Photoelectric Properties of Polyimides Based on N,N',N',N''-Substituted Paraphenylenediamine and Dianhydrides of Aromatic Tetracarboxylic Acids

摘要

The photoelectric characteristics and paramagnetic properties of newly synthesized polyimides (PI) are investigated using the electrophotographic method and an ESR signal study. The polyimides based on N,N,N',N'-substituted paraphenylenediamine and dianhydrides, were prepared via the polycondensation. The polymers exhibit excellent solubility in common organic solvents, and have high thermal stability, T_(d10) = 726K in a nitrogen atmosphere and T_g= 413K. The PI films (3 μm thickness) exhibit photoelectric sensitivity in the UV, and visible spectral regions, due to charge transfer interactions between donor and acceptor fragments of the PI chains (formation of a charge transfer complex, CTC). Study of the photogeneration quantum yield field dependence provides the evidence that the photogeneration mechanism is the field assisted thermo-dissociation of radical ion pairs kinetically associated with the excited CTC. The second important mechanism of photogeneration is photostimulation of long-lived stable cation-radicals of the donor PI fragments, representing the hole (majority carriers) captured by deep centers (photostimulated currents). The accumulation of labile cation-radicals in the dark and photo processes leads to the dependence of photoelectric characteristics on the number of charge-discharge cycles of the sample. Paramagnetism of the air stored PI samples is determined by the air oxidized PI donor fragments (stable radicals). PI ESR spectra in solid state and solution are investigated as well as the effect of light excitation on those spectra. Correlations between ESR signal intensity and photoelectric sensitivity are observed.