Effect of orientation of phosphinate pendant on the transparency of polyimides: Design and synthesis of phosphinated diamines with a bulky ortho substitution for high-Tg, transparent polyimides

Two phosphinated diamines (1-2), designed to reduce the intermolecular interaction, were successfully prepared. PIs based on (1-2) were prepared in NMP/xylene by high-temperature solution polymerization. PIs with an ortho t-butyl or methoxy substitution display much higher transparency and lower cutoff wavelengths than those with ortho-dialkyl substituted PIs. X-ray single crystal diffractograms of (1-2) show the orientation of the phosphinate pendant inclines toward the unsubstitued 4-aminophenyl linkage, which has conformation with much smaller intramolecular interaction than that of a diethyl substituted diamine. It is thought that the improved transparency of PIs 3-4 is related with the reduced intramolecular interaction. To the best of our knowledge, this is the first study discussing the streic hindrance and orientation of a bulky pendant on the transparency of PI films. In addition to good transparency, PIs 3-4 display flexibility, good organo-solubility, highTg values, good dimensional stability and moderate thermal properties. The combination of these characteristics makes PIs 3-4 attractive for applications in microelectronic and optical devices.