Miscible polymer blends, definition

By definition, miscible polymer blends are singlephase mixtures. Miscibility depends on the molecular weight, concentration, temperature, pressure, deformation rate, etc. Flow of these systems can be compared to that of solutions of low molecular weight, miscible components, or to flow of mixtures of polymeric fractions. Both models are far from perfect, but they serve to illustrate the basic behavior of miscible systems. 

As discussed in Chapter 2 of this Handbook, by definition, the miscible polymer blend is characterized by negative value of the free energy of mixing, AG = < 0, and positive value of... 

Sometimes, in analogous to metal alloys, the term polymer alloy is used. According to the lUPAC recommendation, the term polymer alloy for a polymer blend is discouraged. The term polymer alloy should be used for polymeric materials with macroscopically uniform physical properties in their whole volume. This definition includes compatible polymer blends, miscible polymer blends, or multiphase copolymers. 

This article reviews the phase behavior of polymer blends with special emphasis on blends of random copolymers. Thermodynamic issues are considered and then experimental results on miscibility and phase separation are summarized. Section 3 deals with characteristic features of both the liquid-liquid phase separation process and the reverse phenomenon of phase dissolution in blends. This also involves morphology control by definite phase decomposition. In Sect. 4 attention will be focused on flow-induced phase changes in polymer blends. Experimental results and theoretical approaches are outlined. 

By definition, polymer blends are mixtures of at least two macromolecular species, polymers and/or copolymers. For this reason any mixture of polymers should be treated as a blend. Most blends of chemically different polymers are immiscible. However, it has been documented that blends of chemically identical polymers (viz. LLDPE s) can also be immiscible when their macromolecules sufficiently differ in chain configuration. The polymer/polymer miscibility is limited to mixtures of homologous fractions, rare cases of polymeric pairs with strong specific interactions, or systems with the so-called miscibility... 

To make an analogy with metals polymer blends are sometimes referred to as polymer alloys. Thus the term alloy has been used to describe miscible or immiscible mixtures of polymers that are usually blended as melts. Another definition often used for a polymer alloy is that it is an immiscible PB having a modified interface and/or morphology. The general relationship between blends and alloys is shown in Figure 4.39. The term compatibilization in the figure refers to a process of modification of interfadal properties (discussed later) of an immiscible PB, leading to the creation of a polymer alloy. 

Compatibility in the polymer blends is defined by either miscibility on the molecular scale or absence of gross symptoms of phase separation. This definition is phenomenological and empirical but no unanimous theoretical scheme for predicting compatibility has been established. Thermodynamics predicts that exothermic mixtures satisfying Equation 5 would spontaneously mix,... 

Several interesting observations relate to such thermodynamic measurements. For example, the exothermic effects, associated with phase separation in LCST-type polymer blends, showed a correlation between the exothermic enthalpy and the interactions between the components (Natansohn 1985) however, the specific interaction parameter xn was not calculated. In another example, there are definitive correlations between the thermodynamic and the transport properties (see Chap. 7, Rheology of Polymer Alloys and Blends ). Thermodynamic properties of multiphase polymeric systems affect the flow, and vice versa. As discussed in Chap. 7, Rheology of Polymer Alloys and Blends , the effects of stress can engender significant shift of the spinodal temperature, AT = 16 °C. While at low stresses the effects can vary, i.e., the miscibility can either increase or decrease. 

Much of the work that has been done up to this point on high temperature polymer blends is the definition of miscible blend polymer pairs and an understanding of the features that lead to that miscibility. The development of miscible blends often leads to the ability to tailor the properties, including the Tg of mixtures. Such a tailoring is an alternative to the development of entirely new polymeric materials with the desired property profile. One of the advantages of the blend approach is that it is generally faster and less expensive than the synthesis and scale-up of an entirely new polymer. The downside of the blend approach is that it is difficult to define miscible pairs and miscibility is often the situation that is not observed with polymer mixtures. 

While the above definition of miscibility is unequivocal as it corresponds to a precise thermodynamic description of the system, in practical terms the experimental determination of polymer blend miscibility is not free from ambiguities. This is because the experimental determination of homogeneity in a blend is dependent on the experimental probe and, consequently, while a system may appear to be one-phase if examined at sufficiently large length scales, it may not be truly miscible at the molecular level. 

Usually, the presence of two Tg values for polymer blends is taken as an indication of immiscibility. The presence of a single Tg value dependent on the component ratio may signify that either the system is really miscible and consists of one phase, or the domain size in the system is below 15 nm, whereas the system preserves its two-phase state. Thus, the transfer from the system with two glass transition temperatures to the system with a single glass transition temperature may be considered, according to both definitions of compatibilization, as a sign of compatibilization and the only question is what process is under discussion. 

Abstract A review of definitions and the overall rationale for the production of high temperature polymer blends is provided.The discussion is divided essentially into two parts miscible and immiscible blends. It is pointed out that one concern with miscible polymer pairs is that of processing in the miscible state. This phenomenon is dependent on the position of the phase separation temperature relative to the glass transition temperature of the polymer blend. In the case of immiscible blends, the issue of adhesion of the polymers is discussed. Finally, the need for better theoretical models for the prediction of miscibiUty in polymer blends is highhghted and discussed. 

In fact, this topic has evolved into a central area of polymer research during the last 40 years. One of the first ideas, that as a rule polymer blends are immiscible, needs to be reevaluated due to the increasing number of miscible or partially miscible polymer pairs reported in the literature (see, for example, Paul and Newman, 919) Despite this high level of activity, much of the work remains based on art and intuition rather than on science. Most of the work performed on high temperature polymer blends has involved the definition of miscible polymer pairs and their phase separation characteristics. Not much work has been done to predict miscible pairs. In fact, this is an open area of research in the entire area of polymer blends. 

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... 

The definition of compatibility has been differentiated from miscibility since it is concerned with phase-separated polymers and is approached through the attainment of optimum properties for the blend (Bonner and Hope, 1993). Two of the main technologies used to achieve it are the addition of a third component (as discussed above) and reactive blending. The target in using a compatibilizer is the control of the interfacial tension between the components in the melt, translating to interfacial adhesion in the blend after processing. 

Measurements have been made of the size of the polymer coil in miscible blends, ° using neutron scattering. Some have observed expanded coils and others contracted coils. It has been suggested that this should depend on whether the polymer is glassy or rubbery, i.e., the chain flexibility. The results are not consistent but a definitive set of experiments remains to be done. Studies on PVC show the dominant effect that the small crystalline part of this polymer can play in scattering from its blends. The crystalline regions do not mix with the other polymer and this could greatly affect the properties of PVC blends. 

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