Immiscible blends, properties processing

Thus, it appears that chemical reactivity or ionic-cross interactions could lead to in situ compatibilising or miscibility enhancement during melt-mixing. However, several questions remain. How does the reactivity modify the thermodynamic balance, the reciprocal miscibility or the rheological behaviour of the melt Or, how the covalent or ionic bonding influence the interfacial adhesion processability and final mechanical properties of the immiscible blends ... 

Much additional work is still needed, however, to develop reliable rules for predicting (even at a merely qualitative level) the phase structure (the continuities and sizes of phase domains) in immiscible blends from a knowledge of the composition, the component properties and the flow field in a mixing or processing device [47],... 

This particular strategy is limited to those cases in which an immiscible polymer blend contains two semi-crystalline polymers that can co-crystalUze. Nadkarni and Jog [1989, 1991] have reviewed examples of this type of compatibilized blend. Co-crystaUization may also occur as a secondary process in an intimately mixed blend containing a copolymer with concomitant elfects on blend properties as shown in a few of the examples of this review. 

At high dilution the morphology of an immiscible blend is controlled by the viscosity ratio, f, the capillarity number, K, and the reduced time, t, as defined in Eq 9.8. The interfacial and rheological properties enter into K, and t. As the concentration increases, the coalescence becomes increasingly important. This process is also controlled by the interphasial properties. 

Polymer blends can be divided into two groups miscible and immiscible blends. Miscible blends are homogeneous and stable. Their properties tend to be intermediate. However, they are relatively few. Most polymer blends are immiscible. Their properties are strongly affected by their phase morphologies, which are decided by their viscosity, interfacial tension, and processing methods. In this review we will describe polyolefin blends. Many of these blends involve polar polymers with polyolefins. 

We begin in Section 9.2 with the morphology in binary blends of iPP and various rubbery olefin copolymers where we remark the interrelation between the miscibility and dynamic mechanical properties. Section 9.3 describes the molecular orientation behavior under tensile deformation of iPP-based blends, and we compare the differences in deformation behavior between miscible and immiscible blends. Section 9.4 contains the solidification process in iPP-based blends where the effects of miscibility in the molten state on the crystallization of iPP matrix are discussed. 

Compatibilized Polymer Blends. Immiscible blends in which the microstructure and physical properties can be stabilized by adding surface-active species called compatibilizers. These compatibiliz-ers will influence various morphological processes, such as deformation, breakup, and coalescence of droplets. 

PEN and PET copolymers fall into two groups. Low-NDC copolymers contain less than 15% NDC, and high NDC copolymers have 85% or more NDC. Copolymers with intermediate ranges of NDC are not used because they cannot crystallize and therefore have inferior properties. Because homopolymer PET and PEN are immiscible, blends require special mixing techniques to cause sufficient transesterification to occur. This amounts to the production of a copolymer during the extrusion process, as mentioned earlier. Blends of homopolymers with copolymers are easier to process than blends of the homopolymers themselves. Usually low-NDC copolymers are blended with PET, and high-NDC copolymers with PEN. 

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. 

The properties of the finished articles made from immiscible blends are governed by the morphology created as a result of the interplay of processing conditions and inherent polymer characteristics, including crystallizability. Therefore, a scientific understanding of the crystallization behavior in immiscible polymer blends is necessary for the effective manipulation and control of properties by compounding and processing of these blends. 

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