Fluorine-Free ThermoplasticHigh-Frequency Low DielectricPoly(ether imide)s for Flexible Copper Clad Laminates

To meet the requirements of high-frequency signal transmission in 5G or 6G applications, the development of high-performance functional polyimide possessing a low dielectric constant (<i>D</i>_k) and low dielectric loss (<i>D</i>_f) at high frequencies, along with melting processing capabilities and good adhesion to copper foil, is both urgent and highly challenging. In this work, three diamines containing <i>tert</i>-butyl moieties and ether bonds but different substitution positions of amine groups were designed and synthesized, and high-performance poly­(ether imide)­s (PEIs; TBPI, TTBPI, ATBPI, and PTBPI) were obtained from their polycondensation with 4,4′-(4,4′-isopropylidenediphenoxy) diphthalic anhydride (BPADA). The introduction of flexible groups such as ether bond and bisphenol A structure has the potential to improve the flexibility of the molecular chains and reduce the density of the imide ring, the molecular polarity, and the glass transition temperature of the PEIs. The incorporation of the unique spatial structure of the nonpolar <i>tert</i>-butyl side group may help to increase the free volume and exceptional hydrophobicity of the PEIs. Both give the as-prepared fluorine-free PEIs a range of beneficial characteristics, such as melting processing property, excellent solubility, low water absorption (0.42, 0.35, and 0.50% at <i>P</i>/<i>P</i>₀ = 0.50 (50% relatively humidity (RH)), respectively), high-frequency low <i>D</i>_k (2.83, 2.84, and 2.82 at 10 GHz and 50% RH), low <i>D</i>_f (0.00339, 0.00257, and 0.00366 at 10 GHz and 50% RH), and good peel strength with copper foil via hot-pressing molding. The successful development of such functional PEIs enables the preparation of truly adhesive-free flexible copper clad laminates (FCCLs) via hot-pressing molding. This significantly simplifies the preparation process and structure of high-performance FCCLs and enhances the reliability of electronic circuits, thereby presenting attractive application prospects for high-frequency mobile communication.