Synergistic Enhancement of Mechanical and Dielectric Properties in Transparent Polyimides by Regulating Hydrogen Bonding and Microbranched Cross-Linking Structure

The development of polyimide (PI) films with excellent mechanical properties and low dielectric constants is crucial for flexible optoelectronic devices and printed circuit boards. Here, a method to improve the mechanical properties and decrease the dielectric constant of PI films is reported by introducing a synergistic effect between hydrogen bonding (H-bonding) and microbranched cross-linking structures. A triamine monomer (4,4′,4″-(1 H -imidazole-2,4,5-triyl) trianiline, DTI) acting as a hydrogen bond donor was designed and synthesized. It was then in situ polymerized with commercial 3,3′,4,4′-biphenyl tetracarboxylic dianhydride (BPDA), 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 4,4′-oxidianiline (ODA), and 1,4-phenylenediamine (PDA) to obtain four different PI films, BPDA/ODA/DTI, BPDA/PDA/DTI, 6FDA/ODA/DTI, and 6FDA/PDA/DTI. With the introduction of DTI, the corresponding PI films exhibited high modulus and low dielectric constant and coefficient of thermal expansion (CTE). When the DTI content was optimized, several high-performance PI films suitable for electronic applications were achieved. At 10 MHz, the dielectric constant of the 6FDA/PDA series films decreased from 3.39 to 2.89, while the modulus increased from 3.41 to 4.60 GPa. The CTE of the BPDA/PDA series films was reduced from 8.61 to 0.27, a reduction of approximately 97%. Structural characterization, density functional theory (DFT), and molecular dynamics (MD) simulations revealed the synergistic and competitive relationships between hydrogen bonds and branched cross-links within the PI molecular chains. This approach offers a strategy to overcome the performance trade-off in polyimide films.