Immiscible elastomer blends properties

Multiple notable reviews of elastomer blends exist. The first general treatment of the subject by Hess et al. (1993) reviews the applications, analysis, and the properties of the immiscible elastomer blends. Two related treatments by Roland (1989) and Ngai and Roland (2004) exist and describe the physics of mixing immiscible polymer blends and a more recent account of the analytical methods. Mangaraj (2002) has a more detailed review of elastomer blends. Other reviews by Corish (1978) and McDonel et al. (1978) deal with specific aspects of elastomer blends. A publication by Zhang (2009) on specific EPDM blends with NR/BR for tire sidewall approaches this area from the view of a specific application. Less comprehensive accounts of this area are also described for polyolefin elastomer blends by Slusarski et al. (2003) and Feldman (2005). 

Since most polymers, including elastomers, are immiscible with each other, their blends undergo phase separation with poor adhesion between the matrix and dispersed phase. The properties of such blends are often poorer than the individual components. At the same time, it is often desired to combine the process and performance characteristics of two or more polymers, to develop industrially useful products. This is accomplished by compatibilizing the blend, either by adding a third component, called compatibilizer, or by chemically or mechanically enhancing the interaction of the two-component polymers. The ultimate objective is to develop a morphology that will allow smooth stress transfer from one phase to the other and allow the product to resist failure under multiple stresses. In case of elastomer blends, compatibilization is especially useful to aid uniform distribution of fillers, curatives, and plasticizers to obtain a morphologically and mechanically sound product. Compatibilization of elastomeric blends is accomplished in two ways, mechanically and chemically. 

These concepts for formation of miscible blend of elastomers with similar or near equivalence of solubility parameters require the components to be similar in properties. Thus a wide variation in the properties of the elastomer blends by changing the relative amounts of the two elastomers is not typical since it is unlikely that, for example, a nonpolar polyolefin elastomer and a polar elastomer like acrylate would be similar in solubility parameters. This relative invariance in the properties of the blend compared to the components is an inherent limitation on the basic, economic, and technological need for elastomer blends, which is to generate new properties by blends of existing materials. Similar or near equivalence of solubility parameters can be difficult to predict from chemical structure. For example, chemically distinct 1,4-polyisoprene and 1,2-polybutadiene are miscible, but isomeric 1,2-polybutadiene and 1,4-polybutadiene are immiscible. It is illustrative of this concept that an apolar hydrocarbon elastomer and a highly polar elastomer such as an acrylate cannot have, under any practical structural manifestation for either, a similar solubility parameter and thus be miscible. 

While true miscibility may not be required for elastomer properties, adhesion between the immiscible phases is required. Immiscible polymer blends that fulfill this criteria provide a significant opportunity to change the rheological, tensile, and wear properties of elastomer blends compared to miscible blends. 

Where the solubility parameter rule is in error is for natural rubber and polybutadiene. The differential in solubility parameters is around 0.6 but the two polymers are immiscible. Polybutadiene grade IISRP 1207 and an oil extended polymer such as IISRP 1712 have a differential of less than 0.1 and in this case the two elastomers are nearly fully miscible between the lower and upper critical solution temperatures. The blended elastomers mechanical properties then become a function of the filler type, distribution, vulcanization system, and any processing aids present. 

Polymer blends are a mixture of at least two polymers, their combination being supposed to lead to new materials with different properties. The classification of polymer blends into (1) immiscible polymer blends, (2) compatible polymer blends, and (3) miscible polymer blends is given by the thermodynamic properties of the resulting compound by means of the number of glass transition temperatures observed for the final product. To improve the compatibility between the blended polymers, some additives or fillers are used. To the same extent, rubber blends are mixtures of elastomers, which are usually combined to obtain an improved product, with properties derived from each individual component. 

Morphological effects in polymer blends and in block copolymers consisting of partially or completely immiscible components were discussed at a sufficient level of detail earlier in this article. See the paragraphs titled Polymer Blend Properties and Thermoplastic Elastomer Properties in thfe section titled... 

In addition to changes and variation in morphology, immiscible blends show additional, more complex changes due to different chemical properties of the two-component elastomer. This difference in the chemical structure manifests as three distinct but interrelated properties. First, the dissimilar elastomers differ in the retention of the fillers (e.g., carbon black) and plasticizers (e.g.. 

At ambient and processing temperatures, elastomers are viscous fluids with persistent transport phenomenon. In immiscible blends, these lead to change in the size and shape of the elastomer phases and migration of the fillers, plasticizers, and curatives from one phase to another. These changes are accelerated by processing and plasticization but retarded by the ultimate vulcanization. Retention of the favorable properties of a metastable blend, which is often attained only at a select interphase morphology and filler/plasticizer distribution, thus requires careful control of both the processing and the vulcanization procedures. 

Block polymers and polymer blends deserve now a great intere because of their multiphase character and their related properties. The thermodynamic immiscibility of the polymeric partners gives rise indeed to a phase separation, the extent of which controls the detailed morphology of the solid and ultimately its mechanical behavior. The advent of thermoplastic elastomers and high impact resins (HIPS or ABS type) illustrates the importance of the industrial developments that this type of materials can provide. In selective solvents, and depending on molecular structure, concentration and temperature, block polymers form micelles which influence the rheological behavior and control the morphology of the material. 

As noted earlier, compatibility of two elastomers is defined as when the polymers are immiscible but if in combination provide properties or show characteristics that are more useful than the properties of the original polymers 2]. Incompatible polymers when blended will show a dual 7. As discussed earlier, miscible blends are obtained when, at the molecular level, the two polymers are compatible. For example, blends of two highly polar polymers such as PVC and NBR are truly miscible. Polymer compatibility is governed by three parameters ... 

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