Immiscible blend systems

For predicting the phase inversion and co-continuous regime in immiscible blend systems, various empirical and semi-empirical models exist [5], A model, valid for high viscosity ratios and elevated shear rates, was established by Utracki [46] ... 

As previously shown for PPE/SAN blends, the foaming behavior of immiscible blend systems is affected by both the properties of the blend phases and the overall blend structure [1], In the present blend system, the viscosity of one specific blend phase is varied as a result, not only the foaming behavior of the blend phase is altered but also the microstructure of the blend is affected [94]. By investigating blend systems with constant PPE to PS ratios of 75/25 and 50/50, and varying the SAN content in the range of 20-40 wt%, the influence of both the microstructure and the viscosity ratio can be analyzed (Table 3). 

By foaming an immiscible blend system of a poly(ethylene glycol)PEG/ polystyrene (PEG/PS), Taki et al. detected a similar foaming behavior as well as a bimodal cell size distribution [78], While smaller cells formed in the more... 

Selective blending the more viscous PPE phase with PS allowed one to tailor the processing window of the miscible PPE/PS blend phase and the microstructure of the immiscible blend system. Following this approach, simultaneous foaming of both blend phases and more homogeneous cell structures could be achieved. Additionally, the overall foam density could be reduced. 

Gutmann P, Ruckdaschel H, Bangarusampath DS, Altstadt V, Schmalz H, Muller AHE (2009) Influence of the microstructure on the foaming behavior of an immiscible blend system. Antec... 

Over the last decade, the poor economics of new polymer and copolymer production and the need for new materials whose performance/ cost ratios can be closely matched to specific applications have forced polymer researchers to seriously consider purely physical polymer blend systems. This approach has been comparatively slow to develop, however, because most physical blends of different high molecular weight polymers prove to be immiscible. That is, when mixed together, the blend components are likely to separate into phases containing predominantly their own kind. This characteristic, combined with the often low physical attraction forces across the immiscible phase boundaries, usually causes immiscible blend systems to have poorer mechanical properties than could be achieved by the copolymerization route. Despite this difficulty a number of physical blend systems have been commercialized, and some of these are discussed in a later section. Also, the level of technical activity in the physical blend area remains high, as indicated by the number of reviews published recently (1-10). 

The polyacrylate used by Sim et al., (2011) is a thermoplastic acrylic copolymer synthesized (Gan, 2005) by free-radical polymerization of six monomer units added in semi batches to the reactor, as described in detail by Zhou et al., (2004). Schematic representation of the chemical stmcture of PAc showing part of the random distribution of the six monomer units viz. styrene, methyl methacrylate (MMA), butyl acrylate (BA), AA, 2-hydroxy ethylacrylate (2HEA), and isobutyl methacrylate (iBMA) in the backbone of the copolymer is shown in Figure 13 (Sim et al., 2009). The alphabets a-f denote mole fraction of 0.16, 0.17,0.39,0.19,0.06, and 0.03, respectively of each monomer unit in the copolymer. Figure 14 displays the difference in the results between a miscible and an immiscible blend systems. 

PEO/ENR/LiClO and PEO/PAc/LiClO blend systems are depicted in Figure 25. The difference in the and values of the two amorphous polymers, ENR and PAc, are reflected in the conductivity results of the two immiscible blend systems to be discussed later. 

In immiscible blend systems, the accent was put on the fractionated crystallization features. A new and interesting work has been done since the first edition. This is extensively highlighted in the present chapter. The phenomenon is significant when the ciystallizable phase is dispersed in the amorphous phase of the second blend component. Reactively compatibilized blends were compared to uncompatibilized... 

For cases involving a random copolymer or a miscible blend of two amorphous rubbery polymers, the behavior is generally a volume fraction weighted average of the permeabilities of the two homopolymers. On the other hand, the transport properties of immiscible blend systems depend significantly on the relative permeabilities and the morphology of the immiscible blend. 

