Thermodynamics blending miscible high temperature

There have been several miscible high temperature polymer pairs that have been identified in both the patent and open literature. The term thermodynamics invariably brings to mind miscibility. The aim of this section is to discuss the thermodynamic features of polymer blends and highhght studies that have focused on a determination of the features that lead to miscibility. 

There have been several miscible high temperature polymer pairs defined in the literature. Several of these pairs are miscible from solution but are immiscible when processing is attempted in the melt state. These results indicate that the blends phase separate when heated above their glass transition temperature. This further shows that kinetic factors as well as thermodynamic factors are important in the observed miscibility. Also, the role of the solvent in the observed miscibility needs to be better understood. One of the current technical challenges is to widen the temperature range between the glass transition temperature of the blend and its phase separation temperature, to allow miscible blends to be processed in the melt state. 

Abstract The performance and subsequent properties of polymer blends are highly dependent on the blend s phase structure. For example, a miscible mixture of two polymers wiU have different features to an immiscible mixture of the same two polymers. Additionally, the manner in which a transformation from a miscible blend to an immiscible blend occurs will affect the ultimate properties. These features can be categorized under the topic of thermodynamics of polymer blends. This chapter discusses some features of the thermodynamics of blends that contain high temperature polymers, highlighting those which are most important in defining the blend phase structure. A comparison is made with other polymer blends, and important differences are noted. 

The most basic question when considering a polymer blend concerns the thermodynamic miscibility. Many polymer pairs are now known to be miscible or partially miscible, and many have become commercially Important. Considerable attention has been focussed on the origins of miscibility and binary polymer/polymer phase diagrams. In the latter case, it has usually been observed that high molar mass polymer pairs showing partial miscibility usually exhibit phase diagrams with lower critical solution temperatures (LCST). 

Generally, most polymer pairs of high molecular weight are immiscible in the range from glass transition temperature (Tg) to thermal decomposition temperature (T ). It is difficult to mix polymers at a molecular level even for polymer pairs with similar structures, e.g., polyethylene (PE) and polypropylene (PP). To discuss the thermodynamics for the miscibility and the phase diagram of polymer blends, the Flory-Huggins equation has been widely used (Flory 1953),... 

Miscibility in polymer blends has been studied by both theoreticians and experimentalists. The number of polymer blend systems that have been found to be thermodynamically miscible has increased in the past 20 years. Systems have also been found to exhibit the upper or lower critical solution temperatures. So complete miscibility is found only in limited temperature and composition ranges. A large number of polymer pairs form two-phase blends. This is consistent with the small entropy of mixing that can be expected of high polymers. These blends are characterized by opacity, distinct glass transition temperatures, and deteriorated mechanical properties. Some two-phase blends have been made into composites with improved mechanical properties. Often, incompatibility is the general rule, and miscibility or even partial miscibility is the exception. 

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