Thermodynamics of high temperature polymer blends

The performance of polymer blends depends on the properties of the polymeric components, as well as on how they are arranged in space. The spatial arrangement is controlled to a large extent by the thermodynamics of the system. The term thermodynamics invariably brings to mind miscibility . The aim of this chapter is to discuss the thermodynamic features of high temperature polymer blends and highlight studies that have focused on a determination of the features that lead to miscibility in such systems. 

A necessary condition for two polymers to form a miscible mixture is that the free energy of mixing, should be less than zero. In general, is given by the following equation  

Without the satisfaction of this condition, the miscible system is thermodynamically unstable and phase separation will occur. 

One of the most common models for dealing with the free energy of mixing between two polymers is the Flory-Huggins model. That model assumes that the free energy of mixing, AG, for the two polymers is given by  

Flory- first proposed the concept of molecular composites which are systems based on the mixing of a rigid-rod polymer and a random coil polymer, vastly different molecular conformations. The theoretical prediction was made that phase separation is easily induced in such systems. The phase separation in such blend systems is based solely on entropic effects. This is an important difference with random coil mixtures in which 

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There do not appear to be any books devoted solely to high temperature polymer blends, let alone the thermodynamics of such mixtures. A reasonable review article of some of the factors that affect the phase behavior of polymer blends for high temperature applications was provided by Jaffe et al. ... 

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. 

Practically speaking, however, very few polymers can be categorized as rigid rod in conformation, but instead fall into the category of semi-rigid in molecular conformation. In fact, most high temperature polymers that have been blended can best be classihed as semi-rigid. Therefore, the theories that have been developed to define the thermodynamics of mixtures of polymers do not apply to these polymers. 

The flow behavior of the polymer blends is quite complex, influenced by the equilibrium thermodynamic, dynamics of phase separation, morphology, and flow geometry [2]. The flow properties of a two phase blend of incompatible polymers are determined by the properties of the component, that is the continuous phase while adding a low-viscosity component to a high-viscosity component melt. As long as the latter forms a continuous phase, the viscosity of the blend remains high. As soon as the phase inversion [2] occurs, the viscosity of the blend falls sharply, even with a relatively low content of low-viscosity component. Therefore, the S-shaped concentration dependence of the viscosity of blend of incompatible polymers is an indication of phase inversion. The temperature dependence of the viscosity of blends is determined by the viscous flow of the dispersion medium, which is affected by the presence of a second component. 

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). 

Buta et al. (2001) tested the Monte Carlo approach for the lattice cluster theory to derive the thermodynamic properties of binary polymer blends. They considered the two polymers to have the same polymerization indices, i.e., M = 40, 50, or 100. The results confirm that this lattice cluster theory had a higher accuracy compared to the Flory-Huggins theory and the Guggenheim s random mixing approximation. However, some predictions for the specific heat were found to be inaccurate because of the low order cutoff of the high temperature perturbative expansion. 

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),... 

Block polymers and polymer blends deserve now a great intere because of their multiphase character and their related properties. The thermodynamic immiscibility of the polymeric partners gives rise indeed to a phase separation, the extent of which controls the detailed morphology of the solid and ultimately its mechanical behavior. The advent of thermoplastic elastomers and high impact resins (HIPS or ABS type) illustrates the importance of the industrial developments that this type of materials can provide. In selective solvents, and depending on molecular structure, concentration and temperature, block polymers form micelles which influence the rheological behavior and control the morphology of the material. 

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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