Incompatible polyblends

Dynamic-Mechanical Measurement. This is a very sensitive tool and has been used intensively by Nielsen (17) and by Takayanagi (18). When the damping curves from a torsion pendulum test are obtained for the parent components and for the polyblend and die results are compared, a compatible polyblend will show a damping maximum between those of the parent polymers whereas the incompatible polyblend gives two damping maxima at temperatures corresponding to those of the parent components. Dynamic mechanical measurement can also give information on the moduli of the parent polymer and the polyblend. It can be shear modulus or tensile modulus. If the modulus-temperature curve of a polyblend locates between those of the two parent polymers, the polyblend is compatible. If the modulus-temperature curve shows multiple transitions, the polyblend is incompatible. 

One further point should be made. The WLF equation can correctly predict the relaxation behavior of incompatible systems, but for temperature ranges limited to one transition. For incompatible polyblends that exhibit two transitions, the equation will yield satisfactory results if applied to each transition separately. 

Takayanagi et aL (1963) found that the model of Figure 2.1 Ic, equation (2.17), most closely represented the behavior of incompatible polyblends. 

Incompatible polyblends n. Polymers that do not alloy to form stable compositions. 

Reinforcing fibers are sometimes added to incompatible polyblends to increase their resistance to mechanical stress. The fibers are much longer than the dimensions of the incompatible micro-phases, and bridge across the weak interfaces between them, producing much higher strength. 

A variety of other third polymers may be added to incompatible polyblends to improve compatibility. Most of them are random eopolymer structures with flexible or rubbery properties. Their compatibilizing action may be visualized in three ways ... 

More usually, as has been described earlier, polyblends are made from incompatible polymers that give a two-phase structure. These polyblends show two TgS, one for each phase. The temperatures of these transitions correspond closely to the T s of the respective homopolymers. 

To understand the mechanism of polyblending, experiments have been carried out with polymeric solution. W. Borchard and G. Rehage mixed two partially miscible polymer solutions, measured the temperature dependence of the viscosity, and determined the critical point of precipitation. When two incompatible polymers, dissolved in a common solvent, are intimately mixed, a polymeric oil-in-oil emulsion is formed. Droplet size of the dispersed phase and its surface chemistry, along with viscosity of the continuous phase, determine the stability of the emulsion. Droplet deformation arising from agitation has been measured on a dispersion of a polyurethane solution with a polyacrylonitrile solution by H. L. Doppert and W. S. Overdiep, who calculated the relationship between viscosity and composition. 

Appearance of Fused Product. From the fabrication point of view, if two polymers give a smooth band on a two-roll mill, the polyblend is said to be compatible. If the fused product is cheezy, it is said to be incompatible. Frequently, the fused product is pressed into a flat sheet. Transparency of the sheet signifies compatibility, whereas an opaque appearance means incompatibility. Obviously, these criteria are arbitrary and crude. They are subject to great variation owing to difference in individual judgement. In addition, they give no information on the morphological feature of the system. 

Glass Transition Temperature. If the glass transition temperatures of the polymeric components are known and the glass transition temperature of the polyblend is determined, one of two things can happen. If the polyblend shows two distinct transitions corresponding to the parent polymers, it is incompatible. If the polyblend shows one transition only, the system is compatible. Since the glass transition temperature is a measure of the segmental mobility of a polymer, it must be sensitive... 

VVThen two chemically different polymers are mixed, the usual result is a two-phase polyblend. This is true also when the compositional moities are part of the same polymer chain such as, for instance, in a block polymer. The criterion for the formation of a single phase is a negative free energy of mixing, but this condition is rarely realized because the small entropy of mixing is usually insufficient to overcome the positive enthalpy of mixing. The incompatibility of polymers in blends has important effects on their physical properties, which may be desirable or not, depending on the contemplated application. 

Stress Birefringence. Confirmation of the multiphase nature of the polyblends described may be obtained by stress-optical measurements. Since this technique apparently has not been used before to demonstrate incompatibility in polyblends, a brief description is given of the rationale behind the method. 

Where the two phases are completely compatible, a homogeneous polyblend results which behaves like a plasticized resin (one phase). If two polymers are compatible, the mixture is transparent rather than opaque. If the two phases are incompatible, the product is usually opaque and rather friable. When the two phases are partially compatibilized at their interfaces, the polyblend system may then assume a hard, impact-resistant character. However, incompatible or partially compatible mixtures may be transparent if the individual components are transparent and if both components have nearly the same refractive indices. Furthermore, if the particle size of the dispersed phase is much less than the wavelength of visible light (requiring a particle size of 0.1/a or less), the blends may be transparent. 

Rubber as the Disperse Phase. In polyblend systems, a rubber is masticated mechanically with a polymer or dissolved in a polymer solution. At the conclusion of blending, a rubber is dispersed in a resin as particles of spherical or irregular shape. We can further subdivide this system into three classes according to the major intermolecular forces governing adhesion (a) by dispersion forces—e.g., the polyblend of two incompatible polymers, (b) by dipole interaction—e.g., the polyblend of polyvinyl chloride and an acrylonitrile rubber (56), and (c) by covalent bond—e.g., an epoxy resin reinforced with an acid-containing elastomer reported by McGarry (43). 

The marginally compatible polyblend is analogous to the poor solvent case in that presence of the discrete dispersed phase results in less interaction between phases and, therefore, lower melt viscosities. Styrene-butadiene block copolymers have lower melt viscosities than random copolymers of the same composition and yield solutions of lower viscosity at the same concentration. This is because of the incompatibility but inseparability of the segments of the chain. 

Block and graft copolymers (incompatible copolymers) — For block or graft copolymers in which the component monomers are incompatible, phase separation will occur. Depending on a number of factors — for example, the method of preparation — one phase will be dispersed in a continuous matrix of the other. In this case, two separate glass transition values will be observed, each corresponding to the Tg of the homopolymer. Figure 4.6 shows this behavior for polyblends of polystyrene (100) and 30/70 butadiene-styrene copolymer (0). 

When the polymer components in a blend are less miscible, phase separation will form larger domains with weaker interfacial bonding between them. The interfaces will therefore fail under stress and properties of polyblends are thus likely to be poorer than for either of the polymers in the blend. U-shaped property curves (Figure 4.40c) thus provide a strong indication of immiscibility. In most cases they also signify practical incompatibility, and hence lack of practical utility. 

A major problem in polyblend development is trying to predict polymer miscibility. The incompatibility of various pairs of polymers has been correlated with the mutual effects on intrinsic viscosities and dipole moment differences of the component polymers [67,68]. These results can give a guide for finding compatible polymer or polymer pairs or with very low incompatibility. 

Whereas incompatible partners in polyblends usually result in heterogeneous morphologies (e.g., styrene-butadiene SB), and often in highly resilient plastics, compatible partners result in plastics with moderate property profiles in accordance with the mixing percentages (e.g., PC/PBT). 

Equation-5, X and Phi denote the molar and volume fiaetion of the components, respectively. Eor two polymers to be miscible, the free energy of mixing must be negative. If the solubility parameters of the polymer pairs are too far apart, the free energy of mixing becomes positive, and compatibilizers are often needed to reduce the interfacial tension between incompatible components in a blend. In industry, both miscible and immiscible polyblends are important materials because they fill (filFerent market needs. 

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