ABSTRACT:The relationships between the color intensities of polyimide films and the electronic properties of their source materials-aromatic diamines and aromatic tetracarboxylic dianhydrides-are discussed. The arrangement of the diamine moieties in order of color intensity of polyimides shows fairly good agreement with the order of the electron-donating properties of the diamines estimated from 15 N NMR chemical shifts (bN). On the other hand, the arrangement of the dianhydrides moieties in order of color intensity of polyimides agrees with the order of the electron-accepting properties of the dianhydrides estimated from experimental and calculated electron affinity (EA) although systematic inconsistencies are observed for the dianhydrides having-CF 3 groups and a benzophenone carbonyl group. These results are consistent with the formation of charge transfer complex (CTC) and indicate that the electron-donating properties of diamines and electron-accepting properties of dianhydrides are retained to a significant extent even in polyimide molecular chains. KEY WORDSPolyimide / Coloration / Charge Transfer Complex / Dianhydride / Diamine / Nuclear Megnetic Resonance Chemical Shift / MNDO-PM3 / Optical transparency of polyimide films is of special importance in some applications such as flexible solar radiation protectors, 1 orientation films in liquid crystal display devices, 2 optical waveguides for communication interconnects, 3 and optical half-waveplates for planar lightwave circuits. 4 However, most of the conventional polyimide films always show considerable coloration ranging from pale yellow to deep brown. Rogers 5 first reported that optically transparent and colorless polyimides can be synthesized from a dianhydride and a diamine that have hexafluoroisopropyridene (-C(CF 3 ) 2-) groups. We have reported that the fluorinated polyimides derived from 2,2' -bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFDB) exhibit excellent properties needed for optical applications. 6 In particular, the polyimide derived from 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and TFDB show no color, high transparency in the visible region, and low optical transmission losses in the near-infrared region as well as low dielectric constants, low refractive indices, and low water absorption. The idea that fluorination of dianhydride and diamine moieties give colorless polyimides are widely accepted. On the other hand, we have recently reported the first synthesis of perfluorinated polyimides that have very high optical transparency over the entire range of optical communication wavelengths (1.0--1.7 µm) and high glass transition temperatures over 300°C. 7 • 8 Despite the high fluorination of the diamine and dianhydride, the perfluorinated polyimides show considerable coloration ranging from orange to brown. This fact indicates that the fluorination of source materials does not necessarily lead to colorless polyimides. If the coloration of polyimide films can be predicted from the properties of their source materia...

Optical loss and refractive indices of fluorinated polyimides and copolyimides are studied for application to optoelectronic components. These polyimides and copolyimides are prepared from 2,2-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), pyromellitic dianhydride (PMDA), and 2,2'bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFDB). The polyimide 6FDA/TFDB prepared from 6FDA and TFDB cured at 350 °C has an optical loss of 0.7 dB/cm at 0.63 nm, and this is the lowest of all polyimides cured at high temperature. Direct measurement of the optical loss of a polyimide at 1.3 pm was achieved for the first time by using a 6FDA/TFDB block with unequal length, width, and thickness. The optical loss was also low, 0.3 dB/cm. 6FDA/TFDB, PMDA/TFDB, and their copolyimides cured on a silicon substrate at 350 0 C have two different refractive indices, an in-plane refractive index («te) and an out-of-plane refractive index («tm). The wte of the copolyimide at 1.3 pm can be controlled between 1.523 (6FDA/TFDB) and 1.614 (PMDA/TFDB) by changing the 6FDA/TFDB content. It decreases monotonically with increasing 6FDA/ TFDB content. The Rtmi on the other hand, can be controlled between 1.514 (6FDA/TFDB) and 1.521 (copolyimides with 60 mol % 6FDA/TFDB), but it has a maximum value at a 6FDA/TFDB content of 60 mol %. These polyimides and copolyimides prepared on a silicon substrate have birefringences between 0.008 (6FDA/TFDB) and 0.123 (PMDA/TFDB). Low optical loss and precise refractive index control are very important characteristics for optoelectronic components, especially at the telecommunication wavelength of 1.3 pm.

Polymers are expected to be used as media for transmitting near-infrared light in such optical communication applications as the waveguide in optoelectronic integrated circuits (OEIC) and in multichip interconnections.1-2 The current manufacturing process for ICs and multichip modules includes soldering at 260 °C and short-term processes at temperatures of up to 400 °C. Waveguide polymeric materials should therefore have high thermal stability-that is, a high glass transition temperature (Tg) and a high polymer decomposition temperature-as well as high transparency at the wavelengths of optical communication (WOC), 1.0-1.7 #tm.Conventional waveguide polymeric materials, such as poly(methyl methacrylate) (PMMA) or polycarbonates (PC), do not have such thermal stability. In addition, their optical losses at the WOC are much higher than in the visible region (0.4-0.8 pm) because carbon-hydrogen bonds (C-H bonds) harmonically absorb near-infrared radiation.

A series of new polyimides containing fluorinated alkoxy side chains are prepared from novel fluorinated alkoxy diamines. The dieletric constant at 1 kHz in the fluorinated polyimides decreases from 3.3 to 2.6 as fluorine content increases. The refractive index also changes from 1.58 to 1.48, dependent on the fluorine content. In addition, the fluorinated polyimides exhibit lower water absorption than the reference polyimides prepared from m‐phenylenediamine.

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