Immiscible blends, interfacial

Sulfonation has been used to change some characteristics of blends. Poly(2,6-diphenyl-l,4-phenylene oxide) and polystyrene are immiscible. However, when the polymers were functionalized by sulfonation, even though they remained immiscible when blended, the functionalization increased interfacial interactions and resulted in improved properties (65). In the case of DMPPO and poly(ethyl acrylate) the originally immiscible blends showed increased miscibility with sulfonation (66). 

The important factors that affect the rubber toughening are (1) interfacial adhesion, (2) nature of the matrix, (3) concentration of the rubber phase, and (4) shape and size of the rubber particles. In the PS-XNBR blend containing OPS, due to the reaction between oxazoline groups of OPS and carboxylic groups of XNBR, the interfacial adhesion increases and as a result, the minor rubber phase becomes more dispersed. The immiscible blend needs an optimum interfacial adhesion and particle size for maximum impact property. In PS-XNBR, a very small concentration of OPS provides this optimum interfacial adhesion and particle size. The interfacial adhesion beyond this point does not necessarily result in further toughening. 

The applicability of Noolandi and Hong s theory of compatibilization of immiscible blends using block copolymers has been extended to the reactive compatibilization technique by Thomas and coworkers [75,76]. According to Noolandi and Hong [77], the interfacial tension is expected to decrease linearly with the addition... 

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 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. 

The use of copolymers as surfactants is widespread in macromolecular chemistry in order to compatibilize immiscible blends. These additives are sometimes named surfactants , interfacial agents or more usually compatibi-lizers . Their effect on improving different properties is observed interfacial tension and domain size decrease, while there is an increase in adhesion between the two phases and a post-mixing morphology stabilization (coalescence prevention). The aim of the addition of such copolymers is to obtain thermodynamically stable blends, but the influence of kinetic parameters has to be kept in mind as long as they have to be mastered to reach the equilibrium. Introducing a copolymer can be achieved either by addition of a pre-synthesized copolymer or by in-situ surfactant synthesis via a fitted re-... 

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... 

Blend morphology commonly depends on the weight fraction and viscoelastic properties of each component, the interfacial tension between components, the shape and sizes of the discontinuous phase, and the fabrication conditions and setup. Most rheological experiments applied to homogeneous melts can also be similarly applied to these immiscible blends [55,63,88,89]. The viscoelastic properties arising from these studies should be labeled with a subscript apparent since the equations used to translate rheometer transducer responses to properties incorrectly assume that the material is homogeneous. Nevertheless, these apparent properties are often found to be excellent metrics of fabrication performance. 

Figure 16 shows that the best retardation effect is obtained with R22f5 among repulsive block copolymers investigated in this study. Thus, the competitive interactions of the blocks with different homopolymers are shown to promote the retardation. In other words, as the relative repulsion between block C and homopolymer B and between block D and homopolymer A increases, i.e., if the interaction of block C (D) with homopolymer B (A) is more repulsive than that of block C (D) with homopolymer A (B),the rate of phase separation is retarded more effectively. Vilgis and Noolandi [81] also predicted strong interfacial activity of block copolymers with such interactions. Therefore, it can be concluded that the interaction energy between blocks also has an important effect on the phase separation of immiscible blends. 

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 ... 

Impermeable inorganic filled (f) Interfacially stabalized immiscible blend... 

An interesting application of the direct relationship between nucleating interfaces and the total amount of the interfacial contact surface can be found in compatibilized immiscible blends. In these systems, the dispersed phase size becomes much smaller, strongly increasing the total amount of interface at which nucleation can occur. Some authors reported that this could cause an upward shift in the by up to 10°C [Wei-Berk, 1993]. However, other studies in which the crystallization behavior of a compatibilized blend was investigated did not always mention such a clear nucleating activity (Table 3.17). 

A few authors have observed coincident crystallization of both phases in crystalline/crystaUine immiscible blends. This phenomenon was reported for blends in which the minor phase exhibits a higher degree of undercooling for crystallization due to its fine dispersion (see Section 3.4.4.) and the matrix phase crystallizes at its bulk T that is lower than that of the minor phase. An additional factor that should be taken into account is that a heterogeneous nucleation is promoted on surfaces with a high interfacial tension [Helfand and Sapse, 1975] (i.e., a crystal-... 

The structure and morphology of immiscible blends depends on many factors among which the flow history and the interfacial properties are the most important. At high dilution, and at low flow rates the morphology of polymer blends is controlled by three dimensionless microrheologi-cal parameters (i) the viscosity ratio, where r j is the viscosity of the dispersed liquid and r 2 that of the matrix (ii) the capillarity number, k = d / Vj2, where d... 

Addition of a block copolymer, A-B, to immiscible blend of homopolymers A and B reduces the interfacial tension coefficient similarly as addition of a surfactant affects emulsions. Thus, the idea of the critical micelle concentration, CMC, and the limiting value of the interfacial tension coefficient, can be applied to polymer... 

To select proper compatibilizer, it is imperative to know whether the copolymer is capable to (i) engender a hne dispersion during blending, (ii) preferentially migrate to the homopolymers interface, (iii) stabilize the morphology against segregation, and (iv) enhance the adhesion between the phases. It is only when all these conditions are satished that the idea of interfacial activity of classical emulsifiers can be applied to copolymers added to immiscible blends. 

A copolymer of the two immiscible polymers themselves would seem to be ideally suited to act as a compatibilizing agent for an immiscible blend. If the copolymer is at the interface of the two phases, then the segments of the copolymer dissolve in the respective bulk phases of the same identity. The copolymer acts as emulsifying agent for the blend resulting in reduced interfacial energy and improved interphase adhesion. 

Effects of addition of a compatibilizing block copolymer, poly(styrene-b-methyl methacrylate), P(S-b-MMA) on the rheological behavior of an immiscible blend of PS with SAN were studied by dynamic mechanical spectroscopy [Gleisner et al., 1994]. Upon addition of the compatibilizer, the average diameter of PS particles decreased from d = 400 to 120 nm. The data were analyzed using weighted relaxation-time spectra. A modified emulsion model, originally proposed by Choi and Schowalter [1975], made it possible to correlate the particle size and the interfacial tension coefficient with the compatibilizer concentration. It was reported that the particle size reduction and the reduction of occur at different block-copolymer concentrations. 

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. 

As described earlier, compatibilizers can enhance compatibility in a polymer blend by promoting physical or chemical interactions with blend components. If the compatibilizer locates at the interface, it will bind the two components together interlacing their phases. The main effect of interfacial modification on the morphology of an immiscible blend is a reduction on the particle size and a narrowing of the particle size distribution. This reduction in particle size is related with a decrease in the interfacial tension and a reduction in the coalescence process. Interfacial modification seems to be the dominant factor for controlling the dispersed phase size, and the dependence of this phase size... 

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