Monomer ratio-controlled polyimides with enhanced dielectric properties and thermal stabilities through crosslinking network

Polyimides (PI) are considered as one of the most used materials for flexible printed circuit board substrates because of their excellent thermal stabilities, outstanding mechanical properties, and great dielectric properties. Dielectric constants (Dk) of common commercial PIs, however, are still above 3.0 (at 10 GHz), which is still substandard for high-frequency applications. In this study, we develop thermally stable PIs with enhanced dielectric properties by structural design and crosslinking. Stiff aromatic 2,2 '-bis(trifluoromethyl)benzidine (TFMB) and soft aliphatic dimer diamines (DDA, priamine), including priamine 1074 (DDA-1) and priamine 1071 (DDA-2), are used as the diamines, and cyclobutanetetracarboxylic dianhydride (CBDA) is used as the dianhydride. By adjusting the ratio of the diamines, a series of PIs (PI-1-PI-5) with high thermal stabilities and low-Dk values are synthesized. Among these PIs, PI-4 synthesized by diamines with the ratio of TFMB:DDA-1:DDA-2 = 85:10:5, possesses relatively good performances. Therefore, PI-4 is further chosen to be thermally crosslinked. By changing the contents of the crosslinker, triallyl isocyanurate (TAIC), crosslinked PIs (CPIs) are synthesized. The resulting CPIs have a significant improvement in thermal properties, dielectric properties, and mechanical strengths. Especially, for the CPI-4-30, in which 30% of TAIC is used, the glass transition temperature (Tg) increases by 20 degrees C, the Dk decreases from 2.62 to 2.41, and the ultimate tensile strength boosts from 91.65 to 115.16 MPa. The CPIs with low Dk and high Tg values may have great potential as substrate materials for broad applications, such as microelectronics and integrated circuit packages. image