Exploring the Cross-Linking Effect on Decreasing the Dielectric Constant and Dissipation Factor of Poly(ester imide)s at a High Frequency of 10-40 GHz

Polyimides possess excellent thermal-mechanical stability and solution processability and can be widely used in the electronic industry. In particular, they can achieve a decent insulating property at a high frequency spanning the range 10-40 GHz via chemical structural design, so it is important to investigate the structure-dielectric relationship of polyimides. However, it is challenging to reach a low dielectric constant (D-k) as well as a low dissipation factor (Df), simultaneously, owing to the trade-off between the free volume and rigidity of polyimides. Among them, poly(ester imide)s (PEIs) are well-known for their ultralow Df of 0.003 at 10 GHz; however, their high Dk of 3.2 confines the insulating capability of PEIs. To overcome this difficulty, a cross-linking strategy is applied to PEIs to decrease their Dk. In this research, the linear PEI is copolymerized with a series of aromatic crosslinkers comprising melamine, tris( 4-aminophenyl) amine, 1,3,5-tris(4-aminophenyl)benzene, 1,3,5-tris(4-aminophenoxy)benzene, benzene-1,3,5-triyl tris(4-aminobenzoate), and tris(4-aminophenyl)benzene-1,3,5-tricarboxylate. The functional groups of aromatic triamine cross-linkers including ether and ester groups strike the best balance between the polymer's free volume and rigidity. The experimental results revealed that they maintain decent thermal stability with a glass transition point over 350 degrees C and enhance mechanical durability with strength/elongation at a break of 164 MPa and 22%. Notably, the cross-linked PEIs can reduce the Dk to below 3.0 and maintain the Df at approximately 0.003 at a high frequency spanning the range of 10-40 GHz. This study provides an effective approach to control the dielectric properties of polyimides through manipulating the structure rigidity and free volume by using cross-linking.