Yield stress compatibilized blends

Figure 2. Influence of atactic (aPM) and isotactic (iPM) PP-g-MA blend compatibilizers on the morphological and mechanical properties of PP—PA6 (70/30) PA6 domain size (a), Youngs modulus (b), yield stress (c), and notched Charpy impact strength (d).

The general shape of the stress-strain curve is the same for all the blends with Q-series and Kraton compatibilizers. This shape is described by two tangent lines drawn from the initial elastic region and the plastic region, respectively. The intersection of the lines is defined as the yield point and is described by a yield stress (ay) and an apparent yield strain (ey). The stress (a) and strain (e) in the plastic region are related by... 

Figure 7. Yield stress of the compatibilized blends compared with calculations. Key —, a yielding angle of 70° and Hashins upper modulus bound —, Nielsens approach for debonded particles and, Nielsens approach with a debond-...

Fracture Stress and Strain. Yielding and plastic deformation in the schematic representation of tensile deformation were associated with microfibrillation at the interface and stretching of the microfibrils. Because this representation was assumed to apply to both the core-shell and interconnected-interface models of compatibilization, the constrained-yielding approach was used without specific reference to the microstructure of the interface. In extending the discussion to fracture, however, it is useful to consider the interfacial-deformation mechanisms. Tensile deformation culminated in catastrophic fracture when the microfibrillated interface failed. This was inferred from the quasi-brittle fracture behavior of the uncompatibilized blend with VPS of 0.5, which indicated that the reduced load-bearing cross section after interfacial debonding could not support plastic deformation. Accordingly, the ultimate properties of the compatibilized blend depended on interfacial char-... 

Compatibihzation enhances dispersion, increases the total apparent volume of the dispersed phase, rigidifies the interface, and increases interactions not only between the two phases, but also between the dispersed drops. These changes usually increase the blends viscosity, elasticity and the yield stress. The compatibilizer effects are especially evident at low frequencies. There are two mechanisms that may further affect these behaviors (i) the... 

The consequences of Eq. (2.5) are displayed in Figure 2.2. Independently of the compatibilizer efficiency, the postulated parallel concentration dependence of dz and Vi2 is observed. The rheological outcome of compatibilization is related to the increased volume fraction ofthe dispersed phase and enhanced interactions between particles, which often lead to the appearance of yield stress in a compatibilized blend (usually yield stress is absent in non-compatibilized blends). 

Keywords blends, alloys, compatibilizer, rheology, flow, viscosity, extru-date swell, shear modulus, shear flow, elongational flow, orientation in flow, sizing, yield stress, fillers, talc, mica, glass fibers, reinforced system, interlayer slip, log-additivity rule, concentration dependence, Trouton rule. 

Compatibilization, making the interface more rigid causes the constant stress viscosity to increase. Similarly, an increase of the apparent volume of the dispersed phase causes the relative viscosity to increase. Furthermore, increased interactions between the phases reduce the possibility of the interlayer slip and increase formation of an associative network, resulting in systems with increased yield stress. Thus, compatibilization is expected to increase melt viscosity, elasticity and the yield stress. These effects are especially large at low frequencies, but may not be significant at high deformation rates. However, other mechanisms that may reverse this tendency 1. Preferential micellization of compatibilizer, 2. Increase of the free volume in the blend, etc. 

It is known that PLA forms miscible blends with polymers such as PEG [53]. PLA and PEG are miscible with each other when the PLA fraction is below 50 per cent [53]. The PLA/PEG blend consists of two semi-miscible crystalline phases dispersed in an amorphous PLA matrix. PHB/PLA blends are miscible over the whole range of composition. The elastic modulus, stress at yield, and stress at break decrease, whereas the elongation at break increases, with increasing polyhydroxybutyrate (PHB) content [54]. Both PLA/PGA and PLA/PCL blends give immiscible components [55], the latter being susceptible to compatibilization with P(LA-co-CL) copolymers or other coupling agents. 

The dynamic behavior of polymer blends under low strain has been theoretically treated from the perspective of microrheology. Table 2.3 lists a summary of this approach. These models well describe the experimental data within the range of stresses and concentrations where neither drop-breakup nor coalescence takes place. The two latter models yield similar predictions as that of Palierne. The last entry in the Table 2.3 is an empirical modification of Palieme s model by replacement of the volume fraction of dispersed phase by its efiective quantity (Eq. (2.24)), which extends the applicability of the relation up to 0 < 0.449. However, at these high concentrations the drop-drop interactions absent in the Palierne model must complicate the flow and coalescence is expected. The practical solution to the latter problem is compatibilization, but the presence of the third component in blends has not been treated theoretically. 

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