Blending miscible high temperature polymers

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. 

Thus, we have two different types of behavior in blends of high temperature polymers. The first situation is exemphfied by the PBI blends in which specific interactions through the N-H group of the PBI are responsible for the observed miscibility. The other situation is displayed by the mixtures of two LCP s in which entropic effects are important for the miscibility. These types of behavior are the extreme case of behavior. This also suggests that there are blend systems of high temperature polymers in which both enthalpic and entropic effects should be important factors in the miscibihty. 

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. 

Poly(aryl ketones) (PEEK, PEK, and PEKK) are commercial high temperature polymers offering an excellent combination of properties combined with thermoplastic behavior. Poly(aryl ether ketone) PAEK blends have been reviewed by Harris and Robeson [1989]. Miscibility with PEI (Ultem 1000 GE) and other PI containing isopropylidene bridging units was noted. Arzak et al. [1997] reviewed the performance ofPEEK/PEI blends and noted a synergistic behavior in ductility and impact strength as reported earlier. Utility of these blends as a thermoplastic matrix candidate for advanced composites has been proposed [Harris and Robeson, 1989 Davis et al., 1992]. 

It should be noted that there have been attempts to produce molecular modeling results and simulations of high temperature polymer blends. The most extensive of these efforts was published by Jacobson et al. (1992). Those workers used a short chain molecular model that incorporates the effects of both inter- and intra-molecular interactions. Using that model, estimates of the net interaction energies for a series of high temperature polymer blends were calculated and used to predict miscibility. The results were in general agreement with experiments and were used to focus the direction of additional experimental work. 

PBI is one of the most highly studied high temperature polymers in miscible blends. The fundamental reason for the observation of miscibility in PBl-based systems is the presence of the N-H functional group that can interact with the functional groups which are present on the backbone of other polymers. Thus, miscibility in these type of systems is an example of a specific interaction that leads to a negative enthalpy of mixing, a requirement for forming miscible mixtures. 

There are other high temperature polymers that have been shown to form miscible mixtures as well. Several polyimides (Pi s) have already been discussed in blends with PBI. Other miscible PI blends have been reported with PEEK (Kong et al. 1998), polyethersulfone (Liang et al. 1992) and sulfonated PEEK (Karcha and Porter 1989). Although the mechanism for miscibility of PBI/PI mixtures was demonstrated to be related to hydrogen bonding between the chains of the two components, the mechanism for miscibility of these other systems was not so clear. 

This approach was further explored by Hakemi (2000) who prepared blends that contain both a wholly aromatic and an aromatic-aliphatic LCP that are miscible with each other. The ultimate goal of this approach was to develop multi-component blends that have components of thermoplastics. The miscible LCP blends could be useful as reinforcing agents for the thermoplastic matrix polymer and, due to the fact that the LCP s contain some of the components of the thermoplastic polymer, there is expected to be improved adhesion between the LCP portion and the matrix portion of the mixture. This is another example of an attempt to balance the phase separation that is inherent in high temperature polymer blends due to molecular conformation differences by strengthening the enthalpic interaction between the two polymers. 

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. 

As has already been discussed earlier in this chapter, one of the most common ways to achieve miscibility in high temperature polymer blends is through the introduction of specific interactions between the two components in the mixture. This is not surprising, as this is also an often-used approach to develop miscibility in other types of blend systems as well. The one exception to this pattern is the case of blends of two liquid crystal polymers in which miscibility has been determined in systems in which there is no well-defined specific interaction. In those systems, miscibility appears to occur based primarily on similarities in molecular conformation between the two blend components. These results suggest that entropic effects play a significant role in defining the phase behavior of mixtures that contain high performance polymers. 

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. 

Based on the rigid conformation of many high temperature polymers, it is expected that the criteria for miscibility of such polymers will be even stricter than for random coil polymers. However, several high temperature miscible polymer blends have been discovered and reported in the litera-ture2-6. 2-i6 Yvhich are based on poly(2,2 (m-phenylene)-5-5 bibenzimidazole) (FBI), the chemical structure of which is shown in Fig. 1.1. Chapter 7... 

Better theoretical models and predictive tools need to be developed which can be used to understand the structural features in polymers which lead to miscibihty. Presently, most of the information obtained in this regard has been done through trial and error. Then, when a miscible system is found, efforts are expended to rationalize the observed miscibility. A different approach would be to develop theoretical schemes which can predict miscibility beforehand and, then, experimentally produce such systems. Unfortunately, the present computer technology does not yet adequately allow for the incorporation of explicit molecular details in such models. When such developments become available, a major step in the development of high temperature polymer blends will be achieved. 

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. 

From an experimental perspective, it becomes clear that miscible blends offer certain advantages. Further, an obvious way to enhance the possibility of miscibility in mixtures that involve high temperature polymers is through the development of specific interactions. That approach should be further explored with high temperature polymers through the incorporation of chemical groups that can promote such interactions. A caution about that... 

One of the primary rationales for producing blends of polysulfone with other polymers is to use the polysulfone to impart separation and membrane capabilities to the material and for the second polymer to provide higher temperature performance than is possible with the use of the polysulfone alone. For example, in the next section of this chapter, blends of polysulfone with other high temperature polymers such as polyimides (Pis) and polybenzimidazole (PBl) will be discussed. Much of that effort is focused on the production of miscible blends that can be fabricated into both symmetric and asymmetric membranes. Later sections of this chapter will focus on the use of polysulfone in mixtures to modify other properties of polymers, particularly the fracture and impact behaviors. 

Miscible blends of high molecular weight polymers often exhibit LOST behavior (3) blends that are miscible only because of relatively low molecular weights may show UCST behavior (11). The cloud-point temperatures associated with Hquid—Hquid phase separation can often be adequately determined by simple visual observations (39) nevertheless, instmmented light transmission or scattering measurements frequendy are used (49). The cloud point observed maybe a sensitive function of the rate of temperature change used, owing to the kinetics of the phase-separation process (39). 

In Fig. 10.4 there is a temperature range in which two polymers form a miscible blend. Why do the polymers separate into distinct phases at both high and low temperatures ... 

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