Rheological behavior of blends

If the rheology of suspensions and emulsions is difficult to describe theoretically and to determine experimentally, in the case of polymer blends these difficulties reach another order of complication. It suffices to note that in blends both phases are viscoelastic, the viscosity ratio varies over a wide range, and morphology can be very complex. To understand the rheological behavior of blends, it is useful to refer to simpler systems that can offer important insight. The following systems (Table 7.2) are commonly considered and will be treated in the following discussion. 

It is noteworthy that even in miscible polymers of similar molecular structure, viz. 1,4-polyiso-prene with 1,2-polybutadiene, the time-temperature superposition fails. The polymers having glass transition temperatures separated by 60°C preserve their different dynamics in the blends [Kan-nan and Komfield, 1994]. Thus, even miscible systems can be rheologicaUy complex. The rheological behaviors of blends in the vicinity of the phase separation are of great fundamental importance. They will be discussed in Part 7.4.3. 

The rheological behavior of liquid crystal-forming polymer solutions is governed by the phase of the blend solutions. Therefore, first we try to clarify the phase of the solutions at rest and undergoing shear on the basis of optical and rheological data, and then describe the rheological behavior of blend solutions. 

The rheological behavior of the blends and composites was totally different. Addition of LCP reduced the... 

Cook, R.F., Koester, K.J., Macosko, C.W., and Ajbani, M. Rheological and Mechanical Behavior of Blends of Styrene-Butadiene Rubber with Polypropylene, Polym. Eng. Set 45(11), 1487-1497, 2005. 

Oommen, Z., S. Thomas, C. K. Premalatha, and B. Kuriakose, Melt rheological behavior of natural rubber/polyfmethyl methacrylate)/natural rubber-g-poly(methy methacrylate) blends. Polymer, 38(22), 5611-5621 (1997). 

Several factors can be identified as being crucial for the foaming of immiscible polymer blends the blend morphology, the phase size of the blend constituents, the interfacial properties between the blend partners, and, last but not least, the properties of the respective blend phases such as the melt-rheological behavior, the glass transition temperature, the gas solubility, as well as the gas diffusion coefficient. Most of these factors also individually influence the melt-rheological behavior of two-phase blends. 

Most of the methods used to characterize the rheological behavior of butter are empirical and attempt to imitate certain sensory perceptions. They typically involve penetrometry, extrusion or sectility tests (Prentice, 1972). In these tests, the structure of the material is destroyed in order to probe its response to an applied stress or deformation. These methods mostly serve a quality control function. Their results provide an index of consistency to adjust milk-blending operations or to regulate a step in the butter-making process. While the results have practical significance, they often have no theoretical basis. Therefore, attempts have also been made to study the intrinsic properties of plastic fats. In many such cases, small deformation tests, in which the structure of the sample remains intact have been used to probe milk fat rheology. 

Relatively little information is available on the rheological behavior of ABS polymers (4, 5, 7, 8, 9, 10). Particularly little work of fundamental nature seems to have been done on the relation between ABS rheological properties and their composition, probably because of their complex structure. It is therefore difficult, on the basis of the published data, to develop a rational theory on the effect of the dispersed particles on the flow behavior of the blend. A number of papers were published on the viscoelastic properties of two-phase... 

Finally, the rheological behavior of block copolymers serves as a model for well compatibilized blends, with perfect adhesion between the phases. The copolymers provide important insight into the effects of the chemical nature of the two components, and the origin of the yield phenomena. 

Effects of addition of a compatibilizing block copolymer, poly(styrene-b-methyl methacrylate), P(S-b-MMA) on the rheological behavior of an immiscible blend of PS with SAN were studied by dynamic mechanical spectroscopy [Gleisner et al., 1994]. Upon addition of the compatibilizer, the average diameter of PS particles decreased from d = 400 to 120 nm. The data were analyzed using weighted relaxation-time spectra. A modified emulsion model, originally proposed by Choi and Schowalter [1975], made it possible to correlate the particle size and the interfacial tension coefficient with the compatibilizer concentration. It was reported that the particle size reduction and the reduction of occur at different block-copolymer concentrations. 

