Polyimides unique properties

Much attention has been paid to the synthesis of fluorine-containing condensation polymers because of their unique properties (43) and different classes of polymers including polyethers, polyesters, polycarbonates, polyamides, polyurethanes, polyimides, polybenzimidazoles, and epoxy prepolymers containing pendent or backbone-incorporated bis-trifluoromethyl groups have been developed. These polymers exhibit promise as film formers, gas separation membranes, seals, soluble polymers, coatings, adhesives, and in other high temperature applications (103,104). Such polymers show increased solubility, glass-transition temperature, flame resistance, thermal stability, oxidation and environmental stability, decreased color, crystallinity, dielectric constant, and water absorption. 

Since the first commercialization of Kapton polyimide nearly three decades ago, an impressive number of new polyimides of unique properties have been introduced for various industrial and consumer product applications. Commercial and semi-commercial production of numerous new monomers have made it possible to explore seemingly infinite varieties of structurally different polyimides and copolyimides. As a result we seem to have a more finely tuned understanding of structure-property relationship necessary for a specific application. Research and development activities in new polyimides seem to be intensifying rather than reaching its maturity. In this article, recent advances in polyimides are reviewed in the area of scientific activity as well as commercial developments. 

The major thermosetting plastics, in order of decreasing market volume, are polyurethanes, phenol-formaldehyde, urea-formaldehyde, and polyesters. More specialized thermosets include melamine-formaldehyde, furans, vinyl esters, aUyls, epoxy resins, silicones, and polyimides. While they may sometimes compete with each other and with thermoplastics, for the most part, each of them has unique properties and fills unique markets and applications. 

Heat-resistant polyaromatic adhesives also have shown promising low-temperature properties. The shear strength of a polybenzimidazole adhesive on stainless-steel substrates is 5,690 Ib/in at a test temperature of 23°F, and polyimide adhesives have exhibited shear strength of 4,100 Ib/in at -320°F. These unique properties show the applicability of polyaromatic adhesives on structures seeing both very high and low temperatures. 

Considerable attention has been devoted to the preparation of fluorine-containing polymers because of their unique properties and high temperature performance (I). Recently we reported the preparation and characterization of novel fluorine-containing polyimides and polyethers which exhibit low moisture absorption and low dielectric constants (2, 3). Fluorinated polyimides absorb 1 wt% water and have dielectric constants of about 2.8 (all dielectric constants reported in this paper were measured at 10 kHz) vtdiereas their non-fluorinated analogs absorb as much as 3 wt% water and have dielectric constants of about 3.2. Fluorinated polyarylethers, which are free of polar groups such as ketones, imides and sulfones, absorb as little as 0.1 wt% water and have dielectric constants less than 2.8. 

Relatively few processible polyimides, particularly at a reasonable cost and iu rehable supply, are available commercially. Users of polyimides may have to produce iutractable polyimides by themselves in situ according to methods discussed earlier, or synthesize polyimides of unique compositions iu order to meet property requirements such as thermal and thermoxidative stabilities, mechanical and electrical properties, physical properties such as glass-transition temperature, crystalline melting temperature, density, solubility, optical properties, etc. It is, therefore, essential to thoroughly understand the stmcture—property relationships of polyimide systems, and excellent review articles are available (1—5,92). 

Because of their unique and highly desirable properties, polyimides are used for many applications... 

Cyclotrimerization of polyfunctional aryl acetylenes offers a unique route to a class of highly aromatic polymers of potential value to the micro-electronics industry. These polymers have high thermal stability and improved melt planarization as well as decreased water absorption and dielectric constant, relative to polyimides. Copolymerization of two or more monomers is often necessary to achieve the proper combination of polymer properties. Use of this type of condensation polymerization reaction with monomers of different reactivity can lead to a heterogeneous polymer. Accordingly, the relative rates of cyclotrimerization of six para-substituted aryl acetylenes were determined. These relative rates were found to closely follow both the Hammett values and the spectroscopic constants A h and AfiCp for the para substituents. With this information, production of such heterogeneous materials can be either avoided or controlled. 

The second method of surface modification permits the formation of a composite particle, the core of which is composed of polymer (UHMWPE or polyimide) and the surface of which is coated with titanium carbide which is hard and abrasion resistant. The composite particles can be incorporated into any suitable matrix resulting in improved abrasion resistance, lowered fiiction, higher compressive strength, improved creep resistance, etc. This new product is a unique form of raw material which has the potential to improve the properties of many products. 

Du Pom, a leader in reverse osmosis technology built aronnd a unique class of tailored aromeik polyamides, was also an early leeder in the gas separation field.27,1 14,16 Molecuiariy engineered arometic polyimides were found by Du Pont to provide extraordinarily good flux and selectivity properties For hydrogen separations.27 Posttreataiem processes for these membranes were not reported. 

Among various strategies that have been used to synthesize polyimides with lower dielectric constants, the most common approach is to incorporate fluorene, in the form of trifluoromethyl groups, into diamine and dianhydride units that minimize polarizability and increase the free volume [46]. It is well-known that fluorene atom has unique characteristics such as high electronegativity and low electric polarity. These properties give fluorinated polymers (e.g., poly[tetrafluoroethylene]) attractive features such as low water uptake, water and oil repellency, low permittivity, low refractive indices, resistance to wear and abrasion, and thermal and chemical stability. Fluorination is also known to enhance solubility and optical transparency and to lower the moisture absorption of polyimides. Therefore, it is expected that fluorinated polyimides will be widely applied in the electro-optical and semiconductor industries. The polymer series studied was essentially limited mainly to 6F dianhydride because it proved to be the only dianhydride with which many of the fluorinated diamines would form polymer films suitable for physical characterization. 

To summarize, the data generated in this study indicate that polyimide based on PMDA/ODA has outstanding stability up to 425 (under inert conditions) and shows little loss in electrical and mechanical properties even after heating for several hours. There are subtle changes in composition at these temperatures associated with evolution of small amounts of CO2 however, these changes do not appear critical. The exceptional mechanicals with an elongation at break in excess of 50% make this material unique... 

Many high-performance polymer fibres are used in filter media to meet various specific requirements in diverse filtration applications. Filters made from fluoropol-ymer (Polytetrafluoroethylene (PTFE), Polyvinylidene fluoride (PVDF), and Per-fluoroalkoxy alkane (PFA)) fibres, and membranes have inherent, chemical-resistant, and flame-retardant properties, and they are widely employed to filter aggressive chemicals and acids in the manufacture of wafers and microchips in the microelectronics industry. Ethylene ChloroTriFluoroEthylene (E-CTFE) melt blown fabrics have a unique ability to coalesce difficult liquids and can withstand the piranha effect in filtering ozone enriched ultrapure water. Polyphenylene sulfide (PPS) fibres are also chemical resistant, stand high temperature, and are suitable for making baghouse filters. Eilter media made from other high-performance polymer fibres, such as polyamide-imide, polyetherimide (PEI), Polyimide P84 fibre,polyetheretherke-tone, and liquid crystal polymers also appear in the filtration and separation market. 

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