Miscible polymer blends phase morphology

A number of miscible polymer blends are only completely miscible and form one-phase systems over a limited concentration, temperature and pressure range. Under certain conditions of temperature, pressure and composition, miscible binary blends may phase separate into two liquid phases with different compositions. Important characteristics of this type of blends are the overall blend composition, the morphology and the composition of the different phases as well as the nature of the interface between the phases. 

The best commercial advantages of a polymer blend can be summarized by the word versatility [Olabisi et ah, 1979]. Unfortunately, miscible polymer-polymer blends usually show additivity of the component polymers properties, thus their versatility is limited. Furthermore, tike any other single-phase resin, for most apphcations miscible blends need to be toughened and/or reinforced. Thus, with the exception of PMMA/PVDF blends (primarily used for coatings) there are no miscible blends on the market The interest in miscible polymer blends is for the purpose of compatibUization and judicious selection of the processing conditions that may lead to the spinodal decomposition-type morphology (see Chapter 8 Morphology in this Handbook). 

Ternary blends that comprise two immiscible polymers and a copolymer are of a particular interest. They not only represent an ideal model for smdying compatibi-lization of polymer blends, but also they have found direct commercial applications. Phase diagram information can be found in reviews by Ajji and Utracki (1996, 1997) and in Chap. 2, Thermodynamics of Polymer Blends, Chap. 21, Miscible Polymer Blends, and Chap. 8, Morphology of Polymer Blends in this handbook. 

The morphologies of the miscible, immiscible, and partially miscible polymer blends are distinct from each other. In an immiscible blend, two phases are present the discrete phase (domain), which is lower in concentration, and the continuous phase, which is higher in concentration. The miscible polymer blends exhibit singlephase morphology. Partially miscible polymer blends may form completely miscible blends at a different composition. The two phases may not have a well-dehned boundary. Each component of the blend penetrates the other phase at a molecular level. The molecular mixing that occurs at the interface of a partially miscible two-phase blend can stabilize the domains and improve the interfacial adhesion. A compatible blend that has commercial possibilities may be immiscible, and a miscible polymer blend may lack commercial applications due to other factors such as cost, source of raw materials, safety, and environmental issues such as recyclability. 

Over 20 pairs of miscible polymer blends have been reported [4]. Substantial progress has been made in polymer science and engineering since the first review by Krause [5] in understanding the phase behavior of blends [6-10] and interpreting the technology practiced in the industry better. The application of thermodynamics is visible in the improved generation and stabilization of the morphology of blends. 

When polymers undergo phase separation in thin films, the kinetic and thermodynamic effects are expected to be pronounced. Phase morphology with a single characteristic length scale can be synthesized by quenching a partially miscible polymer blend below the critical temperature of demixing. 

In addition, the partially miscible polymer blends are thermorheologically simple, so that all considerations available for homogeneous systems can be applied in their case as well (Utracki 1990). The composition of the two phases from the vicinity of phase separation is not the same as in the case of immiscible mixtures where it is supposed that, if droplets are present in their corresponding morphology, they are composed from one of the mixture components and that their total volume is equal to that of the blend composition. This theory is not applicable to partially... 

When two or more polymers are mixed, the phase structure of the resulting material can be either miscible or inuniscible. Polymer blend can be classified into three basic categories namely miscible polymer blends, compatible polymer blends, and immiscible polymer blends [34]. The majority of polymers are immiseible at molecular level, as given by the laws of thermodynamics [35]. The internal disorder of the polymer system will eventually result in phase separation on a macroscopic scale after some time. The relative miscibility of polymers controls their phase behavior, which determines the final properties. Partially miscible blends show either two phases or single-phase morphology. However, the manifestation of snperior properties depends on the miscibility behavior of homopolymer. 

In a fundamental sense, the miscibility, adhesion, interfacial energies, and morphology developed are all thermodynamically interrelated in a complex way to the interaction forces between the polymers. Miscibility of a polymer blend containing two polymers depends on the mutual solubility of the polymeric components. The blend is termed compatible when the solubility parameter of the two components are close to each other and show a single-phase transition temperature. However, most polymer pairs tend to be immiscible due to differences in their viscoelastic properties, surface-tensions, and intermolecular interactions. According to the terminology, the polymer pairs are incompatible and show separate glass transitions. For many purposes, miscibility in polymer blends is neither required nor de-... 

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