Polyimides have been used widely in fiber-reinforced composite materials for aerospace components and in thin films for packaging of electronic circuitry. Typically, the synthesis of a material of this type involves the condensation of a diamine with a dianhydride. This synthesis also produces low-molecular-weight byproducts, e.g., water, which can cause problems in processing the polyimide that one seeks to produce. In addition, health concerns about the handling of aromatic diamines - which are often toxic, mutagenic, or carcinogenic - give rise to requirements to use costly engineering controls in the workplace to limit exposure to these compounds.
Over the years, other approaches to the synthesis of polyimides have been developed to address these problems and concerns. Diels-Alder polymerizations - usually involving a bismaleimide and a stable bisdiene, such as a bisfuran - have been investigated extensively. More recent work in this area has focused on the use of reactive diene intermediates, such as o-quinodimethanes generated from benzocyclobutenes. To date, most if not all of these approaches have entailed the use of high reaction temperatures (above 200 °C).
A new Diels-Alder route to the synthesis of polyimides involves the use of ultraviolet light, rather than heat, to effect polymerization. This approach is based upon a well-known class of photochemical reactions - the photoenolization of o-methylphenyl ketones - which can be carried out at room temperature. For example (see Figure 1) the irradiation of an o-methylphenyl ketone (compound 1) with ultraviolet light of wavelengths above 300 nm produces a photoenol (compound 2). This photoenol is unstable, but persists long enough to undergo Diels-Alder reactions with typical dienophiles, such as a maleimide (compound 3).
By utilizing a diketone, such as 2,5-dibenzoyl-p-xylene (compound 5) and a bismalemide (compound 6), this chemistry can be used to make polyimides (see Figure 2). A number of polyimides (represented as compound 7), have been prepared by following this approach. Glass-transition temperatures for these polyimides can be as high as 300 °C. These polyimides exhibit modest stabilities in both air and nitrogen. Temperatures of onset of decomposition, measured by thermal gravimetric analysis (TGA), are as high as 400 °C in air and 450 °C in nitrogen. Higher glass-transition temperatures and temperatures of onset of decomposition can be obtained by conversion of compound 7 into compound 8 through acid-catalyzed dehydration followed by dehydrogenation. Polyimides represented as compound 8 have glass-transition temperatures as high as 330 °C and temperatures of onset of decomposition as high as 550 °C in air and 525 °C in nitrogen.
This chemistry has been demonstrated in solution (benzene or cyclohexanone), but should be easily adaptable to achieve solid-state (solvent-free) curing. Such adaptation would make the present approach particularly suitable for thin-film applications (e.g., coatings, electronics packaging, and photonic/optical materials). This ultraviolet-curing approach could offer several advantages over other approaches to the preparation of polyimides. Ultraviolet-cured films should undergo less shrinkage during cure than do those films that are cured at high temperatures. This approach would be useful for the curing of polyimides that contain such thermally sensitive groups or additives as nonlinear optical materials. In addition, this approach does not entail some of the disadvantages of condensation-chemistry-based approaches; namely, the formation of volatiles during cure, health risks associated with the use of diamines, and poor solution stability.
This work was done by Michael A. Meador and Mary Ann B. Meador of Lewis Research Center, Lesley L. Williams of Spelman College, and Jeremy R. Jones of NASA Center for High Performance Polymers and Composites, Clark Atlanta University.
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