell or... 

Rheological properties of filled polymers can be characterised by the same parameters as any fluid medium, including shear viscosity and its interdependence with applied shear stress and shear rate elongational viscosity under conditions of uniaxial extension and real and imaginary components of a complex dynamic modulus which depend on applied frequency [1]. The presence of fillers in viscoelastic polymers is generally considered to reduce melt elasticity and hence influence dependent phenomena such as die swell [2]. 

Thermotropic LCPs have high melt elasticity, but exhibit little extrudate swell. The latter has been attributed to a yield stress and to long relaxation times (60). The relaxation times for LCPs are normally much longer than for conventional polymers. Anomalous behavior such as negative first normal stress differences, shear-thickening behavior and time-dependent effects have also been observed in the. rheology of LCPs (56). Several of these phenomena are discussed for poly(benzylglutamate) solutions in the chapter by Moldenaers et al. 

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. 

The rheological property responsible for the entrance pressure drop has been argued to be melt elasticity [58] ... 

Keywords peroxide, molar weight distribution (MWD), rheology, crystallization, extrusion, melt flow index (MFI), controlled rheology (CR), peroxide-degradation, residence time distribution (RTD), halflifetime of peroxides, melt elasticity, die swell, viscosity curve, shear rate, elongational viscosity, melt fracture, heterophasic PR... 

A segmented thermoplastic elastomer, Hytrel 5526 (HT), as a flexible filler is added to PET phase to modify the rheological behavior of PET/PE MRCs [34]. The flexibility of the microbrils with low HT loading has no obvious influence on the viscosity of MRCs. With the increase of HT content, especially for the 30 wt% HT, the microfibrils flexibility increases, causing the viscosity of MRCs to have a slight decreasing trend shown in Figure 12.19. Moreover, the more flexible microfibrils lead to lower melt elasticity of MRCs. 

In semicrystalline thermoplastic matrix composites, the nucleation and growth of a transcrystalline interface around the reinforcing fiber is thought to have a critical influence on the improvement of the stiffness and tensile strength [63-65]. The rheological behavior of in situ microfibrillar HDPE/PET and HDPE/polycarbonate (PC) polymer blends was investigated in a recent study [66,67]. For both HDPE/PET and HDPE/PC microfibrillar blends, the viscosity increased with PET and PC microfibrils concentration. In another study, Xu et al [68] reported that the flexible microfibrils of PET reduced the melt elasticity and viscosity of HDPE/PET microfibrillar blends. 

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