Coloration of Aromatic Polyimides and Electronic Properties of Their Source Materials

Abstract

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...

Organosoluble, thermally stable and low dielectric constant fluorinated polyimides containing 2,4,5-triphenylimidazole moiety in the main chains

“…The 6FDA is the most popular fluorinated dianhydride, the quaternary carbon brings very unique properties for this dianhydride compared with the other fluorinated dianhydrides. [40] As can be seen in Figure 3, the transparency of the polymer increased when the number of CF 3 groups increased. The PIEI-c produced from TFIA and 6FDA showed more transparency than PIEI-a and PIEI-b due to higher content of -CF 3 groups in the polymer backbone.…”

Absorption and Fluorescence Spectra and Thermal Properties of Novel Transparent Polyimides.

“…It is widely accepted that inter-and intramolecular charge transfer [1][2][3] are formed between imide moieties and diamine moieties in the usual aromatic polyimides which result in the coloration of poly im ides [4,5]. In recent years, with the demand of optical communication [6], liquid crystal display devices [7,8], transparent polyimides have become very important.…”

Synthesis and Properties of Cost-Effective Light-Color and Highly Transparent Polyimide Films from Fluorine-Containing Tetralin Dianhydride and Aromatic Diamines

“…Conventional whollyaromatic PI films exhibit a deep color from deepbrown to brown-yellow due to the presence of charge transfer complexes (CTC) in their molecular structures [4]. It has been now well-established that the formation of CTC can be efficiently prohibited by cutting or weakening the conjugation interactions between the electron-donating amino moiety and electron-withdrawing anhydride moiety [5].…”