Semiaromatic Poly(ester\nimide) Copolymers with Alicyclic\nDiamines for Low‑K Properties at a High Frequency of 10–40\nGHz

Polyimide, a highly promising insulating material for\nhigh-frequency\napplications, has recently been the focus of attention. In particular,\npoly(ester imide) (PEI) has garnered significant interest due to its\nremarkably low dissipation factor (<i>D</i><sub>f</sub> <\n0.002 @ 10 GHz) and coefficient of thermal expansion (CTE < 20\nppm K<sup>–1</sup>). However, its relatively high dielectric\nconstant (<i>D</i><sub>k</sub> > 3.0 @ 10 GHz) has posed\na challenge for its use in high-frequency flexible circuit boards.\nThis study presents the approach by introducing a series of alicyclic\ndiamines with varying steric hindrances into the PEI backbone to disrupt\nthe molecular chain stacking and reduce PEI’s <i>D</i><sub>k</sub>. The copolymerization of PEIs with a series of alicyclic\ndiamines, including 1,4-cyclohexane diamine, 1,4-bis(aminomethyl)\ncyclohexane, 4,4′-methylenebis (cyclohexylamine), bis(aminomethyl)norbornane,\ntricyclodecane diamines (TCDDA), has yielded promising results. Notably,\nTCDDA, a value-added chemical derived from a well-known petroleum\nwaste of dicyclopentadiene, has shown significant potential. The results\ndemonstrate that introducing flexible methyl linkers, bicyclic rings,\nbridged rings, or fused rings effectively reduces <i>D</i><sub>k</sub> by increasing the steric hindrance. Importantly, PEIs\ncan maintain low <i>D</i><sub>f</sub> and decent thermal\nand mechanical properties. Among them, PEI with 10 mol % TCDDA demonstrates\nexcellent properties, including a glass transition temperature higher\nthan 380 °C, low CTE of 15.7 ppm K<sup>–1</sup>, high\nultimate tensile strength of 132.4 MPa, high elongation at break of\n8.5%, and low <i>D</i><sub>k</sub> (2.88/2.67) and <i>D</i><sub>f</sub> (0.0019/0.0025) at 10/38 GHz. The steric hindrance\ngenerated by TCDDA achieves an optimal balance in thermal, mechanical,\nand dielectric properties, making it a promising low-dielectric insulating\nlayer for next-generation communication.