Interlayer slip

The capillary flow. The Interlayer slip, apparent In unconpatlblllzed blend, BL, disappeared upon addition of EP-1 or EP-2. As shown In Fig. 21 the slip affected the entrance-exit pressure drop, P, ("Bagley correction") more than viscosity. 

The concentration dependence of rig conqjuted from Equation 20 Is shown In Fig. 30, where the solid points represent the experimental data and the open points their values corrected for the effects of PP degradation. For System-1 there Is strong negative deviation (NDB) from the log additivity rule, viz. Equation 1, but for System-2 NDB Is visible at low PP content, converting to positive deviation (PDB) at high. It Is worth recalling that ng was computed from corrected for the yield stress values of n. The NDB behavior. Indicative of Interlayer slip, reflects poor miscibility In System-1 and that at low concentration of PP In System-2. The emulslon-llke behavior of Syetem-2 at high PP content reflects a better Interphase Interaction. 

The shear-induced interlayer slip was theoretically predicted — it creates a tree-ring structure in the extrudates [Utracki et al., 1986 Utracki, 1991b Bousmina et al., 1999]. The relation may be used to describe the steady-state viscosity of antagonistically immiscible polymer blends, such as PP/LCP [Ye et al., 1991 Utracki, 1991b]. 

In the absence of interlayer slip, addition of a second phase leads to an increase of viscosity. The simplest way to treat the system is to consider that the relative viscosity as a function of the solids volume fraction, particle aspect ratio and orientation. There is no difference between the flow of suspensions in Newtonian liquids and that of polymeric composites, when the focus is on the Newtonian behavior. The non-Newtonian behavior of suspensions originates either from the non-Newtonian behavior of the medium or from the presence of filler particles. The problems associated with this behavior can originate in inter-particle interactions (viz. yield stress), and orientation in flow [Leonov, 1990 Mutel and Kamal, 1991 Vincent and Agassant, 1991 Shi-kata and Pearson, 1994]. 

It should be noted that the Doi and Ohta theory predicts oifly an enhancement of viscosity, the so called emulsion-hke behavior that results in positive deviation from the log-additivity rule, PDB. However, the theory does not have a mechanism that may generate an opposite behavior that may result in a negative deviation from the log-additivity rule, NDB. The latter deviation has been reported for the viscosity vs. concentration dependencies of PET/PA-66 blends [Utracki et ah, 1982]. The NDB deviation was introduced into the viscosity-concentration dependence of immiscible polymer blends in the form of interlayer slip caused by steady-state shearing at large strains that modify the morphology [Utracki, 1991]. 

In consequence, the flow imposed morphologies can be classified as (i) Dispersion (mechanical compatibilization), (ii) Fibrillation, (iii) Lamellae formation, (iv) Coalescence, (v) Interlayer slip, (vi) Encapsulation, etc. These types will be discussed below under appropriate headings. 

The simplest mechanism that explains the NDB behavior is the interlayer slip, which led to derivation of Eqs 7.123 and Eq 7.124. One may postulate that at constant stress, the net q vs. (() dependence can be written as a sum of two contributions the interlayer slip, expressed by Tjj (calculated from either Eq 7.123 or Eq 7.124), and the emulsion-hke viscosity enhancement given by... 

Figure 7.24. Concentration dependence of blend viscosity at five levels of shear stress, (from top Ojj = 10 to 10 ) indicating a gradual change of dominant fiow mechanism from emulsion-type to interlayer slip.

The rheological consequences of these changes can be predicted from a model system. The emulsion model indicates that making the interface more rigid causes the intrinsic viscosity of the emulsion to increase (see Eq 7.50). Similarly, an increase of the apparent volume of the dispersed phase causes the relative viscosity to increase (see Eqs 7.24-7.25). Furthermore, enhanced interactions between the phases will reduce the possibility of the interlayer slip, and increase formation of associative network formation, which may result in the yield stress. In short, compatibilization is expected to increase melt viscosity, elasticity and the yield stress. 

Immiscible blends shear-induced interlayer slip Utracki, 1987, 1988,... 

On the other hand, if one looks at the whole system, a number of phenomena can take place segregation, encapsulation, interlayer slip or variations of miscibility. Components can segregate during the flow, leading to spatial redistribution, encapsulation of one phase by the other being a special case of flow segregation. A redistribution of the phases was observed for HDPE/PA-6 blends during capillary flow [Dumoulin et al.,... 

Figure 10.4. Micrograph of a freeze-fractured surface of an HDPE/PA-6 extruded blend, showing tree ring structures typical of telescopic flow in the capillary, caused by interlayer slip [Dumoulin et al., 1986].

Flow can impact on the blend s morphology in a number of ways. It can affect either the drops, causing drop deformation to ellipsoids or fibrils, break up or coalescence, or the whole system, inducing then domain segregation, encapsulation, interlayer slip or variations of miscibility. Examples of each of these phenomena abound. 

Shear-induced interlayer slip see Interlayer slip... 

In the absence of interlayer slip, addition of a second phase leads to an increase of viscosity. The simplest way to treat the system is to consider the relative viscosity as a function volume fraction of the solids, < ), particle aspect ratio and orientation. 

The dependence was re-derived later for a telescopic flow of two polymers through a pipe (Heitmiller et al. 1964). The two liquids formed a large number of concentric layers, each of the same cross-sectional areas. The fundamental condition that leads to the fluidity additivity relation was the continuity of the shear stress across the multi-stratified structure. Lin (1979) followed this derivation with an additional assumption that the shear stress of each layer can be modified by the presence of an additional frictional stress, Z = (p — 1)(RAP/2L), where R is the capillary radius, AP is the pressure drop, and p is a characteristic material parameter (interlayer slip factor) in... 

The material parameter 0 in Eq. 7.124 governs the NDB behavior. It was shown that its value is inversely proportional to the thickness of the interphase, Al, and its viscosity, rii terphase (Bousmina et al. 1999). Theoretically, the same molecular mechanism should be responsible for both factors, viz., better miscibility, better interdiffusion, thus higher Al and tiimerphase- However, the low molecular weight components of the blend, that are forced by the thermodynamics to diffuse to the interphase, may not change much the former parameter, but drastically reduce the latter. For immiscible blends, Al is small, typically 2-6 nm. Thus 0 is large, and interlayer slip takes place. For compatibUized blends, the macromolecules of the two phases interact and interlace, which increases both factors thus, the slip effects are negligible. Measured or calculated values of the interphase viscosity are listed in Table 7.10. 

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