The data on the rheological behavior of molten (PP/PE)-g-IA systems suggest a complex nature of influence of the polymers on the course of free-radical chain transformations taking place during reactive extrusion. Small amounts of one or another polymeric component, introduced into a blend, cause some changes in the rheological properties of the molten (PP/PE)-g-IA systems. For instance, a low (below 25 wt%) quantity of PE, contrary to the expectations, would increase MFI while low amounts of PP would decrease MFI for (PP/PE)-g-IA systems against... 

The problem of PP/EPR functionalization is important from a practical standpoint because starting blends are a widely available commercial product EPR is added to PP to increase impact strength and low temperature resistance (53). Functionalized PP/EPR blends have been used as modifiers of impact strength and rheological behavior of engineering thermoplastics melts (54). From the scientific viewpoint, the functionalization of PP/EPR blends attracts attention of researchers, similarly to PP/PE blends, because of the differences in the course of secondary reactions in certain POs that accompany grafting and because of the process selectivity. 

When assessing the rheological behavior of PA/PO blends, a strong effect of shear forces upon should be considered. The reason is a qualitative difference between the flow curves for PO and PA. Aliphatic PAs show an extended Newtonian plateau typical of polymers with a narrow MWD (71). PA6, for instance, can retain the Newtonian pattern of flow (72) up to a shear rate of 10 s . The curve describing the relationship of rj versus y for PO is typical of polymers with a wide MWD the anomaly in viscosity ( j decreases with increase in the shear rate) was observed at a much lower shear rate of < 10 s . That is why the effects of viscosity s growth—in the case of PA6/PO compatibilized blends—manifest themselves to the utmost at relatively low shear rates, upto 10 s . Such shear rates are typical of extrusion of polymer materials (72). 

Some information for comparison of rheological behavior of PA6 blends with pure and grafted PO is given in Table 18.3. Values of MFI < 1 g/10 min for PA blends... 

Table 18.3 Effect of PO Type on Rheological Behavior of PA6/PO Blends as Obtained from Analysis of MFI (Concentration of PO = 30 wt%, P = 21.6 N).

J. L. White, Rheological behavior of molten polymer blends and particle-filled melts, in Polymer Compatibility and Incompatibility, Principles and Practices, K. Sole (ed.), Harwood Academic Publishers, Chur, 1980. 

Rheological Behavior of the Pure Components [SI], [SIS], and Pure Blends... 

In Section 16.4, we will particularly discuss the variations of the secondary plateau as a function of the diblock content in the blend [SIS-SI] and in the full adhesive formulation. The secondary plateau value is really a key parameter for optimizing the rheological behavior of the final adhesives. 

The linear viscoelastic behavior of the pure polymer and blends has already been described quantitatively by using models of molecular dynamics based on the reptation concept [12]. To describe the rheological behavior of the copolymers in this study, we have selected and extended the analytical approach of Be-nallal et al. [13], who describe the relaxation function G(t) of Hnear homopolymer melts as the sum of four independent relaxation processes [Eq. (1)]. Each term describes the relaxation domains extending from the lowest frequencies (Gc(t)) to the highest frequencies (Ghf( )), and is well defined for homopolymers in Ref [13]. 

We have calculated from the model the characteristics of these newly designed structures, which have been synthesized [23], and we have measured their rheological properties at room temperature. As Fig. 16.4 shows, it is possible to mimic the rheological behavior of the [SIS-SI] blend (we have applied a vertical shift on the curves for greater clarity). Also, the rheological behavior of the full formulations (Fig. 16.4) based on these newly designed copolymers yields the same properties as for [SIS-SI], which demonstrates that the tackifying resins act Hke a solvent of the elastomeric part of all the copolymers presented here. 

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