High Thermal and Low Dielectric Polyimides Based on Ether-Linked Xanthone-Based Diamines Having Bulky Trifluoromethyl Groups

Abstract

By controlling/balancing PI backbone rigidity/linearity and polarizability, free volumes can be affected, which leads to a decrease in dielectric properties. In this regard, ether linkages and bulky trifluoromethyl (−CF 3 ) groups containing monomers are good strategies for reducing the dielectric properties. Herein, we develop two xanthone (Xn)-based diamine monomers, Xn-OPh-NH 2 and Xn-OPhF-NH 2 by introducing ether-linkage phenoxy (−OPh) and trifluoromethyl phenoxy (−OPh−CF 3 ) groups on the 3,6-positions of the Xn-core, respectively. The Xn-OPh-NH 2 and Xn-OPhF-NH 2 diamine monomers are synthesized through dehydrocyclization, nucleophilic substitution, and Pd/C-reduction reactions. Additionally, all the reactions are purified through simple recrystallization methods with excellent overall yields. Thereafter, two Xn-series PIs are prepared using a conventional two-step method i.e., ring-opening polycondensation, followed by thermal imidization. The two PI-XnH-/PI-XnF-series PIs consist of diamines Xn-OPh-NH 2 /Xn-OPhF-NH 2 and commercial dianhydrides, such as 4,4′-(4,4′isopropylidenediphenoxy)diphthalic anhydride (BPADA), 4,4-oxydiphthalic anhydride (ODPA), 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA), and 3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride (DSDA). Additionally, a series of PI-Xn-based PIs are also prepared for the comparative study, consisting of 2,7diamino-9H-xanthen-9-one (Xn-NH 2 ) diamine and commercial dianhydrides. Due to rigid/planar aromatic Xn-core, the reference PI-Xn-series PI films enhance the intra/intermolecular interactions, resulting in excellent thermal stabilities and decent optical/ mechanical properties. After introducing ether-linked −OPh/−OPh−CF 3 groups, the molecular rigidity/coplanarity can be adjusted, thus improving the PI molecular backbone flexibility and molar volume/free volume. As a result, the PI-XnH-/PI-XnF-series PIs films display excellent thermal stabilities, optical transparencies, mechanical properties, and low dielectric properties compared with the reference PI-Xn-series PI films. The dielectric properties are in the order of PI-Xn-series > PI-XnH-series > PI-XnF-series. Among the three Xn-series PIs, PI-XnF-series PI films achieve low dielectric constants (D k ) values within 2.82−3.16 and low dissipation factors (D f ) values within 0.0055−0.0141 at 10 GHz frequency, due to a more flexible PI molecular backbone nature with the presence of low-polarized electron-withdrawing −CF 3 groups.