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 (Dk) 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 cross-linkers 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 °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.