High temperature polymer blends

The last several decades have seen an exponential increase in the activity of engineering polymer blends. While this activity will continue, the area that will probably show the most future increase in commercial activity will be in high temperature systems. These blends include LCP and molecular composites as subsections that will be discussed separately. The activity in high temperature polymer blends has been primarily in the patent and published literature. Several examples of developmental and specialty commercial blends have emerged and many more are expected to follow in the future. 

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]. 

Additional polymer blends comprising PAEK s offering property combinations of potential utility include PSF [Robeson and Harris, 1986 Harris and Robeson, 1989] structurally different poly(aryl ketones) [Harris and Robeson, 1986], PAr [Robeson and Harris, 1992], poly(amide-imide) PAI [Harris and Gavula, 1992], PPS [Robeson, 1987], and other PI [Harris et al., 1992]. Mixtures of structurally different PAEK s were noted to be isomorphic within specific limits of ether/ketone ratios [Harris and Robeson, 1987]. Blends of polybenzimidazole, PBI and several commercial PI (Ultem 1000 and Matrimid 5218) have been studied in depth at the University of Massachusetts and found to be miscible. FTIR studies [Guerra et al., 1988 Kim et al., 1993], NMR studies [Grobelny et al., 1990], thermal, dielectric, and mechanical 

Miscibility of PI blends of different structures was reported by Hasegawa et al. [1991] using charge-transfer fluorescence spectra, dynamic mechanical analysis, and phase-contrast microscopy. These blends were BPDA/PDA PI with PMDA/PDA and PMDA/ODA PFs (BPDA = biphenyltetracarboxylic dianhydride PDA = p-phenylene diamine ODA = oxydianUine PMDA = pyromellitic dianhydride). Two patents issued virtually simultaneously noting the utility of miscible PI blends for gas separating membranes [Burgoyne et al., 1991 Kohn et al., 1991]. 

Recently, combinations of polymers meeting the basic concept of molecular composites have been reported. Poly-(p-phenylene terephthalamide) miscibility with PA-6 and PA-66 was reported [Kyu et al., 1989]. The blends were prepared by rapid coagulation of methane sulfonic acid solutions in water. Above 70% of the rigid rod polymer, polyamide crystallization disappears implying a level of intermixing of the blend constituents. However, thermal treatment results in phase separation thus indicating metastability for this combination. 

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Wang YF, Hsu T, Hay AS, Li K, Patel B. High temperature polymer blends of poly(aryl ether ketone phthalazinone). US Patent Application 20110104417, assigned to Polymics Ltd., State College, PA 2011. 

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. 

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. 

M.T. DeMeuse, High Temperature Polymer Blends (Woodhead Publishing, Cambridge, 2014) M.T. DeMeuse, M. Jaffe, Mol. Cryst. Liq. Cryst. 157, 535 (1988)... 

Suitable polymers acrylics, fluorocarbon elastomers, PA6, PA6,6, PBT, PC, PDMS, PEEK, PE, PEI, PEN, PET, PP, PPO, PPS, PS PSF, PVC, SEBS Rnk, J K, High Performance Polymers, William /Andrew, 2008 DeMeuse, M T Kiss, G, High Temperature Polymer Blends, Elsevier, 2014,141-64. 

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. 

Nowhere is that fact more apparent than in blends of liquid crystal polymers (LCPs) with other thermoplastic polymers. It is not the intent of the present chapter to completely review all the work done in the area of blends which contain LCPs. Instead, that will be the focus of Chapter 5. The objective of the present discussion is to make general observations and show how those general phenomena may be applicable to other high temperature polymer blends as well. 

In addition, in blends which contain two-phase separated polymers, adhesion of the two phases is of primary importance for utilizing the blend. Additional research needs to be done to understand the features that provide compatibilization in high temperature polymer blends and the optimum structure of compatibilizing agents for such blends. An example of this has been presented in the case of blends which contain liquid crystal polymers, but the same point is true of all blends based on immiscible high temperature polymers. 

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. 

Characterization methods for high temperature polymer blends... 

Key words high temperature polymer blends, polymer blend degradation, polymer characterization, blending polymers. 

Table 2.2 High temperature polymer blends (HTPBs) arranged in order of increasing continuous use temperature (CUT)... 

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