Polyimides: Versatile Specialty Polymers

Abstract

Polyimide specialty polymers are finding growing usage in applications such as high temperature, thermally stable insulators and dielectrics, coatings, adhesives, and high performance composites. To add to this already rich field of polyimide functions, we wish to review recent research from our laboratory which features the photoconductive and electrochemical behavior of polyimides, areas which have not as yet been widely explored. The first area of research, which illustrates polyimides as electrochemically active materials, is the recent work of S. Mazur and S. Reich. It deals with the specially controlled deposition of very thin silver layers, or certain other metal layers, into a polyimide film prepared from 4,4′-oxydianiline and pyromellitic dianhydride. This is achieved by coating the polyimide film, ~10μm thick, onto a cathode. The cathode is placed in an electrolyte solution containing silver ion. A potential is applied between the solution and the cathode, resulting in the diffusion of silver ions towards the cathode. At steady state, when the flux of electrons (moving away from the cathode) equals that of the silver ions, a well defined, particulate, dense silver layer, ~0.1μm thick, is deposited within the polyimide film. It is shown that the position and thickness of the metal layer as well as its geometrical shape can be controlled very precisely. The second area of research, photoconductivity in polyimides, is the work of S. C. Freilich. It is found that addition of electron donors, such as dimethylaniline, to Kapton® polyimide film results in an enhancement of the photocurrent generated by as much as five orders of magnitude when compared to the undoped polymer. The mechanism of the enhancement is shown to be the result of radiation absorption by a charge transfer complex formed between the added electron donor and the imide portion of the polymer backbone. Excitation is followed by rapid and complete electron transfer from the donor to pyromellitimide to yield the radical anion of the polymer and the radical cation of the donor. These ion pairs are demonstrated to be the carriers of the photocurrent. They also undergo rapid back electron transfer, and the geminate recombination process is consistent with an Onsager model analysis.