The morphology of immiscible polymer blends

There are many ways of starting with a single phase and proceeding to a phase-separated structure figure 4.30 illustrates three of them. In figure 4.30a we presuppose the existence of a phase diagram that has both one-phase and 

To these three possibilities we must add a fourth (figure 4.31) in which the two polymers are mechanically mixed. Work is done by the mixing device, part of which is converted into heat, but some goes into creating domains of one species dispersed in the other. Associated with the extra area of interface created is an interfacial energy that is supplied by the mixing device. 

This is a demanding requirement all polymers have a glass transition temperature below which all motion is frozen out, defining a lower limit on accessible temperatures at higher temperatures chemical degradation of the polymers becomes important. 

Of these methods of reaching a phase-separated state, the first method, via a temperature quench, is probably the least important in practice. Nonetheless, it is the simplest to deal with theoretically and consequently it is the route to phase separation that has been most intensively studied. It is worth taking a more detailed look at the mechanisms by which phase separation occurs, for this emphasises the importance of the interfacial energy in determing the phase morphology. 

Within the spinodal line, any small fluctuation in composition will lead to a lowering of the free energy under these conditions phase separation will proceed immediately via a mechanism of amplification of random composition fluctuations called spinodal decomposition (Binder 1991). In the metastable part of the phase diagram a small composition fluctuation actually raises the free energy and, in order to begin the phase-separation process, a droplet of the minority phase, of a size greater than a critical size, has to be nucleated. Thus this mechanism of phase separation is known as nucleation and growth. 

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The effect of viscosity ratio on the morphology of immiscible polymer blends has been studied by several researchers. Studies with blends of LCPs and thermoplastics have shown indications that for good fibrillation to be achieved the viscosity of the dispersed LCP phase should be lower than that of the matrix [22,38-44]. 

Since the processing conditions and mixing equipment have a crucial effect on the morphology of immiscible polymer blends [45], experiments were carried out in four different types of extruders to find optimal conditions for blend preparation and fibrillation. Nevertheless, the morphologies of PP-LCP blends produced by... 

B. D. Favis, Factors Influencing the Morphology of Immiscible Polymer Blends in melt Processing, in Polymer Blends, Vol. I, D. R. Paul and C. B. Bucknall, Eds., Wiley-Interscience, New York, 1999. 

Meijer et al. [1988] and Elemans et al. [1988] investigated the potential of electron irradiation to stabilize the morphology of immiscible polymer blends. Their concept was that selective crosslinking of a dispersed phase in a matrix that remains unaffected, or degrades, should help fix the morphology of the blend. 

FIGURE 1.5 Variation of the first normal stress difference versus strain for (O) uncompati-bilized blends of polydimethylsiloxane (PDMS) in polyisoprene (PI) and for ( ) 10% com-patibilized blends of polydimethylsiloxane (PDMS) with polyisoprene (PI). (Adapted from Van Hemelrijck Ellen. Effect of physical compatibilization on the morphology of immiscible polymer blends. PhD thesis, K U Leuven, 2005.)... 

Van Hemelrijck Ellen. Effect of physical compatibilization on the morphology of immiscible polymer blends. PhD thesis, K U Leuven, 2005. 

It is well known that the mechanical properties of polymer blends are determined by their composition, domain size, and domain size distribution. The ability to monitor and control the morphology of immiscible polymer blends is very important. For example, Epstein and Carhart [56] used a Newtonian model for emulsions and derived relationships for the attenuation as a function of blend composition, domain size, R, and frequency,/ A longitudinal wave propagating in the matrix approaching an interface with a domain of the second fluid will give rise to reflected, transmitted shear and thermal waves, while excess attenuation can be determined as a linear function of the composition. A similar approach was also described elsewhere [57]. 

The properties of immiscible polymers blends are strongly dependent on the morphology of the blend, with optimal mechanical properties only being obtained at a critical particle size for the dispersed phase. As the size of the dispersed phase is directly proportional to the interfacial tension between the components of the blend, there is much interest in interfacial tension modification. Copolymers, either preformed or formed in situ, can localize at the interface and effectively modify the interfacial tension of polymer blends. The incorporation of PDMS phases is desirable as a method to improve properties such as impact resistance, toughness, tensile strength, elongation at break, thermal stability and lubrication. 

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. 

Effect of Flow Geometty on the Steady-State Shear Viscosity and Morphology of Immiscible Polymer Blends... 

Different monomers may be copolymerized to modify the properties of thermoplastics. For the same purpose, homo- and copolymers are frequently mixed with other substances, including other polymers, various fillers, and nanofillers. The presence of comonomers in macromolecules, as well as interactions between macromolecules in miscible blends, can affect both crystallization and morphology of the polymeric material. Interfaces and the confinement of polymer chains within a finite volume influence the solidification and morphology of immiscible polymer blends and polymer-based composites. They are also of special importance in ultra-thin polymer layers where the thickness is comparable to or smaller than the lamellar crystal thickness itself... 

The question might be addressed now to know whether phase morphology and properties of Immiscible polymer blends can be modified by a way different from the previously described emulsification. 

Process of modification of the interphase in immiscible polymer blends, resulting in reduction of the interfacial energy, development and stabilization of the desired morphology, leading to the creation of a polymer alloys with enhanced performance. 

This chapter, related to the crystallization, morphological structure and melting of polymer blends has been divided into two main parts. The first part (section 3.1) deals with the crystallization kinetics, semicrystalline morphology and melting behavior of miscible polymer blends. The crystallization, morphological strucmre and melting properties of immiscible polymer blends are described in the second part of this chapter (section 3.4). 

Blending offers an interesting means of tailoring product properties to specific applications. However, in the case of immiscible polymer pairs, the desired properties are not achieved readily without a compatibilizer, which enhances the phase dispersion and stability, as well as a good adhesion between the phases. This can be effectuated by physical or reactive methods [Folkes and Hope, 1993]. Compatibilization strongly affects the blend phase morphology and as such, it also may influence the crystallization behavior of the blend [Flaris et al., 1993]. Because both factors are related to the final properties of the blend, it is worth paying attention to these phenomena. 

Before discussing the rheological performance of polymer blends, it is important to recall the basic features of morphology in immiscible polymer blends. 

With the knowledge of the flow behavior of simpler systems, viz. suspensions, emulsions, block copolymers, as well as that of the mumal interactions between the rheology and thermodynamics near the phase separation, one may consider the flow of more complex systems where all these elements may play a role. Evidently, any constitutive equation that may attempt to describe flow of immiscible polymer blends should combine three elements the stress-induced effects on the concentration gradient an orientation function and the stress-strain description of the systems, including the flow-generated morphology. Such a comprehensive description stiU remains to be formulated. 

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]. 

Because of the high interfacial tension, the morphology of the blends is not stable. Coalescence readily occurs in the molten state. As suggested by Macosko et al. (121), in melt mixing of immiscible polymer blends, the disperse phase is first stretched into threads and then breaks into droplets, which can coalesce together into larger droplets. The balance of these processes determines the final dispersed particle sizes. With increase of disperse phase fraction (usually more than 5 wt%), the coalescence speed increases and the dispersed phase sizes increase (121-123). 

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