Semiaromatic Poly(ester imide) Copolymers with Alicyclic Diamines for Low-K Properties at a High Frequency of 10–40 GHz

Polyimide, a highly promising insulating material for high-frequency applications, has recently been the focus of attention. In particular, poly(ester imide) (PEI) has garnered significant interest due to its remarkably low dissipation factor (Df < 0.002 @ 10 GHz) and coefficient of thermal expansion (CTE < 20 ppm K–1). However, its relatively high dielectric constant (Dk > 3.0 @ 10 GHz) has posed a challenge for its use in high-frequency flexible circuit boards. This study presents the approach by introducing a series of alicyclic diamines with varying steric hindrances into the PEI backbone to disrupt the molecular chain stacking and reduce PEI’s Dk. The copolymerization of PEIs with a series of alicyclic diamines, including 1,4-cyclohexane diamine, 1,4-bis(aminomethyl) cyclohexane, 4,4′-methylenebis (cyclohexylamine), bis(aminomethyl)norbornane, tricyclodecane diamines (TCDDA), has yielded promising results. Notably, TCDDA, a value-added chemical derived from a well-known petroleum waste of dicyclopentadiene, has shown significant potential. The results demonstrate that introducing flexible methyl linkers, bicyclic rings, bridged rings, or fused rings effectively reduces Dk by increasing the steric hindrance. Importantly, PEIs can maintain low Df and decent thermal and mechanical properties. Among them, PEI with 10 mol % TCDDA demonstrates excellent properties, including a glass transition temperature higher than 380 °C, low CTE of 15.7 ppm K–1, high ultimate tensile strength of 132.4 MPa, high elongation at break of 8.5%, and low Dk (2.88/2.67) and Df (0.0019/0.0025) at 10/38 GHz. The steric hindrance generated by TCDDA achieves an optimal balance in thermal, mechanical, and dielectric properties, making it a promising low-dielectric insulating layer for next-generation communication.