For polymer blends that are mixtures of two miscible polymers, only one Tg is observed. Thus any analytical technique that can be used to determine the Tg, can also be employed to distinguish between miscible and immiscible blend systems, provided that the 7 of the two individual components are sufficiently far apart. Several equations are available to describe the dependence of the glass transition temperature on blend composition. A simple linear relationship between Tg and composition is very seldom observed [12—15] ... 

Tables 5 and 6 summarize key properties and appHcations for miscible and immiscible blends which are either commercial as of 1996 or were commercialized in the past (2,314—316,342,343). Most of the Hsted blends contain only two primary components, although many are compatibiLized and impact-modified. Consequently, an immiscible system consisting of two primary components or phases may contain impact modifiers for each phase and a compatihilizer copolymer, for a total of five or more components.

Figures 20.13 and 20.14 describe the effect of dibutyltin dilaurate (DBTDL) on the tensile strength and tensile modulus for the 25/75 LCP/PEN blend fibers at draw ratios of 10 and 20 [13]. As expected, the addition of DBTDL slightly enhances the mechanical properties of the blends up to ca. 500 ppm of DBTDL. The optimum quantity of DBTDL seems to be about 500 ppm at a draw ratio of 20. However, the mechanical properties deteriorate when the concentration of catalyst exceeds this optimum level. From the previous relationships between the rheological properties and the mechanical properties, it can be discerned that the interfacial adhesion and the compatibility between the two phases, PEN and LCP, were enhanced. Hence, DBTDL can be used as a catalyst to achieve reactive compatibility in this blend system. This suggests the possibility of improving the interfacial adhesion between the immiscible polymer blends containing the LCP by reactive extrusion processing with a very short residence time.

Thus most of the time one obtains phase-separated systems in which the macromolecules of component A are not at all or only to a limited extent miscible with the macromolecules of component B, i.e., polymer A is incompatible or only partially compatible with polymer B. The synonymical terms polymer blend , polymer alloy , or polymer mixture denote miscible (homogeneous) as well as immiscible (heterogeneous) systems consisting of two or more different polymers. 

The number of PPE particles dispersed in the SAN matrix, i.e., the potential nucleation density for foam cells, is a result of the competing mechanisms of dispersion and coalescence. Dispersion dominates only at rather small contents of the dispersed blend phase, up to the so-called percolation limit which again depends on the particular blend system. The size of the dispersed phase is controlled by the processing history and physical characteristics of the two blend phases, such as the viscosity ratio, the interfacial tension and the viscoelastic behavior. While a continuous increase in nucleation density with PPE content is found below the percolation limit, the phase size and in turn the nucleation density reduces again at elevated contents. Experimentally, it was found that the particle size of immiscible blends, d, follows the relation d --6 I Cdispersed phase and C is a material constant depending on the blend system. Subsequently, the theoretical nucleation density, N , is given by... 

In order to overcome these drawbacks, novel concepts for enhancing the foamability of such immiscible blends need to be introduced. Using the PPE/SAN 60/40 blend as a reference system, the following concepts appear as most promising (Fig. 14) ... 

In this example of model reactive polymer processing of two immiscible blend components, as with Example 11.1, we have three characteristic process times tD,, and the time to increase the interfacial area, all affecting the RME results. This example of stacked miscible layers is appealing because of the simple and direct connection between the interfacial layer and the stress required to stretch the multilayer sample. In Example 11.1 the initially segregated samples do create with time at 270°C an interfacial layer around each PET particulate, but the torsional dynamic steady deformation torques can not be simply related to the thickness of the interfacial layer, <5/. However, the initially segregated morphology of the powder samples of Example 11.1 are more representative of real particulate blend reaction systems. 

Time-temperature superposition works for homopolymers and miscible blends but not for immiscible blends, filled systems (e. g., glass fiber reinforced plastics) or reactive or unstable polymers. 

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