Imides, polymer performance

Scola et al. [67] studied the kinetics of the MW cure of a phenylethyl-terminated imide polymer model compound and an oligomer using a variable frequency MW source and found that the activation energy of the MW cures were 68% and 51% of the thermal cure respectively. It should be noted that the reactions were performed in the liquid phase in the absence of solvent. 

Although epoxy adhesives represent a relatively small proportion of the total adhesives market, there are perhaps more varieties of formulations available from more sources than for any other adhesive class. For many years, they have dominated the high-performance adhesives market, but the desire for increased service temperatures has led to the development of new adhesives based on imide polymers (Polyimides and Bismaleimides). 

PAIs are unique materials that have elements of PA (aka nylon) chemistry, as well as aromatic polyimide chemistry. They have exceptional mechanical, chemical, and thermal properties and are considered by some to be at the top of the thermoplastic performance chart. They have high strength, exceptional high heat capability, and broad chemical resistance. Polyamide-imide polymers are melt processible and can be processed into a wide variety of forms—from injection- or compression-molded parts and ingots— to coatings, films, fibers, and adhesives. PAI is often lower in cost than TPI. 

Polymers based on trimellitic anhydride are widely used in premium electromagnetic wire enamels requiring high temperature performance. Several types of trimellitic anhydride-derived polymers are used as wire enamels poly(amide—imide)s (133), poly(ester—imide)s (134), and poly(amide—imide— ester)s (135). Excellent performance characteristics are imparted by trimellitic anhydride-based polymers for wire enamel requirements of flexibiUty, snap, burnout, scrap resistance, heat shock, and dielectric strength. 

Polyether Imides. Polyether imides (PEIs) are amorphous, high performance thermoplastic polymers that have been in use since 1982. The first commercial polyether imides were the Ultem series developed by the General Electric Co. The first, Ultem 1000 [61128-24-3] is prepared from phthahc anhydride, bisphenol A, and meta-phenylenediamine and has the following stmcture ... 

Post-Curing. Whenever production techniques or economics permit, it is recommended that compounds based on terpolymer grades be post-cured. Relatively short press cures can be continued with an oven cure in order to develop full physical properties and maximum resistance to compression set. Various combinations of time and temperature may be used, but a cycle of 4 h at 175°C is the most common. The post-cure increases modulus, gready improves compresson set performance, and stabilizes the initial stress/strain properties, as chemically the polymer goes from an amide formation to a more stable imide formation. Peroxide-cured dipolymer compounds need not be post-cured. 

The polyetherimides are competitive not only with other high-performance polymers such as the polysulphones and polyketones but also with polyphenylene sulphides, polyarylates, polyamide-imides and the polycarbonates. 

The rates of certain reactions of polymers have been reported to be enhanced by MW under homogeneous conditions at atmospheric pressure. Lewis et al. [64] performed kinetic studies on the imidization of the polymer BDTA-DDS polyamic acid 46 in N-methylpyrrolidone (NMP) giving the polymer 47 (Scheme 4.24) and showed that the apparent activation energy was reduced from 105 kj mol-1 under conventional heating to 55 kj mol-1 under MW heating. Rate enhancements (kMw/kthermai)... 

As part of an effort to develop high-performance, high-temperature-resistant polymers for microelectronics applications, we also recently described a series of both partially fluorinated and nonfluorinated poly(aryl ether ketone)s containing amide, amide-imide, cyano oxadizole, or pyridazine groups and characterized their thermal and electrical properties.11... 

Sulfonated poly(arylene ether)s have shown promise for durability in fuel cell systems, while poly-(styrene)- and poly(imide)-based systems serve as model systems for studying structure-relationship properties in PEMs because their questionable oxidative or hydrolytic stability limits their potential application in real fuel cell systems. Sulfonated high performance polymer backbones, such as poly(phe-nylquinoxaline), poly(phthalazinone ether ketone)s, polybenzimidazole, and other aromatic or heteroaromatic systems, have many of the advantages of poly-(imides) and poly(arylene ether sulfone)s and may offer another route to advanced PEMs. These high performance backbones would increase the hydrated Tg of PEMs while not being as hydrolytically sensitive as poly(imides). The synthetic schemes for these more exotic macromolecules are not as well-known, but the interest in novel PEMs will surely spur developments in this area. 

Besides in the liquid phase, some polyreactions are also performed in the solid state, for example, the polymerization of acrylamide or trioxane (see Example 3-24). The so-called post condensation, for example, in the case of polyesters (see Example 4-3), also proceeds in the solid phase. Finally, ring closure reactions on polymers with reactive heterocyclic rings in the main chain (e.g., poly-imides, see Example 4-20) are also performed in the solid state. 

The ethynyl terminated imide oligomers are very attractive because their cured polymers are thermally stable (131). However, improvements are required in processability. An interesting approach to this problem was the synthesis and use of ethynyl-terminated isoimide (132). If the cyclodehydration of the amide acid intermediate is performed chemically with dicyclohexylcarbodiimide, isoimide is formed in almost quantitative yield. (Fig. 46). It is claimed that the isoimide provides better flow and solubility compared with the corresponding imide. At elevated temperatures, during cure, the isoimide rearranges into the... 

In most of the previous work with polyimide fibers, the fibers were spun from poly(amic acid) precursors, which were thermally imidized in the fiber form. However, high degrees of imidization were not achieved. Thus, tensile properties of these polymers were not as good as those of high-performance fibers. Work in our laboratories has shown that when the fibers are spun directly from preimidized polymers, it is possible to achieve tensile properties that are as good or even better than those of poly(p-phenyleneterephthalamide) (PPTA or Kevlar ) fibers. For example, fibers have been prepared from m-cresol solutions of BPDA-PFMB using a dry-jet wet-spinning method. The as-spun fibers were then extensively drawn and annealed above 400°C to achieve excellent mechanical properties. 

The goal of this research was to shed further light on the solution chemistry of amic acid and imide materials using F-NMR. Several immediate advantages of this technique were obvious in advance. Studies could be performed in any solvent desired, with no spectral interference from the solvents. Industry has demonstrated a preference for W-methylpyrrolidinone (NMP) as a solvent for PAA and, in the case of soluble Pis, for the final polymer. Utilization of an external lock solvent and fluorinated standards would allow the investigations to be carried out in an uncontaminated NMP setting. 

A great deal of literature attention has been devoted to polymers in this section as thermally stable polymers (B-80MI11101). While some very elegant syntheses have been conducted, the resulting polymers have been, for the most part, quite intractable materials not conducive to extensive screening for a variety of applications. Thus, aside from their bulk thermal performance, little else besides the conditions of synthesis is known about most of the polymers shown. Three notable exceptions about which considerable characterization and product information are available are poly(imides), poly(benzimidazoles) and poly(quinoxalines), and a short discussion is included concerning properties and applications of these polymers. 

POLYAMIDE-IMIDE RESINS. An injection-moldable, high-performance engineering thermoplastic, polyamide-imide is the condensation polymer of tnmellitic anhydride and various aromatic diamines with the general structure ... 

As to the polymers, the most important of them - considering production figures - are very probably the poly(ether-imide)s (PEIs), marketed under the trade name Ultem. Neat PEI resins are amorphous, soluble polymers that show Tg values around 220 °C. They can be processed from the melt by conventional means, and offer a price-performance balance that enables them to compete successfully in the market of engineering thermoplastics. 

Yoon TH, McGrath JE (1992) Enhanced adhesive performance of thermoplastic po-ly(imide siloxane) segmented copolymer with peek-graphite composites by gas plasma treatment. High Perform Polym 4(4) 203... 

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