Blend components, immiscible

Although blending is an easy method for the preparation of TPEs, most of the TPE blends are immiscible. Very often the resulting materials exhibit poor mechanic properties due to the poor adhesion between the phases. Over the years different techniques have been developed to alleviate this problem. One way is to alter the blending technique so that the interfacial area between the component phases can be increased. By the proper selection of the processing technique either a co-continuous or... 

In this chapter we have discussed the thermodynamic formation of blends and their behavior. Both miscible and immiscible blends can be created to provide a balance of physical properties based on the individual polymers. The appropriate choice of the blend components can create polymeric materials with excellent properties. On the down side, their manufacture can be rather tricky due to rheological and thermodynamic considerations. In addition, they can experience issues with stability after manufacture due to phase segregation and phase growth. Despite these complications, they offer polymer engineers and material scientists a broad array of materials to meet many demanding application needs. 

Tg of either polymer components. In addition to the two distinguishable Tg signals, the melting endotherm and cold-crystallization exotherm of CA are both detectable for every blend at almost the same temperature positions as those for the CA (DS = 2.95) alone, with a proportional reduction of the respective peak areas. In contrast to the result, the thermograms compiled in Fig. 9a for the pair of CA (DS = 2.70) and P(VP-co-VAc) (VP/VAc = 0.51/0.49) (combination A) indicate a definitely single Tg that shifts to the higher temperature side with increasing CA content. As summarized in Fig. 8, CA/PVAc blends are immiscible irrespective of the DS of the CA component, while PVP forms a miscible monophase with CAs unless the acetyl DS exceeds a value of... 

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. 

We have previously used the term interfacial reaction to describe mixing between two reactive blend components. In reality, as we have seen in the Example 11.2, there is an interphase that is formed on the surface of the dispersed phase where molecules of both components can be found and react (66,67). If the nonfunctionalized blend components have high immiscibility, then the thickness, Si, of the interphase around the droplets, as well as the volume of the interphase, Vh will be small and, thus, the probability of the functional groups to react forming compatibilizing products will be low, giving rise to coarse and not very stable morphologies. Helfand (66) defines Si as... 

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). 

In the field of thermoplastic immiscible blends, the emulsifying activity of block copolymers has been widely used to solve the usual problem of large immiscibility associated with high interfacial tension, poor adhesion and resulting in poor mechanical properties. An immiscible thermoplastic blend A/B can actually be compatibilised by adding a diblock copolymer, poly(A-b-B) whose segments are chemically identical to the dissimilar homopolymers, or poly(X-b-Y) in which each block is chemically different but thermodynamically miscible with one of the blend component. Theoretical... 

As discussed in the first part, blends containing immiscible components such as polyolefins could improve the performances of the inherently brittle sPS. Until now the reported investigations have concerned simple binary blends containing a polyolefin and sometimes SEBS as a compatibilizer. In addition, sPS/ polyurethane and sPS/sulfonated sPS blends were also investigated. All these studies tried to correlate the microscopic features of the blends with their mechanical properties. 

Chen et al. [67,68] further extended the study of binary blends of ESI over the full range of copolymer styrene contents for both amorphous and semicrystalline blend components. The transition from miscible to immiscible blend behavior and the determination of upper critical solution temperature (UCST) for blends could be uniquely evaluated by atomic force microscopy (AFM) techniques via the small but significant modulus differences between the respective ESI used as blend components. The effects of molecular weight and molecular weight distribution on blend miscibility were also described. 

Tliis assumption is not necessarily useful technologically. A mote practical definition would consider components of a mixture compatible if the blend exhibits an initially desirable balance of properties that does not deteriorate over a lime equal to the useful life that is expected of articles made from the mixture. Miscible mixtures are evidently compatible by this criterion. Compatibility is not restricted to such behavior since a blend of immiscible materials can be very useful so long as no significant desegregation occurs while the mixture is being mixed. 

Blends of immiscible polymers exhibit a coarse and unstable phase morphology with poor interfacial adhesion. The ultimate properties of these blends are often poorer than those of either component. The poor mechanical properties can be improved with a small amount of an interfacial agent that lowers interfacial tension in the melt and enhances interfacial adhesion in the solid. High-strain properties, such as strength, tensile elongation, and impact strength, especially benefit from compatibilization (I, 2). 

Blends of immiscible components which undergo phase separation [4]. Blends usually have... 

PP/EVAc blends are immiscible, thus in two-component systems only a small amount of EVAc can be used, e.g., to improve dyeability, flexibility, electrostatic dissipation, or barrier properties. The hydrolyzed EVAc (EVAl) was also used [Minekawa ei al., 1969]. In most cases the PP/EVAc blends are part of more a complex, multicomponent system comprising a reactive compatibilizer (see Table 1.50). 

A large number of polymer blends contain one or two crystallizable components. The crystallization behavior of a polymer component in a blend is expected to be altered by the presence of the second blend component, whether both are completely miscible, partially miscible or totally immiscible. Therefore, a profound scien-... 

From a commercial point of view, semicrystalline polymers are of prime importance. Among the four mostly used commodity plastics (PE, PS, PVC and PP), only PS is completely amorphous. The three semicrystalhne polymers account for the largest volume of the commercial polymer blends. A majority of the polymer blends contains at least one crystalline component. Most polymer blends are immiscible. 

When the blends are immiscible in the molten state, the crystallinity is even more complex function of the ingredients properties, com-patibilization method, processing parameters, and post-processing treatments. The following factors have been identified to play major role (i) molecular constitution and M of the components (ii) composition, (iii) the type of phase morphology and the degree of dispersion (iv) the interphase, thus interactions between the phases, nature of the interface, migration of nuclei from one phase to the other, etc. (v) melt history, in... 

It may be interesting to note that assuming equal degrees of polymerization, N., of both blend components, Eq. 12.25 yield a simple relationship between the binary interaction parameter and the molecular weight (as expressed by R) B N/RT = 2. Thus, within the framework of the Huggins-Flory theory, system will be miscible when B N/RT < 2, and immiscible when B N/RT > 2. This is schematically shown in Figure 12.24. 

These blends are immiscible and their interfaces are unstable. Special interfacial treatments are required to make them suitable as materials of commerce. A second group of blends are those in which the components are all polyolefins. These blends will be miscible or nearly so. Also in this paper the general questions of blend miscibility and interfacial characteristics will be treated. 

Radioluminescence spectroscopy has been used to examine molecular motion, solubility, and morphology of heterogeneous polymer blends and block copolymers. The molecular processes involved in the origin of luminescence are described for simple blends and for complicated systems with interphases. A relatively miscible blend of polybutadiene (PBD) and poly(butadiene-co-styrene) and an immiscible blend of PBD and EPDM are examined. Selective tagging of one of the polymers with chromophores in combination with a spectral analysis of the light given off at the luminescence maxima gives quantitative information on the solubility of the blend components in each other. Finally, it is possible to substantiate the existence and to measure the volume contribution of an interphase in sty-rene-butadiene-styrene block copolymers. 

Compatible Polymer Blends. These are immiscible polymer blends that exhibit macroscopically uniform physical properties caused by sufficiently strong interface interactions between the polymer blend components. 

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