Polyimides with reduced dielectric anisotropy

Ordered polyimides have been the subject of considerable interest for thin film applications due to their excellent balance of final properties, however, these polyimides have been shown to have an exceptionally large dielectric anisotropy. Two approaches were examined as a means of obtaining materials with lowered dielectric anisotropies. ATR spectroscopy was used to measure in-plane and out-of-plane refractive indices from which the directional dielectric constant was estimated. In one case, monomers based on hexafluoroisopropylidine were introduced into PMDA at levels /spl les/20 mole%. This afforded some reduction in the in-plane dielectric constant. A more promising approach involved the use of polyimides derived from 2,2'-bis(trifluoromethyl)benzidine (BTFB). The PMDA-BTFB polyimide displayed the best combination of properties, including an estimated in-plane dielectric constant of 2.9 and an out-of-plane dielectric constant of 2.6 at 1 MHz.<
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