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ABSTRACT:Photoconductivity of a polyimide with an alicyclic diamine [the polyimide film prepared from pyromellitic dianhydride and diaminodicyclo hexylmethane Pl(PMDA/DCHM)], and an aromatic polyimide [the polyimide film prepared from pyromellitic dianhydride and oxydianiline PI(PMDA/ODA)] was investigated by the addition of an electron donor, N,N,N' N' -tetramethyl-p-phenylenediamine (TMPD). The addition of the electron donor increased the photocurrent generation of two polyimide films by about three orders of magnitude shifting to longer wavelength by forming intermolecular charge-transfer complex in the ground state of the polyimide films. The polyimide with the alicyclic diamine doped with the electron donor showed a larger enhancement of photocurrent probably due to the existence of only intermolecular charge-transfer complex in the polyimide film, compared to the case of the aromatic polyimide having charge-transfer complex both intra-and intermolecularly. The mechanism of the enhancement of photocurrent by the addition of the electron donor would be first due to photo-absorption by the intermolecular charge-transfer complex formed between the added electron donor and pyromellitic imide unit of the polymer backbone. Under an applied field, its excitation is followed by rapid electron transfer from the donor to pyromellitic imide to produce the radical anion of the polymer and the radical cation of the donor, resulting in the photoconductivity in the bulk polyimide films.

Transient absorption spectra of an aromatic polyimide, PI(PMDA/ODA), and a polyimide with an alicyclic diamine, PI(PMDA/DCHM), were determined in order to investigate the main active species in the photoconductivity of the polyimide films by picosecond pulse radiolysis method. A model compound, N,N′di-n-butylpyromellitic diimide (DBPMDI), was utilized to investigate the active species of pyromellitic imide moieties in solutions of dichloromethane (DCM) and tetrahydrofuran (THF). In the aromatic polyimide film, the radical anions of the electron-accepting pyromellitic imide moieties at 660 and 720 nm and the radical cation of diphenyl ether moieties at 550 nm were determined by electron transfer from the aromatic ring to imide moieties in the polymer backbone. In the polyimide film with the alicyclic diamine, by utilizing the model system, only the radical anions of pyromellitic imide moieties could be assumed at 660 and 720 nm as the active species in the photoconductivity of the polyimide film. These active species, such as radical cations or radical anions, are expected to be the charge carriers of holes or electrons under the applied electric field, and the transport of these charge carriers would result in the photoconductivity of the polyimide films.

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